2-Allyloxy-5-nitrobenzoic acid

The molecule of the title compound, C10H9NO5, is approximately planar, with the mean planes of the nitro, carboxyl and allyloxy groups rotated by 8.1 (3), 7.9 (3) and 4.52 (18)°, respectively, from the plane of the benzene ring. Bond lengths in the aromatic ring are influenced by both electronic effects and strain induced by ortho-substitution. In the crystal structure, centrosymmetrically related molecules are paired into dimers through strong O—H⋯O hydrogen bonds.


Comment
The unimolecular rearrangement of chorismate to prephenate, catalyzed by chorismate mutase, is a rare case of a [3,3] sigmatropic shift reaction in live organisms and it is a natural target in drug development, since corresponds to a key step in the pathway to form aromatic amino acids in plants, bacteria and fungi (Ziegler, 1977;Castro, 2004;Zhang et al., 2005).
Chorismate mutase catalyzes this intramolecular reaction without formation of an enzyme-substrate covalent intermediate and the proposed transition state structures in the gas phase, water and enzyme are characteristic of a concerted pericyclic rearrangement. Transition state stabilization seems to be responsible for only 10% of the enzymatic advantage over the water reaction (106-fold catalytic effect). Since we are interested in the systematic analysis of the influence of electrostatic stabilization and intramolecular hydrogen bonding in [3,3] sigmatropic Claisen rearrangements, a series of ethers derived from salicylic acid has been synthesized. The 2-allyloxy-5-nitrobenzoic acid (I, scheme 2) is a new synthesized compound and here we report its X-ray crystal structure.
A projection of the molecular structure and the numbering of the non-hydrogen atoms are shown in Fig. 1. Bond length data show that in the aromatic ring the C3-C4, C4-C5 and C5-C6 bonds are the strongest (shortest) C-C ring bonds, as a consequence of both electronic effects and strain induced by ortho-substitution at C1 and C2. The mean plane of nitro, carboxyl and allyloxy groups are deviated from the best plane of the phenyl ring by 8.1 (3)°, 7.9 (3)° and 44.52 (18)°, respectively. The electron withdrawing influence of the carboxyl group, combined with the strain introduced by the allyl ether, weakens the C1-C2 and C1-C6 bonds significantly and makes them longer than the other ring bonds. These effects in both bond lengths and coplanarity of the aromatic ring and the carboxyl group are similar to those found in the crystal structure of 2-allyloxy-5-chlorobenzoic acid (II) (Ferreira et al., 2007). The nitro group in 1 has a small effect on the C3-C4--C5 angle (121.5 (2)°), but the COOH group reduces the C1-C2-C3 angle from 120° (normal benzene ring) to 119.10 (18)°. The effect is practically identical to that found in compound (II, scheme 2), where the C3-C4-C5 angle is 120.4 (2)° and C1-C2-C3 angle is 119.37 (19)°. In both compounds, the observed effect evidently results from the presence of the allyloxy group in (I), lengthening both C1-C2 and C1-C6 bonds, and reducing the C2-C1-C6 angle to 119.09 (19)°. Closely similar effects are observed for 2-methoxymethoxybenzoic acid, where the ortho-substituent is electronically and sterically similar (Jones et al., 1984).
In the carboxylic group, the C-O distances are very similar to each other. This indicates a high degree of delocalization of the π-electrons over the COOH backbone. Once the acid group is protonated, the similarity in bond lengths can be attributed to the very strong hydrogen bond (see Table 1). These intermolecular interactions also induce the formation of dimeric structures through center of symmetry. In the three-dimensional packing, the pairs of molecules are perfectly stacked along crystallographic a axis (Fig. 2).

Experimental
Preparation of (I) followed closely the procedure described by White et al. (1958). A mixture of 9.15 g (0.05 mol) of 5-nitrosalycilic acid, 6.05 g (0.05 mol) of allyl bromide, 8.29 g (0.06 mol) of dry, powdered potassium carbonate, and supplementary materials sup-2 sufficient dry acetone (about 30 ml) to give an easily stirred mass was stirred and refluxed for eight hours. Then the mixture was filtered, acidified with diluted acetic acid and the acetone removed by distillation under reduced pressure. The residue was initially obtained as an amorphous solid and yellow crystals of (I) were grown from aqueous acetone solution by slow evaporation at room temperature (m.p. 120-121°C).

Refinement
All non-H atoms were refined with anisotropic displacement parameters. H atoms were placed at their idealized positions with distances of 0.93 and 0.97 Å and U eq fixed at 1.2 times U iso of the preceding atom for C-H Ar and CH 2 , respectively.
The H atom of the COOH group was found in a Fourier difference map and treated with riding model and its U eq was also fixed at 1.2 times U iso of the parent atom. Fig. 1. The molecular structure of the title compound with the labelling scheme. Displacement ellipsoids are shown at the 40% probability level.