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Sep 7, 2017 - ABSTRACT: A highly efficient strategy for remote reductive cross-electrophile coupling has been developed through the ligand-controlled ...
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Remote Migratory Cross-Electrophile Coupling and Olefin Hydroarylation Reactions Enabled by in Situ Generation of NiH Fenglin Chen,† Ke Chen,† Yao Zhang,† Yuli He,† Yi-Ming Wang,*,‡ and Shaolin Zhu*,† †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China ‡ Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260, United States S Supporting Information *

ABSTRACT: A highly efficient strategy for remote reductive cross-electrophile coupling has been developed through the ligand-controlled nickel migration/arylation. This general protocol allows the use of abundant and bench-stable alkyl bromides and aryl bromides for the synthesis of a wide range of structurally diverse 1,1-diarylalkanes in excellent yields and high regioselectivities under mild conditions. We also demonstrated that alkyl bromide could be replaced by the proposed olefin intermediate while using n-propyl bromide/Mn0 as a potential hydride source.

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C(sp3)−C(sp2) bonds from the corresponding halides has received considerable attention in recent years,6 and encouraging progress for reductive cross coupling has been made using both nickel and palladium catalysis.7 As a variation of this process, a migratory cross-electrophile coupling of aryl and alkyl halides with benzylic selectivity should be of considerable synthetic utility. However, such a process poses a significant challenge and remains unreported. In related work, Baudoin has reported a palladium-catalyzed terminal-selective reductive coupling of moderate regioselectivity using aryl triflates and alkyl bromides as starting materials.2d We recently reported a NiH-catalyzed remote reductive C−H arylation reaction by using hydrosilane as the exogenous reducing agent (Figure 1a).2v Significant drawbacks of this protocol, however, include the requirements for aryl iodides as coupling partners and CsF as the base for regeneration of the nickel hydride (NiH)

INTRODUCTION Cross-coupling chemistry is a powerful technology for the construction of carbon−carbon bonds. In classical crosscoupling reactions, C−C bond formation occurs at the site of reactive functional groups preinstalled on two coupling partners. Complementary techniques that allow the new C− C bond formation at remote, unfunctionalized sites would enable new strategies for complex molecule synthesis to be employed and allow access to structures that would otherwise be difficult to prepare.1 An iterative migratory insertion/βhydrogen elimination process mediated by metal hydride intermediates represents a profitable approach for the activation of remote unfunctionalized sites.2 The combined application of cross-coupling and metal-hydride chemistry3 in remote C−H functionalization, however, is reported in only a few contexts.2m−v,4 As an important class of compounds, alkyl halides are reactive, yet bench-stable and readily available starting materials for synthesis. Nevertheless, their use in transition metal-catalyzed coupling reactions, is often limited by isomerization brought about by β-hydrogen elimination and reinsertion. We felt that this facile and often undesired side reaction could, in fact, be used advantageously in a regioselective, migratory and reductive cross-coupling of alkyl and aryl halides. Over the past decade, catalysis utilizing earth-abundant nickel has emerged as a practical tool for carbon−carbon bond formation, most notably through the Negishi, Suzuki-Miyaura, Kumada, Stille, and Hiyama cross coupling reactions.5 As an alternative approach, reductive cross-electrophile coupling,6 a strategy wherein two shelf-stable electrophiles (typically alkyl or aryl halides) are coupled in the presence of an external reductant, circumvents the synthesis of organometallic reagents of limited stability and commercial availability and is amenable to parallel synthesis. As a consequence, the construction of © 2017 American Chemical Society

Figure 1. Design plan: Migratory cross-electrophile coupling. Received: July 31, 2017 Published: September 7, 2017 13929

