Brominations with Phosphorus Bromides and Other Reagents

Brominations with Phosphorus Bromides and Other Reagents 

2-Hydroxypyrazine with phosphoryl bromide and phosphorus pentabromide has been found to give a mixture of 2-bromopyrazine and 2,6-dibromopyrazine (818, 865) the former was converted to the latter under similar reaction conditions (865). Phenyl-substituted 2-hydroxypyrazines were found to be transformed in good yield by phosphorus tribromide alone to the corresponding 

2- bromopyrazines, but this procedure was not satisfactory when applied to alkylhydroxypyrazines because they formed complexes insoluble in phosphorus tribromide and the yields of bromopyrazines were poor (866). Phosphoryl bromide alone, or with phosphorus tribromide as diluent, was a useful reagent for the synthesis of alkylated 2-bromopyrazines but it complicated the reaction by yielding polybromides as by-products [presumably as a result of free radical bromination of alkyl substituents and of the pyrazine nucleus (866)]. This behavior precluded the use of forcing conditions to increase the yield of monobromopyrazines. 

Refluxing phosphoms tribromide converted 2-bromo-3-hydroxy-5,6-diphenyl-pyrazine and 2-bromo-5-hydroxy-3,6-diphenylpyrazine to the dibromopyrazines 2,3-dichloro-5,6-dimethylpyrazine and 2,5-dichloro-3-phenylpyrazine to the dibromopyrazines (817) 5-chloro-2,3-diphenylpyrazine (and its 6-ethyl derivative) to 5-bromo-2,3-diphenylpyrazine (and its 6-ethyl derivative) (866) and 2-hydroxy- 

3- nitro-5,6-diphenylpyrazine also to 2,3-dibromo-5,6-diphenylpyrazine (834, 841). 23-Dihydroxy-5-phenyl-, 2,5-dihydroxy-3-phenyl-, and 3,5-dihydroxy-2-phenyl-pyrazines were each converted with phosphorus tribromide at 180-200° to the corresponding dibromopyrazine (829). 2-Cyano-3-hydroxy-5,6-diphenyIpyrazine and 2-carbamoyl-3-hydroxy-5,6-diphenylpyrazine with boiling phosphorus tribromide each gave 2-bromo-3-cyano-5,6-diphenylpyrazine (837). 

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F. Brominations with Phosphorus Bromides and Other Reagents... 

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