Copper-catalyzed Ullmann-type synthesis of diaryl ethers assisted by salicylaldimine ligands

Article abstract of DOI:10.1016/j.cclet.2013.11.036

A series of salicylaldimine ligands were designed to promote the copper-catalyzed Ullmann cross-coupling reaction. After a screening process, 2-((2-isopropylphenylimino)methyl)phenol was found to serve as a good supporting ligand for this reaction. Employing this Schiff-base ligand as a new supporting ligand, the copper-catalyzed coupling reactions of aryl bromides and aryl iodides with various phenols successfully proceeded in good yields under mild conditions. Various diaryl ethers were obtained with excellent yields in dioxane in the presence of K₃PO₄ and a catalytic amount of copper(I) salt.

Full text of DOI:10.1016/j.cclet.2013.11.036

G Model
CCLET 2783 1–4
Chinese Chemical Letters xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
journal homepage: www.elsevier.com/locate/cclet
1
2
Original article
3
4
Copper-catalyzed Ullmann-type synthesis of diaryl ethers assisted by
salicylaldimine ligands
a,
a
b
a
a
5
Q1 Cun-Wei Qian *, Wen-Lin Lv , Qian-Shou Zong , Mao-Yuan Wang , Dong Fang
a
6
7
School of Chemistry and Chemical Engineering, Yancheng Teachers University, Yancheng 224051, China
b
School of Biological and Chemical Engineering, Jiaxing University, Jiaxing 314001, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A series of salicylaldimine ligands were designed to promote the copper-catalyzed Ullmann cross-
coupling reaction. After a screening process, 2-((2-isopropylphenylimino)methyl)phenol was found to
serve as a good supporting ligand for this reaction. Employing this Schiff-base ligand as a new supporting
ligand, the copper-catalyzed coupling reactions of aryl bromides and aryl iodides with various phenols
successfully proceeded in good yields under mild conditions. Various diaryl ethers were obtained with
Received 20 July 2013
Received in revised form 29 October 2013
Accepted 6 November 2013
Available online xxx
3 4
excellent yields in dioxane in the presence of K PO and a catalytic amount of copper(I) salt.
Keywords:
ß 2013 Cun-Wei Qian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights
Copper catalyst
Salicylaldimine ligands
Ullmann coupling
Diaryl ethers
reserved.
8
9
1. Introduction
tetraethyl orthosilicate (as solvent) [5q], (2-pyridyl)acetone [5r], 32
n
4
Bu NBr (as a phase transfer catalyst) [5s], picolinic acid [5t], 33
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
Carbon –oxygen bonds extensively exist in all kinds of
biologically active natural products, important pharmaceutical
compounds and polymers [1,2]. The palladium-catalyzed forma-
tion of carbon-oxygen bonds is one of the two major methods
available for aryl ether synthesis. However, palladium-based
protocols, although successful [3], have some inherent limitations
such as moisture sensitivity, costly metal catalysts, and environ-
mental toxicity. These shortcomings limit their large scale
applications in industrial fields, particularly in the pharmaceuti-
cal industry, where metal contamination to the products may
cause serious problems. The other method for ether syntheses is
the copper-mediated Ullmann reaction. The harsh classical
conditions of the Ullmann ether synthesis also have limitations
in its application because of the high temperatures (120–220 8C)
required and the use of stoichiometric amounts of copper reagents
[4]. In the past years, significant advances have been achieved for
the copper-catalyzed synthesis of diaryl ethers using new ligands
such as 1-naphthoic acid [5a], 8-hydroxyquinoline [5b,5c],
2,2,6,6-tetramethylheptane-3,5- dione [5d], amino acids [5e-g],
methenamine [5u] and some copper(I) complexes [5v–x]. With 34
the carefully selected combinations of a catalytic amount of 35
copper sources, bases, and supporting ligands, aryl bromides and 36
aryl iodides had been reported to couple with phenols with 37
excellent yields under mild conditions. Therefore, further 38
discovery of new facile ligands for the copper-catalyzed cross- 39
coupling reactions of phenols with aryl halides is still an area of 40
considerable interest.
41
Salicylaldimine ligands are a type of simple and versatile 42
bidentate ligand, and they can be readily prepared by the 43
condensation of salicylaldehyde with amines. Moreover, the steric 44
and electronic properties of these ligands can be easily tuned using 45
appropriate substituents. These ligand systems have been utilized 46
in main group, transition-metal and lanthanides coordination 47
chemistry, and some of these metal complexes show good activity 48
in homogeneous catalysis [6]. To our knowledge, there are very few 49
examples available in the copper-catalyzed arylation of phenols 50
involving these ligands. Herein we wish to report the efficient use 51
of salicylaldimine ligands (Fig. 1) for the copper catalyzed O- 52
diimine ligands [5h,5i], PPAMP [5j], phosphazene P
4
-t-Bu base
-ketoester [5l], 1,1 -binaphthyl-2,2 -diamine [5m], tripod
ligands [5n], silica supported Cu(II) [5o], aryl boronic acid [5p],
arylation of phenols with aryl halides.
53
0
0
[5k],
b
2. Experimental
54
2
-((2-Isopropylphenylimino)methyl)phenol (15 mol %), CuI 55
(5 mol%), phenol (1.5 mmol, 141 mg) and K PO (2.0 mmol, 56
425 mg) were added to a screw-capped Schlenk tube under 57
*
Corresponding author.
3
4
E-mail address: qiancunwei@163.com (C.-W. Qian).
1
001-8417/$ – see front matter ß 2013 Cun-Wei Qian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Please cite this article in press as: C.-W. Qian, et al., Copper-catalyzed Ullmann-type synthesis of diaryl ethers assisted by
salicylaldimine ligands, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.11.036

