Pyridine hydrobromide perbromide

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... 

The cyclisation of o-alkenylphenols features in two approaches to the chroman ring system. The Hg-mediated cyclisation of (7) affords the chroman-4-ols (8) and (9) which can be separated after debenzylation, providing the chroman unit of the calophyllum coumarins (95S630). 2-Cyclohexenylphenols undergo a 6-endo cyclisation to fused chromans on treatment with pyridine hydrobromide perbromide (95CJC1727). 

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. 

Riordan and Stammer11,12 applied Bergmann s procedure for the syntheses of a series of azlactones. Amino acids, such as phenylalanine, tyrosine, valine, leucine, and isoleucine were converted to A a-methyl-cinnamoyl) derivatives. Azlactonization followed by oxidation using pyridine hydrobromide perbromide gave the corresponding unsaturated azlactone having the Z configuration around the newly formed double bond in the 4-position [Eq. (5)1. 

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

Pyridine hydrobromide perbromide 3-Amino-4-bromotetrahydrothiophene from 4-yinyltbiazolidine ring Stereospecific rearrangement... 

Startg. cw-olefin allowed to react with pyridine hydrobromide perbromide in methanol product. Y 60%. - This is part of a stereospecific total synthesis of d-biotin from L-(+)-cysteine without a diemical resolution sequence characteristic of all previous syntheses. F. e. with a tra 5-olefin s. P. N. Confalone et al.. Am. Soc. 97, 5936 (1975). 

Pyridine hydrobromide perbromide a,p-Ethylenealdehydes from salts of glycidic acids... 

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