The herringbone structure of methyl 1,3-benzoxazole-2-carboxylate is characterized by strong C—H⋯N and weak C—H⋯O hydrogen bonds, and further stabilized by C—O⋯π and π–π interactions.
Keywords: crystal structure, benzoxazole, herringbone arrangement, γ packing type, π–π interactions, strong C—H⋯N hydrogen bonds
Abstract
The title compound, C9H7NO3, crystallizes in the monoclinic (P21) space group. In the crystal, the almost planar molecules display a flattened herringbone arrangement. Stacking molecules are slipped in the lengthwise and widthwise directions and are linked by π–π interactions [d(Cg⋯Cg = 3.6640 (11) Å]. The structure is characterized by strong C—H⋯N and weak C—H⋯O hydrogen bonds, and further stabilized by C–O⋯π interactions.
Chemical context
Benzoxazoles are common in natural products and represent an important class of key structural motifs, often incorporated as building blocks in ligands to target a variety of receptors and enzymes in medicinal chemistry studies (Demmer & Bunch, 2015 ▸; Kamal et al., 2020 ▸). They are also a scaffold of prime importance for fluorescent probes and materials (Carayon & Fery-Forgues, 2017 ▸; Fery-Forgues & Vanucci-Bacqué, 2021 ▸). Methyl-1,3-benzoxazole-2-carboxylate (1) belongs to this family and much attention has been paid to its preparation.
This compound was first prepared by a multi-step synthesis starting from 2,3-dioxo-1,4-benzoxazine (Dickoré et al., 1970 ▸) and 2-cyanobenzoxazole (Möller, 1970 ▸), but it can be obtained much more simply from condensation of 2-aminophenol with methyl 2,2,2-trimethoxyacetate (Musser, Hudec et al., 1984 ▸; Koshelev et al., 2019 ▸). It has been synthesized in high yields by direct carboxylation of benzoxazole using carbon dioxide (CO2) as a naturally abundant and renewable C1 source, with (Zhang et al., 2010 ▸; Inomata et al., 2012 ▸) or without any metal catalyst (Vechorkin et al., 2010 ▸; Fenner & Ackermann, 2016 ▸). Recently, it has been produced by oxidative cyclization of glycine catalysed by copper (Liu et al., 2021 ▸) or induced by irradiation with visible light (Zhu et al., 2021 ▸). The molecule is commercially available. It has been used to complex europium, resulting in a very efficient electroluminescent layer for applications in the field of organic light-emitting diodes (OLEDs) (Koshelev et al., 2019 ▸). Used as a synthetic intermediate, methyl-1,3-benzoxazole-2-carboxylate has led to various pharmacologically active agents with anti-allergic (Musser, Brown et al., 1984 ▸), anti-microbial (Vodela et al., 2013 ▸) and neuro-anti-inflammatory (Shang et al., 2020 ▸) activity, to name just a few.
Structural commentary
The title compound (Fig. 1 ▸) crystallizes in the monoclinic space group P21 and exhibits the expected bond lengths and angles for a benzoxazole. The N1—C1 bond, which corresponds to a double bond, is significantly shorter [1.293 (2) Å] than the other bonds (>1.36 Å) of the oxazole cycle. The molecule is almost planar [N1—C1—C2—O3 = −6.7 (2)°]. The heterocyclic and carbonyl oxygen atoms O1 aand O2, respectively, are located on the same side with respect to the long axis of the molecule.
Figure 1.
The molecular structure of the title compound with the atom numbering. The displacement ellipsoids are drawn at the 50% probability level.
