Silica vanadic acid [SiO2-VO(OH)2] as an efficient heterogeneous catalyst for the synthesis of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-one and 2,4,6-triarylpyridine derivatives via anomeric based oxidation

Article abstract of DOI:10.1039/c5ra21392d

Silica-bonded vanadic acid [SiO₂-VO(OH)₂] (SVA) efficiently catalyzed the synthesis of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-ones via reacting aromatic aldehydes and β-naphthol and urea under solvent-free condition. Additi

Full text of DOI:10.1039/c5ra21392d

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ARTICLE TYPE
Silica vanadic acid [SiO2–VO(OH)2] as an efficient heterogeneous
catalyst for the synthesis of 1,2-dihydro-1-aryl-3H-naphth[1,2-
e][1,3]oxazin-3-one and 2,4,6-triarylpyridine derivatives via anomeric
based oxidation
†
Mohammad Ali Zolfigol,*a Maliheh Safaiee,b Fatemeh Afsharnadery,a Neda Bahraminejad,a Saeed
5
Baghery,a Sadegh Salehzadeha and Farahnaz Malekia
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
tautomeric character of 1,3ꢀO,Nꢀheterocycles suggestions
a great number of synthetic potentials.14ꢀ16 [BMIM]HSO4
as an ionic liquid, nꢀtetrabutylammonium bromide as a
Silicaꢀbonded vanadic acid [SiO2–VO(OH)2] (SVA) efficiently
catalyzed the synthesis of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀ 10
e][1,3]oxazinꢀ3ꢀones via reacting aromatic aldehydes and βꢀ
naphthol and urea under solventꢀfree condition. Additionally,
2,4,6ꢀtriarylpyridine derivatives were prepared through the
condensation of ammonium acetate, aromatic aldehyde and
various acetophenone in the presence of a catalytic amount of 15
[SiO2–VO(OH)2] (SVA) under a same condition. A new
anomeric based oxidation was proposed for the final step of
the 2,4,6ꢀtriarylpyridine derivatives synthesis which was
supported by theoretical studies. So the described new
mechanistic approach can open up a new and promising 20
insight in the course of rational design, synthesis and
applications of novel hydride releasing compounds. The
significant features of the present procedure are short reaction
time, high yields, easy workꢀup, and recyclability of the SVA
phase transfer catalyst in water and sodium hydrogen 50
sulfate (NaHSO4) have been applied as mild catalysts for
the cyclocondensation of formaline, βꢀnaphthol and
aromatic amines to give the corresponding 2,3ꢀdihydroꢀ2ꢀ
phenylꢀ1Hꢀnaphth[1,2–e][1,3]oxazines.17
using the threeꢀcomponent system
Furthermore,
(βꢀnaphthol, 55
benzaldehyde and urea) using HClO4ꢀSiO2.18 Additional
reports exhibited the condensation of βꢀnaphthol with
hetero aryl aldehydes or substituted benzaldehydes using
ammonia.19
Pyridine ring system, mainly 2,4,6ꢀtriarylpyridine is of 60
enormous attention due to its unique position in medicinal
chemistry.20, 21 Moreover, the superb thermal stabilities of
these pyridines have prompted a developing attention for
their use as monomeric building blocks in thin films and
organometallic polymers.22 These compounds have also 65
been evoked significant consideration recently as these
donated with broad range of pharmaceutical activities for
catalyst.
25
Introduction
Solid acids have been used as catalysts in various functional
groups transformations because catalysis has played a
major role in the preventing pollution in our environment.
example
anesthetic,
vasodilator,
antiꢀcovulsant,
antimalarial, antiepileptic and agrochemicals for example
pesticidial, fungicidal and herbicidal.21 These compounds 70
have now been prepared through reaction of Nꢀ
Recently, a wide range of solid acids and their supported 30
forms reported in the last few years for various chemical
processes have been reviewed.1 Owing to their possible
utilizations as an alternative for liquid inorganic acids in
industry, solid acid catalysts have received main attention,
they reveal the advantages of easy recycle of the catalyst 35
from the liquid reaction media, reuseability, least
corrosion, green chemical procedure, and increase product
selectivity.2ꢀ5
phenacylpyridinium salts with α,βꢀunsaturated ketones
24
using ammonium acetate.23,
Though, the pyridinium
salts and the unsaturated ketones have to be prepared first,
so this process is relatively expensive. Lately, numerous 75
novel developed approaches and processes for preparation
of 2,4,6ꢀtriarylpyridines have been studied, such as
reaction of αꢀketoketene dithioacetals with methyl ketones
using ammonium acetate,25 solventꢀfree reaction of
Molecules with oxazine moieties have significant biological
properties, counting antiꢀconvulsant, antiꢀbacterial, antiꢀ 40
tubercular, analgesic and antiꢀcancer,6ꢀ11 and possess
attracted far more attention because of their therapeutic
chalcones
with
NH4OAc,26
reaction
of
Nꢀ 80
phosphinylethanimines with aldehydes,27 solventꢀfree
reaction of benzaldehydes, acetophenones and NH4OAc
using several catalysts, such as I2,28 HClO4ꢀSiO2,29
Preyssler type heteropolyacid30 and the oneꢀpot reaction of
benzaldehydes, acetophenones and NH4OAc under 85
13
potential for the treatment of Parkinson's illness.12,
Moreover, they can be applied as intermediates in the
preparation of Nꢀsubstituted amino alcohols or in 45
enantioselective syntheses of chiral amines. The
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catalystꢀfree microwave irradiation.31
ammonium acetate was established to be satisfactory and
hence the molar ratio of 4ꢀchlorobenzaldehyde and 4ꢀ
methoxyacetophenone to ammonium acetate was studied
Afterward, to overcome those difficulties and in our
continuous attention to the design, synthesis, applications
and knowledgeꢀbased development of solid acids,32
inorganic acid salts,33 nanoꢀsphere silica with acidic ionic
liquid tag,34 novel nano structure, green and benign acidic
ionic liquids, molten salts and organo catalysts for organic
functional group transformations,35 and also our
investigations in the synthesis of aromaticꢀcondensed
at 1:2:5.
