Brönsted acid ionic liquids catalyzed Friedel–Crafts Alkylations of electron-rich arenes with aldehydes

Abstract

Triarylmethanes (TRAMs) and diarylalkanes (DIAAs) are valuable intermediates with wide applications in many fields. TRAMs are usually obtained from the acid-catalysed bisarylation of activated aryl aldehydes. However, the synthesis poses many problems, such as harsh reaction conditions, and the disposal of the excess solvents and/or toxic metal waste. In this study, some functionalized ionic liquids including Brönsted acid ionic liquids (BAILs) and traditional ionic liquids were designed and synthesized. BAILs catalyzed Friedel–Crafts (F–C) alkylation was applied in this specific reaction for the first time. And the BAILs showed bifunctional properties acting as catalyst and solvent. Research shows that BAILs can be used for catalyzing F–C alkylations of electron-rich arenes with aromatic or aliphatic aldehydes successfully under mild reaction conditions. Furthermore, BAILs containing triflic anion has higher activity than other BAILs and traditional ionic liquids. [HSO3-pmim][OTf] gets the highest yields in the presence of 20mol% of BAILs at 40°C to give the corresponding TRAMs derivatives. After five cycles, the yields remain about 93–97%. Finally, according to IR spectrum and the experimental validation, the aromatic electrophilic substitution reaction was considered to be the possible catalysis mechanisms.