Microwave-Assisted Transfer Hydrogenation of Ketones by Ru(xantphos) Arene Complexes

A Ru-catalyzed hydrogen transfer from 2-propanol to acetophenone in the presence of the strong base KOtBu afforded 1-phenyl alcohol in >99?% yield within 5 min under microwave irradiation; no reaction occurred in the absence of the base. A series of catalysts was then prepared in situ from [RuCl2-?6-p-cymene]2 and xantphos family ligands, which were characterized by having natural bite angles ranging from 102.2 to 137.9 degrees. The Ru(xantphos)arene catalyst was used for various substrates to give yields and turnover frequencies between 48 and 99?% and between =1431 and 2982 h-1, respectively, for acetophenone derivatives and >99?% and =2982 h-1 for sterically bulky ketones (benzophenone, 9H-fluoren-9-one, and a-tetralone).