Promoted bromination

The base-promoted bromination of ketones is a second-order process, first order in ketone and first order in base dius v = /c ketone base. The bromine concentration does not appear in die rate law that is, the reaction is zero order in [Br2]. 

Chlorination and bromination of pyridine and some alkylpyridines at the -position can be effected in the liquid phase at -100C using excess AICI3 promoter. -Bromination of pyridine and 2- and 4-picoline is conveniently effected in oleum at 80120C. Bromination kinetics using HOBr in aqueous HCIO4 indicate that the partial rate factor for bromination of the pyridinium cation is 1013 comparable to that for nitration. 

The high temperature experiments [19, 27, 90—94] were performed with all the inert gases as diluents, and a set of precise data has been compared [27] with several theories [95—100] dealing with the collision partner effect. The experimental results showed little variation with regard to the identity of the inert gas and corresponded most closely to the predictions of the Keck—Carrier model [100]. The efficiencies were expressed on a per collision basis, fe,/Z (Kr as reference), at 1500°K He (0.354), Ne (0.830), Ar (0.869), Kr (1.00), Xe (0.870). The efficiency of CO2 in promoting bromine dissociation [101] has been determined to be... 

The PyBroP-promoted bromination was initially accompanied by a significant amount of deoxygenation (to afford 69), which was obviously detrimental to the overall yield of the reaction. During the initial discovery, we noticed the... 

The directing effects of two or more substituents can work with or against one another. Bromoxynii and ioxynil are contact herbicides especially used in spring cereals to control weeds resistant to other weedkillers, and both are synthesized from p-hydroxybenz-aldehyde by double halogenation. The aldehyde directs meta and the OH group directs ortho both effects work together to promote bromination or iodination at the same two positions. 

Like with AICI3, AlBrs promote bromine-flnorine exchange. In a case of perfluo-ropyrazine 150 AlBrs treatment leads to exhanstive bromination affording perbro-mopyrazine 305. For single fluorine exchange to bromine atom another approaches were used (Schane 46, compare with Scheme 39) [57]... 

Fester [18] has made this modification to his recipe by promoting the idea that bubbling HCI gas into the safrole/48% aq HBr reaction mix one can affect dehydration allowing dominant bromination to occur. Strike does not know if that actually works. Could be. But what Strike and Eleusis draw on are the examples given by Vogel [37 p277]. 

Halogenation (e.g., bromination) takes place in chloroform for the 2,4-dialkylthiazoles, and the majority of studies have been of 2,4-dimethylthiazole (227, 228). In other cases and in acetic or stronger acids, substitution occurs at the 5-position and is promoted by electronreleasing groups in the 2-position. When the releasing group is in the 4-(or 5-)-position, steric hindrance may decrease the yield of substitution at the 5- (or 4-) position. Nevertheless, the thiazole nucleus is not very reactive since 4-methylthiazole and 2.5-dimethylthiazole are inert in dilute sulfuric acid with bromine (229-231). 

Magnesium is not attacked seriously by dry chlorine [7782-50-5] iodine [7553-56-2] bromine, or fluorine [7782-41 ] gas. However, the presence of water promotes attack. Similarly, sulfur dioxide, ammonia, and fluorine-containing refrigerants do not attack in the absence of water. 

Autoxidation of alkanes generally promotes the formation of alkyl hydroperoxides, but d4-tert-huty peroxide has been obtained in >30% yield by the bromine-catalyzed oxidation of isobutane (66). In the presence of iodine, styrene also has been oxidized to the corresponding peroxide (44). 

Acetaldehyde can be used as an oxidation-promoter in place of bromine. The absence of bromine means that titanium metallurgy is not required. Eastman Chemical Co. has used such a process, with cobalt as the only catalyst metal. In that process, acetaldehyde is converted to acetic acid at the rate of 0.55—1.1 kg/kg of terephthahc acid produced. The acetic acid is recycled as the solvent and can be isolated as a by-product. Reaction temperatures can be low, 120—140°C, and residence times tend to be high, with values of two hours or more (55). Recovery of dry terephthahc acid follows steps similar to those in the Amoco process. Eastman has abandoned this process in favor of a bromine promoter (56). Another oxidation promoter which has been used is paraldehyde (57), employed by Toray Industries. This leads to the coproduction of acetic acid. 2-Butanone has been used by Mobil Chemical Co. (58). 

In the petroleum (qv) industry hydrogen bromide can serve as an alkylation catalyst. It is claimed as a catalyst in the controlled oxidation of aHphatic and ahcycHc hydrocarbons to ketones, acids, and peroxides (7,8). AppHcations of HBr with NH Br (9) or with H2S and HCl (10) as promoters for the dehydrogenation of butene to butadiene have been described, and either HBr or HCl can be used in the vapor-phase ortho methylation of phenol with methanol over alumina (11). Various patents dealing with catalytic activity of HCl also cover the use of HBr. An important reaction of HBr in organic syntheses is the replacement of aHphatic chlorine by bromine in the presence of an aluminum catalyst (12). Small quantities of hydrobromic acid are employed in analytical chemistry. 

As foretold in the introduction, ring formation via attack on a double bond in the endo-trig mode is not well exemplified. The palladium(II) catalyzed oxidative cyclization of o-aminostyrenes to indoles has been described (78JA5800). The treatment of o-methyl-selenocinnamates with bromine in pyridine gives excellent yields of benzoselenophene-2-carboxylates (Scheme 10a) (77BSF157). The base promoted conversion of dienoic thioamides to 2-aminothiophenes is another synthetically useful example of this type (Scheme 10b) (73RTC1331). 

Thieno[ 1,2,3]diazaborines, dihydro-bromination, 1, 656 deuteration, 1, 658 iodination, 1, 656 nitration, 1, 656 nucleophilic substitution copper-promoted, 1, 658 Thienodiazepines synthesis, 7, 595 Thieno[ 1,2]diazepines synthesis, 7, 598 Thieno[2,3-d][l, 2]diazepines synthesis, 4, 749 Thieno[3,2-d][l,2]diazepines synthesis, 4, 749 Thieno[ 1,3]diazepines synthesis, 7, 607 Thieno[ 1,4]diazepinones as anticonvulsants, 1, 170 Thieno[3,4-d][l,3]dioxol-2-one, 4,6-diphenyl-... 

Bromination with A-bromosuccinimide generally gives the same result as bromination with free bromine or hypobromous acid. The reaction is considered to proceed with a small concentration of free bromine and does not generate an appreciable concentration of acid. Conditions are therefore mild. In addition, A-bromosuccinimide has been used to brominate the allylic position of a, -unsaturated ketones in the presence of free-radical promoters or with irradiation, and thus gives access to dienones by dehydro-halogenation, for exaraple " ... 

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