Aziridinium imides

A comprehensive review on the chemistry of mercapto- and thione-substituted 1,2,4-triazoles and their utility in heterocyclic synthesis has been published <06ARK59>. The thermodynamic parameters, AAH and AAS, were determined for the interception of an intermediate, with the structural characteristics of an aziridinium imide, by nucleophilic solvents during the reaction of 2-methy 1-2-butene with A-phenyltriazolinedione <06TL2961>. 

Assuming the intermediacy of the aziridinium imide from the qualitative analysis of isotope effect values highly asynchronous C-N bond formation can be... 

In the reaction of alkenes with 4-substituted 3//-l,2,4-triazole-3,5(4//)-diones (13), the ene products, the diazetidines, the vinyl hydrazine derivatives, and rearrangement products, all derive from a common intermediate 14 which is thought to be an aziridinium imide or a 1,4-zwitter-ionic intermediate20 23. The intermediate can be trapped by methanol or acetone20 24. 

Whereas in dichloromethane the ene product is obtained from ( )- and (Z)-2-butenes and 4-phenyl-3//-l, 2,4-triazolc-3,5(4//)-dione (6), by performing the reaction in methanol the attack of methanol on the structurally rigid aziridinium imide intermediate allows the diastereoselective preparation of syn- and anri-l-(2-methoxy-l-methylpropyl)-l,2,4-triazolidine-3,5-diones together with minor amounts of the ene products11. Similar results are obtained with linear 1,3-dienes (Section 7.2.10.3.10.1.). 

Dialkyl diazenedicarboxylates do not undergo efficient cycloaddition with vinyl ethers. For example the [2 -I- 2] adduct was obtained in low yield from the cyclopropane-fused dihydropy-ran 11 even at high temperature28. From the same substrate the [2 + 2] cycloadduct with 4-phenyl-3//-1,2,4-triazole-3,5(4//)-dione was obtained quantitatively even at low temperature through a polar mechanism involving an aziridinium imide as the intermediate28. 

Heterocyclic ketene aminals have been found to be good aza-ene compounds which can undergo hetero-ene reactions readily with activated carbonyl compounds and enones " to provide novel and efficient synthetic routes to y-lactam-fused diaza-heterocycles and fused di- and tri-heterocycles, respectively. Such aminals bearing a secondary enamine moiety have been found to undergo an efficient aza-ene reaction with 4-phenyl-l,2,4-triazoline-3,5-dione under very mild conditions. The regio- and diastero-selectivity of the ene reaction of 4-phenyl-l,2,4-triazoline-3,5-dione with chiral allylic alcohols and their derivatives have been studied, " while Singleton and Hang have proposed a new mechanism for the ene reactions of triazolinediones which involves an open biradical as the key intermediate. This biradical is assumed either to form the ene product or to form reversibly an intermediate aziridinium imide which would be a shunt off the main ene mechanistic pathway. 

A study of the reaction between 4-phenyl-l,2,4-triazoline-3,5-dione (PTAD) and tetramethylethylene also shows the solvent influence on ene reactions [662]. Initially believed to proceed via a concerted pericyclic mechanism, experiments have established that the reaction follows a stepwise route. The calculations are in good quantitative accord with experimentally measured free activation energies (AG (kcal/mol) MeCN exp. 15.0 calc. 14.9). A stepwise mechanism was confirmed and the addition of PTAD to the alkene was found to be the rate-determining step. The traditional supposed mechanism in which an aziridinium imide is formed following the rate determining T8 structure could not be proven. Instead, the calculations predict that the reaction proceeds directly to an open intermediate. 

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