G Model
CCLET 2783 1–4
2
C.-W. Qian et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
L : R = H
Table 1
1
L : R = o-Me
a
OH
2
Comparison of various ligands in the coupling reaction.
L : R = p-Cl
3
N
L : R = o-OMe
I
OH
4
L : R = p-OMe
5
O
L : R = o-i-Pr
R
6
CuI, K PO
DMF, 110 C,
ligand, 24 h
3
4
+
o
Fig. 1. Structures of ligand.
1a
2a
3aa
5
5
6
6
6
6
6
6
6
6
8
9
0
1
2
3
4
5
6
7
argon. The tube was then evacuated and backfilled with argon
(three cycles). Iodobenzene (1 mmol, 0.11 mL) and dry dioxane
(0.5 mL) were added by syringe at room temperature. The reaction
mixture was stirred at needed temperature (101 8C) for 24 h. The
reaction mixture was allowed to reach room temperature and
then diluted with dichloromethane (10 mL). The slurry was
filtered, and the filter cake was washed with 10 mL of dichlor-
omethane. The solvent was removed in vacuo, and the residue was
purified by column chromatography on silica gel to afford the
desired product (3aa).
.
b
Entry
Ligand
–
Yield (%)
1
2
3
4
5
6
7
8
18
55
64
66
61
86
L
L
L
L
L
1
2
3
4
5
L6
a
General reaction conditions: 1.5 mmol of phenol, 1.0 mmol of iodobenzene,
PO , 0.05 mmol CuI, 0.05 mmol ligand, 0.5 mL of anhydrous DMF.
Isolated yield.
2
.0 mmol of K
b
3
4
6
8
3. Results and discussion
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
The ligands L
methods (Fig. 1) [7]. These ligands (L
iodobenzene and phenol as model substrates in the presence of
tripotassium phosphate and copper salts at 110 8C for 24 h in DMF.
The results are listed in Table 1. The desired product was obtained
in 8% yields in the absence of the ligand (Table 1, entry 1). Among
1
–L
6
were synthesized by known literature
–L ) are examined using
dioxane as a solvent gave the best result, furnishing diphenyl ether
in 71% yield. DMF, DME, THF, DMSO, toluene and CH CN were not
as good as dioxane (Table 2, entries 3 and 5–9). For comparison on
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
1
6
3
the efficiency of copper salts in the O-arylation reactions, both
Cu
CuCl were moderate (Table 2, entries 9–12). After screening a
series of bases, 2 equiv. of K PO was determined to be most
2 2
O and CuI were found to be highly effective, while CuCl and
the ligands L
This might be due to the suitable steric hindrance of L
Subsequently, L was selected as the ligand for assisting Cu salts
1
–L
6
tested, L
6
is most efficient (Table 1, entries 2–7).
3
4
6
.
effective and gave diphenyl ether in 71% yield at 81 8C (Table 2,
entries 9 vs. 13 and 14). To further optimize the reaction
conditions, increased yields of the coupling products were
observed by simply increasing the reaction temperature to
101 8C (91%, Table 2, entry 15). As previously reported [4], a slight
excess of the phenol was employed (0.5 equiv. excess) since the
use of 1.0 equiv. resulted in very slow conversion as the reaction
6
in catalyzing O-arylation of phenols with aryl halides to further
investigate the effects of the other conditions on the O-arylation,
including alkalis, solvents, copper salt, etc. The results are listed in
Table 2. The results showed the optimal ratio of Cu(I) (5 mmol%) to
6
L used in this study is 1:3 (Table 2, entries 1–4). Then, on turning
our attention to examining the solvent effects at 81 8C, we find that
neared completion. Therefore, the combination of CuI (5%), 15% L
6
Table 2
a
Copper-catalyzed O-arylation of phenol with aryl halide: optimization of the reaction conditions.
I
OH
O
Copper salt, base
+
o
solvent, 81 C,
L6, 24 h
1a
2a
3
aa
.
b
Entry
Ligand (mol%)
Copper salt (mol%)
Solvent
Yield (%)
1
2
3
4
5
6
7
8
9
L
L
L
L
L
L
L
L
L
L
L
L
L
L
L
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
, 5
CuI, 5
CuI, 5
CuI, 5
CuI, 5
CuI, 5
CuI, 5
CuI, 5
CuI, 5
CuI, 5
CuCl, 5
DMF
DMF
DMF
DMF
27
34
62
61
32
27
46
55
71
31
52
73
37
12
91
, 10
, 15
, 20
, 15
, 15
, 15
, 15
, 15
, 15
, 15
, 15
, 15
, 15
, 15
3
CH CN
DMSO
DME
Toluene
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
1
1
1
1
1
1
0
1
2
3
4
5
CuCl
2
, 5
2
Cu O, 2.5
c
CuI, 5
CuI, 5
CuI, 5
d
e
a
b
c
General reaction conditions: 1.5 mmol of phenol, 1.0 mmol of iodobenzene, 0.5 mL of anhydrous solvent.
Isolated yield.
Na
2
CO
CO
At 101 8C.
3
was used in place of K
3 4
PO .
d
e
K
2
3
was used in place of K PO .
3
4
Please cite this article in press as: C.-W. Qian, et al., Copper-catalyzed Ullmann-type synthesis of diaryl ethers assisted by
salicylaldimine ligands, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.11.036

