Zwitterions as Intermediates in 1,3‐Dipolar Cycloadditions of Electrophilic Azides to 2‐Alkylidenetetrahydroimidazoles and 2‐Alkylidenedihydrobenzimidazoles

Cyclic ketene N,N‐acetals derived from imidazolidine (4a, c) and 2,3‐dihydrobenzimidazole (6a–c) add methanesulphonyl azide (2a) or picryl azide (2f) to afford the zwitterions 5 and 7, respectively. The structure of 7d is elucidated by X‐ray crystallography. Reversibility of formation and thermal stability of the N‐sulphonyl zwitterions depend on the substitution pattern at the carbon atom to which the triazenide moiety is attached: In the case of a pair of geminal methyl groups (5a, 7a) formation is irreversible and decomposition by cyclisation and subsequent reactions occurs above −20°C, while in presence of a single alkyl group (7c, d) these processes require heating to 80°C and are accompanied by partial reversion to 2a and ketene N,N‐acetals (6b, c). Cyclisation of the zwitterions yields intermediate spirocyclic [3 + 2] cycloadducts, which may undergo [3 + 2] cycloreversion into N‐sulphonylimine 13 and diazo compound 14 or extrude molecular nitrogen to furnish ring‐expanded 2‐(sulphonylimino)piperazine derivatives (9, 11).