Palladium-Catalyzed Synthesis ofΔ2-Isoxazoline from Toluene Derivatives Enabled by the Triple Role of Silver Nitrate

Article abstract of DOI:10.1021/acs.orglett.5b03059

A palladium-catalyzed direct synthesis ofΔ²-isoxazoline from toluene derivatives has been established. The present reaction proceeds through nondirected Csp³-H activation, benzylic nitration, dehydration, and cycloaddition. This protocol also features the unusual triple role of silver nitrate in a one-pot reaction.

Full text of DOI:10.1021/acs.orglett.5b03059

Letter
pubs.acs.org/OrgLett
2
Palladium-Catalyzed Synthesis of Δ ‑Isoxazoline from Toluene
Derivatives Enabled by the Triple Role of Silver Nitrate
Chengliang Li,†,§ Hongmei Deng, Chunju Li, Xueshun Jia,* and Jian Li*
‡
†
,†
,†
†
Department of Chemistry, Innovative Drug Research Center, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China
Laboratory for Microstructures, Shanghai University, Shanghai, 200444, P. R. China
‡
§
College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, P. R. China
*
S Supporting Information
2
ABSTRACT: A palladium-catalyzed direct synthesis of Δ -
isoxazoline from toluene derivatives has been established. The
3
present reaction proceeds through nondirected Csp −H
activation, benzylic nitration, dehydration, and cycloaddition.
This protocol also features the unusual triple role of silver
nitrate in a one-pot reaction.
oluene and its derivatives are the simplest and most highly
valuable starting materials in nature; hence, developing
not surprising that many efforts have been devoted to the
14
T
construction of the isoxazoline scaffold over the past decades.
2
transformations toward their efficient utilization continues to be
An extensive literature survey revealed that Δ -isoxazolines are
1
a research focus in organic synthesis. On the other hand,
usually prepared from oximes or nitro compounds (benzoylni-
1
5,16
transition-metal-catalyzed cross-dehydrogenative coupling
tromethane, nitroacetate).
To the best of our knowledge,
2
2
(
CDC) has been proven to be a powerful tool in various
the direct construction of Δ -isoxazolines from toluene
derivatives via nondirected C−H activation is unknown. In
2006, Neumann and co-workers discovered that alkylmethanes
experienced oxidation followed by acetolysis to afford benzylic
3
bond-forming reactions. As such, metal-catalyzed benzylic
3
4
Csp −H bond activation of arylmethane has drawn much
attention from organic chemists, thus providing a new
5
17
opportunity to approach diverse organic structures. Of note
acetate as the final product. This reaction was believed to
proceed via a benzylic nitrite intermediate using nitrate as the
catalyst (Scheme 1, eq 1). As a continuation of our interest in
is the palladium-catalyzed processes, which are quite interest-
ing, as toluene and its derivatives are often used as solvent in
many palladium-catalyzed reactions. In 1968, Bryant and co-
workers discovered that toluene could undergo acetoxylation
Scheme 1. Representative Transformation of Arylmethane in
the Presence of Nitrate
6
with the aid of palladium. Of late, Huang and co-workers
reported the oxidative carbonylation of arylmethanes via
7
8a
8b
nondirected activation. In 2013, Sun and Kwong reported
the directed ortho C−H acylation of acetanilides independently.
Additionally, palladium-catalyzed efficient acylation of azoarene,
2
-arylpyridine, and N-nitrosoaniline were also realized using
9
toluene derivatives as an acyl precursor. More recently, the
coupling between carbon nucleophile and toluene derivatives
has also been disclosed by Kozlowski. Unlike the common
radical-mediated process, this protocol underwent palladium-
catalyzed C−H activation via a nonradical pathway. Although
considerable achievements have been made in the palladium-
10
18
synthesizing heterocycles, herein we report a novel palladium-
2
3
catalyzed cascade reaction to synthesize functionalized Δ -
catalyzed reaction involving benzylic Csp −H activation; most
examples were limited to the simple coupling of two different
isoxazoline from toluene derivatives and alkenes (Scheme 1, eq
11
2
).
Initially, mesitylene 1a and tert-butyl acrylate 2a were
selected as model substrates for the reaction optimization
see the Supporting Information for full optimization). In the
C−H bonds or between C−H and X−H bonds. Thus,
3
exploration of the cascade process via a benzylic Csp −H
activation strategy to construct new heterocyclic compounds
remains very attractive.
