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

• Published in: Journal of the American Chemical Society Vol. 142; no. 34; pp. 14649 - 14663
• Main Authors: García-Domínguez, Andrés, West, Thomas H, Primozic, Johann J, Grant, Katie M, Johnston, Craig P, Cumming, Grant G, Leach, Andrew G, Lloyd-Jones, Guy C
• Format: Journal Article
• Language: English
• Published: American Chemical Society 26.08.2020
• Subjects: alkenes; alkynes; density functional theory; fluorides; spectral analysis; toxicity
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/jacs.0c06751

The mechanism of CF2 transfer from TMSCF3 (1), 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/c-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 (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 TMSCF3 (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.