First palladium-catalyzed denitrated coupling reaction of nitroarenes with phenols

Article abstract of DOI:10.1002/aoc.3043

The first palladium-catalyzed protocol for the denitrated coupling reaction of nitroarenes with phenols has been developed, achieving unsymmetrical diaryl ethers in moderate to excellent yields. The cyclopalladated ferrocenylimine (catalyst Ic) exhibited highly catalytic activity for this transformation with low catalyst loading (0.75 mol%) and short reaction time (2 h). The efficiency of this reaction was demonstrated by its compatibility with a range of groups. Moreover, the rigorous exclusion of air or moisture was not required in these transformations. Copyright

Full text of DOI:10.1002/aoc.3043

Full Paper
Received: 16 May 2013
Revised: 24 June 2013
Accepted: 13 July 2013
Published online in Wiley Online Library: 27 August 2013
(wileyonlinelibrary.com) DOI 10.1002/aoc.3043
First palladium-catalyzed denitrated coupling
reaction of nitroarenes with phenols
Hailei Wang, Ajuan Yu*, Aijuan Cao, Junbiao Chang and Yangjie Wu*
The first palladium-catalyzed protocol for the denitrated coupling reaction of nitroarenes with phenols has been developed,
achieving unsymmetrical diaryl ethers in moderate to excellent yields. The cyclopalladated ferrocenylimine (catalyst Ic)
exhibited highly catalytic activity for this transformation with low catalyst loading (0.75 mol%) and short reaction time
(2 h). The efficiency of this reaction was demonstrated by its compatibility with a range of groups. Moreover, the rigorous
exclusion of air or moisture was not required in these transformations. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: cyclopalladated ferrocenylimines; catalysis; denitrated; nitroarene; phenol
(1a) and phenol (2a) was initially chosen as a model reaction in
DMF at 100 °C with 1 mol% catalyst loading. The results are
Introduction
Diaryl ethers represent an important class of intermediates for
pharmaceuticals, agrochemicals and polymers.[1,2] Traditionally, the
synthesis of such ethers has been accomplished via Ullmann cross-
couplings, which employed aryl halides and sodium or potassium
aryloxides in the presence of a stoichiometric (or greater) amount
of a copper species at elevated temperatures (125–220 °C).[3]
Unfortunately, such drawbacks limited their applications to large-
or industrial-scale production. Consequently, several modifications
to the original Ullmann conditions have been developed, such as
rhodium,[4] palladium,[5] copper[6] and iron[7] catalytic systems, which
enable the Ullmann ether formation reaction to be performed under
milder conditions. Generally aryl halides were employed as coupling
partners in these improved methods. Only recently, wu and co-
workers[8] developed rhodium- or copper-catalyzed coupling
reaction of arylboronic acids and phenols using nitroarenes as a
new arylation reagent to produce diaryl ethers. In spite of the
significant improvements achieved, it is still desirable to develop
new, efficient catalytic systems to further improve the efficiency
and generality of the coupling of nitroarenes with phenols.
Palladacycles are one class of the most fascinating organometallic
reagents and efficient catalysts for constructing carbon–carbon and
carbon–heteroatom bonds. During the past decade, there has been
a growing interest in the utility of palladacycles as catalyst
precursors.[9] In our previous work, we focused on the application
of cyclopalladated ferrocenylimines in cross-coupling reactions[10]
and found that they were highly versatile in the Suzuki reaction, Heck
reaction, Kumada reaction, Sonogashira reaction, etc. In our continu-
ation of efforts to develop cyclopalladated ferrocenylimines in cross-
coupling reactions, we herein report the first example of palladacycle
(catalyst Ic)-catalyzed synthesis of unsymmetrical diaryl ethers via
denitrated coupling reaction of nitroarenes with phenols.
summarized in Table 1. To our delight, when 1a and 2a were
reacted in the presence of the tricyclohexylphosphine adduct Ia
as well as 2,2′-bipyridine adduct Ib, employing Cs2CO3 as base
and DMF as solvent, 4-phenoxybenzaldehyde (3aa) was isolated
in good yield (Table 1, entries 1 and 2). Surprisingly, it gave
excellent denitrated product 3aa (up to 97% yield) when
complex Ic was used as catalyst (Table 1, entry 3). It is worth
noting that Pd2(dba)3 and Pd(acac)2 could also be used for this
transformation (Table 1, entries 21 and 22), but the yields were
lower. Therefore, catalyst Ic was chosen as catalyst for the
following study.
