A practical innovative method for highly selective oxidation of alcohols in neat water using water-insoluble iron and manganese porphyrins as reusable heterogeneous catalysts

Article abstract of DOI:10.1016/j.catcom.2011.09.039

A novel heterogeneous method for clean and selective oxidation of alcohols to the aldehydes and ketones using tetrabutylammonium peroxomonosulfate (n-Bu₄NHSO₅) in neat water catalyzed by simple Fe (III) and Mn (III) meso-tetraphenylporphyrin as a water-insoluble catalyst has been developed. The oxidation system proceeds well in the absence of organic co-solvents and surfactants. The presence of an electron-donating group on the phenyl ring of benzyl alcohol increases the reactivity of substrate. The catalysts could easily be recycled and reused without noticeable loss of activity and the oxidant's by-product (n-Bu₄NHSO₄) could also be recycled.

Full text of DOI:10.1016/j.catcom.2011.09.039

Catalysis Communications 16 (2011) 240–244
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
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Short Communication
A practical innovative method for highly selective oxidation of alcohols in neat water
using water-insoluble iron and manganese porphyrins as reusable
heterogeneous catalysts
Abdolreza Rezaeifard ⁎, Maasoumeh Jafarpour ⁎, Atena Naeimi
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science, University of Birjand, Birjand, 97179-414, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 June 2011
Received in revised form 22 September 2011
Accepted 30 September 2011
Available online 7 October 2011
A novel heterogeneous method for clean and selective oxidation of alcohols to the aldehydes and ketones
using tetrabutylammonium peroxomonosulfate (n-Bu NHSO ) in neat water catalyzed by simple Fe (III)
4 5
and Mn (III) meso-tetraphenylporphyrin as a water-insoluble catalyst has been developed. The oxidation
system proceeds well in the absence of organic co-solvents and surfactants. The presence of an electron-
donating group on the phenyl ring of benzyl alcohol increases the reactivity of substrate. The catalysts could easily
be recycled and reused without noticeable loss of activity and the oxidant's by-product (n-Bu
be recycled.
4 4
NHSO ) could also
Keywords:
Porphyrin
Oxidation
© 2011 Elsevier B.V. All rights reserved.
Alcohol
Water
Heterogeneous catalyst
1
. Introduction
The selective oxidation of alcohols to carbonyl compounds is
recent past and has become an active area of research in green chemis-
try [7–12].
4 5
Tetra-n-butylammonium peroxomonosulfate (n-Bu NHSO , TBAOX),
fundamentally vital in life science, and immensely useful in industry
for the preparation of many drugs, vitamins and fragrances [1,2]. It
has been known that cytochrome P-450, a monooxygenase enzyme,
catalyzes oxidation of alcohols to the corresponding carbonyl
compounds [3,4]. Accordingly, metalloporphyrins as biomimetic
catalysts have been employed for this invaluable transformation
using a variety of oxygen donors during the past decades [5,6].
Nevertheless, the extensive use of toxic and volatile organic solvents
in many oxidation systems mediated by metalloporphyrins has reduced
considerably the importance of biologically inspired oxidation catalysis.
Furthermore, during recent years, increasingly demanding legislation as
well as public and economic pressure led to increased interest in new,
which is an organic salt of Oxone® has been introduced for using in
anhydrous conditions without recourse to a buffered two phase system
[18]. The compound is commercially available and also can be prepared
easily by simple mixing of Oxone® and tetrabutylammonium hydrogen-
4
sulfate (TBAHSO ) [19]. It has been employed in limited oxidation
reactions under non-catalytic conditions in different organic solvents
[20–22]. A sluggish catalyst-free method for oxidation of alcohols using
this oxidant in water has also been reported in which, low/moderate
yields of the related carbonyl compounds were achieved after
10 h [23]. Recently, efficient oxidation of alcohols using TBAOX in
the presence of immobilized TEMPO in ionic liquid [bmim][PF6]
has also been reported [24].
