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 (β-lactones) to yield alkenes has been studied using ab initio SCF-MO computational methods. Solvent effects have also been taken into account. The reaction is predicted to be concerted in the gas phase, the corresponding transition structures having non-Woodward−Hoffmann topologies. A set of 10 substituted 2-oxetanones has been studied, and it has been found that donating groups (methyl, vinyl, phenyl) at C4 facilitate the [2+2] cycloreversion. The saddle points which connect substrates and products are predicted to be highly polar in nature, and, therefore, even moderately polar 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 field methods based on multipole expansions of the free energy of solvation 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 mechanisms.