THEORETICAL-STUDY OF THE REACTION-MECHANISM AND MIGRATORY APTITUDE OF THE PINACOL REARRANGEMENT

The reaction pathways of the pinacol rearrangement are examined for some monosubstituted protonated 1,2-ethanediols by means of ab initio SCF MO methods. The relative activation energies are evaluated for both stepwise and concerted pathways. Gas-phase calculations show the concerted mechanism dominates over the traditionally supported stepwise mechanism via a carbonium ion intermediate. These results agree well with experiments in the gas phase and in aprotic solvents. The migratory aptitude is also calculated for some substituents. The high migratory aptitude of vinyl and cyclopropyl groups is compatible with the participation of pi- or pseudo-pi-orbitals to stabilize the transition states. The relatively low migratory aptitudes of alkyl and alkynyl groups are also reproduced. The reasons for the differences of migratory aptitudes of each group are discussed.