One-pot synthesis of 2-arylbenzoxazoles promoted by heteropolyacids

Article abstract of DOI:10.1002/jccs.200800133

Benzoxazole derivatives were obtained in high yields with excellent purity from the condensation of 2-aminophenol with benzaldehydes and benzoic acids in the presence of a catalytic amount of heteropolyacids (HPAs).

Full text of DOI:10.1002/jccs.200800133

8
90
Journal of the Chinese Chemical Society, 2008, 55, 890-895
One-pot Synthesis of 2-Arylbenzoxazoles Promoted by Heteropolyacids
Majid M. Heravi,* Samaheh Sadjadi, Hossein A. Oskooie,
Rahim Hekmat Shoar and Fatemeh F. Bamoharram
Department of Chemistry, School of Sciences, Azzahra University, Vanak, Tehran, Iran
Benzoxazole derivatives were obtained in high yields with excellent purity from the condensation of
-aminophenol with benzaldehydes and benzoic acids in the presence of a catalytic amount of heteropoly-
acids (HPAs).
2
Keywords: Benzoxazole derivatives; Recyclable catalysts; Heteropolyacids.
Benzoxazole ring moieties are often found in com-
H
5
[PMo10
V
2
O
40], H
4
[PMo11VO40] and H
the catalysts (Scheme I, II).
In connection with our program of using heteropoly
3
[PMo12O40] as
pounds that exhibit biological activities, including anti-
1
tumor, antimicrobial, and antiviral properties.
1
1
There are two general methods for synthesizing 2-
substituted benzoxazoles. One is the coupling of 2-amino-
phenols with carboxylic acid derivatives, which is either
acid in organic reactions we wish to report the result of a
study on the use of three Keggin types of HPAs including
4 5 2 3
H [PMo11VO40], H [PMo10V O40] and H [PMo12O40] in
2
catalyzed by strong acids or requires microwave condi-
3
tions. The other is the oxidative cyclization of phenolic
the synthesis benzoxazole derivatives and the effects of re-
action parameters such as the type, amount of HPA and sol-
vent on the yield of reaction. The results of synthesis of
benzoxazole derivatives from reaction of 2-aminophenol
Schiff bases derived from the condensation of 2-amino-
phenols and aldehydes. In the latter reactions, various oxi-
– 7
4
dants such as DDQ, Mn-(OAc)
5
, PhI(OAc)
6
, Th .ClO
+
3
2
4
,
5 2 40
with benzaldehydes and benzoic acids using H [PMo10V O ]
8
, NiO
9
, and Pb-(OAc)
10
BaMnO
4
2
4
have been used. How-
are summarized in Tables 1 and 2 and the comparison of ef-
ficiency of different HPAs in synthesis of benzoxazole de-
rivatives from these reactions are shown in Tables 3 and 4.
Comparison of these two methods for synthesis of
benzoxazole derivatives shows that condensation of 2-ami-
nophenol with benzoic acids using heteropolyacids under
refluxing conditions leads to better yields than benzalde-
hydes. In addition, higher amounts of heteropolyacids and
ever, all of these oxidants are required in stoichiometric or
excess amounts relative to their respective substrates. There-
fore, a more effective process is needed.
