Quantum chemical study on the activity of organic superbase phosphazene t-Bu-P4/DMSO in the autocondensation reaction of ketones

Abstract

This theoretical work is devoted to the study on the activity of phosphazene t-Bu-P4/DMSO in the autocondensation reaction of ketones, which is often an undesirable side process in base-promoted reactions involving ketones. It is shown that the 4-hydroxyenolate stabilized by the phosphazenium cation t-Bu-P4H⁺ is thermodynamically and kinetically most stable in the t-Bu-P4/DMSO system among the condensation products of two acetone molecules. The remaining condensation products are formed only with a slight decrease in free energy. Possible condensation of a larger number of ketone molecules is hindered due to the high barrier and disadvantageous formation of 4-methylpent-3-en-2-one. The formation of the 4-hydroxyenolate from acetone is associated with activation energies greater than the activation energy of the reaction of acetone ethynylation with phenylacetylene. As a result, the use of the t-Bu-P4/DMSO system minimizes or eliminates the formation of ketone condensation by-products in ethynylation reactions. In cascade assemblies of acetylenes with ketones, carried out under more severe conditions, only ketols can be expected as condensation by-products.