Efficient aerobic photooxygenation of aldehydes to carboxylic acids using cobalt(II) phthalocyanine sulfonate as a photosensitizer in organic-water biphasic media

Article abstract of DOI:10.1007/s11243-018-0281-x

The aerobic oxidation of a variety of aromatic aldehydes to the corresponding carboxylic acids by molecular oxygen in the presence of 4-carboxyl tetraphenylporphyrin (H₂TCPP), methylene blue (MB), cobalt(II) phthalocyanine sulfonate (CoPcS) and FeTCPPCl as water-soluble photosensitizers in organic-water biphasic media at room temperature under either visible light or sunlight is described. The products were obtained with 25–100% conversion and 100% selectivity. This method has a wide range of applicabilities, has a straightforward workup procedure, is chemoselective and proceeds under mild reaction conditions. The resulting products were obtained in good yields in reasonable times.

Full text of DOI:10.1007/s11243-018-0281-x

Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0281-x
Efcient aerobic photooxygenation of aldehydes to carboxylic
acids using cobalt(II) phthalocyanine sulfonate as a photosensitizer
in organic‑water biphasic media
Mahdi Hajimohammadi1 · Zahra Ahmadi Khamesi1 · Parisa Nosrati1
Received: 9 August 2018 / Accepted: 6 October 2018
© Springer Nature Switzerland AG 2018
Abstract
The aerobic oxidation of a variety of aromatic aldehydes to the corresponding carboxylic acids by molecular oxygen in the
presence of 4-carboxyl tetraphenylporphyrin (H2TCPP), methylene blue (MB), cobalt(II) phthalocyanine sulfonate (CoPcS)
and FeTCPPCl as water-soluble photosensitizers in organic-water biphasic media at room temperature under either visible
light or sunlight is described. The products were obtained with 25–100% conversion and 100% selectivity. This method has
a wide range of applicabilities, has a straightforward workup procedure, is chemoselective and proceeds under mild reaction
conditions. The resulting products were obtained in good yields in reasonable times.
Introduction
[19]. The oxidation of alkanes to alcohols and ketones has
been accomplished at 80 °C under 10 atm pressure of oxy-
gen [20]. A few examples of photochemical photooxidation
using O2 and a metalloporphyrin catalyst have been reported
[21, 22]. The photosensitized production of singlet oxygen
is relevant to the photooxidation of organic compounds, as
well as to DNA damage and photodynamic therapy [23, 24].
Consequently, a variety of photosensitizers have been devel-
oped and their photochemical and photophysical properties
have been studied extensively [25]. Among such compounds,
tetrapyrrolic compounds such as porphyrins and phthalocya-
nines are promising candidates for photosensitizers due to
their special photophysical properties [26–30]. In particular,
cobalt phthalocyanine has been widely used as a photosen-
sitizer in photocatalytic reactions [31–34]. In our previous
studies, we introduced efcient systems for the aerobic oxi-
dation of alcohols and alkenes in organic solvents under
visible light or sunlight [35–38]. The current study devel-
ops the photocatalytic oxygenation of aldehydes using air
under visible light or sunlight irradiation with water-soluble
photosensitizers in organic-water biphasic media at room
temperature (Scheme 1). The products were obtained with
25–100% conversion and 100% selectivity.
Oxidation is one of the most essential reactions in synthetic
organic chemistry, and for this purpose, a very wide variety
of oxidants have been developed. The oxidation of aldehydes
to carboxylic acids is of abiding interest in synthetic organic
chemistry [1–4]. The popular conventional route to carbox-
ylic acids involves the use of Jones reagent [5–10]. This
is a stoichiometric reaction using highly acidic conditions
which may not be tolerated by acid-sensitive functionalities
in the substrate. Furthermore, the production of Cr-based
side products may be viewed as a potential environmental
hazard. Other efcient reagents that have been reported in
the literature to achieve this transformation include oxone
[11], calcium hypochlorite [12] and 2-hydroperoxyhex-
afuoro-2-propanol [13]. Some interesting methodologies
involving metal-mediated transformation of aldehydes to
the corresponding carboxylic acids have also been recently
reported [14–18]. However, the development of catalytic
systems for hydrocarbon oxidation by O2 under mild condi-
tions is still a challenging problem. Iron porphyrins have
been used as catalysts for alkane hydroxylation by O2 while
consuming a stoichiometric amount of a reducing agent
* Mahdi Hajimohammadi
hajimohammadi@khu.ac.ir
1
Faculty of Chemistry, Kharazmi University, G. C., P. O.
Box 14911-15719, Tehran, Iran
Vol.:(0123456789)
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Transition Metal Chemistry
Scheme 1 Photooxygenation
of aldehydes to carboxylic
acids in the acetonitrile–water
media with diferent kinds of
water-soluble photosensitizers.
a Coph(SO3H)4, b methylene
blue, c H2TCPP, d FeTCPPCl
intensity of CoPcS. At the end of the reaction, the solvent
was removed under vacuum, and the residue was separated
by column chromatography (silica gel, n-hexane/EtOAc,
13:1) to give the corresponding carboxylic acid products.
The identities of the products given in Table 3 were con-
Experimental
Materials and methods
Benzaldehyde, 4-methoxybenzaldehyde, 2,3-dimeth-
oxybenzaldehyde, 3,4-dimethoxybenzaldehyde,
3-chlorobenzaldehyde, 4-chlorobenzaldehyde, anthra-
cene, anthracene-9-carbaldehyde, cinnamaldehyde,
2,3,4,5,6-pentafuorobenzaldehyde, iron(II) chloride and
solvents were purchased from Fluka or Merck and used
without further purification. MB was purchased from
Fluka. H2TCPP, FeTCPPCl and CoPcS were synthesized
according to the literature procedures [39–42].
1
frmed from their melting points and H NMR spectra
(NMR (BRUKER DRX-300 AVANCE spectrometer).
1
Benzoic acid (1): colorless crystals, mp 120–123 °C. H
NMR (300 MHz, CDCl3) δ 8.17 (s, 2H), 7.60 (s, 3H) ppm,
OH not observed. Cinnamic acid (2): colorless crystals,
mp 132–135 °C. 1H NMR (300.13 MHz, CDCl3) δ 10.80
(brs, 1H, –COOH), 7.78 (d, J = 12.05 Hz, 1H), 7.42–7.59
(m, 5H), 6.46 (d, J = 12.05 Hz, 1H) ppm. 4-methoxyben-
1
For generation of the carboxylic acids, 5 × 10−4 mol of
the required aldehyde and 8 × 10−6 mol of water-soluble
photosensitizer were added to acetonitrile (7.5 ml) plus
water (7.5 ml) in a test tube. The samples were irradiated
under visible light (288 power LED lamps, 1 W, 2.3 V,
59,660 Lux for 150 h at room temperature while bub-
bling air through the mixture (20 cm3 min−1). To prevent
evaporation of the solvent, the photoreactor was equipped
with a strong fan that kept the reaction temperature low
and the test tube was closed with paraflm. The stability
of the sensitizer was monitored by UV–Vis spectroscopy
(Cintra 101) during the reaction, by following the Q band
zoic acid (3, 4): colorless crystals, mp 182–185 °C. H
NMR (300.13 MHz, CDCl3) δ 11.85 (brs, 1H, –COOH),
8.07 (d, J = 8.3 Hz, 2H), 6.95 (d, J = 8.3 Hz, 2H), 3.89 (s,
3H) ppm. 2,3,4,5,6-pentafuorobenzoic acid (5): colorless
crystals, mp 100–103 °C. 1H NMR (300.13 MHz, CDCl3)
H of aldehyde (δ 10.01 (1H, s) ppm.) not observed. 3-chlo-
robenzoic acid (6): colorless crystals, mp 154–157 °C. 1H
NMR. (300.13 MHz, CDCl3) δ 11.30 (brs, 1H, –COOH),
8.11 (s, 1H), 8.01 (d, J = 7.6 Hz, 1H), 7.59 (d, J = 7.8 Hz,
1H), 7.42 (t, J = 7.8 Hz, 1H) ppm. Anthracene-9-carbox-
ylic acid (7): colorless crystals, mp 214–217 °C 1H NMR
(300.13 MHz, CDCl3) δ 11.53 (s, 1H, –COOH), 8.97 (d,
J = 9.0 Hz, 2H), 8.7 (s, 1H), 8.05 (d, J = 7.6 Hz, 2H), 7.68
1 3

