Indole Synthesis via Cobalt(III)-Catalyzed Oxidative Coupling of N-Arylureas and Internal Alkynes

Article abstract of DOI:10.1021/acs.orglett.6b00494

A mild Co(III)-catalyzed oxidative annulation of N-arylureas and internal alkynes has been developed. The use of less electrophilic ureas other than acetamides as directing groups is crucial for the reaction. A broad range of synthetically useful functional groups are compatible with this reaction, thus providing a new opportunity for the synthesis of diverse indoles.

Full text of DOI:10.1021/acs.orglett.6b00494

Letter
pubs.acs.org/OrgLett
Indole Synthesis via Cobalt(III)-Catalyzed Oxidative Coupling of
N‑Arylureas and Internal Alkynes
Zhuo-Zhuo Zhang, Bin Liu, Jing-Wen Xu, Sheng-Yi Yan, and Bing-Feng Shi*
Department of Chemistry, Zhejiang University, Hangzhou 310027, China
S
* Supporting Information
ABSTRACT: A mild Co(III)-catalyzed oxidative annulation of N-
arylureas and internal alkynes has been developed. The use of less
electrophilic ureas other than acetamides as directing groups is crucial
for the reaction. A broad range of synthetically useful functional
groups are compatible with this reaction, thus providing a new
opportunity for the synthesis of diverse indoles.
he indole nucleus is prevalent in natural products,
biologically active molecules, and pharmaceuticals.1
Scheme 1. Indole Synthesis via Transition-Metal-Catalyzed
Oxidative Annulation of Anilines with Alkynes
T
Consequently, interest in the synthesis and modification of
this privileged structural motif has constantly grown,2 after Emil
Fischer’s landmark achievement of indole synthesis over a
century ago.3 One of the most powerful synthetic methods to
access these compounds is Larock’s indole synthesis via Pd-
catalyzed coupling of ortho-haloanilines with alkynes.4 More
recently, a wealth of research has been directed toward the
synthesis of indoles via transition-metal-catalyzed C−H
functionalization, and many elegant methods have been
developed.2c,d Particularly, the oxidative annulation of anilines
or their derivatives with alkynes have received tremendous
attention, due to the ready availability of each coupling partner.
In 2008, Fagnou and co-workers reported the first Rh(III)-
catalyzed indole synthesis via the reaction of acetanilides with
internal alkynes.5 Shortly after, Jiao reported a Pd-catalyzed
oxidative coupling of N-nonsubstituted and N-alkyl mono-
substituted anilines with electron-deficient alkynes.6 Ever since
then, indole synthesis via the oxidative annulation of alkynes
with various N-substituted or N-nonsubstituted anilines
catalyzed by second-row transition metal catalysts, such as
Rh(III), Pd(II), or Ru(II), have been well investigated by Li,7
Glorius,8 Lu,9 Ackermann,10a Huang,11 and others (Scheme
1a).12 In contrast, indole synthesis via C−H activation catalyzed
by more sustainable and cost-effective first-row transition
metals is very rare. Ackermann reported a Ni(0)-catalyzed
annulation of N-(2-pyrimidyl)indoles with internal alkynes.10b
During the preparation of this work, Glorius, Jiao, and
Ackermann independently reported the Co(III)-catalyzed
redox-neutral synthesis of indoles using Boc-protected
hydrazines, N-nitroso, or nitrone as oxidizing directing
groups.13
further, we herein report the Co(III)-catalyzed oxidative
annulation of N-arylureas and internal alkynes to access diverse
indoles. The method features high functional group tolerance,
broad substrate scope, high yields, and the use of a cost-
effective, first-row transition metal catalyst. It was noteworthy
that this indole synthesis via oxidative annulation was enabled
by the fine-tuning of the electrophilicity of the directing groups,
which was mechanistically complementary to the synthesis of
quinolines via nucleophilic addition/dehydration recently
reported by Li (Scheme 1d).17
Recently, cobalt-catalyzed C−H functionalization has
received increased attention due to its abundance and
inexpensiveness.14,15 As part of our ongoing studies on first-
row transition-metal-catalyzed C−H functionalization,16 we had
found that a cationic high valent Cp*Co(III) complex
efficiently catalyzed the C2-selective C−H alkynylation of
indoles.15m To expand the utility of the Cp*Co(III) catalysis
Although the oxidative indole synthesis has been well studied
in Rh(III) catalysis using acetanilides and alkynes,5 we failed to
achieve the Co(III) version of this reaction under the same
Received: February 20, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00494
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
reaction conditions by treatment of acetanilide 1aa with
diphenylacetylene 2a. After extensively screening various
oxidants and additivies, the desired oxidative annulation
product 3aa was only obtained in 5% yield when a copper
salt was used, with the nucleophilic addition/dehydration
product 4aa produced predominantly (4aa, 15%, Scheme 2).
