CATALYST SUBSTRATE ADDUCTS IN ASYMMETRIC CATALYTIC-HYDROGENATION - CRYSTAL AND MOLECULAR-STRUCTURE OF [((R,R)-1,2-BIS(PHENYL-ORTHO-ANISOYLPHOSPHINO)ETHANE)(METHYL (Z)-BETA-PROPYL-ALPHA-ACETAMIDOACRYLATE)]RHODIUM TETRAFLUOROBORATE, [RH(DIPAMP)(MPAA)]BF4

[Rh(R,R-DIPAMP)(MeOH)2]+ (DIPAMP = l,2-bis(phenyl-o-anisoylphosphino)ethane), which serves as an asymmetric hydrogenation catalyst for enamides, reacts with methyl (Z)-β-propyl-α-acetamidoacrylate (MPAA) to form the 1:1 adduct, [Rh(R,R-DIPAMP) (MPAA)]+ (1), with a binding constant of 1.4 × 104 M-1 at 25 °C. Crystals of the BF4 salt of the predominant diastereomer of 1 were isolated and subjected to single-crystal X-ray analysis. The structure of 1 resembles those deduced previously for rhodium [(l,2-bis(diphenylphosphino)ethane)(methyl(Z)-α-acetamidocinnamate)]tetrafluoroborate and rhodium [(2S,3S)-2,3-bis(diphenylphosphino)butane)(ethyl (Z)-α-acetamidocinnamate)] perchlorate. In this case also the face of the C═C bond (pro-R) that is coordinated to the Rh atom is opposite to that to which H2 adds to form the predominant (S) enantiomer of the hydrogenated product. Accordingly, it is concluded that the origin of enantioselection in the [Rh(R,R-DIPAMP)]+-catalyzed hydrogenation of MPAA is not the preferred mode of binding of the prochiral substrate but, rather, the higher reactivity toward H2 of the minor, less stable, diastereomer of 1. The structural features of 1 that may be relevant to enantioselection are discussed. © 1990, American Chemical Society. All rights reserved.