Highly enantioselective transfer hydrogenation of aromatic ketones

Enantioselective reduction of prochiral ketones with 2-propanol or formic acid catalyzed by chiral metal complexes has been developed as a synthetic method complementary to asymmetric hydrogenation in the past decade. However, these processes are necessary to be improved for practical use in organic synthesis because of low catalytic activity and insufficient enantioselectivity. In this paper, we wish to report an excellent catalytic system generated in situ from iridium complex IrCl (Ph3P)(3) and chiral tetradentate diaminodiphosphine ligand for asymmetric transfer hydrogenation of various aromatic ketones. For isobutyrophenone transfer hydrogenation, the corresponding optically active alcohol was obtained in 93% yield and with up to 99 % e. e. after 5 h at 28 degreesC. Propiophenone was a better substrate with respect to catalytic activity and enantioselectivity in the presence of base as a co-catalyst. The conversion of propiophenone reached 95. 8 % and the turnover number as high as 3 193 h(-1). These results will provide a useful index for further designing practical chiral catalytic systems.