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Review Electrochemistry and Photoredox Catalysis: A Comparative Evaluation in Organic Synthesis
Rik H. Verschueren and Wim M. De Borggraeve * Department of Chemistry, Molecular Design and Synthesis, KU Leuven, Celestijnenlaan 200F, box 2404, 3001 Leuven, Belgium; [email protected] * Correspondence: [email protected]; Tel.: +32-16-32-7693
Received: 30 March 2019; Accepted: 23 May 2019; Published: 5 June 2019
Abstract: This review provides an overview of synthetic transformations that have been performed by both electro- and photoredox catalysis. Both toolboxes are evaluated and compared in their ability to enable said transformations. Analogies and distinctions are formulated to obtain a better understanding in both research areas. This knowledge can be used to conceptualize new methodological strategies for either of both approaches starting from the other. It was attempted to extract key components that can be used as guidelines to reﬁne, complement and innovate these two disciplines of organic synthesis.
Keywords: electrosynthesis; electrocatalysis; photocatalysis; photochemistry; electron transfer; redox catalysis; radical chemistry; organic synthesis; green chemistry
1. Introduction Both electrochemistry as well as photoredox catalysis have gone through a recent renaissance, bringing forth a whole range of both improved and new transformations previously thought impossible. In their growth, inspiration was found in older established radical chemistry, as well as from cross-pollination between the two toolboxes. In scientiﬁc discussion, photoredox catalysis and electrochemistry are often mentioned alongside each other. Nonetheless, no review has attempted a comparative evaluation of both ﬁelds in organic synthesis. Both research areas use electrons as reagents to generate open-shell radical intermediates. Because of the similar modes of action, many transformations have been translated from electrochemical to photoredox methodology and vice versa. This reciprocity is very interesting as it provides the ideal study ground for analysis and comparison of both ﬁelds. In this review, we look where those research domains interlock and where they diﬀer. Analogies will be drawn between both methods to obtain a better understanding in both areas of research. In an eﬀort to do so, this review will be selective. Transformations were selected which had reports of great similarity to ease comparison. This comparative evaluation can serve as a guidance tutorial review and as an introduction to the ﬁelds of electrochemistry and photoredox catalysis in organic synthesis. More rigorous reviews of the literature can be found in the following accounts and references therein [1–12]. The present study serves to provide a summary of synthetic transformations that can be performed by electrochemistry as well as photoredox catalysis to gain a better understanding of both. Key features are extracted that can be used as guidelines to formulate new ideas. In the following section we will display 10 synthetic transformations that have been performed by both electro- and photoredox catalysis and one photoelectrochemical method integrating these two. Schematic presentations of the conditions, the respective mechanisms (as proposed in literature) and a selected scope will assist in a side by side comparison of the two synthetic methodologies for the same transformation.
Molecules 2019, 24, 2122; doi:10.3390/molecules24112122 www.mdpi.com/journal/molecules Molecules 2019, 24, 2122 2 of 38
2. SideMolecules by Side 2019, Comparison24, x FOR PEER REVIEW of Synthetic Methodologies 2 of 38
2.1. Dehydrogenative2. Side by Side Comparison Lactonization of ofSynthetic C(sp2/sp Methodologies3)-H Bonds The2.1. Dehydrogenative lactonization Lactonization of carboxylic of C(sp acids2/sp has3)-H evolvedBonds as a very convenient approach to integrate lactones asThe structural lactonization subunits of carboxylic in molecules acids has of evolved medicinal as a andvery biologicalconvenient interestapproach [ 13to– integrate18]. Among the establishedlactones as structural approaches, subunits direct in dehydrogenative molecules of medicinal C-H /andO-H biological oxidative interest coupling [13–18]. has Among gained the special interestestablished due to itsapproaches, high atom direct economy dehydrogenative [19–28]. Pioneering C-H/O-H work oxidativ wase reportedcoupling has in 2013 gained by Martinspecial , Wanginterest  and due Gevorgyan to its high atom , whoeconomy independently [19–28]. Pionee developedring work the was intramolecular reported in 2013 oxidative by Martin coupling , of C(sp2Wang)-H bonds  and of aromaticGevorgyan carboxylic , who acidsindependently using Cu- developed and Pd-catalysis. the intramolecular In 2015, oxidative the Xu group coupling reported 2 a mild,of silverC(sp )-H catalyzed bonds of version aromatic of C(spcarboxylic2)-H lactonizationacids using Cu- [23 and]. Nonetheless, Pd-catalysis. mostIn 2015, of thesethe Xu methods group use reported a mild, silver catalyzed version of C(sp2)-H lactonization . Nonetheless, most of these high temperatures, transition-metal catalysts and strong oxidants. In this comparative segment we methods use high temperatures, transition-metal catalysts and strong oxidants. In this comparative will analyze how photoredox catalysis and electrochemistry perform regarding the dehydrogenative segment we will analyze how photoredox catalysis and electrochemistry perform regarding the 2 3 lactonizationdehydrogenative of C(sp lactonization/sp )-H bonds of C(sp (Figure2/sp31)-H). bonds (Figure 1).
FigureFigure 1. Side 1. Side by side,by side, a comparative a comparative visualization visualization of the the photoredox photoredox catalyzed catalyzed and and electrochemical electrochemical dehydrogenativedehydrogenative lactonization lactonization of C-H C-H bonds bonds regard regardinging reaction reaction conditions, conditions, mechanismmechanism and product and productscope. scope. Scope Scopeexamples examples shown to shown work with towork both methods with both in black, methods solely in shown black, by solely photoredox shown by + photoredoxcatalysis catalysisin blue and in blueby electrosynthesis and by electrosynthesis in red. CCE in= constant red. CCE current= constant electrolysis, current Mes-Acr electrolysis, = 9-mesityl-10-methylacridinium. Mes-Acr+ = 9-mesityl-10-methylacridinium. Gonzalez-Gomez was the first to employ a photoredox catalyst to facilitate the transformation Gonzalez-Gomez was the ﬁrst to employ a photoredox catalyst to facilitate the transformation in 2015 . In 2017, Luo disclosed a very similar method using photoredox cobalt co-catalysis and in 2015the [inclusion24]. In of 2017, C(sp Luo2)-X (X disclosed = F, Cl, Br, a OMe, very SMe) similar bond method cleavage using which photoredox also yields the cobalt lactonization co-catalysis 2 and theadducts inclusion . In of 1938, C(sp Fichter)-X (X and= coworkersF, Cl, Br, desc OMe,ribed SMe) the bondfirst anodic cleavage lactonization which also. yieldsThey the lactonizationreported adductsthe electrosynthesis . In 1938, of 5-methyl Fichter andphthalide coworkers from described2,4-dimethyl the benzoic ﬁrst anodic acid. Later, lactonization in 2017, . TheyLam reported and coworkersthe electrosynthesis reported another of 5-methylelectrosynthesis phthalide of phthalides from 2,4-dimethyl from aromatic benzoic carboxylic acid. acids Later, in 2017, Lam and coworkers reported another electrosynthesis of phthalides from aromatic carboxylic acids in combination with aliphatic carboxylic acids . The ﬁrst dehydrogenative lactonization of Molecules 2019, 24, 2122 3 of 38 biphenyl-2-carboxylic acids by anodic oxidation was reported by Tylor and coworkers in 1957 ; however, the electrochemical approach was not the main focus. In 2017 Zeng , closely followed in 2018 by Xu  and Luo , independently reported more elaborate electrochemical dehydrogenative lactonizations of C(sp2/sp3)-H Bonds. For both strategies we will mainly discuss and compare the work of Gonzalez-Gomez and Zeng in more detail (Figure1). Both the photoredox and electrochemical approach omit the use of transition metal catalysts as well as high temperatures. Most of the earlier, transition metal catalyzed procedures use temperatures above 75 ◦C to overcome the large HOMO-LUMO energy gap needed for the reaction to take place [19–21,23]. As can be seen in Figure1, both the photoredox catalyzed and electrochemical reaction mechanistically operate via single electron oxidation-radical cyclization. The 9-mesityl-10-methylacridinium (Acr-Mes+) cation contains one of the most oxidizing triplet states among the organophotocatalysts. Upon irradiation with visible light, the excited photocatalyst generates a carboxylate radical via single electron transfer (SET). The catalyst turnover occurs via reoxidation with a persulfate as the terminal oxidant. In the electrochemical pathway, the same single electron oxidation occurs at the anode by anodic oxidation. At the cathode, hydrogen gas is generated at the same time forming methoxide which can deprotonate the carboxylic acid prior to its oxidation. Following carboxylate radical generation, the lactonization of the C(sp2)-H bond happens by way of a 6-endo-trig cyclization. Finally, rearomatization by another single electron oxidation occurs. The ﬁnal oxidant for the photoredox catalyzed reaction is the sulfate radical-anion [SO4•−] originating from the catalyst regeneration. In the electrosynthesis, again, this happens by anodic oxidation. Comparing both methods, it can be noticed that a common path runs like a thread through the entire mechanism, with the two oxidations being distinct. The advantage with electrosynthesis is that it obviates the need of external oxidants where photoredox catalysis needs a terminal oxidant, even though the strong oxidizing power comes from the photocatalyst. Both methods are uncomplicated and innocuous, having very similar general conditions. An acetonitrile/protic solvent mixture was reported to be the solvent system of choice. Other, similar publications communicate diﬀerent solvent systems which are also suitable for this transformation, with the common denominator of them performing well in acetonitrile and polar protic solvent systems (TFE, HFIP). Both methods show very similar high yields, though the electrochemical method is generally faster in nature and avoids the use of external oxidants. Both procedures demonstrate the ability to cyclize a wide range of carboxylic acids. All literature examples on the photoredox catalyzed and electrochemical lactonization cover a broad collective scope. In Figure1, a selected scope is presented displaying examples that were shown to work with both methods in black, solely shown by photoredox catalysis in blue and by electrosynthesis in red. A wide range of 2-arylbenzoic acids can be cyclized towards the corresponding benzocoumarins. The lactonization of 30-substituted acids poses a regioselectivity problem. Nonetheless, both methods can cyclize these substrates in good yields with regioselectivities ranging from 80:20 to 96:4, being an improvement over some previous radical lactonization methods [19,21,22]. Comparing the analogous 30-substituted substrates in the respective scopes, the photocatalytic strategy is slightly more regioselective. Further, heterocyclic substrates and 2-substituted cinnamic acids are reported for both approaches. In 2017, Luo et al. demonstrated a photoredox cobalt co-catalyzed lactonization including a substrate scope with ortho-enyl benzoic acids to yield the 5-exo adducts and one example that reacted to form the 6-endo adduct without hydrogen evolution. The scope of Zeng et al. included the electrochemical dehydrogenative lactonization of C(sp3)-H bonds. 2-Benzylbenzoic acids exclusively generated phenyl phthalides without observing C(sp2)-H coupling products. Tertiary C(sp3)-H bonds could also be formed, and one example of an aliphatic carboxylic acid is given. In summary, both photoredox catalysis and electrochemistry provide a general and facile access to a multitude of lactones under environmentally benign and mild conditions. The electrosynthesis is preferable when chemical oxidants are to be avoided and fast reactions times are desired. The regioselectivity seems to be superior via the photochemical approach. Finally, both methods have been proven to be scalable on laboratory scale. Molecules 2019, 24, 2122 4 of 38
Molecules 2019, 24, x FOR PEER REVIEW 4 of 38 2.2. Dehydrogenative Lactamization Lactams2.2. Dehydrogenative (as well asLactamization lactones) serve as a very common subunit in pharmaceutical, medicinal and biologicalLactams sca (asﬀolds. well as For lactones) this reason, serve as synthetic a very common strategies subunit aiming in pharmaceutical, at the synthesis medicinal of lactams and have beenbiological widely explored. scaffolds. OfFor thesethis reason, methods, synthetic a straightforward strategies aiming approach at the synthesis is the of direct lactams dehydrogenative have been C-H/N-Hwidely oxidative explored. coupling. Of these Inmethods, 2017 and a straightfo 2018 therward Hong approach and Zeng is groups,the direct respectively, dehydrogenative disclosed a photoredoxC−H/N−H catalyzedoxidative coupling. and electrochemical In 2017 and 2018 lactamization the Hong and [ 32Zeng,33 ].groups, The followingrespectively, segment disclosed aims a to illustratephotoredox the diﬀ erencescatalyzed and and similarities electrochemical of both lactamization methodologies [32,33]. by takingThe following a closer looksegment at the aims suggested to illustrate the differences and similarities of both methodologies by taking a closer look at the mechanisms (Figure2). suggested mechanisms (Figure 2).