60
Table 1. Result of amount of the catalyst and temperature on the condensation
5
reaction of 4ꢀchlorobenzaldehyde, 4ꢀmethoxyacetophenone and ammonium acetate
under solventꢀfree conditions.a
Catalyst amount
(mg)
Reaction temperature
(oC)
Reaction time
(min)
Yieldb
(%)
Entry
oxazinones and 2,4,6ꢀtriarylpyridine derivatives,36 we 10
herein consider an suitable approach for silicaꢀbonded
vanadic acid [SiO2–VO(OH)2] (SVA)37 as an effective
catalyst for the preparation of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀ
naphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone and 2,4,6ꢀtriarylpyridine
1
2
—
—
1
100
130
100
130
r.t.
120
120
120
120
120
120
120
120
60
21
33
45
57
—
18
43
75
88
88
88
75
88
88
3
4
1
5
2
derivatives under solventꢀfree conditions (Scheme 1).
15
20
25
6
2
50
Silica-bonded vanadic acid [SiO2-VO(OH)2] (SVA)
OH
O
7
2
75
V
SVA (0.002 g), Solvent-free, 130 o
C
SiO2
O
HO
8
2
100
125
130
150
100
130
150
NH4OAc
9
2
O
O
1
OH
O
3
10
11
12
13
14
2
45
5
+
O
H
H
+
CH3
2
45
2
2
H2N NH2
6
R
R
R': H, Me, OMe
5
120
45
R'
5
R
5
45
O
Reaction
conditions:
a4ꢀChlorobenzaldehyde
(1
mmol),
4ꢀ
N
O
7
R'
R'
H
N
4
methoxyacetophenone (2 mmol), ammonium acetate (5 mmol); bIsolated 65
R
yields.
Scheme 1. The synthesis of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀ
To compare the result of the solution in comparison with
one and 2,4,6ꢀtriarylpyridine derivatives in the presence of silica vanadic 30
acid [SiO2–VO(OH)2] (SVA) catalyst.
solventꢀfree
conditions,
a
mixture
of
4ꢀ
chlorobenzaldehyde,
4ꢀmethoxyacetophenone
and
ammonium acetate as a typical reaction, in the presence of 70
2 mg of [SiO2–VO(OH)2] as a catalyst in several solvents
for example H2O, C2H5OH, CH3CN, CH3CO2Et, CH2Cl2,
nꢀhexane and toluene was studied under reflux conditions.
Moreover, reaction was done under solventꢀfree condition
Results and discussion
In continuation of our studies on the knowledgeꢀbased
development of new silicaꢀbased resins,32 we synthesized
SVA according to the our previously reported procedure 35
as follow (Scheme 2).37
o
at 130 C. The results are reported in Table 2. As it can be 75
exposed in Table 2, solventꢀfree condition was the best
O
V
O
V
O
V
Cl
Cl
SiO
SiO2
nꢀHexane
r.t
OH
+
air
2
SiO22
O
OH
OH
SiO2
O
Cl
Brönsted acid site
Cl
I
Cl
condition for this purpose.
II
III
Lewis acid site
SVA with a dual Lewis and Bronsted acid ability
Table 2. Reaction of 4ꢀchlorobenzaldehyde, 4ꢀmethoxyacetophenone and
40
ammonium acetate catalyzed by [SiO2–VO(OH)2] (2 mg) in various solvents under
Scheme 2. The synthesis of silica vanadic acid [SiO2–VO(OH)2] (SVA)
reflux conditions. Reaction was done under solventꢀfree condition at 130 oC.a
80
catalyst.
Entry
Solvent
Reaction time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
Solventꢀfree
H2O
45
60
45
60
60
60
60
60
88
45
73
73
67
58
65
63
Initially, to optimize the reaction conditions, the condensation
reaction
of
4ꢀchlorobenzaldehyde,
4ꢀ
methoxyacetophenone and ammonium acetate were 45
selected as a model reaction and different amounts of
[SiO2–VO(OH)2] as a catalyst at temperature range of 25–
150 °C were verified on it under solventꢀfree conditions
(Table 1). As revealed in Table 1, the best results were
achieved when the reaction was attained using 2 mg of 50
SVA catalyst at 130 oC (Table 1, entry 10). No
improvement was known in the yield of reaction through
increasing the amount of catalyst and temperature (Table
1, entries 11ꢀ14). Table 1 visibly exhibitions that in the
absence of catalyst, the product was in low yield 55
synthesized (Table 1, entries 1 and 2). An excess of the
C2H5OH
CH3CN
CH3CO2Et
CH2Cl2
nꢀHexane
Toluene
Reaction
conditions:
a4ꢀChlorobenzaldehyde
(1
mmol),
4ꢀ
methoxyacetophenone (2 mmol), ammonium acetate (5 mmol); bIsolated
yields.
