(2E,6E)-2,6-Bis(4-methylbenzylidene)cyclohex-3-en-1-one

The title compound, C22H20O, shows an approximately planar cyclohexenone ring [maximum deviation = 0.069 (4) Å], with a disordered position of the C=C bond [ratio = 0.71 (2)/0.29 (2)]. The benzene rings of the 4-methylbenzylidene units, attached in the 2- and 6-positions to the cyclohexenone ring, are rotated in the same direction by 28.6 (4) and 22.4 (4)°, with respect to the mean plane of the cyclohexenone ring [fraction 0.71 (2); maximum deviation = 0.06 (3) Å]. In the crystal, molecules are packed in the manner of a distorted hexagonal rod packing with their long axes all aligned along [201]. A number of C—H⋯π interactions stablize the crystal structure.

The title compound, C 22 H 20 O, shows an approximately planar cyclohexenone ring [maximum deviation = 0.069 (4) Å ], with a disordered position of the C C bond [ratio = 0.71 (2)/ 0.29 (2)]. The benzene rings of the 4-methylbenzylidene units, attached in the 2-and 6-positions to the cyclohexenone ring, are rotated in the same direction by 28.6 (4) and 22.4 (4) , with respect to the mean plane of the cyclohexenone ring [fraction 0.71 (2); maximum deviation = 0.06 (3) Å ]. In the crystal, molecules are packed in the manner of a distorted hexagonal rod packing with their long axes all aligned along [201]. A number of C-HÁ Á Á interactions stablize the crystal structure.

Related literature
For background information to aldol condensation reactions in hetero-and homocyclic ketones, see: Abaee et al. (2007). For the crystal structure of the analogous compound with 4methoxybenzylidene substituents in the 2-and 6-positions on the cyclohexenone ring, see: Abaee et al. (2007). For other similar substituted cyclohexenone structures, see: Shi et al.  Table 1 Hydrogen-bond geometry (Å , ).
Cg1, Cg2 and Cg3 are the centroids of the C8-C11,C12a,C13, C1-C6 and C15-C20 rings, respectively.  It crystallizes in the acentric space group P2 1 but the molecule shows pseudosymmetry m (C s ), with the mirror plane perpendicular to the main molecular plane. The C s symmetry is broken by the central symmetry-less cyclohexenone ring, but a second orientation of the molecule generated by this mirror plane appears as an alternative disordered orientation in a ratio of 0.71 (2)/0.29 (2) (Fig. 1). A split atom model was refined [C12a/C12b: occupancies 0.71 (2)/(0.29 (2)] using restraints providing the same bond lengths for corresponding atom pairs, C11-C12a/C13-C12b and C11-C12b/C13-C12a, of 1.475 (5)/1.319 (5)Å. The strong anisotropy of the displacement parameters of the O atom may be associated to this disorder. In addition, one of the terminal methyl groups, C21, showed 1:1 disorder over two orientations [occupancy of 0.5 for each of the six H-atom postions].

D-HÁ
A second pseudosymmetric mirror plane can be found in the main plane of the molecule, the realisation of which would lead to the centrosymmetric space group P2 1 /m. The clear inclination of both benzene rings [II (C1-C6) and III (C15-C20)] by 28.6 (4) and 22.4 (4)°, respectively, to the cyclohexenone ring I (O1,C8-C11,C12a,C13) rules out this possibility. Benzene rings II and III are inclined to one another by 8.66 (13)°.
In the presence of only one O atom besides 22 C and 20 H atoms, the absolute structure could not be determined, and from the synthesis the formation of a racemate is expected. In an analogous compound, with 4-methoxybenzylidene substituents in the 2-and 6-positions on the cyclohexenone ring (Abaee et al., 2007), the benzene rings are rotated in opposite directions with respect to the central ring plane, while in the title compound the rotation is in the same direction ( Fig. 1). In general the bond distances and angles are similar to those observed in analogues structures (Abaee et al., 2007;Shi et al., 2008;Guo et al., 2008).
In the crystal, molecules pack in the manner of a distorted hexagonal rod packing with their long axes all aligned along the [201] direction (Figs. 2 and 3). The intermolecular contacts are reinforced by C-H···π interactions (Table 1).

Experimental
A mixture of cyclohex-2-enone (193 µL, 2 mmol), 4-methylbenzaldehyde (471 µL, 4 mmol), triethylamine (1122 µL, 8 mmol), and ZnBr 2 (900 mg, 4 mmol) in 5 ml dry CH 2 Cl 2 was stirred at room temperature under argon atmosphere for 10 h. The progress of the reaction was checked by TLC using a 1:8 mixture of EtOAc/hexane. At the end of the reaction, the mixture was diluted with CH 2 Cl 2 and washed with brine. The organic layer was dried using Na 2 SO 4 and concentrated under reduced pressure. The product obtained was isolated (540 mg, 90%) by column chromatography over silicagel using a 1:8 mixture of EtOAc/hexane. The solid product was recrystallized from EtOAc to give light-orange block-like crystals of the title compound. and C12b, were set to be equal. In the final cycles of refinement, in the absence of significant anomalous scattering effects, 1386 Friedel pairs were merged and Δf " set to zero. All the H atoms could be located in a difference Fourier map. In the final cycles of refinement they were included in calculated positions and treated as riding atoms: C-H = 0.95 and 0.99 Å for CH and CH 3 H-atoms, respectively, with U iso (H) = k × U eq (parent C-atom), where k = 1.5 for CH 3 H-atoms and k = 1.2 for all other H-atoms.