Deep eutectic solvent catalyzed Friedel–Crafts alkylation of electron-rich arenes with aldehydes

Triarylmethanes (TRAMs) and diarylalkanes (DIAAs) are important intermediates in medicinal, materials chemistry and dye precursors. TRAMs and DIAAs are usually obtained from Friedel–Crafts alkylation reactions. However, because the Friedel–Crafts reaction is catalyzed by Bronsted or Lewis acids, this causes many problems, such as harsh reaction conditions, large amounts of acidic waste liquid after the reaction, etc. All these disadvantages are not good for the environment. In this study, six deep eutectic solvents (DES) were synthesized for this specific reaction. And the DES act as catalyst and solvent. Research showed that DES can be used for catalyzing Friedel–Crafts alkylation of electron-rich arenes with aldehydes under mild reaction conditions. Among the six synthesized DES, a choline chloride–zinc chloride ([ChCl][ZnCl2]2) based DES was shown to be the most efficient catalyst. Using 2.00 equivalents of [ChCl][ZnCl2]2, based on the aldehydes, in the Friedel–Crafts alkylation gave the corresponding TRAMs or DIAAs with the highest yields. The DES was reused directly without any activation process. After five cycles, the yields remained at 89–94%. Finally, a possible mechanism for this reaction is discussed according to the analysis of IR spectrum and further experiments.