Bismuth(III) Triflate Catalyzed Three-Component Reactions of Indoles, Ketones, and α-Bromoacetaldehyde Acetals Enable Indole-to-Carbazole Transformation

Article abstract of DOI:10.1021/acs.orglett.8b01707

A three-component reaction of indoles, α-bromoacetaldehyde acetals, and ketones was developed by using bismuth(III) triflate as the catalyst to realize a straightforward approach for synthesizing carbazole derivatives. The reaction was established mechanistically through the autotandem catalysis of Bi(OTf)₃ in the following two steps: (i) Friedel-Crafts-type alkylation of indole with α-bromoacetaldehyde acetal, which produced a tryptaldehyde-type intermediate and (ii) [4 + 2] annulation of this intermediate with the ketone component.

Full text of DOI:10.1021/acs.orglett.8b01707

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Bismuth(III) Triflate Catalyzed Three-Component Reactions of
Indoles, Ketones, and α‑Bromoacetaldehyde Acetals Enable Indole-
to-Carbazole Transformation
Yanlong Gu,*,†,‡ Wenbo Huang,† Shaomin Chen,† and Xin Wang†
†Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and
Chemical Engineering, Hubei Key Laboratory of Material Chemistry and Service Failure, Huazhong University of Science and
Technology, 430074, Wuhan, China
‡State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, 730000, Lanzhou, China
S
* Supporting Information
ABSTRACT: A three-component reaction of indoles, α-bromoacetal-
dehyde acetals, and ketones was developed by using bismuth(III)
triflate as the catalyst to realize a straightforward approach for
synthesizing carbazole derivatives. The reaction was established
mechanistically through the autotandem catalysis of Bi(OTf)3 in the
following two steps: (i) Friedel−Crafts-type alkylation of indole with α-bromoacetaldehyde acetal, which produced a
tryptaldehyde-type intermediate and (ii) [4 + 2] annulation of this intermediate with the ketone component.
arbazole and its derivatives are important nitrogen-
containing heterocycles because their structural core is
C
omnipresent in natural products and biologically active
compounds.1 Given their structural rigidity and extensive π-
conjugation, carbazole derivatives are widely applied in
preparing organic photoelectronic materials and chromo-
phores.2 In view of their widespread significance, cost-effective
methods must be developed for synthesizing carbazoles. At the
early stage of the process, carbazole synthesis relies heavily on
the Fischer−Borsche method via a multistep procedure.3 In the
past two decades, transition-metal-catalyzed cyclization strat-
egies have been widely applied in carbazole4 and 1-(hetero)aryl
carbazoles synthesis.5 The Cadogan cyclization of 2-nitro-
biaryls with the use of suitable reducing reagents, such as
phosphines, phosphites, and carbon monoxide, has provided
an alternative approach for accessing substituted carbazoles.6
This type of reaction can also be performed in the presence of
PhMgBr under mild and transition-metal-free conditions.7
Carbazoles can also be synthesized from indole derivatives via
different strategies, such as Diels−Alder reaction,8 electrocyclic
reaction,9 and ketosulfoxide cyclization.10 Although these
strategies can synthesize carbazoles, new reaction systems are
being continuously explored to improve efficiency and reduce
cost.
Figure 1. Representative works on indole-to-carbazole transforma-
tions starting from 2,3-unsubstituted indoles.
(SI));11 (ii) [2 + 2′ + 2′] annulation, in which indoles are
reacted with two molecules of alkene or alkyne in the presence
of a transition-metal catalyst to form carbazoles;12 and (iii) [2
+ 2′ + 2″] annulation, in which indoles are reacted with two
different molecules, and each of them contribute two carbons
to construct a carbazole ring.13 Among these three approaches,
the third is considered the most attractive route for carbazole
The direct conversion of 2,3-unsubstituted indoles into
carbazoles has an intrinsically high synthetic potential because
simple indoles are readily available. At present, this trans-
formation can generally be realized through the following three
approaches (Figure 1): (i) [2 + 4] annulation, in which indoles
are reacted with a carbon-based 1,4-biselectrophile, such as a
1,4-dicarbonyl compound, or their alkyne-, allene-, dihydrofur-
an-, and donor-and-acceptor-cyclopropane-type variations,
with the aid of an acid or transition-metal catalyst to create
carbazole scaffolds (see Figure S1 in Supporting Information
Received: May 30, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b01707
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
synthesis not only because it uses easily available substrate and
transition-metal-free reaction conditions but also because it
synthesizes carbazoles with a high potential of molecular
diversity and complexity. However, to date, only a few [2 + 2′
+ 2″] annulation reactions have been reported,13 and the
productivity for creating molecular diversity and complexity
has yet to be fully displayed. In this work, we used α-
bromoacetaldehyde acetal and a simple ketone as a reagent
couple to react with 2,3-unsubstituted indole. The established
[2 + 2′ + 2″] annulation reaction provided a straightforward
approach for accessing various carbazoles that are otherwise
difficult to synthesize through other methods.
