Molecules of the title β-keto acid, 7-oxobicyclo[2.2.1]heptane-1-carboxylic acid, C
8H
10O
3, exhibit chirality due to the bridgehead carboxyl group, which is partially ordered and has a slightly asymmetric conformation. The molecules form centrosymmetric hydrogen-bonded carboxyl dimers [O
O 2.639 (2) Å]. The title alkenoic γ-keto acid, (
)-7-oxobicyclo[2.2.1]hept-5-ene-2-
endo-carboxylic acid, C
8H
8O
3, also forms typical centrosymmetric hydrogen-bonded carboxyl dimers [O
O 2.660 (3) Å]. There is partial disorder of the carboxyl group in each compound.
Supporting information
CCDC references: 143264; 143265
Compound (I) was synthesized by the method of Hatchard & Schneider (1957) and
purified by sublimation. Recrystallization from either benzene or
EtOAc/cyclohexane gave crystals suitable for X-ray analysis (m.p. 433 K).
Compound (II) was prepared by hydrolysis of the corresponding dimethyl ketal,
synthesized as described by Thompson et al. (1985). The crystals used
in this work (m.p. 376 K) were obtained from Et2O-hexane.
For both (I) and (II), all non-carboxyl H atoms were found in electron density
difference maps but replaced in calculated positions and allowed to refine as
riding models with isotropic displacement parameters set at 120% of their
respective C atoms. For both (I) and (II), the disordered pair of carboxyl H
atoms was also found in electron density difference maps and allowed to refine
in idealized positions, with isotropic displacement parameters set at 150% of
their respective O atoms; their occupancies were allowed to refine [values
0.68 (4)/0.32 (4) for (I), 0.65 (5)/0.35 (5) for (II)].
For both compounds, data collection: XSCANS (Siemens, 1996); cell refinement: XSCANS; data reduction: XSCANS; program(s) used to solve structure: SHELXTL (Sheldrick, 1997); program(s) used to refine structure: SHELXTL; molecular graphics: SHELXTL; software used to prepare material for publication: SHELXTL.
(I) 7-Oxobicyclo[2.2.1]heptane-1-carboxylic acid
top
Crystal data top
C8H10O3 | Dx = 1.326 Mg m−3 |
Mr = 154.16 | Melting point: 433 K |
Monoclinic, P21/c | Mo Kα radiation, λ = 0.71073 Å |
a = 6.3714 (9) Å | Cell parameters from 22 reflections |
b = 10.560 (2) Å | θ = 2.6–17.5° |
c = 11.610 (3) Å | µ = 0.10 mm−1 |
β = 98.616 (14)° | T = 293 K |
V = 772.3 (3) Å3 | Block, colorless |
Z = 4 | 0.64 × 0.48 × 0.28 mm |
F(000) = 328 | |
Data collection top
Siemens P4 diffractometer | 960 reflections with I > 2σ(I) |
Radiation source: normal-focus sealed tube | Rint = 0.020 |
Graphite monochromator | θmax = 25°, θmin = 2.6° |
2θ/θ scans | h = −7→1 |
Absorption correction: numerical (SHELXTL; Sheldrick, 1997) | k = −1→12 |
Tmin = 0.96, Tmax = 0.97 | l = −13→13 |
