Synthesis and crystal structures of (2E)-1,4-bis(4-chlorophenyl)but-2-ene-1,4-dione and (2E)-1,4-bis(4-bromophenyl)but-2-ene-1,4-dione

The molecular structure of (2E)-1,4-bis(4-chlorophenyl)but-2-ene-1,4-dione (1) is composed of two p-chloro phenyl rings, each bonded on opposite ends to a near planar 1,4-trans enedione moiety [–C(= O)—CH=CH—(C=O)–]. (2E)-1,4-Bis(4-bromophenyl)but-2-ene-1,4-dione (2) has a similar structure to (1), but with two p-bromo phenyl rings and a less planar enedione group. In the crystal, molecules of (1) exhibit C—Cl⋯Cl type I interactions, whereas molecules of (2) present C–Br⋯Br type II interactions.


Database survey
A search of the Cambridge Structural Database (CSD web interface; Groom et al., 2016) and the Crystallography Open Database (Gražulis et al., 2009) (Xu et al., 2013;CCDC 918566, BZOYEY01) and cis-1,2-dichlorobenzoylethylene (4) (Rabinovich et al., 1970;CCDC 112151, CBOZET) merit discussion because the former has a similar structure to the title compounds, whereas the latter is a stereoisomer of (1). The title compounds adopt an E configuration, similar to (3). They contain halogen atoms in the para position of the phenyl groups, unlike (3), but the rings are nearly planar as are those of (3), whose r.m.s value = 0.008 Å . The r.m.s. value, reflecting the planarity of the enedione moiety, in (1) is different to that of (3) (0.003 vs 0.0035 Å ), and the value determined for (2) (0.011 Å ). The dihedral angles between the ring planes of (1) and (2) are nearly identical to those of (3) [16 (average) vs 15.7 (1) ]. Unlike (1), its diastereomer (4) does not exhibit a planar enedione moiety and its near planar chlorophenyl rings (r.m.s deviation = 0.018 Å ) form a dihedral angle of 77.4 (3) with respect to each other. Superimposition of atom C1 of the E/Z diastereomers through the C7, Cl1, and O1 atoms yields an r.m.s. deviation of 0.033 Å . The remaining parts of the molecules are twisted from each other, with the planes containing the chlorophenyl group and adjoining carbonyl group of each stereoisomer forming a dihedral angle of approximately 79 .

Synthesis and crystallization
The title compounds were synthesized following a modified literature procedure (Sauer et al., 2017 Molecular conformations of (1) and (2) viewed along the b and c axes, respectively, showing type I and II halogen interactions, centroid-tocentroid distances, and short intramolecular HÁ Á ÁH interactions.

Figure 3
Crystal packing of (1) along the vicinity of the a axis. Dashed lines depict run 'neat' with chloro-or bromobenzene used in excess and serving also as the reaction solvent. Under a stream of nitrogen, aluminum chloride (3.6 g, 27 mmol, 2.9 equiv.) was dissolved in chloro-or bromobenzene (9.0 and 9.3 ml, respectively, 89 mmol, 9.6 equiv.) at room temperature. The reaction mixture was subsequently cooled to 273 K and fumaryl chloride (1.0 ml, 9.3 mmol, 1.0 equiv.) was added dropwise under constant stirring, at which point an instantaneous color change from clear to deep red was observed. The reaction mixture was then heated to 333 K for 2-4 days until fumaryl chloride was no longer detected on a TLC plate (SiO 2 , DCM). At the conclusion of the reaction, the mixture was cooled to room temperature, poured into ice-cold aqueous 1 M HCl, and extracted several times with DCM. The combined organic layers were washed with 0.5 M NaOH and dried over Na 2 SO 4 , and the volatiles were removed under reduced pressure. The resulting red-brown solid was recrystallized in DCM, further purified with a series of cold DCM washes, and dried under reduced pressure, affording either compound (1) (burnt orange solid, 1.5 g, 4.9 mmol, 53% yield) or (2) (yellow solid, 1.9 g, 4.8 mmol, 50% yield). Slow evaporation of DCM solutions saturated with either (1) or (2) yielded single crystals suitable for X-ray diffraction.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 1. The hydrogen atoms of both compounds were refined using a riding model with C-H = 0.93 Å and U iso (H) = 1.2U eq (C). Table 1 Experimental details.

(2E)-1,4-Bis(4-chlorophenyl)but-2-ene-1,4-dione (1)
Crystal data Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.