Ligand-free reusable palladium-catalyzed heck-type coupling reactions of hypervalent iodine reagents under mild conditions

Article abstract of DOI:10.1002/adsc.201100008

Highly effective palladium-catalyzed Heck-type couplings of hypervalent iodine reagents are reported for the first time. It is noteworthy that such reactions could be carried out in the absence of ligand at 40 °C and the catalytic system could be easily reused for five times. Copyright

Full text of DOI:10.1002/adsc.201100008

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DOI: 10.1002/adsc.201100008
Ligand-Free Reusable Palladium-Catalyzed Heck-Type Coupling
Reactions of Hypervalent Iodine Reagents under Mild Conditions
a
a
a
a,
Xiaoming Qu, Peng Sun, Tingyi Li, and Jincheng Mao *
a
Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials
Science, Soochow University, Suzhou 215123, Peopleꢀs Republic of China
Fax : (+86)-512-6588-0403; e-mail: jcmao@suda.edu.cn
Received: January 5, 2011; Published online: April 28, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100008.
[
4a]
Abstract: Highly effective palladium-catalyzed
Heck-type couplings of hypervalent iodine reagents
are reported for the first time. It is noteworthy that
such reactions could be carried out in the absence
of ligand at 408C and the catalytic system could be
easily reused for five times.
tween a variety of aryl halides and olefins.
Al-
though the desired products were obtained with high
yields in the presence of low loadings of catalyst,
there is still the problem about how to reuse the cata-
lytic system in terms of potential applications on in-
dustrial and semi-industrial scales.
To meet the demands for recyclability and environ-
mental concerns, a more facile way is to immobilize
the catalyst in a liquid phase by dissolving it into a
non-volatile and low toxicity liquid, such as a poly-
Keywords: aryl halides; coupling reactions; hyper-
valent iodine reagents; olefins; palladium catalysis
[
5]
AHCTUNGTRENNUNG
[
6]
discovery. Thus, we performed the Heck reaction
It is well known that palladium-catalyzed cross-cou- between iodobenzene and methyl acrylate using
pling is a chemically investigative tool that allows sci- PEG-400 as the medium in the presence of 2 mol%
[
7]
entists to develop new drugs and electronics, among of Pd
ACHTUNGTNERUNNG( OAc) at 408C in air. The corresponding
2
other things, by facilitating a more efficient linking of product was obtained with 48% yield. In order to im-
[1]
carbon atoms. Just recently, the 2010 Nobel Prize in prove the catalytic effect, various additives were em-
chemistry was awarded to Ei-ichi Negishi, Akira ployed including H O and NaClO. The results
2
2
[
8]
Suzuki and Richard Heck for the development of pal- showed that (diacetoxy)iodobenzene could afford
ladium-catalyzed cross-couplings, which has stimulat- the desired product in 66% yield as shown in
ed extensive studies on CÀC bond-forming reac- Scheme 1. This raises the question: where does the
[2]
tions. Among these basic types of palladium-cata- additional 18% of yield come from?
lyzed transformations, the Heck reaction and related
In order to find the possible answer, the Heck reac-
chemistry occupy a special place for its resourceful tion between iodoanisole and methyl acrylate in the
[3]
versatility. In the long run, our group has much in- presence of 1 equivalent of (diacetoxy)iodobenzene
terest in cross-couplings using readily available cata- was conducted using 2 mol% of Pd(OAc) as the cata-
AHCTUNGTRENNUNG
2
[4]
lysts. Recently, we have reported that tridentate sali- lyst (Scheme 2). GC-MS indicated that besides the de-
cyaldiminato thiosemicarbazone palladium(II) com- sired product, the product formed from iodobenzene
plexes could catalyze the Heck coupling reaction be- and methyl acrylate was obtained. This suggests that
Scheme 1. The Heck reaction between iodobenzene and methyl acrylate in the presence of PhI ACHTUNGRETNN(UNG OAc) .
