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Synthesis

Unstable radicals zapped into action in microfluidic electrochemical cell

Tight space between electrodes is key to working with short-lived radicals

by Bethany Halford
June 18, 2020 | APPEARED IN VOLUME 98, ISSUE 24

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Chemists can couple short-lived radicals to make myriad types of molecules, thanks to a microfluidic electrochemical cell that places the anode and the cathode just 25 µm apart. The new device offers an alternative to photoredox chemistry, which generates and couples transient radicals in solution using light and a chemical catalyst.

“We’re trying to give synthetic chemists an alternative way of getting these important intermediates,” says Stephen L. Buchwald, a chemist at the Massachusetts Institute of Technology who led the research effort with Klavs F. Jensen, an MIT chemical engineer.

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Credit: Science
Small and large versions of the microfluidic electrochemical cell.

The microfluidic electrochemical cell features a fluorinated ethylene propylene film sandwiched between two glassy carbon electrodes. The electrodes are 25 µm apart, creating a microfluidic channel for the reagents to pass through (Science 2020, DOI: 10.1126/science.aba3823). This slim space is critical, the researchers say, because persistent radicals generated at one electrode don’t have far to travel to meet up with transient radicals generated at the other electrode. If the space were larger, the radicals would decompose before they could couple.

Song Lin, an organic electrochemistry expert at Cornell University, says that using a microfluidic device is a “clever and elegant way” to get around mass transport problems that have made it tough to do electrochemistry with radicals. “This innovation will no doubt further improve the application of electrochemistry in organic synthesis,” he says.

Using glassy carbon for the electrode materials was key, says Yiming Mo, a postdoc in Jensen’s lab who designed and built the cell. “If we use conventional electrode materials, like metals, it’s very hard to get a smooth surface. When you get the electrodes so close, they can easily touch each other and short-circuit the cell,” he says. “But the glassy carbon is extremely smooth and flat.”

As a demonstration of the platform’s synthetic capabilities, the MIT team made more than a gram of the liquid crystal compound 5CB using multiple cells stacked together (reaction shown).

There are no plans to sell the microfluidic electrochemical cells, but Mo says that it’s possible to build one using the instructions in the report’s supporting information. “We hope that people will pick it up and use it and maybe we can get some inspiration for what else you can do,” Jensen says.

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