[4+2]-cycloaddition of 3,6-bis(3,5-dimethyl-4-R-pyrazol-1-yl)-1,2,4,5-tetrazines with alkenes

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.