(9R,9aS,12aR,13S)-9,13-Diphenyl-9,9a,12a,13-tetrahydro-9,13-methanotriphenyleno[2,3-c]furan-10,12,14-trione

Abstract

X-ray crystallography was used to characterise the title compound for the first time, and the ¹H NMR, ¹³C NMR and IR spectroscopic data from earlier reports were also updated.

1. Introduction

The [4 + 2] cycloaddition reaction of phencyclone 1 and maleic anhydride 2 in refluxing benzene [1] or toluene [2] affords cycloadduct 3 as the sole product (Scheme 1).

Cycloadduct 3 was used in studies on the control of reactivity and regioselectivity of cycloaddition reactions of phencyclone with a range of electron-deficient dieneophiles [1]. Compound 3 has also featured in studies of photochemical decarbonylation reactions of norbornene-7-ones [3]. More recently, 3 has been used as an intermediate to prepare maleimides, and the formation of compound 5 is shown as an example in Scheme 2 [2]. Compound 5 is an example of a “molecular balance”: this type of molecule allows interactions between alkyl C—H bonds and aryl π-systems to be investigated [2,4,5,6]. Maleimides such as 5 (Figure 1) exhibit restricted rotation around the N–C bond, and the resulting “unfolded” (5a) and “folded” (5b) conformers can be probed using ¹H NMR spectroscopy.

2. Results

Phencyclone 1 readily reacts with maleic anhydride 2 in refluxing benzene or toluene, however, toluene was preferred for use in this work. The reaction is usually complete within 20–30 min, the end point is readily identified since phencyclone forms a dark green solution in toluene, which fades as the starting material is consumed. Phencyclone can be purchased from commercial suppliers, alternatively, it can easily be prepared from 1,3-diphenylacetone and 9,10-phenanthraquinone [7,8]. Cycloadduct 3 can be stored in a cool, dark place for several years without significant decomposition, however, it is not stable if heated to melting point. Under these conditions, decomposition will occur, and the vigorous release of CO gas will take place as soon as the melting point is reached [1]. The stability of 3 is also limited if stored in hygroscopic solvents, on prolonged storage, the hydrolysis of the cyclic anhydride can take place.

Material that was satisfactory for study by X-ray crystallography was obtained by dissolving samples of compound 3 (0.05–0.1 g) in ethyl acetate (5–10 mL). Where necessary, the mixtures were heated to ensure that all the solid material had dissolved. The samples were stored for several days, until suitable crystals were formed and isolated. The crystal structure of 3 (Figure 2) confirms the results from earlier studies that the compound is isolated as the endo-isomer [1,2]. As noted above, cycloadduct 3 releases carbon monoxide gas when heated to the melting point, and this phenomenon is known to occur in other phencyclone cycloadducts [8,9] and it is attributed to a cheletropic CO extrusion reaction. In a published study, crystallography techniques were used to survey certain structural characteristics of bridged cyclopentenones and comparisons were made to their reactivity in cheletropic decarbonylation reactions [9]. It was concluded that the C=O and highlighted C–C bond distances in bridged cyclopentenones can be useful indicators of the potential for cheletropic decarbonylation to take place. The survey of bridged cyclopentenone structures indicated that C-C bonds broken during the cheletropic reaction are typically longer than those expected for the associated cyclopentanone. In contrast, the C=O bond (that can be lost as CO) is shorter than would be expected in the corresponding bridged cyclopentanone [9]. The crystal structure of 3 has allowed the relevant bond distances to be measured and compared with those expected. In this case, the C1–O1 bond distance is 1.199(4) Å, the C2–C3 bond distance is 1.539(4) Å and the C16–C17 bond distance is 1.535(4) Å. These data are consistent with similar structures reported in the literature [8,9], for comparison, the relevant C–C and C=O bond distances in bridged cyclopentanones are typically 1.519 Å and 1.207 Å, respectively [9].

The IR spectrum of 3 would be expected to show a high-frequency C=O stretch due to bond angle compression in the cyclopentenone ring [10] (the crystal structure shows that the C2–C1–C17 bond angle is 99.4(2)°). There is an absorption at 1792 cm⁻¹ which is consistent with this expectation (an IR spectrum has been provided in the Supporting Information document), however, the anhydride C=O (asymmetric, out of phase stretch) signal also likely occurs in the same region of the IR spectrum [11]. In this case, both C=O signals are likely to be very close together and may not be resolved. The anhydride C=O (symmetric, in-phase stretch) signal is less intense but is visible at 1855 cm⁻¹. The ¹³C NMR spectrum of compound 3 confirms the presence of the ketone and anhydride carbonyl groups (195.0 and 170.6 ppm, respectively, are present).

The NMR spectra indicate that the bridgehead phenyl groups exhibit restricted rotation around the C(sp²)–C(sp³) bond (¹H and ¹³C NMR spectra have been provided in the Supporting Information document). Rapid C–C rotation on the NMR timescale would be expected to lead to the observation of three chemical shift environments associated with the phenyl groups. In this example, five signals in the ¹H NMR spectrum can be attributed to the phenyl ring protons. This phenomenon has previously been noted in the spectra of [4 + 2] cycloadducts obtained from phencyclone and maleimides and detailed spectroscopic studies of these compounds have been reported [12]. The restricted rotation of phenyl groups in this class of compounds was ascribed to steric interactions between the ortho-protons of the phenyl ring and protons in close proximity from the adjacent phenanthrene ring system (Figure 3).

