(±)-3-(3-Oxo­cyclo­hexyl)­propionic acid: dual conformational selection and hydrogen bonding in an [epsilon]-keto acid

The asymmetric unit of the title compound, C₉H₁₄O₃, consists of two mol­ecules having conformations that differ by a rotation of 111.7 (5)° about the equatorial substituent bond, so that the side chains of the two species extend away from the ring in different directions. Each conformer forms centrosymmetric hydrogen-bonded acid-to-acid dimers with its own enantiomer [O...O = 2.681 (3) and 2.698 (4) Å]. There is an intermolecular C—H...O close contact involving the ketone group of one of the conformers.

S1. Comment

The crystal structures of keto carboxylic acids display five known hydrogen-bonding modes. Two of these lack ketone involvement, reflecting the common pairing and much rarer chain modes known for simple acids (Leiserowitz, 1976). Acid-to-ketone chains (catemers) constitute a sizable minority of cases, while intramolecular hydrogen bonds and acid-to-ketone dimers are rarely observed. We have previously provided and cited examples of many of these, along with discussions of the factors that appear to govern the choice of mode (Lalancette et al., 1998, 1999; Brunskill et al., 1999).

We report here the crystal structure of the title compound, (I). The category of ε-keto acids to which (I) belongs includes catemers of the helical, translational and glide types, as well as dimers and hydrated patterns. Fig. 1 presents a view of the asymmetric unit of (I) with its numbering. The two molecules in this unit, (Ic) and (It), differ in the conformations of their side chains, as illustrated in the Scheme. Although the chain extends away from the ring equatorially in a standard staggered-anti fashion in both molecules, C7 has two options for the onward direction the chain can take, differing by a rotation of 111.7 (5)° about C1—C7. For both these conformations, the staggered-anti arrangement of the chain extends into the ring, but in one case [(Ic), `cisoid' carbonyls], this extension includes C2 and C3 (the ketone carbon), while in the other [(It), `transoid' carbonyls], it includes C6 and C5. Except for a 35.5 (6)° difference in rotational conformations for the carboxyl groups relative to their chains [torsion angle O2—C9—C8—C7 = 28.7 (5)° for (Ic) and -6.8 (4)° for (1 t)], the distinction between (Ic) and (It) would disappear in the absence of the ketone function. The ketone O atom is sufficiently remote from the side chain that, without packing forces, enthalpy differences between (Ic) and (It) are negligible; remarkably, of these isoenthalpic conformers, the crystal packing arrangement chooses both.

Although averaging of the C—O bond lengths and C—C—O angles by disorder is common in dimeric carboxyls (Leiserowitz, 1976), neither (Ic) nor (It) displays significant carboxyl disorder. In (Ic), the C—O bond lengths are 1.234 (4)/1.297 (4) Å, with angles of 122.6 (3)/113.8 (3)°; the corresponding values for (It) are 1.228 (3)/1.324 (3) Å, with angles of 124.2 (3)/112.9 (2)°. Typical values cited for highly ordered dimeric carboxyls are 1.21/1.31 Å and 123/112° (Borthwick, 1980).

Fig. 2 illustrates the packing of the cell. Each of the two conformers pairs centrosymmetrically with its own enantiomer. Dimers of the cisoid type, (Ic), are centered on the b edges of the chosen cell, while dimers of the (It) type are centered on the bc faces. The O···O distance and O—H···O angle are 2.698 (4) Å and 167 (4)° for (Ic), and 2.681 (3) Å and 177 (4)° for (It).

In the packing, a 2.64 Å intermolecular C—H···O close contact was found for the ketone function of the transoid species (It) to H8A of a neighboring molecule of type (Ic). This lies within the 2.7 Å range we usually employ for non-bonded C—H···O packing interactions (Steiner, 1997). Using compiled data for a large number of C—H···O contacts, Steiner & Desiraju (1998) have found significant statistical directionality even as far out as 3.0 Å, and conclude that these are legitimately viewed as `weak hydrogen bonds', with a greater contribution to packing forces than simple van der Waals attractions.

The solid-state (KBr) IR spectrum of (I) has a broadened stretching absorption at 1709 cm^-1 for both C═O functions in both species, typical for acids having and ketones lacking hydrogen bonding; a shoulder appears at ca 1730 cm^-1. In CHCl₃ solution, this broad peak appears at 1708 cm^-1, with a typical carboxyl-dilution shoulder at ca 1740 cm^-1.

Related literature

For related literature, see: Borthwick (1980); Brunskill et al. (1999); Lalancette et al. (1998, 1999); Leiserowitz (1976); Steiner (1997); Steiner & Desiraju (1998).

