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Cite This: Acc. Chem. Res. 2019, 52, 161−169
Upgrading Cross-Coupling Reactions for Biaryl Syntheses Yun-Fei Zhang†,‡ and Zhang-Jie Shi*,†,§,∥ †
Department of Chemistry, Fudan University, Shanghai 200433, China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China § State Key Laborotary of Organometallic Chemistry, CAS, Shanghai 200032, China ∥ State Key Laboratory Elemento-Organic Chemistry, Nankai University, Tianjin 300071, China
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‡
CONSPECTUS: Transition-metal catalyzed cross-coupling reactions have emerged as a powerful tool for constructing biaryl compounds. Aryl halides and aryl metallic reagents (typically prepared from aryl halides) are used as coupling partners. It would be desirable to replace either aryl halide or aryl metallic reagents used in cross-couplings reactions with more readily available surrogates. Oxidative dehydrogenative cross-coupling between two different “inert” aryl C−H bonds represents an ideal system that would revolutionize cross-coupling chemistry. Furthermore, crosscoupling reactions might be improved by developing new catalytic protocols based on cheap transition-metal catalysts or even transition-metal-free systems to decrease costs and avoid the use of heavy metal and noble transition metals. It would be desirable to promote both catalytic systems and replace either or both coupling partners. We have used different strategies to improve cross-coupling reactions for constructing biaryls, which we categorized into four groups as follows. First, we focused on developing methodologies to be applied to easily produced and naturally abundant arenol-based electrophiles in cross-coupling via C−O activation. We have extended coupling partners to aryl carboxylates and arenols. Direct application of arenes as surrogates for organohalides and organometallic reagents avoids the tedious preparation of these reagents from arenes and considerably reduces the cost of starting materials. We have also explored cross-coupling reactions of arenes with various organometallic reagents, such as arylboronic acids, arylsilanes, and aryl Grignard reagents. Second, we summarize oxidative cross-coupling reactions based on C−H activation with aryl metallic reagents. On the basis of the reactivity patterns of different organometallic reagents, we adapted different catalytic systems to achieve effective crosscoupling reactions. Third, we improved a well-developed cross-coupling between arenes and organohalides through a strategy of replacing one coupling partner and using a new catalytic system. We have applied earth-abundant transition metals, such as Fe, and Co, and even developed transition-metal-free catalytic systems. Finally, our ultimate goal is to construct biaryls by cross dehydrogenative arylation between two different arenes. Owing to the structural similarity of both arenes, in particular two substituted benzenes, the greatest challenges are not only achieving regio- and chemo-selective C−H activation reactions but also matching both the reactivities and selectivities of both substrates to avoid homocouplings of either arene. Through our efforts, we have developed and applied four different strategies by introducing directing groups, controlling electronic and steric properties, and using dual directing strategies. We hope our studies will stimulate interest and new thinking on cross-couplings reactions for building carbon−carbon bonds from readily available and inexpensive chemicals from basic petroleum chemistry and nature.
1. INTRODUCTION
efficient methods for constructing complex and useful compounds. The use of arenol-based electrophiles or simple hydrocarbons to take the place of either aryl halides or aryl metallic reagents is an attractive strategy from both environment and economic viewpoints. Cross dehydrogenative arylation (CDA) between two different “inert” arenes represents an ideal protocol, which would revolutionize cross-coupling chemistry (Scheme 1). In this Account, we
Cross-coupling chemistry has been well developed and broadly applied as a powerful synthetic method and was the subject of the Nobel Prize for Chemistry in 2010.1,2 Conventionally, aryl halides are used as key starting materials for coupling with aryl metallic compounds, which are generally prepared from aryl halides. The extra energy cost and generation of undesired byproducts over the whole synthetic sequence represent weaknesses of this chemistry, considering current trends toward green and sustainable procedures.3 Recently, considerable attention has been paid to transitionmetal-catalyzed C−H and C−O functionalization as clean and © 2018 American Chemical Society
Received: August 13, 2018 Published: October 30, 2018 161
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Accounts of Chemical Research summarize our recent work on improving cross-coupling reactions to construct biaryl compounds.
