Synthesis of Oxindoles through the gold-catalyzed oxidation of N-arylynamides

Article abstract of DOI:10.1002/ejoc.201300162

α-Oxo gold carbenoids generated by the oxidation of N-arylynamides can be trapped intramolecularly at the ortho position of the aryl ring to give functionalized oxindoles under mild reaction conditions. Pyridine N-oxide works as the oxidant, ligand, and base in this transformation. Copyright

Full text of DOI:10.1002/ejoc.201300162

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201300162
Synthesis of Oxindoles through the Gold-Catalyzed Oxidation of
N-Arylynamides
Liu-Qing Yang,[a] Kai-Bing Wang,[a] and Chuan-Ying Li*[a]
Keywords: Carbenes / Alkynes / Oxidation / Gold / Heterocycles
α-Oxo gold carbenoids generated by the oxidation of N-
arylynamides can be trapped intramolecularly at the ortho
position of the aryl ring to give functionalized oxindoles un-
der mild reaction conditions. Pyridine N-oxide works as the
oxidant, ligand, and base in this transformation.
Introduction
The oxindole scaffold is a privileged structure in modern
synthetic chemistry because it is the core of a range of natu-
ral products and biologically active compounds.[1] Many
synthetic strategies have been established to access this skel-
eton, including the oxidation of indoles,[2] reduction or ad-
dition of isatin,[3] Heck-type reactions,[4] cyclization of o-
haloanilides[5] or α-haloanilides,[6] double C–H activation of
unfunctionalized anilides,[7] and so on. However, general
and convenient methods for the construction of oxindoles
bearing various functional groups with easily available
starting materials are still of great interest.
Scheme 1. Initial hypothesis.
Given the pioneering work of Toste[8] and L. Zhang[9] Results and Discussion
and the efforts of Liu,[10] Shin,[11] Davis,[12] Gagosz,[13]
To our delight, treatment of 1a with Ph3PAuCl/AgSbF6
J. Zhang,[14] Li,[15] Asensio,[16] Skrydstrup,[17] Maulide,[18]
Hashmi,[19] and our group,[20] alkynes can be viewed as pre-
cursors of gold carbenoids. Various transformations such
as insertion, cycloaddition, 1,2-H shift, and ylide formation
have been realized. In line with this strategy, we envisioned
that treatment of N-arylynamide 1 with a gold catalyst and
pyridine N-oxide would afford α-oxo gold carbenoid, which
could undergo sp2 C–H insertion to afford oxindole 2
(Scheme 1). As ynamides 1 can be easily prepared by cop-
per-mediated amidation of bromoalkynes or terminal
acetylenes,[21] this work may provide convenient access to
the oxindole skeleton. Furthermore, in view of the high
electrophilicity of the gold carbene intermediate,[9p] C–H in-
sertion may be realized even if the aryl ring is electron poor.
Thus, a conceptually different approach to achieve various
oxindole derivatives can be established.
(4 mol-%) and pyridine N-oxide (2 equiv.) in ClCH2CH2Cl
at 40 °C afforded oxindole 2a in 50% yield after 25 min,
together with 19% yield of 4a as a byproduct (Table 1, en-
try 1). The formation of 4a can be ascribed to Wolff re-
arrangement of A and subsequent oxidation and hydroli-
zation.[22] Gold catalysts with N-heterocyclic carbene li-
gands are not a good choice for this reaction (Table 1, en-
tries 2 and 3). If a stronger Lewis acidic gold catalyst such
as 5a or AuCl3 was used,[23] the yield of 2a decreased and
a greater amount of 4a was isolated (Table 1, entries 4 and
5). Silver salts, oxidants, and solvents have a great impact
on the yield of 2a (Table 1, entries 6–14). Additional control
experiments showed that the reaction proceeded much more
slowly if Ph3PAuCl or AgSbF6 was used as the catalyst,
which demonstrates that Ph3PAu+ is the most active cata-
lytic species (Table 1, entries 15 and 16). N-Phenyl-N-tosyl-
acetamide was obtained in 98% yield after 0.5 h without
the addition of pyridine N-oxide (Table 1, entry 17). This
result indicates that the addition of H2O to the ynamide
can be hampered by pyridine N-oxide.
[a] Department of Chemistry, Zhejiang Sci-Tech University,
Xiasha West Higher Education District,
Hangzhou 310018, China
E-mail: licy@zstu.edu.cn
Homepage: http://www.chem.zstu.edu.cn/page/
Default.asp?I2.D=208
The scope of this reaction was then examined (Table 2).
This method is compatible with a variety of functional
groups. If ynamides with an electron-rich aryl ring were
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201300162.
Eur. J. Org. Chem. 2013, 2775–2779
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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L.-Q. Yang, K.-B. Wang, C.-Y. Li
SHORT COMMUNICATION
Table 1. Optimization of reaction conditions.[a]
Table 2. Substrate scope.[a]
Entry Catalyst
N-Oxide Solvent
Yield [%][b]
2a
4a
1
2
3
4
5
6
7
8
Ph3PAuCl/AgSbF6
3a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3a
3a
3a
3a
3a
3a
3a
ClCH2CH2Cl
50
23
31
30
27
38
40
25
35
19
29
41
51
54
23
26
18
15
IMesAuCl/AgSbF6
IPrAuCl/AgSbF6
PicAuCl2 (5a)/AgSbF6
AuCl3/AgSbF6
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
toluene
THF
CH3CN
CH2Cl2
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
Ph3PAuCl/AgNTf2
Ph3PAuCl/AgOTf
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgSbF6
Ph3PAuCl/AgSbF6
Ph3PAuCl
9
10
11
12
13
14[c]
15[d]
16[e]
17[f]
trace 35
43
21
31
53
38
20
0
29
16
15
25
28
26
0
AgSbF6
Ph3PAuCl/AgSbF6
[a] [1a] = 0.1 m; the reaction was conducted in the solvent (2 mL)
at 40 °C in an open flask. [b] Isolated yield. [c] The reaction was
conducted at 0 °C. [d] The reaction time was 3 h. [e] The reaction
time was 20 h. [f] No pyridine N-oxide was added; N-phenyl-N-
tosylacetamide was obtained in 98% yield after 0.5 h.
used, the corresponding oxindoles were obtained in moder-
ate yield (Table 2, entries 1–3 and 5). m-Methylphenylyn-
amide 1c was synthesized to test the regioselectivity, and
2c and 2cЈ were isolated as an inseparable mixture with a
para/ortho ratio of 1:0.7 (Table 2, entry 2). Ynamides with
an electron-poor aryl ring led to oxindoles in acceptable
yield (Table 2, entries 6–10). Notably, bromide and iodide
substituents were tolerated in this reaction (Table 2, en-
tries 7 and 8).[24] To our surprise, m-trifluoromethyphenyl
ynamide 1k gave 2k as the sole product, and no regioisomer
was detected (Table 2, entry 10). Other protecting group
such as Ms (methylsulfonyl), SES (2-trimethylsilylethane-
sulfonyl), and p-MeOC6H4SO2 were tolerated under the re-
action conditions, and this provides a choice of conditions
for deprotection (Table 2, entries 11–13). Finally, a terminal
alkyne moiety is essential for this transformation, as in-
ternal ynamide 1o led to α-keto imide 6 under the optimized
reaction conditions [Equation (1)].[20a]
The synthetic utility of this method was further demon-
strated by deprotection of the SES group [Equation (2)] and
by the aldol reaction of 2a with acetone [Equation (3)]. Ox-
indole 7 could be obtained in 60% yield if 2m was treated
with TBAF (tetrabutylammonium fluoride) at room tem-
perature. Condensation of 2a with acetone led smoothly to
[a] [1] = 0.1 m; the reaction was conducted in CH2Cl2 (2 mL) at
0 °C in an open flask.
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Eur. J. Org. Chem. 2013, 2775–2779

