Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

Navakouski, M.; Zhylitskaya, H.; Chmielewski, P. J.; Lis, T.; Cybińska, J.; Stępień, M.
Angew. Chem. Int. Ed. 2019, 58, 4929-4933 | view at publisher's site

Chiral heteroaromatic propellers based on radially π-extended hexapyrrolohexaazacoronenes were obtained in a concise synthesis from suitably functionalized donor–acceptor monopyrroles. To overcome steric hindrance, a new cyclodehydrogenation method was developed, which uses bromine electrophiles as oxidative coupling agents, instead of the commonly employed high-potential oxidants. The new reaction offers high yields of propeller targets even for electron-deficient precursors, and shows electrophile-dependent stereoselectivity, with N-bromosuccinimide and dibromine yielding respectively D6- and C2-symmetric products. The propeller azacoronenes are chiral and can be separated into configurationally stable enantiomers. In addition to providing steric bulk, peripheral functionalization considerably affects the electronic properties of the propellers, which exhibit reduced optical and electrochemical bandgaps, and a more clearly defined electroreduction behavior.