The nature of the transition structures of triazolinedione ene reactions

The ene reactions of triazolinedione (TAD) with propene, trans-and cis-butene, and tetramethylethylene (TME) have been investigated theoretically with abinitio molecular orbital calculations. All geometries were fully optimized at the RHF/6-31G* level, followed by MP2/6-31G* and Becke3LYP/6-31G* single point energy calculations. A stepwise mechanism involving an aziridinium imide (AI) intermediate is predicted. The most stable transition structure for the first step involves a decidedly non-least-motion attack of TAD on the alkene, with methyl group rotation to bring a hydrogen in close proximity to the nitrogen on TAD for favorable electrostatic and secondary orbital interactions. Some isomerization of the AI intermediates is feasible, while reversion to reactants is less favorable than the product-forming hydrogen transfer. The activation energies decrease in the series from propene, to butenes, to TME, as the alkenes become more substituted and electron-rich. Kinetic isotope effects were computed based on the RHF/6-31G* geometries and frequencies, using the Bigeleisen-Mayer equation and the QUIVER program. The calculated isotope effects are in reasonable accord with the experimental measurements. The stabilizing N-H interaction in the first transition structure contributes significantly to the observed isotope effect.