Crystal structure of diethyl 2-acetoxy-2-[3-(4-nitrophenyl)-3-oxo-1-phenylpropyl]malonate

The title compound diethyl 2-acetoxy-2-[3-(4-nitrophenyl)-3-oxo-1-phenylpropyl]malonate possesses a three-dimensional supramolecular structure formed through weak C—H⋯O and C—H⋯π hydrogen bonds.


Chemical context
The formation of C-C bonds by the Michael addition of the appropriate carboanionic reagents to ,-unsaturated carbonyl compounds is one of the most useful methods of remote functionalization in organic synthesis (Mather et al., 2006;Little et al., 1995). In particular, a much studied reaction is the conjugate addition of malonates to chalcones. Compounds with the chalcone backbone were reported to possess a wide range of biological activities, such as nematicidal, antifungal, antiallergenic, antimicrobial, anticancer, antimalarial and antifeedant properties. Malonates are traditionally regarded as important materials for synthesizing the key intermediates of numerous active substances, but are rarely found as pharmacophores belonging to the target compounds (Lopez et al., 2001;Chen et al., 2016). Therefore, a catalytic version of the Michael addition of dialkyl malonates to chalcones in the presence of different catalysts has been studied extensively in recent years. Many phase-transfer-catalyzed methods that are simple and environmentally friendly have been developed for the Michael reaction (Shioiri, 1997). This new racemic compound was prepared in a phase-transfer reaction using a sugar-based crown ether as the catalyst (Rapi et al., 2016).

Structural commentary
The molecular structure of the racemic title compound is shown in Fig. 1. In this molecule, the C4 atom is a chiral centre, ISSN 2056-9890 but no resolution occurred upon crystal preparation, the racemic mixture crystallizing in the centrosymmetric space group P2 1 /n. The dihedral angle between the planes of the two benzene rings is 80.16 (6) and the molecular conformation is stabilized by an intramolecular methylene C5-HÁ Á ÁO5 hydrogen bond (Table 1).

Supramolecular features
Because of the numerous C O acceptor groups and the lack of primary donor groups in the molecule, the main intermolecular interactions in the crystal are weak C-HÁ Á ÁO carboxyl hydrogen bonds (Table 1), having HÁ Á ÁO distances equal to or less than 2.6 Å . However, one of the four interactions (C24-HÁ Á ÁO8 iii ; see Table 1 for hydrogen-bond geometry details and symmetry codes) involves a nitro Oatom acceptor. Intermolecular C15-HÁ Á ÁO7 ii hydrogen bonds form centrosymmetric cyclic dimers (Fig. 2) having the graph-set descriptor (Bernstein et al., 1995) R 2 2 (16). These dimers are linked along the crystallographic a direction through C24-HÁ Á ÁO8 iii hydrogen bonds, forming a chain. These chains are further extended in the crystallographic b direction through C11-HÁ Á ÁO4 i and C12-HÁ Á ÁO6 i interactions, forming a cyclic motif with the graph-set descriptor of R 2 2 (10) (Fig. 3). Despite the presence of two aromatic rings in the molecule, there are no significantinteractions in the crystal lattice. This can be explained by the diverse chain system of the molecule and, therefore, the steric preference of the C-HÁ Á ÁO hydrogen bonds. However, there is a C16-H16Á Á Á interaction across c with the C7-C12 nitrophenyl ring (CÁ Á ÁCg iv = 2.81 Å and C-HÁ Á ÁCg iv = 149 ; Cg is the centroid of the C7-C12 ring) ( Fig. 4

Database survey
The structures of different derivatives of 1,2-diphenylpentan-1-one, carrying methyl or nitrile substituents on the chiral C atom, have been reported, viz. Cambridge Structural Database (CSD; Groom & Allen, 2014)  The molecular structure of the title compound, showing the atom labelling. Displacement ellipsoids are drawn at the 50% probability level.   Table 1 Hydrogen-bond geometry (Å , ).

D-HÁ
in the chiral P2 1 2 1 2 1 and Pca2 1 space groups, respectively, and LAPKEU crystallized as a racemic mixture in the centrosymmetric P2 1 /c space group. Comparing the dihedral angles between the planes of the two benzene rings, the steric effect of the bulky substituents on atom C2 can be seen. This value is 62.5 for the methyl derivative (DULJOK) and 68.4 for the nitrile (LAPKEU), but significantly higher for the bulky methyl-nitro derivative (88.13 ; RULFIN) or the title compound (80.2 ).

Synthesis and crystallization
The title compound was synthesized by the reaction of 4 0 -nitrochalcone [(E)-3-(4-nitrophenyl)-1-phenylprop-2-en-1one] with diethyl 2-acetoxymalonate. The reaction was carried out in a solid/liquid two-phase system [Na 2 CO 3 /tetrahydrofuran (THF)] in the presence of a glucopyranoside-based crown ether catalyst. The compound was isolated by preparative thin-layer chromatography (TLC) (silica gel) in good yield. The structure of the compound was confirmed by 1 H and 13 C NMR and mass spectroscopy (m.p. 366-369 K). The details of the synthesis are presented in Rapi et al. (2016). Single crystals of the title compound suitable for X-ray diffraction analysis were obtained by crystallization from ethanol.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All H atoms were located in difference electron-density maps. However, these atoms were included in the structure refinement at calculated positions, with C-H = 0.95-1.00 Å , and allowed to ride, with U iso (H) = 1.2U eq (C).

Figure 4
The arrangement of four molecules, showing the C-HÁ Á ÁCg interactions (dashed lines).