Mild aerobic oxidation of alcohols catalysed by Fe2(SO 4)3/4-OH-TEMPO/NaNO2

Article abstract of DOI:10.3184/174751913X13700197900636

An environmentally friendly catalytic system of Fe₂(SO ₄)₃/4-OH-TEMPO/NaNO₂ has been developed for aerobic oxidation of alcohols to aldehydes and ketones. The catalytic system allowed efficient and selective oxidation of a wide range of alcohols using oxygen at 1 atmospheric pressure and room temperature. 100% selectivity and excellent conversion rates were observed for benzylic alcohols and cinnamyl alcohol.

Full text of DOI:10.3184/174751913X13700197900636

398 RESEARCH PAPER
JULY, 398–401
JOURNAL OF CHEMICAL RESEARCH 2013
Mild aerobic oxidation of alcohols catalysed by
Fe2(SO4)3/4-OH-TEMPO/NaNO2
Xiang-Jun Shi, Jie Qian, Fan-Fan Tan and Chuan-Ming Yu*
Key Laboratory for Green PharmaceuticalTechnologies and Related Equipment of Ministry of Education, College of Pharmaceutical
Sciences, Zhejiang University ofTechnology, Hangzhou 310014, P. R. China
An environmentally friendly catalytic system of Fe2(SO4)3/4-OH-TEMPO/NaNO2 has been developed for aerobic oxida-
tion of alcohols to aldehydes and ketones. The catalytic system allowed efficient and selective oxidation of a wide
range of alcohols using oxygen at 1 atmospheric pressure and room temperature. 100% selectivity and excellent
conversion rates were observed for benzylic alcohols and cinnamyl alcohol.
Keywords: alcohols, TEMPO, iron salts, aerobic oxidation, oxygen
Selective oxidation of alcohols to the corresponding aldehydes
or ketones is important in chemical syntheses of pharmaceuti-
cals, fragrances food additives and more.1–3 Traditionally, the
alcohol oxidation requires addition of a stoichiometric amount
of inorganic oxidants such as CrO3,4 KMnO4 and MnO2,5 which
generate hazardous by-products and cause environmental
problems, especially when the reaction is carried out on an
industrial scale. To address this issue, many research groups
have been looking for an efficient, high yield and selective
catalyst system for the oxidation of alcohols to the correspond-
ing carbonyl compounds. In 1975, Cella6 and Ganern7 discov-
ered that 2,2,6,6-tetramethyl-piperidyl-1-oxy (TEMPO) could
be used in the alcohol oxidation. Later, numerous TEMPO-
related systems have been reported for the alcohol oxidation.
Among these, the catalytic systems using both transition metals
and TEMPO are of particular interest. Sheldon et al.8 devel-
oped a RuCl2(PPh3)3/TEMPO catalytic system for efficient
aerobic oxidation of primary and secondary alcohols. Stahl
et al.9 reported a CuI/TEMPO catalyst system that could oxi-
dise primary alcohols under an ambient atmosphere of air at
room temperature. Recently, Liang et al.10 reported the first use
of FeCl3/TEMPO/NaNO2 as a catalytic system and achieved
quantitative conversion in alcohol oxidations. This cheap and
environmentally friendly iron/TEMPO catalytic system has
quickly attracted a lot of attention. For further optimisation
of the FeCl3/TEMPO/NaNO2 catalytic system, Liu et al.11
screened a variety of 4-substituted-TEMPO examples and dis-
covered that among these, 4-acetamido-TEMPO was the most
effective one, but 4-OH-TEMPO was the least active one and
showed only a 49% conversion rate. Wang et al.12 used silica
gel to enhance the catalytic activity of various iron salt/TEMPO
systems; catalyst containing FeCl3 showed good activity, but
Fe2(SO4)3 was not effective and yielded only a 8% conversion
rate. Ma et al.13,14 developed a Fe(NO3)3/TEMPO catalytic
system to oxidise alcohols, including allenols and propargyl
alcohols, under aerobic conditions at room temperature; inter-
estingly, a catalytic amount of chloride ion was reported to be
able to expedite the reaction, so sodium chloride was added to
provide an inorganic halide ligand. Furthermore, Aoyama
et al.15 reported a catalytic system using Fe(NO3)3 in a non-
polar solvent and 4-hydroxy-TEMPO-NaCl loaded SiO2 which
could be recovered by filtration and reused; the oxidation
of various alcohols proceeded smoothly at room temperature.
