Diversified Syntheses of Tetrathia[7]helicenes by Metal-Catalyzed Cross-Coupling Reactions

Efficient and versatile synthetic routes to functionalized tetrathia[7]helicenes (7-THs) are described. The key intermediates of these methodologies are 2-bromo-3,3′-bibenzo[1,2-b:4,3-b′]dithiophenes (1), synthesized through a palladium-catalyzed homocoupling reaction between two benzo[1,2-b:4,3-b′]dithiophene units followed by a regioselective α-bromination. Direct palladium-catalyzed annulation of bromides 1 with internal alkynes provides a set of 7,8-disubstituted 7-THs 2 in moderate to good yields (46-80 %). Otherwise, 7-monosubstituted 7-THs 4 have been prepared through Sonogashira coupling of 1 with terminal alkynes, followed by platinum- or indium-promoted cycloisomerization of alkynyl intermediates 6. Finally, the versatility of bromides 1 has also been demonstrated by using them for the preparation of benzo (hetero) fused 7-TH derivatives 7 via Suzuki coupling with (hetero)arylboronic acids and the photocyclization of the obtained intermediates 9.