Kinetics and mechanism of the 1,3-dipolar cycloaddition of nitrilimine with thione-containing dipolarophile: a detailed DFT study

The 1,3-dipolar cycloadditions are extensively used for the preparation of five-membered heterocycles. A mechanism consisting of two pathways was proposed for production of two regioisomers of the 1,3-dipolar cycloaddition of the nitrilimine and a thione-containing dipolarophile. Here, we have investigated the kinetics and mechanism of this reaction using density functional theory. Two possible mechanisms of A and B have been investigated in which Cl is present in the structure of the nitrilimine reactant in the case of mechanism A, while it is absent in the case of mechanism B. Mechanism A, involving Cl, has higher barrier energy than mechanism B, and so is rejected. Mechanism B involves two pathways, I and II, which lead to two regioisomers with different percentages in the products. Both of the pathways are one-step. Pathway II involves the attack of the nitrogen atom of the nitrilimine on the carbon atom of the C=S group of the dipolarophile. The product of pathway II is kinetically and thermodynamically a more favorable product than its regioisomer produced in the other pathway. The obtained results are in agreement with the experimental results.