Alkoxycarbonylation of aryl iodides catalyzed by Pd with a thiourea type ligand under balloon pressure of CO

Article abstract of DOI:10.1016/j.tet.2008.07.053

Palladium-catalyzed alkoxycarbonylation of aryl iodides with a thiourea-oxazoline type ligand has been achieved under mild conditions. Various functional groups were tolerated and the yields were from moderate to excellent.

Full text of DOI:10.1016/j.tet.2008.07.053

Tetrahedron 64 (2008) 9581–9584
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Alkoxycarbonylation of aryl iodides catalyzed by Pd with a thiourea type
ligand under balloon pressure of CO
,
,
b,
*
Jing Liu a b, Bo Liang a, Dongxu Shu a, Yanhe Hu b, Zhen Yang a , Aiwen Lei
*
a Key Laboratory of Bioorganic Chemistry and Molecular Engineering of Ministry of Education and Beijing National Laboratory for Molecular Science (BNLMS),
College of Chemistry, Peking University, Beijing 100871, PR China
b College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Palladium-catalyzed alkoxycarbonylation of aryl iodides with a thiourea-oxazoline type ligand has been
achieved under mild conditions. Various functional groups were tolerated and the yields were from
moderate to excellent.
Received 27 March 2008
Received in revised form 10 July 2008
Accepted 15 July 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Available online 18 July 2008
Keywords:
Palladium
Alkoxycarbonylation
Thiourea
Aryl iodide
Aryl ester
1. Introduction
clusters, which deactivate the catalyst,19–21 and inconvenience of
application in laboratory synthesis as a routine process when high
Palladium-catalyzed carbonylation of aryl and benzyl halides
pressure apparatus are not available or substrates sensitive to high
temperature are involved.
was pioneered by Heck and co-workers in 1970s and is an efficient
1–13
way to prepare carboxylic acid derivatives.
Based on the
Thiourea type ligands, usually insensitive to air and moisture,
mechanistic studies reported so far,14–16 it is generally accepted
that alkoxycarbonylation of aryl halides with CO proceeds through
the following steps (Fig. 1): oxidative addition of an aryl halide
(ArX) to Pd(0)Ln (I) to form intermediate ArPd(II)XLn (II); co-
ordination of CO to the Pd-center and insertion into the Pd–Ar
bond to give acyl palladium IV; reaction of IV with base and al-
cohol to release product ArCOOR and regenerate the Pd(0)
catalyst.
have been reported to be beneficial in CO involved reactions. 22–26
HX+base
ArX
L
(0)Pd
L
ArCOOR
I
CO is a good p
-acid ligand, 17 so its coordination to Pd(0)-species
L
will decrease the electron density in the Pd-center and make the
oxidative addition step (I/II) difficult. 18 Also, because ArPd(II)XLn
(II) is electron-deficient species, coordination of CO to II is more
reluctant compared with that to I. As a result, high pressure of CO is
often needed to facilitate the formation of III. However, high con-
centration of CO might further slow down the oxidative addition
step (I/II) (vide supra). Thus, elevated temperature, usually higher
than 100 ꢀC, is necessary. The harsh conditions result in several
problems, such as agglomeration of Pd atoms and formation of
Ar Pd L
X
II
CO
L
L
Ar Pd X
CO
III
Base+ ROH
L
ArCO Pd L
X
IV
* Corresponding authors. Fax: þ86 27 68754067.
E-mail address: aiwenlei@whu.edu.cn (Z. Yang).
Figure 1. Proposed mechanism of Pd-catalyzed alkoxycarbonylation of aryl halides.
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.07.053

