4,13-Di­acetyl-[2.2]­para­cyclo­phane

The mol­ecule of the title compound, C₂₀H₂₀O₂, displays crystallographic twofold symmetry. The bridgehead bond lengths are 1.584 (3) and 1.590 (3) Å. There is slight distortion at one bridgehead C atom [C2—C3—C4 124.71 (13)° in standard cyclo­phane numbering]. The mol­ecules are linked by a weak hydrogen bond of the form C—H...O to form a layer structure.

Key indicators

• Single-crystal X-ray study
• T = 178 K
• Mean [sigma](C-C) = 0.002 Å
• R factor = 0.045
• wR factor = 0.134
• Data-to-parameter ratio = 16.9

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

Among the chiral [2.2]paracyclophanes relatively little is known about the 4,13-disubstituted derivatives, otherwise known as `pseudo-meta' compounds. If the two substituents are identical, the compounds may in principle display ideal C₂ symmetry. Our interest in these compounds has led us to prepare the title compound, (I), by our established synthetic method (see Experimental); here we report its structure.

The molecule (Fig. 1) displays imposed twofold symmetry, with the twofold axis (at x = 1/2, z = 1/4) passing through the midpoints of C2—C2ⁱ and C9—C9ⁱ; the atom numbering is standard for one half of a cyclophane molecule. The six-membered rings display the distortion towards a boat form that is typical of [2.2]paracyclophanes, whereby the bridgehead atoms C3 and C6 are displaced by 0.182 (2) and 0.174 (2) Å, respectively, from the plane of the remaining four atoms (mean deviation 0.004 Å). Also typical is the lengthening of the bridge bonds to 1.584 (3) and 1.590 (3) Å. The carbonyl group is rotated out of the corresponding ring plane, with a torsion angle C3—C4—C17—O of -32.2 (2)°. The substituent is associated with some distortion at C3, with a C4—C3—C2 angle of 124.71 (13)°.

The molecules are connected by a weak C18—H18a···O hydrogen bond via a twofold screw axis (at x = 3/4, z = 1/4) to form layers parallel to the xy plane (Fig. 2).

S2. Experimental

The title compound was prepared by the standard method (Hopf et al., 1981) by cycloaddition of but-3-yn-2-one (ethynyl methyl ketone) to 1,2,4,5-hexatetraene (biallenyl) in toluene at 348 K. Apart from the title compound, which is formed in 8% yield, other isomers are produced. These were separated by preparative middle pressure chromatography on silica gel with dichloromethane (Hillmer, 1991). Crystals were grown by evaporation from 2-propanol.

S3. Refinement

H atoms on sp² C atoms were included using a riding model, starting from idealized positions. Methyl H atoms were located as rather weak but distinct maxima in difference syntheses, idealized and refined as rigid groups allowed to rotate but not tip.

Computing details

Data collection: P3 (Nicolet, 1987); cell refinement: P3; data reduction: XDISK (Nicolet, 1987); program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: XP (Siemens, 1994); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. The molecule of the title compound in the crystal. Ellipsoids represent 50% probability levels.
	Fig. 2. Packing diagram of the title compound with the view direction perpendicular to the xy plane. The hydrogen bond is indicated by a dashed line; H atoms other than those of the methyl group have been excluded for clarity. There are two such layers, related by inversion symmetry, per z axis repeat. Radii are arbitrary.

(I)

Crystal data

C20H20O2	F(000) = 624
Mr = 292.36	Dx = 1.305 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
a = 15.827 (6) Å	Cell parameters from 50 reflections
b = 9.442 (2) Å	θ = 10–12°
c = 11.423 (4) Å	µ = 0.08 mm^−1
β = 119.34 (2)°	T = 178 K
V = 1488.1 (8) Å^3	Prism, colourless
Z = 4	0.7 × 0.45 × 0.4 mm

Data collection

Nicolet R3 diffractometer	Rint = 0.018
Radiation source: fine-focus sealed tube	θmax = 27.5°, θmin = 3.7°
Graphite monochromator	h = −20→0
ω scans	k = −12→8
3126 measured reflections	l = −12→14
1710 independent reflections	3 standard reflections every 147 reflections
1411 reflections with I > 2σ(I)	intensity decay: none

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)] = 0.045	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.134	H-atom parameters constrained
S = 1.03	w = 1/[σ^2(Fo^2) + (0.0677P)^2 + 1.4684P] where P = (Fo^2 + 2Fc^2)/3
1710 reflections	(Δ/σ)max < 0.001
101 parameters	Δρmax = 0.27 e Å^−3
0 restraints	Δρmin = −0.27 e Å^−3

Special details

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.

