Br+ and I+ Transfer from the Halonium Ions of Adamantylideneadamantane to Acceptor Olefins. Halocyclization of 1,ω-Alkenols and Alkenoic Acids Proceeds via Reversibly Formed Intermediates

The kinetics of the transfer of X + from the bromonium and iodonium ions of adamantylideneadamantane ( 1 -Br + and 1 -I + ) to some 1,ω-alkenols and alkenoic acids in ClCH 2 CH 2 Cl at 25 °C was investigated. In all cases, the expected products of halocyclization were observed. For the iodonium ion transfer the reaction kinetics are second order overall, first order in both 1 -I + and acceptor olefin. Transfer of the bromonium ion from 1 -Br + to these acceptor olefins exhibits different kinetic characteristics. In most cases, the rate of the Br + transfer is subject to strong retardation in the presence of added parent olefin (Ad Ad), suggestive of a common species rate depression. In some cases, such as 4-penten-1-ol ( 2b ) and 4-pentenoic acid ( 4b ), the reaction can be completely suppressed at high [Ad Ad]. In other cases, such as 3-buten-1-ol ( 2a ), 5-hexen-1-ol ( 2c ), cyclohexene, 4-(hydroxymethyl)cyclohexene ( 3 ), and 5- endo -carboxynorbornene ( 5 ), added Ad Ad does not suppress the reaction completely. In the cases of the 1,ω-alkenols, the reactions appear to exhibit kinetic terms that are greater than first order in alkenol. In these cases, alcohols such as 1-pentanol also accelerate the reaction, pointing to the involvement of the hydroxyl group of the second alkenol as a catalytic species. A unifying mechanism consistent with the data that involves two reversibly formed intermediates is presented.