Efficient synthesis of carboxylic esters via palladium(II)-catalyzed direct alkoxycarbonylation of arenes with CO and alcohols

Article abstract of DOI:10.1055/s-0033-1339868

An efficient palladium(II)-catalyzed procedure for the alkoxycarbonylation of arenes and heteroarenes with atmospheric pressure carbon monoxide and alcohols was developed to synthesize aryl carboxylic esters. Georg Thieme Verlag Stuttgart. New York.

Full text of DOI:10.1055/s-0033-1339868

2274▌
LETTER
letter
Efficient Synthesis of Carboxylic Esters via Palladium(II)-Catalyzed Direct
Alkoxycarbonylation of Arenes with CO and Alcohols
Palladium-Catalyzed Direct Carbonylation of Arenes
Bin Liu,a Bing-Feng Shi*a,b
a
Department of Chemistry, Zhejiang University, Hangzhou 310027, P. R. of China
Fax +86(571)87951895; E-mail: bfshi@zju.edu.cn
b
State Key Laboratory of Bioorganic & Natural Products Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science,
Shanghai 200032, P. R. of China
Received: 26.07.2013; Accepted after revision: 28.08.2013
the presence of nucleophiles has received tremendous
Abstract: An efficient palladium(II)-catalyzed procedure for the
interests, and various other directing groups have been
alkoxycarbonylation of arenes and heteroarenes with atmospheric
reported.8 However, despite of these successes, relative-
pressure carbon monoxide and alcohols was developed to synthe-
ly few examples of the direct alkoxylcarbonylation of
arenes with CO and alcohols to form carboxylic esters
have been reported yet (Scheme 1, B).8l–n Herein, we de-
scribe a novel and efficient palladium-catalyzed oxida-
tive alkoxycarbonylation to form esters with
atmospheric pressure CO and alcohol coupled with an
inexpensive terminal oxidant (Scheme 1, B).9h
size aryl carboxylic esters.
Key words: palladium, C–H activation, alkoxycarbonylation, CO,
carboxylic esters
Aryl carboxylic esters are valuable commodity chemicals
and useful synthetic building blocks for agrochemicals,
active pharmaceutical ingredients, and process chemicals.
Transition-metal-catalyzed carbonylation reactions in-
volving the coupling of aryl halides (ArX) or arylmetallic
compounds (ArM) with CO and various nucleophiles,
such as alcohols, amines, and carbon nucleophiles, have
been increasingly explored and received great success in
both academia and industry in the last several decades
(Scheme 1, A).1,2 However, these reactions require multi-
step sequences to prepare the starting materials (ArX or
