Asymmetric hydrogenation enamide 94, reaction pathways

The problems which remain in understanding and interpreting rhodium asymmetric hydrogenation arise from a persistent lack of information on the presumed rhodium dihydride without which the pathway between the enamide complex and the turnover limiting TS for H2 addition (i.e. the step in which the enantioselectivity of the reaction is set) remains opaque, and hence the overall understanding is elusive. [52]... 

The reaction pathway for rhodium-catalyzed asymmetric hydrogenation of enamides is described and intermediates are defined in solution by P-31, C-13, and H-l NMR. The stereochemical relationship of bound enamide to rhodium alkyl and to the product of hydrogenation is demonstrated. Experiments involving the addition of HD to a variety of olefins in the presence of rhodium biphosphine catalysts suggest that a concerted addition of hydrogen to olefin and metal may occur in appropriate cases. 

Scheme 1.26 Different reaction pathways in asymmetric hydrogenation of enamides 13 and 16. (Reprinted with permission from Gridnev, I. D. et al.,/. Am. Chem. Soc., 123, 5268-5276. Copyright 2001 American Chemical Society.)...

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