Caged Lithium Speeds Fullerene Cycloadditions

To get more zip out of Diels-Alder reactions, chemists often employ a Lewis acid catalyst to starve the cycloaddition’s dienophile of electron density as it seeks out the electron-rich diene. One problem with this approach is that a heteroatom is needed in the dienophile to coordinate with certain Lewis acids, such as lithium ions. This coordination adds a steric component to the reaction, making it difficult to separate electronic and steric effects when teasing out the intricacies of the reaction. Seeking a way to remove steric effects from their study of the Diels-Alder reaction, chemists in Japan decided to put their lithium ion in a spot where it could exert only an electronic effect—inside a buckyball (J. Am. Chem. Soc. 2014, DOI: 10.1021/ja505952y). Researchers led by Ken Kokubo of Osaka University and Yutaka Matsuo of the University of Tokyo found that C₆₀ with an encapsulated lithium ion reacts with 1,3-cyclohexadiene (shown) about 2,400 times as fast as empty C₆₀. Kinetic and computational studies indicate that the lithium ion lowers the energy of the transition state and stabilizes the reactant complex and the product through favorable molecular orbital interactions.