Zheng16 filed a patent for citronellal preparation through
citronellol oxidisation using catalytic systems containing
TEMPO or its derivatives, iron-, cuprous- or tin-containing
salts, various ligands and bases.
iron salt/TEMPO catalytic system to increase its selectivity
and efficiency, so the method could be used on an industrial
scale. After screening various combinations of iron salts
and TEMPO derivatives, we discovered that a novel four-
component oxidation system consisting of 1 atm O2, Fe2(SO4)3,
4-OH-TEMPO(TMHPO) and NaNO2 could oxidise most
benzylic alcohols and cinnamyl alcohol with 100% selectivity
and excellent yields. We now report the development of our
four-component oxidation system, its reaction mechanism,
and the oxidation results using various alcohols.
Results and discussion
Oxidation of 4-methylbenzyl alcohol was chosen as a model
reaction to determine the optimal reaction conditions. First, we
screened various iron salts in the presence of 1 atm O2, 5 mol%
TMHPO, 5 mol% NaNO2 at room temperature. As shown
in Table 1 in the presence of the iron salts, the alcohol was
converted to 4-methylbenzaldehyde with 100% selectivity
(Table 1, entries 1–5), and no acid formation was observed.
Among these iron salts, Fe2(SO4)3 was the most efficient cata-
lyst (Table 1, entry 3) which converted the alcohol to the alde-
hyde in 3h with 98.3% alcohol conversion rate. FeSO4•7H2O
(Table 1, entry 4) achieved 93.4% conversion rate in 6h. The
conversion rates obtained in the presence of FeCl3•6H2O,
Fe(NO3)3•9H2O, FeCl2•4H2O were significantly lower (69–
78%, Table 1) and required longer reaction time. Also, these
results showed that Fe3+ salts (Fe2(SO4)3 or FeCl3•6H2O) had
a better catalytic activity than the corresponding Fe2+ salts
(FeSO4•7H2O or FeCl2•4H2O). Interestingly, the counterion of
the iron salts also had some effect on the conversion rate. The
sulfate ion was more effective in this catalytic system than
Table 1 Oxidation of 4-methylbenzyl alcohol in the presence
of various iron salts
Entrya
Iron salt
Time/h Select./%b Conv./%b
1
FeCl3•6H2O
FeCl2•4H2O
12
24
3
100
100
100
100
100
100
100
100
77.5
70.2
98.3
93.4
69.1
93.0
91.3
79.8
2
3c
4
Fe2(SO4)3
FeSO4•7H2O
6
5
Fe(NO3)3•9H2O
Fe2(SO4)3+0.1%NaCl
Fe2(SO4)3+1.0%NaCl
Fe2(SO4)3+15%NaCl
12
3
3
3
6d
7e
8f
a Reaction conditions: 5 mmol 4-methylbenzyl alcohol, 5 mol%
iron salt, 5 mol% TMHPO, 5 mol% NaNO2, 5 mL acetonitrile,
10 mL water, 1 atm O2, room temperature.
b Conversion and selectivity are based on gas chromatography
with area normalisation.
The iron/TEMPO catalytic system showed early promise as
an inexpensive and environmentally friendly synthetic method
for alcohol oxidation. We were interested in optimising the
c Fe2(SO4)3 was analytical grade, the content of chlorine ion was
0.003%.
d 0.1% of the mass of Fe2(SO4)3. e 1% of the mass of Fe2(SO4)3.
f 15% of the mass of Fe2(SO4)3.
