| The transition structures for the (2+1) cycloadditions of dichlorocarbene, chlorofluorocarbene, and difluorocarbene to cyclohexene, 1--hexene, ethylene, and &agr;--chloroacrylonitrile were located using quantum mechanical methods. In addition, transition structures for the (2+1) cycloadditions of chloromethoxycarbene, fluoromethoxycarbene, and dimethoxycarbene to ethylene and &agr;--chloroacrylonitrile were computed. Except for the reactions with ethylene, these cycloadditions were studied experimentally and computationally by Moss and Krogh--Jespersen (Zhang, M.; Moss, R. A.; Thompson, J.; Krogh--Jespersen, K. J. Org. Chem. 2012, 77, 843-850). As a complement to the work of those groups, we have utilized the distortion/interaction model to understand reactivities and selectivities. Computational methods overestimate the entropies of activation for these carbene cycloadditions. Enthalpies, entropies, and free energies of activation for these carbene cycloadditions were computed with a variety of density functionals and ab initio methods relative to carbene-alkene precursor complexes, carbene pyridine ylides, and carbene diazirine ylides. These complexes and ylides are predicted to be unstable in terms of free energy and hence are not a viable explanation for the observed discrepancy between experimental and computed activation parameters. Quantum mechanical/molecular mechanical molecular dynamics simulations were used to determine the timescales of reaction for CCl2/CF2 + ethylene in the gas phase and in explicit pentane solvent. The course of the reactive event is on the order of tens of femtoseconds, which takes place in a frozen solvent configuration. There is no statistically significant difference between timing of bond gap formation or vibrational energy redistribution between carbene cycloadditions in the condensed phase versus the gas phase. |
