Synthesis and acid-responsive electron-Transfer disproportionation of non- and tetramesityl-substituted 1,1',9,9'-bicarbazole

Article abstract of DOI:10.1246/cl.150557

Non-substituted 1,1',9,9'-bicarbazole and 3,3',6,6'-Tetramesityl- 1,1',9,9'-bicarbazole were synthesized through dimerization of carbazole derivatives and oxidative NN bond formation reaction. Non-substituted 1,1',9,9'-bicarbazole formed a stacking packin

Full text of DOI:10.1246/cl.150557

CL-150557
Received: June 6, 2015 | Accepted: June 26, 2015 | Web Released: October 5, 2015
Synthesis and Acid-responsive Electron-transfer Disproportionation
of Non- and Tetramesityl-substituted 1,1¤,9,9¤-Bicarbazole
Palash Pandit,1 Toshikazu Nakamura,1 and Shuhei Higashibayashi*1,2
1Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787
2Japan Science and Technology Agency, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
(E-mail: higashi@ims.ac.jp)
Non-substituted 1,1¤,9,9¤-bicarbazole and 3,3¤,6,6¤-tetrame-
cording to the reported procedure.1 t-Butyl groups of 3a were
removed by refluxing with AlCl3 in benzene, to afford the non-
substituted 3b in 65% yield. Oxidative coupling of 3b afforded
non-substituted 1b in 24% yield. 3,3¤,6,6¤-Tetramesityl-1,1¤,9,9¤-
bicarbazole 1c was synthesized from dibromocarbazole 42
(Scheme 2). Suzuki-Miyaura cross-coupling between 4 and
2,4,6-trimethylphenylboronic acid afforded dimesitylcarbazole
5, which was further converted to bromodimesitylcarbazole 6
by monobromination. Dimer 7 was obtained in 40% yield by
sityl-1,1¤,9,9¤-bicarbazole were synthesized through dimeriza-
tion of carbazole derivatives and oxidative N-N bond formation
reaction. Non-substituted 1,1¤,9,9¤-bicarbazole formed a stacking
packing structure in crystal. Both bicarbazoles were found to
undergo acid-responsive electron-transfer disproportionation.
The radical cation generated from the non-substituted 1,1¤,9,9¤-
bicarbazole was stable in solution under air at room temperature,
even without protecting bulky substituents.
3
oxidative coupling of 6 at nitrogen atom using KMnO4 in
acetone. Ni(0)-mediated reductive cyclization of 7 did not afford
the desired 1c but only 3c in 20% yield, with recovered 7, due
to the reductive cleavage of the N-N bond. The N-N bond of 7
seems to be weaker to the reductive condition than that of the
corresponding derivative with stronger electron-donating t-butyl
substituents.1 Thus, the C-C bond was first connected by Ni(0)-
Recently, we have reported the acid-responsive electron-
transfer disproportionation of a bicarbazole derivative 1a and a
biacridine derivative 2a with t-butyl substituents through the
acid-regulated ring-opening/closing reactions (Figure 1 and
Scheme 1).1 Since the acid-responsive property is a novel
finding, the scope and limitation of the compounds have not yet
been revealed fully. During the course of our study on 1a with t-
butyl substituents, we also investigated non-substituted bicarba-
zole 1b and 3,3¤,6,6¤-tetramesityl-substituted bicarbazole 1c,
and found that they also undergo the acid-responsive electron-
transfer disproportionation. Here, we report the syntheses of 1b
and 1c, the crystal structure of 1b, their acid-responsiveness, and
the stability of the radical cation of 1b.
Br
R
c
d
NH
NH
Me
(HO)2B
Me
Br
R
5 R=mesityl
4
Me
e
1b and 1c were synthesized as shown in Scheme 2.
