An efficient palladium-catalyzed synthesis of cinnamyl ethers from aromatic halides, phenols, and allylic chloride

Article abstract of DOI:10.1002/adsc.201300757

A one-pot, two-step catalytic protocol for the preparation of cinnamyl ethers from simple and readily available aryl halides, phenols and allyl chloride is reported for the first time. This simple and highly efficient palladium nanoparticles catalytic system shows good regio- and stereoselectivities and affords the desired products in good to high yields (49-85%) from aryl iodides. Furthermore, less reactive aryl bromides can also give the cinnamyl ethers in moderate yields (24-72%).

Full text of DOI:10.1002/adsc.201300757

UPDATES
DOI: 10.1002/adsc.201300757
An Efficient Palladium-Catalyzed Synthesis of Cinnamyl Ethers
from Aromatic Halides, Phenols, and Allylic Chloride
Wei Wang,a Rong Zhou,a Zhi-Jie Jiang,a Kun Wang,a Hai-Yan Fu,a Xue-Li Zheng,a
Hua Chen,a and Rui-Xiang Lia,*
a
Key Lab of Green Chemistry and Technology, Ministry of Education; College of Chemistry, Sichuan University, Chengdu
610064, Peopleꢀs Republic of China
Fax : (+86)-28-8541-2904; e-mail: liruixiang@scu.edu.cn
Received: August 21, 2013; Revised: November 12, 2013; Published online: February 6, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300757.
Abstract: A one-pot, two-step catalytic protocol for
the preparation of cinnamyl ethers from simple and
readily available aryl halides, phenols and allyl
chloride is reported for the first time. This simple
and highly efficient palladium nanoparticles catalyt-
ic system shows good regio- and stereoselectivities
and affords the desired products in good to high
yields (49–85%) from aryl iodides. Furthermore,
less reactive aryl bromides can also give the cin-
namyl ethers in moderate yields (24–72%).
plicability of these methods. To avoid using the corre-
sponding cinnamyl derivatives as substrates, the Miz-
oroki–Heck-type reaction of allyl ethers with aryl hal-
ides is extensively applied to synthesize cinnamyl
ethers.[9] Norrby et al.[9c] reported that cinnamyl alco-
hol derivatives could be obtained in high yields by the
palladium-catalyzed Mizoroki–Heck (MH) reaction
of aryl iodides or bromides with 2-(allyloxy)tetrahy-
dropyran. Mino et al.[9e] prepared a series of aryl cin-
namyl ethers in good yields, utilizing the MH reaction
of allyl aryl ethers and aryl iodides with phosphine-
free hydrazone as a ligand. However, in most cases,
high palladium loadings and expensive aryl iodides
were used in these systems. This makes the procedure
less attractive to large-scale reactions and industrial
applications.
Keywords: cinnamyl ethers; Mizoroki–Heck reac-
tion; nanoparticles; one-pot synthesis; palladium
Therefore, the development of a new catalytic
Cinnamyl ethers are important starting materials for system to synthesize cinnamyl ethers from simple and
a variety of organic reactions, including the Claisen inexpensive starting materials with low catalyst load-
rearrangement,[1] [1,3]-sigmatropic shift,[2] and allylic ing is still highly desirable. We envisioned the poten-
substitution reactions,[3] in addition to being an impor- tial of a suitable palladium catalyst that is capable of
tant structural assembly in many natural products and generating allyl ethers in situ from the Williamson-
biologically active molecules.[4] Over the past years, type reaction of allyl chlorides with phenols and, in
lots of synthetic methods have been developed for the the same reaction vessel, then catalyzes the MH reac-
preparation of cinnamyl ethers. The traditional tion of aryl halides (Scheme 2). This would not only
method is the palladium-catalyzed etherification of broaden the range of the MH reaction of allyl ethers,
cinnamyl halides,[5] cinnamyl alcohols,[6] cinnamyl but also circumvent the need for intermittent isolation
esters,[7] or cinnamyl carbonates[8] with phenols and purification.
(Scheme 1). However, the high costs and tedious mul-
tiple synthetic steps involved in the preparation these
cinnamyl derivatives have generally limited the ap-
Scheme 1. Palladium-mediated synthesis of cinnamyl ethers.
Scheme 2. Palladium-catalyzed one-pot reaction.
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UPDATES
An Efficient Palladium-Catalyzed Synthesis of Cinnamyl Ethers
together with palladium precursor was added and re-
sultant mixture further reacted at 1008C for 20 h
under the protection of argon. The reaction was
found to give five products: 3a, 3b, 3d, and 3e (all
generated via the 3-arylation reaction) and the
branched derivative 3c, with 3a as the major product.
