Pyridinium hydrobromide perbromide

As these solid agents, some quaternary ammonium tribromides such as pyridinium hydrobromide perbromide (ref. 1), phenyltrimethylammonium tribromide (ref. 2), tetramethylammonium tribromide (ref. 3), and tetrabutyl-ammonium tribromide (ref. 4) have already been reported as mild and selective brominating agents (Fig. 1). 

Djerassi et al. have already demonstrated the use of pyridinium hydrobromide perbromide for efficient a-bromination of steroid ketones (ref. 23). Cacchi et al. 

Fieser et al. have already found that bromination of trans-stilbene with pyridinium hydrobromide perbromide in acetic acid gave exclusively meso-stilbene dibromide, and have further shown that the agent possesses far greater stereoselectivity than free bromine (ref. 26). Fournier et al. have reported the bromo-addition to double-bond of several alkenes by use of TBA Br3 (ref. 27). Moreover, Bethelot et al. described the bromo-addition to triple-bond of alkynes with TBA Br3 (ref. 28). 

The medicinal importance of 2-aryltryptamines led Chu and co-workers to develop an efficient route to these compounds (130) via a Pd-catalyzed cross-coupling of protected 2-bromotryptamines 128 with arylboronic acids 129 [137]. Several Suzuki conditions were explored and only a partial listing of the arylboronic acids is shown here. In addition, boronic acids derived from naphthalene, isoquinoline, and indole were successfully coupled with 128. The C-2 bromination of the protected tryptamines was conveniently performed using pyridinium hydrobromide perbromide (70-100%). 2-Phenyl-5-(and 7-)azaindoles have been prepared via a Suzuki coupling of the corresponding 2-iodoazaindoles [19]. 

Alkenes can be aziridinated using a variety of nitrogen sources. Among the recently reported systems are Chloramine T (iV-chloro-A -sodio-p-toluenesulfonamide) with pyridinium hydrobromide perbromide catalyst (c.g., 119 120) <99OL705>, the A -chloramine salt of t-... 

D KIE of 6.35 has been observed in the oxidation of a-deuteriomandelic acid by pyri-dinium bromochromate to the corresponding oxo acid. The analysis of the D KIE indicated that the reaction involves a symmetric transition state443. The oxidations of phosphinic and phosphorous acids by pyridinium bromochromate exhibits a substantial primary deuterium KIE444. The hydroxyacids, glycolic, lactic, mandelic and malic acids are oxidized by pyridinium hydrobromide perbromide in acetic acid-water mixtures to oxo acids445. The primary KIE in the oxidation of a-deuteriomandelic acid is kn/kn = 5.07, and it does not exhibit a solvent isotope effect. A mechanism involving hydride ion transfer to the oxidant has been proposed445. 

The oxidation of substituted benzylamines by pyridinium hydrobromide perbromide is of first order with respect to both the oxidant and the amine. The oxidation of deuter-ated benzylamine exhibited a kinetic isotope effect (kH/kD = 3.20 at 303 K.). The rates showed excellent correlations in terms of Taft s and Charton s multiparametric equations. A mechanism has been proposed.135... 

Pyridinium hydrobromide perbromide salt was introduced by Djerassi and Scholz as an alternative brominating agent to bromine in 1948. Salazar and Dorta rationalized that since alkylpyridinium salts are well documented and commercially available room temperature ionic liquids, a combination of an alkylpyridinium cation with tribromide anion 1 should therefore lead to a room temperature ionic liquid bromine analogue (Equation 1). 

Reagents i, vinyl acetatc-hv ii, pyridinium hydrobromide perbromide-THF iii, LiBr-LiC03-DMF iv, KOH-MeOH v, same as reagents xii to xviii in Scheme 2... 

Methoxyisatins can be converted to the corresponding phenolic compounds by the action of pyridinium hydrobromide perbromide. This seems to be the best method for obtaining these derivatives, as aminophenols are not useful substrates for the synthesis of... 

Once again, ( )-carvone was chosen as starting material (Scheme 48) (727). Aldol reaction with formaldehyde led to 6-hydroxymethylcarvone. The primary alcohol was protected as silylether 415 to permit addition of (cyanomethyl)lithium. The 5 1 mixture of epimeric tertiary alcohols was separated and the main product, 416, the alcohol generated by axial attack, was protected stereoselectively as bromoether 417 utilizing pyridinium hydrobromide perbromide. To introduce the necessary alkyne, the nitrile was reduced to the aldehyde with diisobutylaluminum hydride and subsequent hydrolysis. Addition of the alkyne led to a 2 1 mixture of... 

Selective monoprotection of dienes. Since the rate of addition of bromine to a double bond increases with substitution, monoprotection is possible. The selectivity is higher with pyridinium hydrobromide perbromide than with bromine, especially at low temperatures (- 60°). The dibromide can be deprotected by cathodic reduction at the mercury cathode in DMF. 

Hexene is brominated with pyridinium hydrobromide perbromide as in test (E). The reaction mixture is diluted with water. By what physical property can you be sure that a reaction product has been produced and not starting material ... 

Dissolve the crude Z-stilbene (e.g., 1.5 g) in 10 mL of acetic acid and, in subdued light, add double the weight of pyridinium hydrobromide perbromide (e.g., 3.0 g). Warm on the steam bath until the reagent is dissolved, and then cool under the tap and scratch to effect separation of a small crop of plates of the me o-dibromide (10-20 mg). Filter the solution by suction, dilute extensively with water, and extract with ether. Wash the solution twice with water and then with 5% sodium bicarbonate solution until neutral shake with saturated sodium chloride solution, dry over sodium sulfate, and evaporate to a volume of about 10 mL. If a little more of the sparingly soluble me o-dibromide separates, remove it by gravity filtration and then evaporate the remainder of the solvent. The residual dl-dibromide is obtained as a dark oil that readily solidifies when rubbed with a rod. Dissolve it in a small amount of methanol and let the solution stand to crystallize. The t//-dibromide separates as colorless prismatic plates, mp 113-114°C yield, about 0.6 g. 

Other reactions. Pyridinium hydrobromide perbromide is reported to be the most satisfactory reagent for the conversion of indole into 3-bromoindole. The reagent can be used for the selective bromination of cyclic ketals in tetrahydroftirane solution, but phenyltrimethylammonium perbromide is now the preferred reagent. 

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. ... 

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