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Review
Recent Advances in C-H Functionalization Using Electrochemical Transition Metal Catalysis Cong Ma, Ping Fang, and Tian-Sheng Mei ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01697 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018
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Recent Advances in CH Functionalization Using Electrochemical Transition Metal Catalysis Cong Ma, Ping Fang, and Tian-Sheng Mei* State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu, Shanghai 200032, China ABSTRACT: Electrochemical transition metal catalysis is a powerful strategy for organic synthesis because it obviates the use of stoichiometric chemical oxidants and reductants. C–H bond functionalization offers a variety of useful conversions of simple and ubiquitous organic molecules into diverse functional groups in a single synthetic operation. This review summarizes recent progress in merging electrochemistry with transition metal-catalyzed C–H functionalization, specifically C– C, C–X (halogen), C–O, C–P, and C–N bond formation. KEYWORDS: CH functionalization, electrochemistry, transition metal, electrooxidation, mechanism Introduction Electrochemical organic synthesis has a rich history dating back to 1800, when Italian physicist Alessandro Volta invented the voltaic pile—the first device to emit a steady, lasting current.1,2 In 1830, Michael Faraday performed the first electrolysis of acetic acid.3 Subsequent seminal advances through 1950 include Kolbe’s decarboxylative dimerization (1847),4 Tafel’s electrolytic rearrangement of alkylated ethyl acetoacetates (1907),5 and Simons’s electrolytic C–H fluorination using HF (1949).6 Since then, electrochemical organic synthesis has declined in popularity compared to non-electrochemical reactions such that most organic chemists consider synthesis and electrochemistry as separate areas of study. Nowadays, an electrochemical apparatus is rarely encountered in an organic synthesis laboratory, but electrochemical organic synthesis is experiencing a rebirth. Advances in synthetic organic electrochemistry since 2000 have been discussed in a recent review.7 The past decade has witnessed an explosion of literature involving transition metal-catalyzed CH functionalization reactions, constituting a wide variety of useful direct conversions of simple and ubiquitous CH bonds in organic molecules into diverse functional groups in a single synthetic operation.8 Several challenges have limited the practicality or widespread adoption of various catalytic CH functionalization reactions. First, many metal-catalyzed CH functionalizations require a stoichiometric amount of traditional chemical oxidant, such as a peroxide, a hypervalent iodine, and so forth, thus causing the generation of byproducts and reducing atomic economy.7,11 Second, the external chemical oxidant could cause the selective issue of reductive elimination from metal center.9 Third, the byproduct derived from chemical oxidants could
cause the difficulty of separation.9 Finally, limited catalyst turnover is a major problem in C–H functionalization reactions involving a CH activation step.10 With the development of electrochemical methods and techniques, electrochemistry has come to be regarded as a promising atomless alternative to traditional stoichiometric oxidants and reductants, thereby avoiding additive side reactions and byproducts. Thus, the efficiency of a transition metal-catalyzed process might be improved as a result of the absence of traditional oxidants. Another advantage of electrochemistry is the ability to control the oxidative potential as measured by a reference electrode rather than relying on the intrinsic potential of a chemical oxidant. Thus the oxidation of the metal center of a catalyst can be controlled selectively by using a specific oxidative potential. On the other hand, the schedule of the reaction could be controlled by simply flipping the ON/OFF switch that controls the flow of electric current, which is not possible in conventional synthetic chemistry. Thus, the merger of electrochemistry and transition metal-catalyzed CH functionalization for chemo and regioselective reactions is a rapidly growing area of interest that provides synthetic opportunities that conventional chemistry may not have achieved. While previous reviews have focused on the use of anodic oxidation to regenerate a transition metal catalyst,11 this review summarizes the recent progress made in catalytic C–H functionalization reactions using organometallic electrochemistry. It is divided into sections on CC, CX (halogen), CO, CP, and CN bond formation. To aid the reader, a set of “cell notations” has also been devised to graphically represent the electrochemical parameters of each reaction, including cell type (divided vs. undivided),
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electrolytic conditions (constant current or constant potential), and electrode compositions (Figure 1).
Scheme 1. Regeneration of transition metal catalysts upon electrooxidation
Figure 1. Representative experimental reaction cell set ups, and a set of “cell notations”.
1.
