Selectivity of Br/Li Exchange and Deprotonation of 4,4′-Dibromo-3,3′-bithiophene for Synthesis of Symmetrical and Unsymmetrical Dithienoheteroaromatic Rings

The novel selective synthesis of symmetrical and unsymmetrical dithienoheteroaromatic rings (DTHAs) has been developed via intramolecular cyclization of 4,4′-dibromo-3,3′-bithiophene (3). Four reaction conditions including n-BuLi/Et2O, n-BuLi/THF, s-BuLi/Et2O, and t-BuLi/Et2O were employed to react with 3 for selective formation of two types of dicarbanions, which generate the symmetrical and unsymmetrical DTHAs after quenching with three electrophilic reagents (4a–c). The possible mechanism of formation of DTHAs was proposed. In addition, two unsymmetrical DTHAs were confirmed by X-ray single-crystal analyses.