Crystal structure of ethyl (E)-2-cyano-3-(thiophen-2-yl)acrylate: two conformers forming a discrete disorder

The molecular structure of the title compound is characterized by a planarity that allows the formation of (010) sheets. However, the existence of two different conformations of the ethyl fragment introduces the occurrence of discrete disorder due to a molecular overlay.

In the title compound, C 10 H 9 NO 2 S, all the non-H atoms, except for the ethyl fragment, lie nearly in the same plane. Despite the molecular planarity, the ethyl fragment presents more than one conformation, giving rise to a discrete disorder, which was modelled with two different crystallographic sites for the ethoxy O and ethoxy -C atoms, with occupancy values of 0.5. In the crystal, the three-dimensional array is mainly directed by C-HÁ Á Á(O,N) interactions, giving rise to inversion dimers with R 2 2 (10) and R 2 2 (14) motifs and infinite chains running along the [100] direction.

Chemical context
Cyanoacrylate derivatives are organic compounds with a very important industrial interest due to their use as monomers in the production of adhesives and polymer materials (Gololobov & Krylova, 1995). Furthermore, these compounds have been described as promissory intermediates for heterocycle synthesis (Gololobov et al., 1995) and as nitrile-activated precursors in bioreduction reactions (Winkler et al., 2014). Still, their most outstanding application is related to their very attractive absorption properties in the UV-Vis region. This capability has been widely described in the literature where cyanoacrylates were employed as precursors for the synthesis of dye-sensitized photovoltaic materials (Chen et al., 2013;Zietz et al., 2014;Lee et al., 2009) and sensors (Zhang et al., 2010). Considering that the absorption properties are related to the molecular structure of cyanoacrylate compounds (Ma et al., 2014), it is therefore very useful to know their crystal structures in detail in order to have a better understanding of the link between the structures and properties of these derivatives. In this contribution, we present the crystal structure of a thiophene-based cyanoacrylate derivative with promising applications in the synthesis of ligands for metal sensing. Fig. 1 shows the molecule of the title compound. The near planarity of the molecule (r.m.s. deviation of 0.006 Å ) means that nearly all atoms lie in the same plane perpendicular to [010] except for the ethyl ester fragment (O2/C2/O1/C1/C1A), which presents a discrete disorder due to the existence of two conformations of the ethyl moiety that overlay in the same crystallographic site. This disorder was modelled using two sites for the O1, C1 and C1A atoms with occupancy values of 0.5. The split fragment is observed as a reflection of two ethyl moieties in the two opposite sides of the mirror plane that contains the molecule. These atoms lie, respectively, 0.21 (2), 0.340 (7) and À1.010 (10) Å out of this plane. The planarity allows the formation of a weak intramolecular C5-H5Á Á ÁO2 close contact ( Fig. 1 and Table 1), which generates an S(6) motif. This molecule is similar to (E)-ethyl-2-cyano-3-(furan-2-yl)acrylate (Kalkhambkar et al., 2012), differing in the fivemembered ring, which is a furanyl in this compound, and presenting a distorted planarity compared with the title compound [dihedral angles of 177.5-179.0 in the two molecules of the asymmetric unit compared with the value of 180.0 in the C6-C5-C3-C2 fragment of the title compound]. Also, no molecular disorder was reported in the furanyl molecule.

Figure 2
The crystal structure of the title compound, showing the C-HÁ Á Á(O, N) hydrogen-bonding interactions (dotted lines) along the [100] direction.

Figure 1
The molecular structure of the title compound, showing anisotropic displacement ellipsoids drawn at the 50% probability level. The intramolecular C-HÁ Á ÁO hydrogen bond is shown as a dashed line (see Table 1) and the discrete disorder in the ethyl moiety is also observed.

Synthesis and crystallization
All reagents and solvents were purchased from commercial sources and used as received. In a two-necked round-bottom flask equipped with a condenser, thiophene-2-carboxaldehyde (740 mg, 6.6 mmol), cyanoacetic acid ethyl ester (753 mg, 6.6 mmol) and piperidine (6,8 mL, 1% mol) were stirred in ethanol for three h. A yellowish brown solid was obtained and recrystallized from ethanol solution (see Fig. 3). The product was filtered out and then dried under vacuum. The yellowish brown solid was dissolved in methanol and yellow crystals were grown through slow evaporation of the solvent at room temperature with 80% yield.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. H atoms were placed in calculated positions (C-H: 0.93-0.97 Å ) and included as riding contributions with isotropic displacement parameters set at 1.2-1.5 times the U eq value of the parent atom.