FigureFigure 2. Side 2. Side by side,by side, comparative comparative visualization visualization of the photoredox photoredox catalyze catalyzedd and and electrochemical electrochemical dehydrogenative lactamization regarding reaction conditions, mechanism and product scope. dehydrogenative lactamization regarding reaction conditions, mechanism and product scope. Both the photoredox catalyzed and electrochemical approach use the efficacy of nitrogen- Both the photoredox catalyzed and electrochemical approach use the eﬃcacy of nitrogen-centered centered radicals to establish C-N bond formation. The amidyl radical is key in both, but the radicalsgeneration to establish of the C-N reactive bond intermediate formation. is The distinct amidyl. In the radical photochemical is key in both,strategy, but the the cooperation generation of of the reactivea weak intermediate phosphate isbase distinct. and an Inexcited the photochemical photocatalyst *Ir strategy,III homolytically the cooperation cleaves the of N-H a weak bond phosphateof the III baseamide and an substrate. excited This photocatalyst happens in *Ira singlehomolytically step through cleavesa concerted the proton-coupled N-H bond of theelectron amide transfer substrate. This happens(PCET) generating in a single the step key through amidyl radical. a concerted A similar proton-coupled intermediate electronis electrochemically transfer (PCET) generated generating in the keythree amidyl distinct radical. steps. The A similar conjugated intermediate MeO− base is electrochemicallyis generated by cathodic generated reduction in three and distinctwill first steps. The conjugateddeprotonate MeOthe amide− base substrate. is generated The latter by cathodic will be trapped reduction by in and situ will generated ﬁrst deprotonate bromine to form the amidea substrate.nitrogen-bromine The latter willbond. be Finally, trapped homolysis by in situ of generated the N-Br bond bromine yields to formthe key a nitrogen-bromineamidyl radical. The bond. Finally,umpolung homolysis reactivity of the of N-Br the open-shell bond yields species the keycompared amidyl to radical.the classical The nucleophilic umpolung nature reactivity of the of the open-shell species compared to the classical nucleophilic nature of the closed-shell nitrogen species is pertinent for this reaction . The electrophilicity of the amidyl radical eﬀectuates cyclization onto Molecules 2019, 24, 2122 5 of 38 the neighbouring sp2 carbon to form a new C-N bond and an adjacent carbon centered radical. Again, the following oxidation is diﬀerent for both methodologies. In the electrosynthesis, rearomatization probably occurs through deprotonation and consecutive anodic oxidation to yield the lactam product. In the photoredox catalyzed method, the intermediate radical is intercepted by O2 to form a peroxy species. Turn-over of the catalytic cycle and product formation go hand in hand as hydrogen peroxide is eliminated from the product by oxidation with the reduced IrII catalyst. The advantages of this electrochemical and photocatalytic method are that they use either no oxidant, or otherwise readily available oxygen as an oxidant, respectively. In both literature reports, phenanthridinones with a wide variety of aryl substituents have been explored. Nonetheless, their scope is very diﬀerent regarding the type of secondary amide substrates. The Zeng group found that the N-acyloxy group is ideal for lactam formation using their electrochemical method. The Hong group, on the other hand, uses N-aryl amides in their photoredox catalyzed lactamization. Despite this diﬀerence, both demonstrate the ability to synthesize phenanthridinone derivatives bearing a variety of substituents in good yields. Both methods resolve a regioselectivity issue, which is prominent in C-H amination, primarily showcased in the electrochemical method . Meta-substituted substrates resulted in C-N bond formation, either exclusively or predominately at the more sterically accessible aryl C-H bond. This way the more preferable 3-substituted lactams were formed. Besides phenanthridinones, both methodologies have a versatile extension to their scope. With the same photoredox catalyzed method, the synthesis of quinolinone derivatives from N-phenylcinnamamides was made possible. Prior art had shown that (E)-oleﬁns could be eﬃciently and selectively converted into (Z)-oleﬁns by photoisomerization [36,37]. By means of this method, the excited photocatalyst *IrIII performs a dual role in the reaction. First, it achieves the in situ generation of (Z)-N-phenylcinnamamides by eletron transfer for E/Z isomerization. Second, it serves as a photooxidant to generate the amidyl radical in a PCET event. The electrochemical method showed one example of a quinolinone, which was feasible due to its symmetry, circumventing the E-Z isomeric problem. In the electrochemical method, one example of a quinolinone is shown where smart choice of substitution pattern circumvents the E/Z isomerization requirement. Distinctly, the electrochemical strategy has been shown to tolerate a propenyl and alkyl moiety to aﬀord quinolinone and indolinone products. Even the C(sp3)-H amination was made possible given a small modiﬁcation, in spite of lower yet moderate yields. As described above, both strategies are quite similar on a mechanistic level. However, the intricate diﬀerences result in a diﬀerent scope, making the two methods complementary. Yet, primary amides are tolerated in neither of the two methods. The free amide has been reported to undergo a Hofmann rearrangement under near-identical electrochemical conditions . Otherwise, the eﬀective BDFE (bond dissociation free energy) for the N-H bond of a primary amide does not match this speciﬁc photocatalyst/base pair . Even though both methods are unable to use primary amides as substrates, the advantage of the electrochemical method is that the unsubstituted phenanthridinones can be obtained after deprotection of the N-acyloxy group. In case of the p-methoxyphenyl substituted amides, the photoredox catalyzed method provides a way to aﬀord unsubstituted phenanthridinones after deprotection of the PMP group. Needless to say, the desired product is a key deciding factor upon the determination of which methodology to use. Within the given conditions, the discussed strategies could seem restricted in their amide scope. The chosen conditions impose an inherent oxidation potential window, which brings forward a set of substrates that can be oxidized within these limits. Of all the examples within this review, this comparison shows best how dialing into the reactivity of the substrate could impact the scope. In electrochemistry this can be done by changing the active potential and/or the redox mediator. Correspondingly, in photoredox catalysis, the photocatalyst could be selected or designed in order to ﬁt the redox potential of the desired substrate . Here speciﬁcally, using PCET, the eﬀective BDFE of any given oxidant/base pair can be modiﬁed over an arbitrarily wide range of energies. Molecules 2019, 24, 2122 6 of 38
2.3. IntramolecularMolecules 2019, 24, Oxidativex FOR PEER AnnulationREVIEW of N-Aryl Enamines: Indole Synthesis 6 of 38
Indole2.3. Intramolecular represents Oxidat one ofive the Annulation most important of N-Aryl sca Enamines:ﬀolds in Indole bioactive Synthesis compounds and natural products and is consequently a target of extensive methodology research [41–49]. Aside from the Fischer indole Indole represents one of the most important scaffolds in bioactive compounds and natural synthesesproducts [50 –and54], is plenty consequently of novel a moderntarget of synthetic extensive approachesmethodology exist research [42,46 [41–49].,55–57 ].Aside These from approaches the typicallyFischer start indole from synthesesortho-halogenated [50–54], plenty anilines of novel or alternative modern synthe complextic approaches substrates, exist which [42,46,55–57]. restricts these methodologiesThese approaches in their typically versatility start due from to highortho-halogenated cost and limited anilines availability or alternative of starting complex materials. substrates, One of the mostwhich straightforward restricts these methodologies transformations in istheir the versatility direct C-H due activation to high cost of N and-aryl limited enamines. availability Synthetically, of previouslystarting described materials.methods One of the accomplished most straightforwar this byd transformations using stoichiometric is the direct amounts C-H ofactivation external of oxidants N- sucharyl as Cu(II) enamines. salts [58Synthetically,–60], hypervalent previously iodine described reagents [61methods,62] and accomplishedN-bromosuccinimide this by [using63]. Instoichiometric 2014, Rueping amounts reported of external the combined oxidants such palladium- as Cu(II) andsalts photoredox-catalyzed[58–60], hypervalent iodine C-H reagents oleﬁnation [61,62] and N-bromosuccinimide . enabling the synthesis of indole derivatives (Figure3). A catalytic amount of a photoredox active In 2014, Rueping reported the combined palladium- and photoredox-catalyzed C-H olefination iridium complex in the presence of visible light manages the re-oxidation of the palladium catalyst. enabling the synthesis of indole derivatives (Figure 3). A catalytic amount of a photoredox active In thisiridium way, thecomplex typical in the high presence loadings of visible of external light manages oxidants the or re-oxidation metal salts of could the palladium be avoided. catalyst. Further controlIn this experiments way, the typical gave interesting high loadings mechanistic of external insights oxidantsshowing or metal salts that eithercould be (a) avoided. the iridium Further catalyst in thecontrol absence experiments of oxygen gave or interesting (b) the in situmechanistic formed insights superoxide showing anions that either resulting (a) the from iridium regeneration catalyst of the photoredoxin the absence catalyst of oxygen with or oxygen(b) the in function situ formed as thesuperoxide external anions oxidant. resulting For thisfrom reason, regeneration this method of servesthe as photoredox an alternative catalyst to common with oxygen methods function relying as the on external peroxo oxidant. species. For Photoredox this reason, catalysis this method enables to remodelserves this as transformation an alternative to ascommon a more methods atom-eﬃ relyincientg reactionon peroxo under species. mild Photoredox reaction conditionscatalysis enables making it well suitableto remodel for this syntheses transformation using oxidant-sensitive as a more atom-efficient substrates. reaction under mild reaction conditions making it well suitable for syntheses using oxidant-sensitive substrates.
FigureFigure 3. Side 3. Side by side,by side, comparative comparative visualization visualization of th thee visible-light-promoted visible-light-promoted and and electrochemical electrochemical intramolecular oxidative annulation of N-aryl enamines regarding reaction conditions, mechanism intramolecular oxidative annulation of N-aryl enamines regarding reaction conditions, mechanism and and product scope. product scope. In 2017, the Lei group reported the electrocatalytic reaction protocol for achieving the same In 2017, the Lei group reported the electrocatalytic reaction protocol for achieving the same direct direct intramolecular C-H activation of aromatic enamines for the synthesis of indoles. As can be seen intramolecularin Figure 3, C-Htheir activationapproach effectively of aromatic is different. enamines Though for the starting synthesis from ofthe indoles. same substrates As can and be seen in Figureobtaining3, their the approachsame reaction e ﬀ ectivelyproducts, isthe di mechanisﬀerent.tic Though blueprint starting is completely from the different. same substratesWhere the and obtaining the same reaction products, the mechanistic blueprint is completely diﬀerent. Where the photoredox approach maintains the established palladium cycle and improves the re-oxidation step, Molecules 2019, 24, 2122 7 of 38 the electrocatalytic approach works under external oxidant-free and transition-metal-free conditions. Mechanistic insights led to a plausible mechanism where after two anodic oxidations, an I+ intermediate can be generated from an iodide anion. Next, the N-aryl enamine reacts with the in situ generated I+ to form an N-iodo intermediate. The labile N–I bond homolyzes to reveal the high energy N-centered radical and the corresponding iodine radical. This aminyl radical tautomerizes to a lower energy carbon centered radical. This radical can intramolecularly add to initiate the cyclization process. After oxidation and deprotonation, the indole product is formed. This is possible by way of anodic oxidation or the in situ generated iodine radical. Concurrently, cathodic reduction of water produces hydrogen gas. This electrocatalytic approach arguably provides a more environmentally friendly synthesis of indoles as it employs external oxidant-free and transition-metal-free conditions. Although it is tenable that platinum leaching from the electrodes is a possibility, no metal catalysts are used as KI not only acts as the electrolyte but also takes part in the redox processes of the reaction. Markedly, DMF was crucial in both methodologies as the solvent of choice. For the electrochemical method, water was added as the co-solvent to sustain the cathodic reduction, producing hydrogen gas and hydroxide base. Contrastingly, the photoredox-palladium dual catalytic C-H activation requires excess base in order to achieve good yields. Both the electrochemical as the photoredox-palladium dual catalyzed system allow the synthesis of highly functionalized indoles. The substrate scope consists mainly of N-aryl enamine esters. The dual catalytic strategy tolerates both electron withdrawing as donating groups on the N-phenyl ring with slightly lower yields for electron donating groups in the para position. The opposite is true for substitutions in the ortho position. The tricyclic benzodioxole product was isolated as a mixture of regio-isomers. Additionally, diﬀerent indole-3-carboxylate ester products were obtained (Me, Et, t-Bu, Bn). The electrosynthetic method tolerates various substituents on the N-phenyl ring as well as on the phenyl on the 2-position of the indole product. Replacing the C-2 substituent from a phenyl group to the isopropyl group led to decreased reaction eﬃciency. This could suggest that for the synthesis of 2-arylindoles, the electrochemical method is preferable and for the 2-alkylindole products, the dual catalytic strategy is more suitable in terms of isolated yield. In addition, the electrochemical method still worked upon replacing the carboxylic ester by a ketone, notwithstanding the reaction yield decreased as well. Finally, N-pyridyl and N-pyrazinyl enamines could be used to aﬀord imidazo [1,2-a]-pyridine and imidazo[1,2-a]pyrazine products. Even though both methodologies use two very diﬀerent modes of activation, the scopes are quite similar. This might come naturally as both make use of the characteristic reactivity of the vinylic C-H bond in the enamine. The latter originates from the two adjacent functional groups, increasing the polarization of this vinylogous bond.
2.4. Carbazole Synthesis The advances observed in carbazole synthesis were very similar to those in indole research discussed above. Carbazoles have also gained great interest in recent years for their frequent appearance as an important motif in bioactive natural products, pharmaceutical agents, electronic materials, including photoconducting polymers and organic optoelectronic materials [64–75]. For this reason, many eﬀorts have been made towards the synthesis of carbazole derivatives. The transition-metal-catalyzed intramolecular C-H bond amination of N-substituted 2-amidobiaryls has long been the most atom economical method [76–83]. However, some of these transformations use hypervalent iodine as the oxidant via PhI(OAc)2/Pd(OAc)2, PhI(OAc)2/Cu(OTf)2. In 2010, Nishiyama and co-workers obviated the need for a transition-metal catalyst by way of electrochemically generating stoichiometric amounts of hypervalent iodine, which in turn is used to mediate the C-H amination of diarylamines to yield the carbazole products . The iodine(I) precursor, phenyl iodide (PhI), is electrochemically oxidized in the presence of triﬂuoroethanol (TFE) to yield the hypervalent iodine(III) reagent PhI(OCH2CF3)2. It possesses comparable or superior properties to commercially available oxidants, such as PIFA [phenyliodine(III)bis(triﬂuoroacetate)] [85–87]. Diaryl Molecules 2019, 24, 2122 8 of 38 amines are later added to the hypervalent iodine(III) reagent to perform the reaction. A plausible mechanism (Figure4) is initiated by S N2 attack of the substrate’s amide oxygen to the iodine(III) of theMolecules oxidizing 2019, 24 reagent, x FOR PEER to REVIEW give an imidate-type intermediate. Subsequently, the neighbouring8 of 38 nucleophilic aromatic ring can attack the nitrogen to realize the desired cyclization by C-H amination. A plausible mechanism (Figure 4) is initiated by SN2 attack of the substrate’s amide oxygen to the This method eliminates the use of transition metals for the synthesis of carbazoles. Nonetheless, it is iodine(III) of the oxidizing reagent to give an imidate-type intermediate. Subsequently, the currentlyneighbouring impossible nucleophilic to use substoichiometric aromatic ring can amounts attack the of nitrogen mediator to realize because the the desired exposure cyclization of carbazoles by to electrochemicalC-H amination. conditionsThis method results eliminates in a the complex use of transition reaction metals mixture. for the The synthesis electrolysis of carbazoles. of carbazole and itsNonetheless, derivatives it is currently quite complex impossible as anodic to use oxidationsubstoichiometric of carbazoles amounts leads of mediator to homocoupling because the and electropolymerizationexposure of carbazoles [88– to90 ].electrochemical This exhibits conditions a ﬂaw of results this methodology in a complex reaction as it uses mixture. stoichiometric The amountselectrolysis of chemical of carbazole oxidants. and On its thederivatives same note, is quite the usecomplex of redox as anodic mediators oxidation often of implies carbazoles an additionalleads obstacleto homocoupling towards product and electropolymerization separation. After reaction [88–90]. completion,This exhibits thea flaw redox of this mediator methodology/oxidant as it waste mustuses be separated stoichiometric from amounts the reaction of chemical mixture. oxidants. The On same the countssame note, for the use supporting of redox mediators electrolyte, often which implies an additional obstacle towards product separation. After reaction completion, the redox is idiosyncratic to electrochemistry. Except when the redox mediator and electrolyte are recovered mediator/oxidant waste must be separated from the reaction mixture. The same counts for the and reused,supporting they electrolyte, are an associated which is sourceidiosyncratic of waste. to electrochemistry. Francke et al. aspiredExcept when to tackle the redox this by mediator developing a redox-activeand electrolyte supporting are recovered electrolyte and reused, by merging they are an an associated iodine(I/ III)source mediator of waste. with Francke an alkylammonium et al. aspired groupto [91tackle]. This this allowed by developing for the a recovery redox-active and supporting reuse of both electrol components.yte by merging The iodine(I)an iodine(I/III) precursor consistsmediator of an with iodophenyl an alkylammonium moiety, in thegroup same . way This as allowed Nishiyama for the used recovery PhI, whichand reuse is attached of both to a quaternarycomponents. ammonium The iodine(I) group. precursor Both are consists linked of by an aniodophenyl alkyl linker moiety, that in contains the same a way benzylic as Nishiyama carbonyl as a protectingused PhI, group, which is as attached the benzylic to a quaternary position ammonium is prone to group. anodic Both oxidation. are linked The by an trimethylammonium alkyl linker that moietycontains serves a to benzylic provide carbonyl ionic conductivity as a protecting and grou is easilyp, as separated the benzylic from position the mixture is prone in orderto anodic to allow recyclingoxidation. of the The iodine trimethylammonium mediator. This mediator-salt moiety serves concept to provide was ionic applied conductivity to the direct and oxidative is easily C-N separated from the mixture in order to allow recycling of the iodine mediator. This mediator−salt bond formation, amongst others. concept was applied to the direct oxidative C−N bond formation, amongst others.
