Efficient Solar Photooxygenation with Supported Porphyrins as Catalysts

Full text of DOI:10.1002/cctc.201200532

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DOI: 10.1002/cctc.201200532
Efficient Solar Photooxygenation with Supported
Porphyrins as Catalysts
Sꢀnia Ribeiro, Armꢁnio C. Serra, and Antꢀnio M. d’ A. Rocha Gonsalves*[a]
Light can be used to mediate several chemical processes by
changing the reactivity of molecules through their activation,
thereby essentially corresponding to an electronic reorganiza-
tion. This activation can be achieved with the aid of an inter-
mediate molecule or even directly without the need for any
additional reagents. According to the principles of sustainable
chemistry, this latter option is the ideal choice.[1] The involve-
ment of light-activated catalysts that can initiate a chemical re-
action, thereby helping atom transfer or electron transfer, be-
longs to the field of photocatalysis.[2] Recent examples of these
reactions that use solar irradiation include synthetic reactions[3]
and clean degradation processes.[4] Otherwise, photochemical
catalysis depends on the physical transfer of energy from excit-
ed molecules to ground-state molecules that afford excited
states through their own reactivity.[2a] By using the relatively in-
exhaustible solar light, several photochemical processes can be
performed.[5] Recent examples of photochemical synthetic pro-
cesses that are promoted by sunlight include trans-to-cis
double-bond isomerization[6] and the photo-Friedel–Crafts acy-
lation of 1,4-naphthoquinones.[7] The photochemical oxidation
reaction by singlet oxygen is an attractive process, not only be-
cause it provides access to useful derivatives, such as endoper-
oxides and allylic hydroperoxides, but also because it can be
achieved by using molecular oxygen and solar radiation, there-
by making it an example of a “green” process. This reaction re-
quires the use of a soluble photosensitizer[8] or, better, a hetero-
genized form,[9] which improves the isolation procedures and
is much closer to sustainable chemistry principles. Porphyrins
are excellent photosensitizers and their covalent immobiliza-
tion on polymeric matrices has been shown to give very active
singlet-oxygen-generating materials.[10]
synthesis of a non-symmetric halogenated tetra-arylporphyrin
(1) in a reaction sequence that did not involve any chromato-
graphic purification steps. In fact, the chlorosulfonation reac-
tion was performed directly on the mixture that was obtained
from the condensation step. Owing to the different reactivities
of the phenyl and 2,6-dichlorophenyl rings, a selective chloro-
sulfonation of the unsubstituted phenyl ring was achieved to
give the reactive chlorosulfonylporphyrin derivative (2).[12] Sub-
sequent condensation with aminated Merrifield polymers
Mp1–Mp3 gave photosensitizers Ps1–Ps3,[10e,12] with different
spacers between the polymer backbone and the porphyrin
macrocycle.
The level of porphyrin incorporation into the polymer struc-
ture was calculated from elemental analysis of the supported
photosensitizers, taking into account the nitrogen content of
the Merrifield-modified polymers (Mp1–Mp3, Table 1).
Porphyrins with halogen atoms in the meso-phenyl positions
are better singlet-oxygen generators, mainly owing to the
heavy-atom effect.[11] In the case of supported porphyrins, the
type of support and the role of the spacer in photooxygena-
tion reactions have been studied previously by ourselves.[12]
Photosensitizer Ps1 has been shown to be an active catalyst
and, herein, two new supported photosensitizers with different
spacer groups were prepared (Ps2 and Ps3, Scheme 1). The
linking of porphyrins to Merrifield polymers began with the
[a] S. Ribeiro, Dr. A. C. Serra, Prof. A. M. d. A. Rocha Gonsalves
Chemistry Department
Faculdade de CiÞncias e Tecnologia
Universidade de Coimbra
Rua Larga, P-3000 Coimbra (Portugal)
Fax: (+351)239-826069
E-mail: arg@qui.uc.pt
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200532.
