Pyrazole 4.5- dibromo-3-methyl

Janiak et al. found that dibromo(methyl)borane MeBBr reacts with Hpzx at room temperature in the presence of NEt3 and Tl(OEt) to form [Tl(MeTpx)] in high yield.132 For the synthesis of R2B(pzx) species, sufficient pyrazolate ion is required to be present to convert [R3B(pzx)j quickly to [R2B(pzx)2], otherwise the R2B(pzx) species will dimerize to the stable pyrazabole R2B( x-pzx)2BR2. 

Bromination of 1-phenylpyrazoles resembles nitration in that it occurs initially in the benzene ring, and only then in the heterocyclic nucleus to give what is probably 4,5-dibromo-l-(p-bromophenyl)pyrazole [61JCS2769 66AHC(6)391]. 3-Methyl-1-phenylpyrazole was brominated at both C-4 and in the para-position (61JCS2769), and there are other examples (83JHC277). 

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