A Crossover between Conventional Cross-Coupling and Carbene Insertion Reaction Pathways in a Pd Catalyzed C(sp2)-H Insertion

Computational methods (Density Functional Theory) along with experimental tools are used to probe the mechanism and stereocontrol in a Pd(II)-catalyzed enantioselective C–H insertion reaction. The mechanistic aspects reveal a crossover between general transition metal-catalyzed carbene insertion reactions and typical Pd-catalyzed cross-coupling reactions. We demonstrate the intermediacy of a metal hydride species which contrasts the previously studied mechanistic routes taken in the case of all other transition metals (Rh, Fe, Au, Cu, etc.). Detailed investigation into the origin of stereoselectivity again throws up a new paradigm wherein stereocontrol arises during the formation of the metallocarbene itself. Novel mechanistic routes illustrated in this report provide crucial insights into the differential reactivity exhibited by Pd in carbene insertion reactions.