Cobalt and Manganese Salts of p-Aminobenzoic Acid Supported on Silica Gel: A Versatile Catalyst for Oxidation by Molecular Oxygen

Article abstract of DOI:10.1007/s00706-002-0534-3

A 1:1 molar ratio of the cobalt and manganese salts of p-amino benzoic acid supported on silica gel is an effective catalyst for the oxidation of various organic compounds in reasonable yields using molecular oxygen. The catalyst can be reused several times.

Full text of DOI:10.1007/s00706-002-0534-3

Monatshefte f€ur Chemie 134, 411–418 (2003)
DOI 10.1007/s00706-002-0534-3
Cobalt and Manganese Salts
of p-Aminobenzoic Acid Supported
on Silica Gel: A Versatile Catalyst
for Oxidation by Molecular Oxygen
ꢀ
Mohammed M. Hashemi and Yusef Ahmadibeni
Department of Chemistry, Sharif University of Technology, P. O. Box 11365-9516, Tehran, Iran
Received April 30, 2002; accepted (revised) July 16, 2002
Published online December 20, 2002 # Springer-Verlag 2002
Summary. A 1:1 molar ratio of the cobalt and manganese salts of p-amino benzoic acid supported on
silica gel is an effective catalyst for the oxidation of various organic compounds in reasonable yields
using molecular oxygen. The catalyst can be reused several times.
Keywords. Oxidation; Molecular oxygen; Cobalt; Manganese; Silica gel.
Introduction
Solid phase organic synthesis for preparation of various molecules has provided an
attractive option for organic synthesis [1–4]. Modification of the activity and prod-
uct selectivity are the characterizations of this method.
Solid supported reagents for organic reactions were first introduced by
Merrifield [5] and Letsinger [6]. Easy access to the products via a simple filtration
for removing unreacted materials has rendered the method very attractive. Organic
compounds such as polystyrene and its analogs have long been of interest as solid
supports [7, 8]. Inorganic supports such as silica gel, alumina and clays are also of
interest [9, 10].
Results and Discussion
During the course of our systematic study on the catalytic oxidation of organic
compounds with molecular oxygen, we have recently developed a heterogeneous
catalytic method based on Co(II) and Mn(II) salts of p-aminobenzoic acid sup-
ported on silica gel for the oxidation of different functional organic compounds.
ꢀ
Corresponding author. E-mail: mhashemi@sharif.edu

412
M. M. Hashemi and Y. Ahmadibeni
Table 1. Oxidation of Hantzsch 1,4-DHP (1a) to pyridine (2a) using different Co and Mn salts ratio
with molecular oxygen (all reactions in heptane at reflux temperature in 3 min; yields refer to pure
isolated products)
Entry
Mn=Co Salt Ratio
Yield (%)
1
2
3
4
5
6
7
Mn only
Co only
1 : 1
93
96
98
95
92
96
94
1 : 2
2 : 1
1 : 5
5 : 1
Control experiments of oxidation of 1,4-dihydropyridine (1a) to pyridine (2a)
were done under different conditions to determine the effects of cobalt and man-
ganese salts of p-aminobenzoic acid supported on silica gel separately or as a
mixture [11]. The best results were obtained when a mixture of 1:1 ratio of the
cobalt and manganese salts were used. The results are summarized in Table 1.
In this method silica gel is activated in a pyrolysis apparatus at 260ꢁC in 20 mm
Hg to lose its water (5.7%). Then p-aminobenzoic acid is reacted with the activated
silica gel to bind the acid to the surface of the silica gel which was verified by
infrared spectrum [12].
The reaction of the acid bonded to the silica gel with sodium carbonate affords
the sodium salt of the acid, which is then converted to the cobalt or manganese salts
of p-aminobenzoic using cobalt and manganese chloride solutions. The first use of
the mixture of Co(II) and Mn(II) salts of p-aminobenzoic acid supported on silica
gel was the oxidation of alcohols by molecular oxygen to their corresponding
aldehydes and ketones [12]. This oxidation method is selective and only allylic
and benzylic alcoholes can be oxidized, Table 2.
Table 2. Oxidation of benzylic and allylic alcohols to carbonyl compounds (all reactions at reflux
temperature; yields refer to pure isolated products)
Substrate
Product
Time (h)
Yield (%)
C6H5CH2OH
C6H5CHO
27
26
30
30
30
28
21
20
5
80
85
77
75
60
83
88
95
99
37
70
74
4-(OCH3)C6H4CH2OH
4-(NO2)C6H4CH2OH
3-(NO2)C6H4CH2OH
2-(NO2)C6H4CH2OH
4-(CH3)C6H4CH2OH
C6H5CHOHCH3
4-(OCH3)C6H4CHO
4-(NO2)C6H4CHO
3-(NO2)C6H4CHO
2-(NO2)C6H4CHO
4-(CH3)C6H4CHO
C6H5COCH3
(C6H5)2CHOH
(C6H5)2CO
C6H5CHOHCOC6H5
CH2¼CHCH2OH
C6H5COCOC6H5
CH2¼CHCHO
35
23
33
C6H5CH¼CHCH2OH
C6H5CH¼CHCHOHCH
C6H5CH¼CHCHO
C6H5CH¼CHCOCH3

