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.