Evolution of Structure and Reactivity in a Series of Iconic Carbenes

We present experimental activation parameters for the reactions of six carbenes (CCl₂, CClF, CF₂, ClCOMe, FCOMe, and (MeO)₂C) with six alkenes (tetramethylethylene, cyclohexene, 1-hexene, methyl acrylate, acrylonitrile, and α-chloroacrylonitrile). Activation energies range from −1 kcal/mol for the addition of CCl₂ to tetramethylethylene to 11 kcal/mol for the addition of FCOMe to acrylonitrile. A generally satisfactory analysis of major trends in the evolution of carbenic structure and reactivity is afforded by qualitative applications of frontier molecular orbital theory, although the observed entropies of activation appear to fall in a counterintuitive pattern. An analysis of computed cyclopropanation transition state parameters reveals significant nucleophilic selectivity of (MeO)₂C toward α-chloroacrylonitrile.