Pyridine perbromide

When 10-phenylphenothiazine (104) (and 10-phenylphenoxazine) was brominated in acetic acid a number of products were isolated. Pyridine perbromide, though, only brominated the phenyl substituent (Scheme 47). The suggestion that acetic acid bromination might involve the radical cation of the substrate (104) was confirmed by generating the radical cation of the substrate (104) with perchloric acid prior to bromination. Again a 43% yield of the 3-bromo product and multiple bromination products were observed (Scheme 47). The reaction of 10-phenylphenoxazine with pyridine perbromide appeared to be at least partially electrophilic the products... 

A number of N-brominated and N-chlorinated heterocycles also provide sources of electrophilic bromine. Examples include 1-chlorobenzotriazole (82JOC4895 87JOC173 88CHE36) and various HBr and Br2 adducts of pyridines, or pyridine perbromides [84SC939 85JAP(K)60/87264], Polymer-supported reagents of this type include 1-cyclohexylpyridinium perbromide linked to polystyrene, effective for the bromination of 1-methylindole, benzo[fc]furan, and benzo[6]thiophene (89T7869). 

The cyclization of a number of chalcone dibromides to flavones has been achieved by treatment with pyridine (81JOC638). It is thought that the reaction may proceed through debromination and dehydrobromination to the chalcone and bromochalcone, respectively (Scheme 166). The formation of pyridine perbromide would account for the nuclear brominated flavones which sometimes accompany the flavone (63CB913). 

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. ... 

Extremely mild oxidizing agents convert lycorine and several other alkaloids with the same nucleus to oxyphenanthridinium betaines (102). When lycorine was oxidized with selenium dioxide, W-bromosuccini-mide, pyridine perbromide hydrobromide, or f-butyl hypochlorite, the oxyphenanthridinium betaine (XXIX) was isolated. Mercuric acetate... 

Bromine dropped into a stirred mixture of -ethoxypropionaldehyde diethylacetal and pyridine, then kept at 60-65° until, after 2-4 hrs., the red color of the pyridine perbromide has disappeared - abromo-/ -ethoxypropionaldehyde diethylacetal. Crude Y 83%. (W. T. Simpson, Am. Soc. 71, 754 (1949) Directing the bromination by pyridine s. F. Bell and D. B. Mulholland, Soc. 1949, 2020.)... 

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. 

The conversion of indoles to oxindoles can be achieved in several ways. Reaction of indoles with a halogenaling agent such as NCS, NBS or pyridin-ium bromide perbromide in hydroxylic solvents leads to oxindoles[l]. The reaction proceeds by nucleophilic addition to a 3-haloindolenium intermediate. 

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. 

The action of aluminium and its compounds is not fully understood. Pyridine probably acts by the intermediate formation of the perbromide, as explained. 

Bromination of pyridine is much easier than chlorination. Vapour phase bromination over pumice or charcoal has been studied extensively (B-67MI20500) and, as with chlorination, orientation varies with change in temperature. At 300 °C, pyridine yields chiefly 3-bromo-and 3,5-dibromo-pyridine (electrophilic attack), whilst at 500 °C 2-bromo- and 2,6-dibromo-pyridine predominate (free radical attack). At intermediate temperatures, mixtures of these products are found. Similarly, bromination of quinoline over pumice at 300 °C affords the 3-bromo product, but at higher temperatures (450 °C) the 2-bromo isomer is obtained (77HC(32-1)319). Mixtures of 3-bromo- and 3,5-dibromo-pyridine may be produced by heating a pyridine-bromine complex at 200 °C, by addition of bromine to pyridine hydrochloride under reflux, and by heating pyridine hydrochloride perbromide at 160-170 °C (B-67MI20500). 

DetkioketaKzation. Various thioacetals and thioketals are readily hydrolyzed by pyridinium bromide perbromide (1 equivalent) under phase-transfer conditions. Tetrabutylammonium bromide is used as catalyst and aqueous methylene chloride as solvent. The reaction is more efficient in the presence of pyridine as buffer. Yields are generally 75-90%.1... 

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). 

Halogenation of furopyridines using bromine affords either 2,3-dihydro-2,3-dibromofuro-pyridines or unstable perbromides depending on the reaction stoichiometry. Bromination with larger excesses of bromine leads to the more stable dibromo products. Dehydrobromination of the dibromo derivatives gives monobromo products. Bromination results are summarized in Table 42... 

The preparation of the reagent of Step 1 from pyrrolo[2,3-b]pyridine using peridinium perbromide is described by the author. 

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%). ... 

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