Arenes brominations

Dimethyl tartrate, 124 of arenes Bromine, 170 of other substrates N-Bromosuccinimide, 49 Dimethyl tartrate, 124 Tetrabutylammonium tribromide, 287... 

Brominations. Hydrochloric acid is a catalyst for arene bromination in acetone. 

A -Bromination is almost certainly the initial step in a process which results in the overall arene bromination of some macrocyclic complexes of the arylamino type (e.g. equation 2). In this reaction the arylamino nitrogen atoms still bear their pairs of non-bonded electrons and are capable of nucleophilic behaviour. Rearrangement of the Af-bromo compound to the p-bromo isomer would then yield the observed product. 

It is well documented [95] that direct halogenation of arenes, bromination in particular, is one of the most selective electrophilic reactions yielding almost exclusively para-substituted products. Evidently, however, the use of cyclometalated compounds might drastically change the selectivity in favor of ort/io-halogenated compounds, as shown in Eq. (7.43) [76, 96]. This halogenation is also widely applied as a regioselective reaction method, as shown below in Eq. (7.44) [97]. 

There has been a summary of computational and experimental studies of the use of palladium complexes with A -heterocyclic carbenes (NHCs) in the asymmetric coupling of -hybridized carbon-hydrogen bonds with aryl halides. It has been shown that the electronic and catalytic properties of NHCs fused to porphyrins may be modified by varying the inner metal in the porphyrin. A DPT study of the use of palladium-NHC complexes in the asymmetric intramolecular a-arylation of 2-bromoaryl amides to give 3,3-disubstituted oxindoles (101) has been reported. The likely pathway involves insertion of the palladium into the arene-bromine bond to form a palladacycle which deprotonates to give an (9-enolate. Conversion into the C-enolate followed by reductive elimination gives the product. The intramolecular reaction of 0 a cyclopropane carbon-hydrogen bond in a 2-bromoanilide derivative has been used to form cyclopropyloxindoles, (102), in a palladium-catalysed, silver-mediated reaction. 

Halogenation (Section 12 5) Chlorination and bromination of arenes are carried out by treatment with the appropriate halogen in the presence of a Lewis acid catalyst Very reactive arenes undergo halogenation in the absence of a catalyst... 

Chlorination is cariied out in a manner similar- to bromination and provides a ready route to chlorobenzene and related ar-yl chlorides. Fluorination and iodination of benzene and other arenes are rarely perfor-med. Fluorine is so reactive that its reaction with benzene is difficult to control. Iodination is very slow and has an unfavorable equilibrium constant. Syntheses of aryl fluorides and aryl iodides are nor-mally cariied out by way of functional group transformations of arylffluines these reactions will be described in Chapter 22. 

Arenes are unsaturated but, unlike the alkenes, they are not very reactive. Whereas alkenes commonly take part in addition reactions, arenes undergo predominantly substitution reactions, with the TT-bonds of the ring left intact. For example, bromine immediately adds to a double bond of an alkene but reacts with benzene only in the presence of a catalyst—typically, iron(III) bromide—and it does not affect the bonding in the ring. Instead, one of the bromine atoms replaces a hydrogen atom to give bromobenzene, C H Br ... 

The bromination of benzene illustrates the difference between addition to alkenes and substitution of arenes. First, to achieve the bromination of benzene it is necessary to use a catalyst, such as iron(III) bromide. The catalyst acts as a Lewis acid, binding to the bromine molecule (a Lewis base) and ensuring that the outer bromine atom has a pronounced partial positive charge ... 

Our recent studies on effective bromination and oxidation using benzyltrimethylammonium tribromide (BTMA Br3), stable solid, are described. Those involve electrophilic bromination of aromatic compounds such as phenols, aromatic amines, aromatic ethers, acetanilides, arenes, and thiophene, a-bromination of arenes and acetophenones, and also bromo-addition to alkenes by the use of BTMA Br3. Furthermore, oxidation of alcohols, ethers, 1,4-benzenediols, hindered phenols, primary amines, hydrazo compounds, sulfides, and thiols, haloform reaction of methylketones, N-bromination of amides, Hofmann degradation of amides, and preparation of acylureas and carbamates by the use of BTMA Br3 are also presented. 

