Gl. Rusinov et al., [4+2]-cycloaddition of 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines with alkenes, RUSS CHEM B, 49(2), 2000, pp. 355-362
A number of 1,4-dihydropyridazines and pyridazines were prepared by the Die
ls-Alder reaction with an inverse electron demand from cyclic heterodiene s
ystems, 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines, and some
enamines as well as from 4-vinylpyridine. butyl vinyl ether, phenylacetyle
ne, and acrylamide. The reaction of 3,6-bis(3,5-dimethylpyrazol-1-yl)-1,2,4
,5-tetrazine with styrene afforded 4,5-dihydropyridazine, which was readily
oxidized by atmospheric oxygen to form the corresponding pyridazine. Elect
ron-withdrawing substituents (Br or Cl) in the pyrazole rings accelerate [4
+2]-cycloaddition. When heated, 1,4-dihydropyridazines, which were synthesi
zed from tetrazines and enamines, eliminated amine to give pyridazines. The
reactivities of tetrazines were evaluated by quantum-chemical methods.