| Abstract
| - trans-(1-Methyl-2-adamantylidene)-1-methyladamantane (DMAD, 1b) reacts with Br2 in chlorinatedhydrocarbon solvents to give either a bromonium polybromide ion pair or a substitution product,depending on bromine concentration. The first intermediate is a 1:1 π-complex having Kf = 1.85(0.19) × 103 M-1 at 25 °C, which rapidly evolves to the bromonium tribromide ion pair. At highbromine concentration, which shifts all equilibria involving the counteranion of the ion pairintermediate toward the pentabromide species, this bromonium ion is stable and unable to furtherevolve into products. Temperature-dependent NMR spectra indicate chemical exchange of Br+between the sides of the plane containing the two carbons of the bromonium ion. At very low bromineconcentration, no ionic intermediate is detected and the reaction rapidly yields a rearrangedsubstitution product, identified as 10. Under these conditions the disappearance of the π-complexfollows a first-order rate law, and the observed rate constant increases with increasing olefinconcentration, showing that product formation implies Br- as counteranion of the ionic intermediate,whose formation is a reversible process. A comparison of the results reported here for thebromination of 1b with those previously found for the parent olefin, adamantylideneadamantane(1a), shows that steric strain markedly affects the reactivity of the double bond.
|
