The Nature of the Transition Structures of Triazolinedione Ene Reactions

Abstract

The ene reactions of triazolinedione (TAD) with propene, trans- and cis-butene, and tetramethylethylene (TME) have been investigated theoretically with ab initio 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 bas...