German Edition: DOI: 10.1002/ange.201913743Asymmetric CatalysisInternational Edition: DOI: 10.1002/anie.201913743

Nickel-Catalyzed Asymmetric Reductive 1,2-Carboamination ofUnactivated AlkenesJun He, Yuhang Xue+, Bo Han+, Chunzhu Zhang, You Wang,* and Shaolin Zhu*

Dedicated to the 100th anniversary of the School of Chemistry and Chemical Engineering, Nanjing University

Abstract: Starting from diverse alkene-tethered aryl iodidesand O-benzoyl-hydroxylamines, the enantioselective reductivecross-electrophilic 1,2-carboamination of unactivated alkeneswas achieved using a chiral pyrox/nickel complex as thecatalyst. This mild, modular, and practical protocol providesrapid access to a variety of b-chiral amines with an enantioen-riched aryl-substituted quaternary carbon center in good yieldsand with excellent enantioselectivities. This process revealsa complementary regioselectivity when compared to Pd andCu catalysis.

Alkenes are versatile and readily available starting materialsfor many types of transformations, including hydrogenation,[1]

hydrofunctionalization,[2] and difunctionalization.[3] In partic-ular, transition-metal-catalyzed asymmetric difunctionaliza-tion of alkenes,[4, 5] leading to concomitant introduction of twofunctional groups into a C=C bond in a catalytic andenantioselective manner, is a protocol enabling fast increasein molecular complexity and direct access to a diverse set ofdensely functionalized chiral building blocks from simplestarting materials. Substantial efforts to achieve asymmetricalkene 1,2-carboamination using Pd[6] and Cu[7] catalysts havebeen reported but these transformations produce only a-chiral amines in a highly regioselective fashion. Catalyticasymmetric carboamination with opposite regioselectivity toaccess b-chiral amines remains a significant unexploredchallenge (Figure 1a). Recently, b-chiral amines wereobtained from alkenes by Pd[6g] and Cu[7d] catalysts withmoderate ee values, but the alkenes were limited to dihydro-furans and cyclopropenes. b-Chiral amines are common inpharmaceuticals and natural products, and their direct con-struction through enantioselective alkene 1,2-carboaminationis highly desirable. Simultaneous regioselective introductionof an aryl and an amino group into an unactivated 1,1-disubstituted olefin would lead to the synthesis of b-chiralamines with an enantioenriched aryl-substituted quaternary

carbon center at the b-position, a privileged structure found ina number of natural products and pharmaceuticals (Fig-ure 1b).

Although nickel has been widely used over the pastdecade in cross-coupling chemistry,[9] Ni-catalyzed enantiose-lective alkene difunctionalization has been reported onlyrarely.[5a–k] To realize asymmetric 1,2-carboamination,[8] wepostulated that the readily prepared electrophilic nitrogensource,[10] O-benzoyl-hydroxylamine, could be used with anaryl halide, another electrophile, to undergo reductive alkenecarboamination producing the desired b-aryl-substitutedchiral amine. As shown in Figure 1c, we anticipated that thearylnickel(II) species generated through the oxidative addi-tion of aryl iodide with a nickel catalyst could undergo anasymmetric intramolecular cyclization reaction with a teth-ered 1,1-disubstituted alkene. The resulting alkylnickel(II)species with an all-carbon quaternary center at the adjacentposition could be reduced in situ to afford the alkylnickel(I)intermediate and undergo a sequential electrophilic amina-tion reaction[11] to introduce the desired amine. It is critical tothe success of this design that the O-benzoyl-hydroxylamine

[*] J. He, Y. Xue,[+] B. Han,[+] C. Zhang, Dr. Y. Wang, Prof. S. ZhuState Key Laboratory of Coordination Chemistry, Chemistry andBiomedicine Innovation Center (ChemBIC), School of Chemistry andChemical Engineering, Nanjing UniversityNanjing 210093 (China)E-mail: wangyou@nju.edu.cn

shaolinzhu@nju.edu.cn

[++] These authors contributed equally to this work.

Supporting information and the ORCID identification number(s) forthe author(s) of this article can be found under:https://doi.org/10.1002/anie.201913743.

Figure 1. Design plan: Access to b-chiral amines with a b-aryl-substi-tuted quaternary stereocenter by Ni-catalyzed reductive alkene carboa-mination.

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meets two requirements. First, the intramolecular cyclizationmust be rapid compared to the undesired competitivereductive cross-electrophilic coupling. Second, the O-benzo-yl-hydroxylamine must be stable enough to avoid decom-position under the reductive conditions.

