The Baeyer-Villiger reaction: solvent effects on reaction mechanisms.

This study focuses on the Baeyer-Villiger reaction of propanone and performic acid, with formic acid as catalyst. Continuum solvation methods (EIF-PCM and CPCM) and two density functionals (B3LYP and MPWB1K) are used to study solvent effects on two types of reaction mechanisms: concerted non-ionic and stepwise ionic. The ionic mechanism is the one found in most organic chemistry textbooks; it begins with the protonation of the ketone by the acid catalyst, even though this reaction normally takes place in non-polar solvents such as dichloromethane. Our calculations show that the concerted non-ionic pathway, which is the least energetic in non-polar solvents such as dichloromethane, becomes more energetic the more polar the solvent. After investigating a variety of non-ionic and ionic pathways in water, it is found that the addition step seems to be ionic but the migration step, which is rate-determining, is uncatalyzed, non-ionic and fully concerted. These results confirm the experimental findings in solvents of low to medium polarity that the rate constant of the reaction decreases as the solvent polarity increases. Moreover, we find that contrary to what is commonly accepted, in the addition and migration ionic steps the deprotonation of the ionic species occurs in a concerted manner with the other chemical events taking place.