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 [PdII ]-F complexes with the silane- and stannane-based trifluoromethylation agents, R3 SiCF3 and R3 SnCF3 . 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 R3 SiCF3 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 CF2 carbene, which reacts with [PdII ]-F to ultimately generate [PdII ]-CF3.