Pyrazolines oxidation with bromine

A -Pyrazolines are converted into pyrazoles by oxidation with bromine or Pb(OAc)4 and... 

A2-Pyrazolines are converted into pyrazoles by oxidation with bromine or Pb(OAc)4 and they can also be dehydrogenated with sulfur. 3,5-Diphenylpyrazoline (419) on heating with platinum disproportionates to the pyrazole (420) and the pyrazolidine (421) (66AHC(6)347). 

The reaction has proceeded satisfactorily with one or two benzoyl groups present,153,507 but a benzoyl group in position 1 has also been eliminated.153 The oxidation proceeds similarly when position 4 is substituted.608 Pyrazoline itself and monoalkyl- and monoaryl-pyrazolines give the corresponding pyrazoles only in small yield on oxidation with bromine.509,510 In the majority of cases bromination occurs together with oxidation, and often brominated products are the only ones isolated. Thus 1,3,5-triphenylpyrazoline yields a tribromide of unknown constitution.49 The hydro bromide of 4-bromopyrazole (43) was obtained almost quantitatively from 3-butoxy-J1-pyrazoline (42).611A mixture of 1,5-dimethylpyrazole and the 4-bromo compound... 

Some halogenated 2-pyrazolin-5-ones have been prepared by cyclization of various aliphatic compounds. Darapsky, Berger and Neuhaus353 have treated a /J-hydrazino hydrazide with bromine to achieve cyclization, oxidation and halogenation in one step (eq. 207). 

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