Figure 4. Side by side, comparative visualization of the visible-light-promoted and electrochemical Figure 4. Side by side, comparative visualization of the visible-light-promoted and electrochemical approach to carbazole synthesis regarding reaction conditions, mechanism and product scope. Green approach to carbazole synthesis regarding reaction conditions, mechanism and product scope. Green = = regioselective, orange = non-regioselective isomers, red = no mesomeric substituent tolerated. regioselective, orange = non-regioselective isomers, red = no mesomeric substituent tolerated. Opposed to the electrochemical strategy, the photoredox catalyzed variation of this Opposed to the electrochemical strategy, the photoredox catalyzed variation of this transformation transformation is very similar to the indole synthesis discussed in Section 2.3. In 2015, a dual catalytic is verysystem similar was to reported the indole by Cho synthesis and co-workers, discussed where in Section a Pd-catalyzed 2.3. In method 2015, a is dual aided catalytic by visible-light- system was reportedenabled by oxidation Cho and (Figure co-workers, 4) . whereUnder an a Pd-catalyzedaerobic atmosphere, method O2 serves is aided as the by terminal visible-light-enabled oxidant to oxidationreoxidize (Figure the 4reduced)[ 92]. photocatalyst. Under an aerobic This merger atmosphere, of palladium O2 serves and photoredox as the terminal catalysis oxidant circumvents to reoxidize the reduced photocatalyst. This merger of palladium and photoredox catalysis circumvents the use of
Molecules 2019, 24, 2122 9 of 38 stoichiometric amounts of chemical oxidants or potentially toxic chemical additives. As a result, it is a highly atom economical and environmentally benign version of this transformation. The visible-light mediated dual catalytic approach has a substrate scope including several N-substituted amidobiaryls. The N-substitution is of importance as no alkyl groups (Et, Bn) or free aniline could aﬀord the carbamate product. N-sulfonyl (benzene-, p-toluene- and methylsulfonyl) and N-acetyl-substituted amidobiaryl substrates were successfully reacted to produce the corresponding N-substituted carbazoles. The reactivity of N-acetyl substrates was found to be lower than that of the N-benzenesulfonyl analogues. The electronic properties and substitution pattern have no apparent inﬂuence on the reactivity. 30-Substituted substrates could be selectively transformed into single regioisomer products. The indirect electrochemical method, on the other hand, was heavily inﬂuenced by substitution of the two aryl rings and showed diﬀerent functional group tolerance. Regioselectivity was shown to be a problem for 30-substituted amidobiaryl substrates which aﬀorded a mixture of regioisomers. This diﬀerence could be explained because the photochemical approach goes through the least sterically hindered palladium complex. 20-Substituted substrates, on the other hand, did aﬀord the desired product and were not included in the scope of Cho and co-workers. Additionally, electron-donating and –withdrawing substituents para to the acetamide gave complex mixtures. This was believed to be caused by a resonance eﬀect as a mesomeric meta substituent showed no undesired products. A summary of the reactivity is shown in Figure4. The indirect electrochemical method was also shown to work for the intermolecular C-N bond formation. In conclusion, both photoredox catalysis and electrochemistry can be used in carbazole synthesis but both perform a very diﬀerent role in the process. Photoredox catalysis aids palladium catalysis by providing oxidative power but is not directly involved in C-N bond formation. Electrochemistry is also indirectly put to use to pre-electrolyze a (recyclable) mediator, after which the iodine(III) reagent is used for C-N bond formation. Owing to these diﬀerences, they show very few similarities in conditions and reactivity.
2.5. Decarboxylative Sulfonylation of Cinnamic Acids with Aromatic Sulfonylhydrazides to Vinyl Sulfones In the last decade, the decarboxylative coupling reaction has emerged as a powerful synthetic strategy that ﬁnds great use in the formation of novel carbon-carbon and carbon-heteroatom bonds [93–104]. The transformation is very versatile as it starts from inexpensive, easily accessible and highly stable carboxylic acids and typically occurs under mild reaction conditions accessing a broad range of substrates with excellent functional-group tolerance. An example of this is the decarboxylative coupling with sulfonylhydrazides. They make for an interesting coupling partner as they are readily accessible synthetic intermediates that can behave as sulfone sources upon fragmentation by N2 elimination. In 2015, Singh et al. reported this decarboxylative sulfonylation . The reaction is I2-catalyzed with TBHP as the terminal oxidant starting from aromatic sulfonylhydrazides and cinnamic acids as substrates. In an attempt to reduce catalyst loading, to avoid strong oxidants and to create milder reaction conditions, photoredox- and electrocatalysis gave possible solutions for this challenge. In 2016, Weng et al. reported the visible-light-enabled decarboxylative cross-coupling of cinnamic acids with sulfonylhydrazides under transition-metal-free conditions . A year later, in 2017, the Huang group disclosed the electrochemical approach for the same transformation . The mechanistic approach is very similar as they both start with the activation of the sulfonyl hydrazide by oxidation (Figure5). For the photochemical method, the excited eosin Y photocatalyst oxidizes the sulfonyl hydrazide, after which it undergoes sequential N-H abstractions and an oxidation. Oxygen serves as the terminal oxidant by oxidative quenching of the organocatalyst to yield a superoxide radical anion O2•−, which further oxidizes the intermediate. Subsequent loss of N2 yields the key sulfonyl radical. In the electrochemical pathway, deprotonation of the sulfonyl hydrazide followed by quick anodic oxidation and a repeated deprotonation and oxidation yields the same sulfonyl radical Molecules 2019, 24, 2122 10 of 38
after loss of N2. This open shell intermediate subsequently reacts with cinnamate. The resulting adduct intermediate is readily decarboxylated and oxidized to aﬀord the vinyl sulfone product. In the Molecules 2019, 24, x FOR PEER REVIEW 10 of 38 photoredox catalyzed method the sequence of radical addition and oxidation is accompanied by an iodideproduct. anion In I− thecapturing photoredox the adductcatalyzed intermediate. method the sequence Elimination of radical of CO addition2 and I −andalso oxidation gives the is ﬁnal sulfoneaccompanied product. by Besides an iodide a di anionﬀerent I− capturing oxidation the approach, adduct intermediate. both strategies Elimination are very of CO similar.2 and I- Byalso using oxygengives as the the final terminal sulfone oxidant product. or applyingBesides a electricaldifferent oxidation current, bothapproach, methods both precludestrategies theare needvery for a strongsimilar. chemical By using oxidant. oxygen Further, as the the terminal catalyst oxidan loadingt or is lowapplying (1 mol%) electrical or absent current, in theboth electrochemical methods case,preclude which is the a signiﬁcant need for a improvement strong chemical over oxidant. previous Further, methods. the catalyst The advantage loading is oflow the (1 electrochemical mol%) or methodabsent is thatin the it requireselectrochemical neither case, mediator which is nor a significant catalyst. improvement over previous methods. The advantage of the electrochemical method is that it requires neither mediator nor catalyst.
FigureFigure 5. Side 5. Side by side,by side, comparative comparative visualization visualization of of the the photoredox photoredox catalyze catalyzedd and and electrochemical electrochemical decarboxylative sulfonylation of cinnamic acids with aromatic sulfonylhydrazides towards vinyl decarboxylative sulfonylation of cinnamic acids with aromatic sulfonylhydrazides towards vinyl sulfones regarding reaction conditions, mechanism and product scope. (PET: photoinduced electron sulfones regarding reaction conditions, mechanism and product scope. (PET: photoinduced electron transfer). EDG = electron-donating group, EWG = electron-withdrawing group. transfer). EDG = electron-donating group, EWG = electron-withdrawing group. The scope of both procedures is limited by the radical nature of the processes. The key sulfonyl radicalThe scope intermediate of both procedureshas to exist islong limited enough by to the react. radical This nature limits ofthe the scope processes. to aromatic The key sulfonylsulfonylhydrazides. radical intermediate In the case has of to alkyl exist sulfonyl long enoughradicals, toloss react. of SO2 This will generally limits the generate scope toa more aromatic sulfonylhydrazides.stable sp3-carbon derived In the case radical. of alkylThe instability sulfonyl of radicals, phenyl and loss other of SO aryl2 will radicals generally prevents generate this from a more stablehappening. sp3-carbon Furthermore, derived radical. both scopes The are instability very similar. of A phenyl series of and cinnamic other acid aryl derivatives radicalsprevents and aryl this fromsulfonylhydrazides happening. Furthermore, were screened. both Surprisingly, scopes are verythe α similar.,β-unsaturated A series carboxylic of cinnamic acid scope acid showed derivatives and arylvery sulfonylhydrazidesdifferent functional group were tolerance screened. and Surprisingly, reactivity. The the photocatalyticα,β-unsaturated method carboxylic was unable acid to scope showedafford very products diﬀerent in reasonable functional quantities group tolerancewhen an electron-donating and reactivity. group The photocatalytic(i.e., Ph-OMe or methodthienyl) was was present. Alternatively, methoxy-substituted cinnamic acids present good yields using the unable to aﬀord products in reasonable quantities when an electron-donating group (i.e., Ph-OMe or electrochemical equivalent. Also, (strongly) electron-withdrawing groups (CF3, CN) on the cinnamic thienyl) was present. Alternatively, methoxy-substituted cinnamic acids present good yields using the acid decrease the electrochemical reaction yield but perform better when employed in the photoredox electrochemicalcatalyzed method. equivalent. Sulfonylhydraz Also, (strongly)ides could electron-withdrawing be employed with both groups the (CFelectron-donating3, CN) on the cinnamicand acid decreaseelectron-withdrawing the electrochemical substituents reaction to furnish yield the but co performrresponding better products when employedin diverse yields in the with photoredox no catalyzedspecific method. reactivity Sulfonylhydrazidespattern. Other than that, could the photoredox be employed catalyzed with bothtransformation the electron-donating was shown to and electron-withdrawingproduce the oxygenated substituents β-keto sulfone to furnish product the correspondingwhile only changing products the solvent in diverse to DCE yields in 38% with no speciﬁcisolated reactivity yield. This pattern. once Othermore showcases than that, the the import photoredoxance of solvent catalyzed in these transformation reactions. Though was shownboth to producestrategies the oxygenated showed a βdifferent-keto sulfone preference product in the while solvent only studies, changing the the electrochemical solvent to DCE method in 38% also isolated worked decently well when DMF was used.
Molecules 2019, 24, 2122 11 of 38 yield. This once more showcases the importance of solvent in these reactions. Though both strategies showed a diﬀerent preference in the solvent studies, the electrochemical method also worked decently well when DMF was used.
2.6. HydroaminationMolecules 2019, 24, x FOR of Oleﬁns PEER REVIEW 11 of 38
Oleﬁn2.6. Hydroamination hydroamination of Olefins is a very attractive method for C-N bond formation. For this reason, much eﬀort has been put into designing novel synthetic methods for this transformation [108–116]. Olefin hydroamination is a very attractive method for C-N bond formation. For this reason, Possibly the most desirable route is the utilization of nitrogen-centered radicals, which are versatile much effort has been put into designing novel synthetic methods for this transformation [108–116]. intermediatesPossibly the for most the desirable construction route is of the nitrogen-carbon utilization of nitrogen-centered bonds. In 2015, radicals, the Knowles which are and versatile Xu group independentlyintermediates reported for the a construction catalytic oleﬁn of nitrogen-c hydroamidationarbon bonds. using In amidyl 2015, the radicals Knowles [117 and,118 Xu]. Preliminarygroup workindependently had already shownreported that a methodologiescatalytic olefin usinghydroa thesemidation open-shell using intermediatesamidyl radicals have [117,118]. the beneﬁt of anPreliminary extensive scope,work had consistent already shown anti-Markovnikov that methodologies regioselectivity using these andopen-shell low kinetic intermediates barriers have moving towardsthe benefit C-N bond of an formation extensive [scope,119–126 consistent]. Although anti-Markovnikov the oxidative regioselec generationtivity of and these low synthetically kinetic valuablebarriers amidyl moving radicals towards is attractive,C−N bond formation it still remains [119–126]. a challenging Although the task. oxidative In the generation following of section, these the photo-synthetically and electrocatalytic valuable amidyl hydroamination radicals is attractive, of oleﬁns it still are remains discussed a challenging (Figure6 ).task. In the following section, the photo- and electrocatalytic hydroamination of olefins are discussed (Figure 6).
