In this work, mechanistic investigation of the addition-cyclization reaction between thiourea (TU) and dialkyl acetylenedicarboxylate (DAAD) was made theoretically with the B3LYP method using a 6-311++G(2d,2p) basis set. In order to investigate the effect of substituted alkyl groups, the reaction was evaluated in the presence of dimethyl acetylenedicarboxylate, diethyl acetylenedicarboxylate and di-tert-butyl acetylenedicarboxylate. To evaluate the effect of structure and dielectric constant of the solvent, all structures were optimized in different solvent phases such as dichloromethane and acetone at the B3LYP/6-311++G(2d,2p) level of theory. Different orientations of alkyl groups make different kinetic paths. Results indicated that the reaction mechanism did not change with different orientations of COOMe, COOEt or COOtBu groups, but caused significant changes in the potential energy surfaces of the reaction coordinate. The reaction product (alkyl E-2-(2-imino-4-oxo-1,3-thiazolane-5-yliden) acetates) can be converted to different tautomeric forms in two proton transfer processes. So, in order to investigate the effect of different substituted alkyl groups, intermolecular proton transfer in the product was investigated both in gas phase and in the presence of methanol, ethanol and tert-butanol molecules as a bridge to facilitate the proton transfer process.
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