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Oxidative Substitution Reaction of 1-Amino-3,6-di-tert-butyl-9H-carbazole at 4 Position

4 position of 1-amino-3,6-di-tert-butyl-9H-carbazole, which was derived from carbazole through three steps, was oxidatively substituted and a carbazole-1,4-diimine type compound was obtained. The sterically crowded molecular structure of the substituted product was revealed by X-ray crystal structural analysis.

The aminated carbazole 1 was derived from 3,6-di-tert-butyl-1-nitro-9H-carbazole, which was synthesized by nitration of 3,6-di-tert-butyl-9H-carbazole ( Figure 2). The dialkylcarbazole was synthesized by Friedel-Crafts reaction of 9Hcarbazole and tert-butyl chloride catalysed by zinc (II) chloride [7]. For introduction of a nitro group on the heterocyclic compounds, AgNO 3 is sometimes useful as a nitration reagent and the alkylated carbazole was converted to the nitro compound. The nitro group was easily reduced to the amino group by simple catalytic reduction.
As an oxidative substitution reactant, N,N-diiodo-4-nitroaniline was used. Minakata and co-workers reported that the N,N-diiodo aniline derivatives can be obtained by treatment of aniline derivatives with sodium iodide and tert-butyl hypo chloride in THF, and the prepared iodinated compounds were used in situ [11][12][13].
When N,N-diiodo-4-nitroaniline was prepared from 4-nitroaniline and reacted with 1 at 0°C, the 1,4-diimine compound 2 was obtained ( Figure 1). Recrystallization of 2 with CH 2 Cl 2 and n-hexane afforded CH 2 Cl 2 -inclusion crystals. Sometimes the bulky group such as butyl group makes a cavity in the crystal lattice and a small molecule would be included there [14,15]. X-ray crystal structural analysis revealed the sterically crowded molecular structure of 2 and the guest molecule was observed in the lattice. As shown in Figure 3, the nitrogen atom attached to 4 positions is closed to the hydrogen atom on 5 position and the tert-butyl group on 3 position. The mesomeric effect of an amino group on 1 position and the inductive effect of a tert-butyl group on 3 position would enhance the distribution of HOMO on 4 position. Low reaction temperature inhibits a C-H stretching vibration of 5 position and a rotation of the tert-butyl groups. These electric and thermodynamic effects could allow the nitrogen atom of N,N-diiodo-4nitroaniline to attack at crowded 4 position of 1 (Figure 3).
In conclusion, 4 position of 1 was reacted with N,N-diiodo-4-nitroaniline and the oxidative substitution reaction afforded 2 as a carbazole-1,4-diimine compound. The sterically crowded molecular structure of 2 was revealed by X-ray crystal structural analysis. The effects of substituents of 1 and low reaction temperature could allow the reaction. The carbazole-1,4-diimine is rare structure and it would contribute the progress of studies on carbazole derivatives.

MEASUREMENTS
1 H NMR spectra were determined on a Bruker Advance III NMR spectrometer with CDCl 3 and CD 3 COCD 3 as solvents and TMS as internal standard (δ=0 ppm). The high resolution ESI-MS spectra were obtained using a Bruker micrOTOF-Q III.    [16]. Thermal ellipsoids are drawn at the 50% probability level.

3,6-Di-tert-butyl-N 4 -(4-nitrophenyl)-9H-carbazole-1,4-diimine (2)
Sodium iodide (300 mg, 2.0 mmol), p-nitroaniline (208 mg, 1.5 mmol) and 1 (149 mg, 0.51 mmol) were placed in a three neck round bottom flask. Under flowing of nitrogen gas, dried THF (6 mL) was added and attached an ice bath. Then tert-butyl hypochloride (0.23 mL, 2.0 mmol) was added and stirred for 4.5 hours. The reaction mixture was poured into aqueous sodium thiosulfate (1M, 20 mL) and extracted with ethyl acetate. The obtained organic layer was washed with brine, dried over MgSO 4 and concentrated under reduced pressure. The residue was purified by column chromatography on silica gel by eluting with a 1:1 mixture (v/v) of ethyl acetate and n-hexane. The collected fraction was evaporated. Recrystallization of the residue with CH 2 Cl 2 and n-hexane prepared 2 as a CH 2 Cl 2 -inclusion compound (56 mg, 0.12 mmol, 24%); the guest (CH 2 Cl 2 )/host (2) stoichiometric ratio was 1/4. For Z=2 and F.W.=1799.06, the calculated density is 1.193 g/cm 3 . Based on a statistical analysis of intensity distribution, and the successful solution and refinement of the structure, the space group was determined to be: P(-1). Of the 72482 reflections were collected, where 22787 were unique (Rint=0.0621); equivalent reflections were merged. Data were collected and processed using CrystalClear (Rigaku) [17]. The linear absorption coefficient, µ, for Mo-Kα radiation is 1.280 cm -1 . An empirical absorption correction was applied which resulted in transmission factors ranging from 0.902 to 0.994. The data were corrected for Lorentz and polarization effects.
Structure solution and refinement were carried out with the CrystalStructure [18] crystallographic software package except for refinement, which was performed using SHELXL Version 2014/7 [19]. The structure was solved by charge flipping method [20] and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The contributions of highly disordered dichloromethane molecules to the diffraction data were removed with the SQUEEZE procedure in PLATON [21].