Divergent Reactivity of Stannane and Silane in the Trifluoromethylation of PdII: Cyclic Transition State versus Difluorocarbene Release

The transmetalation is a key elementary step in cross-coupling reactions. Yet, the precise nature of its mechanism and transition state geometry are frequently elusive. This report discloses our study of the transmetalation of [Pd-II]-F complexes with the silane-and stannane-based trifluoromethylation agents, R3SiCF3 and R3SnCF3. A divergent reactivity was uncovered, with the stannane showing selective R-group transfer, and the silane selective CF3-group transfer. Using a combined experimental and computational approach, we uncovered a hitherto unrecognized transmetalation mechanism with the widely employed R3SiCF3 reagent, explaining its unique activity in metal-catalyzed trifluoromethylations. While the stannane reacts via a cyclic, 4-membered transition state, the silane undergoes a fundamentally different pathway and releases a difluorocarbene in the transmetalation event. Molecular dynamics studies clearly reinforced the liberation of a free CF carbene, which reacts with [Pd-II]-F to ultimately generate [Pd-II]-CF3.