Mechanistic evaluation of the transfer of Br+ from bis(sym-collidine)bromonium triflate to acceptor alkenes

The kinetics of the reaction of bis(sym-collidine)bromonium triflate (2-Br+/OTf-) with adamantylideneadamantane (Ad=Ad), 4-penten-1-ol (4), and cyclohexene has been investigated in 1,2-dichlorethane at 25 degrees C under a variety of conditions. The rates of all the reactions are shown to be depressed by added collidine, indicating that the first step for all is a reversible dissociation of 2-Br+/OTf- into free collidine and a reactive intermediate, coll-Br+, which is then captured by the alkene. The product of the reaction of Ad=Ad with 2-Br+/OTf- is an Ad=Ad:Br+-coll complex, while that of reaction of 4 with 2-Br+/OTf- is the cyclic ether 2-bromomethyltetrahydrofuran. The reaction with cyclohexene is more complex and involves at least two reversibly formed intermediates, suggested to be coll-Br+ and cyclohexene:Br+-coll, the latter being captured by attack of triflate to give trans-1-bromo-2-trifluoromethanesulfonylcyclohexane. Detailed kinetic analysis shows that the reactions of collidine, Ad=Ad, cyclohexene, and 4-penten-1-ol with the reactive intermediate coll-Br+ are fast but not very sensitive to the nature of the nucleophile, the second-order rate constants being 3 x 10(6), 1.1 x 10(6), 1.5 x 10(5), and 4.5 x 10(4) M-1 s(-1), respectively. H-1 NMR analysis of the reaction of trans-1-bromo-2(trifluoromethanesulfonyl)cyclohexane, produced in situ from cyclohexene and 2-Br+/OTf- in CD2Cl2, with tetrabutylammonium bromide or tetrabutylammonium acetate indicates a very rapid and quantitative production of trans-1,2-dibromocyclohexane and trans-1-bromo-2-acetoxycyclohexane, respectively.