Efficient synthesis of alkyl aryl ketones & ketals via palladium-catalyzed regioselective arylation of vinyl ethers

Article abstract of DOI:10.1039/c001004a

The combination of Pd(OAc)₂ with 1,3-bis(diphenylphosphino) propane (dppp) in ethylene glycol constitutes a high-performance catalytic system for highly regioselective arylation of a range of electron-rich vinyl ethers by aryl bromides to provide, upon hydrolysis, alkyl aryl ketones and cyclic ketals in good yields with up to 3.75 × 10⁵ TON and 15625 h^-1 TOF.

Full text of DOI:10.1039/c001004a

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Efficient synthesis of alkyl aryl ketones & ketals via palladium-catalyzed
regioselective arylation of vinyl ethers†
Mingcui Liu,‡a Zeynab Hyder,‡b Yawei Sun,a Weijun Tang,a Lijin Xu*a and Jianliang Xiaob
Received 18th January 2010, Accepted 12th March 2010
First published as an Advance Article on the web 22nd March 2010
DOI: 10.1039/c001004a
The combination of Pd(OAc)2 with 1,3-bis(diphenylphos-
phino)propane (dppp) in ethylene glycol constitutes a high-
performance catalytic system for highly regioselective aryl-
ation of a range of electron-rich vinyl ethers by aryl bromides
to provide, upon hydrolysis, alkyl aryl ketones and cyclic
ketals in good yields with up to 3.75 ¥ 105 TON and 15625 h-1
TOF.
attributed to promoting the ionic pathway A, which leads to
internal a-arylated vinyl ethers. Earlier studies by Hallberg and
Cabri revealed that the poor regioselectivity could be greatly
improved by employing aryl triflates or by adding stoichiometric
silver or thallium salts when aryl iodides and bromides are chosen.6
However, triflates are thermally labile and are rarely commercially
available, and the inorganic additives create new problems such
as toxicity and high cost. The regioselectivity enhancement has
been observed by changing the reaction medium. For example,
good b-regioselectivities have been achieved with poly(ethylene
glycol) or aqueous DMF as the reaction medium in the presence
of a Pd(OAc)2 or Pd–P(tBu)3 catalyst.7a–b Recent work by Hallberg
and us has shown that the use of imidazolium-based ionic liquids
or DMF–water mixtures in combination with Pd-dppp catalysis
provides excellent a-regioselectivity in the coupling reaction of
vinyl ethers with aryl bromides.7c–e It is assumed that the ionic
or polar environment provided by the solvent enables formation
of the cationic species in a more facile way. The recent kinetic
studies of Amatore, Jutand and co-workers support this view.8
Interestingly it has been found that ammonium salts, such as
[HNEt3][BF4] or [H2NiPr2][BF4], could act as hydrogen-bond
donor to increase both the rate and regioselectivity in the arylation
of vinyl ethers not only in ionic liquids but also in molecular
solvents, presumably by facilitating dissociation of bromide anion
from Pd(II).9a Following this leading report, inexpensive and
environmentally benign alcohol solvents, such as ethylene glycol
(EG) and iPrOH, which possess large hydrogen-bond-donating
ability, have been found to efficiently promote a-regioselectivity
in the arylation of several vinyl ethers with aryl bromides.9b–c In
addition, Larhed et al. reported that ethylene glycol vinyl ether
could undergo fast and highly regioselective a-arylation in water,
although other vinyl ethers failed.9d
Alkyl aryl ketones have been widely applied in the pharmaceutical,
fragrance, dye and agrochemical industries.1 In general the prepa-
ration of these compounds employs the traditional Friedel–Crafts
acylation, which involves handling hazardous reagents and fails
with electron-deficient arenes; thus the development of efficient
alternative routes has been an active topic in both academia
and industry.2 Since the discovery of palladium-catalyzed cross-
coupling reactions, a great deal of interest has been devoted
to the development of Pd-catalyzed protocols to access these
ketones, and a range of efficient routes have been described in
the literature.3–5 Among these reported methods, the Pd-catalyzed
Heck reaction of aryl halides with the electron-rich olefin vinyl
ethers is very attractive due to its simplicity, its tolerance of
various functional groups and the easy availability of reagents.5
However, this arylation is rarely regioselective under normal Heck
conditions, giving rise to a mixture of a- and b-arylated vinyl
ethers and thus hampering its wider application in synthesis.
