Heteroannulation Route to Benzothiophenes from 2-Bromo Alkynylbenzenes

Significance

Reported is the heteroannulation of 2-bromo alkynylbenzenes with sodium sulfide in the presence of copper catalyst to afford benzo­thiophenes. This procedure has been reported before under conditions not involving copper and at 180 ˚C (T. Kashiki et al. Org. Lett. 2009, 11, 2473). In the absence of catalyst and at 80 ˚C, the product is formed in only 18% yield. A variety of copper salts and ligands were tested, and CuI and TMEDA were found to be optimal. Lowering the amount of copper led to lower yields. The substrate scope was well studied, and the yields were found to be lower when R^² is an electron-withdrawing group. Furthermore, steric bulky (R^² = 2-PhC₆H₄) and alkyl groups (R^² = n-Pr) led to failure. The presence of a chloro substituent on either ring was tolerated, but unfortunately use of substrates bearing additional bromo substituents were not reported.