(1R,2S,5R)-5-Methyl-2-[2-(4-nitrophenyl)propan-2-yl]cyclohexyl 2-(4-methoxyphenyl)-2,5-dihydro-1H-pyrrole-1-carboxylate: crystal structure and Hirshfeld analysis

The molecule in the title compound approximates a U-shape with the nitrobenzene and ester substituents lying to the same side of the molecule. In the crystal, linear supramolecular chains are sustained by methylene-C—H⋯O(carbonyl) interactions.


Chemical context
The reaction of an unsaturated halide species with an alkene, in the presence of both a base and a organopalladium catalyst, to form a substituted alkene, is termed the Heck reaction (Heck, 1982;Crisp, 1998). As part of our investigations into the scope of the Heck reaction in the total, enantioselective and efficient synthesis of pyrrolidine alkaloids, such as the natural product (-)-codonopsinine (Severino & Correia, 2001), an enecarbamate containing the chiral auxiliary residue, 8-(4-nitrophenyl)menthol, was submitted to a Heck arylation reaction with 4-methoxyphenyldiazonium tetrafluoroborate. The reaction yielded the title compound, 8-(4-nitrophenyl)menthyl 2-(4-methoxyphenyl)pyrroline-3-carboxylate, (I), as the sole crystalline material (Machado, 2001). Herein, the crystal and molecular structures of (I) are described along with an analysis of the calculated Hirshfeld surfaces.

Structural commentary
The molecular structure of (I), Fig. 1, comprises a 1-, 2-and 5substituted cyclohexyl ring (chair conformation) with the chirality at these equatorially substituted centres, i.e. C14, C15 and C18, established from the synthesis, being R, S and R, respectively. The dihydropyrrole ring is essentially planar, with an r.m.s. deviation of 0.003 Å for the five constituent

Supramolecular features
The molecular packing of (I) features a number of weak noncovalent contacts as discussed below in the Hirshfeld surface analysis (x4). In accord with the distance criteria assumed in PLATON (Spek, 2009), there is only one directional interaction of note, Table 1. Thus, methylene-C19-HÁ Á Á O2(carbonyl) interactions connect molecules into a linear supramolecular chain along the b-axis direction, Fig. 2a. These assemble in the crystal with no directional interactions between them, Fig. 2b.

Hirshfeld surface analysis
The Hirshfeld surfaces calculated for (I) were conducted as reported recently for a related organic molecule (Zukerman-Schpector et al., 2017) and provide information on the influence of short interatomic non-bonded contacts upon the molecular packing.

Figure 1
The molecular structure of (I), showing the atom-labelling scheme and displacement ellipsoids at the 35% probability level.  Table 2 are also viewed as faint-red spots near the O3, H4, O5 and H20B atoms in Fig. 3. The influence of the short interatomic OÁ Á ÁH, CÁ Á ÁH and HÁ Á ÁH contacts in the molecular packing are also illustrated in Fig. 4a and b, which show the Hirshfeld surface mapped over the shape-index property and d norm , respectively. The intramolecular C-HÁ Á Á contact between the pyrrole-H5A atom and the nitrobenzene ring [H5AÁ Á ÁCg(C24-C29) = 2.67 Å , C5Á Á ÁCg(C24-C29) = 3.612 (3) Å and C-H5AÁ Á ÁCg(C24-C29) angle = 163 ] is shown as a black-dotted line within the Hirshfeld surfaces mapped over the electrostatic potential in Fig. 5. The overall two-dimensional fingerprint plot for (I), Fig. 6a, and those delineated into HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/ HÁ Á ÁC contacts (McKinnon et al., 2007) are illustrated in Fig. 6b-d, respectively. The fingerprint plots also reflect the presence of the short interatomic contacts on the packing, Table 2. This is also evident from the percentage contribution from different interatomic contacts to the Hirshfeld surface summarized in Table 3: the HÁ Á ÁH, OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/ HÁ Á ÁC interatomic contacts make the greatest contribution to the Hirshfeld surface and account for 97.9% of the overall surface. The broad feather-like distribution of points with a peak at d e + d i $2.3 Å in the fingerprint plot delineated into HÁ Á ÁH contacts in Fig. 5b represent HÁ Á ÁH contacts in the structure and make the greatest, i.e. 61.7%, contribution to the surface. The interatomic OÁ Á ÁH/HÁ Á ÁO contacts having a 23.9% contribution to the Hirshfeld surface arise from the C-HÁ Á ÁO contact (Table 1) and short interatomic OÁ Á ÁH/HÁ Á ÁO contacts (Table 2), and are viewed as the pair of green aligned points beginning at d e + d i $2.6 Å and a pair of jaw-shaped distribution of points in the range d e + d i $2.5-2.6 Å in Fig. 6c. The points distributed around the pair of forceps-like peaks at d e + d i $2.8 Å in the fingerprint plot delineated into CÁ Á ÁH/ HÁ Á ÁC contacts ( Table 3 Percentage contributions of interatomic contacts to the Hirshfeld surface for (I).

Contact
Percentage contribution

Figure 3
Two views of the Hirshfeld surface for (I) mapped over d norm in the range À0.071 to +1.718 au.
Contact Distance Symmetry operation

Figure 4
Views of Hirshfeld surfaces mapped (a) with shape-index property highlighting short interatomic OÁ Á ÁH/HÁ Á ÁO and CÁ Á ÁH/HÁ Á ÁC contacts by red and sky-blue dashed lines, respectively, and (b) over d norm showing intra-layer interatomic HÁ Á ÁH contacts by black dashed lines. and inter-layer contacts in the crystal. The small contribution from other interatomic contacts summarized in Table 3 appear to have a negligible impact on the molecular packing.

Database survey
The (1R,2S,5R)-menthyl substrate is important as a chiral source for the synthesis of natural products and, as such, has been found in a number of crystal structures related to (I).
Owing to the dictates of the chirality at the C1 and C2 positions, a parallel alignment of the substituents at these positions usually result in U-shaped geometries (Aoyagi et al., 1998;Singh et al., 1990;Streith et al., 1995), except in circumstances where steric hindrance precludes such an arrangement (Comins & Killpack, 1992).

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. The C-bound H atoms were placed in calculated positions (C-H = 0.93-0.98 Å ) and were included in the refinement in the riding model approximation, with U iso (H) set to 1.2-1.5U eq (C).

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.