Reaction Pathways in Carbonates and Esters

Abstract

This review reports on the competition/collaboration among intertwined base-catalyzed acyl cleavage bimolecular mechanism (B_Ac 2)/base-catalyzed alkyl cleavage bimolecular mechanism (B_Al 2) or the related acid catalyzed mechanisms A_Ac 2/A_Al 2 and A_Al 1 concerning Carbonates chemistry also in comparison with Esters reactivity. A consistent analysis of the experimental data so far available in the literature led to proposing a theoretical Model outlining the differences in energy profiles among the above-mentioned reaction mechanisms. The reactions involving Carbonates are so tightly interconnected that the formation of the final product is driven by a precise not interfering sequence of B_Ac 2-B_Al 2 (or A_Al 2-A_Ac 2) mechanisms. When entropic effect (in cyclisations) or an anchimeric effect (mustard carbonates, isosorbide methylation) are involved, the difference in Gibbs activation energy is reduced allowing chemical transformations that would normally require higher temperatures. In these cases (intramolecular alkylation, cyclisation reaction, and alkylation by mustard carbonates) only a catalytic amount of base is required as the leaving group CH₃ OCOO^- decomposes restoring the base. As Green Chemistry is concerned, syntheses with much lower environmental impact are achieved with Carbonates when compared with the corresponding ones involving Chlorine chemistry.

Keywords: CO2 Chemistry; Dialkyl carbonates; Esters; Green Chemistry; Reaction mechanisms.