It is generally believed that this regioselectivity problem arises
from two competing reaction pathways, which result in different
regioisomers (Scheme 1).5b–d Several methods have been success-
fully developed to overcome this problem. The success is usually
Despite all the above advances, it is surprising that less attention
has been placed on enabling the regioselective arylation more
economically and practically.10 In most of the cases, effort has been
made to exclusively deal with the improvement of regioselectivity,
and high catalyst loading is generally required, typically 2.5
mol%, thus limiting its attractiveness in large scale commercial
applications. When compared to the exceedingly high catalytic
efficiency achieved in other Pd-catalyzed coupling reactions,11
such as Heck arylation of electron-deficient olefins and Suzuki
reactions, it appears that there is much room for improvement in
terms of rates and turnover numbers. From the viewpoints of both
academic interest and practical applications, it is highly desirable
to develop high-performance catalytic system for regioselective
arylation of vinyl ethers to prepare alkyl aryl ketones. Following
our continuing interest in the highly regioselective arylation of
electron-rich olefins,7a–b,9a–c herein we report that the combination
Scheme 1 The two competing pathways.
aDepartment of chemistry, Renmin University of China, Beijing, 100872,
China. E-mail: xulj@chem.ruc.edu.cn
bLiverpool Centre for Materials and Catalysis, Department of Chemistry,
University of Liverpool, Liverpool, UK L69 7ZD
† Electronic supplementary information (ESI) available: General experi-
1
mental information and H and 13C NMR spectra of aryl alkyl ketones.
See DOI: 10.1039/c001004a
‡ These authors contributed equally to this work.
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Table 1 Solvent and S/C effect on the Heck arylation of n-butyl vinyl
and excellent regioselectivity albeit with a longer reaction time
being required (Table 1, entry 5). It is worthy to note that the
reaction performed well even under still lower catalyst loading
(S/C = 500000), giving 3.75 ¥105 TON and 15625 h-1 TOF. These
values represent the highest TON and TOF ever reported for the
Heck reaction of electron-rich olefins (Table 1, entry 6). Many
Pd-catalyzed Heck reactions have been reported to suffer from the
fast decomposition of catalyst, and 1–10 mol% catalyst loadings
are usually used.3,5 The thermal stability of the Pd-dppp system
may be partly attributed to EG, which could act as a ligand,
inhibiting decomposition of the catalyst into inactive Pd(0). As
we pointed out before,9b EG is an excellent hydrogen bond donor
and promotes the dissociation of bromide anion from Pd(II),
thus facilitating the arylation. Taken together, these features may
account for the excellent performance of EG at extremely low
catalyst loadings.
ethera
Entry
Solvent
S/C
Temp./◦C
Time/h
Yield [%]b
1
2
3
EG
1000
1000
1000
1000
10000
500000
145
Reflux
115
145
145
0.5
18
12
10
8
90
89
91
90
87
75
Isopropanol
[bmim][BF4]
[bmim][BF4]
EG
4
5c
6c
EG
145
24
a Reaction conditions: 1a (1.0 mmol.), 2a (2.0 mmol),
Pd(OAc)2/dppp=1/2, NEt3 (2.0 mmol), solvent (3 ml), no linear
product observed. b Isolated yield after acid hydrolysis. c 1a (1.0 mmol),
vinyl ether (2.0 mmol), Pd(OAc)2/dppp(1/2) and NEt3 (2.0 mmol) in
2.0 ml EG stock solution.
