Synthesis of Thiophene‐fused Helicenes

Abstract

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.