Palladium-Catalyzed Decarboxylative Ortho-Ethoxycarbonylation of O-Methyl Ketoximes and 2-Arylpyridines with Potassium Oxalate Monoester

Article abstract of DOI:10.1021/acs.orglett.5b02422

A novel method for introducing an ester group via palladium-catalyzed ligand-directed C-H activation has been explored. The ortho-ethoxycarbonylation of O-methyl ketoximes proceeded smoothly with the nontoxic and easily handled reagent potassium oxalate monoester, affording the desired products in moderate to good yields. Furthermore, pyridine could also be employed as a directing group to obtain similar results in this transformation.

Full text of DOI:10.1021/acs.orglett.5b02422

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Decarboxylative Ortho-Ethoxycarbonylation of
O‑Methyl Ketoximes and 2‑Arylpyridines with Potassium Oxalate
Monoester
Zhong-Yuan Li and Guan-Wu Wang*
CAS Key Laboratory of Soft Matter Chemistry, Collaborative Innovation Center of Chemistry for Energy Materials (iChem), Hefei
National Laboratory for Physical Sciences at Microscale, and Department of Chemistry, University of Science and Technology of
China, Hefei, Anhui 230026, P. R. China
S
* Supporting Information
ABSTRACT: A novel method for introducing an ester group
via palladium-catalyzed ligand-directed C−H activation has
been explored. The ortho-ethoxycarbonylation of O-methyl
ketoximes proceeded smoothly with the nontoxic and easily
handled reagent potassium oxalate monoester, affording the
desired products in moderate to good yields. Furthermore,
pyridine could also be employed as a directing group to obtain similar results in this transformation.
n recent decades, the transition-metal-catalyzed ligand-
directed C−H bond activation has aroused extensive attention
potassium phenyltrifluoroborate to react with potassium oxalate
monoester or substituted oxalic acids to achieve an analogous
reaction.21 Nevertheless, the ethoxycarbonylation via C−H
activation with potassium oxalate monoester has never been
reported. As part of our continuing interest in C−H bond
functionalizations,22 herein we disclose the first example of the
decarboxylative ortho-ethoxycarbonylation with potassium ox-
alate monoester via the palladium-catalyzed sp2 C−H activation.
To explore the optimal reaction conditions of this ortho-
ethoxycarbonylation, the coupling of p-methyl O-methyl
ketoxime 1a with potassium oxalate monoester 2 was chosen
as the model reaction (Table 1). At first, K2S2O8 and
ClCH2CH2Cl were employed as the oxidant and solvent,
respectively. Only a trace amount of the desired product was
observed (Table 1, entry 1). When 0.5 equiv of acidic additive
such as p-toluenesulfonic acid (PTSA) or D-camphorsulfonic
acid (D-CSA) was used in this reaction, product 3a was obtained
in 15−21% yield (Table 1, entries 2 and 3). Considering that
silver salts had been utilized frequently in decarboxylative
systems, different types of silver salts were examined in this
reaction. In doing so, the yield of 3a was dramatically increased to
73% when 2.0 equiv of AgOAc was added (Table 1, entry 4).
Encouraged by this result, other silver salts such as Ag2O and
Ag2CO3 were also explored, but the yields were unsatisfactory
(Table 1, entries 5 and 6). When other oxidants were used in this
reaction, no better results were achieved (Table 1, entries 7−9).
A yield of 75% was obtained when 0.5 equiv of D-CSA and 2.0
equiv of AgOAc were added together (Table 1, entry 10).
Gratifyingly, when AgOAc was replaced by Ag2CO3, 3a was
produced in 83% yield (Table 1, entry 11). The yield was reduced
sharply to 33% if the amount of Pd(OAc)2 was decreased from 10
to 5 mol % (Table 1, entry 12). Product 3a was isolated in 69%
I
of a growing number of chemists because of its high efficiency
and good functional group tolerance. Among these important
reactions, a variety of nitrogen-containing functional groups such
as pyridine,1 quinoline,2 imine,3 and others4 are widely utilized.