G Model
CCLET 2783 1–4
C.-W. Qian et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
3
Table 3
a
Cu-catalyzed coupling reaction of aryl iodides(bromides) with phenols (or benzyl alcohol).
CuI, L6
dioxane, 101 C, 24 h
ArX + R-OH
Ar O R
N
o
OH
X = Br, I; R = aryl or benzyl; L6=
.
b
Entry
Aryl halide
Phenol (alcohol)
Product
Yield (%)
1
2
3
4
5
6
7
8
9
1a
2b
2c
2d
2e
2f
3ab
3ac
3ad
3ae
3af
3ag
3ah
3ai
85
91
84
83
92
54
84
47
17
2g
2h
2i
2j
3aj
10
11
12
13
14
15
16
1b
2b
2c
2e
2d
2f
3bb
3bc
3be
3bd
3bf
85
87
80
79
89
47
72
2g
2h
3bg
3bh
1
1
1
2
2
7
8
9
0
1
1c
1d
1e
2b
2c
2f
3cb
3cc
3cf
73
76
79
59
80
2d
2a
3cd
3ca
22
23
24
25
26
2e
2k
2d
2f
3de
3dk
3dd
3df
3dc
87
90
89
93
93
2c
2
2
2
3
3
7
8
9
0
1
2c
2f
3ec
3ef
87
89
85
75
87
2d
2b
2a
3ed
3eb
3ea
a
Unless otherwise stated, general reaction conditions: 1.5 mmol of phenol, 1.0 mmol of ArX, 2.0 mmol of K
3 4
PO , 0.5 mL of anhydrous dioxane under Ar atmosphere for 24 h.
b
Isolated yield.
9
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
9
and 200% K
conditions for further exploration.
3
PO
4
in dioxane at 101 8C was chosen as the optimal
standard reaction conditions when 2,6-dimethylphenol coupled 120
with phenyl iodides (Table 3, entry 8). The results showed that the 121
coupling of a or b-naphthol and aryl iodides (or 4-iodotoluene) 122
00
01
02
03
04
05
06
07
08
09
10
11
12
13
14
15
16
17
18
19
The scope of the copper-catalyzed C–O bond formation was
explored by using a variety of aryl iodides (or bromides) with
substituted phenols under the optimized conditions. As shown in
Table 3, O-arylation of aryl iodides with phenol bearing either
electron-withdrawing or electron-donating substituents was shown
to give the corresponding products in excellent yields of 83–92%
proceeded in moderate to good yields (Table 3, entries 6, 7, 15 and 123
16). The results also revealed that an electron- withdrawing group in 124
the aryl iodides favored the coupling reactions. For example, 1-iodo- 125
4-chlorobenzene successfully coupled with various phenols with 126
higher yields than that for iodobenzene without an electron- 127
withdrawing group (Table 3, entry 22,24–26 vs. 2–5). Similarly, the 128
coupling reaction of benzyl alcohol with 1-iodo-4–chlorobenzene 129
also smoothly afforded the expected ethers (3dk) in 90% yield 130
(Table 3, entry 23). Itwas noteworthy that the coupling reaction of 2- 131
acetylphenyl bromide with various substituted phenols gave good 132
yields (73–90%) under our optimal conditions, although the aryl 133
bromides are less reactive than the aryl iodides for the arylation 134
0
(Table 3, entries1–5) with the exception of4 -hydroxyacetophenone,
whichled toa low yield(17%) for the couplingreaction (Table 3, entry
9). This might be due to the decreased nucleophilicity of the phenolic
hydroxyl group induced by the keto group. Slight electronic effects
were observed when various phenols were submitted to coupling
with para-methyl aryl iodides, because only slight decreases in the
reaction yields were recorded (Table 3, entries 10–16 vs. 1, 2, 4–7).
Whereas the O-arylation of various substituted phenols with 2-
iodotoluene gave only moderate to good product yields (59–80%)
(Table 3, entries17–21), which might be attributable to the influence
of steric hindrance. As mentioned above, the results clearly indicated
that steric hindrance had a huge influence on the coupling reaction.
For instance, only 47% yield was obtained after 24 h under our
reaction (Table 3, entries 27–31).
135
4. Conclusion
136
In conclusion, 2-((2-isopropylphenylimino)-methyl)phenol 137
was found to be an efficient, inexpensive and facile ligand for the 138
Please cite this article in press as: C.-W. Qian, et al., Copper-catalyzed Ullmann-type synthesis of diaryl ethers assisted by
salicylaldimine ligands, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.11.036

