Gold/Copper-Co-catalyzed Tandem Reactions of 2-Alkynylanilines: A Synthetic Strategy for the C2-Quaternary Indolin-3-ones

Article abstract of DOI:10.1021/acs.orglett.7b00200

A new strategy for direct and highly efficient synthesis of 2,3′-bisindolin-3-ones has been developed via a gold/copper-co-catalyzed tandem reactions of 2-alkynylanilines using TBHP as the terminal oxidant and oxygen-atom source. The single-step process involves a novel tandem intermolecular nucleophilic addition, intramolecular cyclization/oxidative cross-dehydrogenative coupling where up to four new bonds and two indole rings were created simultaneously in one-pot manner. The reaction features a broad substrate scope, good functional group tolerance, and high atom-economy.

Full text of DOI:10.1021/acs.orglett.7b00200

Letter
pubs.acs.org/OrgLett
Gold/Copper-Co-catalyzed Tandem Reactions of 2‑Alkynylanilines:
A Synthetic Strategy for the C2-Quaternary Indolin-3-ones
Yi-Jin Li, Na Yan, Chun-Hua Liu, Yang Yu, and Yu-Long Zhao*
Jilin Province Key Laboratory of Organic Functional Molecular Design & Synthesis, Faculty of Chemistry, Northeast Normal
University, Changchun 130024, China
S
* Supporting Information
ABSTRACT: A new strategy for direct and highly efficient
synthesis of 2,3′-bisindolin-3-ones has been developed via a
gold/copper-co-catalyzed tandem reactions of 2-alkynylani-
lines using TBHP as the terminal oxidant and oxygen-atom
source. The single-step process involves a novel tandem
intermolecular nucleophilic addition, intramolecular cycliza-
tion/oxidative cross-dehydrogenative coupling where up to four new bonds and two indole rings were created simultaneously in
one-pot manner. The reaction features a broad substrate scope, good functional group tolerance, and high atom-economy.
2,2-Disubstituted indolin-3-ones are one of the privileged
structural units found in many natural products such as isatisine
A, austamide, brevianamide, fluorocurine, cephalinone, and
notoamide O (Scheme 1) and biologically active products.1 In
catalyzed tandem transformation of acyclic alkynes; however, the
reactions are severely limited to 1-nitroarylalkynes, 2-alkynyl
arylazides and 1-(2-(allylamino)phenyl-4-hydroxybut-2-yn-1-
ones.8 To the best of our knowledge, direct methods for one-
pot synthesis of 2,2-disubstituted indolin-3-ones from simple 2-
alkynylanilines have never been reported. Herein, we report a
new synthetic strategy for the direct, one-pot synthesis of 2,2-
disubstituted indolin-3-ones via gold/copper-co-catalyzed tan-
dem reactions of simple 2-alkynylanilines employing TBHP as
the terminal oxidant and oxygen-atom source.
Scheme 1. Representative Natural Products with a 2,2-
Disubstituted Indolin-3-one Skeleton
As part of our endeavors on carbon−carbon and carbon−
heteroatom bond-forming reactions,9 we recently disclosed
several routes for the synthesis of 1,2,3-triazoles,10a α-amino-
cyclopentenones,10b dihydrotriazinones,10c and dihydropyrida-
zinones10c starting from easily accessible α-amino carbonyl
derivatives. These results and our interests in the synthetic
applications of functionalized alkynes11 prompted us to
investigate the cyclization reaction of N-substituted 2-(2-
(ethynyl)arylamino)acetamides 1a. As a result, we found that
the tandem reaction of 1a (0.2 mmol) proceeded smoothly to
give 2-phenyl-2,3′-biindolin-3-one 2a in 70% yield in the
addition, apart from their use as intermediates in the
construction of various natural products,2 in recent years,
indol-3-one derivatives have also found wide applications in
the areas of fluorescence dyeing and in solar cell applications.3
Accordingly, the development of novel and efficient synthetic
methods for 2,2-disubstituted indolin-3-one derivatives has been
a major topic in synthetic organic chemistry.1−8 For the
construction of the 2,2-disubstituted indolin-3-one framework,
the predominant strategy is direct transformation of indoles,4
3H-indol-3-ones,5 and indolin-3-ones.6 In stark contrast, reports
on the direct and efficient synthesis of 2,2-disubstituted indolin-
3-one derivatives from easily available acyclic starting materials in
a single step are considerably rare.7,8 The Smalley cyclization
