Enantioselective Synthesis of Configurationally Stable [5]helicenes Containing 1,2-Azaborine Units

Abstract

Two different families of BN-doped [5]helicenes have been efficiently synthesized through a highly enantioselective, intramolecular, Au-catalyzed alkyne hydroarylation reaction. Key for the success of the method is the use of BINOL-derived cationic phosphonites as ancillary ligands (BINOL: 1,1-Bi-2-naphthol). The inversion barriers of the structures obtained have been determined both experimentally and theoretically, and are essentially identical to those reported for non-dopped carbo[5]helicenes of otherwise identical structure. Contrarily, the newly prepared BN-doped helicenes exhibit intensified absorption spectra at long wavelength (λ ≈ 400 nm) and fluorescence when compared with their only-carbon counterparts. These effects are particularly pronounced for the naphtho[2,1-c]phenanthro[1,2-e][1,2]azaborinine series, in which the BN-unit is located at the rim of the helix. Preliminary studies on the post-synthetic functionalization of these structures are also described; specifically, the naphtho[2,1-c]phenanthro[1,2-e][1,2]azaborinine structure can be site-selective brominated at position 4. In addition, the unprecedented deborilation of these helices to afford axially chiral anilines has been observed by treatment with DDQ.