2-phenylindole Theory Fischer indole synthesis is an important method for the preparation of substituted indoles. It consists of heating the phenylhydrazone of an aldehyde, ketone, or keto acid in the presence of an acid catalyst. Zinc chloride, hydrochloric acid or glacial acetic acid may be used, but polyphosphoric acid is often preferred. Thus, acetophenone phenylhydrazone gives 2-phenylindole and the reaction sequence involves an intramolecular condensation with the elimination of ammonia as shown in the mechanism. The general key steps in Fischer Indole synthesis are (i) the condensation of arylhydrazines with ketones to form arylhydrazones, (ii) rearrangement of the arylhydrazones to form ene-hydrazines (iii) acid-catalyzed [3,3]-sigmatropic rearrangement followed by intramolecular displacement of NH3 to produce indoles. (first arrow p CH3COOH aur second arrow p H3PO4 likhdio) (R1 ki jagah H aur R2 ki jagah Ph) 2-phenylindole is a reactant or the preparation of organic light emitting diodes (OLEDs), anti-inflammatory agents, antibacterial, antifungal agent and fluorescent probes. It is a reactant in difluorohydroxylation reactions and Mannich-type reactions. Recent advances and greener methods: 1. Modified Fischer Indole method 2. 2-Phenylindoles can be prepared by heteroannulation of 2-haloaniline derivatives and phenylacetylene under mild conditions in a one-pot reaction catalyzed by Pd(PPh3)2Cl2. 3. 2-phenylindole can be synthesized using N-benzoyl-o-toluidine and two equivalents of sodium ethoxide in a heated, airless, reaction (Madelung Synthesis). (R6 ko chodhke saare R ki jagah H aur R6 ki jagah Ph) References 1. Vogel’s Practical Organic Chemistry 2. A Facile, Mild and Efficient One-Pot Synthesis of 2-Substituted Indole Derivatives Catalyzed By Pd(PPh3)2Cl2; Hossien A. Oskooie, Majid M. Heravi and Farahnaz K. Behbahani 3. Org. Synth. 1942, 22, 98; DOI: 10.15227/orgsyn.022.0098 4. Reisenegger, Ber., 16, 662 (1883) 5. Klein D., Organic Chemistry, 6th Edition, Pearson: USA, 2012, pp 1043 Melting point = 188 – 189oC