O-Methyl ketoxime is undoubtedly one of the most popular
directing groups due to its moderate coordination ability to
transition metals. In particular, it is a removable functional group
and easy to transform to other moieties.5 In 2006, Sanford and
co-workers reported the alkoxylation and acetoxylation of O-
methyl ketoximes.6 Then the Che group described the ortho-
amidation of O-methyl ketoximes.7 Many other functionaliza-
tions of O-methyl ketoximes including arylation,1j,8 ethoxycar-
bonylation,9 acylation,10 nitration,5b,11 cyanation,12 and other
transformations13 were also demonstrated.
On the other hand, owing to the prevalence of the carbonyl
group in organic molecules, comprehensive research has been
conducted to introduce a carboxyl or ester group into aromatic
compounds through ligand-directed C−H activation reactions
during the past decade. Yu and other chemists reported the
ligand-directed C−H carboxylation and esterification with CO
and CO2.14 The Yu and You groups utilized the diethyl
azodicarboxylate (DEAD) to realize the ortho-ethoxycarbonyla-
tion.9,15 Many other research groups including the Kakiuchi,16
Shi,17 Tan,18 and Wang19 groups reported this important
alkoxycarbonylation with different kinds of reagents. Although
many protocols to realize the esterification by ligand-directed C−
H activation strategy have been documented, there are still great
challenges to develop a new method using a nontoxic and easily
handled reagent to expand the scope of this important
transformation. In 2009, Fu, Liu, and co-workers synthesized
the aromatic esters via decarboxylative coupling reaction with the
nontoxic and easily handled reagent potassium oxalate
monoester and aryl halides.20 After that, the Ge group utilized
Received: August 21, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b02422
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Table 1. Optimization of the Reaction Conditions
Scheme 1. Pd-Catalyzed Oxidative Ethoxycarboxylation of O-
Methyl Oximes and 2-Arylpyridines
a,b
b
entry
oxidant
K2S2O8
silver salt additive
solvent
yield (%)
1
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
ClCH2CH2Cl
1,4-dioxane
trace
15
2
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
Na2S2O8
(NH4)2S2O8
Oxone
PTSA
D-CSA
3
21
4
AgOAc
73
5
Ag2O
42
6
Ag2CO3
AgOAc
65
7
44
8
AgOAc
21
9
AgOAc
11
10
11
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
AgOAc
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
D-CSA
75
83
c
12
33
d
13
69
e
14
50
f
15
g
41
16
75
h
17
82
18
19
20
21
trace
trace
trace
trace
toluene
DMSO
HOAc
a
Reaction conditions: 1a (0.2 mmol), 2 (0.4 mmol), Pd(OAc)2 (10
mol %), K2S2O8 (0.4 mmol), silver salt (0.4 mmol), additive (0.1
b
c
mmol), 80 °C, 16 h. Isolated yield based on 1a. Pd(OAc)2 (5 mol
d
e
f
%). Additive (0.05 mmol). K2S2O8 (0.2 mmol). 2 (0.3 mmol).
g
h
Ag2CO3 (0.2 mmol). 1a (0.5 mmol), 2 (1.0 mmol), Pd(OAc)2 (10
mol %), K2S2O8 (1.0 mmol), Ag2CO3 (1.0 mmol), D-CSA (0.25
mmol), 90 °C, 12 h.
when the amount of D-CSA was reduced to 0.25 equiv (Table 1,
entry 13). Decreasing the amount of oxidant or 2 provided worse
results (Table 1, entries 14 and 15). Reducing the amount of
Ag2CO3 to 1.0 equiv led to a slight decrease of product yield
(Table 1, entry 16). Other solvents were also examined in this
conversion, but no better result was achieved (Table 1, entries
18−21). When the amount of 1a was amplified to 0.5 mmol, the
temperature had to be raised to 90 °C to keep an 82% yield of
product 3a (Table 1, entry 17). Therefore, the optimized
conditions for the palladium-catalyzed decarboxylative ortho-
ethoxycarbonylation with potassium oxalate monoester were
determined as follows: 10 mol % of Pd(OAc)2 as the catalyst, 2.0
equiv of K2S2O8 as the oxidant, 2.0 equiv of Ag2CO3 as the silver
salt, 0.5 equiv of D-CSA as the additive, and 2.0 equiv of
potassium oxalate monoester 2 as the partner of O-methyl
ketoxime 1a. The reaction performed best at 90 °C with
ClCH2CH2Cl as the solvent.
