Crystal structures of two thiazolidinone derivatives bearing a trichloromethyl substituent at the 2-position

The molecular conformations of the racemic title molecules are almost identical. Each crystal structure features a short C—H⋯O hydrogen bond arising from the chiral carbon atom, which generates monochiral chains, although the overall structures are centrosymmetric.

The title compounds 2-trichloromethyl-3-phenyl-1,3-thiazolidin-4-one (C 10 H 8 Cl 3 NOS), 1 and 3-(4-chlorophenyl)-2-trichloromethyl-1,3-thiazolidin-4one (C 10 H 7 Cl 4 NOS) 2, are structurally related with one atom substitution difference in the para position of the benzene ring. In both structures, the thiazolidinone ring adopts an envelope conformation with the S atom as the flap. The dihedral angles between the rings [48.72 (11) in 1 and 48.42 (9) in 2] are very similar and the molecules are almost superimposable. In both crystal structures, C-HÁ Á ÁO 'head-to-tail' interactions between the chiral carbon atoms and the thiazolidinone oxygen atoms result in infinite monochiral chains along the direction of the shortest unit-cell parameter, namely a in 1 and b in 2. C-HÁ Á Á interactions between the thiazolidinone carbon atom at the 4-position and the phenyl ring of the neighboring enantiomer also help to stabilize the packing in each case, although the crystals are not isostructural.

Chemical context
The title compounds 1 and 2 are unique structures containing a trichloromethyl substituent at the 2-position of the thiazolidinone ring. Their synthesis was first reported as two of only three known 2-alkyl thiazolidin-4-one compounds (Tierney, 1989;Issac et al., 1996). Substituted thiazolidin-4-one compounds are synthesized by reacting an in situ generated imine (Schiff base) with thioglycolic acid and with a mechanism to remove the water byproduct (Surrey, 1947;Erlenmeyer & Oberlin, 1947). Therefore, when chloral is reacted with arylamines, the corresponding imine is formed, which, upon reacting with thioglycolic acid, produces the desired 2-trichloromethyl-3-aryl-thiazolidin-4-one (Issac et al., 1996). It is interesting to note, however, that the reaction of chloral with some alkyl amines results in an N-alkylformamide product when the initially formed aminol loses chloroform instead of water (Mascavage et al., 2010). The loss of chloroform appears to be more facile in electron-rich N-alkylamines that can stabilize the transition state and lower the energy of activation of the elimination step better than the less electronrich N-arylamines. On the other hand, imine formation is favored with arylamines because of the lower pK a of the proton on the nitrogen in the aminol, which facilitates the removal of water to give an imine. As part of our ongoing studies in this area, we now describe the crystal structures of 1 and 2.

Structural commentary
Compounds 1 and 2 are structurally related with one atom substitution difference in the para position of the benzene ring; a hydrogen atom is substituted for a chlorine atom (Figs. 1 and 2). Both contain a stereogenic centre at C1, which is arbitrarily assigned as having an R configuration in the asymmetric units of the centrosymmetric unit cells. In both structures, the thiazolidinone ring adopts an envelope conformation with the S atom as the flap. The sulfur atom is displaced from the thiazolidinone ring plane by 0.35 (2) Å in both structures. The dihedral angles between the thiazolidinone and phenyl rings are 48.72 (11) in 1 and 48.42 (9) in 2. The C1-N1 and C1-S1 bond lengths are 1.445 (2) Å and 1.816 (2) Å , respectively, for structure 1 and 1.4471 (18) Å and 1.8181 (16) Å , respectively, for structure 2. The N-C-S bond angle is found to be 106.52 (12) in structure 1 and 106.23 (10) in structure 2. Overall, the molecular structures of both are almost exactly superimposable (Fig. 3). Bond length and angle values in the thiazolidinone ring in both structures appear to be typical and match currently available data (Yennawar et al., 2015).

Supramolecular features
Both extended structures exhibit C-HÁ Á ÁO 'head-to-tail' intermolecular interactions between the chiral carbon atom C1 and the thiazolidinone oxygen atom (Tables 1 and 2 The molecular structure of compound 2 with displacement ellipsoids drawn at the 50% probability level.

Figure 1
The molecular structure of compound 1 with displacement ellipsoids drawn at the 50% probability level. along the shortest unit-cell dimension, namely a in 1 and b in 2; in both cases adjacent molecules are related only by translational symmetry. The short HÁ Á ÁO distances of 2.30 Å suggest that these interactions are relatively strong. Weak C-HÁ Á Á interactions between the tetrahedral, non-chiral carbon atom (C3) of the thiazolidinone ring and the phenyl ring of the symmetry-related enantiomer are also observed in both structures (Tables 1 and 2). Despite the similar molecular conformations and intermolecular interactions, the crystals are not isostructural (1 is triclinic and 2 is monoclinic).

Synthesis and crystallization
The two compounds were synthesized using previously reported procedures (Tierney, 1989;Issac et al., 1996).
Cg2 is the centroid of the C5-C10 ring Compound 1 was crystallized by vapor diffusion where the sample was dissolved in acetone and placed in a chamber containing hexanes. Compound 2 was crystallized by the same method using methylene chloride as the solvent and a chamber containing hexanes.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atoms were placed in calculated positions with C-H = 0.93-0.98 Å and refined using a riding model with fixed isotropic displacement parameters: U iso (H) = 1.5U eq (C) for the methyl group and U iso (H) = 1.2U eq (C) for the remaining H atoms.

Funding information
We acknowledge NSF funding (CHEM-0131112) for the X-ray diffractometer.

Special details
Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 4 sets of ω scans each set at different φ and/or 2θ angles and each scan (20 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 5.82 cm. (SADABS; Bruker, 2001) was used for absorption correction. R(int) was 0.0816 before and 0.0197 after correction. The Ratio of minimum to maximum transmission is 0.7686. The λ/2 correction factor is 0.0015. 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. 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 > 2sigma(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.

Special details
Experimental. The data collection nominally covered a full sphere of reciprocal space by a combination of 4 sets of ω scans each set at different φ and/or 2θ angles and each scan (10 s exposure) covering -0.300° degrees in ω. The crystal to detector distance was 5.82 cm. SADABS V2.05 (BRUKER, 2001) was used for absorption correction. R(int) was 0.0443 before and 0.0167 after correction. The Ratio of minimum to maximum transmission is 0.8678. The λ/2 correction factor is 0.0015. 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. 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 > 2sigma(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.