Investigation of the Early Steps in Electrophilic Bromination through the Study of the Reaction with Sterically Encumbered Olefins

The electrophilic bromination of an alkene is probably the quintessential reaction of the double bond and is portrayed in undergraduate textbooks as being a wellunderstood process adhering to the generalized mechanism depicted in Scheme 1.1,2 For alkenes that do not form a highly stabilized â-bromocarbocation, the reaction invariably proceeds with the initial instantaneous formation of an olefin-Br2 charge transfer complex3 (CTC) which has been shown to be on the reaction pathway.4,5 Central to any rationalization of the stereochemical and structure/reactivity characteristics of this deceptively simple reaction is the postulate of the three-membered bromonium ion intermediate (1) first proposed by Roberts and Kimball in 1937 to account for the trans mode of addition.6 Generally, these species are too short lived to be detected during the electrophilic addition reaction, and although widely accepted as intermediates, until the late 1960s, virtually no information about their structures or physicochemical properties was available. Olah and co-workers subsequently demonstrated that the these species can be generated under stable ion conditions (SbF5, SO2) and their NMR spectra determined.7 A major breakthrough in the field occurred in 196970 when Wynberg and co-workers reported8 that adamantylideneadamantane (AddAd, 2), when treated with Br2 in chlorinated hydrocarbons, produced a yellow precipitate, then formulated as the world’s first isolable three-membered bromonium/Br3 salt. Subsequent studies9 led to the opinion that several atypical properties of the precipitate, such as the ability to release Br2 and reform AddAd, were more consistent with the behavior of a molecular π-complex, (2-(Br2)2). The identity of the yellow precipitate finally was resolved in 1985 when we reported the crystal structure of 2-Br/Br3.