Electrochemical transition CH functionalization
metal-catalyzed
Electrochemical anodic oxidation provides an atom economical alternative to traditional chemical oxidants. In a transition metal-catalyzed reaction, anodic oxidation can be used either to regenerate the active transition metal catalyst, or to oxidize an organometallic intermediate to a high-valent species, where upon subsequent reductive elimination affords the final product and regenerates the catalyst. The use of anodic electrolysis to recycle the active transition metal catalyst is graphically represented in Scheme 1, where in the intermediate C undergoes a reductive elimination process to give the product and reduce the metal’s valency; then the low valent metal can be oxidized to the active catalyst at the anode to complete the catalytic cycle. In contrast, in Scheme 2, the intermediate C' is oxidized to a high-valent metal intermediate at the anode, and is thereby inclined to undergo reductive elimination to close the catalytic cycle. Additional details of these two reaction types are discussed in the context of real reactions below.
Scheme 2. Generation of a high-valent metal species upon anodic oxidation
2.
Electrochemical CC bond formation
Great progress on electrolytic intramolecular and intermolecular CC bond formation (absent transition metal catalysis) has been made by several groups, including those of Waldvogel,12 Lei,13 Yoshida,14 Xu,15 and Harran,16 and electrooxidative organocatalytic asymmetric dehydrogenative CC bond cross-couplings by the groups of Jorgensen,17 Jang,18 and Luo.19 In 2005, the Moeller group illustrated the electrochemically assisted heck reactions of aryliodide substrates with Pd(OAc)2 as the catalyst.20 In 2007, Amatore, Jutand, and co-workers reported Pd(II) catalyzed CH bond olefination reactions of N-acetylanilines 1 with benzoquinone or hydroquinone as substoichiometric redox mediators (Scheme 3).21 Because benzoquinone can be regenerated electrochemically at an anode, only a catalytic amount is needed to continuously recycle the active Pd(II) species. In contrast, a stoichiometric amount of benzoquinone would be required using nonelectrochemical approaches. Control experiments showed
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that only 30% yield was obtained in the absence of benzoquinone.
Scheme 4. Electrooxidative Pd or Ni-catalyzed ortho-perfluoroalkylation of 2-phenylpyridine (Budnikova et al., 2012)22
Scheme 3. Electrochemical recycling of benzoquinone in the Pd(II)-catalyzed direct Heck-type reaction (Jutand et al., 2007)21
In 2012, Budnikova and co-workers described nickel and palladium-catalyzed electrochemical CH bond perfluoroalkylation of 2-phenylpyridine with perfluoroakyl halides or perfluoroalkyl carboxylic acids (Scheme 4).22 They used cyclic voltammetry to identify higher oxidation states of nickel or palladium species (e.g. Pd(III), Pd(IV), Ni(III), etc.) present in the reaction. The perfluoroalkyl carboxylic acids deliver a perfluoroalkyl functional group upon decarboxylation. In 2014, Kakiuchi and co-workers developed a protocol for palladium-catalyzed regioselective homocoupling of arylpyridines under anodic oxidation with I2 as the electron-transfer mediator (Scheme 5).23 Substrate concentration depenedence studies suggested a doubly chelated Pd(II) species was probably involved in this process.
Scheme 5. Palladium-catalyzed electrooxidative regioselective homocoupling of arylpyridines (Kakiuchi et al., 2014)23
In 2017, the Mei group reported the first Pd-catalyzed C(sp2)–H couplings to benzoyl acetic acids and organoboron reagents using anodic oxidation instead of stoichiometric oxidants, providing ortho-monobenzoylated or ortho-monomethylated products respectively (Scheme 6).24 In this case, palladacycle 12 was prepared, and the structure was unambiguously confirmed by X-ray analysis. In addition, catalytic reactions also suggested that this palladacyle 12 is viable intermediate for C–H cross-coupling reactions. This work demonstrated that transmetallation could proceed under electrochemical conditions, thus offering a novel combination of transition metal-catalyzed direct cross-coupling with conventional electrochemistry.
