Catalytic racemization of secondary alcohols with new (arene)Ru(II)-NHC and (arene)Ru(II)-NHC-tertiary phosphine complexes

Abstract

Five new complexes of the type [RuCl 2 (NHC)(η 6 -arene)] (4, 5, and 6) and [RuCl(NHC)(η 6arene)(PR 3 )]Cl (7 and 8) (NHC=N-heterocyclic carbene=bmim, emim; arene=benzene, pcymene; PR 3 =PPh 3 or pta=1,3,5-triaza-7-phosphaadamantane) were synthetized and applied as catalysts (together with the known [RuCl 2 (bmim)(η 6 -p-cymene)] (3) with and without added PPh 3 ) in racemization of optically active secondary alcohols in toluene. The highest catalytic activity, TOF= 9.3 h -1 (ee as low as 1.3% in 4 h at 95°C) was observed in racemization of (S)-1-phenylethanol with a catalyst (4 mol%) prepared in situ from 3 and 1 equivalent of PPh 3 . It is of practical significance that formation of acetophenone byproduct was suppressed to 3.5% by 17% v/v isopropanol in toluene. DFT calculations revealed that the rate determining step in the suggested reaction mechanism was the agostic coordination of hydrogen on the chiral carbon atom of the alcohol substrate.