Marjan Hashemi-Shahri, Sayyed Mostafa Habibi-Khorassani and Mehdi Shahraki* Pages 210 - 221 ( 12 )
Background: Phosphorus ylides have less nucleophilic and are more stable than arsenic ylides. Due to less interactions of p-carbon orbital with d-arsenic orbital, arsenic ylides react more than phosphorus ylides in the same reactions. Kinetics and a mechanistic investigation of many reactions with TPP have been reported earlier. Herein, we have reported the kinetic results obtained by UV-vis spectrophotometry technique from the reactions between dialkyl acetylenedicarboxylate (DMAD, DEAD and DTAD), triphenylarsine (TPA) as a catalyst and N-H heterocyclic compounds.
Methods: A kinetic study was followed for the present reaction using the UV-vis spectrophotometry method. 0.3 mL aliquot of 10-2 M solution of reactants 1 and 3 were pipetted into a quartz spectrophotometer cell, then 0.3 mL aliquot of 5×10-2 M solution of reactant 2 was added to the mixture according to the stoichiometry of each reactant in the overall reaction. During the whole reaction time, the reaction was supervised by recording scans of the entire spectra at 25°C. The wavelength of 290 nm was chosen to follow the kinetics studies.
Results: The reaction followed second-order kinetics and the partial order with respect to dialkyl acethylenedicarboxylates 1 and N-H heterocycle 3 compounds was one and one. . The fourth step (k4) of the proposed mechanism was recognized as the rate-determining step. This is logical, because in step4 (k4), a tiny protic solvent such as methanol is able to rapid the rate of proton-transfer through a hard non- linear arrangement of TS4 from C1 towards C2. Both steric and inductive effects of different alkyl groups within the structure of dialkyl acetylenedicarboxylates (which participate in the first step1 (k1) of the reaction mechanism) had a significant role on the reduction of the reaction rate, because kobs depends on (k1).
Conclusion: An electron withdrawing substituent group on the second ring of 5-chloro-2- benzoxazolinione speeds up the rate of reaction, opposite the way of 2-benzoxazolinione alone. The magnitude of ΔHǂ is larger than TΔSǂ, so the reactions are enthalpy- controlled. The large and very positive values of ΔSǂ imply that all reactions have dissociative process. In the case of triphenylarsine (TPA), kinetics and reaction mechanism was different from triphenylphosphine (TPP) in a similar reactions.
Dialkyl acetylenedicarboxylates derivative, kinetics, mechanism, N-H heterocycle compound, triphenylarsine.
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