Metal-free electrocyclization at ambient temperature: Synthesis of 1-arylcarbazoles

Article abstract of DOI:10.1055/s-0032-1316743

An efficient novel protocol for the construction of an hyellazole-inspired compound collection is described. Starting from 2,6-diarylacetanilides, the desired products were obtained using hypervalent iodine promoted electrocyclization. The mechanism of pr

Full text of DOI:10.1055/s-0032-1316743

FEATURE ARTICLE
▌2325
feature article
Metal-Free Electrocyclization at Ambient Temperature: Synthesis of
1-Arylcarbazoles
1-Arylcarbazoles by Metal-Free Electrocyclization
Rajarshi Samanta,a Katharina Kulikov,a,b Carsten Strohmann,b Andrey P. Antonchick*a
a
Max Planck Institute of Molecular Physiology, Department of Chemical Biology, Otto-Hahn-Str. 11, 44227 Dortmund, Germany
b
TU Dortmund, Faculty of Chemistry, Otto-Hahn-Str. 6, 44227 Dortmund, Germany
E-mail: andrey.antonchick@mpi-dortmund.mpg.de
Received: 10.05.2012; Accepted after revision: 18.06.2012
gioselective formation of 1-arylcarbazoles from 2,6-di-
arylated acetanilides using C–H bond amination as the
key step toward the synthesis of the compound collection.
Abstract: An efficient novel protocol for the construction of an hy-
ellazole-inspired compound collection is described. Starting from
2,6-diarylacetanilides, the desired products were obtained using hy-
pervalent iodine promoted electrocyclization. The mechanism of
product formation was investigated through intramolecular compe-
tition experiments.
Cl
OMe
OMe
Key words: alkaloids, heterocycles, electrocyclization, hyperva-
lent iodine, biaryls
N
H
N
H
6-chlorohyellazole
hyellazole
Natural products present a rich resource for the discovery
of disease-modulating drug candidates.1 Compounds hav-
ing C–N bonds are widely occurring in nature and have
ubiquitous applications in biological studies.2 Among all
the nitrogen-containing natural products, substances with
the carbazole motif are highly interesting due to their ver-
satile and unique physiological properties, and a variety of
synthetic methods have been reported to date for the con-
struction of this unique structural motif.3 Herein, we re-
port the development of a straightforward approach to 1-
arylcarbazoles from simple chemicals using hypervalent
iodine mediated C–H bond amination.
In continuation of our studies4 on the direct functionaliza-
tion of C–H bonds, we decided to examine a short synthet-
ic route to a natural-product-like compound collection
containing the 1-arylcarbazole scaffold. Hyellazole and 6-
chlorohyellazole (Figure 1) are two unusual, nonbasic,
marine carbazole alkaloids isolated by Moore and co-
workers from the blue-green algae Hyella caespitosa.5
These alkaloids possess structures that are entirely differ-
ent from the carbazole alkaloids isolated from terrestrial
plants. Various strategies have been developed over the
years for the synthesis of these natural products since their
isolation.6 Significantly, Knölker and co-workers have de-
veloped a short and straightforward large-scale synthesis
of hyellazole in eight steps with 63% yield and of 6-chlo-
rohyellazole in 10 steps with 55% yield using iron com-
plex chemistry.6j In both cases, the starting material used
was the inexpensive and commercially available 2,6-di-
methoxytoluene. Based on the limited knowledge on bio-
logical functions of this kind of natural product and the
fact that known syntheses of these natural products are
based on multiple steps, we decided to investigate the re-
Figure 1 Natural products based on the 1-arylcarbazole scaffold
During our studies on the direct functionalization of unac-
tivated C–H bonds, we developed an organocatalytic in-
tramolecular oxidative method of C–N bond formation
(Scheme 1). Using a substoichiometric amount of aryl io-
dide, the desired carbazoles were formed under ambient
conditions in the presence of peracetic acid.4a
R2
R1
R2
R1
ArI (2–10 mol%)
AcOOH
N
NH
Ac
Ac
Scheme 1 Organocatalytic intramolecular oxidative amination
Our retrosynthetic plan was as shown in Scheme 2. The
key reaction is a hypervalent iodine mediated intramolec-
ular amination reaction on compound 1. Compound 1 can
be prepared from different inexpensive, commercially
available anilines in two steps, acetylation followed by
palladium-catalyzed diarylation, in 50–75% yield.7 We
commenced our study by examining the reactions of elec-
tronically variable 2,6-diarylated acetanilides 1 using a
substoichiometric amount of aryl iodide at ambient tem-
perature.
Using various differently substituted derivatives 1, we re-
alized that the use of (diacetoxyiodo)benzene [PhI(OAc)2]
as an oxidant in stoichiometric amounts led to better
yields than the organocatalytic conditions (Table 1).8,9
Having conditions for the synthesis of the 1-arylcarbazole
moiety in hand, we focused on an exploration of the scope
of the reaction. We first examined a variety of symmetri-
cal N-(1,1′:3′,1′′-terphenyl-2′-yl)acetamides 1 with varia-
tions in the outer aryl parts, which we synthesized by a
SYNTHESIS 2012, 44, 2325–2332
Advanced online publication: 11.07.2012
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X
DOI: 10.1055/s-0032-1316743; Art ID: SS-2012-E0422-FA
© Georg Thieme Verlag Stuttgart · New York

2326
R. Samanta et al.
FEATURE ARTICLE
with different electronic and steric properties in various
positions did not have an effect on the formation of the de-
sired product (Scheme 3, products 2a–g). In all cases, ex-
clusive formation of carbazole was observed.
