Highly efficient etherification and oxidation of aromatic alcohols using supported and unsupported phosphorus pentoxide as a heterogeneous reagent

Article abstract of DOI:10.1080/00397911.2010.487982

A highly efficient procedure for etherification and oxidation of aromatic alcohol is described using unsupported and supported P₂O₅ on alumina and/or silica gel. The silica gel and alumina proved to be the most suitable support among the supports examined in our experiments. It was illustrated that the etherification and oxidative performance in reactions depend largely upon variables including reaction temperature, the nature of the P₂O₅ used (supported or unsupported P₂O ₅), and whether solvent-free conditions are applied. It was shown that P₂O₅ not only can convert the aromatic alcohols into corresponding ethers and/or aldehyde and ketone but also can convert aromatic ethers into aldehyde or ketone via oxidative cleavage. Finally, quantum mechanical calculations were performed to rationalized these events, and it was indicated that aldehyde and ketone are more favorable products on the basis of the heat of formation (Hf).

Full text of DOI:10.1080/00397911.2010.487982

1
Synthetic Communications , 41: 1544–1553, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.487982
HIGHLY EFFICIENT ETHERIFICATION AND OXIDATION
OF AROMATIC ALCOHOLS USING SUPPORTED AND
UNSUPPORTED PHOSPHORUS PENTOXIDE AS A
HETEROGENEOUS REAGENT
1
Ardeshir Khazaei, Mohammad Navid Soltani Rad,
2
1
Maryam Kiani Borazjani, Shahnaz Saednia,
1
1
1
Mohammad Kiani Borazjani, Maryam Golbaghi, and
2
Somayeh Behrouz
1
Faculty of Chemistry, Bu-Ali Sina University, Hamedan, Iran
Department of Chemistry, Faculty of Basic Sciences, Shiraz University of
Technology, Shiraz, Iran
2
GRAPHICAL ABSTRACT
Abstract A highly efficient procedure for etherification and oxidation of aromatic alcohol is
2 5
described using unsupported and supported P O on alumina and/or silica gel. The silica gel
and alumina proved to be the most suitable support among the supports examined in our
experiments. It was illustrated that the etherification and oxidative performance in reac-
tions depend largely upon variables including reaction temperature, the nature of the
P
applied. It was shown that P not only can convert the aromatic alcohols into correspond-
O
2 5
2 5
used (supported or unsupported P O ), and whether solvent-free conditions are
2 5
O
ing ethers and/or aldehyde and ketone but also can convert aromatic ethers into aldehyde or
ketone via oxidative cleavage. Finally, quantum mechanical calculations were performed to
rationalized these events, and it was indicated that aldehyde and ketone are more favorable
products on the basis of the heat of formation (DH
f
).
Keywords Alumina; aromatic alcohol; etherification; oxidation; P
2
5
O ; silica gel
INTRODUCTION
The preparation of ethers is an important reaction in organic synthesis. A wide
[
1]
variety of procedures have been established for etherification during recent decades.
Received February 16, 2010.
Address correspondence to Ardeshir Khazaei, Faculty of Chemistry, Bu-Ali Sina University,
P.O. Box 651783868, Hamedan, Iran. E-mail: khazaei_1326@yahoo.com or Mohammad Navid Soltani
Rad, Department of Chemistry, Faculty of Basic Sciences, Shiraz University of Technology, Shiraz
71555-313, Iran. E-mail: soltani@sutech.ac.ir, nsoltanirad@gmail.com
1
544

