Magnetic nanoparticle-supported DABCO tribromide: A versatile nanocatalyst for the synthesis of quinazolinones and benzimidazoles and protection/deprotection of hydroxyl groups

Article abstract of DOI:10.1039/c7nj00479f

1,4-Diazabicyclo[2.2.2]octane tribromide supported on magnetic Fe₃O₄ nanoparticles (MNPs-DABCO tribromide) as a novel heterogeneous tribromide type compound was found to be an efficient and reusable nanocatalyst for the one-pot synthesis of 2-arylquinazolin-4(3H)-ones and 2-aryl-1H-benzo[d]imidazoles through oxidative cyclization of aldehydes with 2-aminobenzamides and 1,2-phenylenediamine, respectively. Also, MNPs-DABCO tribromide catalyzed trimethylsilylation/tetrahydropyranylation and desilylation/depyranylation of a wide variety of alcohols and phenols through changing the solvent medium at room temperature.

Full text of DOI:10.1039/c7nj00479f

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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Magnetic nanoparticles-supported DABCO tribromide: a versatile
nanocatalyst for the synthesis of quinazolinones and
benzimidazoles and protection/deprotection of hydroxyl groups
Received 00th January 20xx,
Accepted 00th January 20xx
Amin Rostami*, Omid Pourshiani, Yahya Navasi, Neda Darvishi and Shaghayegh Saadati
DOI: 10.1039/x0xx00000x
1,4-diazabicyclo[2.2.2] octane tribromide supported on the magnetic nanoparticle Fe3O4 (MNPs-DABCO
tribromide) as a novel heterogeneous tribromide type compound was found to be efficient and reusable
nanocatalyst for one-pot synthesis of 2-arylquinazolin-4(3H)-ones and 2-aryl-1H-benzo[d]imidazoles via
oxidative cyclization of aldehydes with 2-aminobenzamides and 1,2-phenylenediamine, respectively. Also,
www.rsc.org/
MNPs-DABCO
tribromide
catalyzed
trimethylsilylation/tetrahydropyranylation
and
desilylation/depyranylation of a wide variety of alcohols and phenols by changing the solvent medium at
room temperature.
methods which can overcome these drawbacks.
1. Introduction
The use of ‘green oxidants’ such as molecular oxygen and
hydrogen peroxide is attractive, since they are readily available,
Magnetic nanoparticles (MNPs) are ideal supports for the
immobilization of catalysts, since they can be recollected by using
an external magnet. This enables convenient purification avoiding
conventional filtration or centrifugation processes. In view of the
importance of this research area, some reviews on nano-magnetite-
supported catalysts and sustainable protocols are now available [1-
4]. It is an important and urgent need to design magnetically
recyclable nanocatalysts, in benign reaction media.
inexpensive, and environmentally benign, with formation of water
as the only by-product. However, poor oxidizing power of H2O2 is a
main drawback, which needs catalyst in some cases.
The protection and deprotection of functional groups are
indispensable in the synthesis of polyfunctional compounds. Several
chemical conversions are often required for the protection of
hydroxyl
groups
[22].
The
trimethylsilylation
and
tetrahydropyranylation are popular methods widely used for the
protection of the hydroxyl group because of their easy installation,
general stability to most nonacidic reagents, and easy removal
under mild acidic conditions. The protection of the hydroxyl groups
as TMS-ethers and THP-ethers have been achieved in the presence
of various catalysts [23-31]. Although reported catalysts display
considerable progress, the separation and reuse of the catalysts are
difficult.
As an alternative reagent or catalyst for the toxic and corrosive
liquid bromine, a wide variety of organic ammonium tribromides
(OATBs) were designed [5-8]. However, recoverability and
reusability of OATBs catalysts are high challenge for their
application in organic reactions. A possible strategy to circumvent
this problem is immobilization of these reagents or catalysts on
MNPs, which can be easily removed from the reaction mixture by
magnetic separation.
