Rhodaelectro-Catalyzed C–H and C–C Activation

Abstract

Open AccessCCS ChemistryMINI REVIEW1 Feb 2021Rhodaelectro-Catalyzed C–H and C–C Activation Youai Qiu†, Cuiju Zhu†, Maximilian Stangier, Julia Struwe and Lutz Ackermann Youai Qiu† Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Göttingen 37077 , Cuiju Zhu† Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Göttingen 37077 , Maximilian Stangier Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Göttingen 37077 , Julia Struwe Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Göttingen 37077 and Lutz Ackermann *Corresponding author: E-mail Address: [email protected] Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Göttingen 37077 https://doi.org/10.31635/ccschem.020.202000365 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Rhodium(III) catalysis has set the stage for a plethora of oxidative C–H functionalizations over the last decade, which have predominantly employed stoichiometric amounts of toxic and expensive metal oxidants, such as silver(I) salts. In the meantime, electrosynthesis has emerged as an increasingly viable alternative for expensive and toxic oxidants. Recently, significant momentum has been achieved with the merger of electrocatalysis with organometallic C–H activation. However, user-friendly and robust rhodaelectro-catalysis has until very recently proven elusive for oxidative C–H activations. This minireview highlights the current knowledge and recent advances of electrooxidation in rhodium-catalyzed C–H or C–C activations, with a topical focus on contributions from the Ackermann group through July 2020. Download figure Download PowerPoint Introduction Organometallic C–H activation has emerged as one of the most efficient tools for molecular synthesis.1–10 Particularly, rhodium(III) catalysis has received notable attention for the development of oxidative C–H functionalizations.11–16 Despite considerable advances, rhodium(III)-catalyzed oxidative C–H activations heavily rely on stoichiometric amounts of toxic and/or expensive copper(II) and silver(I) salts as sacrificial oxidants.17–21 In the meantime, electrocatalysis has been identified as an increasingly viable strategy for organometallic C–H activations over the last decade.22–36 While general reviews and reports on metallaelectro-catalysis have appeared,37–59 a focus on state-of-the-art rhodaelectro-catalyzed transformations is not yet available,60,61 despite its unique potential for molecular syntheses, pharmaceutical industries, and material sciences.62–65 Herein, we discuss recent developments of rhodaelectro-catalyzed transformations with specific interests on mechanistic aspects. Thus, we specifically summarize our findings on rhodaelectro-catalyzed C–H and C–C activations, which provide a number of useful molecular structures and, more importantly, reveal new synthetic disconnections. Overall, rhodaelectro-catalyzed C−H/C–C activations have set the stage for molecular syntheses with unique levels of resource economy.66 Rhodaelectro-Catalyzed C−H Alkenylation In 2018, a key breakthrough in rhodaelectro-catalyzed C–H activation was established by the Ackermann group (Göttingen, Germany) (Scheme 1).67 Hence, cross-dehydrogenative C–H/C–H alkenylation was achieved with weakly O-coordinating68 benzoic acids 1 and alkenes 2, serving as a proof of concept for the first rhodium electrocatalyzed C–H activation. The optimized reaction conditions were characterized by using Potassium acetate (KOAc) as the additive and a mixture of t-AmOH and H2O as the effective solvent system, delivering the desired products 3 in a user-friendly undivided cell setup. Initially, various substituents in the ortho-, meta-, and para-positions of benzoic acids were employed to probe the robustness of the electrocatalyzed C–H transformation, proceeding with excellent levels of positional, diastereo-, and chemoselectivities. Notably, a variety of valuable electrophilic functional groups, including sensitive esters and ketones, were fully tolerated in this electrooxidative rhodium-catalyzed C–H alkenylations. Likewise, variously substituted acrylates 2 proved