Making anilines from thin air

Making use of an abundant and essentially free resource, chemists have coaxed dinitrogen from air and benzene to react with one another, creating aniline derivatives. Sean F. McWilliams, who helped invent the reaction as part of his doctoral work in Patrick L. Holland’s group at Yale University, presented the discovery in the Division of Inorganic Chemistry at the ACS Fall 2020 Virtual Meeting on Aug. 17.

“There’s a lot of emphasis on trying to find ways to take nitrogen from the atmosphere and turn it into ammonia,” Holland told C&EN. “But people have not really found ways to take nitrogen from the atmosphere and turn it into the wide variety of nitrogen-containing organic compounds,” such as pharmaceuticals and polymers. The new reaction “enables a new way to take a free resource and do something useful with it,” Holland said.

To make N₂ reactive, you need a metal to pump electrons into its strong and highly stable bond. But any organic electrophiles that would react this nucleophilic nitrogen to make an organic compound will instead preferentially react with the metal in the reaction.

Holland said McWilliams, who is now a postdoctoral scholar at the University of North Carolina at Chapel Hill, and Daniël L. J. Broere, who is now an assistant professor at Utrecht University, discovered a way to reverse the reaction. Instead of making it nucleophilic, they make nitrogen electrophilic. The new reaction (shown) uses diketiminate-supported iron, which reacts with one of benzene’s C–H bonds and then forms a complex with N₂. Partial silylation makes the nitrogen electrophilic and therefore susceptible to attack by the benzene in the complex. The chemists recently reported the work in Nature (2020, DOI: 10.1038/s41586-020-2565-5).

“The coupling of N₂ reduction with C–H bond activation is an impressive and significant breakthrough,” said John D. Protasiewicz, an inorganic chemist at Case Western Reserve University, who presided over the session where McWilliams presented the research. Protasiewicz notes that the use of earth-abundant, non-toxic iron for the transformation is noteworthy, as well as the fact that the whole process occurs at a single metal center.

The reaction isn’t yet practical for scale up because it uses sodium metal, bromotrimethylsilane, and must go through several cycles of heating and cooling. “This is the kind of thing that will require a lot more development,” Holland said, but added his group is working on more practical variations.