Synthesis, characterization, and reactivity of ruthenium bis-allyl complexes with chiral phosphine-phosphite ligands

Abstract

A series of ruthenium bis-allyl complexes of formula Ru(η 3- 2-MeC 3H 4) 2(P-OP) (1) which incorporate chiral phosphine-phosphite ligands (P-OP) have been prepared and characterized. Compounds 1 exist in solution as a mixture of two diastereomers (maj and min) differing in the configuration at the metal. Both isomers are in equilibrium in solution, and their ratio varies with the nature of the P-OP ligand. Along the series, higher values of the maj/min ratio were observed with complexes bearing less sterically encumbered phosphite groups. In reactions of compounds 1 with protic reagents removal of one or two allyl ligands has been observed. Thus, reaction of 1h with pentachlorophenol produces the mixed allyl-phenoxide 3h, whereas reaction of 1a or 1h with tiglic acid produces the corresponding tiglates 4a,h, respectively. In addition, compounds 1 generate active catalysts for the hydrogenation of tiglic and 2-methyl-2-pentenoic acids. By appropriate optimization of the structure of the P-OP ligand, enantioselectivities up to 89% ee were obtained in these reactions. In addition, complexes 1 also catalyze the ROMP reaction of norbornene, although low initiation rates have been observed for this process. © 2012 American Chemical Society.