Pyridinium Hydrobromide Perbromide: A Versatile Brominating Agent？ ¹

Pyridinium Hydrobromide Perbromide (PHBP) is a solid complex formed by the reaction of bromine with pyridine hydrobromide salt. It serves as a mild and selective brominating reagent in various reactions. PHBP exhibits high yields and stereo selectivity in α-bromination reactions of ketones and can also facilitate dehydrogenation reactions.

Can PHBP be used for α-bromination of ketones?

PHBP has been proven to be highly effective in α-bromination reactions of ketones. For example, when PHBP reacts with cyclohexanone in acetic acid, it yields α-bromo-cyclohexanone. Interestingly, in the presence of CuBr2, the yield of the reaction is further improved. Similarly, other ketones such as cyclopentenone can also undergo successful bromination with PHBP in the presence of pyridine.

How does PHBP facilitate bromination of aromatic compounds?

PHBP is an ideal single brominating agent for activated aromatic ethers and phenols. It selectively mono-brominates aniline, yielding ortho-substituted products with high yields. PHBP has been widely used for the analysis and detection of aniline and phenol. Alternatively, bromine-1,4-dioxane can also be used for bromination of phenols. PHBP can also be employed for multi-bromination reactions. For instance, 2-hydroxy-4-methoxybenzaldehyde treated with PHBP (2.1 eq. pyridine) yields 3,5-dibromo-2-hydroxy-4-methoxybenzaldehyde, while using 3.5 times the amount of PHBP under the same conditions predominantly produces 2,4,6-tribromo-5-methoxyphenol.

Can PHBP be used for dehydrogenation/oxidation reactions?

PHBP can convert ketones in steroids, tricyclic terpenes, and other polycyclic compounds into phenols.

Furthermore, PHBP can be used for the preparation of alkyl bromides from alcohols and for amine transfer nitrosation. Under phase transfer conditions, PHBP can also oxidize thioaldehydes to aldehydes and ketones. Heating PHBP to 230 ℃ yields 3-bromopyridine (37%) and 3,5-dibromopyridine (33%). Indole, imidazole, and benzothiazine can undergo mono-bromination reactions with PHBP at room temperature or low temperatures.

Can PHBP catalyze the nitrogenation of alkenes?

PHBP, in combination with chloramine-T as a nitrogen source, serves as an effective catalyst for the nitrogenation of alkenes. Both electron-deficient and electron-rich alkenes can undergo this reaction.

References

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