Osmium Pyme Complexes for Fast Hydrogenation and Asymmetric Transfer Hydrogenation of Ketones

The osmium compound trans , cis ‐[OsCl 2 (PPh 3 ) 2 (Pyme)] ( 1 ) (Pyme=1‐(pyridin‐2‐yl)methanamine), obtained from [OsCl 2 (PPh 3 ) 3 ] and Pyme, thermally isomerizes to cis , cis ‐[OsCl 2 (PPh 3 ) 2 (Pyme)] ( 2 ) in mesitylene at 150 °C. Reaction of [OsCl 2 (PPh 3 ) 3 ] with Ph 2 P(CH 2 ) 4 PPh 2 (dppb) and Pyme in mesitylene (150 °C, 4 h) leads to a mixture of trans ‐[OsCl 2 (dppb)(Pyme)] ( 3 ) and cis ‐[OsCl 2 (dppb)(Pyme)] ( 4 ) in about an 1:3 molar ratio. The complex trans ‐[OsCl 2 (dppb)(Pyet)] ( 5 ) (Pyet=2‐(pyridin‐2‐yl)ethanamine) is formed by reaction of [OsCl 2 (PPh 3 ) 3 ] with dppb and Pyet in toluene at reflux. Compounds 1 , 2 , 5 and the mixture of isomers 3 / 4 efficiently catalyze the transfer hydrogenation (TH) of different ketones in refluxing 2‐propanol and in the presence of NaO i Pr (2.0 mol %). Interestingly, 3 / 4 has been proven to reduce different ketones (even bulky) by means of TH with a remarkably high turnover frequency (TOF up to 5.7×10 5  h −1 ) and at very low loading (0.05–0.001 mol %). The system 3 / 4 also efficiently catalyzes the hydrogenation of many ketones (H 2 , 5.0 atm) in ethanol with KO t Bu (2.0 mol %) at 70 °C (TOF up to 1.5×10 4  h −1 ). The in‐situ‐generated catalysts prepared by the reaction of [OsCl 2 (PPh 3 ) 3 ] with Josiphos diphosphanes and (±)‐1‐alkyl‐substituted Pyme ligands, promote the enantioselective TH of different ketones with 91–96 %  ee ( ee =enantiomeric excess) and with a TOF of up to 1.9×10 4  h −1 at 60 °C.