Room-Temperature Negishi Cross-Coupling of Unactivated Alkyl Bromides with Alkyl Organozinc Reagents Utilizing a Pd/N-Heterocyclic Carbene Catalyst

Niloufar Hadei, Eric Assen B. Kantchev, Christopher J. O'Brien, and Michael G. Organ*
Department of Chemistry, York University, 4700 Keele Street Toronto, ON, Canada M3J 1P3
J. Org. Chem., 2005, 70 (21), pp 8503–8507
DOI: 10.1021/jo051304c
Publication Date (Web): September 9, 2005
Copyright © 2005 American Chemical Society

Abstract

Abstract Image

A high-yielding cross-coupling reaction of unactivated alkyl bromides possessing β-hydrogens with alkylzinc halides utilizing a Pd/N-heterocyclic carbene (NHC) catalyst at room temperature is described. A variety of Pd sources, Pd2(dba)3, Pd(OAc)2, or PdBr2, with the commercially available ligand precursor 1,3-bis(2,6-diisopropylphenyl)imidazolium chloride (IPr·HCl) successfully coupled 1-bromo-3-phenylpropane with n-butylzinc bromide in THF/NMP. An investigation of different NHC precursors showed that the bulky 2,6-diisopropylphenyl moiety was necessary to achieve high coupling yields (75−85%). The corresponding ethyl analogue was moderately active (11%). A range of unsymmetrical NHC precursors were prepared and evaluated. The ligand precursor containing one 2,6-diisopropylphenyl and one 2,6-diethylphenyl afforded the coupling product in 47% yield, clearly suggesting a direct relationship between the steric topography created by the flanking N-substituents and catalyst activity. Under optimal conditions, a number of alkyl bromides and alkylzinc halides possessing common functional groups (amide, nitrile, ester, acetal, and alkyne) were effectively coupled (61−92%). It is noteworthy that β-substituted alkyl bromides and alkylzinc halides successfully underwent cross-coupling. Also, under these conditions alkyl chlorides were unaffected.

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History

  • Published In Issue October 14, 2005
  • Received June 25, 2005

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