Catalyst for Oxidation
413
Table 3. Reusability of catalyst for the oxidation of 1-phenylethanol to acetophenone (conditions see
Table 1)
Experimental trial
Yield (%)
1st
88
82
80
79
2nd
3rd
4th
Table 3 shows the reusability of catalyst after 4 replicate oxidations of 1-phen-
ylethanol as model substrate to the corresponding ketone. The catalysts were
recovered via a simple filtration and dried in oven at 135ꢁC for 4 hours.
The oxidation of Hantzsch 1,4-dihydropyridines is one of theuniquitous prob-
lems in organic chemistry and even in recent years several groups have reported-
new methods for aromatization of these compounds. Many methods have been
developed for the oxidation of 1,4-DHPs [13–27] but some of them suffer from
low chemical yields [18, 22], strong oxidative conditions [15, 18, 23], side product
formation or dealkylation at 4-position [13, 21]. In this regard, special attention has
been focused on the developing of heterogeneous systems in order to support of
reagents and to achieve catalytic activities.
We have reported another application of our heterogeneous catalytic method
for the oxidation of Hantzsch 1,4-dihydropyridines to corresponding pyridines
[28]. The aromatization of 4-alky 1,4-dihydropridines were carried out in n-hep-
tane as solvent at 93ꢁC reflux temperature and because of nonsolubility of 4-aryl
and 4-heteroaryl 1,4-dihydropridines in n-heptane, the oxidation of 4-aryl and 4-
heteroaryl 1,4-dihydropridines were carried out in toluene as solvent at 105ꢁC
reflux temperature, Table 4. This reactions are selective and only 4-substituted
pyridine were obtained.
Following the success of oxidation of Hantzsch 1,4-dihydropyridines with Co
and Mn salts=O2 systems we extended this heterogeneous method by raising of
temperature. The oxidation reactions in higher temperature led to an oxidative
dealkylation of 4-alkyl-1,4-dihydropyridines in n-octane at 118ꢁC (reflux tempera-
ture), Table 5. However in the case of 4-aryl and 4-heteroaryl 1,4-dihydropyridines
no dearylation process occurred by raising of temperature to 150ꢁC. At higher
temperatures (up to 150ꢁC) the catalyst was decomposed and the yields dropped
drastically.
Table 6 shows the reusability of the catalyst after 4 replicate oxidations of
ethyl-1,4-dihydro-2,3-dimethyl-3,5-pyridine dicarboxylate and phenyl-1,4-dihy-
dro-2,3-dimethyl-3,5-pyridine dicarboxylate as model substrates to the correspond-
ing pyridines. The catalyst was recovered quantitatively after each experiment.
The oxidation of phenols to quinones is another application of this catalytic
oxidation system. Reagents such as cobalt(II) chelate complex [29], H2O2 cata-
lyzed by three methyl oxorenium(VII) [30], Fermys salts [31, 32], C6H5SeO2H and
(C6H5SeO)2O [33], Pb2O [34], CrO3 [35], DDQ [36], HgO and Hg(OCOCF3) [38],
CAN [39], Ag2CO3=Celite [40], and singlet oxygen [41], were reported for the

