Synthesis of highly functionalized diaryl ethers by copper-mediated O-arylation of phenols using trivalent arylbismuth reagents

Article abstract of DOI:10.1002/chem.201303684

Highly functionalized diaryl ethers were prepared by copper(II) acetate mediated O-arylation reaction of phenols using trivalent organobismuthanes. The reaction is performed under simple conditions and tolerates a wide diversity of functional groups on the phenol and on the organobismuth reagent. Substoichiometric amounts of catalyst can be used by performing the reaction under an oxygen atmosphere. The N-arylation of pyridones is also reported. Highly functionalized diaryl ethers were prepared by a copper(II) acetate mediated O-arylation reaction of phenols using trivalent organobismuthanes (see scheme). The reaction is performed under simple conditions and tolerates a wide diversity of functional groups on the phenol and on the organobismuth reagent. Substoichiometric amounts of catalyst can be used by performing the reaction under an oxygen atmosphere. The N-arylation of pyridones is also reported. FG=functional group.

Full text of DOI:10.1002/chem.201303684

DOI: 10.1002/chem.201303684
Communication
&
Synthetic Methods
Synthesis of Highly Functionalized Diaryl Ethers by Copper-
Mediated O-Arylation of Phenols using Trivalent Arylbismuth
Reagents
Cynthia Crifar, Pauline Petiot, Tabinda Ahmad, and Alexandre Gagnon*[a]
reagents, as reported by Olofsson.[9] However, in this case,
Abstract: Highly functionalized diaryl ethers were pre-
pared by copper(II) acetate mediated O-arylation reaction
of phenols using trivalent organobismuthanes. The reac-
tion is performed under simple conditions and tolerates
a wide diversity of functional groups on the phenol and
on the organobismuth reagent. Substoichiometric
amounts of catalyst can be used by performing the reac-
tion under an oxygen atmosphere. The N-arylation of pyri-
dones is also reported.
a strong base is usually required to promote the reaction.
The O-arylation of phenols using arylmetals remains an at-
tractive strategy to access functionalized diaryl ethers.[10] This
approach has been put into practice not only with boronic
acids,[6,7] but also with potassium trifluoroborates,[11] organo-
lead[12] and organotin[13] reagents. In the 1980s, Barton report-
ed the use of triphenylbismuth diacetate in the O-arylation of
phenols.[14] However, these studies relied on a pre-formed pen-
tavalent organobismuth reagent, did not illustrate the func-
tional group tolerance of the method and involved only the
transfer of an unsubstituted phenyl group. Following the pub-
lication of this pioneering work, few isolated examples of O-ar-
ylation of phenols using trivalent organobismuthanes were re-
ported.[15] To our knowledge, the only comprehensive report of
O-arylation reaction using trivalent organobismuthanes in-
volved hydroxy benzyl acetates and required PhI(OAc)2 as a sto-
ichiometric co-oxidant.[16]
Diaryl ethers are commonly found in natural products[1] and in
medicinally relevant compounds.[2] This motif has also been
identified in natural peptides such as vancomycin, complesta-
tin, teicoplanin, and eurypamide.[3] Due to their diversified bio-
logical activities, efficient methods that allow the preparation
of diaryl ethers is of utmost importance. To be applicable to
the context of natural product synthesis, these methods
should tolerate a wide range of functional groups. Additionally,
to find broad use in medicinal chemistry, these protocols
should operate under simple conditions that can be amenable
to parallel chemistry and should not require complex or costly
ligands or catalysts.[4]
Our group has reported a portfolio of methods for the for-
mation of CÀC and CÀN bonds based on trivalent[17] and pen-
tavalent[18] organobismuthanes. Triarylbismuthanes B are par-
ticularly attractive organometallic reagents since they show
unique reactivity and have low toxicity.[19] These reagents can
be easily prepared by the addition of the corresponding
Grignard reagent A over bismuth chloride (Scheme 1). We
demonstrated that functional group manipulation directly on
The Ullmann reaction, which consists of the coupling of
a phenol with an aryl halide under copper catalysis, was one of
the first methods reported to prepare diaryl ethers.[5] However,
the necessity of using high temperatures prevented its use in
the synthesis of complex molecules. The disclosure by Evans,
Chan, and Lam of conditions for the coupling of arylboronic
acids with phenols provided a remarkable solution to the
problem of diaryl ether synthesis.[6,7] Despite the fact that this
reaction is quite general, excess boronic acid is often required
to reach good yields. Recently, the introduction of copper
complexing ligands by Buchwald allowed catalytic amounts of
copper catalyst to be used in the Ullmann reaction.[8] O-Aryla-
tion of phenols can also be accomplished using aryliodonium
[a] C. Crifar, P. Petiot, T. Ahmad, Prof. A. Gagnon
Dꢀpartement de chimie
Universitꢀ du Quꢀbec ꢁ Montrꢀal
C.P. 8888, Succ. Centre-Ville, Montrꢀal, Quꢀbec, H3C 3P8 (Canada)
Fax: (+1)514-987-4054
E-mail: gagnon.alexandre@uqam.ca
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201303684.
