Reactions of cyclohexenyl iodonium tetrafluoroborate with bromide ion:: Retardation due to the formation of λ3-bromoiodane

[GRAPHICS] The reaction of 4-tert-butylcyclohex-1-enyl(phenyl)iodonium tetrafluoroborate, (1a) and the 4-chlorophenyl derivative (1b) with bromide ion was examined in methanol, acetonitrile, and chloroform. Products include those derived from the intermediate cyclohexenyl cation as well as 1-bromocyclohexene. Kinetic measurements show that the reaction of I is strongly retarded by the added bromide. The curved dependence of the observed rate constant on the bromide concentration is typical of a pre-equilibrium formation of the intermediate adduct with a fast bromide-independent reaction (solvolysis of the iodonium ion). The formation of the adduct, lambda(3)-bromoiodane, was also confirmed by the UV spectral change. The relative reactivity of the iodonium ion and lambda(3)-bromoiodane is evaluated to be on the order of 10(2). The bromide substitution product forms both via the S(N)1 reaction of the free iodonium ion and via the ligand coupling of the iodane.