Intramolecular nucleophilic interception by the sulfonyl group of reaction intermediates arising from electrophilic addition to unsaturated non-conjugated bicyclic sulfones

Electrophilic addition of molecular bromine and related reactions to unsaturated bicyclic sulfones resulting in atypical addition is described. Bromination of the bicyclic sulfone 3 results in a ca. 1 : 1 mixture of exo,exo- and exo,endo-dibromides 4 and 5, respectively. Similarly, acid-catalysed cleavage of epoxide 8 produces a 37 : 63 mixture of exo,exo- and exo,endo-bromohydrins 9 and 10, respectively. In both cases, formation of exo,exo-products is explained in terms of a favourable stabilising effect, which can be quenched by the presence of water in the reaction media, between the polar SO2 group and the incipient carbocation centre as depicted in 6. Also described is formal nucleophilic neighbouring group participation by the endo-sited sulfonyl group in norbornylene systems 12 which leads exclusively to the exo,exo-dibromides 13 upon ionic bromination. In the case of unsaturated sulfone 27, bromination to form the exo,exo-dibromide 30 occurs via. decomposition of an isolable, albeit metastable, tribromide salt 29. Owing to a similar effect, treatment of the related exo-epoxide 16 with hydrogen bromide in acetic acid results in the regio- and stereo-specific formation of exo-bromohydrin 17 as the sole product.