Green and Safe Hydrogenations in Deep Eutectic Solvents

The reduction of nitrogen- and oxygen-containing functional groups, as well as the catalytic semihydrogenation of alkynes to access cis-alkenes, is of great importance in organic synthesis since reduction products are essential structural units in many natural products, pharmaceuticals, and agrochemicals. Hydrogen is an explosive gas, its production needs extensive energy and generates a considerable amount of carbon dioxide. Therefore, the development of cost-effective reduction methods that use safe reagents, environmentally friendly solvents and prevent or minimize waste formation represents a challenge of great interest in sustainable chemistry. Continuing our interest in developing sustainable synthetic methodologies, herein, we describe an alternative and safe palladium-catalyzed hydrogenation reaction in Deep Eutectic Solvents (DESs, Figure 1), unconventional green solvents displaying low toxicity, high biodegradability, and renewability. The use of aluminum powder in combination with water and a base, in DESs, results in an environmentally responsible and controlled in-situ formation of hydrogen. Our optimized protocol was effective for the reduction of a wide range of molecules, containing C–C, C–N, C–O, N–O multiple bonds, as well as, changing the nature of DES components, the stereoselective semihydrogenation of alkynes to cis-alkenes was achieved, leading to the desired products in yield up to 99%. The simplicity, tunability, recyclability and the environmentally benign character of both catalytic system and DESs, offer numerous advantages over the currently available reduction methods, performed in toxic volatile organic solvents and employing external and pressurized dangerous H2 source.