Kinetic Study of the Diels-Alder Reaction of Li~+@C_(60) with Cyclohexadiene: Greatly Increased Reaction Rate by Encapsulated Li~+

摘要

We studied the kinetics of the Diels-Alder reaction of Li~+-encapsulated [60]fullerene with 1,3-cyclo- hexadiene and characterized the obtained product, [Li~+@ C_(60)(C_6H_8)](PF_6~-). Compared with empty C_(60), Li~+@C_(60) 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 C_(60) and Li~+@C_(60) (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~+@C_(60). 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.