ArM) and generate stoichimetric byproducts, which is not
environmentally friendly and atom economic. Evidently,
the direct C–H bond carbonylation of ArH would be an
ideal method to construct aryl carboxylic acids and their
derivatives in terms of atom- and step-economy.
Our studies commenced with examining the oxidative
alkoxycarbonylation of 2-phenylpyridine (1a) with atmo-
spheric pressure CO and n-pentanol in dioxane.10 A vari-
ety of oxidants and additives were screened, and the
results are shown in Table 1. Initially, we found that the
alkoxycarbonylation proceeded in 12% yield in the pres-
ence of BQ and NaHCO3 (Table 1, entry 1). Further opti-
mization revealed that CuBr2 is the ideal oxidant (Table 1,
entry 3). The choice of additives is crucial for the success
of this transformation, and NaOAc gave the best yield
(Table 1, entries 4–7). After extensive screening, we were
delighted to find that oxygen may promote the reac-
tion.6,8m,p When the alkoxycarbonylation reaction was
A) classic alkoxycarbonylation reactions: refs. 1 and 2
O
Recently, great successes have been made in the palladi-
um(II)-catalyzed direct transformation of unactivated
C–H bonds into a variety of functional groups through
the formation of C–C and C–X bonds.3 However, palla-
dium(II)-catalyzed direct carbonylation of C–H bonds
remains an outstanding challenge.4–9 Palladium(II)-cata-
lyzed C–H carbonylation reactions were first reported by
Fujiwara and co-workers in 1980, in which the substrate
was used as the solvent and mixtures of regioisomeric
products were obtained in the case of substituted arenes.5
Little success has been made in this area, until 2004 Ori-
to and co-workers demonstrated the ortho-selective car-
ArX or ArM
+ CO + ROH
Ar
OR
B) Pd-catalyzed alkoxylcarbonylation of C–H bonds
NMe2
CO2R
RO2C
R2
previous
work:
R1
N
R2
R1
R1
CO2R
ref. 8l
ref. 8m
ref. 8n
bonylation of N-alkyl-ω-arylalkylamines
under
this work
atmospheric pressure CO to obtain benzolactams.6 In
2008, the Yu group demonstrated the ortho-carbonyl-
ation of benzoic and phenylacetic acids.7 Ever since
then, the oxidative carbonylation of arenes with CO in
Pd(II)
N
N
O
CO/O2 (4:1, 1 atm)
H
n-pentanol
OC5H11
R
R
up to 95% yield
SYNLETT 2013, 24, 2274–2278
Advanced online publication: 09.10.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
Scheme 1 Palladium-catalyzed alkoxycarbonylation with CO and
DOI: 10.1055/s-0033-1339868; Art ID: ST-2013-W0701-L
© Georg Thieme Verlag Stuttgart · New York
alcohols

