title: Kinetic study of the Diels-Alder reaction of Li⁺@C₆₀ with cyclohexadiene: greatly increased reaction rate by encapsulated Li⁺.
abstract: 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.
authors.affiliation: Division of Applied Chemistry, Graduate School of Engineering, Osaka University , Suita, Osaka 565-0871, Japan.
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Ueno
authors.forename: Hiroki
authors.initials: H
authors.lastname: Kawakami
authors.forename: Koji
authors.initials: K
authors.lastname: Nakagawa
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Okada
authors.forename: Naohiko
authors.initials: N
authors.lastname: Ikuma
authors.forename: Shinobu
authors.initials: S
authors.lastname: Aoyagi
authors.forename: Ken
authors.initials: K
authors.lastname: Kokubo
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Matsuo
authors.forename: Takumi
authors.initials: T
authors.lastname: Oshima