A DFT study of the role of the Mg complex formation on the mechanism of the 1,3-dipolar cycloadditions of benzonitrile oxides with acryloylpyrazolidinone

The role of the Mg complex formation on the mechanism, the reaction rate and regioselectivity of the 1,3-dipolar cycloaddition (13DC) reactions of benzonitrile oxides (BNOs) towards electron-deficient acryloylpyrazolidinones has been theoretically studied using DFT methods at the 6-31G(*) level. These 13DC reactions have asynchronous concerted mechanism with a non-polar character. These cycloadditions present a slight 5-regioselectivity induced by a steric repulsion between the phenyl substituent of the BNOs and the pyrazolidinone appendage present on the acryloyl derivative, which increases with the formation of the bulky Mg-acryloylpyrazolidinone complex. The present DFT study points out that the coordination of the Mg to the two oxygen atoms of the acryloyl derivative does not produce a discernible change in the mechanism and the reaction rate as a consequence of the non-polar character of these cycloadditions, but produces a regio- and stereoselective induction because of the hindrance produced by the bulk chiral magnesium complex. In this study the solvent effect is also examined. (C) 2009 Elsevier B.V. All rights reserved.