S2. Experimental

Compound (I) was prepared by Rh-catalyzed hydrogenation of 3-hydroxycinnamic acid, followed by Jones oxidation. Crystals suitable for X-ray were produced from a cyclohexane–Et₂O solution (m.p. 315 K).

S3. Refinement

All H atoms were found in electron-density difference maps but were placed in calculated positions (0.97 Å for the methylene and 0.98 Å for the methine H atoms) and allowed to refine as riding models on their respective C atoms. Their displacement parameters were fixed at 120% of those of their respective C atoms. The positional parameters of the carboxyl H atoms were allowed to refine, but their displacement parameters were fixed at 150% of those of their respective O atoms.

Computing details

Data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997a); molecular graphics: XPin SHELXTL (Sheldrick, 1997b); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the asymmetric unit of (I) with the atom-numbering scheme; the two molecules differ in the direction of extension of the side chain away from the ring, producing cisoid, (Ic), and transoid, (It), arrangements of the two carbonyl groups, as illustrated in the Scheme. Displacement ellipsoids are drawn at the 20% probability level.
	Fig. 2. A partial packing diagram, showing the dimers formed by centrosymmetric hydrogen-bonded pairing of (Ic) and of (It). Dimers centered on the b edge are of the cisoid type, (Ic), and those on the bc face are of the transoid type, (It). All carbon-bound H atoms has been removed for clarity. Displacement ellipsoids are drawn at the 20% probability level.

(±)-3-(3-Oxocyclohexyl)propionic acid

Crystal data

C9H14O3	F(000) = 368
Mr = 170.20	Dx = 1.213 Mg m^−3
Triclinic, P1	Melting point: 315 K
a = 5.429 (3) Å	Mo Kα radiation, λ = 0.71073 Å
b = 12.005 (7) Å	Cell parameters from 31 reflections
c = 14.570 (8) Å	θ = 4.0–10.1°
α = 87.05 (2)°	µ = 0.09 mm^−1
β = 88.96 (2)°	T = 260 K
γ = 79.39 (2)°	Parallelepiped, colourless
V = 932.1 (9) Å^3	0.46 × 0.16 × 0.07 mm
Z = 4

Data collection

Siemens P4 diffractometer	2085 reflections with I > 2σ(I)
Radiation source: normal-focus sealed tube	Rint = 0.000
Graphite monochromator	θmax = 25.0°, θmin = 2.3°
2θ/θ scans	h = −6→6
Absorption correction: numerical (Sheldrick, 1997)	k = −14→14
Tmin = 0.97, Tmax = 0.99	l = 0→17
3303 measured reflections	3 standard reflections every 97 reflections
3303 independent reflections	intensity decay: variation <1.2%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.063	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.175	H atoms treated by a mixture of independent and constrained refinement
S = 1.04	w = 1/[σ^2(Fo^2) + (0.0751P)^2 + 0.3643P] where P = (Fo^2 + 2Fc^2)/3
3303 reflections	(Δ/σ)max < 0.001
223 parameters	Δρmax = 0.42 e Å^−3
0 restraints	Δρmin = −0.18 e Å^−3

Special details

Experimental. Crystal mounted on glass fiber with epoxy resin.

