4-HALO-4H-PYRAZOLES - CYCLOADDITION WITH CYCLOPENTADIENE TO AZOALKANES OF THE 2,3-DIAZABICYCLO[2.2.1]HEPT-2-ENE TYPE VERSUS ELECTROPHILIC ADDITION WITH CYCLOPENTENE

The acid-catalyzed reactions of the 4-halogen-substituted 4H-pyrazoles 3 with cyclopentadiene and cyclopentene in methylene chloride have be en examined. The 4,4-dichloro-3,5-diphenyl-4H-pyrazole (3a) cycloadded quantitatively with cyclopentadiene to the gem-dichloro-substituted a zoalkane 4a of the 2,3-diazabicyclo[2.2.1]hept-2-ene type, whose struc ture was established by X-ray analysis. The azoalkane 4a was catalytic ally hydrogenated to the azoalkane 6 with reduced C=C double bond and the relatively persistent (toward autoxidation) hydrazine 7 with reduc ed C=C and N=N double bonds. The cycloaddition of 4-chloro-4-methyl-3, 5-diphenyl-4H-pyrazole (3b) with cyclopentadiene gave the azoalkane an ti-4b in high yield and diastereoselectivity. The azoalkanes 4a,b and 6 were reluctant toward photolysis and thermolysis and the expected ho usanes were not produced under these conditions. The acid-catalyzed re action of the 4-bromo-substituted 4H-pyrazoles 3c-e with cyclopentadie ne gave, instead of the desired cycloadducts, complex product mixtures . In the presence of acid, the 4H-pyrazoles 3 gave with cyclopentene t he 2-(alpha-halocyclopentyl)-substituted 2H-pyrazoles 5 as products of electrophilic addition.