Electrocatalytic CâH Activation - ACS Catalysis (ACS Publications)

Review Cite This: ACS Catal. 2018, 8, 7086−7103

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Electrocatalytic C−H Activation Nicolas Sauermann, Tjark H. Meyer, Youai Qiu, and Lutz Ackermann*

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Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany ABSTRACT: C−H activation has emerged as a transformative tool in molecular synthesis, but until recently oxidative C−H activations have largely involved the use of stoichiometric amounts of expensive and toxic metal oxidants, compromising the overall sustainable nature of C−H activation chemistry. In sharp contrast, electrochemical C−H activation has been identified as a more efficient strategy that exploits storable electricity in place of byproduct-generating chemical reagents. Thus, transition-metal catalysts were shown to enable versatile C−H activation reactions in a sustainable manner. While palladium catalysis set the stage for C(sp2)−H and C(sp3)−H functionalizations by N-containing directing groups, rhodium and ruthenium catalysts allowed the use of weakly coordinating amides and acids. In contrast to these precious 4d transition metals, the recent year has witnessed the emergence of versatile cobalt catalysts for C−H oxygenations, C−H nitrogenations, and C−C-forming [4+2] alkyne annulations. Thereby, the use of toxic and expensive silver(I) oxidants was prevented, improving the environmentally benign nature of C−H activation catalysis. Herein, we summarize the recent major advances in organometallic activations of otherwise inert C−H bonds by electrocatalysis through May 2018. KEYWORDS: C−H activation, cobalt, electrochemistry, electrosynthesis, mechanism, pyridines, transition metal photo-induced organic transformations.9−17 However, the direct use of daylight continues to be challenging because of the strong variations in the accessible energy during day and night time. Hence, solar energy is usually first converted into storable electrical energy. In addition, photo-redox catalysis often required precious transition-metal catalysts for strong C−H bond activation,18,19 and largely employed costly reagents as the sacrificial redox mediators.9−17 In sharp contrast, the direct use of storable electricity compares favorably, and arguably represents an attractive means for enabling chemical reactions, with particular potential for redox transformations (Figure 1).20,21 In the past decade, transition-metal-catalyzed C−H activation has emerged as a powerful tool in molecular sciences, which significantly reduces the number of synthetic operations.22−26 At the same time, C−H activation prevents the formation of undesired byproducts, thereby improving the overall footprint of organic syntheses. Yet, despite indisputable advances in the field, redoxidative C−H activation processes were until recently largely limited to stoichiometric quantities of expensive or toxic chemical reagents, such as copper(II) and silver(I) salts, as the sacrificial oxidants, compromising the overall sustainable nature of C−H activation (Scheme 1a). In addition, metal-free, innate transformations were predominantly realized with activated substrates in terms of

1. INTRODUCTION The use of alternative forms of energy, including wind power, solar energy, and hydropower (Figure 1), to establish novel molecular transformations bears great potential for establishing sustainable organic syntheses.1−3 When thermal reactions are performed, electricity is initially transferred in an inefficient manner into thermal energy due to dissipative processes.4−7 As a consequence, recent years have witnessed a renaissance8 of

Received: April 29, 2018 Revised: June 15, 2018 Published: June 18, 2018

Figure 1. Gross electricity generation in Germany in April of 2018, in TWh,20 and overall energy mix.21 © XXXX American Chemical Society

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DOI: 10.1021/acscatal.8b01682 ACS Catal. 2018, 8, 7086−7103

Review

ACS Catalysis Scheme 1. (a) Stoichiometric Oxidants, (b) Electrochemical C−H Activation, and (c) Comparison of Bond Dissociation Energies27−30

