Photooxygenation of alkenes by molecular oxygen in the presence of porphyrins and chlorin sensitizers under visible light irradiation

Article abstract of DOI:10.1142/S1088424610002446

Photooxidation of alkenes by molecular oxygen and visible light in the presence of tetraphenylporphyrin (H₂TPP), tetramesitylporphyrin (H₂TMP), tetrakis pentafluorophenylporphyrin (H₂TPFPP) and tetrakis(2,3-dimetoxyphenyl)porphyrin T(2,3-OMeP)P and metalloporphyrins such as ClFeTPP, ClMnTMP, ClMnTPP, ClMnTPFPP, ClCoTPP and ZnTPP has been performed. Photooxidation of alkenes with tetraphenylchlorin (H₂TPC) as the sensitizer with visible light has also been studied. The conversion rates for alkene oxidation were in the order of free-base porphyrins > chlorin > metalloporphyrins. In the presence of NaN₃/Na₂SO ₃ the yield of oxidation products and conversion percentage is considerably reduced, confirming involvement of ¹O₂ in the mechanism.

Full text of DOI:10.1142/S1088424610002446

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 639–645
DOI: 10.1142/S1088424610002446
Published at http://www.worldscinet.com/jpp/
Photooxygenation of alkenes by molecular oxygen in the
presence of porphyrins and chlorin sensitizers under
visible light irradiation
Mahdi Hajimohammadi and Nasser Safari*◊
Chemistry Department, Shahid Beheshti University, G.C, Evin, Tehran, 19839-63113, Iran
Received 8 May 2010
Accepted 26 June 2010
ABSTRACT: Photooxidation of alkenes by molecular oxygen and visible light in the presence of
tetraphenylporphyrin (H2TPP), tetramesitylporphyrin (H2TMP), tetrakis pentafluorophenylporphyrin
(H2TPFPP) and tetrakis(2,3-dimetoxyphenyl)porphyrin T(2,3-OMeP)P and metalloporphyrins such
as ClFeTPP, ClMnTMP, ClMnTPP, ClMnTPFPP, ClCoTPP and ZnTPP has been performed. Photo-
oxidation of alkenes with tetraphenylchlorin (H2TPC) as the sensitizer with visible light has also been
studied. The conversion rates for alkene oxidation were in the order of free-base porphyrins > chlorin >
metalloporphyrins. In the presence of NaN3/Na2SO3 the yield of oxidation products and conversion
percentage is considerably reduced, confirming involvement of 1O2 in the mechanism.
KEYWORDS: photooxygenation, chlorin, singlet oxygen, visible light.
[10–12]. In addition, the photochemistry of metallopor-
INTRODUCTION
phyrins, porphyrins and chlorins has been the subject of
Porphyrins and metalloporphyrin complexes (MP)
extensive studies such as solar energy in environmentally
have received great attention, due in part to their direct rel-
friendly green chemistry [13]. A few examples of photo-
evance to numerous biological systems [1]. Magnesium
chemical photooxidation using O2 and MP catalyst have
porphyrin has been used as an effective antenna in the
been reported [14, 15]. High turnover number (TON) of
absorbance and transfer of solar energy in photosynthesis
the catalyst has also been recently reported when using
[2]. Metalloporphyrins have been found to be efficient
metalloporphyrins and UV light [16], although the cata-
biomimetic catalysts for hydrocarbon oxidation by using
lyst system, ClMnTPP/1O2 , does not have any product
molecular oxygen and various oxygen transfer reagents
for cyclohexene substrate [17].
[3–5]. However, finding a catalytic system for hydrocar-
In this work we describe the photocatalytic oxidation
bon oxidation by O2 under mild conditions is still a chal-
of cyclooctene, cyclohexene, styrene and cis-stilbene by
lenge in the laboratory and in industry. Iron porphyrins
O2 under white light irradiation in the presence of por-
have been used as catalysts for alkane hydroxylation by
phyrins, metalloporphyrins and chlorin as the sensitizers
O2, with consumption of a stochiometric amount of a
(sens). The effects of solvents, time, metal and ligand in
reducing agent [6]. Oxidation of alkanes to alcohols and
the photooxidation reactions were investigated.
