Understanding the mechanism of stable phosphorus ylides derived from maleimide: a kinetic study

Kinetic studies were made of the reactions between triphenylphosphine 1, dialkyl acetylenedicarboxylates 2 in the presence of NH-acid, such as maleimid (as a protic/nucleophilic reagent) 3. To determine the kinetic parameters of the reactions, they were monitored by UV spectrophotometery. The second order fits were automatically drawn and the values of the second order rate constant (k2) were automatically calculated using standard equations within the program. All reactions were repeated at different temperature range, the dependence of the second order rate constant (ln k2) and (ln k2/T) on reciprocal temperature was in a good agreement with Arrhenius and Eyring equations. This provided the relevant plots to calculate the activation parameters (Ea, ?H?, ?S? and ?G?) of all reactions. Furthermore, useful information was obtained from studies of the effect of solvent, structure of reactants (dialkyl acetylenedicarboxylates) and also concentration of reactants on the reaction rates. The proposed mechanism was confirmed according to the obtained results and steady state approximation, and the first and third steps (k2, k3) of all reactions were recognized as rate determining and fast steps, respectively on the basis of experimental data.