New chiral thioether-phosphite ligands and their rhodium-coordination chemistry: steric and electronic properties, dynamic processes and application in catalysis

Abstract

Thioether-phosphite ligands (SP’s) with axially chiral (S)-H0- or (S)-H8-binaphthyl moiety and their [Rh(COD)(SP)]BF4 complexes have been synthesized to study their coordination chemistry and catalytic features and to compare them to those of the structurally analogous phosphine-phosphites (PP’s). NMR exchange studies on [Rh(COD)(SP)]BF4 complexes showed selective 1,5-cyclooctadiene dynamics. Firm evidence has been found that this fluxional process involves dissociation of the Rh-sulfur bond. Based on in situ NMR studies, SP ligands can form two types of bis-ligated species at ligand-to-metal ratio higher than 1. Unlike bis-ligated phosphine-phosphite complexes, bis-ligated thioether-phosphite Rh-complexes can efficiently catalyze asymmetric hydrogenation reactions due to their distinct coordination chemistry.