2,2-exo-3,5,6-Penta­bromo­bi­cyclo­heptane

The title compound, C₇H₇Br₅, consists of a norbornane skeleton composed of two five-membered rings in envelope conformations or a six-membered ring with five Br atoms held in a boat conformation by a bridging methyl­ene group. The repulsive interactions between the Br atoms affect the topology of the norbornane moieties.

Key indicators

• Single-crystal X-ray study
• T = 294 K
• Mean [sigma](C-C) = 0.016 Å
• R factor = 0.064
• wR factor = 0.133
• Data-to-parameter ratio = 19.8

checkCIF results

No syntax errors found

ADDSYM reports no extra symmetry

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S1. Comment

The constitution and configuration of the products formed by electrophilic addition to norbornane and norbornadienes are interesting (Traylor, 1969). The reaction of these systems has been used as mechanistic probe to elucidate the mechanism of different reactions. Bromonorbornanes are important in the synthesis of bromonorbornadienes which may be used for the other substituted norbornadienes.

Halogenation of norbornadiene has been studied less extensively (Alvernhe et al., 1988; Gregorcic & Zupabn, 1977). Winstein has studied bromination of norbornadiene and has pointed out dangerous properties of the products (Winstein, 1961).

In the course of studying the bromination reactions of unsaturated bicyclic systems, we noticed that the reaction temperature has a dramatic influence on product distribution (Balcı et al., 1992a,b). In connection with our continuing work in the temperature bromination reactions, we carried out brominations in photolytic and high-temperature conditions to prevent the skeletal rearrangement, i.e. to obtain normal addition products.

Addition of bromine to norbornadiene results in the products of Wagner–Meerwein rearrangement and homoallylic conjugation. Whereas, recently we succeeded in obtaining normal addition product, i.e. 2,3,5,6-tetrabromonorbornanes (II), and developed a synthetic methodology leading to dibromonorbornadiene by dehydrobromination of (II) (Tutar et al., 1996). We are now interested in photobromination of tetrabromonorbornanes (II) to obtain further brominated norbornanes and norbornane derivatives.

Bromination of norbornadiene at reflux temperature of solvent (CCl₄) forms three diasteremeric 2,3,5,6-tetrabromonorbornanes [TBN, (II)] and two 3,5-dibromonortricyclane [DBN, (III)] which are easily isolated by simple distillation in two parts in a yield of 40 and 54%, respectively (Tutar et al., 1996). In an initial experiment, photobromination of (II) was carried out at room temperature. After crystallization and combined silica-gel column chromatography eluting with hexane, we isolated three compounds. Because of the very close similarity, we were not able to make a clear-cut differentation between stereochemistries in any of these materials containing five Br atoms. Therefore, we carried out the structure determination of compound (3).

The molecule (Fig. 1) contains the norbornane skeleton, composed of two fused five-membered rings in envelope conformation or of a six-membered ring held in a boat conformation by a bridging methylene group. There is an approximately tetrahedral environment about the C4 atom, but the Br4—C4—Br5 [105.4 (5)°] angle is smaller and Br5—C4—C3 [116.6 (7)°] angle is larger than the conventional value of the tetrahedral angle. The C4—C3—Br3 [117.9 (7)°] angle is also larger than the other angles about C3. This behaviour appears to be a result of a repulsive interaction between Br3 and Br5 atoms. Similarly, the Br1—C1—C2 [117.4 (7)°] and Br2—C2—C1 [117.3 (8)°] angles are larger than the other angles about C1 and C2, respectively, probably because of the repulsion between Br1 and Br2 atoms. The Br—C bond lengths are nearly equal and average 1.95 (1) Å. The C—C single bond lengths range from 1.50 (1) to 1.58 (2) Å, with a mean value of 1.54 (2) Å.

