Solvent-free and selective oxidation of hydroxy groups to their corresponding carbonyl functions with ferric nitrate activated by heteropoly acids

Article abstract of DOI:10.1055/s-2003-37343

Keggin-type heteropoly acids revealed high catalytic activity for swift and selective oxidation of various hydroxy functionalities to the corresponding carbonyl groups using ferric nitrate as an oxidant under mild and solvent-free conditions. We have found that the catalytic activities of the heteropoly acids were much higher than mineral or solid acids such as sulfuric acid, p-toluenesulfonic acid, triflic acid, acidic Amberlyst-15, Montmorillonite-K10 clay, and HY-zeolite.

Full text of DOI:10.1055/s-2003-37343

408
PAPER
Solvent-Free and Selective Oxidation of Hydroxy Groups to their Correspon-
ding Carbonyl Functions with Ferric Nitrate Activated by Heteropoly Acids
Solvent-Free
Oxid
a
ation of
A
lc
b
ohols ib Firouzabadi,* Nasser Iranpoor,* Kamal Amani
Department of Chemistry, Shiraz University, Shiraz 71454, Iran
E-mail: firouzabadi@chem.susc.ac.ir; E-mail: Iranpoor@chem.susc.ac.ir
Received 21 November 2002
acids is very high and it is in the beginning of its growth
in organic chemistry.
Abstract: Keggin-type heteropoly acids revealed high catalytic ac-
tivity for swift and selective oxidation of various hydroxy function-
alities to the corresponding carbonyl groups using ferric nitrate as
an oxidant under mild and solvent-free conditions. We have found
that the catalytic activities of the heteropoly acids were much higher
than mineral or solid acids such as sulfuric acid, p-toluenesulfonic
acid, triflic acid, acidic Amberlyst-15, Montmorillonite-K10 clay,
and HY-zeolite.
We have recently started studies upon new catalytic prop-
erties of heteropoly acids, especially the tungstophospho-
ric acid (H3PW12O40) which is a cheap, reusable,
heterogeneous and easily available catalyst. We have al-
ready used catalytic amounts of this compound for the ox-
idation of aromatic amines to nitro compounds with
sodium perborate in micellar media,11a thioacetalization
and transthioacetalization reactions,11b its cesium salt for
regioselective bromination of aromatic compounds,11c
and selective O-trimethylsilylation of a wide variety of al-
cohols and phenols using easily available hexamethyldi-
silazane (HMDS) under solvent-free conditions.11d
Key words: alcohols, carbonyl compounds, ferric nitrate, het-
eropoly acids, oxidations
Carbonyl compounds in the fine chemicals industry are
precursors with wide applicability from drugs to fragranc-
es. The oxidation of alcohols to carbonyl products is an
important transformation in organic chemistry,1 which
has received the most attention over the years, especially
in search of versatile and selective reagents in catalytic
applications.2 Several methods are known for selective
oxidation of alcohols to the corresponding carbonyl com-
pounds. Swern oxidation,3 Dess–Martin oxidation,4 and
various metal reagents5 are employed for this purpose.
However, they are not free from some disadvantages and
there is still a need for new methods. The main require-
ments are simplicity of the method, selectivity, in particu-
lar with regard to over-oxidation of carbonyl compounds,
effectiveness, and mildness of the reaction.
Metal nitrates supported on various inorganic supports
have been used as oxidizing agents.12 Iron(III) nitrate sup-
ported on silica gel (Silfen),12a–e iron(III) nitrate or cop-
per(II) nitrate supported on K10-clay (Clayfen or
Claycop),12f–i are the reagents employed for the oxidation
processes. However, the stability of these reagents faces
problems.12k,l Recently, ferric nitrate mixed with HZSM-
5 zeolite (Zeofen) has been successfully employed for the
oxidation of various alcohols in solution and also in the
absence of solvent under microwave irradiation.13
Now we report the use of ferric nitrate (1 mol) in the pres-
ence of tungstophosphoric acid (H3PW, 0.1 mol), as a cat-
alyst for the simple, swift, and selective oxidation of a
wide variety of primary, secondary, and benzylic alcohols
into their corresponding carbonyl compounds under mild
and solvent-free conditions.
