Bromination of Stilbenes

Table 7 Product Distribution in the Bromination of Stilbenes in 1,2-Dichloroethane at 25°C. ...

The three-pathway bromination of stilbenes can interestingly be compared with the dehydration of 1,2-diarylethanols (Noyce et al, 1968), which unambiguously takes place through two - and 0-aryl carbocations. The ratios of the two reaction constants, p /pp, are very similar (Table 14), despite large differences in solvents and in the nature of the encounter complexes formed in the step preceeding the ionization. 

Bromonium ion is generated by reactions of both erythro (38) and threo 2-bromo-1,2-diphenylethane with hydrogen bromide95 (in 1,2-dichloroethane and in chloroform) as illustrated by Scheme 15. Stilbene (39) and the 1,2-dibromo derivative are the main reaction products. Product distribution and kinetic investigations on the bromination of stilbenes clearly confirm the reversibility of the bromonium/bromide formation in bromination of olefins, through an olefin/bromine CT complex. 

Draw the mechanism that shows how the bromination of -stilbene produces m o-stilbene dibromide. 

Since elemental bromine (Br2) is volatile and highly corrosive, special systems, for example, pyridinium tribromide and related compound, are commonly used to generate Br2 in situ [2]. A method for stereoselective bromination of stilbene and chalcone in a water suspension medium using pyridinium tribromide has been reported [2]. Bromination reactions of ( )-stilbene and ( )-chalcone in a water suspension medium by salts... 

A catalytic oxidative bromination of arenes, alkenes, and alkynes in aqueous media was achieved by using NH4VO3 catalyst combined with H2O2, HBr, and KBr [14]. Dodecyltrimethylammonium bromide was found to serve as an efficient surfactant to facilitate the NH4V03-catalyzed bromination in aqueous media. A scheme of the oxidative bromination of stilbenes in water in the presence of an additive is shown in Figure 2.2. 

Figure 2.2 Oxidative bromination of stilbenes in water in the presence of an additive [14]. (Reproduced with permission from Elsevier.)...

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