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 II 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 degreesC gave monohydride intermediates 19 and 20; respectively. Tn these monohydrides the Rh atom is bound to the B-carbon. A new effect of the significant decrease, of ee was found for the asymmetric hydrogenation of CH2=C(C6H4OCH3-o)NHCOCH3 (21), when H-2 was substituted for HD Or D-2.