ETHERIFICATION AND OXIDATION OF ALCOHOLS
1545
2 5 2 5 2 2 5 2 3
Scheme 1. Heterogeneous reagent: P O or P O =SiO or P O =Al O .
[
2]
The most commonly used protocol is the Williamson ether synthesis, which requires
the initial transformation of alcohols into their corresponding halides or tosylates fol-
lowed by their displacement with highly basic alkoxides or phenoxides. However, the
strongly basic conditions used in the Williamson ether synthesis can damage complex
molecules carrying base-sensitive functional groups. Thus, etherification by direct
condensation of alcohols has been considered a useful alternative strategy that is con-
ducted in the presence of catalytic amounts of organic or inorganic protic as well as
[
3]
Lewis acids.
Phosphorus pentoxide (phosphoric anhydride) has many applications in vari-
ous organic transformations that can be used as a dehydrating agent for the forma-
[
tion of anhydride from two molecules of an ordinary acid, ketenimines from
4]
[
amides, vinyl ethers from acetals, nitriles from amides, amides from oximes
4]
[4]
[4]
[5]
[6]
Beckmann rearrangement), phenolic esters from carboxylic acids, and so on.
(
Additionally, P O can be used as an activator of DMSO in a Swern type of oxida-
2
5
[
7]
tion of alcohols, namely the Onodera reaction.
P O is an effective reagent for one-pot etherification of alcohols; however, to
2
5
the best of our knowledge, there is no report for oxidation of aromatic alcohols via
P O . Hence, in this context, we reported a mild and convenient procedure for
2
5
one-pot etherification–oxidation of aromatic alcohols using supported and unsup-
ported P O . P O can be used as a both dehydrating and oxidizing agent for attain-
2
5
2
5
ing benzhydryl ether, benzyl ether and aldehydes or ketones from corresponding
aromatic alcohols at different reaction conditions (Scheme 1).
RESULTS AND DISCUSSION
Initially, we decided to investigate the proper control reactions in the presence
of different solid supports. To render these oxidations economically more viable and
simpler, we studied the effect of supported P O on SiO and Al O . These mineral
2
5
2
2
3
supports are nontoxic, widely available, inexpensive, and useful in straightforward
workup procedures. Thus, the benzhydrol was used as a model compound, and
the oxidative effect of various supported P O was studied. The results are depicted
2
5
in Table 1.
As data in Table 1 indicate, supported P O on both Al O and SiO (Table 1,
2
5
2
3
2
entries 1 and 2) afforded the best yields for benzophenone in shortest reaction times,
whereas, other supported-P O reagents were not efficient enough. Although ZnO
2
5
(Table 1, entry 3) results in reasonable yields; however, technical problems were
encountered, as it becomes pasty on mixing with P O . This drawback complicates
2
5
the reaction as well as workup procedures. Therefore, Al O and SiO were the
2
3
2
supports of choice in all our experiments. The impregnation of P O on solid
2
5

1546
A. KHAZAEI ET AL.
Table 1. Reactivity trend of P (0.5 g, 35.2 mmol) on 0.5 g supports for the
oxidation of benzhydrol (1 mmol) under solvent-free conditions
2 5
O
ꢀ
Entry
Support
Time (min)
Temperature ( C)
Isolated yield (%)
1
2
3
4
5
Al
2
O
3
120
100
180
240
240
90
90
90
90
90
97
95
87
48
59
SiO
2
a
ZnO
AlCl
a
3
TiO
2
a
Tedious workup procedures.
supports is effective for oxidation, and we questioned the theory based on the
premise that the oxidation rate enhancements are due to their impregnation on
[4]
the solid supports.
In this case, we investigated the influence of various
parameters on oxidation and etherification of benzhydrol. The results are indicated
in Table 2.
As data in Table 2 indicate, the presence of P O is critically essential because
2
5
using support (SiO and=or Al O ) alone either at room temperature and=or higher
2
2
3
ꢀ
temperature (90 C) did not afford the any product after 12 h under solvent-free con-
ditions (Table 2, entries 1–4). The reaction is achieved at room temperature in the
absence of support in nonpolar n-hexane and in more polar diethylether (Table 2,
entries 5 and 6). The benzhydryl ether was attained exclusively in both solvents; how-
ever, the reaction took longer and produced a lower yield with n-hexane. Changing
Table 2. Optimization of temperature and solvent-free and solvent-assisted conditions on etherification
2 5
and oxidation of benzhydrol (1 mmol) with supported and unsupported P O
Isolated
yield
(%) [Ref.]
Temperature
ꢀ
Time Mixture
(min)
Entry
Reagent
( C)
Condition
color
Product
b
1
2
3
4
5
SiO
2
rt
rt
Solvent free
Solvent free
Solvent free
Solvent free
720
720
720
720
210
White
White
White
White
Orange
—
—
—
—
NR
b
2
Al O
3
NR
b
SiO
Al
2
90
90
rt
NR
b
2
O
3
NR
[8,9]
75
a
P
2
P
2
P
2
P
2
O
O
O
O
5
5
5
5
(0.05 g)
n-Hexane
Benzhydryl
ether
a
[8,9]
6
7
8
(0.15 g)
(0.05 g)
(0.5 g)
rt
Diethylether
Solvent free
Solvent free
180
17
Orange
Orange
Benzhydryl
ether
95
[
8,9]
60
90
Benzhydryl
ether
98
180
Orange- Benzhydryl
Brown
ether (20%) þ
Benzophenone
80%)
—
(
9
P
2
O
5
(0.5 g)=
90
90
Solvent free
Solvent free
100
120
Orange- Benzophenone
Brown
95
97
SiO
2
1
0
P
2
O
5
(0.5 g)=
Orange- Benzophenone
Brown
2 3
Al O
a
b
Anhydrous solvent.
No reaction after 12 h.