Scheme 3. Ni-Catalyzed Cross-Coupling of Aryl Carboxylates with Arylboronic (a) and Arylzinc Reagents (b)
Scheme 1. Routes to Improving Cross-Coupling by Replacing Coupling Partners
operate, some issues must be overcome including the extremely high BDE of C−O(H) and C−O(M) and the strong coordinating ability of the substrates, which hampers C−O cleavage catalyzed by transition metals. Selection of an appropriate metal and ligands are crucial.11,12 Except for a single example in Wenkert’s studies,13 this area was unexplored until we discovered a successful coupling of naphtholate with ArMgBr in the presence of NiF2 and PCy3 in a mixed solvent (Scheme 4).14
2. ARENOL-BASED ELECTROPHILES IN CROSS-COUPLING The application of arenol-based electrophiles in cross-coupling reactions has received great attention. Initially, arenols were transformed into active triflates, sulfonates, and phosphates for coupling reactions. Direct application of readily available and “inert” arenol-based electrophiles, such as carboxylates and anisole derivatives, would avoid these steps. However, this approach faces two major challenges: (1) the relatively high bond dissociation energy (BDE) of these molecules and (2) achieving selectivity among different O-based groups and between the two different bonds associated with the O-group (Scheme 2).4 As pioneered by Wenkert5 and Dankwardt,6
Scheme 4. Ni-Catalyzed Kumada Coupling of Naphtholate
Scheme 2. C−O Activation: (a) Challenges for Reactivity and Selectivity and (b) Earliest Representative Work
A complex of magnesium naphtholate acted as the actual substrate, and coordination between Mg2+ and naphtholate lengthened the C−O bond length, indicating activation of C− O bonds. In the mechanism of the Suzuki−Miyaura coupling, a “mutual activation” model is proposed involving coordination of naphtholate to arylboron reagents. Both C−O and C−B bonds are activated, as supported by X-ray single crystal structures of intermediate complexes. On the basis of this design, coupling through direct cleavage of the sp2 C−O bond of naphtholate has been developed via Ni catalysis (Scheme 5).15 Notably, Et3B promotes the reaction efficacy. Although the system cannot be successfully extended to phenols, an 18% isolated yield from 3-methoxyphenol exhibits great potential.
many researchers have contributed to the field of C−O activation.7 Our major contribution has been to investigate the reactivity patterns of aryl carboxylates and arenols. 2.1. Applying Aryl Carboxylates in Cross-Coupling
Aryl carboxylates are easily prepared and hydrolyzed under mild conditions and have thus been broadly applied as protecting groups. In 2008, our group8 and Garg9 simultaneously applied aryl carboxylates and carbamates in the Suzuki−Miyaura reaction via Ni catalysis (Scheme 3a). More reactive 2-naphthyl acetate was also effective. To expand the scope of cross-coupling with reactive arylzinc reagents, aryl pivalates must be used owing to the high nucleophilicity of arylzinc reagents (Scheme 3b). The high reactivity promotes the coupling by lowering the required catalyst loading and reducing the reaction temperature.10
3. CROSS-COUPLING OF C−H BONDS WITH ORGANOMETALLIC REAGENTS Cross-coupling of aryl C−H bonds with arylmetallic reagents, so-called “oxidative coupling”, has the advantage of avoiding aryl halides and pseudohalides.16 Owing to their instability under oxidative and acidic conditions and potential homocoupling of arylmetallic reagents, investigations on such oxidative
2.2. Arenols in Cross-Coupling
Arenols are considered to be attractive substrates in crosscoupling reactions to construct biaryls. For this chemistry to 162
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Accounts of Chemical Research Scheme 5. Ni-Catalyzed Suzuki−Miyaura Coupling of Naphtholate
Scheme 7. Pd-Catalyzed Oxidative Coupling of Organoboron Reagents with Acetanilides (a) and Benzoic Acids (b)