Our investigations began with the enantioselective 1,2-carboamination of the aryl iodide tethered alkene 1a with O-benzoyl-hydroxylamine (2a ; Table 1). After extensive exami-nation of nickel sources, ligands, reductants, additives, sol-

vents, and temperature, we found that the desired b-chiralamine 3a[12] could be obtained in 74% yield upon isolationand 96 % ee under mild reaction conditions in the presence ofa Ni/pyrox catalyst (entry 1). Use of other nickel precatalysts,including Ni(cod)2, led to a slightly reduced yield butcomparable ee value (entry 2). The use of the structurallysimilar tBu-Pyrox ligand (L2) led to somewhat lower yield(entry 3) and the Ph-Pyrox ligand (L3) provided a poor yieldand ee value (entry 4). Use of the bioxazoline ligands L4 andL5 delivered only traces of the desired product (entry 5). Theaddition of a co-catalyst, cobalt(II) phthalocyanine (CoPc),[13]

improved the yield (entry 6). Replacement of the reductantZn0 by Mn0 led to essentially no coupling product (entry 7).The reactivity could however be significantly improved by theaddition of TMSCl as an additive (entry 1 versus entry 8). N-methyl-2-pyrrolidone (NMP) was shown to be a less effectivesolvent and tetrahydrofuran (THF) was shown to be anunsuitable solvent (entries 9 and 10). Inferior results were

obtained when the cross-coupling was conducted at eitherhigher temperatures or when other amination reagents wereused,[14] revealing the subtle interplay of the reagents in thisreaction (entries 11 and 12). A lower catalyst loading could beused, producing only a slightly reduced yield and comparableee value (entry 13). The reaction was conducted at the 5 mmollevel to determine its scalability (entry 14). At this scale, onlya slightly diminished yield was observed with unchangedenantioselectivity, demonstrating the robustness of this cata-lytic system.

Under the optimal reaction conditions, a broad range ofalkene-tethered aryl iodides were found to undergo thistransformation smoothly (Table 2). Both electron-rich (1b–e)

and electron-deficient (1 f–i) substituents on the aryl ringwere well tolerated, affording the cyclized amination productswith good yields and high enantioselectivities. Of particularinterest is that a potential cross-coupling partner, the arylbromide 1g, remained intact and was available for furtherderivatization. A variety of alkyl-substituted alkenes weresubject to the reaction. These had linear (1j–l and 1n–p) andbranched (1m) alkyl groups, although more steric hinderedalkyl group led to a slightly lower ee value (1m vs. 1j–l). Asanticipated, our protocol is also efficient with alkene sub-strates containing a variety of functional groups, includinga furan (1n), ethers (1o, 1p), and an olefin (1p). Accordingly,alkenes tethered with an all-carbon chain (1q–t) deliveredindanes in good yields with excellent enantioselectivity.

A subsequent survey of hydroxylamine esters revealedthat a range of amino groups can be installed (Table 3).Benzyl-protected hydroxylamine esters were used in most

Table 1: Variation of reaction parameters.

Entry Variation from standard reaction conditions Yield [%][a] ee [%][b]

1 none 78(74) 962 Ni(cod)2, instead of NiI2 69 963 L2, instead of L1 69 964 L3, instead of L1 37 465 L4 or L5, instead of L1 trace –6 w/o CoPc 68 967 Mn, instead of Zn trace –8 w/o TMSCl 23 969 NMP, instead of DMF 68 9610 THF, instead of DMF trace –11 45 88C, instead of 35 88C 66 9612 Bn2NOBz, instead of 2a 69 9613 5 mol% NiI2, 6 mol% L1 63 9614 5 mmol-scale 68 96

[a] Yields determined by GC using n-dodecane as the internal standard,the yield within parentheses is the isolated yield (0.20 mmol scale).[b] Enantioselectivities determined by chiral HPLC analysis; the absoluteconfiguration determined by X-ray crystallography. Ad = 1-adamantyl,DMF= N,N-dimethylformamide, Mes = 2,4,6-trimethylphenyl,TMS= trimethylsilyl.

Table 2: Scope with respect to the alkene-tethered aryl iodide couplingpartners.[a]

[a] The yield of the isolated product and enantioselectivity (ee) for eachproduct is given (0.20 mmol scale, average of two runs).

