Lewis Acid Platinum Complexes of Conformationally Flexible NUPHOS Diphosphines:  Highly Efficient Catalysts for the Carbonyl−Ene Reaction

Enantiopure Lewis acid complexes of conformationally flexible acyclic and monocyclic NUPHOS diphosphines, δ- and λ-[(NUPHOS)Pt(OTf)₂], are efficient catalysts for the carbonyl−ene reaction between various unsymmetrical 1,1‘-disubstituted alkenes and phenylglyoxal or ethyl glyoxylate. While catalyst performance was substrate dependent, ee values as high as 95% and yields up to 90% have been obtained. In a number of cases catalysts generated from δ- and λ-[(NUPHOS)Pt{(S)-BINOL}] showed marked enhancements in enantioselectivity in ionic liquids compared with organic media. Although an enhancement in enantioselectivity was not obtained for all substrate combinations in such cases, the enantioselectivities were comparable to those obtained in dichloromethane. Furthermore, although the ee's are initially comparable in both the ionic liquid and dichloromethane, a gradual erosion of ee with time was found in the organic solvent, whereas the ee remained constant in the ionic liquid. Preliminary kinetic investigations suggest that the decrease in ee may be due to a faster racemization of the catalyst in dichloromethane compared with the ionic liquid.