1a/2a/3a at 2/1/1, 80 °C, and 6 h. It should be noted that,
with this catalyst, reactions scaled up to multigram quantities
provided uniform results (entry 7).
Once a set of optimized conditions was obtained, we probed
the scope of the reaction with respect to both the indole and
ketone components. In particular, we intended to assess the
usefulness of Bi(OTf)3-catalyzed [2 + 2′ + 2″] annulation in
synthesizing N−H carbazoles from the N−H indole, which has
been proven to be difficult for indole-to-carbazole trans-
formation.15 As shown in Scheme 1, when the N−H indole
Scheme 1. Substrate Scope of the Bi(OTf)3-Catalyzed [2 +
2′ + 2″] Annulation Reaction of 2,3-Unsubstitued Indoles,
2a, and Ketones
Our studies started with a three-component reaction of N-
methylindole 1a, α-bromoacetaldehyde ethylene acetal 2a, and
acetophenone 3a. The reaction was performed in acetonitrile
at 80 °C. As shown in Table 1, no reaction occurred in the
Table 1. Condition Optimization of the Reaction between
a
1a, 2a, and 3a
b
entry
catalyst
solvent
MeCN
yield (%)
1
2
3
4
5
6
7
8
−
TfOH
0
c
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
1,4-dioxane
DCE
PhMe
EtOH
MeCN
MeCN
MeCN
MeCN
30 (trace)
Fe(OTf)3
Cu(OTf)2
Al(OTf)3
Sc(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
Bi(OTf)3
<5
55
39
65
h
81 (80)
74
52
15
trace
45
53
49
47
9
10
11
was used as the substrate, the reaction proceeded sluggishly,
and the expected carbazole 4b was isolated only in 31% yield.
The nitrogen in the indole might coordinate with the
bismuth(III) species, thus deactivating the catalyst. This
imposed thus a negative effect on the reaction. When
electron-donating N−H indoles or acetophenones were used
as substrates, the selectivity to the target carbazole was slightly
increased (4c−e). Nonetheless, the reactions exhibited no
yield above 50%.
d
12
e
13
14
f
g
15
a
1a (0.6 mmol), 2a (0.3 mmol), 3a (0.3 mmol), catalyst (0.03
b
mmol), 80 °C, 6 h. Isolated yield, calculated with respect to 2a.
c
d
e
TfOH, 0.003 mmol (1 mol %). 1a/2a/3a is 1/1/1. Bi(OTf)3,
f
g
h
0.015 mmol. 50 °C. 3 h. 10 mmol scale reaction.
The reactions with N-alkylindoles proceeded generally well.
Upon repeating the reaction with 1a and 2a, acetophenones
with different functional groups on the arene ring worked well
under the standard conditions, efficiently offering the
corresponding functionalized carbazoles 4f−p with yields
ranging from 40% to 82%. Various functional groups, such as
fluoro (4m), chloro (4n), bromo (4o), iodo (4p), tert-butyl
(4i), cyclohexyl (4j), and methoxy (4l), can well tolerate the
Bi(OTf)3-promoted conditions. 1-Acetonaphthone and 1-(9H-
fluoren-3-yl)ethanone, which are sterically demanding ketones,
were successfully engaged in this reaction, delivering the
corresponding carbazoles, namely, 4q and 4r, with 75% and
67% yields, respectively. Aryl methyl ketones with heterocyclic
functionalities, such as 3,4-methylenedioxyacetophenone, 2-
acetylthiophene, and 2-acetylbenzofuran, could also be used
uneventfully, and the heterocyclic fragments were delivered
into the product without structural damage (4s−u). Carbazole
synthesis with propiophenone was also successful, albeit with a
absence of catalyst (Table 1, entry 1). Triflic acid can promote
this reaction, and the yield of 4a reached 30% after 6 h (entry
2). Several readily available Lewis acids were examined.
