An unexpected change in reaction mechanism in the homo-Diels-Alder cycloadditions of N-methyl-1,2,4-triazoline-3,5-dione with a series of bicycloalkadienes

Bicycloalkadienes are known to engage in homo-Diels-Alder reactions with potent dienophiles such as N-substituted 1,2,4-triazoline-3,5-diones (TAD). We investigated the reaction of N-methyl TAD (MeTAD) with the three dienes bicyclo[2.2.1]hepta-2,5-diene (norbornadiene), bicyclo[2.2.2]octa-2,5-diene, and bicyclo[3.2.2]nona-6,8-diene experimentally and computationally. Experimentally, the C7 and C8 dienes resulted in the formation of the expected homo-Diels-Alder product. However, the C9 diene formed a rearranged adduct that could be explained by the intermediacy of an aziridinium imide intermediate. These findings were supported by computational studies (omega B97X-D/def2-SVP{CH2Cl2}) in which not only were the observed products predicted but transition states leading to the products were successfully located. Ultimately, it was determined that subtle differences in transition state structures and energies determine the course of the reaction.