trans-2,5-dialkylpyrrolidinyl-containing phosphinamines. Synthetic and mechanistic studies in Pd-catalysed asymmetric allylic alkylation

The preparation of new phosphinamine ligands possessing an enantiopure tran s-2,5-dialkylpyrrolidinyl unit linked by a rigid o-phenylene bridge to a di phenylphosphine is described. Only that route forming the trans-2,5-dialkyl pyrrolidine in the final step from (2-aminophenyl)diphenylphosphine proved successful. The cyclocondensation proceeded in 48% and 27% yields, respecti vely, for the dimethyl- and diethyl-analogues. Their palladium complexes we re prepared and applied to the test of enantioselective alkylation of 1,3-d iphenyl-2-propenyl acetate with dimethyl malonate in high chemical yields b ut with moderate enantioselectivities of up to 34% ee. H-1 NMR spectra of the eta(3)-allyl Pd complexes of four trans-2,5-dialkylp yrrolidine-containing ligands were analysed in an attempt to explain the re sults obtained. In the cases of the 1,3-diphenylallyl complexes, two diaste reomers were observed for all four ligands and their configurations were as signed with the aid of COSY and NOESY experiments. The catalytic results ob tained are best interpreted by the reaction proceeding with nucleophilic at tack on the allyl trans to the phosphorus donor atom of the major diastereo mer. (C) 1999 Elsevier Science Ltd. All rights reserved.