nucleophilicity, oxidation potential, and bond dissociation energies (Scheme 1c).27−30 Since the early reports by Kolbe in the 1840s,31,32 the enormous potential of electro-organic synthesis has been repeatedly exploited.33−42 Hence, electro-transformations of activated substrates have been explored since the 1960s and have proven instrumental for recent elegant developments in electro-organic synthesis, exploiting the innate reactivity profile of a given organic molecule. Hence, electrochemical, metal-free functionalizations have gained considerable recent momentum due to major contributions from Waldvogel,43−49 Baran,50−54 Yoshida,55−61 and Xu,62−68 among others.69−75 While those findings significantly broadened the viable scope of electrosynthesis, these transformations are based on the inherent reactivity and exploit the oxidation or radical formation in somewhat preactivated positions. In contrast, the transformation of strong C−H bonds with a bond dissociation energy BDEC−H > 113.5 kcal/mol30 and pKa > 4476 is challenging under metal-free conditions (Scheme 1c). Here, the enabling merger of metal-catalyzed C−H activation with electrocatalysis constitutes the key to success for the development of environmentally benign syntheses beyond the naturally encoded innate reactivities. Thus, electrochemistry holds great potential to replace costly stoichiometric redox reagents in organometallic C−H activation by userfriendly cost-effective electricity (Figure 2).77 Herein, we summarize the recent merger of electrocatalysis with organometallic C−H activation through Spring 2018.78

Figure 2. Costs of common oxidants and reductants.

Scheme 2. Electrocatalytic Fujiwara−Moritani Reaction

cant potential of merging palladium-catalyzed C−H activation with electrosynthesis. The arylation of acrylates 2 and styrenes 3 was proposed to be initiated by a base-assisted83 C−H activation manifold, followed by a migratory insertion of the double bond into the palladium−carbon bond.79 The subsequent β-elimination liberates the final product 5/6, and reductive elimination of acetic acid generates the key palladium(0) species 10. The palladium(0) intermediate 10 is then reoxidized by BQ 4. The thus formed hydroquinone 11 is itself reoxidized at the carbon anode, while the reduction of protons to molecular hydrogen takes place at the nickel cathode (Scheme 3). In 2009, Kakiuchi disclosed a palladium-catalyzed halogenation using electrochemical C−H functionalization by palladium catalysis (Scheme 4).84 In contrast to the previous Fujiwara−Moritani approach by Amatore and Jutand,79 here

2. PALLADIUM-CATALYZED TRANSFORMATIONS An early example of the combination of palladium-catalyzed C−H activation and electrochemistry was elegantly designed by Amatore and Jutand in 2007.79 In their pioneering contribution, a palladium(II/0)-catalyzed Fujiwara−Moritanitype80−82 C−H alkenylation reaction was devised (Scheme 2),79 using co-catalytic amounts of p-benzoquinone (4, BQ) as a redox mediator.79 Thus, the formed palladium(0) species was not directly oxidized at the electrode surface, but rather the electron shuttle BQ (4) was interacting with the anode. This example illustrated that palladium C−H activation catalysis is compatible with electrosynthesis, while indicating the signifi7087


Review

ACS Catalysis Scheme 3. Proposed Catalytic Cycle for the Electrochemical Fujiwara−Moritani Reaction

Scheme 5. Proposed Catalytic Cycle for the Electrochemical C−X Formation

Scheme 6. (a) Electrochemical Palladium-Catalyzed Iodination and (b) One-Pot Suzuki−Miyaura Coupling Scheme 4. Electrochemical Halogenation of Phenylpyridines 12a

a

PdBr2 (2.0 mol %) and 2M HBr(aq) used.

oxidation of a palladium(0) intermediate was not occurring.84 Instead, the electricity was solely required for the generation of the electrophilic Cl+ cation from mineral acid. This presents an elegant and cost-efficient alternative to the use of reactive halogenation reagents, such as N-halosuccinimides, which drastically improves the efficiency of direct halogenation.84 In contrast to the potentiostatic approach by Amatore and Jutand,79 Kakiuchi used constant current electrolysis (CCE),84 which resulted in an arguably overall more user-friendly protocol. The catalyst’s working mode was proposed to follow the pathway of palladium(II)-catalyzed C−H halogenations by initial coordination through the Lewis-basic nitrogen, followed by a proximity-induced electrophilic C−H palladation (Scheme 5). The cyclometalated intermediate 15 then forms a C−X bond with the electrochemically generated halonium ion, which finally liberates the desired product 13 by ligand exchange at the palladium center. Later, the C−H halogenation strategy was nicely extended to enable C−H iodinations (Scheme 6a).85 Here, elemental iodine proved amenable as the iodonium precursor of choice, while potassium iodide was also found to be a competent iodonium source. Additionally, the authors demonstrated the power of this approach by palladium(0)-catalyzed Suzuki− Miyaura coupling with the thus-formed products in a one-pot

