We studied the kinetics of the Diels-Alder reaction of Li +-encapsulated 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.
ASJC Scopus subject areas
- Colloid and Surface Chemistry