Aryl-palladium-NHC complex: Efficient phosphine-free catalyst precursors for the carbonylation of aryl iodides with amines or alkynes

Article abstract of DOI:10.1039/c4ob01878h

A series of aryl-palladium-NHC compounds was prepared according to the reported methods and their catalytic activity in the carbonylation of aryl iodides to synthesize α-keto amides and alkynones was examined. These practical aryl-palladium-NHC complexes have shown highly efficient catalyzed carbonylation and Sonogashira carbonylation reactions, with high turnover number in synthesis of α-keto amides (TON = 4300) and in synthesis of alkynones (TON = 980).

Full text of DOI:10.1039/c4ob01878h

Organic &
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Aryl-palladium-NHC complex: efficient
phosphine-free catalyst precursors for the
carbonylation of aryl iodides with amines
or alkynes†
Cite this: Org. Biomol. Chem., 2014,
12, 9702
Chunyan Zhang,a,b Jianhua Liu*a and Chungu Xia*a
A series of aryl-palladium-NHC compounds was prepared according to the reported methods and their
catalytic activity in the carbonylation of aryl iodides to synthesize α-keto amides and alkynones was exam-
ined. These practical aryl-palladium-NHC complexes have shown highly efficient catalyzed carbonylation
and Sonogashira carbonylation reactions, with high turnover number in synthesis of α-keto amides
(TON = 4300) and in synthesis of alkynones (TON = 980).
Received 3rd September 2014,
Accepted 16th October 2014
DOI: 10.1039/c4ob01878h
www.rsc.org/obc
The carbonylation reaction represents an organic synthesis
core technology for transforming raw materials and fine
chemicals into a diverse set of useful compounds,1 such as
pharmaceuticals, agrochemicals, cosmetics and materials, in
which CO acts as a most essential C1 building block for the
preparation of carbonyl containing derivatives.2 Among the
Scheme 1 Palladium-catalyzed carbonylation reaction.
carbonylation reactions, the transition metal-catalyzed carbonylation reactions. Although a few examples have been
reported for aryl-Pd-NHC catalyzed coupling reactions,9
carbonylation reaction is a widely used transformation that
has been applied to the synthesis of acids, amides, alkynones,
heterocycles and their derivatives.3
reports on the carbonylation reaction catalyzed by aryl-palla-
dium-NHC complexes are rare. Inspired by these results10 and
in connection with our interests in developing carbonylation,11
herein we disclose an efficient protocol for the aryl-Pd-NHC
Over the past decades, effort has focused on the develop-
ment of new efficient synthetic methods for the carbonylation
reaction.4 Phosphine ligands are usually required to promote complex catalyzed carbonylation of aryl iodides with amines or
the selectivity of the carbonylation reaction.5 However, the air-
alkynes converting to α-keto amides or alkynones.
sensitive phosphine ligands greatly limit the utility of the
Towards this goal, the potential for a palladium-catalyzed
aforementioned transformations, especially on a large scale. iodobenzene carbonylation reaction was investigated (Table 1).
To circumvent this drawback, much attention has been paid to
stable N-heterocyclic carbenes (NHC) as ligands,6 which have
This palladium-catalyzed carbonylation reaction depends
greatly on the palladium species. To understand the nature of
emerged as a class of ligands in transition-metal-catalyzed this reaction and to find the optimal reaction conditions, the
reactions due to their strong σ-donor properties compared
with the phosphine ligand. Indeed, Pd- or Rh-NHC complexes
effect of palladium complexes on the yield of 3aa was exam-
ined. Fortunately, initial treatment of iodobenzene 1a with
have been successfully used as catalysts for some carbonyla- diethylamine in the presence of PdCl2, and K2CO3 in 1,4-
tion reactions (Scheme 1).7
dioxane at 80 °C for 12 hours to give α-keto amide 3aa in 25%
Recently, we disclosed a series of Pd-NHC complexes
yield (Table 1, entry 1). It is noteworthy that when the palla-
according to reported methods8 as efficient catalysts for dium species was replaced by PdCl2(CH3CN)2, the reaction pro-
ceeded with a much higher yield (47%) (Table 1, entry 2). To
our delight, the use of the palladium complex containing
aState Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute
of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, China.
