Mechanochemical Scholl Reaction on Phenylated Cyclopentadiene Core: One-Step Synthesis of Fluoreno[5]helicenes

In this study, we explore feasibility of the mechanochemical approach in the synthesis of tetrabenzofluorenes (fluoreno[5]helicenes). For this, commercially available phenylated cyclopentadiene precursors are subjected to the Scholl reaction in the solid state using FeCl₃ as an oxidant and sodium chloride as the solid reaction medium. This ball milling process gave access to the 5-membered ring containing-helicenes in one synthetic step in high (95–96 %) isolated yields. The solution-phase reactions, however, were found to be moderate to low yielding in this regard (10–40 %).