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Scheme 6. Palladium-catalyzed electrooxidative CC bond formation of oximes (Mei et al., 2017)24
In 2018, the Ackermann group reported an example of a cobalt-catalyzed electrochemically enabled CH/NH annulation of alkynes at room temperature. In this case, the catalytically active cobalt(III) carboxylate complex is regenerated by anodic oxidation, thereby avoiding the use of toxic and expensive metals as sacrificial stoichiometric oxidants (Scheme 7).25 This sustainable cobalt-catalyzed electrooxidative manifold proceeds with excellent levels of chemoselectivity and positional selectivity in a singlepot. Notably, it was conducted at room temperature in water. Ethylene and ethyne are among the simplest two-carbon building blocks, although, methods to directly incorporate ethylene or ethyne into fine chemicals are somewhat uncommon.26 Lei and co-workers demonstrated a cobalt-catalyzed dehydrogenative CH/NH [4+2] annulation of aryl and vinyl amides with ethylene or ethyne (1 atm) using an electrochemical protocol (Scheme 8).27 Like the previous example, this showcased the electrochemical regeneration of an active cobalt catalyst in an oxidative C–H functionalization reaction. Significantly, the electrochemical method provides a reliable and safe way for incorporating gasphase ethylene or ethyne into fine chemicals. Xu and co-workers reported a ruthenium-catalyzed electrochemical dehydrogenative annulation reaction between aniline derivatives and alkynes to synthesize indoles (Scheme 9).28 Electric current is used to recycle the catalytically active Ru(II)-based complex and promote H2evolution, which obviates the need for external oxidants or Hacceptors. Importantly, the electrolysis reaction proceeded efficiently with a simple undivided cell in an aqueous solution exposed to air.
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Scheme 7. Cobalt-catalyzed electrooxidative CH/NH annulation of the alkynes (Ackermann et al., 2018)25
Scheme 8. Cobalt-catalyzed electrooxidative CH/NH [4+2] annulation of ethylene and ethyne (Lei et al., 2018)27
Scheme 9. Ruthenium-catalyzed electrooxidative [3+2] annulation of aniline derivatives with internal alkynes (Xu et al., 2018)28
All of the aforementioned reports utilize nitrogen-centered directing groups. Very recently, the Ackermann group published the first transition metal-catalyzed electrocatalytic CH activation using an oxygen-centered directing group (Scheme 10).29 In this case, a ruthenium(II)
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carboxylate catalyst enabled the electrooxidative CH/OH functionalization for alkyne annulations without external oxidants. Through cyclic voltammetry and electrolysis of the pre-formed ruthenium(0) intermediate, they observed a significant beneficial effect of pivalic acid on the anodic oxidation of the ruthenium(0) intermediates. Mechanistic studies provided strong support for a facile organometallic C–H ruthenation and electrochemical reoxidation of the key ruthenium(0) intermediate.
Scheme 10. Electrooxidative ruthenium-catalyzed CH/OH annulation using an oxygen directing group (Ackermann et al., 2018)29
Carbon–halogen bond formation is important in synthetic organic chemistry due to the multitude of downstream transformations of halogen-containing compounds available for the synthesis of natural products, pharmaceuticals, and materials. Electrochemical oxidation provides an alternative and comparatively environmentally benign way to achieve C–X bond formation via CH functionalization. Recent methods of electrochemical C–X bond formation in the absence of transition metal catalysts have been explored by the Fuchigami33 and Hara34 groups. In 2017, the Lin group reported a Mn-catalyzed electrochemical dichlorination of alkenes with MgCl2 as the chlorine source.35 In 2009, Kakiuchi and co-workers demonstrated a Pdcatalyzed CH halogenation of aryl pyridine derivatives with hydrogen halides using electrochemical oxidation (Scheme 12).36 This method provides an environmentally benign tool for regioselective halogenations of aromatic rings in an efficient and selective manner. Dichlorination was obtained for substrates containing two available orthoCH bonds. In addition, the bromination of CH bonds was also successful when hydrobromic acid and PdBr2 were used.
Very recently, the first electrochemical rhodium(III) catalyzed oxidative CH functionalizations of weaklycoordinating benzoic acids and benzamides employing electricity as the terminal oxidant with H2 as the sole byproduct has been described by Ackermann and co-workers (Scheme 11).30
Scheme 12. Pd-catalyzed electrooxidative CH halogenation of arylpyridine derivatives (Kakiuchi et al., 2009)36
Scheme 11. Electrooxidative rhodium-catalyzed CH/CH cross-dehydrogenative alkenylation (Ackermann et al., 2018)30
After the submition of this review, there were some new examples of electrochemical oxidative CH functionalizations. The Lei group reported cobalt-catalyzed electrochemical oxidative CH/NH carbonylation with carbon monoxide as C1 building block.31 Ackermann and co-workers demonstrated cobalt-catalyzed electro-oxidative CH/NH activation of hydrazides with internal alkynes.32 3.
