Switching up the Kolbe reaction

Back in 1849, German chemist Hermann Kolbe reported that he could form carbon-​carbon bonds by electrolyzing carboxylate salts. On paper, this decarboxylative coupling looks promising, but it typically uses high-density direct current. This current generates strongly oxidizing conditions that destroy most functional groups.

“The classical Kolbe is kind of like taking your normal organic molecule and putting it into the electric chair . . . . You literally zap it to smithereens,” says Phil S. Baran, a chemist at Scripps Research in California.

Chemists led by Baran and his Scripps colleague Yu Kawamata recently discovered that by tweaking the way they apply current during the Kolbe reaction, they can avoid these highly oxidizing conditions and make the transformation practical for coupling many types of primary carboxylates (Science 2023, DOI: 10.1126/science.adf4762).

Instead of using direct current, which is common in electrochemistry, the chemists use rapid alternating polarity, which quickly switches the electrodes’ polarity back and forth. This waveform prevents acidic conditions from developing around the electrodes. In the classical Kolbe reaction, that acidity protonates carboxylates and requires strongly oxidizing conditions to get the reaction to go.

The chemists used the new version of the Kolbe to make molecules that are tough to prepare in other ways. In one example (shown), they made 1,19-eicosadiene—a compound that costs about $10,000 per gram—from 10-undecenoic acid, a molecule that can be isolated from inexpensive castor oil.

The work “demonstrates that electronic waveform is an underappreciated yet enabling handle in modulating reactivity in synthetic electrochemistry,” write Fuzhou University’s Peng Guo and Ke-Yin Ye in a commentary that accompanies the paper.

Baran and Kawamata say they’re excited to see what other electrochemistry they can improve with rapid alternating polarity. For almost 180 years, Kawamata says, “human beings have been studying static electrolysis, and now we have dynamic electrolysis. So the world is completely open to us.”