Ethyl 1H-indole-2-carboxylate

The synthesis and structure of 1H-indole-2-carboxylate is presented.

Our work in the area of synthesis of tris indole compounds as a potential chelator led to the synthesis and crystallization of ethyl 1H-indole-2-carboxylate, C 11 H 11 NO 2 , an indole that was synthesized by the thionyl chloride reaction of 1H-indole-2-carboxylic acid, followed by dissolution in ethanol. The molecular packing exhibits a herringbone pattern with the zigzag running along the b-axis direction; the compound crystallizes as a hydrogen-bonded dimer resulting from OÁ Á ÁH-N hydrogen bonds, between the indole N-H group and the keto oxygen atom, which build centrosymmetric R 2 2 (10) ring motifs in the crystal.

Structure description
Indole esters can easily be prepared from 1H-indole-2-carboxylic acid via an isolated acyl chloride intermediate followed by dissolving the residue in the appropriate alcohol solvent. These indole-type compounds are of interest because of their prevalence in nature (Stempel & Gaich, 2016). Derivatives of this type of compound have also been implicated in a number of biological roles including antifungal (Kipp et al., 1999), antitumor (Lu et al., 2016 and anti-inflammatory (Liu et al., 2016) agents. These types of compounds have also been reported as potential cellular inhibitors of kinase (Jobson et al., 2009) as well as an antagonist for glycine-binding sites (Ohtani et al., 2002). Previous reports include the structures of indole-2-carboxylic acid (Morzyk-Ociepa et al., 2004) and methyl 1H-indole-2-carboxylate (Almutairi et al., 2017).
Herein we report the crystal structure of ethyl 1H-indole-2-carboxylate ( Fig. 1), which forms a hydrogen-bonded dimer. The hydrogen bonding occurs between N atoms of the indole ring and the keto oxygen atoms with an R(10) synthon. The hydrogen bond between N1 and O2 i is characterized by an NÁ Á ÁO separation of 2.877 (3) Å [symmetry code: (i) Àx + 2, Ày + 1, Àz + 1; Table 1], and the ring motifs, R 2 2 (10), are placed on inversion centres in the space group P2 1 /c (Fig. 2). The crystal structure exhibits a classic data reports herringbone pattern (Fig. 2) with the blocks consisting of the hydrogen-bonded dimers, with the zigzag running along the baxis direction. The molecule is nearly planar, with a r.m.s.d. of 0.028 Å for the non-hydrogen atoms. There are no other short contacts orinteractions observed in the crystal.

Synthesis and crystallization
The title compound was synthesized by modification of an early method laid out by Terent'ev et al. (1969). Indole-2carboxylic acid (0.50 g, 3.1 mmol) was dissolved in SOCl 2 (19 ml) at 0 C. After stirring for 1 h, the solution was rotary evaporated and to the resulting oil was added absolute ethanol (17 ml) at room temperature. After stirring overnight, the solution was vacuum filtered to yield ethyl 1H-indole-2carboxylate as a beige solid, which was recrystallized from methanol to yield 0.54 g (2.9 mmol, 93%) of the product. Further recrystallization by slow evaporation from methanol solution resulted in X-ray quality crystals.

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

Figure 1
A view of the molecular structure of the title compound, with the atom labelling. Displacement ellipsoids are drawn at the 50% probability level. Table 1 Hydrogen-bond geometry (Å , ). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.28 e Å −3 Δρ min = −0.16 e Å −3 Special details Refinement. All carbon-bound H atoms were positioned geometrically and refined as riding, with C-H = 0.95, 0.98 or 0.99 Å and U iso (H) = 1.2U eq (C) or U iso (H) = 1.5U eq (C) for C(H) and CH 3 groups, respectively. Hydrogen atom of the N-H group was refined freely.