A Continuous-Flow Fischer Indole Synthesis of 3-Methylindole in an Ionic Liquid

A continuous-flow reactor (Corning G1 reactor) was hereby introduced for Fischer indole synthesis of 3-methylindole by reaction of phenyl hydrazines and propylaldehyde. Stoichiometric Lewis acid, ZnCl2, was supplied as catalyst as well as remover of NH3 which generated during the indole ring formation. Ionic liquid ([EMIM][BF4]) was employed as solvent according to its high-temperature tolerance and good product distribution. After a straightforward extraction process, 3-methylindole could be obtained in a 95.3% yield (96.0% purity) under optimized conditions. Ionic liquid (IL) was then recovered by another extraction process, and the recovered IL acted nearly as efficiently as new IL for this Fischer indole reaction.