Furthermore, in the case of the meta-substituted substrates
1e and 1f, the products, 2e and 2f, were obtained with a
high regioisomeric ratio. In the case of 1g, it was found
that the addition of small amounts of mesitylene to
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) improved the
yield of the desired polysubstituted product 2g from 10%
to 52%. We then tested reactants with a substituent in the
middle aromatic part. In general, the examined substrates
led to the formation of products in 56–88% yield (Scheme
R3
R2
R3
N
COR1
R2
HN
NH
COR1
COR1
R2
R3
R3
2
1
Scheme 2 Retrosynthetic approach
one-step procedure from the corresponding anilides.7 To
our delight, we found that the presence of substituents
Biographical Sketches█
Rajarshi Samanta under- an Institute of Chemical ology, Dortmund, Germany.
took his bachelors (2002) Technology (Osmania Uni- His present research inter-
and masters (2004) studies versity), Hyderabad, India ests include metal and
in chemistry at the Jadavpur in 2010. Since June, 2010 he metal-free C–H functional-
University, Kolkata, India. has been a postdoctoral fel- ization, asymmetric synthe-
He obtained his PhD degree, low in the group of Dr. A. P. sis and the total synthesis of
working with Dr. T. K. Antonchick, Max Planck In- natural products.
Chakraborty, from the Indi- stitute of Molecular Physi-
Katharina Kulikov was school in 2008 and started ished her bachelor’s thesis
born in Uzbekistan in 1987. her course of studies in in the Antonchick group.
In 1996, she moved to Ger- chemical biology at the TU
many where she finished Dortmund. In 2011, she fin-
Carsten Strohmann stud- (1990–1991) with Prof. D. Privatdozent at the Univer-
ied chemistry (1982–1990) Seyferth as a DFG fellow at sity of Saarbrücken and in
at the Universities of Braun- MIT in Cambridge (USA), 1998 he was appointed
schweig and Karlsruhe, he gained his habilitation Privatdozent at the Univer-
Germany. He obtained his (1992–1995) in the group of sity of Würzburg. Since
PhD degree, working with Prof. M. Veith at the Uni- 2008 he has been a Profes-
Prof. R. Tacke, in 1990. Af- versity of Saarbrücken. In sor of Inorganic Chemistry,
ter
postdoctoral
study 1995 he was appointed TU Dortmund.
Andrey P. Antonchick Belarus) and the Max joined the group of Prof. H.
studied chemistry at Belaru- Planck Institute for Chemi- Waldmann at the Max
sian
State
University cal Ecology (Jena, Germa- Planck Institute of Molecu-
(Minsk, Belarus). He re- ny) under the guidance of lar Physiology (Dortmund,
ceived a PhD in bioorganic Prof. V. A. Khripach and Germany). In June 2010 he
chemistry from the Institute P.D. Dr. B. Schneider. After was appointed group leader
of Bioorganic Chemistry of a postdoctoral appointment at the Max Planck Institute
the National Academy of with Prof. M. Rueping at of Molecular Physiology.
Sciences of Belarus (Minsk, Frankfurt University, he
Synthesis 2012, 44, 2325–2332
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
1-Arylcarbazoles by Metal-Free Electrocyclization
2327
Table 1 Comparison of the Organocatalytic and Stoichiometric
Conditions
R1
R2
R3
PhI(OAc)2
(1.5 equiv)
HN
R1
R1
R1
R2
R3
Conditionsa
Yield (%)
R3
N
HFIP (0.1 M)
Cl
Cl
H
H
H
H
H
H
H
H
A
B
A
B
A
B
A
B
49 (2b)
73 (2b)
60 (2f)
71 (2f)
65 (2h)
88 (2h)
53 (2i)
61 (2i)
R1
R2
H
H
1
2
Me
Me
H
H
F
Cl
H
Cl
Cl
i-Pr
i-Pr
H
NAc
NAc
NAc
H
H
F
Cl
a A: 1 (1 equiv), ArI (10 mol%), AcOOH (2.2 equiv), HFIP–CH2Cl2
(1:1) (0.05 M), r.t.; B: 1 (1 equiv), PhI(OAc)2 (1.5 equiv), HFIP (0.1
M), r.t.
2a, 76%, 16 h
2b, 73%, 3 h
2c, 54%, 5 h
tBu
Cl
3, products 2h–l). In the presence of electron-donating or
electron-withdrawing groups, product formation occurred
smoothly. Finally, besides the application of various acet-
anilides, we demonstrated that a 2-methoxyacetamide de-
rivative can also be used to obtain the product of oxidative
amination (Scheme 3, product 2n). We were able to obtain
the crystal structure of 2b which unequivocally confirmed
the structure (Figure 2).10
NAc
NAc
NAc
Cl
2e, 65%,a 3 h
tBu
2d, 75%, 5 h
2f, 71%,b 3 h
Cl
iPr
NAc
NAc
NAc
2g, 52%,c 5 h
2h, 88%, 3 h
2i, 61%, 12 h
tBu
nBu
NAc
NAc
Figure 2 ORTEP plot of compound 2b at the 50% probability level10
2j, 76%, 12 h
2k, 56%, 12 h
After intensive investigation of the symmetrical reactants,
we focused on the application of nonsymmetrical sub-
strates in the synthesis of 1-arylcarbazoles (Table 2). Sub-
strates bearing a deactivating group such as fluorine or
chlorine in the para position led to the formation of a mix-
ture of nonseparable regioisomeric carbazole ring systems
(Table 2, entries 1 and 2). The fluorine-containing deriva-
tive gave a 1:1 mixture, while the chlorine-containing re-
actant led to a 3:1 mixture, where the major product 4b is
derived from functionalization of the unsubstituted part of
the molecule. Interestingly, the shift of chlorine to the
meta position led to the completely selective formation of
isomer 4c in high yield (Table 2, entry 3). Furthermore, a
derivative bearing an activating group (methyl) in the
MeO2C
O
NAc
NAc
N
OMe
2l, 70%, 5 h
2m, 82%, 3 h
2n, 54%, 16 h
Scheme 3 Reaction scope. Yields are for isolated products after col-
umn chromatography. a Major isomer is shown (ratio = 15:1). b Major
isomer is shown (ratio = 10:1). c Using HFIP–mesitylene (7:1) as sol-
vent.
