Synthesis of Thiophene-fused Helicenes

The synthesis of three penta- and three hexahelicenes containing two terminal thiophene units is described. The syntheses of pentahelicenes consist of 1,4-bisalkynylation of a benzene precursor and double Suzuki coupling in 2,3-position to introduce thiophene units. The ortho,ortho’ fusion yielding the final products was achieved with Fürstner's protocol using platinum(II) chloride or JohnPhos-complexed gold(I) as catalysts. A similar approach to hexahelicenes started with a naphthalene derivative, where 2,7-bisalkynylation and subsequent double Suzuki coupling with thiophene-2-boronic acid at 1,8-position furnished precursors, in which ortho,ortho’ fusion to the respective hexahelicenes was achieved with platinum(II) chloride or, favourably, with indium(III) chloride. UV/Vis spectra and cyclic voltammograms were recorded for all helicenes and HOMO/LUMO gaps were calculated with DFT methods.