Supramolecular features
In the crystal structure, molecules are displayed according to the γ packing type, i.e. a flattened herringbone featuring stacks of parallel, translationally related molecules (Desiraju et al., 1989 ▸; Campbell et al., 2017 ▸) (Fig. 2 ▸). Neighboring molecules situated in almost perpendicular planes (84.4°) are linked through C—H⋯N interactions between the heterocyclic nitrogen atom N1 and H9 of an adjacent molecule and weak C—H⋯O hydrogen bonds between O2 and one hydrogen atom of the methyl group (Table 1 ▸, Fig. 2 ▸). Strong C—O⋯π interactions are also important for the stabilization of the structure (Table 2 ▸, Fig. 3 ▸). Stacking molecules are slipped in the lengthwise and widthwise directions and linked by π– π interactions [centroid–centroid distance = 3.6640 (11) Å] (Table 3 ▸).
Figure 2.
C—H⋯N and C—H⋯O hydrogen bonds (blue dotted lines).
Table 1. Hydrogen-bond geometry (Å, °).
| D—H⋯A | D—H | H⋯A | D⋯A | D—H⋯A |
|---|---|---|---|---|
| C9—H9⋯N1i | 0.95 | 2.53 | 3.377 (2) | 149 |
| C3—H3C⋯O2ii | 0.98 | 2.65 | 3.389 (2) | 133 |
Symmetry codes: (i) x-1, y, z; (ii) -x+1, y+{\script{1\over 2}}, -z+1.
Table 2. C—O⋯π interactions (Å, °).
Cg1 is the centroid of the O1/C1/N1/C5/C4 ring and Cg3 is the centroid of the O1/C1/C5–C9 ring.
| X | I | J | I⋯J | X⋯J | X—I⋯J |
|---|---|---|---|---|---|
| C2 | O2 | Cg1ii | 3.2088 (14) | 3.5487 (18) | 96.39 (10) |
| C2 | O2 | Cg3ii | 3.5912 (14) | 3.7321 (17) | 87.29 (10) |
Symmetry code: (ii) x, −1 + y, z.
Figure 3.
π–π and C—O⋯π interactions (green dotted lines). Orange balls represent the ring centroids Cg.
Table 3. π–π interaction (Å, °).
Cg1 is the centroid of the O1/C1/N1/C5/C4 ring and Cg2 is the centroid of the C4–C9 ring. CgI⋯CgJ is the distance between ring centroids. α is the dihedral angle between the planes of the rings I and J. CgI perp and CgJ perp are the perpendicular distances of CgI from ring J and of CgJ from ring I, respectively. CgI Offset and CgJ Offset are the distances between CgI and the perpendicular projection of CgJ on ring I, and between CgJ and the perpendicular projection of CgI on ring J, respectively.
| I | J | CgI⋯CgJ | α | CgI perp | CgJ perp | CgI Offset | CgJ Offset |
|---|---|---|---|---|---|---|---|
| 1 | 2ii | 3.6640 (11) | 0.19 (9) | 3.3115 (7) | 3.3065 (8) | 1.579 | 1.568 |
Symmetry code: (ii) x, −1 + y, z.
Database survey
Benzoxazole-based molecules have given an umpteen number of crystal structures. A search of the Cambridge Structural Database (CSD, version of November 2020; Groom et al., 2016 ▸) found only twelve benzoxazoles substituted by a carbonyl group on the 2-position. In almost half of the cases, the benzoxazole derivative is used as a ligand to complex an Ni, Co or Cu atom (CAYSIG and CAYSOM; Iasco et al., 2012 ▸; LAJNAN; Zhang et al., 2010 ▸), or incorporated in a macromolecule (NESPUY; Lim et al., 2012 ▸; LUYJUL; Osowska & Miljanić, 2010 ▸), resulting in a geometry quite far from that of a small entity. Among the remaining examples, the benzoxazolylcarbonyl moiety may be linked to an aromatic group. When the latter is a phenyl group, the molecule is almost planar (ROFZUJ; Boominathan et al., 2014 ▸). With another benzoxazole heterocycle, the dihedral angle is only around 8° (AGESUD; Boga et al., 2018 ▸). In contrast, this angle almost reaches 71° with a benzoic acid that is involved in many intermolecular interactions (DEJGEE; Ling et al., 1999 ▸), and when the benzoxazole and phenyl derivative moieties are attached via a flexible linker (KONTEP; Deng et al., 2019 ▸). Finally, the benzoxazolylcarbonyl moiety may be linked to an aliphatic moiety, which may be rather bulky like a bornane-1,2-sultam moiety (BAKRIQ; Piątek et al., 2011 ▸), or smaller like a morpholine moiety (JAXMED; Xing et al., 2017 ▸). In both cases, the network is structured by an interaction between the carbonyl oxygen of one molecule and the hydrogen atom borne by the C7 carbon of a neighbouring molecule. Finally, the framework closest to that of the title compound is an isopropyl 4-acetyl-5-hydroxy-1,3-benzoxazole-2-carboxylate (MIMZUG; Tangellamudi et al., 2018 ▸). In this molecule, the hydroxyl and the acetyl substituents form intramolecular hydrogen bonds while the carbonyl oxygen of one molecule interacts with the isopropyl group of the neigbouring one to form some kind of dimer. In general, planar molecules tend to assemble in layers (AGESUD; Boga et al., 2018 ▸; MIMZUG; Tangellamudi et al., 2018 ▸) and even in ribbons (JAXMED; Xing et al., 2017 ▸).