© Georg Thieme Verlag Stuttgart · New York
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2328
R. Samanta et al.
FEATURE ARTICLE
para position resulted in 3d (Table 2, entry 4). In this case, deuterated substrate for the examination of a primary ki-
oxidative amination occurs by selective modification of netic isotopic effect. Competition experiments under opti-
the substituted part of the substrate.
mized oxidative conditions gave a kinetic isotopic effect
of 1.04. Accordingly, if C–N bond formation and C–H
bond cleavage occurred simultaneously, a primary kinetic
isotope effect (KIE) would lead to the preferred reaction
with the unlabeled substrate (KIE >1); however, the cal-
culated KIE = 1.04 shows that C–H cleavage does not oc-
cur as the rate-determining step of the reaction and the
formation of carbazole via a concerted mechanism is not
taking place. Subsequently, we found that the presence of
a radical scavenger does not affect formation of the de-
sired carbazole. Therefore, reaction mechanisms with rad-
ical species cannot be taken into consideration.
Table 2 Investigation of Nonsymmetrical Reactants 1a
R
R
PhI(OAc)2
+
NHAc
NAc
NAc
R
Ph
1
3
4
Entry
R
Time (h)
Yieldb,c
Yieldb,c
(%) of 3
(%) of 4
To further examine the mechanism, a series of symmetri-
cal N-(1,1′:3′,1′′-terphenyl-2′-yl)acetamides 1 substituted
with electron-donating or electron-withdrawing groups
were screened as substrates in competition experiments to
determine the electronic dependency of the formation of
1-arylcarbazoles. The product ratios obtained from a se-
ries of experiments were correlated with the Hammett
constants. The results were analyzed using the Hammett
equation to obtain hints on the reaction mechanism. First
of all, we assumed formation of a nitrenium ion B via in-
termediate A by oxidation of acetamide 1 with (diacetoxy-
iodo)benzene (Scheme 5). The nitrenium ion B is then
involved in electrophilic aromatic substitution leading to
the formation of the carbocation C which subsequently re-
aromatizes to provide product 2 (Scheme 5, path A); how-
ever, this pathway was excluded based on the nonlinear
correlation in the corresponding Hammett plot (see the
Supporting Information for details). The best linear corre-
lations in the corresponding Hammett plots were obtained
for σp and σ+ constants with goodness of the fit (R2) 0.93
and 0.99, correspondingly (see Figure 3 and the Support-
ing Information for details). Linear correlations for σp
constants correspond to an ipso-substitution pathway;
however, this pathway can be excluded due to the unlikely
formation of a conformationally strained four-membered-
ring intermediate. The negative ρ value observed in the
Hammett plot for σ+ constants (ρ = –0.501) reveals that
electron density is leaving the π-system in the rate-deter-
mining step. Taken together, those studies allowed us to
1
2
3
4
p-F
2
22 (3a)
22 (3b)
n.d.
22 (4a)
66 (4b)
92 (4c)
n.d.
p-Cl
2.5
3
m-Cl
p-Me
5
80 (3d)
a Reaction conditions: 1 (1 equiv), PhI(OAc)2 (1.5 equiv), HFIP (0.1
M), r.t.
b Isolated yields after column chromatography. In the case of nonse-
lective formation of the product, the yields of the isomers were calcu-
lated based on 1H NMR spectra of the product mixture.
c n.d. = not detected.
We successfully demonstrated that the protecting group
on the nitrogen atom can be removed under basic condi-
tions (Scheme 4). To our delight, we were able to synthe-
size the unprotected 1-arylcarbazoles 5 in good to
excellent yields.
After the investigation of the generality of the developed
transformation, we performed studies on the mechanism
of the reaction. Initially, we performed experiments using
R1
R1
R2
R2
KOH (1.5 equiv)
MeOH (0.1 M)
reflux, 4 h
NAc
NH
R1
R1
5
2
Cl
R2
NH
NH
NH
Cl
5a, 76%
5c, 65%, R2 = Cl
5d, 98%, R2 nBu
5b, 96%a
=
Scheme 4 Scope of the hydrolysis. Yields are for isolated products af-
ter column chromatography. a Major isomer is shown (ratio = 10:1).