2
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Scheme 2. The Heck reaction between iodoanisole and methyl acrylate in the presence of (diacetoxy)iodobenzene.
Scheme 3. The Heck reaction between (diacetoxy)iodobenzene and methyl acrylate in two separate steps.
iodobenzene was possibly generated in situ from (di- Pd CAHTUNGTRNEU(GN OAc) or K CO as the base (Table 1, entries 2
2 2 3
acetoxy)iodobenzene and then the related Heck cou- and 3). 2 mol% of catalyst also provided the coupling
pling occurred. In addition, further evidence was ac- product with good yield (Table 1, entry 4). If MPEG-
quired that from the GC-MS: iodobenzene could be 400 [methylpoly(ethylene glycol)] was used as the sol-
detected from the control experiment which was car- vent, there was a sudden drop in the yield of the de-
ried out using only (diacetoxy)iodobenzene sired product (only 45% yield) (Table 1, entry 5).
(
Scheme 3). If we added methyl acrylate to this When DMF (dimethylformamide) was employed as
system for another 24 h, the desired product could be the solvent, the corresponding product was obtained
isolated in 51% yield. in 27% yield (Table 1, entry 6). In addition, a simple
Thus, a novel attempt was performed in which (di- glycol with a low molecular weight, such as EG (eth-
acetoxy)iodobenzene was directly used instead of io- ylene glycol), could not afford the desired product
dobenzene in the Heck coupling. To our surprise, the (Table 1, entry 7). The commonly used ether-type sol-
desired product was obtained almost quantitatively vents, such as THF, could not afford the coupling
(
Table 1, entry 1). The control experiments showed product, either (Table 1, entry 8). Under the same
that the coupling could not occur in the absence of conditions, only 48% yield of the desired product was
[a]
Table 1. Screening catalytic conditions in Heck coupling between (diacetoxy)iodobenzene and methyl acrylate.
[b]
Entry
Pd
A
H
U
G
R
N
U
G
Base
Solvent
Yield [%]
2
1
2
3
4
5
6
7
8
4
4
–
2
2
2
2
2
2
4
K CO
–
PEG-400
PEG-400
PEG-400
PEG-400
MPEG-400
DMF
EG
THF
PEG-400
PEG-400
>99
–
–
98
45
27
trace
–
2
3
K CO
2
3
3
3
3
3
3
3
3
K CO
2
K CO
2
K CO
2
K CO
2
K CO
2
[
c]
[
9
1
K CO
48
98
2
d]
0
K CO
2
[
a]
Reaction conditions: (diacetoxy)iodobenzene (0.5 mmol), methyl acrylate (3.0 mmol), K CO (1.0 mmol), PEG-400
2
3
(
2 mL), in air, 18 h, 408C.
[
[
[
b]
Isolated yield [based on (diacetoxy)iodobenzene].
Iodobenzene was employed instead of (diacetoxy)iodobenzene.
The reaction was performed in argon atmosphere.
c]
d]
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Ligand-Free Reusable Palladium-Catalyzed Heck-Type Coupling Reactions of Hypervalent Iodine Reagents
[a]
Table 2. Various Heck reactions between hypervalent iodine reagents and olefins using Pd ACHTNUGTERNN(UNG OAc) as catalyst.
2
1
2
o
[b]
Entry
ArIR R
Olefin
Temperature [ C]
Yield [%]
1
2
3
4
5
6
7
8
9
40
40
40
40
40
60
60
60
60
60
98
98
>99
91
96
99
98
99
99
91
1
1
0
1
60
92
[
[
a]
Reaction conditions: hypervalent iodine reagent (0.5 mmol), Pd
PEG-400 (2 mL), in air, 18 h, 40–608C.
Isolated yield based on hypervalent iodine reagent (average of two runs).
ACHTUNGTRNEN(UG OAc) (4 mol%), olefin (3.0 mmol), K CO (1.0 mmol),
2 2 3
b]
obtained when iodobenzene was replaced (Table 1, other hypervalent aryl iodine reagents were employed
entry 9). This tells us that (diacetoxy)iodobenzene into the Heck reaction. Although there is a slight de-
possibly acts as not only the source of the arylating crease in the yield of corresponding product, both
agent but also as an accelerant of the coupling during [bis(trifluoroacetoxy)iodo]benzene and [hydroxyl-
the process of the reaction, which is different from
ACHTUNGTRENNUNG
[
9]
ACHTUNGTRENNUNG
[
10]
AHCTUNGTRENNUNG
2
[
11]
iodine reagents in this coupling reaction. The reac- K CO /PEG-400 system as shown in Scheme 4. We
2
3