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cases because benzyl groups can be easily removed to give thecorresponding primary and secondary amines. The currentreaction conditions are compatible with a variety of func-tional groups, including an aniline (4b), ethers (4 b, 4d, and4e), a thioether (4c), a nitrile (4h), a boronic acid pinacolester (4 i), a furan (4j), a thiophene (4k), an alkyne (4 l), a Boccarbamate (4q), an ester (4 r), and an olefin (4s). Potentialcoupling motifs, including a boronic acid pinacol ester (4 i)and an aryl chloride (4 f) remained intact and could serve asconvenient handles for further derivatization. Notably,a more sterically hindered amination reagent, substitutedwith a secondary alkyl group (4n), was also well tolerated,and a pre-existing adjacent stereocenter in the aminationreagent was not affected, with the carboamination reactionproceeding in a completely catalyst-controlled manner (4u,4v).

The mechanism shown in Figure 1c was proposed andpreliminary experiments were performed to gain support forit. With stoichiometric amounts of Zn0, both arylnickel(II)and alkylnickel(II) complexes are known to be reduced to thecorresponding NiI species and we sought to determine if thisreduction process takes place before or after the cyclization.Accordingly, a stoichiometric amount of nickel(0) wasemployed (Scheme 1a). The presence of a zinc reductantwas found to be critical for formation of the desiredcarboamination product, suggesting that the amination stepinvolves alkylnickel(I) intermediates, and cyclized side prod-ucts (3q-3 and 3q-4) were observed, even in the absence ofthe zinc reductant. We therefore tentatively propose thatcyclization occurs through the intramolecular migratoryinsertion of arylnickel(II) into the tethered-alkene and thereduction process happens after the cyclization.

To gain insight into the amination step, we first ruled outSN2-type nucleophilic displacement of the potential alkyliodide intermediates by the secondary amine, as no desiredcarboamination product (3a) was observed when a freesecondary amine was used (Scheme 1b). In addition, noaniline product (5) was observed when treating simple aryliodide with O-benzoyl-hydroxylamines under the standardreaction conditions (Scheme 1c), suggesting that anarylnickel(I) species is not formed in our system.[15]

In summary, we have established a practical and efficientreductive 1,2-carboamination process that allows the rapidand enantioselective construction of b-chiral amines with anenantioenriched aryl-substituted quaternary carbon center.Under the mild reaction conditions used, excellent chemo-and enantioselectivity was observed for a wide range of bothalkene-tethered aryl iodide and O-benzoyl-hydroxylaminepartners. Further investigation of the mechanistic details iscurrently in progress.

Acknowledgements

Support was provided by NSFC (21702102, 21772087), NSF ofJiangsu Province (BK20190281), and the FundamentalResearch Funds for the Central Universities (020514380182).

Conflict of interest

The authors declare no conflict of interest.

Keywords: alkenes · asymmetric catalysis · carboamination ·nickel · synthetic methods

How to cite: Angew. Chem. Int. Ed. 2020, 59, 2328–2332Angew. Chem. 2020, 132, 2348–2352

[1] For selected reviews, see: a) W. S. Knowles, Angew. Chem. Int.Ed. 2002, 41, 1998; Angew. Chem. 2002, 114, 2096; b) R. Noyori,

Table 3: Scope with respect to the electrophilic amine coupling partner.[a]

[a] Yield and ee are as defined in Table 2.

Scheme 1. Control experiments.

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[12] CCDC 1955504 (3a) contains the supplementary crystallo-graphic data for this paper. These data can be obtained free ofcharge from The Cambridge Crystallographic Data Centre.

[13] CoPc is proposed to stabilize the alkylnickel(I) intermediates.For examples of CoPc as a co-catalyst to facilitate the Ni-

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catalyzed reductive cross-electrophile coupling, see: a) L. K. G.Ackerman, L. L. Anka-Lufford, M. Naodovic, D. J. Weix, Chem.Sci. 2015, 6, 1115; b) J. L. Hofstra, A. H. Cherney, C. M. Ordner,S. E. Reisman, J. Am. Chem. Soc. 2018, 140, 139.

[14] Y. Yang, S.-L. Shi, D. Niu, P. Liu, S. L. Buchwald, Science 2015,349, 62.

[15] With a Bn-BiOX ligand, arylnickel(II) could be reduced intoarylnickel(I) in situ, the aniline product (5) could be obtained(see the Supporting Information for details).

Manuscript received: October 28, 2019Revised manuscript received: November 20, 2019Accepted manuscript online: November 22, 2019Version of record online: December 27, 2019

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