Figure 3 Hammett plot of log(kX/kH) versus σ+
Synthesis 2012, 44, 2325–2332
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
1-Arylcarbazoles by Metal-Free Electrocyclization
2329
1H and 13C NMR spectra were recorded in CDCl3 on Bruker
DRX400 (400 MHz), Bruker DRX500 (500 MHz) and Varian
Inova 500 (500 MHz) spectrometers. High-resolution mass spectra
were recorded on an LTQ Orbitrap mass spectrometer coupled to an
Accela HPLC system (HPLC column: Hypersil GOLD, 50 mm × 1
mm, 1.9 μm). Fourier transform infrared (FT-IR) spectra were ob-
tained with a Bruker Tensor 27 spectrometer (ATR, neat) and are re-
ported in terms of frequency of absorption (cm–1). Chemical yields
refer to pure isolated substances.
make a conclusion on the mechanism of formation of 1-
phenylcarbazole. Oxidation of 1 with hypervalent iodine
leads to formation of ion D, which is in resonance with ion
B, via the intermediate A (Scheme 5, path B). Ion D un-
dergoes a 4π-electron–5-atom electrocyclic ring closure
to form carbocation C which, in a subsequent aromatiza-
tion, leads to the formation of the desired product 2.11,12
Furthermore, a 4π-electron–5-atom electrocyclization
mechanism correlates well with the results obtained for
nonsymmetrical 2,6-diarylacetanilides (Table 2).
N-Protected 1-Arylcarbazoles 2–4; General Procedure
A 2,6-diarylacetanilide 1 (0.1 mmol) was placed in a 4-mL screw-
capped vial with 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP, 1 mL).
Then, (diacetoxyiodo)benzene (0.15 mmol) was added to the stirred
solution at r.t. The reaction mixture was stirred for 3–16 h. After
completion of the reaction, the mixture was directly concentrated
under reduced pressure and the N-protected 1-arylcarbazole product
was purified by silica gel column chromatography (3–5% EtOAc in
petroleum ether).
path A
path B
1
1
AcOH
AcOH
9-Acetyl-7-fluoro-1-(4-fluorophenyl)-9H-carbazole (2a)
White crystalline solid; yield: 25 mg (76%); mp 160–161 °C [petro-
leum ether (40–60 °C)].
FT-IR: 2924, 1707, 1512, 1439, 1313, 1267 cm–1.
NAc
I
NAc
I
1H NMR (400 MHz, CDCl3): δ = 7.98–7.90 (m, 3 H), 7.60–7.54 (m,
2 H), 7.48 (t, J = 7.6 Hz, 1 H), 7.41 (dd, J = 7.6, 1.2 Hz, 1 H), 7.24–
7.17 (m, 2 H), 7.13 (td, J = 8.8, 2.3 Hz, 1 H), 1.79 (s, 3 H).
R
AcO
R
AcO
A
A
13C NMR (101 MHz, CDCl3): δ = 172.28, 162.87 (d, J = 243.7 Hz),
162.38 (d, J = 248.5 Hz), 140.86 (d, J = 12.7 Hz), 137.14 (d, J = 1.9
Hz), 136.86 (d, J = 3.5 Hz), 129.52 (d, J = 7.9 Hz), 129.23, 128.81,
128.13, 124.73, 121.54, 120.68 (d, J = 10.1 Hz), 119.10, 116.88 (d,
J = 21.5 Hz), 111.52 (d, J = 24.1 Hz), 102.78 (d, J = 28.8 Hz),
26.46.
PhI, AcO–
PhI, AcO–
HRMS: m/z [M + H]+ calcd for C20H14ONF2: 322.10380; found:
322.10378.
N
N
R
Ac
R
Ac
9-Acetyl-7-chloro-1-(4-chlorophenyl)-9H-carbazole (2b)
White crystalline solid; yield: 25.8 mg (73%); mp 224–225 °C [pe-
troleum ether (40–60 °C)–EtOAc].
FT-IR: 2923, 1708, 1586, 1456, 1358, 1217 cm–1.
B
D
electrophilic
aromatic
substitution
4π-electrocyclic
ring closure
1H NMR (400 MHz, CDCl3): δ = 8.24 (d, J = 1.7 Hz, 1 H), 7.97 (dd,
J = 8.2, 1.7 Hz, 1 H), 7.91 (d, J = 8.2 Hz, 1 H), 7.55–7.42 (m, 6 H),
7.38 (dd, J = 8.2, 1.8 Hz, 1 H), 1.81 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.08, 140.56, 139.16, 136.87,
134.00, 133.71, 130.08, 129.32, 129.09 (2 C), 127.94, 124.78,
124.16, 123.81, 120.61, 119.62, 115.34, 26.56.
HRMS: m/z [M + H]+ calcd for C20H14ON35Cl2: 354.04470; found:
354.04483; m/z [M + H]+ calcd for C20H14ON35Cl37Cl: 356.04175;
found: 356.04199; m/z [M + H]+ calcd for C20H14ON37Cl2:
358.03880; found: 358.03864.
C
2
N
H
Ac
R
C
2
Scheme 5 Proposed mechanism of the formation of 1-phenylcarba-
zole
9-Acetyl-7-methyl-1-p-tolyl-9H-carbazole (2c)
White crystalline solid; yield: 17 mg (54%); mp 151–152 °C [petro-
leum ether (40–60 °C)].
FT-IR: 2922, 1703, 1432, 1314, 1272, 1216 cm–1.
