Selectivity bromination

N Bromosuccimmide will be seen again as a reagent for selective bromination in Sec tion 11 12... 

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). 

We present here examples of this condensation with an aromatic aldehyde and a cyclic ketone. Both of these examples are useful because, although other methods are available for their preparation, problems often attend these syntheses. In the synthesis of cyclohexy11deneaceton1tr11e, for example, the standard method results exclusively In the g.y-lsomer and none of the a,g-Isomer. In Part A of this procedure, cyclohexanone Is condensed with acetonitrile to give predominantly the conjugated Isomer (80-83%) whicfi is then separated from the nonconjugated isomer by selective bromination. 

New copolymers based on a copolymerization of isobutylene and p-methyl-styrene with improved heat resistance have been reported [64]. Once copolymerization was accomplished, the polymer was selectively brominated in the p-methyl position to yield a terpolymer called EXXPO. In contrast to butyl and halobutyl, the new terpolymer has no unsaturation in the backbone and therefore shows enhanced thermal stability and resistance to oxidation. Useful solvent-based adhesives can be formulated using the new terpolymer in combination with block copolymers [65]. The hydrocarbon nature of the new terpolymer results in excellent compatibility with hydrocarbon resins and oils. 

This method of bromination has been employed in the selective bromination (777) of the ketone (167). While direct bromination results in bromination not only in the position alpha to the ketone but also in the aromatic ring, bromination of the enamine (168) and subsequent hydrolysis gave only the monobrominated product (169). 

The selective bromination of a ketone in the presence of another susceptible functional group was achieved in a diterpene synthesis 240). A competing bromination of an anisole ring could be avoided here through the use of a pyrrolidine enamine derivative for activation of the methylene group adjacent to the carbonyl function. 

In contrast to porphyrin which is brominated in the meso positions, unsubstituted porphycene (1) is selectively brominated in the 2-position.26... 

Selective bromination of pyrido[2,3-c][l,2-6]thiadiazine 2,2-dioxides (206) gave 6-bromo products in 71-85% yields (91SC827). 

In this article we describe selective bromination of various phenols under mild conditions and discuss their reaction mechanisms. 

Calo et al. (ref. 5) studied solvent effects on selective bromination of phenol with NBS and found the selectivity of bromination depended on the polarity of the solvents. But thereafter no investigation concerning the solvent effects was reported. We report the effects systematically. 

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). 

Fig. 10. Selective bromination of aromatic ethers with BTMA Bt3...

SELECTIVE BROMINATION OF AROMATIC SUBSTRATES Selective Bromination of Toluene... 

Fig. 6. Effect of diethyl ether on the bromination of toluene Selective Bromination of Naphtalene...

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. 

The direct bromination of aniline and phenol derivatives in solution results in polybromination to give a complex mixture. But bromination by gas-solid and solid-solid reactions proceeds more efficiently and selectively. Bromination by gas-solid reaction can be accomplished through a very simple procedure. For example, the powdered crystalline aniline 13 and Br2 were placed in... 

The synthesis of both enantiomers of vasicinone has been carried out using almost entirely polymer-supported reagents. The route was based on functionalisation of deoxyvasicinone by a highly selective bromination then via enantioselective reduction of the derived ketone <06SL2609>. 

Kamikawa et al. chose the first option to generate the benthocyanin skeleton [91]. To begin with, 100 and aniline 126 are transformed into o-nitrodi-phenylamine 127 by intermolecular N-arylation. Reduction of the nitro group and selective bromination produces 128, and this time an intramolecular Buch-wald-Hartwig reaction is used to derive a mixture of the desired phenazine 129 and the elimination product 130. The fundamental problem with this approach relates to the selective introduction of the halogen substituent that is required for the intramolecular N-arylation. 

Relatively few bisindole derivatives having unnatural aromatic substituents have been prepared. The most reactive aromatic center to electrophilic substitution is certainly at C-12, where steric hindrance is minimized and electron density favors stabilization of positive charge. Treatment of vinblastine (1) with less than 1.0 equiv of bromine in dichlo-romethane results in selective bromination at C-12 to give 12 -bromovin-blastine (5) (45,46). If excess bromine is employed, then bromination in the dihydroindole ring is also observed, and mixtures of (5) and 12, 17-dibromovinblastine (6) are obtained (46). 

Thin-Layer Solvent Systems for Selected Brominated Organics... 

Tetrabromocyclohexadienone Selective bromination of polyolefins and cyclization induced by Br+... 

Benzyltrimethylammonium tribromideb Selective bromination of alkenes and carbonyl compounds... 

Selective bromination of monobromo-substituted dithienyl sulfides with iV-bromosuccinimide afforded two more dithienothiophene isomers. From the reaction of 3-thienylIithium with disulfide 182 at —70°, 4-bromo-3,3 -dithienyl sulfide (185) was isolated. Bromination with 7 T-bromosuccinimide provided 2,4 -dibromo-3,3 -dithienyl sulfide (186) dilithiation of the latter followed by oxidative ring closure gave dithienothiophene 9 in 13% overall yield [Eq. (54)]. 

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