Mechanistic Studies in Triazolinedione Ene Reactions

Triazolinediones react with olefins bearing allylic hydrogens in an ene fashion to afford N-allylurazoles. This reaction, apart from considerable mechanistic attention, shows also substantial synthetic utility. Extensive experimental data, such as deuterium kinetic isotope effects, trapping of intermediates, and stereochemical studies, and to a lesser extent computational work, support a two-step mechanism with irreversible formation of a three-membered ring intermediate (aziridinium imide). On the basis of recent experimental and theoretical work, an open biradical has been proposed as the key intermediate.