Temperature-tuned enantioselectivity in transfer hydrogenation: preparation and catalytic property of amino acid-modified benzimidazole

A series of l‐amino acid‐modified benzimidazoles have been synthesized and their application in Ru‐catalysed asymmetric transfer hydrogenation (ATH) of ketones has been evaluated using i‐propanol as hydrogen donor as well as solvent. The enantioselectivity of the product was sensitive to the reaction conditions and showed reversed temperature effect. Better reaction activity and enantioselectivity were obtained at higher reaction temperature and higher molar ratio of substrate to catalyst (S/C). At an S/C of 2000:1 and 5000:1, the turnover frequency (TOF) and the enantiomeric excess (ee) value of the ATH of acetophenone promoted by the combination of l‐proline benzimidazole 7c and [RuCl2(p‐cymene)]2 were up to 6880, 13 000 h−1 and 67%, 51% respectively. The ee value dropped off after the reaction reached equilibrium. The high activity and enantioselectivity of [RuCl2(p‐cymene)]2/7c was primarily influenced by the (S,R) configuration, whereas the (S,S) configuration had low activity and enantioselectivity. Copyright © 2012 John Wiley & Sons, Ltd. Asymmetric transfer hydrogenation of acetophenone promoted by 8/[RuCl2(p‐cymene)]2 displayed unique reverse temperature effect on enantioselectivity. The higher reaction temperature and higher the molar ratio of substrate to catalyst would result in better activity and enantioselectivity. The TOF and ee value were up to 13000 h−1, 51% at S/C/KOH of 5000:1:7.