to be amenable, including an oxidation-sensitive aliphatic hydroxy group. Furthermore, the procedure proved to be applicable to amides and indoles. Specifically, endogenous steroid pregnenolone 2k could be efficiently converted to the desired products 3k without racemization of the stereogenic centers. It is worth noting that electrochemical vinyl C–H activation was initially realized in this study, as well.69,70 Scheme 1 | Rhodaelectro-catalyzed C–H alkenylation. Download figure Download PowerPoint Competitive experiments showed a clear preference in favor of the more electron-rich benzoic acids 1. The experiment was conducted by an analysis of the initial rates for electron-rich and electron-deficient benzoic acids 1c and 1d in independent reactions (Scheme 2a). This observation is in good agreement with a base-assisted intramolecular electrophilic-type substitution (BIES)71–78 C–H activation manifold. Furthermore, deuteration studies using CD3OD as the cosolvent suggested a facile and reversible C–H activation event, while highlighting an organometallic C–H activation mechanism (Scheme 2b). A minor kinetic isotopic effect illustrated that C–H rhodanation is not the rate-determining step, providing an additional support for fast C–H scission (Scheme 2c). Scheme 2 | (a–c) Summary of key mechanistic findings. Download figure Download PowerPoint On the basis of the mechanistic findings, a catalytic cycle depicted in Scheme 3 has been proposed. Initially, carboxylate-assisted BIES C−H activation delivers cyclometalated intermediate 4b. Next, migratory alkene insertion generates the catalytically competent rhodium(III) species 4d. Thereafter, β-hydride elimination and reductive elimination deliver the desired products 3. Finally, the key anodic oxidation of the reduced rhodium(I) intermediate 4e regenerates the catalytically active rhodium(III) species 4a via an anodic single-electron transfer (SET) event. Scheme 3 | Plausible catalytic cycle for alkenylation. Download figure Download PowerPoint In contrast to the alkenylation of α,β-unsaturated carbonyl compounds under rhodaelectro-catalysis, Ackermann recently reported an intriguing alkenylation reaction using unactivated alkenes 6 with weakly coordinating benzamides (Scheme 4).79 Here, the dehydrogenative alkenylation products 7 were obtained by using NaOPiv instead of the previously reported KOAc additive. The rhodaelectro-catalyzed C–H alkenylation was shown to proceed with ample substrate scope, including heterocycles and valuable electrophilic functional group, such as chloro, bromo, and nitrile. Likewise, various alkenes 6 proved to be amenable, including oxidation-sensitive hydroxyl substituents. A gram-scale reaction highlighted the synthetic utility of the rhodaelectro-catalyzed C–H activation. Scheme 4 | (a–c) Rhodaelectro-catalyzed C–H alkenylation and gram-scale reaction. Download figure Download PowerPoint Rhodaelectro-Catalyzed C−H Alkynylation The strategy of rhodaelectro-catalytic C−H activation proved to be broadly applicable and gave access to synthetically useful polycyclic aromatic hydrocarbons (PAHs)80–84 through a two-step sequential dehydrogenative annulation electrocatalysis (Scheme 5a).85 Thus, electrooxidative C−B/C−H [2 + 2 + 2] cyclization was realized with a variety of boronic acids featuring versatile rhodium catalysis. The C−B/C−H annulation was efficiently established with ample scope and remarkable levels of functional group tolerance, such as chloro, ester, and cyano substituents, in a user-friendly undivided cell setup. Notably, the chemoselectivity of the conversion of sensitive iodo-substituted boronic acids could be significantly improved as compared with transformations with typical chemical oxidants, AgOAc and Cu(OAc)2 (Scheme 5b). Scheme 5 | (a and b) Rhodaelectro-catalyzed C–B/C–H [2 + 2 + 2] annulation. Download figure Download PowerPoint Further transformation of substituted tetraphenyl naphthalenes into π-conjugation PAHs proved viable in the presence of 20 mol % of 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) in a divided electrochemical cell at room temperature (Scheme 6).86,87 Thereby, a set of useful late-stage diversification reactions provided access to important PAHs derivatives. The unambiguous structure of cyclodehydrogenated product 11 was confirmed by X-ray diffraction analysis, revealing a structurally nonplanar PAH. In addition, the conducted photoabsorption and cyclic voltammetry (CV) measurements reflected the new optoelectronic properties of the electrochemically generated PAHs (Scheme 7).88,89 Scheme 6 | Late-stage diversification by DDQ-catalyzed electrochemical cyclodehydrogenation. Download figure Download PowerPoint Scheme 7 | (a) Photoabsorption and (b) CV of 11e and 11f. Download figure Download PowerPoint Importantly, Ackermann also established robust flow rhodaelectro-catalyzed alkyne annulations using aryl imidates 12 as the substrates (Scheme 8).90 It is particularly noteworthy that C−H/N−H alkyne annulations were amenable to electroflow technology using a slightly modified IKA setup.91–98 This strategy represents a user-friendly tool for the efficient upscaling of a reaction with significantly improved control of heat and mass transfer. The scope for this challenging flow rhodaelectro-catalyzed alkyne annulation gave access to isoquinolines, as well as azo-tetracycles, by an intramolecular reaction. Scheme 8 | (a–c) Flow-rhodaelectro-catalyzed alkyne annulations.a [Cp*Rh(CH3CN)3](SbF6)2 (5.0 mol %) as the catalyst, 50 °C, 10 h, batch reaction. Download figure Download PowerPoint In-depth mechanistic studies were performed to probe the catalyst's modus operandi (Scheme 8). Thus, the stoichiometric synthesis of the two novel cyclometalated rhodium(III) complexes 17a and 17b from imidates 12a was accomplished (Scheme 9a). The well-defined rhodium(III) complexes were found to be competent in the catalytic C–H annulation (Scheme 9b). Notably, the formation of well-characterized rhodium(III)-heptacycle 18 was observed when treating complex 17b with alkyne 13a, whereas alkyne 13a underwent an insertion reaction (Scheme 9c). The formation of product 14a was observed when electricity was applied, thus providing support for an oxidation-induced reductive elimination within a unusual rhodium(III/IV/II) regime (Scheme 9d).99,100 In addition, the Ackermann group conducted CV experiments to gain further insights into the role of sodium salt. These studies indicated that the additive, NaOPiv, accelerated the product formation from rhodacycle 18 upon electrolysis. Scheme 9 | (a–d) Synthesis of rhodacyles 17a, 17b, 18, and applications to C–H activation catalysis. Download figure Download PowerPoint In addition, computational studies rationalized a favorable Rh(III/IV/II) manifold with an activation barrier of 15.2 kcal mol–1 for the oxidatively induced reductive elimination step (Figure 1). These computational studies were in good agreement with the experimental findings. Figure 1 | Gibbs free-energy profile (in kcal mol–1) comparing the direct reductive elimination and oxidatively induced reductive elimination at the B3LYP-D3(BJ)/6-311++G(d,p),SDD(Rh)+SMD(methanol)//B3LYP-D3(BJ)/6-31G(d,p),SDD(Rh) level of theory. Nonparticipating hydrogen atoms were omitted for clarity. Download figure Download PowerPoint On the basis of these mechanistic studies, a plausible catalytic cycle was proposed to feature carboxylate-assisted C−H rhodanation to deliver the cyclometallated rhodium complex 17 (Scheme 10). Thereafter, migratory insertion and anodic SET generated rhodium(IV) complex C to subsequently undergo oxidatively induced reductive elimination from intermediate C. Thereby, product 14 is released, while the catalytically competent species A was regenerated. Scheme 10 | Proposed catalytic cycle for flow-rhodaelectro-catalyzed alkyne annulation. Download figure Download PowerPoint Electrooxidative alkyne annulation was recently merged with a multiple C−H domino strategy (Scheme 11).101 In contrast to the previous transformation,85 the use of easily accessible imidamides 19 enabled the challenging formation of various aza-PAHs. The rhodaelectro-catalyzed cascade C−H activations were efficiently realized with ample scope and