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.