10,10'-Dimethoxy-9,9'-biazuleno[2,1-c]phenanthrene

Abstract

The title compound was prepared by the regioselective homocoupling of 10-methoxyazuleno[2,1-c]phenanthrene in the presence of ammonium persulfate. The structure of the synthesized compound was assigned on the basis of its ¹H-NMR, FT-IR, and mass spectral data. Its crystal structure and electrochemical properties are also reported.

Azulene and its derivatives, which are brilliant blue nonbenzenoid aromatic hydrocarbons, have garnered significant attention because of their remarkable electronic and optical properties. Significantly, the parent molecule exhibits a large dipole moment due to charged aromatic partial structures. Hence, azulene possesses donor-acceptor characteristics that may be exploited in advanced functional electronic, optoelectronic, and electrochromic devices. Among azulene derivatives, expansion of the π-conjugated system of 1,1'-biazulene derivatives has attracted significant interest owing to their unique spectral and redox properties [1,2,3]. In that context, we recently reported that azulene-fused helicenes could be formed through 1-functionalized [5]helicenes by Pt(II)-catalyzed cycloisomerization [4,5].

As a part of our research program, we herein report the synthesis of 10,10'-dimethoxy-9,9'-biazuleno[2,1-c]phenanthrene (2) using 10-methoxyazuleno[2,1-c]phenanthrene (1) and ammonium persulfate (APS) as an radical initiator (Scheme 1) [6]. The observed regioselectivity was attributed to the electron-rich 9-position of 1. The structure of 2 was assigned on the basis of its ¹H-NMR, FT-IR, and mass spectral data. Green crystals of 2 were obtained by recrystallization from dichloromethane/diethyl ether (1:1) (Figure 1) [7]. Intermolecular interactions were found in the crystals of enantiomers of 2, ((P, P, S) and (M, M, R)) (Figure S1).

Compound 2 was investigated by cyclic voltammetry in o-dichlorobenzene (Figure S2). The cyclic voltammogram of 2 revealed reversible two-stage one-electron oxidation processes (E₁^OX = 0.46, E₂^OX = 0.75 V (vs Fc/Fc⁺)) despite the twisted geometry of the neutral species. Such redox behavior may be attributed to the planar conformation, and may lead to spin and/or charge delocalization over the entire π-system [8,9]. The title compound is thus a good candidate for constructing reversible multistage redox systems.

Experimental Section

General Information

NMR spectra were recorded in CDCl₃ at ambient temperature on Bruker Avance III (500 MHz) spectrometer. The chemical shifts (δ) were recorded in ppm with residual CHCl₃ signal referenced to 7.26 ppm. Coupling constants (J) are reported in Hz and refer to the apparent peak multiplicities. The following abbreviations were used to explain the multiplicities: s = singlet, d = doublet, t = triplet, m = multiplet, br s= broad singlet, br d = broad doublet, br t = broad triplet. FT-IR spectra were recorded on a Perkin-Elmer Spectrum One (FT-IR) spectrometer (Yokohama, Japan). FT-IR absorptions were recorded in cm⁻¹. Melting point was recorded on a Yanako MP-13 (Tokyo, Japan). High resolution mass spectra were recorded on a Bruker micrOTOF (ESI-TOF) spectrometer (Yokohama, Japan).

Synthesis of 10,10'-Dimethoxy-9,9'-biazuleno[2,1-c]phenanthrene (2)

10-Methoxyazuleno[2,1-c]phenanthrene (1) was synthesized according to our previously reported method [3]. Under an argon atmosphere, a mixture of compound 1 (50 mg, 0.16 mmol) and APS (183 mg, 0.80 mmol) in degassed toluene (25 mL) was heated at 100 °C for 1 h. Additional APS (183 mg, 0.80 mmol) was added and the mixture was stirred for 2 h at 100 °C. After cooling to room temperature, the reaction mixture was washed with brine, and extracted with CHCl₃. The combined organic layers were dried over anhydrous MgSO₄, filtered, and concentrated under reduced pressure. The crude product was purified by flash column chromatography on silica gel 60N (spherical neutral, 40–50 µm, Kanto Chemical Co., Inc., Hexane/AcOEt = 50:1) to give 2 (40 mg, 80% yield) as a green solid.

Melting point: no change until 300 °C.

FT-IR (neat): ν_max (cm⁻¹): 2922, 1571, 1499, 1446, 1261, 1218, 954, 834, 789, 758, 729, 697, 682.

¹H-NMR (CDCl₃, 500 MHz): δ9.54 (d, J = 7.7 Hz, 2H), 9.35 (d, J = 8.2 Hz, 2H), 7.99 (dd, J = 8.0, 0.9 Hz, 2H), 7.86 (br d, J = 8.0 Hz, 2H), 7.80 (d, J = 8.5 Hz, 2H), 7.74 (br s, 2H), 7.60 (dt, J = 7.0, 1.0 Hz, 2H), 7.52 (t, J = 7.4 Hz, 2H), 7.34 (br t, J = 9.8 Hz, 4H), 6.75 (t, J = 9.7 Hz, 2H), 6.55 (br s, 2H), 3.12–3.25 (m, 6H).

HRMS (ESI-TOF): [M]⁺ calcd for C₄₆H₃₀O₂ 614.2240; found 614.2210.