Silver-prompted carbonitration of acrylamides for the synthesis of nitrating oxindoles

Article abstract of DOI:10.1055/s-0034-1380514

A silver-prompted carbonitration of alkenes involving concomitant direct C-H functionalization and C-N bond formation to synthesize nitrating oxindoles has been developed. The CR TH2 receptor antagonist skeleton can be obtained from one of the products with further modification.

Full text of DOI:10.1055/s-0034-1380514

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Georg Thieme Verlag Stuttgart · New York
2015, 26, 1417–1421
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letter
Synlett
X.-H. Wei et al.
Letter
Silver-Prompted Carbonitration of Acrylamides for the Synthesis
of Nitrating Oxindoles
Xiao-Hong Weia
Quan-Xiang Wua
Shang-Dong Yang*a,b
R2
AgNO3 (10 mol%)
TsOH•H2O (2.0 equiv)
H
NO2
O
R3
+
R3
Mg(NO ) •6H O
3
2
2
O
MeCN, 110 °C
N
R1
(1.0 equiv)
N
R1
a
State Key Laboratory of Applied Organic Chemistry,
R2
Lanzhou University, Lanzhou 730000, P. R. of China
State Key Laboratory for Oxo Synthesis and Selective
b
26 examples
yield up to 88%
Oxidation, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, P. R. of China
yangshd@lzu.edu.cn
Received: 05.02.2015
Accepted after revision: 06.03.2015
Published online: 02.04.2015
tive radical cyclization leading to functionalized oxindoles
has also been performed by Li and coworkers. Based on
6
DOI: 10.1055/s-0034-1380514; Art ID: st-2015-w0083-l
this considerable progress, numerous groups have inde-
pendently developed some new methods of metal-cata-
lyzed or metal-free oxidative carbo–hetero or carbo–carbo
difunctionalization of arylacrylamides, which involved all
Abstract A silver-prompted carbonitration of alkenes involving con-
comitant direct C–H functionalization and C–N bond formation to syn-
thesize nitrating oxindoles has been developed. The CR TH2 receptor
antagonist skeleton can be obtained from one of the products with fur-
ther modification.
5
a,7
kinds of functional groups, such as trifluoromethyl,
al-
kyl,6 phosphoryl,
,8
7l,m,9
azidyl,
7l,m,10
hydroxyl, carbonyl,
11
12
nitro,13 sulfonyl, and (trifluoromethyl)thio
14
7m,15
in the past
Key words silver, N-methyl-N-arylacrylamide, carbonitration, nitrat-
ing oxindoles
few years. We wish to apply this strategy for the selective
synthesis of nitrated oxindoles.
In classical methods, the simplest pathway to obtain the
aliphatic nitro compounds was through the direct addition
of nitrogen dioxide to a C–C multiple bond, but the problem
Oxindoles play an important role in natural products,
pharmaceuticals, agrochemicals, and the key structural
motifs of numerous natural products. Additionally, nitro
1
compounds are important synthetic intermediates in phar-
maceuticals, chemical industry, and organic synthesis be-
cause they can be easily converted into various amines and
ketones. Representative oxindole derivatives such as hors-
filine, CR TH2 receptor antagonist, and alstonisine (Figure
F
O
Cl
O
2
N
NH
Cl
MeO
O
1) can be easier afforded from nitrating oxindoles. There-
O
N
fore, the selective assembly of nitrated oxindoles from read-
ily available precursors is a prominent objective in chemical
research. Recently, transition-metal-catalyzed simple
alkene difunctionalization to synthesize different oxindoles
represents a highly attractive alternative. For example, the
N
H
HOOC
horsfiline (1)
CR TH2 receptor antagonist (2)
O
3
Grigg group reports the palladium-catalyzed difunctional-
H
ization of alkenes with aryl iodides and CO through intera-
molecular Heck coupling and succeeds in obtaining ox-
indoles derivatives. Subsequently, the Zhu group applies the
intramolecular Heck coupling with o-haloaniline to oxin-
doles synthesis and gets the best advancements.4 Very
recently, a new tactic of direct 1,2-difunctionalization of
alkenes for the synthesis of oxindoles by the palladium-
catalyzed C–H oxidative activation has been developed
NH
O
O
N
alstonisine (3)
Figure 1 Representative oxindole derivatives
5
firstly by the Liu group. Moreover, an iron-catalyzed oxida-
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X.-H. Wei et al.