LETTER
Palladium-Catalyzed Direct Carbonylation of Arenes
2275
conducted under an atmospheric pressure of the mixture ate to good yields under this protocol (Table 2, entries 10–
of CO and O2 (about 4:1, v/v), the yield increased to 48% 12). As mentioned previously, additives are crucial for
(Table 1, entry 8). Various reaction parameters were stud- this reaction and Li2CO3 was found to be a better additive
ied next, and we found that the aryl carboxylic ester 2a than NaOAc in the reaction of 1i (Table 2, entry 9). It is
could be afforded in 80% yield under our optimized con- interesting to note that the monocarbonylation products
ditions: 10 mol% Pd(OAc)2, 1.0 equivalent CuBr2, 1.0 were obtained in all cases. Even with pyrimidine as the di-
equivalent NaOAc, and 15 equivalents n-pentanol in 1,4- recting group, the monocarbonylation product 2l was
dioxane under CO/O2 (v/v, 4:1, 1 atm) at 100 °C (Table 1, formed exclusively (Table 2, entry 12). Furthermore,
entry 10).11
products 2b,d–2j,l were all obtained with high regioselec-
tivities. In these cases, the alkoxycarbonylation occurs se-
lectively to the less sterically hindered site (Table 2,
entries 2, 4–6, and 10–12).
Table 1 Optimization of Reaction Conditionsa
Pd(OAc)2 (10 mol%)
N
Finally, a plausible mechanism for the present palladi-
um(II)-catalyzed alkoxycarbonylation of 2-phenylpyri-
dine (1a) is proposed on the basis of previous studies
(Scheme 2). Coordination of 1a to palladium acetate is
followed by C–H activation to form the dimeric pallada-
cycle I.10a Subsequently, CO and alcohol coordinate to
palladium via ligand exchange and release of AcOH.
There are two possible pathways for the formation of car-
boxylic ester 2a from intermediate II: 1) migration inser-
tion of CO into the Pd–C bond to form intermediate III,
followed by C–O bond reductive elimination; or 2) migra-
tion insertion of CO into the Pd–OR bond to form inter-
mediate IV, followed by C–C bond reductive elimination.
Although theoretical studies have shown that migration
insertion of CO into Pd–OR bonds was preferred under
certain conditions,12 either of these pathways can be ruled
out at this stage.
N
oxidant, additive
O
+
OH
3
CO (1 atm)
OC5H11
1,4-dioxane, 24 h
1a
2a
Entry
Oxidant
Additive
Alcohol
Time
(°C)
Yield
(equiv)
(%)b
1
2
BQ
NaHCO3
NaHCO3
NaHCO3
NaBF4
5.0
5.0
130
130
130
130
130
130
130
130
130
100
12
10
32
<5
20
34
18
48
66
80
Oxone
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
CuBr2
3
5.0
4
5.0
5
KOAc
5.0
6
NaOAc
K3PO4
5.0
In conclusion, we have developed an efficient protocol for
the preparation of aryl carboxylic esters via palladium(II)-
catalyzed oxidative alkoxycarbonylation of arenes and
heteroarenes with atmospheric pressure CO and alcohol.13
The reaction shows high regioselectivity and reactivity
(up to 95% yield). Moreover, this protocol might be fur-
ther explored to the alkoxycarbonylation of other synthet-
ically useful substrates.
7
5.0
8c
9c
10c
NaOAc
NaOAc
NaOAc
10.0
15.0
15.0
a Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (10 mol%), oxidant
(1.0 equiv), additive (1.0 equiv), and n-pentanol in 1,4-dioxane (2
mL) under CO (1 atm).
Cu(II), O2
b Isolated yield.
1a
Py
Pd(OAc)2
c CO/O2 = 4:1 (v/v, 1 atm).
CO2C5H11
Pd(0)
C–H
activation
AcOH
2a
With the optimized conditions in hand, we then explored
the scope of pyridine derivatives. Generally, the reactions
proceeded smoothly at 100 °C, and the yield of the alk-
oxycarbonylation product varied from 38% to 95%, de-
pending on the nature of the pyridine derivatives (Table
2). The reaction delivered an array of carboxylic esters
with various functional groups, such as carboxylic esters
(Table 2, entries 4), methoxyl (Table 2, entries 5 and 10–
12), chloride (Table 2, entry 6, and fluoride (Table 2, entry
10). Heteroarene is also tolerated, albeit affording the de-
sired product in a reduced yield (Table 2, entry 9). Ethanol
and tert-amyl alcohol exhibited no reactivity under the
carbonylation reaction conditions. Under the optimal con-
ditions, substrates containing different directing groups,
including substituted pyridines and pyrimidine, were ex-
amined for this oxidative alkoxycarbonylation reaction.
All of them generate the carbonylation products in moder-
reductive
elimination
N
N
Pd
Pd
N
C5H11
O
AcO
O
IV
2
Pd
O
I (ref.10)
OR
III
migratory
insertion into
Pd-OR bond
ligand
exchange
CO
ROH
migratory
insertion into
Pd–C bond
N
AcOH
Pd
OC
OC5H11
II
Scheme 2 Plausible mechanism
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2274–2278

