Complexes and Negative Activation Energies in Arylhalocarbene/Alkene Additions: Activation Parameter Dependence on Alkane Solvent Chain Length.

Activation parameters for the additions of PhCCl, F-PhCCl, and 3,5-dinitro-PhCCl to tetramethylethylene, cyclohexene, and 1-hexene have been determined in decane. With the exception of two carbene/alkene combinations, Arrhenius correlations of ln k vs 1/T were unimodal and linear, affording negative activation energies and entropies. The additions of PhCCl or F-PhCCl to 1-hexene gave bimodal Arrhenius correlations. Comparisons to the analogous experimental data obtained in pentane and computational studies help to elucidate the observed behavior. Activation entropies decrease in parallel with activation enthalpies going from pentane to decane solvent, suggesting that enthalpy-entropy compensation is operative in these carbene additions. The bimodal Arrhenius behavior is proposed to result from carbene-alkene additions taking place intrinsically or extrinsically to decane solvent cage assemblies.