couplings through transition-metal-catalyzed C−H activation are lacking. Before we initiated these studies, a few examples had been reported. Chatani and Kakiuchi reported the first elegant example from arenes and arylboronic acids directed by an acyl group through Ru catalysis.17 Yu and co-workers first explored C−H coupling with alkylboronic acids via Pd catalysis; however, reactions to construct biaryl are lacking.18 We aimed to address the challenges of constructing biaryls from arenes and different aryl metallic reagents; specially, (1) a proper catalytic system must make the C−H activation more favorable than the homocoupling of arylmetallic reagents; (2) the selected oxidants should regenerate catalysts while avoiding oxidation and homocoupling of starting materials. 3.1. Coupling with Arylsilanes
arylsilanes, N-alkylacetanilides are better substrates. This chemistry also showed broad group compatibility. Simultaneously, Yu and co-workers reported the same oxidative coupling with benzoic acids (Scheme 7b).22 Through this strategy, we extended oxidative couplings directed by oxime23 and N,N-dimethylamino groups24 in addition to the acetamino group. In fact, a directing group is not a requirement. We first used both heterocycles and nondirected electron-rich arenes as coupling partners (Scheme 8).25 With heterocycles, the coupling conditions were simple in the presence of only Pd(OAc)2 as a catalyst and O2 as an oxidant. Various
Owing to the low reactivity of arylsilanes, oxidative coupling reactions to construct biaryls between aryl C−H and arylsilanes face various challenges. Motivated by observation of Pd(II)-catalyzed ortho-halogenation of acetanilides,19 in which we isolated the palladacycle and confirmed it to be a key intermediate, we successfully developed palladium catalyzed oxidative coupling of acetanilides with trialkyloxy(aryl)silanes in the presence of Cu(OTf)2 as the best oxidant and AgF as the promotor (Scheme 6).20 We tested different trialkyloxyScheme 6. Pd-Catalyzed Oxidative Arylation of Acetanilides with Organosilanes
Scheme 8. Pd-Catalyzed Oxidative Arylation of Arenes and Heteroarenes with Phenylboronic Acid
(aryl)silanes, which showed excellent reactivity regardless of their electronic properties. Furthermore, different acetanilides with a free N−H group reacted well. These studies indicated the feasibility of oxidative coupling between arenes and different arylmetallics under the proper conditions. 3.2. Coupling with Arylboron Reagents
The oxidative coupling of aryl C−H bond and arylboron reagents would be a useful transformation. We have previously reported Pd-catalyzed oxidative coupling between acetanilides and arylboronic acids to construct biaryls (Scheme 7a).21 In this case, the protonation, oxidation and homocoupling of arylboron reagents were inhibited. Unlike the coupling with 163
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Accounts of Chemical Research heterocycles, including indole, pyrrole, furan, and thiophene derivatives, were suitable. For electron-rich arenes, Cu(OAc)2 was essential with O2 as a terminal oxidant. Some important points were noted: (1) Acidity is not only beneficial for C−H activation but also determines the C2-position selectivity in the arylation of indole derivatives, which was further confirmed by the isolation of Pd-indole intermediates in latter studies.26 (2) The acidic conditions help to inhibit homocoupling of arylboronic acids.
Scheme 10. C−H Arylation with Aryltin Reagents (a) and Arylzinc Reagents (b)
3.3. Coupling with Grignard Reagents
Direct oxidative coupling of Grignard reagents with arenes is difficult because of their good nucleophilicity and reducibility. Owing to the high reactivity of aryl Grignard reagents, compared with “unreactive” arylsilanes and arylboronic acids, late-transition metal catalysts may not be suitable. Stimulated by the unique catalyticity of Co complexes,27 we successfully applied Co catalysis in the oxidative coupling of aryl C−H bonds with Grignard reagents directed by a pyridyl group.28 The coupling occurred smoothly in the presence of Co(acac)3 assisted by TMEDA (N,N,N′,N′-tetramethylethylenediamine) and DCB (2,3-dichlorobutane) as an oxidant in THF at room temperature (Scheme 9). Both alkyl and aryl Grignard reagents
materials and the catalytic system. Thus, we developed new catalysts to construct biaryls from arenes. 4.1. From Heavy Transition Metals to Earth-Abundant Transition Metals
The normal electronic property patterns in cross-coupling reactions between arenes and aryl halides have seen the widespread development of cross-coupling reactions based on heavy transition-metal catalysis and in particular Pd catalysts (Scheme 11a).32 Our studies were stimulated by Ni and Cu