Herein we report a simple method for synthesis of
benzoxazole derivatives from reaction of 2-aminophenol
with benzaldehydes and benzoic acids using catalytic
amounts of three different Keggin types of HPAs including,
Scheme I
Scheme II
*
Corresponding author. E-mail: mmh1331@yahoo.com

Synthesis of 2-Arylbenzoxazoles Using Heteropolyacids
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 891
Table 1. Synthesis of benzoxazole derivatives from reaction of 2-
aminophenol with benzoic acids using H [PMo V O ]
Table 4. Synthesis of benzoxazole derivatives from reaction of 2-
aminophenol with benzaldehydes using various hetero-
polyacids under refluxing in THF
5
10
2
40
under refluxing in THF
a
Mp/ref. (°C)
Found Reported
Entry
R
Time (h)
Catalyst
Yield %
a
Entry
R
Time (h) Yield %
1
2
3
4
5
6
7
8
9
Ph
Ph
7
7.5
7.5
7
H [PMo V O ]
92
90.2
88.2
90
5
4
3
5
4
10
2
40
1
8
H
H
H
H
[PMo11VO40
]
1
2
3
4
Ph
5
5
7
6
94.8
92.7
85.7
94
102
113
268
101
102
1
8
Ph
[PMo12O ]
40
4-MeC H
6
113-114
266-268
101-103
4
1
1
8
8
4-MeC
4-MeC
6
H
H
4
[PMo10
V
2
O
40
]
4-NO C H
4
2
6
7.5
7.5
9
[PMo11VO40
]
88.5
87
6
4
4-OMeC H
6
4
4-MeC H4
6
H [PMo O ]
3 12 40
a
Yields are analyzed by GC
4-NO C H
6
H [PMo V O ]
2
82.5
81.4
80
2
4
4
4
5
10
40
4-NO C H
2
10
10
6.5
7
H [PMo VO ]
4 11 40
6
4-NO C H
2
H [PMo O ]
3 12 40
6
Table 2. Synthesis of benzoxazole derivatives from reaction of 2-
aminophenol with benzaldehydes using
1
0
4-ClC H4
6
H [PMo V O ]
5
88
10
2
40
11
4-ClC H4
6
H [PMo VO ]
40
85
4
11
H [PMo V O ] under refluxing in THF
5
10
2
40
12
4-ClC H4
6
7
H [PMo O ]
3 12 40
83.4
91
1
3
4-OMeC H4
6
7.5
8
H [PMo V O ]
5
4
10
2
40
Mp/ref. (°C)
a
Entry
R
Time (h) Yield %
14
4-OMeC H
6
4
H
[PMo11VO40
]
88
Found Reported
1
5
4-OMeC H4
6
8
H [PMo O ]
3 12 40
85.3
1
8
1
2
3
4
5
Ph
7
7
92
90
102
113
268
148
101
102
a
Yields are analyzed by GC
1
8
4-MeC H
6
113-114
266-268
4
1
8
4-NO C H
4
9
82.5
88
2
6
18
4-ClC H
6
6.5
7.5
147
101-103
4
Table 5. Comparison of synthesis of 2-phenyl-benzoxazole from
reaction of 2-aminophenol with benzaldehyde and
1
8
4-OMeC H
6
91
4
a
Yields are analyzed by GC
benzoic acid using H [PMo V O ] as catalyst under
2
5
10
40
refluxing in THF
Catalyst amount
a
Table 3. Synthesis of benzoxazole derivatives from reaction of 2-
aminophenol with benzoic acids using various hetero-
polyacids under refluxing in THF
Entry
Reagent
Time (h)
Yield %
(
mol %)
1
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
benzoic acid
benzaldehyde
3
3
0.3
0.3
0.5
0.5
0.3
0.3
0.5
0.5
0.3
0.3
0.5
0.5
0.3
0.3
0.5
0.5
85
78
2
a
Entry
R
Time (h)
Catalyst
Yield %
3
3
87
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
5
5.5
5.5
5
H [PMo V O ]
40
94.8
92.5
90.7
92.7
91.2
90
4
3
80
5
10
2
H [PMo VO ]
4
5
5
94.8
83
11
40
H [PMo O ]
3 12 40
6
5
4-MeC H4
6
H [PMo V O ]
40
7
5
94.9
86
5
10
2
4-MeC H4
6
6
H [PMo VO ]
4
8
5
11
40
4-MeC H4
6
6
H [PMo O ]
3 12 40
9
7
94.8
90
4-NO C H
2
7
H [PMo V O ]
5
85.7
84
10
11
12
13
14
15
7
6
4
4
4
10
2
40
4-NO C H
2
8
H [PMo VO ]
4
7
95
6
11
40
4-NO C H
2
8
H [PMo O ]
3 12 40
81.6
94
7
92
6
1
0
4-OMeC H4
6
6
H [PMo V O ]
5
10
10
10
10
95
10
2
40
1
1
4-OMeC H4
6
6.5
6.5
H [PMo VO ]
4
92.7
91
92
11
40
1
2
4-OMeC H4
6
H [PMo O ]
3 12 40
95
1
6
92.2
a
Yields are analyzed by GC
a
Yields are analyzed by GC
a longer reaction time is required for the latter method.
For deeper investigation of the differences of these
two methods, synthesis of 2-phenyl benzoxazole from con-
densation of 2-aminophenol with benzoic acids and benz-
shown in this table the sever condition is required for syn-
thesis of 2-phenyl benzoxazole from reaction of 2-amino-
phenol with benzaldehydes.
aldehyde using H
5
[PMo10
V
2
O
40] as catalyst was selected as
The efficiency of H
and H 40] were also studied.