DOI: 10.1021/jacs.7b08064 J. Am. Chem. Soc. 2017, 139, 13929−13935

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Journal of the American Chemical Society Scheme 1. Variation of Reaction Parametersa,b

species, both of which reduce the practicality of the protocol. To address these shortcomings, we sought to develop a procedure for the coupling of alkyl and aryl bromides under mild and base-free conditions. In order to achieve high yields such a process, conditions for the preferential oxidative addition of a low-valent nickel species to an alkyl bromide in the presence of an aryl bromide needed to be uncovered. As first demonstrated by Weix and coworkers,6b,d Ni(0) complexes will generally react preferentially with an aryl bromide before engaging an unactivated alkyl bromide to produce the direct coupling product (Figure 1b, left). The complementary selective oxidative addition of an unactivated alkyl bromide in preference to an aryl bromide is currently unknown.8 If this inversion of selectivity could be achieved, a NiH species and olefin could be generated in situ by β-hydrogen elimination of an alkylnickel species derived from a widely-available and bench-stable alkyl bromide.4,11d The in situ-generated NiH would trigger an iterative elimination/ reinsertion process to potentially allow for functionalization at remote sites of an alkyl chain. In particular, we postulated that functionalization at a benzylic site could occur with high regioselectivity, allowing for a subsequent nickel-catalyzed cross-coupling reaction to generate 1,1-diarylalkanes (Figure 1b, right side), a privileged class of compounds in medicinal chemistry and materials science.9 In this article, we report the successful implementation of this strategy using a nickel catalyst supported by 6,6′-dimethyl-2,2′-bipyridyl as the ligand. The mild (room temperature and base free), regioselective, and efficient conditions reported herein were applicable to a wide range of both simple and functionalized alkyl and aryl bromide substrates, potentially serving as a useful and complementary addition to previous methods for the synthesis of 1,1diarylalkanes. In addition, some insights gleaned from preliminary mechanistic studies are also presented.

a

Yields were determined by GC using dodecane as the internal standard, the yield in parentheses was isolated yield of purified product and is an average of two runs (0.2 mmol scale). brr refers to regioisomeric ratio, represents the ratio of the major (1,1-diarylalkane) product to the sum of all other isomers as determined by GC analysis (See SI for details). PMHS, polymethylhydrosiloxane; DMA, N,Ndimethylacetamide; PMP, 4-methoxyphenyl.

with other polar aprotic solvents (entries 5, 6), raising the reaction temperature (entry 7), or reducing the number of equivalents of aryl bromide (entry 8) all led to somewhat lower yields. Moreover, use of the more reactive 4-iodoanisole in place of the 4-bromoanisole led to poor yields with almost complete loss of regioselectivity; the homocoupled biaryl was observed to be the major side product (entry 9). It is worth pointing out that methyl groups of L1 were found to be critical to the success of the reaction.11 Use of the parent bpy (L2) or the 6,6′-dimethoxy derivative (L3) resulted in only the linear coupling product 3A (entries 10, 11, see SI for details). In line with our expectations, formation of the desired product was not observed in control experiments in which Ni, ligand, or Mn0 were omitted. Finally, using NiI2·xH2O lead to diminished yield for the current combination of substrates, but was beneficial in the case of electron-poor aryl bromides (entry 12). We next turned our attention to the substrate scope of the transformation. Evaluation of a wide range of starting materials revealed the broad applicability of this protocol. In terms of electronic properties, both electron-rich and electron-withdrawing aryl bromides could undergo the desired migratory coupling with excellent regioselectivity (3a and 3w, Table 1). For the alkyl bromide substrate, a wide variety of substituents on the remote aryl ring were also well tolerated, including those that were electron withdrawing (3o−q) or electron donating (3s, 3t). Since the optimized protocol avoided the addition of exogenous acids or bases as additives, a broad range of sensitive functional groups were expected to be compatible. Indeed, under these exceptionally mild reaction conditions, a tertiary amine (3b), esters (3c, 3w, and 3y), a nitrile (3h), a Boc carbamate (3j), a silane (3s), and a ketone (3z), an aryl chloride (3o), a phosphate ester (3e), a boronic acid pinacol ester (3x), and an aldehyde (3a′) were all left intact. Notably, various basic and acidic functional groups, including a primary