Subsequently recognizing the optimized reaction conditions,
2
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the investigation was followed by performing the reaction
of aromatic aldehydes with acetophenones and ammonium
acetate. To demonstrate the general applicability of this
process, several aldehydes were efficiently reacted with
acetophenones and ammonium acetate under similar
conditions. These results encouraged us to investigate the
scope and the outline of this procedure for numerous
aldehydes under optimized conditions. As shown in Table
properties of the substituents on the aromatic ring effect
the alteration rate, and aromatic aldehydes having
electronꢀwithdrawing groups on the aromatic ring react 15
faster than electronꢀdonating groups. Also, according to
the reaction time and isolated yields, the order of
reactivity in the acetophenones are as follows: 4ꢀ
5
methoxyacetophenone
>
4ꢀmethylacetophenone
>
acetophenone. Inorder to evaluate the efficiency of SVA 20
catalyst on the synthesis of 2,4,6ꢀtriarylpyridines TOF
values were calculated via the equation TOF= Yield
(%)/[Time (min)×Catalyst amount (mol%)] (2 mg of
3,
a wide range of aromatic aldehydes underwent
electrophilic substitution reaction with acetophenones and 10
ammonium acetate to afford various substituted 2,4,6ꢀ
triarylpyridines in good yields. The nature and electronic
catalyst is equivalent to 0.0156 mol% of catalyst37).
Table 3. Synthesis of 2,4,6ꢀtriarylpyridines in the presence of [SiO2–VO(OH)2] as a catalyst.a
25
Entry
Aldehyde
Ketone
Time (min)
Yieldb (%)
TOF
M.p (oC) [Lit.]Ref.
1
45
88
125.36
110ꢀ112 [113.8ꢀ115]38a (Yellow solid)
2
3
4
45
45
60
85
81
83
121.08
115.38
88.68
198ꢀ200 [200.6ꢀ202]31 (Yellow solid)
133ꢀ135 [129ꢀ130]27 (Yellow solid)
135ꢀ137 [136ꢀ137]38b (Yellow solid)
5
6
7
60
60
45
82
82
88
87.61
87.61
175ꢀ177 [178ꢀ180]38c (Yellow solid)
153ꢀ15538d (Yellow solid)
125.36
140ꢀ142 [143.1ꢀ144.7]31 (Yellow solid)
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8
45
85
121.08
233ꢀ235 (Green solid)
9
60
60
81
81
86.54
86.54
153ꢀ155 [156ꢀ157.5]38e (Yellow solid)
10
136ꢀ138 [134ꢀ135]26 (White solid)
Reaction conditions: aAldehydes (1 mmol), acetophenones (2 mmol), ammonium acetate (5 mmol), [SiO2–VO(OH)2] as a catalyst (2 mg), Solventꢀfree, 130 oC;
bIsolated yields.
Acording to the previously reported method, a reasonable
mechanism for the preparation of 2,4,6ꢀtriarylpyridines
as anomeric effect. As it can be seen in the proposed
mechanism, the chalcone intermediate (11) is performed in
the route of reaction, which is stable and considered as a
byꢀproduct. Excess ammonium acetate is used to convert 45
chalcone into 2,4,6ꢀtriarylpyridine derivatives (4) and to
improve reaction yields. Also, because of high
temperature of reaction (considering conversion of
ammonium acetate in the route of reaction to ammonia and
acetic acid) it is used to prevent evaporation and 50
elimination of ammonia to improve reaction yields.
using [SiO2–VO(OH)2] as
a
catalyst was suggested
5
(Scheme 3).39 Firstly, acetophenone 3 using SVA as a
catalyst is converted into its enol form 10 (via
tautomerization), which affords nucleophilic addition on
the intermediate 8 (SVA as a catalyst activates the
carbonyl group of the aromatic aldehyde 2 to give 10
intermediate 8) to give chalcone product 11 (via aldol
condensation). Then second molecule of enol form 10
undergoes Michael addition reaction with 11 to form the
1,5ꢀdiketone intermediate 12. This 1,5ꢀdiketone 12 on
reaction with ammonium acetate followed by cyclization 15
gives intermesiate 13 and its dehydration affords
intermesiate 14. Previously reported studies have been
proposed aerobic auto oxidation of intermediate (15) to its
corresponding 2,4,6ꢀtriarylpyridines (4). In contrast to the
previously reported mechanistic description for the final 20
step of the above described organic synthesis,28ꢀ31, 38, 39 we
believed that, this step might be progress through
uncommon hydride transfer as well as Cannizzaro reaction
(Scheme 4)40 and H2 releasing from tricyclic orthoamide
which presented in Scheme 5.41 Very recently, we 25
proposed an anomeric based oxidation for the final step in
mechanistic pathways of 1,4ꢀdihydropyranoꢀ[2,3ꢀc]ꢀ
pyrazole derivatives synthesis (Scheme 6).42 For
improving of this idea, reaction was occurred under
nitrogen atmosphere and in the absence of any molecular 30
oxygen. It was detected that, the reaction progressed under
atmosphere of nitrogen as well as normal reaction
condition using oxygen. By considering the aboveꢀ
mentioned evidence, conversion of intermediate (15) to its
corresponding 2,4,6ꢀtriarylpyridines (4) might be 35
happened through uncommon hydride transfer and
releasing of molecular hydrogen (H2). The CꢀH bond is so
weakend via electron donation from the nitrogen lone
Recently, Taherpour et al reported theoretical studies on the
rotamers and dynamic behaviors dihydropyridines.43
Herein, in order to prove the above suggested mechanism
we performed
a
DFT calculation. Starting from 55
intermediate 15, different orientations of two H atoms
attached to N1 and C4 atoms on 2,4,6ꢀtriarylpyridine ring
with respect to each other give cisꢀ and transꢀlike isomers.
