Modular Synthesis of Organoboron Helically Chiral Compounds: Cutouts from Extended Helices

Two types of helically chiral compounds bearing one and two boron atoms were synthesized by a modular approach. Formation of the helical scaffolds was executed by the introduction of boron to flexible biaryl and triaryl derived from small achiral building blocks. All-ortho-fused azabora[7]helicenes feature exceptional configurational stability, blue or green fluorescence with quantum yields (Φfl) of 18-24% in solution, green or yellow solid-state emission (Φfl up to 23%) and strong chiroptical response with large dissymmetry factors of up to 1.12×10-2. Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting Φfl of up to 47% in CH2Cl2 and 25% in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than of H1. Single crystal X-ray analysis shows clear differences in the packing arrangement of methyl and phenyl derivatives. These molecules are proposed as primary structures of extended helices.