Among the Keggin heteropolyacids, H
4 5 2
[PMo11VO40], H [PMo10V O40],
a model reaction. Different conditions for both reactions
were tested. The results are summarized in Table 5. As
3
[PMo12O
3
40
[PMo12O ]

8
92 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Heravi et al.
and Keggin-type vanadium-substituted heteropolymolyb-
dates: H [PMo11VO40] and H 40], the last one
gave the best results in reactions. The Keggin anions have
an assembly of 12 corner-shared octahedral MoO from
trimetallic groups [Mo 13] around a heteroatom tetrahe-
dron PO . The introduction of vanadium(V) into the Keggin
framework of [PMo12
catalysts in this reaction. We believe there is a complex re-
lationship between the activity and structure of polyanion.
Transition metal cations have an important effect on the
catalytic properties of these compounds when they substi-
tute molybdenum cations in the Keggin units. The case of
vanadium, which can occupy both anionic and cationic po-
sitions, is more complex. It is suggested that the interac-
tions of the polarized polyanion with substrate and the num-
ber of vanadium atoms are important factors in catalytic ac-
tivity in our reaction.
4
5 2
[PMo10V O
6
3
O
4
3
-
O
40] is beneficial for catalysis reac-
tions. Usually positional isomers are possible and coexist
when two or more vanadium atoms are incorporated into
the Keggin structure. Studies on these isomers in catalytic
reactions indicate that different isomers cause different re-
activities to show.
Besides the energy and composition of the LUMO
and abundance of different isomers, the presence of both
Bronsted acidity and vanadium in the structure of the men-
tioned heteropolyacids is responsible for catalytic activity.
A greater number of protons may lower the activation en-
ergy barrier and a greater number of vanadium atoms may
With respect to the catalytic performances for these
catalysts and the overall effects of all isomers, for synthe-
sizing them, we can not control the reaction conditions for
synthesis of positional vanadium-substituted isomers sepa-
rately, revealing the relationship between the structures of
1
2-17
provide many sites for catalytic reaction.
H
3+xPMo12-xVxO40 (x = 1, 2) and hence study of their cata-
As described before in the synthesis of benzoxazole
lytic activity is difficult. However, because the metal sub-
stitution may modify the energy and composition of the
LUMO and redox properties, for the mentioned hetero-
polyacids with different charges, the energy and composi-
tion of the LUMOs have significant effects on the catalytic
activity. Substitution of vanadium ions into the molybde-
num framework stabilize the LUMOs because these orbit-
als derive in part from vanadium d-orbitals which have
been assumed to be more stable than those of molybdenum
and tungsten. The abundance of different isomers may also
play an important role in catalytic performance. In addi-
tion, different positional Mo atom(s) substituted by the V
derivatives from reaction of 2-aminophenol with benzalde-
4
-8
hydes, different oxidants can be used. However, all these
processes require stoichiometric or excess amounts of oxi-
dants. Many of these procedures produce toxic or environ-
mentally problematic by-products, often require laborious
work up and purifications (i.e., to remove quinones and re-
lated products), and/or suffer from low isolation yields. As
mentioned above, the Keggin type of heteropolyacids show
oxidative and acidic characteristics. Hence, by using these
catalysts in this reaction, no more oxidant is required.
To optimize the amount of catalyst, the yields of reac-
tion using various amounts of catalysts were obtained. The
results for synthesis of benzoxazole derivatives from reac-
tion of 2-aminophenol with benzaldehydes and benzoic ac-
3
-
40
atom(s) in [PMo12O ] may create different vanadium
chemical environments, thus causing these catalysts to
exhibit varying catalytic performances.
5 2
ids using different amounts of H [PMo10V O40] are shown
The introduction of vanadium(V) into the Keggin
framework is beneficial for redox catalysis, shifting its re-
activity from acid-dominated to redox-dominated. In addi-
tion the amount of introduced vanadium(V) has a dramatic
effect on the yields. One of the difficulties encountered in
interpreting data obtained from reactions of vanadomo-
lybdophosphate anions is that in solution, a mixture of
heteropoly anions are usually present. In addition posi-
tional isomers of the polyvanadium anions are also appar-
ent. Another complication inherent in the study of multi-
electron oxidations by polyvanadium-containing anions is
the capacity of these oxidants to be reduced by one or more
electrons (reduction of each V(V) ion to V(IV). However it
is difficult to clarify the different activities between these
in Tables 6 and 7. It is clear in this table that the yields of re-
actions in very small amounts of catalysts are very high and
the optimum amount of catalyst was selected as 0.03 mmol
for reaction of 2-aminophenol with benzoic acids and 0.05
mmol for reaction of 2-aminophenol with benzaldehydes.
