Indoles from anilines
(Rated as: excellent)
Ruthenium and Tin-Catalyzed Synthesis of Indoles from Anilines and Ethylene Glycol, Bull. Korean Chem. Soc., 1999, Vol. 20, No. 1, pp 119
Typical procedure for ruthenium-catalyzed synthesis of indoles
A mixture of aniline (0.931g, 10 mmol), ethylene glycol (0.062g, 1 mmol), RuCl3 × nH2O (0.021g, 0.1 mmol), SnCl2 × 2H2O (0.226g, 1 mmol) and PPh3 (0.079g, 0.3 mmol) in 10mL dioxane was placed in a pressure vessel. After the system was flushed with argon, the mixture was stirred at 180° C for 20 hours. The reaction mixture was filtered through a short column (silica gel, dichloromethane) and evaporated under reduced pressure. To the residual oily material was added 30mL of dichloromethane and washed two times with 30mL of aqueous 5% HCl to remove excess aniline. The organic layer was dried over anhydrous Na2SO4. Removal of the solvent under reduced pressure left an oil which was separated by column chromatography (ethyl acetate/hexane = 1/20) to give indole (0.047g, 40% yield).
o-Toluidine -> 7-Methylindole, 32% yield
m-Toluidine -> 4- and 6-Methylindole, 33% yield
p-Toluidine -> 5-Methylindole, 40% yield
p-Chloroaniline -> 5-Chloroindole, 18% yield
p-Anisidine -> 5-Methoxyindole, 17% yield
2,3-Dimethylaniline -> 6,7-Dimethylindole, 60% yield
2,5-Dimethylaniline -> 4,7-Dimethylindole, 56% yield
3,5-Dimethylaniline -> 4,6-Dimethylindole, 61% yield
All reactions were carried out with anilines (10 mmol), ethylene glycol (1 mmol), RuCl3 × nH2O, PPh3 (0.3 mmol) and SnCl2 × 2H2O (1 mmol) in dioxane (10mL) at 180° C for 20 h.
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