1H NMR (400 MHz, CDCl3): δ = 8.07 (s, 1 H), 7.93 (dd, J = 7.8, 1.7
Hz, 1 H), 7.88 (d, J = 7.8 Hz, 1 H), 7.49 (d, J = 8.1 Hz, 2 H), 7.47–
7.39 (m, 2 H), 7.30 (d, J = 7.8 Hz, 2 H), 7.22 (d, J = 7.8 Hz, 1 H),
2.54 (s, 3 H), 2.42 (s, 3 H), 1.74 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.99, 140.56, 138.26, 138.11,
137.60, 136.92, 130.45, 130.29, 128.66, 128.63, 127.74, 124.74,
124.31, 122.99, 119.42, 118.71, 115.22, 26.59, 22.35, 21.38.
In conclusion, we have developed a highly efficient, gen-
eral method for the preparation of a compound library
based on the hyellazole scaffold. The mechanism of
1-arylcarbazole formation was examined. Our results sug-
gest a 4π-electron–5-atom electrocyclization mechanism
for hypervalent iodine mediated intramolecular oxidative
amination of 2,6-diarylacetanilides.
Unless otherwise noted, all commercially available compounds
were used as provided without further purification. Solvents for
chromatography were technical grade and distilled prior to use. Col-
umn chromatography was performed using Merck silica gel 60 (par-
ticle size: 0.040–0.063 mm). Solvent mixtures are volume/volume.
HRMS: m/z [M + H]+ calcd for C22H20ON: 314.15394; found:
314.15398.
© Georg Thieme Verlag Stuttgart · New York
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2330
R. Samanta et al.
FEATURE ARTICLE
9-Acetyl-7-tert-butyl-1-(4-tert-butylphenyl)-9H-carbazole (2d)
HRMS: m/z [M + H]+ calcd for C20H15ON35Cl: 320.08367; found:
320.08367; m/z [M + H]+ calcd for C20H15ON37Cl: 322.08072;
found: 322.08065.
Light yellow oil; yield: 29.8 mg (75%).
FT-IR: 2959, 1704, 1361, 1276, 1244 cm–1.
1H NMR (500 MHz, CDCl3): δ = 8.34 (s, 1 H), 7.99–7.89 (m, 2 H),
7.57–7.41 (m, 7 H), 1.72 (s, 3 H), 1.45 (s, 9 H), 1.38 (s, 9 H).
9-Acetyl-3-isopropyl-1-phenyl-9H-carbazole (2i)
Light yellow oil; yield: 19.9 mg (61%).
13C NMR (126 MHz, CDCl3): δ = 173.03, 151.78, 150.98, 140.64,
138.13, 137.28, 130.27, 128.71, 128.62, 127.58, 126.61, 124.32,
123.01, 121.27, 119.18, 118.80, 111.90, 35.59, 34.81, 31.86, 31.50,
26.51.
FT-IR: 2959, 1701, 1437, 1401, 1360, 1272 cm–1.
1H NMR (400 MHz, CDCl3): δ = 8.24 (d, J = 8.3 Hz, 1 H), 8.01 (d,
J = 7.6 Hz, 1 H), 7.85 (d, J = 1.5 Hz, 1 H), 7.64–7.59 (m, 2 H),
7.54–7.45 (m, 3 H), 7.39 (t, J = 7.6 Hz, 2 H), 7.34 (d, J = 1.5 Hz, 1
H), 3.17–3.09 (m, 1 H), 1.75 (s, 3 H), 1.40 (d, J = 6.6 Hz, 6 H).
HRMS: m/z [M + H]+ calcd for C28H32ON: 398.24784; found:
398.24695.
13C NMR (101 MHz, CDCl3): δ = 172.54, 145.40, 141.35, 140.61,
135.30, 130.09, 129.72, 128.78, 128.15, 127.94, 127.73, 127.71,
125.58, 123.49, 119.66, 116.72, 115.17, 34.24, 26.43, 24.53.
HRMS: m/z [M + H]+ calcd for C23H22ON: 328.16959; found:
328.16954.
9-Acetyl-6-chloro-1-(3-chlorophenyl)-9H-carbazole (2e) (Ma-
jor Isomer)
White amorphous solid; yield: 23 mg (65%).
FT-IR: 2924, 1707, 1591, 1563, 1437, 1263, 1170 cm–1.
1H NMR (500 MHz, CDCl3): δ = 8.13 (d, J = 8.8 Hz, 1 H), 8.00–
7.95 (m, 2 H), 7.60 (s, 1 H), 7.52–7.36 (m, 6 H), 1.85 (s, 3 H).
9-Acetyl-3-tert-butyl-1-phenyl-9H-carbazole (2j)
Light yellow oil; yield: 25.9 mg (76%).
13C NMR (101 MHz, CDCl3): δ = 171.85, 142.53, 138.49, 137.07,
135.76, 130.97, 129.86, 129.26, 128.95, 128.06, 127.94, 127.86,
127.63, 126.64, 125.92, 124.71, 120.04, 119.72, 116.18, 26.49.
HRMS: m/z [M + H]+ calcd for C20H14ON35Cl2: 354.04470; found:
354.04470; m/z [M + H]+ calcd for C20H14ON35Cl37Cl: 356.04175;
found: 356.04169; m/z [M + H]+ calcd for C20H14ON37Cl2:
358.03880; found: 358.03837.
FT-IR: 2958, 2926, 1702, 1479, 1394, 1271, 1195 cm–1.
1H NMR (400 MHz, CDCl3): δ = 8.24 (d, J = 8.2 Hz, 1 H), 8.03 (dd,
J = 7.6, 0.5 Hz, 1 H), 8.00 (d, J = 1.9 Hz, 1 H), 7.64–7.60 (m, 2 H),
7.54–7.45 (m, 4 H), 7.42–7.37 (m, 2 H), 1.75 (s, 3 H), 1.48 (s, 9 H).
