Similarity and Differences in the Regioselectivities of Thermal and Acid-Catalyzed van Alphen–Hüttel Rearrangements in the Series of 3,3-Diphenyl-3H-pyrazoles Containing Electron-Withdrawing Substituents on C4 and C5

Abstract

3,3-Diphenyl-3H-pyrazoles containing an electron-withdrawing substituent in the 5-position undergo van Alphen–Hüttel 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–Hüttel rearrangements have been proposed.