1957 measured reflections | 3 standard reflections every 97 reflections |
1362 independent reflections | intensity decay: variation < 1.2% |
Refinement top
Refinement on F2 | Primary atom site location: structure-invariant direct methods |
Least-squares matrix: full | Secondary atom site location: difference Fourier map |
R[F2 > 2σ(F2)] = 0.047 | Hydrogen site location: inferred from neighbouring sites |
wR(F2) = 0.117 | H-atom parameters constrained |
S = 1.05 | w = 1/[σ2(Fo2) + (0.0373P)2 + 0.2509P] where P = (Fo2 + 2Fc2)/3 |
1362 reflections | (Δ/σ)max < 0.001 |
103 parameters | Δρmax = 0.23 e Å−3 |
0 restraints | Δρmin = −0.14 e Å−3 |
Crystal data top
C8H10O3 | V = 772.3 (3) Å3 |
Mr = 154.16 | Z = 4 |
Monoclinic, P21/c | Mo Kα radiation |
a = 6.3714 (9) Å | µ = 0.10 mm−1 |
b = 10.560 (2) Å | T = 293 K |
c = 11.610 (3) Å | 0.64 × 0.48 × 0.28 mm |
β = 98.616 (14)° | |
Data collection top
Siemens P4 diffractometer | 960 reflections with I > 2σ(I) |
Absorption correction: numerical (SHELXTL; Sheldrick, 1997) | Rint = 0.020 |
Tmin = 0.96, Tmax = 0.97 | 3 standard reflections every 97 reflections |
1957 measured reflections | intensity decay: variation < 1.2% |
1362 independent reflections | |
Refinement top
R[F2 > 2σ(F2)] = 0.047 | 0 restraints |
wR(F2) = 0.117 | H-atom parameters constrained |
S = 1.05 | Δρmax = 0.23 e Å−3 |
1362 reflections | Δρmin = −0.14 e Å−3 |
103 parameters | |
Special details top
Experimental. Crystal mounted on glass fiber using 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 (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
O1 | 0.2895 (3) | 0.69608 (19) | 0.19048 (14) | 0.0877 (6) | |
O2 | 0.2533 (2) | 0.55160 (16) | 0.45722 (16) | 0.0678 (5) | |
O3 | 0.5521 (2) | 0.65951 (17) | 0.46628 (18) | 0.0788 (6) | |
C1 | 0.2352 (3) | 0.76269 (19) | 0.38760 (16) | 0.0455 (5) | |
C2 | −0.0021 (3) | 0.7712 (2) | 0.3960 (2) | 0.0618 (6) | |
C3 | −0.0955 (4) | 0.8551 (2) | 0.2912 (2) | 0.0737 (7) | |
C4 | 0.0973 (4) | 0.8898 (2) | 0.2339 (2) | 0.0671 (7) | |
C5 | 0.2403 (4) | 0.9781 (2) | 0.3142 (2) | 0.0759 (7) | |
C6 | 0.3359 (4) | 0.8921 (2) | 0.4172 (2) | 0.0626 (6) | |
C7 | 0.2210 (3) | 0.7683 (2) | 0.25455 (19) | 0.0560 (6) | |
C8 | 0.3496 (3) | 0.64972 (19) | 0.44093 (17) | 0.0471 (5) | |
H2 | 0.3369 | 0.4994 | 0.4900 | 0.102* | 0.32 (4) |
H3 | 0.6024 | 0.5908 | 0.4889 | 0.118* | 0.68 (4) |
H2A | −0.0245 | 0.8098 | 0.4690 | 0.074* | |
H2B | −0.0668 | 0.6879 | 0.3904 | 0.074* | |
H3A | −0.1993 | 0.8086 | 0.2377 | 0.088* | |
H3B | −0.1622 | 0.9303 | 0.3172 | 0.088* | |
H4A | 0.0658 | 0.9169 | 0.1524 | 0.081* | |
H5A | 0.1591 | 1.0469 | 0.3411 | 0.091* | |
H5B | 0.3510 | 1.0132 | 0.2749 | 0.091* | |
H6A | 0.4892 | 0.8876 | 0.4229 | 0.075* | |
