Synthesis of Racemic, Diastereopure, and Enantiopure Carba- or Oxa[5]‑, [6]‑, [7]‑, and -[19]helicene (Di)thiol Derivatives

A series of carba- or oxa[5]-, [6]-, [7]-, and -[19]­helicene (di)­thiols was prepared. The Miyazaki–Newman–Kwart rearrangement of (dimethylcarbamothioyl)­oxy (oxa)­helicenes in a flow reactor or nucleophilic substitution of dichloro (oxa)­helicenes with alkanethiolates were used in the sulfanylation step. Despite the high temperatures employed in this key step, no conformational scrambling was observed during the asymmetric synthesis of the diastereo- and enantiopure oxahelicenes. Single-molecule conductivity of the longest oxa[19]­helicene dithiol derivative was studied by the scanning tunneling microscopy break-junction method.