文摘
The mechanism of electrophilic aromatic substitution (SEAr) is still matter of debate and interest in the literature. In this work, the Friedel-Crafts alkylation and the acylation in the gas phase were investigated in the context of the unified mechanism for SEAr. In this unified proposal three kinds of intermediates can potentially be formed: oriented and unoriented π-complexes, intimate single electron transfer (SET) intermediates and σ-complexes. Quantum chemical calculations at M06-2X/6–311++G(d,p) level were carried out for the investigation of the reaction of benzene with acetyl and tert-butyl ions as model non-oxidant electrophiles for acylation and alkylation, respectively, in the gas phase. It was found that both the tert-butyl and the acetyl cations prefer to form oriented π-complexes. Both electrophiles do not react through a SET pathway with benzene. The π-complex between tert-butyl cation and benzene can evolve to a σ-complex, while in the case of the acetyl cation and benzene the σ-complex was not found as a minimum on the potential energy surface. Instead, it corresponds to a transient species or a very shallow minimum. The outcome of this is that the π-complex would only react with the aromatic ring evolving to the product with nucleophilic assistance by a species of the reaction medium, in either through a concerted mechanism or a specific interaction. This is also observed for aromatics with low ionization energies/nucleophilicities. However, very electron rich aromatic systems afford σ-complexes, and as their ionization energies increases (i.e., less nucleophilic), the more the resulting complex resembles a π-complex, more or less continuously. This suggests out that electrophilic aromatic substitution reactions cannot be rationalized within a single mechanistic framework. Instead, a continuum of mechanistic possibilities may be involved.
