2,11-Dibromo-5,8-dibutyl[4]helicene

A racemic mixture of the title compound, C26H26Br2, a brominated [4]helicene, crystallizes, forming columns of stacked molecules. There are two crystallographically unique molecules in the asymmetric unit, both with the same helical handedness. As is typical with helicene congeners, the unique molecules show short interatomic contacts between H atoms at the fjord region, with H⋯H distances of 1.87 and 1.94 Å. Molecules with the same helical handedness segregate in the crystal packing, forming homochiral columns. The stacked molecules are piled in a column with alternate orientations. The shortest C⋯C distance in the stacked molecules is 3.306 (4) Å.

A racemic mixture of the title compound, C 26 H 26 Br 2 , a brominated [4]helicene, crystallizes, forming columns of stacked molecules. There are two crystallographically unique molecules in the asymmetric unit, both with the same helical handedness. As is typical with helicene congeners, the unique molecules show short interatomic contacts between H atoms at the fjord region, with HÁ Á ÁH distances of 1.87 and 1.94 Å . Molecules with the same helical handedness segregate in the crystal packing, forming homochiral columns. The stacked molecules are piled in a column with alternate orientations. The shortest CÁ Á ÁC distance in the stacked molecules is 3.306 (4) Å . This study was partly supported by KAKENHI (21685005, 20108015, 22550094) and the Asahi Glass Foundation. We thank Professor T. Iwamoto (Tohoku University) for the X-ray instrument and JEOL for the DART MS instrument. A generous gift of HFIP from Central Glass Co. is also gratefully acknowledged.

Related literature
substituents for the stacking assembly. However, various forms of the stacking structures are found even among these rare examples. We found that additional effects from the substituents other than the halogen can affect the packing structure.
The asymmetric unit of the title compound is shown in Fig. 1. A set of alternate molecular pairs in a column is observed as non-equivalent molecules in the asymmetric unit. The interatomic distances between the hydrogen atoms at the fjord region are 1.87 Å and 1.94 Å. The packing structures are shown in Fig. 2 and 3. Each of the enantiomers segregate to form homochiral columns (Fig. 2). Unlike 5,8-dimethyl derivative that formed the stack with a synchronized orientation (Isobe et al., 2009), the title compound with 5,8-dibutyl substituents formed the stack with an alternating orientation ( Fig. 3) with the shortest intermolecular C···C distance of 3.306 (4) Å between C1 and C42 ( Fig. 1).

Experimental
The title compound was synthesized from difluoroalkene through acid-mediated intramolecular cyclization and dehydrogenative aromatization as reported in the literatures (Ichikawa et al., 2008;Nakanishi et al., 2011). A single-crystal suitable for X-ray crystallographic analysis was obtained by slow evaporation from a mixture of hexane and 2-propanol.

Refinement
H atoms were included in calculated positions and treated as riding atoms, with C-H = 0.95, 0.99 and 0.98 Å for CH (aromatic), CH 2 and CH 3 , respectively, with U iso (H) = 1.2U eq (C) for CH (aromatic) and CH 2 , and U iso (H) = 1.5U eq (C) for CH 3 .

Figure 1
Molecular structures of the title compound with displacement ellipsoids drawn at the 50% probability level. Two nonequivalent molecules for (P)-form are shown. Note that the two structures differ only slightly at the methylene chain and that the enantiomeric (M)-form can be found as the mirror geometries.

Figure 2
Packing structure of the title compound, viewed along the b axis. The carbon atoms of enantiomers are colored differently. Color code: C for (P)-form = green, C for (M)-form = blue, Br = brown. Hydrogen atoms are omitted for clarity. Note that one colomn comprise single enantiomeric molecules with an alternate stack of two crystallographically unique molecules.  Stacking structure of the (P)-form. Hydrogen atoms are omitted for clarity.

2,11-Dibromo-5,8-dibutyl[4]helicene
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 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 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.