Synthesis of 2,3-Disubstituted Indoles via Palladium-Catalyzed Annulation of Internal Alkynes

The palladium-catalyzed coupling of 2-iodoaniline and the corresponding N-methyl, -acetyl, and -tosyl derivatives with a wide variety of internal alkynes provides 2,3-disubstituted indoles in good-to-excellent yields. The best results are obtained by employing an excess of the alkyne and a sodium or potassium acetate or carbonate base plus 1 equiv of either LiCl or n-Bu4NCl, occasionally adding 5 mol % PPh3. The yields with LiCl appear to be higher and more reproducible than those obtained with n-Bu4NCl. The process is quite general as far as the types of substituents which can be accommodated on the nitrogen of the aniline and the two ends of the alkyne triple bond. The reaction is quite regioselective, placing the aryl group of the aniline on the less sterically hindered end of the triple bond and the nitrogen moiety on the more sterically hindered end. This methodology readily affords 2-silylindoles, which can be easily protodesilylated, halogenated, or reacted with alkenes and Pd(OAc)2 to produce 3-substituted indoles, 2-haloindoles, or 2-(1-alkenyl)indoles, respectively. The presence of alcohol groups in the alkyne seems to have a particularly strong directing effect, perhaps due to coordination with palladium. This catalytic process apparently involves arylpalladium formation, regioselective addition to the C−C triple bond of the alkyne, and subsequent intramolecular palladium displacement.