Efficient Asymmetric Carbonyl-Ene Reactions Catalyzed by Platinum Metal Lewis Acid Complexes of Conformationally Flexible NUPHOS Diphosphines: A Comparison with BINAP

Abstract:

A comparative study of the Lewis acid-catalyzed carbonyl-ene reaction of a range of monosubstituted, unsymmetrical 1,1-disubstituted and trisubstituted alkenes with ethyl trifluoropyruvate revealed that platinum complexes of enantiopure conformationally flexible tropos NUPHOS diphosphines rival or outperform their atropisomeric enantiopure BINAP counterpart. The stereochemical integrity of these NUPHOS diphosphines remains intact over extended periods, as evidenced by the high ee’s obtained for an unreactive substrate requiring >20 h to reach good conversions and the presence of a single diastereoisomer after addition of (S,S)-DPEN to the reaction mixture. The absolute and relative stereochemistry of a number of the ene products has been determined by single-crystal X-ray crystallography. The sense of asymmetric induction, the regioselectivity, and the exo diastereoselectivity are consistent with a stereochemical model based on a square-planar catalyst-pyruvate adduct. The allylbenzene derivatives required for this study were conveniently prepared by the palladium-catalyzed cross-coupling between the corresponding aromatic bromide and allylmagnesium bromide using a catalyst mixture based on Pd₂(dba)₃ and a NUPHOS diphosphine.