Diels-Alder and Retro-Diels-Alder Cycloadditions of (1,2,3,4,5-Pentamethyl)cyclopentadiene to La@C2v-C82: Regioselectivity and Product Stability

Abstract

One of the most important reactions in fullerene chemistry is the Diels-Alder (DA) reaction. In two previous experimental studies, the DA cycloaddition reactions of cyclopentadiene (Cp) and 1,2,3,4,5-pentamethylcyclopentadiene (Cp*) with La@C(2v)-C(82) were investigated. The attack of Cp was proposed to occur on bond 19, whereas that of Cp* was confirmed by X-ray analysis to be over bond o. Moreover, the stabilities of the Cp and Cp* adducts were found to be significantly different, that is, the decomposition of La@C(2v)-C(82)Cp was one order of magnitude faster than that of La@C(2v)-C(82)Cp*. Herein, we computationally analyze these DA cycloadditions with two main goals: First, to compute the thermodynamics and kinetics of the cycloadditions of Cp and Cp* to different bonds of La@C(2v)-C(82) to assess and compare the regioselectivity of these two reactions. Second, to understand the origin of the different thermal stabilities of the La@C(82)Cp and La@C(82)Cp* adducts. Our results show that the regioselectivity of the two DA cycloadditions is the same, with preferred attack on bond o. This result corrects the previous assumption of the regioselectivity of the Cp attack that was made based only on the shape of the La@C(82) singly occupied molecular orbital. In addition, we show that the higher stability of the La@C(82)Cp* adduct is not due to the electronic effects of the methyl groups on the Cp ring, as previously suggested, but to higher long-range dispersion interactions in the Cp* case, which enhance the stabilization of the reactant complex, transition state, and products with respect to the separated reactants. This stabilization for the La@C(82)Cp* case decreases the Gibbs reaction energy, thus allowing competition between the direct and retro reactions and making dissociation more difficult.