Electrochemical CX (halogen) bond formation
In 2012, the same group employed the same strategy to develop a Pd-catalyzed protocol for ortho-selective electrochemical CH iodination of arylpyridines using I2 (Scheme 13).37 The reaction proceeded via dual activation of each substrate by a palladium catalyst and an electrode. An investigation of substrate scope showed that a substituent at the 3-position of the pyridine ring or the ortho position or the benzene ring played a key role in achieving high yield of the monoiodination product. In the same paper, Kakiuchi and co-workers also reported a palladium-catalyzed electrochemical one-pot arylation of arylpyridines (Scheme 14). Various aryl groups were introduced at the ortho positions of aryl pyridines by ON/OFF switching of two different catalytic cycles using the same palladium catalyst in a one-pot arylation process. With the electricity on, ortho-selective CH iodination was executed. When the
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electricity was switched off, a Suzuki coupling was used to deliver the corresponding arylation products. This offers a convenient way to obtain arylation products.
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5.1 Electrochemical C(sp2)H/CO bond formation In 2015, Budnikova and co-workers reported the electrochemical Pd-catalyzed CH oxygenation of 2-phenylpyridine with perfluoroalkyl carboxylic acids (Scheme 16).46 The electochemical oxidation of palladium acetate or palladium perfluoroacetate in the presence of 2-phenylpyridine promotes catalytic ortho CH substitution. NMR and electrochemical data suggested that the reactions proceed through monometallic Pd(II) intermediates (in acetonitrile), or bimetallic Pd(II) intermediates (in dichloromethane).
Scheme 13. Pd-catalyzedelectrooxidativeCH iodination of arylpyridines with I2 (Kakiuchi et al., 2012)37
Scheme 16. Electrochemical palladium-catalyzed orthooxygenation of 2-phenylpyridine with perfluorocarboxylic acids (Budnikova et al., 2015)46 Scheme 14. Pd-catalyzedelectrochemical one-pot C–H arylation of arylpyridines (Kakiuchi et al., 2012)37
The breadth of electrochemical chlorination reactions was extended further when the Kakiuchi group reported palladium-catalyzed ortho-selective chlorination of Nquinolinylbenzamide derivatives under anodic oxidation conditions (Scheme 15).38 They used 5,7-dichloro-8-quinoline as a directing group, and the reaction afforded high yields of ortho-chlorination products in the presence of various electron-donating and electron-withdrawing substituents. The authors applied their method as a step in an efficient synthesis of vismodegib, which was completed after two additional steps.
In 2017, Mei and co-workers developed an efficient electrochemical method for the palladium-catalyzed C(sp2)H bond acetoxylation under constant-current electrolysis conditions at 1.0 mA (Scheme 17).46 The acetoxylation of both electron-rich and electron-deficient oxime substrates proceeded smoothly to provide the corresponding products in moderate to high yields.
Scheme 17. An electrochemical method for palladiumcatalyzed C(sp2)H bond acetoxylation (Mei et al., 2017)47
Scheme 15. Electrochemical Pd-catalyzed chlorination of N-quinolinylbenzamide derivatives (Kakiuchi et al., 2017)38
5.
In a very recent example, Ackermann and co-workers demonstrated the application of anodic oxidation to facilitate cobalt catalyzed C(sp2)H alkoxylation with the assistance of a bidentate N,O directing group for the first time (Scheme 18).48 The method is procedurally simple and suitable for a wide range of alcohols and benzamide substrates bearing a variety of electron-rich and electron-poor groups. The protocol featured high levels of chemo, regio, and diastereoselectivity under exceedingly mild conditions at 23 °C. The direct use of anodic electrons as oxidant avoids the requirements of silver(I) salts
Electrochemical CO bond formation
Electrochemical C–H functionalization also provides new opportunities for the construction of CO bonds. In the last few years, the Moeller,39 Zeng,40 Chiba,41 Waldvogel,42 Lee,43 Royer,44 and Okimoto45 groups have demonstrated various CO couplings.