Synthesis and crystallization
The title compound was synthesized according to a variant of the procedure described by Jacobs et al. (2017 ▸) (Fig. 4 ▸). To a mixture of 5-aminophenol (1.09 g, 0.01 mol) and triethylamine (2.02 g, 0.02 mol) in anhydrous tetrahydrofuran (40 mL) at 263 K was added slowly methyl oxalyl chloride (1.34 g, 0.011 mol). The mixture was stirred at room temperature for 3 h and then cooled onto an ice–water bath. Triphenylphosphine (5.64 g, 0.0215 mol), diisopropyl azodicarboxylate (2.25 g, 0.011 mol) and tetrahydrofuran (50 mL) were then added. The solution was allowed to stir at room temperature for 16 h and concentrated in vacuo. The crude product was purified by column chromatography (SiO2, petroleum ether/dichloromethane 70/30 v/v until 60/40 v/v) to give a white solid (1.2 g) in 83% yield. 1H NMR (300 MHz, CDCl3): δ = 7.90 (ddd, J = 7.9, 1.5, 0.8 Hz, 1H), 7.67 (ddd, J = 8.1, 1.2, 0.8 Hz, 1H), 7.57–7.44 (m, 2H), 4.10 (s, 3H). 13C NMR (75 MHz, CDCl3): δ = 156.9, 152.5, 150.9, 140.5, 128.2, 125.8, 122.2, 111.7, 53.7.
Figure 4.
Synthesis route to methyl-1,3-benzoxazole-2-carboxylate.
Single crystals of the title compound, suitable for X-ray analysis, were grown by slow evaporation of a dichloromethane solution.
Refinement
Crystal data, data collection and structure refinement details are summarized in Table 4 ▸. All H atoms were fixed geometrically and treated as riding atoms with C—H = 0.95 Å (aromatic) or 0.98 Å (CH3), with U iso(H) = 1.2U eq(C) or 1.5U eq(CH3).
Table 4. Experimental details.
