Nuclear magnetic resonance and crystallographic studies of chiral η3-allyl palladium(II) complexes and asymmetric allylic alkylation of propen-2-yl acetates

Abstract

The chiral complexes [Pd(η³-allyl)(P–P′)]X [P–P′= the chiral chelating ligand (S)(N-diphenylphosphino)(2-diphenylphosphinoxymethyl)pyrrolidine; allyl = MeCHCHCHMe, 2; or PhCHCHCHPh 3; X = BF₄^– or PF^–₆] have been prepared and analysed by NMR spectroscopy. Their diastereoisomeric composition have been determined on the basis of ¹H, ¹³C, ³¹P and two-dimensional ¹H–X (X =¹³C or ³¹P) correlation NMR spectra. Since retention of the preferred syn disposition of the allylic substituents occurs in both cases the reaction product is a mixture of two isomers which differ in allylic chirality. The relative absolute configurations have been assigned by two-dimensional nuclear Overhauser effect measurements and confirmed by the X-ray crystallographic determination of the structure of the major diastereoisomer of 2: the allyl fragments have (1S,3R) absolute configuration. The reaction of racemic 1,3-dimethylprop-2-enyl acetate in the presence of 2 and of 1,3-diphenylprop-2-enyl acetate in the presence of 3 with sodium dimethyl malonate gives the allylic alkylation products in 20 and 30% enantiomeric excess respectively.