Heck Reactions of α‐ or β‐Substituted Enol Ethers with Aryl Bromides Catalysed by a Tetraphosphane/Palladium Complex – Direct Access to Acetophenone or 1‐Arylpropanone Derivatives

Abstract

Abstractcis,cis,cis‐1,2,3,4‐Tetrakis(diphenylphosphanylmethyl)cyclopentane/[PdCl(C3H5)]2 efficiently catalyses the Heck reaction of α‐ and β‐substituted enol ethers with aryl bromides. The arylation of 1‐phenyl‐1‐(trimethylsilyloxy)ethylene led directly to the 2‐aryl‐1‐phenylethanones. Similar reaction rates were observed with electron‐rich, electron‐deficient or sterically congested aryl bromides. Heck reaction with benzyl isopropenyl ether gave a mixture of isomers. However, this mixture gave selectively the 1‐arylpropanones after hydrolysis. Employing β‐methoxystyrene, 3‐ethoxyacrylonitrile or methyl 3‐methoxyacrylate, the regioselective α‐arylation of these enol ethers was observed in all cases, but mixtures of (Z) and (E) isomers were generally obtained, which in many cases yielded a single ketone product after acid treatment. The stereoselectivity of this reaction depends on steric and electronic factors, and better stereoselectivities in favour of (Z) isomers were observed with electron‐rich or sterically congested aryl bromides. Higher yields were obtained for this reaction with electron‐rich or sterically congested aryl bromides than with electron‐poor aryl bromides. These observations suggest that the rate‐limiting step of the catalytic cycle is not the oxidative addition of the aryl bromide to the palladium complex with these substituted enol ethers.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)