aziridines

  The aziridine moiety A (Fig. 1) represents one of the most valuable threemembered ring systems in organic chemistry, due to the uncommon combination of reactivity, synthetic flexibility, and atom economy. Indeed, ring strain renders aziridines susceptible to ring-opening reactions that dominate their chemistry and makes them useful synthetic intermediates in the arsenal of the organic chemist. The regio-controlled ring opening of C-substituted aziridines constitutes a powerful approach toward the preparation of a large variety of functionalized nitrogen-containing target compounds. The ring opening of activated aziridines, i.e., aziridines with an electron-withdrawing group at the nitrogen atom, has shown to be quite straightforward, mostly involving the nucleophilic attack at the less-hindered aziridine carbon atom.

Fig. 1 Aziridines A, aziridinium ions B and C, and aziridinium ylides D

  On the other hand, nonactivated aziridines, which do require quaternization toward an aziridinium intermediate B for nucleophilic ring-opening reactions due to an electron-donating group at the nitrogen atom, often have different reactivity and applications. They provide interesting opportunities for the selective synthesis of a variety of functionalized amines. It should be noted that aziridinium ions not only can be obtained through N-functionalization of neutral aziridines but also through intramolecular substitution of amines bearing a leaving group at the β-position.

  Their are different types of strain and different methods to determine ring strain are described and a rationalization of the ring strain of different aziridinium ions will be given based on theoretical calculations. Next, a short overview will be given of the formation of aziridinium ions B from both the rearrangement of β-amino alcohols and the ring opening of nonactivated aziridines, and the reactivity of aziridinium ions will be investigated theoretically. In addition, the role of bicyclic aziridinium ions C as intermediates in the ring expansion of aziridines to azetidines will be unraveled. In the last part, we highlight the importance of spiro aziridinium ylides D as intermediates in ene reactions involving the highly reactive electrophile triazolinedione.