The diyne reaction of 3,3'-bis(phenylethynyl)-2,2'-bithiophene derivatives via rhodium complexes: A novel approach to condensed benzo[2,1-b:3,4-b']dithiophenes

The syntheses of benzo-fused benzo[2,1-b: 3,4-b']dithiophenes 1 and benzo[2,1-b :3,4-b': 5,6-c'']trithiophenes 2 are described. The treatment of easily available 3,3'-bis(phenylethynyl)-2,2'-bithiophene derivatives 5a and.6 (via Pd-II-catalyzed alkynylation of the corresponding 3,3'-dibromo-2,2'-bithiophenes; see Scheme I) with chlorotris(triphenylphosphine)rhodium(I) yields the corresponding cyclic rhodium complexes 7 (Scheme 2) which smoothly react with acetylenes and sulfur to give 1 and 2 in good yields (Schemes 3-5).