In continuation of our studies on preparation and application of
magnetic nanocatalysts [32-34], herein, we report the catalytic
Heterocycle compounds such as quinazolinones and
benzimidazoles are very important building blocks found in
biologically and pharmacologically active compounds, natural
products, and functional materials [9-10]. Therefore, the
development of efficient methods for the synthesis of these
valuable compounds is very important. The cyclocondensation of
aldehydes with o-anthranilamides and o-phenylenediamine,
followed by subsequent oxidation is the main synthetic approach
for the synthesis of quinazolinones [11-14] and benzimidazoles [15-
20], respectively. The major disadvantages of these methods
include recovery and reuse of the expensive catalysts, the use of
excess amounts of toxic and expensive oxidants (especially for
synthesis of quinazolines), metal residues in the products, the
formation of large amounts of toxic waste and requiring tedious
purification processes. Hence, there is a need to develop new
activity of MNPs-DABCO tribromide as
a first example of
magnetically recoverable OATBs type catalyst for the synthesis of
quinazolinones and benzimidazoles and protection/deprotection of
hydroxyl groups.
2. Results and discussions
2.1. Characterization of MNPs-DABCO tribromide
The catalyst was characterized by FT-IR, EDX, TGA, XRD, SEM and
VSM techniques. The FT-IR spectrum of MNPs-DABCO tribromide
shows peaks that are characteristic of DABCO tribromide-
unfunctionalized Fe3O4 magnetic nanoparticles (Fig. S1). The EDX
spectrum shows the elemental composition (Fe, O, Si, N, Br and Cl)
of the MNPs-DABCO tribromide (Fig. S2). One indication of bond
formation between MNPs and the DABCO tribromide can be
inferred from TGA. The TGA curve of the MNPs-DABCO
tribromide show the mass loss of the organic functional group
Department of Chemistry, Faculty of Science, University of Kurdistan, Zip Code 66177-
15175, Sanandaj, Iran(E-mail: a_rostami372@yahoo.com, Tel: +989183730910; Fax:
+988733624004)
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as it decompose upon heating (Fig. S3). The loading of the H2O2 as green oxidant in the presence of MNPs-DABCO tribromide
DABCO tribromide function in MNPs-DABCO tribromide can be
calculated from TGA curve, which confirmed a loading of
approximately 0.22 mmol/g. The position and relative
intensities of all peaks in the XRD pattern of MNPs-DABCO
tribromide confirm well with standard XRD pattern of Fe3O4,
indicating retention of the crystalline cubic spinel structure
during functionalization of MNPs (Fig. S4) [35]. Fig. S5 shows
the SEM image of the MNPs-DABCO tribromide. It was
confirmed that the catalyst was made up of uniform
nanometer-sized particles less than 26 nm. The VSM pattern of
as a magnetically reusable nanocatalyst was investigated (Scheme
1).
Scheme 1. MNPs-DABCO tribromide catalyzed the synthesis of 2-
arylquinazolin-4(3H)-one in the presence of H2O2
In order to optimize the reaction conditions, the influence of
MNPs-DABCO tribromide
properties of magnetic nanoparticles (Fig. S6) and can be catalyst type, temperature, solvent and different amounts of
indicate superparamagnetic
efficiently attracted with a small magnet.
hydrogen peroxide and catalyst on the synthesis of 2-
phenylquinazolin-4(3H)-one was evaluated (Table 1). As shown in
Table 1, employing 50 mg of MNPs-DABCO tribromide as a
catalyst, 2.5 equiv. of H2O2 as an oxidant and EtOH as a solvent
under reflux conditions gave the best results (Table 1, entry 6).
2.1. The catalytic applications of MNPs-DABCO tribromide for the
synthesis of 2-arylquinazolin-4(3H)-one and 2-arylbenzimidazole
compounds
Initially, the synthesis of 2-arylquinazolin-4(3H)-one derivatives
from anthranilamide and aldehydes via oxidative cyclization using
Table 1. Synthesis of 2-phenylquinazolin-4(3H)-one under different conditions
Entry
Catalyst (mg)
H2O2 (equiv.)
