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.