E-mail: jhliu@licp.cas.cn, cgxia@licp.cas.cn; Fax: +86-931-827-7088
Bmim and PPh3 ligands as a catalyst efficiently improved the
yield of 3aa from 47% to 81% (Table 1, entry 3). Other palla-
dium complexes containing NHC and N-ligand, such as B and
C were also active, giving 3aa in 90% and 74% yields, respect-
ively (Table 1, entries 4 and 5). Further study revealed that pal-
ladium complex D containing IMes and aryl ligand, was also
bUniversity of Chinese Academy of Sciences, Beijing, 100039, People’s Republic
of China
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4ob01878h
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Table 1 Screening [Pd] catalysts of double carbonylation
Table 2 Substrate scope of palladium-catalyzed double carbonylation
of aryl iodidesa
Entry
R1
Yield (%)
1
2
H
4-CH3
3aa, 92
3ba, 79
3
4
3-CH3
2-CH3
3ca, 73
3da, 95
5
6
7
8
2,6-(CH3)2
4-CH3O
3-CH3O
2-CH3O
4-F
4-Cl
4-Br
4-OH
4-NH2
2-NH2
4-NHAc
4-COOCH3
4-NO2
1-Naphthyl
3-Pyridinyl
4-Pyridinyl
2-Thiophenyl
3ea, 12
3fa, 78
Entry
[Pd]
Yielda (%)
3ga, 70
3ha, 98 (10 : 1)b
3ia, 95
1
2
3
4
5
6
7
PdCl2
25
47
81
74
90
86
91
PdCl2(CH3CN)2
Bmim-Pd-PPh3-I2
IPr-Pd-Im-Cl2
IPr-Pd-(3-Cl-pyridinyl)-Cl2
IMes-Pd-dmba-Cl
IPr-Pd-dmba-Cl
9
10
11
12
13
14
15
16
17
18
19
20
21
3ja, 97
3ka, 87 (3ka′, 11)c
3la, <5
3ma, 16
3na, <5
3oa, 47
3pa, 98
3qa, 58
3ra, 92
3sa, 92
a General conditions: 1a (1.0 mmol), 2a (5.0 mmol), [Pd] (0.5 mol%),
K2CO3 (2.0 mmol), 1,4-dioxane (5.0 mL), CO (4.0 MPa), 80 °C, 12 h.
Isolated yield.
3ta, <5
3ua, 99 (9 : 1)b
effective giving 3aa in 86% yield (Table 1, entry 6).12 Mean-
while, the hindrance on the phenyl ring of NHC had a slight
effect on the efficiency. Gratifyingly, the aryl palladium
complex E containing the sterically demanding IPr-ligand is
the most effective in forming 3aa with 91% isolated yield
(Table 1, entry 7).
a General conditions: 1 (1.0 mmol), 2a (2.0 mmol), [Pd] (0.5 mol%),
K2CO3 (2.0 mmol), 1,4-dioxane (5.0 mL), CO (4.0 MPa), 80 °C, 12 h.
Isolated yield. b Ratios of regioisomers was determined by 1H NMR
analysis. Major isomers are shown. c 3ka′: 2,2′-(1,4-phenylene)bis(N,N-
diethyl-2-oxoacetamide).
With the optimized reaction conditions in hand, we next
investigated the substrate scope of this carbonylation.10 As
shown in Table 2, electron-donating and electron-withdrawing
groups contained in the phenyl ring were tolerated well. A
series of functional groups, such as methyl, methoxy, halide,
methoxycarbonyl, acetamide and nitro were compatible with
the optimized aryl-Pd-NHC catalytic system, and the desired
products were isolated in good to excellent yields (3aa–3qa),
which indicated that the present reaction had a good func-
tional group tolerance. Among these functional groups, the
surviving halide groups could be valuable for further manipu-
lation (3ia–3ka). An obvious steric hindrance effect on the reac-
tivity was observed, which was demonstrated by the reactivity
of 1b–d vs. 1e. Using very highly sterically hindered 1e as the
substrate, the single carbonylation product and deiodination
product of m-xylene were also detected by GC-MS under stan-
dard conditions. Notably, 1-bromo-4-iodobenzene containing
dihalide groups survived well in the standard procedure,
leading to 3ka in excellent 87% yield and the symmetrical
product, 2,2′-(1,4-phenylene)bis(N,N-diethyl-2-oxoacetamide),
3ka′ in 11% yield. Although free phenolic hydroxyl and amino
groups, such as 4-iodophenol and 2-iodoaniline, did not react
with amines under these conditions and single carbonylation
products and 4-amino-N-(4-iodophenyl)benzamide, 4-iodo-
phenyl 4-hydroxybenzoate could detected by GC-MS as by-pro-
ducts, 4-iodoaniline and N-(4-iodophenyl)acetamide are
suitable for this reaction, affording the corresponding pro-
ducts 3ma and 3oa, respectively, in decent yields. These
results indicated that anilines and phenols could act as stron-
ger nucleophilic reagents for this the double carbonylation
reaction. It is worth noting that heterocyclic derivatives are
usable for the present reaction, affording the corresponding
products 3sa and 3ua in excellent yields as well. The reaction
of 1-iodo-2-methoxybenzene 1h and 2-iodothiophene 1u
afforded 3ha and 3ua as a mixture of regioisomers (double
carbonylation : single carbonylation) in a ratio of 10 : 1 and
9 : 1.
