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Synthesis

Frustrated Lewis Pairs Go Hydrogenating

Lewis acid-base pairs with metal-like reactivity add new facet to synthetic chemistry

by Stephen K. Ritter
December 24, 2012 | APPEARED IN VOLUME 90, ISSUE 52

In 2006, Douglas W. Stephan of the University of Toronto and his coworkers took the concept of Lewis acid-base pairs to a new level when they devised a chemical construct known as a frustrated Lewis pair. Lewis acid-base pairs are common in chemistry: An electron-deficient Lewis acid readily shares a Lewis base’s spare electrons. However, when the Lewis acid and base each have bulky substituents, their ability to form a close relationship is denied, and the pair is said to become “frustrated.” The unquenched pair garners penned-up reactivity, comparable to that of an organometallic catalyst. Several research groups have shown that frustrated pairs can trap small molecules such as H2, NO, N2O, SO2, and CO2 and subsequently use them in metal-free reactions to functionalize imines, silyl ethers, and N-heterocyclic compounds. This year, a bevy of new metal-free reactions made possible by frustrated pairs suggest that they are not “a one-trick pony,” Stephan says. Hydrogenations in particular came to the fore. In one example, Stephan and his coworkers tackled aromatic hydrogenations by using B(C6F5)3 as the Lewis acid and various anilines as the Lewis base (C&EN, Feb. 27, page 8; J. Am. Chem. Soc., DOI: 10.1021/ja300228a). The pair splits H2 and then adds the hydrogen to aniline’s aromatic ring, which ultimately leads to cyclohexylamines (shown). This example stands out because hydrogenation of an aromatic compound is hard to accomplish, even with the best transition-metal catalysts. Among other reactions reported this year, Stephan’s group and its collaborators showed that a frustrated pair can hydrogenate polyaromatic hydrocarbons (Chem. Commun., DOI: 10.1039/c2cc37190a) and substituted olefins (C&EN, Sept. 10, page 27; Angew. Chem. Int. Ed., DOI: 10.1002/anie.201204007).

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