Our calculations show that the transꢀlike isomer is about
0.14 kcal/mol more stable than cis one. As illustrated in 60
Fig. 1, by addition of SVA catalyst and through the
formation of transition structures, TS-1 and TS-2, and
then removing the molecular hydrogen (H2) the
zwitterionic intermediate 16 will be formed. According to
values of calculated Gibbs free energies, this reaction is 65
about ꢀ13.18 kcal/mol exothermic for trans isomer. There
are two pathways that lead to the formation of
intermediate 16. Fig. 1 indicates that these to be centered
on the stereo orientation of SVA catalyst with
intermediate 15, which in turn leads to formation of TS-1 70
and TS-2 structures. The TS-1 pathway independent of
orientation of substituents of 15 are plausible for both cis
and trans isomers (ꢀG‡ is 39.59 and 39.73 kcal/mol for cis
and trans isomers, respectively). The weak hydrogen bond
between the oxygen atom of SVA catalyst and the H atom 75
attached to atom N1 of triarylpyridine ring in TS-2 (see
Fig. 2) allows a lowꢀenergy pathway to complete removal
of H2, starting just from cis isomer. So TS-2 pathway by
ꢀG‡ = 36.50 kcal/mol is more favorable pathway. The
important step of this newly proposed mechanism is the 80
pairs into the antiꢀbonding of CꢀH (σ*
orbital) which it
CꢀH
can be broken by reaction with a proton to give molecular 40
hydrogen. As we know, this phenomenon has been named
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best results were attained when the reaction was achieved 55
using 2 mg of SVA catalyst at 130 oC (Table 5, entry 10). No
improvement was recognized in the yield of reaction via
increasing the amount of the SVA catalyst and the
temperature (Table 5, entries 11ꢀ14). Table 5 clearly exhibits
that in the absence of catalyst, the product was not formed 60
(Table 5, entries 1 and 2). A slight excess of the urea was
recognized to be acceptable and hence the molar ratio of
aromatic aldehyde and βꢀnaphthol to urea was studied at
1:1:1.2.
direct elimination of H2 molecule from 15 and SVA
catalyst via both TS-1 and TS-2 (Fig. 1) in which the CꢀH
bond in compound 15 is so weakened due to the
delocalization of π electron of adjacent CꢀC double bond
to σ* CꢀH bond (the anomeric effect) and therefore the
activation energy is relatively small. The NBO analysis of
donor–acceptor interactions identified that the anomeric
effect due to delocalization of π electron of CꢀC double
bond to σ* CꢀH bond for intermediate 15 is 6.56 and 4.84
kcal/mol for cis and trans isomers, respectively.
5
10
The summation of the different delocalizations from the lone
pair of nitrogen atom to σ* and π* of CꢀC double bond is
57.78 and 57.60 kcal/mol for cis and trans isomers,
respectively. The main donorꢀacceptor interaction of
To compare the proficiency of the solution versus solventꢀfree 65
conditions, a mixture of 4ꢀchlorobenzaldehyde, βꢀnaphthol
and urea as model reaction, using 2 mg of SVA as a catalyst
in various solvents for example H2O, C2H5OH, CH3CN,
CH3CO2Et and CH2Cl2 was tested under reflux conditions.
Also reaction was done under solventꢀfree condition at 130 70
oC. The results are summarized in Table 6. As it can be
recognized in Table 6, solventꢀfree condition was the best
conditions in this reaction.
Consequently optimization of the reaction conditions, to consider
the efficiency and the scope of the presented procedure, many 75
1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone
derivatives were prepared via the oneꢀpot threeꢀcomponent
condensation reaction between aromatic aldehydes with βꢀ
naphthol and urea using catalytic amounts of [SiO2–
2,4,6ꢀtriarylpyridine ring for both cis and trans isomers of 15
intermediate 15 is shown in Table 4.
In the final step of the reaction the zwitterionic intermediate
16 converts to 2,4,6ꢀtriarylpyridine (4) through an
exothermic process (ꢀG = ꢀ3.48 kcal/mol). In conclusion,
the whole process of conversion of 15 to 4 through the 20
releasing molecular hydrogen is exothermic (ꢀG is ꢀ16.80
and ꢀ16.66 kcal/mol for cis and trans isomers,
respectively). Thus the above theoretical studies, similar
to our previous work,42 support our suggested mechanism
and shows that releasing molecular hydrogen (H2) is quite 25
possible in such systems. The optimized structures of all
compounds involved in our suggested mechanism are
shown in Fig. 2.
VO(OH)2] as
a
catalyst under solventꢀfree reaction 80
conditions. The results have been shown in Table 7. The
effect of substituents on the aromatic ring was assessed strong
effects in terms of yields under described reaction conditions.
Both class of aromatic aldehydes containing electronꢀ
releasing, electronꢀwithdrawing substituents on their aromatic 85
ring gained the suitable products with high yields. The
reaction times of aromatic aldehydes having electron
withdrawing groups were rather faster than electron donating
groups.
It should be noted that in the above calculations, the solvent
effect was not investigated, because the reaction has been 30
performed under solventꢀfree conditions.
Furthermore, reusability of the [SiO2–VO(OH)2] as a catalyst was
confirmed via its recycling and reusing in condensation of 4ꢀ
chlorobenzaldehyde, 4ꢀmethoxyacetophenone and ammonium
acetate. At the end of the reaction, diethyl ether was added to 35
the reaction mixture to extract product from SVA catalyst.
