Structure of (R,R)-4-bromo-2-{4-[4-bromo-1-(4-toluenesulfonyl)-1H-pyrrol-2-yl]-1,3-dinitrobutan-2-yl}-1-(4-toluenesulfonyl)-1H-pyrrole, another ostensible by-product in the synthesis of geminal-dimethyl hydrodipyrrins

The crystal structure of a by-product in the synthesis of geminal-dimethyl hydrodipyrrins is reported; (R,R)-4-bromo-2-{4-[4-bromo-1-(4-toluenesulfonyl)-1H-pyrrol-2-yl]-1,3-dinitrobutan-2-yl}-1-(4-toluenesulfonyl)-1H-pyrrole (1, C26H24N4O8S2Br2). Generated through a nitronate-mediated dimerization, this compound presents unforeseen enantiomeric resolution, something previously not noted in its singular prior report. This crystal adds to the ever-growing library of by-products arising from these syntheses.


Chemical context
geminal-Dimethyl hydroporphyrins were first made a reality via the de novo syntheses of (AE)-bonellin presented in the 1980s and 1990s (Dutton et al., 1983;Montforts & Schwartz, 1991). However, for modern oxidation-resistant chlorins, we look to the Lindsey group (Lindsey, 2015). Beginning at the turn of the century , their extension of Battersby's thermal route has become the go-to synthesis for oxidation-resistant hydroporphyrins. Since its inception there have been multiple refinements (Ptaszek et al., 2005;Laha et al., 2006;Krayer et al., 2009). Subsequently, this synthesis has found applications in understanding the electronics of the chlorin macrocycle (Mass et al., 2009), the generation of Ering-functionalized hydroporphyrins (Ptaszek et al., 2010), the generation of hydroporphyrin dimers and arrays (Meares et al., 2015), and taking steps towards generating N-confused oxidation-resistant hydroporphyrins (Xiong et al., 2019).
Noted only once previously is the formation of a byproduct, 1 (Krayer et al., 2009). Through our own ventures into the world of hydroporphyrins (Melissari et al., 2020;Kingsbury et al., 2021), we have in one instance generated a suitable amount of dimeric by-product 1, and single crystals therefrom. The crystal structure of this elusive by-product, obtained in the synthesis of geminal-dimethyl hydrodipyrrins and hydroporphyrins, is described in this work. The structure presented in this work adds to an ever-increasing library of by-products from this field, which includes tricyclic undecane (CSD refcode CAJVUF; Taniguchi et al., 2001) and dihydrooxazine (BESZEI; Tran et al., 2022).

Structural commentary
The title compound 1 presents an asymmetric unit of one molecule of the title compound with no solvate. Compound 1 was found to crystallize in the orthorhombic system (Pbca, Z = 8). Although a chiral compound, this is a racemate and the asymmetric unit is shown in Fig. 1 as (R,R)-stereochemistry. In 1 H NMR spectroscopy, along with the respective 2D NMR with analyses undertaken of the same sample, we observe only one set of resonances for the aliphatic nitrobutane system (full 1 H, 13 C and 1 H-13 C HSQC NMR spectra are presented in the supporting information). The implication herein is that the sample presented contains the enantiomers (R,R) and (S,S) only, with no other diastereomers present; see Fig. 2 for the synthetic pathway.
Both pyrrole rings are essentially planar, with RMSD values of 0.009 Å in both instances, and exhibit bond distances comparable with previous data (Kingsbury et al., 2021). Both tosyl groups also exhibit the same conformation, i.e. with the p-tolyl ring coming out of the plane of the pyrrole ring, when viewing the respective pyrrole ring face on, as shown in Fig. 1, with N-S-C angles of 104.36 (9) and 105.26 (10) , with the larger angle arising in the motif exhibiting an intramolecular C sp 3-HÁ Á ÁO sulfonyl interaction (see Table 1). Despite the hydrogen-bonding interactions present, the O S O angle changes minimally 120.34 (10) , in comparison to 120.86 (11) for the non-interacting tosyl moiety. The dihedral angle between the pyrrole rings is 72.00 (12) . The bond distances are within normal ranges (Groom et al., 2016).
Lacking any protic donor or more traditional strong supramolecular interactions, this structure is dominated by weaker C-HÁ Á ÁO interactions; see Table 1   Synthesis of dimeric by-product 1 through the reduction of 2 to yield 3. Reagents are non-specific given the number of differing procedures in the literature. and labels added to heighten the disymmetry of 1. bifurcated C8Á Á ÁO22 sulfonyl and C8Á Á ÁO31 sulfonyl interactions of 3.071 (3) and 3.072 (3) Å , we observe seven-membered ring formation. In another bifurcated intramolecular interaction, C12Á Á ÁO15 nitro and C12Á Á ÁO31 sulfonyl , 2.719 (2) and 2.913 (3) Å differing sized rings are formed, with the interaction between methine and nitro motifs yielding a five-membered ring, and a six-membered ring between the methine and sulfonyl motifs. With C13 sp 3Á Á ÁO10 nitro at 3.038 (3)Å , we observe one of the two nitro groups forming a six-membered ring with an opposing nitromethyl motif.
We have no mechanistic evidence to rationalize the generation of 1, be it through a non-stereoselective nitronate addition followed by kinetic precipitation to yield 1, or simply through the impossibility of the formation of (R,S)-1 or (S,R)-1 as a direct result of steric interactions between two 1,2,4trisubstituted pyrrolic motifs.

Supramolecular features
Regarding intermolecular interactions, there are several C-HÁ Á ÁO synthons present involving the nitro motifs. The first is seen with the opposite oxygen to the intramolecular synthon described above, with the bromopyrrole linking to the adjacent nitro group, C21Á Á ÁO11 ii , 3.459 (3) Å . The second involves the other nitromethyl motif which exhibits a C3Á Á ÁO15 i interaction of 3.194 (3) Å with an adjacent molecule of the title compound arising from the 5-pyrrolyl position. The other nitro oxygen is involved with the methyl group on the tosyl phenyl ring with C38 methyl Á Á ÁO16 iv , 3.478 (3) Å and this also brings the methyl group into alignment with a neighbouring bromine, C38Á Á ÁBr1 v , 3.639 (3) Å . These two interactions propagate along the crystallographic c-axis direction, which is shown in Fig. 3, forming loosely associated sheets. These sheets are weakly connected by C27 tosyl Á Á Á O10 nitro iii , 3.413 (3) Å .

Synthesis and crystallization
Compounds 2 and 3 were synthesized following the reported procedures (Krayer et al., 2009). For 1, crystals were generated via slow evaporation at room temperature of a saturated solution of 1 in CDCl 3 . We have previously described the crystallization of 2 (Kingsbury et al., 2021) and currently no structure of 3 has been reported. Compound 1 was obtained in 10% yield from 2, with yields for 3 we typically observe approx. 69%, close to those previously reported (Laha et al., 2006).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Hydrogen atoms were positioned geometrically and refined isotropically using a riding model with C-H = 0.93-0.98 Å and U iso (H) = 1.2-1.5U eq (C).

Funding information
Funding for this research was provided by: H2020 Marie Skłodowska-Curie Actions (grant No. 764837).   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.