Effect of beryllium bonds on the keto–enol tautomerism of formamide derivatives: a subtle basicity–acidity balance

Abstract

The effects of the association of BeH2 to formamide derivatives have been investigated through the use of G4 high-level ab initio calculations. The association takes place preferentially at the carbonyl group of the amide in the keto tautomer and to the imino group in the enol form. In both cases, the complexes formed are stabilized through the formation of beryllium and dihydrogen bonds. The relative stability of these complexes is the result of a subtle balance between the changes induced by the formation of the beryllium bond on the intrinsic basicity and acidity of the amide. One of the main consequences of this balance is the significant stabilization of the enol tautomer due to the concomitant increase in the basicity of the imino group with respect to the carbonyl group and the significant acidity enhancement of the OH group, which leads to the formation of very strong BeH···HO dihydrogen bonds in the enol complexes. For the Cl-, Br- and NO2-formamide derivatives, this dihydrogen bond is so strong that a spontaneous formation of hydrogen molecule takes place. The formation of the beryllium bond not only stabilizes the enol forms, but also leads to a significant decrease in the activation barriers involved in the enolization process.