Also, the catalyst was filtered and washed with CH3CN. It has
been known that the catalytic activity of the catalyst was
restored within the limits of the experimental errors for seven
continuous runs (Fig. 3). Deactivation of the SVA catalyst is 40
low, while reactant was anticipated. The reaction was scaled
It is obvious that the SVA catalyst activates the carbonyl group of 90
the aromatic aldehyde 2 to give intermediate 8. Therefore, the
reaction firstly includes the creation of ‘fitted ion pair
complex’ intermediate 8 between SVA catalyst (because of
proton releasing of SVA) and nonꢀbonded electrons on the
oxygen of the carbonyl group of the aldehydes 2. 95
Consequently, carbon of the carbonyl group becomes more
electrophilic toward nucleophile for the attack via π electrons
of βꢀnaphthol 5. Later, the attack of π electrons of βꢀnaphthol
5 happens more easily to form adduct 22 which loses one
water molecule and results in intermediate o-QMs: 23 which 100
further involvements nucleophilic attack of NH2 of urea to
form adduct 24. After again the ‘ion pair complex’ made
because of the coordination of proton and oxygen of the urea
carbonyl favors the nucleophilic attack of electrons on the
phenolic OH group and results into the creation of the 105
analogous compounds 7, which was represented in Scheme
7.44a, 45
up
to
10
mmol of
4ꢀchlorobenzaldehyde, 4ꢀ
methoxyacetophenone and ammonium acetate in the presence
of 20 mg of SVA catalyst at 130 oC. The yield of the reaction
was 88% in 45 minutes and 79% after the seventh run. The 45
results were summarized in Fig. 3.
Subsequently the synthesis of 2,4,6ꢀtriarylpyridines, in the
additional study for the synthesis of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀ
naphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone derivatives, to optimize the
reaction conditions, the condensation reaction of 4ꢀ 50
chlorobenzaldehyde, βꢀnaphthol and urea was chose as a
typical reaction and different amounts of [SiO2–VO(OH)2]
catalyst at temperature range of 25–150 °C were used under
solventꢀfree conditions (Table 5). As exposed in Table 5, the
110
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Table 4. The main second order perturbation energies (kcal/mol) calculated for 2,4,6ꢀtriarylpyridine ring in intermediate 15.
H6
Ph
Donorꢀacceptor interactions
cis
trans
C3
3.28
3.28
2.42
2.42
π C1ꢀC2 → σ* C3ꢀH6
π C4ꢀC5 → σ* C3ꢀH6
C2
C1
C4
C5
1.37
1.37
27.52
27.52
1.49
1.49
27.31
27.31
LP N → σ* C1ꢀC2
LP N → σ* C4ꢀC5
LP N → π* C1ꢀC2
LP N → π* C4ꢀC5
N
Ph
Ph
H
5
10
15
20
Fig. 1. Energy profile calculated for synthesis of 2,4,6ꢀtriarylpyridine (4) beginning from compound 15 (see scheme 3). The relative Gibbs free energies and the total electronic
energies (Eel + ZPE, figures in parentheses) obtained from the B3LYP/SVP calculations both are given in kcal/mol. In the calculations, we used Si(OH)3 group as a model for
silica moiety.
(15-cis)
(15-trans)
(16)
(TS-1)
(TS-2)
(4)
6
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Fig. 2. The optimized structure of all compounds involved in conversion of 15 to 4 according to the mechanism suggested in Fig. 1.
OH
O
O
O
SiO2
OH
O
V
O
O
SiO2
V
H
O
V
H
O
O
O
H
SiO2
O
HO
O
9
CH2
CH3
3
H
C
10
R'
R'
H2
R'
Silica-bonded vanadic acid
[SiO2-VO(OH)2] (SVA)
H
OH
O
O
O
SiO2
V
OH
O
O
SiO2
V
O
O
O
H
O
O
V
O
H
SiO2
O
HO
H
H
2
11
8
OH
SiO2
R'
R
O
R
R
V
O
O
H
O
CH2
10
NH4OAc
1
R
NH
R
R'
NH3
AcOH
O
O
O
13
12
R'
R'
R'
R'
Cyclization
R
HO
O
O
O
OH
O
O
O
V
SiO2
SiO2
V
R
R
H
H
H
H
Tautomerization
R'
15
R'
R'
R'
R'
R'
N
H
N
14
N
H
15
Anomeric effect
lead to hydrid transfer
H2
R
R
Driving force
of aromatization
R'
R'
R'
R'
SVA cat
N
4
N
H
OH
O
16
V
SiO2
O
O
Scheme 3. The proposed mechanism for the synthesis of 2,4,6ꢀtriarylpyridines using [SiO2–VO(OH)2] as a catalyst.
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5
Scheme 4. The proposed mechanism for the inꢀsitu oxidationꢀreduction in Cannizzaro reaction through unusual hydride transfer via anomeric effect.40
10
Scheme 5. A striking example which had been observed for an unusual hydride transfer from tricyclic orthoamide (16) through anomeric effect.41
Scheme 6. An anomeric based oxidation for the final step in mechanistic pathways of 1,4ꢀdihydropyranoꢀ[2,3ꢀc]ꢀpyrazole derivatives synthesis.42
20
15
25
Fig. 3. Reusability of [SiO2–VO(OH)2] as a catalyst in the synthesis of 2,4,6ꢀtriarylpyridine in 45 minutes.
Table 5. Result of amount of the catalyst and temperature in the condensation reaction of 4ꢀchlorobenzaldehyde, βꢀnaphthol and urea under solventꢀfree conditions.a
30
Entry
Catalyst amount (mg)
Reaction temperature (oC)
Reaction time (min)
Yieldb (%)
1
2
3
—
—
1
100
130
100
120
120
120
—
—
31
8
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4
5
6
7
8
1
2
2
2
2
2
2
2
5
5
5
130
r.t.
50
120
120
120
120
120
60
45
45
120
45
45
49
—
—
—
42
87
88
88
42
88
88
75
100
125
130
150
100
130
150
9
10
11
12
13
14
Reaction conditions: a4ꢀChlorobenzaldehyde (1 mmol), βꢀnaphthol (1 mmol), urea (1.2 mmol); bIsolated yield.
