Methyl 2,2-dimethoxy-5,5-bis(methylsulfanyl)-3-oxopent-4-enedithioate

In the title molecule, C10H16O3S4, a short intramolecular S⋯O(=C) distance [2.726 (2) Å] indicates the presence of a nonbonding attractive interaction. In the crystal, molecules are linked into centrosymmetric dimers via weak intermolecular C—H⋯O and S⋯S [3.405 (3) Å] interactions. These dimers are linked by further weak C—H⋯O interactions into columns along the a axis.


Mohammad Hassan Ghorbani Comment
In preparation of α-oxoketene dithioactal (I) (Mahata et al., 2003) (Fig. 1), a red-crystalline compound sometimes is derived as a byproduct of the reaction particularly, when NaH, CS 2 and CH 3 I are used in excess amounts. In order to identify this compound, the red-crystals were investigated using elemental analysis and convenient spectroscopic methods. These investigations show that the byproduct should be a methyl dithioformated derivative of the α-oxoketene dithioactal (I) and it may be one of the two probable structures (II) and (III) (see Fig .1). In order to find out the exact structure of byproduct, the red-prismatic crystals were investigated using single crystal X-ray crystallography.
In the molecule, the bond length of CH 3 -S4 (1.813 (4) Å) is slightly longer than the similar CH 3 -S3 bond (1.784 (3) Å). In addition to a short distance between non-bonded atoms S4 and O3, this observation shows that the intramolecular S···O interaction, might be responsible for lengthening the C6-S4 bond length in the molecule.
In the crystal, molecules are linked into centrosymmetric dimers via weak intermolecular C-H···O and S···S interactions [intermolecular S···S distance is 3.405 (3)Å] (Fig. 3). These dimers are linked by further weak C-H···O interactions into columns along the a axis.

Experimental
The title compound was produced as a byproduct of the reaction of synthesis of α-oxoketene dithioactal (I) (Mahata et al., 2003), when NaH, CS 2 and CH 3 I were used in excess amounts. The melting point of the title compound is 392-394K.
The suitable single crystals for X-ray analysis were obtained from ethyl acetate solution at room temperature.

Refinement
All hydrogen atoms were positioned geometrically with C-H distances = 0.93-0.96 Å and included in a riding model approximation with U iso (H) = 1.2 or 1.5U eq (C). A rotating group model was applied to the methyl groups.

Figure 1
Reaction scheme and possible products.

Special details
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. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.