General, mild, and intermolecular Ullmann-type synthesis of diaryl and alkyl aryl ethers catalyzed by diol-copper(I) complex

Article abstract of DOI:10.1021/jo900438e

(Chemical Equation Presented) A wide range of diaryl ethers and alkyl aryl ethers are synthesized through intermolecular C(aryl)-O bond formation from the corresponding aryl iodides/aryl bromides and phenols/alcohols through Ullmann-type coupling reaction in the presence of a catalytic amount of easily available (±)-diol L3-CuI complex under very mild reaction conditions. Less reactive aryl bromides can also be used for O-arylation of phenols under the same reaction conditions without increasing the reaction temperature, catalyst loading, and time. The catalytic system not only is capable of coupling hindered substrate but also tolerates a broad range of a series of functional groups.

Full text of DOI:10.1021/jo900438e

General, Mild, and Intermolecular Ullmann-Type Synthesis of
Diaryl and Alkyl Aryl Ethers Catalyzed by Diol-Copper(I)
Complex†
Ajay B. Naidu, E. A. Jaseer, and Govindasamy Sekar*
Department of Chemistry, Indian Institute of Technology Madras, Chennai, Tamil Nadu 600036, India
gsekar@iitm.ac.in
ReceiVed August 12, 2008
A wide range of diaryl ethers and alkyl aryl ethers are synthesized through intermolecular C(aryl)-O
bond formation from the corresponding aryl iodides/aryl bromides and phenols/alcohols through Ullmann-
type coupling reaction in the presence of a catalytic amount of easily available (()-diol L3-CuI complex
under very mild reaction conditions. Less reactive aryl bromides can also be used for O-arylation of
phenols under the same reaction conditions without increasing the reaction temperature, catalyst loading,
and time. The catalytic system not only is capable of coupling hindered substrate but also tolerates a
broad range of a series of functional groups.
1. Introduction
multistep processes involved in the synthesis of these ligands
have rendered Pd unpopular, particularly for large scale reac-
tions.4 Copper-catalyzed Ullmann coupling between an aryl
halide and phenol/alcohols is an alternate for palladium catalyzed
diaryl ether/alkyl aryl ether synthesis. However, harsh reaction
conditions such as high temperatures (120-220 °C), the usual
requirement of stoichiometric quantities of copper catalysts, and
low to moderate yields have generally limited the utility of
copper catalyst.5 In fact, only in the past few years have the
considerable efforts taken to improve the efficiency of this
reaction started to bear fruit with the use of copper salts with
several ligands such as 1-naphthoic acid,6 1,10-phenanthroline,7
neocuproine,8 triphenylphosphine,9 2,2,6,6-tetramethylheptane-
Numerous synthetically challenging and medicinally impor-
tant chemicals contain diaryl ethers and alkyl aryl ethers as
structural units.1,2 Palladium-catalyzed diaryl ether and alkyl
aryl ether formation from the corresponding aryl halide and
phenol/alcohols or its sodium salt used to be the method of
choice.3 However, the high costs of palladium salts, high
oxophillicity associated with phosphine ligands, and tedious
† This paper is dedicated to Prof. Veejendra K. Yadav.
(1) (a) Theil, F. Angew. Chem., Int. Ed. 1999, 38, 2345–2347. (b) Zhu, J.
Synlett 1997, 133–144. (c) Boger, D. L.; Patane, M. A.; Zhou, J. J. Am. Chem.
Soc. 1994, 116, 8544–8546.
(2) (a) Yamazaki, N.; Washio, I.; Shibasaki, Y.; Mitsuru Ueda, M. Org. Lett.
