Successful Computational Modeling of Isobornyl Chloride Ion-Pair Mechanisms

Along with the directly related Wagner−Meerwein camphene
hydrochloride−isobornyl chloride rearrangement, the racemization
of isobornyl chloride involves intermediate carbocation−anion
ion pairs; both processes have become mechanistic icons in organic
chemistry. The two known racemization pathways, involving either a
hydride transfer or a methyl migration, are observed to be concurrent.
However, prior quantitative computational modeling has not been able
to reproduce the fine kinetic balance of these processes. We demonstrate
that a density functional approach, which includes two explicit solvent
molecules embedded in a continuum solvent field, coupled with full
geometric optimization using smoothed solvent cavities and free energy
calculation, yields results in accord with experiment. Alternative
racemization routes also have been explored.