(R,R)-1-Acetyl-1′-(2,4,6-trinitrophenyl)-2,2′-bipyrrolidine

The structure of the title molecule, C16H19N5O7, is mainly determined by the steric effect of a bulky 2,4,6-trinitrophenyl group attached to the N atom of a pyrrolidine ring. Both pyrrolidine rings adopt an envelope conformation, with one of the methylene C atoms as the flap in each case, and the N—C—C—N torsion angle along the bond connecting the two pyrrolidine rings is −174.9 (2)°. The benzene ring of the 2,3,5-trinitrophenyl substituent is deformed and the r.m.s. deviation of its six atoms from the best plane is 0.026 Å. The N atoms of the two nitro groups in the ortho positions deviate from the best plane of the benzene ring by −0.033 (5) and 0.385 (5) Å. These groups, as well as the pyrrolidine ring, are twisted relative to the aromatic ring in the same direction, their best planes forming dihedral angles of 30.2 (2), 64.8 (1) and 46.6 (2)°, respectively, with the ring. An intramolecular C—H⋯O hydrogen bond occurs. In the crystal, there is a short [O⋯C = 3.019 (4) Å] contact between a nitro O atom and a C atom of the benzene ring bearing the nitro group and a C—H⋯O interaction between a methyl H atom and another nitro O atom.


Comment
The title compound was synthesized as a part of a project aiming at the application of 2,4,6-trinitrophenyl chromophore for determination of absolute configuration of secondary diamines. The molecular structure of the title compound is shown in Fig. 1. Both pyrrolidine rings adopt an envelope conformation with the methylene C4 and C9 atoms forming a flap in each of the five-membered rings, respectively. The N7-C6-C2-N1 torsion angle along the bond connecting two pyrrolidine rings is -174.9 (2)°.
The benzene ring of the 2,3,5-trinitrophenyl substituent shows large deformation from planarity with r.m.s. deviation of 0.026 Å for the six fitted atoms and the maximum deviation from the best plane of 0.038 (2) Å for C11. Whereas N3 and N4 atoms of the nitro groups are vitrually in the mean plane of the benzene ring [their deviations from the plane being -0.050 (5), -0.033 (5) Å, respectively] the N1 atom from the pyrrolidine substituent and the N2 atom from one of the ortho nitro groups deviate strongly from this plane [deviations of -0.168 (4) and 0.385 (5) Å, respectively] reflecting steric effects within this overcrowded molecule. The nitro groups attached to C12 and C16 of the benzene ring are twisted in the same direction as the pyrrolidine ring attached to C11 forming the fragment of a propeller. The dihedral angles formed by these nitro groups and the planar C11, N1, C2, C5 fragment are 30.2 (2), 64.8 (1) 46.6 (2)°, respectively. The nitro group attached to C14 is only slightly twisted relative to the benzene ring with the dihedral angle of 4.9 (2)°. The conformation adopted by the molecule leads to two short intermolecular contacts between the pyrrolidine ring H atoms and O toms of the ortho nitro-groups (Table 1). Interestingly, the release of strain in the title molecule occurs differently than in 1-pyrrolidino-2,4,6-trinitrobenzene (Baggio et al., 1997) where the benzene ring adopted a sofa form with the flap formed by the C atom to which the pyrrolidine ring was attached. On the other hand, the release of strain is similar to that observed for N, N-dimethyl-2,4,6-trinitroaniline (Butcher et al., 1992), 1-piperidylo-2,4,6-trinitrobenzene and 1morpholino-2,4,6-trinitrobenzene (Baggio et al., 1997) Two short intermolecular contacts are observed in this crystal structure. One, O1···.C16(1/2 + x, 3/2 -y, 2 -z) of 3.019 (4) Å, is formed between the nitro group O atom and the carbon atom of the benzene ring bearing the nitro group.
The second one, H18C···.O2(2 -x, 1/2 + y, 3/2 -z) of 2.51 Å, is formed between the methyl group H atom and the nitro group O atom. The crystal packing in the studied crystal is shown in Fig. 2.

Experimental
A mixture of (R,R)-2,2′-bipyrrolidine hydrochloride (280 mg, 1.31 mmol), 1-chloro-2,4,6-trinitrobenzene (650 mg, 2.63 mmol) and anhydrous sodium acetate (860 mg, 10.50 mmol) in anhydrous ethanol (10 ml) was heated under reflux for 30 min. The resulting suspension was cooled to room temperature and water (15 ml) was added into it. The aqueous layer was extracted with dichloromethane (2 x 15 ml). The combined organic extracts were dried over anhydrous magnesium sulfate. Filtration of the drying agent and removal the solvent in vacuo afforded the crude product, which was purified by means of column chromatography on silica gel using ethyl acetate to yield 0.12 g (23%) of product as a yellow solid.

Refinement
All H atoms were located in electron-density difference maps, however for further refinement their positions were determined geometrically with C-H bond lengths of 0.93 -0.97 Å. All H atoms were refined in the riding-model approximation, with U iso (H)=1.5U eq (C methyl ) or U iso (H)=1.2U eq (C) for the remaining H atoms. In the absence of significant anomalous dispersion effects, 1319 Friedel pairs were merged.

Figure 1
The molecular structure of the title compound with displacement ellipsoids shown at the 50% probability level.   Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.