N-Bromosuccinimide (NBS)-Catalyzed C-H Bond Functionalization: An Annulation of Alkynes with Electron Withdrawing Group (EWG)-Substituted Acetyl Indoles for the Synthesis of Carbazoles

Article abstract of DOI:10.1021/acs.orglett.7b03021

An N-bromosuccinimide-catalyzed intermolecular annulation of acetyl indoles with alkynes was developed, allowing for regioselective formation of valuable carbazoles through direct C-H bond functionalization. The readily available catalyst, wide substrate scope, gram scale synthesis, and mild conditions make this method practical. Mechanistic investigations indicate that the bromination of acetyl indole takes place to generate a bromide intermediate, followed by coupling with an alkyne and intramolecular cycloaromatization to furnish carbazole products.

Full text of DOI:10.1021/acs.orglett.7b03021

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX-XXX
N‑Bromosuccinimide (NBS)-Catalyzed C−H Bond Functionalization:
An Annulation of Alkynes with Electron Withdrawing Group (EWG)-
Substituted Acetyl Indoles for the Synthesis of Carbazoles
Han Wang, Zhen Wang, Yi-Long Wang, Rui-Rui Zhou, Guang-Chuan Wu, Si-Yao Yin, Xu Yan,
and Bin Wang*
State Key Laboratory of Medicinal Chemical Biology, College of Pharmacy and Tianjin Key Laboratory of Molecular Drug Research,
Nankai University, Haihe Education Park, 38 Tongyan Road, Tianjin 300353, People’s Republic of China
S
* Supporting Information
ABSTRACT: An N-bromosuccinimide-catalyzed intermolecular annulation of
acetyl indoles with alkynes was developed, allowing for regioselective formation
of valuable carbazoles through direct C−H bond functionalization. The readily
available catalyst, wide substrate scope, gram scale synthesis, and mild
conditions make this method practical. Mechanistic investigations indicate
that the bromination of acetyl indole takes place to generate a bromide
intermediate, followed by coupling with an alkyne and intramolecular
cycloaromatization to furnish carbazole products.
he wealth of carbazoles in biologically active molecules1 and
organic light-emitting materials2 has driven organic
strategy employs a variety of O- or N-containing compounds as
reaction partners to generate intermediates of cyclization.
Although some elegant metal-free examples have been
reported,15f−p these reactions are often accompanied by the loss
of large molecular fragments, and some of them are actually one-
potmultistepreactions rather thanone-steptransformations. The
preparationofsomeunusualstartingmaterialsisneededincertain
cases of path b. By comparison, path a is a more efficient
cyclization strategy with higher atom economy since it is based on
the direct cross-dehydrogenative coupling (CDC) reaction of
multiple C−H bonds.16 Another advantage of path a is that
various functionalized alkenes or alkynes are readily available.
Thus, replacement of metal catalysts to common organic catalysts
will make this approach more attractive. Very recently, simple
iodine reagents such as molecular iodine and iodide salts have
been intensively studied as an environmentally friendly
alternative to metal catalysts in C−H bond functionalization.17
In sharp contrast, many cheap and readily available bromine
reagents are well-known for their bromination and oxidation
reactions,18 but their catalytic applications in organic synthesis,19
especially in the direct C−H bond functionalization for C−C
bond formation, are extremely rare.19f Herein, we report an N-
bromosuccinimide (NBS) catalyzed intermolecular annulation of
functionalized indoles with alkynes through direct C−H bond
functionalization (Scheme 1). This highly efficient and practical
method allows for the gram scale synthesis of carbazoles with
complete regioselectivity under mild conditions, providing a
useful alternative to existing protocols.
T
chemists to continue developing novel methods toward their
synthesis. Apart from the traditional Fischer−Borsche,3 Bucher-
er,4 Cadogan,5 and Graebe−Ullmann6 carbazole synthesis, recent
work has enabled carbazole construction to be performed with a
growing number of starting materials.7 Of these processes,
intermolecular annulation has received much attention because it
allows for the atom-economical synthesis of functionalized
carbazoles directly from simple indole or its easily available
C2(3)-functionalized derivatives (Scheme 1). The benzannula-
Scheme 1. Annulation for Synthesis of Carbazoles
tion of indoles typically occurs by two pathways in terms of the
reaction partner used. The first proceeds through transition-
metal-catalyzed C−H bond activation of indoles and their
reaction partners alkenes or alkynes (path a). Transition metal
catalysts such as Pd,8 Au,9 Ag,10 Ir,11 Mn,12 bimetallic Pd−Cu,13
and trimetallic Pd−Cu−Ag14 were revealed to catalyze these [4 +
2] or [2 + 2 + 2] cycloadditions to produce various carbazoles.
The second proceeds via tandem reaction of indoles with diene or
