Theoretical study of the mutarotation of erythrose and threose: acid catalysis

Abstract

The acid catalysis of the mutarotation mechanism in the two aldotetroses,D-erythrose andD-threose, hasbeen studied at B3LYP/6-311++G(d,p) computational level in gas phase and in solution employing thePCM–water model. The open-chain, the furanose and the connecting TS structures have been character-ized. To study the enhancing effect of acid groups on the electrophilicity of the carbonyl carbon atom, foursituations have been considered: (i) a classical Lewis acid as BH3; (ii) a classical hard-Pearson acid as Na+;(iii) classical Brønsted acids as H+and H3O+; and (iv) a combined strategy using H3O+and one bridge-H2Omolecule as assistant in the proton transfer process. All the acidic groups reduce the activation energywith exception of the Na+, which can act, in some cases, as inhibitor. It is greatly reduced by the presenceof Brønsted acids (iii) and through the combined strategy (iv). For this last mechanism, the electronic acti-vation energies are between 12 and 43 kJ mol 1in vacuum and between 13 and 25 kJ mol 1in watersolution, through the use of the PCM model.