The ene reaction of triazolinediones (TADs), singlet oxygen (¹O₂), and nitrosoarene (ArNO) with alkenes bearing allylic hydrogens have attracted much attention over the year from both synthetic and mechanistic points of view. Furthermore, they have recently been reviewed. The reactions proceed via stepwise processes through a three-membered ring intermediate, namely a perepoxide, a diarizinium imide, and an aziridine-N-oxide, as evidence by isotope effect studies. Recently, on the basis of computational results, a biradical intermediate was proposed in the ene reactions of TADs, which was consequently challenged by experimental evidence from stereochemical and stereoisotopic studies. This issue remains open for additional structural studies in the future.

The thermodynamic parameters, ∆∆H^≠ and ∆∆S^≠, were determined for the interception of an intermediate, with the structural characteristics of an aziridinium imide, by nucleophilic solvents during the reaction of 2-methyl-2-butene with N-phenyltriazolinedione. The experimentally parameters were found to be in favour of an S_N2-'like' transition state and showed strong dependence on the bulkiness of the incoming molecule of the nucleophile solvent.

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[2] H. R. Hoffman, Angew. Chem., Int. Ed. Engl. 1969, 8, 556-577.

[3] Y. Elemes, M. Orfanopoulos, Tetrahedron Lett. 1991, 32, 2667-2670.

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