Table 6. Reaction of 4ꢀchlorobenzaldehyde, βꢀnaphthol and urea catalyzed by [SiO2–VO(OH)2] (2 mg) in various solvents under reflux conditions. Reaction was done under
solventꢀfree condition at 130 oC.a
Entry
Solvent
Reaction time (min)
Yieldb (%)
1
2
3
4
5
6
Solventꢀfree
H2O
C2H5OH
CH3CN
CH3CO2Et
CH2Cl2
45
88
—
31
35
12
41
120
120
120
180
120
Reaction conditions: a4ꢀChlorobenzaldehyde (1 mmol), βꢀnaphthol (1 mmol), urea (1.2 mmol); bIsolated yields.
5
Table 7. Synthesis of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone derivatives using [SiO2–VO(OH)2] as a catalyst.a
Entry
Aldehyde
Time (min)
Yieldb (%)
TOF
M.p (oC) [Lit.]Ref.
1
45
88
125.36
208ꢀ210 [209ꢀ212]44a (Yellow solid)
169ꢀ171 [172ꢀ174]44b (Yellow solid)
181ꢀ183 [180]44c (Red solid)
2
3
4
5
45
60
60
60
88
81
83
82
125.36
86.54
88.68
87.61
182ꢀ184 [185ꢀ187]44a (Yellow solid)
169ꢀ171 [170ꢀ172]44b (White solid)
6
45
84
89.74
224ꢀ226 [226ꢀ227]44d (Brown solid)
7
8
45
45
82
83
116.81
118.23
253ꢀ255 [251ꢀ253]44e (Yellow solid)
211ꢀ213 [214ꢀ217]44a (Yellow solid)
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9
60
45
82
87
116.81
123.93
217ꢀ219 [218ꢀ220]44c (Yellow solid)
10
218ꢀ220 [220ꢀ223]44c (White solid)
Reaction conditions: aAldehyde (1 mmol), βꢀnaphthol (1 mmol), urea (1.2 mmol), [SiO2–VO(OH)2] as a catalyst (2 mg), Solventꢀfree, 130 oC; bIsolated yields.
Scheme 7. The plausible mechanism for the synthesis of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone derivatives using [SiO2–VO(OH)2] as a catalyst.
Additionally, reusability of the SVA catalyst was studied in the
condensation of 4ꢀchlorobenzaldehyde, βꢀnaphthol and urea
as a typical reaction. At the end of the reaction, acetonitrile
was added to the reaction mixture and heated to extract
product from SVA catalyst. Also, the catalyst was filtered and
washed with CH3CN. It has been identified that the catalytic
and ammonium acetate under N2 atmosphere at 130 oC
was applied as a model (Table 8).
5
25
30
activity of the catalyst was restored within the limits of the 10
experimental errors and negligible loosing catalytic activities
for seven continuous runs (Fig. 4). Deactivation of the SVA
catalyst is low, while reactant was anticipated. The reaction
was scaled up to 10 mmol of 4ꢀchlorobenzaldehyde, βꢀ
o
35
naphthol and urea using 20 mg of SVA catalyst at 130 C. 15
The yield of the reaction was 88% in 45 minutes and 79%
after the seventh run. The results were summarized in Fig. 4.
In continuation of our consider into the use of SVA catalyst in
the synthesis of 2,4,6ꢀtriarylpyridine derivatives, we
considered the efficiency of SVA catalyst (an agreement 20
of proposed mechanism) is comparable with numerous
catalysts. To optimize the reaction conditions, the reaction
between 4ꢀnitrobenzaldehyde, 4ꢀmethoxyacetophenone
Fig. 4. Reusability of [SiO2–VO(OH)2] as a catalyst on the synthesis of 1,2ꢀ 40
dihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone in 45 minutes.
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ARTICLE TYPE
Table 8. The synthsis of 2,4,6ꢀtriarylpyridine using several catalyst for an agreement of proposed mechanism.a
Entry
Catalyst
Catalyst loading
Time (min)
Yieldb (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
SVA
HBF4
Fe(HSO4)3
Ca(HSO4)2
Zn(HSO4)2
Oxone
Ce(HSO4)3.7H2O
Al(HSO4)3
Bi(HSO4)3
CeO2
2 mg
45
60
90
90
90
90
90
90
90
90
90
60
60
60
60
60
90
60
88
77
41
27
43
35
44
45
25
35
20
85
88
83
83
73
25
78
10 mol%
12 mol%
15 mol%
10 mol%
5 mol%
10 mol%
12 mol%
12 mol%
10 mol%
10 mol%
2 mg
1 mol%
1 mol%
1 mol%
10 mol%
10 mol%
1 mol%
PbO2
Silica sulfuric acid
[MIMPS]3PW12O40
NanoꢀZnO
[MSIM]Cl
NH2SO3H
ZrOCl2
H3PW12O40
b
Reaction conditions: a4ꢀNitrobenzaldehyde (1 mmol), 4ꢀmethoxyacetophenone (2 mmol), ammonium acetatee (5 mmol), under N2 atmosphere, 130 oC; Isolate
yield.
To compare the efficiency of our catalyst with some reported
catalysts for the synthesis of 2,4,6ꢀtriarylpyridines and
1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone
derivatives, the results of using these catalysts in the
condensation of benzaldehyde, acetophenone and
ammonium acetate (in the synthesis of 2,4,6ꢀ
triarylpyridine) and 4ꢀchlorobenzaldehyde with βꢀnaphthol 10
and urea (in the synthesis of 2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀ
naphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone) are presented in Table 9.
As it can be seen, the SVA catalyst has remarkably
improved the synthesis of products in different terms.
5
Table 9. Comparison of the results in the synthesis of model product by SVA catalyst with those attained via the reported catalysts.