A series of aryl bromides were then subjected to olefination
by 1a at a high S/C ratio ranging from 100000 to 500000, and
the desired methyl aryl ketones were isolated after acidification of
the reaction mixture.12 The results obtained are listed in Table 2.
As can be seen, although necessitating longer reaction time (24
h) for completion, both electron-rich and electron-deficient aryl
bromides examined underwent successful olefination by 1a to
provide aryl methyl ketones in moderate to good yields, and in
no case was the linear product detected or isolated. Of particular
note are aryl bromides 2b–e, which can be smoothly olefinated
at a high S/C ratio of 500000, resulting in up to 355000 TON
and 14791 h-1 TOF (Table 2, entries 1–4). The coupling procedure
also tolerated the presence of the ortho- and meta-substituents
(Table 2, entries 9–13). Dual transformation could be achieved
in the case of dibromide 2p to give the diketone in good yield
(Table 2, entry 14). With palladacycle or PdCl2 as the catalyst,
Herrmann et al. achieved up to 400 TON in the Heck coupling
of 1a with aryl bromide in ionic liquid [NBu4]Br; but only poor
regioselectivity was observed.10a Santelli and co-workers reported
a Pd-tetraphosphine catalytic system, which allowed the Heck
reaction of aryl halides with vinyl ethers to proceed in DMF at
low catalyst loadings, affording up to 61000 TON; but the reaction
suffered from poor regioselectivity.10b–c Our reaction is not limited
to 1a, and the hydroxyalkyl vinyl ethers 1b could also be arylated to
give cyclic ketals at low catalyst loadings without adding any acid,
and again no linear product was observed (Table 2, entries 15–16).
It is notable that the reaction of 1-bromonaphthalene with 1b led
to a remarkable TON of 3.75 ¥105, and the TOF thus reached
15625 h-1 (Table 2, entry 15). This is the best result reported to
date in the regioselective arylation of hydroxyalkyl vinyl ethers.
Most of the reported studies have so far focused on the regio-
selective arylation of alkyl vinyl ethers by aryl halides to provide
methyl aryl ketones as the products.6–9 In contrast, the reaction
of 2-substituted vinyl ethers with aryl halides to access products
other than methyl ketones has been rarely investigated.10c–d To
further test the effectiveness of our catalytic system, the arylation
of 2-substituted vinyl ethers with aryl bromides was then carried
out at low catalyst loading; this would lead to products other
than methyl ketones after hydrolysis. Since these olefins are less
reactive than 1a, a higher catalyst loading was required.10c–d As can
be seen in Table 3, the Pd-dppp catalyst in EG allowed complete
reactions of aryl bromides with 2-substituted vinyl ethers 1c–f
in 24 h regardless of the nature of substituents on the aryl ring,
of Pd(OAc)2 and dppp in ethylene glycol results in a high-
performance catalytic system, which allows for regioselective a-
arylation of a series of vinyl ethers, producing alkyl aryl ketones,
after aqueous hydrolysis, and cyclic ketals in acceptable to good
yields with up to 3.75 ¥ 105 TON and 15625 h-1 TOF.
We first examined the feasibility of arylation of vinyl ether
under a low catalyst loading of 0.1 mol%. Under the previously
established conditions,9 butyl vinyl ether 1a was arylated by 2-
bromo-naphthalene 2a in EG at 145 ◦C with NEt3 as the base,
with the catalyst generated in situ from Pd(OAc)2 and 2 equiv.
dppp. In order to prevent the formation of palladium black, the
olefin was added 3–4 min after the mixture of other reagents
in EG had been heated.9b To our delight, 1a was completely
arylated by 2a in just 0.5 h without any linear product detected,
and the subsequent hydrolysis afforded the corresponding methyl
ketone in 90% yield (Table 1, entry 1). For comparison, the same
reaction was also carried out in iPrOH under identical conditions.
