The first intermediates in the bromination of bicyclo[3.3.1]nonylidenebicyclo[3.3.1]nonane, combination of experiments and theoretical results.

Bicyclo[3.3.1]nonylidenebicyclo[3.3.1]nonane (1) and adamantylideneadamantane (Ad=Ad) are two caged olefins with closely related structures at the double bond. Both compounds react instantaneously with Br2 in chlorinated hydrocarbon solvents to give mixtures of olefin-Br2 aggregates identified as the 1:1 pi-complex and bromonium tribromide, bromonium pentabromide ion pairs. The stoichiometry, formation constants and the electronic spectra of all the species present at equilibrium (pi-complex and bromonium ions), obtained by addition of bromine to alkene 1, have been determined in 1,2-dichloroethane at 25 degrees C and compared with the values that characterize the corresponding aggregates arising from Ad=Ad. The absence of the two bridging CH2 groups in 1 significantly affects all the formation constants. Moreover, at variance with Ad=Ad, olefin 1 reacts with bromine to give, depending on reagent concentration, a substitution product. DFT (B3LYP) and ONIOM computations of 1:1 Br2-olefin complexes for 1 and Ad=Ad confirm that the association energy is larger for the complex 1-Br2. The higher stability of this species seems to be correlated to the greater IP of 1 with respect to Ad=Ad which is able to compensate the reduced polarizability. The experimental value of the formation constant found for the complex 1-Br2, 643 vs 289 M(-1) further supports the primary role exerted by dispersion interactions in alkene-Br2 pi-complexes.