To select more active and selective palladium nano-
particle catalyst systems, several palladium sources
were investigated in this one-pot reaction. As shown
in Table 1, with 1 mol% of catalyst loading, all of the
palladium precursors exhibited good activity and af-
forded a mixture of regio- and stereoisomeric prod-
ucts in 94–100% yields (Table 1, entries 1–5). While
the selectivity towards the desired (E)-cinnamyl ether
Scheme 3. One-pot reaction of iodobenzene, phenol, and
allyl chloride.
isomer 3a was quite different. The [PdACTHNURTGNENG(U COD)Cl2]
proved to be the most selective catalyst, affording 3a
with the highest yield (87%). It is noteworthy that the
Our continuous efforts in the area of palladium-cat- yield of product 3a decreased remarkably to 68%
alyzed MH reactions prompted us to explore the utili- when the reaction was carried out in air, under other-
ty of phosphine-free and highly efficient palladium wise identical conditions (Table 1, entry 6). In case of
nanoparticles catalyst[10] for the one-pot reaction of the lower palladium loading (0.1 mol% catalyst), the
allyl chloride, phenols, and aryl halides resulting in decrease was even more significant, from 77% to 8%
the construction of cinnamyl ethers. This research (Table 1, entries 7 and 8). The above results revealed
demonstrated that the reaction conditions of the gen- that the protection of argon is very important for
eration of allyl ethers could be used in the subsequent maintaining the palladium catalyst’s activity, which is
MH reaction, which was also crucial to this one-pot well consistent with our previously reported results.[10]
reaction. On the basis of these results, we wish to Is [Pd
ACHTUNGTRENNUNG
report herein an efficient palladium-catalyzed one-pot cles like [PdAHCTNUGTRENNNUG
MH reaction for the synthesis of cinnamyl ethers conversion vs. time plot and Hg poisoning test were
from the readily available substrates – allyl chloride, investigated, which are typical characteristics of nano-
phenols, and aryl haldies.
particle catalysis (Figure 1). Furthermore, transmis-
We firstly attempted to synthesize (E)-cinnamyl sion electron microscopy (TEM) imaging demonstrat-
phenyl ether 3a from phenol 1a, allyl chloride, and io- ed that palladium is present in the form of nanoparti-
dobenzene 2a by a one-pot reaction, where the Wil- cles with an average particle size of 3.76 nm
liamson and MH reactions took place sequentially (Figure 2).
(Scheme 3). Phenol was first allowed to react with
Although there have been a few reports by Kçhler
allyl chloride at 1008C for 4 h in the presence of and Zotto wherein PdO hydrate behaved as an effi-
K3PO4 and (n-Bu)4NBr (TBAB), then iodobenzene cient precatalyst in the MH and Suzuki–Miyaura
Table 1. Effect of catalyst precursors and air on this one-pot reaction.[a]
Entry
Pd source
Conditions
S/C
Yield [%][b]
Total
3a
3b
3c
3d
3e
1
2
3
4
5
6
7
8
PdCl2
Ar
Ar
Ar
Ar
Ar
Air
Ar
air
air
Ar
100
100
100
100
100
100
1000
1000
1000
–
94
100
96
100
100
85
100
8
98
82
80
81
84
87
68
77
8
1
2
1
2
0
3
2
0
3
0
3
5
3
4
4
3
5
0
5
0
4
5
7
5
5
5
6
0
8
0
4
8
4
5
4
6
10
0
6
0
Pd
Pd(dba)2
(C3H5)Cl]2
(COD)Cl2]
(COD)Cl2]
(COD)Cl2]
(COD)Cl2]
(COD)Cl2]
A
E
[Pd
[Pd
[Pd
[Pd
[Pd
[Pd
–
N
9[c]
10[d]
76
0
0
[a]
1a (2 mmol), 2a (1 mmol), allyl chloride (2 mmol), K3PO4 (3 mmol), DMA (4 mL), TBAB (0.75 mmol), 1008C, 20 h.
Yields are determined by GC-MS and H NMR.
PEG-400 (1 mL) was used.
[PdACHTUNGTRENNUNG(COD)Cl2] was not added.
[b]
[c]
[d]
1
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Wei Wang et al.
Figure 1. Conversion vs. time plot for this one-pot reaction.
