Fe-catalyzed novel domino isomerization/cyclodehydration of substituted 2-[(indoline-3-ylidene)(methyl)]benzaldehyde derivatives: An efficient approach toward benzo[ b ]carbazole derivatives

Article abstract of DOI:10.1021/ol500505k

A new and efficient protocol to synthesize substituted benzo[b]carbazole derivatives has been demonstrated involving iron-catalyzed domino isomerization/cyclodehydration sequences from substituted 2-[(indoline-3- ylidene)(methyl)]benzaldehyde derivatives. The substrates could be easily made via Pd-catalyzed domino Heck-Suzuki coupling from 2-bromo-N-propargylanilide derivatives in high yields. Notably, the generality and efficiency of this two-stage domino strategy was further exemplified by the synthesis of a polycyclic benzofuran derivative.

Full text of DOI:10.1021/ol500505k

Letter
pubs.acs.org/OrgLett
Fe-Catalyzed Novel Domino Isomerization/Cyclodehydration of
Substituted 2‑[(Indoline-3-ylidene)(methyl)]benzaldehyde
Derivatives: An Efficient Approach toward Benzo[b]carbazole
Derivatives
Kartick Paul, Krishnendu Bera, Swapnadeep Jalal, Soumen Sarkar, and Umasish Jana*
Department of Chemistry, Jadavpur University, Kolkata 700 032, West Bengal, India
S
* Supporting Information
ABSTRACT: A new and efficient protocol to synthesize substituted
benzo[b]carbazole derivatives has been demonstrated involving iron-catalyzed
domino isomerization/cyclodehydration sequences from substituted 2-[(indo-
line-3-ylidene)(methyl)]benzaldehyde derivatives. The substrates could be
easily made via Pd-catalyzed domino Heck−Suzuki coupling from 2-bromo-N-
propargylanilide derivatives in high yields. Notably, the generality and
efficiency of this two-stage domino strategy was further exemplified by the
synthesis of a polycyclic benzofuran derivative.
he construction of polycyclic carbazoles has attracted
considerable attention because of their remarkable
luminescent, hole-transporting, and host materials in organic
light-emitting diodes (OLEDs).8 Consequently, a number of
useful synthetic strategies are available now for the synthesis of
a benzo[b]carbazole scaffold.2,9
T
biological and pharmacological activities and applications in
material science.1−3 Very recently, several benzo- and
naphthocarbazole analogues have been explored as potential
anticancer agents.4 Numerous synthetic methods for the
construction of these annulated carbazole ring systems have
been developed in the past decades.2 In particular, the synthesis
of benzo[b]carbazole A framework and related structures
received much attention because a number of biologically and
pharmacologicaly active alkaloids contain this unit. For
instance, naturally occurring alkaloids such as ellipticine B
and 9-methoxyellipticine are isosteric with benzo[b]carbazole
and have shown promising antitumor activity,5 and few of them
have been the subject of clinical trials.6 Due to the structural
similarity of 5H-benzo[b]carbazole to the ellipticines, this
coplanar tetracyclic molecule appears to be an interesting lead
structure for the development of novel antineoplastic agents.7a
For instances, carbazole C possesses cytostatic activity
against leukemia type L 1210 cell culture,7b and D represents
a potential bifunctional nucleic acid intercalating agent.7c
Moreover, benzo[b]carbazole scaffolds such as E and related
structures are widely used as molecular platforms for
Generally, these tetracyclic structures are commonly made
from either prefunctionalized indoles9 or naphathlene10
derivatives. A few new approaches have been developed to
access benzo[b]carbazole derivatives such as cycloaromatization
of N-[2-(1-alkynyl)phenyl]ketenimines,11a,b intramolecular de-
hydro Diels−Alder reactions of N-(o-ethynyl)arylyanimides,11c
Cu(II)-catalyzed annulation of 2-alkynylcarbazole-3-carbalde-
hydes,11d Pd(II)-catalyzed reaction between 2-alkynylbenzalde-
hydes and indoles,11e and cyclization of 2-ethynyl-N-triphenyl-
phosphoranylidine anilines with α-diazoketones.11f However,
many of these protocols have some limitations, such as lengthy
synthetic sequences, harsh conditions, and low chemical yields
with restricted substitution patterns. Accordingly, the develop-
ment of new approaches for the construction of benzo[b]-
carbazole derivatives with specific substitution patterns is highly
desirable.
