Application Of Deep Eutectic Solvents As Reaction Medium Or Catalyst In Organic Synthesis

Deep eutectic solvents(DESs),also known as eutectic ionic liquids,is a new type of ionic liquids with similar physical and chemical properties to the traditional ionic liquid(ILs).Compared with traditional ionic liquids,DESs also have many distinctive properties and advantages,such as low cost,easy preparation,good biodegradability and biocompatibility.In recent years,DESs as green and sustainable reaction mediums and catalysts have been widely used in organic catalysis,metal catalysis,organic synthesis and electrochemistry,and good results have been achieved.Based on the above,DESs ChCl/p-Ts OH and ChCl/urea are respectively used as catalyst component and reaction medium in the synthesis of nitriles from alcohols,and the synthesis of imides from anhydrides to expand the application scopes of DESs.First,various catalyst systems were constructed from different DESs in combination with 2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)and Na NO₂,which were applied to the one pot two-step synthesis of benzyl nitriles from aerobic oxidation of benzyl alcohols to benzaldehydes,followed by condensation of the generated benzaldehydes with hydroxylamine hydrochloride.The effcts of various DESs,solvents,reaction temperature and the loading of each component in the catalytic systems were evaluated with benzyl alcohol as a model substrate.It was found that the catalyst system derived from choline chloride(ChCl)and p-toluenesulfonic acid(p-Ts OH)with a ratio of 1:1 in combination with TEMPO and Na NO₂ showed good performances in the reaction.The optimum reaction conditions were determined by systematic experiments,which were:benzyl alcohol 5 mmol,Na NO₂ 5 mol%,TEMPO 5 mol%,ChCl/p-Ts OH 30 mol%,CH₃CN 3 m L,oxidation temperature 25 ^oC;6.5 mmol hydroxylamine hydrochloride,condensation temperature 120^oC.Under the optimal conditions,various benzyl alcohols were converted to their corresponding benzyl nitriles over the catalyst system.This strategy avoids the use of toxic metal cyanides and additional transition metal,which represents a green route in synthesis of nitriles.Next,various N-unsubstituted cyclic imides were readily synthesized from cyclic anhydrides and urea in deep eutectic solvent composed of choline chloride(ChCl)and urea with molar ratio of 1:2.Herein,urea factionalized as both a component of the DES and a nitrogen source in the reaction.A series of aromatic,naphthalene and aliphatic cyclic anhydrides(including pyromellitic dianhydride)were converted to the corresponding imides with good to excellent yields in the DES.In addition,medium yield was obtained with aliphatic anhydrides as substrates.This protocol shows the advantages of smooth reaction,easy product separation,and simple recovery and recycling of ChCl/urea.ChCl/urea can be reused for 5 times without significant loss of its functions as both reaction medium and catalyst.