“
clean” methods for the production of chemical compounds. Discarding
Followed by initial promising report on the alkene epoxidation
and C-H bond hydroxylation with TBAOX catalyzed by Mn(TPP)OAc
in dichlorometane [25], various organic substrates including alcohols
have been successfully subjected to oxidation using this versatile
oxidant under catalytic conditions [26–30]. Nevertheless, iron-
porphyrins virtually showed no catalytic activity in this oxidation
system in organic solvents. Our attempts to overcome this limitation
led to an interesting strategy for efficient and selective epoxidation
of olefins and oxygenation of hydrocarbons in neat water using
Fe(III) and Mn (III) porphyrins as water-insoluble catalysts in the
absence of any organic co-solvents and surfactants under hetero-
geneous conditions [31]. A salient feature of this oxidation method
is the high/excellent selectivity for the formation of ketones which
of harmful organic solvents is the major problem in chemical industries
which accounts around 80% of their wastes. Thus, a new challenge is
to make such processes by using non-toxic solvents in particularly
aqueous media [7–12]. Moreover, it has been found that reactions in
water can facilitate access to different reactivity and selectivity patterns
compared with those observed in common organic solvents due to its
unique physical and chemical properties [13–17]. In this regard, the
use of water as a reaction solvent has attracted great attention in the
⁎
Corresponding authors. Tel.: +98 561 2502516; fax: +98 561 2502515.
E-mail addresses: rrezaeifard@birjand.ac.ir, rrezaeifard@gmail.com (A. Rezaeifard),
mjafarpour@birjand.ac.ir (M. Jafarpour).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.09.039

A. Rezaeifard et al. / Catalysis Communications 16 (2011) 240–244
241
is higher than that found for the alcohols in the oxidation of C-H bonds
prompting us to apply it for the oxidation of alcohols to carbonyl
compounds. Herein, we wish to describe the merit of this sustainable
biomimetic protocol in efficient and highly selective oxidation of various
alcohols to the corresponding carbonyl compounds in neat water
(
Scheme 1). The catalyst and the reduction product n-Bu
4 4
NHSO
(
TBAHSO ) from the oxidant TBAOX were easily recovered and reused
4
in the procedure.
2
. Experimental
2
.1. General remarks
Fig. 1. The influence of catalyst contents on the oxidation of benzyl alcohol using
TBAOX catalyzed by Fe(TPP)Cl in water.
Purity determinations of the products were accomplished by GC-
FID on a Shimadzu GC-16A instrument using a 25 m CBP1-S25
0.32 mm ID, 0.5 μm coating) capillary column. IR spectra were
(
recorded on a Perkin Elmer 780 instrument. UV–vis spectra were
2.4. General procedure for competitive reactions
recorded on a 160 Shimadzu spectrophotometer. NMR spectra
were recorded in CDCl
NMR 250 MHz instrument. The residual CHCl
atom% CDCl gives a signal at δ=7.26 ppm, which was used for cali-
bration of the chemical shift scale. Mass spectra were recorded on a
Shimadzu GC-MS-QP5050A. The free base porphyrin: TPPH [32],
3
solutions with a Brucker Avance DPX FT-
To an equimolar mixture of two benzyl alcohols (1 mmol) in distilled
water (10 ml) containing Fe(TPP)Cl complex (7 mg, 0.01 mmol) was
added freshly prepared TBAOX (0.7 g, 2 mmol). The reaction mixture
was stirred under air at 25 °C.
3
in conventional 99.8
3
2
and their iron and manganese complexes were prepared and puri-
fied by methods reported previously [33,34]. The freshly prepared
TBAOX was a much stronger oxidant than commercially available
samples.
2.5. Recycling procedure
After isolation of products, the reaction mixture was centrifuged
and the solid catalyst was separated, washed with petroleum ether,
dried and reused for the subsequent reaction under the similar reaction
conditions. The aqueous phase was lyophilized to obtain TBAHSO
which can be reused in the preparation of TBAOX.