13C NMR (101 MHz, CDCl3): δ = 172.54, 147.73, 141.55, 140.62,
134.97, 129.73 (2 C), 128.48, 127.99, 127.71, 127.68, 127.13,
125.72, 123.49, 119.61, 115.76, 115.18, 34.99, 31.88, 26.43.
HRMS: m/z [M + H]+ calcd for C24H24ON: 342.18524; found:
342.18541.
9-Acetyl-6-methyl-1-m-tolyl-9H-carbazole (2f) (Major Isomer)
Light yellow oil; yield: 22.2 mg (71%).
FT-IR: 2923, 1702, 1393, 1361, 1274, 1196 cm–1.
9-Acetyl-3-butyl-1-phenyl-9H-carbazole (2k)
Light yellow oil; yield: 19 mg (56%).
FT-IR: 2926, 1701, 1438, 1399, 1271 cm–1.
1H NMR (400 MHz, CDCl3): δ = 8.11 (d, J = 8.4 Hz, 1 H), 7.98–
7.93 (m, 1 H), 7.80 (s, 1 H), 7.48–7.35 (m, 5 H), 7.30 (dd, J = 8.4,
1.1 Hz, 1 H), 7.19 (d, J = 6.7 Hz, 1 H), 2.53 (s, 3 H), 2.42 (s, 3 H),
1.75 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.57, 141.02, 139.47, 138.44,
137.12, 133.15, 130.44, 129.64, 129.10, 128.94, 128.54, 128.49 (2
C), 125.47, 124.95, 124.23, 119.83, 119.06, 114.75, 26.43, 21.73,
21.47.
1H NMR (400 MHz, CDCl3): δ = 8.24 (d, J = 8.4 Hz, 1 H), 7.99 (d,
J = 7.6 Hz, 1 H), 7.80 (s, 1 H), 7.60 (d, J = 7.6 Hz, 2 H), 7.53–7.44
(m, 3 H), 7.38 (dd, J = 10.8, 4.1 Hz, 2 H), 7.30 (d, J = 1.3 Hz, 1 H),
2.86–2.79 (m, 2 H), 1.80–1.70 (m, 5 H), 1.50–1.40 (m, 2 H), 0.98 (t,
J = 7.3 Hz, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.54, 141.25, 140.59, 139.35,
135.22, 130.02, 129.90, 129.71, 128.78, 127.92, 127.72, 125.51,
123.49, 119.67, 118.80, 115.17, 35.67, 34.19, 26.42, 22.63, 14.17.
HRMS: m/z [M + H]+ calcd for C22H20ON: 314.15394; found:
314.15400.
HRMS: m/z [M + H]+ calcd for C24H24ON: 342.18524; found:
342.18533.
9-Acetyl-8-(3,5-dimethylphenyl)-1,3-dimethyl-9H-carbazole
(2g) (Major Isomer)
Light yellow oil; yield: 17.7 mg (52%).
FT-IR: 2922, 1731, 1702, 1601, 1454, 1393, 1361, 1240, 1203 cm–1.
Methyl 9-Acetyl-1-phenyl-9H-carbazole-3-carboxylate (2l)
White crystalline solid; yield: 24 mg (70%); mp 206–207 °C [petro-
leum ether (40–60 °C)–EtOAc].
FT-IR: 2924, 1728, 1701, 1432, 1356, 1272, 1224 cm–1.
1H NMR (500 MHz, CDCl3): δ = 8.70 (d, J = 1.1 Hz, 1 H), 8.21–
8.16 (m, 2 H), 8.08 (d, J = 7.7 Hz, 1 H), 7.62 (d, J = 7.7 Hz, 2 H),
7.56–7.49 (m, 3 H), 7.47–7.39 (m, 2 H), 4.00 (s, 3 H), 1.77 (s, 3 H).
13C NMR (126 MHz, CDCl3): δ = 172.35, 167.10, 140.58, 140.19,
139.60, 130.46, 129.88, 129.81, 128.48, 128.45, 128.24, 127.95,
126.20, 124.86, 123.93, 121.07, 120.17, 114.83, 52.44, 26.68.
1H NMR (400 MHz, CDCl3): δ = 7.90–7.86 (m, 1 H), 7.63 (s, 1 H),
7.42–7.37 (m, 2 H), 7.23 (s, 2 H), 7.08 (s, 1 H), 7.03 (s, 1 H), 2.52
(s, 3 H), 2.48 (s, 3 H), 2.38 (s, 6 H), 1.81 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 174.14, 140.43, 138.98, 138.88,
138.59, 132.99, 131.71, 130.41, 129.32, 128.81, 127.21, 126.35 (2
C), 126.16, 123.87, 118.79, 117.55, 27.32, 21.62, 21.58, 21.24.
HRMS: m/z [M + H]+ calcd for C24H24ON: 342.18524; found:
342.18535.
HRMS: m/z [M + H]+ calcd for C22H18O3N: 344.12812; found:
344.12823.
9-Acetyl-3-chloro-1-phenyl-9H-carbazole (2h)
Colorless oil; yield: 28 mg (88%).
FT-IR: 2924, 1707, 1470, 1388, 1271 cm–1.
9-Acetyl-1-phenyl-9H-carbazole (2m)
Colorless oil; yield: 23.4 mg (82%).
FT-IR: 2925, 1702, 1476, 1445, 1401, 1269 cm–1.
