π-Extended Diaza[7]helicenes by Hybridization of Naphthalene Diimides and Hexa-peri-hexabenzocoronenes

Abstract

The synthesis of an unprecedented, π-extended hexabenzocorene (HBC)-based diaza[7]helicene is presented. The target compound was synthesized by an ortho-fusion of two naphthalene diimide (NDI) units to a HBC-skeleton. A combination of Diels-Alder and Scholl-type oxidation reactions involving a symmetric di-NDI-tolane precursor were crucial for the very selective formation of the helical superstructure via a hexaphenyl-benzene (HPB) derivative. The formation of the diaza[7]helicene moiety in the final Scholl oxidation is favoured, affording the symmetric π-extended helicene as the major product as a pair of enantiomers. The separation of the enantiomers was successfully accomplished by HPLC involving a chiral stationary phase. The absolute configuration of the enantiomers was assigned by comparison of circular dichroism spectra with quantum mechanical calculations.

Keywords: chirality; helicene; hexabenzocoronene; naphthalene; π-extension.

Figure 1

General structure of a chiral, π‐extended diaza[7]helicene based on HBC and NDI.

Scheme 1

Synthetic concept to generate 7‐helical HBC 3.

Scheme 2

Synthesis of symmetric and asymmetric di‐NDI HPBs 2 and 10. a) PdCl₂(PPh₃)₂, CuI, Et₃N, toluene, 80 °C, 18 h, yield 82 %. b) TFA, DCM, RT, 3 h. c) DMF, 140 °C, 18 h, yield 56 %. d) Tetrakis‐(tert‐butyl)tetracyclone, toluene, 220 °C, 24 h, yield 29 % (2) and 30 % (10).

Scheme 3

Synthesis of the naphthalene imide benzimidazole isomers 5 (orange) and 12 (black) and their ¹H NMR and UV/Vis absorption spectra. a) Imidazole, 160 °C, 3 h, yield 36 % (5) and 42 % (12).

Figure 2

Crystal structure of 4‐bromo‐1,2‐naphthaleneimidebenzimidazole (5).

Scheme 4

Final Scholl‐oxidation of HPB 2 leading to the three possible isomers symmetrical HBC 13, 7‐helical HBC 3, and asymmetrical HBC 14; hydrogen atoms without COSY‐correlation: H1 highlighted in orange a) FeCl₃, CH₃NO₂, DCM, 0 °C, o.n. (overnight), yield 43 %.

Figure 3

Important regions of the ¹H‐NMR spectrum of 7‐helical HBC 3.

Figure 4

Comparison of UV/Vis absorption spectra of HPB 2 (blue) and 7‐helical HBC 3 (green).

Figure 5

A) (M)‐ and (P)‐configuration of 7‐helcial HBC 3, tBu groups and swallow tail are omitted for clarity; B) chiral separation of 3 (Chiralpak 1BN‐5, n‐heptane/EtOH/diethylamine (DEA) 70:30:0.1, 4mLmin⁻¹, RT); C) CD spectroscopy measurements of (M)‐3 (black) and (P)‐3 (blue) and the corresponding calculated spectra (sTD‐CAM‐B3LYP/def2‐TZVP with D3: grey, light blue; without D3 red, green).

Figure 6

Geometry‐optimised structures of HBC 3 at B3LYP/def2‐SVP without dispersion correction; hydrogen atoms, tBu groups and swallow tail are omitted for clarity; A) top view with relevant distances between the two NDI arms; B) torsion angles of the inner helix; C) side view with interplanar angle of the inner helix; D) representation of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).