Solvent
EtOH
EtOH
ETOH
EtOH
EtOH
EtOH
EtOH
T (˚C)
reflux
reflux
reflux
reflux
rt
Time (h)
40
Isolated Yield (%)
1
2
3
4
5
6
7
Catalyst-free
2.4
2.4
2.4
-
-
Fe3O4 MNPs (50)
24
75
DABCO-bromine (50)
MNPs-DABCO tribromide (50)
MNPs-DABCO tribromide (50)
MNPs-DABCO tribromide (50)
MNPs-DABCO tribromide (30)
24
60
36
Trace
Trace
90
2.4
2.4
2.4
24
reflux
reflux
9
17
75
Solvent-
free
8
MNPs-DABCO tribromide (50)
2.4
reflux
24
20
9
MNPs-DABCO tribromide (50)
MNPs-DABCO tribromide (50)
2.4
2.4
H2O
reflux
reflux
24
24
20
40
10
MeOH
In order to generalize the scope of the reaction, a series of to oxidative cyclization under optimized reaction conditions, and
structurally diverse aldehydes with anthranilamide were subjected the results are presented in Table 2.
2 | J. Name., 2012, 00, 1-3
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Table 2. MNPs-DABCO tribromide (50 mg) catalyzed the synthesis of 2-phenylquinazolin-4(3H)-one derivatives using 30% H2O2 (2.4
equiv.) in EtOH (2 mL) under reflux conditions
Entry
Aldehyde
Product
Time (h)
Isolated yield (%)
Mp /°C (Lit.)
1
9
90
239-240 (237-238[11])
2
48
76
235-239 (239-240 [37])
3
48
79
229-231
4
5
17
48
80
62
236-238 (238-239 [11])
269-271(268-269 [36])
6
7
17
24
80
68
200-203 (196-197[12])
215-217 (223-226[37])
8
48
70
314-316 (> 300[11])
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The proposed mechanism for the preparation of 2-
In another effort, we studied the catalytic application of MNPs-
arylquinazolinones is shown in Scheme 2. On the basis of our DABCO tribromide for the synthesis of 2-arylbenzimidazole
previously reported mechanism [32], we believe that MNPs-DABCO derivatives from 1,2-phenylenediamine and aromatic aldehydes via
tribromide act as a bromine source. The bromine can act as both oxidative cyclization reaction in CH3CN under reflux conditions
Lewis acid catalyst and oxidant, in the cyclization step, bromine act (Scheme 3).
as Lewis acid catalyst and in oxidation step it is an oxidizing agent.
That the actual oxidant is Br2 and not H2O2 was confirmed in control
experiment [14]. When the reaction was performed in the absence
Scheme 3. The synthesis of 2-arylbenzimidazole derivatives in the
presence of MNPs-DABCO tribromide as a catalyst
of MNPs-DABCO tribromide no product was obtained even after
prolonged reaction time (Table 1, entry 1).
The investigation of the reaction conditions for the synthesis of
2-phenyl benzimidazole from the reaction of 1,2-phenylenediamine
and benzaldehyde demonstrated that 50 mg of the catalyst in
CH3CN under reflux conditions was optimal for obtaining the
desired reaction (Table 3, entry 5).
Scheme 2. Proposed mechanism for the synthesis of 2-arylquinazolinones
from 2-aminobenzamide and aldehydes in the presence of MNPs-DABCO
tribromide and H2O2
Table 3. Optimization of the reaction conditions for the synthesis of 2-phenyl benzimidazole from the
reaction of 1,2-phenylenediamine (1 mmol) and benzaldehyde (1 mmol)
Entry
Catalyst (mg)
Solvent
T (˚C)
Time (h)
Isolated yield (%)
1
2
none
CH3CN
CH3CN
reflux
reflux
10
5
80
80
Fe3O4 MNPs (50)
3
DABCO-bromine (50)
CH3CN
reflux
5
90
4
5
MNPs-DABCO tribromide (50)
MNPs-DABCO tribromide (50)
CH3CN
CH3CN
r.t.
10
-
reflux
2.5
91
6
MNPs-DABCO tribromide (40)
CH3CN
reflux
4
85
Under the optimized conditions, reactions of various aromatic arylbenzimidazoles (Table 4). Substitution groups on the aromatic
aldehydes bearing either electron-donating groups or electron- ring have no obvious effect on the yields and reaction times. In all
withdrawing groups on the aromatic ring with 1,2- cases, the reactions gave the corresponding products in short
phenylenediamine were studied to afford a diverse range of 2- reaction times and good to excellent yields.
