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Updated: Apr 15, 2026

Isolating Free Carbenes, their Mixed Dimers and Organic Radicals
Published on: April 19, 2019
Absolute reactivity of (N-methyl-3-pyridinium)chlorocarbene
Abstract:
(N-Methyl-3-pyridinium)chlorocarbene tetrafluoroborate (MePyr(+)CCl BF4(-), 4) is generated by laser flash photolysis (LFP) of the corresponding diazirine (5) and reacted with tetramethylethylene, cyclohexene, 1-hexene, 2-ethyl-1-butene, methyl acrylate, and acrylonitrile. Absolute rate constants are measured for these carbene-alkene addition reactions, and activation parameters are obtained for additions of MePyr(+)CCl BF4(-) to tetramethylethylene, cyclohexene, and 1-hexene. MePyr(+)CCl BF4(-) is computed to be a highly reactive, electrophilic, singlet carbene, and experiments are in accord with expectations. Its activation parameters are compared with those of CF3CCl, CCl2, CClF, and CF2. In all cases, enthalpy-entropy compensation is observed, with ΔH(‡) and ΔS(‡) decreasing in tandem as carbenic stability decreases. A qualitative explanation is offered for this phenomenon.
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