alkene precursors to furnish the carbazoles (path b).15 This
Thestudybeganbyinvestigatingthereactionof3-(1H-indol-3-
yl)-3-oxopropanenitrile 1a with phenylacetylene 2a using TBHP
Received: September 26, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b03021
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Screening of the Reaction Conditions
Scheme 2. Variation of the Alkyne Component
entry
cat. (x mol %)
yield (%)
1
2
3
4
5
6
Bu4NI (10)
KI (10)
trace
trace
24
I2 (10)
Bu4NBr (10)
NBS (10)
NBS (30)
NBS (30)
−
16
66
90
b
7
b
99
8
trace
a
Reaction conditions: 1a (0.5 mmol), 2a (1.0 mmol), TBHP (3.0
b
equiv, 70% aqueous solution), under air; isolated yields. TBHP (3.5
equiv, 70% aqueous solution).
as an oxidant. Attempts to realize the intermolecular annulation
using 10 mol % of Bu4NI or KI only led to a trace amount of the
desired product 3aa (Table 1, entries 1 and 2), while I2 led to 3aa
in24% isolated yield(entry 3), accompanied bytheoxyiodination
of alkyne 2a, which converted to a complex mixture of side
products.20 Considering that bromide (Br−) has a higher
oxidation potential than iodide (I−),21 we then used bromides
instead of iodides to avoid the side reactions. However, no yield
improvement was observed with Bu4NBr (entry 4). We were
delighted to find that NBS could promote the annulation with
high catalytic efficacy to afford 3aa in 66% yield (entry 5).
Increasing the amount of NBS to 30 mol % gave a very high yield
(90%) of 3aa (entry 6), and a quantitative yield was further
achieved in the presence of 3.5 equiv of TBHP (entry 7). It is
worth mentioning that the annulation of 1a and 2a yielded only
trace product 3aa in the absence of NBS (entry 8).
a
Reaction conditions: 1a (0.5 mmol), 2 (1.0 mmol), NBS (30 mol %),
TBHP (3.5 equiv, 70% aqueous solution); isolated yield.
subsequently functionalized. When methyl acetylenecarboxylate
2v and cyclopropylacetylene 2w were subjected to the reaction
conditions, the corresponding carbazole 3av and 3aw were
obtained in 25% and 29% yields, respectively. An alkyne with a
long alkyl chain could be similarly employed, albeit in a low yield
(3ax, 12%).
The scope of the indoles was next examined using phenyl-
acetylene 2a as the starting material (Scheme 3). The electron-
a
Scheme 3. Variation of the indole component
Encouraged by the excellent yield, we further performed the
annulation in gram scale under the optimal conditions, and 1.3 g
of 3aa was isolated in 76% yield starting from 1.5 g of 1a and 0.4 g
of 2a (Scheme 2, 3aa). Moreover, this operationally simple
cycloaromatization reaction installs a cyano group at C3 and a
hydroxyl at C4, which provides the possibility of modifying
carbazoles. With the established conditions in hand, various
alkynes were first investigated to explore the applicability of the
reaction. As shown in Scheme 2, an array of diversely substituted
terminal arylacetylenes underwent the annulations to afford the
corresponding carbazoles 3aa−aq in good to excellent yields.
More than half of them afforded the yields over 90%, and even
several alkynes gave almost quantitative yields (Scheme 2, 3aa,
3al, 3an, and 3ap). As is known, the use of terminal alkynes in the
transition-metal-catalyzed annulation is often problematic due to
the homocoupling. Furthermore, some annulations produced a
mixture of regioisomers without selectivity when unsymmetrical
alkynes were used as reaction partners. The present metal-free
process avoids the homocoupling of these terminal alkynes and
leads to a completely regioselective construction of carbazole in
whichthearylgroupislocatedattheC1position. Forproduct 3al,
its constitution was unambiguously established by single-crystal
X-ray analyses.22 The reaction conditions could also be extended
to the regioselective formation of a thiophene substituted
carbazole in 66% yield, using 2-ethynylthiophene 2r (Scheme 2,
3ar). Unfortunately, the low yields were observed in the reactions
of internal alkynes (3as−au). The annulation of 1-bromo-2-
phenylacetylene 2u yields a carbazole bromide 3au that can be
a
Reaction conditions: 1 (0.5 mmol), 2a (1.0 mmol), NBS (30 mol %),
TBHP (3.5 equiv, 70% aqueous solution); isolated yield.
donating or -withdrawing character of the substituents on the
indoles were well tolerated (3ba−ma), providing the products in
moderate to excellent yields (62−99%). It should be mentioned
that electron-donating groups somewhat retarded the annulation,
and 6-OMe substrate 1d gives only a 62% yield of 3da. We were
pleased to find that not only N−H indoles but also N-methyl and
N-benzyl indoles reacted smoothly to yield the corresponding
carbazoles in high yields (3na and 3oa). Interestingly, the
annulation of N-acetyl indole 1p produced 3aa rather than 3pa.
Obviously, the Ac group was released in this case. In addition to
the cyano group, other electron-withdrawing groups in the side
B
DOI: 10.1021/acs.orglett.7b03021