The Bradsher reaction, involving Lewis or Brønsted acid
catalyzed cyclodehydration of o-formyl/acyldiarylmethanes,
offers a useful strategy to a wide array of polycyclic aromatic
hydrocarbons (Scheme 1).12
Scheme 1. Bradsher Reaction for the Synthesis of
Polynuclear Hydrocarbon
Received: February 27, 2014
Published: March 28, 2014
Figure 1. Structure of some important benzo[b]carbazole molecules.
© 2014 American Chemical Society
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Letter
However, the difficulty of preparation, the starting materials,
and harsh reaction conditions limit the synthesis of
benzoannulated carbazole derivatives using this strategy.
Considering our interest in the synthesis of heterocyclic
compounds, we anticipated that the domino Heck−Suzuki
coupling reaction13 of 2-bromo-N-propargylanilide derivatives
1 with 2-formyl phenylboronic acid followed by isomerization
could easily yield the Bradsher reaction precursor 2aa. Finally,
annulated carbazoles 3 could be achieved from 2aa by a
Bradsher reaction (Scheme 2). Herein, we report our results for
the construction of libraries of benzo[b]carbazole derivatives.
Table 1. Preparation of Substrates via Domino Heck−Suzuki
Reaction
b
entry
R1
R2
R3
time (h) yield (%)
(E/Z)
1
H
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ts
Ms
Ts
Ts
Ph
2
2.5
3
5
5
4
4
5
4
4
5
5
3
2
2
2a
2b
2c
2d
2e
2f
85 (1:0)
78 (1:0)
76 (1:0)
54 (1:1)
60 (3:1)
86 (0:1)
74 (1:0)
75 (1:0)
70 (1:0)
73 (1:0)
63 (1:0)
68 (5:1)
75 (6:1)
83 (1:0)
57 (1:0)
2
H
p-MeC6H4
p-MeOC6H4
p-MeCOC6H4
p-EtO2CC6H4
3-thienyl
Scheme 2. Synthetic Strategy to the Synthesis of
Benzo[b]carabazole Derivatives
3
H
4
H
5
H
6
H
7
H
4-biphenyl
1-naphthyl
p-ClC6H4
p-MeOC6H4
Ph
2g
2h
2i
8
H
9
p-Me
p-Me
p-F
p-F
H
10
11
12
13
14
15
2j
2k
2l
p-MeC6H4
p-ClC6H4
Me
2m
2n
2o
H
H
(CH2)3CH3
a
To test the above idea, we first attempted to prepare 2-
[(indoline-3-ylidene)(methyl)]benzaldehyde 2a via Heck−
Suzuki coupling of 2-bromo-N-propargylanilide 1a with 2-
formylphenylboronic acid. Although the domino Heck−Suzuki
coupling reaction is reported for the synthesis of 2-oxindoles
derivatives,13a−c it has not been reported with 2-bromo-N-
propargylanilide systems to synthesize 2,3-dihydroindole
derivatives. Therefore, at first we began to optimize the
domino Heck−Suzuki coupling between 1a and 2-formylphe-
nylboronic acid to find the best reaction conditions. After a
great deal of study using various solvents, catalysts, bases,
ligands, and temperatures (see the Supporting Information), we
observed that 5 mol % of Pd(OAc)2, 10 mol % of
tricyclohexylphosphine (PCy3), and 2.5 M K2CO3 in
combination with ethanol and toluene at 90 °C gave the best
results. Under these reaction conditions, the yield of the desired
product 2a was 85% (Table 1, entry 1). The reaction proceeds
through an intramolecular syn-carbopalladation step via a 5-exo-
dig process with the alkyne unit to give a σ-alkylpalladium(II)
intermediate, and a subsequent intermolecular Suzuki coupling
with phenylboronic acid derivatives gave the desired product 2a
in a stereoselective fashion.
A large number of substrates were prepared under the
optimized reaction conditions; the results are shown in Table 1.
This transformation was found to be very general, and the
desired 2-[(indoline-3-ylidene)(methyl)]benzaldehydes 2a−o
were obtained in reasonably good yields. Aniline derivatives
possessing electron-rich groups, such as methyl, and electron-
poor groups, such as fluoride, were coupled under standard
reaction conditions in very good yields (Table 1, entries 9−12).