fashion, thereby highlighting the versatility of the halogenated arenes 13 as transient functional groups86 in organic synthesis (Scheme 6b). In addition to C−H halogenations, direct C−H oxygenations87,88 have been of interest in electrochemically guided metal-catalyzed C−H activation. In 2013, the group of Budnikova found that palladium complexes are competent catalysts in the perfluoroalkoxylation of phenylpyridines 12 (Scheme 7a).89 While the scope of this C−H functionalization appeared to be thus far limited to perfluoroalkylated acids 18, the authors provided invaluable information on the reaction mechanism, including the isolation of key intermediates (Scheme 7b) as well as informative cyclic voltammetry (CV) studies. Based on their findings on electrochemical C−H halogenations,84,85 Kakiuchi devised reaction conditions for the homodimerization of phenylpyridines 12 in the presence of stoichiometric or co-catalytic amounts of iodine (Scheme 8).90 It is noteworthy that a remarkable regioselectivity was achieved with meta-substituted arenes 12, which cannot be matched by chemical oxidation conditions with oxone.91 Also, the reaction 7088


Review

ACS Catalysis Scheme 7. (a) Palladium-Catalyzed Electrochemical C−H Perfluorooxygenation and (b) Stoichiometric Studies

Scheme 9. Electrochemical C−H Chlorination of Benzamides 23 by Bidentate Assistance

Scheme 8. Homodimerization of Phenylpyridines 12

chlorination of the quinoline in C-5 and C-7 position led to mixtures of multichlorinated products.100 The practical relevance of the electrochemical chlorination of phenylpyridines 12 and benzamides 23 was mirrored by an elegant synthesis of vismodegib (27),100 a marketed drug for cancer treatment (Scheme 9b).105,106 To gain further insights into the reaction mechanism, cyclometalated palladium(II) complex 28 was isolated and shown to undergo electrophilic chlorination under the electrochemical conditions in a stoichiometric fashion (Scheme 9c).100 This finding is suggestive of the complex 28 being a kinetically relevant, on-cycle intermediate of the electrocatalytic transformation. Thus far, electrochemical C−H activation was severely limited to C(sp2)−H transformations. In sharp contrast, the use of electricity in palladium-catalyzed C(sp3)−H oxygenation was achieved by Mei (Scheme 10),107 whichto the best of our knowledgerepresented the first C(sp3)−H activation by electrochemical transition-metal catalysis. Mei achieved the desired reaction with Pd(OAc)2 as the catalyst in a divided cell setup, using carboxylic acids as the solvent and the corresponding sodium salts as the base. The broad scope as to the substitution pattern at the oxime 29 is noteworthy. Valuable functional groups, such as ester, nitrile, and halides, were also well tolerated. Likewise, the enantiomerically enriched oxazoline 32 underwent the C−H oxygenation without racemization of the stereogenic center. The reaction mechanism was proposed to initiate by a proximity-induced base-assisted C−H metalation at the neighboring primary C(sp3)−H bond (Scheme 11). The

featured a good tolerance of valuable functional groups, such as ester and bromide substituents.90 More recently, a C−H phosphorylation was disclosed by Budnikova by means of electrocatalysis.92,93 The generated products are of considerable interest for their coordination abilities and their potential application as ligands. In these mechanistic studies, crucial intermediates could be isolated and were shown to be competent in stoichiometric settings. Moreover, detailed CV studies provided strong support for the involvement of a palladium(IV) species within the catalytic cycle. Budnikova also reported on the perfluoroalkoxylation of phenylpyridines 12 by means of electrochemistry, during which considerable amounts of perfluoroalkylated product were observed as a byproduct at higher currents.89 The same group disclosed an improved protocol, and perfluoroalkyl bromides as well as perfluoroalkylcarboxylic acids 18 proved to be viable substrates likewise.94−96 This approach was suggested not to be limited to palladium catalysis.94,97−99 Until recently, electrochemical palladium-catalyzed C−H activation was largely restricted to strongly coordinating phenylpyridines 12 and electron-rich anilides 1. In contrast, Kakiuchi established C−H chlorinations with synthetically useful electron-withdrawing benzamides 23 by bidentate chelation assistance (Scheme 9a).100 Here, the bidentate 8aminoquinoline directing group (8-AQ)101−104 had to be adjusted, because otherwise side reactions of undesired 7089


