Enantioselective catalysis XVI: regio- and enantioselectivity in nickel-catalysed cross-coupling reactions of allylic substrates with Grignard reagents

Abstract

An investigation of the asymmetric synthesis of 3-phenyl-1-butene by the cross-coupling reaction of linear 2-butene-1-yl compounds and of branched 1-butene-3-yl substrates with phenylmagnesium halides is presented. Nickel and palladium complexes bearing chiral 3,4-bis(phosphanyl)pyrrolidine or 3-(diphenylphosphanyl)pyrrolidine ligands have been employed as catalysts. The quantitative analysis and the determination of enantiomeric composition of the catalytic samples were accomplished using enantioselective gas-liquid chromatography separations. Regio- and enantioselectivity can be influenced by the choice of the leaving group present in the allylic substrates and additionally by the choice of the ligand to nickel ratio in the case of monophosphane ligands. The influence of tertiary phosphanyl groups in three 3,4-bis(phosphanyl)pyrrolidine ligands, differing in basicity and molecular volume on chemical yield and on regio- and enantioselectivity is investigated. High chemical and optical yields (up to 83.5% ee) can be exclusively obtained with 3,4-bis(diphenylphosphanyl)pyrrolidine ligands. This conclusion contrasts with the results of another cross-coupling reaction yielding 3-phenyl-1-butene ((1-phenylethyl)magnesium halides and vinyl halides as starting compounds). In this catalysis nickel complexes bearing 3-(diphenylphosphanyl) pyrrolidine ligands were the most enantioselective catalysts.