(
2
presence of a catalytic amount of Pd(OAc) , heating the
Δ -Isoxazoline derivatives represent an important synthetic
2
2
mixture of 1a and 2a afforded Δ -isoxazoline 3a in 43% yield
skeleton that is frequently found in a wide variety of natural
products and biologically active molecules. More importantly,
these compounds also serve as important building blocks as
12
Received: October 21, 2015
13
well as asymmetric ligands in organic synthesis. It is therefore
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b03059
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
using AgNO as the oxidant (Table 1, entry 1). To our delight,
the yield of 3a was increased to 58% when 4.0 equiv of AgNO3
Scheme 2. Scope of the Reaction with Respect to the
a
3
Arylmethane Substrate 1
a
Table 1. Optimization of Reaction Conditions
b
entry
1
catalyst
solvent
oxidant
AgNO3
yield (%)
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(PPh ) Cl
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
mesitylene
C H Cl
43
58
78
51
0
c
d
2
AgNO3
AgNO3
AgTFA
Ag CO
d
3
e
4
e
5
2
3
e
6
e
Ag O
2
<5
0
7
K S O
8
2
2
e
8
TBHP
0
d
d
d
d
d
d
9
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
AgNO3
58
<5
76
40
32
66
6
5
1
1
1
1
1
0
1
2
3
4
DMF
t-AmOH
t-AmOH
t-AmOH
t-AmOH
3
2
2
Pd(PPh3)4
Pd(TFA)2
a
Unless otherwise noted, all reactions were carried out with mesitylene
a (1 mmol), tert-butyl acrylate 2a (3 mmol), catalyst (5 mol %),
1
a
Reaction conditions: arylmethane 1 (1 mmol), tert-butyl acrylate 2a
3.0 mmol), Pd(OAc) (5 mol %), AgNO (4.0 equiv), p-TsOH·H O
oxidant (2.0 equiv), solvent (3 mL), 120 °C in a sealed tube using p-
(
(
TsOH·H O (0.5 equiv) as an additive. And 1 mmol 2a was used when
2
3
2
2
b
b
0.5 equiv), t-AmOH (3 mL), 120 °C in a sealed tube, 24 h. Isolated
mesitylene was used as the solvent. Yields of product after silica gel
chromatography. No additive was added. Another 2.0 equiv AgNO
were added as an extra nitrogen source. 4.0 equiv of NaNO were
c
d
yield on a 10 mmol scale is given in parentheses.
3
e
3
added as an extra nitrogen source.
all reactions proceeded smoothly to give the desired products 3.
Reactions with substrates 1 bearing a substituent at the para-
position were first conducted (3b−3g). Other toluene
derivatives 1 having a substituent at the meta- and ortho-
position were next subjected to the optimal conditions to
produce the desired compounds 3h−3l. The experimental
results showed that electron-withdrawing groups on the
aromatic ring decreased the reaction reactivity slightly, whereas
electron-donating groups seemed to favor the formation of
product 3. Furthermore, di- and trisubstituted arylmethanes 1
were also used to yield the corresponding adducts 3m and 3n
in satisfactory yields. Remarkably, the present reaction was not
limited to simple benzene-based substrates; pyridyl-, naphthyl-,
and biphenyl-substituted substrates were also found to be
compatible in the present transformation (3o−3q).
were used (Table 1, entry 2). Encouraged by these
experimental results, the reaction parameters including the
additive, the oxidant, the catalyst, and the solvent were
subsequently screened to improve the reaction performance.
For instance, the employment of p-TsOH as additive (see the
Supporting Information for details) was found to be particularly
effective to increase the yield of 3a from 58% to 78% (Table 1,
entry 3). We then focused our attention on the variation of
different oxidants. Of the silver oxidants examined [AgTFA,
Ag CO , Ag O], only AgTFA showed discernible catalytic
2
3
2
activity (Table 1, entries 4−6). As shown in Table 1, no
reaction occurred when oxidants K S O and TBHP were used
2
2
8
(Table 1, entries 7−8). In addition, the experimental outcome
displayed that solvent also had significant impact on the
reaction performance and some representative results were
summarized in Table 1. At the beginning, mesitylene was used
as reactant as well as solvent. Lower yield was observed when
mesitylene was replaced by C H Cl, whereas the employment
To further explore the versatility of the above-mentioned
cascade reaction strategy, we turned our attention to the
possibility of substituted alkenes 3. As shown in Scheme 3,
various alkyl acrylates 2 containing Me, Et, n-Bu, and Bn groups
were used to react with mesitylene and its derivatives to
produce the desired products 4a−4g. For instance, reactions of
methyl vinylketone (MVK) and its analogs furnished 4h and 4i
in 51% and 55% yields, respectively. The expected cyclo-
addition adducts 4j and 4k were successively obtained when
styrene and norbornylene reacted with xylene under the
optimized conditions. It was also worthy to note that excellent
regioselectivity was exemplified by the employment of 1,1-
disubstituted olefins to yield 4l and 4m.
6
5
of other solvents such as DMF only led to very poor yields (see
the Supporting Information for details). Pleasingly, the
employment of t-AmOH as solvent gave comparable yield
with mesitylene (Table 1, entry 11). Subsequent investigation
of the catalyst revealed that the yield of 3a decreased when
other catalysts Pd(PPh ) Cl , Pd(PPh ) , Pd(TFA) were used
3
2
2
3 4
2
(
Table 1, entries 12−14).