15
Entry
Reaction condition
Catalyst loading
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
9
SVA cat, Solventꢀfree, 130 oC
DPAT, Solventꢀfree, 120 oC
2 mg
6 mg
60
240
210
120
180
180
45
180
60
600
60
81
96
98
89
88
85
88
89
88
75
90
88
This work
46
30
39
47
H14[NaP5W30O110], Solventꢀfree, 120 oC
Bi(OTf)3, Neat, 120 oC
100 mg
5 mol%
20 mol%
7 mol%
2 mg
1 mmol
40 mg
40 mg
0.3 eq
[HO3S(CH2)4MIM][HSO4], Solventꢀfree, 120 oC
nanocrystalline MgAl2O4, Solventꢀfree, 120 oC
SVA cat, Solventꢀfree, 130 oC
48
This work
49
17
50
51
44a
H3Mo12O40P, DMF, 100 oC
HClO4ꢀSiO2, Solventꢀfree, 150 oC
RuCl2(PPh3)3, Toluene, Reflux
10
11
12
ZnO NPs, Solventꢀfree, 150 oC
Cellulose sulphuric acid, Water, SDS, 80 oC
50 mg
150
promising insight in the course of rational design, synthesis 30
and applications of novel hydride releasing compounds. The
suggested mechanisms exposed that the dual acidic ability of
SVA (Lewis and Brönsted acid), probably plays the important
roles in the defined reactions. Lastly, noteworthy advantages
of the described procedure and study are reasonably simple 35
workꢀup, cleaner reaction profile, recycle and reusability of
the SVA catalyst which makes it in close impact with the
green chemistry disciplines.
Conclusion
In conclusion, a mild, recyclable and reusable silica vanadic acid
[SiO2–VO(OH)2] (SVA) catalyst was applied in the synthesis
of
1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀnaphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone
derivatives under solventꢀfree conditions via threeꢀcomponent 20
condensation reaction of various aromatic aldehydes, βꢀ
naphthol and urea. Subsequently, 2,4,6ꢀtriarylpyridines were
also prepared by oneꢀpot threeꢀcomponent condensation
reaction between several aromatic aldehydes, acetophenone
derivatives and ammonium acetate under similar reaction 25
conditions. A new anomeric based oxidation was proposed
for the final step of the 2,4,6ꢀtriarylpyridine derivatives
synthesis which was supported by theoretical studies. So the
described new mechanistic approach can open up a new and
Experimental
General procedure for the preparation of silicaꢀbonded 40
vanadic acid [SiO2–VO(OH)2] (SVA):
The silicaꢀbonded vanadic acid [SiO2–VO(OH)2] (SVA) as a
catalyst was synthesized and prepared according to our
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previously reported process with a slightly modification.37
In a suction flask (100 mL), nꢀhexane (25 mL) and silica
gel (4.0 g) were stirred for 10 min. It was equipped with a
constantꢀpressure dropping funnel containing of vanadium
oxytrichloride (I) (4.0 g) in nꢀhexane (25 mL) and gas
inlet tube for conducting HCl gas over an adsorbing
solution i.e. water. Then it was added drop wise to silica
gel suspension (15ꢀ20 min). Subsequently addition of
vanadiumoxytrichloride, the redꢀbrown mixture was
for alternative reaction. The solvent of organic layer was
evaporated and the crude product was purified by
recrystallization from ethanol/water (10:1). In this work, 60
SVA catalyst was recycled and reused for seven times
without significant loss of its catalytic activity.
5
Spectral data analysis for compounds should be added.
2,6ꢀBis(4ꢀmethoxyphenyl)ꢀ4ꢀ(pꢀtolyl)pyridine (Table 3, entry
o
6): Yellow solid; M.p: 153ꢀ155 C; Yield: 82%; IR (KBr): 65
1
υ 3033, 1671, 1599, 1575, 1402, 1183, 809 cmꢀ1; H NMR
stirred for 10 h. Formerly, solvent was evaporated and the 10
residue (the redꢀbrown powder (II) attained after the
grafting procedure) was filtered, washed with dry nꢀ
(400 MHz, DMSOꢀd6): δppm 2.40 (s, 9H, —CH3), 7.39 (d,
6H, J = 8.4 Hz, ArH), 7.95 (d, 2H, J = 8.0 Hz, ArH), 8.10
(s, 2H, ArH), 8.23 (d, 4H, J = 8.0 Hz, ArH); 13C NMR
(100 MHz, DMSOꢀd6): δppm 20.8, 20.9, 115.5, 126.8, 70
127.1, 129.3, 129.6, 134.8, 136.1, 138.7, 138.9, 149.2,
156.3.
hexane
numerous
times
to
remove
unreacted
vanadiumoxytrichloride, dried under vacuum, and stirred
in the air for 72 h to promote the hydrolysis of VꢀCl 15
bonds. At the moment, the redꢀbrown powder transformed
to dark green powder (III) gradually (Scheme 2).
Additional amounts of HCl produced in the synthesis of
SVA catalyst is neutralize in the presence of a certain
concentration of sodium hydroxide. The SVA catalyst was 20
characterized by FTꢀIR, thermal gravimetric (TG),
derivative thermal gravimetric (DTG), Xꢀray diffraction
patterns (XRD), scanning electron microscopy (SEM) and
energyꢀdispersive Xꢀray spectroscopy (EDX) analysis (Fig
4ꢀ(4ꢀNitrophenyl)ꢀ2,6ꢀdiꢀpꢀtolylpyridine (Table 3, entry 8):
Green solid; M.p: 233ꢀ235 oC; Yield: 85%; IR (KBr): υ
1
3031, 1669, 1598, 1513, 1341, 1175, 832 cmꢀ1; H NMR 75
(400 MHz, DMSOꢀd6): δppm 2.39 (s, 6H, —CH3), 7.36 (d,
4H, J = 8.4 Hz, ArH), 8.18 (s, 2H, ArH), 8.23 (d, 4H, J =
8.4 Hz, ArH), 8.30 (d, 2H, J = 8.8 Hz, ArH), 8.36 (d, 2H,
J = 8.8 Hz, ArH); 13C NMR (100 MHz, DMSOꢀd6): δppm
20.9, 21.0, 116.1, 123.9, 126.8, 128.7, 129.3, 135.7, 138.9, 80
144.2, 147.0, 147.7, 156.6.
