Asymmetric Synthesis of Nonracemic 2-Amino[6]helicenes and Their Self-Assembly into Langmuir Films

Alternative ways of preparing nonracemic 2-amino[6]helicene derivatives were explored. The enantioselective [2 + 2 + 2] cycloisomerization of a nonchiral triyne under Ni(cod)2/(R)-QUINAP catalysis delivered the enantioenriched (+)-(P)-2-aminodibenzo[6]helicene derivative in 67% ee. An ultimate "point-to-helical" chirality transfer was observed in the cyclization of enantiopure triynes mediated by Ni(CO)2(PPh3)2 affording (−)-(M)- or (+)-(P)-7,8-bis(p-tolyl)hexahelicen-2-amine in >99% ee as well as its benzoderivative in >99% ee. The latter mode of stereocontrol was inefficient for a 2-aminobenzo[6]helicene congener with an embedded five-membered ring. The rac-, (−)-(M)-, and (+)-(P)-7,8-bis(p-tolyl)hexahelicen-2-amines formed Langmuir monolayers at the air–water interface featuring practically identical surface pressure vs mean molecular area isotherms. The corresponding Langmuir–Blodgett films on quartz or silicon substrates were characterized by UV–vis/ECD spectroscopy and AFM microscopy, respectively.