Rhodium catalysts for asymmetric ketone reduction

Asymmetric hydrosilylation of ketones has developed enormously since these early reports and probably there are few catalytic reactions for which the variety in ligand structure is so immense as for this reaction. Numerous reports have been published and in general oxazoline-based ligand systems seem to give the highest enantioselectivities. In the following we will mention a few 

We start the scheme after the oxidative addition of diphenylsilane and coordination of acetophenone has taken place, after the classic mechanism by Ojima [28], The formation of enol ethers proves, that at least for this part of the products formed (up to 22%, Brunner [27], 40% [29]), the reaction proceeds 

With the use of NMR spectroscopy Giering and coworkers studied the reaction of acetophenone and HSiBu3 in more detail. The catalyst was [(1,5-cod)RhCl]2 and R-BINAP [36], They noted that the silyl enol ether by-product was formed mainly at the beginning of the reaction and thus this must form via an independent pathway. The common intermediate for silyl ether product and 

10 Harrod, J. F. Chalk, A. J. in Organic Synthesis Via Metal Carbonyls Wender, I. Pino, P. Eds. Wiley, New York, 1977, p 673. 

22 Ojima, I. Kogure, T. Nagai, Y. Chem. Lett. 1973, 541. Tetrahedron Lett. 1972, 5035. 

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