Oxindoles through the Au-Catalyzed Oxidation of N-Arylynamides
Without pyridine N-oxide, 9 decomposed completely within
13 h, but only 15% of 2a was isolated [Equation (6)]. How-
ever, the yield of 2a could be increased to 62% when
2 equivalents of pyridine N-oxide were added [Equa-
tion (7)]. This information clearly asserts the intermediacy
of gold carbenoid A and the importance of pyridine N-ox-
ide. Thus, a plausible mechanism was proposed. Gold car-
benoid A can be obtained after the expulsion of pyridine.
Owing to its high electrophilicity, the carbene is attacked
by the ortho position of the aryl ring to afford intermediate
B. After the loss of a proton, intermediate C is formed,[26]
which undergoes protodeauration to afford final product 2a
(Scheme 2).
Scheme 2. Proposed mechanism.
Conclusions
A mild and convenient method to synthesize function-
alized oxindoles by using N-arylynamides has been devel-
oped. Various functional groups were tolerated in this
transformation. Gold carbenoid proved to be the key inter-
mediate, and pyridine N-oxide acted as not only the oxidant
but also as the ligand and base. In view of the easy access
to N-arylynamides from cheap anilines and ethynyltrimeth-
ylsilane, this work provides a simple but versatile approach
to the oxindole skeleton.
3-alkenyloxindole 8 in 52% yield. Considering the fact that
a large number of synthetic strategies have been established
in recent years[25] to functionalize oxindoles at the 3-posi-
tion, we believe this method will find its utility in the syn-
thesis of oxindole derivatives.
Experimental Section
General Procedure for the Gold-Catalyzed Reaction: Dichlorometh-
ane (0.5 mL) was added to a reaction flask charged with Ph3PAuCl
(4.0 mg, 4 mol-%) and AgSbF6 (2.7 mg, 4 mol-%). The reaction
mixture was cooled to 0 °C. Then, a solution of N-arylynamide 1
(0.2 mmol) and pyridine N-oxide (3a, 0.4 mmol) in CH2Cl2
(1.5 mL) was added dropwise over 15 min. The reaction mixture
was stirred at 0 °C for 17 h and then warmed to room temperature.
When TLC analysis showed that the starting material was com-
pletely consumed, the reaction mixture was filtered through a short
plug of silica gel. The mixture was concentrated, and the residue
was purified by flash chromatography to give product 2.
Deuterated samples 1a-d and 1aЈ-d were prepared to ex-
plore the mechanism. Compound 2a-d with a deuteration
level of 45% at the 7-position of the oxindole was isolated
under the optimized reaction conditions [Equation (4)],
which indicates that the isotope effect is not significant and
that a stepwise electrophilic aromatic substitution process
may be involved. Compound 1aЈ-d, which was 100% deu-
terated at the alkyne terminus, led to 2a in 44% yield and
2aЈ-d was not detected [Equation (5)]. This result indicates
that pyridine N-oxide can act as a base to facilitate H–D
exchange in the presence of a gold catalyst. To further ex-
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, compound characterization, and 1H
plore the mechanism, diazo compound 9 was synthesized. NMR and 13C NMR spectra.
Eur. J. Org. Chem. 2013, 2775–2779
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SHORT COMMUNICATION
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This work was generously supported by the National Natural Sci-
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[24] For excellent examples exploring the orthogonality of gold and
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Received: January 30, 2013
Published Online: April 9, 2013
Eur. J. Org. Chem. 2013, 2775–2779
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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