Synthesis of 5,9‐Diaza Analogues of [5]‐ and [6]Helicene and their Chiroptic and Photophysical Characterization

Abstract

6,10‐Dipropyl‐5,9‐diaza[5]‐ and ‐[6]helicene were synthesized by ortho,ortho’ fusion of ortho‐teraryldicarboxamides. Key steps in the synthesis of the teraryls are azide formation with subsequent reduction and amidation, followed by Suzuki cross couplings. The ortho fusions were achieved with phosphorus pentoxide and phosphoryl oxide. The total syntheses could be accomplished with 10% and 3%, respectively, in seven consecutive steps starting with meta‐dibromobenzene. The helicenes were investigated by UV/Vis and fluorescence spectroscopy and by quantum chemical calculation of, inter alia, the HOMO‐LUMO gaps. Racemization barriers of the helicenes were calculated, whereupon the optical resolution of 5,9‐diaza[6]helicene was attempted and carried out successfully; ECD spectra were measured of its enantiomers.