remarkable functional group tolerance. Scheme 11 | Rhodaelectro-catalyzed domino annulation. Download figure Download PowerPoint Having demonstrated the versatility of the rhodaelectro-catalyzed C−H annulation, Ackermann was encouraged to investigate its mechanism (Scheme 12). It is noteworthy that the electrosynthesis occurred in the presence of well-defined rhodacycles 21 and 22 as the catalysts. These findings provide support for the order of the three subsequent C–H activation events. An additional application for a unique dendrimer 23 through electrooxidative assembly of protected d-lactone 24 was accomplished (Scheme 13). Scheme 12 | (a–c) Key mechanistic findings. Download figure Download PowerPoint Scheme 13 | Late-stage functionalization of aza-PAHs. Download figure Download PowerPoint Rhodaelectro-Catalyzed C−H Phosphorylation Recently, the Xu group102 concurrently disclosed a mechanistically related phosphorylation using a N-coordinating directing group (Scheme 14). The broadly applicable concept of rhodaelectro-catalysis was further utilized for effective C−H phosphorylation using diphenylphosphines 26. To prove scalability, a decagram scale reaction was successfully performed, illustrating the potential for future industrial applications. Scheme 14 | (a and b) Rhodaelectro-catalyzed C−H phosphorylation. Download figure Download PowerPoint The catalyst's modus operandi was interrogated by detailed mechanistic studies. Specifically, two well-defined rhodium(III) complexes 28 and 29 were prepared. Both complexes 28 and 29 proved to be catalytically active in catalytic settings (Scheme 15a). Thus, the rhodium(III) complex 28 undergoes ligand exchange to form the more oxidizable key intermediate 29, which is followed by oxidation-induced reductive elimination to generate the desired product 27 (Scheme 15b). Scheme 15 | (a and b) Key mechanistic findings and plausible catalytic cycle for rhodaelectro-catalyzed C−H phosphorylation. Download figure Download PowerPoint Notably, electrochemistry is an ideal platform for mechanistic studies. In a proof-of-concept study, the Chang group103 recently probed the viability of cyclometallated rhodium complexes for oxidatively induced reductive elimination steps studies. First, two fully characterized stable cyclometallated rhodium complexes 33 and 35a were successfully prepared and their electrochemical properties investigated by CV studies (Scheme 16a). An irreversible oxidation potential for 35a was observed at Epa = 0.331 V versus Fc/Fc+ in tetrahydrofuran (THF), which can be further oxidized by silver salt to generate a putative high-valence rhodium species according to the known oxidation potential of AgI (E1/2 = 0.41 V vs Fc/Fc+ in THF). Indeed, the desired arylated and methylated products 37a and 37b were obtained, while reductive elimination did not take place even at 80 °C under oxidant-free conditions. These findings confirm that reductive elimination was induced by single-electron oxidation (Schemes 16b and 16c). Scheme 16 | (a–c) Oxidatively induced reductive elimination of rhodium complexes. Download figure Download PowerPoint Rhodaelectro-Catalyzed C−C Alkenylation The versatile electrochemical rhodium catalysis manifold is not limited to C−H transformations. Hence, Ackermann disclosed a rhodaelectro-catalyzed C−C alkenylation, representing the proof-of-concept for organometallic C–C functionalization104 by electrocatalysis.105 Within the organometallic C–C activation manifold, electrochemical chelation-assisted C–C functionalizations were demonstrated to proceed with ample substrate scope and outstanding levels of chemo- and position-based selectivities (Scheme 17a). In addition, competition experiments between the C–C and C–H functionalizations revealed a preferential reactivity of the C–C activation manifold, thus position selectively furnishing densely decorated 1,2,3-substituted arenes, not accessible by more common C−H activation strategies (Scheme 17b). Scheme 17 | (a and b) Rhodaelectro-catalyzed C−C alkenylation and position selectivity. Download figure Download PowerPoint The catalyst's mode of action was