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RESULTS On the basis of our previously reported coupling of olefins with aryl iodides, we initially investigated the use of 1-iodo-4phenylbutane and 4-iodoanisole as substrates. As shown in Scheme 1a, we were pleased to find that the desired migratory coupling product 3a could indeed be formed in moderate isolated yield (47%) with good regioselectivity [rr (1,1diarylalkane: all other isomers) = 17:1] under our previous remote hydroarylation reaction conditions.2v On the basis of this preliminary result, we next turned our attention to the coupling of the less reactive bromide substrates 1-bromo-4phenylbutane (1a) and 4-bromoanisole (2a). Although these previously reported conditions were almost completely ineffective in the case of bromide substrates (entry 2, Scheme 1b), a thorough optimization of the nickel source, solvent, reductant, and a range of other parameters afforded a mild protocol for efficient and selective coupling: under optimized conditions, 3a was obtained in good isolated yield (81%) and with excellent regioselectivity [rr = 38:1] (entry 1). Interestingly, it was found that an inorganic base was not required for catalytic turnover in this system. Postulating that it would act as an additional hydride source, we incorporated n-PrBr (0.5 equiv) into the reaction mixture as an additive. Indeed, improved yields were observed (entry 1 vs 3).4,9d Notably, arylation of n-PrBr itself was not observed, demonstrating the excellent chemoselectivity of this crosscoupling system. Zinc as a reducing agent gave poor results compared to manganese (entry 4), while replacement of DMA 13930

DOI: 10.1021/jacs.7b08064 J. Am. Chem. Soc. 2017, 139, 13929−13935

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Journal of the American Chemical Society Table 1. Cross-Electrophile Coupling: Scope of the Alkyl Bromide/Aryl Bromide Couplinga,b

a

Isolated yields on 0.20 mmol scale (average of two runs). brr refers to regioisomeric ratio, represents the ratio of the major (1,1-diarylalkane) product to the sum of all other isomers as determined by GC analysis; ratios reported as >20:1 were determined by crude 1H NMR analysis. cNiI2· xH2O used. d1.5 equiv aryl bromide used. See Supporting Information for experimental details.

Table 2. Remote Hydroarylation: Scope of the Olefin/Aryl Bromide Coupling

a

0.10 mmol 4b used. bNiI2·xH2O used. Yield and rr are as defined in Table 1. See Supporting Information for experimental details.

aniline (3d), a secondary amine (3k), a primary alcohol (3g), and a phenol (3r), were also compatible with the current protocol, thereby eliminating the need for efficiency-reducing protection/deprotection sequences often required by traditional cross-coupling reactions using stoichiometric organometallic reagents. Additionally, heterocycles frequently found in medicinally active agents, such as an indole (3i, 3j), a benzofuran (3l), a furan (3u), and pyridines (3b′, 3c′), were also competent coupling partners. Importantly, both primary and secondary alkyl bromides were suitable partners under our standard conditions (Table 1b), and high selectivity for migration/arylation at the benzylic position was observed, regardless of the starting position of the C−Br bond (3a, 3m,

3n, 3r, 3v, 3f′). Finally, we found that 3a could be prepared on a 10 mmol scale with only a slightly diminished yield and unchanged regioselectivity, demonstrating the robustness and practicality of the procedure. In light of these results and our proposed reaction pathway (Figure 1b), we questioned whether readily available olefins, which are proposed intermediates in the above process,2v,4 could be used directly through the addition of a stoichiometric quantity of n-propyl bromide to regenerate the NiH species (via β-H elimination) needed to initiate the chainwalking process after every turn of the catalytic cycle.4,11d As illustrated in Table 2, this turned out to be a viable strategy. As before, migratory arylation of n-PrBr was not observed, again 13931

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Journal of the American Chemical Society

of the migratory arylation product with excellent selectivity for arylation at the carbon α to the heteroatom, producing αarylether 8 and α-arylamine 10, respectively (Figure 2c). Finally, since they may serve as inexpensive chemical feedstocks, regioisomeric mixtures of alkyl halides are of considerable interest as starting materials for the migratory cross-coupling reaction. To demonstrate the potential applicability of our protocol to mixtures of alkyl halides, an equimolar mixture of 1- and 2-bromo-4-phenylbutane was treated with aryl bromide 2y under catalytic conditions. This process retained much of its chemical efficiency and regioselectivity (Figure 2d), as compared to the same transformation using regioisomerically pure alkyl halide as starting material (Table 1).