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
O1	0.3531 (5)	0.1644 (2)	0.46678 (16)	0.0671 (7)
O2	0.0112 (5)	0.4559 (2)	0.89251 (15)	0.0656 (7)
O3	0.3059 (5)	0.4071 (2)	0.99924 (16)	0.0616 (7)
C1	0.4486 (5)	0.2449 (2)	0.69517 (19)	0.0401 (7)
C2	0.3881 (6)	0.2785 (3)	0.59365 (19)	0.0443 (7)
C3	0.4816 (6)	0.1858 (3)	0.5291 (2)	0.0425 (7)
C4	0.7478 (6)	0.1255 (3)	0.5426 (2)	0.0506 (8)
C5	0.8117 (6)	0.0956 (3)	0.6444 (2)	0.0561 (9)
C6	0.7285 (6)	0.1968 (3)	0.7045 (2)	0.0523 (8)
C7	0.3519 (6)	0.3463 (3)	0.7546 (2)	0.0504 (8)
C8	0.3822 (7)	0.3192 (3)	0.8576 (2)	0.0582 (9)
C9	0.2166 (7)	0.4022 (3)	0.9179 (2)	0.0481 (8)
O1'	−0.0001 (5)	0.8195 (2)	1.01843 (17)	0.0805 (8)
O2'	0.1602 (4)	0.58615 (17)	0.54830 (13)	0.0456 (6)
O3'	−0.1838 (4)	0.52836 (19)	0.60580 (14)	0.0497 (6)
C1'	0.1728 (6)	0.7993 (2)	0.78151 (18)	0.0390 (7)
C2'	0.1517 (6)	0.7407 (3)	0.87663 (19)	0.0465 (8)
C3'	0.1433 (6)	0.8200 (3)	0.9545 (2)	0.0469 (8)
C4'	0.3325 (6)	0.8971 (3)	0.9504 (2)	0.0508 (8)
C5'	0.3486 (7)	0.9555 (3)	0.8554 (2)	0.0562 (9)
C6'	0.3813 (6)	0.8693 (3)	0.7807 (2)	0.0489 (8)
C7'	0.2047 (6)	0.7162 (3)	0.7034 (2)	0.0471 (8)
C8'	−0.0158 (5)	0.6570 (2)	0.69202 (18)	0.0398 (7)
C9'	−0.0015 (5)	0.5877 (2)	0.60849 (19)	0.0356 (7)
H3	0.205 (8)	0.466 (3)	1.036 (3)	0.092*
H1	0.3572	0.1842	0.7140	0.048*
H2A	0.2079	0.3004	0.5873	0.053*
H2B	0.4618	0.3442	0.5757	0.053*
H4A	0.8602	0.1733	0.5169	0.061*
H4B	0.7749	0.0563	0.5093	0.061*
H5A	0.7302	0.0336	0.6661	0.067*
H5B	0.9912	0.0701	0.6502	0.067*
H6A	0.7651	0.1732	0.7682	0.063*
H6B	0.8233	0.2557	0.6871	0.063*
H7A	0.4410	0.4073	0.7371	0.060*
H7B	0.1758	0.3734	0.7418	0.060*
H8A	0.3459	0.2440	0.8714	0.070*
H8B	0.5558	0.3174	0.8734	0.070*
H3'	−0.173 (7)	0.491 (3)	0.552 (3)	0.075*
H1'	0.0150	0.8526	0.7707	0.047*
H2'A	0.0010	0.7080	0.8790	0.056*
H2'B	0.2938	0.6792	0.8852	0.056*
H4'A	0.4958	0.8533	0.9665	0.061*
H4'B	0.2875	0.9544	0.9954	0.061*
H5'A	0.1970	1.0111	0.8441	0.067*
H5'B	0.4892	0.9950	0.8533	0.067*
H6'A	0.5410	0.8184	0.7890	0.059*
H6'B	0.3852	0.9090	0.7212	0.059*
