A series of halogen-free quaternary ammonium Brønsted acidic ionic liquids (ILs) were prepared and used for Biginelli reaction among aromatic aldehydes, urea or thiourea and β-dicarbonyl compounds. The influencing factors, such as the type and amount of catalyst, solvent, reaction time and reaction temperature, were investigated. The results demonstrated that the 3-(N,N-dimethylhexadecylammonium) propanesulfonic acid toluene sulfate ([DHPA][Tos]) provide the best catalytic activity, with good catalytic performance and reusability under solvent-free conditions. A possible mechanism that accounted for the [DHPA][Tos]-catalyzed reaction was proposed.
Abstract
A highly efficient and green protocol for the synthesis of pyrimido[4,5-b]quinolines has been described. The one-pot multicomponent reaction of dimedone with arylaldehydes and 6-amino-1,3-dimethyluracil in the presence of N,N-diethyl-N-sulfoethanaminium chloride ([Et3N–SO3H][Cl]) as an ionic liquid (IL) catalyst under solvent-free conditions afforded the mentioned compounds in high yields and short reaction times. Our protocol is superior to many of the reported protocols in terms of two or more of these factors: the reaction times, yields, conditions (solvent-free versus usage of organic solvents), temperature and catalyst amount.
Synthesis of Quinoxaline Derivatives using Sulfonic Acid Functionalized Imidazolium Salts as Highly Efficient and Reusable Bronsted Acidic Ionic Liquids Catalysts under Solvent-free Conditions