4
2
.2. Typical procedure for oxidation of benzyl alcohol
To a mixture of benzyl alcohol (5 mmol, 0.54 g) and Fe(TPP)Cl
3. Results and discussions
complex (35 mg, 0.05 mmol) in distilled water (10 ml) was added
freshly prepared TBAOX (3.5 g, 10 mmol). The reaction mixture was
stirred under air at 25 °C. After completion of the reaction which was
monitored by TLC and GC, the mixture was washed with petroleum
ether (3×2 ml), and the organic phase was separated and evaporated
3.1. Preliminary experiments
The oxidation of benzyl alcohol (1 mmol, 0.108 g) using two
equivalents of TBAOX (0.7 g) in water which proceed sluggishly in
the absence of catalyst was used as a model reaction (43% after 10 h).
Preliminary catalytic experiments were addressed to the oxidation of
benzyl alcohol (1 mmol) with TBAOX (2 mmol, 0.7 g) in the presence
of a minute amount of Fe(TPP)Cl (0.5 mol%) in neat water at 25 °C
under air which led to formation of benzaldehyde in 52% yield after
−1
1
giving benzaldehyde in 94% yield (0.5 g). IR, υCO=1703 cm
NMR, δ=10, 7.5–7.7 ppm.
; H
2
.3. General procedure for TOF determination
1
5 min based on the starting alcohol according to GC analysis. Similar
To a mixture of benzyl alcohols (25 mmol) and Fe(TPP)Cl complex
1.75 mg, 0.0025 mmol) in distilled water (50 ml) was added freshly
prepared TBAOX (17.5 g, 50 mmol). The reaction mixture was stirred
under air at 25 °C.
result was obtained (51% benzaldehyde) in the absence of air (Ar
atmosphere), so autoxidation processes was excluded. The addition
of imidazole (45% benzaldehyde) and pyridine (51%) as common
additives, the use of micellar soultion of sodium dodecyl sulfate (SDS)
(
N
N
N
M
N
R
R'
R
M=Fe, Mn
O
HO
n-Bu NHSO , Water
4
5
R'
Scheme 1. Oxygenation of alcohols using TBAOX catalyzed by Fe(TPP)Cl in water.

242
A. Rezaeifard et al. / Catalysis Communications 16 (2011) 240–244
Table 1
Table 1 (continued)
a
Oxidation of alcohols using TBAOX catalyzed by Fe(TPP)Cl in water .
Entry Alcohols
Product
Yield%
(
Time
temperature/°C) b (min)
Entry Alcohols
Product
Yield%
(
Time
temperature/°C) b (min)
1
4
88 (70 °C)
15
1
94 (25 °C)
15
10
15
5
2
3
4
95 (25 °C)
87 (25 °C)
96 (25 °C)
15
79 (70 °C )
90 (70 °C )
15
15
1
6
7
OH
OH
OH
OH
(
δ
C–H oxiran ring
=
=
O
2.7 ppm)
1
94 (70 °C )
(δ
C–H oxiran ring
15
15
O
2.5 ppm)
OH
O
5
6
7
8
93(25 °C)
85 (25 °C)
78 (25 °C)
56 (25 °C)
15
25
25
60
1
8
92 (70 °C )
δ=2–2.4, 6,
.1 ppm)
(
7
a
The reactions were run under air and the molar ratio of alcohol:TBAOX:catalyst
was 100:200:1.
b
Yields of isolated products. The 100% selectivities were obtained for all products
1
according to GC and H NMR analysis.
at its CMC conditions (43%) and also the sulfonation of porphyrin [35]
54%), did not improve the conversion rate in this oxidation method.
Performing the reactions at controlled pH conditions did not affect on
the rate and yield of benzaldehyde (50, 48, 53 and 51% respectively at
pH=2, 4, 6, 9).
