Copper-catalyzed regio- and stereoselective O-arylation of enolates

Article abstract of DOI:10.1021/ol501434d

Copper-catalyzed O-arylation of enolates with diaryliodonium salts as arylating reagents was realized successfully. As important building blocks, β-aryloxy carbonyl compounds were obtained in up to 98% yield under mild conditions, and complete control of

Full text of DOI:10.1021/ol501434d

Letter
pubs.acs.org/OrgLett
Copper-Catalyzed Regio- and Stereoselective O‑Arylation of Enolates
Ze-Feng Xu, Chen-Xin Cai, Min Jiang, and Jin-Tao Liu*
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Science, 345 Lingling
Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: Copper-catalyzed O-arylation of enolates with diaryliodonium salts as arylating reagents was realized successfully.
As important building blocks, β-aryloxy carbonyl compounds were obtained in up to 98% yield under mild conditions, and
complete control of O-arylation and Z-stereoselectivity were achieved. The origin of the selectivity was also discussed.
Scheme 1. Proposal of O-Arylation of Enolates
he regioselective control of the C-functionalization or O-
Tfunctionalization of enolate is a historical research area and
still very attractive.1 The regioselectivity is sensitive to both
reaction conditions and the properties of enolate and electro-
phile. In general, the softer electrophiles react more easily with
the carbon center, whereas the harder electrophiles have stronger
affinity to the oxygen center.2 Although the control of the
regioselective functionalization of the C/O center has been
achieved for some reactions,3 such as C/O-alkylation,3a,b C/O-
fluoroalkylation,3c,d and C/O-acylation,3e,g and the C-arylation
reaction has also been developed very well,4 no regioselective O-
arylation of enolates has been reported. Considering the
importance of aryl ethers, especially the existence of β-aryloxy
carbonyl moiety in many useful molecules,5 it would be of great
significance to develop a practical method for the regioselective
O-arylation of enolates.
dicarbonyl compounds using diaryliodonium salt as arylating
reagent and sodium hydride as base (Scheme 1).7l
Considering the important role of catalyst in the regioselective
arylation reaction involving diaryliodonium salt, we assumed that
if a proper metal catalyst was used, the regioselectivity of the
reaction might reverse and the O-arylation product would be
obtained regioselectively (Scheme 1). This would provide a facile
synthesis of β-aryloxy-α,β-unsaturated carbonyl compounds
which were obtained by the addition reaction of phenols and
propiolates only.5h Herein, we report our realization of the
copper-catalyzed regioselective O-arylation of enolates derived
from 1,3-dicarbonyl compounds with diaryliodonium salts.
In our initial experiment, ethyl acetoacetate (2a) was chosen as
a model substrate to react with Ph2IOTf (1a) in the presence of
CuI (20 mol %), proline (40 mol %), and Cs2CO3 (4.0 equiv) in
DMSO at 40 °C (Table 1, entry 1).4g Unfortunately, no expected
O-arylation product 3a was formed, and the C-arylation product
4a was generated in 52% yield after 24 h. When Li2CO3 was used
as base, the yield of 4a was reduced and the desired product 3a
was obtained in 3% yield (entry 2).
As an appealing arylating reagent, diaryliodonium salt 1,6 easily
prepared in one step from commercially available materials, is air-
and moisture-stable and has been used as an efficient arylating
reagent in the arylation of O-center,7a−g such as alcohols,
phenols, carboxylates and hydroxylamines, and the arylation of
C-center,7g−t such as carbonyl compounds and their derivatives.