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Table 4. MNPs-DABCO tribromide catalyzed the synthesis of 2-arylbenzimidazoles in CH3CN under reflux conditionsa
Entry
Aldehyde
Product
Time (h)
Isolated yield (%)
Mp /˚C (Lit.)
1
2:30
91
291-292 (290-292 [16])
2
3
1:50
90
80
250-256 (181–184 [16])
228-229 (232 [15])
6
4
5
6
7
1:20
3:25
4:30
2:45
92
89
90
91
285-287 (287-288 [16])
222-224 (226 [16])
295-297 (292-293 [16])
244-247 (248 [15])
8
9
1
84
82
88
224-227 (224-226 [17])
263-265 (264 [20])
198-200 (203 [15])
2:20
3:30
10
11
12
7
3
84
46
229-232 (231 [18])
312 (>300 [19])
aReaction conditions: aldehyde (1 mmol), 1,2-phenylenediamine (1 mmol), MNPs-DABCO tribromide (50 mg), CH3CN (2 mL), reflux.
To check the leaching of catalyst, hot filtration test was performed the reaction mixture after 1 h in a hot conditions and the mixture
for the synthesis of 2-phenylbenzimidazole from benzaldehyde (1 was allowed to react further for 2:30 h. The GC yield of the product
mmol) and 1,2-phenylenediamine (1 mmol) in the presence of at this stage (1h) was 50%. After 1 h, there was no significant
MNPs-DABCO tribromide (50 mg) in CH3CN (2 mL) under reflux increase in the yield of the product. This study clearly demonstrated
conditions. The catalyst was removed by an external magnet from that there was no significant amount of leaching.
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We examined also the recycling of MNPs-DABCO tribromide for the
synthesis of 2-phenyl benzimidazole. After the completion of the
reaction, the catalyst was separated easily and rapidly from the
product by exposure to an external magnet (within 5s), and the
reaction solution was decanted. The remaining magnetic
nanoparticles were further washed with the CH3CN (5 mL), dried
under vacuum, and subjected to the next run. The recycled catalyst
was used for 9 runs without any significant loss of activity (Fig. 1).
The investigation of the reaction conditions for the protection of
benzyl alcohol as a model with HMDS in the presence of catalytic
amount of MNPs-DABCO tribromide demonstrated that 10 mg of
the catalyst and 1 mmol of HMDS under solvent-free conditions at
room temperature were optimal for the desired trimethylsilylation
(Table 5, entry 4).
Table 5. Trimethylsilylation of benzyl alcohol (1 mmol) using HMDS, in the
presence of various catalysts under
temperature
a variety conditions at room
HMDS
Time
(min)
Isolated
yield (%)
Entry
Catalyst (mg)
Solvent
SF
(mmol)
1
2
Fe3O4 MNPs (10)
1
120
120
95
95
DABCO-bromine
(10)
1
1.2
1
SF
MNPs-DABCO
3
4
5
6
7
8
9
SF
SF
50
50
95
95
95
95
95
95
75
tribromide (10)
MNPs-DABCO
tribromide (10)
Fig. 1. Recycling of MNPs-DABCO tribromide for the reaction of 1,2-
phenylenediamine and benzaldehyde in CH3CN under reflux conditions for
2:30 hour
MNPs-DABCO
tribromide (5)
1
SF
100
150
25
On the basis of previously reported mechanisms, one explanation
for this process is that MNPs-DABCO tribromide act as a source of
Br2 which makes the carbonyl and imine groups susceptible to
nucleophilic attack (act as Lewis acid catalyst) for cyclization step
and in the second step the actual oxidant is air [38-39]. Another
explanation is that the oxidation step proceeds via anomeric based
oxidation [40].
MNPs-DABCO
0.7
1
SF
tribromide (10)
MNPs-DABCO
CH3CN
tribromide (10)
MNPs-DABCO
1
CH3COCH3
CH2Cl2
25
tribromide (10)
MNPs-DABCO
1
160
tribromide (10)
Scheme 4.
