Pyridine hydrobromide

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. 

A-Homo-estra- Q>),2,Aa-triene-4, l-dione (45) from the l-ketal mono-dibromocarbene adduct (43b). A solution of monoadduct 17-ketal (43b 0.46 g) and dry pyridine (20 ml) is heated at reflux for 2 hr. After cooling the reaction mixture, the pyridine hydrobromide (0.1 g) is removed by filtration and the filtrate is concentrated under reduced pressure. The resultant gum is dissolved in ether and washed successively with water, ice-cold 4 N hydrochloric acid, water, 5% aqueous sodium bicarbonate solution, water, saturated salt solution and dried over anhydrous magnesium sulfate. Evaporation of the solvent at reduced pressure gives 3-bromo-4-methoxy-A-homo-estra-2,4,5(10)-trien-17-one (44 0.22 g) mp 158-162° after crystallization from ether. 

The hydrobromide was hydrolyzed to the free base as follows 0.119 kg of 2-(2-bromoacet-amido-5-bromobenzoyl)-pyridine hydrobromide was stirred with 1.2 liters of cold water for 3.5 hours. The mixture was chilled and filtered, and the residue washed with cold water and dried to give 2-(2-bromoacetamido-5-bromobenzoyl)-pyridine, MP 101°C (sinters), 103 -106°C, dec. 

Pyridino-bromo-titanic Acid, (CsHsN)2.H2TiBr6, is formed by saturating a solution of pyridine hydrobromide and titanic acid in alcoholic hydrobromic acid with hydrogen bromide.1... 

To a stirred mixture of 20.8 gm (0.25 mole) of pyridine and 40.5 gm (0.25 mole) of ethyl orthoacetate is added dropwise 40 gm (0.25 mole) of bromine over a period of hr while the temperature is kept at 10°C. The pale yellow brominated ester is filtered, the pyridine hydrobromide washed with ether, the ether washings combined with the bromo ester and then distilled to afford 44 gm (74 %), b.p. 77°-79°C (9 mm), ng 1.4393, d 1.2639. A small amount of ethyl orthodibromoacetate, b.p. 102°-104°C (8 mm), 1.469, d 1.5272, is also isolated. 

The direct electrochemical oxidation of aliphatic alcohols occurs at potentials which are much more positive than 2.0 V w. SCE. Therefore, the indirect electrolysis plays a very important role in this case. Using KI or NaBr as redox catalysts those oxidations can be performed already at 0.6 V vs. SCE. Primary alcohols are transformed to esters while secondary alcohols yield ketones In the case of KI, the iodo cation is supposed to be the active species. Using the polymer bound mediator poly-4-vinyl-pyridine hydrobromide, it is possible to oxidize secondary hydroxyl groups selectively in the presence of primary ones (Table 4, No. 40) The double mediator system RuOJCU, already mentioned above (Eq. (29)), can also be used effectively Another double mediator system... 

Certain convenient brominating agents, such as pyridine hydrobromide perbro-mide (PyHBr3) and tetramethylammonium tribromide, may be used to transform alkenes to vicinal dibromides. They often give better yields than does liquid bromine, but may react by different mechanisms. Isomeric 1-phenylpropenes, for instance, react nonstereoselectively with bromine, but exhibit near-exclusive anti addition with the other reagents 244... 

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. 

A further small quantity of impure glycine can be isolated by concentrating the filtrates obtained on recrystallization. The liquors from the first precipitation, containing ammonium bromide, pyridine hydrobromide, excess pyridine, and alcohol-soluble by-products, may be treated as follows The mixture is first acidified with mineral acid and the methyl alcohol recovered by distillation with a column. The residue is rendered strongly alkaline and the bulk of the ammonia removed by boiling under a reflux condenser wet pyridine can be recovered by downward distillation and treatment of the distillate with solid sodium hydroxide. 

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

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. 

Tin isopropyl tribromide forms deliquescent, pale yellow, prismatic tal)lets, melting about 112 C. Its aqueous solution is strongly acid. Witli pyridine hydrobromide it Ibrms the complex, CjjHy.SnBrg. 2Cr,H5N.2llBr, winch is unchanged at 300 C. 

The formation of a ketene acetal similar to that suggested by Freudenberg and Scholz was also assumed by Pacsu in order to explain the appearance of crystalline pyridine hydrobromide in the pyridine solu-... 

The synthesis of the preceding vinyl amino sulfone offers a very good example of a stereospecific sulfide-directed epoxidation (Scheme 76).Oxidation of optically active sulfide alcohol (12 readily made from epoxycyclopentadiene and resolved) with MCPBA affords the corresponding sulfoxide, which is in equilibrium with the sulfenate ester. Treatment with pyridine hydrobromide and then phenyl disulfide and bromine gives the bromodiol (13), which is simply cyclized to the epoxide with aqueous sodium hydroxide solution (83% overall yield from the sulfoxide). Treatment of the P-epoxy sulfone with DBU followed by in situ silylation with f-butyldiphenylsilyl chloride affords an 86% yield of vinyl sulfone (14). Mesylation of the alcohol moiety followed by immediate treatment with dimethylamine produces the amino vinyl sulfone via a syn Sn2 substitution. ... 

Bromination. Wilson stirred a solution of 1-methylaminoanthraquinone in pyridine under reflux while bromine was run in in 9-10 min., and heated the mixture on the steam bath with stirring for 6 hrs. On cooling, pyridine hydrobromide separated along with the 4-bromo derivative, and hence the precipitate was collected and... 

These salts must be distinguished from pyridinium tribromide, obtained by treating pyridine hydrobromide with bromine, which does not contain an iV-halogen bond, but does have 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. 

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. 

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