Synthesis and crystal structure of allyl 7-(diethylamino)-2-oxo-2H-chromene-3-carboxylate

The crystal structure of the title compound, C17H19NO4, consists of nearly planar molecules that are linked by intermolecular C—H⋯O hydrogen bonding into chains along the b-axis direction.

The title compound, C 17 H 19 NO 4 , was synthesized by the reaction of 7-(diethylamino)-2-oxo-2H-chromene-3-carboxylic acid with allyl bromide and purified by flash column chromatography on silica gel. Crystals suitable for single-crystal X-ray diffraction were obtained by recrystallization from acetone. The aromatic core of the molecule is not planar with the diethylamino group and with the carboxyl group that are rotated out of the 2-oxo-2H-chromene plane by 6.7 (2) and 11.4 (2) . The NC 2 unit of the diethylamino group is planar with an angle sum close to 360 . Intermolecular C ar -HÁ Á ÁO carbonyl interactions lead to the formation of chains parallel to the b axis. X-ray powder diffraction analysis proves that the title compound was obtained as a pure phase.

Chemical context
Coumarins or 2H-1-benzopyran-2-ones are fluorophores with a wide range of biological and chemical applications (Bardajee et al., 2006a). One of the most important aspects is the detection of enzymatic activity from bacteria like Enterococci or Streptococci (Devriese et al., 1999). Within the enzymatic reaction, naturally occurring aesculin is hydrolysed with a concomitant loss of fluorescence (Edberg et al., 1976). In addition, (coumarin-4-yl)methyl esters are often used as a photocleavable protecting group that could be useful for proton detection in biological processes (Geissler et al., 2005). Another emerging field of application is photoelectricity such as in organic light-emitting diodes (OLEDs) or laser dyes (Bardajee et al., 2006a;Jones et al., 1985;Jones & Rahman, 1992, 1994Cui et al., 2018). In this context, Cui et al. (2018) developed two coumarines that show solid-state fluorescence influenced by NH 3 or HCl gas.
In a current research project, we planed to insert a coumarin moiety as part of a pH-sensitive polymer to visualize ISSN 2056-9890

Structural commentary
The molecular structure of the title compound, C 17 H 19 NO 4 , consists of a central 2-oxo-2H-chromene (2-benzopyrane) unit with a carboxylic acid allyl ester in 3-position and a diethylamino group in 7-position. All atoms of the molecule are in general positions (Fig. 2). The 2H-chromene unit is essentially planar with a maximum deviation for O2 of 0.1021 (6) Å from the least-squares plane calculated through C1-C7 and O1 and O2. The carboxyl group (C10,O3,O4) is slightly twisted from the 2-oxo-2H-chromene unit, with the dihedral angle between the plane calculated through the ring system and that of the carboxyl group being 6.7 (2) (Fig. 3). The NC 3 unit (N1,C7,C14,C16) of the diethylamino group is nearly planar with a maximum deviation of the N atom from the mean plane of 0.0873 Å ; planarity is also obvious from the sum of the C-N-C angles of 358.9 . This unit is rotated from the 2-oxo-2Hchromene plane by 11.4 (2) (Fig. 3), which points to conjugation between the ring system and the diethylamino group. The latter feature is also reflected by the C7-N1 bond length of 1.3597 (12) .

Supramolecular features
In the crystal structure of the title compound, the molecules are linked by intermolecular C-HÁ Á ÁO hydrogen bonding between one of the aromatic hydrogen atoms of a 2-oxo-2H- Molecular structure of the title compound with atom labelling and displacement ellipsoids drawn at the 50% probability level.

Figure 3
The orientation of the substituents in the molecular structure of the title compound.

Figure 4
The formation of C-HÁ Á ÁO hydrogen-bonded chains in the title compound in a view along the crystallographic c axis. Hydrogen bonds are shown as dashed lines. chromene unit and a carbonyl oxygen atom of a neighbouring molecule into chains extending parallel to the crystallographic b axis ( Fig. 4; Table 1). The C-HÁ Á ÁO angle is close to linearity, indicating that this is a relatively strong interaction. The molecules are additionally stacked into columns that are directed along the crystallographic c axis but the mean planes of the 2H-chromene rings of neighbouring molecules are not parallel (Fig. 5). They are rotated by 33.2 , which preventsinteractions.

Synthesis and crystallization
All reagents and solvents were commercially available and were used without further purification: allyl bromide (abcr), 7-(diethylamino)-2-oxo-2H-chromene-3-carboxylic acid (Fluorochem). For the reaction, flasks were flame-dried, evacuated and flooded with a stream of nitrogen. The NMR spectra were measured with a Bruker AvanceNeo 500 ( 1 H NMR: 500 MHz, 13 C NMR: 125 MHz) in dimethylsulfoxide-d 6 (deutero) as solvent. TMS was used as reference. The melting point was measured with a Melting Point Apparatus from Electrothermal. The mass spectrum was measured in the positive mode with an AccuTOF GCV 4G (Jeol, EI, 70 eV). R f values were determined by thin-layer chromatography using ALUGRAMM 1 Xtra Sil G/UV 254 plates (Machery-Nagel). Flash column chromatography was performed using cartridge SNAP Ultra 25 g (Biotage 1 ) on a Isolera one (Biotage 1 ). Infrared spectroscopy was performed on a Perkin-Elmer 1600 series FTIR spectrometer. An AG531-G Golden-Gate-Diamond-ATR unit was used. The elemental analysis was performed with a vario MICRO CUBE (Elementar). The probe was put into a zinc container and was burned in an oxygen atmosphere.

Figure 5
Packing of molecules in the crystal structure of the title compound in a view along the crystallographic b axis. Intermolecular C-HÁ Á ÁO hydrogen bonding is shown as dashed lines.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The C-H hydrogen atoms were located in difference maps but were positioned with idealized geometry (methyl H atoms allowed to rotate but not to tip) and refined isotropically with U iso (H) = 1.2U eq (C) (1.5 for methyl H atoms) using a riding model.

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.