Synthesis and crystal structure of 2-(1,3-dithietan-2-ylidene)cyclohexane-1,3-dione

In the title compound, the dihedral angle between the mean planes of the cyclohexane and 1,3-dithietane rings is 9.1 (3)°. A short S⋯O contact is observed in the crystal.


Chemical context
Ketene dithioacetals are useful intermediates in organic synthesis and have been used for the preparation of heterocyclic compounds (Kolb, 1990;Ila et al., 2001). The synthesis of trifluoromethyl ketene dithioacetals has applications in the field of pharmaceuticals and agrochemicals (Gouault-Bironneau et al., 2012;Timoshenko & Portella, 2009). The functionalization of ketene dithioacetals provides more powerful tools for the development of new intermediates (Wang et al., 2011;Gao et al., 2010;Hu et al., 2012). The direct formation of a C-C bond has been carried out by reacting a cyano ketene dithioacetal and Morita-Baylis-Hillman (MBH) alcohols resulting from the reaction of acrylonitrile and aryl aldehydes. This reaction led to the corresponding 1,4-pentadiene derivatives (Zhao et al., 2007). Fiala et al. (2007) have studied the inhibitive action of some synthetic ketene dithioacetal derivatives towards the corrosion of copper in aerated nitric acid solutions. They concluded that these compounds are good inhibitors of copper corrosion in this medium. In the present study, we report the synthesis, crystal structure and Hirshfeld surface analysis of the new title 1,3-dithian-2-ylidene derivative, C 8 H 8 O 2 S 2 , (I).

Hirshfeld surface analysis
The nature of the intermolecular interactions in (I) has been computed by CrystalExplorer17.5 (Turner et al., 2017), using Hirshfeld surface (HS) analysis (Spackman & Jayatilaka, 2009) and two-dimensional fingerprint plots (McKinnon et al., 2007). The d norm plot (Fig. 3) shows red spots corresponding to the C5-H5AÁ Á ÁS2 hydrogen bond and short S2Á Á ÁO2 contact. A list of the relative percentage contributions of the close contacts to the HS of (I) are given in Table 2 and the overall two-dimensional fingerprint plot is shown in Fig. 4a Interactions of the type HÁ Á ÁH appear in the middle of the scattered points in the fingerprint plots with a pair of spikes at d e + d i = 2.5 Å and comprise 25.9% of the entire surface ( Fig. 4c); the van der Waals radius for this interaction is 2.4 Å , which means it is a weak interaction. The SÁ Á ÁH/HÁ Á ÁS contacts (Fig. 4d), which account for 23.8% of the Hirshfeld surface, are displayed on the fingerprint plot as a pair of long Table 1 Hydrogen-bond geometry (Å , ).

Figure 1
The molecular structure of (I) with displacement ellipsoids drawn at the 50% probability level. The short intramolecular SÁ Á ÁO contacts are shown as dashed lines. spikes at d e + d i = 2.7Å . This distance differs by 0.3 Å from the sum of the van der Waals radii, which means it is the strongest interaction present. The SÁ Á ÁC/CÁ Á ÁS (4.0%, Fig. 4f) and SÁ Á ÁO/O Á Á ÁS (3.3%, Fig. 4g) contacts are seen as pairs of spikes at d e + d i = 3.2 and 3.05 Å , respectively. These distances are shorter than the sums of the van der Waals radii of 3.5 and 3.32 Å , respectively. The CÁ Á ÁO/OÁ Á ÁC interactions make a contribution of 0.7% to the Hirshfeld surface ( Fig. 4h), their interatomic distances (d e + d i = 3.3 Å ) being larger than the sum of the van der Waals radius (3.22 Å ), so this interaction is very weak in this structure. The fingerprint plot corresponding to CÁ Á ÁH/HÁ Á ÁC contacts (Fig. 4e) shows a fin-like distribution of points with the edges at d e + d i = 2.8 Å .

Synthesis and crystallization
Potassium carbonate (0.3 mol, 42 g) in DMF (50 ml) was well stirred at room temperature. To this mixture, cyclohexane-1,3dione (0.1 mol) was added and the resultant solution stirred at room temperature for 20 min. Carbon disulfide (0.15 mol, 9.0 ml) was then added in one lot. The reaction mixture was stirred and kept for 10 min at room temperature. Diiodomethane (0.12 mol) was added dropwise over 20 min and the reaction mixture stirred for 7 h at room temperature. Icewater (500 ml) was added to the reaction mass, the solid was filtered and washed with water, dried and recrystallized from ethanol solution to give (

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.