Olofsson and co-workers reported that in the C-arylation
reaction of enolate, [2,3] rearrangement of the O−I intermediate
was slightly favored compared to the [1,2] rearrangement of the
C−I intermediate7s (Scheme 1). However, Gaunt and co-
workers reported that the catalyst is very important to the
regioselectivity of the arylation reaction of diaryliodonium salt.8
For example, using 1 as the arylating reagent, Pd salts could
catalyze the C-2 arylation of indol, whereas Cu salts achieved the
C-3 arylation. In the case of acyl-protected aniline, the ortho-
arylation products were obtained with Pd catalysts, while Cu salts
promoted the meta-arylation. In 1999, Oh and co-workers
reported the efficient C-arylation of enolate derived from 1,3-
Without proline, the yield of 3a was increased to 21% at 60 °C
(entry 3). Further studies indicated that the amount of Li2CO3
Received: April 18, 2014
© XXXX American Chemical Society
A
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Letter
a
Table 1. Optimization of O-Arylation of Ethyl Acetoacetate
Scheme 2. O-Phenylation of Enolates
b
yield (%)
temp
time
(h)
entry
base (x)
solvent
(°C)
3a
4a
52
c
1
Cs2CO3 (4.0)
Li2CO3 (4.0)
Li2CO3 (4.0)
Li2CO3 (3.0)
Li2CO3 (3.0)
Li2CO3 (3.0)
Li2CO3 (3.0)
Li2CO3 (3.0)
Li2CO3 (3.0)
Li2CO3 (3.0)
Li2CO3 (2.0)
Li2CO3 (1.0)
Li2CO3 (2.0)
Li2CO3 (2.0)
Li2CO3 (2.0)
Li2CO3 (2.0)
Li2CO3 (2.0)
Li 2CO3 (2.0)
Li2CO3 (2.0)
Li2CO3 (2.0)
DMSO
DMSO
DMSO
DMSO
DMF
40
40
60
60
60
60
60
60
60
70
70
70
70
70
80
80
70
70
70
70
70
70
24
24
24
24
24
24
24
24
36
36
36
36
48
48
48
48
60
60
60
60
60
60
c
2
3
5
3
21
27
12
8
3
4
2
5
18
2
6
DMAC
NMP
7
11
6
4
8
MeCN
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
dioxane
trace
3
9
79
80
84
75
83
87
5
10
11
12
trace
trace
trace
trace
trace
d
13
e
yields of the reaction, the excellent selectivity remained
unaffected. When R2 = H, R3 = Me, and R1 = Et, Me, or i-Pr,
the corresponding products were obtained in excellent yields
(3a−c 96−98%, Scheme 2A). Good yields were also achieved
with tert-butyl (3d, 88%) and allyl (3e, 94%) acetoacetate,
indicating that the steric effect of the R1 substituent had little
influence on the reaction because it was far away from the
reaction center. In contrast, the R2 substituent had much more
influence on the reaction. As shown in Scheme 2B, lower yields
were obtained when a substituent was introduced to the α-
carbon of substrate 2 (3f−h, 75−88%), and the yield dropped
sharply with the increase of the steric hindrance of substituent
(3i,j, R2 = Bn, n-C7H15). Substituent R3 also had some influence
on the yield of the reaction. When R3 = Pr or i-Pr, the yields of 3k
and 3l were a little lower than that of 3a (Scheme 2C).
Furthermore, the reaction of cyclic substrate also proceeded well
to give the corresponding products in good yields (86−88%,
Scheme 2D).
14
f
15
e
16
78
98
19
68
trace
24
6
trace
N.D.
3
e
g
17
h
18
i
19
11
j
20
trace
5
e
21
Na2CO3 (2.0) dioxane
K2CO3 (2.0) dioxane
e
22
10
a
Reaction scale: 1a (0.32 mmol, 1.0 equiv), 2a (0.38 mmol, 1.2 equiv),
b
1
solvent (1.0 mL). Determined by H NMR with 1,3,5-trimethylben-
c
zene as internal standard. Proline (40 mol %) was used as ligand.
d
e
CuI (10 mol %) was used as catalyst. CuI (5 mol %) was used as
f
g
h
catalyst. No CuI was used. Isolated yield. CuCl (5 mol %) was used
i
j
as catalyst. Cu(OTf)2 (5 mol %) was used as catalyst. Pd(OAc)2 (5
mol %) was used as catalyst. temp = temperature, time = reaction time,
DMSO = dimethyl sulfoxide, DMF = dimethylformamide, DMA =
dimethylacetamide, NMP = N-methyl-2-pyrrolidinone, N.D. = not
determined.
