Chemists have a gleam in their eyes---from the reflection of chrome bumpers that adorn classic roadsters. The raw material at the root of the bumpers' sheen, chromium, is helping University of Rochester chemists leapfrog into the future by providing an automated way to create hundreds of thousands of compounds in a few easy steps. The classic compound, long used in a variety of chemical reactions, now is part of a new process to create materials known as anilines that are used in the manufacture of drugs, dyes, plastics and film.
The findings published recently in the chemistry journal Angewandte Chemie are part of a research program in combinatorial chemistry, a field where chemists create and screen thousands or even millions of compounds simultaneously for a given application. A paper by post-doctoral researcher Anitha Hari and Benjamin Miller, assistant professor of chemistry, shows how chromium chloride makes it possible to use combinatorial chemistry in the manufacture of anilines. Such a development could speed up the screening of the compounds for a wide variety of industrial products, says Miller.
The Rochester chemists have shown how chromium chloride can be used to energize a rare solid-to-solid catalytic reaction that makes it possible to produce a huge variety of anilines simultaneously. Chromium chloride shuttles back and forth between two solid substrates, converting raw materials to anilines.
"You can't just bang two solids up against each other and have them react," says Miller, whose research project was supported by Eastman Kodak Co. "You need a catalyst that can migrate from one material to the other."
Traditionally scientists produce anilines by starting with powdery compounds known as nitroarenes, which are dissolved in a solvent like a packet of Kool-Aid powder dissolves in water. Then chemists use palladium as a catalyst to convert the nitroarenes to anilines, which chemists separate out and purify in a time-consuming process.
In Miller's configuration, the raw nitroarenes are fixed on polymer beads that are then immersed in a solvent; the material clings to the beads like algae to a set of buoys. Also in the solvent is a strip of manganese. Chromium chloride acts as a catalyst, going back and forth between the strand of polymer beads and the strip of manganese and converting nitroarenes to anilines. Schematically the process is a reminder of the old video game Pong, where a ball bounces back and forth between two paddles; in this case the "ball" is a chromium chloride molecule that goes back and forth between the polymer beads and the manganese strip. The nitroarenes are converted to anilines right on the polymer beads, and the chromium is continually regenerated by the manganese.
By slightly altering the properties of the polymer beads, each about the size of a grain of sand, Miller can produce a huge variety of anilines, even hundreds of thousands of different types in just one experiment. The system acts as a tiny factory producing different chemical compounds that a drug company might then test in a hunt for a new drug. The anilines are very easy to separate out---chemists simply pull out the polymer beads and wash off the anilines. Because the chromium, which is very expensive and toxic in high amounts, is recycled by the manganese, there's little residue left on the anilines.