Ethylenehalonium ions

Ethylenehalonium Ions. The parent ethylenehalonium ions 158 were obtained when 1-halo-2-fluoroethanes were ionized in SbFs-S02 solution at — 60°C386 [Eq. (4.114)]. The 1H NMR spectra of the ethylenebromonium and -iodonium ions show a singlet at 8 H 5.53 and 5.77, respectively. Under similar experimental conditions, when 1,2-dichloroethane and 1-chloro-2-fluoroethane were treated with SbF5-S02 solution, only donor-acceptor complexes 159 and 160 were formed instead of the ethylenechloronium ion. 

Halonium ions are another group of intermediates that have been characterized by spectral methods. Peterson et al. have shown that five- and six-membered cyclic halonium ions are reasonably stable species capable of being isolated, stored, and subjected to reaction with a variety of nucleophiles. Three-membered cyclic halonium ions (ethylenehalonium ions) are not so stable, yet because of their long-postulated involvement in electrophilic addition reactions with alkenes and in the nucleophilic displacement of j8-substituted leaving groups, they have attracted a significant amount... 

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