2006, 8, 2321–2324. (b) Asano, M.; Inoue, M.; Katoh, T. Synlett 2005, 2599–
2602. (c) Jiang, H.; Leger, J.-M.; Huc, I. J. Am. Chem. Soc. 2003, 125, 3448–
3449. (d) Bolm, C.; Hildebrand, J. P.; Mun˜iz, K.; Hermanns, N. Angew. Chem.,
Int. Ed. 2001, 40, 3284–3308. (e) Gu, W. X.; Jing, X. B.; Pan, X. F.; Chan,
A. S. C.; Yang, T. K. Tetrahedron Lett. 2000, 41, 6079–6082. (f) Matsumoto,
Y.; Uchida, W.; Nakahara, H.; Yanagisawa, I.; Shibanuma, T.; Nohira, H. Chem.
Pharm. Bull. 2000, 48, 428. (g) Cao, B.; Park, H.; Joullie, M. M. J. Am. Chem.
Soc. 2002, 124, 520–521. (h) Temal-La¨ıb, T.; Chastanet, J.; Zhu, J. J. Am. Chem.
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(3) Sawyer, J. S. Tetrahedron 2000, 56, 5045–5065. For some recent reports
on palladium-catalyzed aryl ether synthesis, see: (a) Vorogushin, A. V.; Huang,
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Shelby, Q.; Stambuli, J.; Hartwig, J. J. Org. Chem. 2002, 67, 5553–5566. (c)
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10.1021/jo900438e CCC: $40.75
Published on Web 04/10/2009
2009 American Chemical Society
J. Org. Chem. 2009, 74, 3675–3679 3675

Naidu et al.
SCHEME 1
FIGURE 1. Oxygen-based ligands for copper-catalyzed C-O bond
formation.
3,5-dione,10 tripod ligands,11 tetraethyl orthosilicate (as sol-
vent),12 silica supported Cu(II) complex,13 aryl boronic acid,14
N,N-dimethylglycine,15 diimine ligands,16 ꢀ-keto ester,17 and
bipyridyl complex.18 Very recently diketone along with iron19,20
has also been used as efficient catalyst for the formation of
C(aryl)-O bond.
TABLE 1. Effect of the Ratio of Ligand and CuI
Recently we have shown that 1,1′-binaphthyl-2,2′-diamine
(BINAM)-Cu(OTf)2 complex can be used as an efficient catalyst
for diaryl ether and alkyl aryl ether synthesis through C-O bond
formation in dioxane from aryl iodides and phenols/alcohols at
110 °C.21 As a part of our ongoing research toward finding more
efficient copper catalyst for diaryl ether and alkyl aryl ethers
synthesis from aryl halides and phenols/alcohols, herein for the
first time we report an easily available racemic anthracene-based
diol, trans-9,10-dihydro-9,10-ethanoanthracene-11,12-dimetha-
nol22 L3-CuI complex, as an efficient catalyst for Ullmann
coupling for the formation of diaryl and alkyl aryl ethers through
C-O bond formation in acetonitrile under very mild conditions
(82 °C). This procedure is very simple, mild, clean and works
efficiently (Scheme 1).
entry
ligand
Cul
time (h)
yield (%)
1
2
3
4
5
6
7
8
L1 (20 mol %)
L2 (20 mol %)
L3 (20 mol %)
L4 (20 mol %)
L5 (20 mol %)
L3 (10 mol %)
L3 (20 mol %)
L6 (20 mol %)
L6 (20 mol %)
L6 (7.5 mol %)
L6 (7.5 mol %)
-
20 mol %
20 mol %
20 mol %
20 mol %
20 mol %
10 mol %
10 mol %
20 mol %
20 mol %
2 mol %
2 mol %
-
28
28
17
30
30
24
24
16
17
18
18
32
32
55
49
97
00
45
76
33
55
80a
75a
42
00
39
9
10
11
12
13
-
20 mol %
a Dioxane was used as solvent.
2. Results and Discussion
yield to 80% (entry 9). Then the reaction was carried out with
different ratios of L3 and CuI, and it was found that a 20 mol
% ligand L3-copper combination was the most effective catalytic
system. The coupling reaction did not provide even a trace
amount of diaryl ether when the reaction was carried out without
L3-CuI (entry 12). When the reaction was carried out only with
CuI without L3, the reaction provided only 39% yield for the
coupling product (entry 13), which shows that ligand is
mandatory for quantitative yield of the product.
