The Diels−Alder (DA) reactions of triazolinedione (TAD) with s-cis- and s-trans-butadiene have been investigated theoretically with density functional theory. All geometries were fully optimized at the Becke3LYP/6-31G* level. For butadiene, the asynchronous concerted pathway dominates; only stereospecific DA products should be formed. The endo transition structure is 9.7 kcal/mol lower in energy than the exo TS; this exo-lone-pair preference comes from the electrostatic repulsion of the lone pairs on nitrogens with the π-system of butadiene. The stepwise mechanism leads to an aziridinium imide (AI) intermediate. The stepwise formation of AI intermediates is only 2.9−6.0 kcal/mol higher in energy than the asynchronous concerted mechanism. The AI intermediates can open to form a diradical or zwitterion which can react to yield nonstereospecific Diels−Alder, ene, or solvent-trapping products.