demonstrating the excellent chemoselectivity for this coupling process. We were pleased to find that a wide array of alkenes and aryl bromides can be efficiently coupled.12 A range of alkenes, including terminal olefins (6a−f), unactivated internal olefins (6g−j), both E and Z olefins, as well as E/Z mixtures (6g−i) were suitable partners, regardless of the starting position of the CC bond. Electron donating (6b, 6f) and electron withdrawing substituents (6c−e) on the remote aryl ring were all well-tolerated. Encouraged by these results, further interrogation of the reaction scope also demonstrated the broad scope of aryl bromide partner. Of particular note, a triflate (6k), ketones (6n, 6q), and an acetal (6r) could be used. Moreover, heteroaromatic bromides, such as those containing a pyridine (6s) and a pyrimidine (6t) in place of the aryl group were competent substrates for the current protocol. Interestingly, an alkyl bromide with internal branching (a tertiary sp3 carbon atom) along the alkyl chain (1g′, Figure 2a)

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DISCUSSION A working model of the catalytic cycle for our migratory crosselectrophile coupling reaction is depicted in Figure 3. In

Figure 3. Proposed pathway of migratory cross-electrophile coupling. Figure 2. Expanded substrate scope and regioconvergent experiment.

contrast to the majority of previously developed nickelcatalyzed reductive coupling reactions,6b,d the process is initiated by selective oxidative addition of ligand-bound Ni(0) complex I to the unactivated alkyl bromide, rather than the aryl bromide, to form an alkylnickel(II) intermediate (II). Reduction of II by Mn0 and β-hydrogen elimination of the resultant Ni(I)-alkyl (III) forms key the nickel(I) hydride species, IV.8c,d A series of iterative insertion and elimination reactions ultimately provides access to benzylnickel(I) intermediate VI. Selective reaction of VI with aryl bromide 2, followed by reductive elimination of nickel(III) adduct VII10,13 then delivers the migratory arylation product 3 and nickel(I) bromide VIII, the reduction of which regenerates I to complete the catalytic cycle. Using alkenes in place of alkyl bromides as starting materials (Table 2), a stoichiometric amount of n-PrBr is necessary, in order to generate IV via a nickel(I) hydride intermediate. Oxidative addition of propyl bromide to LNi0 (I) followed by β-hydrogen elimination affords the propene adduct

could also undergo migratory cross-coupling under the current conditions. Coupling product 3g′ was obtained with high regioselectivity. Yield and overall conversion, however, were modest in this case. Furthermore, branched and functionalized olefins could also be employed (Figure 2b), including a 1,1disubstituted alkene (4u) and a free allylic alcohol (4w). Notably, even a trisubstituted internal alkene (4v), a challenging substrate for transition metal catalysis in general, could also undergo remote hydroarylation, although under the current conditions, the desired process only progressed to about 30% conversion. Surprisingly, excellent benzylic selectivity was still observed when methyl cinnamate (4x), an α,β-unsaturated ester, was employed.2v We could also extend this chemistry to heteroatomsubstituted cyclic olefins. In preliminary studies, the use of 2,5-dihydrofuran and a N-phenyl-3-pyrroline led to formation 13932

DOI: 10.1021/jacs.7b08064 J. Am. Chem. Soc. 2017, 139, 13929−13935

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Journal of the American Chemical Society of a NiH species. Alkene exchange of this complex with the olefin substrate (4), presumably by a dissociative mechanism, then provides IV to initiate the chainwalking process (Figure 3a). With these preliminary proposals as a starting point, a series of mechanistic experiments were conducted to gain a more accurate and detailed understanding of the migratory cross coupling reactions. To gain some insight into the nature of the species involved in the chainwalking olefin isomerization, olefin 4a was subjected to the usual reaction conditions in the absence of aryl bromide. A mixture of olefins arising from olefin isomerization, as well as a small amount of homocoupling product, was observed to form within several hours (Figure 4).14

Figure 4. Olefin isomerization in the absence of aryl bromide.

