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

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.