Helical fused 1,2:8,9-dibenzozethrene oligomers with up to 201° end-to-end twist: “one-pot” synthesis and chiral resolution

Twisted polyarenes with persistent chirality are desirable but their synthesis has remained a challenge. In this study, we present a “one-pot” synthesis of 1,2:8,9-dibenzozethrene (DBZ) and its vertically fused dimers and trimers using nickel-catalyzed cyclo-oligomerization reactions. X-ray crystallographic analysis confirmed highly twisted helical structures that consist of equal parts left- and right-handed enantiomers. Notably, the end-to-end twist between the terminal anthracene units measured 66°, 130°, and 201° for the DBZ monomer, dimer, and trimer, respectively, setting a new record among twisted polyarenes. Furthermore, the chiral resolution by HPLC yielded two enantiomers for the fused DBZ dimer and trimer, both of which maintained stable configurations and showed absorption dissymmetry factors of around 0.008–0.009. Additionally, their optical and electrochemical properties were investigated, which exhibited a chain-length dependence.


General
All reagents and starting materials were obtained from commercial suppliers and used without further purification unless otherwise noted. Compounds 4 and 6 were synthesized according to previous literatures. [1] Anhydrous THF were distilled from sodium-benzophenone immediately prior to use. All reaction conditions dealing with air-and moisture sensitive compounds were carried out in a dry reaction vessel under an argon atmosphere. The 1 H NMR and 13 C NMR spectra were recorded in deuterated solvents on Bruker DPX400/DPX500 NMR spectrometer. All chemical shifts are quoted in ppm, relative to tetramethyl silane, using the residual solvent peak as a reference standard. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, m = multiplet, t = triplet. Atmospheric Pressure Chemical Ionization Mass Spectrometry (APCI MS) measurements were performed on a Finnigan TSQ 7000 triple stage quadrupole mass spectrometer. UV-vis-NIR absorption spectra were recorded on a Shimadzu UV-3600 spectrophotometer. Cyclic voltammetry measurements were performed in dry DCM on a CHI 620C electrochemical analyzer with a three-electrode cell, using 0.1 M n-Bu4NPF6 as supporting electrolyte, AgCl/Ag as reference electrode, gold disk as working electrode, Pt wire as counter electrode, with a scan rate of 50 mV/s. The potential was externally calibrated against the ferrocene/ferrocenium (Fc/Fc + ) couple. The HOMO and LUMO energy levels were calculated according to the equations: HOMO = -(4.8 + Eox onset ) eV and LUMO = -(4.8 + Ered onset ) eV, where the Eox onset and Ered onset are the onset potentials of the first oxidative and reductive redox wave, respectively. Chiral HPLC analysis was conducted on a Shimadzu Prominence 2000 instrument.
CD spectra were obtained on JASCO J-1500 at 25 ℃.

Synthesis of compound 5a:
S4 Phenylacetylene (3.27 g, 32.0 mmol) was dissolved in 200 mL of anhydrous THF and the solution was cooled down to 0 ℃ with an ice bath. Ethylmagnesium bromide (10.0 mL, 30.0 mmol, 3 M in diethyl ether) was added dropwise and the mixture was allowed to stir for 5 hours.
Compound 4′ (4.54 g, 10 mmol) was then added dropwise to the freshly prepared Grignard reagent at 0 ℃. The mixture was then slowly warmed up to room temperature and stirred for 14 hours.
The mixture was then slowly warmed up to room temperature and stirred for 14 hours.

Synthesis of compound 7a:
Phenylacetylene (6.55 g, 64.0 mmol) was dissolved in 200 mL of anhydrous THF and the solution was cooled down to -78 ℃ with an acetone/dry ice bath. n-BuLi (30.0 mL, 60.0 mmol, 2 M in cyclohexane) was added dropwise and the mixture was allowed to stir for 3 hours. Then 1,5diiodoanthraquinone (4.6 g, 10.0 mmol) was added dropwise to the freshly prepared lithium reagent at -78 ℃. The mixture was then slowly warmed up to room temperature and stirred overnight. Subsequently, a solution of SnCl2 (9.45 g, 50.0 mmol) in 40 mL of 3M HCl was added to the reaction mixture, and the solution was further stirred for 3 hours at room temperature.
Afterwards, the mixture was poured into 200 mL of water and the solid was filtered and collected.

Synthesis of compound 7b:
1-Hexyne (5.32 g, 64 mmol) was dissolved in 200 mL of anhydrous THF and the solution was cooled down to -78 ℃ with an acetone/dry ice bath. n-BuLi (30.0 mL, 60.0 mmol, 2 M in cyclohexane) was added dropwise and the mixture was allowed to stir for 3 hours. Then 1,5diiodoanthraquinone (4.6 g, 10.0 mmol) was added dropwise to the freshly prepared lithium reagent at -78 ℃. The mixture was then slowly warmed up to room temperature and stirred overnight. Subsequently, a solution of SnCl2 (9.45 g, 50.0 mmol) in 40 mL of 3M HCl was added to the reaction mixture, and the solution was further stirred for 3 hours at room temperature.