Benzenes Reduced By Super Electron Donors

Benzene and other arenes are normally among the most challenging organic species to reduce. But not so for John A. Murphy of the University of Strathclyde, in Scotland. Murphy and his colleagues have devised powerful new versions of neutral organic molecules that readily give away their electrons to reduce arenes and form reactive intermediates, he reported yesterday at the American Chemical Society national meeting in Philadelphia. These reagents, called super electron donors, are on track to become important synthetic tools for organic chemists.

Few chemical reagents have the wherewithal to add an electron to a ground-state aromatic ring to form a radical anion. This type of electron transfer has traditionally required the most brutish of metal-based reducing agents, such as sodium metal dissolved in liquid ammonia.

Murphy’s group originally discovered that a simple organic molecule, a bisimidazolylidene, was up to the task of reducing iodoarenes and arenesulfones. In Philly, Murphy’s team reported that shining ultraviolet light on the bisimidazolylidene or a bispyridine analog (both shown) promotes an electron to a higher energy level in the donor, generating the most powerful super electron donors to date (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201200084). The light-activated bisimidazolylidene reduces the more difficult chlorobenzenes to nonchlorinated benzenes, and both light-activated super electron donors can ring-open diphenylcyclopropanes (shown).

“This is the first time any neutral organic reducing reagent has tackled these highly formidable reduction reactions,” Murphy told C&EN. “The sky is now the limit for what simple organic donors can reduce.”

Murphy has set his sights on applying the new reagents to challenging industrial processes such as reducing carbon dioxide to methane and nitrogen to ammonia.

Armido Studer, a expert in radical chemistry based at the University of Münster, in Germany, is intrigued by the potential of the light-activated approach. “Arene reductions by electron transfer are generally conducted with alkali metals dissolved in liquid ammonia,” Studer told C&EN. “That type of reaction, known as the Birch reduction, is well-known in industry and in academia. It’s difficult to compete with the Birch reduction. But just shining light on a molecule to increase its reduction power is certainly easier than playing around with substituent effects. Murphy and his team have shown that light can bring us closer to the established reagents.”