In situ Acidic Carbon Dioxide/Ethanol System for Selective Oxybromination of Aromatic Ethers Catalyzed by Copper Chloride

An environmentally benign carbon dioxide/ethanol reversible acidic system was developed for the copper(II)‐catalyzed regioselective oxybromination of aromatic ethers without the need of any conventional acid additive and organic solvent. Good to excellent yields together with very good regioselectivity were achieved when easily available cupric chloride dihydrate was used as catalyst and lithium bromide as the cheap and easy‐to‐handle bromine source under supercritical carbon dioxide conditions. Notably, the catalytic system worked well for a wide range of aromatic ethers including sulfides, resulting in the formation of the mono‐brominated products in high yields and exclusive regioselectivity. The alkylcarbonic acid in situ formed from ethanol and carbon dioxide is assumed to play the crucial role in the Brønsted acid‐promoted reaction, which could probably act as the proton donator, and was studied employing in situ FT‐IR technique under carbon dioxide pressure by monitoring the vibration shift of the hydroxy group of ethanol. Given with excellent bromine atom efficiency as well as no need of neutralization in waste disposal, this approach thus represents a greener pathway for the aerobic bromination of aromatic ethers. A possible catalytic cycle for the in situ alkylcarbonic acid‐assisted oxybomination and the effect of supercritical carbon dioxide, i.e., activation of alcohol and enhancement of mass transfer are also discussed.