Bromination electrophilic aromatic

The X-ray structure of the dibromine complex with toluene (measured at 123 K) is more complicated, and shows multiple crystallographically independent donor/acceptor moieties [68]. Most important, however, is the fact that in all cases the acceptor shows an over-the-rim location that is similar to that in the benzene complex. In both systems, the acceptor is shifted by 1.4 A from the main symmetry axis, the shortest Br C distances of 3.1 A being significantly less than the sum of the van der Waals radii of 3.55 A [20]. Furthermore, the calculated hapticity in the benzene/Br2 complex (x] = 1.52) is midway between the over-atom (rj = 1.0) and over-bond (rj = 2.0) coordination. In the toluene complex, the latter varies from rj = 1.70 to 1.86 (in four non-equivalent coordination modes) and thus lies closer to the over-bond coordination model. Moreover, the over-bond bromine is remarkably shifted toward the ortho- and para-carbons that correspond to the positions of highest electron density (and lead to the transition states for electrophilic aromatic bromination [12]). Such an experimental location of bromine is in good agreement with the results of high level theoretical... 

Brominated PPO. The electrophilic aromatic bromination of the PPO was carried out in chloroform solution to yield polymers with different degrees of substitution. For the highly brominated structures the TG expanded over a considerable temperature range. A maximum value of 273 C was obtained for 100 mol % brominated PPO. 

Organolithium reagent 35 was added to aldehyde 31 (Scheme 7.6) to obtain alcohol 36 as an inconsequential 1 1 mixture of diastereomers. The benzylic alcohol was removed using a Barton two-step radical deoxygenation protocol, followed by electrophilic aromatic bromination to provide the desired coupling partner 37. 

Exercise 22-13 Arrange the following bromine-containing species in order of their expected reactivity in achieving electrophilic aromatic bromination HOBr, Br2, Br , Br9, HBr, H2OBr , BrCI. 

It is of interest to observe that the kinetic behaviour (and the effect of changes of temperature on the rate of reactions) of brominations of olefins and of electrophilic aromatic brominations confirms the presence of pre-associative processes77,130,131 on the reaction pathway, as well as that observed for some nucleophilic aromatic substitutions132. 

Click Mechanisms in Motion to view Electrophilic Aromatic Bromination. 

Aromatic bromides (3, 286). The definitive paper on electrophilic aromatic bromination with bromine and thallium(lll) acetate has been published. The two most oul.standingfeaturc.s are I) monobromination is observed in almost all cases, and 2) exclusive para substitution is observed with almost all monosubstituted benzenes. Electron-withdrawing groups inhibit bromination of monosubstituted benzenes. It... 

An excellent alternative to the classical Hunsdiecker reaction and its variants, which totally avoids the use of heavy metal salts and potent electrophilic reagents, consists of the simple photolysis or thermolysis of Barton esters in refluxing bromotri-chloromethane for the bromides or tetrachloromethane for the chlorides [4], The analogous decarboxylative iodination can also be achieved using iodoform as the reagent in a benzene/cyclohexene solvent system (Scheme 5). For the cases of vinylic and aromatic acids, where the usual problems of chain efficiency are encountered, the addition of azobisisobutyronitrile (AIBN) is also required [10]. Nevertheless, since this method can operate on both electron-rich and electron-poor aromatic systems, and moreover does not suffer from the competitive electrophilic aromatic bromination found with electron rich aromatics under normal Hunsdiecker conditions, this route to synthetically useful aryl iodides and bromides should find widespread application. 

P G. Dumanski, C. J. Easton, S. F. Lincoln, J. S. Simpson, Effect of cyclodextrins on electrophilic aromatic bromination in aqueous solution, Aust. J. Chem., 2003, 56, 1107-1111. 

The Lewis acids typically used are aluminum chloride (AICI3) and iron chloride (FeCIa) for chlorination, and iron bromide (FeBts) for bromination. The purpose of the Lewis acid is to make the halogen a stronger electrophile. A mechanism for electrophilic aromatic bromination is shown here. 

Electrophilic Aromatic Bromination 673 Nitration of Benzene 675 Sulfonation of Benzene 676 Friedel-Crafts Alkylation 677 Friedel-Crafts Acylation 680 Benzylic Halogenation 701 Birch Reduction 710... 

Thallation of thiophens in the 2-position with thallium(m) trifluoro-acetate in trifluoroacetic acid is complete within a few minutes at room temperature. The thallium derivative reacts in situ with aqueous potassium iodide solution to give a convenient and high-yield synthesis of iodo-thiophens. A mixture of thallium(m) acetate has been shown to be a mild and efficient reagent for electrophilic aromatic bromination. Thiophen yields 2-bromothiophen in 82% yield and very little dibromothiophen. 3-Methylthiophen appears to be selectively brominated in the 2-position and 2-methylthiophen in the 5-position in 70—75% yield. The direct thiocyanation of thiophen and some alkylthiophens with thiocyanogen under various conditions using a variety of Friedel-Crafts catalysts has... 

Purpose To evaluate the relative reactivities of different arenes in electrophilic aromatic bromination. 

Divide all values of k by the smallest value of this rate constant to obtain the relative reactivities of the substrate toward electrophilic aromatic bromination. 

Why is the first step in an electrophilic aromatic bromination reaction so much more endothermic than the first step in an electrophilic addition to an alkene ... 

Sodium bisulfite is used to discharge the yellow color produced in the electrophilic aromatic bromination of acetanilide. 

Compared to Br2/HOAc, Br2/FeBrj is a more powerful combination of reagents for effecting electrophilic aromatic bromination. What difficulty might arise if this combination were used for brominating acetanilide ... 

The high reactivity of phenols toward electrophilic aromatic bromination is discussed in Section 25.12A. This reaction can be used to prepare bromophenols, which may serve as solid derivatives. 

When the cyclobutenone 33 was treated with a rhodium catalyst, the C(arene) -C(carbonyl) bond was site-selectively cleaved and the alkene moiety was intramolecularly inserted therein to construct the tetracyclic skeleton of 34. The isopropyl group was introduced by electrophilic aromatic bromination. 

Armed with the kuowledge of the structure of A, we can now formulate the possible products of its electrophilic aromatic bromination ... 

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