S_EAr Mechanism of the Products of 1,2-Dimethoxybenzene and a Captodative Olefin: A Theoretical Approach

Abstract

In this work, a conceptual DFT investigation is carried out to study the electrophilic aromatic substitution reaction (S_EAr) of 1,2-dimethoxybenzene and 3-(p-nitrobenzoyloxy)-but-3-en-2-one (a captodative olefin). Herein, we have studied the regioselectivity of such reactions considering the effect of solvents of different polarities and the presence of BF₃ as the catalyst. Understanding the effect of the solvent and the role of the Lewis catalyst on the pathway of Friedel-Crafts reactions is important to further facilitate the introduction of side chains in aromatic rings with captodative olefins, and thus be able to synthesize compounds analogous to natural products, e.g., α-asarone. Global and local reactivity descriptors were obtained, finding a key role when these reactions take place in the presence of nonpolar solvents. In addition, the Intrinsic Reaction Coordinate diagrams (IRCs) were calculated. Such results of the free activation energy (ΔG^‡) clearly show that this reaction is entirely regioselective, forming the unique product in the para position, in agreement with our predictions of the local reactivity descriptors obtained from the Parr functions, wherein the first reaction step, the carbon C₄ of the aromatic compound 1,2-dimethoxybenzene is favored. Moreover, from the IRCs, we found that the reactivity of the para adduct increases in the presence of nonpolar solvents. Interestingly, considering a polar solvent (MeCN), the intermediate formed (σ-complex) is more stable since it presents a more significant charge transfer with the solvent than the intermediate in the presence of a nonpolar solvent, making a reaction more challenging to reach when the reaction is carried out in the presence of MeCN because of the increasing of the energetic barrier from σ-complex to the TS₂ in the intrinsic reactive coordinate diagram. Therefore, the polarity of the solvent plays an important role, particularly in the activation energy of the TS₂. Our computational results explained our experimental results quite well, confirming the importance of the solvent’s polarity to this S_EAr reaction and explaining why, experimentally, the nonpolar solvent drove the reaction under catalyzed conditions.