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 (phi(fl)) of 18-24 % in solution, green or yellow solid-state emission (phi(fl) up to 23 %), and strong chiroptical response with large dissymmetry factors of up to 1.12x10(-2). Azabora[9]helicenes consisting of angularly and linearly fused rings are blue emitters exhibiting phi(fl) 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 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.