| Crystal data | |
| Chemical formula | C9H7NO3 |
| M r | 177.16 |
| Crystal system, space group | Monoclinic, P21 |
| Temperature (K) | 193 |
| a, b, c (Å) | 6.8165 (3), 4.4676 (2), 13.2879 (6) |
| β (°) | 95.1319 (16) |
| V (Å3) | 403.04 (3) |
| Z | 2 |
| Radiation type | Mo Kα |
| μ (mm−1) | 0.11 |
| Crystal size (mm) | 0.40 × 0.30 × 0.10 |
| Data collection | |
| Diffractometer | Bruker D8-Venture Photon III detector |
| Absorption correction | Multi-scan (SADABS; Krause et al., 2015 ▸) |
| T min, T max | 0.698, 0.746 |
| No. of measured, independent and observed [I > 2σ(I)] reflections | 9084, 1954, 1860 |
| R int | 0.022 |
| (sin θ/λ)max (Å−1) | 0.667 |
| Refinement | |
| R[F 2 > 2σ(F 2)], wR(F 2), S | 0.030, 0.077, 1.10 |
| No. of reflections | 1954 |
| No. of parameters | 119 |
| No. of restraints | 1 |
| H-atom treatment | H-atom parameters constrained |
| Δρmax, Δρmin (e Å−3) | 0.20, −0.16 |
Supplementary Material
Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989021010094/dj2033sup1.cif
Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989021010094/dj2033Isup3.hkl
Supporting information file. DOI: 10.1107/S2056989021010094/dj2033Isup4.cml
CCDC reference: 2112709
Additional supporting information: crystallographic information; 3D view; checkCIF report
supplementary crystallographic information
Crystal data
| C9H7NO3 | F(000) = 184 |
| Mr = 177.16 | Dx = 1.460 Mg m−3 |
| Monoclinic, P21 | Mo Kα radiation, λ = 0.71073 Å |
| a = 6.8165 (3) Å | Cell parameters from 6695 reflections |
| b = 4.4676 (2) Å | θ = 3.3–28.2° |
| c = 13.2879 (6) Å | µ = 0.11 mm−1 |
| β = 95.1319 (16)° | T = 193 K |
| V = 403.04 (3) Å3 | Plate, colourless |
| Z = 2 | 0.40 × 0.30 × 0.10 mm |
Data collection
| Bruker D8-Venture Photon III detector diffractometer | 1860 reflections with I > 2σ(I) |
| Radiation source: Fine-focus sealed tube | Rint = 0.022 |
| Phi and ω scans | θmax = 28.3°, θmin = 3.3° |
| Absorption correction: multi-scan (SADABS; Krause et al., 2015) | h = −8→9 |
| Tmin = 0.698, Tmax = 0.746 | k = −5→5 |
| 9084 measured reflections | l = −17→17 |
| 1954 independent reflections |
Refinement
| Refinement on F2 | Primary atom site location: dual |
| Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
| R[F2 > 2σ(F2)] = 0.030 | H-atom parameters constrained |
| wR(F2) = 0.077 | w = 1/[σ2(Fo2) + (0.0432P)2 + 0.0403P] where P = (Fo2 + 2Fc2)/3 |
| S = 1.10 | (Δ/σ)max < 0.001 |
| 1954 reflections | Δρmax = 0.20 e Å−3 |
| 119 parameters | Δρmin = −0.16 e Å−3 |
| 1 restraint |
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. |
Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2)
| x | y | z | Uiso*/Ueq | ||
| O1 | 0.25255 (16) | 0.3885 (3) | 0.69933 (9) | 0.0329 (3) | |
| O2 | 0.43981 (19) | −0.0004 (3) | 0.57993 (9) | 0.0382 (3) | |
| O3 | 0.73220 (17) | 0.2017 (3) | 0.63833 (9) | 0.0341 (3) | |
| N1 | 0.54541 (19) | 0.5493 (3) | 0.77177 (10) | 0.0277 (3) | |
| C1 | 0.4528 (2) | 0.3789 (4) | 0.70473 (12) | 0.0296 (3) | |
| C2 | 0.5378 (2) | 0.1703 (4) | 0.63294 (12) | 0.0297 (3) | |
| C3 | 0.8319 (3) | 0.0125 (4) | 0.57015 (13) | 0.0381 (4) | |
| H3A | 0.794777 | −0.196824 | 0.579791 | 0.057* | |
| H3B | 0.974729 | 0.034297 | 0.584461 | 0.057* | |
| H3C | 0.793506 | 0.072127 | 0.500188 | 0.057* | |
| C4 | 0.2149 (2) | 0.5919 (4) | 0.77264 (12) | 0.0291 (3) | |
| C5 | 0.3947 (2) | 0.6919 (4) | 0.81779 (11) | 0.0273 (3) | |
| C6 | 0.4014 (2) | 0.9009 (4) | 0.89607 (13) | 0.0347 (4) | |
| H6 | 0.522730 | 0.971603 | 0.928121 | 0.042* | |
| C7 | 0.2220 (3) | 0.9998 (4) | 0.92465 (14) | 0.0401 (4) | |
| H7 | 0.220300 | 1.141802 | 0.977785 | 0.048* | |
| C8 | 0.0429 (3) | 0.8959 (5) | 0.87719 (15) | 0.0406 (4) | |
| H8 | −0.076652 | 0.971435 | 0.898830 | 0.049* | |
| C9 | 0.0342 (2) | 0.6878 (5) | 0.80018 (14) | 0.0370 (4) | |
| H9 | −0.087048 | 0.615698 | 0.768384 | 0.044* |
Atomic displacement parameters (Å2)
| U11 | U22 | U33 | U12 | U13 | U23 | |
| O1 | 0.0259 (5) | 0.0362 (6) | 0.0359 (6) | −0.0064 (5) | −0.0013 (4) | 0.0001 (5) |
| O2 | 0.0406 (6) | 0.0349 (6) | 0.0380 (6) | −0.0087 (5) | −0.0024 (5) | −0.0035 (5) |
| O3 | 0.0315 (6) | 0.0342 (6) | 0.0364 (6) | −0.0025 (5) | 0.0009 (4) | −0.0061 (5) |
| N1 | 0.0241 (6) | 0.0274 (6) | 0.0312 (6) | −0.0017 (5) | 0.0008 (4) | 0.0002 (5) |
| C1 | 0.0288 (7) | 0.0286 (7) | 0.0311 (7) | −0.0048 (6) | 0.0007 (6) | 0.0048 (6) |
| C2 | 0.0329 (8) | 0.0264 (7) | 0.0292 (7) | −0.0042 (6) | −0.0006 (6) | 0.0039 (6) |
| C3 | 0.0384 (9) | 0.0380 (10) | 0.0381 (8) | 0.0030 (8) | 0.0047 (7) | −0.0042 (8) |
| C4 | 0.0259 (7) | 0.0305 (8) | 0.0308 (7) | −0.0049 (6) | 0.0010 (5) | 0.0063 (6) |
| C5 | 0.0233 (7) | 0.0278 (7) | 0.0306 (7) | −0.0019 (6) | 0.0013 (5) | 0.0057 (6) |
| C6 | 0.0311 (8) | 0.0357 (8) | 0.0366 (8) | −0.0030 (7) | −0.0006 (6) | −0.0014 (7) |
| C7 | 0.0438 (10) | 0.0370 (9) | 0.0407 (9) | 0.0020 (8) | 0.0096 (7) | −0.0015 (8) |
| C8 | 0.0302 (8) | 0.0431 (10) | 0.0505 (10) | 0.0040 (8) | 0.0146 (7) | 0.0107 (9) |
| C9 | 0.0220 (7) | 0.0429 (9) | 0.0461 (9) | −0.0035 (7) | 0.0032 (6) | 0.0104 (8) |
Geometric parameters (Å, º)
| O1—C1 | 1.3610 (19) | C4—C9 | 1.384 (2) |