A possible mechanism for the synthesis of 2-phenyl-1H-
benzo[d]imidazole catalyzed by MNPs-DABCO tribromide
With the optimal reaction conditions in hand, we then preceded
to investigate the scope of this new silylation protocol for other
hydroxyl groups (Table 6). As shown in Table 6, primary benzylic
(including electron releasing or withdrawing groups and acid
sensitive groups), allylic and linear alcohols were silylated with good
2.2. The catalytic application of MNPs-DABCO tribromide in the
protection and deprotection of hydroxyl groups as trimethylsily
ether
Based on our previous experience about application of N-halo
catalysts [41-42] in the protection and deprotection hydroxyl
groups, we have tested the catalytic activity of MNPs-DABCO to high yields (entries 1-10). The benzylic and cyclic secondary
tribromide in the trimethylsilylation of a wide range of alcohols and
phenols using HMDS; a cheap, stable and readily available silylating
agent, under solvent-free conditions and their deprotection to the
corresponding parent compounds in methanol at room
temperature (Scheme 5).
alcohols are successfully protected using this method by increasing
the molar ratio of HMDS (entries 11-13). This method was also
found to be useful for trimethylsilylation of phenolic compounds
(entries 15-20). Tertiary alcohol did not react using this method
even after prolonged reaction time (entry 14). Unlike some acid
catalysts that have been reported for the protection of hydroxyl
group, no elimination and rearrangement by-products were
observed at all. The substrates with multiple bonds (entry 8) were
trimethylsilylated without any addition product of bromine as side
products. The possibility of deprotection of trimethylsilyl ethers
using MNPs-DABCO tribromide as catalyst was also investigated. It
was found that the trimethylsilyl group could be removed efficiently
in the presence of a catalytic amount of MNPs-DABCO tribromide in
Scheme 5. MNPs-DABCO tribromide catalyzed the protection/deprotection
of the hydroxyl group as TMS-ether.
6 | J. Name., 2012, 00, 1-3
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methanol at room temperature in good to excellent yields (Table 6).
It is interesting to note that the deprotection of the most
compounds reported in the present study was completed within 5
min.
Table 6. MNPs-DABCO tribromide catalyzed trimethylsilylation and desilylation of alcohols and phenols.
Entry
Substrate
TMS protectiona,b
Deprotectionb,c
Time (min)
Isolated yield (%)
Time (min)
Isolated yield (%)
1
2
PhCH2OH
4-(MeO) C6H4CH2OH
4-(F) C6H4CH2OH
2-(Cl) C6H4CH2OH
2,4-(Cl)2C6H4CH2OH
4-(NO2) C6H4CH2OH
Furfuryl alcohol
PhCH=CHCH2OH
PhCH2CH2OH
50
130
40
95
90
5
5
5
5
5
5
5
5
5
5
5
96
80d
90
3
97
4
80
93
92
5
150
250
30
95
97
6
94
96
7
90e
85d,e
89
93
8
20
92
9
130
480
360
90
10
11
CH2(CH2)6CH2OH
1-Indanol
87
80d
80d
92d
12
13
120
150
90d
95d
5
5
85d
50d
e
-
14
480
-
-
15
16
17
18
19
20
C6H5OH
60
60
90
93
96
97
90
94
5
5
92
92
94
95
96
95
4-(OMe) C6H5OH
4-(Br) C6H5OH
2,4-(Cl)2 C6H5OH
4-(NO2) C6H4OH
2-Naphthol
60
10
5
80
230
150
5
5
aReaction conditions: alcohol/phenol (1 mmol), HMDS (1 mmol), catalyst (10 mg), solvent-free, r.t. bAll products were
characterized by comparison of their spectral (1H NMR and FT-IR spectroscopies) with those of authentic samples [23, 25].
cReaction conditions: trimethylsilylether (1 mmol), catalyst (10 mg), MeOH (2 mL), r.t. dReaction was not completed even
after prolong time.e3 mmol of HMDS was used.
