Aromaticity interplay between quinodimethanes and C-60 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 C-60 by the AM1 model and limited ab initio and DFT techniques. All reactions are shown to proceed through a concerted tra nsition state possessing a considerable net aromaticity as evidenced from b ond orders and magnetic criteria such as the magnetic susceptibility exhalt ations (MSE) and nucleus independent chemical shifts (NICS) and produce dif ferent kinds of aromatic stabilized fullerene cycloadducts. Computations sh ow that a strong LUMO-dienophile control of C-60 is realized by the influen ce of pyramidalization, but its high reactivity over alkene appears to be g overned 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-C-60 reactions as compared to the butadiene-C-60 reaction. In fact, the simultaneously existi ng aromatic destabilization in fullerene indicates its opposite effect to t he resonance stabilization in diene; it is thus fully restricted when the g ained aromaticity is transmitted from the nucleophilic QDM to the fullerene electrophile in a push-pull manner. However, the overall aromaticity effec t shown by the aromatization as well as the aromaticity of C-60 seems to ac celerate these reactions at an increased rate.