Controlling the Helicity and Handedness of Polyaromatics with Isobenzofuranophane

Abstract

The extent of helicity in nonplanar polyaromatic materials affects their electronic, optical and chiroptical properties. However, controlling helicity in different systems typically requires different multistep synthetic approaches for each target compound, often with significant complexity, thereby limiting its applicability. Here, we introduce a helical synthon, isobenzofuranophane, with either a short or long tether, which enables late-stage helicity induction and control in aromatic frameworks. We demonstrate the applicability of this synthon by reacting it with a [5]helicene aryne precursor, where both the handedness and helicene pitch are remotely-controlled by the tether length, directly affecting the (chiro)optical properties of the helicene. The X-ray structures support these findings with up to 35° torsion for a single benzene ring, demonstrating the potential of isobenzofuranophane to induce significant curvature to polyaromatic molecules.

Keywords: Chirality; Circular dichroism; Circularly; Cyclophanes; Helicenes; Twistacenes.

Figure 1

a) The dihedral ABCD angle accounts for end‐to‐end twist and pitch in twistacene and helicene, respectively. b) Induction of twistacene handedness using helicene. c) Axially‐chiral benzyne synthon. d) The synthesis of helically‐locked tethered twistacene, which involves multiple steps. e) Axially chiral aryne synthon. f) The concept of a tethered dienophile as a “synthon” to induce specific helicity and handedness. g) The current work: utilizing tethered isobenzofuran as a synthon to control the degree of helicity and the handedness of helicene.

Figure 2

The synthesis of isobenzofuranophane synthons 1‐Cn and the helicene‐twistacene products HT‐Cn. Reagents and conditions: a). (1) phenol, AlCl₃, C₂H₂Cl₄, 120 °C, 48 hr, (2) KOH, H₂O, NaBH_4, rt_, 24 h, 50% yield; b). 2‐(hydroxy) phenylboronic acid, Pd(PPh₃)₄, Na₂CO₃, H₂O/dioxane (1:4), 100 °C, 24 hr, 45% yield; c). Br‐(CH₂)_n‐Br, K₂CO₃, DMF, 60 °C, 48 hr (n = 4, 8), 46% yield; d). (1) phenylboronic acid, Pd(PPh₃)₄, K₂CO₃, H₂O: dioxane (1:4), 100 °C, 24 hr; (2) PhMgBr, THF, −40 °C→ 0 °C, 1 hr; (3) CF₃CO₂H, 0 °C→25 °C, 1.5 hr, 44% yield; e). CsF, trifluoromethanesulfonate, acetonitrile/DCM (1:4), rt, 12 hr,; f. NaI, trimethylsilyl chloride, 0 °C, 30 min 42%–65% yields.

Figure 3

a) X‐ray and b) calculated (B3LYP/6–311G(d)) structures of isobenzofuranophene synthon 1‐C8 and 1‐C4 (P enantiomers), respectively. c) HOMO and LUMO energy levels of naphthalene versus isobenzofuran upon twisting, calculated at the B3LYP/6–311G(d) level. d) ECD spectra of P‐1‐C8 and M‐1‐C8.

Figure 4

a) Top and c) side views of the X‐ray structures (left to right) for HT‐C4, HT‐C8, HT‐C0 and CA‐C4 (the phenyl rings and tether in the side view are omitted for clarity). b) Side view of the central ring, shared between the helicene and anthracene (denoted in blue).

Figure 5

a) ECD spectrum of HT‐Cn in chloroform at rt. Calculated spectra for M‐HT‐C4 and the three main lowest‐energy transitions (purple lines). Right: electron density maps for the main lowest‐energy transitions (TDDFT/CAM‐B3LYP/6–311G(d)), yellow→blue represents the difference in electron density for S0→Sn. b) VT‐ECD of HT‐C8 (red) and HT‐C4 (blue). Measured in tetrachloroethane at −80 and 25 °C, and in toluene at 90 °C. c) Relative free energies (ΔG ^‡ and ΔG ⁰, in kcal mol⁻¹) for the isomerization process of HT‐C8 (red) and HT‐C4 (blue). Calculated at the B3LYP/6–311G(d)/D3 level.

Figure 6

a) Calculated relative P/M helicene populations and energies measured at different angles of the naphthalene moiety. b) EXSY NMR of CA‐C4, showing correlation between two diastereomers, (the correlation between protons at the 1 and 14 positions, designated in red, is displayed). c) ECD spectra of the non‐tethered HT‐C0 in tetrachloroethane at 90 °C measured at intervals of 10 min, showing racemization. The calculated barrier is 25 kcal mol⁻¹.