Similarly, a wide number of functional groups on the
aromatic ring of the alkyne terminus were studied. Aromatic
rings possessing electron-donating groups such as p-Me and p-
OMe (Table 1, entries 2, 3, 10, and 12) could be efficiently
transformed to the desired product in good yields (68−78%).
Alkynes bearing a number of electron-withdrawing groups such
as p-COMe, p-CO2Et, and p-Cl (Table 2, entries 4, 5, 9, and
13) were also investigated and gave the desired products in
Reaction conditions: substrate 1 (0.5 mmol), 2-formylphenylboronic
acid (0.75 mmol), Pd(OAc)2 (5 mol %), PCy3 (10 mol %), toluene (2
mL), ethanol (2 mL), and 2.5 M K2CO3. Isolated yield.
b
Table 2. Optimization of Reaction Conditions for the
Domino Isomerisation/Cyclodehydration Process
b
temp
(°C)
time
(h)
yield
(%)
entry
catalyst
FeCl3
solvent
CH2Cl2
1
40
rt
6
70
95
48
78
83
nr
2
FeCl3
ClCH2CH2Cl
CH3CN
3
3
FeCl3
80
60
60
60
60
60
60
60
60
85
85
85
60
60
8
4
FeCl3
MeNO2
5
5
FeCl3
toluene
6
6
FeCl3
tetrahydrofuran
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
6
7
FeCl3,6H2O
FeBr3
1
90
63
90
78
90
8
0.5
1.5
6
9
InCl3
10
11
12
13
14
15
16
In(OTf)3
Ag(OTf)
AlCl3
1.5
5
c
50
ZnCl2
5
30
nr
CuCl
6
p-TsOH
TfOH
5
85
90
1.5
a
Reaction conditions: substrate 2a (0.21 mmol), solvent (2 mL),
b
c
catalyst (10 mol %). Isolated pure yield. 2.5 equiv of AlCl3 used.
moderate to good yields. Notably, other aromatic rings
connected to alkynes such as 3-thienyl, 4-biphenyl, and 2-
naphthyl rings were also efficiently converted in this domino
Heck−Suzuki coupling reaction and furnished the desired
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Letter
products in very good yields of 86%, 74%, and 75%,
respectively (Table 2, entries 6−8). Interestingly, this strategy
was also applicable to the alkyne unit bearing alkyl groups such
as -Me and -Bu and afforded the desired product in 83% and
57% yields, respectively (Table 1, entries 14 and 15). In most
cases, the desired compounds were isolated as a single isomer,
and in a few cases, a mixture of stereoisomers was obtained
(Table 2, entries 4, 5, 12, and 13). The next step was conducted
without separation of the isomers.
Scheme 3. Iron-Catalyzed Synthesis of Substituted
Benzo[b]carbazole Derivatives
a
After having a series of key substrates 2a−o, next we focused
on the development of reaction conditions for the isomer-
ization of 2a as the model substrate. Because of our ongoing
interest in the area of development of new reactions catalyzed
by iron(III) salts,14 we first investigated the isomerization of 2a
to 2aa in the presence of iron(III) chloride.
Serendipitously, we observed that the formation of benzo-
[b]carbazole 3a occurred in one pot via domino isomerization/
cyclodehydration sequences in 70% yield when the substrate 2a
was treated with 10 mol % of FeCl3 in dichloromethane at 40
°C (Table 2, entry 1). Inspired by this result, some reaction
parameters such as solvent effects and role of catalysts were
systematically investigated for the purpose of improving the
yield. The results are summarized in Table 2. To our delight,
the yield of 3a was significantly increased to 95% when the
reaction was carried out in 1,2-dichloroethane at room
temperature in 3 h (Table 2, entry 2). Further study showed
that MeCN, MeNO2, and toluene were less efficient (Table 2,
entries 3−5), while in THF the reaction did not proceed at all,
even after heating at 60 °C (Table 2, entry 6). Coordinating
solvents may have reduced the activity of iron(III) and, hence,
were not suitable for this reaction. Next, different metal
catalysts such as hydrated FeCl3, FeBr3, InCl3, In(OTf)3, and
AgOTf were also evaluated for this process using the substrate
2a in 1,2-dichloroethane. All these catalysts showed high
catalytic activity, but with regard to temperature and yields,
they were inferior to anhydrous FeCl3. Lewis acids such as
ZnCl2 gave only 30% yield of the desired product (Table 2,
entry 13), whereas catalytic anhydrous AlCl3 (10 mol %) did
not furnish any product but gave 50% yield in the presence of
2.5 equiv of AlCl3 (Table 2, Entry12). Brønsted acids such as p-
TsOH and TfOH (Table 2, entries 15 and 16) worked at 60 °C
and afforded lower yields of 85% and 90%, respectively. The
reaction did not proceed in the absence of catalyst and was
sluggish in reducing the amount of catalyst loading. Thus, FeCl3
(10 mol %) in 1,2-dichloroethane was defined as the standard
reaction conditions for the additional study.
