A THEORETICAL-STUDY OF THE FAVORSKII REARRANGEMENT - CALCULATION OF GAS-PHASE REACTION PATHS AND SOLVATION EFFECTS ON THE MOLECULAR MECHANISM FOR THE TRANSPOSITION OF THE ALPHA-CHLOROCYCLOBUTANONE

The molecular mechanism of the alpha-chlorocyclobutanone transposition to yield cyclopropanecarboxyIic acid, as a model of the Favorskii rea rrangement, has been theoretically characterized in vacuo by means of ab initio molecular orbital procedures at the Hartree-Fock (HF) level of theory with the 6-31G and 6-31+G* basis sets. The electron correla tion has been estimated at the MP2/6-31G level and calculations based on density functional theory, BLYP/6-31G. The solvent effects are in cluded at HF/6-31G level by means of a polarizable continuum model. T he questions related to the two accepted molecular mechanisms, the sem ibenzilic acid and the cyclopropanone transpositions, as well as the c ompetition between both reaction pathways are addressed in this invest igation. The dependence of the geometries of the stationary structures along the corresponding reaction pathways and the transition vectors associated with the transition structures upon theoretical methods is discussed. The analysis of the results shows that the electrostatic so lute-solvent interactions modify appreciably the topology of the poten tial energy surface. The cyclopropanone mechanism is stabilized with r espect to the semibenzilic acid mechanism, but this latter remains the energetically favorable reactive channel both in vacuo and in solutio n. The semibenzilic acid mechanism is a two-step process and the rate- limiting step corresponds to the nucleophilic attack of the hydroxyl i on on the carbon atom of the carbonyl group belonging to the alpha-chl orocyclobutanone ring. In the cyclopropanone mechanism three transitio n structures appear along the energy profile and the rate-limiting ste p is the dehydration process of the bicyclo[1.1.0]2-butanone intermedi ate with concomitant ring contraction and formation of the cyclopropan ecarboxylic acid product.