(±)-2-Oxocyclopentaneacetic acid: catemeric hydrogen bonding in a γ-keto acid

The title racemate, C7H10O3, aggregates in the solid as acid-to-ketone hydrogen-bonding catemers [O⋯O = 2.7050 (13) Å and O—H⋯O = 166.1 (17)°] having glide-related components. Four such heterochiral chains, paired centrosymmetrically about (, , ) in the cell, proceed through the cell in the 010 direction, with alignment with respect to the c axis of ++−−.

HWT is grateful to Professor Gree Loober Spoog for helpful consultations. The authors acknowledge support by NSF-CRIF grant No. 0443538. EG and HWT express their gratitude to Sanofi-Aventis for a grant in support of undergraduate research in organic synthesis. This paper is dedicated to the memory of HWT; he was a dedicated mentor, teacher and friend at Rutgers University-Newark for over 40 years.
Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: FL2241).

Comment
Our study of the crystal structures of ketocarboxylic acids explores their five known H-bonding modes. Two of these do not involve the ketone, corresponding to the common pairing and much rarer chain modes in simple acids. Acid-to-ketone chains (catemers) constitute a sizable overall minority of cases, while acid-to-ketone dimers and intramolecular H-bonds are rarely observed. We have presented examples of many of these and have discussed factors that contribute to the choice of mode (Coté et al., 1996;Newman et al., 2002;Lalancette et al., 2006;DeVita Dufort et al., 2007).
An issue of interest is the minimum requirements for catemer formation. However, the very smallest molecules offer several experimental problems (volatility, low crystallinity, little structural variability), so that few C 3 -C 6 keto-acids have been previously reported (Harata et al., 1977;Malak et al., 2006;Efthimiopoulos et al., 2009). We now report the crystal structure of the title C 7 γ-keto acid (I), among the smallest found to aggregate in the solid as a catemer. The category of γ-keto acids is especially rich in H-bonding types, embracing internal H bonds and catemers of the screw, translation and glide types, as well as dimers and hydrated patterns. The intra-chain glide relationship found is considerably rarer than either screw or translational schemes generally, and is shared with three other γ-keto acids of our experience (Barcon et al., 1998(Barcon et al., , 2002DeVita Dufort et al., 2007). In solution, full rotation about both C-C bonds in the side-chain is possible; however, in the solid the staggering requirements about C1-C6 allow few real options. The observed C2-C1-C6-C7 torsion angle of 57.31 (14)° places the carboxyl group maximally away from the ring plane, and the carboxyl is rotated so that its carbonyl is essentially coplanar with the C1-C6 bond [O2-C7-C6-C1 = -7.31 (18)°]. The intramolecular dihedral angle between the carboxyl and ketone planes is 75.03 (5)°.
Averaging of C-O bond lengths and C-C-O angles by disorder, although common in carboxyl dimers, is not seen in acid-to-ketone catemers, whose geometry cannot support any of the averaging mechanisms required. In (  We characterize the geometry of H bonding to carbonyls by 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 O in terms of its deviation from, respectively, C=O axiality (ideal = 120°) and planarity with the carbonyl (ideal = 0°). In (I) the values for these two angles are 125.2 (5) & -10.2 (7)°.
No intermolecular C-H···O contacts were found within the 2.6 Å range we routinely survey for such close non-bonded polar interactions (Steiner, 1997).
Among the factors disfavoring standard dimeric carboxyl H bonding, we have identified low availability of alternative conformations. The conformational flexibility associated with cyclopentane rings is a solution characteristic; in the crystal, the requirements disfavoring hydrogen eclipsing and favoring pseudo-equatorial substituents leaves a system like (I) with few actual conformational options. As a result (I) joins a number of nominally flexible cyclic molecules we have found that behave much more like rigid systems and adopt catemeric H-bonding modes (Barcon et al., 2002;Malak et al., 2006;Lalancette & Thompson, 2003).
Because of the similar shifts produced by ketone ring-strain and by H bonding, the solid-state versus liquid IR spectra of carboxycyclopentanones are typically ambiguous regarding H bonding in the crystal. The solid-state (KBr) spectrum of (I) has C=O stretching absorptions at 1735 (acid) and 1721 cm -1 (ketone), consistent with known shifts produced when Hbonding is removed from carboxyl C=O and added to a ketone. In CHCl 3 solution these peaks appear, presumably reversed, at 1736 and 1714 cm -1 .

Experimental
The ethyl ester of 2-oxocyclopentaneacetic acid, prepared via the enamine (Stork et al., 1963), was hydrolyzed by refluxing with conc. HCl. Distilled keto acid was recrystallized from ether-hexane to give material suitable for X-ray, mp 327 K.

Refinement
All H atoms for (I) were found in electron density difference maps. The carboxyl H was refined positionally with U iso (H) = 1.5U eq (O). 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 of (I) with its numbering scheme. Displacement ellipsoids are drawn at the 40% probability level for non-H atoms. Fig. 2. A packing diagram, illustrating the four heterochiral catemers created by acid-toketone H bonds proceeding along chains of molecules glide-related in the 010 direction. The handedness of the molecules is differentiated by the shading of the bonds. Starting at the origin, the order of the directional alignment of the four chains with respect to the c axis is + + --. Displacement ellipsoids are drawn at the 40% probability level for non-H atoms.