Intermediates in asymmetric hydrogenation : The structure and 31P NMR spectra of rhodium enamide complexes containing 1R, 2R-trans-1,2-bis (diphenylphosphinomethyl) cyclo buta

Hydrogenation of bicyclo2.2.1]heptadiene-1R,2R-(trans-1,2-bis(diphenylphosphinomethyl)cyclbutane rhodium (I) tetrafluoroborate in methanol gives a solvent adduct which reacts with N-acyldehydroamino acids and their esters to give air-unstable scarlet to yellow complexes. Effects of structure variation in the enamide on the ³¹P NMR spectra of these species are reported and discussed with reference to (a) the equilibrium between methanol and enamide complexes; (b) the ratio of diastereomeric enamide complexes formed; (c) the temperature dependence of this ratio; (d) the rate of complexation equilibria and (e) the structure of enamide complexes, which are normally bidentate with binding via the olefin and amide groups. In certain cases the complex may be terdentate and E-enamides bind through the olefin and carboxyl groups. Each mode of binding gives rise to characteristic ³¹P NMR spectra with regard to P-P and P-Rh coupling constants.