G Model
CCLET 2783 1–4
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C.-W. Qian et al. / Chinese Chemical Letters xxx (2013) xxx–xxx
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c) C.W. Qian, S.J. Xu, Q.S. Zong, D. Fang, Copper-catalyzed synthesis of triaryla-
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CuX-catalyzed O-arylation of phenols with aryl iodides and
bromides. The present protocol is applicable to a variety of phenols
and aryl iodides containing electron-withdrawing, electron-donat-
ing, and sterically demanding substrate combinations under mild
conditions. Further study on the design and application of these new
ligands in copper based Ullmann-type coupling reactions is
currently ongoing.
mines from aryl halides and arylamines, Chin. J. Chem. 30 (2012) 1881–1885;
(
d) E. Buck, Z.J. Song, D. Tschaen, et al., Ullmann diaryl ether synthesis: rate accelera-
tion by 2,2,6,6-tetramethylheptane-3,5-dione, Org. Lett. 4 (2002) 1623–1626;
e) D. Ma, Q. Cai, H. Zhang, N,N-dimethyl glycine-promoted Ullmann coupling reac-
tion of phenols and aryl halides, Org. Lett. 5 (2003) 3799–3802;
f) Q. Cai, B. Zou, D.W. Ma, Mild Ullmann-type biaryl ether formation reaction by
(
(
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China and Key Laboratory of Organic Synthesis of Jiangsu Province
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3
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Please cite this article in press as: C.-W. Qian, et al., Copper-catalyzed Ullmann-type synthesis of diaryl ethers assisted by
salicylaldimine ligands, Chin. Chem. Lett. (2013), http://dx.doi.org/10.1016/j.cclet.2013.11.036