The reaction was screened with several copper salts, solvents,
and bases to increase the efficiency of the coupling reactions,
and the results are summarized in Table 2. Although several
copper salts catalyzed the reaction, CuI turned out to be the
copper salt of choice in view of yield (Table 2, entry 1).
Similarly acetonitrile was the best solvent among those exam-
ined. Cs2CO3 as base gave the best yields of product in
comparison with bases such as Na2CO3 and K2CO3.
Using the above-mentioned optimized conditions, we initiated
our investigations into the scope of the L3-CuI catalyzed
Ullmann-type coupling reaction, and the results are summarized
in Table 3. Various aryl iodides and phenols reacted to give
the corresponding diaryl ethers under very mild reaction
conditions. We found that iodobenzene containing electron-
releasing groups as well as electron-withdrawing groups reacted
with phenols to give corresponding diaryl ethers.
The yields were different with electron-rich and electron-
deficient phenols. The presence of electron-releasing groups such
as methyl and methoxy groups in phenol at para and meta
positions increased the yield of the diaryl ethers by 5-23%
(entries 2 vs 1, 4, 5, and 8), whereas an electron-withdrawing
group such as a chloro group at the para position of phenol
decreased the yield to 64% yield (entries 1 vs 10). The presence
of an electron-releasing group such as a methoxy or methyl
In preliminary studies, we used 20 mol % 1,1′-binaphthyl-
2,2′-diol L1 (BINOL) (Figure 1) as ligand with 20 mol % CuI
for the coupling of iodobenzene with p-cresol in acetonitrile at
82 °C. After 28 h the coupling reaction provided 55% yield for
the corresponding diaryl ether (Table 1, entry 1). When the
BINOL L1 was replaced by diester ligand L2, the reaction
provided only 49% yield of diaryl ether. Surprisingly, when
diol L3 was used as ligand along with CuI, it provided 97%
yield (entry 3). However, replacing L3 by L4, the reaction failed
to provide any coupling product (entry 4), and using (+)-diethyl
tartarate L5 as ligand gave only 45% yield for corresponding
diaryl ether (entry 5). The C(aryl)-O bond formation between
iodobenzene and p-cresol in the presence of 20 mol % N,N-
dimethylglycine L6-CuI gave only 55% yield in acetonitrile at
82 °C (entry 8). Changing the solvent to dioxane increased the
(10) Buck, E.; Song, Z. J.; Tschaen, D.; Dormer, P. G.; Volante, R. P.; Reider,
P. J. Org. Lett. 2002, 4, 1623–1626.
(11) Chen, Y. J.; Chen, H. H. Org. Lett. 2006, 8, 5609–5612.
(12) Zhao, Y.; Wang, Y.; Sun, H.; Li, L.; Zhang, H. Chem. Commun. 2007,
3186–3188.
(13) Miao, T.; Wang, L. Tetrahedron Lett. 2007, 48, 95–99.
(14) Evans, D. A.; Katz, J. L.; West, T. R. Tetrahedron Lett. 1998, 39, 2937–
2940.
(15) Ma, D.; Cai, Q. Org. Lett. 2003, 5, 3799–3802.
(16) Cristau, H. J. A.; Pascal, P. C.; Hamada, S.; Francis, J. S.; Taillefer, M.
Org. Lett. 2004, 6, 913–916.
(17) Lv, X.; Bao, W. J. Org. Chem. 2007, 72, 3863–3867.
(18) Jiajai, N.; Zhou, H.; Li, Z.; Xu, J.; Hu, S. J. Org. Chem. 2008, 73,
7814–7817.
(19) Bistri, O.; Correa, A.; Bolm, C. Angew. Chem.Int. Ed. 2008, 47, 586–
588.
(20) Xia, N.; Taillefer, M. Chem.-Eur. J. 2008, 14, 6037–6039.
