Evidence for cyclic bromonium ion transfer in electrophilic bromination of alkenes: Reaction of ω-alkenyl glycosides with aqueous N-bromosuccinimide

Evidence is provided to support the theory that intermolecular Br+ transfer from a cyclic bromonium ion to an alkene occurs readily and can indeed overwhelm alternative reaction pathways. In the course of a study to determine which ω-alkenyl glycosides could serve as glycosyl donors, it was found that upon treatment with N-bromosuccinimide (NBS) in aqueous acetonitrile, under conditions in which an n-pentenyl glycoside underwent oxidative hydrolysis to the corresponding hemi-acetal, allyl, butenyl, and hexenyl analogs gave bromohydrin addition products. It was further found that when pentenyl and hexenyl analogs were made to compete for an insufficient amount of NBS, the former reacted while the latter was apparently recovered unchanged. However, both reacted independently at similar rates. In addition, the phenomenon was found to be concentration dependent. These results are consistent with the intermolecular, non-degenerate transfer of Br+ from cyclic bromonium ion to alkene.