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of C(sp2)H and C(sp3)H bonds (Scheme 20).51 They investigated the mechanism of the process by decoupling the CH activation and oxidation steps and using cyclic voltammetry and bulk electrolysis of a preformed palladacycle of 8-methylquinoline, respectively. Furthermore, they have shown that chemical and electrochemical oxidation affords different reaction outcomes. This transformation was also applied to the acetoxylation of both C(sp2)−H and C(sp3)−H bonds using various directing groups.
Scheme 20. Palladium-catalyzed CH bond acetoxylation via electrochemicaloxidation (Sanford et al., 2018)51 Scheme 18. Electrochemical cobalt-catalyzed CH alkoxylation (Ackermann et al, 2017)48
5.2 Oxidation of C(sp3)H Bonds Selective functionalization of saturated alkanes is an active area of CH activation. Bercaw and co-workers demonstrated the first selective anodic homogeneous functionalization of alkanes, where in p-toluenesulfonic acid is electrochemically hydroxylated to the alcohol pmethoxyphenylsulfonic acid using aqueous PtCl42- as CH activation catalyst and phosphomolybdic acid as a redox mediator.49 The Mei group recently developed an electrochemical strategy that leverages anodic oxidation of Pd(II) to induce selective CO reductive elimination of a variety of oxyanion coupling partners (Scheme 19).50 This process offers an alternative to conventional methods that require harsh chemical oxidants, and represents an environmentally benign tool for coupling various oxygen anions, including acetates, toslyate, and alkoxides, to C(sp3)H bonds.
Scheme 19. Electrochemical palladium-catalyzed C(sp3)H bond acetoxylation (Mei et al., 2017)50
Methane is an abundant and cheap carbon-based feedstock, but its chemical inertness and propensity for uncontrolled oxidation impedes its widespread utilization as a precursor to liquid fuels and commodity chemicals.52 Surendranath and co-workers elegantly established electrochemical oxidation as a versatile new strategy for accessing high-valent methane monofunctionalization (Scheme 21).53 They provided evidence for the electrochemical oxidation of PdSO4 in concentrated sulfuric acid electrolytes to generate a putative Pd2(III,III) species in an alloxidic ligand field. This electrically-generated high-valent Pd complex rapidly activates methane with a low barrier of 25.9 (±2.6) kcal/mol, generating methanol precursors methylbisulfate (CH3OSO3H) and methanesulfonic acid (CH3SO3H) via concurrent faradaic and non-faradaic reaction pathways. This work enables new electrochemical approaches for promoting rapid methane monofunctionalization.
Scheme 21. Electrochemical palladium-catalyzed methane monofunctionalization (Surendranath et al., 2017)53
6. Electrochemical CP bond formation
Later, the Sanford group described the development of a method for the Pd-catalyzed electrochemical acetoxylation
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Li, Lessard, and co-workers reported electrochemical oxidative phosphonation of N-phenyltetrahydroisoquinoline in an imidazoliumionic liquid in the absence of transition metal catalysts using a cation pool strategy.54 Khrizanforov and coworkers proposed a new approach to the phosphorylation of benzenes bearing both electronwithdrawing and electron-donating substituents on the ring, as well as some coumarins (coumarin, 6-methylcoumarin, 7-methylcoumarin).55 In this case, the reaction was carried out using dialkyl H-phosphonates, (RO)2P(O)H (R = Et, n-Bu, i-Pr), via catalytic oxidation of the arene and H-phosphonatein a 1:1 ratio under mild electrochemical conditions using the bimetallic catalyst system MnIIbpy/NiIIbpy (1%). Budnikova and co-workers developed electrooxidative Pd-catalyzed directed ortho CH phosphonation of 2-phenylpyridine. The electrochemical synthesis proceeds using HP(O)(OEt)2 as the phosphorylating reagent under mild conditions at potentials sufficient to generate high-valent Pd(III) or Pd(IV) without added oxidizing agents (Scheme 22).56 To gain insight into the process, an intermediate binuclear phosphonate palladacycle [(PhPy)Pd(EtO)2P(O)]2 was isolated. Direct electrochemical oxidation of this complex can quantitatively reductively eliminate the phosphonation product, suggesting it is an intermediate in the catalytic cycle.
Scheme 22. Electrochemical palladium-catalyzed phosphonation of pyridines (Budnikova et al., 2015)56
7.