414
M. M. Hashemi and Y. Ahmadibeni
Table 4. Oxidation of Hantzsch 1,4-DHPs using molecular oxygen catalyzed by Co and Mn salts (all
reactions at reflux temperature; yields refer to pure isolated products)
1,4-DHPs
4-R
Solvent
Time (min)
Pyridines
Yield (%)
1a
1b
1c
1d
1e
1f
H
n-heptane
n-heptane
n-heptane
n-heptane
n-heptane
n-heptane
n-heptane
toluene
3
15
8
2a
2b
2c
2d
2e
2f
98
92
98
97
97
95
93
95
91
93
92
96
92
95
CH3
CH2CH3
CH2CH2CH3
CH2CH(CH3)2
CH2C6H5
CH2CH2C6H5
C6H5
8
12
10
10
8
1g
1h
1i
2g
2h
2I
p-(CN)C6H4
m-(NO2)C6H4
o-(OCH3)C6H4
p-(OCH3)C6H4
2-furyl
toluene
12
12
15
15
10
8
1j
toluene
2j
1k
1l
toluene
2k
2l
toluene
1m
1n
toluene
2m
2n
2-thienyl
toluene
Table 5. Oxidative dealkylation of Hantzsch 1,4-DHPs at high temperature using molecular oxygen
catalyzed by Co and Mn salts (all reactions at reflux temperature; yields refer to pure isolated products)
1,4-DHPs
R
Time (min)
Yield (%) of Pyridines
1b
1c
1d
1e
1f
CH3
15
8
2b (40)
2c (34)
2d (35)
2e (43)
2f (58)
2g (45)
2a (52)
2a (63)
2a (60)
2a (51)
2a (35)
2a (50)
CH2CH3
CH2CH2CH3
CH2CH(CH3)2
CH2C6H5
8
12
10
10
1g
CH2CH2C6H5
oxidation of phenols to quinones. Our Co and Mn salts=O2 system can also oxidize
phenols to quinones [42], Table 7.
Silyl ethers, acetals, and ketals not only are used for the characterization and
purification of alcohols and carbonyl compounds, but also play an important role in
the protection of organic compounds [43, 44]. A number of methods have been
developed for the deprotection of silyl ethers to the alcohols, [45–47] and oxidative
cleavage of acetals and ketals to the corresponding carbonyl compounds [48–50].
But some of them have limitations for silyl ethers and some of them, in spite of

