Aromaticity Interplay between Quinodimethanes and C60 in Diels−Alder Reactions:  Insights from a Theoretical Study

A theoretical study is performed of the Diels−Alder reactions of various o-quinodimethanes (QDM) with C60 by the AM1 model and limited ab initio and DFT techniques. All reactions are shown to proceed through a concerted transition state possessing a considerable net aromaticity as evidenced from bond orders and magnetic criteria such as the magnetic susceptibility exhaltations (MSE) and nucleus independent chemical shifts (NICS) and produce different kinds of aromatic stabilized fullerene cycloadducts. Computations show that a strong LUMO-dienophile control of C60 is realized by the influence of pyramidalization, but its high reactivity over alkene appears to be governed by the global aromaticity on fullerene rather than its strain. The aromatic functionalization occurring in QDM upon cycloaddition drastically increases the reaction rate and exothermicity of all QDM-C60 reactions as compared to the butadiene-C60 reaction. In fact, the simultaneously existing aromatic destabilization in fullerene indicates its opposite effect to the resonance stabilization in diene; it is thus fully restricted when the gained aromaticity is transmitted from the nucleophilic QDM to the fullerene electrophile in a push−pull manner. However, the overall aromaticity effect shown by the aromatization as well as the aromaticity of C60 seems to accelerate these reactions at an increased rate.