The importance and full characterization of various het-
eropoly acids (HPAs) and their properties have been re-
viewed.6 A large number of recent reports concerning the
catalytic activity of Keggin-type heteropoly acids and
polyoxometalates for the oxidation of organic substrates
are available in the literature.7 Solutions of these materials
exhibit very strong Brønsted acidity with acid strengths
more than mineral acids.8 Solid HPAs possess also Brøn-
sted acidity and are stronger than conventional solid acids
such as SiO2/Al2O3, H3PO4/SiO2, and those of zeolites in-
cluding HX, HY and H-ZSM-5 (Si/Al = 40).9 Among het-
eropoly acids, polytungstic acids are the most widely used
catalysts owing to their high acid strengths, thermal stabil-
ities, and low reducibilities. Most of HPA catalysts are en-
vironmentally friendly and regarded as promising green
catalysts.10 It is evident that the application of heteropoly
Effect of Catalyst
Ferric nitrate nonahydrate, an abundant, cheap, non-toxic
and commercially available can be used as an oxidant in
the presence of catalytic amounts of heteropoly acids or
polyoxometalates. Primarily, oxidation of benzyl alcohol,
as a model compound, was performed in the presence of
several heteropoly acids or their salts, and some other
common or solid acids, with this oxidant under solvent-
free conditions (Table 1). The results show that in the ab-
sence of the catalysts, the oxidation reaction did not pro-
ceed after 30 minutes at room temperature, but in the
presence of heteropoly acids this conversion proceeded
efficiently in a few minutes under similar reaction condi-
tions (Table 1, entries 2–6). In order to show the strong
Synthesis 2003, No. 3, Print: 28 02 03.
Art Id.1437-210X,E;2003,0,03,0408,0412,ftx,en;T08802SS.pdf.
© Georg Thieme Verlag Stuttgart · New York
ISSN 0039-7881

PAPER
Solvent-Free Oxidation of Alcohols
409
catalytic activity of heteropoly acids with the other acid Table 2 Oxidation of Benzyl Alcohol with Various Metal Nitrates
in the Presence of H3PW at Room Temperature in a Solventless
catalysts, we have studied the catalytic activities of Am-
berlyst-15, Montmorillonite-K10 Clay, HY-zeolite, sul-
Systema
furic acid, p-toluenesulfonic acid, and triflic acid for this
En- Metal Nitrate
try
Subst./
H3PW/
Metal
Time
(min)
Conv. GC
purpose. We found that none of them was an effective cat-
alyst for the oxidation of benzyl alcohol to benzaldehyde
in the presence of ferric nitrate nonahydrate (Table 1, en-
tries 9–14). Therefore, we chose tungstophosphoric acid;
H3PW12O40 that is a cheap and a commercially available
compound as a catalyst for further oxidation studies.
