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

cis, cis, cis- 1, 2,3,4 -Tetrakis(diphenylphosphanylmethyl) cyclopentane/[PdCl(C3H5)](2) efficiently catalyses the Heck reaction of alpha- and beta-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 beta-methoxystyrene, 3-ethoxyacrylonitrile or methyl 3-methoxyacrylate, the regioselective alpha-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.