H6B | 0.2999 | 0.9235 | 0.4902 | 0.075* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.1086 (15) | 0.0994 (14) | 0.0569 (10) | 0.0306 (12) | 0.0186 (10) | −0.0119 (10) |
O2 | 0.0523 (9) | 0.0532 (10) | 0.0942 (13) | −0.0099 (8) | −0.0014 (9) | 0.0134 (9) |
O3 | 0.0441 (9) | 0.0685 (12) | 0.1207 (16) | −0.0055 (8) | 0.0020 (9) | 0.0260 (11) |
C1 | 0.0455 (11) | 0.0464 (12) | 0.0450 (11) | −0.0031 (9) | 0.0081 (9) | −0.0004 (9) |
C2 | 0.0530 (13) | 0.0631 (14) | 0.0723 (15) | 0.0020 (11) | 0.0188 (11) | 0.0025 (12) |
C3 | 0.0591 (15) | 0.0732 (17) | 0.0881 (18) | 0.0137 (13) | 0.0092 (13) | 0.0065 (14) |
C4 | 0.0736 (16) | 0.0728 (16) | 0.0549 (14) | 0.0152 (14) | 0.0098 (12) | 0.0149 (12) |
C5 | 0.0839 (18) | 0.0555 (15) | 0.0908 (18) | 0.0003 (14) | 0.0211 (15) | 0.0158 (14) |
C6 | 0.0720 (15) | 0.0507 (13) | 0.0643 (14) | −0.0102 (12) | 0.0077 (12) | −0.0031 (11) |
C7 | 0.0564 (13) | 0.0639 (14) | 0.0483 (13) | 0.0029 (12) | 0.0096 (10) | −0.0012 (11) |
C8 | 0.0446 (11) | 0.0495 (12) | 0.0465 (11) | −0.0080 (10) | 0.0044 (9) | 0.0001 (10) |
Geometric parameters (Å, º) top
O1—C7 | 1.193 (3) | C1—C7 | 1.535 (3) |
O2—C8 | 1.233 (2) | C2—C3 | 1.550 (3) |
O3—C8 | 1.284 (2) | C3—C4 | 1.526 (3) |
C1—C8 | 1.484 (3) | C4—C7 | 1.505 (3) |
C1—C6 | 1.527 (3) | C4—C5 | 1.522 (3) |
C1—C2 | 1.532 (3) | C5—C6 | 1.551 (3) |
| | | |
C8—C1—C6 | 117.51 (17) | C5—C4—C3 | 109.1 (2) |
C8—C1—C2 | 116.63 (18) | C4—C5—C6 | 104.10 (19) |
C6—C1—C2 | 108.58 (19) | C1—C6—C5 | 104.32 (18) |
C8—C1—C7 | 113.72 (17) | O1—C7—C4 | 132.6 (2) |
C6—C1—C7 | 98.69 (16) | O1—C7—C1 | 129.1 (2) |
C2—C1—C7 | 98.75 (17) | C4—C7—C1 | 98.28 (17) |
C1—C2—C3 | 104.45 (18) | O2—C8—O3 | 122.6 (2) |
C4—C3—C2 | 103.90 (18) | O2—C8—C1 | 121.0 (2) |
C7—C4—C5 | 99.98 (19) | O3—C8—C1 | 116.4 (2) |
C7—C4—C3 | 99.34 (19) | | |
| | | |
C8—C1—C2—C3 | −154.57 (19) | C5—C4—C7—C1 | 55.1 (2) |
C6—C1—C2—C3 | 69.9 (2) | C3—C4—C7—C1 | −56.3 (2) |
C7—C1—C2—C3 | −32.4 (2) | C8—C1—C7—O1 | −0.6 (3) |
C1—C2—C3—C4 | −1.3 (3) | C6—C1—C7—O1 | 124.7 (3) |
C2—C3—C4—C7 | 35.4 (2) | C2—C1—C7—O1 | −124.9 (3) |
C2—C3—C4—C5 | −68.7 (2) | C8—C1—C7—C4 | 179.36 (18) |
C7—C4—C5—C6 | −33.2 (2) | C6—C1—C7—C4 | −55.4 (2) |
C3—C4—C5—C6 | 70.4 (2) | C2—C1—C7—C4 | 55.1 (2) |
C8—C1—C6—C5 | 156.72 (19) | C6—C1—C8—O2 | 153.7 (2) |
C2—C1—C6—C5 | −68.2 (2) | C2—C1—C8—O2 | 22.2 (3) |
C7—C1—C6—C5 | 34.1 (2) | C7—C1—C8—O2 | −91.8 (2) |
C4—C5—C6—C1 | −1.1 (2) | C6—C1—C8—O3 | −27.7 (3) |
C5—C4—C7—O1 | −125.0 (3) | C2—C1—C8—O3 | −159.2 (2) |
C3—C4—C7—O1 | 123.6 (3) | C7—C1—C8—O3 | 86.8 (2) |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C6—H6B···O1i | 0.97 | 2.65 | 3.362 (3) | 130 |
C4—H4A···O2ii | 0.98 | 2.65 | 3.369 (3) | 131 |
O3—H3···O2iii | 0.82 | 1.82 | 2.639 (2) | 173 |
Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) −x, y+1/2, −z+1/2; (iii) −x+1, −y+1, −z+1. |