H7'A	0.2314	0.7574	0.6462	0.056*
H7'B	0.3538	0.6591	0.7149	0.056*
H8'A	−0.0281	0.6076	0.7461	0.048*
H8'B	−0.1681	0.7138	0.6890	0.048*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O1	0.0621 (16)	0.0818 (17)	0.0584 (15)	−0.0127 (14)	−0.0055 (13)	−0.0124 (13)
O2	0.0818 (19)	0.0675 (15)	0.0400 (13)	0.0074 (14)	−0.0035 (13)	−0.0068 (11)
O3	0.0621 (16)	0.0698 (16)	0.0518 (14)	−0.0060 (13)	0.0020 (12)	−0.0155 (12)
C1	0.0385 (17)	0.0409 (16)	0.0409 (16)	−0.0084 (14)	0.0059 (13)	−0.0009 (13)
C2	0.0367 (17)	0.0475 (17)	0.0462 (17)	−0.0024 (14)	0.0031 (14)	−0.0003 (14)
C3	0.0425 (18)	0.0483 (18)	0.0375 (16)	−0.0116 (15)	0.0034 (14)	0.0010 (14)
C4	0.0459 (19)	0.0544 (19)	0.0482 (18)	0.0004 (16)	0.0074 (15)	−0.0102 (15)
C5	0.047 (2)	0.062 (2)	0.055 (2)	0.0033 (17)	0.0037 (16)	0.0002 (16)
C6	0.0461 (19)	0.065 (2)	0.0435 (18)	−0.0049 (16)	−0.0025 (15)	−0.0018 (16)
C7	0.053 (2)	0.0495 (18)	0.0472 (18)	−0.0070 (16)	0.0067 (16)	−0.0038 (15)
C8	0.061 (2)	0.062 (2)	0.0473 (19)	−0.0011 (18)	0.0032 (17)	−0.0014 (16)
C9	0.058 (2)	0.0447 (18)	0.0400 (18)	−0.0090 (17)	0.0054 (16)	0.0059 (14)
O1'	0.096 (2)	0.096 (2)	0.0601 (16)	−0.0412 (17)	0.0226 (16)	−0.0191 (14)
O2'	0.0462 (13)	0.0553 (13)	0.0403 (11)	−0.0188 (10)	0.0058 (10)	−0.0187 (10)
O3'	0.0529 (14)	0.0635 (14)	0.0407 (12)	−0.0277 (12)	0.0053 (10)	−0.0189 (10)
C1'	0.0425 (17)	0.0415 (16)	0.0342 (15)	−0.0091 (14)	0.0024 (13)	−0.0085 (12)
C2'	0.056 (2)	0.0513 (18)	0.0386 (16)	−0.0257 (16)	0.0008 (14)	−0.0074 (14)
C3'	0.055 (2)	0.0553 (19)	0.0329 (15)	−0.0163 (16)	0.0039 (15)	−0.0060 (14)
C4'	0.056 (2)	0.056 (2)	0.0449 (18)	−0.0196 (17)	−0.0037 (16)	−0.0178 (15)
C5'	0.062 (2)	0.062 (2)	0.052 (2)	−0.0288 (18)	−0.0005 (17)	−0.0138 (16)
C6'	0.054 (2)	0.0565 (19)	0.0432 (17)	−0.0264 (16)	0.0082 (15)	−0.0116 (15)
C7'	0.0501 (19)	0.0562 (18)	0.0407 (17)	−0.0210 (16)	0.0064 (15)	−0.0199 (14)
C8'	0.0387 (17)	0.0470 (16)	0.0350 (15)	−0.0096 (14)	0.0003 (13)	−0.0101 (13)
C9'	0.0339 (16)	0.0372 (15)	0.0360 (15)	−0.0058 (13)	−0.0049 (13)	−0.0057 (12)

