Base-catalyzed C-alkylation of potassium enolates with styrenes via a metal-ene reaction: a mechanistic study.

Base-catalyzed, C-alkylation of potassium (K) enolates with styrenes (CAKES) has recently emerged as a highly practical and convenient method for elaboration or synthesis of pharmaceutically-relevant cores. K enolate-type precursors such as alkyl-substituted heterocycles (pyridines, pyrazines and thiophenes), ketones, imines, nitriles and amides undergo C-alkylation reactions with styrene in the presence of KOtBu or KHMDS. Surprisingly, no studies have probed the reaction mechanism beyond the likely initial formation of a K enolate. Herein, a synergistic approach of computational (DFT), kinetic and deuterium labelling studies rationalizes various experimental observations and supports a metal-ene-type reaction for amide CAKES. Moreover, our approach explains experimental observations in other reported C-alkylation reactions of other enolate-type precursors, thus implicating a general mechanism for CAKES.