Racemic 6-exo-hydroxy­bi­cyclo­[2.2.1]­heptane-2-exo-carboxyl­ic acid

The crystal structure of the title cage hydroxy acid, C₈H₁₂O₃, has confirmed the exo configuration for both the carboxyl­ic acid and hydroxy substituents, as previously determined from chemical and ¹³C NMR evidence. Both the carboxyl­ic acid and the hydroxy O atoms are involved in intermolecular hydrogen-bonding interactions, giving a centrosymmetric 12-membered cyclic ring system which extends via hydrogen bonding into an infinite two-dimensional network.

Key indicators

• Single-crystal X-ray study
• T = 295 K
• Mean [sigma](C-C) = 0.003 Å
• R factor = 0.043
• wR factor = 0.132
• Data-to-parameter ratio = 12.1

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

Racemic 6-exo-hydroxybicyclo[2.2.1]heptane-2-exo-carboxylic acid, (I), was first reported in 1969 together with the isomeric 5-exo-hydroxy isomer (Beckmann et al., 1969) with which it forms in the normal synthetic route. The hydroboration procedure used here in the preparation (Fischer et al., 1980) gives predominantly the 6-exo-hydroxy isomer, whereas the oxymercuration/demercuration procedure of Beckmann et al. (1969) gives predominantly the 5-exo-hydroxy isomer. The crystal structure determination of (I) shows the relatively inflexible norbornane cage which is similar to other norbornane carboxylic acids (Apgar & Ludwig, 1972; Albinati et al., 1973), with both the carboxylic acid and hydroxy groups exo-related (Fig. 1). The torsion angles C1–C2–C21–O212 and C2–C1–C6–O6 are 80.9 (2) and 173.6 (1)°, respectively. The carboxylic acid O atoms associate with the hydroxy groups of two different acids [O212—H212···O6ⁱ 2.665 (2) Å and O—H···O 167 (2)°; O6—H6···O211ⁱⁱ, 2.752 (2) Å, O—H···O, 170 (2)°; symmetry codes: (i) 1/2 + x, 1/2 - y, 1 + z; (ii) 1/2 - x, -1/2 + y, 1 - z], forming cyclic centrosymmetric 12-membered R⁴₄(12) rings (Fig. 2). These then form into an infinite two-dimensional structure. This association is different from the common R²₂(8) cyclic A—A dimers or the more unusual catemer structures found among the monofunctional carboxylic acids (Leiserowitz, 1976).

S2. Experimental

The title compound was synthesized using a modification of the method of Fischer et al. (1980) by the hydroboration of the unsaturated ester precursor followed by hydrolysis with sodium hydroxide. The modification (Vogel, 1989) involved the generation of diborane in situ by addition of the boron trifluoride–dietherate to a mixture of sodium borohydride and the ester precursor under a nitrogen atmosphere (Wermuth, 1995). Fractional crystallization of the isomeric mixture from acetone gave the 5-hydroxy and 6-hydroxy isomers. Crystals of the 6-hydroxy isomer (the title compound) suitable for X-ray analysis were obtained by slow evaporation from acetone.

S3. Refinement

All H atoms were refined; the range of C—H distances was 0.95 (2)–1.03 (2) Å.

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1999a); cell refinement: MSC/AFC Diffractometer Control Software; data reduction: TEXSAN for Windows (Molecular Structure Corporation, 1999b); program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON for Windows (Spek, 1999); software used to prepare material for publication: TEXSAN for Windows.

Figures

	Fig. 1. The molecular configuration and atom-numbering scheme for the title compound. Atoms are shown as 30% probability ellipsoids (Spek, 1999).
	Fig. 2. Perspective view of the two-dimensional polymer structure. Hydrogen bonds are shown as broken lines (Spek, 1999).

Racemic 6-exo-hydroxybicyclo[2.2.1]heptane-2-exo-carboxylic acid

Crystal data

C8H12O3	F(000) = 336
Mr = 156.18	Dx = 1.320 Mg m^−3
Monoclinic, P21/a	Melting point: 434–435 K K
Hall symbol: -P 2yab	Mo Kα radiation, λ = 0.71069 Å
a = 11.821 (2) Å	Cell parameters from 25 reflections
b = 10.7618 (18) Å	θ = 13.1–16.3°
c = 6.3146 (11) Å	µ = 0.10 mm^−1
β = 101.860 (14)°	T = 295 K
V = 786.2 (2) Å^3	Prism, colourless
Z = 4	0.50 × 0.35 × 0.20 mm