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 4. Control Experiments
Scheme 5. Proposed Mechanism
Regeneration of the radical B is achieved through SET oxidation
of D by bromide A to the corresponding cationic intermediate E,
which upon deprotonation eventually provides carbazole 3
(outside cycle). The released Br− is oxidized by tBuOOH to
regenerate “Br+”. Alternatively, the radical D can first be
deprotonated to a radical anion [D]•−, which then reduces the
bromide A to give radical anion [A]•−, thereby forming the
product 3. Radical anion [A]•− subsequently fragments to radical
B and Br− (inside cycle).26 In addition, it is also possible that the
radical−radical coupling of D with more stabilized radical
tBuOO· produces intermediate F, which forms carbazole 3
through ionic elimination of tBuOOH.27
chain of acetyl indoles were also tested in the reaction (3qa−sa).
The substrates with an ester, an amide, and a ketone converted to
the desired carbazoles smoothly. Especially regarding 1r, a
quantitative yield was observed (3ra). When EWG was replaced
with a phenyl, the annulation did not occur and the starting
materials were recovered.
The mechanism of the annulation was investigated after we
explored the substrate scope (Scheme 4). In the cycloaromatiza-
tion reaction of 1q, the bromination of 1q occurred
simultaneously, and the corresponding bromide 4 was isolated
in 10% yield (eq 1). Independently prepared 4 was then subjected
to reaction. Regardless if whether NBS was used or not, the
reactionproceededsmoothlytoyieldthedesired3qa(eq2).Even
in the absence of NBS and TBHP, 3qa was obtained in 10% yield
(eq 2). Moreover, 1.0 equiv of NBS could promote the reaction of
1awith 2atogive 3aain13% yieldinthe absence of TBHP(eq 3).
Similar control experiments were carried out with 1-(bromoe-
thynyl)-4-ethylbenzene 2u, which could be another possible
intermediate generated by bromination of the alkyne. The
carbazole bromide 3au was isolated in 22% yield, and another
potential product 3al generated from the release of Br was not
detected (eq 4). These results suggest that the α-bromination of
acetyl indoles, rather than the bromination of alkynes, is involved
in this cycloaromatization reaction. The free radical inhibition
studywasthen carriedout inthepresence ofTEMPO(3.5 equiv).
The reaction was completely retarded by TEMPO, and neither
3aa nor brominated intermediate 5 was detected (eq 5). We
further performed afree radical inhibition studyon the reaction of
bromide 4 with 2a. Product 3qa was not detected either (eq 6).
Obviously, TEMPO not only inhibited the whole reaction but
also destroyed the annulation of the resulting intermediate 4 with
In summary, we have developed an efficient and scalable
method for the intermolecular annulation of acetyl indoles with
alkynes through direct C−H bond functionalization. The
carbazoles are smoothly formed under mild and metal-free
conditions. The use of NBS as the catalyst allows the formation of
3
2
C−C bonds from Csp , Csp , and Csp bonds with complete
regioselectivity. Further development and applications of this
cycloaromatization in (hetero)aromatic rings synthesis are
ongoing in our laboratory. We hope that the simple and
commercially available bromine reagents will find new
applications in organic synthesis, especially in green and
sustainable chemistry.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.7b03021.
Detailed experimental procedures and spectral data (PDF)
AUTHOR INFORMATION
alkyne 2a. Based on the above results and previous reports,11,23
a
■
Corresponding Author
*E-mail: wangbin@nankai.edu.cn.
ORCID
mechanistic pathway is proposed (Scheme 5). First, an oxidation
of NBS occurs to produce active species “Br+” (Br2, BrOH, BrO−,
24
+
andBrOH2 ), aswell astBuO·and tBuOO·via theSETprocess.
These “Br+” species undergo bromination with 1 to give
intermediate A.21,25 The initiation reaction provides radical B,
which is generated through hemolysis of the C−Br bond of
intermediate A or through H-abstraction from 1. Next, radical B
enters an innate chain cycle26 and couples with alkyne 2 yielding
vinyl radical C,11,23a,b whose accessibility and stability determine
the regioselectivity of annulation. That is, the stable arylvinyl
radicals are formed preferentially in the cases of arylacetylenes,
leading to carbazoles with complete regioselectivity. An intra-
molecular homolytic aromatic substitution (HAS) of C
subsequently takes place to provide radical intermediate D.11,23
Bin Wang: 0000-0002-3674-8980
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the National Natural Science
Foundation of China (Nos. 21172120 and 21472093), Tianjin
Municipal Science and Technology Commission (No.
14JCYBJC20600), and the University Student Innovation
Program of Tianjin (No. 201710055342) for funding support.
C
DOI: 10.1021/acs.orglett.7b03021
Org. Lett. XXXX, XXX, XXX−XXX

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