investigated by detailed mechanistic studies (Scheme 18). Competition experiments showed that electron-rich arenes and olefins were preferentially converted (Scheme 18a). The C–C and C–H functionalization competition experiment showed that the C–C activation occurred faster than the C–H activation (Scheme 18b). The use of isotopically labeled [D]1-tAmOD and D2O did not lead to a significant deuteration in the unreacted starting material or the obtained product. These findings are indicative of a slow C–C scission (Scheme 18c). The formation of hydrogen as the sole byproduct of cathodic proton reduction was confirmed by headspace gas chromatographic analysis (Scheme 18d). Notably, the two well-defined rhodium complexes 42a and 42b proved to be competent catalysts for the organometallic nature of the electrooxidative C–C alkenylation (Scheme 18e). Scheme 18 | (a–e) Key mechanistic findings of rhodaelectro-catalyzed C−C alkenylation. Download figure Download PowerPoint Their findings were rationalized by a plausible catalytic cycle depicted in Scheme 19. Initiated by the formation of active catalyst 4a, the seven-membered rhodacycle 43 was formed upon chelating with the nitrogen and oxygen of substrate 38 with the rhodium(III) catalyst. Thereafter, migratory alkene insertion occurred to form the key seven-membered intermediate 46. Finally, reductive elimination furnished the desired product 40, and the active catalyst 4a was regenerated upon anodic oxidation of the rhodium(I) intermediate 47. Scheme 19 | Plausible catalytic cycle of rhodaelectro-catalyzed C−C alkenylation. Download figure Download PowerPoint Conclusions In recent years, rhodaelectro-catalyzed C−H activation has emerged as a powerful platform for molecular synthesis, employing sustainable electricity as the terminal oxidant and avoiding the use of stoichiometric amounts of sacrificial chemical oxidants. Since the first example of rhodaelectro-catalyzed C−H activation with weakly O-coordinating benzoic acids was described by the Ackermann group, numerous elegant transformations applying rhodaelectro-catalyzed C−H or C−C activations have been established. Key breakthroughs in the understanding of the catalytic mode of action and overall catalysis have been achieved by among others headspace gas chromatographic analyses, CV, and computation, prominently featuring oxidation-induced reductive elimination pathways. A flow metallaelectro-catalyzed C−H activation was realized in terms of robust rhodaelectro-catalyzed alkyne annulations. In addition, the electrochemical assembly of PAHs was proved viable via rhodaelectro-catalyzed cascade C−H annulations. Furthermore, electrochemical C−C activations were accomplished by expedient oxidative rhodium(III) catalysis. Given the sustainable nature of metallaelectro-catalyzed C−H activation reaction, exciting future advances are expected in this rapidly evolving research area, which should address enantioselective metallaelectro-catalysis,106 photoelectrochemical transformations,107–112 and organic materials electrochemical syntheses. Conflict of Interest The authors declare no conflict of interest. Acknowledgments Generous support by the Deutsche Forschungsgemeinschaft (German Research Foundation) (Gottfried-Wilhelm-Leibniz award to L.A.) and the China Scholarship Council (fellowship to C.Z.) is gratefully acknowledged. References 1. Park Y.; Kim Y.; Chang S.Transition Metal-Catalyzed C–H Amination: Scope, Mechanism, and Applications.Chem. Rev.2017, 117, 9247–9301. Google Scholar 2. Gandeepan P.; Ackermann L.Transient Directing Groups for Transformative C–H Activation by Synergistic Metal Catalysis.Chem2018, 4, 199–222. Google Scholar 3. Piou T.; Rovis T.Electronic and Steric Tuning of a Prototypical Piano Stool Complex: Rh(III) Catalysis for C–H Functionalization.Acc. Chem. Res.2018, 51, 170–180. Google Scholar 4. Wei Y.; Hu P.; Zhang M.; Su W.Metal-Catalyzed Decarboxylative C–H Functionalization.Chem. Rev.2017, 117, 8864–8907. Google Scholar 5. He J.; Wasa M.; Chan K. S. L.; Shao Q.; Yu J.