3,3¤,6,6¤-Tetra-t-butyl-1,1¤-bicarbazole (3a) was prepared ac-
R
R
R
R
R
Br
Br
R
2
R
2'
NH
N
N
R
3
R
3'
4
4'
1
1'
Br
R
9
9'
N
N
6 R=mesityl
7 R=mesityl
N
N
10 10'
9
9'
g
f
8
8'
7
7'
6'
6
R
R
R
R
R
R
R
R
1a R=C(CH3)3
1b R=H
2a R=C(CH3)3
b
1c R=2,4,6-Me3C6H2
N
N
(mesityl)
NHHN
R
R
Figure 1. 1,1¤,9,9¤-Bicarbazole derivatives 1 and 4,4¤,10,10¤-biacri-
dine derivatives 2.
R
R
3a R=C(CH3)3
3b R=H
3c R=mesityl
1b R=H
1c R=mesityl
a
R
R
R
R
R
R
Acidification
Scheme 2. Reagents and conditions: (a) AlCl3 600 mol %, benzene,
80 °C, 3 h, 65%; (b) 3b, Bu4NMnO4 220 mol %, pyridine, 70 °C, 24 h,
24%, 3c, Bu4NMnO4 200 mol %, pyridine, 70 °C, 24 h, 49%; (c) 2,4,6-
Me3C6H2B(OH)2 300 mol %, [Pd(PPh3)4] 2 mol %, Cs2CO3 300 mol %,
toluene:EtOH:H2O = 3:1:1, 100 °C, 24 h, 68%; (d) N-bromosuccin-
imide 110 mol %, SiO2, CH2Cl2, rt, 6 h, 90%; (e) KMnO4 250 mol %,
acetone, 60 °C, 6 h, 40%; (f) Ni(1,5-cyclooctadiene)2 150 mol %, 1,5-
cyclooctadiene 150 mol %, 2,2¤-bipyridyl 150 mol %, THF, 55 °C,
24 h; (g) Ni(1,5-cyclooctadiene)2 300 mol %, 1,5-cyclooctadiene
300 mol %, 2,2¤-bipyridyl 300 mol %, THF, 80 °C, 3 h, 89%.
3 CF3CO2H
+
3
2
N
N
N
N
NH HN
H
R
R
R
R
R
R
CF3CO2
CF3CO2
1
-
1
•+ CF3CO2
3·CF3CO2H
Scheme 1. Acid-responsive electron-transfer disproportionation of
bicarbazole 1.
1336 | Chem. Lett. 2015, 44, 1336–1338 | doi:10.1246/cl.150557
© 2015 The Chemical Society of Japan

a
top view
b
0 mol% TFA
1000 mol% TFA
5000 mol% TFA
10000 mol% TFA
15000 mol% TFA
450 nm
503 nm
0.4
0.3
0.2
0.1
0
621 nm
side view
9'
9
8a'
9
9'
8a
8'
8a
8a'
8
8
8'
400 500 600 700
Wavelength /nm
400 500 600 700 800 900 1000
Wavelength /nm
Figure 2. ORTEP drawings of 1b at 50% probability level obtained
by X-ray crystallographic analysis.
Figure 4. (a) UV-vis-NIR spectra of 1b (0.05 mM) and 1b with
1000, 5000, 10000, and 15000 mol % CF3CO2H in CH2Cl2. (b)
Emission spectrum of 1b (0.02 mM) in CH2Cl2 (excited at 450 nm).