ketones was preformed at 80 °C under 10 atm pressure
of oxygen [7]. Porphyrins and metalloporphyrins have
also been used in photosensitizers, especially in photo-
EXPERIMENTAL
dynamic therapy (PDT) [8, 9]. Bacteriochlorin A (BCA)
H2TPC was synthesized and purified according to a
is also used in the treatment of many cancers in PDT
literature method [18]. H2TPP, H2TMP, T(2,3-OMeP)P
and H2TPFPP were synthesized, purified using the
Lindsey method [19, 20] and metalated according to
SPP full member in good standing
the Adler method [21]. Metalated and non-metaled
*Correspondence to: Nasser safari, email: n-safari@cc.sbu.
ac.ir, tel: +98 21-22401765, fax: +98 21-22403041
sensitizers were characterized by UV-vis and 1H NMR.
Copyright © 2010 World Scientific Publishing Company

640
M. HaJIMOHaMMaDI anD n. SafarI
O
O
All solvents were purchased from Merck
and distilled prior to use. Cyclooctene was
obtained from Fluka and cyclohexene, cis-
stilbene and styrene were obtained from
Merck. UV-vis spectra were recorded on a
Shimadzu 2100 spectrophotometer. Analysis
of the oxidation products were performed
by a Finnigan gas chromatograph equipped
with a capillary column CP-Sil 5 CB.
GC-Mass analysis was done by a Thermo-
Quest Finnigan GC-MS.
In a typical experiment, 1 mL dichlo-
romethane solution of porphyrins, metal-
loporphyrins or chlorin and 7.7 × 10-4 mol
cyclooctene were added to 14 mL of ace-
tonitrile in a quartz tube. Oxygen was bubbled
through the solution and the sample was irra-
diated using visible light (fluorescent circular
lamp, 22 W and 230 V) and (λ > 350 nm) for
16–72 h at 40 °C; the sample was held 3 cm
HO
hν
sens
+
+
+
+
O2
+
+
OH
O
O
hν
sens
+
+
O2
O2
O2
O
O
hν
sens
+
O
O
hν
sens
+
Scheme 2. Photocatalytic oxidation of various alkenes by molecular
oxygen in the presence of porphyrin or chlorin sensitizers in acetonitrile
at 40 °C
from center of lamp. The products were identified by GC-
Mass and gas chromatography. Yields and conversions of
generated products were determined by the coefficient
factor method [22].
The conversions of cyclooctene and the oxidation
product ratios with different kinds of sensitizers are listed
in Table 1. The conversion rates of cyclooctene are in the
order H2TPFPP > H2TPP > H2TMP > T(2,3-OMeP)P.
After 72 hours of irradiation all the cyclooctene is con-
verted into the oxidation products by H2TPP, H2TMP and
H2TPFPP. Therefore, fluorinated porphyrin has shown
higher stability and turnover number for alkene oxidation
in the presence of visible light.
H2TPP was chosen to study the effects of porphy-
rins and solvents on the photooxidation reaction.
Cyclooctene was chosen for optimization of the reac-
tion conditions such as time of reaction, metal, ligand
structureandtypeofsolvent.Figure1showsthediagram
of conversion of cyclooctene in an oxygenated solution
of CH3CN under visible light irradiation in the pres-
ence of H2TPP. Soret band of the H2TPP was
RESULTS AND DISCUSSION
The photocatalytic oxidation of alkenes by molecular
oxygen under irradiation with white light in the presence
of porphyrins, metalloporphyrins and chlorin sensitizers
was studied. Scheme 1 shows the structures of sensitiz-
ers employed and Scheme 2 summarizes the variation of
substrates and major photooxidation products for the ole-
fins used in this study.
R4
H
R2
H
H
R3
R2
R3
R4
H
monitored at 418.5 nm by UV-vis method.
This method showed that sensitizer bleach-
ing for H2TPP occurred after 72 hours under
our reaction conditions. After the disap-
pearance of the porphyrin Soret band, for-
mation of oxidation products stopped and
the reaction conversion remained constant.
