Reaction Paths of Keto-Enol Tautomerization of β-Diketones

Abstract

Density functional theory calculations were conducted on the title reactions with water molecules. Malonaldehyde, acetylacetone, and malonic acid were adopted as reactants. A reaction of malonaldehyde and (2 + 2) water molecules was found to proceed by a small (ca. 7 kcal/mol) activation energy. Two are reactant and two are catalyst molecules, respectively. The intramolecular hydrogen bond in the enol form is disrupted by the intermolecular one. Tautomerization transition-state geometries for acetylacetone and malonic acid are similar to that of malonaldehyde. Without the enhanced reactivity of water hydrogen-bond networks, tautomerization reactions undergo large activation energies. Various reaction models of (malonaldehyde)n (n = 2, 3, and 4) have given the energetic result. The keto-enol tautomerization has an analogy with the bimolecular nucleophilic elimination (E2) mechanism.