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previous work:
t-BuNO2 (2.5 equiv)
Jiao's group
H
R2
O
AgNO3 (3.0 equiv)
AcOH (10.0 equiv)
NO2
R3
R3
O
N
R1
Li's group
N
R2
NaNO2 (2.0 equiv)
K2S2O8 (2.0 equiv)
R1
this work:
our group
R2
H
N
AgNO3 (10 mol%)
TsOH•H2O (2.0 equiv)
NO2
O
R3
R3
O
+
Mg(NO3)2•6H2O
1.0 equiv)
N
R1
MeCN, 110 °C
R1
R2
(
Scheme 1 Different methods related to carbonitration of alkenes
R2
H
AgNO3 (10 mol%)
NO2
O
TsOH⋅H2O (2.0 equiv)
R3
R3
O
+
Mg(NO3)2⋅6H2O
1.0 equiv)
N
R1
MeCN, 110 °C
N
R1
(
R2
1a–x
2a–y, 2aa
NO2
NO2
NO2 MeO
NO2
O
O
O
O
N
N
N
N
R1
2
2
2
2
a R1 = Me, 82%
2e, 62%
2k, 84%
2l, 75%
1
b R = Bn, 82%
1
NO2
F
NO2 Cl
NO2
c R = Ph, 82%
Ph
1
d R = H, 82%
O
O
O
NO2
N
N
N
R3
O
2m, 82%
2n, 85%
2o, 88%
N
OMe
CO2Et
NO2
Br
NO2
NO2
3
2
f R = H, 46%
O
O
O
3
2
2
2
2
g R = OMe, 41%
N
N
N
MeO
3
h R = F, 60%
3
i R = Cl, 66%
2
p, 83%
2q, 74%
2r, 43%
3
j R = Br, 63%
Bn
NO2
NO2
NO2
NO2
O
O
O
O
N
N
N
N
2
(
8
s
2t
1.6:1)
0%
Br
2u, trace
2v, 68%
OAc
OH
CO2Me
NO2 Cl
CO2Me
NO2
NO2
NO2
MeO
O
O
O
O
N
N
N
N
2w, 53%
2x, 47%
2y, 19%
2aa, 37%
Scheme 2 The scope of substrates investigation; isolated yields are given. Reagents and conditions: 1a (0.2 mmol), Mg(NO ) ·6H O (1 equiv), TsOH·H O
3
2
2
2
(
2 equiv) and AgNO (0.1 equiv), MeCN (2 mL), 110 °C, 24 h. Yields for 2f–j are for a 36 h reaction.
3
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is the difficulty of handling NO exhaust and the inefficien-
Table 1 Optimization of the Reaction Conditionsa,b
2
16
cy of reaction efficiency. Therefore, the development of an
efficient nitrating method is always a considerable chal-
lenge. Very recently, the oxidative metal-free difunctional-
ization of N-aryl acrylamides 1 to construct nitro-substitut-
H
NO2
O
AgNO3 (10 mol%)
solvent
+
XNOy
O
N
N
13b
ed oxindole derivatives 3 has been reported by Jiao and
Me
Me
2a
13c
Li groups (Scheme 1). Our group has also disclosed an ef-
ficient synthesis of nitro-containing oxindoles and dihydro-
quinolin-2(1H)-ones by oxidative arylnitration of alkenes
1a
Entry XNO
1 equiv)
y
Additive
(2 equiv)
Solvent
Yield
(%)
(
13a
via nitration and C–H functionalization. However, these
aforementioned methods are the equivalent reactions.
