A DFT study of the role of Lewis acid catalysts in the mechanism of the 1,3-dipolar cycloaddition of nitrile imines towards electron-deficient acryloyl derivatives

The role of the Mg complex formation in the reaction rate and regioselectivity of the 1,3-dipolar cycloaddition (13DC) reactions of nitrile imines (NIs) towards electron-deficient methacryloylpyrazolidinones has been theoretically studied using DFT methods at the B3LYP/6-31G* level. These 13DC reactions have an asynchronous concerted mechanism with a low polar character, displaying a high C regioselectivity induced by a steric repulsion between the phenyl substituent present at the C3 carbon of the NIs and the pyrazolidinone appendage present in the methacryloyl derivative, which increases with the formation of the bulky Mg–methacryloylpyrazolidinone complex. Solvent effect has been considered in this study by means of the polarizable continuum model. It was found that, in dichloromethane as solvent, the reaction profiles are similar to these obtained for the reaction in gas phase. Analysis of the reactivity indexes of the reagents explains and interprets the low polar character of the investigated reactions.