4-(3-Methoxyphenyl)-2,6-dimethylcyclohex-3-enecarboxylic acid

The racemic title compound, C16H20O3, was synthesized to study the hydrogen-bonding interaction of the two enantiomers in the solid state. In the crystal structure, R and S pairs of the racemate are linked by pairs of intermolecular O—H⋯O hydrogen bonds, producing centrosymmetric R 2 2(8) rings.

The racemic title compound, C 16 H 20 O 3 , was synthesized to study the hydrogen-bonding interaction of the two enantiomers in the solid state. In the crystal structure, R and S pairs of the racemate are linked by pairs of intermolecular O-HÁ Á ÁO hydrogen bonds, producing centrosymmetric R 2 2 (8) rings.

Related literature
For similar compounds in which the racemates also consist of carboxylic acid RS dimers, see: Xie et al. (2002Xie et al. ( , 2007aXie et al. ( , 2008a. For the structure of the precursor of the title compound, which is achiral and forms hydrogen-bonded dimers, see: Xie et al. (2007b). The chirality of the title compound is solely generated by the presence of the double bond in the cyclohexene ring, see: Xie et al. (2004). For hydrogen-bond motifs, see: Bernstein et al. (1995).  Table 1 Hydrogen-bond geometry (Å , ).

Comment
The title carboxylic acid was prepared to study the interaction of the two enantiomers in the solid state. We have previously reported the structure of its precursor, which is achiral and forms hydrogen-bonded dimers (Xie et al., 2007b). The chirality of the title compound is solely generated by the presence of the double bond in the cyclohexene ring (Xie et al., 2004).
The resultant racemate is made up of carboxylic acid RS dimers (Xie et al., 2002(Xie et al., , 2007a(Xie et al., , 2008a. The structure and atom numbering are shown in Fig. 1, which illustrates the half-chair conformation of the cyclohexene ring. The torsion angles involving atoms C4, C5, C6, C1, and C2 are near 180°. The carboxyl group is almost perpendicular to the cyclohexene ring with an angle of 85.3° between the O1-C14-O2-C3 plane and the C1-C6 ring. The double bond between C5-C6 is not fully conjugated with the aromatic ring as shown by the C1-C6-C5 plane to benzene ring angle of 38.7°. Unlike other previously reported para substituted analogs and like other previously reported meta substituted analogs (Xie et al., 2008b), the molecule also has a chiral axis due to the meta methoxy substituent on the aromatic ring. Fig. 2 shows the hydrogen bonding scheme. Atom O2 acts as a donor in an intermolecular hydrogen bond to atom O1, producing an R22(8) ring (Bernstein et al., 1995), thus creating a hydrogen-bonded dimer. There is no evidence to suggest that weak directional interactions interconnect the dimers. Hydrogen bond geometry is given in Table 1.

Experimental
The title carboxylic acid was synthesized following the similar method reported by Xie et al., 2002. Purified compound was recrystallized from hexane-dichloromethane as colorless plates (m.p. 415-417 K).

Refinement
The data collection was carried out using synchrotron radiation (λ= 0.44280, diamond 111 monochromator, two mirrors to exclude higher harmonics) with a frame time of 2 second and a detector distance of 6.0 cm. A randomly oriented region of reciprocal space was surveyed to the extent of a hemisphere. Two major sections of frames were collected with 0.50°s teps in φ and a detector position of -20° in 2θ. Data to a resolution of 0.84 Å were considered in the reduction. Final cell constants were calculated from the xyz centroids of 2804 strong reflections from the actual data collection after integration (SAINT, Bruker Analytical X-Ray Systems, Madison, WI, 2008). The intensity data were corrected for absorption (SADABS) (Blessing, 1995).
The space group Pbca was determined based on intensity statistics and systematic absences. The structure was solved using SIR-2004(Burla et al., 2005 and refined with SHELXL-97 (Sheldrick, 2008). A direct-methods solution was calculated, which provided most non-hydrogen atoms from the E-map. Full-matrix least squares / difference Fourier cycles were performed, which located the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters with the exception of the hydroxyl hydrogen atom, which was refined for all parameters. The final supplementary materials sup-2 full matrix least squares refinement converged to R1 = 0.0368 and wR2 = 0.1190 (F 2 , all data). The structure was found as proposed. The remaining electron density is minuscule and located on bonds. Fig. 1. The molecular structure and atom numbering scheme.

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.
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.