The mechanism of the pinacol-pinacone rearrangement. V. Carbon isotope effects

The intermolecular isotope effects of methyl-labelled and alcoholic carbon-labelled pinacol converted to pinacone have been studied over a temperature range of 60-113.5 �C. Tests were made to establish the validity of the results by using two starting materials for synthesizing methyl-labelled pinacol, and several different methods of analysing the products. The isotope effects were determined by measurements made both on (i) pinacol and (ii) pinacone. The results above 80 �C can be interpreted in terms of the synartetic ion mechanism provided both the carbonium ion and the transition state are largely unhydrated, and both the formation of the carbonium ion and the methyl migration are slow steps. Below 60 �C an alternative mechanism is operative. Unless unsuspected exchange reactions are present, the results suggest that the transition state is not so greatly distorted from a simple ethane-type configuration as in the high temperature reaction. Results at intermediate temperatures indicate that the alternative route proceeds via a stable intermediate.