Review

ACS Catalysis Scheme 10. Palladium-Catalyzed Primary C(sp3)−H Oxygenation

Scheme 12. Palladium-Catalyzed C−H Alkylation

Scheme 13. Proposed Catalytic Cycle for the Electrochemical C−H Methylation of Oximes 36

Scheme 11. Plausible Mechanism for the PalladiumCatalyzed C(sp3)−H Oxygenation

Pd(IV) species. This key step could take place either by transmetalation under anodic oxidation conditions with MeBF3K (37) or by the attack of a radical generated in situ from the same reagent at the anode surface. Finally, the product 38 is obtained by the oxidation-induced reductive elimination. Moreover, Mei established the palladium-catalyzed C−H benzoylation of oximes 36 (Scheme 14).108 Yet, the reaction thus far featured 2-oxo-2-phenylacetic acid (41) as the sole coupling partner. Based on his previous finding on C(sp3)−H oxygenation,107 Mei also developed the oxygenation of aromatic C−H bonds with a related catalytic system using oximes 36 as the directing group (Scheme 15a).109 The reaction temperature could be slightly decreased. Ortho-substituted arenes 36 performed somewhat less efficiently. A range of valuable functional groups was tolerated, and likewise alkenes 43 were identified as viable substrates. Independently, Sanford found an electrochemical C−H oxygenation (Scheme 15b), broadening the substrate scope to include quinoline 45 and pyrazine 46 directing groups.110 Electrophilic functionalities, such as esters and nitro

intermediate 33 is then likely oxidized at the anode to palladium(IV) complex 34, which can subsequently undergo reductive elimination to form the C−O bond, followed by a ligand exchange to liberate the desired oxygenated product 30. Besides kinetic isotope effect (KIE) studies (kH/kD ≈ 3.7 in parallel experiments), the authors also conducted detailed CV experiments of their reaction to substantiate the proposed palladium(IV) intermediate 34. Mei also discovered a transformative C−C formation in terms of oxidative C−H methylation with methyltrifluoroborates 37 (Scheme 12a).108 The authors disclosed a wide oxime 36 scope, while the C−C-forming manifold seemed mostly applicable to methylations. A KIE of kH/kD ≈ 1.2 was determined, and the cyclometalated complex 39 was isolated and unambiguously characterized (Scheme 12b). The complex 39 was further shown to be a competent catalyst for the C−H functionalization reaction. With this information in hand, the authors proposed a catalytic cycle (Scheme 13), which was suggested to proceed via base-assisted metalation83 of the oxime substrate 36, followed by the generation of a Pd(III) or 7090


Review

ACS Catalysis Scheme 14. Palladium-Catalyzed Oxidative Benzoylation

Scheme 16. Electrochemical Rhodium-Catalyzed C−H Alkenylation

Scheme 15. Palladium-Catalyzed C−H Oxygenation

benzoic acids 51, benzamide 53 and pyrimidylindole 55 were shown to be competent substrates likewise (Scheme 17). Scheme 17. Electrochemical Rhodium-Catalyzed C−H Alkenylation of Benzamide 53 and Indole 55

The rhodium catalyst’s mode of action was investigated in detail (Scheme 18).125 A competition experiment revealed the Scheme 18. Key Mechanistic Findings substituents, were well tolerated, while oximes 29 showed a reduced efficacy under these reaction conditions.

