Role of Lewis acid(AlCl₃)-aromatic ring interactions in Friedel-Craft's reaction: An ab initio study

Electrophilic aromatic substitution reactions, which are highly exemplified by the widely used Friedel-Craft's reaction, have been extensively studied using theoretical and experimental techniques. A number of elegant mechanisms have been proposed for the Friedel-Craft's reaction till date. In all the proposed mechanisms, the role of the Lewis acid has been limited to the generation of the electrophile, which subsequently attacks the aromatic system to form either a π or σ complex. A recent experimental report on the reaction of CO with benzene in zeolite catalysts intrigued us because the presence or absence of AlCl₃ was found to govern the reaction product. These clearly indicated that AlCl₃ has an additional role in the reaction. We probed this role theoretically by performing high-level ab initio calculations on two model systems C₆H₆-BCl₃ and C₆H₆-AlCl₃. Our results clearly indicate that one of the benzene carbon tends to become highly nucleophilic, thereby facilitating an attack by an incipient electrophile. There appear unusual molecular orbital interactions including the loss of the benzene nodal plane and back-donation from Cl 3p orbital to the benzene HOMO. In what could be the first high-level theoretical study of Lewis acid-aromatic reactions, we believe our results could help understand the nature of the intermediates in electrophilic aromatic substitution reactions.