Pyridine hydrobromide perbromide, bromination

Pyridine hydrobromide perbromide (pyridinium bromide perbromide) [39416-48-3] M 319.9, m 130 (dec), 132-134 (dec). It is a very good brominating agent - liberating one mol. of Br2. Purified by recrystn from glacial acetic acid (33g from lOOmL of AcOH). [Fieser and Fieser Reagents for Organic Chemistry Vol 1 967 1967.]... 

Pyridine hydrobromide perbromide (Pyr HBr Br2) has been employed as a convenient, easily weighed source of bromine for small scale-reactions. The chlorine analog has also been used in this connection. 

Since the reactivity of double bonds in electrophilic bromination increases drastically with increasing alkyl substitution, selective monoaddition to the more substituted double bond in nonconjugated dienes can be accomplished with pyridin hydrobromide perbromide.264 Chlorination265 and bromination266 of c/s,cis-1,5-cyclooctadiene lead to the expected dihalogen and tetrahalogen derivatives. 

Another approach in the study of the mechanism and synthetic applications of bromination of alkenes and alkynes involves the use of crystalline bromine-amine complexes such as pyridine hydrobromide perbromide (PyHBts), pyridine dibromide (PyBn), and tetrabutylammonium tribromide (BiMNBn) which show stereochemical differences and improved selectivities for addition to alkenes and alkynes compared to Bn itself.81 The improved selectivity of bromination by PyHBn forms the basis for a synthetically useful procedure for selective monoprotection of the higher alkylated double bond in dienes by bromination (Scheme 42).80 The less-alkylated double bonds in dienes can be selectively monoprotected by tetrabromination followed by monodeprotection at the higher alkylated double bond by controlled-potential electrolysis (the reduction potential of vicinal dibromides is shifted to more anodic values with increasing alkylation Scheme 42).80 The question of which diastereotopic face in chiral allylic alcohols reacts with bromine has been probed by Midland and Halterman as part of a stereoselective synthesis of bromo epoxides (Scheme 43).82... 

In furan and thiophene, the halogen enters the alpha position. The vapor-phase reaction of pyridine and bromine at 500° furnishes 2-bromo- and 2,6-dibromo-pyridines, and at 300°, 3 bromo- and 3,5-dibtomo-pyridines. 3-Bromopyridine is more conveniently prepared by pyrolysis of pyridine hydrobromide perbromide (40%). Similarly, quinoline yields 3-bromo-quinoline at 300° and 2-bromoquinoline at 500°. Pyrolysis of iso-quinoline hydrobromide perbromide gives the 4-bromo derivative (53%). ... 

The bromine may be added in a stream of nitrogen which also serves to remove the liberated hydrogen halide. In the bromination of pinacolone, aluminum amalgam or aluminum chloride is used as a catalyst, " Phosphorus pentabromide, N-btomosuccinimide, " and pyridine hydrobromide perbromide have been used as brominating agents. 

Triethylene diamine bromine complex Pyridine hydrobromide-perbromide Diphenyliodonium iodide... 

These salts must be distinguished from pyridinium bromide perbromide , obtained by treating pyridine hydrobromide with bromine, which does not contain an A -halogen bond, but does include a trihalide anion. The stable, crystalline, commercially available salt can be used as a source of molecular bromine especially where small accurately known quantities are required. 

In a somewhat analogous fashion, pyridine hydrobromide on treatment with sodium nitrite and bromine gives a perbromide which decomposes to 2-bromopyridine (92%). ... 

V. Braun reaction Cyanogen bromide. Phosphorus tribromide. Piperidine. Thionyl chloride. Bromination Aluminum bromide. Aluminum chloride. Boron tribromide. Bromine chloride. N-Bromocaprolactam. N-Bromosuccinimide. Bromotrichloromethane. Cupric bromide. Dibenzoyl peroxide. l,3-Dibromo-5,5-diraethylhydantoin. 1,2-Dibromotetrachloromethane. HBr-scavengers acetamide and potassium chlorate. Iodine. Iodine monobromide. Iron. Mercuric acetate. Phenyl trimethylammonium perbromide. Phosphorus trichloride. Pyridine. Pyridine perbromide. Pyridinium hydrobromide perbromide. Silver sulfate. Sodium acetate. Sodium hypobromite. Sulfur. Sulfur monochloride. Tetramethylaramonium tribromide. M-Tribromoacetophenone. Trichloromethane sulfonyl bromide. Trilluoroacetyl hypobromite. Triphenylphosphine dibromide. 

The only further mention of the reagent we have discovered is unfavorable. Djerassi and Scholz, in introducing pyridinium hydrobromide perbromide as a reagent for the bromination of ketones, state that attempts to employ pyridine perbromide, a reagent which should bind any hydrogen bromide liberated, proved disappointing. ... 

The formation of diequatorial and diaxial dibromides from 3- and 4-sub-stituted cyclohexenes has prompted examination of the factors which may influence the steric course of the reactions. All additions of bromine are anti and all adducts are formed under kinetic control. The diequatorial diastereo-isomer (207) content of the reaction product decreases when a basic solvent such as diethyl ether is used the same result pertains in the presence of tertiary amines or when the brominating agent is either pyridine perbromide or pyri-dinium hydrobromide perbromide. Of the olefins examined, (205b) yields the highest amount of diequatorial adduct. 

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