Asymmetric Hydrogenation of Enamides with Rh-BisP* and Rh-MiniPHOS Catalysts. Scope, Limitations, and Mechanism

The asymmetric hydrogenation of aryl- and alkyl-substituted enamides catalyzed by Rh-BisP* complex
affords optically active amides with very high ee values. The Rh-MiniPHOS catalyst gives somewhat less
satisfactory results. Hydrogenation of the aryl-substituted enamides with (S,S)-BisP*-Rh catalyst gives R-amides,
whereas the t-Bu- and 1-adamantyl-substituted enamides give S-products with 99% ee. Reaction of [Rh(BisP*)(CD3OD)2]BF4 (11) with CH2C(C6H5)NHCOCH3 (5) gives two diastereomers of the catalyst−substrate
complex (12a,b), which interconvert reversibly by both intra- and intermolecular pathways as shown by EXSY
data. Only one isomer in equilibrium with solvate complex 11 was detected for each of the catalyst−substrate
complexes 17 and 18 obtained from CH2C(t-Bu)NHCOCH3 (6) or CH2C(1-adamantyl)NHCOCH3 (7).
Hydrogenation of these equilibrium mixtures at −100 °C gave monohydride intermediates 19 and 20,
respectively. In these monohydrides the Rh atom is bound to the β-carbon. A new effect of the significant
decrease of ee was found for the asymmetric hydrogenation of CH2C(C6H4OCH3-o)NHCOCH3 (21), when
H2 was substituted for HD or D2.