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Synthesis, Structure and Reactivity of Cyclopenta‐annulated 1,2,3,4‐Tetrazines
Author(s) -
Mackert Peter J.,
Hafner Klaus,
Nimmerfroh Norbert,
Banert Klaus
Publication year - 1994
Publication title -
chemische berichte
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 0009-2940
DOI - 10.1002/cber.19941270822
Subject(s) - chemistry , bicyclic molecule , protonation , methyllithium , reactivity (psychology) , ring (chemistry) , nuclear magnetic resonance spectroscopy , electrophile , medicinal chemistry , aryl , ketene , electrophilic substitution , stereochemistry , crystallography , organic chemistry , medicine , ion , alkyl , alternative medicine , pathology , catalysis
The 2‐aryl‐2 H ‐cyclopenta[ e ]‐1,2,3,4‐tetrazines 3a ‐ n are formed by coupling of the diazocyclopentadienes 1a and 1b with arenediazonium salts and subsequent reversible electrocyclization of the primary coupling products 2a ‐ n. From the solutions of the equilibrium mixtures of 2a ⇌ 3a ‐ 2n ⇌ 3n the tetrazines 3a ‐ d , h ‐ k and the arylazo‐diazocyclopentadienes 2e ‐ g and 21 ‐ n crystallize. The 2‐methyl‐2 H ‐cyclopenta[ e ]‐1,2,3,4‐tetrazines 3o and 3p are obtained by addition of methyllithium to 1a and 1b followed by a diazo transfer reaction and cyclization. In solutions of 3o and 3p the ring‐opened isomers 2o and 2p could not be detected. X‐ray analyses of 3h and 3p prove their bicyclic planar geometry in the solid state. 15 N‐NMR and temperature‐dependent 1 H‐NMR spectroscopy have enabled a detailed study of the reversible ring closure reaction in the case of 2d ⇌ 3d. Reaction of 2‐phenyl‐2 H ‐cyclopenta[ e ]‐1,2,3,4‐tetrazine ( 3b ) with tetrafluoroboric acid results in the formation of the protonated monocyclic salt 4. Furthermore 3b undergoes electrophilic substitution reactions preferably at C‐7, as demonstrated by bromination, formylation, and trifluoroacetylation. Photolysis of solutions of 2i / 3i , 2k / 3k , and 21 / 31 leads to the ketene imines 11a‐c. The structure of 11c has been determined by X‐ray crystallography.