Nickel-catalyzed Suzuki–Miyaura cross-coupling reactions in deep eutectic solvents

Recently, deep eutectic solvents (DES) have been received great attention and considered as greener and environmentally benign solvent system because they have indispensable advantages to replace conventional organic solvents and ionic liquids in scientific domain.1 They are low cost, ease of synthetic accessibility, environmentally benign nature, negligible volatility, non-flammable, renewability, and biodegradability. Generally, DES are obtained through the complexation of hydrogen-bond acceptors such as quaternary ammonium salts with a hydrogen bond donor such as urea, carboxylic acids, amino acids, sugars, and glycerol.2 Moreover, DESs have been intensively used for many chemical processes including organic functional group transformations and metal catalyzed cross-coupling reactions. Among them, the Suzuki-Miyaura coupling reactions continue to play a vital role in the construction of biaryl and polyaryl compounds that are frequently found in natural products, pharmaceuticals, and functional materials. In this context, various phosphine-based palladium catalytic systems have been developed for the cross-coupling of arylboronic acids with a range of electrophilic partners, including aryl chlorides, bromides, iodides, triflates, and sulfonates. To avoid high-cost palladium catalysts, Miyaura and co-workers explored for the first time Ni-catalyzed system for the coupling of arylboronic acids with aryl chlorides. Since then, various cheaper nickel complexes have also been investigated for the Suzuki-Miyaura coupling reaction. Among them, transition-metal-catalyzed cross-coupling reactions are one of the most powerful and widely used transformation in organic chemistry for the formation of carbon-carbon and carbon-heteroatom bonds. However, to the best of our knowledge, there are few methods are reported the use of DESs for the Suzuki coupling reaction.3 Although the significant results are observed in the reported methods, there is a need to develop a facile and efficient catalytic system for metal catalyzed Suzuki-Miyaura reactions under DES medium and ligand-free conditions. Here, we have introduced an efficient and straightforward Ni-catalyzed system for the Suzuki coupling of arylboronic acids with aryl chlorides in choline chloride: urea as medium reusable solvent medium