Rhodium-Catalyzed CH Annulation of Nitrones with Alkynes: A Regiospecific Route to Unsymmetrical 2,3-Diaryl-Substituted Indoles

The direct CH annulation of anilines or related compounds with internal alkynes provides straightforward access to 2,3‐disubstituted indole products. However, this transformation proceeds with poor regioselectivity in the synthesis of unsymmetrically 2,3‐diaryl substituted indoles. Herein, we report the rhodium(III)‐catalyzed CH annulation of nitrones with symmetrical diaryl alkynes as an alternative method to prepare 2,3‐diaryl‐substituted N‐unprotected indoles with two different aryl groups. One of the aryl substituents is derived from NC‐aryl ring of the nitrone and the other from the alkyne substrate, thus providing the indole products with exclusive regioselectivity. They know their place: Unsymmetrically 2,3‐diaryl‐substituted N‐unprotected indoles were synthesized by the rhodium(III)‐catalyzed annulation of nitrones with symmetrical diaryl alkynes. Exclusive regioselectivity was observed, as one of the two aryl substituents on the resulting indole ring is derived from the C‐aryl ring of the nitrone, and the other from the alkyne.