Scheme 9. Co-Catalyzed C−H Arylation with Grignard Reagents
Scheme 11. Different Transition-Metal Catalyzed CrossCouplings of C−H Bonds with Aryl Halides
were applicable. Notably, the survival of free hydroxyl or tertiary amino groups is a useful feature for a broad range of potential applications. Mechanistic studies have indicated that cleavage of C−H bonds is not involved in the rate-determining step, unlike Pd catalyzed oxidative coupling reactions. Oxidative coupling with other aryl metallic reagents and arenes has also been developed by other groups, including aryltin reagents reported by Inoue29 and arylzinc reagents reported by Nakamura30 (Scheme 10). Oxidative coupling reactions between aryl carboxylic acids and arenes have been developed in an intra- and intermolecular manner.31
catalysis to approach the same goal (Scheme 11b).33−36 We have applied more earth-abundant transition metals in this transformation. Most transition metals are found to be active centers in natural enzymes for promoting biological transformations through SET. We envisioned that these metals might also catalyze the desired cross-coupling reactions. On this basis, we explored the cross-coupling of benzene with 4-bromoanisole in the presence of different transitionmetal species. Most metals showed effective catalytic activity. For example, Co(acac)3 could be used to catalyze the coupling of 4-bromoanisole with benzene in the presence of TMEDA and KOtBu at 80 °C (Scheme 11c),37 which was consistent with the report by Lei and co-workers (Scheme 11d).38 With proper ligands, the rarely used Nb and Mo salts also showed
4. IMPROVING CATALYTIC SYSTEMS FOR CROSS-COUPLING ARENES AND ARYL HALIDES A major challenge in cross-coupling chemistry is avoiding the use of heavy transition metal catalysts. This same challenge applies to C−H activation. One of our goals is to improve cross-coupling reactions by replacing both the starting 164
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Accounts of Chemical Research good catalyticity. These studies might provide a new approach to exploring catalysis by transition metals. Moreover, C−H coupling with the C−O bond is desirable but suffers from activation of both C−H and C−O bonds under the same conditions. Following the finding of effective cross-coupling between aryl pivalates or sulfonates with (hetero)arenes by Itami39 and Ackermann,40 respectively, we recently developed a C−O/C−H coupling of polyfluoroarenes with aryl carbamates through cooperative Ni/Cu catalysis (Scheme 12).41
Scheme 13. Direct Arylation of Benzene with Aryl Halides with the Aid of Organocatalysts
Scheme 12. Earth-Abundant Transition-Metal-Catalyzed C−H/C−O Coupling
4.2. From Earth-Abundant Transition-Metal to Transition-Metal-Free Systems
Many transition metals can catalyze cross-coupling reactions and in some cases the transition metal might not be an essential part of the reaction. Control experiments have shown this to be the case. Cross-coupling reactions that typically require transition-metal catalysts at low temperature (80 °C), occurred with more reactive 4-iodoanisole at the same temperature in the absence of any added transition metals. At 100 °C, the coupling with 4-bromoanisole occurred in the presence of KOtBu and phenanthroline, which was unexpected at that time,.42 Previous reports by Daugulis43 and Itami44 on KOtBu-promoted intramolecular arylation of phenols and electron-deficient N-heterocycles prompted us to confirm these studies (Scheme 13a). Through our efforts, we achieved direct arylation of benzene with aryl halides with organocatalysts. Aryl halides bearing different substituents, such as methoxy, fluoro, chloro, and cyano, were compatible (Scheme 13b). This chemistry was extended to an intramolecular reaction to form fused ring systems (Scheme 13c).45 Simultaneously, the Kwong/Lei group46 and the Shirakawa/ Hayashi group,47 independently reported similar systems (Scheme 13d). These studies give strong support to each other. Further mechanistic studies have shown that aryl radicals are the key intermediates, which are generated from aryl halide radical anions. This chemistry has been developed into an effective route to building up biaryl scaffolds and substituted arenes through homolytic aromatic substitution (HAS).