In the case of H2TPFPP the UV-vis and dia-
R5
N
R1
H
H
H
H
R1
R5
N
R5
R1
N
H
H
H
H
H
N
M
N
N
N
H
N
R4
R3
R2
H
H
R1
R5
H
H
R3
R2
R4
H
H
gram of time vs. yield still confirmed the
presence of this sensitizer, showing fur-
ther stability and reactivity of the H2TP-
H
tetraphenylchlorin (H2TPC)
H
H2TPP: R1=R2=R3=R4=R5=H
H2TMP: R1=R3=R5=CH3, R2=R4=H
T(2,3-OMeP)P: R1=R2=OMe, R3=R4=R5=H
H2TPFPP: R1=R2=R3=R4=R5=F
FPP sensitizers. Higher reactivity for the
fluorinated porphyrin may be related to the
long-lived triplet states of halogenated por-
phyrin, due to the heavy atom effect [23],
which results in a higher quantum yield for
singlet oxygen production. It is important
to note the oxidation of substrate does not
continue in the absence of porphyrin or
when irradiation is interrupted (conversion
ClFeTPP: R1=R2=R3=R4=R5=H, M=FeCl
ClMnTPP: R1=R2=R3=R4=R5=H, M=MnCl
ClCoTPP: R2=R3=R4=R5=H, M=Zn
ZnTPP: R1=R2=R3=R4=R5=H, M=Zn
ClMnTMP: R1=R3=R5=CH3, R2=R4=H, M=MnCl
ClMnTPFPP: R1=R2=R3=R4=R5=F, M=MnCl
Scheme 1. Free-base (H2TMP, H2TPP and H2TPFPP) and metalated (Fe,
Mn, Zn and Co) porphyrins and tetraphenylchlorin (H2TPC) sensitizers
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 640–645

PHOtOOxygenatIOn Of alkeneS by MOleCular Oxygen
641
Table 1. Photooxidation of cyclooctene in acetonitrile by O2 in the presence of various porphyrins and metalloporphyrins as the
photosensitizer under visible lighta
Entry
Catalyst
Conversion, Cyloocteneoxide, Cycloocten-1-ol, Cycloocten-1-on, Pysical qenching
TON
%
%
%
%
kp, M-1.s-1 c
1
2
H2TPP
H2TPPd
100 (88)b
40 ( 1)
1.5 ( 1)
41 ( 3)
20 ( 1)
20 ( 2)
4 ( 1)
8 ( 2)
5 ( 1)
trace
10 ( 2)
0.3 ( 1)
30 ( 2)
7 ( 1)
15 ( 1)
7 ( 1)
5 ( 1)
0
50 ( 2)
2 ( 1)
29 ( 2)
72 ( 4)
48 ( 1)
16 ( 2)
21 ( 3)
8 ( 2)
trace
0.09 × 107
770 ( 50)
53 ( 30)
770 ( 70)
770 ( 60)
639 ( 40)
254 ( 40)
269 ( 60)
100 ( 30)
trace
7
100 (82)b
100 (100)b
83 (40)b
33
—
—
3
H2TMP
4
H2TPFPP
T(2,3-OMeP)P
ZnTPP
—
5
—
4 × 107
6
7
ClMnTPFPP
ClMnTPP
ClMnTMP
ClCoTPP
ClFeTPP
35
—
8
13
—
9
1.5
0
—
200 × 107
10
11
10
5 ( 2)
trace
0
5 ( 1)
trace
77 ( 30)
trace
1.5
0
—
a
b
1 × 10-6 mol porphyrin, 7.7 × 10-4 mol cyclooctene (sub/cat = 770) in 15 mL acetonitrile and 72 h reaction time. Conversion
after 48 h irradiation. c Rate constant of the physical qenching of singlet oxygen in CCl4 (Krasnovskii values) [27]. d 1 × 10-6 mol
porphyrin, 7.7 × 10-4 mol cyclooctene (sub/cat = 770) in 15 mL acetonitrile, 1 × 10-3 mol sodium azide, 1 × 10-3 mol sodium
sulfite.
the reactive oxo intermediate (P+MOIV) related to cyto-
chrom P-450 is reported by interaction of UV light, O2
and metalloporphyrins [17], it seems visible light has no
potential to generate a reactive intermediate (P+MOIV)
[26].