Transition Metal Chemistry
(m, 2H), 7.53(m, 2H). 4-chlorobenzoic acid (8): colorless
crystals, mp 237–240 °C. 1H NMR (300.13 MHz, CDCl3)
δ 8.02 (d, J = 7.9 Hz, 2H), 7.43 (d, J = 8.2 Hz, 2H) ppm,
OH not observed. 3,4-dimethoxybenzoic acid (9): color-
Hence, the photosensitizer, light and O2 are all essential for
oxidation of aldehydes.
According to the literature, there are two major pathways
for photooxygenation in the presence of photosensitizers,
designated as Type I and Type II (Scheme 2) [45]. For inves-
tigation of the Type I mechanism, we performed our reac-
tions in the presence H2O2, OH· and O2·−. In the presence of
these reactive oxygen species, the yield of the reaction was
signifcantly decreased (Table 1, entries 8–10).
1
less crystals, mp 179–182 °C. H NMR (300.13 MHz,
CDCl3) δ 7.77 (d, J = 8.3 Hz, 1H), 7.61 (s, 1H), 6.92 (d,
J = 8.4 Hz, 1H), 3.96 (s, 3H), 3.95 (s, 3H) ppm, OH not
observed. 2,3-dimethoxybenzoic acid (10): colorless crys-
1
tals, mp 121–124 °C. H NMR (300.13 MHz, CDCl3) δ
7.70 (d, J = 6.9 Hz, 1H), 7.09–7.22 (m, 2H), 3.98 (s, 3H),
Singlet oxygen generation (Type II) and its subsequent
reaction with the substrate is the principal mechanism under
our reaction conditions, since a very characteristic reaction
of singlet oxygen is the [4+2] cycloaddition to conjugated
cyclic dienes and polycyclic aromatic hydrocarbons such as
3.91(s, 3H) ppm, OH not observed.
Results and discussion
The photooxygenation of 4-methoxybenzaldehyde was
chosen as a model reaction in order to evaluate the dif-
ferent factors involved in the catalytic reaction. Table 1
gives the yields of 4-methoxybenzoic acid with various
photosensitizers. The yield of 4-methoxybenzoic acid was
CoPcS > MB > FeTCPP > H2TCPP for the same reaction
time (Table 1, entries 1–4). It is worth noting that after 150 h
of photooxygenation of 4-methoxybenzaldehyde in the pres-
ence of MB, FeTCPP and H2TCPP, the color of the solution
had completely disappeared. Although the amount of singlet
oxygen that was produced in the presence of CoPcS was
low, because of lower degradation of CoPcS compared to
MB, FeTCPP and H2TCPP, it could be used to convert alde-
hydes to carboxylic acids over a longer period of time. It is
important to note that the oxidation of substrate ceased in the
absence of photosensitizer, light or air (Table 1 entries 5–7).
Scheme 2 Alternative mechanisms for the production of reactive
oxygen species in the presence of photosensitizers (sen). A is oxidiz-
able substrate
Table 1 Photooxygenation of
Entry
Condition
Conversion (%)
Quantum yield for
4-methoxybenzaldehyde under
1O2 generation (ΦΔ)
diferent reaction conditionsa
1
Light+air+CoPcS
Light+air+MB
Light+air+FeTCPP
Light+air+H2TCPP
Light+air
100
28
23
8
Trace
Trace
Trace
Trace
Trace
Trace
Trace
0.3 [43]
0.49 [25]
2
3b
4b
5
0.76 [25]
6
7
Air+CoPcS
Light+CoPcS+nitrogen
8c
9d
10e
11
OH·
O·2−
H2O2
Light+air+NaN3− +CoPcS
a8×10−6 mol photosensitizer, 5×10−4 mol 4-methoxybenzaldehyde, air (1Atm) and 288 power LED
lamps, 1 W, 2.3 V (59,660 LUX) in 1:1 CH3CN/H2O
bphotosensitizer was dissolved in KCl–NaOH bufer (0.2 M, pH=12.0)
cOH· produced from photolysis of 2.5×10−3 mol 30% H2O2 using light from a high-pressure 500 W mer-
cury lamp (λ=200–280 nm)
dO·2− was prepared by dissolving K2O in dried DMSO [44]
e4 mmol 30% H2O2 dissolved in the reaction solution
1 3

