Iron-catalyzed arylation of benzoazoles with aromatic aldehydes using oxygen as oxidant

Article abstract of DOI:10.1039/c2gc35457h

An iron-catalyzed arylation of azoles with aromatic aldehydes using oxygen as oxidant has been discovered. The reaction proceeded well for a range of different substrates under oxidative conditions. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2gc35457h

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Iron-catalyzed arylation of benzoazoles with aromatic aldehydes using oxygen
as oxidant†
Saiwen Liu,a Ru Chen,a Xiangyu Guo,b Huiqiong Yang,a Guojun Deng*a and Chao-Jun Li*b
Received 27th March 2012, Accepted 24th April 2012
DOI: 10.1039/c2gc35457h
H bonds via double C–H activations.16 However, the direct ary-
An iron-catalyzed arylation of azoles with aromatic alde-
lation of benzoazoles using aromatic aldehydes as aryl sources
remains a challenge. Aromatic aldehydes are cheap, commer-
cially available and easy to handle and thus can be potentially
used as the ideal arylation reagents. Very recently, we and others
developed various catalytic systems for direct aryl ketone for-
mation between arene C–H bonds and aromatic aldehydes.17 We
also developed a rhodium-catalyzed oxidative C–H arylation of
2-arylpyridine derivatives via the decarbonylation of aromatic
aldehydes using peroxides as oxidants.18 In this reaction, various
aromatic aldehydes act as the aryl sources. It would be highly
desirable to develop a process using inexpensive and non-toxic
metal such as iron as catalyst for the preparation of aryl-substi-
tuted benzoazoles using benzoazoles and aromatic aldehydes as
starting materials.19,20 Herein, we report an iron-catalyzed aryla-
tion of benzoazoles with aldehydes in the presence of oxygen,
affording the aryl-substituted benzothiazoles and benzoxazoles
in high yields (Scheme 1).
hydes using oxygen as oxidant has been discovered. The
reaction proceeded well for a range of different substrates
under oxidative conditions.
The biaryl motif is an important substructure of many bioactive
and functional molecules and has been the focus of organic che-
mists for over a century.1 Over the last few decades, transition-
metal-catalyzed cross-coupling reactions have proved to be effec-
tive synthetic tools for the formation of biaryls between aryl
halides and organometallics.2 A variety of other arylation
methods using organic substrates instead of organometallics has
also been successfully explored.3 Among them, the decarboxyla-
tive coupling reactions developed by Goossen et al. have
emerged as powerful alternatives for selective biaryl formation
using cheap arenecarboxylates as the coupling partners.4 In the
meantime, the direct conversion of C–H bonds into C–C bonds
can potentially lead to a more efficient synthesis with a reduced
number of synthetic operations and thus has attracted great inter-
est in recent years. Since the pioneering efforts of Murai et al.,5
great progress has been achieved in the transition-metal-cata-
lyzed activation and subsequent reaction of C–H bonds
especially for biaryl synthesis.6
Aryl-substituted benzoazoles are common building blocks for
the synthesis of pharmaceuticals, natural products, functional
materials, and many other biologically active molecules.7 The
conventional methods for the synthesis of these important com-
pounds typically involve either the metal-catalyzed intramolecu-
lar cyclization of thioformanilides or the cross-coupling of
carboxylic acids or aldehydes with 2-aminophenol (or 2-ami-
nothiophenol),8,9 but such methods suffer from difficulties in the
preparation of readily oxidized 2-aminothiophenols. Alterna-
tively, aryl-substituted benzoazoles have been successfully syn-
thesized by direct C–H activation and subsequent C–C bond
formation with aryl halides,10 arylsilanes,11 aromatic carboxylic
acids,12 aryl boronic acids,13 sodium sulfinates14 and aryl
triflates (or mesilates and sulfamates),15 and even with arene C–
We began our study by examining the reaction of benzothia-
zole (1a) with benzaldehyde (2a) in H2O–diglyme by using
oxygen (1 atm) as oxidant at 130 °C. When benzaldehyde
reacted with 1.5 equivalents of benzothiazole in the absence of
any catalyst, the desired product 3aa was obtained in 6% yield
1
as determined by GC and H NMR methods (Table 1, entry 1).
Then various iron salts were investigated for this reaction under
similar reaction conditions. Only a trace amount of product was
observed when Fe(acac)2 and Fe2O3 were used (entries 2 and 3).
