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 ).