Difluorocarbene Generation from TMSCF_3: Kinetics and Mechanism of Nal-Mediated and Si-Induced Anionic Chain Reactions

摘要

The mechanism of CF_2 transfer from TMSCF_3 (1), mediated by TBAT (2-12 mol %) or by NaI (5-20 mol %), has been investigated by in situ/stopped-flow ~(19)F NMR spectroscopic analysis of the kinetics of alkene difluorocyclopropanation and competing TFE/c-C_3F_6/homologous perfluoroanion generation, ~(13)C/~2H KIEs, LFERs, CF_2 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_3 anionoid to TMSCF_3 (1) 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_3 (1) 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.