observed when the amount of Na2CO3 was increased from 0.4
to 1.0 equiv (entry 4). NaH2PO4 was found to be the optimal
additive, giving indole 3ab in 44% yield (entry 5). Next, we
increased the quantity of the AgBF4 from 40 to 100 mol %, and
a significant improvement was achieved (entry 6). Further
tuning the amount of oxidants and additives revealed that the
reaction proceeded better in the presence of 2 equiv of
NaH2PO4 and 1.2 equiv of Ag2CO3 (entries 7−9). In addition,
the reaction temperature was also examined, and the yield
could be improved to 84% at 130 °C (entry 10). Finally, the
indole product 3ab could be obtained in 94% isolated yield by
changing the ratio of 1ab and 2a (entry 11). As expected, when
switching an N-phenylurea to the acetanilide as a substrate
under these coupling conditions, indole 3aa was obtained in a
reduced yield (29%) along with the nucleophilic addition/
dehydration product 4aa (entry 12).
Scheme 2. Effect of Directing Groups
With the optimal reaction conditions in hand, we next tested
the scope of the N-arylureas using diphenylacetylene 2a as the
annulation partner (Figure 1). In general, indoles were
This is likely ascribable to the intrinsic property of cationic
Co(III) complexes in activating the carbonyl group toward
nucleophilic attack of organocobalt species.15b,e,17 We reasoned
that lowering the electrophilicity might shift the selectivity to
indole formation. Consistent with this hypothesis, we were
pleased to find that switching acetanilide to an N-phenylurea
(3a) resulted in the isolation of the indole product (3ab) in
25% yield under our previous conditions. Other commonly
used directing groups gave unsatisfactory results (Scheme 2).
Inspired by this promising result, we next sought to improve
the efficiency using 1,1-dimethyl-3-phenylurea 1a and diphenyl-
acetylene 2a as coupling partners (Table 1). The screening of
oxidants revealed that the yield could be increased to 35% by
using AgBF4 (entries 1−3). A slight improvement in yield was
a
Table 1. Optimization of the Reaction Conditions
Figure 1. Scope of N-arylureas.
b
entry
R
oxidant (equiv) additive (equiv) t (°C) yield (%)
1
2
3
4
5
6
7
8
9
10
NMe2
NMe2
NMe2
NMe2
NMe2
NMe2
NMe2
NMe2
NMe2
NMe2
NMe2
Me
Ag2O (0.4)
Na2CO3 (0.4)
Na2CO3 (0.4)
Na2CO3 (0.4)
Na2CO3 (1)
110
110
110
110
110
110
110
110
110
130
130
130
19
28
35
40
44
65
70
76
80
84
94
Ag2CO3 (0.4)
AgBF4 (0.4)
AgBF4 (0.4)
AgBF4 (0.4)
AgBF4 (1)
successfully obtained from a broad range of substrates in
moderate to high yields with ortho-regioselectivity. Both
electron-donating and -withdrawing substituents were tolerated.
A number of synthetically useful functional groups, such as
fluoro (3ba), bromo (3ca), methoxycarbonyl (3da and 3fa),
and methoxy (3ha), were compatible with this reaction. The
tolerance of halides was particularly important, since such
substituents could be further transformed to other functional
groups via a traditional cross-coupling reaction. We also found
that this protocol tolerates the presence of ortho substituents
well. As demonstrated, N-arylureas bearing methyl and
methoxyl in the ortho position reacted smoothly to give the
desired indole products 3ga and 3ha. Furthermore, the
annulation of disubstitued substrate 1ia could also afford the
desired product 3ia in moderate yield. Moreover, the reaction is
not limited to the N,N-dimethyl ureas. When N-pyrrolyl urea
NaH2PO4 (1)
NaH2PO4 (1)
NaH2PO4 (1)
NaH2PO4 (2)
NaH2PO4 (2)
NaH2PO4 (2)
NaH2PO4 (2)
NaH2PO4 (2)
Ag2CO3 (1)
Ag2CO3 (1)
Ag2CO3 (1.2)
Ag2CO3 (1.2)
Ag2CO3 (1.2)
Ag2CO3 (1.2)
c
d
11
c
d
d
12
3aa, 29
4aa, 16
a
Reaction conditions: 1a (0.10 mmol), 2a (0.15 mmol), [CoCp*-
b
(CO)I2] (10 mol %), AgSbF6 (20 mol %) in DCE (1 mL). 1H NMR
c
yield using 1,3,5-trimethoxybenzene as the internal standard. 1a (0.15
mmol), 2a (0.10 mmol). Isolated yield.
d
B
DOI: 10.1021/acs.orglett.6b00494
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1ja was reacted with diphenylacetylene 2a, indole 3ja was
obtained in 74% yield.
