Highly selective, economical and efficient oxidation of alcohols to aldehydes and ketones by air and sunlight or visible light in the presence of porphyrins sensitizers

Article abstract of DOI:10.1039/c0gc00910e

In this work, a new aerobic route is introduced for the selective oxidation of a variety of aromatic and aliphatic alcohols to the corresponding aldehyde and ketone derivatives by molecular oxygen in the presence of free base porphyrins and metalloporphyrins as sensitizers using white light or sunlight in an organic solvent. The method shows a wide scope, exhibits chemoselectivity and proceeds under mild reaction conditions. The resulting products are obtained in good conversions in a reasonable time. The Royal Society of Chemistry.

Full text of DOI:10.1039/c0gc00910e

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PAPER
Highly selective, economical and efficient oxidation of alcohols to aldehydes
and ketones by air and sunlight or visible light in the presence of porphyrins
sensitizers
Mahdi Hajimohammadi, Nasser Safari,* Hamid Mofakham and Farzad Deyhimi
Received 11th December 2010, Accepted 31st January 2011
DOI: 10.1039/c0gc00910e
In this work, a new aerobic route is introduced for the selective oxidation of a variety of aromatic
and aliphatic alcohols to the corresponding aldehyde and ketone derivatives by molecular oxygen
in the presence of free base porphyrins and metalloporphyrins as sensitizers using white light or
sunlight in an organic solvent. The method shows a wide scope, exhibits chemoselectivity and
proceeds under mild reaction conditions. The resulting products are obtained in good conversions
in a reasonable time.
1-oxyl (TEMPO)/Br2/NaNO2 aerobic oxidation system re-
Introduction
ported by Liu et al.,7 in which TEMPO was used as a
The oxidation of alcohols to carbonyl compounds plays an
homogeneous catalyst, resulted in the in situ generation of NO,
important role in organic synthesis not only in the laboratory,
which played a crucial role in the activation of O2. However, to
but also at the industrial level.1 In recent years the demands for
date there have been no reports on the selective aerobic oxidation
environmentally benign and eco-conscious chemical processes
of alcohols using the above systems. Molecular oxygen does
have also emerged.2 In particular, there has been an increasing
indeed have considerable oxidation potential, but does not react
interest in the oxidizing properties of organohypervalent iodine
with organic compounds owing to the spin rule. To achieve
reagents.3 Catalytic methods for this reaction are intensively
selective oxidations with oxygen, transition-metal complexes
being investigated to replace stoichiometric oxidation processes
have been used as homogeneous catalysts that could overcome
that generate large amounts of heavy metal and solvent waste.4 In
the spin rule.8
this regard, aerobic oxidation processes have received increasing
Metalloporphyrins have been found to be efficient biomimetic
attention, as oxygen or air are used as the terminal oxidant to
catalysts for hydrocarbon oxidation when using molecular
replace the stoichiometric metal oxides such as chromates and
oxygen and various oxygen transfer reagents.9 However, finding
manganese oxides. Furthermore the use of oxygen has great
a catalytic system for hydrocarbon oxidation by O2 under mild
benefits from both economic and green chemistry viewpoints,
conditions is still a challenging issue. Iron porphyrins have
because oxygen is relatively cheap and produces water as
been used as catalysts for alkane hydroxylation by O2 while
the only by-product. Accordingly, varied heterogeneous and
consuming a stochiometric amount of a reducing agent. The
homogenous catalysts derived from precious metals have been
oxidation of alkanes to alcohols and ketones was preformed
developed for this purpose,5 but due to their rarity, high price and
at 80 ◦C under a 10 atm pressure of oxygen. Oxidation of
toxicity, precious metals are sometimes impractical for industrial
use of O2 activation.