top view
side view
b
b
experiment
simulation
experiment
simulation
a
b
c
a
P
M
P
M
340
342
344
346
348
340
342
344
346
348
Magnetic Field / mT
Magnetic Field / mT
Figure 5. (a) ESR spectrum of 1b (1.0 mM) with 2000 mol %
CF3CO2H in CH2Cl2 at room temperature (X-band, ¯ = 9.639864
GHz, g = 2.00290) and the simulated spectrum (S = 1/2, hyperfine
coupling constants a = 6.1 G with 2 nitrogens and 2.2, 1.8, 1.6, 1.1,
0.3, 0.2, 0.1 G with 14 hydrogens) by EasySpin.4 (b) ESR spectrum of
1c (1.0 mM) with 2000 mol % CF3CO2H in CH2Cl2 at room temper-
ature (X-band, ¯ = 9.637984 GHz, g = 2.00302) and the simulated
spectrum (S = 1/2, hyperfine coupling constants a = 6.0 G with
2 nitrogens and 1.8, 1.4, 0.2, 0.2, 0.2 G with 10 hydrogens) by
EasySpin.4
M
Figure 3. Top view of packing and side view of stacked column
of 1b.
Table 1. Structural parameters of 1a and 1b
1aa
1.41 1.40
48 32
3.33 3.25
1ba Calculated 1bb
Distance of N9-N9¤/¡
Dihedral angle of ¾C8aN9N9¤C8a¤/°
Distance of C8-C8¤/¡
b
1.39
42
3.26
tion upon the addition of acid. UV-vis-NIR spectrum of the
yellow-colored solution of 1b in CH2Cl2 showed absorption at
450 nm (Figure 4a), and the emission maximum was 503 nm
(Figure 4b). The absorption band at 450 nm was decreased by
the addition of CF3CO2H (TFA) and a new absorption maximum
at 621 nm of 1b•+ was observed (Figure 4a), concomitant with
the disproportionation of 1b. The formation of 1b•+ was
confirmed by the observation of the ESR spectrum (Figure 5a).
The quenching experiment of the mixture by hydrazine afforded
1b in 64% NMR yield and 3b in 35% NMR yield, which reflects
the formation of 1b•+ and protonated 3b through the dispro-
portionation reaction under acidic conditions. Similar results
were observed for 1c by the addition of CF3CO2H (Figures 5b
and 6). Meanwhile, some difference in the acid responsiveness
among 1a, 1b, and 1c was observed, as shown in the generation
of the radical cations depending on the amount of CF3CO2H
in the UV-vis-NIR spectra (Figures 4a and 6a).1 The order
of the acid responsiveness was 1a, 1c, and 1b, which could
be attributed to the difference in the basicity caused by the
substituents.
aX-ray structures. ωB97Xd/6-31G(d).
mediated reductive coupling of 6, giving the desired 3c in 89%
yield. Oxidative N-N bond formation of 3c by KMnO4 in
acetone gave the desired 1c in 7% yield. The conditions using
Bu4NMnO4 in pyridine improved the yield up to 49%.
A single crystal of 1b for X-ray crystallographic analysis
was obtained (Figures 2, 3, and Supporting Information). The
structural parameters of the X-ray structures of 1a1 and 1b, and
those of the calculated 1b (ωB97Xd/6-31G(d)) are shown in
Table 1. The dihedral angle ¾C8aN9N9¤C8a¤ of 1b is significantly
smaller than those of 1a and the calculated 1b. This difference is
attributed to the effect of stacking of 1b in crystal. While 1a is
not stacked in crystal due to the steric repulsion of the t-butyl
groups,1 1b is stacked with the same enantiomers in crystal,
giving slipped columns (Figure 3). DFT calculation (ωB97Xd/
6-31G(d)) shows that the racemization energy barrier of 1b is
only 5.2 kcal mol¹1, indicating the flexibility of the helical
structure.
