Difluorocarbene Generation from TMSCF: Kinetics and Mechanism of NaI-Mediated and Si-Induced Anionic Chain Reactions.

The mechanism of CF transfer from TMSCF (), mediated by TBAT (2-12 mol %) or by NaI (5-20 mol %), has been investigated by in situ/stopped-flow F NMR spectroscopic analysis of the kinetics of alkene difluorocyclopropanation and competing TFE/CF/homologous perfluoroanion generation, C/H KIEs, LFERs, CF transfer efficiency and selectivity, the effect of inhibitors, and density functional theory (DFT) calculations. The reactions evolve with profoundly different kinetics, undergoing autoinhibition (TBAT) or quasi-stochastic autoacceleration (NaI) and cogenerating perfluoroalkene side products. An overarching mechanism involving direct and indirect fluoride transfer from a CF anionoid to TMSCF () has been elucidated. It allows rationalization of why the NaI-mediated process is more effective for less-reactive alkenes and alkynes, why a large excess of TMSCF () is required in all cases, and why slow-addition protocols can be of benefit. Issues relating to exothermicity, toxicity, and scale-up are also noted.