Figure 6. Side by side, comparative visualization of the photoredox catalyzed and electrochemical Figure 6. Side by side, comparative visualization of the photoredox catalyzed and electrochemical hydroamination of olefins regarding reaction conditions, mechanism and product scope. hydroamination of oleﬁns regarding reaction conditions, mechanism and product scope. All previous methods had to rely on uncommon and expensive prefunctionalized substrates or required either stoichiometric or excess amounts of strong oxidants. In this way, catalytic amidyl
Molecules 2019, 24, 2122 12 of 38
All previous methods had to rely on uncommon and expensive prefunctionalized substrates or required either stoichiometric or excess amounts of strong oxidants. In this way, catalytic amidyl radical generation using amide substrates under redox-neutral conditions considerably beneﬁts the applicability and atom economy of these transformations. Knowles et al. accomplished this using a triple catalytic system that enables proton-coupled electron transfer (PCET) . An iridium photocatalyst and weak phosphate base cooperatively cause PCET homolysis of the strong anilide N-H bond, generating the amidyl radical. This reactive species then cyclizes onto an adjacent oleﬁn to form a new C-N bond. The following radical intermediate is reduced by thiophenol (PhSH) which acts as a hydrogen atom transfer (HAT) catalyst to yield the product. The PCET and HAT systems cooperatively accomplish each other’s turn-over, closing the dual catalytic cycle. This nicely shows the advantage of this redox neutral method. Shortly after, in 2016, the Xu group reported the electrocatalytic version of the same transformation . As can be seen in Figure6, the general layout of the mechanism is parallel to that of the photoredox catalyzed method. The ﬁrst important diﬀerence is that in this electrocatalytic mechanism, a deprotonation is followed by the single-electron oxidation of the conjugate base rather than a concerted PCET activation step. The carbon anode, being the working electrode, oxidizes the redox catalyst, ferrocene (Fc), while the platinum counter electrode conjoins the reaction by reducing MeOH to H2 and MeO−. After the methoxide base deprotonates the amide, it will be rapidly oxidized to furnish the amidyl radical. The second crucial diﬀerence is that a sacriﬁcial HAT reagent is used to reduce the radical intermediate instead of using a HAT catalyst in the photoredox process. This shows that photoredox catalysis is more able to take advantage of the redox-neutral nature of this transformation, as it does not need any terminal oxidants or reductants. Unless an oxidative and a reductive event are coupled, this cannot be realized with electrocatalysis. This strategy is called paired electrolysis and shows comparable utility to redox-neutral and other multicatalytic photoredox chemistry. Nonetheless, it is often hard to pair two half-reactions in a clean manner. For the model reaction, Xu reports 80% isolated product in the absence of 1,4-cyclohexadiene, compared to 90% in its presence. They propose that H-atom abstraction from a solvent molecule is also possible as the ﬁnal step. As both strategies are very similar in nature, they aﬀord an analogous scope. A selected scope is highlighted in Figure6. Besides general amide substrates, ureas and carbamates were also shown to cyclize to the desired products by both methodologies. Moreover, a thiazolidinone product was synthesized from a thiocarbamate by photocatalysis. Both strategies display a multitude of reactions with mono-, di- and trisubstituted oleﬁns. The hydroamination of a tetrasubstituted oleﬁn was also shown to be possible by the visible-light mediated approach. Diﬀerent substitution patterns on the N-aryl group showcasing a wide variety of steric and electronic properties were viable. Included are some oxidation-sensitive functional groups (thioethers, primary alcohols, sulfonamides and competing amides), of which some are generally incompatible with the other modern methods. The two methods experienced no chemoselectivity problems transforming polyoleﬁns. The synthesis of biologically relevant molecules was incorporated in both scopes. The Knowles group performed their hydroamination method on a progesterone- and gibberellic acid derived carbamate yielding high diastereoselectivities and good yields. The Xu group improved the synthesis of an androgen receptor modulator, reducing the number of steps from 7 to 4, including the key hydroamination step. A multitude of cyclic oleﬁns were transformed to provide polycyclic products. Additionally, the electrosynthesis of polycyclic N-heterocycles was demonstrated by a radical cascade reaction allowing tandem cyclization of diene substrates or the oxidative addition to the N-phenyl group. Both methodologies are restricted to secondary amide derivatives. Nevertheless, oxidative cleavage of N-PMP containing products allows for the access to N-unsubstituted product classes. Generally good to high diastereomeric ratios are obtained, being comparable between both methods. Molecules 2019, 24, 2122 13 of 38
2.7. Cross-Dehydrogenative Coupling of Tertiary Amines with Ketones The direct assembly of carbon-carbon (C-C) bonds starting from two simple carbon-hydrogen (C-H) bonds is the ideal scenario for fundamental synthetic transformations. This direct catalytic cross-coupling between two hydrocarbons is termed cross-dehydrogenative coupling (CDC), for which many advances have been reported in the last decade [127–132]. An example is the oxidative coupling Molecules 2019, 24, x FOR PEER REVIEW 13 of 38 of tertiary amines with ketones. In 2017, the Luo group reported a photoredox catalyzed as well as electrochemicalcoupling of versiontertiary ofamines this transformationwith ketones. In [2013317,,134 the]. Luo For group this reason, reported it isa anphotoredox ideal study catalyzed case for as this review.well as electrochemical version of this transformation [133,134]. For this reason, it is an ideal study Bothcase for methods this review. employ the same general strategy to enable asymmetric cross-dehydrogenative coupling (FigureBoth methods7). Organocatalysis employ the same is used general to gain strategy enantiocontrol to enable asymmetric over the reaction. cross-dehydrogenative In particular, aminocatalysiscoupling (Figure is operated 7). Organocatalysis in synergy withis used established to gain enantiocontrol C-H activation over the cycles reaction. to enable In particular, highly stereoselectiveaminocatalysis C-H is functionalizations. operated in synergy Chiral with primaryestablished amine C-H catalysisactivation iscycles coupled to enable with eitherhighly photoredox-cobaltstereoselective dual C-H catalysis functionalizations. or electrochemistry Chiral topr gainimary access amine toalkylated catalysis tetrahydroisoquinolines.is coupled with either In bothphotoredox-cobalt cases, the interaction dual betweencatalysis the or chiral electr amineochemistry catalyst andto ketonegain willaccess generate to alkylated a chiral enaminetetrahydroisoquinolines. intermediate. The iminium In both generation cases, the frominteraction the tertiary between amine the substratechiral amine is where catalyst the and diﬀ erenceketone lies. Firstwill ogenerateﬀ, this is a performed chiral enamine by cooperative intermediate. photoredox The iminium Ru/Co generation catalysis with from a substoichiometricthe tertiary amine amountsubstrate of nitrobenzoic is where the acid difference as a hydrogen lies. First acceptor off, this (yet,is performed not in terms by cooperative of redoxcount). photoredox The Ru/Co latter acts ascatalysis the sacriﬁcial with a substoichiometric oxidant, obviating amount the need of nitrobenzoic for excess amounts acid as a of hydrogen strong oxidants acceptor and(yet, averts not in oxidativeterms consumption of redox count). of theThe organocatalyst.latter acts as the sacri Theficial electrochemical oxidant, obviating method the generatesneed for excess the iminium amounts intermediateof strong by oxidants a twofold and anodic averts oxidation. oxidative Subsequently,consumption of the the iminium organocatalyst. cation is The intercepted electrochemical by the method generates the iminium intermediate by a twofold anodic oxidation. Subsequently, the enamine intermediate through the sterically and electrostatically favoured transition state (2S, 3S)‡. iminium cation is intercepted by the enamine intermediate through the sterically and electrostatically After hydrolysis, the desired product is formed, and the chiral amine can take part in the next favoured transition state (2S, 3S)‡. After hydrolysis, the desired product is formed, and the chiral catalytic cycle. amine can take part in the next catalytic cycle.
FigureFigure 7. Side 7. bySide side, by side, comparative comparative visualization visualization of theof the photoredox photoredox catalyzed catalyzed and and electrochemical electrochemical cross-dehydrogenativecross-dehydrogenative coupling coupling of tertiaryof tertiary amines amines with with ketones ketones regarding regarding reaction reaction conditions, conditions, mechanismmechanism and and product product scope. scope.ee ee= = enantiomeric excess, excess, d.r. d.r. = dias= tereomericdiastereomeric ratio, EDG ratio, = EDGelectron-= electron-donatingdonating group, group, EWG EWG = elec=tron-withdrawingelectron-withdrawing group. group.
The two distinct systems were optimized separately and showed markedly different performance under similar conditions. This can likely be explained by the nature of the oxidation process being so different. Whereas one occurs in a dual catalytic homogeneous fashion, the other oxidation event takes place at an electrode. Both strategies obviate the need for stoichiometric or excess amounts of strong oxidants by using a substoichiometric amount of a nitro-compound acting as the hydrogen acceptor and electricity as a green oxidant, respectively. After iminium ion
Molecules 2019, 24, 2122 14 of 38
The two distinct systems were optimized separately and showed markedly diﬀerent performance under similar conditions. This can likely be explained by the nature of the oxidation process being so diﬀerent. Whereas one occurs in a dual catalytic homogeneous fashion, the other oxidation event takes place at an electrode. Both strategies obviate the need for stoichiometric or excess amounts of strong oxidants by using a substoichiometric amount of a nitro-compound acting as the hydrogen acceptor and electricity as a green oxidant, respectively. After iminium ion generation the catalytic enamine intermediate takes stereo control over the reaction. The visible light mediated approach requires decreased temperature ( 10 C) and 20 mol% of organocatalyst to achieve excellent enantioselectivities. − ◦ Here lies an advantage of the electrochemical method, which can operate at room temperature and only needs 10 mol% of organocatalyst to yield similar—but lower—ee’s (vide infra). Acetonitrile proved to be the ideal solvent in both optimizations. For the electrochemical approach, triﬂuoroethanol (TFE) was found to be a crucial additive. It is proposed to capture the iminium ion intermediate to produce a more stable hemiaminal intermediate, which will further facilitate the C-C bond formation. By preventing the use of strong chemical oxidants, both methodologies ensure high functional group tolerance and good chemoselectivity. For both strategies, the cross-dehydrogenative coupling was optimized using N-phenyl-tetrahydroisoquinoline and cyclohexanone as the model reaction. Here, the numbers, already point out that the photoredox-cobalt-organo triple catalytic method is superior over the electrocatalytic-organo catalytic method in both isolated yield (85% vs. 75%), diastereoselectivity (9:1 d.r. vs. 7:1 d.r.) and enantioselectivity (99% ee vs. 95% ee). This general trend is also reﬂected in the rest of the analogous scope. A sequence of cyclic and a few acyclic ketones were shown to form the syn-Mannich-type adducts with good yields and high diastereo- and enantioselectivity. Cyclic ketones, from 4- to 7-membered, were shown to aﬀord the products in moderate to high yields (except for cyclobutanone in the electrosynthetic method). However, enantio- and diastereoselectivity were troublesome with the CDC reaction of cyclobutanone and cyclopentanone. Cycloheptanone gave lower yields for both reactions and hardly any diastereoselectivity and reduced ee for the electrochemical method. Both methodologies tolerated N-aryl-tetrahydroisoquinolines substituted with electron-donating or electron-withdrawing moieties on the N-phenylgroup. The visible-light-promoted strategy is not restricted to cyclic ketones, as tetrahydropyranone and tetrahydrothiopyranone were also used as nucleophiles in the CDC reaction. Notwithstanding, the obtained products were found to easily racemize upon isolation by silica gel chromatography. In total, three acyclic ketones were applied in both catalytic systems resulting in good yields but low enantioselectivities (25–74% ee). As a conclusion, the photochemical method has the upper hand regarding both the extensiveness of the scope and stereoselectivity. Nonetheless, the electrosynthetic method is oxidant free and less complex, being monocatalytic compared to the triple catalytic process. Here, the direct anodic oxidation as a feature is substrate dependent and requires some optimization for each instance. On the other hand, upon optimization, the electrosynthesis is more conveniently scalable (a 1 mmol scale reaction was performed with no loss of stereoselectivity). Furthermore, many similar transformations are known in both research ﬁelds that use the same approach of iminium ion generation and trapping thereof [1–3,135]. Because they could be analyzed analogously to this this section, the summation of these transformations is not included in the scope of this review.
2.8. Nitrate Radical Induced Alkyne Oxygenation to 1,2-Diketones and Other Transformations
The nitrate radical (NO3•) is an inorganic radical which has interesting reactivity towards organic species. Its high reactivity can be ranked between that of the hydroxyl (OH•) and sulfate (SO4•−) radical . Despite the versatility of this reactive species, few applications in organic synthesis have been reported so far. For laboratory use, the only ordinarily used strategies for nitrate radical generation are the reaction of nitrogen dioxide and ozone , decomposition of N2O5 [138–140], and the electrooxidation and photoinduced electron transfer of inorganic nitrates or nitric acid [141,142]. The two former strategies are not ideal due to the use of toxic gases. Moreover, the presence of the Molecules 2019, 24, 2122 15 of 38 highly reactive gases will promote an abundance of unwanted side reactions. In 2007, Wille and Andropof demonstrated the use of the nitrate radical for the oxygenation of alkynes . The nitrate radicals were both electro- and photochemically generated and compared as viable techniques. For the latter, ceric(IV)ammonium nitrate (CAN) is used, which under irradiation with UV-light undergoes photo-induced electron transfer. The electrochemical strategy directly generates the nitrate radicals from nitrate anions via anodic oxidation.
WhenMolecules NO2019,3 24•, xwas FOR PEER reacted REVIEW with aromatic alkynes, 1,2-diketones were formed as15 the of 38 major product (Figure8). Surprisingly, considerable amounts of the benzophenone were also detected. After experimentalWhen NO3•and was reacted computational with aromatic investigation, alkynes, 1,2-di theketones following were formed mechanism as the major was proposedproduct to explain(Figure both 8). products; Surprisingly, in spite consider of aable diﬀ erenceamounts in of nitrate the benzophenone radical generation, were also the detected. mechanistic After path experimental and computational investigation, the following mechanism was proposed to explain followed by the substrate to give both products is the same for both methods. The addition of NO3• to both products; in spite of a difference in nitrate radical generation, the mechanistic path followed by diarylacetylenes leads to a nitrate adduct vinyl radical. This highly reactive and unstable species ﬁnds the substrate to give both products is the same for both methods. The addition of NO3• to endo one ofdiarylacetylenes two ways to react. leads Itto can a nitrate undergo adduct 5- vinylcyclization radical. This to highly the ﬁve-membered reactive and unstable intermediate, species finds which in its turnone undergoes of two ways the to react. loss of It nitrogencan undergo oxide 5-endo NO cyclization• to yield to the the 1,2-diketone five-membered product. intermediate, Alternatively, which the vinylin radical its turn can undergoes lose a nitrite the loss radical of nitrogen NO2• oxidevia γ NO-fragmentation.• to yield the 1,2-diketone Wolﬀ-rearrangement product. Alternatively, of the resulting α-oxothe carbene vinyl radical to the ketenecan lose and a nitrite subsequent radical oxidationNO2• via γ and-fragmentation. decarboxylation Wolff-rearrangement gives the benzophenone of the product.resulting Photogeneration α-oxo carbene of NOto the3• usingketene CAN and wassubsequent found to oxidation result in poorerand decarboxylation 1,2-diketone/ benzophenonegives the ratios,benzophenone compared to product. the electrochemical Photogeneration method. of NO3• They using suggest CAN was that found the aromaticto result in diketones poorer 1,2- absorb diketone/benzophenone ratios, compared to the electrochemical method. They suggest that the light at the same wavelength as the NO3• precursor CAN and could therefore photochemically • decompose.aromatic Fordiketones this reason, absorb electrochemical light at the same generation wavelength of as nitrate the NO radicals3 precursor was CAN concluded and could to be the therefore photochemically decompose. For this reason, electrochemical generation of nitrate radicals preferable method. was concluded to be the preferable method.
