Bromination shape-selective

Shape-selective bromination of either linear alkenes or branched and cyclic alkenes in the presence of zeolites has been reported.246 In the so-called... 

In one final example from the work carried out in our laboratories, it is interesting to consider the possibility of shape selection between chemically similar functional groups. This would be especially useful when catalytic sites within the pores of a zeolite were essential for reaction, but the selectivity could still be demonstrated even on uncatalysed reactions. We therefore considered the addition of bromine to alkenes. 

Zeolites can facilitate the shape-selective bromination of olefins. When a mixture of cyclohexene and oct-2-ene are reacted with bromine the two alkenes are brominated to a similar extent. However, when the zeolite catalyst silicalite-1 is present the reaction becomes selective [143]. This selectivity depends upon the order in which the reactants are introduced to the catalyst. If the alkene mixture is stirred with the zeolite prior to the addition of bromine then the straight chain octene enters the zeolite pores. The more bulky cyclohexene remains in solution and is halogenated in preference to the octene when the bromine is introduced. If the bromine is pre-absorbed on the zeolite before the alkene mixture is added then the selectivity of the process is reversed [144]. 

Next we must select a hybridization scheme for the Br atom that is compatible with the predicted shape. It turns out that only sp3d hybridization will provide the necessary trigonal bipyramidal distribution of electron pairs around the bromine atom. In this scheme, one of the sp3d hybrid orbitals is filled while the remaining four are half-occupied. 

Radical bromination of the V-shaped diaryl 52 gave the single exo-bis-bromo derivative 53. The explanation for the high selectivity was attributed to the requirement of the formed radical orbital to be delocalized in the aromatic plane, thereby acting as a radical conformational anchor. ... 

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