Several studies of carbon catalyst materials have already
revealed the possibility of replacing metal-based catalysts in
hydrocarbons by metalloporpyrins that have a strong affinity
for the generation of metal-oxo species (M O) were investi-
gated with chemical oxidants such as meta-chloroperbenzoic
acid (m-CPBA).10 In all of these metalloxo generations, a
several important transformations, such as the Friedel–Crafts
stochiometric amount of an strong oxidant is needed.11 The
reaction and the oxidative dehydrogenation of aromatic hydro-
photosensitized production of singlet oxygen has significance
carbons and alkanes. Interestingly, carbon catalysts with an or-
in the photooxidation of organic compounds, DNA damage
dered nano shell structure or doped with heteroatoms have been
and photodynamic therapy.12 Therefore, a variety of these,
reported to possess surprisingly high oxygen reduction reaction
photosensitizers have been developed and their photochemical
(ORR) activities.6 In addition the 2,2,6,6-tetramethylpiperidine-
and photophysical properties have been extensively studied.13
Among them, tetrapyrrolic compounds such as porphyrins and
phthalocyanines, are promising candidates for photosensitizers
due to their unique photophysical properties.14
Department of Chemistry, Shahid Beheshti University, G. C. P. O. Box
19396-4716, Tehran, Iran. E-mail: n-safari@cc.sbu.ac.ir
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However, if solar radiation and oxygen can be used to mediate
the efficient oxidation of alcohols, an environmentally benign
process will be established. This concept dates back to the origin
of organic photochemistry in the late 19th century, but was not
adequately persuaded.15
Table 2 shows the oxidation of alcohols in the presence of
ClFeTMP with different substrates. The reaction condition
remained the same as that of the aforementioned H2TMP
reactions.
It is interesting to note that the photooxidation of 1,4-
phenylenedimethanol with H2TMP (Table 1, entry 2) produced
two products, while ClFeTMP acts more selectivity and only
gave one product (Table 2, entry 3).
Results and discussion
In our previous studies, an efficient system for the porphyrins-
sensitized aerobic oxidation of aldehydes and alkenes has
been developed in the presence of visible light.16 Herein, in
continuation of our studies we report on a new and highly
efficient system for the aerobic oxidation of alcohols by using
porphyrins as sensitizers (sens). In the following study, alcohol
and its substituted derivatives selectively convert to aldehydes or
ketones smoothly by oxygen and sunlight or white light under
mild reaction conditions. Scheme 1 shows the structures of the
sensitizers employed and the reaction conditions. Photooxida-
tion was performed in an aerobic solution of CH3CN under
visible light or sunlight in the presence of sensitizers (porphyrins)
under 1 atm pressure of air at room temperature. Relatively high
conversion and excellent selectivity was obtained.
The other advantage of iron porphyrins is that in the presence
of the sensitizer and O2/hu, over oxidation of aldehydes to
acids did not occur. In order for the free base porphyrins to
avoid conversion of aldehyde to acids, exact stochiometry of the
reactants is needed, since after all of the alcohol is converted
to the related aldehyde, if any sensitizer is still available the
formation of acids from aldehyde will begin. In spite of this, free
base porphyrins are more reactive and give a higher conversion
than that of iron porphyrins.
The effect of light sources on the aerobic oxidation of alcohols
to aldehydes catalyzed by H2TMP and ClFeTMP were also
investigated. As shown in Table 1 (entries 1, 5, 6 and 16) and
Table 2 (entries 1, 2, 3 and 8), it seems that the conversion of
alcohols was closely related with white light. It is clear that these
reactions proceed under solar radiation because the spectral
distribution curve of a fluorescent lamp is similar to that of
sunlight. Furthermore, in accordance with the concept of “green
chemistry” solar radiation is an infinite source of clean energy.
Fig. 1 shows the reaction of 2-bromobenzaldehyde formation
versus time in an oxygenated solution of CH3CN under visible
light irradiation in the presence of H2TMP and ClFeTMP.
The Soret band of the H2TMP and ClFeTMP was followed
at 415 and 418 nm by a UV-Vis method. This method showed
that sensitizer bleaching for H2TMP was completed after 16
h in our reaction conditions. After the disappearance of the
porphyrin Soret band, the formation of the oxidation products
stopped. If an excess of H2TMP is remaining after all the
alcohol has been converted to aldehyde, further oxidation
of aldehyde to corresponding acids will start. While in the
presence of ClFeTMP the final product is aldehyde or ketone
regardless of the reaction time or what concentration of
Scheme 1 Oxidation of alcohols to aldehydes.
Oxidation of 2-bromobenzyl alcohol applying nonmetallated
porphyrins were investigated as a typical benchmark reaction
for oxidation at room temperature, with air (1 atm) as the
oxidant with white light and CH3CN as the solvent. The reaction
proceeded smoothly under these conditions, with a conversion
of 100%. The selectivity for aldehyde formation was 99%, with
2-bromobenzoic acid being the trace by-product; Table 1.