The topology of the norbornane moieties in the title compound, (I), is consistent with that reported in other norbornanes. The value of the C5—C7—C6 [95.9 (9)°] angle is markedly different from the tetrahedral value. In other norbornane derivatives, the corresponding angle is reported as 93.3 (8)° in exo,exo-2,3-endo,endo-5,6-tetrabromobicycloheptane (Hökelek et al., 1998), 94.3 (7)° in 2,2-exo-3,5,5-exo-6-hexabromobicycloheptane (Akkurt et al., 2000) and 101.0 (9)° in endo,exo-9,11-dibromotricyclo-[6.3.1.0^2,7]dodeca-2(7),3,5-triene-10-one (Büyükgüngör et al., 1989).

The Br—C—C bond angles are between 106.8 (7) and 117.9 (7)°, with an average value of 112.6 (7)°, which is 111.2 (3)° in exo,endo,endo-9,9,10,11,12-pentabromotricyclo[6.2.2.0^2,7]dodeca-2(7),3,5- triene (Hökelek et al., 1990), 115.1 (6)° in exo,exo-9,10,12-tribromotricyclo[6.3.1.0^2,7]dodeca-2(7),3,5,10-tetraene (Hökelek et al., 1991) and 113.9 (7)° in exo,exo-2,3-endo,endo-5,6-tetrabromobicycloheptane (Hökelek et al., 1998).

The deviations of the Br1, Br2, Br3, Br4 and Br5 atoms from the best least-squares plane passing through the C1/C2/C3/C4 atoms are 0.827 (1), 0.781 (1), 0.603 (2), -1.828 (1) and 0.883 (1) Å, respectively. An examination of the deviations from the least-squares planes through the individual fragments shows that the fragments A(C5—C1—C2—C6) and B(C5—C4—C3—C6) are nearly planar. The dihedral angles between A, B and C(C5,C6,C7) are A/B = 68.3 (5), A/C = 56.1 (7) and B/C = 55.7 (6)°.

The strain in the structures containing one atom on the bridge is more predominant than in structures containing two carbon atoms on the bridge (Hökelek et al., 1990).

S2. Experimental

A solution of excess Br₂ (4 equivalents) was added to a solution of tetrabromonorbornadiene (4.12 g, 10.0 mmol) in CCl₄ (40 ml), in an internal-type photochemical reaction apparatus. The reaction mixture was irradiated with projector lamp (350 W) and reaction progress was monitored by NMR. The resulting mixture was crystallized from methylene chloride–hexane and chromatographed with silica gel, eluting with hexane. Compound (III) was isolated in a yield of 4% (196 mg). It was recrystallized from a methylene chloride–petroleum ether mixture over a period of 12 h (m.p. 403 K).

S3. Refinement

The positions of the H atoms were calculated geometrically at distances of 0.98 (CH) and 0.97 Å (CH₂) from the corresponding atoms, and a riding model was used during the refinement process.

Computing details

Data collection: MolEN (Fair, 1990); cell refinement: MolEN; data reduction: MolEN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976); software used to prepare material for publication: MolEN.

Figures

Fig. 1. An ORTEPII (Johnson, 1976) drawing of the title molecule with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level.

(I)

Crystal data

C7H7Br5	F(000) = 896
Mr = 490.63	Dx = 2.888 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
a = 6.765 (2) Å	Cell parameters from 25 reflections
b = 13.822 (2) Å	θ = 10–18°
c = 12.114 (3) Å	µ = 17.75 mm^−1
β = 94.92 (2)°	T = 294 K
V = 1128.6 (5) Å^3	Rod-shaped, colorless
Z = 4	0.30 × 0.10 × 0.10 mm

Data collection

Enraf-Nonius CAD-4 diffractometer	1112 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.015
Graphite monochromator	θmax = 26.3°, θmin = 3.4°
ω/2θ scans	h = 0→8
Absorption correction: ψ scan (MolEN; Fair, 1990)	k = 0→17
Tmin = 0.137, Tmax = 0.169	l = −15→15
2599 measured reflections	3 standard reflections every 120 min
2291 independent reflections	intensity decay: 1%

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)] = 0.064	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.133	H-atom parameters constrained
S = 0.98	w = 1/[σ^2(Fo^2) + (0.0557P)^2] where P = (Fo^2 + 2Fc^2)/3
2201 reflections	(Δ/σ)max < 0.001
111 parameters	Δρmax = 1.04 e Å^−3
0 restraints	Δρmin = −0.91 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.