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CH amination (Scheme 23).68 This environmentally benign electrocatalysis can be conducted under ambient air for sustainable aminations in an atom- and step-economical manner with H2 as the sole byproduct, thus avoiding undesired stoichiometric metal-contaminated byproducts. Cyclic voltammetry analysis implicated a single-electron oxidation of cobalt(II) and the formation of a key cobalt(III) intermediate.
Scheme 23. Electrochemical Co-catalyzed CH amination (Ackermann et al., 2018)68
Meanwhile, the Lei group published another environmentally friendly electrochemical protocol for cobalt-catalyzed CH amination of various arenes, which offers a simple way toaccess synthetically useful arylamines (Scheme 24).69 A variety of arenes and alkylamines were examined to afford CN products in divided cells without using solution-phase oxidants. Kinetic isotope effect experiments suggested that CH bond cleavage may not be the rate-limiting step.
Electrochemical CN bond formation
Anodic oxidation offers a convenient avenue to access Nradicals directly from amines and amides, which can then undergo facile radical reactions. The Moeller,57 Xu,58 Zeng,59 Yoshida,60 Boydston,61 Huang,62 Francke,63 and Yudin64, Lei65 groups have developed useful electrochemical oxidative aminations of different substrates. The Baran group explored nickel-catalyzed amination of a variety of aryl donors (ArCl, ArBr, ArI, ArOTf) under mild reaction conditions. Notably, amine types (primary and secondary), and even alternative XH donors (alcohols and amides) were all tolerated in the process.66 The use of transition metal catalysts could significantly increase the chemoselectivity of reactions involving N-radicals. In 2017, Lin and co-workers demonstrated that manganese-catalyzed reactive N3•—which could be generated electrochemically under very mild conditions—reacts with alkenes with high chemoselectivity.67 Very recently, Ackermann and co-workers again bridged electrochemistry and cobalt catalysis, this time to drive
Scheme 24. Electrochemical cobalt-catalyzed CH amination (Lei et al., 2018)69
8.
Conclusions and Outlook
In this review, numerous elegant transformations have been presented that involved electrochemical transition metal-catalyzed CH bond functionalization. Still, there are many remaining challenges to overcome before such methods are widely adopted in academic or industrial settings, including: (1) the recycling of electrolytes, (2) increasing the variety and quantity of effective transfor-
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mations of CH bonds, (3) increasing the variety of transition metals, and (4) developing asymmetric electrochemical CH bond functionalization. In regard to the first challenge, although electrochemical synthesis is an environmentally friendly and sustainable method for organic synthesis, in our experience the electrolytes required to maintain a certain current or voltage could not be recycled from the mixture, therefore the exploration of reusable electrolytes which could be separated from the reaction mixture easily are essential for extensive large-scale application. Another method to reduce the amount of supporting electrolyte required is the application of electrochemical microreactors,70 which would inherently requireless electrolyte. Towards the second goal, CH bond functionalization is a rapidly growing area of interest as it provides a powerful means that enables the conversion of CH bonds of organic molecules into diverse functional groups in a single synthetic operation. Traditional catalytic CH bond functionalization has emerged as an increasingly powerful tool for fundamental research and pharmaceutical development, as well as materials applications.71 However, achievements and applications of effective transformations of CH bonds in various molecules by electrochemical organometallic CH bond functionalization remain limited. Thirdly, while CH functionalizations have thus far exploited palladium, ruthenium, cobalt, nickel, and rhodium, the exploration of earth-abundant and less toxic 3rd row “base metals” require the attention. Lastly, toward the fourth challenge, various groups have developed asymmetric chiral amine organo catalyzed electrooxidative C–H functionalization.1618 However transition metal chemistry has inherent advantages in its recognized ability to control the enantioselectivity of CH transformations via chiral ligands. Thus the merger of electrochemistry with asymmetric transition metal-catalyzed CH bond functionalization offers interesting opportunities that conventional asymmetric transition metal catalysis may not have achieved. Looking forward, it is our belief that the merger of electrochemical methods and transition metal-catalyzed CH bond functionalization will continue to develop as a dynamic and promising field of research for modern synthetic methodology, and such reactions will be embraced by more and more chemists.
the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant XDB20000000).
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AUTHOR INFORMATION 11.
Corresponding Author * E-mail:
[email protected] Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT
12.
This work was financially supported by the “1000-Youth Talents Plan”, NSF of China (Grant 21572245, 21772222, 21772220), S&TCSM of Shanghai (Grant 17JC1401200, 18JC1415600), and
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