1H NMR (400 MHz, CDCl3): δ = 8.24 (d, J = 8.3 Hz, 1 H), 8.09–
7.96 (m, 2 H), 7.64–7.58 (m, 2 H), 7.54–7.46 (m, 5 H), 7.45–7.36
(m, 2 H), 1.75 (s, 3 H).
1H NMR (500 MHz, CDCl3): δ = 8.20 (d, J = 8.3 Hz, 1 H), 7.99–
7.95 (m, 2 H), 7.60–7.58 (m, 2 H), 7.55–7.48 (m, 3 H), 7.46 (d,
J = 2.1 Hz, 1 H), 7.44–7.38 (m, 2 H), 1.74 (s, 3 H).
13C NMR (101 MHz, CDCl3): δ = 172.19, 140.67, 139.80, 135.30,
131.41, 129.98, 129.88 (2 C), 128.83, 128.58, 128.36, 127.81,
124.40, 123.79, 120.00, 119.00, 115.08, 26.50.
Synthesis 2012, 44, 2325–2332
© Georg Thieme Verlag Stuttgart · New York

FEATURE ARTICLE
1-Arylcarbazoles by Metal-Free Electrocyclization
2331
13C NMR (101 MHz, CDCl3): δ = 172.61, 141.01, 140.24, 136.88,
130.29, 129.77, 129.27, 128.57, 127.89 (2 C), 127.82, 125.35,
124.39, 123.57, 119.79, 119.29, 115.02, 26.45.
HRMS: m/z [M + H]+ calcd for C20H16ON: 286.12264; found:
286.12266.
J = 8.2 Hz, 2 H), 7.43–7.37 (m, 2 H), 7.33 (t, J = 7.5 Hz, 1 H), 7.23
(d, J = 8.2 Hz, 1 H).
13C NMR (126 MHz, CDCl3): δ = 140.06, 137.46, 137.25, 133.85,
131.86, 129.73, 129.66, 126.08, 124.15, 123.42, 122.27, 121.44,
120.63, 120.50, 119.87, 110.95.
GC-MS: m/z (%) = 313 (73) [M+], 311 (100), 275 (27), 241 (52),
213 (6), 155 (9), 137 (8), 119 (15).
9-(Methoxyacetyl)-1-phenyl-9H-carbazole (2n)
Colorless oil; yield: 17 mg (54%).
FT-IR: 2926, 1704, 1502, 1446, 1266, 1122 cm–1.
6-Methyl-1-m-tolyl-9H-carbazole (5b) (Major Isomer)
Light green oil; yield: 26 mg (96%).
1H NMR (400 MHz, CDCl3): δ = 8.16 (d, J = 8.3 Hz, 1 H), 8.04–
7.98 (m, 2 H), 7.64 (d, J = 7.2 Hz, 2 H), 7.59–7.36 (m, 7 H), 3.61 (s,
2 H), 2.90 (s, 3 H).
FT-IR: 3432, 3028, 2919, 2857, 1605, 1503, 1477, 1400, 1333,
1293, 1232 cm–1.
13C NMR (101 MHz, CDCl3): δ = 173.20, 140.43, 139.91, 136.35,
130.20, 129.96, 129.05, 128.46, 128.18, 127.98, 127.94, 125.58,
124.50, 123.76, 119.89, 119.34, 114.79, 74.23, 58.88.
1H NMR (500 MHz, CDCl3): δ = 8.21 (s, 1 H), 8.05 (d, J = 7.7 Hz,
1 H), 7.92 (s, 1 H), 7.53–7.50 (m, 2 H), 7.48–7.41 (m, 2 H), 7.32 (d,
J = 7.6 Hz, 2 H), 7.28–7.24 (m, 2 H), 2.56 (s, 3 H), 2.50 (s, 3 H).
HRMS: m/z [M + H]+ calcd for C21H18O2N: 316.13321; found:
316.13325.
13C NMR (126 MHz, CDCl3): δ = 139.25, 139.07, 137.83, 137.75,
129.25, 129.23, 128.93, 128.39, 127.40, 125.66, 125.52, 125.24,
123.88, 123.66, 120.49, 119.78, 119.42, 110.45, 21.75, 21.60.
HRMS: m/z [M + H]+ calcd for C20H18N: 272.14338; found:
272.14328.
9-Acetyl-7-methyl-1-phenyl-9H-carbazole (3d)
Light yellow oil; yield: 23.9 mg (80%).
FT-IR: 2923, 1703, 1429, 1396, 1314, 1272, 1216, 1175 cm–1.
1H NMR (500 MHz, CDCl3): δ = 8.07 (s, 1 H), 7.95 (dd, J = 7.8, 1.7
Hz, 1 H), 7.88 (d, J = 7.8 Hz, 1 H), 7.62–7.59 (m, 2 H), 7.52–7.43
(m, 4 H), 7.41–7.36 (m, 1 H), 7.22 (dd, J = 7.8, 0.5 Hz, 1 H), 2.54
(s, 3 H), 1.74 (s, 3 H).
3-Chloro-1-phenyl-9H-carbazole (5c)
Colorless oil; yield: 18 mg (65%).
FT-IR: 3441, 3058, 2925, 1622, 1584, 1494, 1475, 1444, 1402,
1308, 1247, 1232, 1148, 1112 cm–1.
13C NMR (126 MHz, CDCl3): δ = 172.75, 141.12, 140.62, 138.33,
136.91, 130.27, 129.76, 128.74, 128.73, 127.94, 127.78, 124.80,
124.34, 123.00, 119.44, 118.98, 115.28, 26.45, 22.35.
