SYNTHESIS, STRUCTURE AND REACTIVITY OF CYCLOPENTA-ANNULATED 1,2,3,4-TETRAZINES

The 2-aryl-2H-cyclopenta[e]-t,2,3,4-tetrazines 3a-n are formed by coup ling of the diazocyclopentadienes 1a and 1b with arenediazonium salts and subsequent reversible electrocyclization of the primary coupling p roducts 2a-n. From the solutions of the equilibrium mixtures of 2a hal f arrow right over half arrow left 3a - 2n half arrow right over half arrow left 3n the tetrazines 3a-d, h-k and the arylazo-diazocyclopenta dienes 2e-g and 21-n crystallize. The 2-methyl-2H-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 solu tions of 3o and 3p the ring-opened isomers 2o and 2p could not be dete cted. X-ray analyses of 3h and 3p prove their bicyclic planar geometry in the solid state. N-15-NMR and temperature-dependent H-1-NMR spectr oscopy have enabled a detailed study of the reversible ring closure re action in the case of 2 d half arrow right over half arrow left 3 d. R eaction of 2-phenyl-2H-cyclopenta[e]-1,2,3,4-tetrazine (3b) with tetra fluoroboric acid results in the formation of the protonated monocyclic salt 4. Furthermore 3b undergoes electrophilic substitution reactions preferably dt C-7, as demonstrated by bromination, formylation, and t rifluoroacetylation. Photolysis of solutions of 2i/3i, 2k/3k, and 21/3 1 leads to the ketene imines 11a-c. The structure of 11c has been dete rmined by X-ray crystallography.