An MNDO SCF-MO study of the mechanism of the benzilic acid and related rearrangements

Abstract

Mechanisms for the benzilic acid and related rearrangements have been studied using the MNDO SCF-MO method. The barriers to concerted closed shell [1,2] migration of a substituent R in the initially formed intermediate (2) were found to display a much smaller range of values than is commonly found in rearrangements involving cations or radicals. Both the formation of (2) and its subsequent rearrangement are predicted to be enhanced if either R or the non-migrating group X bears electron-withdrawing substituents. The calculated hydrogen isotope effect and the Bell tunnelling correction for the rearrangement of glyoxal are both significantly larger than for the analogous hydride transfer in the Cannizzaro reaction of formaldehyde. Alternative open shell pathways involving intramolecular single-electron-transfers (SET) to the carbonyl group are relatively high in energy if the non-migrating substituent X is not capable of stabilising an adjacent radical centre (i.e. X = H), but are much more favourable for e.g. the case of X = phenyl. When R bears an electron-withdrawing substituent, electron-transfer to this group is favoured. We suggest on this basis that an SET mechanism may explain the migration of hydrogen rather phenyl in the reaction of phenylglyoxal. Electron-transfer is also favoured for the rearrangement of benzoin to benzilic acid itself, and is predicted to result in the formation of low-energy cyclic intermediates. Intervention of a counter-ion was modelled with Li(H₂O)₂⁺ co-ordinating to two oxygen atoms during migration. The migratory barriers were actually increased as a result, and it is suggested that the role of the counter-ion in non-polar solvents is to alter the pre-equilibrium in favour of the intermediate (2).