5′,11′-Dihydrodispiro[cyclohexane-1,6′-indolo[3,2-b]carbazole-12′,1′′-cyclohexane]

The title compound, C28H30N2, is a symmetrical 2:2 product from the condensation of indole and cyclohexanone. It is the only reported 5,11-dihydroindolo[3,2-b]carbazole compound in which the spiro atoms are quaternary C atoms. Crystals were grown by vapor diffusion in a three-zone electric furnace. The molecule resides on a crystallographic inversion center. The cyclohexyl rings are in a slightly distorted chair conformation, whereas the indole units and the spiro-carbons are coplanar within 0.014 Å.

The title compound, C 28 H 30 N 2 , is a symmetrical 2:2 product from the condensation of indole and cyclohexanone. It is the only reported 5,11-dihydroindolo[3,2-b]carbazole compound in which the spiro atoms are quaternary C atoms. Crystals were grown by vapor diffusion in a three-zone electric furnace. The molecule resides on a crystallographic inversion center. The cyclohexyl rings are in a slightly distorted chair conformation, whereas the indole units and the spiro-carbons are coplanar within 0.014 Å .

Experimental
EC thanks Professor Wayland E. Noland for his assistance in initiating this project.

Comment
Condensations of indole with cyclohexanone yield a variety of products, often 1:1 or 1:2 structures in which the carbonyl carbon is attached to the 3-position of one or more indole moieties (Yadav et al., 2001). These products are often useful as pharmaceutical intermediates, and a variety of catalysts and reaction conditions have been explored. A particularly interesting example of indole-ketone condensation chemistry is vinylindole formation followed by the Diels-Alder reaction (Noland et al., 1993). While investigating this reaction we isolated highly symmetrical 2:2 condensation products from preparations containing indole and a cyclic ketone (cyclohexanone, tert-butyl cyclohexanone, or cyclopentanone) using hydrochloric acid as a catalyst. These 2:2 condensation products possess unusual physical properties for this class of compound, including limited solubility in most solvents, decomposition without melting at temperatures over 300 °C, and fluorescence despite the absence of extended intramolecular conjugation. Due to the lack of a suitable recrystallization solvent we employed the vapor-phase diffusion approach of Kloc et al. (1997) Data mining of the Cambridge Structural Database (CSD; November 2011 update; Allen, 2002) revealed that (I) is the only crystallographically characterized condensation product of indole and cyclohexanone in which the spiro atoms are quaternary carbon atoms.
The molecule of (I) resides on a crystallographic inversion center. The amino hydrogen atoms do not participate in hydrogen bonding interactions due to the lack of acceptors. In contrast, the related compound trans-6, 12-diphenyl-5,6,11,12tetrahydroindolo[3,2-b]carbazole dimethyl sulfoxide tetrahydrofuran solvate (II) which also has hydrogen atoms on nitrogen atoms forms hydrogen bonding interactions with the oxygen atoms of the dimethyl sulfoxide solvent molecules (Gu et al., 2009).
A Mogul (Bruno et al., 2002) structural check confirmed that the geometrical parameters of (I) are typical except for the C10-C9-C14 angle. The latter measures 111.20 (10)° and is more obtuse than the average angle of 108.3 (9)° computed for related compounds. The difference is statistically significant. The two cyclohexyl substituents most closely resemble a chair conformation. The 5,11-dihydroindolo[3,2-b]carbazole core is planar within 0.0142 Å.

Experimental
Indole (3 g) was dissolved in 25 ml of cyclohexanone. Approximately 0.25 ml of concentrated HCl was added and the mixture was stirred at room temperature for 7-14 days. The resulting pink-white precipitate was isolated using vacuum filtration and washed in refluxing acetonitrile for 60 min.
In an alternative preparation, indole (3 g) and cyclohexanone (2.5 g) were dissolved in 25 ml of acetonitrile. Approximately 0.1 ml of concentrated HCl was added and the mixture was heated to reflux for 24 h. The product was recovered from an accompanying intractable tarry material by washing with acetone followed by reflux in fresh acetonitrile.
supplementary materials sup-2 Crystals were grown in a three-zone electric furnace. A 1 g sample of the material was placed on a microscope cover slip and inserted into a 25 mm quartz tube. The tube was placed in the furnace and connected to a supply of argon. The argon flow was adjusted to 2 ml/min, the tube was purged of air and heated in three zones. The first zone contained the initial sample and was heated to 308-310 °C to promote volatilization. The second zone was the region of molecular transport and was heated to 280-290 °C. The third zone was heated to 200 °C to encourage crystal deposition. These heating and gas flow conditions yielded needle-shaped crystals approximately 500 µm in the short dimensions and 10-15 mm long over 14 h.

Refinement
All H-atoms attached to carbon atoms were placed in idealized locations and refined as riding with appropriate thermal displacement coefficients Uĩso(H) = 1.2 times Ueq(bearing atom). Default effective X-H distances for T = -173.0°C C(sp 3 )-2H=0.99, C(sp 2 )-H=0.95. The hydrogen atom attached to N1 was located in the difference map and refined independently. Fig. 1. Molecular structure of (I) (Brandenburg, 1999). Displacement ellipsoids are shown at the 50% probability level and C-bound H atoms are omitted. The compound resides on a crystallographic inversion center. Symmetry transformations used to generate equivalent atoms: (i) -x + 1,-y,-z.