Crystal structure of 2-[chloro(4-methoxyphenyl)methyl]-2-(4-methoxyphenyl)-5,5-dimethylcyclohexane-1,3-dione

One of the methyl groups and the 4-methoxyphenyl substituent are in axial positions and the chloro(4-methoxyphenyl)methyl substituent is in the equatorial position of the cyclohexane ring which adopts a chair conformation. The packing features inversion-symmetric dimeric units and strands along [100] and [010] established by weak C—H⋯O and C—H⋯Cl contacts.


Chemical context
Iodonium ylides, a subclass of hypervalent iodine compounds (Zhdankin & Stang, 2008), have a variety of synthetic applications due to their versatile reactivity pattern. The known transformations of these reagents include decomposition (Moriarty et al., 2008;Lee & Jung, 2002) in various solvents, transylidation reactions (Hadjiarapoglou & Varvoglis, 1988), C-H insertion reactions (Adam et al., 2003;Batsila et al., 2003) and intra-and intermolecular cycloaddition reactions under photochemical, thermal, or metal-catalysed activation (Goudreau et al., 2009). During our studies on the reactions of iodonium ylides with stabilized carbenium ions, we obtained the title compound, the structure of which provides valuable information on the mechanism of these reactions that will be discussed in a separate paper.

Supramolecular features
The packing of the title compound manifests weak C-HÁ Á ÁO and C-HÁ Á ÁCl contacts (Table 1), while -stacking and C-HÁ Á Á interactions are not present. Pairs of contacts of the type C14-H14Á Á ÁO2 between the benzene ring and a ketogroup lead to the formation of inversion dimers with an R 2 2 (14) ring motif (Fig. 2). Strands along [010] are established by weak C8-H8CÁ Á ÁCl1 contacts between the axial-oriented methyl substituent of the cyclohexane ring and the chloro substituent (Fig. 3). Finally, strands along [100] are formed by C19-H19Á Á ÁO3 contacts between the benzene ring (C17-C22) and the methoxy group on benzene ring C10-C16 (Fig. 4). The full packing including cell outlines is shown in Fig. 5.

Database survey
A CSD database (Version 5.36; Groom & Allen, 2014) search has been conducted for the three structure fragments A, B and C depicted in the following scheme. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 50% probability level.

Figure 2
A view of the inversion dimer formed by a pair of weak C-HÁ Á ÁO contacts (blue dotted lines).
The search for fragment A yielded 21 hits; however, in 20 of them the cyclohexane ring is part of an annulated ring system and in the remaining hit it is part of a spiro-compound. Since none of the hits is really closely related to the title compound, they are not cited in detail. The search for fragment B led to six hits with the CSD refcodes CBZPOX (Noordik & Cillissen, 1981), IYISAL (Sparr & Gilmour, 2011), PAQKAV (Nair et al., 2012), POMZOH (Unruh et al., 2008), UREKEI (Betz et al., 2011) and YUZPOZ (Kalyani et al., 2010). Finally, the search for fragment C comprising the 5,5-dimethylcyclohexane-1,3-dione moiety produced 25 hits. In merely two of them fragment C is part of a non-spiro compound comparable to the title compound: CSD refcodes CETMCD (Roques et al., 1976) and FAWDEM (Ochiai et al., 1986).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were positioned geometrically (C-H = 0.98 Å for methyl-H, 0.99 Å for C-H 2 , 1.00 Å for aliphatic C-H, 0.95 Å for aromatic H) and treated as riding on their parent atoms, with U iso (H) = 1.2U eq (C) or 1.5U eq (C) for methyl H atoms. The methyl groups were allowed to rotate along the C-C bonds to best fit the experimental electron density.

Special details
Experimental. Absorption correction: CrysAlis PRO (Agilent, 2014), Empirical absorption correction using spherical harmonics, implemented in SCALE3 ABSPACK scaling algorithm. Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.