Transition Metal Chemistry
Table 2 Efect
of
solvent
on
4-methoxybenzaldehyde
anthracene [46–49]. Hence, singlet oxygen generation by
CoPcS is evidenced by chemical trapping of 1O2 with anthra-
cene. The UV–Vis spectra of anthracene as function of time
under photoirradiation with CoPcS as a photosensitizer are
displayed in Fig. 1. A decrease in intensity of the absorp-
tion band of anthracene (λmax=375 nm) was observed over
time. This response can be attributed to anthracene-9,10-en-
doperoxide formation (see Fig. 1). Moreover, the oxidation
reaction did not occur under dark conditions, confrming
that the anthracene oxidation is mediated by singlet oxygen
under visible irradiation.
photooxygenationa
Entry
Solvent
Conversion (%)
1b
2c
3d
4e
5
(H2O:CH3CN) 1:1
(H2O:CH3CN) 2:1
(H2O:CH3CN) 1:2
(H2O:EtOH) 1:1
H2O
100
80.1
68.5
76.0
Trace
a8×10−6 mol CoPcS, 5×10−4 mol 4-methoxybenzaldehyde, air
(1Atm) and 288 power LED lamps, 1 W, 2.3 V (59,660 LUX)
Furthermore, in the presence of N3−, which is a well-
known singlet oxygen scavenger [50], conversion was inhib-
ited (Table 1, entry 11). The photolysis was also studied in
diferent solvent mixtures with variable water-to-acetonitrile
ratios. Optimum conversion was observed in 1:1 acetonitrile/
water (Table 2). The lifetime of singlet oxygen in acetonitrile
is 65 μs and 38 μs in ethanol [51–53], which corresponds
with the results in Table 2, entries 1 and 4, again providing
evidence for generation of singlet oxygen in the reaction
media.
b7.5 ml CH3CN and 7.5 ml H2O
c5 ml CH3CN and 10 H2O
d10 ml CH3CN and 5 ml H2O
e7.5 ml EtOH and 7.5 ml H2O
shows that sensitizer bleaching for CoPcS was almost com-
plete after 150 h under our reaction conditions. After loss of
the Q band of CoPcS, the oxidation reaction ceased, and the
yield of product remained constant. Therefore, the reaction
time was chosen as 150 h in these studies.
High turnover number (TON) for the photooxygenation
of aldehydes was observed with CoPcS under visible light
(Table 3 entries 1–10). It is clear that this reaction will also
proceed under solar radiation because the spectral distribu-
tion curve of the lamp is similar to that of sunlight (Table 3
entry 4).
A plausible mechanism for this reaction is shown in
Scheme 3. The catalyst is activated by visible light to give
singlet oxygen. The singlet oxygen then undergoes insertion
into the C–H bond of the aldehyde 1 to form a peracid 3, via
the acyl and hydroperoxyl radicals as intermediates [61]. In
the fnal stage of reaction, a second aldehyde molecule reacts
with the peracid 3 to generate the adduct 4. Compound 4
then decomposes by a Baeyer–Villiger type rearrangement
to the fnal product 5 [61].
Figure 2 shows a plot of 4-methoxybenzoic acid forma-
tion versus time in oxygenated CH3CN under visible light
irradiation in the presence of CoPcS. The Q band of CoPcS
was followed at 660 nm by UV–Vis spectroscopy. The plot
Fig. 1 a UV–visible spectra for the photooxygenation of anthracene
(λmax=375 nm) in the presence of CoPcS as a photosensitizer using
simulated solar light (288 power LED lamps, 1 W, 2.3 V (59,660
Lux)) under 1Atm of bubbling air in acetonitrile. b UV–Vis spectra
of CoPcS (λmax=660 nm) before and after 180 min photooxygena-
tion
1 3