Reaction conditions: 1a (1.5 mmol), 2a (1 mmol), allyl chlo-
ride (1.5 mmol), [Pd(COD)Cl2] (0.01 mmol), K3PO4
AHCTUNGTRENNUNG
(3.5 mmol), DMA (4 mL), TBAB (0.75 mmol), in argon,
&
*
1008C. ( ) Control experiment; ( ) Hg(0) (500:1 Hg/Pd)
poisoning experiment.
cross-coupling reactions,[11] it was also demonstrated
that the true active species is present in the solution
phase and most probably it arises from nanosized col-
loidal Pd(0) species by dissolution of Pd species from
the support.[12] In this case, when the nanoparticles
were exposed to an oxygen-containing environment,
the Pd(0) active species was likely converted into
PdO species,[13] which inhibited the reaction process
and led to a very low yield. However, the addition of
PEG-400 as a reductive agent could make the reac-
tion proceed rapidly in air with 0.1 mol% of catalyst
loading (Table 1, entry 9). Thus, the results proved the
viewpoint that nanosized colloidal Pd(0) is the active
species.[12] A blank experiment, carried out under the
identical conditions but without any palladium source,
led to no conversion of the starting substrates
Figure 2. TEM micrographs and nanoparticles size distribu-
tions: [Pd
10 min. Reaction conditions: 1a (1.5 mmol), 2a (1 mmol),
allyl chloride (1.5 mmol), [Pd(COD)Cl2] (0.01 mmol),
ACHTUNGTERNUN(NG COD)Cl2] as catalyst precursor and reaction for
AHCTUNGTRENNUNG
K3PO4 (3.5 mmol), DMA (4 mL), TBAB (0.75 mmol), in
argon, 1008C.
(Table 1, entry 10). Thus, we chose [PdACTHNUTRGENUG(N COD)Cl2] as
the catalyst precursor under argon for the following
experiments.
To optimize the reaction conditions, a series of ex- scope of this one-pot reaction with the palladium
periments was conducted with the model reaction, nanoparticles catalytic system generated from
with variation of reaction parameters, such as the [PdACTHNUGTRENUNG(COD)Cl2] in situ. Firstly, the scope of the phenol
amount of substrates, base, solvent etc. (see the Sup- was tested and the results are summarized in Table 2.
porting Information). It was found that a combination The one-pot reaction of various phenols with allyl
of 1.5 equiv. of phenol, 1.5 equiv. of allyl chloride, chloride and aryl iodides proceed smoothly to give
3.5 equiv. of K3PO4, and 4 mL of DMA is ideal for good to high isolated yields in the presence of
a good yield and selectivity. Nevertheless, the yield of 0.5 mol% [PdACTHNURGTNEUNG(COD)Cl2]. It was found that an influ-
product 3a decreased remarkably to 65% when all ence of the substituent groups in the phenols on the
three starting materials were mixed at once (see the reaction activity was evident. The phenols bearing
Supporting Information, Table S1).
electron-rich groups, such as methyl, ethyl, and me-
Using the above-mentioned optimized conditions, thoxy afforded the desired products in 62–84% yields
we initiated our investigations into the substrate (Table 2, entries 3–9), whereas electron-poor groups
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An Efficient Palladium-Catalyzed Synthesis of Cinnamyl Ethers
Table 2. Palladium-mediated reaction of aryl iodides, phenols and allyl chloride.[a]
Entry
1, R1
2, R2
S/C
Conversion
Product
Yield of a [%][b]
1
2
3
4
5
6
1a, H
1b, 4-Cl
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2a, H
2b, 4-OCH3
2b, 4-OCH3
2b, 4-OCH3
2b, 4-OCH3
2b, 4-OCH3
2c, 4-CH3
2c, 4-CH3
2d, 4-COMe
2d, 4-COMe
2e, 2-OCH3
2e, 2-OCH3
200
200
200
200
200
200
100
100
100
100
100
100
200
200
200
200
200
200
200
200
200
200
200
100
98
100
100
100
96
100
100
100
0
3a
4a
5a
6a
7a
8a
9a
10a
11a
–
–
–
12a
13a
14a
15a
16a
17a
18a
19a
20a
21a
22a
85 (82)
78 (74)
67 (64)
83 (78)
84 (78)
77 (70)
63 (60)
62 (59)
66 (63)
0
1c, 4-OCH3
1d, 4-CH3
1e, 3-CH3
1f, 2-CH3
1g, 2-Et
1h, 2-i-Pr
1i, 2-t-Bu
1j, 4-NO2
1k, 4-CHO
1l, 4-COMe
1a, H
7[c]
8[c]
9[c]
10[c]
11[c]
12[c]
13
14
15
16
17
18
19
20
21
22
23
0
0
0
0
100
95
100
100
98
100
100
100
100
97
72 (66)
77 (70)
80 (75)
79 (74)
59 (57)
85 (78)
74 (68)
(60)
1b, 4-Cl
1d, 4-CH3
1e, 3-CH3
1f, 2-CH3
1a, H
1f, 2-CH3
1a, H
1f, 2-CH3
1a, H
1f, 2-CH3
(49)
67 (65)
58 (56)
96
[a]
1 (1.5 mmol), 2 (1 mmol), allyl chloride (1.5 mmol), K3PO4 (3.5 mmol), DMA (4 mL), TBAB (1 mmol), 1008C, 16 h.
Isolated yields.
24 h.
