Azine-fused [6]carbohelicenes: synthesis, crystal structure, stereoisomerism and optical properties

Novel azine-fused [6]helicenes were prepared in five synthetic steps starting from commercially available 2,3-dihaloazines (2,3-dibromopyridine, 2,3-dichloropyrazine and 2,3-dichloroquinoxaline). The X-ray structures, UV-vis absorption and fluorescence spectra of the helicenes were investigated and compared to those of the parent [6]carbohelicene. It was shown that pyrazine-fused [6]helicenes, bearing a pyren-1-yl or 1,8-bis(dimethylamino)naphthalen-4-yl side substituent, contain both helical and axial stereogenic elements and exist as a mixture of stable (P,S_a)-, (M,R_a)-, (P,R_a)- and (M,S_a)-isomers at room temperature. The barriers to R/S- and P/M-isomerizations, as well as the relative stabilities of the isomers, were estimated theoretically.