Osmium pyme complexes for fast hydrogenation and asymmetric transfer hydrogenation of ketones

The osmium compound trans,cis-[OsCl2(PPh3)(2)(Pyme)] (1) (Pyme = 1-(pyridin-2-yl)methanamine), obtained from [OsCl2(PPh3)(3)] and Pyme, thermally isomerizes to cis,cis-[OsCl2(PPh3)(2)(Pyme)] (2) in mesitylene at 150 degrees C. Reaction of [OSCl2(PPh3)(3)] with Ph(2)p(CH2)(4)PPh2 (dppb) and Pyme in mesitylene (150 degrees C, 4 h) leads to a mixture of trans-[OsCl2(dppb)(Pyme)] (3) and cis-[OsCl2(dppb)(Pyme)] (4) in about an 1:3 molar ratio. The complex trans-[OsCl2(dppb)(Pyet)] (5) (Pyet= 2-(pyridin-2-yl)ethanamine) is formed by reaction Of [OsCl2(PPh3)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 NaOiPr (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 x 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, 5.0atm) in ethanol with KOtBu (2.0mol%) at 70 degrees C (TOF up to 1.5x 104 h(-1)). The in-situ-generated catalysts prepared by the reaction of [OsCl,(PPh3)(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 X 10(4) h(-1) at 60 degrees C.