Theoretical studies of halocarbene cycloaddition selectivities: A new interpretation of negative activation energies and entropy control of selectivity

Abstract

Ab initio RHF calculations with the 3–21G basis set have been carried out on cycloadditions of CF 2 and CCl 2 with ethylene. Although π-complex intermediates are predicted for both reactions at this level, MP2/3-21G calculations imply that there are no complexes in reactions of CCl 2 or more reactive carbenes with ethylene or substituted alkenes. Nevertheless, negative activation energies can be observed, since ΔG reaches a maximum due to the increase in — TΔ S for these bimolecular reactions. The apparent “entropy control” for reactive carbenes results from the rapid decrease in ΔH. As the reactivity of the alkene increases, the transition state shifts to an earlier point on the free energy surface, where — TΔ S ‡ is lower, but Δ H ‡ is higher than for less reactive alkenes. Model potentials are developed for ΔH and — TΔ S which reproduce experimental behavior, without the assumption of intermediates.