* Correspondent. E-mail: pharmlab@zjut.edu.cn

JOURNAL OF CHEMICAL RESEARCH 2013 399
Table 2 Influence of the co-solvent/water mixture on the
a slight decrease of conversion (Table 2, entry 3). These indi-
cated that water might play a key role in improving the cata-
lytic activity. The addition of water might help by forming a
more homogeneous reaction system which could accelerate the
oxidation reaction. We also tested three other water-miscible
solvent mixtures (DMF/water, THF/water and acetone/water
solvent mixture) at a ratio 1/2 and 2/1 (Table 2, entries 4–9),
and all the conditions gave 4-methylbenzaldehyde in moderate
conversion.
oxidation of 4-methylbenzyl alcohol
Entrya Solvent mixture (v/v)
Time/h Select./%b Conv./%b
1
2
3
4
5
6
7
8
9
Acetonitrile
Acetonitrile/water (1/2)
Acetonitrile/water (2/1)
DMF/water(1/2)
12
4
Trace
100
100
100
100
100
100
100
100
100
4
91.4
89.3
76.2
79.3
92.2
63.8
65.0
8
24
9
24
24
24
DMF/water(2/1)
THF/water(1/2)
THF/water(2/1)
Acetone/water (1/2)
Acetone/water (2/1)
The optimised reaction conditions were applied to the oxi-
dation of a variety of alcohols in order to explore the versatility
of this aerobic catalytic system. As revealed in Table 3, most
benzylic alcohols were converted into the corresponding
aldehydes or ketones in high yield and excellent selectivity
(Table 3, entries 1–7, 9, 10, 17–19 and 21). However, the elec-
tronic property of the substituents on the benzene ring had
an apparent effect; benzylic alcohols with electron-donating
substituents (Table 3, entries 2–5) were converted to the cor-
responding aldehydes in shorter time with higher conversion
rate, in comparison with those bearing electron-withdrawing
substituents (Table 3, entries 6–11). In the case of benzyl alco-
hols containing the electron-donating methoxy group (Table 3,
entries 3–5), the substitution position affected the reaction
rate; the o- and p- substituted compounds (Table 3, entries 3
and 5) reacted faster than that of the m-substituted one
(Table 3, entry 4). Perhaps, in this case, the electronic effects
are dominant over the steric effects. In the case of benzyl alco-
hols substituted with the electron-withdrawing fluoride group
(Table 3, entries 6–8), the m- and p-substituted compounds
showed much better results than the o-substituted example.
This might be due to the effect of the intermolecular hydrogen
bonding between the fluoride atom at o-position and the
hydroxyl group. Secondary benzylic alcohols could be con-
verted to the corresponding ketones (Table 3, entries 17–21),
but 1-(2-chlorophenyl)ethanol gave only a 30% isolated yield
a Reaction conditions: 5 mmol 4-methylbenzyl alcohol, 5 mol%
Fe2(SO4)3, 5 mol% TMHPO, 5 mol% NaNO2, 1 atm O2, room
temperature.
b Conversion and selectivity are based on gas chromatography
with area normalisation.
chloride ion and nitrate ion. Ma and Aoyama13–15 reported
that chloride ion could promote the catalytic activity, but in
our catalytic system addition of chloride ion decreased the
conversion rate (Table 1, entries 6–8).