tion performed in an nargon atmosphere also afford- can see that almost full conversion of the reaction
ed the desired product in high yield (Table 1, was observed and an a/b product ratio of 1/6.9 was
entry 10).
obtained. Furthermore, a trans/cis isomer ratio of 0.38
1
The optimized procedure was subsequently used to for the b-product was calculated by H NMR. The b-
investigate the scope of the coupling between various arylated product could be isolated as phenylacetalde-
[
12]
commercially available hypervalent aryl iodine re- hyde following acidification.
agents and olefins (Table 2). It can be seen that for
the couplings of (diacetoxy)iodobenzene, different
As presented in Scheme 5, the coupling of (diacet-
AHCTUNGTREGoNNUN xy)iodobenzene and methyl methacrylate using our
types of olefins, such as acrylates, styrenes and acryl- catalytic system was performed. As expected, full con-
1
amide, afforded the desired products in excellent version could be observed. H NMR and GC suggests
yields (91–99%) (Table 2, entries 1–9). In addition, that there are two possible pathways, one leading to
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Xiaoming Qu et al.
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Scheme 4. Coupling of (diacetoxy)iodobenzene and butyl vinyl ether using the Pd ACHTUNGTRNENUG( OAc) /K CO /PEG-400 system.
2 2 3
Scheme 5. Coupling of (diacetoxy)iodobenzene and methyl methacrylate using Pd ACHTUNGTRNENUG( OAc) /K CO /PEG-400 system.
2 2 3
Scheme 6. Coupling of (diacetoxy)iodobenzene and styrene using the Pd CAHTUNGTRNEU(GN OAc) /K CO /PEG-400 system.
2 2 3
a-methylcinnamic acid ester with an internal double
bond (29% selectivity), and another that leads to a-
benzylacrylate with a terminal double bond (45% se-
lectivity), which is susceptible to a further arylation
[
13]
process (26% selectivity).
Analogous to the reactions with methacrylate,
higher selectivity was observed for styrene albeit with
slower reaction rates as shown in Scheme 6. No
doubly arylated product was obtained in this case.
1
H NMR and GC suggests that the E-regioisomer
with an internal double bond was observed (29% se-
lectivity) and the terminal regioisomer was also found
(
71% selectivity).
As far as the reaction mechanism is concerned, it
[
14]
possibly involves a Pd
A
H
U
G
E
N
N
(II/IV) catalytic cycle.
In
order to explain the high efficiency of this coupling
reaction, the nature of the catalytic system was inves-
tigated in situ using TEM. From Figure 1, the TEM
micrograph showed that the average diameter of the
palladium nanoparticles was about 100 nm. The
system of the in situ generated palladium nanoparti-
cles in PEG-400 in aerobic conditions was stable for
several months and no palladium black was observed.
This can be explained in that one of the possible roles
of the iodine reagent is to avoid the aggregation of Figure 1. The TEM image of catalytic system (2 h after reac-
[
15]
palladium nanoparticles as well as that of air. Fur- tion in air).
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Adv. Synth. Catal. 2011, 353, 1061 – 1066

Ligand-Free Reusable Palladium-Catalyzed Heck-Type Coupling Reactions of Hypervalent Iodine Reagents
thermore, these palladium nanoparticles usually Program of Jiangsu Province (BZ2010048) and and the Key
Laboratory of Organic Synthesis of Jiangsu Province for fi-
nancial support.
showed a much higher catalytic activity than common
[7b,16]
palladium.
At the same time, this catalytic system
could be reused for 5 times. The desired product
could be easily isolated by simple extraction with di-
ethyl ether, and the Pd ACHTGNUTRNEUNG( OAc) /K CO /PEG-400
2 2 3
system was subjected to the second run by charging it References
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ium-Catalyzed Heck-Type Coupling Reactions of
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(
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