FigureFigure 8. Comparative 8. Comparative visualization visualization of of the the photochemicalphotochemical and and electrochemical electrochemical oxygenation oxygenation of alkynes of alkynes towards 1,2-diketones regarding reaction conditions, mechanism and product scope. towards 1,2-diketones regarding reaction conditions, mechanism and product scope. A few years later, Wille collaborated with the König group to develop a novel methodology for A few years later, Wille collaborated with the König group to develop a novel methodology for the the photochemical nitrate radical generation . The revival of photoredox catalysis gave an photochemicalopportunity nitrate to revise radical this generation transformation [144 ].an Thed make revival the of need photoredox for excess catalysis of CAN gave and an UV opportunity light to reviseunnecessary. this transformation As the photocatalyst, and make 9-mesityl-10-methylacridinium the need for excess of CAN and(Mes-Acr UV light+) was unnecessary. chosen for its As the + photocatalyst,strong oxidizing 9-mesityl-10-methylacridinium power in the excited state (Mes-Acr . After) was irradiation chosen for by its blue strong light, oxidizing the excited power in the excitedphotocatalyst state [145 is capable]. After of irradiation oxidizing the by nitrate blue light,anion theto elegantly excited photocatalystgenerate the nitrate is capable radical. of In oxidizing this the nitrateway, not anion only to the elegantly stoichiometric generate excess the nitrateof cerium radical. reagent In and this the way, use not of only UV light the stoichiometric is replaced by excessa of ceriumcatalytic reagent amount and of the organocatalyst use of UV light and islower replaced energy by blue a catalytic LEDs, but amount also the of NO organocatalyst3• can be generated and lower from simple inorganic nitrate salts such as LiNO3. It was demonstrated that this method could energy blue LEDs, but also the NO3• can be generated from simple inorganic nitrate salts such as LiNO3. oxygenate alkynes with improved yields and selectivity towards the desired 1,2-diketone product over the former photochemical method. Nonetheless, the electrochemical method has a better 1,2-diketone/benzophenone ratio. The nitrate radical has also proven to be useful in other work from Wille and co-workers for the self-terminating radical oxidative cyclization to tetrasubstituted tetrahydrofurans using either CAN photolysis or electrochemical oxidation of LiNO3 [146,147] and was repeated with the photoredox catalyzed version .
Molecules 2019, 24, 2122 16 of 38
It was demonstrated that this method could oxygenate alkynes with improved yields and selectivity towards the desired 1,2-diketone product over the former photochemical method. Nonetheless, the electrochemical method has a better 1,2-diketone/benzophenone ratio. The nitrate radical has also proven to be useful in other work from Wille and co-workers for the self-terminating radical oxidative cyclization to tetrasubstituted tetrahydrofurans using either CAN photolysis or electrochemical oxidation of LiNO3 [146,147] and was repeated with the photoredox catalyzed version . In conclusion, this section nicely shows how photoredox catalysis and electrochemistry are able to generate the same radical species, which in turn performs a speciﬁc transformation, yet side reactions innate to the approach make one preferable over the other. In the present case, photodegradation of the product highlights a ﬂaw of photoredox catalysis and photochemistry in general.
2.9. Vicinal Difunctionalization of Alkenes: Chlorotriﬂuoromethylation Vicinal difunctionalization of alkenes is an eﬀective and valuable method to introduce two new adjacent functional groups [148–152]. Stereocontrol can often be exerted, particularly if both bonds are formed simultaneously or when activated double bonds show distinct philicity towards nucleophiles or electrophiles . This makes this strategy highly reliable. In this section we will discuss the chlorotriﬂuoromethylation enabled by both photoredox catalysis and electrocatalysis. In 2014, the Han group reported the photoredox-catalyzed vicinal chlorotriﬂuoromethylation of alkenes using a ruthenium photocatalyst [Ru(Phen)3Cl2]. CF3SO2Cl is used as the reagent, which is activated by the excited photocatalyst, so both the CF3 radical as the chlorine source Cl− are discharged, releasing SO2 as the only byproduct. The electrophilic CF3• reacts with the electron 3+ rich-alkene substrate giving the radical intermediate. The [Ru(Phen)3] catalyst is regenerated by oxidizing said intermediate into a carbocation, which in turn is trapped by Cl− to yield the product. This redox neutral process has the advantage of not requiring an external oxidant or reductant. It is accompanied by the disadvantage of needing CF3SO2Cl which is highly reactive and delicate to handle, which could be problematic on a bigger scale (bp: 29–32 ◦C). In 2018, the Lin group reported the electrochemical approach to the chlorotriﬂuoromethylation of alkenes . They opted to use the Langlois reagent (CF3SO2Na) as their triﬂuoromethyl source, combined with MgCl2 as the chloride source. Both are attractive solids to use because they are commercially available and bench-stable. The challenge here is to get both to react in a predictable manner. Where the photochemical strategy started oﬀ by reducing its CF3 source, the Langlois reagent has to be oxidized in order to generate CF3•. The consecutive electrophilic addition to the alkene is the same, giving the corresponding radical intermediate. Wanting a chloride anion Cl− to react with said intermediate, one has to oxidize either of both. For this reason, Lin used the concept of parallel anodic oxidation (or anodically coupled electrolysis), where a completely separate, additional anodic event B takes place (Figure9). In addition to MgCl 2, a catalytic amount of Mn(OAc)2 is used which is anodically oxidized to generate [MnIII]-Cl. It reacts with the intermediate radical as the chlorinating agent. In this way chemo- and stereoselectivity can be achieved. The transient CF3 radical adds faster than the persistent metal-based transient radical ([MnIII]-Cl) after which the radical cross-coupling of a transient and a persistent radical outcompetes that of two transient carbon-centered radicals. For this reason, the heterodifunctionalization is favoured in the stereoselectivity in which it is observed. The two independent optimizations both resulted in acetonitrile being the favoured solvent. In spite of the fact that both reports opt for a very diﬀerent approach, they are able to generate a very similar scope. A selected scope is shown in Figure9. The molecules displayed in black illustrate the lapped substrates between both reports, which include some terminal alkenes with ester, amide and amine functionalities. Both strategies were also able to aﬀord the desired products from 1,2-disubstituted and trisubstituted internal alkenes. The Lin group also reported the electrochemical difunctionalization of a tetrasubstituted system. In addition, they demonstrated that, in their hands, this transformation only aﬀorded a small amount of product using the photochemical method of the Lin group. The same exercise was done on a few other substrates such as a styrene and phenylacetylene Molecules 2019, 24, 2122 17 of 38 among others, with the same conclusion. Other notable examples from the electrochemical scope are cyclic alkenes, being converted to the trans products in high diastereoselectivities, and several natural products (or analogues) in which with secondary alcohol, sensitive tertiary amine and quinoline moieties subsist. The photochemical method also reports the difunctionalization of some biologically active compounds such as rotenone and (+)-nootkatone showing good functional group tolerance. Further, in these conditions the quinoline moiety was shown to withstand CF3 addition to the aromatic Molecules 2019, 24, x FOR PEER REVIEW 17 of 38 ring . Additionally, radical clock experiments showed interesting ring rupture and cyclization adductsshowed from vinyl interesting cyclopropane ring rupture and and bisallylamine cyclization adducts substrates, from vinyl respectively. cyclopropane An and allylpropargylamine bisallylamine also underwentsubstrates, tandem respectively. cyclization-functionalization. An allylpropargylamine Finally,also theunderwent photochemical tandem methodcyclization- has shown ability tofunctionalization. be extendable Finally, to bromotriﬂuoromethylation. the photochemical method has shown ability to be extendable to bromotrifluoromethylation.
Figure 9.FigureSide 9. bySide side, by side, comparative comparative visualization visualization of the the photoredox photoredox catalyze catalyzedd and electrochemical and electrochemical cross-dehydrogenative coupling of tertiary amines with ketones regarding reaction conditions, cross-dehydrogenative coupling of tertiary amines with ketones regarding reaction conditions, mechanism and product scope. mechanism and product scope. For this segment, these two methodologies were chosen because they were the first of their kind Forwithin this both segment, fields. Though, these two in 2015, methodologies Dolbier et al. and were Reiser chosen et al. reported because similar they vicinal were photoredox the ﬁrst of their kind withinmediated both chlorotrifluoromethylati ﬁelds. Though, inons. 2015, The Dolbier former uses et al. Cu(dap) and2 ReiserCl as a etphotocatalyst al. reported and similarfocused vicinal on chlorotrifluoromethylations of α,β-unsaturated amides and esters (and other EWG’s) . photoredox mediated chlorotriﬂuoromethylations. The former uses Cu(dap)2Cl as a photocatalyst and focusedAdditionally, on chlorotriﬂuoromethylations a variety of other fluoroalkylsulfonyl of α,β-unsaturated chlorides amides were andemployed. esters The (and latter other also EWG’s) uses . Cu(dap)2Cl as the photocatalyst of choice and demonstrates it as a unique catalyst for the Additionally, a variety of other ﬂuoroalkylsulfonyl chlorides were employed. The latter also trifluoromethylchlorosulfonylation of unactivated alkenes because it suppresses SO2 extrusion . uses Cu(dap)Net addition2Cl as of theCF3SO photocatalyst2Cl instead of CF of3Cl choice provides and the demonstratesformation of a different it as a product unique class. catalyst These for the triﬂuoromethylchlorosulfonylationother two photoredox catalyzed approaches of unactivated complement alkenes the becausescope of the it suppressesinitial photochemical SO2 extrusion report . Net additionsignificantly. of CF 3SO2Cl instead of CF3Cl provides the formation of a diﬀerent product class. These otherThistwo discussion photoredox shows catalyzed how a approachestransformation complement can be operated the scope by two of thedifferent initial toolboxes photochemical applying two completely different strategies to difunctionalize olefins, yielding a very similar scope. report signiﬁcantly. The photoredox strategy couples two redox events to total a redox neutral series of events. On the other hand, the electrocatalytic method makes strategic use of anodically coupled electrolysis by merging two distinct oxidative events.
Molecules 2019, 24, 2122 18 of 38
This discussion shows how a transformation can be operated by two diﬀerent toolboxes applying two completely diﬀerent strategies to difunctionalize oleﬁns, yielding a very similar scope. The photoredox strategy couples two redox events to total a redox neutral series of events. On the other hand, the electrocatalytic method makes strategic use of anodically coupled electrolysis by merging two distinct oxidative events.
2.10. Oxygenation of Unactivated C-H Bonds Molecules 2019, 24, x FOR PEER REVIEW 18 of 38 Functionalization of unactivated carbon-hydrogen bonds is one of the more challenging tasks in organic2.10. Oxygenation synthesis. of In Unactivated polar chemical C-H Bonds conditions, these unactivated C-H bonds are extremely inert making themFunctionalization disinclined of to unactivated engage in carbon-hydrogen proton or hydride bonds transfer. is one of Rather the more than challenging overcoming tasks the in high acidorganic dissociation synthesis. constants In polar (pKa), chemical the homolysis conditions,of these the C-Hunactivated bond has C-H been bonds the are preferred extremely method inert in a lot ofmaking research them regarding disinclined this to topic.engage Nonetheless,in proton or hydride it is still transfer. demanding Rather tothan achieve overcoming enough the oxidizinghigh acid dissociation constants (pKa), the homolysis of the C-H bond has been the preferred method in a power to oxidatively cleave bonds with bond dissociation energies (BDE) this high. At the same time, lot of research regarding this topic. Nonetheless, it is still demanding to achieve enough oxidizing the oxidation must be selective in order for it to occur prior to the degradation of other moieties. power to oxidatively cleave bonds with bond dissociation energies (BDE) this high. At the same time, Typically,the oxidation catalytic must systems be selective using various in order transition for it to occur metal prior catalysts to the were degradation employed of other to accomplish moieties. such transformationsTypically, catalytic [158–161 systems]. The using need various for high transition temperatures, metal strongcatalysts and were complex employed oxidants to accomplish impose scope limitationssuch transformations and impede general [158–161]. adoption The need of these for high methods. temperat Hydrogen-atomures, strong and transfer complex (HAT) oxidants makes for an idealimpose way scope to activate limitations C-H and bonds. impede In general the 1990 adop0s, Hilltion etof al.these introduced methods. Hydrogen-atom the possibility oftransfer activating nonactivated(HAT) makes C-H for bonds an ideal using way a powerfulto activate UV-mediatedC-H bonds. In polyoxometalatethe 1990′s, Hill et HATal. introduced catalyst, the namely possibility of activating nonactivated4 C-H bonds using a powerful UV-mediated polyoxometalate decatungstate (DT) [W10O32] − . Others later described possibility for DT to photocatalyze the 4− oxygenationHAT catalyst, of cyclohexane namely decatungstate [163,164]. The(DT) following[W10O32] . comparative Others later segment described discusses possibility the oxygenation for DT to photocatalyze the oxygenation of cyclohexane [163,164]. The following comparative segment of C-H bonds via photo- and electrocatalytic methods (Figure 10). discusses the oxygenation of C-H bonds via photo- and electrocatalytic methods (Figure 10).
