Ru-Catalyzed Heteroannulation to Indoles, Dihydroisoquinolines, and Dihydroquinolines

Significance

Terminal alkynes are attractive starting materials for various heterocyclizations. However, the present approach appears not to have been utilized for the synthesis of quinolines or isoquinolines (although the use of substituted alkynes resulting in 2-substituted quinolines has been reported). The present paper fills this lacuna in the chemical literature, and is utilized for the synthesis of indoles, dihydroisoquinolines, and ­dihydroquinolines (only one example) using ruthenium catalysis. After some optimization, the conditions of choice as shown in the scheme afforded the desired products in good to excellent yields. The substrate scope was well studied in terms of N-substitution, less so in terms of substituents on the aromatic ring. Noteworthy is the use of less harmful toluene or THF as solvents, albeit with lower yields and longer reaction times.