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.