X+ transfer from the halonium ions of adamantylideneadamantane to acceptor olefins. The possibility of chiral induction in the transfer process

Article abstract of DOI:10.1139/v97-617

The bromonium ion of adamantylideneadamantane (Ad=Ad-Br⁺) has been used to induce the bromocyclization of a 4-pentenyl glycoside (10) and a 5-hexenyl glycoside (11) in dichloroethane. The kinetics of these processes have been studied at 25°C in the presence of varying [Ad=Ad] and, in the case of the transfer to 10, in the presence of pentanol. The second-order rate constants for bromocyclization of these two alkenes are (1.04 ± 0.06) × 10^-1 M^-1 s^-1 and (5.34 ± 0.2) × 10^-2 M^-1 s^-1, respectively, and in no case does added Ad=Ad or pentanol alter the reaction rate. The kinetic behavior is interpreted in terms of cyclization occurring directly from a 1:1 complex of Ad=Ad-Br⁺ and 10 or 11. The chiral induction for the bromocyclization of 10 promoted by AdAd-Br⁺ was measured at 20% e.e., the (-)-(S)-tetrahydrofurfuryl bromide being the dominant stereoisomer. Ad=Ad molecules substituted at one of the homoallylic carbons by an axial methyl group (12), or by two methyl groups (axial and equatorial), were synthesized and the ¹H NMR spectra of their bromonium ions is given. These materials are not stable for prolonged times at room temperature. A limited kinetic study of the reaction of 12-Br⁺ and 4-pentenol indicated that the Br⁺ transfer is 500 times faster than the comparable transfer from Ad=Ad-Br⁺ to 4-pentenol. The possibility of using these materials to induce chiral bromocyclization is discussed.