Fe(NO3)3 and ferrocene were also inefficient catalysts for this
kind of transformation (entries 4 and 5). The use of FeCl3 and
FeCl2 improved the reaction yield to 28% and 55%, respectively
(entries 6 and 7). Among the various iron salts examined, FeSO4
was the most effective, and its use resulted in the formation of
3aa in 77% yield (entry 8). The choice of solvents was crucial
for this reaction. Moderate yields were obtained when reactions
were carried out in pure organic solvents such as NMP, ethanol
and diglyme (entries 9–11). Interestingly, 60% yield was
achieved when water was used as the sole solvent and the reac-
tion completed in 4 h, which is much faster than those in organic
aKey Laboratory for Environmentally Friendly Chemistry and
Application of Ministry of Education, College of Chemistry, Xiangtan
University, Xiangtan 411105, China. E-mail: gjdeng@xtu.edu.cn;
Fax: (+86) 731-58292477
bDepartment of Chemistry, McGill University, Montreal, QCH3A2K6,
Canada. E-mail: cj.li@mcgill.ca; Fax: (+1) 514-398-3797
†Electronic supplementary information (ESI) available: NMR and MS
analysis for all products. See DOI: 10.1039/c2gc35457h
Scheme 1 Iron-catalyzed arylation of benzoazoles with aldehydes.
Green Chem., 2012, 14, 1577–1580 | 1577
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Table 1 Optimization of the reaction conditionsa
coupling of heteroaromatic aldehydes such as picolinaldehyde
and furan-2-carbaldehyde with 1a afforded 3ap and 3aq in 78%
and 53% yields, respectively. Aliphatic aldehydes such as
octanal also coupled with 1a and afforded the alkylated product
3ar in 30% yield.
The reaction results of various azoles with benzaldehyde 2a
are also investigated. Benzothiazoles bearing electron-donating
substituents at the phenyl ring proved to be good substrates for
this transformation, affording the corresponding products 3ba,
3ca, and 3da in good yields. Having an electron-withdrawing
substituent at the phenyl ring, 6-nitro-2-phenylbenzothiazole
gave a slightly lower yield of 3ea. Unfortunately, benzoxazoles
are not good substrates for this transformation due to the
hydrolysis side reaction in aqueous media. To find reaction con-
ditions suitable for benzoxazoles, we reinvestigated the reaction
conditions systematically. Fe2(SO4)3·xH2O–P(C6F5)3 was found
to be a good catalyst system and diglyme was a good solvent.
When benzoxazole reacted with benzaldehyde at 110 °C with an
extended reaction time to 36 h, the desired product 3fa was
obtained in 70% yield. Various functional groups including
methyl, chloro, and nitro at the phenyl ring were well tolerated
under the optimal reaction conditions.
To get more information about the reaction mechanism,
several control experiments were set up under the standard con-
ditions. A 13C-labeled product was detected as the major product
by GC-MS when 13C-labeled benzaldehyde was reacted with 1a
and 1f (Scheme 2, 1 and 4). This means for the arylation of ben-
zoazoles with aldehydes, the reaction did not take a decarbonyla-
tive and C–H activation pathway. The reaction mainly took a
ring-opening pathway and iron mainly acted as a Lewis acid cat-
alyst. The reaction of 2-aminothiophenol with benzaldehyde (2a)
also gave the desired product 3aa in 79% yield, which was
slightly lower than the reaction yield of benzothiazole with ben-
zaldehyde (Scheme 2, 2). However, the reaction of 2-amino-
phenol with benzaldehyde did not result in the desired product
in aqueous or organic solvent systems (Scheme 2, 3). Although
the exact reaction mechanism is not clear for the arylation of
benzoazoles with aldehydes at this stage.22 This reaction is still
an attractive method since cheap and non-toxic iron was used as
the catalyst and oxygen was used as a green oxidant.20
T
Yieldb
(%)
Entry Catalyst
1
Oxidant (°C) Solvent
O2
130 H2O–diglyme (1 : 1)
6
130 H2O–diglyme (1 : 1) Trace
130 H2O–diglyme (1 : 1) Trace
130 H2O–diglyme (1 : 1) 11
2
3
Fe(acac)2 (10) O2
Fe2O3 (10) O2
4
5
Fe(NO3)3 (10) O2
ferrocene (10) O2
130 H2O–diglyme (1 : 1)
9
6
7
8
9
10
11
12
13
14
15
FeCl3 (10)
FeCl2 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
130 H2O–diglyme (1 : 1) 28
130 H2O–diglyme (1 : 1) 55
130 H2O–diglyme (1 : 1) 77
130 NMP
56
49
51
60
36
28
68
130 C2H5OH
130 Diglyme
130 H2O
130 H2O–DMA (1 : 1)
130 H2O–DMF (1 : 1)
130 H2O–1,4-dioxane
(1 : 1)
130 H2O–C2H5OH (1 : 1) 64
130 H2O–DMSO (1 : 1) 70
130 H2O–NMP (1 : 1)
16
17
18
19
20
21
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (10)
FeSO4 (20)
FeSO4 (20)
O2
O2
O2
O2
O2
Air
72
150 H2O–diglyme (1 : 1) 84
150 H2O–diglyme (1 : 1) 94
150 H2O–diglyme (1 : 1) 45
a Conditions: 1a (0.3 mmol), 2a (0.2 mmol), solvent (0.4 mL), 20 h.