Although virtually the same yield of methyl ketone was obtained,
the reaction time was much longer, at 18 h (Table 1, entry 2). It
has been reported that the ionic liquid [bmim][BF4] is capable of
facilitating highly regioselective arylation of 1a in the presence of
0.1 mol% Pd-dppp catalyst and 1.5 equiv. hydrogen-bond donor
◦
[HNEt3][BF4] at 115 C.9a We then examined the reaction of 1a
with 2a in a mixture of [bmim][BF4] and [HNEt3][BF4] under the
same reaction conditions. Indeed, excellent regioselectivity and a
slightly better yield were achieved; however, the reaction went to
completion in a much longer time of 12 h (Table 1, entry 3). When
increasing the reaction temperature to 145 ◦C, it still required
10 h to finish but with retained regioselectivity (Table 1, entry 4).
Clearly, EG proved to be much more effective than other solvents.
The good performance of Pd-dppp in EG prompted us to
further enhance the catalytic efficiency. It is important to note
when carrying out the regioselective Heck reactions in a dilute
solution, naked Pd may catalyze the arylation in a “ligand free”
fashion, producing the linear product. In order to avoid the
formation of linear product, a stock solution containing Pd/dppp
was prepared in advance by heating Pd(OAc)2 with dppp in EG
for 12 h at 60 ◦C under an inert atmosphere. As can be seen from
Table 1, when the substrate-to-catalyst (S/C) ratio was raised to
10000, the reaction still proceeded smoothly to provide high yield
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Table 2 Heck arylation of vinyl ethers 1a–b in EG under low catalyst
Table 3 Heck arylation of 2-substituted vinyl ethers 1c–f in EG at low
loadingsa
catalyst loadingsa
Entry Olefin
1
Ar–Br S/C
Product
Yield [%]b
74
1a
2b
2c
5 ¥ 105
3b
Yield
2
1a
5 ¥ 105
3c
71
Entry
1
Olefin
Ar–Br S/C
1 ¥ 105
Product
[%]b
3
4
1a
1a
2d
2e
5 ¥ 105
5 ¥ 105
3d
3e
3f
71
69
1c 2a
3ac
3bc
71
2
3
1c
1c
2b
2g
1 ¥ 105
1 ¥ 105
73
69
5
6
1a
1a
2f
1 ¥ 105
1 ¥ 105
72
71
3gc
2g
3g
3h
4
5
1c
1c
2i
2j
1 ¥ 105
1 ¥ 105
3ic
3jc
3kc
3bd
69
70
6
1a
2h
1 ¥ 105
70
1 ¥ 105
5 ¥ 103
65
72
7
1a
1a
1a
1a
1a
1a
2i
1 ¥ 105
1 ¥ 105
1 ¥ 105
1 ¥ 105
1 ¥ 105
1 ¥ 105
3i
73
72
71
73
75
76
6
7
1c
2k
8
2j
3j
1d 2b
9
2k
2l
3k
8
1d
2d
1e 2a
2b
5 ¥ 103
5 ¥ 103
5 ¥ 103
3dd
3ae
3be
74
76
78
10
11
12
3l
11
12
2m
2n
3m
1e
1e
1e
3n
13
2c
5 ¥ 103
3ce
73
13
1a
1a
2o
1 ¥ 105
3o
73
14
15
2j
5 ¥ 103
2 ¥ 102
3je
77
69
14c]
15d
2p
2e
1 ¥ 105
5 ¥ 105
3p
3eb
73
75
1f 2b
3bf
1b
16d
1b
2c
5 ¥ 105
3cb
70
16
1f
2g
2 ¥ 102
3cf
65
a Conditions: aryl bromide (1.0 mmol), vinyl ether (2.0 mmol),
Pd(OAc)2/dppp(1/2) and NEt3 (2.0 mmol) in 2.0 ml EG stock solution,
145 ◦C, 24 h; the ketone was obtained after acidification. All reactions
gave >99/1 regioselectivity as determined by 1H NMR. b Isolated yield
after acid hydrolysis.
