An Efficient Synthesis of Bis-indolylindane-1,3-diones, Indan-1,3-diones, and Indene-1,3(2H)-denies Using [Hbim]BF4 Ionic Medium

We prepared a brand new molecule in one step for the synthesis of bis-indolylindane-1,3-dione and indan-1,3-diones from the reaction of ninhydrin and 3 substituted/unsubstituted indoles using [Hbim]BF4 ionic liquid in excellent yields. The method was also used for the synthesis of novel indene-1,3(2H)-denies derivatives. 1. Introduction In recent times, ionic liquids have gained recognition as possible environmentally benign alternatives to the more volatile organic solvents [1]. Ionic liquids possess many attractive properties, such as wide liquid range, negligible vapor pressure, ease of recyclability, high thermal stability, and good solvating ability in a wide range of substrates and catalysts, which alleviate some of the environmental issues. Their nonvolatile nature can reduce the emission of organic compounds and facilitate the separation of products and/or catalysts from the reaction solvents. Furthermore, ionic liquids are found to be an efficient reaction medium for the immobilization of transition metal-based catalysts, Lewis acids, and enzymes [2]. The hallmark of such ionic liquids is the ability to alter their properties as desired by manipulating their structure with respect to the choice of organic cation or anion and side chain attached to the organic cation. Important pharmaceuticals often possess heterocyclic moieties as their building blocks [3]. The extensive use of heterocyclic compounds in the pharmaceutical industry is perhaps attributable to the availability of ample range of reactions that facilitate subtle structural modifications in heterocyclic compounds [4–7]. Since indole and its derivatives possess various biological activities [8], development of new methodologies for the synthesis of indole derivatives, which will yield subsets of heterocycles having potentiality to serve as templates for new biologically active molecules, is of great importance. In this context, we wish to describe a convenient and simple methodology for the synthesis of bis-indolylindane-1,3-dione (by reacting ninhydrin with 3 substituted/unsubstituted indoles), 2-(1′,3′-dihydro-1H-[2,3′]biindolyl-2′-ylidene)-indan-1,3-diones, indene-1,3(2H)-denies (from the reaction of ninhydrin, 1,2-phenylendiamine, and indole), and 2,2-bis(4-(dimethylamino)phenyl)-1H-indene-1,3(2H)-diones (from the reaction of ninhydrin with N,N-dimethylaniline). The reactions were carried out using [Hbim]BF4 ionic liquid as green solvent. The novelty of the methodology lies in its eco-friendly operation, the formation of structurally unique molecules, short reaction time, and