Thoughts about the electrophilic aromatic substitution mechanism: the Friedel-crafts alkylation and acylation of benzene with acetyl and t-butyl cations in the gas phase

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• 作者：Fernanda G. Oliveira ; Fabio L. Rodrigues ; Alline V. B. de Oliveira…
• 关键词：Density functional calculations ; Electrophilic aromatic substitution ; Friedel ; crafts ; Alkylation ; Acylation
• 刊名：Structural Chemistry
• 出版年：2017
• 出版时间：April 2017
• 年：2017
• 卷：28
• 期：2
• 页码：545-553
• 全文大小：
• 刊物类别：Chemistry and Materials Science
• 刊物主题：Computer Applications in Chemistry; Physical Chemistry; Theoretical and Computational Chemistry;
• 出版者：Springer US
• ISSN：1572-9001
• 卷排序：28

文摘

The mechanism of electrophilic aromatic substitution (Sb>Eb>Ar) 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 Sb>Eb>Ar. 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.