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 Br(2) in chlorinated hydrocarbon solvents to give mixtures of olefin - Br(2) 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 CH(2) 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 Br(2) - olefin complexes for 1 and Ad = Ad confirm that the association energy is larger for the complex 1 - Br(2). 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 - Br(2), 643 vs 289 M(-1) further supports the primary role exerted by dispersion interactions in alkene-Br(2) pi-complexes.