5. CROSS DEHYDROGENATIVE ARYLATION (CDA) BETWEEN TWO ARENES Cross dehydrogenative arylation (CDA) between two arenes is the most desirable transformation for constructing biaryls by avoiding both aromatic halides and aryl metallics. In the past, considerable advances have been achieved in homocoupling of arenes.48 Intramolecular oxidative couplings, such as the Scholl reaction, have also been conducted and broadly used in fused ring synthesis with stoichiometric amounts of oxidants.49 In comparison, CDA between different arenes, in particular two structurally similar substituted benzenes remains a challenging target. Other than elegant chemistry in CDC of sp3 C−H bonds pioneered by Li and co-workers,50 the CDA was first reported between benzene and naphthalene by Lu, albeit in a low efficiency and selectivity.51 We mainly focused on the CDA between two substituted benzenes because the solution of this fundamental problem might be extended to other systems. CDA between heterocycles and benzene derivatives have also been developed by Fagnou52 and DeBoef.53 5.1. CDA Controlled by Directing Group and Steric Hindrance
Following former studies, we proposed that in situ generated palladacycles in oxidative couplings might further attack arenes to form biaryl palladium species, which can undergo reductive elimination to produce biaryls. Owing to the complexity of palladacycles, the second aryl C−H activation could be 165
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Accounts of Chemical Research controlled by steric hindrance from substituents present on the arenes. We tested the oxidative coupling of N-acetyltetrahydroquinoline with o-xylene in the presence of Pd(OAc)2 and Cu(OTf)2 as cocatalysts under a dioxygen (O2) atmosphere and found the CDA proceeded smoothly with a very small amount of the homocoupling byproduct of o-xylene (Scheme 14a).54 As predicted, mechanistic studies showed that the
Scheme 15. Pd-Catalyzed Oxidative Coupling between Polyfluoroarenes and Simple Arenes
Scheme 14. Pd-Catalyzed CDA of N-Acetanilides (a) and Benzo[h]quinoline (b)
achieved, our studies demonstrate an elegant strategy for approaching this goal. 5.3. Intramolecular CDA
Intramolecular oxidative coupling reactions to form fused ring systems are useful for constructing π-extended structures. In previous work, we developed a successfully cascade reaction to produce polysubstituted 9H-fluoren-9-ones through C−H arylation of aryl aldoximes with arylboronic acids followed by addition of TfOH and HCl in one pot.23 Our group60 and Cheng’s group61 further simultaneously developed the Pdcatalyzed dual C−H functionalization of benzophenones to form fluorenones (Scheme 16). Owing to electron-deficiency, Scheme 16. Pd-Catalyzed CDA of Benzophenone
selectivity was controlled by the directing group at the first stage and by steric hindrance at the second step. Because the redox potential of both the cocatalyst Cu(OTf)2 and oxidant O2 might not lead to oxidation of Pd(II) to Pd(IV) complexes,55 the Pd(II)/Pd(0) catalytic cycle was preferred. N-Acetylindoline and acetanilides with free acidic NH were suitable. Notably, the electronic properties of the arenes have a considerable influence on the reactivity, indicating that the second C−H activation might go through CMD. Combining the intramolecular C−H amidation,56 we successfully applied this chemistry to synthesize the carbazoles from two substituted benzenes. Meanwhile, Sanford and co-workers developed CDA directed by pyridyl groups based on the same strategy (Scheme 14b).57 5.2. CDA Controlled by Electronic Properties
There are few examples of CDA in two benzene derivatives in the absence of directing groups, and new methods are highly desired. Su58 and our group59 used the same strategy of discriminating two benzenes with completely different electronic property patterns (one electron rich and the other electron deficient). The CDA of polyfluoroarenes with simple arenes was accomplished in the presence of Pd(OAc)2 as the catalyst and Ag2CO3 as the oxidant (Scheme 15). Interestingly, in our case, we found iPr2S to be essential for the reaction. Although practically applicable methods have yet to be
traditional oxidative coupling reactions are not applicable.21 Benzophenones bearing various substitutes, such as methyl, methoxy, fluoro, chloro, and even free hydroxyl groups were well tolerated, providing an efficient and concise route to construction of fluorenones. 166
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Accounts of Chemical Research
developed.64 Hopefully this strategy can be extended to numerous functionalized benzene derivatives, providing an efficient method for synthesizing polyfunctionalized biaryls.