4a and 5a, using n-C3D7Br as the alkyl bromide additive. Consistent with its proposed role, deuterium transfer from the alkyl bromide to the coupling product was observed, with incorporation of deuterium into all positions along the hydrocarbon chain of 6a (eq 3).8c,16 Notably, only small amount deuterium incorporation at the benzylic position in 6aD was observed, suggesting that β-migratory insertion with the styrenic intermediate to form the benzylnickel species occurs with high regioselectivity. Moreover, we sought to determine whether NiH remains ligated on the substrate throughout the chainwalking process. Thus, a mixture of an alkyl bromide and a sterically and electronically similar alkene were subjected to the coupling conditions. When a mixture of alkyl bromide 1i′ and alkene 4a were subjected to Ni salt, ligand, and manganese metal, both coupling products 3i′ and 6a could be detected in roughly equal amounts during all stages of the coupling process (eq 4). If the initially formed alkylnickel species (II in Figure 3) underwent chainwalking without dissociation of NiH, then the coupling product 3i′ derived from 1i′ should form exclusively in the initial stages of the reaction. The observed formation of both 3i′ and 6a in comparable amounts, even at the start of the reaction, supports a mechanism in which chainwalking occur with rapid dissociation and reassociation of free NiH, allowing for rapid formation of alkylnickel species derived from both alkyl halide and olefin starting materials. Interestingly, in this case, as well as when olefin and aryl bromide alone were used as the starting materials (see Scheme 2b below), an induction period was observed, with no reaction or olefin isomerization occurring in the first hour, while no induction period was observed when alkyl bromide and aryl bromide alone were used. To further understand the behavior of NiH, we monitored the material balance over time for both the standard migratory cross-electrophile coupling and olefin remote hydroarylation reactions (Scheme 2). As depicted in Scheme 2a and 2b, during partial conversion, small amount of both positional isomers of the olefin (4a′ and 4a″) were observed, providing additional evidence of the dissociation of the NiH species from the olefin during chainwalking. In addition, the data in Scheme 2b suggests that olefin isomerization is rapid compared to the subsequent coupling process. As observed for the process depicted in eq 4, there was an induction period for this process,

These results indicate that olefin isomerization does not depend on the presence of the aryl bromide to occur, and suggest that chainwalking precedes oxidative addition by the aryl bromide. As further evidence against an alternative scenario in which chainwalking occurs after oxidative addition of the aryl bromide, we note that regioselectivity increases dramatically from 1:1 to 38:1 rr upon switching from aryl iodide to aryl bromide as the coupling partner (Scheme 1b, entry 1 vs 9). If chainwalking was to occur after oxidative addition of aryl halide for both aryl bromide and iodide substrates, we would expect only a small difference, if any, in the regioselectivities obtained for these reactions. On the other hand, the large difference in regioselectivity is consistent with a mechanism wherein oxidative addition of aryl halide (at least in the case of aryl bromides) follows chainwalking and serves as a regiodetermining step. In the presence of a large excess of manganese metal, Ni(II) complexes are known to be reduced to the corresponding Ni(I) species.15a Moreover, Ni(I) hydride and alkyl species have previously been proposed as intermediates in nickel-catalyzed olefin isomerization reactions.8c,15b We therefore tentatively propose that chainwalking occurs via iterative migratory insertion/β-hydrogen elimination of Ni(I)-hydride and -alkyl intermediates. However, at the moment, we cannot rule out the involvement of Ni(II) species in this process.15c To examine the hypothesis that olefin isomerization occurs through rapid and reversible β-hydrogen elimination/reinsertion steps, we investigated alkyl bromide and alkene substrates bearing an oxygen atom (1h′) or an isopropylidene (4y) spacer in the chain between the aryl group (and its neighboring benzylic position) and the bromide substituent or the double bond. Since the spacer groups lack a β-hydrogen, the chainwalking that leads to the putative benzylnickel intermediates is blocked. Indeed, in accord with the proposed mechanism for chainwalking, neither of the coupling products that could arise from these intermediates was observed under the optimized protocols (eqs 1 and 2). To gain more insight into the chainwalking process, deuterium-labeling experiments were performed for the olefin coupling protocol using perdeuterated propyl bromide. The standard protocol was followed for the preparation of 6a from 13933

DOI: 10.1021/jacs.7b08064 J. Am. Chem. Soc. 2017, 139, 13929−13935

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Journal of the American Chemical Society Scheme 2. Tracing Experimentsa

Figure 5. Mechanistic study.

transformation is under investigation in our laboratory, and progress in this area will be reported in due course.