FigureFigure 10. Side 10. bySide side, by comparativeside, comparative visualization visualization of the photochemicalof the photochemical and electrochemical and electrochemical oxygenation oxygenation of unactivated C-H bonds regarding reaction conditions and mechanism. RFOH of unactivated C-H bonds regarding reaction conditions and mechanism. RFOH represents HFIP represents HFIP (hexafluoroisopropanol) in the mechanism. (hexaﬂuoroisopropanol) in the mechanism. In 2017 and 2018, Schultz et al. and the Noël group independently reported the first synthetically useful methodologies for the selective C(sp3)-H oxygenation using decatungstate photocatalysis (Figure 10) [165,166]. The main differences are the use of a different DT salt (sodium vs. tetrabutyl- ammonium), another general oxidant (H2O2 vs. O2) and acid used (H2SO4 vs. HCl). But most importantly, the former focuses on the remote functionalization of aliphatic amines and the latter
Molecules 2019, 24, 2122 19 of 38
In 2017 and 2018, Schultz et al. and the Noël group independently reported the ﬁrst synthetically useful methodologies for the selective C(sp3)-H oxygenation using decatungstate photocatalysis (Figure 10)[165,166]. The main diﬀerences are the use of a diﬀerent DT salt (sodium vs. tetrabutyl-ammonium), another general oxidant (H2O2 vs. O2) and acid used (H2SO4 vs. HCl). But most importantly, the former focuses on the remote functionalization of aliphatic amines and the latterMolecules provides 2019, 24 a, x general FOR PEER method REVIEW for the C-H oxidation of both activated and unactivated19 aliphatic of 38 bonds. In order to access remote C(sp3)-H sites within unprotected amines, prior art for the deactivation provides a general method for the C-H oxidation of both activated and unactivated aliphatic bonds. of the amine α-C-H bonds was used. By protonation of the amine, the less activated, remote C-H In order to access remote C(sp3)-H sites within unprotected amines, prior art for the deactivation of bondsthe amine become α-C-H susceptible bonds was to oxidation used. By protonation . To get of excited, the amine, and the ultimately less activated, accumulate remote enough C-H bonds energy tobecome cleave asusceptible strong C-H to oxidation bond, decatungstate . To get hasexcited, to be and irradiated ultimately by accumulate UV light. enough The initial energy excited to statecleave is produced a strong C-H by oxygenbond, decatungstate to tungsten has charge to be transfer irradiated and by decays UV light. fast The to initial form excited the longer-lived state is speciesproduced designated by oxygen as wOto tungsten [168,169 ].charge The lattertransfer is and the reactivedecays fast catalytic to form species the longer-lived that performs species HAT withdesignated the substrate, as wO generating [168,169]. The a carbon-centered latter is the reacti radical.ve catalytic In reaction species withthat performs oxygen via HAT several with the steps, mostsubstrate, probably generating involving a acarbon-centered peroxo species, radical. the ketone In reaction product with is formed. oxygen Reoxidation via several ofsteps, the reducedmost intermediateprobably involving will regenerate a peroxo the species, catalyst. the Toketone achieve product the sameis formed. carbon-centered Reoxidation radicalof the reduced via direct electrolysis,intermediate other will functionalities, regenerate the solvents catalyst. and To additives achieve arethe expectedsame carbon-centered to be oxidized radical prior to via the direct desired C-Helectrolysis, bond. other functionalities, solvents and additives are expected to be oxidized prior to the desiredIn 2017, C−H the bond. Baran group tackled this problem by using quinuclidine as a redox mediator in the electrosynthesisIn 2017, the Baran to circumvent group tackled said this degradation problem by [170 using]. The quinuclidine mediator willas a redox be anodically mediator oxidized, in the generatingelectrosynthesis a reactive to circumvent radical cation said degradation which could . facilitate The mediator substrate will oxidation be anodically through oxidized, eﬃcient homogeneousgenerating a electronreactive transferradical cation rather which than acould heterogeneous facilitate substrate process occurringoxidation through at the anode efficient [171 ]. Moreover,homogeneous the quinuclidine electron transfer mediator rather allows than electrochemical a heterogeneous oxidation process occurring to take place at the at aanode relatively . low potential.Moreover, The the key quinuclidine HAT activation mediator step allows is operated electrochemical by the anodically oxidation generated to take place quinuclidine at a relatively radical low potential. The key HAT activation step is operated by the anodically generated quinuclidine cation, in analogy with the excited decatungstate photocatalyst. Consequently, a range of functional radical cation, in analogy with the excited decatungstate photocatalyst. Consequently, a range of groups are tolerated under the given reaction conditions. Figure 11 communicates a selected scope for functional groups are tolerated under the given reaction conditions. Figure 11 communicates a the three methodologies discussed above. selected scope for the three methodologies discussed above.
FigureFigure 11. 11.Selected Selected examples examples from from thethe scopescope of the thr threeee discussed discussed methodology methodology reports reports discussed discussed in in thisthis section. section. For For further further details, details, examine examine thethe respectiverespective articles. articles.
TheThe photochemical photochemical method method from from the the NoëlNoël group des describescribes the the selective selective aerobic aerobic oxidation oxidation of ofboth both 3 activatedactivated and and unactivated unactivated C(sp C(sp3)-H)-H bonds. bonds. Examples Examples of activated of activated scaﬀ oldsscaffolds include include allylic allylic and benzylic and positionsbenzylic and positionsα-alkoxy and C-H α-alkoxy bonds. C-H Yet, bonds. a Boc-protected Yet, a Boc-protected pyrrolidine pyrrolidine was shown was toshown undergo to undergo selective selective oxidation at the α-C-H bond in a Shono-type fashion to afford the protected 2-pyrrolidone oxidation at the α-C-H bond in a Shono-type fashion to aﬀord the protected 2-pyrrolidone in moderate in moderate yield (23%) (a similar electrochemical Shono-type oxidation was recently disclosed ). Notably, the photochemical method from Schultz et al. is able to produce the protected 3-pyrrolidone from pyrrolidine because the protonation of the free amine makes the aforementioned Shono-type oxidation impossible. Amino acids were also suitable substrates towards oxygenation. (S)-proline methyl ester, for example, undergoes oxygenation exclusively at the C4-position. This schowcases some unique regioselectivity for these specific distal (sp3)-H bonds. Other substrates, on
Molecules 2019, 24, 2122 20 of 38 yield (23%) (a similar electrochemical Shono-type oxidation was recently disclosed ). Notably, the photochemical method from Schultz et al. is able to produce the protected 3-pyrrolidone from pyrrolidine because the protonation of the free amine makes the aforementioned Shono-type oxidation impossible. Amino acids were also suitable substrates towards oxygenation. (S)-proline methyl ester, for example, undergoes oxygenation exclusively at the C4-position. This schowcases some unique regioselectivity for these speciﬁc distal (sp3)-H bonds. Other substrates, on the other hand, produced a mixture of carbonyl products. For instance, oxygenation of piperidine gave a 2:1 mixture of the δ/γ ketone products. They also noticed that the presence of a remote tertiary C(sp3)-H bond leads to hydroxylation of that bond. Likewise, the quinuclidine-mediated approach can also be used for the electrosynthesis of tertiary alcohols in the presence of tertiary C(sp3)-H bonds. Regarding the scope of activated C-H bond oxygenations, the electrochemical method is capable of tackling similar allylic and benzylic positions and α-alkoxy C-H bonds. With unactivated aliphatic systems, both the photo- and electrochemical methods induce oxidation of the methylene units most distal to electron-withdrawing functionalities. Moreover, here, indistinct selectivity for the methylene units could lead to a mixture of ketone products. Both approaches will selectively oxidize distal C-H bonds in substrates where the benzylic position is either unavailable or deactivated (e.g., pyridine). Other examples include bioactive compounds such as (1R)-(+)-camphor, (+)-sclareolide, artemisinin and an isosteviol derivative. The scalability of the electrochemical method was shown by the oxygenation of sclareolide on a 50 g scale and incorporated to in the synthesis of (+)-2-oxo-yahazunone. Though the scalability of photoredox catalysis is often questioned, the two photochemical strategies also demonstrated to be scalable with a 5 g and 5 mmol scale synthesis by way of a continuous-ﬂow setup. As a conclusion, electrochemistry and photoredox catalysis compare very well to each other in regard to this transformation, as they both employ a similar mode of activation (through HAT) and share analogous selectivity, which is not the case comparing it to other C-H oxygenations using metal complexes or strong oxidants (e.g., TFDO).
2.11. Photoelectrochemical Chemical Oxidant-Free Minisci-Type C-H Alkylation of Heterocycles From the previous examples it becomes clear how photoredox catalysis excels in redox-neutral transformations. The nature of the catalytic cycle provides the system with an oxidant as well as a reductant with each turnover of the photocatalyst. On the other hand, net reductive and net oxidative transformations can be performed under electrochemical conditions without the need of a chemical reductant or oxidant as it is substituted by electrical power. Nevertheless, the generated radicals can undergo undesirable over-reduction and over-oxidation, radical homocoupling and electrode passivation. The transient and more diﬀuse nature of redox mediators, excited photocatalysts and radical species preclude these phenomena. The combination of photoredox catalysis and electrochemistry could lead to a constructive merger ﬁlling each other’s ﬂaws by their respective advantages. Recently, Xu and co-workers demonstrated this concept by merging photoredox catalysis with electrocatalysis to enable a chemical oxidant-free Minisci-type C-H alkylation of heterocycles with organotriﬂuoroborates as shown in Figure 12[ 173]. As suggested, the photocatalyst is mechanistically involved in three processes. After irradiation, the triﬂuoroborate undergoes single electron oxidation by way of the excited photocatalyst (Ered = 2.06 V vs. SCE) . After Minisci-type radical addition the heterocycle is reoxidized by the mild ground-state catalyst Mes-Acr+ (Ered = 0.57 V vs. SCE)  to aﬀord the product. Both oxidation processes produce the stable acridinyl radical Mes-Acr•, which is in turn anodically oxidized regenerating the ground-state photocatalyst. Concurrent cathodic reduction of protons generates hydrogen gas. A wide range of aromatic heterocycles were shown in this nucleophilic radical substitution reaction. Several bioactive substrates were included, such as camptothecin, quinine and purine producing a monoalkylated product with good chemo- and regioselectivity. The organotriﬂuoroborate scope included a variety of cyclic and acyclic primary, secondary and tertiary triﬂuoroborates. The corresponding radicals include tertiary and α-alkoxyl radicals which have oxidation potentials Molecules 2019, 24, 2122 21 of 38 much lower than their representative organotriﬂuoroborates and are thereby susceptible to overoxidation to carbocations. Due to this merger of the two catalyst systems and altered conditions this problem was overcome. The photoelectrosynthesis of oxindole was shown as a proof of concept for another chemical oxidant-free transformation by oxidative tandem addition-cyclization of acrylamide. Molecules 2019, 24, x FOR PEER REVIEW 21 of 38
FigureFigure 12. 12.Photoelectrochemical Photoelectrochemical chemicalchemical oxidant-free Minisci-type Minisci-type C-H C-H alkylation alkylation of ofheterocycles heterocycles includingincluding mechanism mechanism and and selected selected examples. examples.
3.3. Discussion Discussion
184.108.40.206. Underlying Underlying Physicochemical Physicochemical StructureStructure andand Mode of Activation Activation TheThe mechanistic mechanistic processes processes andand thethe playersplayers in it, it, examined examined in in Section Section 2,2 ,make make it itapparent apparent that that photoredoxphotoredox catalysis catalysis and and electrochemistry electrochemistry shareshare aa principal mode mode of of action. action. This This being, being, the the activation activation ofof substrates substrates by by single single electron electron transfer transfer (SET) (SET) by by assistance assistance of of an an alternative alternative energy energy source. source. InIn FigureFigure 13, this13, is this visually is visually depicted depicted by theby the corresponding corresponding simpliﬁed simplified energy energy statestate diagrams. When When no no external external energyenergy source source is is allowed,allowed, the the molecular molecular orbitals orbitals of a ofground a ground state photocatalyst state photocatalyst PC and PCenergy and levels energy levelsof the of theelectrodes electrodes are arepositioned positioned unfavorable unfavorable towards towards oxidation oxidation and and reduction reduction of ofa hypothetical a hypothetical substrate S. The following paragraph is intended to provide a conceptual understanding and does substrate S. The following paragraph is intended to provide a conceptual understanding and does not include an exhaustive discussion of the respective processes. Further, exceptions to these general not include an exhaustive discussion of the respective processes. Further, exceptions to these general descriptions exist, including chain mechanisms  and indirect electrolysis using mediators , descriptions exist, including chain mechanisms  and indirect electrolysis using mediators , but these representations are descriptive for the majority of photoredox catalyzed and but these representations are descriptive for the majority of photoredox catalyzed and electrochemical electrochemical transformations. For an excellent discussion on these concepts, the following transformations. For an excellent discussion on these concepts, the following literature exists for both literature exists for both photoredox catalysis [1,2,176] and electrochemistry [177,178]. photoredoxAs depicted catalysis in [Figure1,2,176 13A,] and the electrochemistry external energy [ 177input,178 via]. irradiation will excite a ground state As depicted in Figure 13A, the external energy input via irradiation will excite a ground state electron in the photocatalyst PC (S0) so that it is promoted to a higher energy level (S1). The electronchromophore in the photocatalyst could return PCdirectly (S0) so to that its original it is promoted ground to state a higher (S0) by energy either level nonradiative (S1). The or chromophore radiative coulddecay. return Yet, directlya good photocatalyst to its original is ground defined state by it (Ss0 )ability by either to undergo nonradiative spin forbidden or radiative intersystem decay. Yet, a goodcrossing photocatalyst (ISC, S0 → isT1) deﬁned to afford by an its excited, ability tolong-lived, undergo spintriplet forbidden state photocatalyst intersystem PC* crossing (T1). The (ISC, S molecularT ) to aorbitalsﬀord an of excited, the excited long-lived, photocatalyst triplet PC* state are photocatalystnow positioned PC* thermodynamically (T ). The molecular favorable orbitals 0 → 1 1 oftowards the excited both photocatalystsingle electron PC*oxidation are now and positionedreduction of thermodynamicallya hypothetical substrate favorable S. Both the towards loss and both singleacceptance electron of oxidationan electron and are reductionnow beneficial of a hypotheticalcompared to the substrate situation S. before Both the irradiation loss and (depicted acceptance in of anFigure electron 13A, are by now green beneﬁcial and red compared arrows). toQuenchin the situationg of the before excited irradiation photocatalyst (depicted PC* inby Figure electron 13 A, donation to substrate S or an oxidant is called oxidative quenching. Reductive quenching occurs upon quenching of PC* by acceptance of an electron from S or a reductant. The overall process of consecutive photoexcitation of a photoredox catalyst PC and electron transfer with a substrate in the ground state, is called photoinduced electron transfer, or PET.
Molecules 2019, 24, 2122 22 of 38 by green and red arrows). Quenching of the excited photocatalyst PC* by electron donation to substrate S or an oxidant is called oxidative quenching. Reductive quenching occurs upon quenching of PC* by acceptance of an electron from S or a reductant. The overall process of consecutive photoexcitation of a photoredox catalyst PC and electron transfer with a substrate in the ground state, is called photoinduced electron transfer, or PET. Molecules 2019, 24, x FOR PEER REVIEW 22 of 38
FigureFigure 13. 13.Simpliﬁed Simplified MO MO energy energy diagrams. diagrams. (A) Photophysical processes processes occurring occurring in inthe the photoredox photoredox catalyzedcatalyzed PET PET activation activation of ofsubstrate substrate moleculesmolecules (S) by a a photocatalyst photocatalyst (PC). (PC). (B ()B )Depiction Depiction of ofthe the electrochemical activation of substrates (S) by ET. Red = non-favorable ET, green = favorable ET. electrochemical activation of substrates (S) by ET. Red = non-favorable ET, green = favorable ET.
