Consecutively fused single‐, double‐, and triple‐expanded helicenes

Abstract

Abstract We report the quantitative, chemoselective, and diastereospecific formation of the defect‐free C‐shaped single and S‐ and M‐shaped consecutively fused double‐ and triple‐expanded homochiral helicenes (all‐right (P) or all‐left (M) handed), in which two and three expanded helicene subunits were exclusively fused together in a helically winding manner, respectively, through the trifluoroacetic acid‐promoted intramolecular multistep cascade alkyne benzannulations of anthracene‐based cyclization precursors composed of a large number of complicated dynamic stereoisomers due to axial chirality. The resulting racemic expanded helicenes were all separated into enantiomers by chiral chromatography and showed intense circular dichroism and circularly polarized luminescence (CPL) with the absorption and luminescence dissymmetry factors of up to 1.5 × 10−2 and 0.94 × 10−2, respectively. Moreover, the triple‐expanded helicene showed the highest CPL brightness (255 M−1·cm−1) among all the reported CPL‐active carbo‐ and heterohelicenes and ‐helicenoids. The helicity inversion barriers remarkably increased with the increasing number of incorporated helicene subunits, and the racemization of the triple‐expanded helicene was completely suppressed even at 80°C, providing the first example of an optically active expanded helicene with a static helical chirality. Key Points Defect‐free synthesis of single‐, double‐, and triple‐expanded helicenes with C‐, S‐, and M‐shaped fully conjugated helical ladder structures, respectively. Quantitative and chemoselective acid‐catalyzed diastereospecific multistep cascade cyclizations. Circularly polarized luminescence of configurationally stable optically pure expanded helicenes.

Keywords

• alkyne benzannulations
• chirality
• circularly polarized luminescence
• diastereospecific synthesis
• helicene
• ladder formation