Unique versatility of ionic liquids as clean decarboxylation catalyst cum solvent: A metal- and quinoline-free paradigm towards synthesis of indoles, styrenes, stilbenes and arene derivatives under microwave irradiation in aqueous conditions

Ionic liquids have been found to provide a new platform for metal- and quinoline-free decarboxylation of various N-heteroaryl and aryl carboxylic acids under microwave irradiation in aqueous condition. The method was found to possess a wide substrate scope towards the synthesis of various pharmacologically and industrially important aromatic compounds including indoles, styrenes, stilbenes, and nitro- or hydroxyarene derivatives. The decarboxylation of indole and α-phenylcinnamic acids proceeded well without addition of any catalyst in neat 1-hexyl-3-methylimidazolium bromide ([hmim]Br) and 1-methylimidazolium p-toluenesulfonic acid ([Hmim]PTSA), respectively, while addition of a mild base like aqueous sodium hydrogen carbonate (NaHCO₃) to [hmim]Br further improved the decarboxylation of hydroxylated cinnamic and aromatic acid substrates. The developed methodology not only precludes the usage of toxic metal/quinoline and harsh organic bases but also offers several inherent benefits like recyclability of reagent system, reduction in waste and hazards, short reaction time besides ease of product recovery.