TheThe mode mode of of activation activation that that characterizescharacterizes electrosynthesiselectrosynthesis can can easily easily be be compared compared to tothat that of of photoredoxphotoredox catalysis, catalysis, described described above.above. Just Just like like the the unfavorable unfavorable positioning positioning of molecular of molecular orbitals orbitals of the substrate S with respect to those of the ground state photocatalyst, the energy levels of the of the substrate S with respect to those of the ground state photocatalyst, the energy levels of the electrode metal electrons without an applied voltage are also inopportunely positioned (Figure 13B, electrode metal electrons without an applied voltage are also inopportunely positioned (Figure 13B, red arrows). Upon application of an external energy source, an electric potential (UE), the energy red arrows). Upon application of an external energy source, an electric potential (U ), the energy levels levels of the cathode metal electrons are raised while the anode metal electrons areE simultaneously of the cathode metal electrons are raised while the anode metal electrons are simultaneously lowered lowered in energy (the same applies to non-metal electrodes, e.g., graphite). At this point, the cathode inand energy anode (the bear same a negative applies and to non-metalpositive potential, electrodes, respectively. e.g., graphite). While driving At this the point, potential, the cathodethe energy and anodelevelbear of anode a negative electrons and will positive decrease potential, to such arespectively. degree that the While HOMO driving electrons the from potential, a hypothetical the energy substrate S in solution will relocate to a more favorable energy level on the anode via an electron transfer process (Figure 13B, green arrow). This is called anodic oxidation of the substrate and brings forth an oxidation current from the solution to the electrode. Alternatively, while the potential at the cathode decreases, the energy levels of the electrode electrons will elevate to a point where they can transfer into the LUMO’s of substrate molecules. This process is termed cathodic reduction, generating a reduction current from electrode to solution. To conclude, external energy input via
Molecules 2019, 24, 2122 23 of 38 level of anode electrons will decrease to such a degree that the HOMO electrons from a hypothetical substrate S in solution will relocate to a more favorable energy level on the anode via an electron transfer process (Figure 13B, green arrow). This is called anodic oxidation of the substrate and brings forth an oxidation current from the solution to the electrode. Alternatively, while the potential at the cathode decreases, the energy levels of the electrode electrons will elevate to a point where they can transfer into the LUMO’s of substrate molecules. This process is termed cathodic reduction, generating a reduction current from electrode to solution. To conclude, external energy input via irradiation and applied voltage drive the activation, by SET of substrate molecules (oxidation as well as reduction) for photoredox catalysis and electrochemistry, respectively. For both electrosynthesis and photoredox catalyzed reactions, transport of molecules towards and away from their respective “areas of activation” is an important consideration [177,179,180]. For reactions to occur, substrate molecules need to be activated. For electrosynthesis, mass transfer from the bulk solution towards the electrode surface is of fundamental importance. As discussed in Section 3.2, the penetration depth of light is limited, resulting in an analogous scenario where substrate molecules must enter the reactive ﬁlm in which photochemical activation takes place. As a result, mass transfer from the bulk solution towards these respective areas of activation could limit the rate of the reaction. Diﬀusion and convection are two basic mechanisms of mass transport. Diﬀusion is the net transport of molecules from a region of higher concentration to regions of lower concentration. Convection causes the movement of molecules through hydrodynamic stirring. In electrosynthesis, migration, the movement of charged molecules in the electric ﬁeld is also an important mechanism that contributes to mass transfer. When transport of molecules towards their respective areas of activation leading to the reaction is fast, this leaves the electron transfer reaction as the rate limiting factor. In an ideal scenario the mass transport is high so that it provides substrate molecules fast enough to be activated and react. Additionally, the fast evacuation of product molecules away from the electrode surface or heterogeneous photocatalyst into the bulk of the solution will minimize side reactions (overoxidation, overreduction, etc.). Under the same consideration, redox mediators (indirect electrolysis) and homogeneous photocatalysts are sometimes preferred over their heterogeneous counterparts (vide infra).
3.2. Tunability, Advantages and Restrictions When the mode of electrochemical activation is understood, it is easy to understand that electrochemistry is very tunable. For a given transformation, the redox potentials of various substrates can vary signiﬁcantly. Yet, for many syntheses, the oxidative or reductive power is statically ﬁxed (dependent on oxidant/reductant) after optimization of reaction conditions. Consequently, the reaction times and/or yields can vary signiﬁcantly over a set of diﬀerent substrates. Some substrates will simply be ruled out because the redox activation is thermodynamically impossible. An electrochemical set-up is very tunable, as a potentiostat essentially allows to minutiously regulate the active potential. This enables electrosynthesis by dialing in the redox potential needed for a particular substrate. Because of this, potentiostatic control permits the functionalization of complex synthetic intermediates with great chemoselectivity. To a similar extent, the tunability in photoredox catalysis is also possible. Recently, signiﬁcant eﬀort has been spent to tune the electrochemical and photophysical properties of photocatalysts [40,176,181– 190]. Diﬀerent substitution patterns and ligand usage may lead to diﬀerent excitation maxima, redox potentials and improved excited-state (and other redox) lifetimes, chemical stability and solubility. As a result of this, the plethora of photocatalysts that are now available provide a means of dialing into the redox potential of substrates to a certain extent. Obviously, the ease of, and precision in which this is possible is still greater with potentiostatic electrochemistry. Cyclic voltammetry provides a quantitative electrochemical analysis as a powerful instrument for investigating the electron transfer processes in reactions [191,192]. Nonetheless, the tunability is not unconditional as it is restricted by “redox potential windows”. In the ﬁeld of electrochemistry, redox potential windows are termed electrochemical Molecules 2019, 24, 2122 24 of 38 windows and are deﬁned as the diﬀerence between the cathodic and anodic limits. These are the potentials at which reduction and oxidation of a given component takes place, respectively . In photoredox catalysis, this window is deﬁned as the reduction potential window attainable by the + excited photocatalyst PC* and its oxidized/reduced counterpart (PC• and PC•−). Since both systems are more complex in terms of redox considerations, other boundaries have to be taken into consideration. A common restrictive potential window for both photoredox catalysis and electrochemistry (and other redox chemistry for that matter) is the solvent window [193,194]. An alternate solvent choice can broaden the permissible potential window and/or lower the potential of the desired species, while raising others . For this reason, it is not surprising that for most transformations discussed in this review, the same solvent systems proved to perform best for both methodological approaches. In the case of electrosynthesis, the electrolyte selection also sets an anodic and cathodic limit where both the cation and the anion must be considered. Finally, electrode selection is crucial because they all have their own window of operation, and at a certain point will become sacriﬁcial and dissolve. Broad potential windows are found with platinum, gold and boron-doped diamond (BDD) electrodes. These concepts are useful to discuss separately, but it is ﬁnally the interplay of the electrolyte solutions and electrodes that deﬁnes the speciﬁc electrochemical window. In general, electrochemistry has the advantage of having a broader accessible potential window, as photocatalysts have a conﬁned achievable potential range. As discussed in Section 2.10, direct electrolysis of substrates can produce some problems. These are often circumvented by using a mediator . As a result, the electron transfer process changes from a heterogeneous process taking place at the electrode (direct electrolysis) towards a homogeneous electron transfer process. This redox catalyst, in analogy with a photoredox catalyst, is an oxidant or reductant by proxy and ultimately also introduces a redox potential boundary. This is not necessarily a disadvantage as it can increase the selectivity of the desired transformation (Section 2.10). Where electrode deactivation by passivation is an inherent problem associated with electrochemistry, photobleaching of the catalyst can similarly pose a signiﬁcant problem with photoredox catalysis [144,195,196]. One of the factors that help minimize this is the use of light emitting diodes (LEDs). LEDs allow narrow wavelength selection to eﬀectively dial into the absorption spectrum of the photocatalyst towards selective excitation, maximizing energy-eﬃciency. Nonetheless, the series of wavelength-chromophore couples is still incremental compared to the continuous control over the current and voltage via electrochemistry. Additionally, the attainable redox potential window is inseparable from the wavelength of the light source, which exposes another boundary. UV LEDs are available, yet currently still expensive. Moreover, non-redox photochemical side reactions become more prevalent at shorter wavelengths. This can make it challenging to use certain substrate classes in photoredox catalysis. Problematic compounds include photo-active compounds, which decrease the eﬃciency of the system by competitive absorption of the light (Section 2.8), undergoing photocycloadditions, bond homolysis or rearrangements. At the same time, a welcome advantage could be the autocatalytic eﬀect of a photoactive product as a (co-)catalyst . In a similar way, some transformations currently seem very challenging via an electrochemical approach. Examples are the usage of carbazoles due to electropolymerization (Section 2.4)[84,88–91] and the diﬃculty of employing aldehydes as acyl radical precursors  compared to the photochemical approach . As discussed above, potentiostatic control allows tunability in electrochemical transformations. Constant potential electrolysis (CPE) generally aﬀords better chemoselectivity than constant current electrolysis (CCE, galvanostatic). However, potentiostatic set-ups require a reference electrode (e.g., SCE = saturated calomel electrode or Ag/AgCl) to properly measure the potential at the working electrode, making the set-ups more complex compared to CCE. Alternatively, a standardized instrument could alleviate this inconvenience . Apart from this, CPE is often accompanied by lower conversion rates than CCE. With controlled potential experiments, the current in the cell decreases over time as a result of the depletion of the substrate at the working electrode. Under galvanostatic conditions, Molecules 2019, 24, 2122 25 of 38 however, the current is kept constant . As the substrate concentration decreases, the potential across the cell must increase in order to maintain a constant electric current. The increasing voltage that is applied now also consumes the current in side reactions originating outside the target voltage (overoxidation-overreduction, oxidation-reduction of solvent molecules, etc.). The compromise of controlled potential experiments is, as the current decreases, the decrease of reaction rate and thus consumption of the starting material. This usually makes it diﬃcult to operate a potentiostatic experiment to completion. Additionally, the reaction times to realize full conversion increase immensely. Nonetheless, potentiostatic electrolysis consumes the current more eﬃciently. In summary, the overall judgement on whether to use CCE or CPE is a settlement between experimental simplicity vs. current eﬃciency and fast/complete conversion vs. chemoselectivity. Moreover, with controlled potential, Molecules 2019, 24, x FOR PEER REVIEW 25 of 38 it is important to know the active potential at the working electrode, rather than the applied voltage. Ohmicexperimental drop (∆Eohm simplicity) or ohmic vs. polarizationcurrent efficiency or IR an dropd fast/complete is the potential conversion drop between vs. chemoselectivity. the reference and workingMoreover, electrode with in controlled an electrochemical potential, cellit is (Figureimportant 14 )[to200 know]. Because the active of thispotential voltage at the drop, workinga diﬀerent potentialelectrode, is experienced rather than atthe the applied working voltage. electrode. Ohmic drop An extra(ΔEohm reason) or ohmic why polarization electrosynthesis or IR drop could is be carriedthe out potential under drop galvanostatic between the control reference is and because workin theg electrode potential in between an electrochemical the reference cell (Figure electrode 14) and working. electrode Because of is this only voltage measured drop, a rather different than pote controlled.ntial is experienced The ohmic at the drop working is proportional electrode. An to the distanceextra between reason why the twoelectrosynthesis electrodes (d),could the be magnitude carried out ofunder the currentgalvanostatic (I) and control inversely is because proportional the potential between the reference electrode and working electrode is only measured rather than to the conductivity of the solution (ρ). For this reason, in electrosynthesis, the voltage drop is controlled. The ohmic drop is proportional to the distance between the two electrodes (d), the ρ generallymagnitude counteracted of the current by the (I) additionand inversely of supporting proportional electrolytes to the conductivity to increase of the the solution conductivity (ρ). For ( ). Whilethis solving reason, the in challenge electrosynthesis, of theohmic the voltage drop, itdrop often is introducesgenerally co otherunteracted problems. by the Separation addition of of the productssupporting from these electrolytes supporting to increase electrolytes the conductivity following (ρ the). While electrolysis solving entailsthe challenge a substantial of the ohmic drawback. Withdrop, the exception it often ofintroduces scenarios other after whichproblems. the electrolyteSeparation canof the be recuperatedproducts from and these reused, supporting a substantial amountelectrolytes of waste following is being the generated. electrolysis Additionally,entails a substantial the recovery drawback. is With often the arduous exception as of the scenarios ionic salts are inherentlyafter which very the electrolyte soluble in can conventional be recuperated solvents. and reused, As a discussed substantial in amount Section of 2.4waste, Francke is being et al. tackledgenerated. this problem Additionally, by designing the recovery a multipurpose is often arduous redox as mediatorthe ionic salts by integrationare inherently of very the supportingsoluble electrolytein conventional into the iodine(I)solvents. As precursor discussed [ 91in ].Section By means 2.4, Francke of such et al. a method, tackled this recovery problem and by designing reuse of both a multipurpose redox mediator by integration of the supporting electrolyte into the iodine(I) mediator and electrolyte becomes possible in a single step. Other approaches to reduce electrolyte precursor . By means of such a method, recovery and reuse of both mediator and electrolyte usagebecomes generally possible often in also a single rely onstep. multifunctional Other approaches reaction to reduce components. electrolyte usage In the generally Kolbe electrolysis often also for example,rely on the multifunctional carboxylic acid reaction substrate components. together In with the aKolbe base electrolysis will form carboxylatefor example, saltsthe carboxylic that function as substratesacid substrate as well together as electrolytes. with a base Following will form carboxyl reactionate completion, salts that function the base as cansubstrates simply as bewell removed as (e.g.,electrolytes. ﬁltration, extraction).Following reaction Tajima, completion, Fuchigami the and base co-workers can simply showed be removed that also (e.g., solid-supported filtration, basesextraction). could serve Tajima, as a similar Fuchigami solution and co-workers by in situ sh generationowed that ofalso the solid-supported electrolyte from bases methanol could serve or HFIP as multifunctionalas a similar solution solvents by in situ [201 generation–204]. Ionic of the liquids electrolyte can from also methanol be used asor HFIP the solvent as multifunctional as well as the electrolytesolvents in electrosynthesis[201–204]. Ionic liquids [205,206 can]. also be used as the solvent as well as the electrolyte in electrosynthesis [205,206].
