Construction of P-stereogenic center by selective ligation of NPN type ligands and application to asymmetric allylic substitution reactions

Abstract

Chiral bisoxazolylphosphine ligands 1 [( S, S)-PhP(Ox-R) 2: R=Me, i Pr, t Bu] with an NPN backbone were prepared from chiral 4-alkyl-2-phenyl-4,5-dihydrooxazole compounds. These ligands coordinated to Pd(II) ion as bidentate ligands selectively to give a stereogenic phosphorus atom. The Pd-( S, S)-PhP(Ox- t Bu) 2 1c catalyst evoked high enantioselectivity in asymmetric allylic substitutions of acyclic substrates using dimethyl sodiomalonate as nucleophile. In the reaction of 3-penten-2-yl acetate, which affords a π-allyl intermediate with a small steric factor, the Pd- 1c catalyst successfully induced very high enantioselectivity (94% ee) indicating the effectiveness of the P-stereogenic center formed by selective ligation of ligand 1. In the reaction of cyclic substrates, moderate to high enantioselectivity was obtained by Pd- 1 catalyst using the sodium salt of dimethyl methylmalonate.