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

Five new complexes of the type [RuCl2(NHC)(eta(6)-arene)] (4, 5, and 6) and [RuCl(NHC)(eta(6)-arene)(PR3)]Cl (7 and 8) (NHC=N-heterocyclic carbene = bmim, emim; arene = benzene, p-cymene; PR3 = PPh3 or pta = 1,3,5-triaza-7-phosphaadamantane) were synthetized and applied as catalysts (together with the known [RuCl2(bmim)(eta(6)-p-cymene)] (3) with and without added PPh3) 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 hat 95 degrees C) was observed in racemization of (S)-1-phenylethanol with a catalyst (4 mol%) prepared in situ from 3 and 1 equivalent of PPh3. 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. (C) 2017 Elsevier B.V. All rights reserved.