Diels–Alder Cycloaddition of Cyclopentadiene to C₆₀ and Si₆₀ and Their Endohedral Li⁺ Counterparts

Both silicon and carbon are elements located in group 14 on the periodic table. Despite some similarities between these two elements, differences in reactivity are important, and whereas carbon is a central element in all known forms of life, silicon is barely found in biological systems. Here, we investigate the Diels–Alder cycloaddition reaction of cyclopentadiene (CP) and cyclopentasildiene (CP_Si) with fullerenes C₆₀, Li⁺@C₆₀, Si₆₀, and Li⁺@Si₆₀ using density functional theory methods. The results reveal distinct kinetic and thermodynamic trends that govern the reactivity and selectivity. For C₆₀, the [6,6] pathway is kinetically and thermodynamically favored, whereas for Si₆₀, the [5,6] pathway is preferred thermodynamically but not kinetically. The introduction of lithium cations increases the reactivity of both C₆₀ and Si₆₀. Energy decomposition analysis (EDA) unveils the importance of the components of the interaction energy between CP_Si and the corresponding fullerenes. The findings provide insights into the interplay of electronic structure, substrate reactivity, and fullerene electrophilicity in cycloaddition reactions.