Benzannulated Double Aza[9]helicenes: Synthesis, Structures, and (Chir)optical Properties

A benzannulated double aza[9]helicene was successfully synthesized via a one-pot oxidative fusion reaction, which was further derivatized to its N-alkylated ones. The X-ray structures showed double helical structures with overlapping terminal benzene rings. The double helicenes exhibit red-shifted absorption and fluorescence spectra compared to single azahelicene. Enantiomers of N-alkyl double aza[9]helicenes were optically resolved by HPLC. The CPL brightness of 19.2 M⁻¹cm⁻¹ was recorded.

Abstract

A benzannulated double aza[9]helicene 1 was successfully synthesized via a one-pot oxidative fusion reaction. 1 was derivatized to N-alkylated double aza[9]helicene 1-Et and 1-Bu, whose structures were determined by X-ray diffraction analysis. 1-Et and 1-Bu exhibited red-shifted absorption and fluorescence spectra compared to single aza[9]helicene. The double aza[9]helicenes were expected to have two different conformers. Consistent with solid-state structure, the chiral-isomer was estimated to be more stable by 16 kcal/mol relative to meso-isomer. Indeed, enantiomers of 1-Et and 1-Bu were optically resolved by HPLC and showed mirror-imaged CD and CPL spectra with the CPL brightness up to 19.2 M⁻¹cm⁻¹ for 1-Bu.