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ASSOCIATED CONTENT

S Supporting Information *

a

Yields were determined by GC using dodecane as the internal standard, and is an average of two runs (0.2 mmol scale).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b08064. Table S1, Schemes S1−S7, experimental procedures, characterization data for all compounds (PDF)

with no isomerization of the olefin or coupling with the aryl bromide observed during the first hour (also see Scheme S5 in SI for details). The origin of this induction period is still under investigation. Deuterium-labeling experiments were also carried out to probe the stereochemical course of the reaction and gain insight into the C−C bond forming process. Interestingly, when indene and 5a were used as the substrates, with C3D7Br as the additive, a 1:1 diastereomeric mixture of deuterated 6z was produced. The D incorporation into the 2-position of the indene was nearly complete (96% overall). We attribute this result to homolysis of the L(Ar)(2-indanyl)Ni(III)Br intermediate, so that the Ni(III) species exists in rapid equilibrium with the Ni(II) complex and the benzylic radical, resulting in a diastereomeric mixture of isotopically labeled products.10b,c Despite this evidence for a radical intermediate,17,18 the addition of radical inhibitors like BHT (2,6-di-tert-butyl-4methylphenol) or DHA (9,10-dihydroanthracene) to the reaction system had a negligible effect on the result, suggesting that homolysis and radical recombination are likely rapid and occurs in-cage (Figure 5b). Additional studies to fully elucidate the reaction pathway are underway.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Yi-Ming Wang: 0000-0001-6414-0908 Shaolin Zhu: 0000-0003-1516-6081 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research reported in this publication was supported by the “1000-Youth Talents Plan”, NSFC (21602101, 21772087), NSF of Jiangsu Province (BK20160642). Y.-M.W. thanks the University of Pittsburgh for the generous provision of startup funds.

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REFERENCES

(1) For selected reviews on C−H activation in total synthesis, see: (a) Godula, K.; Sames, D. Science 2006, 312, 67. (b) Gutekunst, W. R.; Baran, P. S. Chem. Soc. Rev. 2011, 40, 1976. (c) McMurray, L.; O’Hara, F.; Gaunt, M. J. Chem. Soc. Rev. 2011, 40, 1885. (d) Yamaguchi, J.; Yamaguchi, A. D.; Itami, K. Angew. Chem., Int. Ed. 2012, 51, 8960. (e) Hartwig, J. F. J. Am. Chem. Soc. 2016, 138, 2. (2) For a review on remote functionalization through alkene isomerization, see: (a) Vasseur, A.; Bruffaerts, J.; Marek, I. Nat. Chem. 2016, 8, 209 and citations therein. For recent selected examples using Pd, see: (b) Patel, H. H.; Sigman, M. S. J. Am. Chem. Soc. 2016, 138, 14226. (c) Lin, L.; Romano, C.; Mazet, C. J. Am. Chem. Soc. 2016, 138, 10344. (d) Dupuy, S.; Zhang, K.-F.; Goutierre, A.-S.; Baudoin, O. Angew. Chem., Int. Ed. 2016, 55, 14793. (e) Hamasaki, T.; Aoyama, Y.; Kawasaki, J.; Kakiuchi, F.; Kochi, T. J. Am. Chem. Soc. 2015, 137, 16163. (f) Mei, T.-S.; Patel, H. H.; Sigman, M. S. Nature 2014, 508, 340. (g) Larionov, E.; Lin, L.; Guénée, L.; Mazet, C. J. Am. Chem. Soc.

CONCLUSION In conclusion, we have developed a mild and highly robust nickel-catalyzed migratory remote cross-electrophile coupling reaction via the formation of NiH in situ from alkyl halides. Excellent regio- and chemoselectivity were observed, in terms of both alkyl bromides and alkenes precursors. This versatile protocol provides a versatile and synthetically valuable addition to the current processes for reductive cross-coupling. Mechanistic studies are consistent with a mechanism in which a nickel(I) hydride species effects the rapid isomerization of olefin isomers, with the ultimate regiochemical outcome likely determined by the subsequent oxidative addition of the aryl halide. Developing an asymmetric version of the current 13934

DOI: 10.1021/jacs.7b08064 J. Am. Chem. Soc. 2017, 139, 13929−13935

Article

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DOI: 10.1021/jacs.7b08064 J. Am. Chem. Soc. 2017, 139, 13929−13935