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)(CD(3)OD)(2)]BF(4) (11) with CH(2)=C(C(6)H(5))NHCOCH(3) (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 CH(2)=C(t-Bu)NHCOCH(3) (6) or CH(2)=C(1-adamantyl)NHCOCH(3) (7). Hydrogenation of these equilibrium mixtures at -100 degrees C gave monohydride intermediates 19 and 20, respectively. In these monohydrides the Rh atom is bound to the beta-carbon. A new effect of the significant decrease of ee was found for the asymmetric hydrogenation of CH(2)=C(C(6)H(4)OCH(3)-o)NHCOCH(3) (21), when H(2) was substituted for HD or D(2).