Structural effects on kinetics and a mechanistic investigation of the reaction between DMAD and N–H heterocyclic compound in the presence of triphenylarsine: spectrophotometry approach

Kinetics and a mechanistic investigation of the reaction between dimethyl acetylenedicarboxcylate (DMAD) and saccharin (N-H heterocyclic compound) has been spectrally studied in methanol environment in the presence of triphenylarsine (TPA) as a catalyst. Previously, in a similar reaction, triphenylphosphine (TTP) (instead of triphenylarsine) has been employed as a third reactant (not catalyst) for the generation of an ylide (final product) while, in the present work the titled reaction in the presence of TPA leaded to the especial N-vinyl heterocyclic compound with different kinetics and mechanism. The reaction followed second order kinetics. In the kinetic study, activation energy and parameters (Ea, ΔH‡, ΔS‡ and ΔG‡) were determined. Also, the structural effect of the N-H heterocyclic compound was investigated on the reaction rate. The result showed that reaction rate increases in the presence of isatin (N-H compound) that participates in the second step (step2), compared to saccharin (another N-H compound). This was a good demonstration for the second step (step2) of the reaction that could be considered as the rate- determining step (RDS). As a significant result, not only a change in the structure of the reactant (TPA instead of TPP) creates a different product, but also kinetics and the reaction mechanism have been changed.