Solvation and nucleophilic reactivity of conjugate-base anions of 5-methyl Meldrum’s acid and of 3,3-dimethylbarbituric acid in acetonitrile–methanol

Abstract

Conjugate-base anions of 5-methyl Meldrum’s acid and of 1,3-dimethylbarbituric acid give comparable nucleophilic reactivity toward ethyl iodide in acetonitrile and also have specific interaction enthalpies, Δ_tH_SI^AN→MeOH comparable with those of 3,5-dinitrobenzoate ion and of phthalimidide ion, while pK_a values in the aqueous phase vary significantly among the series. The results lend support to the view that nucleophilic reactivity in acetonitrile is controlled mainly by the partial desolvation of nucleophilic anions accompanying activation. Variation of the specific interaction enthalpy for the present reactions on going from the initial to the transition-state is much smaller by comparison to those for imidide ion and carboxylate ion reactions. Theoretical analysis of the enthalpy with use of MNDO/PM3 procedures indicates that the steric inhibition of the approaching solvent molecule to the carbonyl oxygen by the group coordinated to the central atom is the main factor bringing about the smaller variation.