The various reaction conditions were examined by fixing 1 mol%
loading of catalyst Ic. It was found that the base played a critical
t
role in catalytic efficiency. Stronger bases, e.g. KOH and BuOK,
afforded the desired product in 58% and 9% yields, respectively
(Table 1, entries 4 and 5). A variety of weak bases such as Na2CO3,
K3PO4, K2CO3, AcOK and KF were all unsatisfactory (Table 1, entries
6–10). With these partially optimized denitrated conditions, the
effects of the solvents were investigated (Table 1, entries 11–15).
The results revealed that the use of DMF as solvent achieved the
best result (Table 1, entry 3). Other solvents, including DMSO,
toluene, dioxane, THF and pyridine, did not favor this reaction.
The catalyst loading, reaction temperature, the equivalents of base
and reaction time were also surveyed (Table 1, entries 16–19). The
influence of the loading of catalyst Ic could be reduced to 0.75 mol
% with no significant change in yield (Table 1, entries 3 vs. 16 and
17). When reducing the reaction temperature, the yield dropped
*
Correspondence to: Ajuan Yu and Yangjie Wu, Key Laboratory of Chemical
Biology and Organic Chemistry of Henan, College of Chemistry and Molecular
Engineering, Zhengzhou University, Zhengzhou 450052, People’s Republic of
China. Email: yuajuan@zzu.edu.cn; wyj@zzu.edu.cn
Results and Discussion
Key Laboratory of Chemical Biology and Organic Chemistry of Henan, College of
Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450052,
People’s Republic of China
To evaluate the activity of the cyclopalladated ferrocenylimine
adducts Ia–c (Scheme 1), the reaction of p-nitrobenzaldehyde
Appl. Organometal. Chem. 2013, 27, 611–614
Copyright © 2013 John Wiley & Sons, Ltd.

H. Wang et al.
Table 2. Scope of the denitrated coupling reaction of nitroarenes
with phenolsa
Scheme 1. Catalysts Ia–Ic.
Entry
Ar1
Ar2
Product Yieldb (%)
1
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
4-CHOC6H4 (1a)
2-CHOC6H4 (1b)
4-CNC6H4 (1c)
4-CNC6H4 (1c)
4-CNC6H4 (1c)
4-CNC6H4 (1c)
4-CNC6H4 (1c)
4-CNC6H4 (1c)
4-CNC6H4 (1c)
2-CNC6H4 (1d)
2-CNC6H4 (1d)
2-CNC6H4 (1d)
2-CNC6H4 (1d)
2-CNC6H4 (1d)
2-CNC6H4 (1d)
2-CNC6H4 (1d)
4-NO2C6H4 (1e)
4-NO2C6H4 (1e)
4-NO2C6H4 (1e)
4-NO2C6H4 (1e)
4-NO2C6H4 (1e)
4-NO2C6H4 (1e)
4-NO2C6H4 (1e)
4-ClC6H4 (1f)
C6H5 (2a)
2-CH3C6H4 (2b)
3-CH3C6H4 (2c)
4-CH3C6H4 (2d)
2-OCH3C6H4 (2e)
4-OCH3C6H4 (2f)
4-(t-C4H9)C6H4 (2g)
4-ClC6H4 (2h)
2-BrC6H4 (2i)
3aa
3ab
3ac
3ad
3ae
3af
94
75
85
91
86
93
92
90
71
98
87
78
75
97
79
75
93
98
95
84
86
73
87
94
94
98
98
91
78
95
94
96
94
94
87c
99c
58d
2
Table 1. Optimization of reaction conditionsa
3
4
5
6
7
3ag
3ah
3ai
8
Entry
Palladium
source
Base
Solvent
T
(h)
Yieldb
(%)
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
4-BrC6H4 (2j)
3aj
1
Cat. Ia (1 mol%)
Cat. Ib (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (1 mol%)
Cat. Ic (0.75 mol%)
Cat. Ic (0.5 mol%)
Cat. Ic (0.75 mol%)
Cat. Ic (0.75 mol%)
Cat. Ic (0.75 mol%)
Cs2CO3
Cs2CO3
Cs2CO3
KOH
tBuOK
Na2CO3
K3PO4
K2CO3
AcOK
DMF
4
90
92
97
58
9
2,4-(CH3)2C6H3 (2k)
2,5-(CH3)2C6H3 (2l)
C6H5 (2a)
3ak
3al
2
DMF