Data collection

Rigaku AFC-7R diffractometer	Rint = 0.033
Radiation source: Rigaku rotating anode	θmax = 27.5°, θmin = 2.6°
Graphite monochromator	h = −8→15
ω–2θ scans	k = 0→13
2170 measured reflections	l = −8→8
1805 independent reflections	3 standard reflections every 150 reflections
1216 reflections with I > 2σ(I)	intensity decay: 1.2%

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.043	All H-atom parameters refined
wR(F^2) = 0.132	w = 1/[σ^2(Fo^2) + (0.0615P)^2 + 0.1309P] where P = (Fo^2 + 2Fc^2)/3
S = 1.04	(Δ/σ)max < 0.001
1805 reflections	Δρmax = 0.29 e Å^−3
149 parameters	Δρmin = −0.17 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.023 (5)

Special details

Geometry. Bond distances, angles etc. have been calculated using the rounded fractional coordinates. All e.s.d.'s are estimated from the variances of the (full) variance-covariance matrix. The cell e.s.d.'s are taken into account in the estimation of distances, angles and torsion angles.

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
O6	0.05140 (11)	0.20706 (13)	0.0912 (2)	0.0482 (4)
O211	0.39684 (14)	0.48698 (13)	0.6884 (2)	0.0665 (5)
O212	0.42875 (16)	0.28610 (14)	0.6870 (3)	0.0725 (5)
C1	0.20068 (13)	0.32132 (14)	0.3489 (2)	0.0347 (4)
C2	0.32589 (14)	0.37061 (15)	0.3625 (3)	0.0378 (5)
C3	0.30626 (17)	0.49228 (18)	0.2292 (3)	0.0482 (6)
C4	0.17567 (16)	0.49212 (16)	0.1404 (3)	0.0479 (6)
C5	0.14741 (17)	0.3864 (2)	−0.0220 (3)	0.0503 (6)
C6	0.16179 (14)	0.26883 (16)	0.1199 (3)	0.0392 (5)
C7	0.13049 (16)	0.44147 (17)	0.3333 (3)	0.0463 (6)
C21	0.38711 (13)	0.38930 (16)	0.5946 (3)	0.0415 (5)
H1	0.1935 (15)	0.2647 (17)	0.461 (3)	0.040 (5)*
H2	0.3715 (17)	0.3094 (18)	0.301 (3)	0.048 (5)*
H4	0.144 (2)	0.577 (2)	0.081 (4)	0.068 (6)*
H6	0.059 (2)	0.140 (2)	0.159 (4)	0.066 (7)*
H31	0.3284 (19)	0.564 (2)	0.328 (4)	0.062 (6)*
H32	0.3488 (19)	0.494 (2)	0.114 (4)	0.062 (6)*
H51	0.068 (2)	0.3914 (19)	−0.105 (4)	0.067 (6)*
H52	0.200 (2)	0.3834 (19)	−0.123 (4)	0.068 (7)*
H61	0.2205 (17)	0.2087 (17)	0.087 (3)	0.043 (5)*
H71	0.0479 (18)	0.4247 (18)	0.305 (3)	0.048 (5)*
H72	0.1523 (17)	0.4918 (18)	0.462 (4)	0.054 (6)*
H212	0.464 (3)	0.299 (3)	0.826 (5)	0.095 (9)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
O6	0.0447 (7)	0.0445 (7)	0.0461 (7)	−0.0082 (6)	−0.0123 (5)	0.0026 (6)
O211	0.0797 (11)	0.0528 (8)	0.0564 (9)	0.0017 (7)	−0.0109 (7)	−0.0170 (7)
O212	0.0922 (12)	0.0495 (8)	0.0545 (8)	0.0039 (8)	−0.0343 (8)	0.0018 (7)
C1	0.0381 (8)	0.0324 (8)	0.0303 (7)	−0.0010 (6)	−0.0005 (6)	0.0029 (6)
C2	0.0330 (8)	0.0391 (9)	0.0379 (8)	0.0013 (7)	−0.0003 (6)	0.0001 (7)
C3	0.0461 (10)	0.0453 (10)	0.0491 (10)	−0.0076 (8)	0.0002 (8)	0.0101 (8)
C4	0.0457 (10)	0.0373 (9)	0.0560 (11)	0.0029 (7)	−0.0004 (8)	0.0145 (8)
C5	0.0472 (10)	0.0627 (12)	0.0361 (8)	−0.0044 (9)	−0.0032 (7)	0.0119 (8)
C6	0.0355 (8)	0.0433 (9)	0.0340 (8)	0.0015 (7)	−0.0039 (6)	−0.0034 (7)
C7	0.0424 (10)	0.0421 (10)	0.0535 (10)	0.0058 (8)	0.0077 (8)	−0.0027 (8)
C21	0.0338 (8)	0.0415 (9)	0.0438 (9)	−0.0032 (7)	−0.0044 (6)	−0.0005 (7)

