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.