ETHERIFICATION AND OXIDATION OF ALCOHOLS
1547
ꢀ
the reaction conditions into solvent-free status using unsupported P O at 60 C
2
5
afforded the benzhydryl ether in less reaction time and excellent yield (Table 2,
entry 7). Interestingly, when similar reaction conditions were used at increased
ꢀ
temperature up to 90 C, a mixture of products consisting of benzophenone and
benzhydryl ether were simultaneously obtained in 80% and 20% yields respectively.
ꢀ
Surprisingly, using the silica- or alumina-supported P O at 90 C afforded the ben-
2
5
zophenone exclusively in excellent yields (Table 2, entries 9 and 10).
The schematic detail of the reaction is shown in Scheme 2.
To realize the versatility as well as generality of this method, we extended the
optimized conditions for other benzhydrol derivatives to study the products in these
derivatives. Thus, the effects of unsupported P O , P O -Al O , and P O -SiO were
2
5
2
5
2
3
2
5
2
investigated on 2- and 4-chlorobenzhydrol as well as 4-nitrobenzhydrol. The results
are shown in Table 3.
As shown in Table 3, in general the same results were attained as in Table 2
with the exception that 4-nitrobenzhydrol merely afforded the corresponding ether
ꢀ
at 60 C in the presence of supported and unsupported P O (Table 3, entries 7–9)
2
5
ꢀ
while increment the temperature up to 90 C changed the product into 4-nitrobenzo-
phenone (Table 3, entries 10 and 11).
Encouraged by the results obtained for benzhydrol and its derivatives, we
decided to study the effect of supported and unsupported P O on etherification
2
5
and potential oxidation of benzyl alcohols. Hence, we employed the same conditions
as for benzhydrol. The results are presented in Table 4.
We studied the etherification and oxidation of 4-chlorobenzyl alcohol via
supported and unsupported P O as a control reaction. As can be seen in Table 4,
various conditions were employed for etherification and oxidation of 4-chlorobenzyl
2
5
ꢀ
alcohol. Using unsupported P O either at room temperature or 90 C afforded the
2
5
corresponding ether in good yields (Table 4, entries 1 and 2). Furthermore, using
anhydrous diethylether as a solvent at room temperature also afforded the
good yield of ether, but the reaction time was long (Table 4, entry 3). Using
2 5
Scheme 2. Conversion of benzhydrol to benzhydryl ether and benzophenone using P O under solvent-
free conditions.