(%)
Yield
(%)b
Nitrate
1
2
3
4
5
6
7
8
9
none
1:0.1:0
30
15
15
15
15
15
15
10
30
15
4
0
16
0
NaNO3
1:0.1:2.5
1:0.1:1
10
Mg(NO3)2 6H2O
Zn(NO3)2 6H2O
Cu(NO3)2 3H2O
Ce(NH4)2(NO3)6
Cr(NO3)3 9H2O
Bi(NO3)3 5H2O
Zr(NO3)4
47
29
Table 1 Oxidation of Benzyl Alcohol with Fe(NO3)3 9H2O in the
Presence of Various Heteropoly Acids, their Salts and other Common
or Solid Acids at Room Temperature in a Solventless Systema
1:0.1:1
50
32
1:0.1:1
63
42
En- Catalyst
try
Subst./
Cat./
Oxidant
Time Conv. GC
1:0.1:0.5
1:0.1:0.5
1:0.1:0.5
1:0.1:0.5
1:0.1:0.5
1:0.1:0.5
63
56c
61
(min) (%)
Yield
(%)b
0
61
1
2
3
4
5
6
7
8
9
none
1:0:0.5
30
4
0
100
100
100
100
100
68
100
3
99
H3PW12O40
1:0.1:0.5
1:0.1:0.5
1:0.1:0.5
1:0.1:0.5
>98
>98
>98
>98
95
3
H3PW12O40 nH2O
H5PV2Mo10O40 nH2O
H4SiW12O40
6
10 Fe(NO3)3 9H2O
11 Fe(NO3)3 9H2O
12 Fe(NO3)3 9H2O
95
93d
quan.
quan.
5
100
100
8
1:0.05:0.5 15
(NH4)2HPW12O40
α-K6P2W18O62 14H2O
Na2WO4 2H2O, (CO2H)2
amberlyst-15H+c
1:0.1:0.5 10
1:0.05:0.5 30
1:1:1:0.5 30
a Reaction conditions: Metal nitrate (1 mmol), H3PW (0.2 mmol),
benzyl alcohol (2 mmol).
43
b n-Octane was used as an internal standard.
c We have also tried this reaction with ca. 015 equiv of
Ce(NH4)2(NO3)6. Reaction proceeded under longer reaction time (ca.
2 h) with only 45% conversion to the aldehyde plus unreacted starting
material.
11
11
1:-:0.5
1:-:0.5
1:-:0.5
30
30
30
22
20
10 montmorillonite-K10c
11 HY-Zeolitec
12 TfOH
14
14
d Catalyst, ferric nitrate and acetone (10 mL) was mixed and stirred
for 10 min. The solvent is then removed under reduced pressure at
<50 °C and the obtained slurry mixture was employed for the oxida-
tion reaction under solvent-free conditions.
9
9
1:0.3:0.5 30
1:0.3:0.5 30
1:0.2:0.5 30
44
35
13 TsOH
18
15
14 H2SO4
40
33
Effect of Solvent
a Reaction conditions: Fe(NO3)3 9H2O (1 mmol), catalyst (0.05–1
mmol), benzyl alcohol (2 mmol).
The oxidation of benzyl alcohol was also conducted in
different solvents. The results show that the efficiency and
the yield of the reaction in solutions were much less than
that observed under solvent-free conditions (Table 3).
b n-Octane was used as an internal standard.
c 0.2 g of the material was used.
Therefore, we have conducted the oxidation of structural-
ly different alcohols with this oxidation system under sol-
vent-free conditions and the results are compared with
some of those reported using Silfen, Clayfen and Zeofen
(Table 4).
Effect of Countercation
We also investigated other metal nitrates for the oxidation
of benzyl alcohol to benzaldehyde under similar reaction
conditions (Table 2). The results revealed that most of the
metal nitrates were effective oxidants for this transforma-
tion, but iron and bismuth(III) nitrates gave better results.
Fe(NO3)3 9H2O/H3PW12O40 (with a 10:1 molar ratio) is a
stable mixture and does not need any precautions that are
required by Clayfen,12k,l which should be prepared and
used freshly and should be kept under n-pentane.
According to the results, our method is suitable for the ox-
idation of primary and secondary benzylic and secondary
saturated alcohols. Allylic and propargylic alcohols af-
forded unidentified polymeric materials. Primary saturat-
ed alcohols were also oxidized to their aldehydes in low
yields without over-oxidation to the carboxylic acids.