(21) (a) Naidu, A. B.; Raghunath, O. R.; Prasad, D. J. C.; Sekar, G.
Tetrahedron Lett. 2008, 49, 1057–1061. (b) Naidu, A. B.; Sekar, G. Tetrahedron
Lett. 2008, 49, 3147–3151.
(22) Diol ligand L3 was easily synthesized from known literature procedure.
See Supporting Information for experimental details.
3676 J. Org. Chem. Vol. 74, No. 10, 2009

Ullmann-Type Synthesis of Diaryl and Alkyl Aryl Ethers
TABLE 2. Effect of Cu Salts, Solvents, and Bases
TABLE 3. Coupling Reaction of Aryl Iodide with Phenols in the
Presence of Diol L3-CuI Catalyst
entry
Cu salt
Cul
solvent
time (h)
yield (%)
1
2
3
4
5
6
7
8
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
acetonitrile
toluene
DMF
DMSO
xylene
dioxane
17
15
14
18
16
20
14
24
18
20
30
17
36
30
30
97
63
73
49
67
Cu(OTf)2
CuBr
CuCl2
CuCl
Cu(OAc)2
CuSO4
Cul
Cul
Cul
Cul
Cul
52
68
52a
32a
45a
15a
90a
5a
9
10
11
12
13
14
15
Cul
Cul
Cul
chlorobenzene
acetonitrile
acetonitrile
0b
38c
a Reaction was carried out at 110 °C. b Na2CO3 was used as base.
c K2CO3 was used as base.
group in iodobenzene moiety at the para position decreased
the yield of the product by 4-22% (entries 1 vs 11 and 2 vs
12).
As expected, the presence of electron-withdrawing groups
in aryl iodide moiety drastically increased the yield as well as
the reaction rate for the coupling reaction with phenols for C-O
bond-formation reaction. When a very strong electron-with-
drawing group such as a nitro group is present at the para
position of iodobenzene, the reaction proceeded even without
L3-CuI complex, showing that the iodide is activated and the
reaction is taking place through a nucleophilic addition elimina-
tion mechanism rather than a copper complex catalyzed coupling
reaction (entry 20). To our surprise when the same electron-
withdrawing nitro group is present at the meta position of the
iodobenzene, the reaction did not provide even a trace amount
of the corresponding diaryl ether without L3-CuI complex, and
the same reaction provided 85% of yield of the corresponding
diaryl ethers in the presence of our L3-CuI in acetonitrile at 82
°C (entry 19). Also, even in the presence of an ortho-substituted
phenol (which is capable of providing scope for steric bias),
the reaction preceded smoothly to give the diaryl ethers in good
yields (entries 3, 7, and 9). The highest yield that is quantitative
yield was obtained when m-methoxy iodobenzene was reacted
with p-cresol (entry 16). This clearly shows that the presence
of an electron-donating methoxy group at the meta position of
iodobenzene increases the efficiency of the reaction, whereas
the presence of a methoxy group at the para position of
iodobenzene decreases the yield of the coupling reaction
drastically (entries 11-14 vs 15 and 16).
We were pleased to note that under our optimized reaction
conditions, aryl bromides also reacted with phenols without
increasing catalyst loading and reaction temperature to provide
the corresponding diaryl ethers in good yields (Table 4). Both
electron-releasing and electron-withdrawing groups containing
aryl bromides, such as 1-bromo-4-methylbenzene, 1-bromo-4-
methoxy benzene, 1-bromo-2-methoxy benzene, and 1-bromo-
4-cyanobenzene, reacted with a variety of phenols to give the
corresponding diaryl ethers in good yields. Even sterically
hindered ortho-substituted aryl bromide reacted with phenol,
a Reaction takes place without diol 3-Cu complex.
and the reaction proceeded smoothly to give the diaryl ether in
good yield (entry 10). Similarly, ortho-substituted phenol reacted
with 1-bromo-4-methylbenzene to give the corresponding diaryl
ether with 65% yield (entry 7). It is very important to mention
that, in general, aryl bromides are less reactive than aryl iodides
J. Org. Chem. Vol. 74, No. 10, 2009 3677

Naidu et al.