Catalyst for Oxidation
415
Table 6. Reusability of catalyst for the oxidation of Hantzsch 1,4-DHPs (conditions see Table 3)
Experimental trial
Starting Material
Product
Yield (%)
1st
1c
1c
1c
1c
1c
1h
1h
1h
1h
1h
2c
2c
2c
2c
2c
2h
2h
2h
2h
2h
98
95
93
91
90
95
90
87
85
84
2nd
3rd
4th
5th
1st
2nd
3rd
4th
5th
Table 7. Oxidation of phenols to quinones using Co and Mn salts=O2 system (all reactions at reflux
temperature; yields refer to pure isolated products)
Substrate
Solvent
Product
Yield (%)
phenol
benzene
1,4-benzoquinone
64
53
57
39
36
65
3-methylphenol
3,5-dimethylphenol
2-chlorophenol
2-bromophenol
1-naphthol
toluene
2-methyl-1,4-benzoquinone
2,6-dimethyl-1,4-benzoquinone
2-chloro-1,4-benzoquinone
2-bromo-1,4-benzoquinone
1,4-naphthoquinone
toluene
ethylbenzene
ethylbenzene
benzene
Table 8. Oxidative deprotection of silyl ethers, acetals and ketals by Co and Mn salts=O2 system
(yields refer to pure isolated products)
Substrate
Product
Time (h)
Yield (%)
C6H5CH2OSi(CH3)3
C6H5CH2OH
12
15
8.5
12
13.5
8
90
95
96
88
91
83
86
88
66
75
C6H5CH¼CHCH2OSi(CH3)3
(C6H5)2CHOSi(CH3)3
C6H5CH¼CHCH2OH
(C6H5)2CHOH
CH2¼CHCH2OSi(CH3)3
4-(Cl)C6H4CH2OSi(CH3)3
(C6H5)2C(OCH2CH2O)
CH2¼CHCH2OH
4-(Cl)C6H4CH2OH
(C6H5)2CO
C6H5CH3C(OCH2CH2O)
4-(OCH3)C6H4CH3C(OCH2CH2O)
4-(NO2)C6H4CH(OCH2CH2O)
(CH2)5CH(OCH2CH2O)
C6H5CH3CO
10
11
10
10
4-(OCH3)C6H4CH3CO
4-(NO2)C6H4CHO
(CH2)5CHO
generality, have the drawback of using expensive oxidants and tedious work up,
low chemical yields, strong oxidative conditions or side product formation.
Another application of our heterogeneous catalytic system is oxidative depro-
tection of three methyl silyl ethers to their corresponding original alcohols. Oxi-
dative deprotection of acetals and ketals to aldehydes and ketones can also be done
with the Co and Mn salts=O2 oxidant system under mild and nonaqueous condition
[51], Table 8.

416
M. M. Hashemi and Y. Ahmadibeni
Experimental
Chemicals were purchased from Merck, Aldrich, Fluka and Riedel Dehaen AG chemical companies
and were used without further purification. IR spectra were recorded on a FT-IR Unicam Mattson 1000
spectrophotometer. 1H-NMR spectra were recorded on a Brucker AC-80 (80 MHz) spectrometer
in CDCl3. All products are known compounds and they were identified by their mp’s or b p’s, IR
1
and H-NMR spectroscopic properties. All yields refer to pure isolated products.
Preparation of Activated Silica Gel
In a pyrolysis apparatus silica gel (5g) was heated at 260ꢁC under reduced pressure (20mm Hg) using
phosphorus pentoxide (5g) as desiccant. The silica gel was reduced by 0.28 g in weight after 10h
(5.7wt%).
Reaction of Activated Silica Gel and p-Aminobenzoic Acid
To a mixture of ethyl acetate (70 cm3) and p-aminobenzoic acid (2.13 g, 15 mmol) was added activated
silica gel (4.79 g). The reaction mixture was then heated at reflux temperature with magnetic stirring at
80ꢁC for 16h and then cooled to room temperature and filtered. The solid mass was washed with ethyl
acetate for 10 h in a Soxhlet apparatus in order to remove unreacted p-aminobenzoic acid. The solid
was then dried in the oven at 100ꢁC to afford a pale yellow solid (6.05 g); 1.33g (9.7mmol) of the acid
had reacted.
Preparation of Cobalt and Manganese Salts of p-Aminobenzoic Acid
Supported on Silica Gel
The sodium salt of p-amino benzoic acid (5g) supported on silica gel was prepared via reaction of the
acid and a solution of sodium carbonate to give (5.25g, 11mmol, 4.76wt% increase) of the sodium
salt. The presence of sodium salts was verified by X-Ray Fluorescence (XRF) analysis to show a 4.4%
wt increase.
Similarly manganese and cobalt salts of the acid were prepared by using manganese chloride and
cobalt chloride solutions to give 5.39 g of manganese salt as a white solid (6.1% wt increase) and 5.33g
of cobalt salt as a pink solid (6.0% wt increase). These results were verified by XRF analysis.
Oxidation of p-Methoxybenzyl Alcohol to p-Methoxybenzaldehyde (Typical Procedure)
To a stirred solution of p-methoxybenzyl alcohol (1mmol) in 25ml chlorobenzene was added the
cobalt salt (90 mg 0.l mmol) and manganese salt (90 mg 0.l mmol). The reaction mixture was refluxed
during bubbling through oxygen at a rate of 15cm3 minꢂ 1 into the solution. The progress of the
reaction was monitored by TLC (CCl4=Et2O, 1:3). After 26h, the reaction mixture was cooled to room
temperature and filtered to recover the solid material (Co and Mn salts). The filtrate was evaporated and
the residue was purified by column chromatography on silica gel (CCl4=Et2O, 1:3) to obtain
p-methoxy benzaldehyde in 85% yield.
Reusability of Catalyst for the Oxidation of 1-Phenylethanol
to Acetophenone (Typical Procedure)
Oxidation of 1-phenylethanol as model substrate to corresponding ketone was carried out as described
above to give 88% yield of acetophenone. The catalyst was recovered via a simple filtration and dried
in the oven at 135ꢁC for 4 h. The reaction was repeated using the recovered catalyst to give 82% of

