An alternative route to electrophilic substitution .2. Aromatic alkylation in the ion neutral complexes formed upon addition of gaseous arenium ions to olefins

The joint application of mass spectrometric and radiolytic techniques has allowed demonstration of an alternative route to electrophilic aromatic substitution operative in the gas phase, where the Wheland intermediate is formed within the ion-neutral complex (INC) formed upon addition of an arenium ion to an olefin. Experiments involving several representative pairs of arenium ions XC(6)H(6)(+) (X = H, CH3, CF3) and olefins (C3H6, iC(4)H(8), cC(5)H(8)) have provided conclusive evidence for the operation of the above reaction sequence and hence for the occurrence of intracomplex alkylation. The peculiar mechanistic features of the reaction fit a model that identifies the relative basicity of the arene and of the olefin associated in any given INC as the key kinetic factor. The model accounts for the seemingly paradoxical observation that the alkylated products from less activated substrates are formed at relatively higher rates and for the occurrence of consecutive alkylation steps, yielding polyalkylated products of the less activated substrates. The relevance of the work to the theory of the aromatic substitution as a new entry into the reaction manifold of Friedel-Crafts alkylation and to gas-phase ion chemistry, as an irrefutable demonstration of the kinetic role of INCs is briefly discussed.