2276
B. Liu, B.-F. Shi
LETTER
Table 2 Palladium(II)-Catalyzed Alkoxycarbonylation of Aromatic C–H Bonds with CO and Alcohola
N
Pd(OAc)2 (10 mol%)
CuBr2, NaOAc
N
O
+
OH
3
CO/O2 (4:1, 1 atm)
1,4-dioxane, 100 °C
OC5H11
R
R
1
2
Entry
Substrate
Product
Time (h)
24
Yield (%)b
Py
Py
1
80
CO2C5H11
1a
2a
2b
Py
Py
2
3
48
72
63
91
CO2C5H11
1b
Py
Py
CO2C5H11
1c
2c
Py
Py
CO2C5H11
4
24
55
CO2Me
MeO2C
1d
2d
MeO
Py
MeO
Py
5
6
7
24
72
72
70
38
87
CO2C5H11
1e
2e
Cl
Py
Cl
Py
CO2C5H11
1f
2f
Py
Py
CO2C5H11
1g
2g
N
N
S
8
9
96
24
95
C5H11O2C
1h
1i
2h
S
Py
Py
58c
CO2C5H11
2i
Synlett 2013, 24, 2274–2278
© Georg Thieme Verlag Stuttgart · New York

LETTER
Palladium-Catalyzed Direct Carbonylation of Arenes
2277
Table 2 Palladium(II)-Catalyzed Alkoxycarbonylation of Aromatic C–H Bonds with CO and Alcohola (continued)
N
Pd(OAc)2 (10 mol%)
CuBr2, NaOAc
N
O
+
OH
3
CO/O2 (4:1, 1 atm)
1,4-dioxane, 100 °C
OC5H11
R
R
1
2
Entry
Substrate
Product
Time (h)
Yield (%)b
F
F
MeO
2j
MeO
10
48
76
N
N
CO2C5H11
1j
Cl
Cl
MeO
MeO
N
11
12
48
24
70
77
N
CO2C5H11
1k
2k
MeO
Py
MeO
Py
CO2C5H11
1l
2l
a Reaction conditions: 1a (0.2 mmol), Pd(OAc)2 (0.02 mmol), CuBr2 (0.2 mmol), NaOAc (0.2 mmol), and n-pentanol (3.0 mmol) in 1,4-dioxane
(2 mL) under CO/O2 (1 atm, v/v = 4:1) at 100 °C.
b Isolated yields.
c NaOAc was replaced by Li2CO3.
P.; Lei, A. J. Am. Chem. Soc. 2008, 130, 9429. (h) Liu, Q.;
Li, G.; He, J.; Liu, J.; Li, P.; Lei, A. Angew. Chem. Int. Ed.
2010, 49, 3371.
Acknowledgment
Financial support from the National Science Foundation of China
(21002087, 21272206), the Fundamental Research Funds for the
Central Universities (2010QNA3033), Zhejiang Provincial NSFC
(Z12B02000), Qianjiang Project (2013R10033), and Specialized
Research Fund for the Doctoral Program of Higher Education
(20110101110005) is gratefully acknowledged.
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m
o
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 2274–2278

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B. Liu, B.-F. Shi
LETTER
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(11) General Procedure for Alkoxycarbonylation of 1 with
CO and Alcohols
A mixture of substrate 1 (0.2 mmol), Pd(OAc)2 (10 mmol%),
CuBr2 (0.2 mmol), NaOAc (0.3 mmol), alcohol (3.0 mmol),
and dioxane (2.0 mL) in a 50 mL Schlenk tube (purged with
CO/O2 = 4:1) was heated at 100 °C for 24 h. The reaction
mixture was cooled to r.t. and concentrated in vacuo. The
residue was purified by chromatography on silica gel to
afford the desired product 2.
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Pentyl 4-Methyl-2-(pyridin-2-yl)benzoate (2b)
Compound 2b was prepared in 63% yield according to the
general procedure as an oil. 1H NMR (400 MHz, CDCl3): δ
= 8.64 (d, J = 4.8 Hz, 1 H), 7.77 (d, J = 7.6 Hz, 1 H), 7.72 (d,
J = 7.6 Hz, 1 H), 7.41 (d, J = 7.6 Hz, 1 H), 7.34 (s, 1 H), 7.27
(d, J = 7.6 Hz, 2 H), 4.04 (t, J = 6.8 Hz, 2 H), 2.43 (s, 3 H),
1.41–1.35 (m, 2 H), 1.25–1.19 (m, 2 H), 1.11–1.04 (m, 2 H),
0.83 (t, J = 7.2 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ =
168.6, 159.2, 149.0, 141.6, 141.2, 135.9, 130.7, 130.1,
128.9, 123.0, 121.9, 55.0, 28.0, 22.3, 21.4, 13.9. HRMS (EI-
TOF): m/z calcd for C18H21NO2 [M+]: 283.1572; found:
283.1573.
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4547.
(13) While this manuscript was under revision, a Pd-catalyzed
direct C–H activation–carbonylation of 2-arylphenols was
reported: Luo, S.; Luo, F.-X.; Zhang, X.-S.; Shi, Z.-J.
Angew. Chem. Int. Ed. 2013, 52, 10598.
Synlett 2013, 24, 2274–2278
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