The reusability of MNPs-DABCO tribromide was investigated on recyclability. After the first use of MNPs-DABCO tribromide, the
the trimethylsilylation of benzyl alcohol as model reaction. We have reaction mixture was diluted with Et2O and the catalyst was easily
found that this catalyst demonstrated remarkably excellent and rapidly separated from the product by exposure to an external
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magnet and decantation of the reaction solution. Then, the reaction
Owing to the success of MNPs-DABCO tribromide for carrying out
vessel was charged with fresh substrate and HMDS and subjected to different reactions, we have studied the possibility of applying this
the next under similar reaction conditions. The recycled catalyst catalyst for the tetrahydropyranylation/depyranylation of various
was used for up to 10 runs with little loss of activity (Fig. 2).
alcohols and phenols (Scheme 6).
Scheme 6. Tetrahydropyranylation and depyranylation of alcohols and
phenols in the presence of catalytic amount of MNPs-DABCO tribromide.
A wide range of various alcohols and phenols with different
substituents were converted to their corresponding THP ethers by
the treatment of 3,4-dihydro-2H-pyran (DHP) with MNPs-DABCO
tribromide catalyst in dichloromethane medium at room
temperature in moderate to excellent yields, as shown in Table 7.
The deportation of THP ethers can be carried out to obtain the
corresponding alcohols and phenols by merely changing the solvent
from dichloromethane to methanol (Table 7).
Fig. 2. The recycling experiment of MNPs-DABCO tribromide for the
trimethylsilylation of benzyl alcohol (1 mmol) using HMDS (1 mmol) under
solvent-free conditions at room temperature for 50 min.
2.3. The catalytic application of MNPs-DABCO tribromide in the
tetrahydropyranylation/depyranylation of hydroxyl groups
Table 7. MNPs-DABCO tribromide catalyzed tetrahydropyranylation and depyranylation of alcohols and phenols.
Entry
Substrate
THP protectiona,b
Deprotectionb,c
Time (min)
Isolated yield (%)
Time (min)
Isolated yield (%)
1
2
PhCH2OH
4-(MeO) C6H4CH2OH
4-(F) C6H4CH2OH
2-(Cl) C6H4CH2OH
2,4-(Cl)2 C6H4CH2OH
4-(NO2) C6H4CH2OH
Furfuryl alcohol
PhCH=CHCH2OH
PhCH2CH2OH
100
240
300
360
240
360
120
240
480
150
420
93
92
95
93
95
85 d
-
180
270
240
180
180
240
-
94
95
3
80d
95
4
5
97
6
76d
-
7
8
60 d
93
76 d
95
180
240
240
60
50 d
92
9
10
11
CH2(CH2)6CH2OH
1-Indanol
80 d
83
12
120
87
240
90
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Table 7 (Continued)
Entry
Substrate
THP protectiona,b
Deprotectionb,c
Isolated yield (%)
Time (min)
Isolated yield (%)
Time (min)
13
14
360
-
95
-
360
-
25 d
-
15
16
17
18
19
20
C6H5OH
480
480
480
480
180
60
94
92
130
180
120
5
95
93
94
95
45d
95
4-(OMe) C6H5OH
4-(Br) C6H5OH
2,4-(Cl)2 C6H5OH
4-(NO2) C6H4OH
2-Naphthol
96
55
57 d
56d
180
60
aReaction conditions: alcohol/phenol (1 mmol), DHP (1.5 mmol), catalyst (50 mg), CH2Cl2, r.t. bAll products were
characterized by comparison of their spectral (1H NMR and FT-IR spectroscopies) with those of authentic samples [24, 27].
cReaction conditions: tetrahydropyranyl ether (1 mmol), catalyst (30 mg), MeOH (2 mL), r.t. dReaction was not
completed even after prolong time
.