Scheme 1. Reaction sequence that leads to photosensitizers Ps1–Ps3.
ChemCatChem 2013, 5, 134 – 137 134
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version of a-terpinene was achieved and NMR analysis of the
reaction product showed that, besides ascaridol (4), which was
the expected [4+2] cycloadduct, about 13% of p-cymene (5)
was present (Scheme 2).
Table 1. Nitrogen content in the MpX and PsX polymers.
Entry Polymer Nitrogen Polymer Nitrogen Porphyrin incorporation
content
[%]
content [mmolgÀ1
[%]
]
The usefulness of the photooxygenation system for a-terpi-
nene, citronellol, and linalool substrates was evaluated with all
of the photosensitizers. After the completion of the reactions,
the mixtures were filtered to isolate the catalyst, evaporated,
1
2
3
Mp1
Mp2
Mp3
1.34
1.18
3.45
Ps1
Ps2
Ps3
1.54
1.58
3.65
0.0357
0.0714
0.0365
The order of porphyrin load-
Table 2. Solar photooxygenation reactions with supported photosensitizers Ps1–Ps3.[a]
ing (mmolporphyrin per gram
of polymer) was Ps2 (0.0714)>
Ps1 (0.0365)>Ps3 (0.0357). Al-
though being less reactive, the
arylated diamine gave greater
incorporation of the porphyrin
macrocycle, thus suggesting
that the polymer with the bi-
phenyl spacer (Mp2) gave more-
favorable conditions for the in-
corporation of porphyrin deriva-
tive 2. The polymers that were
modified with a C12 chain or
[b]
Entry
PSX
Substrate
nPsX/nsubs
t [h]
Yield[c] [%]
Selectivity[d] [%]
4 (84)
1
2
3[e]
4
5
6
7
8
PS1
PS1
PS1
PS1
PS1
PS1
PS2
PS2
PS2
PS3
PS3
PS3
PS3
–
3
3
3
3
6
9
3
6
9
3
3
6
9
3
1:10000
1:30000
1:30000
1:60000
1:10000
1:10000
1:30000
1:10000
1:10000
1:10000
1:30000
1:10000
1:10000
1:10000
3
4
5
8
7
8
5
5
6
3
4
8
6
6
87
80
89
92
99
99
98
99
99
95
89
99
97
9[g]
5 (16)
5 (39)
5 (29)
5 (69)
8 (53)
11 (53)
5 (30)
8 (58)
11 (58)
5 (13)
5 (13)
8 (52)
11 (48)
5 (91)
4 (61)
4 (71)
4 (31)
7 (47)
10 (47)
4 (70)
7 (42)
10 (42)
4 (87)
4 (87)
7 (48)
10 (52)
4 (9)
9
10
11
12
13
14[f}
with
a
pentaethylene chain
[a] CHCl3 was used as the solvent, with a balloon that was filled with O2. [b] Molar ratio of PsX to the substrate.
[c] Combined yield of the isolated products. [d] Relative amounts of the products, as determined by NMR spec-
troscopy of the reaction mixture. [e] Under a flow of air. [f] The same amount of compound 3 was used as in
entry 1. [g] The remainder of the product was reagent 3.
showed almost the same values
for porphyrin incorporation.
For the solar experiments, we
used a very simple apparatus
that consisted of a two-necked
round-bottomed flask (250 mL) that was adapted with an ice
condenser and a balloon that was filled with oxygen. The mix-
ture was exposed to solar irradiation on the balcony of the
Coimbra Chemistry Department (40815’N08827’W), with a solar
fluence of 45–55 WcmÀ2, and was subjected to constant mag-
netic stirring (see the Supporting Information).
and the residues were analyzed by NMR spectroscopy
(Table 2).
