Synthesis, Structures, Resolution, and Chiroptical Properties of 1,16‐Diaryl‐Substituted Benzo[5]helicene Derivatives

Abstract

An efficient route to the synthesis of benzo[5]helicene derivatives functionalized on the interior side of the helix was developed, and resulted in a series of 1,16-diaryl-substituted benzo[5]helicene derivatives starting from easily available 7-methoxytetralone. X-ray crystal structures showed that the benzo[5]helicene derivatives had highly helical, twisted structures, and could all create hierarchical packing architectures with alternating P and M layers in the solid state. Moreover, seven pairs of enantiomers based on 1,16-diaryl-substituted benzo[5]helicene derivatives were also obtained by efficient resolution through HPLC with semipreparative chiral columns. The enantiomers all showed clear mirror-image circular dichroism (CD) spectra and high specific optical rotations, and their absolute configurations were determined by X-ray crystallography. Interestingly, a helical nanotubular structure was formed by the self-assembly of one enantiomer through halogen bonding. Furthermore, the enantiomers were found to have high racemization barriers and thermostability, which might be caused by the introduction of aryl substituents at the C1(C16) position.

Preparation and Physicochemical Properties of [6]Helicenes Fluorinated at Terminal Rings

“…It is well documented that helicenes racemize. ,− In the case of [6]­helicene, the conformational interconversion takes place at elevated temperature and is characterized by the Gibbs free energy barrier Δ G ⧧ (503 K) = 36.5 kcal mol –1 (152.8 kJ mol –1 ) at 230 °C . On the other hand, the racemization barrier of 1,2,3,4-tetrafluoro[6]­helicene ( 6 ) can be significantly affected by the introduction of bulkier fluorine substituents into the interior of the helix .…”

Frontiers in circularly polarized luminescence: molecular design, self-assembly, nanomaterials, and applications

“…It is worth noting that much attention has been paid to the novel CPL materials from helicene-based derivatives due to their twisted extended π-conjugated molecular structures and strong helical chirality properties [32][33][34][35][36][37][38][39]. Since 2016, Chen's group [32][33][34][35][36][37][38] developed versatile CPL-active emitters based on optically stable enantiomeric hydro [5]helicene and [5]helicene derivatives. Compared with the relatively flexible compound 8a, the rigid helicene 8b could exhibit stronger CPL (Figure 3a) [37].…”

Photochemical Functionalization of Helicenes

“…It is to be noted here that Fukuzumi, Nicewicz, König, and many others have already explored this photochemical strategy for the functionalization of arenes and heteroarenes. Direct C−H helicene bond functionalization is of synthetic interest, but challenging given that there are not many methods known apart from electrophilic substitution for such transformations …”

“…Direct CÀH helicene bond functionalization is of synthetic interest, but challenging given that there are not many methods known apart from electrophilic substitution for such transformations. [21,[49][50][51] The examples of direct C(sp 2 )ÀH helicene bond functionalization using 2-methoxy [6]helicene (3) as a model substrate are depicted in Figure 4. Simple nucleophiles served as precursors for the desired functional groups, including bromination.…”