TABLE 4. Ullmann Coupling of Aryl Bromides with Phenols in
the Presence of L3-CuI Catalyst
TABLE 5. Ullmann Coupling of Aryl Iodides with Aliphatic
Alcohols in the Presence of L3-CuI Catalyst at 82 °C
and require much more drastic reaction conditions for arylation.
However, in the presence of diol ligand 3-CuI catalyst even a
variety of aryl bromides reacted with phenols to give good yields
of the expected diaryl ethers through C-O bond formation
without increasing the catalyst loading, reaction temperature,
and time.
most of other copper-catalyzed reactions require higher
temperature.6-14 It is very important to mention that arylation
of phenols with less reactive aryl bromides also carried out
without increasing the catalytic loading and reaction temperature.
There are only very few reports in the literature for arylation
of phenols at lower temperature. Ma et al. carried out the
coupling reaction at 90 °C, and they had to increase catalytic
loading for the arylation of phenols with aryl bromide.15 The
same catalyst (20 mol % N,N-dimethylglycine L6-CuI) provided
only 55% yield for the coupling of iodobenzene and p-cresol
in acetonitrile at 82 °C (Table 1, entry 8). Although ꢀ-keto
ester17 as ligand with copper provided coupling reaction at
60-80 °C, requirement of high boiling solvent such as DMSO
and limited substrate scope encourages for better alternative.
Similarly, Salox ligand along with Cu2O catalyzed the coupling
reaction at 82 °C but needed longer reaction time (g24 h), and
particularly in the case of aryl bromide the reaction required
higher temperature16 (110 °C).
2.1. Coupling of Aryl Iodides with Aliphatic Alcohols.
After successful completion of diaryl ether formation at 82 °C
from corresponding aryl halide and phenols in the presence of
L3-CuI, we applied the same protocol for coupling of aryl
iodides with aliphatic alcohols for the synthesis of alkyl aryl
ethers at 82 °C (Table 5). It was found that primary alcohols
such as methanol, ethanol, n-propanol, n-butanol, and allylic
alcohol can be successfully coupled with various iodobenzenes
with good yields. In the case of the aryl iodide component, the
presence of both electron-releasing groups such as a methoxy
group or electron-withdrawing group such as a nitro group were
tolerated to give the corresponding alkyl aryl ethers in good
yields. Moving the electron-releasing methoxy group in iodo-
benzene from the para position to the meta position decreased
the yield of the coupling reaction (entries 1 vs 5). The presence
of an ortho substitution in iodobenzene (which is capable of
providing scope for steric bias) also provided the C-O bond-
forming reactions smoothly to give 61% and 50% yields,
respectively (entries 6 and 8).
3. Conclusion
In summary, we have described an efficient and experimen-
tally simple copper-catalyzed O-arylation of phenols and
aliphatic alcohols with aryl halides. The presence of an electron-
releasing group in phenol increased the yield of the coupling
reaction, whereas an electron-withdrawing group in aryl halide
increased the yield of the coupling reaction. The presence of
The advantage of our catalytic system is that it provides the
arylation of phenols and aliphatic alcohols at 82 °C, whereas
3678 J. Org. Chem. Vol. 74, No. 10, 2009

Ullmann-Type Synthesis of Diaryl and Alkyl Aryl Ethers
General Procedure for the Coupling of Aryl Iodides with
Aliphatic Alcohols. An oven-dried screw-cap pressure tube fitted
with a septum was charged with Cs2CO3 (325 mg, 1 mmol), CuI
(19 mg, 0.1 mmol), L3 (26.6 mg, 0.1 mmol), p-mehoxyiodobenzene
(117 mg, 0.5 mmol), and a magnetic stir bar. The pressure tube
was evacuated and backfilled with anhydrous nitrogen. Ethanol (1.5
mL) was added, and the reaction mixture was stirred at 82 °C for
22 h (the progress of the reaction was followed by TLC). After the
complete disappearance of p-mehoxyiodobenzene (by TLC), the
reaction mixture was allowed to cool to room temperature, and the
crude product was purified by column chromatography on silica
gel using ethyl acetate/hexane solvent mixture to give 4-ethoxya-
nisole23 (73 mg, 92%) as a white solid; Rf 0.46 (in hexanes); FTIR
(neat) 1229, 2929, 2980 cm-1; 1H NMR (400 MHz, CDCl3) δ 1.38
(t, J ) 6.8 Hz, 3H), δ 3.76 (s, 3H), 3.97 (q, J ) 6.8 Hz, 2H), δ
6.83 (s, 4H); 13C NMR (100 MHz, CDCl3) δ 15.1, 55.9, 64.2, 114.8,
115.7, 153.3, 153.9; HRMS [MH+] calcd for C9H13O2 153.0916,
found 153.0916.