Furthermore, the scope of the amines was also explored
and the results are shown in Table 3. Symmetrical secondary
amines containing alkyl, allyl and benzyl groups reacted
smoothly to give the corresponding products in high yields
(3aa–3ae). Notably, the surviving allyl group could be valuable
for further manipulation. Furthermore, the challenging cyclic
secondary amines successfully underwent this carbonylation
reaction to provide the desired products 3af–3ai in 48–92%
yields. Among these secondary amines, morpholine showed
the highest reactivity with an almost quantitative conversion of
iodobenzene in 92% yield for the desired α-keto amide. The
unsymmetrical secondary amines, such as 2i and 2j were also
successfully transformed into the corresponding products in
good yields. In addition to secondary amines, primary
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thesize alkynones catalyzed by the aryl-Pd-NHC. With previous
experiences in mind, we initially conducted the reaction of
iodobenzene 1a and ethynylbenzene 3a, with triethylamine
(2 equiv.) as the base.
Table 3 Substrate scope of palladium-catalyzed double carbonylation
of aminesa
Although the polymer form of palladium chloride did not
react with ethynylbenzene, PdCl2(CH3CN)2 is suitable for this
reaction, affording the corresponding products 4aa in 27%
yield (Table 4, entries 1 and 2). Much to our delight, the reac-
tion efficiency was improved to a good yield (81%) by using
Bmim-Pd-PPh3-I2 (Table 4, entry 3). Other palladium species,
such as IPr-Pd-Im-Cl2 and IPr-Pd-(3-Cl-pyridinyl)-Cl2 did not
effectively improve the carbonylation reactivity (Table 4, entries
4 and 5). Further investigation of the palladium catalyst pre-
cursors revealed that the reactivity was affected by the nature
of the NHC ligands, and the best result was achieved with IPr-
Pd-dmba-Cl as the catalyst precursor (Table 4, entries 6 and 7).
After screening reaction conditions, we can concluded that the
reaction was carried out by stirring a toluene solution of iodo-
benzene, ethynylbenzene, 0.1 mol% of aryl-Pd-NHC, and two
equivalents of Et3N at 100 °C under 2 MPa of CO for 18 hours
to give the product in the best yield.12 Notably, this result re-
presents a TON of 980, which is the highest among those
reported for aryl Pd-NHC catalyzed Sonogashira carbonylation
reaction.
As shown in Table 5, the reactions of aryl iodides 1 bearing
an electron-donating or electron-withdrawing group at the
ortho, meta, or para position of the phenyl ring proceeded
smoothly to give the corresponding products 4aa–4la in 39 to
98% yields. An obvious steric hindrance effect on the reactivity
was observed, which was demonstrated by the reactivities of
1c vs. 1d and 1f vs. 1g. Fortunately, the free amino group sur-
vived well in the standard procedure, leading to carbonylation
product 4ha in excellent yield. This result revealed that aniline
acted as a weak nucleophilic reagent for the Sonogashira
carbonylation reaction. In addition to substituted aryl iodides,
naphthyl-substituted iodide was also compatible with this
process, furnishing the desired product 4oa in good yield. It is
worth noting that heterocyclic derivatives are suitable for the
Entry
R3
R2
Yield (%)
1
2
3
4
5
6
7
8
9
Et
n-Pr
n-Bu
Allyl
Et
3aa, 92
3ab, 85
3ac, 84
3ad, 82
3ae, 63
3af, 69
3ag, 48
3ah, 92
3ai, 66
n-Pr
n-Bu
Allyl
Bn
Bn
Pyrrolidinyl
Piperidinyl
Morpholino
3,4-Dihydroisoquinolin-
2(1H)-yl
n-Pr
H
H
10
11
12
Bn
n-Bu
Ph
3aj, 53
3ak, 83
3al, 31b
a General conditions: 1 (1.0 mmol), 2a (2.0 mmol), [Pd] (0.5 mol%),
K2CO3 (2.0 mmol), dioxane (5.0 mL), CO (4.0 MPa), 80 °C, 12 h.
Isolated yield. b 3al: N-phenylbenzamide.
alkyamine was also compatible with this process, furnishing
the desired product 3ak in good yield. However, when aniline
2l was employed as the carbonylation partner, only the single
carbonylation product N-phenylbenzamide 3al was obtained in
31% yield.