a
Reaction conditions: substrates 2 (0.21 mmol), FeCl3 (10 mol %),
and 1,2-dichloroethane (2 mL).
Next, the present protocol was extended to the other
aromatics and heteroaromatic ring at the olefinic center (R3),
and they also worked efficiently and gave good to excellent
yield of the desired products such as 3-thienyl (Scheme 3, 3f,
94%), p-biphenyl (Scheme 3, 3g, 68%), and 1-naphthyl
(Scheme 3, 3h, 84%). Similarly, this strategy was also applicable
to the alkyl group bearing 2,3-dihydroindole derivatives such as
2n and 2o and gave the target benzo[b]carbazole 3n and 3o in
80% and 90% yields, respectively (Scheme 3).
Finally, the generality of this strategy was also verified by the
synthesis of fused benzofuran derivative 5 in 94% yield from the
Heck−Suzuki coupling product 4 (Scheme 4).
Scheme 4. Synthesis of Polycyclic Benzofuran Derivative
To demonstrate the generality of this new reaction,
substrates 2b−o were examined under the optimized reaction
conditions. As shown in Scheme 3, the transformation
proceeded quite smoothly and afforded the desired benzo[b]-
carbazole derivatives in excellent yields (68−98%). The
reaction was found to tolerate various functional groups such
as -Me, -OMe, -COMe, -CO2Et, and -Cl at the para-position of
the aromatic ring (R3) of 2,3-dihydroindole derivatives and
afforded the corresponding products 3b−e,i,l,m in 80−98%
yields. Both electron-donating groups such as p-Me and
electron-withdrawing groups such as p-F on the phenyl ring
of the aniline moiety could be well-tolerated in this trans-
formation and afforded the corresponding products in 80−95%
(Scheme 3, 3i−l). Therefore, the yields of the products are not
significantly sensitive to the electronic effects of the different
groups; however, heating is required in a few cases for
completion the reactions (Scheme 3, 3e,g,h,j,l).
Based on our experimental observations and following the
mechanism of the Bradsher reaction,12 a plausible reaction
pathway is delineated in Scheme 5. The isomerization reaction
did not work in the absence of catalyst and substrate without
bearing a 2-formyl group in 2. Therefore, it was concluded that
isomerization leading to the intermediate G must be driven by
complexation of the carbonyl group with FeCl3. Then,
isomerization and protonation of G, furnishing the intermediate
H, followed by intramolecular Fridel−Crafts alkylation and
subsequent aromatization by dehydration of water, afforded the
benzo[b]carbazole derivatives 3. All the structures of benzo-
[b]carbazoles 3a−n and benzofuran derivative 5 were
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Scheme 5. Plausible Mechanism for the Isomerization/
Cyclization/Aromatization Sequence
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ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedure, copies of NMR spectra, and X-ray
crystallographic data of 3b (CIF). This material is available for
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
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Cossy, J. Org. Lett. 2004, 6, 2511. (f) Arcadi, A.; Blesi, F.; Cacchi, S.;
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Sarkar, S.; Jana, U. Org. Biomol. Chem. 2014, 12, 57. (c) Bera, K.;
Sarkar, S.; Jalal, S.; Jana, U. J. Org. Chem. 2012, 77, 8780. (d) Bera, K.;
Sarkar, S.; Biswas, S.; Maiti, S.; Jana, U. J. Org. Chem. 2011, 76, 3539.
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*E-mail: umasish@gmail.com; jumasish2004@yahoo.co.in.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
K.P., K.B., S.J., and S.S. are thankful to the CSIR, New Delhi,
India, for their fellowships.
■
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