(
3.2. Effect of catalyst contents
9
94 (25 °C)
15
As depicted clearly in Fig. 1, the reaction rate and conversion
enhances with increasing catalyst loading up to 1 mol%, affording to
the full conversion of alcohol within 15 min and 94% benzaldehyde
was secured after a simple extraction from aqueous phase. Neverthe-
less, an increase in this ratio up to 5 mol% did not improve the conver-
sion rate (Fig. 1). Similar to epoxidation reactions in previous report
1
1
0
1
80 (25 °C)
88 (25 °C)
20
15
[31], oxidation of alcohol proceeds also well in the presence of Mn
(
TPP)OAc as catalyst with no need to any additives (93% benzaldehyde
was isolated after 15 min). It is worthy to mention that the over oxida-
tion reaction was controlled completely and no trace of benzoic acid
was observed. In addition, compared with other iron and manganese
cationic salts, porphyrin complexes were proven to have the significant
unique properties to achieve the selective oxidation of alcohols. The
iron salts could not trigger the oxidation reaction. Also only trace
amount of benzyl alcohol could be converted to benzaldehyde (8%)
1
1
2
3
91(25 °C)
15
when the catalyst was replaced to Mn(OAc)
catalytic amount as metalloporphyrins.
2 2
.4H O with the same
3.3. Catalytic oxidation of alcohols in neat water
5
4 (25 °C), [67
(50 °C)] {95
70 °C)}
60,
[60],
{15}
As demonstrated in previous section both iron and manganese
(
porphyrins displayed notable catalytic activity in the oxidation of
benzyl alcohol with aqueous solution of TBAOX. Nevertheless, to ex-
plore different factors that may affect the reactivity and selectivity
of oxidation of alcohols in neat water we continued this study by
iron analogue having a sluggish activity in organic solvents. In order
to establish the general applicability of the present catalytic system,

A. Rezaeifard et al. / Catalysis Communications 16 (2011) 240–244
243
demonstrate the involvement of a bulky and weak electrophilic six co-
ordinate adduct [(L)PorM-O-OHSO ] during the oxidation reaction [30].
3
The high turnover frequencies obtained in the oxidation of different
benzylic alcohols highlights the high efficiency of Fe(TPP)Cl as a biomi-
metic catalyst in the present oxidation method in aqueous media.
Then, we applied this catalytic system in the oxidation of primary
and secondary aliphatic alcohols. They were less reactive than benzylic
ones toward this catalytic system and oxidized to the corresponding
aldehydes or ketones in prolonged reaction times (Table 1, entries
1
3–15). Nevertheless, based on our previous report on oxygenation
of hydrocarbons, [31] we increased the temperature up to 70 °C
which led to high yields of oxidation products within 15 min with no
formation of carboxylic acid resulting from over-oxidation of aldehyde.
The chemoselectivity of the procedure was notable. While, the
primary alcohols containing C–C double bonds (allylic and homo-
allylic alcohols) oxidized completely to the corresponding epoxides
under the influence of this catalytic system (entries 16,17), secondary
one converted to the related α, β-unsaturated ketone as sole product
Fig. 2. The influence of electronic and steric demands on the oxidative activity of benzylic
alcohols using TBAOX catalyzed by Fe(TPP)Cl in water. The molar ratio for alcohol:TBAOX:
catalyst was 10000:20 000:1.
(
entry 18).
a wide range of primary and secondary aliphatic and benzylic alco-
hols were subjected to oxidation under the optimized conditions in
the presence of 1 mol% of Fe(TPP)Cl in water (Table 1). Benzylic alco-
hols are generally excellent substrates for this system (Table 1, en-
tries 1–10) and no over-oxidation to the related carboxylic acids
was observed. It seems that efficiency of oxidation in this catalytic
system is dependent on the electronic and steric demands of the sub-
strate. This was confirmed by determining the turnover frequency of
Fe(TPP)Cl catalyst (TOF/1 min) in the oxidation of different benzyl-
ic alcohols under similar conditions using 10000:20000:1 molar ratio
for alcohol:TBAOX:catalyst (Fig. 2). It was observed that the presence
of electron-donating –OMe group on the para-position of benzyl alcohol
enhanced the TOF in comparison with molecule having no substituent
on the ring (9700 for 4-OMe-benzyl alcohol vs. 7300 for benzyl alcohol).
