2-SUBSTITUTED PYRAZOLE 1-OXIDES - PREPARATION AND REACTION WITH ELECTROPHILIC REAGENTS

1-Substituted pyrazoles have been oxidized by peracids to 2-substituted pyrazole 1-oxides. This leads to a change in reactivity towards electrophilic attack. 2-Benzyl-pyrazole 1-oxide is brominated, chlorinated, and nitrated selectively at C-3. The 3-halo-substituted pyrazole 1-oxides are nitrated selectively at C-4 while their halogenation proceeds with poor mono- and regio-selectivity producing 3,4-dihalo-, 3,5-dihalo- and 3,4,5-trihalo-derivatives. Bromine at C-3 and C-5 can be used for protection since it is removed by treatment with sodium sulfite. The reactions may be followed by deoxygenation. The total sequences provide effective routes for the regioselective introduction of halogen and nitro groups into the pyrazole nucleus.