Herein, we would like to present a practical and an efficient
radical difunctionalization of alkenes to synthesize nitrat-
ing oxindoles via silver-prompted carboazidation and C–H
functionalization of arylacrylamides. In the scope of what
we know, silver-catalyzed difunctionalization of alkenes to
form nitrating oxindoles is the first example.
1c
2
3
4
5
6
7
8
9
Mg(NO
Mg(NO
KNO3
)
)
·6H
·6H
O
O
HO(O)PPh
HO(O)PPh
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MePh
DMF
62
3
2
2
2
2
2
3
2
77
23
27
29
25
50
73
60
11
n.d.
52
61
82
29
37
46
43
29
11
32
55
59
51
trace
50
HO(O)PPh2
NH
4
NO
3
HO(O)PPh
HO(O)PPh
HO(O)PPh
HO(O)PPh
HO(O)PPh
HO(O)PPh
HO(O)PPh
HO(O)PPh
HO(O)PPh
AcOH
2
2
2
2
2
2
2
2
2
NaNO
3
NaNO
2
Y(NO
3
)
3
·6H
2
O
In an initial study, we focused on N-methyl-N-arylacryl-
amide 1a as the model substrate with Mg(NO ) ·6H O as ni-
Co(NO
Bi(NO
Ce(NO
(NH
Zr(NO
Mg(NO
Mg(NO ) ·6H O
3
)
3
·5H
·5H
·6H
2
O
3
2
2
trating source, and diphenylphosphinic acid as acid in the
3
)
3
2
O
presence of 10 mol% AgNO in MeCN at 90 °C. To our de-
3
1
0
1
2
3
3
)
3
2
O
light, the carbonitration of alkene really works, and the de-
sired product 2a was obtained in 62% yield (Table 1, entry
1
1
1
4
)
2
Ce(NO
3
)
3
·6H
2
O
3
)
4
·5H
2
O
1). When the temperature was increased to 110 °C, the
yield of 2a reached 77% (Table 1, entry 2). Encouraged by
these results, we further screened several nitro resources
3 2 2
) ·6H O
14
15
3
2
2
PTSA
(
Table 1, entries 3–12) and found that Mg(NO ) ·6H O is
3 2 2
Mg(NO ) ·6H O
TFA
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
still the best choice. Based on our previous report, we were
strongly aware of the vital role of acid playing in the reac-
tion. The best yield (82%) was obtained in the presence of
p-toluenesulfonic acid (PTSA, Table 1, entries 14–18).
Meanwhile, different solvents were also examined, and
MeCN was found to be the best choice (Table 1, entries 19–
16
17
18
Mg(NO
Mg(NO
Mg(NO
Mg(NO
)
)
)
)
·6H
·6H
·6H
·6H
O
O
O
O
PrivOH
boric acid
H(O)P(OEt)
2
19
20
PTSA
Mg(NO ) ·6H O
3
3
3
3
3
3
2
2
2
2
2
2
2
2
2
2
2
2
PTSA
2
5
4). Furthermore, the control experiment showed that only
0% yield of the desired product was obtained in the ab-
21
22
23
24
Mg(NO
Mg(NO
Mg(NO
Mg(NO
Mg(NO
)
)
)
)
)
·6H
·6H
·6H
·6H
·6H
O
O
O
O
O
PTSA
DCE
PTSA
TFE
sence of AgNO (Table 1, entries 25 and 26).
