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.