an electron-releasing group such as a methoxy group in the para
position of aryl halide decreased the yield of coupling reaction,
whereas the presence of the same methoxy group at the meta
position of aryl halide increased the yield. Less reactive aryl
bromides can also be used for the O-arylation of phenols under
the same reaction conditions without increasing the reaction
temperature, catalyst loading, and time. The catalytic system
not only is capable of coupling hindered substrate but also
tolerates a broad range of a series of functional groups.
3. Experimental Section
General Procedure for the Coupling of Aryl Bromides and
Iodides with Aromatic Phenols. Cs2CO3 (325.8 mg, 1 mmol), CuI
(19 mg, 0.1 mmol), and L3 (26.6 mg, 0.1 mmol) were taken in a
10 mL reaction tube equipped with a septum. The reaction tube
was evacuated and backfilled with nitrogen. Dry acetonitrile (2.5
mL) was added to the reaction mixture at room temperature. To
the resulting solution was added iodobenzene (102 mg, 0.5 mmol)
followed by p-cresol (64.8 mg, 0.6 mmol), and the reaction mixture
was heated for 17 h at 82 °C. After complete disappearance of
iodobenzene (the progress of the reaction was followed by TLC),
the reaction mixture was allowed to cool to room temperature and
the solvent was evaporated. The crude residue was directly purified
by column chromatography on silica gel using ethyl acetate/hexanes
as the eluent to afford 1-phenoxy-4-methylbenzene15 (89.2 mg,
97%) as a colorless oil. Rf 0.73 (in hexanes); FTIR (neat) 1233,
Acknowledgment. We thank DST, New Delhi (Project No.
SR/S1/OC-06/2008) for financial support. A.B.N. thanks UGC,
New Delhi, and E.A.J. thanks CSIR, New Delhi for SRF. We
thank DST, New Delhi, for funding towards the 400 MHz NMR
machine to the Department of Chemistry, IIT Madras under the
IRPHA scheme and ESI-MS facility under the FIST program.
Supporting Information Available: Experimental proce-
dures and characterization data for all new and known com-
pounds. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
2922, 3029 cm-1; H NMR (400 MHz, CDCl3) δ 2.32 (s, 3H),
6.92 (d, J ) 7.6 Hz, 2H), 6.97 (d, J ) 8.4 Hz, 2H), 7.05 (t, J ) 7.4
Hz, 1H), 7.12 (d, J ) 8.4 Hz, 2H), 7.29 (t, J ) 8.0 Hz, 2H); 13C
NMR (100 MHz, CDCl3) δ 20.7, 118.3, 119.1, 122.8, 129.6, 130.2,
132.8, 154.7,157.8; HRMS (MH+) calcd for C13H13O 185.0966,
found 185.0974.
JO900438E
(23) Siegman, J. R.; Houser, J. J. J. Org. Chem. 1982, 47, 2773–2779.
J. Org. Chem. Vol. 74, No. 10, 2009 3679