Asymmetric Platinum Group Metal-Catalyzed Carbonyl-Ene Reactions: Carbon−Carbon Bond Formation versus Isomerization

A comparative study of the carbonyl-ene reaction between a range of 1,1‘-disubstituted or trisubstituted
alkenes and ethyl trifluoropyruvate catalyzed by Lewis acid−platinum group metal complexes of the
type [M{(R)-BINAP}]2+ (M = Pt, Pd, Ni; BINAP is 2,2‘-bis(diphenylphosphino)-1,1‘-binaphthyl) revealed
subtle but significant differences in their reactivity. For instance, the palladium-based Lewis acid [Pd{(R)-BINAP}]2+ catalyzes the ene reaction between methylene cycloalkane to afford the expected
α-hydroxy ester in good yield and excellent diastereo- and enantioselectivity. In contrast, under the same
conditions, the corresponding [M{(R)-BINAP}]2+ (M = Pt, Ni) catalyzes isomerization of methylene
cycloalkane and the ene reaction of the resulting mixture of methylene cycloalkane and 1-methylcycloalkene at similar rates to afford a range of α-hydroxy esters in high regioselectivity, good
diastereoselectivity, and good to excellent enantioselectivity. In addition, [Pt{(R)-BINAP}]2+ also catalyzes
postreaction isomerization of the ene product as well as consecutive ene reactions to afford a double
carbonyl-ene product. The sense of asymmetric induction has been established by single-crystal X-ray
crystallography, and a stereochemical model consistent with the formation of (S)-configured α-hydroxy
ester has been proposed; the same model also accounts for the observed exo-diastereoselectivity as well
as the level of diastereoselectivity.