Therefore, we assume that under our reaction condi-
tions the mechanism employed is the generation of a
singlet oxygen since (i) according to the data in Table 1,
all the free-base porphyrins have higher conversion in
comparison to metalloporphyrins; (ii) Krasnovskii et al.
determined the values of singlet oxygen physical
quenching (Table 1) [23, 27] and our conversion per-
centage makes good correlation with those values
(Table 1, entries 1, 6 and 10); (iii) According to the lit-
erature data DMSO is a singlet oxygen scavenger [28,
29]; the conversion of cyclooctene in DMSO solvent
is considerably diminished (Table 2). It is also found
that in the presence of the [30] m-THPC (5,10,15,20-
tetra(m-hydroxyphenyl)chlorin) sensitizer the lifetime
of the singlet oxygen in DMSO solvent is 19 µs but in
the acetonitrile solvent the lifetime of the singlet oxygen
was reported to be 65 µs.
Fig. 1. Conversion vs. time of irradiation for H2TPP in ace-
tonitrile (series 1). Percent of remaining of H2TPP vs. time
(series 2)
of cyclooctene was traced). Therefore, the presence of
porphyrin, light and O2 are essential for the conver-
sion of cyclooctene.
Metalloporphyrins were less reactive than free-base
porphyrins under our reaction conditions. The order of
reactivity is free-base porphyrins > Zn > Mn > Co >
Fe. Zn porphyrin is known to quench singlet oxygen by
using the charge transfer mechanism (Table 1, entry 6)
[24]. Paramagnetic metals are claimed to quench sin-
glet oxygen by applying an energy transfer mechanisem
from oxygen to the low-lying d electron levels (Table 1,
entry 10) [24]. In addition, paramagnetic metallopor-
phyrins have a much shorter triplet lifetime compared
with diamagnetic complexes, and have smaller quantum
yield for 1O2 production [25]. Although involvement of
1
Further proof for O2 mechanism is obtained by per-
-
forming photooxygenation in the presence of N3 anion,
which is a competitor for reaction with 1O2 [31]. Entry 2
in Table 1 shows that conversion of substrate, degrada-
tion of the porphyrin and production of products are all
profoundly inhibited.
Photooxygenation of alkenes with H2TPP
Photocatalytic oxidation of cyclohexene, cis-stilbene
and styrene were accomplished in a fashion similar to
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 641–645

642
M. HaJIMOHaMMaDI anD n. SafarI
Table 2. Effect of solvent on the photooxidation of cyclooctene in acetonitrile by O2 in the presence of H2TPPa
Entry
Solvent
Conversion,
%
Yield of
cycloocteneoxide, %
Yield of
cycloocten-1-ol, %
Yield of
cycloocten-1-on, %
TON
1
2
3
acetonitrile
100
96
40
40
10
0
50
56
770
739
dimethyl formamide
dimethyl sulfoxide
trace
trace
trace
trace
trace
a 1 × 10-6 mol porphyrin, 7.7 × 10-4 mol cyclooctene in 72 h reaction time.
Table 3. Photooxidation of various alkene in acetonitrile by O2 in the presence of H2TPP under visible lighta
Entry
1
Substrate
Conversion, %
100
Product
Yield of product, %
Time
TON
cyclooctene
cyclooctene oxide
cycloocten-1-ol
cyclooctene-1-on
cyclohexeneoxide
cyclohexene-1-ol
cyclohexene-1-on
benzaldehyde
40 ( 1)
10 ( 2)
50 ( 2)
8 ( 1)
72 h 770 ( 50)
72 h 562 ( 40)
72 h 400 ( 20)
2
cyclohexene
73
6 ( 1)
59 ( 2)
33 ( 1)
18 ( 1)
7 ( 1)
3
4
cis-stilbene
52
10
epoxy cis-stilbene
benzaldehyde
styrene
72 h
77 ( 20)
styreneoxide
3 ( 1)
a 1 × 10-6 mol H2TPP, 7.7 × 10-4 mol substrate (sub/cat = 770) in 15 mL acetonitrile with visible light.
that described for cyclooctene oxidation with H2TPP as
catalyst. Table 3 lists the conversion, yields and TON for
the photooxidation of four different substrates. Conver-
sion of substrates decreases in the order of cyclooctene >
cyclohexene > cis-stilbene > styrene. Cyclooctene is the
most reactive olefin in the series because of its strong
nucleophilicity and more flexible geometry relative to
cycloohexene [32]. Olefins bearing aromatic substituents
are also less reactive because their allylic forms are less
stable [33].
absorbing visible light [34]. However, to our knowlege,
the photooxygenation of alkenes with chlorins sentitizers
has not been reported yet.
