Bromine scavengers

Not every excess acidity mechanistic analysis has been an outstanding success. For instance, several enolization studies have used this technique. The enolization of acetophenone was one of the reactions originally studied by Zucker and Hammett 146 their sulfuric acid rate constant data, obtained by iodine scavenging (the reaction is zero-order in halogen), was used in an excess acidity analysis,242 together with additional results obtained for some substituted acetophenones (using bromine scavenging).243... 

Rates of acid-catalysed enolization of isobutyrophenone and its ot-d analogue have been measured in H2O and D2O, by bromine scavenging.1403 Results include a solvent isotope effect, ku /kDi, of 0.56, and a substrate isotope effect, h/ d, of 6.2 (both for the enolization reaction). Combination of the data with that for ketonization in D2O140b gives the first isotope effect for the keto-enol equilibrium of a simple ketone e(H20)/ e(D20) = 0.92. The results are discussed in terms of the isotopic fiuctionation factors and the medium effect. 

In the formation of the ylide from CBr4, two equivalents of triphenylphosphine are used. One equivalent forms the, ylidfe while the other acts as reducing agent and bromine scavenger. -... 

The yield of BrCH2CH2CH Br found experimentally in bromine-scavenged cyclopropane is less than 1 % of the CHf Br yield. Thus ring-opening has some influence on this system but not to any significant extent. 

Bromine scavenger. When phenol is added as bromine scavenger in the pyrolysis of the tetrabromide (1), the yield of product (2) is 92%. In the absence of phenol, (2) is obtained in yield of 48% along with unidentified products. 

The hydrobromic acid-acetic acid method works well using 33-48% of hydrobromic acid in acetic acid (which can be purchased) with phenol as a bromine scavenger. Phenol is not necessary. The reaction can be carried out at room temperature for an extended period of time, under reflux, refluxed in a sealed tube for an extended period of time, or in a sealed tube at 120°C. Isolation of the product is easier than with the use of concentrated sulfuric acid. Usually, the addition of a large amount of an organic solvent such as ether causes the product to separate as the hydrobromide salt. 

It may be noted that F, G and OA are not Bromine scavengers. However, acetone acts as a Bromine scavenger. Examination of the above scheme shows that... 

For substrates which react readily with bromine atoms, succinimidyl mediated brominations are carried out in the presence of bromine scavengers (ethylene or Jt-butylethylene), employing a good solvent for NBS such as methylene chloride. In these circumstances bromination occurs with a selectivity which closely resembles Cl mediated chlorinations. 

Chlorine and Bromine Scavenger. AcCl (cat. H2SO4, 40-70 °C) scavenges CI2 efficiently (to afford chloroacetyl chloride in 87.1% yield). AcCl also scavenges Br2 efficiently at 35... 

However, in some countries such as Germany there is considerable reservation to adding scavengers because of their possible contribution to dioxin emissions. Furthermore, for lead contents of 0.15 g/1, the need for scavengers is questionable. It is possible that the leaded fuels sold in the coming years will contain neither chlorine nor bromine. 

Much of the bromine output in the U.S. was used in the production of ethylene dibromide, a lead scavenger used in making gasoline antiknock compounds. Lead in gasoline, however, has been drastically reduced, due to environmental considerations. This will greatly affect future production of bromine. 

Aromatic brominations have been mainly carried out using X, Y and L zeolites. Improved para/ortho ratios have been observed upon brominating halobenzenes, benzyl halides, and biphenyl. The side product HBr leads to decreased activity and selectivity. This problem has been addressed by adding scavengers, by working in the gas phase, and by applying oxidative bromination. 

De la Vega and Sasson (ref. 20) added propene oxide as a HBr-scavenger in the bromination of toluene over NaY in CCI4 and obtained an initial p/o-ratio... 

Our group suppressed (ref. 19) the deactivation by HBr by the addition of NaHC03 as a scavenger for HBr and zeolite KA for removal of water formed in the latter reaction. With these additives a p/o-selectivity of 97/3 was achieved in the bromination of bromobenzene over CeY. 

Finally we mention in this section the non-catalytic selective bromination of aniline by the application of a zeolite pre-loaded with Bt2 as a slow release reagent (ref. 27). Aniline, dissolved in CCI4 was treated with Br2 adsorbed onto various zeolites and zeolite CaA was found to be most selective for monosubstitution (92%). The addition of organic bases improved the performance, probably due to scavenging of HBr. Also the toluidines could be monobrominated with this system with >95% selectivity. 

N2, and bromine trifluoride at 25-35°C " are also highly regioselective for tertiary positions. These reactions probably have electrophilic, not free-radical mechanisms. In fact, the success of the F2 reactions depends on the suppression of free-radical pathways, by dilution with an inert gas, by working at low temperatures, and/or by the use of radical scavengers. 

Some time ago, Holliman and co-workers illustrated a method for synthesizing polysubstituted phenazines by reductive cyclization of o-nitrodiphenylamine. However, the yield was poor when competitive cyclizations occurred <70CC1423>. Recently, Kamikawa and co-workers reported a more efficient method to synthesize phenazines using sequential aniline arylation, which was first introduced by Buchwald <97JOC1264>. Regioselective bromination of o-nitrodiphenylamine 226 with bromine in the presence of sodium bicarbonate yielded 227 which was subjected to the Buchwald conditions to provide the desired phenazine 228 and the eliminated product 229 <00TL355>. The former compound is a proposed intermediate for the synthesis of the radical scavenger benthocyanin A. 

Other products can also be obtained since Me2GeBr2 is a germylene scavenger it gives rise to bromine-containing oligogermanes (n = 1, 2) ... 

Analogously, 5-tributylstannylimidazole 29 was easily obtained from the regioselective deprotonation of 1,2-disubstituted imidazole 28 at C(5) followed by treatment with tributyltin chloride [24]. In the presence of 2.6 equivalents of LiCl, the Stille reaction of 29 with aryl triflate 30 afforded the desired 1,2,5-trisubstituted imidazole 31 with 2,6-di-tert-butyl-4-methylphenol (BHT) as a radical scavenger. Reversal of the nucleophile and electrophile of the Stille reaction also provided satisfactory results. For example, the coupling reaction of 5-bromoimidazole 33, derived from imidazole 32 via a regioselective bromination at C(5), and vinylstannane 34 produced adduct 35 [24],... 

The Ru(m)-catalyzed oxidation of glycerol by an acidified solution of bromate (BrCfi ) at 45 °C consumes the required amount of 2 moles of bromate to obtain pure glyceric acid. Traces of Hg(OAc)2 were used as scavenger for potentially formed bromide, thereby eliminating the formation of bromine (formed by reaction of bromide and bromate) as an alternative oxidant [96]. The reaction is first order in Ru(m) (0.58 ms-1 at 45 °C) and zero order in substrate and protons. The addition of RuC163 to protonated bromate is assumed to be rate limiting. Similar catalytic chemistry is obtained with Rh(m)Cl3 [97]. 

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