(II) (±)-7-oxobicyclo[2.2.1]hept-5-ene-2-
endo-carboxylic acid
top
Crystal data top
C8H8O3 | Dx = 1.440 Mg m−3 |
Mr = 152.14 | Melting point: 376 K |
Orthorhombic, Pbca | Mo Kα radiation, λ = 0.71073 Å |
a = 6.0039 (9) Å | Cell parameters from 20 reflections |
b = 10.5565 (14) Å | θ = 11.4–14.9° |
c = 22.151 (2) Å | µ = 0.11 mm−1 |
V = 1403.9 (3) Å3 | T = 293 K |
Z = 8 | Parallelepiped, colorless |
F(000) = 640 | 0.50 × 0.50 × 0.35 mm |
Data collection top
Siemens P4 diffractometer | Rint = 0.032 |
Radiation source: normal-focus sealed tube | θmax = 25°, θmin = 1.8° |
Graphite monochromator | h = −7→1 |
2θ/θ scans | k = −1→12 |
1719 measured reflections | l = −1→26 |
1231 independent reflections | 3 standard reflections every 97 reflections |
827 reflections with I > 2σ(I) | intensity decay: variation < 1% |
Refinement top
Refinement on F2 | Secondary atom site location: difference Fourier map |
Least-squares matrix: full | Hydrogen site location: inferred from neighbouring sites |
R[F2 > 2σ(F2)] = 0.048 | H-atom parameters constrained |
wR(F2) = 0.136 | w = 1/[σ2(Fo2) + (0.0578P)2 + 0.5877P] where P = (Fo2 + 2Fc2)/3 |
S = 1.03 | (Δ/σ)max < 0.001 |
1231 reflections | Δρmax = 0.20 e Å−3 |
104 parameters | Δρmin = −0.18 e Å−3 |
0 restraints | Extinction correction: SHELXTL (Sheldrick, 1997), Fc*=kFc[1+0.001xFc2λ3/sin(2θ)]-1/4 |
Primary atom site location: structure-invariant direct methods | Extinction coefficient: 0.006 (2) |
Crystal data top
C8H8O3 | V = 1403.9 (3) Å3 |
Mr = 152.14 | Z = 8 |
Orthorhombic, Pbca | Mo Kα radiation |
a = 6.0039 (9) Å | µ = 0.11 mm−1 |
b = 10.5565 (14) Å | T = 293 K |
c = 22.151 (2) Å | 0.50 × 0.50 × 0.35 mm |
Data collection top
Siemens P4 diffractometer | Rint = 0.032 |
1719 measured reflections | 3 standard reflections every 97 reflections |
1231 independent reflections | intensity decay: variation < 1% |
827 reflections with I > 2σ(I) | |
Refinement top
R[F2 > 2σ(F2)] = 0.048 | 0 restraints |
wR(F2) = 0.136 | H-atom parameters constrained |
S = 1.03 | Δρmax = 0.20 e Å−3 |
1231 reflections | Δρmin = −0.18 e Å−3 |
104 parameters | |
Special details top
Experimental. Crystal mounted on glass fiber using 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 (Å2) top | x | y | z | Uiso*/Ueq | Occ. (<1) |
O1 | 1.0504 (3) | 0.0811 (2) | 0.33012 (10) | 0.0586 (6) | |
O2 | 0.4114 (3) | 0.3839 (2) | 0.45480 (9) | 0.0610 (7) | |
O3 | 0.7598 (3) | 0.4300 (3) | 0.47880 (10) | 0.0709 (8) | |
C1 | 0.9472 (4) | 0.2644 (2) | 0.39442 (11) | 0.0387 (7) | |
C2 | 0.6964 (4) | 0.2604 (3) | 0.41086 (11) | 0.0384 (7) | |
C3 | 0.5778 (4) | 0.2520 (3) | 0.34818 (11) | 0.0423 (7) | |
C4 | 0.7744 (4) | 0.2472 (3) | 0.30315 (11) | 0.0440 (7) | |