Transition Metal Chemistry
Table 3 Oxidation of aldehydes to carboxylic acids in the presence of CoPcSa
Entry Aldehyde
1
Carboxylic acid
Yield (%) CoPcS (mol) TONb TOFc (h−1
)
mp (°C)
120–123
Lit. mp (°C)
121–123 [54]
100
7 × 10−6
71.4
0.72
2
100
6 × 10−6
83.3
0.55
132–135
132–134 [55]
3
100
83
8× 10−6
1× 10−5
7 × 10−6
62.5
41.5
71.4
0.42
0.28
0.48
182–185
182–185
100–103
178–180 [54]
178–180 [54]
102–104 [56]
4d
5
100
6
7
100
100
7 × 10−6
8 × 10−6
71.4
62.5
0.48
0.42
154–157
214–217
155 [57]
217 [58]
8
9
75.5
44.4
7 × 10−6
7 × 10−6
53.9
31.7
0.36
0.21
237–240
179–182
241 [59]
179–182 [54]
10
25.4
5 × 10−6
25.4
0.17
121–124
122 [60]
a5 × 10−4 mol aldehyde, CoPcS, 7.5 ml CH3CN and 7.5 ml H2O, air (1Atm), 288 power LED lamps, 1 W, 2.3 V (59,660 LUX)
bTurnover number of catalyst
cTurnover frequency of catalyst
dSunlight (91,000 LUX)
in the presence of white light or sunlight and O2 in an
organic–water biphasic media under very mild conditions.
CoPcS was identifed as the optimal catalyst for the oxi-
dation reaction. This procedure is very simple and works
efciently at room temperature, giving good to excellent
yields for a range of substrates.
Conclusion
In conclusion, we have developed a new approach for
the aerobic oxidation of aldehydes to the corresponding
carboxylic acids using water-soluble photosensitizers
1 3

Transition Metal Chemistry
Fig. 2 a Conversion versus time for 4-methoxybenzaldehyde, degradation percent and absorption of CoPcS in 4-methoxybenzaldehyde pho-
tooxygenation. b UV–Vis spectra of CoPcS (λmax=660 nm) over time
Scheme 3 Proposed mechanism for photooxygenation of aldehydes by molecular oxygen in the presence of singlet oxygen in acetonitrile/water
biphasic media
Acknowledgements Financial support of this work by Iran National
Science Foundation (INSF) no. 96005616, and Research Council of
Kharazmi University are gratefully acknowledged.
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