Photooxygenation of cyclooctene with H2TPC in a
fashion similar to that described for cyclooctene oxida-
tion with H2TPP catalyst was performed and the results
can be seen in Table 4.
Figure 2 shows the diagram of time vs. conversion
for H2TPP relative to H2TPC. Initial rate for H2TPC was
higher than for H2TPP since H2TPC is better at harvesting
visible light. However, as time went on, the H2TPP gained
moreconversionpercent.Thisobservationisexplainedby:
(i) a lower triplet-state lifetime of sensitizer and a higher
rate constant of quenching of singlet oxygen in H2TPC
(0.2 × 107 M-1.s-1) [27] relative to H2TPP (0.09 × 107
M-1.s-1) [27]; (ii) higher stability of H2TPP in comparison
to H2TPC. Also noteworthy is the UV-vis spectrum shows
that some H2TPC converts to H2TPP under our reaction
Photooxygenation of alkenes with tetraphenylchlorin
(H2TPC)
Tetraphenylchlorin (H2TPC) is the reduced from
of tetraphenylporphyrin (H2TPP) [18]. This dye has a
strong absorbance in the red region (652 nm) in addi-
tion to a Soret band at 418 nm and is a better antenna for
Table 4. Photooxidation of cyclooctene in various concentration of tetraphenylchlorin (H2TPC) in acetonitrile
by O2 under visible light
Entry Catalyst Conversion,
%
Yield of
Yield of
Yield of
TON
cycloocteneoxide, % cycloocten-1-ol, % cycloocten-1-on, %
1
2
H2TPCa
H2TPCb
68
4
5
59
64
523
385
100
23
12
a 1 × 10-6 mol tetraphenylchlorin (H2TPC). b 2 × 10-6 mol tetraphenylchlorin (H2TPC).
Copyright © 2010 World Scientific Publishing Company
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PHOtOOxygenatIOn Of alkeneS by MOleCular Oxygen
643
Fig. 2. Variation conversion percent of cyclooctene in the presence of 1 × 10-6 mol H2TPP (a) and in the presence of 1 × 10-6 mol
H2TPC with time of the reaction (b)
Fig. 3. UV spectra of H2TPP (-------), H2TPC after 6 h irradiation (––––––) and H2TPC (······). This spectra shows that H2TPC is
oxidized to H2TPP during photooxygenation reaction
conditions (Fig. 3), which means that in the oxidation
enviroment H2TPC is oxidized to H2TPP. The initial rate
of the alkene oxidation is higher for H2TPC and there is
no inhibition period before product formation. Conver-
sion of H2TPC to porphyrin was possibly accompanied
by some decomposition of H2TPC and this reduced the
rate of the oxidative products as time goes on.
Based on the variation in the oxidation products, pro-
duction of singlet oxygen in our reaction condition and
the literature [35–39], the proposed mechanism of O2
1
reaction with substrates is shown in Scheme 3.
CONCLUSION
InTable4,chlorinshowssomeconcentration-dependent
products which can be due to parallel reactivity of H2TPC
and H2TPP formed during the reaction period. Their con-
centration ratio change during reaction period.
Free-base porphyrins, chlorin and metalloporphyrins
have been used as photosensitizer for the oxidation of
alkenes by molecular oxygen under visible light. Pho-
tooxygenation with chlorin sensitizer had higher initial
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 643–645

644
M. HaJIMOHaMMaDI anD n. SafarI
O
O
O
O
(1O2)
H
O
O
O
Ene reaction
OH
+
n
n
n
H
O
n
n
n
H
O
O
H
O
H
n = 0, 2
- O-OH
O
O
H
O
O
H
O
O
O
O
O O
+
- O-OH
H
O
O
O
O
O
O
hv
- O-OH
H
O
O
2
+
O
CHO
O
O
O
O
2
+
Scheme 3. Mechanism of photooxygenation of alkenes by molecular oxygen in the presence of sensitizers under visible light in
acetonitrile solvent
rates and lower turnover number in comparison with
analogous free-base porphyrins.
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MAS, Arnaut LG, Sa GFF, Silva EFF, Rocha LB,
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We gratefully acknowledge financial support from the
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