Chiral Picene Synthesis and Unexpected Transformations Catalyzed by Brønsted Acid

We report the first synthesis of an axially chiral picene via sequential Sonogashira and Suzuki-Miyaura cross-couplings followed by Brønsted acid-catalyzed alkyne benzannulation. Unexpected reactivity during the final step led to the formation of additional products through aryl migration and subsequent acid-promoted transformations. Mechanistic investigations revealed the concurrent formation of chiral picene and benzo[4]helicene, while stronger acid promoted the further transformation of chiral picene through a dihydrodibenzopentaphene intermediate, ultimately yielding a planar polycyclic aromatic hydrocarbon through the elimination of anisole. These findings highlight competing reaction pathways in alkyne benzannulation and reveal unexpected reactivity in the synthesis of smaller chiral polycyclic aromatic hydrocarbons. Compared with our previously reported chiral peropyrene, chiral picene exhibits an ~100 nm hypsochromic shift in absorption and emission, together with a reduced end-to-end twist angle of 20° versus 28° for chiral peropyrene. These differences reflect the reduced π-extension of the picene framework and its distinct steric constraints. X-ray crystallography unambiguously confirmed the chiral picene and additional product structures, revealing distinct molecular geometries and providing insight into the structural consequences of the competing reaction pathways.