C5 | 0.8909 (5) | 0.3717 (3) | 0.30560 (12) | 0.0470 (8) | |
C6 | 0.9906 (4) | 0.3825 (3) | 0.35875 (13) | 0.0428 (7) | |
C7 | 0.9441 (4) | 0.1748 (3) | 0.34063 (12) | 0.0408 (7) | |
C8 | 0.6155 (4) | 0.3664 (3) | 0.44957 (10) | 0.0401 (7) | |
H2 | 0.3888 | 0.4408 | 0.4793 | 0.091* | 0.35 (5) |
H3 | 0.6967 | 0.4839 | 0.4993 | 0.106* | 0.65 (5) |
H1A | 1.0528 | 0.2448 | 0.4268 | 0.046* | |
H2A | 0.6682 | 0.1810 | 0.4324 | 0.046* | |
H3A | 0.4869 | 0.1763 | 0.3453 | 0.051* | |
H3B | 0.4849 | 0.3257 | 0.3411 | 0.051* | |
H4A | 0.7420 | 0.2135 | 0.2629 | 0.053* | |
H5A | 0.8933 | 0.4319 | 0.2749 | 0.056* | |
H6A | 1.0736 | 0.4517 | 0.3717 | 0.051* | |
Atomic displacement parameters (Å2) top | U11 | U22 | U33 | U12 | U13 | U23 |
O1 | 0.0510 (12) | 0.0465 (12) | 0.0784 (15) | 0.0101 (11) | 0.0038 (11) | −0.0068 (11) |
O2 | 0.0412 (12) | 0.0892 (18) | 0.0526 (13) | 0.0042 (11) | 0.0018 (10) | −0.0224 (11) |
O3 | 0.0457 (12) | 0.0923 (18) | 0.0747 (15) | −0.0107 (12) | 0.0069 (12) | −0.0401 (14) |
C1 | 0.0321 (13) | 0.0465 (15) | 0.0375 (14) | 0.0028 (12) | −0.0069 (11) | 0.0010 (12) |
C2 | 0.0350 (14) | 0.0436 (14) | 0.0365 (13) | −0.0026 (13) | 0.0007 (11) | 0.0042 (12) |
C3 | 0.0327 (13) | 0.0471 (15) | 0.0472 (15) | −0.0001 (13) | −0.0044 (12) | −0.0067 (13) |
C4 | 0.0403 (15) | 0.0574 (17) | 0.0343 (13) | 0.0065 (14) | −0.0036 (12) | −0.0049 (13) |
C5 | 0.0461 (17) | 0.0504 (17) | 0.0447 (15) | 0.0098 (14) | 0.0095 (14) | 0.0102 (13) |
C6 | 0.0337 (14) | 0.0419 (15) | 0.0528 (16) | −0.0009 (12) | 0.0053 (13) | −0.0016 (13) |
C7 | 0.0336 (14) | 0.0385 (14) | 0.0504 (15) | 0.0022 (13) | 0.0037 (12) | 0.0012 (13) |
C8 | 0.0382 (15) | 0.0516 (16) | 0.0304 (13) | −0.0047 (13) | 0.0000 (12) | 0.0000 (12) |
Geometric parameters (Å, º) top
O1—C7 | 1.200 (3) | C2—C8 | 1.491 (4) |
O2—C8 | 1.245 (3) | C2—C3 | 1.563 (3) |
O3—C8 | 1.273 (3) | C3—C4 | 1.546 (3) |
C1—C6 | 1.498 (4) | C4—C5 | 1.490 (4) |
C1—C7 | 1.522 (4) | C4—C7 | 1.520 (4) |
C1—C2 | 1.550 (3) | C5—C6 | 1.326 (4) |
| | | |
C6—C1—C7 | 96.1 (2) | C7—C4—C3 | 100.1 (2) |
C6—C1—C2 | 108.4 (2) | C6—C5—C4 | 108.7 (2) |
C7—C1—C2 | 98.9 (2) | C5—C6—C1 | 108.6 (2) |
C8—C2—C1 | 115.5 (2) | O1—C7—C4 | 131.7 (3) |
C8—C2—C3 | 113.9 (2) | O1—C7—C1 | 131.1 (3) |
C1—C2—C3 | 103.63 (19) | C4—C7—C1 | 97.1 (2) |
C4—C3—C2 | 103.12 (19) | O2—C8—O3 | 123.0 (3) |
C5—C4—C7 | 96.3 (2) | O2—C8—C2 | 119.1 (2) |
C5—C4—C3 | 107.8 (2) | O3—C8—C2 | 117.8 (2) |
| | | |
C6—C1—C2—C8 | −62.1 (3) | C5—C4—C7—O1 | −123.7 (3) |
C7—C1—C2—C8 | −161.7 (2) | C3—C4—C7—O1 | 126.9 (3) |
C6—C1—C2—C3 | 63.2 (3) | C5—C4—C7—C1 | 52.8 (2) |
C7—C1—C2—C3 | −36.4 (3) | C3—C4—C7—C1 | −56.6 (2) |
C8—C2—C3—C4 | 128.3 (2) | C6—C1—C7—O1 | 124.2 (3) |
C1—C2—C3—C4 | 2.0 (3) | C2—C1—C7—O1 | −126.1 (3) |