3. RHODIUM-CATALYZED C−H ACTIVATION Rhodium(III)-catalyzed C−H transformations111,112 have proven instrumental for the oxidative synthesis of carbo- and heterocycles, with key contributions by Miura and Fagnou, among others.113−118 Despite major advances,119−124 oxidative rhodium-catalyzed C−H transformations were largely restricted to the use of copper(II) or silver(I) salts as the stoichiometric oxidants. In contrast, Ackermann recently reported on the first electrochemical C−H activation by rhodium catalysis to realize cross-dehydrogenative alkenylations with weakly coordinating benzoic acids 51 (Scheme 16).125 The versatility of the electrochemical rhodium-catalyzed C− H alkenylation includes sensitive ketones and esters.125 It is noteworthy that more sterically hindered ortho-substituted arenes 51 proved to be viable substrates as well. In addition to

slight preference for more electron-rich benzoic acids 51b, being in agreement with a base-assisted internal electrophilic substitution (BIES) mechanism.126−131 Furthermore, in studies with [D]4-MeOD as the solvent, deuteration in the ortho positions of the re-isolated benzoic acid [D]n-51e as well as in the product [D]n-52e was noted.125 In agreement with 7091


Review

ACS Catalysis this finding, a KIE was not observed. Further, the reaction was studied by CV in terms of its microscopic reverse, and calculations on the oxidation potential of the proposed intermediate 61 were performed. Based on these studies, a plausible catalytic cycle was put forward (Scheme 19).125 After the formation of rhodium

Scheme 20. Ruthenium-Catalyzed Electrochemical Indole Synthesis

Scheme 19. Proposed Catalytic Cycle for the RhodiumCatalyzed Electrochemical Alkenylation

Scheme 21. Ruthenium-Catalyzed Electrochemical Isoquinoline Synthesis biscarboxylate complex 57, a facile C−H activation leads to the formation of five-membered rhodacycle 58. Coordination of the alkene 2 and insertion into the rhodium−carbon bond lead to intermediate 60, which after β-hydride elimination and reductive elimination forms the proposed rhodium(I) sandwich complex 61. Ligand exchange and anodic oxidation of the rhodium center regenerate the active catalyst 57 and furnish the desired phthalide 52.

Scheme 22. Key Mechanistic Findings

4. RUTHENIUM-CATALYZED C−H ACTIVATION Ruthenium catalysts feature unique chemoselectivities132−142 for sustainable C−H activation processes.143−146 Intriguing studies on the use of ruthenium catalysts in electrochemical C−H activation were recently reported by Xu using a ruthenium complex in an alkyne annulation by pyrimidylanilines 62,147 which was previously achieved with chemical oxidants (Scheme 20).148 The substrate scope of the electrooxidative reaction was generally broad, tolerating a range of valuable functional groups in various positions.147 Additionally, unsymmetrical internal alkynes 63 were converted with good levels of regiocontrol. Furthermore, preliminary findings indicated that a ruthenium-catalyzed [4+2] annulation of benzylamine 65 to isoquinolines 66 is viable, albeit as of yet with lower catalytic efficiency (Scheme 21). Key mechanistic findings by Xu included a reversible H/D exchange in the product 64 as well as the re-isolated starting material 62 (Scheme 22).147 Additionally, a preference for the more electron-rich substrates 62 was observed in competition experiments. An inhibitory effect of high concentrations of the alkyne 63 was observed, which was rationalized by the reversible formation of an off-cycle ruthenium−alkyne complex 69. Finally, a reaction with stoichiometric amounts of ruthenium complex 67 in the absence of electricity led to

comparable yields, suggesting that electricity is needed to regenerate the active ruthenium(II) catalyst. Hence, the mechanism depicted in Scheme 23 was proposed, commencing with the in situ formation of the active ruthenium(II) carboxylate complex 68. Substrate 62 coordination and subsequent C−H activation lead to the sixmembered ruthenacycle intermediate 71, which can then undergo migratory insertion with the alkyne 63. From the formed intermediate 73, indole 64 is liberated by reductive elimination, followed by anodic oxidation of the ruthenium(0) intermediate 74. The inhibition by the alkyne 63 is rationalized by an off-cycle ruthenium alkyne complex 69, formed by 7092


Review

ACS Catalysis Scheme 23. Plausible Mechanism for the Electrochemical Ruthenium-Catalyzed Indole Synthesis

