Norbomenes bromination

The addition of bromine fluoride to cyclohexene occurs highly stereospecifically to yield the fran.v-adduct. Further examples of the bromofluorination of cyclic alkenes with various reagents are reported in Table 21, and also given by the reactions of norbornene (2), 70,21 5 228 benzo-norbomene (4).2 2 8 and norbornadiene (5).112-215... 

To elucidate the steric factors in halogen addition Caple studied the bromination of 7-anti-bromobenzonorbornadiene >. The bulky substituent at C favours an endo attack on C and C the torsional factors in this system are smaller than in norbomene. Nevertheless the bromine was attached at an exo-position, i.e. irrespective of the steric factors. These results exclude a rearrangement since this would have resulted in a product of the same structure. For the stereoselectivity of the nucleophilic exo attack a nonclassical ion or a rapid equilibration of classical ions is necessary the reason for the exo attack of the electrophile is not quite clear. 

The failure of bromination under ionic conditions at room temperature contrasts markedly with the occurrence of smooth ionic bromination of norbomene itself at — 78 °C and may be caused by a through space (field) —/effect. ... 

The sequential one-pot synthesis of the same graft copolymer also proved successful, but required the optimisation of some experimental parameters such as the reaction time for both the copolymerisation of norbomene and its brominated derivative, and the conversion of the copolymer growing chains (RuCl2[=CH-poly(NB-co-NB-Br)](PPh3)2) into the methylene-endcapped poly(NB-co-NB-Br) and the free RuCl2(=CHOEt)(PPh3)2 complex. 

Azides are traditionally hard to incorporate into monomers for ROMP because of their adverse interactions with the catalyst. However, postfunctionalization of an alkyl bromide may be used. As an example, a norbomene bearing a dendrimer capped with an alkyl bromine was successfully used to form diblocks, which upon polymerization was substituted for the azide. Click chemistry with Cgo formed dendritic side chains bearing up to three Cso units though only half of the potential sites were found to be reactive this was attributed to steric bulk of the Ceo groups as similar chemistry with zinc porphyrins went to completion (Figure 28). 

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