Green synthesis and crystal structure of 3-(benzothiazol-2-yl)thiophene

A solvent-free microwave-assisted synthesis of the title compound is presented together with its crystal structure which is characterized by the herringbone motif in the packing.


Structural commentary
The title compound crystallizes in the monoclinic space group P2 1 /c with four molecules in the unit cell. The structure exhibits whole-molecule disorder by a rotation of approximately 180 around an axis running close to the S and N atoms of the benzothiazole ring, resulting in two orientations (A and B) of about the same shape ( Fig. 1). In addition, orientations A and B both have similar occupancies of 0.4884 (10) and 0.5116 (10), respectively. All the heterocyclic rings are almost planar, with r.m.s. deviations of 0.017 (thiophene ring S1-C5), 0.004 (thiophene ring S15-C19), 0.010 (benzothiazole ring C6-N14) and 0.021 Å (benzothiazole ring C20-N28). For orientation A, the angle between the best planes through the thiophene and benzothiazole rings is 10.02 (18) . In orientation B, this angle is 12.54 (19) . The relatively planar structure of the compound results in intramolecular SÁ Á ÁH contact distances shorter than the sum of the van der Waals radii of S and H (S7Á Á ÁH2 = 2.849 Å and S21Á Á ÁH16 = 2.824 Å ).

Supramolecular features
The crystal packing of the title compound shows a herringbone motif (Fig. 2). This motif is built up by slippedstacking between the aromatic rings and C-HÁ Á Á interactions. The shortest centroid-centroid distances (CgÁ Á ÁCg) 1648 Nguyen Ngoc et al. Crystal packing of the title compound shown in projection down the c axis. Orientation A of the disordered compound (occupancy factor 0.488) is shown in orange.

Figure 1
View of the asymmetric unit of the title compound, showing the atomlabelling scheme. Displacement ellipsoids are drawn at the 50% probability level. H atoms are shown as small circles of arbitrary radii. Orientation A of the disordered compound (occupancy factor 0.488) is shown in orange.
observed in thestacking for orientation B are shown in Fig. 3 and are listed in Table 1 for both orientations. The stacking molecules interact further with neighbouring molecules through C-HÁ Á Á interactions ( Fig. 3 and Table 2). In addition, infinite chains running in the [201] direction are formed through C-HÁ Á ÁN and C-HÁ Á ÁS interactions ( Fig. 4 and Table 2). The crystal packing contains no voids. Wholemolecule disorder is usually caused by a packing which is determined by van der Waals interactions only or by a lack of directional interactions in the packing. However, the crystal packing of the title compound shows several directional interactions, and hence the whole-molecule disorder is the consequence of the very similar interations with neighbouring molecules for the two orientations. Additional insight into the intermolecular interactions was obtained from an analysis of the Hirshfield surface and twodimensional fingerprint plots using CrystalExplorer (McKinnon et al., 2007;Spackman & Jayatilaka, 2009). Fig. 5 illustrates the Hirshfeld surfaces mapped over d norm for both orientations. The bright-red spots near atoms H9 and N14 for orientation A and near atoms H26 and S15 for orientation B are indicative for the hydrogen bonds given in Table 2

Figure 4
Infinite chain formation through C-HÁ Á ÁN (blue dashed lines) and C-HÁ Á ÁS (yellow dashed lines) interactions in the crystal packing of the title compound. Orientation A of the disordered compound (occupancy factor 0.488) is shown in orange. [Symmetry codes: orientation A, the red spots near atoms S1 and C12 refer to short CÁ Á ÁS/SÁ Á ÁC contacts and in the case of S1 also SÁ Á ÁS contacts. The red spots for orientation B near atoms N28 and H16 characterize short NÁ Á ÁH/HÁ Á ÁN contacts, and near atoms H19 and C24 indicate short HÁ Á ÁC/CÁ Á ÁH contacts. The relative distributions from the different interatomic contacts to the Hirshfeld surfaces are summarized in Table 3. The largest contributions are contacts in which H atoms are involved. The largest differences between both orientations are observed for HÁ Á ÁS/SÁ Á ÁH (9.5%), HÁ Á ÁH (5.7%), SÁ Á ÁS (3.3%) and CÁ Á ÁS/ SÁ Á ÁC (3.1%) contacts, and are caused by the presence of the C26-H26Á Á ÁS15 ii hydrogen bond in orientation B.

Synthesis and crystallization
The reaction scheme to synthesize the title compound is given in Fig. 6. The reaction mechanism is similar to that described by Mukhopadhyay & Datta (2007) for the synthesis of 2-arylbenzothiazoles. A reaction mixture of thiophene-3-carbaldehyde (2 mmol) and o-aminothiophenol (2 mmol) was heated for 4 min in a domestic microwave (Sanyo EM-S1065, 800 W) at medium power level (400 W). The progress of the reaction was monitored with thin-layer chromatography (TLC) every minute. The mixture was cooled to room temperature and then dissolved in an n-hexane-ethyl acetate mixture (5:1 v/v) to obtain a solid product, which was further crystallized in the same solvent to give 0.38 g (yield 87%) of the title product as pale-yellow crystals (m.p. 386 K). IR (Nicolet Impact 410  Reaction scheme for the title compound.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4. The molecule is disordered over two positions (A and B)

Funding information
Funding for this research was provided by: VLIR-UOS (award No. ZEIN2014Z182 to LVM).

3-(Benzothiazol-2-yl)thiophene
Crystal data 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.