Base-Free Asymmetric Transfer Hydrogenation of Aromatic Ketones

Key words

asymmetric hydrogen transfer - aromatic ketones - base-free hydrogenation - tetradentate PN ligands

Significance

A highly enantioselective hydrogenation of aromatic ketones using IrH(CO)(PPh₃)₃ and a tetradentate PNNP-type ligand is described. The ability to perform the reaction in the absence of base allows a wider substrate scope. A variety of other ligands (binary and PNNP-type) and metal hydride complexes were investigated.

Comment

In general, asymmetric transfer hydrogenation in 2-propanol requires use of a strong base such as KOH or KOt-Bu. Not only does this procedure require optimization for each specific substrate (in terms of adding the appropriate amount of base) but this method precludes the use of base-sensitive substrates. Very few reports exist on the use of asymmetric transfer hydrogenation under base-free conditions although Noyori and Morris have recently contribuated to this area (K. Abdur-Rashid, S. E. Clapham, A. Hadzovic, J. N. Harvey, A. J. Lough, R. H. Morris J. Am. Chem. Soc. 2002, 124, 15104-15118; T. Ohkuma, M. Koizumi, K. Muniz, G. Hilt, C. Kabuto, R. Noyori J. Am. Chem. Soc. 2002, 124, 6508-6509). In this article, various substituted aromatic ketones undergo reduction with very high yields and enantio­selectivity including sterically hindered ketones such as 2,2-dimethylpropiophenone.