Enantioselective enolate protonations: Evaluation of achiral templates with fluxional brønsted basic substituents in conjugate addition of malononitriles

Enoates derived from α−substituted acrylates containing 3-pyrazolidinone achiral template with Brønsted basic fluxional substituents were evaluated in chiral Lewis acid catalyzed enantioselective protonation reactions. The tandem 1,4-conjuagte addition/enantioselective protonation reaction catalyzed by chiral Lewis acids was found to depend on the fluxional group present on the achiral template. The enantioselectivity of the protonation step is highly dependent on the position of the Brønsted basic atom with respect to the pyrazolidinone nitrogen. A comparison of fluxional substituents without a Brønsted basic site and their impact on selectivity is also reported here. The results we detail here are in sharp contrast to our previous studies on enantioselective conjugate addition of malononitriles. The reaction showed reasonable substrate scope for varying α−substitution. The reactions gave conjugate adducts in good yields and good enantioselectivities up to 83 %.