Chiral Dibenzopentalene-Based Conjugated Nanohoops through Stereoselective Synthesis

Abstract

Conjugated nanohoops allow to investigate the effect of radial conjugation and bending on the involved π-systems. They can possess unexpected optoelectronic properties and their radially oriented π-system makes them attractive for host-guest chemistry. Bending the π-subsystems can lead to chiral hoops. Herein, we report the stereoselective synthesis of two enantiomers of chiral conjugated nanohoops by incorporating dibenzo[a,e]pentalenes (DBPs), which are generated in the last synthetic step from enantiomerically pure diketone precursors. Owing to its bent shape, this diketone unit was used as the only bent precursor and novel "corner unit" in the synthesis of the hoops. The [6]DBP[4]Ph-hoops contain six antiaromatic DBP units and four bridging phenylene groups. The small HOMO-LUMO gap and ambipolar electrochemical character of the DBP units is reflected in the optoelectronic properties of the hoop. Electronic circular dichroism spectra and MD simulations showed that the chiral hoop did not racemize even when heated to 110 °C. Due to its large diameter, it was able to accommodate two C60 molecules, as binding studies indicate.

Keywords: antiaromaticity; chiral macrocycles; chiral resolution; cycloparaphenylenes; fullerenes.

Figure 1

A) DBP‐based chiral nanohoop 1 synthesized herein (and derivatives 2‐1 and 2‐2 used for calculations); B) Synthetic strategy using chiral, enantiopure diketone units as precursors to bent DBP units.

Scheme 1

Racemic resolution of 2,7‐dibromo‐diketone 3 and molecular structure of bis‐adduct 6 in the solid state (displacement ellipsoids are shown at 50 % probability; hydrogen atoms and co‐crystallized chloroform molecule are omitted for clarity).

Scheme 2

Stereoselective synthesis of the chiral, DBP‐based nanohoops (+)‐1 and (−)‐1 (the stereocenters assigned by (R) or (S) are indicated by an asterisk, for 14, 16 and 1, the structures of the (R,R)⁶/(+)‐isomers are shown).

Scheme 3

Synthesis of reference compound 7.

Figure 2

Selected regions of the ¹H NMR spectra of hoop (+)‐1 and reference compound 7 (CDCl₃, 500 MHz, 300 K).

Figure 3

Calculated structure of 2‐1 (PBEh‐3c, H‐atoms are omitted for clarity) and geometrical parameters.

Figure 4

14 possible diastereomers 1 a–1 n of hoop 1 with relative energies in kcal mol⁻¹ (B97‐3c//GFN2‐xTB; for the pairs of enantiomers, one enantiomer each is shown).

Figure 5

A) Electronic circular dichroism (ECD) spectra of (+)‐1 and (−)‐1 (CHCl₃, 20 °C) with spectrum simulated for enantiomer 1 a (sTDA‐xTB level out of 100 snapshots of a GFN2‐xTB/GBSA(toluene) MD‐simulation of 100 ps length); B–D) ECD‐spectra of (+)‐1 (3.34×10⁻⁵  m in PhCl) during (B) heating at a rate of 2 °C min⁻¹, (C) holding the temperature at 110 °C and (D) cooling at a rate of 2 °C min⁻¹.

Figure 6

A) UV/Vis absorption spectra of (+)‐1 and reference compound 7 (CH₂Cl₂) and photos of (+)‐1 as powder and in solution; B) cyclic voltammogram of (+)‐1 (0.16 mm in CH₂Cl₂ with 0.1 m n‐Bu₄NPF₆, scan rate 0.1 V s⁻¹, glassy carbon electrode); C) Selected orbital energies and D) plots for 2 (B3LYP‐D3/def2‐TZVP//PBEh‐3c).

Figure 7

A) Selected regions of ¹H NMR spectra during titration of (+)‐1 with C₆₀ (0 to 12 equiv. from bottom to top, [D₈]toluene, 300 MHz, 300 K); B) Calculated equilibrium structures of 1⊃C₆₀ (GFN2‐xTB/GBSA (toluene)).