Synthesis, crystal structures and Hirshfeld surface analysis of 1,4-dibenzyl-6-methyl-1,4-dihydroquinoxaline-2,3-dione

In the title quinoxaline molecule, the dihedral angle angle between the benzene rings is 72.54 (15)°. In the crystal, molecules are connected into chains extending parallel to (10) by weak C—H⋯O hydrogen bonds.


Chemical context
Given their importance in the pharmaceutical, chemical and industrial fields, the synthesis of quinoxaline and its derivatives has been a goal of chemists in recent years. Quinoxaline derivatives find use as anticancer (Noolvi et al., 2011), antimalarial (Guillon et al., 2004), antifungal (Xu & Fan, 2011), antiviral (Cai et al., 2008) and anti-inflammatory (Yan et al., 2007) agents. Some quinoxaline derivatives have also been reported to be corrosion inhibitors for steel in an acidic medium (Zouitini et al., 2018(Zouitini et al., , 2019El Janati et al., 2020). In this work, we report the synthesis and structure of the title compound obtained by the action of benzyl chloride on 6-methyl-1,4-dihydroquinoxaline-2,3-dione in the presence of potassium carbonate and a catalytic quantity of tetra-nbutylammonium bromide. A Hirshfeld surface analysis was also performed.

Hirshfeld surface analysis
The CrystalExplorer17.5 (Turner et al., 2017) program was used to analyse the interactions within the crystal. The donoracceptor groups are visualized using a standard (high) surface resolution and d norm surfaces mapped over a fixed colour scale of À0.140 (red) to 1.358 (blue) a.u., as illustrated in Fig. 4. Red spots on the surface of the d norm plot indicate intermolecular contacts involving the hydrogen bonds. The red spots identified in Fig. 4(a) correspond to the intermolecular C-HÁ Á ÁO bonds. Regions close to the sum of the van der Waals radii are shown in white. Fig. 4(b) shows the shape-index surface, which can be used to detect the presence of -stacking interactions. The molecular structure of the title compound. Displacement ellipsoids are drawn at the 40% probability level. Table 1 Hydrogen-bond geometry (Å , ).

Figure 2
View of a portion of a chain along the a-axis direction with C-HÁ Á ÁO hydrogen bonds depicted by dashed lines.

Figure 4
Hirshfeld surface mapper over (a) d norm and (b) shape-index to visualize the interactions in the title compound.

Figure 3
Packing viewed along the a-axis direction with C-HÁ Á ÁO hydrogen bonds and C-HÁ Á Á(ring) interactions depicted, respectively, by black and green dashed lines.
The absence of characteristic triangles indicates that no significantinteractions are present. Two-dimensional fingerprints were also generated in the range À1 to 1 Å (Fig. 5

Synthesis and crystallization
To a solution of 6-methyl-1,4-dihydroquinoxaline-2,3-dione (0.3 g, 1.73 mmol) in DMF (15 mL), were added potassium carbonate (0.47 g, 3.61 mmol) and tetra-nbutylammonium bromide (0.07g, 0.23 mmol). After stirring for 10 min, 0.5 mL (4.32 mmol) of benzyl chloride was added and the mixture was stirred at room temperature for 6 h. After filtration of the salts, the DMF was evaporated under reduced pressure and the residue obtained was dissolved in dichloromethane. The organic phase was then dried over Na 2 SO 4 and concentrated.
The mixture obtained was chromatographed on a silica gel column [eluent: hexane/ethylacetate (2/1)]. The crude product was recrystallized from ethanol as yellow crystals suitable for X-ray analysis (m.p. 493.5 K).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms treated as riding: C-H = 0.97 Å and U iso (H) = 1.5U eq (C) for methyl, C-H = 0.96 Å and U iso (H) = 1.2U eq (C) for methylene, C-H = 0.93 Å and U iso (H) = 1.2U eq (C) for aromatic and C-H = 0.98 Å and U iso (H) = 1.2U eq (C) for methine H atoms.

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.