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Zwitterions as Intermediates in 1,3‐Dipolar Cycloadditions of Electrophilic Azides to 2‐Alkylidenetetrahydroimidazoles and 2‐Alkylidenedihydrobenzimidazoles
Author(s) -
Quast Helmut,
Ach Manfred,
Ivanova Svetlana,
Peters EvaMaria,
Peters Karl,
von Schnering Hans Georg
Publication year - 1996
Publication title -
liebigs annalen
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 0947-3440
DOI - 10.1002/jlac.199619961009
Subject(s) - chemistry , ketene , azide , geminal , electrophile , diazo , medicinal chemistry , moiety , cycloaddition , imidazolidine , thermal decomposition , alkyl , stereochemistry , ring (chemistry) , organic chemistry , catalysis
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 ).