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Caged Lithium Speeds Fullerene Cycloadditions

Encapsulated ion acts as a Lewis acid catalyst

by Bethany Halford
July 21, 2014 | A version of this story appeared in Volume 92, Issue 29

A reaction scheme showing a cycloaddition to a buckyball with lithium inside it.

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.


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