Intramolecular 1,3-Dipolar Ene Reactions of Nitrile Oxides Occur by Stepwise 1,1-Cycloaddition/Retro-Ene Mechanisms

Density functional theory studies of intramolecular ene-like (or the so-called 1,3-dipolar ene) reactions between nitrile oxides and alkenes (Ishikawa, T.; Urano, J.; Ikeda, S.; Kobayashi, Y.; Saito, S. Angew. Chem., Int. Ed. 2002, 41, 1586) show that this reaction is a three-step process involving a stepwise carbenoid addition of nitrile oxide to form a bicyclic nitroso compound, followed by a retro-ene reaction of the nitrosocyclopropane intermediate. The competitive reactions, either the intramolecular (3+2) reactions between nitrile oxides and alkenes or the intermolecular dimerizations of nitrile oxides to form furoxans, can overwhelm the intramolecular 1,3-dipolar ene reactions when the tether joining the nitrile oxide and alkene is elongated or some substituents such as trimethylsilyl are absent.