In general, carbazole radicals are not stable enough to stay
in solution under ambient atmosphere at room temperature,
particularly without protecting substituents at the 1,3,6,8-
Both 1b and 1c exhibited acid-responsive property similar
to that of 1a, which underwent electron-transfer disproportiona-
Chem. Lett. 2015, 44, 1336–1338 | doi:10.1246/cl.150557
© 2015 The Chemical Society of Japan | 1337

a
b
a
b
1b
0.5
0.4
0.3
0.2
0.1
0
519 nm
0.4
0.3
0.2
0 mol% TFA
500 mol% TFA
1000 mol% TFA
2000 mol% TFA
3000 mol% TFA
1b+TFA, 1 d
1b+TFA, 2 d
1b+TFA, 3 d
1b+TFA, 10 d
450 nm
621 nm
464 nm
683 nm
541
nm
0.1
0
400 500 600 700 800 900 1000
Wavelength /nm
500 600 700
Wavelength /nm
400 500 600 700 800 900 1000
Wavelength /nm
Figure 7. (a) UV-vis-NIR spectra of 1b (0.05 mM) with 10000
mol % CF3CO2H in CH2Cl2 after 1, 2, 3, and 10 d at 20 °C under air.
(b) Calculated spin density distribution of BC•+ [UωB97XD/
6-31G(d)]. Blue and pink colors indicate the positive and negative
spin density, respectively.
Figure 6. UV-vis-NIR spectra of 1c (0.03 mM) with 0, 500, 1000,
2000, and 3000 mol % CF3CO2H in CH2Cl2. (b) Emission spectrum
of 1c (0.02 mM) in CH2Cl2 (excited at 464 nm).
positions.3,5 On the other hand, 1b•+ without substituents at the
3,3¤,6,6¤-positions showed high stability comparable to 1a•+.1
UV-vis-NIR spectrum of 1b•+ generated from 1b and CF3CO2H
scarcely changed even after 10 days at room temperature under
air (Figure 7a). The stability of 1b•+ under these conditions is
comparable to that reported for 1a•+. The calculated spin density
is delocalized over the bicarbazole skeleton (Figure 7b), which
contributes to the thermodynamic stability of 1b•+ even without
kinetic stabilization by substituents at the 3,3¤,6,6¤-positions. 1c
also showed comparable high stability (Supporting Information).
In summary, non-substituted (1b) and tetramesityl-substi-
tuted (1c) 1,1¤,9,9¤-bicarbazole were synthesized and found to
undergo acid-responsive electron-transfer disproportionation,
similar to tetra-t-butyl-substituted 1,1¤,9,9¤-bicarbazole 1a. The
generated radical cation of 1b showed high stability comparable
to 1a without kinetic stabilization by substituents. 1b without
bulky substituents formed a π-π stacking columnar crystal
structure, in contrast to the non-stacking crystal structure of 1a.
The pristine 1,1¤,9,9¤-bicarbazole 1b would be useful as a
synthetic intermediate for other substituted 1,1¤,9,9¤-bicarbazoles
as well.
This work was supported by the Japan Society for the
Promotion of Science (JSPS) KAKENHI Grant Numbers
25410124 (S. H.), 26102002 (S. H.), and 25287091 (T. N.).
The computations were performed using Research Center for
Computational Science, Okazaki, Japan. We thank S. Masaoka
(IMS) for measurements of UV-vis-NIR spectra.
Supporting Information is available electronically on J-STAGE.
References
1
P. Pandit, K. Yamamoto, T. Nakamura, K. Nishimura, Y.
Kurashige, T. Yanai, G. Nakamura, S. Masaoka, K.
Furukawa, Y. Yakiyama, M. Kawano, S. Higashibayashi,
Chem. Sci. 2015, 6, 4160.
2
3
L. Zhang, S. Zeng, L. Yin, C. Ji, K. Li, Y. Li, Y. Wang, New
J. Chem. 2013, 37, 632.
F. A. Neugebauer, H. Fischer, S. Bamberger, H. O. Smith,
Chem. Ber. 1972, 105, 2694.
4
5
S. Stoll, A. Schweiger, J. Magn. Reson. 2006, 178, 42.
J. F. Ambrose, L. L. Carpenter, R. F. Nelson, J. Electrochem.
Soc. 1975, 122, 876.
1338 | Chem. Lett. 2015, 44, 1336–1338 | doi:10.1246/cl.150557
© 2015 The Chemical Society of Japan