Crystal structure of dimethyl 3,4,5,6-tetraphenylcyclohexa-3,5-diene-1,2-dicarboxylate

In the title compound, C34H28O4, the cyclohexadiene ring has a screw-boat conformation. All four phenyl rings in the two independent molecules are arranged in a propeller-like conformation. The two molecules exhibit S,R- and R,S- chirality and are connected via C—H⋯O intermolecular interactions.


Chemical context
Addition reactions of tetraphenylcyclopentadienone, often abbreviated to 'tetracyclone', were reviewed by Allen (1945Allen ( , 1962. Tetracyclone reacts with unsaturated anhydrides, acids and esters, forming a number of polyfunctional carbonylbridge compounds. These species easily loose carbon monoxide to form dihydrobenzene (cyclohexadiene) derivatives. It was found that the use of maleic and fumaric esters yields various stereoisomers. The photochemical behavior of these compounds was studied (Fuchs & Yankelievich, 1968), showing a number of products including dimethyl tetraphenylphthalate. The relative simplicity of these reactions and the rich organic chemistry and spectroscopy of appropriate products make them attractive for use in undergraduate organic chemistry teaching laboratories.
This study provides an opportunity to investigate the geometry of 1,3-cyclohexadiene rings surrounded by bulky substituents with no strong intermolecular interactions.

Database survey
Conjugation of two double bonds favors a coplanar -system with a dihedral angle close to zero. However, in cyclic 1,3- ISSN 2056-9890 cyclohexadiene molecules angle strain and steric effects promote a non-planar structure (Rabideau & Sygula, 1989). Even for non-cyclic systems, because of steric effects, the geometry of the higher energy non-trans conformer of 1,3-butadiene in the gas phase is non-planar s-gauche (De Maré et al., 1997). Addition of bulky substituents to the 1,3butadiene molecule changes the conformational preference from trans to gauche even in the ground state.
948 Greenberg and Nazarenko C 34 H 28 O 4 research communications Figure 1 Numbering scheme of the title compound with 50% probability elipsoids (S,R-isomer).

Figure 2
Numbering scheme of the title compound with 50% probability elipsoids (R,S-isomer).

Figure 3
Overlay of the two independent molecules, after inversion.

Structural commentary
There are two independent molecules (Figs. 1 and 2) in the asymmetric unit of the title compound, with S,R-chirality and R,S-chirality, respectively (Figs. 1, 2). After inversion they demonstrate a good overlay ( Fig. 3) with an average deviation of 0.14 Å .
Torsion angles between Csp 3 atoms indicate a gauche conformation; the dihedral angles between the two double bonds are 15.2 (3) and À15.3 (3) for the two independent molecules (see Table 2). These values are practically the same as observed for free 1,3-cyclohexadiene in the gas phase: one can argue that the much lower values reported for 1,3-cyclohexadienes in the crystal state are caused by intermolecular interactions which may favor a flat butadiene fragment.
All six substituents are practically flat. Both ester fragments are almost perpendicular to the mean plane of the cyclohexadiene ring (Table 3). All four phenyl rings in both molecules are arranged in a propeller-like formation with angles between 46 and 74 (see Table 3 for exact numbers) from the mean plane of the cyclohexadiene ring. This propeller-like formation is probably inherited from the precursor tetracyclone molecule (refcode KIKTUT02; Pal et al., 2014). Because of the large angles between the planes of the double bonds and each phenyl ring, very little conjugation may be expected. Therefore, substituents serve mainly as bulky decoration, protecting the cyclohexadiene ring from external steric influences.

Supramolecular features
There are no usual hydrogen-bonding or stacking interactions in this structure.

Figure 5
Short C-HÁ Á ÁO contacts connecting two molecules into a weakly bonded dimer in the crystal.
The corresponding hydrogen atoms of the other molecule (H1 and H2) do not have acceptors available for such bonding. These intermolecular interactions, however weak they are, keep together a pair of molecules with opposite chirality. Two short intramolecular C-HÁ Á ÁO contacts within each molecule are also observed and may influence the molecular conformation. There are no other bonding short contacts between the weakly bound dimers and they form a usual molecular crystal.

Synthesis and crystallization
The title compound was obtained by reaction of tetraphenylcyclopentadienone (common name tetracyclone) with dimethylmaleate following Allen & Sheps (1934). GC-MS analysis of the colorless crystalline product dissolved in dichloromethane shows one main compound with a parent peak at 500 which is consistent with the formula weight of the title compound. Because all precursor compounds were non-chiral and synthetic conditions should not induce chirality, we expected to see a racemic product. Crystallization from acetonitrile resulted in several hexagonal flakes, mostly with intergrown smaller crystals. Several crystals were tested, all resulting in essentially the same chiral trigonal structure. The highest quality structure, from a partial racemically twinned crystal, is reported here.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The structure was refined as a twocomponent inversion twin. Cyclohexadiene hydrogen atoms H1, H2, H101 and H102 were refined in isotropic approximation with U iso = 1.2U iso (C). All aromatic hydrogen atoms were refined with riding coordinates with C-H = 0.95-0.98 Å and U iso = 1.2U iso (C). Idealized methyl groups were refined as rotating groups with U iso = 1.5U iso (C).