The type of solvent was reported to have a great influence
on the oxidation of alcohols when a transition metal catalytic
system was used.17–19 In our early trials, we chose acetonitrile
as the solvent because of its excellent dissolution properties to
most organics (Table 2, entry 1). However the alcohol substrate
remained unreactive probably due to the poor solubility of
Fe2(SO4)3 in acetonitrile. Therefore, a mixture of acetonitrile/
water was employed in the reaction. As shown in Table 2,
100% conversion of the alcohol to 4-methylbenzaldehyde with
100% selectivity was achieved in a 1:2 (v/v) acetonitrile/water
solvent mixture after 4h reaction (Table 2, entry 2). Also,
increasing the percentage of acetonitrile in the solvent led to
Table 3 Catalytic aerobic oxidation of alcohols in the presence of 4-OH-TEMPO, Fe2(SO4)3 and NaNO2a
Entry
R1
R2
Products
Time/h
Conv./%b
Select./%b
Yield/%c
1
2
3
C6H5
4-CH3C6H4
4-CH3OC6H4
3- CH3OC6H4
2- CH3OC6H4
4-FC6H4
3- FC6H4
2- FC6H4
4-ClC6H4
4-BrC6H4
4-NO2C6H4
C6H5CH=CH
3-Pyridyl
2-Furyl
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
1b
2b
3b
4b
5b
6b
7b
8b
9b
10b
11b
12b
13b
14b
15b
16b
17b
18b
19b
20b
21b
8
4
3
8
5
12
12
24
12
12
12
24
24
24
24
24
3
98.8
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
50
100
100
100
100
100
100
89
93
95
96
93
95
85
45
93
92
40
92
21
60
–
30
95
94
93
30
94
4
5
6d
100
7d
91.8
53.1
98.2
97.2
46.2
96.6
32.9
77.3
12.7
42.7
100
8d
9d
10d
11d
12
13d
14d
15d
16d
17
18
19
20
21d
C6H5CH2
Cyclohexanol
4-CH3C6H4
4-CH3OC6H4
4-FC6H4
CH3
CH3
CH3
CH3
C6H5
3
8
24
24
100
100
37.7
100
2-ClC6H4
C6H5
a Reaction conditions: 5 mmol alcohol, 5 mol% Fe2(SO4)3, 5 mol% TMHPO, 5 mol% NaNO2, 1 atm O2, room temperature.
b Conversion and selectivity are based on gas chromatography (GC) with area normalisation.
c isolated yield.
d 5 mmol alcohol, 8 mol% Fe2(SO4)3, 8 mol% TMHPO, 10 mol% NaNO2, 1 atm O2, room temperature.

400 JOURNAL OF CHEMICAL RESEARCH 2013
Scheme 1 Proposed mechanism for Fe2(SO4)3/4-OH-TEMPO/NaNO2-catalysed aerobic oxidation of alcohols.
equipped with a HP-5 column using N2 as carrying gas. Melting points
were determined with a Büchi B-540 capillary melting point
apparatus.
(Table 3, entry 20). This perhaps was also due to the intermo-
lecular hydrogen bonding effect. Cinnamyl alcohol could
be oxidised into cinnamaldehyde smoothly in 92% isolated
yield (Table 3, entry 12). 2-Furan methanol was converted into
2-furaldehyde in good conversion (Table 3, entry 14), but
2-pyridine methanol only gave a low conversion rate (Table 3,
entry 13). As shown in the Table 3, the aliphatic alcohols were
less active than the benzylic alcohols. For example, phenyleth-
anol showed a very low conversion rate and 50% selectivity
(Table 3, entry 15), and cyclohexanol gave only a 30% yield of
cyclohexanone (Table 3, entry 16).
The oxidation of alcohols was carried out under O2 in a 50-mL
two-necked, round-bottom flask equipped with a magnetic stirrer.
Typically, Fe2(SO4)3 (0.25 mmol) and TMHPO (0.25 mmol) were
added to the flask, followed by 15 mL of a CH3CN/H2O (1:2) solvent
mixture. After stirring for 5 min, the alcohol (5 mmol) was added,
followed by NaNO2 (0.25 mmol). The resulting mixture was stirred at
room temperature and 1 atm pressure of oxygen. When the reactions
were completed, the reaction mixture was transferred to a separating
funnel and extracted with dichloromethane. The organic layer was
dried over anhydrous Na2SO4 and concentrated and further purified by
flash chromatography to give the desired product.
On the basis of some literature data10,20 and the preliminary
studies, the catalytic cycle was proposed as shown in Scheme
1. TMHPO first coupled with Fe(III) to give intermediate I,
which then reacted with the alcohol to form intermediate II.
Intermediate II might undergo β-H elimination to form inter-
mediate (III) and then an intramolecular one-electron transfer
occurred to generate the corresponding aldehyde, TMHPOH
(IV) and Fe(II). TMHPOH (IV) and Fe(II) could be re-
oxidised by NO2 to regenerate TMHPO and Fe(III). NaNO2
could be a source of NO2.