9582
J. Liu et al. / Tetrahedron 64 (2008) 9581–9584
H
N
H
N
N
H2N
NH2
H2N
H2N
N
N
N
N
N
Ph
O
S
1
S
2
S
3
S
4
S
6
S
5
N
Figure 2. Different thiourea type ligands.
Recently, we have investigated palladium-catalyzed dicarboxy-
10–12), and sodium ethoxide furnished almost quantitative
product in 3 h (Table 1, entry 9), much faster than the 24 h needed
when NEt3 was the base.
lation reaction of olefins with this type of ligands, and achieved
27
moderate to excellent yields under mild conditions.
We spec-
ulate that thioureas might be able to tune the electronic properties
of the Pd(II)-species by electron resonance from the nitrogen
atoms in the backbone of the ligand to the metal center, which
might help the coordination of CO to the Pd(II)-species to form
intermediate III, and finally facilitate the carbonylation of aryl
halides with CO. These important features, together with the
unique capability of thiourea ligands to stabilize palladium cata-
lysts28–36 inspired us to investigate the application of thiourea
type ligands in Pd-catalyzed alkoxycarbonylation of aryl halides.
We report herein our recent exploration in this important
reaction.
When the catalyst loading was lowered to 0.5 mol % with so-
dium ethoxide as the base, the reaction of ethyl o-iodobenzoate
was completed in 6 h, and the isolated yield was 79% (Table 2,
entry 1). Further lowering the catalyst loading to 0.1 mol % resul-
ted in 57% product within 20 h (Table 2, entry 2). However, when
p-iodoanisole was treated under the same condition, the conver-
sion was 100%, but there was only 8% of the desired product to-
gether with 44% of anisole as the side product (detected by GC).39
Lowering the temperature to 25 ꢀC improved the yield to 59%
(Table 2, entry 3). Two more substrates, p-methyliodobenzene and
ethyl p-iodobenzoate were tested at room temperature, and only
38 and 58% products were obtained, respectively (Table 2, entries
4 and 5).
2. Result and discussion
When the ethoxycarbonylation of p-iodoanisole was carried
out in unpurified ethanol with NEt3 as the base, 65% of ethyl
p-methoxybenzoate was isolated with 85% conversion after 24 h,
and trace anisole was detected (Table 3, entry 1). Thus NEt3 was
chosen as the base and the scope of this catalytic system was
examined with aryl iodides bearing various functional groups.40
The yields of different electrophiles varied from 60 to 99%.
Functional groups such as alkoxy, cyano, ketone, ester, and nitryl
groups were well tolerated (Table 2, entries 2, 5–8). ortho-
Substituted substrates and 1-iodonaphthalene were also effec-
tively converted into the corresponding carbonylative products in
high yields (Table 2, entries 4 and 8). Biaryl compound 4-iodobi-
phenyl was effectively converted. 2-Iodothiophene only resulted
in moderate yield, probably because of the volatility of the
product (Table 3, entries 9 and 10). The catalyst worked better
with substrates bearing electron-withdrawing groups, especially
when the group was at the ortho position (Table 1, entry 8 vs
Table 2, entry 5).
Commercial available monodentate thiourea ligands 1–4 and
ligands 5 and 627,37 developed by our laboratory were taken for
initial studies (Fig. 2). The model reaction involved ethyl o-iodo-
benzoate as the electrophile and 2 mol % palladium as the catalyst
at 60 ꢀC under balloon pressure of CO. We firstly identified that
38
base was necessary in this transformation (Table 1, entry 1).
When triethylamine was used, the reaction catalyzed by
PdCl2(CH3CN)2 afforded the desired product in 77% yield (Table 1,
entry 2). Then several thiourea ligands were tested. Mono-
thioureas 1–5 (Fig. 2) lowered the reaction yields (Table 1, entries
3–7), while bidentate thiourea-oxazoline ligand 6 improved the
yield to 99% (Table 1, entry 8). Other bases such as K2CO3, DBU,
and DABCO afforded moderate to good yields (Table 1, entries
Table 1
Evaluation of ligands and basesa
I
COOEt
COOEt
PdCl2(CH3CN)2, ligand
base, EtOH,
CO(balloon), 60 °C
COOEt
7a
8a
Table 2
Pd-6 catalyzed ethoxycarbonylation with EtONa as the basea
Entry
Ligand
Base
Yieldb (%)
PdCl2(CH3CN)2 (2 mol %),
1
2
3
4
5
6
7
8
d
d
0
77
0
28
0
35
55
I
COOEt
6 (2.1mol %), EtONa (1.2 equiv)
d
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
NEt3
EtONa
K2CO3
DBU
R
R
EtOH, CO (balloon)
1 (4 mol %)
2 (4 mol %)
3 (4 mol %)
4 (4 mol %)
5 (4 mol %)
6 (2.1 mol %)
6 (2.1 mol %)
6 (2.1 mol %)
6 (2.1 mol %)
6 (2.1 mol %)
7
8
Entry
R
PdCl2(CH3CN)2
6
Yieldb (%)
99 (96c)
98d
56
70
84
1
2
3
4
5
o-COOEt
o-COOEt
p-OMe
0.5 mol %
0.1 mol %
2 mol %
2 mol %
2 mol %
0.5 mol %
0.1 mol %
2.1 mol %
2.1 mol %
2.1 mol %
79c
9
57c
10
11
12
8c (59d)
38d
p-Me
DABCO
p-COOEt
58d
a
a
Reaction conditions: PdCl2(CH3CN)2 (2 mol %), ligand, base (1.2 equiv), 7a
Reaction conditions: PdCl2(CH3CN)2 (2 mol %), 6 (2.1 mol %), EtONa (1.2 equiv), 7
(0.5 mmol), EtOH (2 mL), CO (balloon pressure), 60 ꢀC, 24 h.
(0.5 mmol), EtOH (2 mL), CO (balloon pressure).
b
b
GC yield determined with naphthalene as the internal standard.
Isolated yield.
Time: 3 h.
Isolated yield.
Temperature: 60 ꢀC.
Room temperature.
c
c
d
d