Synthesis 2003, No. 3, 408–412 ISSN 0039-7881 © Thieme Stuttgart · New York

410
H. Firouzabadi et al.
PAPER
Table 3 Oxidation of Benzyl Alcohol with Fe(NO3)3 9H2O in the
Selectivity
Presence of Tungstophosphoric Acid (H3PW) in Various Solventsa
In order to show the chemoselectivity of the method for
the oxidation of different hydroxyl groups, we have per-
formed a number of competitive reactions as shown in
Scheme 1.
En- Solvent
try
Temp. Time
Conv. GC
(°C)
(min)
(%)
25
6
Yield (%)
1
2
3
4
5
6
7
8
1,2-dichloroethane 55–60
60
17
5
CHCl3
55–60 150
reflux 120
In conclusion, we have introduced a new catalyst for the
activation of ferric nitrate nonahydrate for the oxidation of
primary and secondary saturated and benzylic alcohols
into their corresponding carbonyl compounds. Over-oxi-
dation did not occur by using this method. The handling
of the mixture of ferric nitrate nonahydrate and tungsto-
phosphoric acid is easy and does not require any precau-
tion for its storage. Stability of the oxidation system, high
reaction rates, high yields of the products, solvent-free
and mild reaction conditions are worthy of mention as ad-
vantages of the presented protocol.
CH2Cl2
30
75
0
30
23
0
EtOAc
50
r.t.
r.t.
r.t.
r.t.
150
15
acetone
MeCN
150
30
70
100
100
39
73
76
petroleum ether
n-hexane
15
a Reaction conditions: benzyl alcohol (2 mmol), ferric nitrate
(1 mmol), H3PW (0.2 mmol), solvent (10 mL).
Table 4 Oxidation of Various Alcohols with Ferric Nitrate Nonahydrate Catalyzed By the Tungstophosphoric Acid (H3PW)
Entry
Alcohol
Subst./H3PW/
Fe(NO3)3
Product
Time (min) Temp. (°C) Yielda
(%)
Yield (%)
(other methods)
1
2
PhCH2OH
PhCH2OH
1:0.1:0.5
PhCHO
PhCHO
5
r.t.
95
(75),b (81),c (90)d
1:0.05:0.5
15
r.t.
96
–
3
4-MeOC6H4CH2OH 1:0.1:0.5
4-MeOC6H4CHO 10
r.t.
97
(66)b
4
4-MeC6H4CH2OH
4-ClC6H4CH2OH
2-NO2C6H4CH2OH
2-HOC6H4CH2OH
Ph2CHOH
1:0.1:0.5
4-MeC6H4CHO
4-ClC6H4CHO
2-NO2C6H4CHO
2-HOC6H4CHO
Ph2CO
10
20
15
10
15
5
r.t.
96
(88)d
5
1:0.1:0.5
r.t.
97
–
6
1:0.1:0.625
1:0.1:0.5
55–60
r.t.
95
(73),b (85)d
7
23
–
8
1:0.1:0.625
1:0.1:0.5
55–60
r.t.
96
(88),c (82)d
9
PhCH(OH)Me
PhC(O)Me
95
(45),b (81),c (80)d
10
11
12
13
14
15
PhCH(OH)Et
1:0.1:0.5
PhC(O)Et
10
10
60
90
90
24 (h)
60
r.t.
94
–
D-PhCH(OH)CO2H
PhCH(OH)C(O)Ph
PhCH2CH2OH
1:0.1:0.75
1:0.1:1
PhC(O)CO2H
PhC(O)C(O)Ph
PhCH2CHO
55–60
55–60
r.t.
93
–
95e
60f
56f
10–15f
52f
92
–
1:0.1:0.625
1:0.1:0.625
1:0.1:0.625
(63)b
PhCH2CH2CH2OH
Me(CH2)6CH2OH
PhCH2CH2CHO
Me(CH2)6CHO
r.t.
–
r.t.
–
55–60
r.t.