-Q.Palladium-Catalyzed Transformations of Alkyl C–H Bonds.Chem. Rev.2017, 117, 8754–8786. Google Scholar 6. Wencel-Delord J.; Glorius F.C–H Bond Activation Enables the Rapid Construction and Late-Stage Diversification of Functional Molecules.Nat. Chem.2013, 5, 369–375. Google Scholar 7. Arockiam P. B.; Bruneau C.; Dixneuf P. H.Ruthenium(II)-Catalyzed C–H Bond Activation and Functionalization.Chem. Rev.2012, 112, 5879–5918. Google Scholar 8. Ackermann L.Carboxylate-Assisted Transition-Metal-Catalyzed C–H Bond Functionalizations: Mechanism and Scope.Chem. Rev.2011, 111, 1315–1345. Google Scholar 9. Davies H. M. L.; Manning J. R.Catalytic C–H Functionalization by Metal Carbenoid and Nitrenoid Insertion.Nature2008, 451, 417–424. Google Scholar 10. Bergman R. G.C–H Activation.Nature2007, 446, 391–393. Google Scholar 11. Shin K.; Kim H.; Chang S.Transition-Metal-Catalyzed C–N Bond Forming Reactions Using Organic Azides as the Nitrogen Source: A Journey for the Mild and Versatile C–H Amination.Acc. Chem. Res.2015, 48, 1040–1052. Google Scholar 12. Sperger T.; Sanhueza I. A.; Kalvet I.; Schoenebeck F.Computational Studies of Synthetically Relevant Homogeneous Organometallic Catalysis Involving Ni, Pd, Ir, and Rh: An Overview of Commonly Employed DFT Methods and Mechanistic Insights.Chem. Rev.2015, 115, 9532–9586. Google Scholar 13. Kuhl N.; Schroeder N.; Glorius F.Formal SN-Type Reactions in Rhodium(III)‐Catalyzed C–H Bond Activation.Adv. Synth. Catal.2014, 356, 1443–1460. Google Scholar 14. Song G.; Wang F.; Li X.C–C, C–O and C–N Bond Formation via Rhodium(III)-Catalyzed Oxidative C–H Activation.Chem. Soc. Rev.2012, 41, 3651–3678. Google Scholar 15. Colby D. A.; Bergman R. G.; Ellman J. A.Rhodium-Catalyzed C–C Bond Formation via Heteroatom-Directed C–H Bond Activation.Chem. Rev.2010, 110, 624–655. Google Scholar 16. Satoh T.; Miura M.Oxidative Coupling of Aromatic Substrates with Alkynes and Alkenes Under Rhodium Catalysis.Chem. Eur. J.2010, 16, 11212–11222. Google Scholar 17. Liu Y.; Yang Y.; Shi Y.; Wang X.; Zhang L.; Cheng Y.; You J.Rhodium-Catalyzed Oxidative Coupling of Benzoic Acids with Terminal Alkynes: An Efficient Access to 3-Ylidenephthalides.Organometallics2016, 35, 1350–1353. Google Scholar 18. Kudo E.; Shibata Y.; Yamazaki M.; Masutomi K.; Miyauchi Y.; Fukui M.; Sugiyama H.; Uekusa H.; Satoh T.; Miura M.Oxidative Annulation of Arenecarboxylic and Acrylic Acids with Alkynes Under Ambient Conditions Catalyzed by an Electron‐Deficient Rhodium(III) Eur. Google Scholar 19. Gandeepan P.; P.; Cheng C. Annulation of Aromatic and Acids with An Efficient and Eur. Google Scholar Zhang G.; Yang L.; Wang Y.; Y.; Efficient for Oxidative C–H for to Rh(III) by Chem. Google Scholar K.; Satoh T.; Miura Efficient Oxidative Coupling via C–H Bond of Benzoic Acids with Alkynes and Under Google Scholar C.; Stangier M.; J. C. A.; L.; Ackermann C–H Mechanistic into for Eur. Google Scholar P.; Xu with Chem. Google Scholar J. B.; N.; of as a to the Synthesis of 29, Google Scholar M.; E.; T.; A.; S. for the Synthesis of Organic Chem. Google Scholar 26. N.; H.; Y.; Ackermann C–H Google Scholar Liu Y.; Oxidative with A and for Bond 4, Google Scholar J. A.; K. Reactions and the Mechanistic that Chem. Google Scholar M.; Y.; P. Organic Methods Since On the of a Rev.2017, 117, Google Scholar Xu on the Synthesis of Oxidative Annulation Google Scholar P.; Catalyzed Functionalization with Google Scholar J.; P. Organic An and 2, Google Scholar R. Catalysis in Organic and Soc. Google Scholar S. of Methods for the Construction of Chem. Google Scholar of to Organometallic Catalysis.Chem. Google Scholar K.; in Google Scholar Ackermann C–H Activation by and Chem. Google Scholar Y.; A.; H.; Ackermann C–H of Eur. Google Scholar A.; M.; L.; M. C–H Bond via Google Scholar Xu F.; Li C.; Xu Google Scholar Y.; C.; L.; T.; Ackermann Annulation by Chem. Google Scholar H.; J. C. A.; S. C.; J.; Ackermann by Mild C–H Google Scholar C.; L.; H.; Ackermann Activation by Catalysis at Chem. Google Scholar N.; J. C. A.; Ackermann for Activation with Chem. Google Scholar G.; N.; I.; for the of Chem. Google Scholar 46. X.; Wang P.; L.; Electrooxidative C–H of with Chem. Google Scholar 47.

Topics & Concepts