Abstract
3,3-Diphenyl-3H-pyrazoles containing an electron-withdrawing substituent in the 5-position undergo van Alphen–Huttel rearrangement with migration of one phenyl group to C4 on heating in an aprotic solvent (benzene, toluene), as well as on keeping at 20°C in acetic acid in the presence of a catalytic amount of concentrated sulfuric acid. Under similar conditions, 3,3-diphenyl-3H-pyrazoles with a strong electronwithdrawing group (sulfo or cyano) on C4 isomerize to 1H-pyrazoles via migration of one phenyl group to N2. If a moderate electron-withdrawing substituent (alkoxycarbonyl or acetyl group) is present in the 4-position, the acid-catalyzed phenyl group migration is directed mainly to the C4 atom, while thermal migration, toward N2. Probable reasons for the observed similarity and differences in the regioselectivities of the thermal and acidcatalyzed van Alphen–Huttel rearrangements have been proposed.
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