Synthesis and crystal structure of methyl 3-(3-hydroxy-3-phenylprop-2-enoyl)benzoate

A non-symmetric aromatic β-diketone enol bearing a carboxymethyl group has been synthesized and characterized by X-ray crystallography, 1H and 13C NMR spectroscopy, elemental analysis, UV–Vis spectroscopy and cyclic voltammetry.


Chemical context
The high complexing ability via O-donor atoms and excellent optical properties of aromatic -diketones make them practically irreplaceable in the creation of efficient emitters [as lanthanide or iridium(III) complexes] for application in OLEDs (organic light-emitting diodes; Eliseeva & Bü nzli, 2010;Bü nzli, 2015). In addition, -diketone-based Ir III complexes have attracted particular attention as promising photosensitizers in dye-sensitized solar cells (Baranoff et al., 2010). Surprisingly, aromatic -diketones functionalized by anchoring COOH groups have not been considered as a possible alternative to traditional anchoring 4,4 0 -dicarboxy-2,2 0 -bipyridine groups.
Herein we report on the crystal structure as well as optical and electrochemical properties of a non-symmetric aromatic -diketone with formula C 17 H 14 O 4 , bearing a carboxymethyl group. ISSN 2056-9890

Structural commentary
A 1 H NMR study of the prepared -diketone showed that it appears exclusively as an enol tautomer in solution (CDCl 3 ). Single-crystal X-ray diffraction analysis also confirmed unambiguously that the compound exists in the enol form in the solid state (Fig. 1a). In the molecular structure, an intramolecular resonance-assisted hydrogen bond (for related structures, see: Gilli et al., 2004) connects the two oxygen atoms of the keto-enol moiety with the O3Á Á ÁO4 distance as short as 2.4358 (10) Å (Table 1). The hydrogen atom involved in this interaction is disordered over two sites (H21 and H22) with almost equal occupancies. The virtual HÁ Á ÁH distance of 0.625 (1) Å is a result of the simultaneous presence of two enol forms, O3-HÁ Á ÁO4 and O3Á Á ÁH-O4, respectively, in an approximate 1:1 ratio in the crystal. The title molecule is almost planar with a variation of the dihedral angles between phenyl rings and the keto-enol plane between 5.65 (4) and 11.05 (4) .

Figure 1
(a) Molecular structure of 3-(3-hydroxy-3-phenylprop-2-enoyl)benzoate. Displacement ellipsoids are shown at the 50% probability level; (b) difference-density map in the plane of the hydrogen-bonded ring. This map was computed after least-squares refinement without the hydrogen atoms H21 and H22 involved in the hydrogen bond. Contours are drawn at 0.04 e Å À3 intervals.

Synthesis and crystallization
There are some synthetic difficulties encountered in preparation of carboxylated -diketones according to the common Claisen condensation. Fortunately, the desired compounds can be obtained under mild conditions via an MgBr 2 ÁEt 2 O-assisted acylation of ketones by benzotriazole amides of the corresponding diesters (Lim et al., 2007). The title compound was prepared as follows: To a suspension of MgBr 2 ÁEt 2 O (0.73 g, 2.8 mmol) in dry CH 2 Cl 2 (16 ml), acetophenone (0.35 ml, 3.0 mmol) was added and the mixture was sonicated for a minute. N,N-Diisopropylethylamine (0.52 ml, 3.0 mmol) was added to the mixture and it was sonicated for a minute. The resulted suspension was added quickly to a solution of the methyl ester of isophtalic acid benzotriazole amide (1.15 g, 4.0 mmol) in dry CH 2 Cl 2 (16 ml) and the mixture was stirred at 293 K for 34 h. The reaction mixture was treated by a 2 M HCl solution (40 ml) and stirred vigorously for 1 h. The organic layer was separated and the aqueous layer extracted with CH 2 Cl 2 (3 Â 20 ml). The combined organic extracts were washed with water (1 Â 20 ml) and brine (1 Â 20 ml) and filtrated through paper followed by evaporation of the solvent. The resulting oil was crystallized from CH 3 OH solution at 255 K to give a lightyellow powder, which was purified by column chromatography (SiO 2 , CHCl 3 /hexane 1/3 v/v) and dried in vacuo. Yield 457 mg (54%). Single crystals suitable for X-ray analysis were grown by slow evaporation of the solvent from a solution of the substance in chloroform.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. All hydrogen atoms were located from a difference-density map and refined freely. The disordered hydrogen atoms H21 and H22 were clearly discernible from a difference-density map (Fig. 1b). Their occupancies refined to a ratio of 0.44 (7):0.56 (7)

Methyl 3-(3-hydroxy-3-phenylprop-2-enoyl)benzoate
Crystal data 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.