[English name] Pyridinium Hydrobromide Perbromide
[Molecular formula] C5H6Br3N
[Molecular weight] 319.86
[CAS][39416-48-3]
[Abbreviation and alias] PHBP, pyridinium tribromide
[Physical properties] mp 101~103 oC (39.7% Br); 132~134 oC (47% Br), soluble in acetic acid, methanol, Ethanol, n-butanol, THF, ether (water); insoluble in water, carbon tetrachloride, ethyl bromide, anhydrous ether, benzene, toluene, light petroleum, petroleum ether; purity can be determined by iodometric titration.
【Preparation and Commercial Products】Prepared from pyridine, 48% hydrobromic acid and bromine. The amount of bromine varies depending on the preparation method; the product can be purified by recrystallization from ethanol. Recrystallization with acetic acid can reduce the amount of bromine. Add bromine to the product until a sticky substance appears. The excess bromine can be evaporated to obtain a pure product.
【Notes】Should avoid contact with water, strong acids and alkalis; it is toxic and should be used in a fume hood.
PHBP is a solid complex of bromine and pyridine hydrobromide that can be used as a source of bromine in reactions. PHBP is a milder brominating reagent than pure bromine. It can undergo selective bromination and dehydrogenation reactions.
Commonly used reagents-α-bromination of pyridinone tribromide
PHBP can α-brominate sterones with high yield and stereoselectivity. Tetrahydroindole can react with PHBP to obtain α-bromination product (formula 1), but if CuBr2 is added to the reaction, the yield will be higher [1]. The reaction between PHBP and cyclohexanone in acetic acid gives equatically substituted α-bromocyclohexanone; conversely, in carbon tetrachloride, an ortho-substituted product is obtained. In the presence of an equal amount of pyridine, cyclopentenone can also be successfully brominated with PHBP.
Phenyltrimethylammonium bromide is also a good acetal bromination reagent, and it has better solubility in THF at low temperatures. Bromination of acetals of benzophenone derivatives occurs more readily with bromo-1,4-dioxane than with PHBP.
Commonly used reagents-bromination of pyridine tribromide aromatic compounds
For activated aromatic ethers or phenols, reducing PHBP is an ideal monobromination reagent. Selective monobromination of aniline yields ortho-substituted products in 75% to 95% yield [2]. PHBP has been used to analyze and detect aniline and phenol. In the bromination of phenols, bromo-dioxane can also be used for the bromination of phenols. PHBP can also be used in polybromination reactions. 2-Hydroxy-4-methoxybenzaldehyde can generate 3,5-dibromo-2-hydroxy-4-methoxybenzaldehyde under the action of PHBP (2.1 eq. pyridine); while using 3.5 times the amount of PHBP , under the same reaction conditions, 2,4,6-tribromo-5-methoxyphenol (formula 2) was mainly obtained. This is the bromination of formyl group [3].
Heating PHBP to 230 ℃ can obtain 3-bromopyridine (37%) and 3,5-dibromopyridine (33%). Indole, imidazole, phenothiazine, etc. can undergo monobromination reaction under the action of PHBP at room temperature or low temperature.
Commonly used reagents–dehydrogenation/oxidation of pyridine tribromide
PHBP can convert ketones in steroids, tricyclic diterpenoids and other polycyclic compounds into phenols (Formula 3)[4].
PHBP can be used in the reaction to prepare alkyl bromides from alcohols, and can also be used in the phase transfer nitrosation of amines. Thioacetals can also be oxidized by PHBP to aldehydes and ketones under phase transfer conditions. Lactone (Formula 4)[5] can be obtained by heating dihydropyridine and PHBP through the bromomethyl intermediate. This intermediate is stable at low temperatures and can react with nucleophiles to obtain differently substituted dihydropyridines.
In addition, using chloramine-T as the nitrogen source, PHBP is also an effective catalyst for the aziridination of alkenes. This reaction can occur in both electron-deficient alkenes and electron-rich alkenes (Formula 5)[6].
References
1. Giordano, C.; Coppi, L. J. Org. Chem., 1992, 57, 2765.
2. Reeves, W. P.; King, R. M. Synth. Commun., 199 Sodium carbonate 3, 23, 855.
3. Cordoba, R.; Plumet, J. Tetrahedron Lett., 2002, 43, 9303.
4. Meyer, W. L.; Clemans, G. B.; Manning, R. A. J. Org. Chem., 1975, 40, 3686.
5. Alker, D.; Swanson, A. G. Tetrahedron Lett. Carbonic Acid, 1990, 31, 1479.
6. Ali, S. I.; Nikalje, M. D.; Sudalai, A. Org. Lett., 1999, 1, 705.
