Deep eutectic solvents (DES), which are readily prepared by combining a cationic salt with a hydrogen-bond donor, have invoked interest as sustainable and potential solvents or catalysts in recent years. DESs are biodegradable, nontoxic, and inexpensive. One of the most commonly used examples is the mixture of one mole of choline chloride (ChCl) with two moles of urea, which provides the liquid DES (ChCl/urea) with a room temperature melting point.

Ming-Yang He and co-workers from Changzhou University, China, used ChCl/urea DES as a catalyst to achieve the rapid transformation of aryl/heteroarylboronic acids into phenols in water with hydrogen peroxide as the oxidant. These reactions were conducted at room temperature and provided the corresponding phenols within a few minutes. The catalyst was also easily recovered and reused several times without obvious loss of activity.

• Rapid, Sustainable, and Gram-Scale Synthesis of Phenols Catalyzed by a Biodegradable Deep Eutectic Mixture in Water,
  Liang Wang, Dong-Yan Dai, Qun Chen, Ming-Yang He,
  Asian J. Org. Chem. 2013.
  DOI: 10.1002/ajoc.201300192