b GC yield based on 1a.
solvents (entry 12). The combination of water with other organic
solvents all resulted in lower yields (entries 13–18). The reaction
temperature is another important factor for the yield of the
product. The reaction yield could be improved to 84% when the
reaction temperature was increased to 150 °C (entry 19). The use
of 20 mol% FeSO4 further improved the reaction yield to 94% in
the absence of any ligand (entry 20).21 However, a much lower
yield was obtained when the reaction was carried out under an
atmosphere of air (entry 21).
Under the optimized reaction conditions, the scope and gener-
ality of the oxidative cross-coupling reaction was explored
(Table 2). The reactions with aromatic aldehydes bearing elec-
tron-donating groups at the aromatic ring proceeded smoothly to
give the desired products (3ab–3ae) in good to excellent yields.
However, the reaction yield decreased significantly when 4-
(dimethylamino)benzaldehyde was reacted with benzothiazole in
water–diglyme solvent system. The yield of the desired product
3af could be improved to 52% when a mixture of DMSO–H2O
was used as the solvent. Aldehydes possessing electron-with-
drawing groups on the phenyl ring also reacted smoothly with
benzothiazole and afforded the desired arylated products (3ag–
3aj) in good yields. The position of substituents on phenyl ring
of aldehydes affected the reaction yield slightly (3ak–3am). A
good yield was achieved when 3,4,5-trimethoxybenzaldehyde
was used, and the desired product 3an was obtained in 83%
yield. Functional groups such as fluoro, chloro, bromo were well
tolerated under the optimized conditions. A free hydroxy group
was also tolerated, and the reaction of 4-hydroxy-3-methoxyben-
zaldehyde reacted with 1a gave 3ao in good yield. Notably, the
In summary, we have demonstrated a direct arylation of ben-
zoazoles with aromatic aldehydes. The cheap and non-toxic iron
catalyst efficiently catalyzed the coupling reactions in good to
excellent yields using oxygen as oxidant under base-free con-
ditions. Solvent played an important role in this transformation
and better yields were obtained in a mixture of water–diglyme
than in organic solvents when benzothiazoles were used as sub-
strates. Functional groups such as methyl, methoxy, fluoro,
chloro, bromo and nitro were all well tolerated under the opti-
mized reaction conditions. This method affords a cheap and
efficient alternative route for the synthesis of heteroaromatic
biaryls. The scope, mechanism, and synthetic applications of this
reaction are under investigation.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (21172185, 20902076), the Hunan Provincial
1578 | Green Chem., 2012, 14, 1577–1580
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Table 2 Arylation of benzoazoles with various aldehydesa
a Conditions: 1 (0.3 mmol), 2 (0.2 mmol), FeSO4·7H2O (0.04 mmol), H2O–diglyme (0.4 mL, 1 : 1), 150 °C, 20 h under oxygen, isolated yield.
b DMSO–H2O (0.4 mL, 1 : 1). c Fe2(SO4)3·xH2O (0.04 mmol), P(C6F5)3 (0.02 mmol), diglyme (0.4 mL), 110 °C, 36 h under air. d 2 equiv. of octanal
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1580 | Green Chem., 2012, 14, 1577–1580
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