Ruthenium(II)-catalyzed Oppenauer-type oxidation of secondary alcohols

Highly efficient ruthenium-catalyzed Oppenauer-type oxidations of secondary alcohols to ketones have been developed. The catalytic system consists of [(PPh(3))(3)RuCl2] (1) and K2CO3 or [(C(4)Ph(4)COHOCC(4)Ph(4))(mu-H)(CO)(4)Ru-2] (2) in refluxing acetone. The catalytic reaction is of high efficiency and permits a catalyst:substrate ratio of 1:1000 at 56 degrees C. In some cases the initial turnover rate exceeds 1500 h(-1). The reaction was found to be general and compatible with double bonds and oxidation-sensitive aromatics. With some allylic alcohols, isomerization to saturated ketones took place. The reaction proceeds by ruthenium-catalyzed dehydrogenation of the alcohol and subsequent hydrogen transfer to acetone. No primary kinetic isotope effect was observed for the catalytic reaction when alpha-deuterated 1-phenylethanol was employed as substrate (k(H)/k(D) = 1.1); this shows that beta-hydride elimination from a ruthenium alkoxide intermediate is not the rate-limiting step.