Since the β-aryl, β-aryloxycarbonyl moiety is a very important
scaffold in many active molecules, the O-arylation of substrate 2
with an aryl substituent in the β-position was also investigated.
However, the reaction of 2o (R1 = Et, R2 = H, R3 = Ph) was very
slow, and less than 5% yield was achieved in 60 h under the
optimal reactions. Carrying out the reaction under reflux
improved the yield of 3o to 83% (Scheme 2E). Under these
conditions, 3p and 3q were prepared in 80% and 75% yield,
respectively (Scheme 2E). Obviously, aryl substituents decreased
the reactivity of enolates to some extent.
Next, a variety of diaryliodonium salts 1 were tested in the
reaction (Scheme 3). With symmetrical diaryliodonium salts, the
reaction worked very well, and good to excellent yields were
achieved (86−98%). In the case of unsymmetrical diary-
liodonium salts, even though the dummy aryl 2,4,6-trimethyl-
phenyl (Mes-) was utilized, the selectivity of the aryl transfer was
not as good as reported in other reactions.8a,b,10 Therefore, the
yields of the desired products were relatively lower (71−82%)
and 3′ was obtained as a byproduct. However, these compounds
could be prepared in high yields using the corresponding
symmetric salts, as illustrated by a comparison of the yields for
some products obtained from symmetric and unsymmetric salts,
respectively (see the Supporting Information). Diaryliodonium
salt containing a 3-thiophene-yl group also behaved well to give
could be reduced to 3.0 equiv (27% yield, entry 4). Examination
of the solvent effect (entries 5−9) revealed that when dioxane
was used as solvent, 3a was obtained in 79% yield and 4a was
formed in only 3% yield (entry 2). Further screening of the
reaction conditions using dioxane as solvent (entries 10−17)
showed that 98% isolated yield of 3a was achieved at 70 °C in the
presence of 5 mol % of CuI and 2.0 equiv of Li2CO3 for 60 h, as
indicated in entry 17, which was selected as the optimal reaction
conditions. It is worth mentioning that only 5% yield of 3a was
obtained without CuI (entry 15), and other catalysts such as
CuCl, Cu(OTf)2, and Pd(OAc)2 gave poor results (entries 18−
20), demonstrating the important role of CuI in this reaction.
Furthermore, poor performance was also observed with other
bases, such as Na2CO3 and K2CO3, under the optimal conditions
(entries 21 and 22). The stereochemistry of the double bond in
3a was determined as Z by comparing its analytical data with
those in the literature.9
With the optimized conditions in hand, the scope of substrates
2 was evaluated (Scheme 2). In general, the reaction was
extremely regio- and stereoselective. Only O-arylation occurred,
and the Z-isomer was obtained as the sole product. Although the
substituents in β-ketonesters have different influences on the
B
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Letter
obtained with diaryliodonium salts containing a 3-thiophene-yl
group (6g 53%).
Scheme 3. O-Arylation of Ethyl Acetoacetate
Cyclic dicarbonyl substrates were also tested under the
standard conditions (eq 1). Although the corresponding C-
arylation products (monoarylated product and diarylated
product) were obtained as the major products in the absence
of copper catalyst as reported,7g cyclic 5,5-dimethylcyclohexane-
1,3-dione (7a) was an appropriate substrate for this reaction, and
its O-arylation product 8a was obtained in 98% yield. However,
the cyclic Meldrum’s acid (7b) failed to afford the desired
product after 60 h, and 7b decomposed completely.7i
the desired product 3x in 56% yield. In all reactions, only O-
arylation products with a Z-configuration were formed
selectively.