1548
A. KHAZAEI ET AL.
2 5
Table 3. Etherification and oxidation of benzhydrols (1 mmol) with supported and unsupported P O
under solvent-free condition
Isolated
yield (%)
[Ref.]
Temperature Mixture
ꢀ
Time
(min) Product
Entry
1
Reagent
Alcohol
( C)
color
[8]
93
P
P
P
2
2
O
5
(0.07 g) 2-Cholorobenzhydrol
60
Yellow-
orange
Yellow-
orange
Yellow-
orange
Brown
Brown
15
15
20
Ether
2
3
O
5
(0.5 g)= 2-Cholorobenzhydrol
90
90
Ketone
Ketone
87
88
SiO
2
2
O
5
(0.5 g)= 2-Cholorobenzhydrol
2 3
Al O
[8]
4
5
P
P
2
O
O
5
(0.07 g) 4-Cholorobenzhydrol
(0.5 g)= 4-Cholorobenzhydrol
60
90
15
15
Ether
Ketone
95
90
2
5
SiO
2
6
P
2
O
5
(0.5 g)= 4-Cholorobenzhydrol
90
Brown
20
Ketone
93
2 3
Al O
[
8]
7
8
P
P
2
O
O
5
(0.07 g) 4-Nitrobenzhydrol
(0.5 g)= 4-Nitrobenzhydrol
60
60
Brown
Brown
20
15
Ether
Ether
90
88
[8]
2
5
SiO
2
[
8]
9
P
P
P
2
O
5
(0.5 g)= 4-Nitrobenzhydrol
60
90
90
Brown
Brown
Brown
20
15
20
Ether
90
2 3
Al O
1
0
1
2
O
5
(0.5 g)= 4-Nitrobenzhydrol
Ketone
Ketone
90
92
SiO
2
1
2
O
5
(0.5 g)= 4-Nitrobenzhydrol
2 3
Al O
Table 4. Effect of temperature, solvent, and supported and unsupported P
4-cholorobenzylalcohol (1 mmol) into corresponding ether and aldehyde
2
O
5
on conversion of
Isolated
yield (%)
[Ref.]
Temperature
ꢀ
Time Mixture
Condition (min) color
Entry
Reagent
( C)
Product
[
[
10]
10]
1
2
3
4
P
2
P
2
P
2
P
2
O
5
O
5
O
5
O
5
(0.5 g)
(0.5 g)
(0.5 g)
(0.5 g)=
90
rt
Solvent free
Solvent free
Diethylether
Solvent free
15
35
60
40
Grey Ether
Grey Ether
Grey Ether
Grey Ether
70
70
75
87
a
[10]
rt
60
[
10]
SiO
2
[10]
88
5
6
7
P
P
P
2
O
5
(0.5 g)=
60
rt
Solvent free
Diethylether
Diethylether
50
270
300
Grey Ether
2 3
Al O
a
a
2
O
5
(0.5 g)=
Grey Ether (65%)
—
—
SiO
2
þ aldehyde (35%)
2
O
5
(0.5 g)=
rt
Grey Ether (65%)
þ aldehyde
Al
2
O
3
(35%)
Grey Aldehyde
8
P
2
O
5
(0.5 g)=
120
120
Solvent free
Solvent free
50
65
88
88
SiO
2
9
P
2
O
5
2 3
(0.5 g)=Al O
Grey Aldehyde
a
Anhydrous Solvent.

ETHERIFICATION AND OXIDATION OF ALCOHOLS
1549
Table 5. Effect of temperature, and solvent and supported P
(1 mmol) into 4-nitrobenzyaldehyde
O
2 5
on conversion of 4-nitrobenzylalcohol
Isolated
Temperature
ꢀ
Time Mixture
(min)
yield
(%)
Entry
Reagent
( C)
Condition
color
Product
a
1
2
3
4
5
6
7
P
2
P
2
P
2
P
2
P
2
P
2
P
2
O
5
O
5
O
5
O
5
O
5
O
5
O
5
60
60
60
rt
Solvent free
Solvent free
Solvent free
240
240
250
270
290
60
Yellow
Yellow
Yellow
Brown
Brown
Brown
Brown
—
—
NR
a
(0.5 g)=SiO
2
NR
a
(0.5 g)=Al
(0.5 g)=SiO
(0.5 g)=Al
(0.5 g)=SiO
(0.5 g)=Al
2
O
3
3
3
—
NR
2
Dry diethylether
Dry diethylether
Solvent free
Aldehyde
Aldehyde
Aldehyde
Aldehyde
20
30
94
92
2
O
rt
2
120
120
2
O
Solvent free
65
a
No Reaction.
ꢀ
P O -Al O and=or P O -SiO merely provided the ether in excellent yields at 60 C
2
5
2
3
2
5
2
(Table 4, entries 4 and 5) while using these supports in anhydrous Et O at room tem-
perature led to a mixture of corresponding ether and aldehyde in 65% and 35% yields
2
respectively (Table 4, entries 6 and 7). However, the increment in temperature up to
ꢀ
1
20 C shifted the product into 4-chlorobenzaldehyde in excellent yields (Table 4,
entries 8 and 9).
The optimized reaction conditions were also applied for the conversion of
-nitrobenzylalcohol into products (Table 5); however, all of our attempts for syn-
4
thesis of corresponding ether failed (Table 5, entries 1–3). Nevertheless, using silica-
ꢀ
or alumina-supported P O and increasing the temperature up to 120 C remarkably
2
5
enhanced the formation of 4-nitrobenzaldehyde from 4-nitrobenzylalcohols (Table 5,
entries 6 and 7). Additionally, using silica- or alumina-supported P O in anhydrous
Et O at room temperature exclusively provided 4-nitrobenzaldehyde in low yields
2
5
2
(
Table 5, entries 4 and 5).
The schematic detail of the reaction is shown in Scheme 3.
The reusability of both reagents on oxidation of benzhydrol into benzophe-
ꢀ
none was studied at 90 C under solvent-free conditions for five successive runs. In
2 5
Scheme 3. Conversion of benzyl alcohol to benzyl ether and benzaldehyde using P O under solvent-free
conditions.