C2—C3—C4—C5 | −66.8 (3) | C6—C1—C7—C4 | −52.3 (2) |
C2—C3—C4—C7 | 33.2 (3) | C2—C1—C7—C4 | 57.4 (2) |
C7—C4—C5—C6 | −34.1 (3) | C1—C2—C8—O2 | 165.9 (2) |
C3—C4—C5—C6 | 68.6 (3) | C3—C2—C8—O2 | 46.1 (3) |
C4—C5—C6—C1 | 0.5 (3) | C1—C2—C8—O3 | −18.4 (4) |
C7—C1—C6—C5 | 33.3 (3) | C3—C2—C8—O3 | −138.2 (3) |
C2—C1—C6—C5 | −68.3 (3) | | |
Hydrogen-bond geometry (Å, º) top
D—H···A | D—H | H···A | D···A | D—H···A |
C1—H1A···O2i | 0.98 | 2.68 | 3.339 (3) | 125 |
C3—H3B···O1ii | 0.97 | 2.71 | 3.580 (4) | 149 |
O3—H3···O2iii | 0.82 | 1.84 | 2.660 (3) | 173 |
Symmetry codes: (i) x+1, y, z; (ii) −x+3/2, y+1/2, z; (iii) −x+1, −y+1, −z+1. |
Experimental details
| (I) | (II) |
Crystal data |
Chemical formula | C8H10O3 | C8H8O3 |
Mr | 154.16 | 152.14 |
Crystal system, space group | Monoclinic, P21/c | Orthorhombic, Pbca |
Temperature (K) | 293 | 293 |
a, b, c (Å) | 6.3714 (9), 10.560 (2), 11.610 (3) | 6.0039 (9), 10.5565 (14), 22.151 (2) |
α, β, γ (°) | 90, 98.616 (14), 90 | 90, 90, 90 |
V (Å3) | 772.3 (3) | 1403.9 (3) |
Z | 4 | 8 |
Radiation type | Mo Kα | Mo Kα |
µ (mm−1) | 0.10 | 0.11 |
Crystal size (mm) | 0.64 × 0.48 × 0.28 | 0.50 × 0.50 × 0.35 |
|
Data collection |
Diffractometer | Siemens P4 diffractometer | Siemens P4 diffractometer |
Absorption correction | Numerical (SHELXTL; Sheldrick, 1997) | – |
Tmin, Tmax | 0.96, 0.97 | – |
No. of measured, independent and observed [I > 2σ(I)] reflections | 1957, 1362, 960 | 1719, 1231, 827 |
Rint | 0.020 | 0.032 |
(sin θ/λ)max (Å−1) | 0.595 | 0.595 |
|
Refinement |
R[F2 > 2σ(F2)], wR(F2), S | 0.047, 0.117, 1.05 | 0.048, 0.136, 1.03 |
No. of reflections | 1362 | 1231 |
No. of parameters | 103 | 104 |
H-atom treatment | H-atom parameters constrained | H-atom parameters constrained |
Δρmax, Δρmin (e Å−3) | 0.23, −0.14 | 0.20, −0.18 |
Selected geometric parameters (Å, º) for (I) topO2—C8 | 1.233 (2) | O3—C8 | 1.284 (2) |
| | | |
O2—C8—C1 | 121.0 (2) | O3—C8—C1 | 116.4 (2) |
Hydrogen-bond geometry (Å, º) for (I) top
D—H···A | D—H | H···A | D···A | D—H···A |
C6—H6B···O1i | 0.97 | 2.65 | 3.362 (3) | 130 |
C4—H4A···O2ii | 0.98 | 2.65 | 3.369 (3) | 131 |
O3—H3···O2iii | 0.82 | 1.82 | 2.639 (2) | 173 |
Symmetry codes: (i) x, −y+3/2, z+1/2; (ii) −x, y+1/2, −z+1/2; (iii) −x+1, −y+1, −z+1. |
Selected geometric parameters (Å, º) for (II) topO2—C8 | 1.245 (3) | O3—C8 | 1.273 (3) |
| | | |
O2—C8—C2 | 119.1 (2) | O3—C8—C2 | 117.8 (2) |
Hydrogen-bond geometry (Å, º) for (II) top
D—H···A | D—H | H···A | D···A | D—H···A |
C1—H1A···O2i | 0.98 | 2.68 | 3.339 (3) | 125 |
C3—H3B···O1ii | 0.97 | 2.71 | 3.580 (4) | 149 |
O3—H3···O2iii | 0.82 | 1.84 | 2.660 (3) | 173 |
Symmetry codes: (i) x+1, y, z; (ii) −x+3/2, y+1/2, z; (iii) −x+1, −y+1, −z+1. |
