SELECTIVE a-BROMINATION

Ketimines, enamines from, in ketones alkylation, 54, 48 Ketone, aralkyl, selective a-bromination of, 53, 111 Ketone, a,a -dibromodineopen-tyl-, preparation of, 54,... 

Addition to alkenes1 and alkynes.2 The reagent converts (E)-alkenes into meso- or erythro-dibromides in CHC13 in 80-95% yield. It is also useful for selective a-bromination of ketals (75-80% yield). 

SELECTIVE a-BROMINATION OF AN ARALKYL KETONE WITH PHENYLTRIMETHYLAMMONIUM TRIBROMIDE 2-BROMOACETYL-6-METHOXYNAPHTHALENE AND 2,2-DIBROMOACETYL-6-METHOXYNAPHTHALENE... 

Treatment with bromine and a catalytic amount of phosphorus leads to the selective a-bromination of carboxylic acids. 

Sebacic add dinitrile, 50, 20 SELECTIVE a-BROMINATION OF AN ARALKYL KETONE WITH PHENYLTRIMETHYLAMMO-NIUM TRIBROMIDE 2-BRO-MOACETYL-6-METHOXY-NAPHTHALENE AND 2,2-DI-BROMOACETYL-6-METHOXY-NAPHTHALENE,53, 111 Shikimic add, SO, 27 Silver benzoate, as catalyst in decomposition of diazoketones, 50,... 

Bromination. The reagent is useful for selective a-bromination of ketones. It is converted into N,N-dimethylcyanoacetamide, which can be recycled. 

Similar selectivity is obtained with a homogeneous solution of the reagent in dioxane. A limitation of the reaction is observed with 2 -hydroxy-4, 6 -dimethoxyacetophenone, which undergoes aromatic nuclear bromination with CuBr2. Steroidal ketones have been selectively a-brominated with CuBr2 in the presence of a double bond without bromination of the alkene (eq 2), while y-bromination occurs in other steroidal enones. ... 

Copper(II) bromide has been used to a-brominate diketotetraquinanes and to introduce a double bond into a prostanoid nucleus in a one-pot bromination-elimination procedure (eq 3). 3,7-Dibromo-2//,6//-benzodithiophene-2,6-diones (eq 4) and 5-bromo-4-oxo-4,5,6,7-tetrahydroindoles (eq 5) are prepared by the selective a-bromination of their respective ketone starting materials without bromination of the aromatic or heterocyclic rings. 4-Carboxyoxazolines are converted to the corresponding oxazoles using a mixture of CuBra and l,8-diazahicyclo[5.4.0]undec-7-ene (eq 6). ... 

The procedure can be extended to achieve selective a-bromination and iodination of carboxylic acids and the general mechanism of the a-halogenation is outlined in Chapter 5, p 170. The autocatalytic effects in the selective a-chlorination of propionic acid to the 2-chloro and 2,2-dichloro acids have been studied in a semibatch reaction at 90-130 °C. The reaction was performed in the presence of chlorosulfonic acid and dichloroacetic acid as catalysts and oxygen as the radical scavenger. Kinetic experiments indicated autocatalytic formation of both products and that the selectivity was independent of the chlorine concentration in the liquid phase. The results confirmed the validity of the proposed reaction scheme which involved formation of the reaction intermediate, propanoyl chloride from propionic acid and chlorosulfonic acid, the acid-catalysed enolization of the acid, and a hydroxyl-chlorine exchange reaction. The acid-catalysed enolization was the rate-determining step in the reaction sequence. ... 

Halogen-substituted succinimides are a class of products with important appHcations. /V-Bromosuccinimide [128-08-5] mp 176—177°C, is the most important product ia this group, and is prepared by addition of bromine to a cold aqueous solution of succinimide (110,111) or by reaction of succinimide with NaBr02 iu the presence of HBr (112). It is used as a bromination and oxidation agent ia the synthesis of cortisone and other hormones. By its use it is possible to obtain selective bromine substitution at methylene groups adjacent to double bonds without addition reactions to the double bond (113). 

