exo-10,11-Dibromotricyclo[6.3.1.02,7]dodeca-2,4,6,9-tetraene

The title compound, C12H10Br2, is a bridged ring system based on a homobenzonorbornadiene framework. The exo configuration of one of the Br atoms was previously assigned via NMR correlations and has now been confirmed by the geometry of the solid-state structure. The compound features a Br—C—C—Br torsion angle of 66.68 (12)°, whereby the C atoms in the calculation are respectively sp 3- and sp 2-hybridized.

The title compound, C 12 H 10 Br 2 , is a bridged ring system based on a homobenzonorbornadiene framework. The exo configuration of one of the Br atoms was previously assigned via NMR correlations and has now been confirmed by the geometry of the solid-state structure. The compound features a Br-C-C-Br torsion angle of 66.68 (12) , whereby the C atoms in the calculation are respectively sp 3 -and sp 2hybridized.

Comment
In the title molecule (1) ( Fig. 1), an exo-configuration of Br2, which was originally assigned from NMR correlations (Kitahonoki et al., 1969), has now been confirmed by the X-ray data. As such, the stereochemistry of the two enantiomers that assemble in each unit cell can be unambiguously assigned as RRR and SSS (Fig. 2).
A dihedral angle of 66.7 (1)° was calculated for the Br1-C10-C11-Br2 torsion between the exo-configured C11-Br2 and the sp 2 hybridized C10-Br1. Additionally, the compound contains a six-membered ring (C8-C9-C10-C11-C1-C12) that exhibits an interesting envelope-type conformation. This conformation matches that observed in other structures based on a homobenzonorbornadiene framework (Balci et al., 1996). The structure reported by Hokelek et al. (1991) differs from 1 only by the presence of a third bromine atom attached to C12 and directed towards the cyclohexene ring (i.e. replacing H12B). However, the precision in C-C bond distances in 1 is on average five times better resulting in a considerably more reliable geometry than that reported for the tribromide.
Compound 1 has proven to be a convenient starting material for the preparation of a variety of substituted benzobarrelenes (Cakmak & Balci, 1989). For example, 1 can be readily converted to 2-bromo-3-deuteriobenzobarrelene (2) over several synthetic steps (Bender et al., 2003) as outlined in Figure 3.

Experimental
The dibromide (1) was prepared in a one-step process (Kitahonoki et al., 1969) from the addition of dibromocarbene to benzonorbornadiene (3, Figure 3). The desired compound (1, m.p. 355-356 K) was obtained in 27% yield from the reaction mixture by distillation under reduced pressure and recrystallization from acetone-pentane.

Refinement
Although hydrogen atoms were visible on the Fourier map, they were included at geometrically idealized positions with isotropic displacement parameters and refined in riding mode on their parent atoms with distance constraints: C-H = 1.00, 0.99 and 0.95 Å for methine, methylene and aromatic-type H-atoms, respectively, and U iso (H) = 1.2U eq (C). The highest residual peak had a fraction of the electron density of a single H atom and was located 0.77 Å from Br1.    -3,4-dibromo-6,7-benzobicyclo[3.2.1]octa-2,6-diene (1) and its conversion to 2-bromo-3-deuteriobenzobarrelene (2).
10,11-dibromotricyclo[6.3.1.0 2,7 ]dodeca-2,4,6,9-tetraene 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 > σ(F 2 ) is used only for calculating Rfactors(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.