Acidic DESs as Active Media for a Sustainable Synthesis of Biindoles Starting from 2,2’‐Diaminotolanes and Aldehydes

We describe an alternative and more sustainable method for the synthesis of 2,2′-disubstituted 3,3′‑biindoles starting from 2,2’-diaminotolanes and (hetero)arylaldehydes. The key feature of the approach is the use of an acidic DES able to exploit a double activity, i.e. solvent and BA catalyst, avoiding the use of VOCs as solvents and additional acid catalysts. By this way, we synthesized twenty-five biindoles, including eighteen new compounds. The reactions run quickly under mild conditions by microwave heating at 70 °C, with yields ranging from good to very good. The reaction scope is rather broad. The medium demonstrated to be reusable at least five times with only a slight reduction in yields. Based on experimental evidence and previous literature findings, a plausible mechanism is proposed.