Platinum-Catalysed Allylic Alkylation: Reactivity, Enantioselectivity, and Regioselectivity

Abstract

The use of platinum complexes as catalysts for allylic substitution has been studied. A variety of different complexes catalyse the reaction, and several substrates have been tested. In the alkylation of mono(alkyl)-substituted allylic acetates, regioselectivity is highly dependent on ligand choice. By using tricyclohexylphosphine as the ligand, almost complete formation of branched products is observed. The development of a highly enantioselective (ca. 80-90% ee) reaction that makes use of chiral diphenylphosphinooxazoline ligands (abbreviated as (S)-PN) is also described. The enantioselectivity is highly dependent on the ratio of ligand to platinum (when the ratio ligand/Pt is greater than 1:1, the ee drops off dramatically). This is in contrast to palladium and is interpreted in terms of differing coordination chemistry for the two metals ((S)-PN is hemilabile when complexed to platinum) and should be of significance to future systems that utilise heterobidentate ligands. The crystal structures of two isoelectronic platinum and palladium complexes [[(S)-PN]MCl2] are also described.