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
Br1	−0.57855 (18)	0.31259 (9)	0.66308 (10)	0.0379 (4)
Br2	−0.5429 (2)	0.14816 (10)	0.46669 (11)	0.0510 (5)
Br3	0.1130 (2)	−0.01854 (11)	0.71188 (14)	0.0572 (5)
Br4	−0.32120 (19)	0.06880 (10)	0.92682 (11)	0.0430 (4)
Br5	0.09378 (19)	0.15630 (11)	0.89648 (11)	0.0479 (4)
C1	−0.4579 (15)	0.1892 (8)	0.7079 (9)	0.023 (3)
H11	−0.5304	0.1606	0.7665	0.03 (3)*
C2	−0.4354 (17)	0.1148 (9)	0.6183 (9)	0.032 (3)
H21	−0.4934	0.0535	0.6400	0.01 (3)*
C3	−0.1403 (15)	0.0443 (9)	0.7237 (9)	0.030 (3)
H31	−0.2399	−0.0060	0.7325	0.01 (3)*
C4	−0.1545 (15)	0.1181 (8)	0.8148 (9)	0.023 (3)*
C5	−0.2389 (14)	0.2093 (8)	0.7539 (8)	0.022 (3)
H51	−0.2193	0.2696	0.7959	0.04 (3)*
C6	−0.2041 (16)	0.1022 (9)	0.6169 (9)	0.031 (3)
H61	−0.1580	0.0767	0.5482	0.02 (3)*
C7	−0.1331 (15)	0.2048 (9)	0.6499 (10)	0.032 (3)
H71	0.0099	0.2089	0.6644	0.08 (4)*
H72	−0.1799	0.2533	0.5959	0.08 (5)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Br1	0.0452 (8)	0.0306 (7)	0.0368 (7)	0.0158 (6)	−0.0030 (6)	0.0002 (7)
Br2	0.0747 (10)	0.0388 (8)	0.0343 (8)	0.0099 (8)	−0.0250 (7)	−0.0086 (7)
Br3	0.0438 (8)	0.0496 (9)	0.0779 (11)	0.0250 (7)	0.0029 (7)	−0.0085 (9)
Br4	0.0537 (8)	0.0409 (8)	0.0360 (7)	−0.0030 (7)	0.0119 (6)	0.0148 (7)
Br5	0.0360 (7)	0.0543 (9)	0.0501 (9)	−0.0046 (7)	−0.0163 (6)	−0.0038 (8)
C1	0.026 (6)	0.018 (6)	0.026 (6)	−0.001 (5)	0.005 (5)	−0.007 (6)
C2	0.042 (7)	0.029 (7)	0.024 (7)	−0.003 (6)	−0.003 (5)	0.009 (6)
C3	0.018 (6)	0.039 (8)	0.032 (7)	−0.002 (6)	−0.008 (5)	−0.005 (6)
C5	0.028 (6)	0.017 (6)	0.020 (6)	0.004 (5)	0.003 (5)	0.000 (6)
C6	0.042 (7)	0.037 (8)	0.018 (6)	0.004 (6)	0.016 (5)	0.000 (6)
C7	0.016 (6)	0.036 (8)	0.044 (8)	−0.009 (5)	0.003 (5)	0.002 (7)

Geometric parameters (Å, º)

Br1—C1	1.947 (11)	C3—C4	1.512 (15)
Br2—C2	1.971 (11)	C3—C6	1.551 (15)
Br3—C3	1.937 (11)	C3—H31	0.9800
Br4—C4	1.959 (11)	C4—C5	1.546 (14)
Br5—C4	1.948 (10)	C5—C7	1.503 (14)
C1—C2	1.513 (15)	C5—H51	0.9800
C1—C5	1.562 (13)	C6—C7	1.539 (16)
C1—H11	0.9800	C6—H61	0.9800
C2—C6	1.576 (15)	C7—H71	0.9700
C2—H21	0.9800	C7—H72	0.9700
C2—C1—C5	103.1 (8)	C3—C4—Br4	110.3 (7)
C2—C1—Br1	117.4 (7)	C5—C4—Br4	113.8 (7)
C5—C1—Br1	107.8 (7)	Br5—C4—Br4	105.4 (5)
C2—C1—H11	109.4	C7—C5—C1	101.5 (8)
C5—C1—H11	109.4	C7—C5—C4	100.6 (9)
Br1—C1—H11	109.4	C1—C5—C4	109.0 (8)
C1—C2—C6	104.1 (9)	C7—C5—H51	114.7
C1—C2—Br2	117.3 (8)	C1—C5—H51	114.7
C6—C2—Br2	107.7 (8)	C4—C5—H51	114.7
C1—C2—H21	109.2	C7—C6—C3	101.6 (9)
C6—C2—H21	109.2	C7—C6—C2	100.6 (9)
Br2—C2—H21	109.2	C3—C6—C2	104.7 (9)
C4—C3—C6	103.3 (9)	C7—C6—H61	115.9
C4—C3—Br3	117.9 (7)	C3—C6—H61	115.9
C6—C3—Br3	110.7 (8)	C2—C6—H61	115.9
C4—C3—H31	108.2	C5—C7—C6	95.5 (9)
C6—C3—H31	108.2	C5—C7—H71	112.7
Br3—C3—H31	108.2	C6—C7—H71	112.7
C3—C4—C5	104.2 (9)	C5—C7—H72	112.7
C3—C4—Br5	116.6 (7)	C6—C7—H72	112.7
C5—C4—Br5	106.8 (7)	H71—C7—H72	110.1
C5—C1—C2—C6	−2.5 (10)	Br4—C4—C5—C7	−158.3 (7)
Br1—C1—C2—C6	115.8 (8)	C3—C4—C5—C1	68.1 (10)
C5—C1—C2—Br2	−121.3 (8)	Br5—C4—C5—C1	−168.0 (6)
Br1—C1—C2—Br2	−3.0 (12)	Br4—C4—C5—C1	−52.2 (10)
C6—C3—C4—C5	3.9 (10)	C4—C3—C6—C7	30.7 (10)
Br3—C3—C4—C5	126.4 (8)	Br3—C3—C6—C7	−96.4 (9)
C6—C3—C4—Br5	−113.5 (8)	C4—C3—C6—C2	−73.6 (10)
Br3—C3—C4—Br5	9.0 (12)	Br3—C3—C6—C2	159.2 (7)
C6—C3—C4—Br4	126.4 (7)	C1—C2—C6—C7	−31.9 (10)
Br3—C3—C4—Br4	−111.1 (8)	Br2—C2—C6—C7	93.2 (9)
C2—C1—C5—C7	37.3 (11)	C1—C2—C6—C3	73.2 (11)
Br1—C1—C5—C7	−87.5 (9)	Br2—C2—C6—C3	−161.7 (7)
C2—C1—C5—C4	−68.3 (10)	C1—C5—C7—C6	−56.4 (10)
Br1—C1—C5—C4	166.9 (7)	C4—C5—C7—C6	55.7 (9)
C3—C4—C5—C7	−38.1 (10)	C3—C6—C7—C5	−53.6 (9)
Br5—C4—C5—C7	85.9 (8)	C2—C6—C7—C5	54.0 (9)