Crystal structure of (Z)-2,3-dichloro-1,4-bis(4-methoxyphenyl)but-2-ene-1,4-dione

The title compound, C18H14Cl2O4, adopts a Z conformation around the cental C=C bond. The two aromatic rings of the molecule are nearly perpendicular to each other, with a dihedral angle between of 86.22 (14)°. The methoxy substituents lie close to the plane of the attached benzene rings. The C(ar)—C(ar)—O—C(Me) torsion angles are −2.4 (7) and 7.5 (6)°. Weak C—H⋯O interactions link the molecules forming a three-dimensional network. The crystal packing also displays short [3.160 (3) Å] Cl⋯O halogen-bonding contacts between molecules related by the screw axis. The structure exhibits disorder of one carbonyl O atom with a refined occupancy ratio of 0.21 (6):0.79 (6).


S1. Chemical context
Trichloromethyl groups are known to generate free radicals through homolysis of a C-Cl bond with relative ease in presence of radical initiators or UV-light or redox active metal salts (such as CuCl) or its complexes. A number of research papers are available that show radical intermediates produced from trichloromethyl group containing compounds. For example, CuCl or its complexes with bipyridine or bi-or tridendentate amine ligands have been used with trichloromethyl groups on a variety of organic substrates under non reducing conditions to generate free radicals.
Such radicals have been reported to undergo intramolecular and intermolecular cyclization or addition reactions along with mono-and/or-di redution products (Clark, 2002;Ram et al.,2007). The radicals produced in these reactions can also acts as a radical initiator in ATRP (Pintauer & Matyjaszewski, 2008). However, the presence of relatively better leaving group at the β-position of the radical centre leads to predominantly rearrangement and/or fragmentation reactions through the intermediate formation of contact ion pairs (Ram & Tittal, 2014a,b). It is important to mention here that 2,2,2-trichloroethylalkyl ethers and trichloromethyl carbinols with no leaving group at β-position to the trichloromethyl around carbon carbon double bond undergoes 1,2-H shift under similar conditions through the intermediate copper-carbenoid species (Ram & Tittal, 2014b). Keeping in view the above discussion, we have decided to study the behavior of the radicals produced from substituted trichloromethyl compounds with no suitably located carbon-carbon double bond and leaving group or any hydrogen atom at the β-position of the radical centre in order to restrict the intermolecular or intramolecular addition; ATRP; rearrangement and/or fragmentation or 1,2-H shift. The major product obtained under such conditions is reported here.

S2. Structural commentary
In the asymmetric unit (

S3. Supramolecular features
The crystal packing is stablized by short halogen bond Cl···O interactions ( Fig. 2

S5. Synthesis and crystallization
A two-neck round bottom flask fitted with a rubber septum was charged with CuCl (0.8 g, 0.008mol), 2,2′-bipyridine (bpy, 1.25 g, 0.008 mol) under continuous flow of nitrogen followed by addition of 1 mL dry dichloroethane (DCE) or benzene to ensure CuCl-bpy complex formation. To this reaction flask a solution of the 2,2,2-trichloro-1-(4-methoxyphenyl)-ethanone (0.004 mol) in dry DCE or benzene (5 mL) was injected through the septum with the help of a syringe and the reaction mixture was heated at reflux with stirring under a slow and continuous flow of nitrogen. The completion of the reaction was indicated by TLC (1-2 h). The reaction mixture was cooled and filtered through a celite pad. The filtrate was evaporated under reduced pressure and purified on a silica gel column chromatography using n-hexane and ethylacetate mixture as the solvent to get title compound in 60 or 70% isolated yields in DCE or benzene, respectively.

S6. Refinement
H atoms were placed in calculated positions and refined using riding model, with C-H 0.93 Å and U iso (H) = 1.2U eq (C) for aromatic C-H. Methyl groups were refined as idealised rotating groups, with C-H = 0.96 Å and U iso (H) = 1.5U eq (C). Disorder was modeled for O4 atom of the carbonyl group over two sets of sites with minor:major occupancy ratio of 0.21 (6): 0.79 (6). Similarity restraints were used for the C═O bond distances using SADI. Anisotropic displacement parameters of the minor component of oxygen atom O4 were constrained to those of the major component using EADP.

Figure 1
Molecular structure of the title compound, with atom labels and 50% probability displacement ellipsoids for non-H atoms.

Figure 2
A view of the crystal packing of the title compound. A weak C-H···O hydrogen bond is shown as a dashed line.

Figure 3
Part of the structure of the title compound showing O····Cl and C-H····O interactions.