Fig. 12. Bromination of acetanilides with BTMA Br3 Bromination of arenes...

Combined effect of BTMA Br3 and ZnCl2 in acetic acid provides a new excellent bromination procedure for arenes. That is, while such reactive aromatic compounds as phenols, aromatic amines, aromatic ethers, and acetanilides have been easily brominated by BTMA Br3 in dichloromethane in the presence of methanol, the reaction of arenes, less reactive compounds, with BTMA Br3 in dichloromethane-methanol did not proceed at all, even under reflux for many hours. However, arenes could be smoothly brominated by use of this agent in acetic acid with the aid of the Lewis acid ZnCl2 (Fig. 13) (ref. 16). 

Fig. 13. Bromination of arenes with BTMA Bt3 Bromination of thiophene derivatives...

SIDE-CHAIN BROMINATION OF AROMATIC COMPOUNDS Benzylic bromination of arenes... 

Usually, a-bromo-substituted arenes have been prepared through the reaction of arenes with bromine under ultraviolet irradiation. In the presence of benzoyl peroxide, N-bromosuccinimide can also be used for this purpose. 

Avramoff et al. have already reported that the reaction of hydrocarbons such as toluene with tetramethylammonium tribromide (TMA Br3) in benzene, in the presence of benzoyl peroxide at room temperature gave benzylic bromination products (ref. 21). However, TMA Br3 is not easy to handle in comparison with the stable BTMA Br3 because of its hydroscopic character. Furthermore, as shown in their literature, a large excess of TMA Br3 is necessary to brominate arenes. 

The primary step is the absorption of a quantum, which results in an excitation of the Ti-orbital. This singlet orbital can undergo intersystem crossing with spin change to a triplet stage ((t, (t), which has been found in brominated arenes (ref. 29). The photolytic reaction in methanol is much slower compared to hexane (ref. 27). This is shown for 1,2,3,4-TBDD, for which the decay constants ha/e been determined in both solvents ... 

PART 1 HALOGENATED ARENES AND CARBOXYLATES WITH CHLORINE, BROMINE, OR IODINE SUBSTITUENTS... 

The only reported X-ray structure of a it-bonded diiodine exists in the 12/coronene associate [75], which shows the I2 to be located symmetrically between the aromatic planes and to form infinite donor/acceptor chains. -Coordination of diiodine over the outer ring in this associate is similar to that observed in the bromine/arene complexes (vide supra), and the I - C separation of 3.20 A is also significantly contracted relative to the stun of their van der Waals radii [75]. For the highly reactive dichlorine, only X-ray structures of its associates are observed with the n-type coordination to oxygen of 1,4-dioxane [76], and to the chlorinated fullerene [77]. 

Additional publications from Sanford et al. describe the full exploration of palladium-catalyzed chelate-directed chlorination, bromination, and iodination of arenes using N-halosuccinimides as the terminal oxidant <06T11483>. Moreover, an electrophilic fluorination of dihalopyridine-4-carboxaldehydes was reported by Shin et al. <06JFC755>. This was accomplished via transmetalation of the bromo derivative, followed by treatment with A-fluorobenzenesulfinimide as the source of electrophilic fluorine. 

The remarkable generality of FERET to predict the behavior of Br2 in aromatic bromination as well as in olefin addition is established by the single linear correlation in Fig. 21b when the solvation difference (see equation (103)) between the cation radicals of arene and olefin donors is taken into account.263... 

The dehalogenation of several types of polyhalogenated arenes has been studied by Hofs group [61-67]. Brominated thiophenes and bithiophenes were re-... 

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