reaction of α-azidoaryl sec-alkyl ketones is an efficient and
classical route to 2,2-disubstituted indolin-3-ones.7 Recently,
three elegant methods for the synthesis of 2,2-disubstituted
indolin-3-ones have been reported via the transition-metal-
presence of X-PhosAuCl (1.5 mol %), NaBArF (3.0 mol %),
4
CuO (10 mol %), and tert-butyl hydroperoxide (1.5 equiv) in
DCE (1.0 mL) at 80 °C for 15 min along with 1H-indole 3a in 9%
yield (Table 1, entry 5).12 Decreasing the amount of CuO, X-
PhosAuCl, and NaBArF4 led to comparatively lower yields of 2a
(Table 1, entries 1−4). Other copper catalysts, such as CuOAc·
H2O and CuBr, were less effective (Table 1, entry 9) or
ineffective (Table 1, entry 10). Different oxidants were also
screened; however, no product 2a was obtained when 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) and tert-butyl-
peroxybenzoate (TBPB) were used as the oxidants (Table 1,
entries 11 and 12). In addition, when the reaction was carried out
Received: January 19, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b00200
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Letter
a
Table 1. Optimization of Reaction Conditions
Scheme 2. Synthesis of 2,2′-Diphenyl-2,3′-biindolin-3-ones
a,b
2a−i
b
yield (%)
oxidant
entry
1
catalyst (mol %)
(equiv)
solvent
DCE
2a 3a
CuO (5)/X-PhosAuCl
(1)/NaBArF4 (2)
TBHP
(1.5)
51
59
61
64
70
69
70
69
49
28
28
30
20
9
2
3
4
5
6
7
8
9
CuO (10)/X-PhosAuCl
(1)/NaBArF4 (2)
TBHP
(1.5)
DCE
DCE
DCE
DCE
DCE
DCE
DCE
DCE
CuO (15)/X-PhosAuCl
(1)/NaBArF4 (2)
TBHP
(1.5)
CuO (10)/X-PhosAuCl
(1.5)/NaBArF4 (2)
TBHP
(1.5)
c
c
CuO (10)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBHP
(1.5)
CuO (15)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBHP
(1.5)
9
CuO (10)/X-PhosAuCl
(2)/NaBArF4 (3)
TBHP
(1.5)
10
10
42
CuO (10)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBHP
(2.0)
Cu(OAc)2H2O (10) /X-
PhosAuCl (1.5)
TBHP
(1.5)
/NaBArF4 (3)
10
11
12
13
14
CuBr (10)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBHP
(1.5)
DCE
trace
CuO (15)/X-PhosAuCl
(1.5)/NaBArF4 (3)
DDQ
(1.5)
DCE
CuO (15)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBPB
(1.5)
DCE
CuO (10)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBHP
(1.5)
toluene
DMF
trace
trace
95
36
CuO (10)/X-PhosAuCl
(1.5)/NaBArF4 (3)
TBHP
(1.5)
a
Reaction conditions: 1a (0.2 mmol), CuO (0.01−0.03 mmol), X-
PhosAuCl (0.002−0.004 mmol), NaBArF (0.004−0.006 mmol),
4
solvent (1.0 mL), at 80 °C for 15−30 min. X-Phos =2-
dicyclohexylphosphino-2′,4′,6′-triisopropylbiphenyl, NaBArF = so-
4
a
dium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate, TBHP = tert-
butyl hydroperoxide, TBPB = tert-butylperoxybenzoate, DDQ = 2,3-
Reaction conditions: 1 (0.4 mmol), CuO (0.04 mmol), X-PhosAuCl
(0.006 mmol), NaBArF (0.012 mmol), DCE (2.0 mL), TBHP (0.6
mmol), at 80 °C for 15−30 min. Isolated yield. 2.0 equiv of TBHP
was used. 0.1 equiv of Cu(OAc)2·H2O and 4.0 equiv of TBHP were
4
b
dichloro-5,6-dicyano-1,4-benzoquinone. Estimated by 1H NMR
b
c
d
spectroscopy using dimethyl phthalate as an internal standard.
c
e
Isolated yield.
used. 0.1 equiv of Cu(OAc)2·H2O was used.
in toluene or DMF (Table 1, entries 13 and 14), only a trace
amount of 2a was observed (TLC).
under the optimal conditions, furnishing 1.03 g of the desired
product 2g in 64% isolated yield.
Under the optimal reaction conditions (Table 1, entry 5), the
scope and generality of the reaction were examined, and the
results are summarized in Scheme 2. It is obvious that the tandem
reaction showed broad tolerance for various R substituents at the
nitrogen. All selected 2-(phenylethynyl)anilines 1a−f, bearing
various alkyl and benzyl R groups, could efficiently undergo the
tandem reaction to give the corresponding 2,2′-diphenyl-2,3′-
biindolin-3-ones 2a−f in good to high yields. More importantly,
in the case of N-unprotected 2-(phenylethynyl)aniline 1g, the
tandem reaction also worked well, yielding the desired product
2g in 66% yield. In addition, substrates 1h and 1i possessing
electron-withdrawing and electron-donating R1 groups on the
aminophenyl ring produced the desired 2,2′-diphenyl-2,3′-
biindolin-3-ones 2h and 2i in 80% and 60% yields, respectively.