3
PTSA
DME
With the optimal reaction conditions in hand (Table 1,
entry 14), the reaction scope was examined as shown in
Scheme 2. Varying the different substituent of N-protecting
groups with an electron-donating group gave the product
in good yield, such as methyl, benzyl, and phenyl (2a–c),
but an N-free substituent under identical conditions failed
to produce the desired product 2d. Moreover, tricyclic oxin-
dole 2e was transformed from cyclization of tetrahydro-
quinoline derivative with a moderate yield. Meanwhile,
many substrates with arylacrylamide as N-protecting group
were also tested; the substituent on the phenyl group with
electron-withdrawing or electron-donating groups provid-
ed the corresponding oxindoles in moderate to good yields
PTSA
dioxane
MeCN
MeCN
25
d
26
3 2 2
Mg(NO ) ·6H O
PTSA
a
y
Reaction conditions: 1a (0.2 mmol), XNO (1 equiv), additive (2 equiv),
MeCN (2 mL), 110 °C, 24 h.
Isolated yield.
b
c
90 °C.
d
No AgNO3.
t). However, trace product was obtained when an ortho-
substituent was present on the phenyl group (2u). Next,
various α-substituted groups of alkenes bearing different
functional groups were additionally examined. Analogous
to substrate 1a, substrates with benzyl (2v), acetoxymethyl
(2w), hydroxyl (2x), and ester (2y and 2aa) groups at this
position generated the corresponding products in moderate
to good yields.
(
2f–j). When the N-protecting group was changed to meth-
yl, different substituent on the aryl ring afforded the de-
sired products in good to excellent yields, which are bear-
ing electron-withdrawing or electron-donating groups (2k–
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X.-H. Wei et al.
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F
O
R2
Cl
NO2
N
1
) Raney Ni, H2
EtOAc–EtOH (1:1), r.t.
NH2
R3
O
HCO2NH4, Pd/C
MeOH, r.t.
Cl
N
O
O
3 4
2) CuI, K PO
N
N
N,N-dimethylglycine
DMF, 140 °C
2
3
R = Me, R = H, 2a
2
3
R = CO2Me, R = 4-Cl, 2aa
4
aa, 24%
3a, 86%
Scheme 3 Application of our protocol
N
O
1a
H
NO2
NO2
NO2
•
[SET]
•NO2
•
O
O
2A
N
O
N
N
2B
2C
thermal
decomposition
2a
AgNO3
Mg(NO3)2•6H2O
Mg(NO3)2•6H2O + TsOH
HNO3
Scheme 4 Proposed mechanisms
It is worthy of note that the product of 2aa can be easily
converted into spiropyrrolidinyloxoindoles, such as the CR
TH2 receptor antagonist skeleton, and nitro compounds can
be transferred into amine or amide because they exist in an
extensive array of natural products and pharmaceuticals.
Our group has been reported that compound 2a can be re-
duced by Pd/C to form primary amine 3aa in moderate
carbonitration and C–H functionalization. Our transforma-
tion employs an inexpensive and nontoxic silver salt as
metal catalysts.
Acknowledgment
We are grateful for the NSFC (Nos. 21272100 and 21202075), Program
for Changjiang Scholars and Innovative Research Team in University
(PCSIRT: IRT1138), and Program for New Century Excellent Talents in
University (NCET-11-0215 and lzujbky-2013-k07) for financial sup-
port.
13a
yields, and what is more, spiro-4-pyrrolidinone skeleton
aa was obtained through use of the reported method (see
3
17
the Supporting Information). And then CR TH2 receptor
antagonist skeleton 4aa can be used to synthesize the CR
TH2 receptor antagonist with further modification (Scheme
3
).
Supporting Information
Based on our previous work,9,13a a proposed mechanism
Supporting information for this article is available online at
http://dx.doi.org/10.1055/s-0034-1380514.
for silver-catalyzed carbonitration of arylacrylamide is de-
scribed in Scheme 4. Initially, Mg(NO ) ·6H O decomposed
S
u
p
p
o
nrtIo
i
g
f
rm oaitn
S
u
p
p
ortioIgnfmr oaitn
3
2
2
to produce the nitro radical 2A under heating conditions.
Then 2A was added to alkene 1a to form the alkyl radical
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