Geometric parameters (Å, º)

O6—C6	1.442 (2)	C4—C7	1.528 (3)
O6—H6	0.83 (2)	C5—C6	1.540 (3)
O211—C21	1.200 (2)	C1—H1	0.951 (18)
O212—C21	1.303 (2)	C2—H2	0.98 (2)
O212—H212	0.90 (3)	C3—H31	0.99 (2)
C1—C6	1.532 (2)	C3—H32	0.97 (2)
C1—C2	1.558 (2)	C4—H4	1.03 (2)
C1—C7	1.529 (2)	C5—H51	0.98 (2)
C2—C21	1.509 (3)	C5—H52	0.98 (2)
C2—C3	1.548 (3)	C6—H61	1.00 (2)
C3—C4	1.530 (3)	C7—H71	0.97 (2)
C4—C5	1.522 (3)	C7—H72	0.97 (2)
C6—O6—H6	109.6 (17)	C3—C2—H2	112.7 (11)
C21—O212—H212	111 (2)	C21—C2—H2	106.3 (11)
C6—C1—C7	101.39 (12)	C2—C3—H31	109.0 (14)
C2—C1—C6	105.96 (13)	C2—C3—H32	112.5 (13)
C2—C1—C7	102.27 (13)	C4—C3—H31	110.3 (13)
C3—C2—C21	113.82 (14)	C4—C3—H32	111.5 (14)
C1—C2—C21	111.07 (14)	H31—C3—H32	110.3 (19)
C1—C2—C3	102.91 (14)	C3—C4—H4	113.4 (13)
C2—C3—C4	103.03 (15)	C5—C4—H4	113.9 (14)
C3—C4—C5	108.23 (15)	C7—C4—H4	116.6 (14)
C3—C4—C7	101.53 (15)	C4—C5—H51	111.9 (13)
C5—C4—C7	101.90 (15)	C4—C5—H52	112.5 (13)
C4—C5—C6	103.86 (15)	C6—C5—H51	109.6 (13)
C1—C6—C5	102.83 (14)	C6—C5—H52	110.1 (13)
O6—C6—C1	112.13 (14)	H51—C5—H52	109 (2)
O6—C6—C5	108.47 (14)	O6—C6—H61	108.9 (11)
C1—C7—C4	94.54 (14)	C1—C6—H61	110.3 (11)
O212—C21—C2	112.52 (16)	C5—C6—H61	114.1 (11)
O211—C21—O212	122.43 (18)	C1—C7—H71	111.5 (12)
O211—C21—C2	125.05 (16)	C1—C7—H72	111.9 (12)
C2—C1—H1	114.0 (11)	C4—C7—H71	115.0 (11)
C6—C1—H1	114.3 (11)	C4—C7—H72	113.2 (13)
C7—C1—H1	117.4 (11)	H71—C7—H72	110.0 (17)
C1—C2—H2	110.1 (12)
C6—C1—C2—C21	−163.00 (13)	C3—C2—C21—O211	17.1 (3)
C6—C1—C2—C3	74.84 (15)	C1—C2—C3—C4	−4.73 (17)
C7—C1—C6—C5	36.32 (16)	C1—C2—C21—O211	−98.5 (2)
C2—C1—C7—C4	53.59 (14)	C1—C2—C21—O212	80.92 (18)
C7—C1—C2—C3	−30.96 (16)	C2—C3—C4—C5	−67.85 (18)
C7—C1—C2—C21	91.20 (15)	C2—C3—C4—C7	38.93 (17)
C2—C1—C6—O6	173.56 (13)	C3—C4—C5—C6	72.99 (18)
C2—C1—C6—C5	−70.12 (16)	C7—C4—C5—C6	−33.54 (18)
C7—C1—C6—O6	−80.00 (16)	C5—C4—C7—C1	54.67 (16)
C6—C1—C7—C4	−55.73 (15)	C3—C4—C7—C1	−57.01 (15)
C3—C2—C21—O212	−163.50 (16)	C4—C5—C6—C1	−1.71 (18)
C21—C2—C3—C4	−125.02 (15)	C4—C5—C6—O6	117.20 (16)