To investigate the effect of solvent in these reactions,
the reactions were done in different solvents (ethanol, THF,
acetonitrile). The results are reported in Tables 8 and 9. The
results show that the efficiency of solvents vary as THF >>
acetonitrile > ethanol.
One of the important factors affecting the oxidation
capacity of polyanions is the energy gap between the high-
est occupied molecular orbital (HOMO) and the lowest un-
occupied orbital (LUMO). Because of the large negative

Synthesis of 2-Arylbenzoxazoles Using Heteropolyacids
J. Chin. Chem. Soc., Vol. 55, No. 4, 2008 893
Table 6. The results of using different amounts of
H [PMo V O ] in synthesis of benzoxazole
Table 8. Synthesis of benzoxazole derivatives from reaction of 2-
aminophenol with benzoic acids using H [PMo V O ]
5
10
2
40
5
10
2
40
derivatives from reaction of 2-aminophenol with
benzoic acids under refluxing in THF
under refluxing in different solvents
a
Entry
R
Solvent
Yield %
Catalyst amount
a
Entry
R
Yield (%)
1
2
Ph
Ph
Ph
THF
acetonitrile
ethanol
THF
94.8
90
(
mol %)
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
0.5
0.3
0.1
0.5
0.3
0.1
0.5
0.3
0.1
0.5
0.3
0.1
94.9
94.8
93.2
93
3
87
4
4-MeC H4
6
92.7
89
5
4-MeC H4
6
acetonitrile
ethanol
THF
4-MeC H
6
6
4-MeC H4
6
86
4
4
4
4-MeC H
6
92.7
91
7
4-NO C H
6
85.7
82
2
4
4
4
4-MeC H
6
8
4-NO C H
2 6
acetonitrile
ethanol
THF
4-NO C H
2
86
9
4-NO C H
2 6
79
6
4
4
4
4-NO C H
2
85.7
84
10
11
12
a
4-OMeC H4
6
94
6
4-NO C H
2
4-OMeC H4
6
acetonitrile
ethanol
90
6
1
0
4-OMeC H
6
94.2
94
4-OMeC
6
H
4
87
4
4
4
11
4-OMeC H
6
Yields are analyzed by GC
1
2
4-OMeC H
6
92.2
a
Yields are analyzed by GC
Table 9. Synthesis of benzoxazole derivatives from reaction of 2-
aminophenol with benzaldehydes using
Table 7. The results of using different amounts of
H [PMo V O ] in synthesis of benzoxazole
H [PMo V O ] under refluxing in different solvents
40
5
10
2
5
10
2
40
a
Entry
R
Solvent
Yield %
derivatives from reaction of 2-aminophenol with
benzaldehydes under refluxing in THF
1
Ph
Ph
Ph
THF
acetonitrile
ethanol
THF
92
89
2
Catalyst amount
a
Entry
R
Yield (%)
3
85.3
90
(
mol %)
4
4-MeC H4
6
1
2
3
4
5
6
7
8
9
Ph
Ph
Ph
0.7
0.5
0.3
0.7
0.5
0.3
0.7
0.5
0.3
0.7
0.5
0.3
0.7
0.5
0.3
92.3
92
5
4-MeC H4
6
acetonitrile
ethanol
THF
87
6
4-MeC H4
6
83
90
7
4-NO C H
2
82.5
80
6
4
4
4
4-MeC H
6
90.2
90
4
4
4
8
4-NO C H
2
acetonitrile
ethanol
THF
6
4-MeC H
6
9
4-NO C H
2
77
6
4-MeC H
6
88
10
11
12
13
14
15
4-ClC H4
6
88
4-NO C H
2
82.7
82.5
80
6
4
4
4
4-ClC H4
6
acetonitrile
ethanol
THF
86
4-NO C H
2
6
4-ClC H4
6
83.3
91
4-NO C H
2
6
4-OMeC H4
6
1
0
4-ClC H
6
88.3
88
4
4
4
4-OMeC H4
6
acetonitrile
ethanol
89
11
4-ClC H
6
4-OMeC H4
6
85
1
1
1
1
2
3
4
5
4-ClC H
6
86.3
91.4
91
a
Yields are analyzed by GC
4-OMeC H
6
4
4
4
4-OMeC H
6
4-OMeC H
6
89
The solvent molecules can place these molecular orbitals at
the appropriate level. Therefore both yields and reaction
times can be affected by solvent.
a
Yields are analyzed by GC
charge of polyoxoanions, all of the HOMOs and LUMOs
have very high energy levels.
