Origin of the Regioselectivity in the Gas-Phase Aniline+CH₃⁺ Electrophilic Aromatic Substitution

Nonadiabatic ab initio molecular dynamics simulations are carried out to monitor the attack of CH₃⁺ on aniline in the gas phase to form the corresponding σ complexes. The reaction is ultrafast and is governed by a single electron transfer within 30 fs, which involves two sequential conical intersections and finally produces a radical pair. Positive-charge allocation in the aromatic compound is found to govern the substitution pattern in ortho, meta, or para position. Although the major products in the first step of the electrophilic aromatic substitution are the ortho and para σ complexes, initially 26% of the simulated trajectories also form meta complexes, which then undergo H shifts, mainly to the para position. Which is formed? The mechanism of the electrophilic aromatic substitution of aniline by CH₃⁺ to give the ortho, meta, or para adduct (see picture) is analyzed by means of nonadiabatic molecular dynamics simulations. Regioselectivity, with preference for the ortho product, is observed. CI: conical intersection.