Kinetic study of the diels-alder reaction of Li⁺@C₆₀ with cyclohexadiene: Greatly increased reaction rate by encapsulated Li ⁺

We studied the kinetics of the Diels-Alder reaction of Li ⁺-encapsulated [60]fullerene with 1,3-cyclohexadiene and characterized the obtained product, [Li⁺@C₆₀(C ₆H₈)](PF₆^-). Compared with empty C₆₀, Li⁺@C₆₀ reacted 2400-fold faster at 303 K, a rate enhancement that corresponds to lowering the activation energy by 24.2 kJ mol^-1. The enhanced Diels-Alder reaction rate was well explained by DFT calculation at the M06-2X/6-31G(d) level of theory considering the reactant complex with dispersion corrections. The calculated activation energies for empty C₆₀ and Li⁺@C₆₀ (65.2 and 43.6 kJ mol^-1, respectively) agreed fairly well with the experimentally obtained values (67.4 and 44.0 kJ mol^-1, respectively). According to the calculation, the lowering of the transition state energy by Li⁺ encapsulation was associated with stabilization of the reactant complex (by 14.1 kJ mol^-1) and the [4 + 2] product (by 5.9 kJ mol^-1) through favorable frontier molecular orbital interactions. The encapsulated Li⁺ ion catalyzed the Diels-Alder reaction by lowering the LUMO of Li⁺@C₆₀. This is the first detailed report on the kinetics of a Diels-Alder reaction catalyzed by an encapsulated Lewis acid catalyst rather than one coordinated to a heteroatom in the dienophile.