Catalyst for Oxidation
417
product. Reusing of the catalyst for third and fourth times gave 80% and 79% yields, respectively. The
catalyst was recovered quantitatively in each experiment.
Oxidation of Phenols to Quinones (General Procedure)
To a stirred solution of phenol (1mmol) in 25cm3 solvent (Table 7) was added the cobalt salt (46 mg,
0.05mmol) and manganese salt (45 mg, 0.05mmol). The reaction mixture was heated at reflux tem-
perature for 8 h during which oxygen was bubbled at a rate of 15cm3 minꢂ 1 into the solution and the
progress of the reaction was monitored by TLC (hexane=EtOAc, 4:1). Then the reaction mixture was
cooled to room temperature and filtered to recover the solid material (Co and Mn salts). The filtrate was
evaporated and the residue was purified by column chromatography on silica gel (hexane=EtOAc, 4:1)
to obtain the desired product.
Oxidative Deprotection of Silyl Ethers, Acetals, and Ketals
To a stirred solution of substrate (1mmol) in 25cm3 of hexane was added the cobalt salt (46mg,
0.05mmol) and manganese salt (45 mg, 0.05mmol). The reaction mixture was heated at reflux tem-
perature during which oxygen was bubbled at a rate of 15 cm3 minꢂ 1 into the solution and the progress
of the reaction was monitored by TLC (hexane=Et2O, 4:1). Then the reaction mixture was cooled to
room temperature and filtered to recover the solid material (Co and Mn salts). The filtrate was
evaporated and the residue was purified by column chromatography on silica gel (hexane=Et2O,
4:1) to obtain the desired product.
Oxidation of Hantzsch 1,4-Dihydropyridines (General Procedure)
To a stirred solution of 1,4-dihydropyridines (1 mmol) in 25cm3 of solvent was added the cobalt salt
(90 mg, 0.l mmol) and manganese salt (90 mg, 0.l mmol). The reaction mixture was heated at reflux
temperature during which oxygen was bubbled at a rate of 15cm3 minꢂ 1 into the solution. Progress of
the reaction was monitored by TLC (CH2Cl2=EtOAc 98:2). The reaction mixture was then cooled to
room temperature and filtered to recover the solid material (Co and Mn salts). The filtrate was
evaporated and the residue was purified by column chromatography on silica gel (CH2Cl2=EtOAc
98:2) to obtain the desired product.
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