Difluorocarbene Generation from TMSCF₃: Kinetics and Mechanism of NaI-Mediated and Si-Induced Anionic Chain Reactions

Abstract

The mechanism of CF₂ transfer from TMSCF₃ (<b>1</b>), mediated by TBAT (2-12 mol %) or by NaI (5-20 mol %), has been investigated by in situ/stopped-flow ¹⁹F NMR spectroscopic analysis of the kinetics of alkene difluorocyclopropanation and competing TFE/<i>c-</i>C₃F₆/homologous perfluoroanion generation, ¹³C/²H KIEs, LFERs, CF₂ 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 CF₃ anionoid to TMSCF₃ (<b>1</b>) 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 TMSCF₃ (<b>1</b>) 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.