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 theoreti cally with abinitio molecular orbital calculations. All geometries wer e fully optimized at the RHF/6-31G level, followed by MP2/6-31G* and Becke3LYP/6-31G single point energy calculations. A stepwise mechanis m involving an aziridinium imide (AI) intermediate is predicted. The m ost stable transition structure for the first step involves a decidedl y non-least-motion attack of TAD on the alkene, with methyl group rota tion to bring a hydrogen in close proximity to the nitrogen on TAD for favorable electrostatic and secondary orbital interactions. Some isom erization of the AI intermediates is feasible, while reversion to reac tants is less favorable than the product-forming hydrogen transfer. Th e activation energies decrease in the series from propene, to butenes, to TME, as the alkenes become more substituted and electron-rich. Kin etic isotope effects were computed based on the RHF/6-31G geometries and frequencies, using the Bigeleisen-Mayer equation and the QUIVER pr ogram. The calculated isotope effects are in reasonable accord with th e experimental measurements. The stabilizing N-H interaction in the fi rst transition structure contributes significantly to the observed iso tope effect.