J. Liu et al. / Tetrahedron 64 (2008) 9581–9584
9583
Table 3
further purification. Ethanol used in the reaction with sodium
ethoxide as the base was distilled from magnesium and iodide.
Triethylamine was used as received.
Pd-6 catalyzed ethoxycarbonylation with NEt3 as the basea
PdCl2(CH3CN)2 (2 mol %),
6 (2.1mol %), NEt3 (1.2 equiv)
I
COOEt
R
R
EtOH, CO
(balloon), 70 °C
4.2. Typical procedure A
7
8
In a 10-mL Schlenk tube containing a Teflon-coated stirring bar
was placed PdCl2(CH3CN)2 (2.6 mg, 0.01 mmol), ligand 6 (4.0 mg,
0.021 mmol), and ethanol (0.5 mL) under nitrogen. The mixture
was stirred at 70 ꢀC for 10 min. Then p-iodoanisole 7b (0.5 mmol)
and ethanol (1.5 mL) were added successively. The system was
charged with CO (balloon) and triethylamine was added by syringe.
Then the system was degassed three times and held at 70 ꢀC for
24 h with continuous stirring. After cooling to room temperature,
the reaction mixture was diluted with ether (10 mL) and washed
with saturated aqueous solution of NH4Cl (5 mL). The aqueous layer
was extracted with ether (3ꢁ10 mL). The combined organic phases
were dried over NaSO4, filtered, and concentrated. The residue was
purified by a flash chromatography (PE/EtOAc¼30:1) on silica gel to
afford the product as a colorless oil (65%).
Entry
1
7
Yieldb (%)
65 (8b)
MeO
I (7b)
I (7c)
2
3
85 (8c)
63 (8d)
MeO
I (7d)
I (7e)
4
99 (8e)
5
6
7
91 (8f)
93 (8g)
83 (8h)
EtOOC
I (7f)
4.3. Typical procedure B
NC
O
I (7g)
I (7h)
In a 10-mL Schlenk tube containing a Teflon-coated stir bar was
placed PdCl2(CH3CN)2 (2.6 mg, 0.01 mmol), ligand
6 (4.0 mg,
0.021 mmol), and ethanol (0.5 mL) under nitrogen. The mixture
was stirred at 60 ꢀC for 15 min. Then ethyl o-iodobenzoate 7a
(0.5 mmol) was added. The reaction mixture was charged with CO
(balloon), followed by the addition of sodium ethoxide (1.5 mmol)
in ethanol (1.5 mL). Then the system was degassed three times and
stirred at 60 ꢀC for 2 h. After cooling to room temperature, the re-
action mixture was diluted with ether (10 mL) and washed with
saturated NH4Cl aqueous solution (5 mL). The aqueous layer was
extracted with ether (3ꢁ10 mL). The combined organic phases
were dried over NaSO4, filtered, and concentrated. The residue was
purified by a flash chromatography (PE/EtOAc¼20:1) on silica gel to
afford the product 8a as light yellow oil (98%).
I (7i)
NO2
8
9
95 (8i)
99 (8j)
60 (8k)
I (7j)
I (7k)
10
S
a
Reaction conditions: PdCl2(CH3CN)2 (2 mol %), 6 (2.1 mol %), NEt3 (1.2 equiv), 7
(0.5 mmol), EtOH (2 mL), CO (balloon pressure), 70 ꢀC.
4.3.1. Diethyl phthalate 8a43,44
b
Isolated yield.
1H NMR (300 MHz, CDCl3):
d 7.79–7.67 (m, 2H), 7.60–7.48 (m,
2H), 4.37 (q, J¼7.1 Hz, 2H), 1.37 (t, J¼7.1 Hz, 3H).
3. Conclusion
4.3.2. Ethyl 4-methoxybenzoate 8b13
In summary, Pd-6 catalyst system was efficient for the ethoxy-
carbonylation of aryl iodides in unpurified ethanol under balloon
pressure of CO with NEt3 as the base. The protocol provides a con-
venient option for this type of transformation. Dehalogenated side
products were detected under certain conditions. Further studies
concerning the mechanism of the reactions are ongoing in our
laboratory and will be reported in due course.
1H NMR (300 MHz, CDCl3):
d
8.00 (d, J¼8.7 Hz, 2H), 6.91 (d,
J¼8.7 Hz, 2H), 4.35 (q, J¼7.2 Hz, 2H), 1.38 (t, J¼7.2 Hz, 3H).
4.3.3. Ethyl 3-methoxybenzoate 8c45
1H NMR (300 MHz, CDCl3):
7.34 (dd, J¼8.1, 7.8 Hz, 1H), 7.11 (dd, J¼8.1, 1.8 Hz, 1H), 4.38 (q,
J¼6.9 Hz, 2H), 1.40 (t, J¼6.9 Hz, 3H).
d
7.65 (d, J¼7.8 Hz, 1H), 7.57 (s, 1H),
4. Experimental
4.1. General
4.3.4. Ethyl 4-methylbenzoate 8d13
1H NMR (300 MHz, CDCl3):
d
7.81 (d, J¼7.9 Hz, 2H), 7.08 (d,
J¼7.9 Hz, 2H), 4.23 (q, J¼7.2 Hz, 2H), 2.25 (s, 3H), 1.25 (t, J¼7.2 Hz,
3H).
Column chromatography was performed using EM silica gel 60
(230–400 mesh). 1H NMR spectra were recorded on Mercury
300 MHz. All 1H NMR experiments were reported in parts per
million (ppm) downfield of TMS. GC–MS spectra were recorded on
a Varian GC–MS 3900-2100T. Gas chromatographic analyses were
performed on Varian GC 2000 gas chromatography instrument
with a FID detector and naphthalene was added as the internal
standard. Ethyl o-iodobenzoate,41 ethyl p-iodobenzoate41 and
4.3.5. Ethyl 1-naphthoate 8e43
1H NMR (300 MHz, CDCl3):
d
8.92 (d, J¼8.4 Hz, 1H), 8.16
(d, J¼7.2 Hz,1H), 7.97 (d, J¼8.1 Hz,1H), 7.84 (d, J¼8.1 Hz,1H), 7.59 (t,
J¼7.2 Hz, 1H), 7.52–7.43 (m, 2H), 4.45 (q, J¼7.2 Hz, 2H), 1.43 (t,
J¼7.2 Hz, 3H).
4.3.6. Diethyl terephthalate 8f12
42
PdCl2(CH3CN)2 were prepared following the literature methods.
All other aryl iodides were obtained from Acros and used without
1H NMR (300 MHz, CDCl3):
d
8.10 (s, 4H), 4.40 (q, J¼7.2 Hz, 2H),
1.41 (t, J¼7.2 Hz, 3H).

