Comparison of Ethynylation Mechanisms of Ketones with Acetylenes in the Presence of Organic t-Bu-P4/DMSO and Inorganic KOH/DMSO Superbases: A Detailed Quantum Chemistry Study

We report the first detailed investigation of the mechanism of ethynylation reaction of acetone with phenylacetylene in the presence of organic superbase phosphazene t-Bu-P4 performed using the combined quantum chemistry approach MP2/6–311+G**//B3LYP/6–31+G*. It is shown that the mild experimental conditions of the reaction (r.t., 2 h) relate to the low activation barrier of the limiting stage (∆G^‡ = 19.7 kcal/mol). A comparative analysis of the reaction mechanisms in the presence of t-Bu-P4/DMSO and KOH/DMSO revealed some distinctive features of the action of these superbases. The “soft” t-Bu-P4H⁺ cation only slightly stabilizes the pre-reaction complex and the alcoholate ion, slightly destabilizing the transition state. The reaction in the presence of t-Bu-P4/DMSO can be considered to proceed as with the free phenylethynide ion, which suggests the possibility of regeneration of t-Bu-P4 and its use in catalytic quantities. In contrast, the “hard” potassium cation exerts strong stabilization of both the pre-reaction complex and the transition state, leading to a decrease in the activation barrier. Our results revealed a very strong stabilization of the alcoholate ion by the potassium cation (∆∆G = −13.0 kcal/mol), which would inhibit regeneration of the KOH base, suggesting it should be used in stoichiometric quantities.