To check the generality of the method, a variety of primary
and secondary alcohols were oxidized to carbonyl compounds
with H2TMP in visible light and at room temperature in
moderate to excellent yields. The reactions were performed
in a tube without any particular precautions. Under these
conditions primary alcohols (Table 1, entries 1–10) were oxidized
to the corresponding aldehydes with 33–100% conversions
without any noticeable over oxidation to the carboxylic acids.
Secondary alcohols (Table 1, entries 11–18) were oxidized to the
corresponding ketones with 49–100% conversions.
Fig. 1 (A) Conversion of 2-bromobenzyl alcohol (0.25 ¥ 10-3 mol)
to 2-bromobenzaldhyde vs. time of irradiation with ClFeTMP (0.5 ¥
10-6 mol) in CH3CN; (B) percent of remaining ClFeTMP vs. time of
irradiation; (C) conversion of 2-bromobenzyl alcohol (0.25 ¥ 10-3 mol)
to 2-bromobenzaldhyde vs. time of irradiation with H2TMP (0.5 ¥ 10-6
mol) in CH3CN; (D) percent of remaining H2TMP vs. time of irradiation;
(E) conversion of 2-bromobenzaldhyde to 2-bromobenzoic acid with
H2TMP vs. time.
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Table 1 Oxidation of alcohols to aldehydes and ketones in the presence of H2TMP under visible light and sunlight irradiationa
Entry
1
Substrate
Product
Time (h)
72
Conversion (%)
Selectivity (%)
TONc/TOFd
70
>99
175/2.4
177/2.5
71b
2
72
75e
33e
42e
>44
>59
165/2.3
105/1.5
3
150
82
82
>99
205/2.0
205/1.4
4
5
6
7
150
16
>99
>99
>99
>99
95
950/59.4
950/59.4
95b
16
12
80
100
100b
77
1000/62.5
1000/83.3
192/2.4
8
9
164
80
80
75
>99
>99
200/1.2
375/4.7
10
11
120
140
68
45
>99
>99
170/1.4
122/0.9
12
13
140
140
53
>99
>99
132/0.9
250/1.8
100
14
15
140
140
70
71
>99
>99
175/1.2
177/1.3
16
17
20
95
>99
>99
475/23.8
500/25.0
100b
100
100
666/6.7
18
20
100
>99
500/25.0
a 1 ¥ 10-6 mol of H2TMP and 0.25 ¥ 10-3 mol of alcohol substrate, air (1 atm) and fluorescent circular lamp, six 22 W lamps (32400 LUX). b Sunlight
(62000 LUX) in the CH3CN solvent. c Turn over number of the catalyst. d Turn over frequency (h-1). e This reaction gave two products.
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Table 2 Oxidation of alcohols to aldehydes and ketones in the presence of ClFeTMP under visible light and sunlight irradiationa
Entry
1
Substrate
Product
Time (h)
120
Conversion (%)
Selectivity (%)
TONc/TOFd
65
>99
162/1.4
175/1.4
70b
2
3
16
16
100
>99
>99
1000/62.5
1000/62.5
100b
73
182/11.4
187/11.7
75b
4
5
80
43
80
>99
>99
214/2.7
100
200/2.0
6
7
140
140
45
60
>99
>99
166/1.2
150/1.1
8
32
74
>99
370/11.6
500/15.6
100b
a 1 ¥ 10-6 mol of ClFeTMP, 0.25 ¥ 10-3 mol of alcohol substrate, air (1 atm) and fluorescent circular lamp, six 22 W lamps (32400 LUX). b Sunlight
(62000 LUX) in the CH3CN solvent. c Turn over number of the catalyst. d Turn over frequency (h-1).
Table 3 Effect of nonmetallated porphyrins on the photooxidation of
sensitizer is used. Therefore the reaction time was chosen to
be 32 h for data in Fig. 1. This figure also indicates that the
reaction rate for ironporphyrin is higher than that of free base
porphyrins.