5.4. CDA with Dual Directing Strategy
CDA has become a powerful methodology; however, controlling the selectivity of these reactions remains a key issue. Except for intramolecular CDA, few strategies have been developed, not only because of selectivity issues but also because of the requirement for a large excess of one arene. We envisioned that a dual directing strategy might address these issues.62 Not only would this strategy enable the ideal ratio of the two substrates to be used, but introducing two different groups also increases potential for constructing highly functionalized biaryls. However, introducing two directing groups into both benzene derivatives makes the substrates more similar, which complicates the CDA. Structural similarity also makes it more difficult to avoid homocoupling. Moreover, the proper binding modes are important because of the limited coordination number and the binding sites of the transitionmetal center are essential. Furthermore, aside from reactivity and selectivity considerations of either C−H activation, the reactivity and selectivity of both C−H activations should also be matched. To achieve an ideal reactions, we considered two completely different and well-studied directing groups in the substrates. The CDA between benzyl thioethers and benzoic acids occurred, followed by intramolecular lactonization to produce dibenzooxepinones via Rh catalysis (Scheme 17a).62 Our
6. CONCLUSIONS AND PERSPECTIVE We present our vision of improving cross-couplings in biaryl construction and related efforts on different approaches. Our aim is to provide direction for researchers who are interested in this active area of research. Our efforts and those of other groups have seen O-based electrophiles applied in crosscoupling reactions as potential partners. Oxidative couplings between aryl C−H bonds with aryl metallics have also been successfully developed with and without directing groups. Furthermore, earth-abundant transition metals and even organocatalysts have shown catalytic activity in cross-coupling between arenes and aryl halides. From the perspective of atom and step economy, the most concise route to construct biaryl scaffolds is CDA between two arenes. Different strategies, including combinations of directing control and electronic as well as steric controls, have been applied to realize these goals. The limitations of those strategies has led to the development of dual directing strategies in CDA, which have been successfully applied to the synthesis of dibenzooxepinones from very simple starting materials. Although great progress has been made, there are still substantial hurdles to be overcome. For example, the substrate scope needs to be expanded, the reaction conditions need to be further optimized, and mechanisms need to be investigated in greater detail. Recent new developments in technology and chemistry will help to promote this whole field. For example, the combination of transition-metal catalysis and photocatalysis has been developed to achieve CDA with release of H2,65 although such chemistry cannot be used to realize CDA of two structurally similar benzene derivatives. Rapidly developing electrochemistry might provide another direction for development in this field.66 Moreover, expanding CDA to phenyl C−H bonds and “inert” sp3 C−H bonds will be the next goal. We have already seen the effectiveness of intramolecular CDA of phenyl alkyl ethers in constructing dihydrobenzofurans manipulated by ligands (Scheme 18).67 Cross-coupling through C−C bond cleavage might be another approach to improving cross-coupling reactions through the reorganization of carbon skeletons.68
Scheme 17. Rh-Catalyzed CDA through Dual Directing Strategy
Scheme 18. Pd-Catalyzed Oxidative Coupling of sp2 and sp3 Carbon−Hydrogen Bonds
mechanistic studies indicated that the CDA occurred before the esterification and thus confirmed the concept. Different dibenzooxepinones were obtained in moderate to good yields. Notably, Li’s group developed a homocoupling of aromatic carboxylic acid using a similar strategy (Scheme 17b).63 We proposed that the first thioether-directed C−H bond activation occurs to form a rhodacyclic intermediate. This step is followed by ligand exchange with carboxylate and the second C−H cleavage takes place directed by carboxylate. Subsequent reductive elimination produces the biaryl scaffold and releases Rh(I) complexes. The lactonization was conducted with the assistance of the Rh or silver salt. Using the same strategy, Pdcatalyzed CDA of benzoic acids and phenols has also been
At this stage, developed methods show great possibilities but are still not ready for applications. We hope our studies will inspire chemists to reread existing chemistry and pay attention to developments of novel catalytic systems to realize green and sustainable chemistry.
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Accounts of Chemical Research ORCID
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Zhang-Jie Shi: 0000-0002-0919-752X Notes
The authors declare no competing financial interest. Biographies Yun-Fei Zhang was born in 1991 in Anhui, China. He received his B.S. at China Agricultural University in 2013. He is currently a fifthyear graduate student with Professor Shi at Peking University. His current research interests focus on transition-metal-catalyzed inert C− H bond activation. Zhang-Jie Shi was born in 1974 in Anhui, China. He obtained his B.S. at East China Normal University in 1996 and Ph.D. with Professor Shengming Ma in SIOC in 2001. After his postdoctoral work with Professors Gregory L. Verdine at Harvard University and Chuan He at the University of Chicago, he joined the chemistry faculty of Peking University in 2004, where he was promoted to a full Professor in 2008. In 2017, he moved to Fudan University. His current research interests focus on “inert” chemical bond activations.
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ACKNOWLEDGMENTS Support of this work by the “973” Project from the MOST (2015CB856600) and NSFC (Nos. 21332001, 21431008, 91645111, and 21761132027) is gratefully acknowledged. Over the last 13 years, generous support from Peking University is greatly appreciated by Z.S., which enabled gradual progress towards our goals. We thank Andrew Jackson from Liwen Bianji, Edanz Group China (www.liwenbianji.cn/ ac), for editing the English text of a draft of this manuscript.
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