4,12-Bis(2,2-dibromovinyl)[2.2]paracyclophane

In the title compound, C20H16Br4, both vinylic substituents were introduced by a Corey–Fuchs reaction using 4,12-diformyl[2.2]paracyclophane as starting material. The title compound may be used as a valuable precursor for the synthesis of diethynyl[2.2]paracyclophane. The title molecule is centrosymmetric with a crystallographic center of inversion between the centers of the two phenyl rings. A strong tilting is observed with an interplanar angle between the best aromatic plane and the vinyl plane of 49.4 (5)°. No significant intermolecular interactions are found in the crystal.

In the title compound, C 20 H 16 Br 4 , both vinylic substituents were introduced by a Corey-Fuchs reaction using 4,12diformyl[2.2]paracyclophane as starting material. The title compound may be used as a valuable precursor for the synthesis of diethynyl[2.2]paracyclophane. The title molecule is centrosymmetric with a crystallographic center of inversion between the centers of the two phenyl rings. A strong tilting is observed with an interplanar angle between the best aromatic plane and the vinyl plane of 49.4 (5) . No significant intermolecular interactions are found in the crystal.

Comment
In the context of our research in developing novel π-conjugated functionalized [2.2]paracyclophanes and ferrocenes (Clément et al., 2007a) for potential applications in coordination chemistry, we have recently reported an alternative to the classical Sonogashira synthesis (Morisaki et al., 2003) for the synthesis of ethynyl functionalized [2.2]paracyclophanes (Scheme 2). Our route involves a Corey-Fuchs reaction (Clément et al., 2007b) and subsequent dehydobromation induced by a strong base. In the first step, an ylide species, formed in situ by the interaction of zinc dust, CBr 4 and PPh 3 , reacts with the formyl derivative 1a or 1b leading to the dibromoolefin derivatives 2a or 2b. The molecular structure of the vinylic intermediate 2b was elucidated by an single-crystal X-ray diffraction study ( Figure 1).
2 b possesses a crystallographic center of inversion in the middle of the cyclophane framework. Bond lengths and angles may be considered as normal. Distortions typical of [2.2]paracyclophane systems, e.g. lengthened C-C bonds and widened angles in the bridges, narrowed ring bond angles at the bridgehead atoms, and boat-like distortion of the rings (the bridgehead atoms lie significantly out of the plane of the other four ring atoms) are observed. As previously noticed for the monodibromoolefin compound 2a, the alkenyl unit of 2b is strongly tilted. The two best planes of the arene (C3, C4, C5, C6, C7, C8; plane 1) and the vinyl-group (Br1, Br2, C1, C2; plane 2) possess a cutting angle of the normals of 49.4 (5)°C ontrary to a recently published, related system, no significant intermolecular interactions are observed for 2b due to inproper orientation of the molecules relative to each other (Hopf et al., 2007).
In the light of our recent work on the reactivity of (2,2-dibromovinyl)ferrocene towards thiolates (Clément et al., 2007b), the vinylic intermediates 2a and 2b should be promising starting materials for building new ligand systems. Synthesis, reactivity and photochemical properties of these new compounds will be reported in due course.

Experimental
PPh 3 (4.20 g, 16.0 mmol), CBr 4 (5.31 g, 16.0 mmol) and zinc dust (1.05 g, 16.0 mmol) are placed in a Schlenk tube and CH 2 Cl 2 (45 ml) is added slowly. The mixture is stirred at room temperature for 28 h. Then, 1 b (1.05 g, 4.00 mmol), dissolved in CH 2 Cl 2 (20 ml), is added and stirring is continued for 2 h. The reaction mixture is extracted with three 50 ml portions of pentane. CH 2 Cl 2 is added when the reaction mixture became too viscous for further extractions. The extracts are filtered and evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel with CH 2 Cl 2 /petroleum ether (1:1). Slow evaporation afforded white crystals of 2 b (Yield: 94%). mp 183°C, 1 H NMR

Refinement
Refinement was accomplished by full-matrix least-squares methods (based on Fo2, SHELXL97); anisotropic thermal parameters for all non-H atoms in the final cycles; hydrogen atoms were placed in calculated positions and refined using a riding model with U iso (H) = 1.2 U eq (C). Fig. 1. ORTEP plotof 2b with 30% probability level.   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Figures
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )