2-exo,5-endo,8,8,10-Pentachlorobornane

The title compound, C10H13Cl5, is a polychlorinated monoterpene and a Toxaphene congener. This compound is also the only pentachlorinated derivative of camphene formed via ionic chlorination. Previously, the title compound was thought to be 2-exo,5-endo,9,9,10-pentachlorobornane, but X-ray structural analysis showed it to have a different structure and rather to be 2-exo,5-endo,8,8,10-pentachlorobornane. The title compound shows static disorder and almost half the molecule was divided in two partitions with an occupancy ratio of 0.575 (major) to 0.425 (minor). The repulsive close contacts of Cl atoms could possibly be the cause for this disorder.

The title compound, C 10 H 13 Cl 5 , is a polychlorinated monoterpene and a Toxaphene congener. This compound is also the only pentachlorinated derivative of camphene formed via ionic chlorination. Previously, the title compound was thought to be 2-exo,5-endo,9,9,10-pentachlorobornane, but X-ray structural analysis showed it to have a different structure and rather to be 2-exo,5-endo,8,8,10-pentachlorobornane. The title compound shows static disorder and almost half the molecule was divided in two partitions with an occupancy ratio of 0.575 (major) to 0.425 (minor). The repulsive close contacts of Cl atoms could possibly be the cause for this disorder.

Comment
The title compound (Fig 1) of this study is a polychlorinated monoterpene and a Toxaphene congener. It is prepared via ionic chlorination of 6-exo-chlorocamphene in solution in carbon tetrachloride, followed by chlorination of intermediate dichlorocamphene in presence of Lewis acid (Fig 2). The last transformation obviously involves a series of carbocationic rearrangements, but details of the exact mechanism remain unknown. Based on analytical and mechanistic data the compound was previously proposed to be 2-exo, 5-endo,9,9,10-pentachlorobornane (Nikiforov et al., 2001;Nikiforov et al., 2000;Nikiforov et al., 1999), but this was not verified by crystallographic data and so we decided to perform a detailed single-crystal difffraction analysis to unambiguously identify the nature of the pentachlorinated compound. The structure obtained, however, was found to be different from the previously suggested one. The dichloromethyl and methyl groups at carbon atom C7 exhibit a different orientation than previously presumed for 2-exo, 5-endo,9,9,10-pentachlorobornane, and the correct name of the title compound should thus be 2-exo, 5-endo,8,8,10-pentachlorobornane. There are only few weak intermolecular C-H···Cl and Cl···Cl contacts found in the structure. The static disorder of the present structure may be a consequence of intermolecular repulsive interactions between chlorines, found between two major components, but no unarguable evidence of this was found from the close contacts. However, the major component presented in Fig 1 has a close Cl···Cl contact of 3.49 Å between Cl2 and Cl5 of its enantiomer at −x, 2 − y, 1 − z. The same contanct is found also between Cl2 of the enantiomer and Cl5 in Fig 1, but between the minor components the contact (Cl2···Cl5b) is a bit longer (3.60 Å). The second difference found in the close contacts between major and minor components was another similar double interaction of major components between Cl5 and H10A at −x, 1 − y, 1 − z. This weak contact (2.59 Å) was not found between the minor components, in which the distance Cl5b···H10c is 3.60 Å.

Experimental
The title compound was prepared via the following steps presented in Fig 2. First 6-exo-chlorocamphene was prepared according to a literature method (Jennings & Herschbach, 1965). Then crude 6-exo-chlorocamphene (5 g, 29 mmol) was dissolved in 50 ml of carbon tetrachloride. Chlorine gas was passed through this solution with stirring. After the solution obtained showed a persistent green colour, SnCl 4 (1 g, 4 mmol) was added to the reaction mixture and passing of chlorine and stirring was continued for 4 h. The reaction mixture was washed with water and dried over calcium chloride. The solvent was removed in vacuo and the residue crystallized twice from hexane. The yield of the title compound was 2.2 g (25%, The title compound shows static disorder and it was necessary to divide almost half the molecule in two partitions (Fig 3) as well as to use a large number of restraints (see below). The low data quality seems to be, however, not directly linked to the disorder. Refinement with or without the disorder taken into account (the latter with unacceptably asymmetric thermal ellipsoids) did result in nearly the same refined R values. We thus decided to test for various types of twinning, but the crystals did not appear to be twinned. DIRAX (Duisenberg, 1992) found the correct cell with less than one hundred fitting reflections. With relaxed conditions more than three hundred reflections were fitting the lattice. The same cell was also found using reflections that did not fit the first unit cell found using the strict values, but these lattices were rotated with respect to the first by less than 2 ° and attempts to refine the structure as non-merohedrally twinned using the hklf 5 routine failed.
Visually the crystals seem to be of good quality with no evident fragmentation, but some fragmentation can be seen when cutting the large crystals The multiple unit cells found have thus been attributed to fragmentation of the single-crystal rather than non merohedral twinning. Several data collection endeavours with different crystals resulted in very similar results.
Very large and asymmetric thermal ellipsoids for several atoms indicated static disorder and atoms Cl1, Cl5, C1, C2, C3 and C10 and hydrogen atoms bonded to C2, C3, C4, C6 and C10 were refined as disordered over two partially occupied positions (Fig 3), with an occupancy ratio of 0.575 (major) to 0.425 (minor). The interatomic distances of non-hydrogen atoms of both partitions were restrained to be similar (SADI restraints with default standard deviations). The ADPs of atoms C1, C2, C3, C4, C6, C10, Cl1 and Cl2 (major) and the corresponding atoms of the minor component were restrained to be similar to those of their neighbors (SIMU and DELU restraints with default standard deviations). The ADPs of the disordered atoms were also restrained to be close to isotropic (ISOR restraints with s equal to 0.01 for C1, C2, C3, C10 (major), C1b, C2b, C3b and C10b (minor) and equal to 0.1 for Cl1, Cl5, Cl1b and Cl5b). The anisotropic displacement parameters of C1 and C1b set to be identical.
All H atoms were visible in electron density maps, but were placed in idealized positions and allowed to ride on their parent atoms at C-H distances of 0.98 (methyl), 0.99 (methylene) and 1.00 Å (methine), with U iso (H) of 1.5 (methyl) and 1.2 times U eq (C).

Special details
Geometry. All e.s. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2sigma(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.