Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Abnormal-NHC-Supported Nickel Catalysts for Hydroheteroarylation of Vinylarenes Gonela Vijaykumar, Anex Jose, Pavan K. Vardhanapu, Sreejyothi P, and Swadhin K. Mandal* Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India S Supporting Information *
ABSTRACT: Herein we report the hydroheteroarylation of vinylarenes with benzoxazole in the presence of a free abnormal N-heterocyclic carbene and Ni(COD)2, resulting in 1,1-diarylethane products exclusively. In an attempt to understand the mechanism of this catalytic reaction, two abnormal-NHC (aNHC)-coordinated Ni(II) cyclooctenyl complexes were isolated and their solid-state structures were determined by X-ray crystallographic studies. These Ni(II) cyclooctenyl complexes act as active catalyst precursors to generate in situ aNHC-Ni(0) species, which undergo oxidative addition with heteroarene to form Ni(II) hydride intermediates.
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lysts14 since early reports by Crabtree, Nolan, and co-workers.15 The chemistry of aNHC gained momentum since the seminal effort by Bertrand and co-workers to isolate the first aNHC in 2009.16 Recently, we have reported the same isolated aNHC, derived from 1,3-bis(2,6-diisopropylphenyl)-2,4-diphenylimidazolium salt, as an impressive ligand for convenient metal-free catalysts17 and organometallic catalysts18 for a variety of organic conversions. As a part of our ongoing interest to develop catalysts based on an aNHC ligand,17,18 herein we report that the Ni(COD)2/ aNHC combination can promote hydroheteroarylation of vinylarenes with benzoxazole to afford selectively 1,1-diarylalkane derivatives. Further, in an effort to isolate the in situ generated catalyst, the present study shows that a Ni(II) cyclooctenyl complex is formed in solution. We have been able to isolate two such aNHC-coordinated Ni(II) cyclooctenyl complexes and characterized them by single-crystal X-ray crystallography. On the basis of this observation, as well as spectroscopic evidence from the use of stoichiometric reactions, we propose a mechanistic pathway which involves activation of the C−H bond of the heteroarene to form a Ni(II)-H complex, assisted by flexible coordination adjustment by an abnormal NHC ligand.
ransition-metal-catalyzed hydroheteroarylation of olefins through C−H bond activation provides a highly atom economical approach for the synthesis of 1,1- or 1,2diarylalkanes1 that are found in many pharmaceuticals and biologically active molecules.2 Over the past decade, a variety of transition-metal catalysts has shown significant activity in the selective hydroheteroarylation of alkenes.3−8 There have been intense efforts in recent years to develop hydroheteroarylation catalysts with cost-efficient and cheaper metals. For example, in 2011, Yoshikai and co-workers have developed catalysts based on Co-PCy3 as well as Co-NHC (NHC = 1,3-dimesitylimidazol-2-ylidene (IMes)) for hydroheteroarylation of vinylarenes where the use of ligands can control the regioselectivity of the reaction.9 Among the base-metal-catalyzed reactions, Ni(COD)2 (COD = 1,5-cyclooctadiene) and N-heterocyclic carbene (NHC) based combinations have been the most well studied for hydroheteroarylation, which was first introduced by Nakao and Hiyama after their initial report on hydroheteroarylation of vinylarene in the presence of Ni(COD)2 and an NHC ligand (1,3-dimesitylimidazol-2-ylidene (IMes)) that exclusively afforded a 1,1-diarylethane derivative (branched product).10 Ong and co-workers later reported Ni(COD)2 and amino NHC based ligand combination in the presence as well as in the absence of a Lewis acid (AlMe3) to understand the influence of a Lewis acid on regioselectivity.11 The same group in 2015 reported p-C−H alkylation of pyridine with allylbenzene using a Ni(COD)2/NHC system.12 Recently, Hartwig and co-workers investigated the anti-Markovnikov hydroheteroarylation of aliphatic alkenes with a Ni(COD)2/ NHC based catalyst.13 All of these reports involving Ni(COD)2/NHC based catalysts postulate involvement of a Ni(0)-NHC complex as the active catalyst generated in situ, which undergoes oxidative addition with heteroarene. However, a similar report on hydroheteroarylation catalysis with Ni(COD)2 is not known using an abnormal NHC (aNHC). It may be noted that aNHCs are considered to be excellent building blocks in designing numerous organometallic cata© XXXX American Chemical Society
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RESULTS AND DISCUSSION Optimization of the catalytic study began by screening the reaction of benzoxazole with styrene in the presence of [Ni(COD)2] (5 mol %) and the aNHC ligand 1,3-bis(2,6diisopropylphenyl)-2,4-diphenylimidazol-5-ylidene (1) or 1(2,6-diisopropylphenyl)-2,3,5-triphenylimidazol-4-ylidene (2) (Table 1) under different conditions. The product formation was monitored by 1H NMR spectroscopy. We observed full conversion using aNHC ligand 1 in hexane at 80 °C, to afford Received: August 18, 2017
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DOI: 10.1021/acs.organomet.7b00628 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 1. Evaluation of Ligands, Solvents, and Temperatures for the Hydroheteroarylation of Styrene with Benzoxazolea
entry
aNHC
solvent
temp (°C)
yield (%)b
1 2 3 4 5 6 7d
1 2 1 1 1 1 −
hexane hexane benzene toluene THF dioxane hexane
80 80 90 110 85 90 80
98 (95)c 65 90 85
