A quantum-chemical DFT study of the mechanism and regioselectivity of the 1,3-dipolar cycloaddition reaction of nitrile oxide with electron-rich ethylenes

The 1,3-dipolar cycloaddition (13DC) reactions of nitrile-oxide NO 1 with two ethylenes, enamine 2a and enamine 2b, were computationally studied using B3LYP/6-31G(d) DFT methods. The two possible ortho and meta regioselective channels were characterized and analyzed. The moderate polarity of these 13DC reactions is related to the high nucleophilic character of both ethylenes, and the moderate electrophilic nature of the NO 1, that accounts for the relatively low calculated activation energies. Analysis of different forms of energies along the different reaction channels indicates that the present 13DC reactions are completely ortho regioselective, accordingly to the experimental outcomes. Electron localization function analysis indicates that these 13DC reactions proceed via a nonconcerted (two-stage) one-step mechanism.