Scheme 1. Synthesis of highly functionalized organobismuthanes by func-
tional group manipulation. FG=functional group.
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the organobismuth species is possible, giving access to highly
functionalized organobismuthanes C bearing groups that oth-
erwise cannot be present on the organomagnesium species
A.[17b]
Table 1. Optimization of reaction conditions for the O-phenylation of 1.
We would like to report herein our results on the copper-
catalyzed O-arylation of phenols using highly functionalized or-
ganobismuthanes. Since pentavalent organobismuth species
are usually prepared from their corresponding trivalent ana-
logues, one of our main goals was to develop conditions that
would operate directly with trivalent organobismuthanes. An-
other objective was to develop a protocol that would allow
the use of substoichiometric amounts of catalyst and that
would tolerate a wide diversity of functional groups on both
coupling partners. With these goals in mind, we began by opti-
mizing the conditions for the O-phenylation of 4-cyanophenol
1 (Table 1). Starting with conditions similar to those that we
previously reported for the N-cyclopropylation of azoles,[18] we
obtained the desired diaryl ether 3 in 65% yield (entry 1), dem-
onstrating that the reaction can in fact be performed directly
with trivalent organobismuthanes. While the addition of mo-
lecular sieves did not improve the efficiency of the transforma-
tion (entry 2), we found that conducting the reaction in non-
anhydrous dichloromethane under ambient air gave a superior
yield of product 3 (entry 3), showing that anhydrous and inert
conditions are not required to attain good yields. Erosion in
the yield of the reaction was observed upon using 0.7 equiva-
lents of triphenylbismuth, indicating that only one phenyl
group can be transferred during the process (entry 4). While
triethylamine could be replaced by pyridine (entry 5), the use
of an inorganic base almost completely shut down the reac-
tion (entry 6). Dichloromethane proved superior to other sol-
vents such as toluene and methanol (entries 7 and 8). Replac-
ing the catalyst with copper bis(trifluoroacetate) or reducing
its loading negatively impacted the yield of the reaction (en-
tries 9 and 10). However, the cat-
Change from “standard conditions”
Yield [%][a]
1
2
3
No change
65
57
73
40
57
2
26
11
0
4 ꢁ molecular sieves added
Non-anhydrous CH2Cl2, ambient air, 3 h
0.7 equiv of Ph3Bi instead of 1.0 equiv
Pyridine instead of Et3N
K2CO3 instead of Et3N
Toluene instead of CH2Cl2
Methanol instead of CH2Cl2
Cu(OC(O)CF3)2 instead of Cu(OAc)2
0.7 equiv of Cu(OAc)2 instead of 1.0 equiv
0.7 equiv of Cu(OAc)2 under O2
0.3 equiv of Cu(OAc)2 under O2
4[b]
5[b]
6[b]
7[b]
8[b]
9[b]
10[b]
11
39
76
81
12
[a] Isolated yield of pure product 3. [b] Reaction performed under ambi-
ent air for 3 h using non-anhydrous dichloromethane.
of phenols possessing electron withdrawing (5e–g) and donat-
ing groups (5h). A preliminary investigation showed that nitro
groups (5 f), ketones (5g), and a,b-unsaturated esters (5j) are
well tolerated and that the reaction also functions with 3-hy-
droxypyridines (5i). Similar yields were generally obtained
using the method involving stoichiometric or substoichiomet-
ric amounts of copper acetate (methods A and B, respectively).