a Conditions: aryl bromide (1.0 mmol), vinyl ether (2.0 mmol),
Pd(OAc)2/dppp(1/2) and NEt3 (2.0 mmol) in 2.0 ml EG stock solution,
145 ◦C, 24 h; the ketone was obtained after acidification. All reactions
gave >99/1 regioselectivity as determined by 1H NMR. b Isolated yield
after acid hydrolysis. c 4 Equiv. olefin was employed. d Conditions were the
same as for a, but omitting aqueous acidic work-up.
and productivity (TON up to 3.75 ¥105). It is noteworthy that the
efficacy of the chemistry stems from the use of EG as a solvent,
which may play a role in stabilizing the Pd-dppp catalyst and
facilitating the formation of the ionic palladium intermediate. The
latter feature enables the reaction to proceed via the ionic route
and hence to preferentially afford the branched products. The
chemistry provides a straightforward and convenient route to the
synthesis of alkyl aryl ketones and cyclic ketals, displaying great
potential for practical applications.
and the subsequent hydrolysis provided alkyl aryl ketones with
moderate to good yields. As with the arylation of 1a in EG, only
a-arylated products were detected. The reaction is sensitive to the
nature of substituents of 2-substituted vinyl ethers, however, and
this is exemplified by the observation that lower S/C ratios ranging
from 5000 to 200 are required to efficiently arylate the bulkier vinyl
ethers 1d–1f (Table 3, entries 7–16), whilst the arylation of 1c could
proceed smoothly at a higher S/C ratio of 100000 (Table 3, entries
1–6). The best result was observed in the coupling reaction of 1c
with 2b, affording a TON of 73000 (Table 3, entry 2).
Acknowledgements
In summary, the results presented in this paper show that Pd-
dppp in EG is a high-performance catalyst capable of catalyzing
highly regioselective arylation of a series of electron-rich vinyl
ethers, showing remarkably high activity (TOF up to 15625 h-1)
Financial supports from National Natural Science Foundation of
China (20873179) and Renmin University of China are gratefully
acknowledged. We also thank the ESPRC for a studentship (Z.H.).
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12 General procedurefor regioselective Heck arylation of vinyl ethers:
A stock solution of 0.1 mol% (1 mmol, 2.2 mg) palladium acetate
together with 0.2 mol% dppp (2 mmol, 8.2 mg) was prepared in 10 mL
degassed ethylene glycol, which was used as the mother solution to
which subsequent dilutions were made. To ensure the mixture was
homogeneous, the solution was left stirring under nitrogen at 60 ◦C
for 12 h. The solution turned deep orange, with no sign of residue
indicating that the Pd/dppp was dissolved homogeneously. To make a
1 ¥ 10-2mol% stock solution, 1 mL was taken from this mother solution
and added (under nitrogen) to 9 mL of degassed ethylene glycol.
This procedure was followed to make more dilution solutions when
necessary. An oven-dried, two necked round-bottom flask containing a
stirrer bar was charged with an aryl halide (1.0 mmol), Pd-dppp stock
solution (2.0 mL) under nitrogen atmosphere at room temperature.
After degassing three times, NEt3 (2.0 mmol) was injected. The vinyl
ether (2.0 mmol) was added after 2-4 min of pre-mixing at 145 ◦C in an
oil bath. After an appropriate reaction time, the flask was removed from
the oil bath and cooled to room temperature. For products requiring
acid hydrolysis, aqueous HCl (5%, 5 mL) was added and following
stirring for 0.5-3 h, CH2Cl2 (2.0 mL) was added. After separation of
the CH2Cl2 phase, the aqueous layer was extracted with CH2Cl2 (3 ¥
5 mL), and the combined organic layer was washed with water until
neutrality, dried (Na2SO4), filtered and concentrated in vacuo. The
product was purified via flash chromatography on silica gel using a
mixture of ethyl acetate and hexane (for details, please see Electronic
Supplementary Information†).
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