9584
J. Liu et al. / Tetrahedron 64 (2008) 9581–9584
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1H NMR (300 MHz, CDCl3):
d
8.15 (d, J¼6.8 Hz, 2H), 7.75 (d,
J¼6.8 Hz, 2H), 4.42 (q, J¼7.2 Hz, 2H), 1.41 (t, J¼7.2 Hz, 3H).
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1H NMR (300 MHz, CDCl3):
d
8.13 (d, J¼7.8 Hz, 2H), 8.01 (d,
J¼7.8 Hz, 2H), 4.41(q, J¼6.9 Hz,2H), 2.65(s, 3H),1.42(t, J¼6.9 Hz,3H).
4.3.9. Ethyl 2-nitrobenzoate 8i47
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7.92 (d, J¼7.5 Hz, 1H), 7.76 (d,
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d
8.11 (d, J¼8.7 Hz, 2H), 7.64 (t,
J¼8.8 Hz, 4H), 7.47 (t, J¼7.2 Hz, 2H), 7.40 (m, 1H), 4.40 (q, J¼7.2 Hz,
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d
7.80 (d, J¼3.0 Hz, 1H), 7.55 (d,
J¼4.5 Hz, 1H), 7.10 (dd, J¼3.0, 4.5 Hz, 1H), 4.36 (q, J¼7.2 Hz, 2H), 1.38
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Acknowledgements
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