With a-terpinene, the supported photosensitizers Ps1–Ps3
showed very good efficiency, with complete conversion even
at high molar ratios (1:60000; Table 2, entry 4). With this sub-
strate, the amount of p-cymene increased with the reaction
time, which showed that the oxygen-mediated oxidation reac-
tion proceeded parallel to the photooxidation reaction, as con-
firmed by a blank experiment (Table 2, entry 14). In air (Table 2,
entry 3) the reaction is slower
In the first experiment, the photooxidation of a-terpinene
(3, 75 mm), the photosensitizer (Ps1)-to-substrate ratio was
1:1000, and CHCl3 was the solvent. After 1.5 h, complete con-
but, as expected, is more selec-
tive for ascaridole (4). With a sub-
strate that is more difficult to ox-
idize, such as citronellol (6), the
reaction is slower; however,
complete conversion is achieved
within 5 h with catalyst Ps2 at
a
1:10000 catalyst-to-substrate
molar ratio. This high activity is
similar to a dendrimer porphyrin
system,[13] but much more
simple. The main interest in the
photooxidation of citronellol is
related to the production of rose
oxide from the allylic alcohol
that is derived from compound
7.[14] With this system, photooxi-
Scheme 2. Photooxidation of compounds 3, 6, and 9 with photosensitizers Ps1–Ps3.
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dation with compounds Ps1–Ps3 gives hydroperoxides 7 and
8 in nearly equal amounts; this situation has also been ob-
served with other photooxidation systems.[8b,10b,12a,13,15]
Linalool (9) is a monoterpene that contains one allylic-alco-
hol functionality and one trisubstituted double bond and it is
found in many flavors and fragrances.[16] Photooxidation with
a catalyst-to-substrate ratio of 1:10000 occurred efficiently (6–
8 h) and only at the more electron-rich double bond, thereby
affording two regioisomeric hydroperoxides (10 and 11) in
almost the same amounts. One reaction batch gave 1.0 g of
the products with only 25 mg of the supported photo-
sensitizer.
Table 4. Solar photooxygenation reactions with photosensitizer Ps3 in
EtOH.[a]
[b]
Entry
Substrate
nPs/nsubs
t [h]
Yield[c] [%]
Selectivity[d] [%]
1
2
3[e]
4[f]
5
3
3
3
3
3
6
1:10000
1:30000
1:30000
1:30000
1:60000
1:10000
1.5
2.5
3
3.5
6
94
92
86
89
90
99
4 (99)
4 (99)
4 (99)
4 (99)
4 (99)
7 (49)
5 (1)
5 (1)
5 (1)
5 (1)
5 (1)
8 (51)
6
7
[a] EtOH was used as the solvent, with a balloon that was filled with O2.
[b] Molar ratio of Ps3 to the substrate. [c] Combined yield of the isolated
products. [d] Relative amounts of the products, as determined by NMR
spectroscopy of the reaction mixture. [e] First recycling experiment,
second cycle. [f] Second recycling experiment, third cycle.
The possibility of catalyst recycling was analyzed for sensitiz-
ers PS1 and PS3 by using compound 3 as the substrate in
a 1:30000 catalyst-to-substrate ratio (Table 3).
Table 3. Catalyst-recycling experiments with photosensitizers Ps1 and
Ps3 as the catalyst and compound 3 as the substrate.[a]
tries 2–4). At a ratio of 1:60000, only 6 h were needed to com-
plete the reaction; this result corresponds to the formation of
3.2 g of product with 18 mg of the catalyst. With citronellol (6),
the reaction occurred a little faster (7 h versus 8 h), but with
the same diastereomeric preference as that observed in CHCl3.
The less-favorable lifetime of singlet oxygen in EtOH is
known (12 ms compared with 60 ms in CHCl3[18a]); thus, another
favorable factor must be operating in this system. The similar
solubility of oxygen in these two solvents[19] excludes the con-
centration factor. One possible explanation could be the pho-
todecomposition of CHCl3 catalyzed by the porphyrin,[20] which
is likely to have two main consequences: 1) It quenches the ex-
cited porphyrin, thereby diminishing the amount that is in-
volved in the singlet-oxygen formation and 2) the production
of hydrochloric acid causes the formation of porphyrin dicat-
ion, which is not so reactive.