Our continuing interest in the crystallography of keto acids lies in mapping the molecular characteristics that control their five known hydrogen-bonding modes. Among several small cyclic systems we have investigated, carboxy bicyclo[2.2.1]heptanones stand out as producing significant proportions of catemeric acid-to-ketone hydrogen bonding (Lalancette et al., 1997). We present here the crystal structures of two further compounds in this series, a β-keto acid, 7-oxobicyclo[2.2.1]heptane-1-carboxylic acid, (I), and a γ-keto acid,(±)-7-oxobicyclo[2.2.1]hept-5-ene-2-endo-carboxylic acid, (II).
Although their geometry appears favorable for internal hydrogen bonding, the few β-keto acids whose X-ray structures have been determined display scant tendency toward that hydrogen-bonding mode (Thompson et al., 1996), opting instead for catemeric (Lalancette et al., 1991) or dimeric aggregation (Lalancette et al., 1999). Among simple keto acids, internal hydrogen bonding is better served by seven-membered geometry, and, indeed, occurs most often among γ-keto acids. Fig. 1, showing a pair of molecules of (I) hydrogen-bonded together as a centrosymmetric dimer, reveals that (I) is not internally but dimerically hydrogen bonded. Although the homologous 9-oxobicyclo[3.3.1]nonane-1-carboxylic acid (Thompson et al., 1992) possesses a plane of symmetry, (I) is found to have conformational chirality, even though the molecule is not inherently chiral. A major source of this asymmetry in (I) is the partial order in the carboxyl group. Some averaging of carboxyl bond lengths and angles by disorder is common in acid dimers (Leiserowitz, 1976), and the C=O and C—O bond lengths in (I) are 1.233 (2) and 1.284 (2) Å, respectively, with angles of 121.0 (2) and 116.4 (2)° for C—C=O and C—C—O, respectively. Values cited as typical for highly ordered dimeric carboxyls are 1.21 and 1.31 Å, and 123 and 112°, respectively (Borthwick, 1980). Consistent with this, partial carboxyl H atoms were found in electron density difference maps and were refined to an occupancy ratio of 0.68 (4):0.32 (4) for H atoms bonded to O3 and O2, respectively. The carboxyl group is also rotated very slightly from orthogonality relative to the ketone plane. This departure of the carboxyl group from the `symmetric' arrangement is indicated by the C1—C7—C4 versus C8—O2—O3 dihedral angle of 87.2 (2)° and by a C7—C1—C8—O2 torsion angle of -91.8 (2)°. Additional chirality is imparted by a very slight twist of the bicycloheptane system: torsion angle C1—C2—C3—C4 = -1.3 (2)°, while angle C1—C6—C5—C4 = -1.1 (2)°. The packing for (I) involves typical centrosymmetric dimers centered at (1/2,1/2,1/2), with a second screw-related set centered on the a edge of the chosen cell; the O···O distance is 2.639 (2) Å.