Benzaldehyde (1b): Colourless liquid; 1H NMR (CDCl3, 400 MHz):
δ 10.00 (s, 1H), 7.88–7.84 (m, 2H), 7.64–7.59 (m, 1H), 7.52 (t, J =
8.0 Hz, 2H).
4-Methylbenzaldehyde (2b): Light yellow liquid; 1H NMR (CDCl3,
400 MHz): δ 9.93 (s, 1H), 7.75 (d, J = 8.0 Hz, 2H), 7.31 (d, J = 8.0 Hz,
2H) 2.43 (s, 3H).
4-Methoxybenzaldehyde (3b): Light yellow liquid; 1H NMR
(CDCl3, 400 MHz): δ 9.86 (s, 1H), 7.84–7.79 (m, 2H), 7.01–6.96
(m, 2H) 3.88 (s, 3H).
1
3-Methoxybenzaldehyde (4b): Pale yellow oily liquid; H NMR
(CDCl3, 400 MHz): δ 9.95 (s, 1H), 7.44–7.42 (m, 2H), 7.37 (d, J =
4.0 Hz, 1H), 7.16 (dt, J = 8.0, 4.0 Hz, 1H), 3.86 (s, 3H).
Conclusion
2-Methoxybenzaldehyde (5b): Pale yellow solid; m.p. 38–39 °C
In conclusion, we have further explored the iron salts/TEMPO
catalytic systems and developed a novel catalytic system using
Fe2(SO4)3/TMHPO/NaNO2 for aerobic oxidation. This novel
four component oxidation system used oxygen as a clean
oxidant and inexpensive Fe2(SO4)3/4-OH-TEMPO/NaNO2 as
catalyst, and has achieved high selectivity in alcohol oxidation
at room temperature and high yield when benzylic alcohols
and cinnamyl alcohols were used. These desirable features
made the method attractive for potential industrial usage.
1
(lit.21 37–39 °C); H NMR (CDCl3, 400 MHz): δ 10.44 (s, 1H), 7.81
(dd, J = 8.0, 4.0 Hz, 1H), 7.57–7.49 (m, 1H), 6.99 (dd, J = 16.0,
4.0 Hz, 2H), 3.92 (s, 3H).
4-Fluorobenzaldehyde (6b): Yellow liquid; 1H NMR (CDCl3,
400 MHz): δ 9.95 (s, 1H), 7.90 (dd, J = 8.0, 8.0 Hz, 2H), 7.20 (t,
J = 8.0 Hz, 2H).
3-Fluorobenzaldehyde (7b): Yellow liquid; 1H NMR (CDCl3,
400 MHz): δ 9.98 (s, 1H), 7.67 (d, J = 8.0 Hz, 1H), 7.58–7.49 (m, 2H),
7.33 (td, J = 8.0, 4.0 Hz, 1H).
2-Fluorobenzaldehyde (8b): Light yellow liquid; 1H NMR (CDCl3,
400 MHz): δ 10.35 (s, 1H), 7.88–7.83 (m, 1H), 7.62–7.56 (m, 1H),
7.29–7.23 (m, 1H), 7.20–7.13 (m, 1H).
Experimental
4-Chlorobenzaldehyde (9b): Pale yellow solid; m.p. 46–47 °C (lit.21
All reagents were purchased from commercial sources and were used
1
1
as received. H NMR spectra were recorded on a Varian 400 MHz
45–48 °C); H NMR (CDCl3, 400 MHz): δ 9.96 (s, 1H), 7.81 (d,
instrument using CDCl3 as the solvent, and chemical shifts were
expressed in parts per million (ppm) using TMS as an internal
standard. GC analyses for determining the conversion and selectivity
of the reactions were performed on an Agilent 6890N GC system
J = 8.0 Hz, 2H), 7.50 (d, J = 8.0 Hz, 2H).
4-Bromobenzaldehyde (10b): White solid; m.p. 56–58 °C (lit.22
1
57 °C); H NMR (CDCl3, 400 MHz): δ 9.95 (s, 1H), 7.73 (d, J =
8.0 Hz, 2H), 7.67 (d, J = 8.0 Hz, 2H).

JOURNAL OF CHEMICAL RESEARCH 2013 401
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Received 20 February 2013; accepted 13 April 2013
Paper 1301820 doi: 10.3184/174751913X13700197900636
Published online: 18 July 2013

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