Enantioselective Hydrogenation Using a PEG-Supported Biphep-Ru Complex

Significance

The (R)-MeO-PEG-supported Biphep-RuBr₂ complex (R)-1 was prepared. The hydrogenation of 3a-e was performed with (R)-1 and H₂ in EtOH to give 4a-e in 52-100% yield with 85% to >99.5% ee. The reaction of 3a with (R)-1 gave 4a in 98% yield with 92% ee while the reaction with (R)-2 under similar conditions afforded 4a in only 21% yield with 94% ee. After the first reaction of 3a with (R)-1, the recovered catalyst was successively subjected to a second and third series of the reaction under similar conditions to afford 90% (36 h) and 46% (60 h) yield of 4a, respectively.