Cycle number
PsX
t [h]
Yield[b] [%]
Selectivity[c] [%]
1
2
3
1
2
3
Ps1
Ps1
Ps1
Ps3
Ps3
Ps3
4
5
8
4
4.5
4
80
86
83
89
91
91
4 (61)
5 (39)
5 (47)
5 (50)
5 (33)
5 (34)
5 (17)
4 (53)
4 (50)
4 (67)
4 (66)
4 (73)
[a] CHCl3 was used as the solvent, with a balloon that was filled with O2;
the substrate/catalyst ratio was 30000:1. [b] Combined yield of the isolat-
ed products. [c] Relative amounts of the products, as determined by NMR
spectroscopy of the reaction mixture.
The results of the recycling experiments show that catalyst
Ps3 is more active than catalyst Ps1 and, more importantly, no
apparent deactivation is observed after three consecutive runs.
Moreover, the selectivity for ascaridole (4) is higher with this
catalyst. With this catalytic system, at the end of the third
cycle, 6 g of products can be obtained with 13 mg of the cata-
lyst, which is an outstanding performance.
In support of this theory, we detected the presence of the
porphyrin dication during the reaction in CHCl3 and, thus,
a small portion of solid sodium hydrogen carbonate was
added. The polymer matrix influences the photooxygenation
process by lowering the quantum yield of singlet oxygen.[10f]
This fact points to another possible explanation of this solvent
effect, which is related to the solvation process and the greater
or smaller amount of solvent molecules that are present at the
polymer surface where the oxidative process occurs. This solva-
tion process has been implicated in the observed diastereose-
lectivities of ene reactions with supported catalysts[21] and may
also be active in this case. However, the most important differ-
ence with the CHCl3 system is that, with EtOH, almost no p-
cymene is formed. Taking into account that the source of p-
cymene is the oxygen oxidation of compound 3, with its corre-
sponding reduction into water, the presence of acid is crucial
for that reaction to occur. Again, this result points to the inter-
vention of hydrochloric acid from the decomposition of CHCl3
as being responsible for the difference in the observed reac-
tion rate.
To attempt a further step forward, we moved towards
“greener” conditions by replacing the solvent with EtOH be-
cause CHCl3 is not environmentally friendly,[17] although it pro-
vides a high lifetime for singlet oxygen.[18] The results of the
photooxygenation reactions with Ps3 in EtOH are presented in
Table 4.
Unexpectedly, with EtOH as the solvent, the reaction pro-
ceeded faster than in CHCl3 (Table 4, entry 1). The increase in
the reaction rate was compared with unsupported porphyrin.
The photooxygenation of a-terpinene with a substrate-to-por-
phyrin ratio of 1:10000 was complete after 1.5 h and with 99%
selectivity for the ascaridole product. This result confirms the
high efficiency of this kind of supported catalyst. The increased
catalyst efficiency with EtOH as the solvent was also clear at
a Ps3/a-terpinene (3) ratio of 1:30000 and was more evident
in the case of the recycling experiments (Table 4, entry 3 and
4). In that particular case, 22 mg of the catalyst gave a total of
6 g of ascaridole after the three experiments (Table 4, en-
In conclusion, meso-phenyl-substituted porphyrins can be
easily supported on aminated Merrified polymers through a se-
lective chlorosulfonation reaction of the porphyrin macrocycle.
These supported porphyrins act as efficient photosensitizers in
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at ambient temperature on a laboratory-built photoreactor (Fig-
ure S1 in the Supporting Information). A two-necked round-bot-
tomed flask was charged with the substrate (5 mmol), the photo-
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and solvent (65 mL). The mixture was purged with O2 for a few mi-
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Acknowledgements
The authors would like to thank Chymiotechnon and the FCT
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Keywords: oxygenation · photochemistry · porphyrinoids ·
singlet oxygen · supported catalysts
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Received: August 3, 2012
Published online on November 26, 2012
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