Subsequently, we evaluated the substrate scope with respect
to internal alkynes (Figure 2). The reaction was compatible
[d5] was then treated and gave a consistent value of kH/kD = 3.3
(Scheme 3b), indicating that C−H activation is likely turnover-
limiting in coupling systems. To check the reversibility of the
C−H activation step, 1ab-[d5] was heating in MeOH under the
oxidative coupling conditions in the absence of diphenyl-
acetylene, and H/D scrambling was not observed here (Scheme
3c). This suggested irreversibility of C−H activation. This
observation is in sharp contrast to Glorius and Jiao’s
observations.13a,b
On the basis of our mechanistic experiments and previous
reports,5b,12d,15i a plausible catalytic cycle is depicted in Scheme
4. First, an active cationic Co(III) catalyst is generated upon
Scheme 4. Proposed Reaction Mechanism
Figure 2. Scope of alkynes.
with electron-donating substituents at the para-position on the
benzene rings of internal alkynes, such as Me-, MeO-, nBu-, and
tBu-groups (5a−5d), providing the corresponding indoles in
generally good yields (42%−75%). In addition, halogen-
substituted benzene rings of internal alkynes proceeded to
give the desired product in moderate yields (5e, 63% and 5f,
48%). Furthermore, a heterocyclic thiophene motif also served
as a suitable substrate as exemplified by the synthesis of 5g. In
addition, 1-phenyl-1-propyne coupled in high regioselectivity,
and only one single regioisomer was detected (5h).
Unfortunately, terminal alkynes were not compatible with this
protocol.
To gain some insight into the mechanism, a series of
deterium labeling experiments were carried out (Scheme 3).
Two separate experiments using 1ab and 1ab-[d5] were carried
out, from which a kH/kD value of 1.6 was found on the basis of
1H NMR (Scheme 3a). Further, a 1:1 mixture of 1ab and 1ab-
treatment of [Cp*Co(CO)I2] with AgSbF6. Cobalt-mediated
irreversible C−H bond activation of N-arylurea affords a six-
membered metallacyclic intermediate A.15i Alkyne coordination
and migratory insertion of the aryl group generates a Co(III)
alkenyl species B. Intermediate C is formed via base-assisted
elimination of HX. C−N bond reductive elimination delivers
indole 3aa along with a Co(I) species D. The catalytic cycle is
completed when Co(I) is reoxidized to Co(III) via Ag2CO3 and
O2. When acetanilide 1aa was used as a substrate, quinolone
4aa was generated via nucleophilic addition of metallacyclic B
followed by dehydration.17
In conclusion, we have developed the Cp*Co(III)-catalyzed
oxidative annulation of N-arylureas and internal alkynes for the
construction of indoles. The use of less electrophilic ureas other
than acetamides as directing groups is crucial for the reaction.
This reaction proceeds under mild conditions and tolerates a
wide range of functional groups, providing a straightforward
access to diverse indoles. In addition, when unsymmetrical
internal alkynes were employed, the annulation adducts were
obtained with high regioselectivity.
Scheme 3. Mechanism Studies
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.6b00494.
Experimental details and spectral data for all new
compounds (PDF)
C
DOI: 10.1021/acs.orglett.6b00494
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
11202. [Pd]: (l) Manna, M. K.; Hossian, A.; Jana, R. Org. Lett. 2015,
17, 672.
(13) (a) Lerchen, A.; Vasquez-Cespedes, S.; Glorius, F. Angew.
AUTHOR INFORMATION
■
Corresponding Author
Chem., Int. Ed. 2016, 55, 3208. (b) Liang, Y. J.; Jiao, N. Angew. Chem.,
*E-mail: bfshi@zju.edu.cn.
Notes
Int. Ed. 2016, 55, 4035. (c) Wang, H.; Moselage, M.; Gonzal
́
ez, M. J.;
Ackermann, L. ACS Catal. 2016, 6, 2705.
(14) For recent reviews on cobalt-catalyzed C−H functionalization,
see: (a) Kozhushkov, S. I.; Potukuchi, H. K.; Ackermann, L. Catal. Sci.
Technol. 2013, 3, 562. (b) Gao, K.; Yoshikai, N. Acc. Chem. Res. 2014,
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Am. Chem. Soc. 2014, 136, 5424. (c) Yu, D.-G.; Gensch, T.; de
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the National Basic Research Program of
China (2015CB856600), the NSFC (21572201, 21422206,
21272206), and the Fundamental Research Funds for the
Central Universities (2015XZZX004-03) is gratefully acknowl-
edged.
́ ́
Azambuja, F.; Vasquez-Cespedes, S.; Glorius, F. J. Am. Chem. Soc.
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