Higher-Order Cyclopropenimine Superbases: Direct Neutral Bronsted Base Catalyzed Michael Reactions with α-Aryl Esters

The synthesis and characterization of six new classes of higher-order superbases, including five that incorporate cyclopropenimine functionality, has been achieved. We propose a nomenclature that designates these as the CG(2), GC(2), PC3, PC1, C-3, and GP(2) classes of superbases. The pK(BH+) values were measured to be between 29.0 and 35.6 in acetonitrile. Linear correlations of ten superbase basicities vs that of their substituents demonstrated the insulating effect of the cyclopropenimine core. The molecular structures of several of these materials were obtained by single-crystal X-ray analysis, revealing interesting aspects of conformational bias and non-covalent organization. The types of superbasic cores and substituents were each reliably shown to affect selectivity for deprotonation over alkylation. Higher-order cyclopropenimine and guanidine superbase stability to hydrolysis was found to correlate to basicity. Finally, a GC(2) base was found to catalyze conjugate additions of alpha-aryl ester pronudeophiles, representing the first report of a neutral Bronsted base to catalyze such reactions.