Crystal structure of 2-(2-aminophenyl)-1,3-benzoxazole

The two molecules of 2-(2-aminophenyl)-1,3-benzoxazole in the asymmetric unit feature an intramolecular N—H⋯N hydrogen bond, which closes an S(6) ring and therefore establishes a syn relationship for the N atoms. In the crystal, molecules are linked by N—H⋯N hydrogen bonds, generating [100] chains containing alternating A and B molecules.


Chemical context
Benzimidazole, benzoxazole, and benzothiazole derivatives are key components in many bioactive compounds of both natural and synthetic origin; many are active components of biocides such as bactericides, fungicides, insecticides and anticarcinogens (Kumar-Samota & Seth, 2010). Benzoxazole derivatives have been used as building blocks for biochemical and pharmaceutical agents, as well as dyes, fluorescent brightening agents, biomarkers and biosensors (Costa et al. 2007 andTong et al. 2005).
In this context, 2-(2-aminophenyl)benzoxazole has shown considerable growth inhibition with respect to fungi and grampositive and gram-negative bacteria (Elnima et al. 1981). For this reason, several methods have been described for the synthesis of these heterocyclic compounds, some of which are summarized in the Scheme, which shows the retrosynthesis for the preparation of the title compound, (I). For example, Gajare et al. (2000) described a procedure for the preparation of 2-(o-aminophenyl)oxazolines from isatoic anhydride and 2-aminoalcohols at reflux of PhCl mediated via a natural kaolinitic clay catalyst; a slightly modified procedure has been describe by Button & Gossage (2003) using zinc chloride as a catalyst. Qiao et al. (2011) described the synthesis of benzoxazole via the reaction of anionically activated trifluoromethyl groups with amino nucleophiles under mild aqueous conditions. Recently, Khalafi-Nezhad & Panahi (2014) reported an efficient approach for the preparation of benzoxazole derivatives, via acceptorless dehydrogenative coupling of alcohols with 2-aminophenol using an Ru catalytic system.
In the present work, as part of our ongoing studies of heterocyclic compounds (Ló pez-Ruiz et al., 2011, 2013de la Cerda-Pedro et al., 2014), we report the synthesis of 2-(2aminophenyl)benzoxazole, we analyse its molecular structure, as well as its weak intermolecular interactions in molecular packing, which could be useful in the understanding of their mode of action in pharmaceutical science, as well as in the design of materials with specific functions. The title compound has been previously reported by Button & Gossage (2003) from isatoic anhydride and 2-aminophenol but its crystal structure has not been described.

Structural commentary
Compound (I) crystallized in the monoclinic space group P2 1 /c with two independent molecules (A and B) in the asymmetric unit (Fig. 1). The orientation of the amino group can be described using as a basis the carbon atom C9, this orientation The asymmetric unit of (I) with displacement ellipsoids drawn at the 50% probability level (left: molecule A and right: molecule B) is syn to the nitrogen atom N3 and anti for the oxygen atom O1.
The skeleton of each molecule is practically planar: to analyse the planarity of the molecule we use the torsion angle N3-C2-C8-C9, indicating the rotation of the aromatic ring C8-C13: these angles are À1.2 (2) and 0.9 (2) for molecules A and B, respectively. The dihedral angles between the benzene ring and the fused ring system are 0.74 (8) and 0.67 (6) for molecules A and B, respectively. The two independent molecules are very similar, with an r.m.s. overlay fit of 0.019 Å .
The single crystal used in the experiment was obtained by the method of liquid-liquid diffusion by slow evaporation. The pure compound was dissolved in the minimum amount of dichloromethane to be added by the walls of the tube the same amount of acetone followed by methanol. The tube was sealed to leave the solution in a vibration-free environment at room temperature. After a few days, the solution had evaporated, leaving colourless blocks of the title compound.