Electrochemical Cyclization of 2-Alkynylanilines to Indole Derivatives

Significance

Reported is the synthesis of 2-substituted indoles from Sonogashira coupling derived 2-alkynylanilines involving an electrochemically generated cyanomethyl anion, which induces the cyclization. TEATFB was chosen as the electrolyte, due to its relatively low solubility in organic solvents, simplifying the workup process and, at times, avoiding the use of column chromatography. Various protected and unprotected alkynyl­anilines were cyclized in good to excellent yields. The substrate scope was fairly well studied. In the case of alkynylanilines bearing R¹,R² = H and R³ = Ph substitution, the reaction fails; however, the desired product is obtained when the N-ethyl carbamate is used. Interestingly, the carbamate group is cleaved under the reaction conditions. A large-scale experiment was also carried out in high yields. A catalytic mechanism was suggested for which no evidence was offered.