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 (Φ _fl ) of 18-24 % in solution, green or yellow solid-state emission (Φ _fl 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 Φ _fl of up to 47 % in CH ₂ Cl ₂ and 25 % in the solid state. As revealed by the DFT calculations, their P-M interconversion pathway is more complex than that 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.
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