With the optimized conditions in hand, the substrate scope of
O-methyl ketoximes 1 was examined with 2 in this reaction. As
shown in Scheme 1, O-methyl ketoximes having both electron-
rich and electron-deficient groups on the aryl ring could react
with 2 to afford the ethoxycarbonylation products 3a−q in
moderate to good yields (55−89%). When the strong electron-
donating groups OMe and OEt were substituted at the para-
position, products 3b and 3c were obtained in good yields (85%
and 89%) under our conditions. Similarly, O-methyl ketoximes
a
1a (0.5 mmol), 2 (1.0 mmol), Pd(OAc)2 (10 mol %), K2S2O8 (1.0
mmol), Ag2CO3 (1.0 mmol), D-CSA (0.25 mmol), 90 °C, 12 h.
b
c
Isolated yield based on 1a. The reaction was performed for 36 h.
d
The reaction was performed for 14 h.
with a phenyl group or no substituent groups at the para-position
gave 3d and 3e in 83% and 78% yields. Electron-withdrawing
groups such as chloro and ester groups located at the para-
position were inferior in this transformation. When the time was
prolonged to 36 h, the desired products 3f and 3g could be
isolated in 71% and 55% yields. Notably, meta-substituted
ketoximes exhibited good regioselectivities and efficiencies in the
reaction to give products 3h, 3i, and 3j in 85%, 84%, and 69%
yields, respectively. Furthermore, the reaction could be applied
to ortho-substituted and disubstituted O-methyl ketoximes,
affording 3k and 3l in 73% and 77% yields. As anticipated, 2-
naphthalenyl ketoxime 1m gave the corresponding product 3m
at the less sterically hindered position in 85% yield. When 1n was
used to replace the structurally similar 1a, 3n was obtained in a
yield (84%) close to that of 3a (82%). Moreover, oximes with a
bicyclic scaffold such as 1o and 1p could also undergo facile C−H
B
DOI: 10.1021/acs.orglett.5b02422
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ortho-ethoxycarbonylation (75% and 70%). It is worth
mentioning that in the case of 1q with four ortho-C−H bonds
only monoethoxycarbonylation took place to provide 3q in 78%
yield. In addition, pyridine derivatives were utilized to replace
ketoximes, and products 3r, 3s, 3t, and 3u were obtained in 41−
74% yields under the same conditions.
Scheme 4. Plausible Reaction Mechanism
To demonstrate the synthetic utility of this reaction, we chose
3a as a representative example to remove its directing group. In
the event, 4a could be isolated in 72% yield. Then, NaBH4 was
employed to reduce 4a in EtOH, and lactone 5a was obtained in
73% yield (Scheme 2).
Scheme 2. Removal of Directing Group
Further experiments were conducted to obtain insight into the
reaction mechanism. The ethoxycarbonylation of 1e under the
optimized conditions gave only trace amounts of 3e in the
presence of 1.0 equiv of 2,2,6,6-tetramethyl-1-piperidinyloxy
(TEMPO). This result demonstrated that the reaction may
involve a radical pathway. The intermolecular kinetic isotope
effect (kH/kD) of 1e to 1e-d5 for the C−H ethoxycarbonylation
was determined to be 1.6 (Scheme 3), indicating that the C−H
bond cleavage may not be the rate-limiting step.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b02422.
■
S
Detailed experimental procedures; H NMR and 13C
NMR spectra of 3a−u, 4a, and 5a (PDF)
1
AUTHOR INFORMATION
Corresponding Author
■
Scheme 3. Investigation on the Effect of Kinetic Isotope
*E-mail: gwang@ustc.edu.cn.
Notes
The authors declare no competing financial interest.
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DOI: 10.1021/acs.orglett.5b02422
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