| O1—C4 | 1.373 (2) | C4—C5 | 1.390 (2) |
| O2—C2 | 1.200 (2) | C5—C6 | 1.395 (2) |
| O3—C2 | 1.3281 (19) | C6—C7 | 1.385 (3) |
| O3—C3 | 1.452 (2) | C6—H6 | 0.9500 |
| N1—C1 | 1.293 (2) | C7—C8 | 1.402 (3) |
| N1—C5 | 1.395 (2) | C7—H7 | 0.9500 |
| C1—C2 | 1.488 (2) | C8—C9 | 1.380 (3) |
| C3—H3A | 0.9800 | C8—H8 | 0.9500 |
| C3—H3B | 0.9800 | C9—H9 | 0.9500 |
| C3—H3C | 0.9800 | ||
| C1—O1—C4 | 103.51 (12) | C9—C4—C5 | 123.92 (17) |
| C2—O3—C3 | 115.17 (14) | C4—C5—N1 | 108.64 (15) |
| C1—N1—C5 | 103.74 (13) | C4—C5—C6 | 120.36 (15) |
| N1—C1—O1 | 116.33 (15) | N1—C5—C6 | 131.00 (14) |
| N1—C1—C2 | 128.06 (14) | C7—C6—C5 | 116.58 (16) |
| O1—C1—C2 | 115.61 (13) | C7—C6—H6 | 121.7 |
| O2—C2—O3 | 126.82 (17) | C5—C6—H6 | 121.7 |
| O2—C2—C1 | 123.11 (16) | C6—C7—C8 | 121.71 (18) |
| O3—C2—C1 | 110.07 (14) | C6—C7—H7 | 119.1 |
| O3—C3—H3A | 109.5 | C8—C7—H7 | 119.1 |
| O3—C3—H3B | 109.5 | C9—C8—C7 | 122.32 (17) |
| H3A—C3—H3B | 109.5 | C9—C8—H8 | 118.8 |
| O3—C3—H3C | 109.5 | C7—C8—H8 | 118.8 |
| H3A—C3—H3C | 109.5 | C8—C9—C4 | 115.10 (16) |
| H3B—C3—H3C | 109.5 | C8—C9—H9 | 122.4 |
| O1—C4—C9 | 128.30 (15) | C4—C9—H9 | 122.4 |
| O1—C4—C5 | 107.78 (14) | ||
| C5—N1—C1—O1 | 0.05 (19) | C9—C4—C5—N1 | 179.93 (15) |
| C5—N1—C1—C2 | −179.34 (15) | O1—C4—C5—C6 | −179.61 (14) |
| C4—O1—C1—N1 | 0.03 (18) | C9—C4—C5—C6 | 0.2 (2) |
| C4—O1—C1—C2 | 179.50 (13) | C1—N1—C5—C4 | −0.11 (17) |
| C3—O3—C2—O2 | 1.7 (2) | C1—N1—C5—C6 | 179.60 (17) |
| C3—O3—C2—C1 | −178.99 (13) | C4—C5—C6—C7 | −0.2 (2) |
| N1—C1—C2—O2 | 172.61 (18) | N1—C5—C6—C7 | −179.92 (16) |
| O1—C1—C2—O2 | −6.8 (2) | C5—C6—C7—C8 | −0.1 (3) |
| N1—C1—C2—O3 | −6.7 (2) | C6—C7—C8—C9 | 0.6 (3) |
| O1—C1—C2—O3 | 173.92 (14) | C7—C8—C9—C4 | −0.6 (3) |
| C1—O1—C4—C9 | −179.89 (17) | O1—C4—C9—C8 | 180.00 (16) |
| C1—O1—C4—C5 | −0.10 (16) | C5—C4—C9—C8 | 0.2 (3) |
| O1—C4—C5—N1 | 0.14 (17) |
Hydrogen-bond geometry (Å, º)
| D—H···A | D—H | H···A | D···A | D—H···A |
| C9—H9···N1i | 0.95 | 2.53 | 3.377 (2) | 149 |
| C3—H3C···O2ii | 0.98 | 2.65 | 3.389 (2) | 133 |
Symmetry codes: (i) x−1, y, z; (ii) −x+1, y+1/2, −z+1.
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Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Crystal structure: contains datablock(s) I. DOI: 10.1107/S2056989021010094/dj2033sup1.cif
Structure factors: contains datablock(s) I. DOI: 10.1107/S2056989021010094/dj2033Isup3.hkl
Supporting information file. DOI: 10.1107/S2056989021010094/dj2033Isup4.cml
CCDC reference: 2112709
Additional supporting information: crystallographic information; 3D view; checkCIF report