2-bromobenzyl alcohola
Conversion of 2-bromobenzyl
Entry
Catalyst
alcohol (%)
The catalytic activity and selectivity of different metallo-
porphyrins and structurally different free base porphyrins for
the oxidation of alcohol to carbonyl compounds by molecular
oxygen were investigated by using 2-bromobenzyl alcohol as a
model substrate. The engaged metals for the oxidation reactions
were Fe, Mn and Zn. (H2TPP), (H2TMP) and (H2TPFPP) were
employed to cover different electronic properties. The results are
summarized in Table 3. The oxidation of 2-bromobenzyl alcohol
catalyzed by these catalysts produced 2-bromobenzaldehyde as
the main product. The catalytic activity of metalloporphyrins
for oxidation appears to be dependent on the nature of the
central ions. ClFeTMP porphyrin showed the best activity
among the three simple structure metalloporphyrin catalysts,
and presented excellent catalytic performance for the oxidation
of 2-bromobenzyl alcohol in mild conditions. The catalytic
activation of the three metalloporphyrins were in the sequence
of ClFeTMP > ClMnTMP > ZnTMP. The order for reactivity
of the free base ligands is also H2TMP > H2TPP > H2TPFPP
showing that electron donation to the porphyrin rings facilitates
the conversion of alcohol under our reaction conditions. In our
previous work we also showed that the oxidations of aldehydes
with paramagnetic metals (Fe and Mn) is negligible.16a It is very
interesting that metalloporphyrins can perform the selective
oxidation of alcohols to aldehydes and ketones without any
carboxylic acid as a side product.
1
2
3
4
5
6
7
8
9
H2TMP
H2TPP
100
60
Trace
100
85
Trace
30
23
H2TPFPP
ClFeTMP
ClMnTMP
ZnTMP
ClFeTPP
ClFeTPFPP
—
0
a 1 ¥ 10-6 mol of porphyrin and 0.25 ¥ 10-3 mol of 2-bromobenzyl alcohol,
16 h irradiation, air (1 atm) and fluorescent circular lamp, six 22 W lamps
(32400 LUX) in CH3CN solvent.
Table 4 indicates that a 1O2 scavenger quenches aldehyde
formation for free base porphyrins. According to the litera-
ture, DMSO, H2O and DMF are singlet oxygen quenchers.17
Acetonitrile is a good solvent for oxidation reactions since it
is relatively inert toward oxidation. In addition, it is confirmed
that the lifetime of 1O2 in acetonitrile is high. For the m-THPC
(5,10,15,20-tetra(m-hydroxyphenyl)chlorin) sensitizer, the sin-
glet oxygen lifetime in DMSO is 19 ms compared to 65 ms in
CH3CN.18 The main mechanism which takes place under our
reaction conditions is possibly the generation of singlet oxygens
and their reaction with the substrates. Further evidence for a 1O2
mechanism was obtained by performing the photooxygenation
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Table 4 Effect of solvent on the photooxidation of 2-bromobenzyl
alcohola
Conversion (%)
Entry
Solvent
H2TMP
ClFeTMP
1
2
3
4
5
Acetonitrile
100
100
Dimethyl formamide
Dimethyl sulfoxide
NaN3/Na2SO3/Acetonitrile
H2O
Trace
Trace
11
Trace
Trace
100
Trace
Trace
a 1 ¥ 10-6 mol of porphyrins and 0.25 ¥ 10-3 of mol 2-bromobenzyl
alcohol, 16 h irradiation.
Fig. 2 Visible spectra of, (A) 2-bromobenzyl alcohol, H2TMP, HRP
-
and air in 1 atm, at reaction time of 0, (B) standard reaction of H2O2
with Emerson–Trinder reaction, (C) same solution as A after irradiation
for 10 h.
in the presence of N3 which is known as a singlet oxygen
scavenger.19 Entry 4 in Table 4 shows that the yield of free base
porphyrin was reduced considerably under these conditions. In
the presence of N3 , degradation of the porphyrin sensitizers was
-
the Emerson–Trinder reaction indicator. Insert A in Fig. 2 is
for the standard reaction medium before any radiation. Insert
B is for the standard reaction of H2O2 with Emerson–Trinder
indicator and indicates absorbance at 500 nm. Insert C is for the
reaction mixture after 10 h of light irradiation. As it is evident
from these spectra, the formation of H2O2 is confirmed by the
absorbance at 500 nm which is for the quinoneimine formation
during the reaction time.
The proposed mechanism for the ClFeTMP sensitizer is
presented in Scheme 4. It seems that the singlet oxygen route seen
for free base porphyrins does not apply here, since a) the reaction
did not quench in the presence of NaN3/Na2SO3/CH3CN; and
b) in Table 3, although the yield in the presence of the Zn is low, it
is high in the presence of iron and manganese metallopophyrin.
Krasnovskii et al. have also determined the values of singlet
oxygen physical quenching, and they confirmed that in a
singlet oxygen reaction, zinc porphyrin has a higher singlet
oxygen lifetime and a higher yield compare to corresponding
paramagnetic metal complexes.22
also inhibited.
