Synthetic and mechanistic studies in enantioselective allylic substitutions catalysed by palladium complexes of a modular class of axially chiral quinazoline-containing ligands

Abstract

The application of palladium complexes of a modular series of axially chiral phosphinamine ligands, the Quinazolinaps, to the enantioselective alkylation of 1,3-diphenyl-2-propenyl acetate with dimethyl malonate and methyl dimethyl malonate is described. Complete conversions and enantiomeric excesses of up to 91% were obtained. To elucidate the solution structure of these complexes and their dynamic behaviour, 2D COSY and NOESY NMR experiments were carried out. An X-ray crystal structure of a palladacycle derived from 2-phenylQuinazolinap which possesses two Pd3Cl5 units is shown. Computational studies were also undertaken to allow qualitative predictions of diastereomeric ratios. The observed enantioselectivity was then rationalised in terms of combined spectroscopic and theoretical data. The catalytic results obtained are best interpreted by the reaction proceeding with nucleophilic attack on the allyl trans to the phosphorus donor atom of the major diastereomeric intermediate.