Scheme 25. Ruthenium-Catalyzed Electrochemical C−H/ N−H Functionalization and CV Studies

coordination of the alkyne 63 to the ruthenium(II)carboxylate 68. Concurrently, Ackermann independently developed a ruthenium-catalyzed synthesis of isocoumarins 75 (Scheme 24).149 Particularly, the use of weakly coordinating benzoic Scheme 24. Ruthenium-Catalyzed Electrochemical C−H/ O−H Functionalization

revealing a significant influence of the acid concentration150,151 on the oxidation potential.149 Based on these findings, a plausible catalytic cycle was collaborated (Scheme 26), which includes first the formation of the five-membered ruthenacycle 78 by BIES C−H Scheme 26. Plausible Mechanism for the RutheniumCatalyzed C−H/O−H Annulation

acids 51 is worthy of note,143 featuring for the first time electrochemical C−H activation of substrates being devoid of strong N-coordination. The ruthenium-catalyzed electro-oxidative C−H activation was characterized by high catalytic efficacy and good functional group tolerance that included chloride, bromide, esters, and nitriles.149 Moreover, benzamides 53 were found to be viable substrates, yielding the corresponding isoquinolones 76 (Scheme 25a). Mechanistic studies revealed significant deuterium incorporation in the presence of an isotopically labeled cosolvent and a preference for electron-rich benzoic acids 51, both of which render a BIES-type126−131 C−H activation likely to be operative (Scheme 25b). Additionally, the proposed ruthenium(0) intermediate 77 was independently prepared, and its key oxidation was probed by CV, 7093


Review

ACS Catalysis metalation,126−131 followed by migratory insertion of the alkyne 63.149 The resulting seven-membered ruthenacycle 80 undergoes reductive elimination to generate the ruthenium(0) sandwich complex 77, which upon anodic oxidation liberates the desired product 75 and regenerates the active ruthenium species 68.

The electrochemical cobalt-catalyzed C−H oxygenation proved viable on electron-rich as well as electron-deficient benzamides 81 using various alcohols 84 (Scheme 28).177 Scheme 28. Cobalt-Catalyzed C−H Oxygenation of Benzamides 81

5. COBALT-CATALYZED C−H FUNCTIONALIZATIONS While notable recent progress has been achieved using precious 4d and 5d transition metals in C−H activation, earth-abundant base metals have emerged as cost-effective alternatives in the field (Figure 3).102,103,152−160 While

Valuable functional groups, such as tertiary amines, thioether, ester, ketone, nitrile, bromo, and iodo substituents, were fully tolerated, being a strong testament to the robust nature of the cobalt-catalyzed electrochemical C−H activation regime (Scheme 29).177 It is especially noteworthy that the

Figure 3. Costs of common metals in C−H activation.176

Scheme 29. Electrochemical C−H Oxygenation of Alkenes 89

reductive couplings using 3d metals had been reported earlier,161−168 electrochemical C−H activation had been limited to expensive noble metal catalysts. In sharp contrast, Ackermann identified cobalt complexes as the catalyst of choice in electrochemical C−H activation. While cobaltcatalyzed transformations had previously been realized with chemical oxidants,153,156 usually169 super-stoichiometric amounts of expensive silver(I) or copper(II) salts were needed as terminal oxidants to facilitate these C−H transformations.170−175 In 2017, Ackermann reported on the first electrochemical C−H activation by cost-effective176 cobalt catalysis.177 Here, the cobalt-catalyzed C−H oxygenation of benzamides 81 was realized, using the bidentate pyridine-N-oxide group, which proved to be essential for high catalytic performance, while the 8-AQ was shown to be inferior (Scheme 27).

smooth conversion of oxidation-sensitive benzylic alcohols as well as of citronellol proved viable. Besides aromatic benzamides 81, also alkenes 89 could be C−H oxygenated in a diastereoselective fashion, further highlighting the versatile nature of the cobalt-catalyzed electrochemical C−H oxygenation. Detailed mechanistic studies revealed an inherent preferred reactivity of the more electron-rich arenes 81 within competition experiments (Scheme 30a).177 A minor KIE of kH/kD ≈ 1.05 was calculated, indicating a facile C−H cleavage (Scheme 30b). By detailed CV studies, Ackermann and coworkers showed that the direct oxidation of the cobalt catalyst occurred at a potential of 1.19 VSCE in MeOH (Scheme 30c). This finding provided support for a single electron transfer

