Pyridinium hydrobromide

Pyridinium hydrobromide dibromide reactions, 2, 34 Pyridinium hydrochloride structure, 2, 109... 

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 primary aliphatic alcohols by l w(2,2,-bipyridyl)copper(ll) permanganate (BBCP) in aqueous acetic acid leads to the formation of the corresponding aldehydes446. The oxidation of [l,l-2H2]ethanol exhibited446 a kn/kn of 4.50. The formation constants for BBCP-alcohol complexes and the rates of their decomposition have been evaluated. Aliphatic aldehydes are oxidized by pyridinium hydrobromide... 

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). 

Figure 12.25 shows how acetals can be brominated electrophihcally because of the (weakly) acidic reaction conditions. Proper acidity and electrophihcity is ensured by the use of pyri-dinium tribromide (B). This reagent is produced from pyridinium hydrobromide and one equivalent of bromine. Pyridinium tribromide is acidic enough to cleave the acetal A into the enol ether G. This cleavage succeeds by way of an El elimination like the one encountered in Figure 9.32 as an enol ether synthesis. The enol ether G reacts with the tribromide ion via the bromine-containing oxocarbenium ion H and the protonated acetal D to form the finally isolated neutral bromoacetal C. (The reaction can be conducted despite the unfavorable equilibrium between the acetal A and the enol ether G, since G continuously reacts and is thus eliminated from the equilibrium.)... 

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. 

Pyridine forms a number of complexes and salts which are useful synthetic reagents. These include pyridinium hydrobromide perbromidc (CjHjNH Brj ), which is a convenient crystalline, easily handled, bromi-nating agent, the pyridine sulfur trioxide complex (C H NrSOj) for sul-fonations, pyridine borane (C HjNrBH,) for hydroborations and the pyridine-chromium(VI) oxide complex (CjHjNiCrOj) for oxidations. 

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