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

Abstract

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 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.

Modulare Synthese helikal‐chiraler Organobor‐Verbindungen: Ausschnitte verlängerter Helices

“…Die Absorptionsbanden von H1 mit der niedrigsten Energie befinden sich bei 426-432 nm, mit gut aufgelçsten vibronischen Progressionen bei 407-412 nm, und entsprechen der gelben Farbe von CH 2 Cl 2 -Lçsungen (Abbildung 3a). Absorptionsmaxima von H1 sind bathochrom gegenüber reinen Kohlen-Abbildung 2. a) Molekülstrukturen von H1-Me 2 , H1-Ph 2 , H2-Me 4 und H2-Ph 4 , [47]…”

“…Abbildung 2. a) Molekülstrukturen von H1-Me 2 , H1-Ph 2 , H2-Me 4 und H2-Ph 4 ,[47] bestimmt durch Rçntgenstrukturanalysebei 100 K. ORTEP-Zeichnungen werden mit 50 %W ahrscheinlichkeit gezeigt. Es sind nur (P)-Enantiomere dargestellt.…”

B,N-Embedded Double Hetero[7]helicenes with Strong Chiroptical Responses in the Visible Light Region

“…31 Among various heterohelicenes and multiple heterohelicenes, only a few examples involving both boron (B) and nitrogen (N) atoms have been developed by employing either B-N covalent bonds or B→N coordinate bonds. [32][33][34][35][36][37] In view of the growing interest in B,N-doped PAHs as novel π-conjugated materials with intriguing optoelectronic properties, [38][39][40][41][42] B,N-incorporated (multiple) heterohelicenes have seemingly been underexplored, with the reported gabs only on the order of 10 -3 . [33][34][35] Figure 1.…”

“…[32][33][34][35][36][37] In view of the growing interest in B,N-doped PAHs as novel π-conjugated materials with intriguing optoelectronic properties, [38][39][40][41][42] B,N-incorporated (multiple) heterohelicenes have seemingly been underexplored, with the reported gabs only on the order of 10 -3 . [33][34][35] Figure 1. Molecular design of B,N-embedded double hetero [7]helicenes (1a-c) in this work.…”

Future prospects in boron chemistry: new boron compounds and Lewis acids for catalysis and materials science

“…The recent development of polyfluorinated triphenyldiphenyl boranes tolerant to air and water [Figure 1A] [19] will undoubtedly open new opportunities for improving well-established boron-mediated catalytic processes [20] . Powerful boron Lewis acids and superacids with ever increasing Lewis acidity are also gaining increasing interest from the perspective of the recent intense development of new types of inorganic analogues, including diboraanthracene featuring carborane substituents [Figure 1B] [21] , pyramidal yet trivalent boron Lewis acids embedded in cage-shaped triptycene cores [Figure 1C] [22][23][24] and antiaromatic perfluorinated boroles [Figure 1D] [25] . These unprecedented boron compounds with new structures and reactivities will enable the observation of new coordination modes at boron, in the isolation of long sought after reaction intermediates and new classes of boron-mediated reactions in the near future.…”