Fig. 2 shows a pair of molecules for (II), hydrogen-bonded together as a centrosymmetric dimer. Despite the rigidity of the system, eclipsing strains around C2—C3 are relieved by a significant twist of that bridge, producing a C8—C2—C3—C4 torsion angle of 128.3 (2)° (ideal value 120°). The carboxyl group is rotated as shown [O2—C8—C2—C3 46.1 (3)°] and displays significant disorder. The C=O and C—O bond lengths in (II) are 1.245 (3) and 1.273 (3) Å, respectively, with angles of 119.1 (2) and 117.8 (2)° for C—C=O and C—C—O, respectively. The partial H atoms found for (II) in difference maps were refined to an occupancy ratio of 0.65 (5):0.35 (5) for H atoms bonded to O3 and O2, respectively.
Compound (II) belongs to the category of γ-keto acids which is especially rich in hydrogen-bonding types, embracing dimers, internal hydrogen bonds and carboxyl-to-ketone catemers. The `linearly aligned anti' arrangement seen here for the ketone and acid groups is one which often leads to translational catemers in other systems (Barcon et al., 1998; Zewge et al., 1998). Nevertheless, in (II) the result is again a packing typical for centrosymmetric dimerization, with an O···O distance of 2.660 (3) Å. Parallel screw-related sets of dimers are centered at (1/2,1/2,1/2) and the a edge of the chosen cell, while a second parallel screw-related set, oriented at an angle and screw-related to the first, is centered on the c cell edge.
For both (I) and (II) several close intermolecular C—H···O contacts (2.65–2.71 Å; Tables 2 and 4) were found within the range often used as the `van der Waals limit' (Steiner, 1997) for non-bonded H···O packing interactions. Steiner & Desiraju (1998) have compiled data for a large number of C—H···O contacts and found significant statistical directionality even at cutoff distances as great as 3.0 Å, leading to the conclusion that these may be legitimately viewed as `weak hydrogen bonds', presumably contributing significantly more to packing forces than simple van der Waals attractions. Comparison of our distances and angles (Tables 2 and 4) with such data for other Csp3—H···O=C cases suggests that these interactions are important in the packing of (I) and (II).
Part of the reluctance of (I) and (II) to adopt hydrogen-bonding modes involving the ketone may be attributable to lowered ketone basicity due to angle strain. Carbonyl basicity has been shown to correlate positively with ring size for the seven-, six-, five- and four-membered simple cycloalkanones, whose pKa values (for the protonated species) are -6.6, -6.8, -7.5 and -9.5, respectively (Campbell & Edwards, 1960; Butler, 1976). The ketone angles for these cycloalkanones are cited as 121, 116, 101 and 82°, respectively (Avram & Mateescu, 1972), and the observed bridge angles in (I) and (II) are 98.3 (2) and 97.1 (2)°, respectively. The above trend in basicity also appears generally consistent with hydrogen-bonding solvent shifts for the n → π* absorptions of these same cycloalkanones (Wheeler, 1957; Lambert et al., 1998), which are related to charge distribution in the ground versus excited states (Pimentel, 1957; Jaffé & Orchin, 1962). Since ring strain directly affects IR C=O stretching frequencies (see below), these are also correlatable with pKa, as has been shown for an homologous series of lactams (Huisgen et al., 1954), and basicity has been directly related to νC=O even for non-cyclic cases (Stewart & Yates, 1958).
The solid-state (KBr) IR spectrum of (I) has C=O absorption peaks at 1772 and 1693 cm-1, for free ketone and hydrogen-bonded carboxyl, respectively. In CHCl3 solution the same absorptions are seen at 1776 and 1704 cm-1, with the usual carboxyl-dilution shoulder around 1735–1740 cm-1. Compound (II) gives similar data, the values being 1771 and 1691 cm-1 for KBr, and 1781 and 1710 cm-1 for CHCl3.