In addition, it should be noted that this tandem reaction is easy to
scale-up. A gram-scale reaction of 1.55 g of 1g was carried out
Next, we turned to extending the scope of the reaction by
varying the substituents on the alkyne moiety of 2-alkynylanilines
1. As shown in Scheme 3, all selected substrates 1j−n, bearing
electron-rich and electron-deficient aryl, heteroaryl, and alkyl R2
groups at the alkyne terminus, could provide the desired 2,3′-
diindolin-3-ones 2j−n in good to high yields. Similarly, the
desired 2,3′-diindolin-3-ones 2o−r were obtained in good to
high yields from the substrates 1o−r with different substituents
on the alkyne and aryl moieties.
To further probe the mechanisms for formation 2, some
control experiments were designed and investigated. When 1g
was treated with CuO (10 mol %) under otherwise identical
conditions but in the absence of X-PhosAuCl and NaBArF , no
4
desired product 2g was produced and only coupling product 4g
was generated in 35% yield along with the recovery of 1g in 55%
yield (Scheme 4, a), which demonstrates that X-PhosAuCl and
B
DOI: 10.1021/acs.orglett.7b00200
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Letter
a,b
Scheme 3. Synthesis of 2,3′-Diindolin-3-ones 2j−r
Scheme 4. Control Experiments for Mechanistic Studies
observed when indole 5g was used as a partner under the optimal
reaction conditions (Scheme 4, h).
On the basis of the above experimental results together with
related reports,13,14 a possible mechanism for the formation of 2
is proposed in Scheme 5. Initially, the attack of the hydroxy group
Scheme 5. Proposed Mechanisms for the Formation of 2
a
Reaction conditions: 1 (0.4 mmol), CuO (0.04 mmol), X-PhosAuCl
(0.006 mmol), NaBArF (0.012 mmol), DCE (2.0 mL), TBHP (0.6
mmol), at 80 °C for 30−35 min. Isolated yield. 2.0 equiv of TBHP
was used.
4
b
c
NaBArF play an important role for the cyclization trans-
4
formation. The desired product 2g was obtained in 20% yield
along with the formation of indole 5g in 64% yield under
otherwise identical conditions but in the absence of CuO
(Scheme 4, b). In addition, the reaction of 1g in dry DCE did not
provide [18O]-2g in the presence of H218O (2.0 equiv) under
otherwise identical conditions (Scheme 4, c). When the reaction
was carried out under an atmosphere of nitrogen, the desired
product 2g was produced in 68% yield (Scheme 4, d). However,
no desired product 2g was generated, and only indole 5g was
obtained in 83% yield when the reaction was performed under
otherwise identical conditions but in the absence of TBHP
(Scheme 4, e). These results indicate that H2O and O2
atmospheres are not involved in the tandem reaction and
TBHP participated in the transformation as the oxidant source.
The formation of product 2g was suppressed under the
optimized conditions in the presence of 2.0 equiv of 2,6-di-tert-
butyl-4-methylphenol (BHT) as a radical inhibitor (Scheme 4, f),
indicating that a radical process was involved in the reaction. In
addition, the reaction of 6g (0.2 mmol) and 5g (0.2 mmol)
proceeded smoothly to give the desired product 2g in 60% yield
in the presence of CuO (10 mol %) and TBHP (1.5 equiv) in
DCE (1.0 mL) at 80 °C for 1.5 h (Scheme 4, g). No reaction was
of TBHP on the electrophilic π-alkyne moiety of gold complex A
generates intermediate B,13 which undergoes an intramolecular
amination cyclization reaction in a 5-endo-dig fashion, followed
by elimination of t-BuOH and protonation to give intermediate
D.13 Immediately, intermediate D is oxidized by CuO through a
single-electron transfer (SET) process to generate the
ammoniumyl radical cation intermediate E, which undergoes
hydrogen radical transfer to form iminium salt intermediate F.14
Finally, indole 5, generated in situ via gold-catalyzed intra-
molecular cyclization of 1,13 undergoes nucleophilic addition
with iminium salt intermediate F, followed by loss of a proton to
produce the corresponding indolin-3-ones 2 (Scheme 5).
In conclusion, we have developed a new strategy for the direct
and highly efficient synthesis of 2,3′-bisindolin-3-ones via a gold/
copper-cocatalyzed tandem reactions of simple 2-alkynylanilines
using TBHP as the terminal oxidant and oxygen-atom source.
The single-step process involves a novel tandem intermolecular
nucleophilic addition, intramolecular cyclization/oxidative cross-
dehydrogenative coupling where up to four new bonds and two
C
DOI: 10.1021/acs.orglett.7b00200
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.7b00200.
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AUTHOR INFORMATION
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*E-mail: zhaoyl351@nenu.edu.cn.
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Yu-Long Zhao: 0000-0001-6577-1074
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support of this research by the National Natural
Sciences Foundation of China (21472017), the Natural Sciences
Foundation of Jilin Province (20150101065JC), and the
Thirteenth Five-Year Sci-Tech Research Guidelines of the
Education Department of Jilin Province are greatly acknowl-
edged.
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