1550
A. KHAZAEI ET AL.
each run, the reaction was stopped after 120 min, cooled, and suspended in anhy-
drous CH Cl , the suspension was then filtered, and the reagent was washed with
2
2
anhydrous CH Cl (2 ꢁ 50 mL). The reagents allowed to be dried in vacuum oven
2
2
ꢀ
(
50 C, 15 min) and immediately were used for the subsequent run. The results are
depicted in Table 6. As Table 6 indicates, because of the strongly hygroscopic nature
of both reagents, their reactivity remarkably decreased after the first run. This is due
to the ready conversion of P O into phosphoric acids upon absorption of water
2
5
from reactants and the environment.
One of the interesting events that we observed in our experiments is the oxidat-
ive cleavage of benzhydryl as well as benzyl ether derivatives into their correspond-
ing ketones and aldehydes accordingly (Schemes 2 and 3).
As is shown in Scheme 2, the benzohydol derivatives were primarily converted
ꢀ
to their corresponding benzhydryl ethers using unsupported P O at 60 C under
2
5
ꢀ
solvent-free condition. However, elevating the reaction temperature up to 90 C
readily converted the benzhydrol ethers into benzophenone derivatives.
ꢀ
In the same circumstance, as Scheme 3 indicates, supported P O at 60 C
2
5
converted the benzyl alcohols into benzyl ethers; nevertheless, the increase in tempe-
ꢀ
rature up to 120 C converted the benzyl ethers to corresponding benzaldehydes.
Which potential factors influence oxidative cleavage of aromatic ethers is an
interesting question. We thought that the atmospheric oxygen may be responsible
for such an oxidative cleavage. Therefore, we conducted the reaction under inert
argon or nitrogen atmosphere, but surprisingly we witnessed again the oxidative
cleavage of aromatic ethers into aldehydes or ketones. Interestingly, using P O
2
5
either in supported or unsupported form has no effect on oxidative cleavage of aro-
matic ether to aldehyde and ketone at higher temperature. Thus, we supposed that
P O could be the reason. Water abstraction via P O can occur from a single
2
5
2
5
alcohol or double alcohols, which results in aldehyde or ketone and also ether
accordingly. As matter of fact, P O played three different roles as a powerful dehy-
2
5
drating agent in our experiment. A tentative mechanism showing the three functions
of P O in our study is presented in Scheme 4.
The first reaction (i) expresses the route by which the etherification is achieved
2
5
via dehydration using P O . The second reaction (ii) is the oxidative route, which can
2
5
occur via the dehydration from single alcohol, and the third reaction (iii) is a poten-
tial oxidative route for cleavage of the aromatic ether into aldehyde or ketone. To
rationalize the oxidative cleavage of aromatic ether into aldehyde and ketone, we
employed the semi-empirical mechanical quantum calculations. The semi-empirical
Table 6. Reactivity study of recycled P
2
O
5
-SiO
2
and P
benzophenone under solvent-free condition after 120 min
2
O
5
-Al
2
O
3
on conversion of benzhydrol into
ꢀ
a
Yield (%) (P
a
Yield (%) (P
Run
Temperature ( C)
2
O
5
2
-SiO )
O
2 5
-Al O
2 3
)
1
2
3
4
5
90
90
90
90
90
95
62
48
31
22
97
53
35
26
15
a
Isolated yield.