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
2
2
2
3
DMF
3ba
3ca
3cb
3cc
3cd
3cf
4
DMF
C6H5 (2a)
5
DMF
2-CH3C6H4 (2b)
3-CH3C6H4 (2c)
4-CH3C6H4 (2d)
4-OCH3C6H4 (2f)
4-ClC6H4 (2h)
4-BrC6H4 (2j)
6
DMF
18
89
78
17
7
7
DMF
8
DMF
9
DMF
3ch
3cj
10
11
12
13
14
15
16
17
18c
19d
20
KF
DMF
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
DMSO
Toluene
Dioxane
THF
62
26
18
15
16
94
72
71
95
94
57
21
C6H5 (2a)
3da
3db
3dc
3dd
3df
3dh
3dj
3ea
3eb
3ec
3ed
3ef
2-CH3C6H4 (2b)
3-CH3C6H4 (2c)
4-CH3C6H4 (2d)
4-OCH3C6H4 (2f)
4-ClC6H4 (2h)
4-BrC6H4 (2j)
Pyridine
DMF
DMF
DMF
C6H5 (2a)
DMF
2-CH3C6H4 (2b)
3-CH3C6H4 (2c)
4-CH3C6H4 (2d)
4-OCH3C6H4 (2f)
4-ClC6H4 (2h)
4-BrC6H4 (2j)
DMF
DMF
21 Pd2(dba)3 (0.75 mol%)
22 Pd(acac)2 (0.75 mol%)
DMF
3eh
3ej
aUnless otherwise noted, the reaction conditions were as follows:
4-nitrobenzaldehyde (0.5 mmol), phenol (0.5 mmol), palladium
source, base (0.75 equiv.) and solvent (3 ml), under air.
C6H5 (2a)
3fa
bIsolated yields.
cAt 80 °C.
4-BrC6H4 (1g)
C6H5 (2a)
3ga
3ha
37 4-COOCH2CH3C6H4
C6H5 (2a)
d1.0 equiv of base was used.
(1h)
aReaction conditions: nitroarene (0.5 mmol), phenol (0.5 mmol),
catalyst Ic (0.75 mol %), Cs2CO3 (0.75 equiv) and DMF (3 ml), air,
100 °C, 2 h.
bIsolated yields.
c1-Nitro-4-phenoxybenzene was isolated.
dFor 7 h.
sharply (Table 1, entries16vs.18).However,increasingtheamountof
Cs2CO3to1.0equivalenthadlittleeffectontheyield(Table1,entries16
vs.19);2 hwasappropriateforthesereactions(Table1,entries3vs.20).
Under the optimized reaction conditions, the scope and gener-
ality of the coupling reaction using nitroarenes (1) with phenol
(2) to unsymmetrical diaryl ethers (3) were investigated (Table 2).
To our delight, all the reactions proceeded smoothly and
tolerated a wide range of functional groups, including methoxy,
chloro, bromo, formacyl, cyano and nitro groups.
coupling of 1a with para-, meta- and ortho-methylphenol was exam-
ined; 91% of 3ad and 85% of 3ac were isolated, while the yield of
3ab was decreased to 75% (Table 2, entries 2–4). The same phenom-
enon was observed in the reaction of 1a with para- and ortho-
methoxyphenol (Table 2, entries 5 and 6). However, the reaction
was not significantly affected by the electronic effect of the phenols
First, the mono-substituent positions at the aryl moiety of phenols
were evaluated, and the results demonstrated that steric properties
of substituents had some effects on the reaction. For example, the
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2013, 27, 611–614

Palladium-catalyzed denitrated coupling reaction of nitroarenes with phenols
processes mediated by catalyst Ic and to expand the scope of the
coupling of unactivated nitroarenes.
Experimental
General
1H NMR and 13C NMR spectra were recorded on a Bruker DPX-400
spectrometer with CDCl3 as solvent and tetramethylsilane as
internal standard. None of the solvents in this reaction were
purified before use. Other chemicals were obtained from commer-
cial sources and used as received unless otherwise noted. Catalysts
Ia,[11] Ib[11] and Ic[10a] were prepared according to the references.