Geometric parameters (Å, º)

O1—C3	1.221 (4)	C2—H2A	0.9700
O2—C9	1.234 (4)	C2—H2B	0.9700
O3—C9	1.297 (4)	C4—H4A	0.9700
C1—C6	1.529 (4)	C4—H4B	0.9700
C1—C7	1.538 (4)	C5—H5A	0.9700
C1—C2	1.538 (4)	C5—H5B	0.9700
C2—C3	1.503 (4)	C6—H6A	0.9700
C3—C4	1.504 (4)	C6—H6B	0.9700
C4—C5	1.539 (4)	C7—H7A	0.9700
C5—C6	1.530 (4)	C7—H7B	0.9700
C7—C8	1.524 (4)	C8—H8A	0.9700
C8—C9	1.518 (4)	C8—H8B	0.9700
O1'—C3'	1.203 (4)	O3'—H3'	0.91 (4)
O2'—C9'	1.228 (3)	C1'—H1'	0.9800
O3'—C9'	1.324 (3)	C2'—H2'A	0.9700
C1'—C6'	1.528 (4)	C2'—H2'B	0.9700
C1'—C2'	1.534 (4)	C4'—H4'A	0.9700
C1'—C7'	1.538 (4)	C4'—H4'B	0.9700
C2'—C3'	1.513 (4)	C5'—H5'A	0.9700
C3'—C4'	1.503 (4)	C5'—H5'B	0.9700
C4'—C5'	1.527 (4)	C6'—H6'A	0.9700
C5'—C6'	1.525 (4)	C6'—H6'B	0.9700
C7'—C8'	1.517 (4)	C7'—H7'A	0.9700
C8'—C9'	1.501 (4)	C7'—H7'B	0.9700
O3—H3	0.98 (4)	C8'—H8'A	0.9700
C1—H1	0.9800	C8'—H8'B	0.9700
C6—C1—C7	114.7 (3)	C1—C6—H6A	109.2
C6—C1—C2	109.4 (2)	C5—C6—H6A	109.2
C7—C1—C2	109.7 (2)	C1—C6—H6B	109.2
C3—C2—C1	113.9 (2)	C5—C6—H6B	109.2
O1—C3—C2	122.0 (3)	H6A—C6—H6B	107.9
O1—C3—C4	121.9 (3)	C8—C7—H7A	108.7
C2—C3—C4	116.0 (3)	C1—C7—H7A	108.7
C3—C4—C5	112.7 (3)	C8—C7—H7B	108.7
C6—C5—C4	112.0 (3)	C1—C7—H7B	108.7
C1—C6—C5	112.0 (3)	H7A—C7—H7B	107.6
C8—C7—C1	114.0 (3)	C9—C8—H8A	108.6
C9—C8—C7	114.8 (3)	C7—C8—H8A	108.6
O2—C9—O3	123.5 (3)	C9—C8—H8B	108.6
O2—C9—C8	122.6 (3)	C7—C8—H8B	108.6
O3—C9—C8	113.8 (3)	H8A—C8—H8B	107.5
C6'—C1'—C2'	110.3 (2)	C9'—O3'—H3'	110 (2)
C6'—C1'—C7'	112.0 (2)	C6'—C1'—H1'	107.1
C2'—C1'—C7'	113.0 (2)	C2'—C1'—H1'	107.1
C3'—C2'—C1'	113.3 (2)	C7'—C1'—H1'	107.1
O1'—C3'—C4'	121.1 (3)	C3'—C2'—H2'A	108.9
O1'—C3'—C2'	122.5 (3)	C1'—C2'—H2'A	108.9
C4'—C3'—C2'	116.4 (3)	C3'—C2'—H2'B	108.9
C3'—C4'—C5'	112.4 (3)	C1'—C2'—H2'B	108.9
C6'—C5'—C4'	110.9 (3)	H2'A—C2'—H2'B	107.7
C5'—C6'—C1'	113.1 (3)	C3'—C4'—H4'A	109.1
C8'—C7'—C1'	114.4 (2)	C5'—C4'—H4'A	109.1
C9'—C8'—C7'	114.7 (2)	C3'—C4'—H4'B	109.1
O2'—C9'—O3'	123.0 (2)	C5'—C4'—H4'B	109.1
O2'—C9'—C8'	124.2 (3)	H4'A—C4'—H4'B	107.9
O3'—C9'—C8'	112.9 (2)	C6'—C5'—H5'A	109.5
C9—O3—H3	113 (2)	C4'—C5'—H5'A	109.5
C6—C1—H1	107.6	C6'—C5'—H5'B	109.5
C7—C1—H1	107.6	C4'—C5'—H5'B	109.5
C2—C1—H1	107.6	H5'A—C5'—H5'B	108.0
C3—C2—H2A	108.8	C5'—C6'—H6'A	109.0
C1—C2—H2A	108.8	C1'—C6'—H6'A	109.0
C3—C2—H2B	108.8	C5'—C6'—H6'B	109.0
C1—C2—H2B	108.8	C1'—C6'—H6'B	109.0
H2A—C2—H2B	107.7	H6'A—C6'—H6'B	107.8
C3—C4—H4A	109.1	C8'—C7'—H7'A	108.7
C5—C4—H4A	109.1	C1'—C7'—H7'A	108.7
C3—C4—H4B	109.1	C8'—C7'—H7'B	108.7
C5—C4—H4B	109.1	C1'—C7'—H7'B	108.7
H4A—C4—H4B	107.8	H7'A—C7'—H7'B	107.6
C6—C5—H5A	109.2	C9'—C8'—H8'A	108.6
C4—C5—H5A	109.2	C7'—C8'—H8'A	108.6
C6—C5—H5B	109.2	C9'—C8'—H8'B	108.6
C4—C5—H5B	109.2	C7'—C8'—H8'B	108.6
H5A—C5—H5B	107.9	H8'A—C8'—H8'B	107.6
C6—C1—C2—C3	51.8 (3)	C6'—C1'—C2'—C3'	48.7 (4)
C7—C1—C2—C3	178.4 (3)	C7'—C1'—C2'—C3'	174.9 (3)
C1—C2—C3—O1	136.4 (3)	C1'—C2'—C3'—O1'	136.5 (3)
C1—C2—C3—C4	−46.9 (4)	C1'—C2'—C3'—C4'	−46.0 (4)
O1—C3—C4—C5	−139.1 (3)	O1'—C3'—C4'—C5'	−135.9 (3)
C2—C3—C4—C5	44.2 (4)	C2'—C3'—C4'—C5'	46.6 (4)
C3—C4—C5—C6	−48.3 (4)	C3'—C4'—C5'—C6'	−50.6 (4)
C7—C1—C6—C5	179.4 (3)	C4'—C5'—C6'—C1'	56.8 (4)
C2—C1—C6—C5	−56.9 (3)	C2'—C1'—C6'—C5'	−55.4 (4)
C4—C5—C6—C1	56.2 (4)	C7'—C1'—C6'—C5'	177.9 (3)
C6—C1—C7—C8	−61.5 (4)	C6'—C1'—C7'—C8'	−171.5 (3)
C2—C1—C7—C8	175.0 (3)	C2'—C1'—C7'—C8'	63.3 (4)
C1—C7—C8—C9	−160.7 (3)	C1'—C7'—C8'—C9'	172.3 (2)
C7—C8—C9—O2	28.7 (5)	C7'—C8'—C9'—O2'	−6.8 (4)
C7—C8—C9—O3	−154.2 (3)	C7'—C8'—C9'—O3'	173.6 (3)