Ethyl (2E)-2-(hydroxyimino)propanoate

The molecule of the title compound, C5H9NO3, is essentially planar [the maximum deviation for a non-H atom from the mean plane is 0.021 (3) Å] due to the π-conjugation of the hydroxyimino and carbonyl groups, which are trans to each other; ab initio calculations in vacuo at the DFT (B3LYP/6–311G**++) level of theory confirmed that E conformer is indeed the lowest in energy. The packing in crystal structure is influenced by strong intermolecular O—H⋯N hydrogen-bonding interactions between oxime groups and also by π-stacking of the molecules due to the carbonyl and oxime group orbital overlap [interplanar distance between adjacent molecules = 3.143 (4) Å]. Jointly, these factors afford infinite 6.32 Å thick molecular sheets, where the plane of each molecule is perpendicular to the plane of the sheet. Seen from above, the molecules within the sheet are arranged in a herringbone pattern. Such sheets form a stack due to weak van der Waals interactions; the gap between adjacent sheets is 2.07 Å.

The molecule of the title compound, C 5 H 9 NO 3 , is essentially planar [the maximum deviation for a non-H atom from the mean plane is 0.021 (3) Å ] due to the -conjugation of the hydroxyimino and carbonyl groups, which are trans to each other; ab initio calculations in vacuo at the DFT (B3LYP/6-311G**++) level of theory confirmed that E conformer is indeed the lowest in energy. The packing in crystal structure is influenced by strong intermolecular O-HÁ Á ÁN hydrogenbonding interactions between oxime groups and also bystacking of the molecules due to the carbonyl and oxime group orbital overlap [interplanar distance between adjacent molecules = 3.143 (4) Å ]. Jointly, these factors afford infinite 6.32 Å thick molecular sheets, where the plane of each molecule is perpendicular to the plane of the sheet. Seen from above, the molecules within the sheet are arranged in a herringbone pattern. Such sheets form a stack due to weak van der Waals interactions; the gap between adjacent sheets is 2.07 Å .

Comment
Although the preparation of (I) is well documented (see the Related Literature), no direct structural study has been reported so far. In this communication the molecular and crystal structure of the title compound, determined by a single crystal X-ray diffraction, is presented.
In recent years we became involved in the synthesis and thermodynamic studies of the ligands with chelating oximeand-amide moieties, as well as their complexes with transition metals. The title compound is the key intermediate for the preparation of such ligands via a condensation route with a suitable diamine (Armand & Guette, 1969).

Molecular Structure:
The molecule of (I) is profoundly planar (Fig. 1). Maximum deviation for a non-hydrogen atom from the average plane is 0.021 Å. We attribute this to the stabilizing effect of π-conjugation between the hydroxyimino and carbonyl groups. Such interpretation is supported by the ab initio quantum mechanical modeling at the DFT (B3LYP/ 6-311G**++) level of theory (JAGUAR and MAESTRO;Schrödinger, 2008).
In solid state (I) exists as an E-isomer, with the oxime and carbonyl groups trans to each other. Ab initio calculations for (I) in vacuum confirmed that planar E-isomer is indeed lower in energy than any of the Z-conformers. The only difference of the solid state structure from the lowest energy conformer in vacuum is the orientation of the methyl group riding C1-atom; computed energy of the conformer where H5-atom in plane with the carbonyl group is pointing towards it is 1.71 kJ mol -1 lower than for the conformer where such hydrogen atom is pointing away from it. Computationally, planar E-conformer is 6.98 kJ mol -1 lower in energy than similar Z-conformer. When the dihedral angle N1-C1-C2-O3 is varied from 180°t o 0°, a potential barrier of 16.6 kJ mol -1 is encountered.
Geometric parameters are representative of the hydroxyimino esters. They are in close agreement with the computed ones. For example, the largest difference in the bond length is 0.023 Å (the computed length is longer) for the C8-C5 bond.
Crystal Structure: A packing diagram for the crystal structure of (I) is shown in Fig. 3. The spacial arrangement of molecules is influenced by two factors: a) strong intermolecular hydrogen bonding interactions between oxime groups (O2···N1 i : 2.778 (4) Å, O2···H9-N1 i : 148.4 °; symmetry code: (i) -x+1, -y+2, -z+2), Fig.2, and b) π-stacking of the molecules due to the carbonyl and oxime group orbital overlap (Fig. 4). The former factor causes the formation of dimers, while the latter one is responsible for a "staircase" structure, where the distance between average planes of adjacent molecules is 3.143 (4) Å. Jointly, these factors afford infinite molecular sheets, where the plane of each individual molecule is perpendicular to the plane of the sheet (Fig. 5). Seen from above, the molecules in the sheet are arranged in a herring-bone pattern.
The thickness of such sheets, measured as the distance between two planes drawn through the most external carbon atoms, is 6.32 Å. They form a stack due to weak van der Waals interactions. Measured as above, the gap between adjacent parallel sheets in the stack is 2.07 Å.
supplementary materials sup-2 Experimental Compound (I) was synthesized following a modified procedure of Ponzio & Ruggeri (1925). The reaction between ethylpyruvate and hydroxylamine hydrochloride was carried out at room temperature in aqueous solution. In a typical preparation, hydroxylamine hydrochloride (7.45 g; 105 mmol) was dissolved in 200 ml of water. Sodium carbonate (5.3 g, 50 mmol) was added and the solution stirred for about five minutes. Strong effervescence (evolution of CO 2 ) was observed initially.
Thereafter ethyl pyruvate (11.3 ml; 100 mmol) was added drop-wise and the solution was left to stir for half an hour.
After about 20 min, large quantity of a flaky white precipitate was observed. The precipitate was subsequently filtered off, rinsed with cold water, and dried on a watch glass. Remaining in aqueous layer (I) was extracted with dichloromethane (2×100 ml). The organic fractions were combined, dried over magnesium sulphate, and the solvent removed. The solid recovered was combined with the primary precipitate. This crude product was recystallised from hot ethanol, affording nearly quantitative yield (typical figures: 95-98 %).
Colorless silky crystals in the shape of elongated prisms were characterized by the melting point determination, FTIR, NMR, GCMS, MS/ToF, and X-ray diffraction.  Assignment of chemical shifts in the NMR-spectra is based on the analysis of one-dimensional ( 1 H, 13 C, dept) and correlation two-dimensional (gCOSY, ghmqc, ghsqc) spectra.
Fragmentation in the GCMS spectrum is mainly due to the McLafferty rearrangement of (I); the masses of expected fragments are: 28, 58, 73, 85, and 103.

Refinement
All H atoms were positioned geometrically and allowed to ride on their parent atoms, with C-H = 0.93-0.98 Å and U iso (H) = 1.2-1.5 U eq (C). Fig. 1. A view of the molecular structure of the title compound. Displacement ellipsoids (Mercury 2.2) are drawn at the 50% probability level.
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.
Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conven-  (14)