Fe(OTf)3 was proven to be an ineffective catalyst (entry 3).
Cu(OTf)2, Al(OTf)3, and Sc(OTf)3 can promote the reaction,
but the maximum yield reached only 65% (entries 4−6). The
best result was obtained when Bi(OTf)3 was used as the acid
catalyst (entry 7).14 The results indicate that better results
were obtained with dipolar and aprotic solvents, such as
acetonitrile or 1,4-dioxane (entries 7−11). Further inspection
revealed that the amount of 1a also markedly affected the
reaction. The optimal yield was achieved in the presence of 2.0
equiv of 1a in the reaction mixture (entries 7 and 12). The
yield of 4a decreased when the reaction temperature or catalyst
amount was reduced or when the reaction time was shortened
(entries 13−15). Thus, the optimal conditions were confirmed
as follows: Bi(OTf)3 catalyst (10 mol %), acetonitrile solvent,
B
DOI: 10.1021/acs.orglett.8b01707
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
slightly lower efficiency (4v). The results for the use of other
N-alkylindoles, such as N-ethylindole and N-butylindole, were
mostly similar to those for N-methylindole. Nonetheless, the
presence of an electron-withdrawing group in the indole ring
was detrimental for the carbazole synthesis. For example, when
5-bromo-1-methylindole was used, only 36% of yield was
obtained under identical conditions (4x).
Then, we attempted to use an aliphatic ketone in the three-
component reaction. Although the result obtained for the
linear aliphatic ketone was not as good as that for the aryl
methyl ketone, the attained synthetic efficiency was acceptable.
For example, when acetone was reacted with 1a and 2a, the
expected product, 4w, was obtained in 50% yield (Scheme 1).
A number of cyclic aliphatic ketones, such as cyclopentanone,
cyclohexanone, and cycloheptanone, participated readily in this
reaction, providing the expected carbazoles with generally good
yields (Scheme 2). Given that 4ab-type carbazoles can be
conditions, both sides of α-bromoacetaldehyde ethylene acetal
2a, namely, bromomethyl and aldo-carbonyl groups, can react
with a nucleophile. Given that the nucleophilicity of 1a was
considerably higher than that of 3a, 1a should exhibit the
priority to react with an electrophile. If the carbazole formation
was triggered by a reaction of the bromomethyl side of 2a, then
(i) a tryptaldehyde should be generated as an intermediate and
(ii) replacing 2a with α-chloroacetaldehyde ethylene acetal 2b
should notably influence the synthetic efficiency. Therefore,
control experiments were performed. As shown in Scheme 4, in
Scheme 4. Control Experiments
Scheme 2. Reactions of 1a, 2a, and Cyclic Ketones
the presence of Bi(OTf)3 and HBr, a tryptaldehyde derivative
5a can react with cyclohexanone to form a carbazole derivative
4ab in 63% yield (eq 3). Here, HBr was added, on purpose, to
mimic a real reaction system, in which HBr is generated as a
concomitant of the Friedel−Crafts-type alkylation of 1a and
2a. The three-component reaction of 1a, 2b, and 3a also
proceeded under the optimal conditions, producing 4a in 75%
yield. However, by contrast, the use of α-chloroacetaldehyde
2c instead of 2b drastically reduced the reaction yield, as
inseparable gum-like products were formed (eq 4). These
results imply that the acetal structure of 2b weakened the
reactivity of the aldehyde side, thereby ensuring the
chloromethyl group exhibited the priority to react with indole
to form tryptaldehyde. This phenomenon can also be verified
by the results in Figure S2 in the SI. When α-
bromoacetaldehyde diethyl acetal 2d and α-bromoacetalde-
hyde dimethyl acetal 2e were used to synthesize 4a, the
obtained maximum yields were substantially lower than that by
the reaction with 2a, although the maximum yields were
reached earlier under identical conditions. Compared with the
cyclic acetal 2a, 2d and 2e were less stable under acidic
conditions and tended to give α-bromoacetaldehyde via a
deacetalization reaction in the presence of water (exist in the
solvent and air), thereby destroying the selectivity to 4a.