1H NMR (500 MHz, CDCl3): δ = 8.26 (s, 1 H), 8.06 (d, J = 7.8 Hz,
1 H), 8.03 (d, J = 1.9 Hz, 1 H), 7.68–7.64 (m, 2 H), 7.57 (dd,
J = 10.5, 4.9 Hz, 2 H), 7.50–7.38 (m, 4 H), 7.30–7.24 (m, 1 H).
HRMS: m/z [M + H]+ calcd for C21H18ON: 300.13829; found:
300.13829.
13C NMR (126 MHz, CDCl3): δ = 140.06, 137.93, 135.79, 129.53,
128.42, 128.22, 126.77, 126.30, 125.71, 125.50, 124.96, 122.87,
120.76, 120.05, 119.17, 111.04.
HRMS: m/z [M+] calcd for C18H12N35Cl: 277.06528; found:
277.06525; m/z [M+] calcd for C18H12N37Cl: 279.06233; found:
279.06253.
9-Acetyl-1-(3-chlorophenyl)-9H-carbazole (4c)
Light yellow oil; yield: 29.3 mg (92%).
FT-IR: 2923, 1704, 1591, 1562, 1448, 1393, 1268, 1219, 1196 cm–1.
1H NMR (400 MHz, CDCl3): δ = 8.20 (d, J = 8.3 Hz, 1 H), 8.05–
8.00 (m, 2 H), 7.62 (t, J = 1.6 Hz, 1 H), 7.55–7.35 (m, 7 H), 1.88 (s,
3 H).
3-Butyl-1-phenyl-9H-carbazole (5d)
Light green oil; yield: 29.3 mg (98%).
FT-IR: 3432, 3055, 3030, 2925, 2854, 1605, 1497, 1449, 1411,
1389, 1315, 1258, 1234 cm–1.
13C NMR (101 MHz, CDCl3): δ = 171.99, 142.89, 140.14, 136.69,
135.63, 130.86, 129.17, 128.88, 128.70, 128.00, 127.86, 126.00,
125.93, 125.35, 124.47, 123.70, 119.91, 119.83, 115.00, 26.57.
HRMS: m/z [M + H]+ calcd for C20H15ON35Cl: 320.08367; found:
320.08374; m/z [M + H]+ calcd for C20H15ON37Cl: 322.08072;
found: 322.08067.
1H NMR (500 MHz, CDCl3): δ = 8.22 (s, 1 H), 8.12 (d, J = 7.8 Hz,
1 H), 7.92 (d, J = 1.5 Hz, 1 H), 7.75–7.71 (m, 2 H), 7.61–7.55 (m, 2
H), 7.49–7.38 (m, 3 H), 7.32 (d, J = 1.5 Hz, 1 H), 7.29–7.24 (m, 1
H), 2.90–2.86 (m, 2 H), 1.83–1.74 (m, 2 H), 1.49 (dq, J = 14.7, 7.4
Hz, 2 H), 1.02 (t, J = 7.4 Hz, 3 H).
13C NMR (126 MHz, CDCl3): δ = 139.95, 139.42, 135.82, 134.74,
129.33, 128.50, 127.56, 126.75, 125.87, 124.80, 123.99, 123.66,
120.50, 119.45, 118.98, 110.77, 35.92, 34.68, 22.63, 14.21.
HRMS: m/z [M + H]+ calcd for C22H22N: 300.17468; found:
300.17465.
1-Arylcarbazoles 5 by Hydrolysis of N-Acetyl-1-arylcarbazoles
2; General Procedure
The N-acetyl-1-arylcarbazole 2 (0.1 mmol) was taken up in MeOH
(1 mL) in a 4 mL screw-capped vial, and KOH pellets (1.5 mmol)
were added. Then, the reaction mixture was heated to reflux under
continuous stirring. After 4 h, the mixture was cooled and neutral-
ized with sat. aq NH4Cl soln, then extracted with CH2Cl2 (3 × 20
mL). The extracts were washed with H2O (15 mL) and brine (10
mL), dried (anhyd Na2SO4), concentrated under reduced pressure
and purified by silica gel column chromatography (2–3% EtOAc in
petroleum ether).
Acknowledgment
We thank Prof. Dr. Herbert Waldmann for his generous support.
7-Chloro-1-(4-chlorophenyl)-9H-carbazole (5a)
Supporting Information for this article is available online at
White amorphous solid; yield: 23.7 mg (76%).
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
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FT-IR: 3443, 3061, 2925, 1622, 1595, 1502, 1484, 1442, 1423,
1390, 1334, 1312, 1293, 1177, 1132 cm–1.
Primary Data for this article are available online at
1H NMR (500 MHz, CDCl3): δ = 8.18 (s, 1 H), 8.03 (d, J = 7.5 Hz,
1 H), 7.99 (d, J = 8.3 Hz, 1 H), 7.59 (d, J = 8.2 Hz, 2 H), 7.53 (d,
http://www.thieme-connect.com/ejournals/toc/synthesis and can be
cited using the following DOI:mPirDayrmaPtirDyar
a
t
0.4125/pd0032th.mPirDayraZtahenl1
0
1Chemyrtsi
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2012, 44, 2325–2332

2332
R. Samanta et al.
FEATURE ARTICLE
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Synthesis 2012, 44, 2325–2332
© Georg Thieme Verlag Stuttgart · New York