3-Oxocyclobutanecarboxylic acid: hydrogen bonding in a small-ring γ-keto acid

The title ketocarboxylic acid, C5H6O3, is the smallest carboxycyclanone to have its crystal structure determined. It adopts a chiral conformation, by rotation of its carboxyl O atoms away from the plane of skeletal symmetry that passes through the carboxyl carbon and both atoms of the ketone carbonyl. The four-membered ring is non-planar, with a shallow fold of 14.3 (1)° along a line connecting the two α-carbons of the ketone group. In the crystal, the molecules are linked by centrosymmetric hydrogen-bond pairing of ordered carboxylic acid groups [O⋯O = 2.6392 (12) Å and O—H⋯O = 175.74 (15)°], yielding two different sets of dimers, related by by a 21 screw axis in c, in the cell. A C—H⋯O interaction is also present.

The title ketocarboxylic acid, C 5 H 6 O 3 , is the smallest carboxycyclanone to have its crystal structure determined. It adopts a chiral conformation, by rotation of its carboxyl O atoms away from the plane of skeletal symmetry that passes through the carboxyl carbon and both atoms of the ketone carbonyl. The four-membered ring is non-planar, with a shallow fold of 14.3 (1) along a line connecting the two -carbons of the ketone group. In the crystal, the molecules are linked by centrosymmetric hydrogen-bond pairing of ordered carboxylic acid groups [OÁ Á ÁO = 2.6392 (12) Å and O-HÁ Á ÁO = 175.74 (15) ], yielding two different sets of dimers, related by by a 2 1 screw axis in c, in the cell. A C-HÁ Á ÁO interaction is also present.

Comment
Our study of crystalline ketocarboxylic acids explores their five known hydrogen-bonding modes. The most frequently encountered overall is carboxyl pairing, but acid-to-ketone catemers constitute a sizable minority of cases, followed by the three remaining, rarely observed patterns, consisting of acid-to-acid catemers, internal H-bonds and carboxyl-to-ketone dimers. Of significant interest is the behavior of keto acids in the smallest size range, where aggregation influences other than H-bonding are minimized. These may offer insights into the minimum requirements for specific aggregation modes.
Among simple C 3 -C 5 monocarboxyketones, only the (dimeric) crystal structure of pyruvic acid (Harata et al., 1977) has been reported to date, while 3-oxocyclopentanecarboxylic acid (Malak et al., 2006), a catemer, is the sole published example of a C 6 keto acid. the O2-C5-C1-C4 torsion angle = 78.70 (14) Å. As is normal in cyclobutanes, the ring is not planar but flexed to ease the eclipsing strain that arises when the torsional angles for substituents on adjacent carbons approach zero. This nonplanarity in (I) may be envisioned as the result of folding the ring along a line connecting C2 to C4; the ketone and carboxyl halves of the ring lie in planes at a mutual dihedral angle of 14.3 (1)°. Because of the presence of the ketone, this dihedral is significantly smaller than those typically seen in cyclobutanes containing only sp 3 carbons, which range from 19.1° in trans-pinononic acid (Barcon et al., 1999) to 35° in cyclobutane itself (Meiboom & Snyder, 1967). When any four-sided figure departs from planarity, the internal angles are no longer constrained to an average of 90°, but may approach zero.
Although disorder-averaging of C-O bond lengths and C-C-O angles is common in dimeric carboxyls, these lengths and angles are not significantly averaged in (I), but conform to values typical of highly ordered cases (Borthwick, 1980).
Within the 2.6 Å range we standardly survey for C-H···O packing interactions (Steiner, 1997), a single close intermolecular contact was found, involving the ketone (Table 1).
supplementary materials sup-2 Experimental Compound (I) was synthesized in low yield by the method of Pigou & Schiesser (1988); when the final extract failed to crystallize spontaneously on concentration, (I) was isolated by sublimation. Recrystallization from hexane-ether provided material suitable for X-ray, mp 342 K. The solid-state (KBr) infrared spectrum of (I) features widely separated carbonyl peaks for strained ketone and carboxyl dimer, at 1786 & 1696 cm -1 , respectively. In CHCl 3 solution, where dimers predominate, the separation is nearly identical, although the peaks are somewhat shifted, at 1797 & 1709 cm -1 .

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. The asymmetric unit for (I). Displacement ellipsoids are drawn at the 40% probability level. Fig. 2. A partial packing diagram, illustrating the centrosymmetric pairing of the asymmetric units, with dimers centered at 1/2,1/2,1/2 and at 1/2,0,0 of the chosen cell. Displacement ellipsoids are drawn at the 40% probability level.