[b]
[c]
such as nitro, formyl, and acetyl groups in the phenols the reaction (Table 2, entries 6–9), and even the much
led to no conversion even with the amount of catalyst more sterically demanding 2-isopropylphenol and 2-
increased to 1 mol% (Table 2, entries 10–12). Accord- tert-butyl phenol could furnish the expected products
ing to the literature,[8a,14] in basic conditions related to in moderate yields (Table 2, entries 8 and 9). The
the low activity of these electron-poor phenols in Wil- scope of the aryl iodides was also examined. Both the
liamson-type reaction, the more readily formed phe- electron-rich and electron-deficient iodobenzene de-
nolate usually is a poor nucleophile. Indeed, the cor- rivatives were suitable substrates. In general moderate
responding allyl phenyl ethers were observed with to good isolated yields (49–78%) were obtained
phenols bearing electron-deficient groups as etherifi- (Table 2, 13–23).
cation reagents. Meanwhile, the vast phenols did not
To further extend the scope of the aryl iodides, the
react with allyl chloride under the reaction conditions. one-pot reaction of 2-iodothiophene and allyl chloride
However, further studies showed that electron-with- with various phenols was investigated. The results
drawing groups in phenols not only prevent the for- listed in Scheme 4 showed that 2-iodothiophene
mation of allyl phenyl ethers, but also inhibit the pro- worked well with various phenols and allyl chloride in
cess of the MH reaction. For example, when the elec- this one-pot reaction system, affording the corre-
tron-deficient allyl 4-nitrophenyl ether was allowed to sponding ethers in satisfactory yields (55–63%). It
react directly with iodobenzene in the same catalyst should be noted that when 1-iodonaphthalene was
system, still no corresponding cinnamyl ether was ob- used as arylating agent, in most cases, enol ethers
tained.
(30d–36d) instead of allyl aryl ethers were obtained as
In addition, we were pleased to find that the pres- the major products. Also, a combination of 4-methox-
ence of ortho substituents in phenols did not inhibit yphenol, 1-iodonaphthalene and allyl chloride still
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UPDATES
Wei Wang et al.
Scheme 4. Palladium-mediated reaction of 2-iodothiophene (or 1-iodonaphthalene), phenols, and allyl chloride.
Scheme 5. Palladium-mediated reaction of aryl bromides, phenols, and allyl chloride.
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UPDATES
An Efficient Palladium-Catalyzed Synthesis of Cinnamyl Ethers
R. D. Dura, S. G. Nelson, J. Am. Chem. Soc. 2010, 132,
11875–11877; c) H. Kobayashi, B. Driessen, D. J. G. P.
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provided the normal allyl aryl ether product in 61%
yield.
The wider applicability of aryl bromides is undis-
puted in cross-coupling reactions, whereas few results
have been described for the MH reaction of aryl bro-
mides with allyl ethers.[9c,e] We were pleased to note
that under our optimized reaction conditions aryl bro-
mides also reacted with phenols and allyl chloride
albeit with increasing catalyst loading and reaction
temperature to provide the desired products in mod-
erate yields (Scheme 5). Fortunately, less active heter-
oaryl bromides such as 2-bromothiophene also gave
55% yield.
In summary, we have developed a new and highly
efficient palladium-catalyzed one-pot reaction for the
preparation of cinnamyl ethers from aryl halides, phe-
nols, and allyl chloride. The reaction shows good
regio- and stereoselectivities when electron-rich phe-
nols and aryl iodides were used. Steric effects seem
very weak since phenols having substituents in the
ortho position reacted well. Moreover, the catalytic
system also gives moderate yields with the less active
aryl bromides.
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Experimental Section
General Procedure for the Synthesis of Cinnamyl
Ethers
Phenol (141 mg, 1.5 mmol), K3PO4 (739 mg, 3.5 mmol), and
TBAB (322 mg, 1 mmol) were added into a dried Schlenk
tube with a magnetic stirrer bar, which was subjected to
evacuation/flushing with dry argon five times, and then allyl
chloride (125 mL, 1.5 mmol) and DMA (4 mL) were added
successively into it. The reaction mixture was heated in
1008C for 4 h. After the reaction mixture had cooled to
room temperature, iodobenzene (112 mL, 1 mmol) and the
solution of [PdACHTUNGTRENNUNG(COD)Cl2] (100 mL, 0.005 mmol) in DMA
(1 mL) were added. The reaction temperature was raised to
1008C. After being stirred for 16 h, the mixture was diluted
with ethyl acetate (3ꢂ5 mL) and water (5 mL). The organic
layer was washed with brine (3ꢂ5 mL), dried with anhy-
drous MgSO4, and concentrated under vacuum. The residue
was purified by silica gel chromatography (petroleum ether)
to give the product 3a.
Acknowledgements
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We gratefully acknowledge support from the Natural Science
Foundation of China (No. 21202104).
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