Thus, a plausible mechanism for the reaction of 1a, 2a, and
3a was proposed, and it is depicted in Scheme 5. The initial
converted easily into benzo[α]carbazoles through dehydrogen-
ative aromatization reaction,16 and that many cyclohexanone
derivatives are commercially available, this result provides
another significant feature for extending our protocol to the
synthesis of benzo[α]carbazoles. 2-Bromopriopionaldehyde
diethyl acetal was also used instead of 2a in the model
reaction. However, the expected product was not obtained.
In addition, we performed experiments in which 1,3-
dicarbonyl compounds served as the ketone components in
the three-component reaction. As shown in Scheme 3, ethyl
Scheme 3. Reactions of Indoles, 2a, and 1,3-Dicarbonyls
acetoacetate reacted smoothly with 1a and 2a in the presence
of the Bi(OTf)3 catalyst, affording the corresponding carbazole
derivative 4ad in 86% yield (eq 1). The use of N−H indole
achieved nearly the same results as that with 1a for carbazole
synthesis, indicating that the high electrophilicity of the
ketocarbonyl in the β-ketoester remedied the poor reactivity of
the N−H indole. 4,4-Dimethoxy-2-butanone 3d can also react
with 1b and 2a with the aid of a catalytic amount of Bi(OTf)3
to afford 3-acetylcarbazole 4af in 71% yield (Scheme 3, eq 2).
These results suggest that the current three-component
reaction is compatible with structurally distinct substrates
with different electronic properties. It is therefore an efficient
and practical protocol for accessing substituted carbazoles with
high atom and step economy.
Scheme 5. Plausible Mechanism
To elucidate the mechanism, additional experiments were
conducted. From a theoretical perspective, under strong acidic
C
DOI: 10.1021/acs.orglett.8b01707
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
event should be an acid-catalyzed Friedel−Crafts alkylation of
1a with 2a, which would generate a tryptaldehyde-type acetal
5a. Given that the solvent and reagents were not absolutely
dry, trace amounts of water were in the reaction system. As
such, 5a underwent deacetalization to give a tryptaldehyde
derivative 5b. Then, the enol form of 3a trapped 5b to give a
C3-vinylndole derivative I. Finally, 4ab was formed through an
intramolecular condensation reaction. These experimental
results implied that (i) the tryptaldehyde derivative 5b may
be unstable, as it can react with itself or 1a,17 and the acetal 5a
acted like a reservoir of 5b that gradually released it in the
course of the reaction, thereby maximizing the reaction
selectivity; and (ii) the last step of the reaction involved the
formation of a π-expanded carbazole ring, which conceivably
shifted the reaction equilibrium to the product side. Its
efficiency depended not only on the electrophilicity of the
carbonyl group in the ketone component but also on the
nucleophilicity of the C2 position of indole system. Therefore,
N−H indoles and electron-poor indoles were reluctant to
participate in the reaction.
of the large number of commercially available ketones and the
easy access to 2,3-unsubstituted indoles, the proposed method
is a promising strategy for the synthesis of carbazole libraries
with high diversity.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.8b01707.
Experimental procedures, spectroscopic data of the
1
obtained products, and copies of H and 13C NMR
spectra (PDF)
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: klgyl@hust.edu.cn.
ORCID
The utility of this approach in constructing carbazole
scaffolds was also demonstrated in the synthesis of two
compounds. A carbazole derivative 4ag was used as the
intermediate to synthesize a europium complex, which is a
promising candidate for use as a visible-light excitable red
phosphor for luminescence applications.18 The previous
method for synthesizing this compound was established
based on the three-step synthesis with isolation and
purification of intermediates at each step. It also involved the
use of expensive reagents and catalysts, such as 4-
acetylphenylboronic acid and Pd(PPh3)4. By using three easily
available chemicals, such as N-ethylindole 1c, 2a, and 3d, as
starting materials, 4ag can be synthesized in 60% yield through
a one-step reaction (Scheme 6). Similarly, a carbazole
Yanlong Gu: 0000-0001-5130-3026
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the National Natural Science Foundation of
China (21761132014), the Fundamental Research Funds for
the Central Universities of China (2016YXZD033) and
Opening fund of Hubei Key Laboratory of Material Chemistry
and Service Failure (No. 2017MCF01K) for the financial
support. The Cooperative Innovation Center of Hubei
province is also acknowledged.
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