Recent Advance on the Synthesis of 3,3'-Bisindolylmethane Derivatives under Transition-Metal-Free Catalytic Conditions

3,3'-Bisindolylmethanes (3,3'-BIMs) compounds are important indole alkyloads and their units are widely found in various natural products, functional materials and synthetic pharmaceutical compounds. Due to diverse biological activities and functionalities, for instance, antioxidant, anti-inflammatory, antiangiogenic, anti-bacterial and anti-cancer etc., the construction of 3,3'-BIMs is raised considerable concerns. Conventional methods especially focused on symmetrical 3,3'-BIMs were the condensition of indoles with carbonyl compounds via Friedel-Crafts pathway in the presence of Bronsted acids or Lewis acids. However, the utilization of transtion metals led the residue into the products and environmental contamination. The recent advance on the synthesis of 3,3'-BIMs since 2010, mainly concerned on the approaches and corresponding mechanism to prepare symmetrical and unsymmetrical 3,3'-BIMs under transition-metal-free conditions is summarized and discussed, aiming to provide important theoretical evidence and techinical support for further biological evaluations on desired compounds.