(60)b
16
Me(CH2)5CH(OH)Me 1:0.1:0.625
Me(CH2)5C(O)Me 3 (h)
15
–
–
–
–
55–60
55–60
55–60
90
17
18
norborneol
1:0.1:0.625
1:0.1: 0.5
norcamphor
15
10
94
PhCH(OH)CH2OH
PhC(O)CH2OH
78
a Yields refer to isolated pure products. The products were characterized by IR and 1H NMR spectral data.
b Conditions: Silfen, CHCl3, reflux, 4–5 h.12a
c Conditions: Clayfen, n-pentane or n-hexane, reflux, 1.5–3 h.12j,k
d Conditions: Zeofen, CH2Cl2, r.t., 20–40 min.13
e In the presence of 10 drops of EtOAc.
f Yields were determined by 1H NMR spectroscopy.
Synthesis 2003, No. 3, 408–412 ISSN 0039-7881 © Thieme Stuttgart · New York

PAPER
Solvent-Free Oxidation of Alcohols
411
Scheme 1
lution of NaHCO3 (2 × 5 mL) (except for entry 11, Table 4), H2O
(2 × 25 mL), and dried (Na2SO4). Evaporation of the solvent under
reduced pressure gave the almost pure carbonyl compound. When
necessary, further purification was performed by column chroma-
tography on silica gel (70–230 mesh) or by preparative TLC (silica
gel plates) using n-hexane–EtOAc as eluents (Table 4, entries 11,
12, 18).
Tungstophosphoric acid (H3PW12O40 nH2O) was purchased from
Merck and was purified by extraction with Et2O from an aqueous
solution of the acid. After evacuation at 150–300 °C for 1–2 h under
reduced pressure, pure H3PW12O40 was obtained.14 Tungstosilicic
acid, H4SiW12O40 nH2O was purchased from Merck and dehydrated
by evacuating at 150–300 °C. The acidic salts of heteropoly acids
were prepared according to the known procedures.15
H5V2Mo10O40 nH2O and α-K6P2W18O62 14H2O were prepared by
known procedures.16,17 All products are known and commercially
available and hence their physical data are not given. The products
were purified by column chromatography or TLC on silica gel,
when necessary, the purity determination of the products was ac-
complished by GC on a Shimadzu model GC-14A instrument or by
TLC on silica gel polygram SIL G/UV 254 plates. The IR spectra
were recorded on a Perkin Elmer 781 spectrophotometer. The NMR
spectra were recorded on a Bruker Avance DPX 250 MHz spec-
trometer.
Table 4, entry 6
Anal. Calcd for C7H5O3N: C, 55.62; H, 3.31. Found: C, 55.45; H,
3.24.
Table 4, entry 12
Anal. Calcd for C14H10O2: C, 80.00; H, 4.76. Found: C, 79.8; H,
4.78.
Acknowledgment
The authors are thankful to the Shiraz University Research Council
for the partial support of this work.
Oxidation of Alcohols with Ferric Nitrate Nonahydrate in the
Presence of Tungstophosphoric Acid; General Procedure
Ferric nitrate nonahydrate (0.404–0.808 g, 1–2 mmol,) and tungsto-
phosphoric acid (0.288–0.576 g, 0.1–0.2 mmol) were mixed togeth-
er in a test-tube and was ground gently by a glass rod. Then the
corresponding alcohol (2 mmol) was added and the resulting mix-
ture was thoroughly mixed by a glass rod for the appropriate reac-
tion time and temperature (see Table 4). After completion of the
reaction (monitored by TLC or GC, and also completion of the re-
action was clearly visible by discontinuation of the evolution of red-
dish nitrogen dioxide fumes), the test-tube was laid aside for 10–15
min in order to expel thoroughly the nitrogen dioxide fumes. The
mixture was extracted repeatedly with CH2Cl2 (3 × 20 mL). The
combined organic solutions were washed repeatedly by aq sat. so-
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Synthesis 2003, No. 3, 408–412 ISSN 0039-7881 © Thieme Stuttgart · New York