STRUCTURAL AND SOLVENT EFFECTS ON THE MECHANISM OF THE THERMAL DECARBOXYLATION OF 2-OXETANONES - A LIMITING CASE BETWEEN CONCERTED AND STEPWISE PATHWAYS IN PERICYCLIC-REACTIONS

The thermal decarboxylation of 2-oxetanones (beta-lactones) to yield a lkenes has been studied using ab initio SCF-MO computational methods. Solvent effects have also been taken into account. The reaction is pre dicted to be concerted in the gas phase, the corresponding transition structures having non-Woodward-Hoffmann topologies. A set of 10 substi tuted 2-oxetanones has been studied, and it has been found that donati ng groups (methyl, vinyl, phenyl) at C4 facilitate the [2+2] cycloreve rsion. The saddle points which connect substrates and products are pre dicted to be highly polar in nature, and, therefore, even moderately p olar solvents such as o-dichlorobenzene accelerate the reaction. trans -2-Oxetanones are predicted to react faster than their cis isomers if solvent effects are taking into account. Self-consistent reaction fiel d methods based on multipole expansions of the free energy of solvatio n tend to overestimate the stability of zwitterionic intermediates at the Hartree-Fock level. This results in stepwise mechanisms in several cases, although higher levels of theory predict again concerted mecha nisms.