Ethyl 2-amino-4-methylthiophene-3-carboxylate

The title compound crystallizes with two molecules, A and B, in the asymmetric unit. Each molecule features an intramolecular N—H⋯O hydrogen bond and the same H atom is also involved in an intermolecular N—H⋯S bond to generate A + B dimers. Further N—H⋯O hydrogen bonds link the dimers into a [010] chain.

The title compound, C 8 H 11 NO 2 S, crystallizes with two molecules, A and B, in the asymmetric unit. Each molecule features an intramolecular N-HÁ Á ÁO hydrogen bond and the same H atom is also involved in an intermolecular N-HÁ Á ÁS bond to generate A + B dimers. Further N-HÁ Á ÁO hydrogen bonds link the dimers into a [010] chain.
Many 2-,3-aminothiophene derivatives have been prepared so far and the structures of more than 25 of them have been published (see, e.g. : Ç oruh et al., 2003;Nirmala et al., 2005;Bourgeaux & Skene, 2007;Akkurt et al., 2008;Zhang & Jiao, 2010;Ghorab et al., 2012). Crystal structures of several thiophenes have been determined in which different functional groups are attached in place of NH 2 at the 2-position of the ring (Yan & Liu, 2007;Mukhtar et al., 2012;de Oliveira et al., 2012;Mabkhot et al., 2013;Kaur et al., 2014). Compounds are known in which the replacement of NH 2 group by iodine resulted in a cyclomer by the association of two monomers through a weak intermolecular CNÁ Á ÁI data reports Lewis acid-base interaction (Moncol et al., 2007). In the crystal structure of another compound, which is a derivative of piperidine containing aminothiophenes, a dimer is formed by the intermolecular C-HÁ Á ÁS interaction between the piperidine and thiophene rings (Al-Adiwish et al., 2012).
We report herein the synthesis, characterization and crystal structure of the title compound, 2-amino-4-methylthiophene-3-carboxylate (1) (Fig. 1), which crystallizes in the triclinic space group P1 with four molecules in the unit cell (Z 0 = 2). The two molecules in the asymmetric unit are labelled as A and B. In both A and B, the thiophene ring and the directly attached atoms are all coplanar within experimental error [for A: the r.m.s. deviation of the thiophene moiety is 0.003 (1) Å with N1, C5, and C6 at 0.044 (3), 0.005 (3) and 0.011 (3) Å , respectively; for B the r.m.s. deviation is 0.001 (1) Å with N1, C5 and C6 at 0.009 (4), 0.009 (4), and 0.003 (3) Å , respectively]. For A the dihedral angle between the thiophene ring and the NH 2 substituent is 12.5 (18) while for the C7, O1 and O2 moiety, this angle is 1.65 (10) , indicating that this group is almost exactly coplanar with the ring. For B the corresponding values are 11 (2) and 2.1 (2) .
A search for structures containing a 2-amino-thiophene-3carboxylate moiety gave 45 hits, two of which are particularly relevant to the current reported structure, viz. ethyl 2-amino-4-isobutylthiophene-3-carboxylate (KIKPIE; Liao et al., 2007) and ethyl 2-amino-4-phenylthiophen-3-carboxylate (VIWPUM; Dufresne & Skene, 2010). The only difference between these structures and that of 1 is in the substituent at the 3-position on the ring which are 2-methylpropyl and phenyl for KIKPIE (Liao et al., 2007) and VIWPUM (Dufresne & Skene, 2010). In both cases the metrical parameters are similar as well as the planarity of the substituents.
As far as the packing of the molecules is concerned, there is both intra-and intermolecular hydrogen bonding. This links the molecules into a C 2 4 (12) chain in the b-axis direction (Etter et al., 1990). In addition, there are R 3 2 (6) interactions involving the NH 2 and S moieties with a bifurcated hydrogen bond from H1BA to S1A and O1B, which links the A and B molecules (

Figure 2
Diagram showing both intra-and intermolecular hydrogen bonding, which links the molecules into a C 2 4 (12) chain in the b-axis direction, and R 3 2 (6) interactions involving the NH 2 and S moieties with a bifurcated hydrogen bond from H1BA to S1A and O1B which links the A and B molecules. Hydrogen bonds are shown with dashed lines. Atomic displacement parameters are at the 30% probability level.

Figure 3
Packing diagram viewed along the a axis. Hydrogen bonds are shown with dashed lines.

Figure 1
Diagram showing the two molecules A and B with atom labelling. R 3 2 (6) interactions involving the NH 2 and S moieties with a bifurcated hydrogen bond from H1BA to S1A and O1B links the A and B molecules. Hydrogen bonds are shown with dashed lines. Atomic displacement parameters are at the 30% probability level. the literature (Zhang et al., 2010). A mixture of acetone (0.5 mmol) and ethylcyanoacetate (0.5 mmol) in absolute ethanol (2 ml) was added to a solution of elemental S (0.5 mmol) and diethylamine (0.5 mmol) in absolute ethanol (2 ml) and stirred constantly for 3 h at 50 C. The reaction completion was confirmed by using pre-coated silica gel 60 F254 MERCK (20Â20 cm). The reaction mixture was quenched with ice-cold water and extracted with ethyl acetate. The organic layer was separated, dried over anhydrous sodium sulfate and concentrated. The crude product was purified using silica gel column chromatography (100-200 mesh) using hexane/ethyl acetate (7:3) mixture solution. Yellow crystals were obtained by slow evaporation of a saturated solution in ethyl acetate and the crystals were used for X-ray diffraction studies.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2.

Funding information
RJB wishes to acknowledge NSF award 1205608, Partnership for Reduced Dimensional Materials, for partial funding of this research. 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.