Treatment of pyrrole, 1-methyl-, 1-benzyl- and 1-phenyl-pyrrole with one mole of A -bromosuccinimide in THF results in the regiospecific formation of 2-bromopyrroles. Chlorination with IV-chlorosuccinimide is less selective (8UOC2221). Bromination of pyrrole with bromine in acetic acid gives 2,3,4,5-tetrabromopyrrole and iodination with iodine in aqueous potassium iodide yields the corresponding tetraiodo compound. 

The interconversion with di- and tri-bromothiophenes is very selective. Thus 2,3,5-tribromothiophene gives exclusively 3,5-dibromo-2-thienyllithium, the hydrolysis of which affords a very convenient synthesis of 2,4-dibromothiophene. Similarly, 2,4- and 2,3-di-bromothiophene give interconversion selectively with the a-bromine. 

Olefins react with bromine by addition of the latter to the carbon-carbon double bond. In contrast the Wohl-Ziegler bromination reaction using N-bromosuccinimide (NBS) permits the selective substitution of an allylic hydrogen of an olefinic substrate 1 by a bromine atom to yield an allylic bromide 2. 

Fig. 1. Plots of dielectric constant vs selectivity of bromination of phenol with Br2 (left) and NBS (solid powder) (right). ort/io-Bromophenol (A), para-Bromophenol ( ).

As described above, it was shown that A,iV-dibromomethylamine was effective for orr/io-dibromination of phenol (ref. 7). We also carried out a bromination using NBB as A-bromoamine analogue. One molar amount of NBB did not give 2-bromophenol selectively, but gave a mixture of o/t/io-monobromophenol and 2,6-dibromophenol, and a considerable amount of phenol was recovered. On the other hand, 2,6-dibromophenol was obtained in an 81.7 % yield when two molar amounts of NBB were used. These results suggested that 7V-bromoamines were the best reagents for orf/io-bromination of phenol. However N-bromoamines were very unstable and decomposed explosively in less than a day at room temperature. 

The authors (ref. 19) managed to perform this reaction selectively as telomerization at the C-Br bond of bromoform using initiating system Fe(CO)5 + DMF, which facilitates a bromine transfer at a step of a chain transfer (ref. 19). In this case only one row of telomers is formed which contain three bromine atoms in molecules ... 

Halogenatlon. Poly(2,6-dimethyl- and 2,6-diphenyl-l,4-phenylene ether) can be aryl-brominated simply by exposure to a bromine solution no catalyst is required.6 In fact, the use of Lewis acid catalysts to promote the chlorination of poly(2,6-dimethy1-1,4-phenylene ether) leads to substantial degradation of the molecular weight of the chlorinated products.7 Membranes produced from ring brominated PPO (40% wt Br) exhibited enhanced permeability to CHi and CO2 and proved to be more selective in separating CH4/CO2 mixtures.8... 

We first selected the bromine-substituted TTF-type donor EDT-TTFBr2 (4,5-dibromo-TTS -ethylenedithiotetrathiafulvalene) [68] to achieve strong n-d interaction [69]. This donor molecule has two bromine atoms that cause attractive interaction with a halogen atom of MX4 anions. In this donor molecule the... 

Next we must select a hybridization scheme for the Br atom that is compatible with the predicted shape. It turns out that only sp3d hybridization will provide the necessary trigonal bipyramidal distribution of electron pairs around the bromine atom. In this scheme, one of the sp3d hybrid orbitals is filled while the remaining four are half-occupied. 

Boron trifluoride etherate, co-catalyst, 53, 30, 32 a-Bromination, selective of aralkyl ketone, 53, 111 Bromine, with 3-chlorocyclo-butanecarboxylic acid and mercuric oxide to give 1-bromo-3-chlorocyclobutane, 51, 106... 

J. M. East and A. G. Lee, Lipid selectivity of the calcium and magnesium ion dependent adenosinetriphosphatase, studied with fluorescence quenching by a brominated phospholipid, Biochemistry 21, 4144-4151 (1982). 

In polyfunctional molecules, the elec-trophore with the least negative reduction potential is selectively cleaved [164]. A bromine atom at a carbon atom a to a carbonyl group is fairly easily reducible therefore, cpe at the potential in which this C— Br bond is reduced proceeds highly chemoselectively (Fig. 35) [164]. 

Under carefully controlled conditions the selective mono bromination of an electronically diverse range of acetophenone derivatives can be realized using the polymer-supported pyridinium perbromide reagent. The resultant a-bromoketone... 

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