Chemists report a simple, low-temperature method for synthesizing aryl ethers, an important class of compounds used in fragrances, cosmetics, and pharmaceuticals. The key to the method is using light to push a nickel catalyst to perform a type of reaction it generally struggles to do.
This general photocatalytic strategy could be used to expand the types of possible reactions other metal catalysts can handle, the researchers say.
Palladium and copper catalysts can form C–O bonds to create aryl ethers, but these reactions typically require high temperatures and strong bases, which can adversely affect sensitive substituents. Ni in low oxidation states can catalyze such reactions too, but only in rare cases. Ni in higher oxidation states, Ni(III) or Ni(IV), can catalyze a wider range of C–O bond forming reactions, but chemists have to use either noncatalytic reagents, which can be costly and reduce synthetic efficiency, or strong bases to get the metal into those states.
David W. C. MacMillan of Princeton University and coworkers have now used light and a separate catalytic cycle to oxidize Ni(II) to Ni(III), allowing the metal to catalyze C–O bond formation between aryl bromides and alcohols at room temperature.
Alkoxide and aryl groups from the reagents first coordinate with Ni(II). Visible light then activates an iridium(III) photocatalyst so it can oxidize the Ni(II) complex to the Ni(III) state. This allows Ni to perform a reductive elimination to form the aryl ether. Finally, the Ir and Ni catalysts react together to return to their starting oxidation states, readying them for another catalytic cycle.
The reaction has broad substrate scope, coupling a variety of arene and heteroarene bromides with diverse primary and secondary alcohols to form a wide range of aryl ether products (Nature 2015, DOI: 10.1038/nature14875).
“We get a Ni(II) alkoxide to move to a very high-energy Ni(III) species at room temperature, which allows it to readily participate in reductive elimination, a reaction it is famous for not being able to do with alcohols,” MacMillan says. “This metallo-photoredox concept might be used to switch on mechanisms or reactions that were previously impossible for all sorts of cheap and broadly available metal catalysts.”
For many years, chemists have been thinking about how to perform catalytic versions of reductive eliminations with Ni complexes, “and MacMillan and coworkers now have elegantly shown how to do so with a photoredox system,” comments John F. Hartwig of the University of California, Berkeley, a specialist in transition metal-catalyzed reactions.
“This is a very important paper,” says C–O bond formation expert Stephen L. Buchwald of MIT. The corresponding Pd- and Cu-catalyzed methods “have significant limitations,” and “the development of basic methods that can facilitate reductive elimination is potentially a game changer for many related reactions.”