Scheme 27. Directing Group Power in C−H Oxygenation

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ACS Catalysis Scheme 30. Summary of Mechanistic Findings for the Electrochemical Cobalt-Catalyzed C−H Oxygenation

Scheme 32. Electrochemical C−H/N−H Annulation of Benzamides 81

alkenes as amenable substrates. Furthermore, a variety of alkynes 63 could be used, ranging from diversely decorated phenylacetylenes to alkyl-substituted alkynes, with excellent levels of regiocontrol. The reaction also proceeded in nontoxic H2O180 as the sole reaction medium (Scheme 33a), avoiding the use of organic

(SET)178 oxidation of the cobalt salt, followed by an organometallic C−H activation event. Based on these mechanistic findings, a plausible catalytic cycle was proposed (Scheme 31),177 commencing with the

Scheme 33. Electrochemical Alkyne Annulation in H2O

Scheme 31. Plausible Catalytic Cycle of the Electrochemical Cobalt-Catalyzed Oxygenation

electrochemical oxidation of the cobalt(II) salt to generate the active cobalt(III) catalyst 91, along with the subsequent C−H metalation of the benzamide 81. The C−O formation occurs in a chelation-assisted fashion, and proto-demetalation releases the final product 94. The versatile cobalt-catalyzed C−H activation catalyst was not limited to C−O construction. Indeed, Ackermann highlighted that challenging C−C and C−N bonds could be formed by earth-abundant base metal-catalyzed C−H activation with electricity as the sole oxidant. The proof of concept was provided by a cobalt-catalyzed synthesis of isoquinolones 95 (Scheme 32).179 The versatility of the cobalt catalysis regime was reflected by benzamides, heterocycles, and

solvents in an environmentally benign fashion.179 Furthermore, detailed mechanistic studies unraveled a preference of the more electron-rich substrate for both coupling partners, being in good agreement with a BIES-type126−131 mechanism (Scheme 33b). The mechanism of this C−C/N−C formation was proposed to proceed via chelation-assisted C−H metalation, followed by the regioselective insertion of the alkyne 63 into the C−Co bond, to generate the seven-membered intermediate 98.179 The product is then liberated by reductive elimination 7095


Review

ACS Catalysis followed by SET anodic oxidation to regenerate the active cobalt catalyst 96 (Scheme 34).

Scheme 36. Plausible Catalytic Cycle for the Electrochemical C−H Activation/Annulation Using Ethylene (99)

Scheme 34. Proposed Mechanism for the Cobalt-Catalyzed C−H Activation/[4+2]Annulation

Scheme 37. Electrochemical Cobalt-Catalyzed C−H Amination

This approach was thereafter extended by Lei to include the challenging annulation of gaseous ethylene (99) and ethyne (100) (Scheme 35).181 The obtained yields were found largely Scheme 35. Electrochemical C−H Activation/Annulation Using Ethylene (99) or Ethyne (100)

Kinetic experiments revealed a facile C−H cleavage, and CV studies supported a SET-type oxidation of cobalt(II) to cobalt(III). A reaction profile was elaborated by detailed in situ ReactIR technology (Scheme 38a) and clearly showed the absence of a significant initiation period. Furthermore, the formation of molecular hydrogen as the sole byproduct by cathodic reduction was confirmed by headspace gas chromatographic (GC) analysis (Scheme 38b).182 Hence, a proposed catalytic cycle featured the generation of the active catalyst 109 by anodic oxidation, along with chelation-assisted C−H metalation and C−N formation by reductive elimination (Scheme 39).182 In an independent report, Lei disclosed a similar protocol for electrochemical C−H amination, using the 8-AQ177 directing group (Scheme 40).184 Although an elevated reaction temperature of 65 °C and a higher catalyst loading of 20

moderate to good, and the general catalytic system featured minor variations from the Ackermann conditions.179 The report illustrated detailed kinetic analysis, indicating the reaction rate as only being dependent on the operating current. Thereby, a plausible reaction mechanism was proposed (Scheme 36).181 The electrochemical cobalt-catalyzed C−N formation was not limited to an intramolecular179 scenario. Ackermann disclosed the C−H amination of benzamides 81 by electrochemical cobalt catalysis (Scheme 37).182 This report not only highlighted the first electrochemical C−H amination but also described the first use of a renewable biomass-derived solvent in electrocatalysis.183 The amination proceeded smoothly with secondary amines 107 and tolerated aryl halides, as well as esters and various heterocyclic substrates. 7096


