Bicyclo[2.2.1]hept-2-en-7-yl 4-bromobenzoate

The structure of the title compound, C14H13BrO2, which contains a norbornenyl group and a 4-bromobenzoate ester at the single C-atom bridge, has been redetermined [see McDonald & Trotter (1965 ▶). Acta Cryst. 19, 456–463] to modern standards to establish high-precision geometrical data to compare with norbornyl and other tetracyclic 4-bromobenzoates. Possible structural evidence is sought to help explain solvolytic reactivities.


Related literature
For the previous structure determination of the title compound, see: McDonald & Trotter (1965). For a discussion, see: Coots (1983); Lloyd et al. (1995). For an analogous pnitrobenzoate structure, see: Jones et al. (1992). For related tetracyclic 4-bromobenzoate structures, see: Lloyd et al. (2000) and references therein. For a theoretical discussion, solvolysis rates and molecular orbital calculations, see: Chow (1998). For further synthetic details, see: Coots (1983); Lloyd et al. (1993). Considerably improved precision is obtained for the present, low temperature structure of the title compound 1 over earlier structures. An ORTEP-3 drawing of 1 is shown in Fig. 1, and a cell packing diagram is shown in Fig. 2.
The 2:4 angle shows that C2 and C3 are pyramidalized similarly as in other norbornenyl containing 4-bromobenzoate structures. The larger 1:2 and smaller 1:3 angle in 1 versus 2 may be a consequence of substituting an etheno bridge for an ethano bridge. The C1-C2, C2=C3, and C3-C4 bonds are shorter in 1 versus 2 as expected, but C1-C7 and C4-C7 are longer in 1 than in 2. These longer bonds possibly compensate for what might otherwise be even closer C2···C7 and C3···C7 intramolecular contacts in 1 (Table 3). A wider 1:2 angle in 1 versus 2 should also help relieve these contacts.

Experimental
Anti-7-norbornenyl 4-bromobenzoate (title compound 1) was prepared (Coots, 1983) from anti-7-norbornenol, which was made from 7-norbornenone reduction (Lloyd et al., 1993, and references therein). In 25 ml of freshly distilled (from KOH under N 2 ) dry pyridine was dissolved 0.700 g of sublimed (373 K, 1600 Pa) anti-7-norbornenol and 1.80 g of sublimed (373 K, 7 Pa) 4-bromobenzoyl chloride was added with stirring. The mixture was heated to 373 K for 5 min and set in a refrigerator overnight. The mixture was poured into 100 ml of cold water, and extracted three times with 100 ml of ether.

Refinement
A colorless prism shaped crystal 0.35 × 0.33 × 0.30 mm in size was mounted on a glass fiber with traces of viscous oil and then transferred to a Nonius KappaCCD diffractometer equipped with Mo Kα radiation (λ = 0.71073 Å). Ten frames of data were collected at 150 (1) K with an oscillation range of 1 °/frame and an exposure time of 20 sec/frame (Nonius, 1998). Indexing and unit cell refinement based on all observed reflections from those ten frames, indicated a monoclinic P lattice. A total of 5345 reflections (Θ max = 27.46°) were indexed, integrated and corrected for Lorentz, polarization and absorption effects using DENZO- SMN and SCALEPAC (Otwinowski & Minor, 1997). Post refinement of the unit cell gave a = 14.0633 (2)    Packing diagram for the title compound.

Figure 3
Compounds 1 and 2. Special details Experimental. The program DENZO-SMN (Otwinowski & Minor, 1997) uses a scaling algorithm which effectively corrects for absorption effects. High redundancy data were used in the scaling program hence the 'multi-scan′ code word was used. No transmission coefficients are available from the program (only scale factors for each frame). The scale factors in the experimental table are calculated from the 'size′ command in the SHELXL-97 input file. 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 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 > 2σ(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.