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(C6H8)]­(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.