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The kinetics of the reaction between triphenylphosphine, dialkyl acetylenedicarboxylates and pyrrole in dry organic solvents at different temperatures have been studied spectrophotometrically. The observed overall second rate constant (k 2 ) for reaction decreases with decreasing solvent dielectric constant and media temperature; k 2 follows the Arrhenius equation, and the overall reaction is first order in both the dialkyl acetylenedicarboxylate and triphenylphosphine concentrations. The proposed mechanism has been evaluated, and the activation parameters DG 6 ¼ , DS 6 ¼ and DH 6 ¼ for the first, rate-determining step, as an elementary reaction have been determined on the basis of Eyring equation. In addition, the Z-and E-isomers have been optimized at HF and B3LYP levels with the 6-311þþG** basis set; the more stable is the E-isomer, in agreement with experimental data. Furthermore, to better understand the intramolecular interactions, atoms in molecules (AIM) calculations and natural population analysis (NPA) methods have been carried out. The 1 H, 13 C, and 31 P NMR data of these ylides are consistent with results obtained from theoretical calculations.

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