Least-squares planes (x,y,z in crystal coordinates) and deviations from them (* indicates atom used to define plane)

6.0871 (0.0081) x + 0.0101 (0.0084) y - 11.3445 (0.0044) z = 1.7688 (0.0045)

* -0.0041 (0.0006) C4 * 0.0041 (0.0006) C5 * -0.0041 (0.0006) C7 * 0.0041 (0.0006) C8 - 0.1819 (0.0019) C3 - 0.1738 (0.0019) C6

Rms deviation of fitted atoms = 0.0041

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
C2	0.48519 (12)	−0.03085 (16)	0.17294 (15)	0.0320 (4)
H2A	0.5306	−0.0922	0.1597	0.038*
H2B	0.4193	−0.0713	0.1201	0.038*
C3	0.48651 (10)	0.11654 (15)	0.12126 (13)	0.0254 (3)
C4	0.57137 (10)	0.19324 (15)	0.15119 (13)	0.0239 (3)
C5	0.56930 (9)	0.34162 (15)	0.14949 (13)	0.0239 (3)
H5	0.6264	0.3926	0.1688	0.029*
C6	0.48496 (10)	0.41606 (15)	0.11998 (13)	0.0246 (3)
C7	0.39885 (10)	0.33973 (16)	0.05875 (13)	0.0275 (3)
H7	0.3387	0.3886	0.0167	0.033*
C8	0.39995 (10)	0.19236 (16)	0.05849 (13)	0.0279 (3)
H8	0.3403	0.1423	0.0145	0.033*
C9	0.48858 (11)	0.56335 (15)	0.17402 (14)	0.0288 (3)
H9A	0.4255	0.6106	0.1178	0.035*
H9B	0.5390	0.6189	0.1671	0.035*
C17	0.66673 (10)	0.12126 (15)	0.19799 (15)	0.0294 (3)
C18	0.73574 (11)	0.18517 (18)	0.15758 (18)	0.0382 (4)
H18A	0.7760	0.2565	0.2239	0.046*
H18B	0.6989	0.2299	0.0692	0.046*
H18C	0.7773	0.1107	0.1534	0.046*
O	0.68907 (9)	0.01412 (13)	0.26574 (14)	0.0462 (4)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C2	0.0436 (8)	0.0235 (7)	0.0339 (8)	−0.0042 (6)	0.0230 (7)	−0.0040 (6)
C3	0.0329 (7)	0.0246 (7)	0.0214 (6)	−0.0018 (5)	0.0154 (5)	−0.0042 (5)
C4	0.0279 (7)	0.0258 (7)	0.0204 (6)	0.0021 (5)	0.0137 (5)	−0.0002 (5)
C5	0.0263 (6)	0.0258 (7)	0.0228 (6)	0.0000 (5)	0.0144 (5)	0.0008 (5)
C6	0.0300 (7)	0.0250 (7)	0.0221 (6)	0.0032 (5)	0.0153 (5)	0.0048 (5)
C7	0.0260 (7)	0.0344 (8)	0.0212 (6)	0.0044 (6)	0.0108 (5)	0.0045 (5)
C8	0.0276 (7)	0.0338 (8)	0.0211 (6)	−0.0052 (6)	0.0111 (5)	−0.0036 (5)
C9	0.0338 (7)	0.0222 (7)	0.0352 (8)	0.0042 (5)	0.0207 (6)	0.0048 (5)
C17	0.0316 (7)	0.0262 (7)	0.0314 (7)	0.0026 (6)	0.0163 (6)	−0.0041 (6)
C18	0.0301 (8)	0.0373 (9)	0.0507 (10)	0.0019 (6)	0.0226 (7)	−0.0012 (7)
O	0.0469 (7)	0.0346 (7)	0.0616 (8)	0.0158 (5)	0.0300 (6)	0.0152 (6)