FigureFigure 14. Inﬂuence 14. Influence of “distance”of “distance” (d) (d) on on two two fundamentalfundamental physical physical properties properties regarding regarding photoredox photoredox catalysiscatalysis and electrochemistry.and electrochemistry. The The light light absorption absorption (A)(A) and ohmic drop drop (Δ (E∆ohmEohm) are) areboth both proportional proportional to theto pathlength the pathlength of light of propagationlight propagation (d) and (d) interelectrode and interelectrode distance distance (d). ε =(d).molar ε = extinctionmolar extinction coeﬃcient,
c = molarcoefficient, concentration, c = molar Iconcentration,0 = incident intensity, I0 = incident I = transmittedintensity, I = intensitytransmitted (left), intensity I = electric (left), I current = electric (right), current (right), Rcell = electrical resistance of the cell, ρ = solution specific conductance, A = area of Rcell = electrical resistance of the cell, ρ = solution speciﬁc conductance, A = area of working electrode. working electrode. As mentioned, ohmic drop is also subject to the distance between the working and reference As mentioned, ohmic drop is also subject to the distance between the working and reference electrode. The smaller the electrode gap, the lower the voltage drop in the electrolyte will be. electrode. The smaller the electrode gap, the lower the voltage drop in the electrolyte will be. The The application of an electrochemical ﬂow microreactor can dismiss the resistance associated with the application of an electrochemical flow microreactor can dismiss the resistance associated with the electrolyte solution as it typically has a very small interelectrode distance [180,207]. For this reason, microflow electrolysis becomes possible in dilute electrolyte solutions or even obviates the need for supporting electrolytes, depending on the innate conductivity of the solvent system. Additionally, constant potential electrolysis becomes more feasible compared to batch, as good selectivity can still be accompanied by good conversion rates as well as high control of the degree of functionalization. A substrate’s degree of functionalization can be manipulated by managing the number of electrons supplied (F/mol).
Molecules 2019, 24, 2122 26 of 38 electrolyte solution as it typically has a very small interelectrode distance [180,207]. For this reason, microﬂow electrolysis becomes possible in dilute electrolyte solutions or even obviates the need for supporting electrolytes, depending on the innate conductivity of the solvent system. Additionally, constant potential electrolysis becomes more feasible compared to batch, as good selectivity can still be accompanied by good conversion rates as well as high control of the degree of functionalization. A substrate’s degree of functionalization can be manipulated by managing the number of electrons supplied (F/mol). Similar to how ohmic drop impedes electrosynthesis, the Beer–Lambert–Bouguer law limits photochemical transformations (Figure 14). As the attenuation eﬀect of photon transport—or absorption (A)—is proportional to the penetration depth (d), photochemical reactions only occur in a limited ﬁlm (millimeter magnitude) on the outside of the reactor [208–211]. This limited transmittance of irradiation due to absorption results in extended reaction times, especially in batch. This is a serious problem, and probably the biggest criticism on photoredox catalysis from a process chemistry point of view. Comparing photoredox catalyzed and electrochemical transformations in organic synthesis in this review, two observations are that generally the reaction times are longer and/or the scale in which they are performed are smaller for the photochemical approaches in comparison to the electrochemical. This is most likely a result of the limitation that the Beer–Lambert–Bouguer law sets. Standard upscaling of batch reactions will decrease the surface-to-volume ratio, which in turn will lengthen reaction times to impractical scale and increase the probability of side reactions. Nonetheless, scale-up of photoredox catalyzed reactions is possible, as displayed in Section 2.10[ 165,166] and elsewhere [209,210]. These issues can be circumvented by the same technology electrosynthesis uses to cope with ohmic drop. Continuous-ﬂow reactors have good surface-area-to-volume ratios and allow for more eﬃcient irradiation of the reaction mixture opposed to a dimension-enlarging strategy for scale-up [208–212]. Scalability of electrochemical transformations is possible in batch (Section 2.10) as the addition of supporting electrolytes can counteract ohmic voltage drop. However, this is not an ideal solution because of the associated cost, substantial source of waste and obstacles with puriﬁcation (vide supra). Flow microreactors can alleviate these issues for electrochemical methodologies as well .
3.3. Remoteness of Redox Events and Overall Redox Demand of Transformations Even though the gap between the anode and the cathode can be signiﬁcantly reduced, the anodic oxidation and cathodic reduction are still separated on a macroscopic level. With photoredox catalysis, a homogeneous photocatalytic system allows for oxidation and reduction events to happen uniformly. This has signiﬁcant consequences on the diﬀerent types of redox transformations. In this section we will diﬀerentiate between net oxidative, net reductive and redox-neutral transformations. Net oxidative redox reactions are transformations for which an electron acceptor is required to act as the terminal oxidant. The opposite is true for net reductive redox reactions, demanding an electron donor as the terminal reductant. Redox-neutral transformations are characterized to have both oxidation and reduction steps involved in the same mechanism while retaining overall redox neutrality. From a mechanistic point of view, this can be more arduous than net oxidative or reductive transformations because both donation and acceptance of electrons is necessary at distinct positions in the reaction mechanism. For this reason, it is more facile for photoredox catalysis to execute and manage overall redox-neutral transformations than electrochemistry can. Due to the short-lived nature of the excited state photocatalyst and quenched, redox-active counterpart, both oxidants and reductants will be transiently present in the same reactive system. This temporal and disperse presence of both electron donors and acceptors allows for redox-neutral reactions to be possible. This is an advantage that photoredox catalysis has over methods utilizing stoichiometric chemical oxidants and reductants, as they are typically incompatible with one another. Achieving redox-neutral transformations via an electrochemical approach is very challenging as well because the oxidation and reduction events are macroscopically separated (mm-cm) due to the gap between anode and cathode, respectively. In Section 2.6, a redox-neutral transformation was displayed by both photoredox and electrocatalysis. Molecules 2019, 24, 2122 27 of 38
Nonetheless, for the electrochemical method, a stoichiometric sacriﬁcial HAT reagent is used to aﬀord the ﬁnal product instead of employing a catalytic HAT source in the photoredox process. This demonstrates that photoredox catalysis is more capable of taking advantage of the redox-neutral nature of such transformations. The two half-reactions occur within the same catalytic cycle for photoredox catalysis, while the oxidation and reduction half-reactions take place at remotely distinct locations in the electrochemical approach (Figure 15). As a result, most electrochemical transformations reportedMolecules this 2019 far, 24 are, x netFORoxidative PEER REVIEW and net reductive in nature (with the majority anodic oxidations)27 of [3 –385 ]. In these methodologies, typically only one of the half-reactions aﬀects substrates of interest. Yet, when thehalf-reactions transformation affects is executed substrates at of the interest. working Yet, electrode, when the thetransformation inverse half-reaction is executed simultaneouslyat the working electrode, the inverse half-reaction simultaneously occurs at the counter-electrode. In most of the occurs at the counter-electrode. In most of the recent approaches, this implicates the ineﬀectual recent approaches, this implicates the ineffectual redox conversion of solvent, electrolyte or other redox conversion of solvent, electrolyte or other sacriﬁcial species. To maximize energy eﬃciency, sacrificial species. To maximize energy efficiency, both electrodes could be engaged by executing two both electrodes could be engaged by executing two desirable processes simultaneously [3,213]. This is desirable processes simultaneously [3,213]. This is called paired electrolysis and allows valuable called paired electrolysis and allows valuable products to be formed at both electrodes. products to be formed at both electrodes.
FigureFigure 15. Visualization15. Visualization to emphasize to emphasize the di ﬀtheerence difference in remoteness in remoteness of redox eventsof redox between events photoredox between catalysisphotoredox and electrochemistry. catalysis and electrochemistry.
UnderstandingUnderstanding their their underlying underlying physicochemical physicochemical structure,structure, itit makesmakes sense to, to, figuratively ﬁguratively speaking,speaking, have have these these systems systems run run on on their their own own “redox-fuel”, “redox-fuel”, toto maximizemaximize energy efficiency eﬃciency and and minimizeminimize waste waste production. production. This This entails entails keeping keeping the photoredox the photoredox catalytic catalytic cycle acycle closed a circuit,closed withoutcircuit, without adding external oxidants or reductants. The electrochemical half-reactions are ideally both adding external oxidants or reductants. The electrochemical half-reactions are ideally both beneﬁtted benefitted from, to afford valuable products at both electrodes (paired electrolysis). This implies that from, to aﬀord valuable products at both electrodes (paired electrolysis). This implies that photoredox photoredox catalysis is ideal for conducting redox-neutral transformations due to the homogeneous catalysis is ideal for conducting redox-neutral transformations due to the homogeneous presence and presence and transient nature of the redox-active excited photocatalyst and complementary transient nature of the redox-active excited photocatalyst and complementary oxidized/reduced state. oxidized/reduced state. Electrochemistry shows its full potential in net oxidative and net reductive Electrochemistrytransformations shows as electrons its fullpotential are inherently in net clean, oxidative renewable and net and reductive inexpensive transformations reactants. The as adoption electrons are inherentlyof electric clean,current renewable circumvents and the inexpensive use of conven reactants.tional Thechemical adoption oxidants of electric and currentreductants circumvents and the the usewaste of source conventional that goes chemical along with oxidants them, and increasi reductantsng atom and economy. the waste Consequently, source that electrosynthesis goes along with them,can increasing be used as atoma methodological economy. Consequently, strategy towards electrosynthesis green chemistry can [213,214]. be used This as does a methodological not preclude strategyphotoredox towards catalysis green chemistry from being [213 ,214a viable]. This opti doeson not towards preclude green photoredox net oxidative catalysis or from reductive being a viabletransformations. option towards Due green to the net external oxidative energy or reductive input via transformations. light irradiation, Due the to terminal the external oxidant energy or inputreductant via light can irradiation, be exceedingly the terminal mild in oxidant nature. A or good reductant example can is be the exceedingly use of oxygen mild as inthe nature. sole terminal A good exampleoxidant is thewhere use the of oxygenphotocatalyst as the acts sole as terminal the active oxidant oxidant where (Sections the photocatalyst2.2–2.5, 2.8, 2.10). acts They as the serve active as oxidantmild (Sections alternatives 2.2– to2.5 transformations, 2.8 and 2.10). Theythat woul served as otherwise mild alternatives need strong, to transformations waste-intensive that oxidants. would otherwiseMoreover, need similar strong, to waste-intensive electrons, photons oxidants. are read Moreover,ily available, similar clean toelectrons, and renewable photons reagents are readily for available,organic clean synthesis. and renewable Due to the reagents nature for of organic the energy synthesis. operated Due on, to themoving nature towards of the energy a future operated with on, movingrenewable towards energy a will future cause with both renewable electrochemistry energy will and causephotoredox both electrochemistry catalysis to become and intrinsically photoredox catalysiseven greener. to become intrinsically even greener.
3.4. Product Scope, Reactivity, Selectivity and Functional Group Tolerance When both photoredox catalyzed and electrochemical methodologies exist for the same transformation, similarities in scope are often found, yet not trivially so. Opposite functional group tolerance and reactivity were also showcased in this review. As discussed above, electrochemistry and photoredox catalysis often compare very well to each other regarding their mode of activation. This can result in analogous reactivity, selectivity and functional group tolerance as extensively
Molecules 2019, 24, 2122 28 of 38
3.4. Product Scope, Reactivity, Selectivity and Functional Group Tolerance When both photoredox catalyzed and electrochemical methodologies exist for the same transformation, similarities in scope are often found, yet not trivially so. Opposite functional group tolerance and reactivity were also showcased in this review. As discussed above, electrochemistry and photoredox catalysis often compare very well to each other regarding their mode of activation. This can result in analogous reactivity, selectivity and functional group tolerance as extensively reviewed in Section2 (Side by Side Comparison of Synthetic Methodologies). Conversely, opposite reactivity and functional group tolerance were highlighted as well. A commonality among these latter examples is the diﬀerent—or absent—nature of a co-catalyst, when comparing both methodologies. For example, it is hardly surprising that visible-light mediated palladium co-catalysis leads to diﬀerent reactivity than mediated electrosynthesis (Sections 2.3 and 2.4). Similarly, the dehydrogenative lactamization performs better on diversely substituted amides, depending on the mode activation of the N-H bond (Section 2.2). The activation via the discussed PCET approach performs best on N-aryl benzamides, while the electrosynthesis of lactams was most productive for the N-acyloxy species. Additionally, the synthesis of complex quinolinone derivatives from N-phenyl cinnamamides was only possible via the photochemical approach due to the intrinsic added value it provides, being E/Z isomerization. Opposite functional group tolerance and reactivity was also observed for the decarboxylative sulfonylation (Section 2.5). Again, the involvement of an additive (KI) in the proposed mechanism could possibly explain this. Many discussions that focus on achievable product scope have been discussed in the paragraphs above, as they are often innately linked to their modes of activation and restrictions. For the majority of the transformations discussed in this review, photoredox catalysis displayed either similar or superior selectivities in comparison with the electrochemical method. The only exception in this investigation is the alkyne oxygenation (Section 2.8). It is conceivable that an overarching explanation to this phenomenon exists, yet at this point, the coincidence hypothesis cannot be unambiguously discarded. A larger study would be necessary to back this claim, though in their current development, electrochemical methods appear to deliver faster and more robust substrate conversions compared to photoredox catalysis, which in turn demonstrates a more mild and selective reactivity proﬁle.
4. Conclusions In this review, we summarized and discussed 10 transformations that were performed by both photoredox catalysis and electrochemistry, and one photoelectrochemical method integrating these two. The two methodologies were subjected to a comparative investigation concerning the general reaction conditions, mechanism and product scope. The ﬁndings from these individual comparisons were compiled and used as basis for analysis to investigate where both research domains interlock and where they diﬀer. Analogies were drawn between both methods to get a better understanding of both areas of research. While this survey does not indicate an overall preference for either of the two synthetic approaches, it does demonstrate the situations in which they manifest authority or restrictions over the other. In particular, a methodological design can be built around the approach that is theoretically favorable in terms of the overall redox demand of the transformation under investigation. Conceptually, photoredox catalysis is generally preferable for redox-neutral transformations. Conversely, electrochemistry excels when net reductive and oxidative transformations are contemplated. Nonetheless, it was clearly shown that both methods are able to bring about the same synthetic transformation, for which a very similar mode of activation is often the main reason. Due to this fundamental resemblance, this study also serves to provide a tool for the design of novel synthetic methodologies in both research areas, starting from previous knowledge of the other. However, we realize a case-by-case approach is still necessary. Moreover, appreciation of the underlying physicochemical and mechanistic structures will help to reﬁne and innovate these two disciplines of organic synthesis. Additionally, an example was displayed where the combination of photoredox catalysis and electrochemistry leads to a constructive merger, where the shortcomings of one technique Molecules 2019, 24, 2122 29 of 38 are complemented by the other. We believe this holistic eﬀect has the potential to drive a powerful interdisciplinary addition to the synthetic toolbox. Both methodologies are still facing, often similar, challenges. Going forward, we recognize that both research areas will have to solve similar problems in response to these challenges. The adoption of electrons and photons as readily available, clean and renewable reagents circumvents the use of conventional chemical oxidants and reductants and their concomitant waste. Consequently, photoredox catalysis and electrosynthesis can oﬀer mild, alternative methodological strategies towards green chemistry.
Author Contributions: The manuscript was written by R.H.V. with input from W.M.D.B. Funding: This research was funded by FWO Vlaanderen, grant number 11D6218N. Acknowledgments: R.H.V. thanks FWO Vlaanderen for the fellowship received (aspirant FWO Vlaanderen). Conﬂicts of Interest: The authors declare no conﬂict of interest.
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