With the optimized conditions in hand, we attempted to
establish the scope and limitation of the present reaction.
Reactions of various toluene derivatives 1 and tert-butyl acrylate
To gain further insight into the reaction mechanism, several
mechanistic experiments were subsequently performed
(Scheme 4). Since oximes commonly reacted with alkenes to
2
a under optimal conditions were subsequently examined, and
2
the representative results were shown in Scheme 2. Pleasingly,
prepare Δ -isoxazolines, we initially thought that oxime might
B
DOI: 10.1021/acs.orglett.5b03059
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Scheme 3. Scope of the Reaction with Respect to the Alkene
Substrate 2
Scheme 5. Proposed Mechanism
a
In the presence of TsOH, the OTs anion is possible to be
attached to Pd(II) in the form of Pd(OTs) , thus enchancing
2
19
its reactivity in the following C−H activation step. Then
toluene undergoes C−H activation to afford benzyl Pd(II)
10
intermediate A. The reductive elimination step essentially
leads to the formation of intermediate B, which is then
transformed to intermediate C in the presence of silver
20
nitrate. The second catalytic circle presumably starts from the
interaction between silver nitrate and intermediate C to
generate D, which is converted to silver salt E after dehydration
16a
process. In the last step, the cycloaddition of intermediate E
with olefin 2 takes place to yield the final product 3 or 4, which
also delivers the silver nitrate to finish the second catalytic
circle.
a
Reaction conditions: arylmethane 1 (1 mmol), alkene 2 (3.0 mmol),
Pd(OAc) (5 mol %), AgNO (4.0 equiv), p-TsOH·H O (0.5 equiv),
t-AmOH (3 mL), 120 °C in sealed tube, 24 h.
2
3
2
To verify some processes in the above-mentioned mecha-
nism, two controlled experiments were next carried out. As
shown in Scheme 6, in the absence of palladium, heating the
Scheme 4. Preliminary Mechanistic Study
Scheme 6. Controlled Experiments
mixture of 7 and 2a essentially afforded 3r in 87% yields, which
indicate that 7 was another possible reaction intermediate.
Furthermore, the transformation from 7 to 6 also proceeded
well under the same conditions, thus lending more support to
our proposed mechanism.
be generated as an intermediate from the present trans-
formation. However, the reaction of oxime 5 and tert-butyl
acrylate 2a failed to give the desired product 3f (Scheme 4, eq
3
). Thus, the possibility of oxime as an intermediate could be
ruled out. To find out the real reaction intermediate, we heated
the solution of xylene 1f and tert-butyl acrylate 2a for 5 h under
optimal conditions and then stopped the reaction. From the
resultant reaction mixture we isolated 35% 3f and 6% side
product 6a, which indicated that (nitromethyl)benzene 6 was a
possible reaction intermediate (Scheme 4, eq 4). To verify this
hypothesis, we conducted the experiments with tert-butyl
acrylate 2a and (nitromethyl)benzene 6a. Pleasingly, 3f was
isolated in 90% yield in the absence of palladium (Scheme 4, eq
In conclusion, we have described a palladium-catalyzed direct
2
synthesis of Δ -isoxazolines from toluene derivatives and
olefins. Experimental results indicated that the present cascade
3
reaction might proceed through nondirected Csp −H
activation, benzylic nitration, and dehydration followed by
cycloaddition. The versatility of silver nitrate to serve as an
oxidant for the generation of palladium(II), the nitrogen
source, and the dehydration reagent is believed to be the key to
the present transformation. Despite its popularity in palladium-
catalyzed transformation as an oxidant, the present unprece-
dented triple role of silver nitrate has never been exploited in
previous one-pot reactions.
5
), which suggested that the present reaction might proceed
through intermediate 6. We also found that when TEMPO (4.0
equiv) was added as a radical scavenger, the reaction of 1f and
2a was completely surpressed.
On the basis of the above-mentioned experimental results
ASSOCIATED CONTENT
Supporting Information
and mechanistic findings, a possible mechanism is proposed in
Scheme 5 to explain the formation of products 3 and 4. The
importance of p-TsOH as an additive has been well established
during the conditions optimization. According to our analysis,
the beginning of the reaction may involve the anion exchange.
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.5b03059.
C
DOI: 10.1021/acs.orglett.5b03059
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Experimental procedures and full characterization of all
Letter
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Faghih, N.; Khan, K. M. Org. Lett. 2015, 17, 414.
1
13
compounds, spectral data, and H and C NMR spectra
for all products (PDF)
2
AUTHOR INFORMATION
́
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Corresponding Authors
E-mail: xsjia@mail.shu.edu.cn.
E-mail: lijian@shu.edu.cn.
Notes
The authors declare no competing financial interest.
(b) Arai, M. A.; Kuraishi, M.; Arai, T.; Sasai, H. J. Am. Chem. Soc. 2001,
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DOI: 10.1021/acs.orglett.5b03059
Org. Lett. XXXX, XXX, XXX−XXX