rac-1-(4-tert-Butylphenyl)-5-ethyl-4-ferrocenyl-5-hydroxy-1H-pyrrol-2(5H)-one

The title compound,which is produced by the oxidation of 1-(4-tert-butylphenyl)-2-ethyl-3-ferrocenylpyrrole, crystallizes as a racemic mixture in the centrosymmetric space group P21/n. In the crystal, molecules with the same absolute configuration are linked into infinite chains along the b-axis direction by O—H⋯O hydrogen bonds between the hydroxy substituent and the carbonyl O atom of the adjacent molecule.


Chemical context
In a series of recent publications, we were able to show that the ruthenium-catalysed four-component reaction of an ,unsaturated aldehyde with a primary amine (producing an intermediate imine), carbon monoxide and ethylene produces a library of chiral 1,3-dihydropyrrolones and pyrroles, respectively (Biletzki & Imhof, 2011). The ratio of those two products is highly dependent on the relative permittivity of the solvent used, with the yield of the pyrrole increasing with the polarity of the solvent (Gillies et al., 2007). We were also able to show that the oxidation of the resulting pyrroles with oxygen leads to the formation of derivatives of the title compound (Dö nnecke & Imhof, 2003). There are some similar reactions reported in the literature where a pyrrole was transformed into a hydroxy-pyrrolone by oxidation with O 2 , but the reaction mixture had to be irradiated in the presence of a photosensitizer, or radical initiators such as AIBN had to be added in order to induce the reaction (Machida et al. 1982;Dannhardt & Steindl 1985, 1986Takechi et al. 1988;Boger & Baldino 1991;Procopiou & Highcock 1994;Gonzalez et al. 1999). Therefore, a radical mechanism cannot be ruled out for the formation of the title compound, although no addition of any typical initiator is necessary. So overall, depending on the reaction conditions, either chiral 1,3-dihydropyrrolones, chiral 5-hydroxy-1,5-dihydropyrrolones or 2,3-disubstituted pyrrole derivatives might be the main products of the catalytic synthetic methodology developed in our lab (Biletzki & Imhof, 2011;Gillies et al., 2007;Dö nnecke & Imhof, 2003).

Structural commentary
The title compound, rac-1-(4-tert-butylphenyl)-5-ethyl-4ferrocenyl-5-hydroxyl-1H-pyrrol-2(5H)-one, C 26 H 29 FeNO 2 , is derived from 1-(4-tert-butylphenyl)-2-ethyl-3-ferrocenylpyrrole by an oxidation reaction. Therefore, a new centre of chirality is created at C1, which used to be an sp 2 carbon atom in the starting compound. Since no chiral reaction conditions were applied, a racemate of the title compound is produced. The title compound also crystallizes as a racemic mixture in the centrosymmetric space group P2 1 /n. The molecular structure of the S-enantiomer is shown in Fig. 1. The central heterocyclic ring system N1/C1-C4 shows torsional angles of 13.7 (2) with respect to the attached cyclopentadienyl ring and of 43.6 (7) with the major component of the disordered phenyl group bound to N1. The 4-tert-butylphenyl group, as well as the non-substituted Cp ring, are disordered with s.o.f. values of 0.589 (16) and 0.411 (16). Bond lengths and angles are of expected values with the C2-C3 bond length of 1.336 (5) Å , clearly indicating a double bond. In addition, the N1-C4 bond [1.366 (5) Å ] is shortened with respect to the other nitrogen carbon bonds, as is typical for amides.

Supramolecular features
In the crystal, molecules with the same absolute configuration at C1 are linked into infinite chains along the b-axis direction by O-HÁ Á ÁO hydrogen bonds of the C(6) type (Bernstein et al., 1995) between the hydroxy substituent and the carbonyl oxygen atom of an adjacent molecule (Fig. 2, Table 1). In addition, there are weak contacts between carbon atoms of the phenyl ring and H3A and H23A.
Compounds with related heterocyclic systems such as ferrocenyl-substituted maleimides or a 1,5-dihydro-2Hpyrrole-2-one with an imino substituent at C5 have also been reported (  Molecular structure of the S-enantiomer of the title compound showing the numbering scheme. Non-hydrogen atoms are drawn as displacement ellipsoids at the 50% probability level.

Figure 2
Infinite chain of the S-enantiomers along the b-axis. followed by thin layer chromatography and it could be observed that the reaction was finished after approximately 8 days. The reaction mixture was transferred to a Schlenk tube, the solvent was removed in vacuo and the remaining oily residue was purified by column chromatography (10 Â 2 cm, silica) using CH 2 Cl 2 as the eluent. Slow evaporation of the solvent at ambient temperature led to the formation of crystalline material of the title compound (yield 183 mg, 83%

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. The hydrogen atom of the hydroxy substituent (H1O2) was located in a difference-Fourier map and refined freely. All carbon-bound hydrogen atoms were placed in idealized positions and refined using a riding model with isotropic displacement parameters U iso (H) = 1.2U eq (C) for methylene and aromatic hydrogen atoms and H3 and U iso (H) = 1.5U eq (C) for methyl groups. The p-t BuC 6 H 4 and Cp groups are disordered over two positions and were found to refine well with only one free variable. The proportion of the two positions is 58.94:41.06%. SIMU, RIGU, SAME, SADI and FLAT instructions were used to restrain the geometry and displacement parameters of the disordered moieties.

Funding information
TB gratefully acknowledges a PhD grant from the Deutsche Bundesstiftung Umwelt. The publication was funded by the Open Access Fund of Universitä t Koblenz.
Acta Cryst. (2023). E79, 264-266 research communications   used to refine structure: SHELXL2019/1 (Sheldrick, 2015); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: SHELXL2018/3 (Sheldrick, 2015). 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.