Frans-Stilbene, bromination

Table 9 Solvent-dependence of the stereochemistry of dibromides from frans-stilbene bromination." cis- and...

The claims of exclusive formation of rac-stilbene dichloride upon gas-solid addition of chlorine to frans-stilbene (103) [71] and of meso-stilbene dibromide in the gas-solid addition of bromine to trans- or czs-stilbene [54] could not be verified. Scheme 12 shows the results of more detailed studies indicating the mesolrac ratios on the solid-state chlorination and bromination of trflns-stilbene (103) and some variations when the crystal size was changed [58, 60-61]. There is a risk of partial transient liquefaction if the chlorine is added too rapidly, due to initially heavy reaction. But even at the start with a stoichiometric amount of chlorine at 0.1 bar and 0 °C, a persistent product layer forms on the unground crystal powder of 103 that cannot be disintegrated by the ultrasound of a cleaning bath at 20 °C for 60 h (only 7% conversion with mesolrac ratio of 11 89 under these conditions) [22]. It is therefore unavoidable to mill the crystals of 103 to sizes <1 pm in order to overcome these rare diffi-... 

It was snggested that halogenation-dehalogenation with diaryltellurium derivatives is an eqnilibrinm process" since, at identical concentrations, debromination of erythro-l, 2-dibromo-l,2-diphenylethane with diaryltellurides and bromination of fran -stilbene with dibromodiaryltellurides gives identical product mixtures. 

Most studies of bromine addition to alkenes have presumed that the intermediate proceeds to product and does not revert to alkene and bromine. Brown and co-workers determined that bromonium ions generated from the solvolysis of the fra s-2-bromo-l-brosylates of wclohexene or cyclopentene could react with added Br to produce Br2- Furthermore, erythro-2-hTomo-l, 2-diphenylethanol was foimd to react with anhydrous HBr (in 1,2-dichloroethane or chloroform) to produce both frans-stilbene and meso-l,2-dibromo-l,2-diphenylethane. The reaction of the erythro diastereomer can be explained by a mechanism involving anchimeric assistance in departure of water, which leads to a bromonium ion that reverts to the stilbene, as shown in Figure 9.10. ... 

The bromination of alkenes is known to be a stereospedfic reaction. An example and detailed discussion of this addition to frans-stilbene is given in Experiment [A2b], and another example is illustrated in Experiment [F2], In the present case, bromine undergoes a similar addition, and the halogenated product obtained is the erythro diastereomer of 2,3-dibromo-3-phenylpropanoic add, which is produced as a racemic mixture of the two enantiomers. 

Experiments have shown that the bromination of cis- and frans-phenylpropenes in CC14 is non-selective.134 For stilbene, trims addition occurs in solvents having low dielectric constants, but the reaction loses stereoselectivity if e rises above 35.135... 

The result of the halogenation of frans-cinnamic acid, as in the case of (E)-stilbene in Experiment [A2b], is an anti addition of molecular bromine. The enantiomeric products are shown here ... 

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