In summary, the first X-ray crystal structure of 3 was obtained which confirms the findings of previous studies that endo-cycloadduct 3 is the product from the [4 + 2] cycloaddition reaction of phencyclone 1 with maleic anhydride 2. The crystallographic data provides useful insight into certain spectroscopic properties and thermal decomposition behaviour of the title compound.

3. Experimental Section

Melting points were recorded on an SMP3 melting point apparatus and are uncorrected. IR spectra were recorded on a Perkin Elmer Spectrum Two instrument with DTGS detector and diamond ATR attachment. NMR spectra were obtained for ¹H at 500 MHz and for ¹³C at 125 MHz using a Bruker AVIII 500 instrument. Spectra were run at 25 °C in CD₃SOCD₃. Chemical shifts are reported in ppm to high frequency of the reference and coupling constants J are reported in Hz.

(9R,9aS,12aR,13S)-9,13-Diphenyl-9,9a,12a,13-tetrahydro-9,13-methanotriphenyleno[2,3-c]furan-10,12,14-trione (3)

A solution of phencyclone 1 (0.5 g, 1.3 mmol) and maleic anhydride 2 (0.32 g, 3.25 mmol) in toluene (10 mL) was heated under reflux for 15–20 min (until the green colour due to dissolved phencylone was no longer visible). Upon cooling to room temperature, methanol (5 mL) was added to the reaction flask and the resulting mixture was cooled in an ice bath for 15 min. A colourless solid was formed, which was filtered off and washed with ice-cold methanol (3 × 5 mL) to afford product 3 (0.45 g, 72%) as a colourless solid, mp 298–300 °C (lit. [1] 296–298 °C). IR (ATR) 3035 (ArCH) 1855 (C=O), 1792 (C=O), 1498, 1448, 900, 770, 758, 695, 508 cm⁻¹; ¹H NMR (500 MHz, CD₃SOCD₃); 8.93 (2H, d, J = 8.4 Hz, ArH), 8.18 (2H, d, J = 7.8 Hz, PhH), 7.82 (2H, apparent t, J = 7.7 Hz, PhH), 7.71–7.55 (4H, m, overlapping ArH and PhH), 7.50 (2H, apparent t, J = 7.5 Hz, PhH), 7.30 (2H, apparent t, J = 7.8 Hz, ArH), 7.23 (2H, d, J = 7.8 Hz, PhH), 7.06 (2H, d, J = 8.4 Hz, ArH), 5.17 (2H, s, CH); ¹³C NMR (125 MHz, CD₃SOCD₃) 195.0 (C=O), 170.6 (C=O), 133.9 (ArCq), 133.7 (ArCq), 131.4 (ArCq), 131.4 (ArCH), 129.8 (ArCH), 129.0 (overlapping, 3 × ArCH), 127.9 (ArCH), 127.1 (ArCH), 126.0 (ArCq), 125.4 (ArCH), 124.3 (ArCH), 63.0 (C-Ph), 46.9 (CH).

Colourless X-ray quality crystals of 3 were grown from ethyl acetate solution. X-ray diffraction data for compound 6 were collected at 173 K using a Rigaku FR-X Ultrahigh Brilliance Microfocus RA generator/confocal optics with XtaLAB P200 diffractometer [Mo Kα radiation (λ = 0.71073 Å)]. Intensity data were collected using ω steps accumulating area detector images spanning at least a hemisphere of reciprocal space. Data were collected using CrystalClear [13] and processed (including correction for Lorentz, polarization and absorption) using CrysAlisPro [14]. The structure was solved by dual-space methods (SHELXT) [15] and refined by full-matrix least-squares against F2 (SHELXL-2018/3) [16]. Non-hydrogen atoms were refined anisotropically, and hydrogen atoms were refined using a riding model. All calculations were performed using the Olex2 [17] interface. The showed structure was refined as a two-component, non-merohedral twin, with refined twin fractions of 0.704(5):0.296(5). CCDC 2192702 contains supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.

Crystal data for C₃₃H₂₀O₄ (M = 480.49): triclinic, space group P$\overline{1}$ (no. 2), a = 9.9060(6), b = 11.1504(7), c = 11.9180(11) Å, α = 117.827(8), β = 94.797(6), γ = 100.060(5)°, V = 1125.47(16) ų, Z = 2, T = 173 K, μ(Mo Kα) = 0.093 mm⁻¹, ρ (calc) = 1.418 g/cm³, 14793 reflections measured (3.936° ≤ 2θ ≤ 58.010°), 4911 unique (R_int = 0.0472, R_sigma = 0.2), which were used in all calculations. The final R₁ [I > 2σ(I)] was 0.0757 and wR₂ (all data) was 0.2567.