We next turned our efforts toward investigation of the prac-
ticality of this aryl-Pd-NHC-catalyzed double carbonylation
reaction (Scheme 2). This transformation provides many
opportunities for applications in organic synthesis. Notably,
we conducted the reaction of iodobenzene 1a and diethyl-
amine 2a on a 10 mmol scale with a catalyst loading of
0.05 mol% and elongated time, and α-keto amide 3aa was iso-
lated in a very good 90% yield (1.8 g), which greatly increased
the practicality of this palladium catalyzed carbonylation reac-
tion. And importantly, when we further decreased the catalyst
loading to 0.01 mol%, α-keto amide 3aa was obtained in a very
good (43%) yield (2.2 g). Notably, this result represents a TON
of 4300, which is the highest among those reported for the aryl
Pd-NHC catalyzed carbonylation reaction.
Table 4 Screening [Pd] catalysts of single carbonylation
To extend the scope of the aryl-Pd-NHC catalytic system, we
also tested carbonylation of aryl iodides with alkynes to syn-
Entry
[Pd]
Yield 4aaa (%)
1
2
3
4
5
6
7
PdCl2
NR
27
81
63
79
87
98
PdCl2(CH3CN)2
Bmim-Pd-PPh3-I2
IPr-Pd-Im-Cl2
IPr-Pd-(3-Cl-pyridinyl)-Cl2
IMes-Pd-dmba-Cl
IPr-Pd-dmba-Cl
a General conditions: 1a (1.0 mmol), 3a (1.2 mmol), [Pd] (0.1 mol%),
triethylamine (2.0 mmol), toluene (5.0 mL), CO (2.0 MPa), 100 °C,
18 h. Isolated yield.
Scheme 2 Determination in TON of palladium-catalyzed double
carbonylation of amines.
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Table 5 Substrate scope of palladium-catalyzed carbonylationa
aryl iodide to a Pd(0) species formed in situ from catalyst pre-
cursors of aryl-Pd-NHC reductively eliminates the dmba to give
aryl palladium species A. Then CO insertion into the Pd–C
bond of
A gives aroyl palladium species B. In double
carbonylation, CO insertion into B gives the intermediate C.
Subsequently, the α-keto amide or alkynone is produced by
nucleophilic attack in the palladium species C or B and regen-
erates the active Pd(0) species for the next catalytic cycle.
In summary, we have developed an efficient and facile aryl
Pd-NHC catalytic system that allows the direct formation of
α-keto amide and alkynones from commercially available aryl
iodide via carbonylation under mild conditions. A series of
aryl palladium NHC compounds was prepared and their cata-
lytic activity in the carbonylation of aryl iodides to synthesize
α-keto amides and alkynones was examined. The method is
compatible with a variety of functional groups and can be used
to prepare a range of α-keto amide and alkynone derivatives.
Notably, this aryl Pd-NHC-catalyzed carbonylation exhibits
high reactivities, which afford TONs of 4300 and 980, respect-
ively. The protocol would be practical for use as an economical
synthetic method and offers an alternative synthetic strategy
for the practical construction of α-keto amide and alkynone
derivatives. Further investigations on application of the aryl
Pd-NHC catalytic system in other carbonylation reactions are
currently under way in our laboratory.
Entry
R1
R4
Yield (%)
1
2
3
4
5
6
7
8
H
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4aa, 98
4ba, 82
4ca, 96
4da, 63
4ea, 60
4fa, 83
4ga, 71
4ha, 90
4ia, 39
4ja, 78
4ka, 72
4la, 92
4ma, 40
4na, 49
4oa, 82
4pa, 54
4ab, 97
4ac, 93
4ad, 65
4ae, 95
4af, 96
4-CH3
3-CH3
2-CH3
4-OCH3
3-OCH3
2-OCH3
2-NH2
4-NHAc
4-F
9
10
11
12
13
14
15
16
17
18
19
20
21
4-Cl
4-NO2
4-C(O)CH3
4-C(O)OCH3
1-Naphthyl
3-Pyridine
H
H
H
H
H
4-CH3Ph
4-CH3OPh
4-n-Bu-Ph
3-F-Ph
4-CN-Ph
a General conditions: 1 (1.0 mmol), 2a (1.2 mmol), [Pd] (0.1 mol%),
triethylamine (2.0 mmol), toluene (5.0 mL), CO (2.0 MPa), 100 °C,
18 h. Isolated yield.
This research was supported by the Chinese Academy of
Sciences and the National Natural Science Foundation of
China (21373248, 21133011 and 21073209).
present reaction, affording the corresponding product 4pa in
high yield as well.
To extend the scope of our catalytic system, we also tested
carbonylation of 1a with alkynes under the standard pro-
cedure. Alkynes containing electron-rich or -deficient func-
tional groups reacted smoothly to give the corresponding
products in excellent yields. Typical functional groups such as
methyl, methoxy, halide and cyano groups were compatible
with the reaction conditions.
On the basis of the results we obtained here and previous
reports, a plausible mechanism for the present process can be
proposed as shown in Scheme 3. Initial oxidative addition of
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