A Green Synthesis of Substituted Bis(indolyl)methanes Using Chlorosulfonic Acid Supported Piperidine-4-carboxylic acid (PPCA) Functionalized Fe3O4 Nanoparticles (Fe3O4-PPCA) as Recyclable Catalyst

Bis(indolyl)methane and their derivatives are members of an important class of heterocycliccompounds that display diverse biological properties, and can act as a selective colorimetricsensor for F− and also as a highly selective fluorescent molecular sensor for Cu2+ [1]. In thepast years, various methods were mentioned for the synthesis of bis(indolyl)methanes, generally,these compounds could be obtained by the cascade reaction between indole and aromatic(or aliphatic) aldehydes in the present of protic or Lewis acids, such as I2, Ionic liquids,CeCl3·7H2O, AuCl, SBA-15/SO3H and zeolites [2-4]. However, all of the synthetic protocolsreported so far suffer from disadvantages such as, use of metal and expensive reagent, prolongedreaction time, use of organic solvent, harsh reaction condition, use of excess indoleand low yield. In continuation with the search for simple non-hazardous methods for thetransformations in organic synthesis, herein we report a highly versatile and efficient synthesisof bis(indolyl)methanes (3) from indole (1), aldehyde (2a-j) and catalytic amounts of nanoFe3O4-PPCA under mild reaction in high yields (Fig. 1). This method provides several advantages, such as safety, mild condition, short reaction times,neutral conditions, non-toxic, environment friendly solvent, high yields and easy workup procedurecompared to the traditional method of synthesis.