It is suggested that the solvent effects are dominated
by the interactions of the polarized polyanions with the sol-
vent to place the molecular orbitals at the appropriate level
These highly charged heteropolyanions do not exist
in the gas phase, and the external field generated by the sol-
vent is crucial to stabilize them. With regard to the hetero-
polyanions which are easily reducible and required in cata-
lytic reactions, the energy of the LUMO must be suffi-
ciently low to accept the electrons in catalytic reactions.
1
9
and/or to lower the activation energy.
It can also be presumed that in ethanol as a polar and
protic solvent, the amine group in aniline can be proton-
ated, and it shows the lowest affinity toward reaction. In
less polar solvents, higher yields of reactions were obtained.

8
94 J. Chin. Chem. Soc., Vol. 55, No. 4, 2008
Heravi et al.
EXPERIMENTAL
Table 10. The results of using H [PMo V O ] in synthesis of
5
10
2
40
benzoxazole derivatives from reaction of 2-amino-
phenol with benzoic acids in THF five times
Chemicals and Apparatus
All the chemicals were obtained from Merck Com-
a
Yield % (after different times of recycling)
pany and used as received. H
and H 40] were prepared according to reports in
the literature.
4
5 2 40
[PMo11VO40], H [PMo10V O ]
Entry
R
3
[PMo12
O
9,20
First Second Third Fourth Fifth
1
Melting points were measured using
1
2
3
4
a
Ph
94.8
92.7
85.7
94
93
91
91.7
89.4
81
90
88
85.4
78
Barnstead Electro thermal. Yields are based on GC/mass
analysis using an Agilent 6890 GC system Hp-5 capillary
4-MeC
6
H
4
87.6
79.5
88.2
4-NO C H
4
83.6
92.2
2
6
4-OMeC H4
6
90
85.5
3
0 m × 530 mm × 1.5 mm nominal.
Yields are analyzed by GC
General Procedure
Synthesis of benzoxazole derivatives from the reac-
tion of 2-aminophenols with benzaldehydes
Table 11. The results of using H [PMo V O ] in synthesis of
10
5
2
40
A mixture of 2-aminophenol (10 mmol), benzalde-
hydes (10 mmol) and heteropolyacid (0.03 mmol) was
refluxed in proper solvent (10 mL). The progress of the re-
action was monitored by TLC and GC. After completion of
the reaction, the catalyst was filtered off. The pure products
were obtained by column chromatography. All products
were identified by comparison of their physical and spec-
troscopic data with those reported for authentic samples.
To obtain the yields of reactions by GC analysis the
following procedure was used. Different concentrations of
benzoxazole derivatives were prepared and used as stan-
dard samples for GC analysis. The integral of each curve
was calculated automatically. The standard diagram of con-
centration versus area under the curve (integral) for each
benzoxazole derivative was obtained. The concentration of
synthetic benzoxazole samples was found, using the corre-
sponding standard diagram. Knowing the volume of sol-
vent in GC samples the mass of synthesized benzoxazole
derivatives was estimated. By comparing the mass of re-
agents and those of corresponding products, the yield of
reactions can be calculated.
benzoxazole derivatives from reaction of 2-amino-
phenol with benzaldehydes in THF five times
a
Yield % (after different times of recycling)
Entry
R
First Second Third Fourth Fifth
1
2
3
5
6
Ph
92
90
90.5
87.6
82.2
86.2
89.3
88
85
85.3
83.2
78.6
83.3
85
83.2
81
4-MeC H
6
4
4-NO C H
4
82.5
88
80.3
84
76.8
81.5
84
2
6
4-ClC H
6
4
4-OMeC
H
91
87
6
4
a
Yields are analyzed by GC
ported. As shown in Tables 10 and 11 the yields of reac-
tions after using H
reduction.
5 2
[PMo10V O40] five times show a slight
ACKNOWLEDGMENTS
The authors are thankful for the partial financial as-
sistance from Alzahra University Research Council.
Received February 21, 2008.
Synthesis of benzoxazole derivatives from the reac-
tion of 2-aminophenols with benzoic acids
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fluxed in proper solvent (10 mL). The above procedure was
followed to monitor and purify the products.
1
Ishikawa, M.; Nizato, T.; Suzuki, K.; Konno, F. J. Med.
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