S13† to Fig S17†).
25
General procedure for the synthesis of 2,4,6ꢀtriarylpyridines:
Computational details
To the mixture of aromatic aldehydes (1 mmol), acetophenone
All of the calculations were performed with the Gaussian09
program.52 Full geometry optimization was carried out with
the B3LYP hybrid density functional and the SVP basis set 85
for all atoms. Frequency calculations at the same level of
theory have also been performed to identify all of the
stationary points as minima (zero imaginary frequencies) or
transition structures (one imaginary frequency). In addition,
the nature of transition structures was confirmed by intrinsic 90
reaction coordinate (IRC).53 In order to investigate the
mechanism of the synthesis of 2,4,6ꢀtriarylpyridine
derivatives in the presence of silica vanadic acid [SiO2–
VO(OH)2] (SVA) catalyst we used the total electronic
energies (Eel + ZPE) and the Gibbs free energies. To make the 95
calculations possible and less expensive the Si(OH)3 group
was used as a model for silica moiety. Natural bond orbital
(NBO)54 analysis was employed to evaluate the
intramolecular interactions.
derivatives (2 mmol) and ammonium acetate (5 mmol) in a
round bottom flask, was added SVA (2 mg, 0.0156 mol%
of V) as a catalyst and the resulting mixture was firstly 30
stirred magnetically under solventꢀfree conditions at 130
oC in an oil bath for the suitable time (Table 3). After
completion of the reaction, as checked via TLC nꢀ
hexane/ethyl acetate (15:1), mass was cooled to room
temperature and then diethyl ether was added to the 35
reaction mixture to extract product from SVA catalyst.
Also, the catalyst was filtered and washed with CH3CN.
The remained catalyst was applied for alternative reaction.
The solvent of organic layer was evaporated and the crude
product was purified by recrystallization from ethanol. 40
More purification can be happened by recrystallization in
nꢀhexane. In this work, SVA catalyst was recycled and
reused for seven times without noteworthy loss of its
catalytic activity.
General procedure for the synthesis of 1,2ꢀdihydroꢀ1ꢀarylꢀ3Hꢀ 45
naphth[1,2ꢀe][1,3]oxazinꢀ3ꢀone derivatives:
Acknowledgements
100
The authors gratefully acknowledge the BuꢀAli Sina and
Nahavand universities, University Research Councils, and
also Center of Excellence in Development of
Environmentally Friendly Methods for Chemical
Synthesis (CEDEFMCS), for providing support to this 105
work.
To the mixture of aromatic aldehydes (1 mmol), βꢀnaphthol (1
mmol) and urea (1.2 mmol) in a round bottom flask, was
added SVA (2 mg, 0.0156 mol% of V) as a catalyst and
the resulting mixture was firstly stirred magnetically under 50
solventꢀfree conditions at 130 oC in an oil bath for the
appropriate time (Table 7). After completion of the
reaction, as known by TLC nꢀhexane/ethyl acetate (5:2),
mass was cooled to room temperature and then acetonitrile
was added to the reaction mixture and heated to extract 55
product from SVA catalyst. Also, the catalyst was filtered
and washed with CH3CN. The remained catalyst was used
Notes and references
a
Department of Organic Chemistry, Faculty of Chemistry, BuꢀAli Sina
University, Hamedan 6517838683, Iran
b
Department of Aromatic and Medicinal Plants, Nahavand University, 110
P.O. Box 6591678546, Nahavand, Iran
12 | Journal Name, [year], [vol], 00–00
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*Corresponding Author: Tel: +988138282807, Fax: +988138380709
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DOI: 10.1039/C5RA21392D
Graphical Abstract
Silica vanadic acid [SiO2–VO(OH)2] as an efficient heterogeneous catalyst for the synthesis
of 1,2-dihydro-1-aryl-3H-naphth[1,2-e][1,3]oxazin-3-one and 2,4,6-triarylpyridine
derivatives via anomeric based oxidation
Mohammad Ali Zolfigol,*a Maliheh Safaiee,b Fatemeh Afsharnadery,a Neda Bahraminejad,a Saeed
Baghery,a Sadegh Salehzadeh,a Farahnaz Malekia
a Department of Organic Chemistry, Faculty of Chemistry, BuꢀAli Sina University, Hamedan 6517838683, Iran
b Department of Aromatic and Medicinal Plants, Nahavand University, P.O. Box 6591678546, Nahavand, Iran
*Corresponding Author: Tel: +988138282807, Fax: +988138380709
Eꢀmail: zolfi@basu.ac.ir and mzolfigol@yahoo.com (M.A. Zolfigol).
† Electronic Supplementary Information (ESI) available: [details of any supplementary information available
should be included here]. See DOI: 10.1039/b000000x/
Silica vanadic acid [SiO2–VO(OH)2] (SVA) efficiently catalyzed the synthesis of 1,2-dihydro-1-aryl-3H-
naphth[1,2-e][1,3]oxazin-3-one and 2,4,6-triarylpyridine derivatives via anomeric based oxidation under
solvent-free condition.