Effect of Reactivity on Kinetics and a Mechanistic Investigation of the Reaction between Dimethyl Acetylenedicarboxylate and 1,3-Dicarbonyl Compounds in the Presence of a Catalyst: A Spectrophotometric Approach

Abstract

A kinetic and mechanistic investigation, using conventional UV-Vis spectrophotometry, of the reaction between dimethyl acetylenedicarboxylate (DMAD) and 1,3-dicarbonyl compounds including acetylacetone (ACAC) and dibenzoylmethane (DBM), has been conducted in a methanol environment with triphenylarsine (TPA) acting as a catalyst. Previously, in a similar reaction, triphenylphosphine (TPP) (instead of TPA) had been employed as a reactant (not a catalyst) for the generation of an ylide (final product). In the present work, of significance is the differential behaviour of TPA which, as a catalyst in the reaction environment, leads to a cyclopropane compound. Of other significance is the different behaviours of the two reactants in the kinetics and mechanism of the reaction. In previous work, TPP acted as a weak nucleophile (a reactant), so the first step of the reaction was recognised as the rate-determining step (RDS). Here, TPA reacts as a stronger nucleophile and a catalyst, resulting in the fourth step of the reaction (step4, k4, a proton transfer process) being recognised as the RDS. The reaction followed second-order kinetics. The proposed mechanism was adapted in accord with the experimental results and the steady-state assumption. The results showed that the reaction rate decreases in the presence of DBM, which participates in the second step (step2), compared to ACAC when it is present as another 1,3-dicarbonyl compound (structural effect). In addition, in previous work, the partial order of the reaction with respect to the 1,3-dicarbonyl compound was zero, while it is one in the present work. As a significant result, not only did a change in the structure of one of the reactants (TPA instead of TPP) create a different product, but also the kinetics and reaction mechanism changed. In addition, the reaction is enthalpy-controlled.