On the basis of the above results and those reported in the
literature,14 a possibility for the controlling mode of the high
selectivity of this reaction was proposed as outlined in Scheme 5.
The synthesis of organofluorine compounds has been a
consistent interest in our group. Because of the high electro-
negativity, hydrophobicity, metabolic activity, and bioavailability,
fluorinated compounds, especially trifluoromethylated com-
pounds, have recently attracted significant interest and become
very important synthetic targets in laboratories and industries.11
Among them, compounds containing the 3,3,3-trifluoropropenyl
(CF3CC) founctional group12 have found important
applications in various fields such as medicine, pharmaceuticals,
and functional materials.11 Therefore, the O-arylation reaction of
ethyl trifluoroacetoacetate (5) was investigated. Under the above
conditions, however, compound 5 decomposed owing to
defluorination. Fortunately, further optimization of reaction
conditions showed that the reaction occurred smoothly in DCE
(1,2-dichloroethane) at 60 °C, and the desired O-arylation
product 6a was obtained in 93% yield selectively. The structure
and configuration of 6a was determined according to the
analytical data and the work of Laurent (see details in the
Supporting Information).13
Scheme 5. Discussion of O-Arylation Selectivity of Enolate
Both Hartwig and Lei pointed out that species 9 is the initial
intermediate in copper-catalyzed C-arylation, and there is an
equilibrium between anionic species 9 and C-bounded species
10. The addition of iodoarene (ArI) to species 10 generates
another Cu(III) species 11, which undergoes a reductive
elimination to give the C-arylation product. It is inferred that
the softer copper center in species 10 would be easier to react
with the soft reagent ArI to generate intermediate 11, resulting in
the formation of the C-arylation product. In this work, Ar2IOTf is
a harder reagent compared with ArI and prefers to add to the
harder copper center in intermediate 12 to give intermediate 13.
Reductive elimination of 13 affords the O-arylation product.
Regarding the important role of lithium cation in this
regioselective reaction as showed in Table 1 (entries 17, 21,
and 22), it is assumed that lithium cation might act as a critical
countercation in the intermediates in the reaction. For example,
it might coordinate with oxygen atoms in both TfO group and
the carbonyl compound in intermediate 13, giving a more stable
species 13′. This also helps to achieve the high Z-stereoselectivity
of the reaction.
We further examined the scope of diaryliodonium salts in the
O-arylation reaction of 5. As shown in Scheme 4, the reaction
Scheme 4. O-Arylation of Ethyl 4,4,4-Trifluoroacetoacetate.
gave the expected products in high yields (6a,c−e) with
symmetrical diaryliodonium salts (Ar1 = Ar2), although the
reactivity of electron-deficient ones was a little lower and longer
reaction time was required. For unsymmetrical diaryliodonium
salts containing the dummy aryl group (Ar1 = Mes),8a,b,10
product 6b with a p-methoxyl substituent was obtained in 91%
yield. In the case of electron-deficient aryl groups, the yield of the
corresponding product decreased a little bit because of the
formation of byproduct 6′ (6f 84%). Moderate yields were
In summary, a copper-catalyzed O-arylation reaction of
enolates with diaryliodonium salts as arylating reagents has
been developed. The features of this protocol are as follows: (1)
the reaction is very efficient with up to 98% yield; (2) both the
regio- and stereoselectivity are excellent, and only O-arylation
product with Z-configuration was obtained; (3) the reaction
takes place under mild conditions and tolerates many functional
groups, making it possible to further functionalize the products
and prepare more complicated compounds. These features
C
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ASSOCIATED CONTENT
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■
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1
Complete experimental details, spectral data, and copies of H
NMR, 19F NMR, and 13C NMR spectra for new compounds.
This material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: jtliu@sioc.ac.cn.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Financial support from the National Natural Science Foundation
of China (No. 21172243) is gratefully acknowledged.
■
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