(–)-(S)-N,N′-Bis[1-(1-naphthyl)ethyl]oxalamide

The title molecule, C26H24N2O2, displays C 2 symmetry, with the molecule located on a twofold axis perpendicular to the plane of the oxalamide unit –NH—CO—CO—NH–. The oxalamide core deviates from planarity, as reflected by the O=C—C=O and N—C—C—N torsion angles of 164.3 (5) and 163.2 (5)°, respectively. The naphthyl groups are oriented toward the same face of the oxalamide mean plane and make a dihedral angle of 43.76 (8)°. This conformation is suitable for the formation of intermolecular N—H⋯O hydrogen bonds, giving noncentrosymmetric dimers incorporating R 2 2(10) ring motifs. These nonbonding interactions propagate along the 61 screw axis normal to the molecular twofold axis, resulting in a single-stranded right-handed helix parallel to [001]. In the crystal, Δ helices are arranged side-by-side and interact through π–π contacts between naphthyl groups. The shortest centroid–centroid separation between interacting benzene rings is 3.623 (4) Å.

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2006); software used to prepare material for publication: SHELXL97. properties. In continuation of this work, we synthesized the title chiral compound, under the solvent-free approach, because the reactions occur under mild conditions and usually require easier workup procedures and simpler equipment. Likewise, reducing the use of organic solvents and minimizing the formation of waste are worth-considering points to keep in mind by using this protocol (Jeon et al., 2005;Noyori, 2005).
The title molecule belongs to the oxalamide family, a well studied class of compounds, which are useful in many areas (e.g. Montero-Vázquez et al., 2008). The molecule is placed on a twofold axis, passing by the midpoint of the central C1-C1 i bond (symmetry code i: x-y, -y, 1 -z). The twofold axis is perpendicular to the mean plane of the core oxalamide group. In contrast to other oxalamide derivatives (e.g. Zhang et al., 2006) the oxalamide group in the title compound is not planar. As a consequence, trans angles O1-C1-C1 i -O1 i and N2-C1-C1 i -N2 i deviate significantly from 180°. N atoms are substituted by chiral groups including naphthyl cycles, which make a dihedral angle of 43.76 (8)° (Fig. 1).
The C 2 -symmetric molecules are connected in the crystal through N2-H2···O1 ii hydrogen bonds (symmetry code ii: y, -x + y, z -1/6), which generate R 2 2 (10) ring motifs (Fig. 2). Such motifs have been observed in related chiral (Štefanić et al., 2003) and achiral (Lee & Wang, 2007) oxalamides, but gave different supramolecular structures, depending of the ability of terminal groups to be involved in hydrogen bonds. In the case of the title oxalamide, hydrogen bonds form a supramolecular structure based on a single stranded helix using the screw axis 6 1 as backbone (Fig. 2, inset). The molecular and supramolecular axes are thus perpendicular. Although the space group is enantiomorphic, the helicity of the supramolecular structure can be assigned, since the chirality of the asymmetric unit is known (Ha & Allewell, 1997). The S configuration of chiral center C3 affords right-handed helix oriented along [001]. It is worth noting that such chiral supramolecular structures cannot be achieved for centrosymmetric oxalamide derivatives (Zhang et al., 2006;Lee, 2010).
The crystal structure of the title compound is build up on densely packed parallel Δ helix, which interact through π-π contacts involving terminal naphthyl groups. The shortest intermolecular separation between symmetry-related benzene rings is 3.623 (4) Å [Cg···Cg iii , where Cg is the centroid of ring C9···C14 and iii is symmetry code: 1 -y, 1 -x, 5/6 -z]. This contact should however be regarded as a secondary interaction compared to hydrogen bonds forming the one-dimensional supramolecular structure. Although the ring approach is short, constraints induced by molecular conformation reduce the efficiency of such contacts. For instance, π systems involved in π-π contacts are not parallel, making a dihedral angle of 11.7 (2)°.

Experimental
Under solvent-free conditions, a mixture of oxalyl chloride (0.2 g, 1.

Refinement
All C-bonded H atoms were placed in idealized positions and refined as riding to their carrier C atoms, with bond lengths fixed to 0.93 (aromatic CH), 0.96 (methyl CH 3 ), and 0.98 Å (methine CH). Atom H2 bonded to N2 was found in a difference map and refined freely, although N-H bond length was restrained to 0.91 (1) Å. Isotropic displacement parameters were calculated as U iso (H) = 1.5U eq (carrier atom) for the methyl group and U iso (H) = 1.2U eq (carrier atom) otherwise. The absolute configuration was assigned from the known configuration of the chiral amine used as starting material, and measured Friedel pairs (705) were merged. Fig. 1. The title molecule with displacement ellipsoids for non-H atoms shown at the 30% probability level. Non labeled atoms are generated by symmetry operator x-y, -y, 1 -z.