3.4. The recycling of catalyst and oxidant by-product
Encouraged by the impressive TOFs obtained for simple and un-
immobilized Fe(TPP)Cl indicating well its relative stability in this
heterogeneous oxidation system, the recycling of the catalyst was
investigated. For this purpose, benzyl alcohol as a model compound
was oxidized with TBAOX in the presence of 1 mol% of Fe(TPP)Cl at
25 °C in water. After completion of the reaction, the solid catalyst
was separated by centrifuging and was reused for consecutive
runs. The averaged isolated yield of benzaldehyde for seven runs
was 91.6% (Fig. 3). Similar results were obtained for reusability of
Mn(TPP)OAc complex (Fig. 3) under the same conditions (92.4%).
4
Moreover, oxidant's by-product (TBAHSO ) was separated by lyophilizing
of aqueous phase and reused in the preparation of TBAOX. These
advantages for this high yielding oxidation method offered ready
scalability. In the oxidation of benzyl alcohol as substrate in a semi
scale-up procedure (25 mmol), 91% yield of benzaldehyde was secured
after simple extraction and evaporation of the solvent.
Whereas, electron-withdrawing –Cl and especially –NO
phenyl ring retarded the oxidation reaction (5400 and 840 for 4-Cl and
-NO -benzyl alcohols, respectively). The influence of steric factors in
2
groups on the
4
2
this catalytic system is also notable. The lower TOF of ortho-substituted
alcohols such as 2-OMe- (7800), 2-Me- (6200) and 2-Cl-benzyl alcohol
(
(
4300) in comparison with the corresponding non-hindered alcohols
9700, 8000 and 5400 for 4-OMe-, 4-Me- and 4-Cl-benzyl alcohol
4. Conclusion
respectively) at similar reaction times may be related to the steric
hindrance at the Fe center of active oxidant in this catalytic oxidation
method. More evidences for these effects have been obtained by
performing competitive reactions between different benzylic alcohols
In conclusion, simple water-insoluble Fe(TPP)Cl and Mn(TPP)OAc
are excellent biomimetic catalysts for activation of TBAOX in neat
water as a standard “green” solvent towards oxidation of alcohols to
the carbonyl compounds in high yields and excellent selectivity with
no need to organic co-solvents, surfactants and nitrogenous bases.
(Table 2). We have found a reasonable selectivity among the electroni-
cally and sterically different alcohols. Hindered (entries 1, 2) and
electron-deficient molecules (entries 3, 4) displayed lower activity
than those of non-hindered and electron-rich ones. These results
Table 2
The competitive reactions between different benzylic alcohols using TBAOX catalyzed
a
by Fe(TPP)Cl in water .
Entry
1
Substrates
Aldehyde yield%b
54 30
4-Me-benzyl alcohol+2-Me-benzyl alcohol
2
3
4
4-OMe-benzyl alcohol+2-OMe-benzyl alcohol
4-OMe-benzyl alcohol+Benzyl alcohol
60 40
68 10
45 5
2
Benzyl alcohol+4-NO benzyl alcohol
a
The reactions were run under air and the molar ratio for alcohols:TBAOX:Fe(TPP)Cl
is100:200:1.
b
GC yield after 5 min.
Fig. 3. Recycling of the catalytic system for the oxidation of benzyl alcohol in water.

244
A. Rezaeifard et al. / Catalysis Communications 16 (2011) 240–244
[
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demonstrate high efficiency and also relative stability of catalyst
towards oxidative degradation. The presence of an electron-
donating group on the phenyl ring of benzyl alcohol increases
the reactivity of substrate. The catalysts could easily be recycled
and reused without noticeable loss of activity and the oxidant's
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