Substituted triarylbismuth reagents 6 bearing functional
groups were then prepared by adding the corresponding orga-
nomagnesium reagent A over bismuth chloride as illustrated
in Figure 1.
Functionalized organometallic reagents are extremely useful
for the transfer of substituted groups on medicinally relevant
alyst loading could be reduced
to substoichiometric amounts by
performing the reaction under
oxygen (entries 11 and 12). Re-
ducing the loading further
proved detrimental to the reac-
tion.
Using our optimal conditions,
we next studied the O-phenyla-
tion of diversely substituted phe-
nols and observed that while the
reaction is quite tolerant to para
and meta substitution (5a and
b), the introduction of a methyl
group at the ortho position leads
to a modest reduction in the
yield of the reaction (5c;
Scheme 2). This effect was slight-
ly accentuated by flanking the
phenol by two methyl groups
Scheme 2. O-Phenylation of substituted phenols. [a] Isolated yields. Method in parentheses. Conditions: Method
A: Ph3Bi (1.0 equiv), Cu(OAc)2 (1.0 equiv), Et3N (3 equiv), CH2Cl2, 508C, air, 3 h; Method B: Ph3Bi (1.0 equiv),
(5d). Good to excellent yields
were obtained for the arylation Cu(OAc)2 (0.3 equiv), Et3N (3 equiv), CH2Cl2, 508C, O2, 16 h.
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Scheme 3. Synthesis of tris(3-formylphenyl)bismuthine 6i.
phenol 4l (entry 12) and 4b (entry 13) by reacting them with
organobismuthane 6d and g, respectively.
Contrary to O-arylation reactions involving aryl halides, this
protocol allows the presence of a bromide on the phenol 4c.
Also, in contrast to reactions involving pentavalent organobis-
muthanes, alcohols 4d and ketones 4i are not arylated under
these conditions.[21,22] Most of the diaryl ethers that were gen-
erated using this protocol are highly functionalized and can be
further derivatized using a wide range of conditions, including
palladium and copper(I)-catalyzed reactions.
Figure 1. Functionalized organobismuthanes used in the O-arylation reaction
of phenols.
scaffolds.[20] Introduction of functional groups that are not tol-
erated on the Grignard reagent was accomplished by trans-
forming the functional group directly on the organobismuth
reagent. As a representative example, tris(3-formylphenyl)bis-
muthine 6i was prepared on a multi-gram scale by hydrolyzing
tris(3-(diethoxymethyl)phenyl)bismuthine (6h; Scheme 3). The
organobismuth reagents 6a–i are air and moisture stable and
can be manipulated in the air without any precaution.
We then turned our attention to the coupling of these func-
tionalized organobismuth reagents with diversely substituted
phenols (Table 2). In the event, the para and meta tolyl re-
agents 6a and b reacted smoothly to give the corresponding
diaryl ethers in good to excellent yields (entries 1–4). A good
yield was also obtained when the triarylbismuthane was sub-
stituted at the ortho position (6c), showing that the reaction is
only moderately affected by
We then compared the ability of boronic acid 8 and bismu-
thane 6i to arylate 3-hydroxy methylbenzoate (4m; Scheme 4).
Using conditions from the literature,[6a] we obtained the diaryl-
ether 7m in 41% yield after 22 h at ambient temperature. The
same compound was obtained in 64% yield after 6 h using
bismuthane 6i and 0.3 equivalents of copper acetate. Using
our conditions, the coupling of 4m with 8 provided 7m in
only 14% yield.
