Cyclophanes, Part 73: diastereoselective dimerization of an α,β-unsaturated ketone. Structure of all-trans-1,3-dibenzoyl-2,4-di([2.2]paracyclophan-4-yl)cyclobutane

1 Introduction [1]

While studying the behaviour of the α,β-unsaturated ketone 1 as a Michael acceptor towards enolates derived from ketones such as acetophenone, we noted that, upon refluxing a solution of 1 in benzene for several days, a dimer 2 was formed in small yields (up to ca. 10 %), the structure of which, according to its spectroscopic and analytical data, seemed to be that of a [2+2]dimer of the substrate 1 [2]. To corroborate our structural proposal, and to establish the stereochemical relationships between its four substituents, we carried out an X-ray structural diffraction study of the dimer.

2 Results and discussion

The structure determination of 2 (Fig. 1) confirms that it is the expected dimer with all-trans configuration. It is well known that α,β-unsaturated ketones can be (photo)dimerized [3], whereby diastereomeric mixtures of photoproducts are usually formed. In our case the steric bulk of the 4-[2.2]paracyclophanyl substituent evidently leads to a process with high diastereoselectivity. If the above reaction is indeed a photochemical process, normal daylight is clearly sufficient to initiate it.

Fig. 1:

Molecular structure of compound 2 in the crystal. Ellipsoids represent 30 % probability levels, H atoms are drawn as spheres with arbitrary radii.

The formal twofold axis of the formula diagram of 2 is not reflected in any crystallographic or even approximate twofold symmetry in the solid state, because the substituents at the cyclobutane ring are differently oriented.

The cyclophane units display the usual features reflecting the strain of the system (see e.g., [4]); thus the single bonds in the bridges are elongated (av. 1.580 Å) except for the possibly disordered bridge (see below), the sp³ bridge angles are widened (av. 113.3°), the rings display a flattened boat conformation with narrow sp² angles (av. 116.9°) at the bridgehead atoms, and the non-bonded distances between the rings are short.

The geometry of cyclobutane rings has been discussed in detail by Allen [5, 6]. Compound 2 is consistent with the general pattern, involving slightly lengthened C–C bonds (av. 1.559 Å) and a non-planar ring (fold angle 27.9(1)°, av. torsion angle 19.7°).

3. Spectroscopic and analytical data

For dimer 2 the following spectroscopic and analytical data were recorded [2]: ¹H NMR (400.1 MHz, CDCl₃, int. TMS): δ = 2.34–2.44 (m, 2 H), 2.74–3.11 (m, 14 H), 3.34–3.38 (m, 1 H), 4.36–4.41 (m, 2 H), 4.81–4.85 (m, 1 H), 5.49–5.51 (dd, J₁ = 1.60, J₂ = 7.79 Hz, 2 H), 6.22 (d, J = 7.66 Hz, 2 H), 6.29–6.31 (dd, J₁ = 1.57, J₂ = 7.87 Hz, 2 H), 6.32–6.38 (m, 6 H), 6.44 (br. s, 2 H), 7.17–7.21 (m, 2 H), 7.37–7.41 (m, 1 H), 7.51–7.53 (m, 2 H), 7.72–7.81 (m, 3 H), 8.53–8.55 ppm (m, 2 H). – ¹³C NMR (100.6 MHz, CDCl₃): δ = 200.80, 199.87 (s), 140.05 (s), 139.76 (s), 139.45 (s), 138.66 (s), 138.06 (s), 137.67 (s), 137.02 (s), 135.33 (d), 133.91 (d), 132.91 (d), 132.58 (d), 131.68 (d), 131.34 (d), 130.38 (d), 130.10 (d), 129.24 (d), 128.71 (d), 128.47 (d), 128.10 (d), 54.55, 42.68 (d), 43.08 (d), 35.12, 35.06 (t), 34.00 (t), 33.65 ppm (t). – IR (KBr): ν˜ = 3058 (vw), 2930 (m), 1678 (C=O, vs), 1594 (m), 1445 (m), 1344 (m), 1238 (s), 798 (m), 718 (s), 696 cm^-1 (s). – UV (acetonitrile): λ_max (lg ε) = 196 (5.03), 228 (4.60), 250 nm (4.49). – MS (EI, 70 eV): m/z (%) = 676 (16, M⁺), 571 (12), 338 (65), 105 (100). – C₅₀H₄₄O₂ (676.90): calcd. C 88.99, H 6.27; found C 88.56, H 6.59. – m. p. 265 °C.

4 Crystal structure determination

Suitable single crystals of dimer 2 in the form of colorless prisms were obtained, following NMR investigations [2], by slow evaporation of the solvent (CDCl₃) from a corresponding solution.

Crystal data: C₅₀H₄₄O₂, M_r = 676.85, monoclinic, space group P 2₁/n, T = –143 °C, a = 11.4067(8), b = 25.2008(16), c = 13.4861(10) Å, β = 113.443(3)°, V = 3556.7 ų, Z = 4, F(000) = 1440 e, λ(MoK_α) = 0.71073 Å, μ = 0.08 mm^–1, D_x = 1.264 g cm^–3. Data collection: A colourless tablet ca. 0.3 × 0.3 × 0.15 mm was mounted in inert oil on a glass fibre and transferred to the cold gas stream of a Bruker SMART 1000 CCD diffractometer. A total of 43 989 data were recorded to 2θ = 55°, of which 8139 were unique (R_int = 0.069). Structure refinement: The structure was refined anisotropically on F² using Shelxl-97 [7, 8]. Cyclobutane hydrogens were refined freely; other H were included using a riding model. The final wR 2 (all reflections) was 0.132 for 8139 intensities and 485 parameters, with R 1 (I > 2 σ(I)) = 0.049; S = 1.03, Δρ(max) = 0.52 e Å^–3.

The bridge atoms C11 and C12 display somewhat larger U values, especially laterally, than the remainder of the structure, and the largest difference peaks are observed near these atoms; furthermore the bond C11–C12 is shorter than expected. These effects all imply possible libration or disorder effects, but despite much effort no convincing disorder model of the bridge could be refined.

CCDC 1048234 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre viawww.ccdc.cam.ac.uk/data_request/cif.