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ACS Catalysis

mol % were used, the catalyst performance was inferior.184 Notably, ortho-substituted benzamides 23 were efficiently converted despite their increased steric bulk.184 A plausible catalytic cycle was proposed, based on KIE experiments, which revealed a KIE of kH/kD ≈ 1.13 for intermolecular reactions and 1.70 for the intramolecular reaction (Scheme 41).184

Scheme 38. Kinetic Profile and Headspace GC Analysis

Scheme 41. KIE Experiments for the Electrochemical Cobalt-Catalyzed C−H Amination

Scheme 39. Proposed Catalytic Cycle for the CobaltCatalyzed C−H Amination

Chronoamperometric studies suggested an initial formation of cobalt(III). The reaction is initiated either by the formation of the active cobalt(III) species by anodic oxidation or by an oxidation of the cobalt(II)−substrate complex, both of which result in the same intermediate 115. The intermediate 115 is proposed to undergo base-assisted C−H cobaltation, resulting in the formation of complex 116. From this species 116, formation of the C−N bond is possible by reductive elimination, generating cobalt(I), which is subsequently reoxidized at the anode (Scheme 42).184 Scheme 42. Plausible Mechanism for the Electrochemical Cobalt-Catalyzed Amination of Quinolinamides 23

Scheme 40. Electrochemical Cobalt-Catalyzed Amination of Quinolinyl Amides 23

6. CONCLUSION Electrochemical metal-catalyzed C−H activation has emerged as a powerful tool in molecular synthesis. Thus, palladium catalysts set the stage for electro-oxidative Fujiwara−Moritani reactions as well as C−H oxygenations, methylations, and halogenations. An early report featured the use of benzo7097


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ACS Catalysis

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quinone as redox mediator in palladium(II/0) catalysis, while subsequent studies have illustrated the power of palladium(III) and/or palladium(IV) intermediates for electro-oxidative C−H activation catalysis. Despite significant advances in C(sp2)−H and C(sp3)−H activation, the palladium catalysts were restricted to strongly N-coordinating directing groups. This limitation was recently successfully addressed by means of rhodium(III) and cost-effective ruthenium(II) catalysis. These strategies enabled the use of challenging, weakly coordinating amides and acids. Until very recently, electrochemical C−H activation was realized by precious 4d transition-metal catalysts. However, very recent findings have indicated the unique potential of inexpensive, earth-abundant cobalt catalysis. Hence, C−H oxygenations, C−H nitrogenations, and [4+2] alkyne annulations by C−H/N−H functionalization proved viable in a most user-friendly, undivided cell setup. The chemoselectivity and sustainability of the cobalt-catalyzed C− H activation were, among others, reflected by C−H functionalizations in nontoxic H2O and biomass-derived reaction media, respectively. Thereby, the use of stoichiometric amounts of chemical oxidants, such as expensive and toxic silver(I) salts, could be prevented. Given the sustainable nature of electrochemical C−H activation, notable further progress is expected in this rapidly evolving research area. Thus, further advances using base metals other than cobalt161,162 and stereoselective185−191 C−H transformations are particularly in high demand. The same holds true for addressing the catalyst performance, the viable solvents, suitable supporting electrolytes, and the development of effective scale-up protocols.192

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Lutz Ackermann: 0000-0001-7034-8772 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous support by the DFG (Gottfried-Wilhelm-Leibniz award) and the European Research Council under the Seventh Framework Program of the European Community (FP720072013, ERC Grant Agreement No. 307535) is gratefully acknowledged.

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