A possible mechanism for the formation of products with
H2TMP porphyrin is shown in Scheme 2. Thus, in the presence
of singlet oxygen, there may be a new photochemical pathway
that involves the overall insertion of molecular oxygen into the
C–H bond of the alcohols 1 to form a hydroperoxymethanol 2.
Furthermore, independent photolysis of hydroperoxymethanol
2 in CH3CN results in the formation of 3, which is the only
product.20 Compound 3 decomposes further to the final product
4 and hydrogen peroxide which is a by-product.
Scheme 2 Proposed mechanism.
Formation of hydrogen peroxide was confirmed by the
Emerson–Trinder test.21 This enzymatic method is highly at-
tractive due to its high selectivity, sensitivity and rapidity. The
peroxidase (horseradish peroxidase) catalyzed H2O2 oxidation
reaction takes place in the presence of a suitable peroxidase
substrate electron donor. This indicator acts based on a three-
substrate catalyzed reaction, in which the oxidation of a phenol
(or a wide variety of other H donor organic compounds) is per-
formed by H2O2 in the presence of 4-aminoantipyrine (4-AAP)
hydrogen donor and peroxidase enzyme with the production of
a red-purple chromogenic quinoneimine compound which has
an absorption band with a lmax about 500 nm (see Scheme 3 and
Fig. 2).
Scheme 4 Proposed mechanism.
We propose formation of (TMPFeIV O)+2 as the active
catalyst in the case of iron porphyrin. Highly coordinated
solvents such as DMSO inhibit the formation of an active
intermediate by preventing oxygen coordination with the six
coordinated iron porphyrins formed in the solution and also
quenching aldehyde formation, Table 4, entries 2 and 3 for
ClFeTMP. Nam et al. have also shown that (PFeIV O)+2 can
selectively convert alcohol to aldehyde.23 Other works related to
the formation of metaloxo intermediates in the presence of light
and oxygen have also been reported.14b
Scheme 3 Confirmed formation hydrogen peroxide.
Fig. 2 shows the spectral changes of the reaction of 2-
bromobenzyl alcohol in an oxygenated solution of CH3CN
under visible light irradiation in the presence of H2TMP and
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The presence of an iron-oxo porphyrin was confirmed by UV-
vis spectra for the oxidation of 4-tert- butylcyclohexanol, as
demonstrated in Fig. 3. In this figure lines A and B show the
spectra of solutions at the beginning of the reaction, during the
reaction and right after the irradiation had stopped, respectively.
The strength of the Soret band of ClFeTMP at 418 nm decreased
and the band at 476 nm increased, suggesting the formation
of the oxidant active species (TMPFeIV O)+2 in the reaction
medium.23
through the solution and the sample was irradiated using
visible light [fluorescent circular lamp, 22 W, 230V, 32400
LUX, intensity determined with light meter (LT lutron model
YK-2500LX) and l > 350 nm] or sunlight (62000 LUX)
for 12–164 h. The consumption of the starting alcohols and
the formation of the corresponding carbonyl compounds were
monitored by ThermoQuest Finnigan AQA GC-MS, Agilent
gas chromatograph and spectral changes were monitored with
Shimadzu UV-2100.
Acknowledgements
We gratefully acknowledge financial support from the Research
Council of Shahid Beheshti University.
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pathway that involves the formation of intermediate 6 from
photo-homolysis of 5 which was formed through the reaction of
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Conclusions
In conclusion, we have developed a novel catalytic system for
selective aerobic oxidation of alcohols to the corresponding alde-
hydes or ketones using a catalytic amount of a nonmetallated
and metallated porphyrins in the presence of sunlight or white
light and O2 in CH3CN. This novel method is thought to be
convenient and green in view of the use of catalyst, and mild
reaction conditions.
Experimental section
General procedure for the synthesis of porphyrins
H2TPP, H2TMP, H2TPFPP, ClFeTMP, ClFeTPP, ClFeTPFPP
and ZnTMP were synthesized and purified using the Lindsey’s
Method and metallated according to Adler.25
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Chem., 2006, 118, 4579, (Angew. Chem., Int. Ed., 2006, 45, 4467);
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General procedure for the synthesis of aldehydes and ketones:
the alcohol (0.25 mmol) and a 1 ml CH2Cl2 solution of the
porphyrin (1.00 ¥ 10-3 mmol) were dissolved in 14 mL of
CH3CN in a quartz tube. Air at 1 atm pressure was bubbled
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