General Procedure for Palladacycle-Catalyzed Denitratied
Coupling Reaction
Scheme 2. Plausible reaction mechanism.
A reaction vessel was charged with a mixture of nitroarenes 1
(0.5mmol), phenols 2 (0.5mmol), catalyst Ic (0.75 mol %) and
Cs2CO3 (0.75 equiv.) in DMF (3ml) at room temperature. The
mixture was heated to 100 °C and incubated in an oil bath for 2 h
in air atmosphere. After completion of the reaction, the reaction
mixture was diluted with ethyl acetate and washed with brine three
times. The combined organic solution was dried with Na2SO4,
filtered and evaporated under reduced pressure. The residue was
purified by thin-layer chromatography on silica gel GF 254 (ethyl
acetate/petroleum ether ) to give the pure product.
because both electron-rich and electron-deficient phenols could give
similarly moderate or good results (Table 2, entries 2–10), which was
different from existing reports.[8c] It is noteworthy that hindered
disubstituted methyl moieties in phenols 2k–2l were also tolerated
and afforded a novel route to compounds 3ak–3al in good yields,
making further elaborations of the corresponding biaryl products
possible (Table 2, entries 11 and 12).
Subsequently, we explored the scope of nitroarenes with dif-
ferent substituents on the phenyl ring (Table 2, entries 13–37).
The results showed that the coupling reaction in this case could
be applied to nitroarenes bearing various electron-withdrawing
functional groups (e.g., formyl, cyano, nitro and ethoxycarbonyl)
producing unsymmetrical diaryl ethers in good to excellent yields.
Intriguingly, the mono-coupling products 3ea–3ej were selectively
produced in excellent yields when 1,4-dinitrobenzene was
employed as a coupling partner (Table 2, entries 28–34). Notably,
the chloro and bromo moieties (commonly used for cross-coupling
reactions) in phenols were all well tolerated in our catalytic system
(Table 2, entries 33 and 34). However, nucleophilic displacement of
the chloro and bromo groups with phenol was observed when 1f
and 1g were used under standard conditions, affording 1-nitro-
4-phenoxybenzene in 87% and 99% yields, respectively (Table 2,
entries 35 and 36). When 4-(ethoxycarbonyl)nitrobenzene (1h)
was subjected to the same conditions, a lower yield was
obtained even when prolonging the reaction time to 7 h (Table 2,
entry 37).
The products 3aa,[8c] 3ab,[8c] 3ac,[8c] 3ad,[8c] 3ae,[8c] 3af,
[13]
[8c] 3ag,[12] 3ah,[8c] 3ai,[8c] 3aj,[8c] 3ak,
3al,[14] 3ba,[8c]
3ca,[8c] 3cb,[8c] 3cc,[8c] 3cd,[8c] 3cf,[15] 3ch,[15] 3cj,[8c]
3da,[16] 3db,[15] 3dc,[17] 3dd,[18] 3df,[19] 3dh,[20] 3dj,[21] 3ea,[22]
3eb,[23] 3ec,[24] 3ed,[8c] 3ef,[22] 3eh,[8c] 3ej,[24] 3fa,[22] 3ga[22]
and 3ha[25] were characterized by comparing their 1H NMR
and 13C NMR spectra with those mentioned in the references.
Detailed characterization of the compounds is available in the
supporting information.
Supporting Information
Supporting information may be found in the online version of
this article.
Acknowledgements
We are grateful to the National Natural Science Foundation of
China (21205107, 21172202, 21172200) and Postdoctoral Scholar-
ship of China (2012M511592) for financial support of this research.
Detailed characterization of the compounds is available in the
supporting information.
A possible mechanism for the formation of the diaryl ethers
was proposed as shown in Scheme 2. First, the palladacycle
was pre-activated to form a. Oxidative addition of nitroarene
formed arylpalladium species b. In the presence of inorganic
base, the phenol was transformed into phenoxide intermediate
c. Then phenoxide c could be converted to intermediate d by
nucleophile displacement of a nitro group. The reductive elim-
ination of d gave the diaryl ether e and regenerated the palla-
dium catalyst a.
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Appl. Organometal. Chem. 2013, 27, 611–614