Selectivity of a method is important in the multistep preparation observed under reaction conditions (Table 8, entries 1-3). Benzyl
of organic compounds and provides a broad spectrum for its uses in alcohol was also protected in the presence of 1-indanol (secondary
functional group transformation reactions. As shown in Table 7, the alcohol) with the ratio of 100:0 (Table 8, entry 4). In order to
reaction time and product yield depend on the nature of the establish the chemoselective THP-protection of alcohols in the
hydroxyl compound used as substrate, these result led us to carry presence of phenolic hydroxyl group, we reacted 2-(hydroxymethyl)
out competitive reactions in order to evaluate the selectivity of the phenol with DHP under reaction conditions. As shown in Table 8
protocol. The benzyl alcohol was surprisingly converted to the (entry 5), alcoholic -OH has been protected and phenolic -OH
corresponding THP-ether in 90% yield, while only 10 % of the THP- remained intact during the protection reaction.
ether of 4-nitrobenzyl alcohol, cinamylalcohol, or 1-octanol was
Table 8. Selective tetrahydropyranylation of a benzyl alcohol (1 mmol) in the presence of primary, secondary
alcohols or phenol (1 mmol) using DHP (1.5 mmol) catalyzed by MNPs-DABCO tribromide (30 mg) in CH2Cl2 (1
mL).
Entry
Substrate
THP-ether
Time (min)
GC Yield
90
1
100
10
90
2
100
10
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Table 8 (Continued)
Entry
Substrate
THP-ether
Time (min)
100
GC Yield
90
3
4
10
100
100
0
100
5
180
0
3. Conclusions
In conclusion, MNPs-DABCO tribromide as
a
robust, stable,
heterogeneous, excellent bromine carrier and magnetically OATBs
compound catalyzed for the synthesis of 2-arylquinazolin-4(3H)-
ones and 2-aryl-1H-benzo[d]imidazoles and the selective protection
and deprotection of alcohols and phenols as TMS-ethers or THP-
ethers. These procedures have several advantages: (1) the
preparation of MNPs-DABCO tribromide is simple from easily
available compound; (2) the separation of MNPs-DABCO tribromide
is rapid, simple and efficient by using an appropriate external
magnet, which minimizes the loss of catalyst during separation; (3)
the green and inexpensive oxidants (H2O2 and O2) are used; (4)
MNPs-DABCO tribromide is reused for several times with little loss
of activity; these significant features make it as a promising material
for practical and large-scale applications. Studies aimed at
extending the scope of MNPs-DABCO tribromide to other useful
transformations are currently underway in our laboratories.
Scheme 7. The preparation of MNPs-DABCO tribromide
4.2. General procedures for the synthesis of 2-arylquinazolin-
4(3H)-one compounds
Aldehyde (1 mmol) and 2-aminobenzamide (1 mmol) were added
to a mixture of hydrogen peroxide 30% (2.4 equiv.) and MNPs-
DABCO tribromide (50 mg) in EtOH (5 mL); then, the mixture was
refluxed at 78 °C for the specified time (Table 2). The progress was
monitored by TLC (n-hexane/EtOAc, 2/1). After the completion of
the reaction, the catalyst was separated from the product by an
external magnet, and the mixture was washed with EtOH (2×2 mL)
and decanted. After the evaporation of EtOH under reduced
pressure, a crude product was obtained. The crude products were
purified by preparative thin-layer chromatography using (n-
hexane/ethylacetate, 2:1) as an eluent system. The pure products
were obtained with a yield of 62-90% and were characterized by FT-
IR, 1H NMR, and by melting point comparisons with those of
authentic samples [11, 12, 36, 37].
4. Experimental
4.1. Preparation of MNPs-DABCO tribromide
MNPs-DABCO tribromide, as an organic-inorganic hybrid
heterogeneous nanocatalyst, is prepared by a concise route from
inexpensive commercially available starting material (Scheme 7)
[32].
4.3. General procedure for synthesis of 2-(phenyl)-1H-
benzo[d]imidazole compounds
10 | J. Name., 2012, 00, 1-3
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MNPs-DABCO tribromide (50 mg) was added to a mixture of
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We are grateful to the University of Kurdistan Research Councils
aromatic aldehyde (1 mmol) and 1,2-phenylenediamine (1 mmol) in for partial support of this work.
acetonitrile (2 mL) and the mixture was stirred and heated in an oil
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Acknowledgments
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Table of contents entry
MNPs-DABCO tribromide as a magnetically reusable tribromide type nanocatalyst couples the
advantages of heterogeneous and homogeneous tribromide based systems