4-Oxocyclohexaneacetic acid: catemeric hydrogen bonding and spontaneous resolution of a single conformational enantiomer in an achiral ∊-keto acid

The asymmetric unit of the title compound, C8H12O3, consists of a single conformational enantiomer, which aggregates in the catemeric acid-to-ketone hydrogen-bonding mode [O⋯O = 2.682 (4) Å and O—H⋯O = 172 (6)°]. Four hydrogen-bonding chains of translationally related molecules pass through the cell orthogonal to the 43 screw axis along c, alternating in the 110 and the 10 direction, with alignment with respect to this axis of + + − −. Successive chains are rotated by 90° around the c axis. One C—H⋯O=C close contact, involving the carboxyl group, exists.

The asymmetric unit of the title compound, C 8 H 12 O 3 , consists of a single conformational enantiomer, which aggregates in the catemeric acid-to-ketone hydrogen-bonding mode [OÁ Á ÁO = 2.682 (4) Å and O-HÁ Á ÁO = 172 (6) ]. Four hydrogenbonding chains of translationally related molecules pass through the cell orthogonal to the 4 3 screw axis along c, alternating in the 110 and the 110 direction, with alignment with respect to this axis of + + À À. Successive chains are rotated by 90 around the c axis. One C-HÁ Á ÁO C close contact, involving the carboxyl group, exists.

Comment
The similar carbonyl basicities of carboxylic acids and ketones allow the two to compete as hydrogen-bond acceptors. Hence, simple keto acids display three known solid-state H-bonding modes beyond those seen in functionally unelaborated acids.
The averaging of C-O bond lengths and C-C-O angles by disorder, common in carboxyl dimers, is not observed in catemers whose geometry cannot support the underlying averaging mechanisms involved. Here, these lengths and angles are typical of those for highly ordered dimeric carboxyls and catemers (Borthwick, 1980). We characterize the geometry of H bonding to carbonyls using a combination of H···O=C angle and H···O=C-C torsion angle. These describe the approach of the acid H atom to the receptor O in terms of its deviation from, respectively, C=O axiality (ideal = 120°) and coplanarity with the carbonyl (ideal = 0°). Here, these two angles are 128.6 (16) and -1.2 (19)°.
Within the 2.6 Å range we survey for non-bonded C-H···O packing interactions (Steiner, 1997), only one close contact was found (see Table 2).
Although crystallization in space group P4 3 is itself quite unusual (ca 0.12% of compounds in the Cambridge database), an equally unusual aspect of the packing is the presence of only a single chiral conformer. Preferential crystallization of one chiral conformer from solutions of an inherently achiral molecule is very rare but far from unknown (Jacques et al., 1981;Desiraju, 1989). Among keto acids, five cases are known of this phenomenon: [Cambridge Structural Database (CSD, Version 5.28, update of Nov., 2006;Allen, 2002) refcodes CUHCUD (Kawai et al., 1985), JISVAI (Abell et al., 1991), KICRIX (Halfpenny, 1990) & ZEMJIK (McGuire et al., 1995)], plus the case of mesitylglyoxylic acid (Chen et al., 2000).
The particular antipode crystallizing from such a solution may depend merely on which one chances to crystallize first, and it has been shown in a similar case that stirring seeds the solution and may largely or entirely prevent the enantiomeric supplementary materials sup-2 species from crystallizing (Kondepudi et al., 1990). In the present instance, the Flack parameter allows us to assign a specific hand to (I), so the antipode actually illustrated is the correct one. Also, the octant rule predicts that the conformer should have (-) rotation.

Experimental
The solid-state (KBr) infrared spectrum of (I) has C=O stretching absorptions at 1726 and 1685 cm -1 , consistent with known shifts produced when H-bonding is removed from carboxyl C=O and added to a ketone, respectively. In CHCl 3 solution, these bands coalesce to a single absorption at 1707 cm -1 , with a typical carboxyl-dilution shoulder around 1755 cm -1 .
The title compound was prepared by Jones oxidation of the product obtained by catalytic hydrogenation of p-hydroxyphenylacetic acid over a Rh/C catalyst. The crystal used was obtained from Et 2 O/cyclohexane (60:40 v/v) by evaporation, mp 345 K.

Refinement
All H atoms for (I) were found in electron density difference maps. The hydroxyl H was fully refined. The methylene and methine Hs were placed in geometrically idealized positions and constrained to ride on their parent C atoms with C-H distances of 0.99 and 1.00 Å, respectively, and U iso (H) = 1.2U eq (C). Fig. 1. A view of the asymmetric unit with its numbering. Displacement ellipsoids are set at the 40% probability level. Fig. 2. A packing diagram, with extracellular molecules, showing the four catemers created by acid-to-ketone H bonds proceeding along chains of translationally related molecules, advancing alternately along the 110 and the -110 directions. These chains have alignment with respect to the c axis of + + --. All non-carboxyl H atoms have been removed for clarity. Displacement ellipsoids are set at the 30% probability level.