Aromatic substitution in the gas phase. Alkylation of arenes by C4H9+ ions from the protonation of C4 alkenes and cycloalkanes with gaseous Brønsted acids

Abstract

Use of a reactive aromatic substrate (o-xylene) to sample the isomeric population of the butyl ions from the gas-phase protonation of the C₄H₈ hydrocarbons shows that the primary product from linear olefins and methylcyclopropane is the s-butyl ion, or at least an s-butylating charged species displaying a positional selectivity very similar to that measured for the thermal s-butyl cations obtained from n-butane. A fraction of the primary C₄H₉⁺ ions isomerizes to the most stable tertiary structure, to an extent which depends, inter alia, on the exothermicity of the proton transfer process. Protonation of cyclobutane gives an alkylating reagent whose s-butylation/t-butylation ratio and positional selectivity set it apart from the C₄H₉⁺ reagents obtained from the other C₄H₈ hydrocarbons, suggesting the intervention of a different electrophile, conceivably protonated cyclobutane. Finally, protonation of isobutene yields exclusively a t-butyl ion that does not isomerize despite the large exothermicity of its formation.