Synthesis of Azahelicene Derivatives by Utilizing Functionalities of Pyrrolic NH Sites

Synthesis of ever-longest carbo- or hetero-helicenes is a challenging task in synthetic organic chemistry, but it often involves handling problems such as insolubility in common organic solvents and oxidative degradation under aerobic conditions. We designed benzo-annulated aza[n]helicenes ([n]AH), which benefit from both the suppressed elevation of HOMO energies and high solubility due to hydrogen-bonding with solvent molecules at the pyrrole NH sites. This strategy enabled the synthesis of six single aza[n]helicenes (n=9, 11, 13, 15, 17, 19) and double aza[9]helicene (D[9]AH) via intramolecular oxidative fusion reactions. The solubilities were further tuned by N-alkylation reaction. Aza[9]helicenes with terminal substituents were also synthesized to investigate the substituent effects. The structures of all the synthesized aza[n]helicenes were determined by X-ray diffraction analysis, which revealed triple-layered structures of [17]AH and [19]AH as well as chiral (P,P) and (M,M) structures of D[9]AH-Et as a racemic mixture. Their electrochemical and (chir)optical properties have been comprehensively studied by cyclic voltammetry, UV/Vis absorption, emission, CD, and CPL spectra. These systematic studies provide an insight on the structure-property relationships of azahelicenes.