Abstract
The transfer of Br+ from the bromonium ion of adamantylideneadamantane (1-Br+/CF3SO3−) to the reactive alkenes cyclohexene, ω-alken-1-ols, 3a–c, and 4-pentenoic acid, 4, in 1,2-dichloroethane at 25 °C has been investigated. 1H NMR analysis of the reactions of equimolar 1-Br+/CF3SO3− and olefin indicates the products to be the expected cyclic ones in the cases of 3a–c and 4, and trans-2-bromocyclohexyltrifluoromethane sulfonate in the case of cyclohexene, along with adamantylideneadamantane (Ad=Ad). Detailed kinetic analysis shows that the reaction rates are suppressed by the addition of free Ad=Ad, suggestive of a common species rate depression. In the case of certain of the cyclization reactions such as that of 4-penten-1-ol, 3a, the cyclization reaction is bimolecular in acceptor olefin. A primary deuterium kinetic isotope effect of 2.0 on the cyclization of 3a-OH relative to 3a-OD exists. The role of the second molecule appears to be that of a general base in assisting the cyclization of an intermediate ω-hydroxy cyclic bromonium ion. A general mechanistic scheme for Br+ transfer from 1-Br+ to the reactive alkenes is proposed.
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