Alkylation of potassium ethyl acetoacetate: HSAB versus Marcus theory

Abstract

© 2015 Elsevier B.V. Investigation of the reactions of potassium ethyl acetoacetate (KEAE) with C<inf>2</inf>H<inf>5</inf>X (X=Cl, Br, and I) in the gas-phase, tetrahydrofuran, and hexamethylphosphoramide was conducted at the B3LYP/6-311+G(d,p) level of theory. The C3- and O2-ethylations were examined to find out whether the Marcus or HSAB theory can explain the ambident reactivity of this nucleophile. It was found that the O2-ethylation of KEAE with all three halides is both kinetically and thermodynamically unfavourable in all media. Higher activation energy (as well as intrinsic activation barrier) in the O2 position is a consequence of the attractive forces between the oxygens and potassium, which are often unjustifiably neglected. This finding demonstrates that the Marcus theory is not applicable to the investigated reaction. On the other hand, the reaction is highly dependable on charge distribution and HOMO-LUMO gap in the reactant complexes, thus confirming the applicability of the HSAB rationale. The following conclusion was made: the smaller atomic number of the halogen in ethyl halide and more polar reaction medium, the more favourable charge distribution for the O2-ethylation of KEAE; the larger atomic number of the halogen and less polar reaction medium, the smaller HOMO-LUMO gap and more favourable the C3-ethylation.