Finally, we investigated the arylation of 2(1H)-pyridones
using organobismuthanes (Scheme 5). In this particular case,
the underlying question was to determine if the arylation
would proceed on the oxygen through the hydroxypyridine 9
or on the nitrogen through the pyridone 9’. Using our optimal
conditions, we obtained exclusively the N-arylation product
steric factors (entry 5). The trans-
fer of aryl fragments possessing
a fluorine, a methoxy group, an
acetal or an aldehyde furnished
the corresponding diaryl ethers
in good to excellent yields (en-
tries 6–12). When considering
the synthesis of diaryl ethers,
two different disconnections can
be envisaged, leading to two
complementary combinations of
phenol and organobismuthane.
For instance, diaryl ether 7l was
Scheme 4. Comparison of O-arylation of 3-hydroxy methyl benzoate 4m with organoboronic acid 8 and organo-
conveniently prepared from bismuthane 6i.
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Table 2. O-Arylation of highly functionalized phenols using substituted triarylbismuthanes.
ArOH
Ar3Bi
Product
Yield[a] [%]
Method[b]
(A)
1
2
3
4a
4b
6a
7a
7b
66
85
87
6a
6a
(A)
(A)
4c
7c
4d
7d
94
82
(A)
(B)
4
5
6
6b
6c
6d
4e
7e
75
79
(A)
(A)
4 f
4 f
7 f
7g
75
61
(A)
(B)
7
8
9
6e
6 f
6h
4h
4i
7h
7i
55
82
(A)
(B)
10
11
12
13
4j
6h
6i
7j
80
(B)
4k
7k
71
87
(A)
(B)
4l
6d
6g
7l
7l
98
52
(B)
(A)
4b
[a] Isolated yield. [b] Conditions: Method A: Ar3Bi (1.0 equiv), Cu(OAc)2 (1.0 equiv), Et3N (3 equiv), CH2Cl2, 508C, air, 3 h; Method B: Ar3Bi (1.0 equiv),
Cu(OAc)2 (0.3 equiv), Et3N (3 equiv), CH2Cl2, 508C, O2, 16 h.
10b, albeit in modest yield. Fortunately, the yield could be
considerably improved by using pyridine as the base, provid-
ing 10b in 98% yield. The O-arylation product 10a was not
detected under our reaction conditions.23 In order to demon-
strate the applicability of this method in the arylation of other
pyridones, compounds 10c–e, which are all densely functional-
ized, were prepared in good to excellent yields.
agents. This reaction operates under simple conditions and tol-
erates numerous functional groups on both coupling partners,
giving access to highly functionalized diaryl ethers. Substoi-
chiometric amounts of catalyst can be used when the reaction
is performed under oxygen. The arylation reaction of pyridones
provided exclusively the corresponding N-arylated products.
Applications to other arylation reactions and to parallel
chemistry are in progress in our laboratory and results will be
reported in due course.
In summary, we developed an efficient O-arylation reaction
of phenols using functionalized trivalent organobismuth re-
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Typical procedure for the O-arylation of phenols using orga-
nobismuthanes:
Method A:
In a sealed tube, the phenol (0.28 mmol) was dissolved in non-an-
hydrous solvent grade dichloromethane (3 mL). The organobismu-
thane (1.0 equiv) was added followed by copper (II) acetate
(1.0 equiv) and triethylamine (3.0 equiv). The tube was sealed and
heated at 508C until completion as indicated by TLC analysis. The
reaction mixture was cooled to room temperature and silica gel
was added. The mixture was concentrated under reduced pressure
and the crude product was purified by flash column chromatogra-
phy using the indicated solvent system. The pure fractions were
concentrated under reduced pressure to afford the desired pure
product.
Method B:
Same as method A, except that 0.3 equivalent of copper (II) ace-
tate was used. The reaction tube was purged with 99.6% extra dry
oxygen for 1 min, sealed and heated at 508C until completion as
indicated by TLC analysis. The reaction mixture was treated as in
method A.
Acknowledgements
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This work was supported by the Natural Sciences and Engi-
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Keywords: copper catalysis · functionalized diaryl ethers · O-
arylation · phenols · triarylbismuthanes
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Received: September 19, 2013
Published online on February 12, 2014
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