Geometric parameters (Å, º)

C2—C3	1.516 (2)	C7—C8	1.392 (2)
C2—C2^i	1.584 (3)	C7—H7	0.9500
C2—H2A	0.9900	C8—H8	0.9500
C2—H2B	0.9900	C9—C9^i	1.590 (3)
C3—C8	1.393 (2)	C9—H9A	0.9900
C3—C4	1.4126 (19)	C9—H9B	0.9900
C4—C5	1.401 (2)	C17—O	1.2162 (19)
C4—C17	1.4949 (19)	C17—C18	1.505 (2)
C5—C6	1.3959 (19)	C18—H18A	0.9800
C5—H5	0.9500	C18—H18B	0.9800
C6—C7	1.390 (2)	C18—H18C	0.9800
C6—C9	1.511 (2)
C3—C2—C2^i	112.31 (7)	C8—C7—H7	119.7
C3—C2—H2A	109.1	C7—C8—C3	121.53 (13)
C2^i—C2—H2A	109.1	C7—C8—H8	119.2
C3—C2—H2B	109.1	C3—C8—H8	119.2
C2^i—C2—H2B	109.1	C6—C9—C9^i	112.88 (7)
H2A—C2—H2B	107.9	C6—C9—H9A	109.0
C8—C3—C4	116.08 (13)	C9^i—C9—H9A	109.0
C8—C3—C2	118.33 (13)	C6—C9—H9B	109.0
C4—C3—C2	124.71 (13)	C9^i—C9—H9B	109.0
C5—C4—C3	119.75 (12)	H9A—C9—H9B	107.8
C5—C4—C17	118.17 (12)	O—C17—C4	122.24 (14)
C3—C4—C17	121.81 (13)	O—C17—C18	120.02 (14)
C6—C5—C4	121.32 (12)	C4—C17—C18	117.73 (13)
C6—C5—H5	119.3	C17—C18—H18A	109.5
C4—C5—H5	119.3	C17—C18—H18B	109.5
C7—C6—C5	116.50 (13)	H18A—C18—H18B	109.5
C7—C6—C9	120.95 (12)	C17—C18—H18C	109.5
C5—C6—C9	121.63 (12)	H18A—C18—H18C	109.5
C6—C7—C8	120.62 (13)	H18B—C18—H18C	109.5
C6—C7—H7	119.7
C2^i—C2—C3—C8	−95.12 (19)	C6—C7—C8—C3	1.3 (2)
C2^i—C2—C3—C4	73.7 (2)	C4—C3—C8—C7	−17.11 (19)
C8—C3—C4—C5	16.01 (18)	C2—C3—C8—C7	152.65 (13)
C2—C3—C4—C5	−153.02 (13)	C7—C6—C9—C9^i	89.40 (18)
C8—C3—C4—C17	−170.06 (12)	C5—C6—C9—C9^i	−79.23 (19)
C2—C3—C4—C17	20.91 (19)	C5—C4—C17—O	141.82 (15)
C3—C4—C5—C6	0.63 (19)	C3—C4—C17—O	−32.2 (2)
C17—C4—C5—C6	−173.52 (12)	C5—C4—C17—C18	−38.66 (18)
C4—C5—C6—C7	−16.51 (18)	C3—C4—C17—C18	147.32 (14)
C4—C5—C6—C9	152.61 (12)	C3—C2—C2^i—C3^i	14.1 (3)
C5—C6—C7—C8	15.59 (18)	C6—C9—C9^i—C6^i	−5.6 (2)
C9—C6—C7—C8	−153.60 (13)

Symmetry code: (i) −x+1, y, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C18—H18A···O^ii	0.98	2.48	3.288 (2)	139

Symmetry code: (ii) −x+3/2, y+1/2, −z+1/2.