ETHERIFICATION AND OXIDATION OF ALCOHOLS
1551
2 5
Scheme 4. A tentative mechanism showing the three functions of P O .
Austin model 1 (AM1) and parameterized model 3 (PM3) calculations were run on
MOPAC in CS Chem 3D Ultra 8 (2004 Cambridge Soft) and Hyperchem
(
Hypercube Inc., version 7). The heat of formation or DH is a criterion to estimate
f
the stability of molecules in comparison with a molecule. The molecule that has a
lower DH value has more stability in comparison with related molecules and thus
f
thermodynamically is more prone to be produced. Therefore, DH values were calcu-
f
lated for 4-chloro benzaldehyde, 4-chlorobenzophenone, 4-chlorobenzylether, and
4-chlorobenzhydryl ether using both PM3 and AM1 semi-empirical calculations.
The results are depicted in Table 7.
f
Table 7. Comparison of DH values attained from PM3 and AM1 calculations for oxidative–
etherification products of 4-chlorobenzaldehyde and 4-chlorobenzhydrol
Heat of formation (DH
PM3
f
) (Kcal=mol)
Entry
1
Structure
AM1
ꢂ17.27
ꢂ15.81
2
3
10.62
13.33
ꢂ1.17
ꢂ8.86
4
59.27
56.20

1552
A. KHAZAEI ET AL.
As shown in Table 7, both aldehyde and ketone (Table 7, entries 1 and 2) have
lower DH values in comparison with their corresponding ethers, and these results
f
indicate that there should be a greater tendency for formation of aldehyde and
ketone rather than their corresponding ethers. As the results in Tables 2–5 also indi-
cate, formation of corresponding ethers was observed predominantly in the early
stages of reaction while increasing the temperature and prolonging the reaction time
shifted the products to aldehydes and ketones. It seems that formation of ethers is
controlled by kinetic factors while synthesis of aldehydes and=or ketones is con-
trolled by thermodynamic factors. Furthermore, it is rationalized as well that the
conjugation via carbonyl bond in aldehyde and ketone can be considered as a driving
force for synthesis of aldehyde and ketone at higher temperature using supported
and unsupported P O . Additionally, it is observed that benzhydrol ether deriva-
2
5
tives are more prone to oxidative cleavage via P O rather than benzyl ethers. Both
2
5
experimental observations and calculations (Table 7, entries 3 and 4) confirmed
this fact.
CONCLUSIONS
In summary, a highly efficient procedure has been established for etherification
and oxidation of aromatic alcohols using supported or unsupported P O under
2
5
solvent-free conditions. It was shown that the reaction trends toward etherification
and oxidation of aromatic alcohols are dependent upon various factors including
temperature, support, and solvent or solvent-free conditions. Furthermore, semi-
empirical quantum mechanical calculations have been achieved that confirmed the
propensity of reaction toward the synthesis aldehydes and ketones as oxidative
products.
EXPERIMENTAL
All products were characterized by comparison of their physical and spectral
1
data (mp, H NMR, and IR) with those of authentic samples.
Preparation of Supported P O on Alumina or Silica Gel [50% (W/W)]
2
5
P O (5 g, 35 mmol) was added to anhydrous alumina (5 g, 50 mmol) (neutral,
2
5
0
.063–0.200 mm) or anhydrous silica gel (5 g, 83 mmol) (0.040–0.063 mm), which was
ꢀ
previously dried and kept at 120 C for 24 h in vacuum oven, and mixed. The
obtained white solid was corked and stored at room temperature in vacuum-
desiccator flask for subsequent use.
General Procedure for Etherification or Oxidation of Aromatic
Alcohols Using Supported P O on Alumina or Silica Gel
2
5
Supported P O on alumina or silica gel (0.67 g, 50%w=w) was added to an aro-
5
2
matic alcohol (2 mmol). The mixture was kept at an appropriate temperature (see
Tables 2–5). When thin-layer chromatography (TLC) monitoring indicated no
further progress in reaction, then the reaction was stopped, cooled, and diluted with

ETHERIFICATION AND OXIDATION OF ALCOHOLS
1553
anhydrous Et O (100 mL). The Et O solution was dried on anhydrous Na SO and
2
2
2
4
filtered, and the filterate was evaporated to give an almost pure product.
ACKNOWLEDGMENT
The authors acknowledge to Bu-Ali Sina University Research Council and
Center of Excellence in Development of Chemical Methods (CEDCM) for support
of this work.
REFERENCES
1
2
. March, J. Advanced Organic Chemistry, 4th ed.; John Wiley & Sons: Singapore, 2005.
. Smith, K. (Ed). Solid Supports and Catalysts in Organic Synthesis; Prentice Hall: New
York, 1992.
3
4
5
6
7
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