From N-sulfonyl,C-homoallyl-hydrazones to pyrazole and pyridazine (N2)-heterocycles: the ultimate aromatization process

Isomeric six- and five-membered (N2)-aromatics, 6-methylpyridazines and 5-vinylpyrazoles, which energetic topological aromaticity is comparable to that of benzene, are shown to be efficiently produced by sequential isomerization–elimination processes from the corresponding 6-methylidene-1,4,5,6-tetrahydropyridazines and 5-vinylpyrazolines, respectively. The latter precursors are available from the same N-sulfonyl,C-homoallyl-hydrazone substrates by a suitable choice of previously reported conditions for Pd-catalyzed CH-oxidative C,N-ring closing processes. The generality of these cyclization, isomerization, and aromatization reactions, for which detailed mechanisms are proposed, provides a systematic access to wide ranges of 3,4,6-trisubstituted 6-methyl-1,4-dihydropyridazines and 6-methylpyridazines, and their 3,4,5-trisubstituted 5-vinylpyrazole isomers.

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