Photoinitiated catalysis in nafion membranes containing palladium(II) meso-tetrakis(N-methyl-4-pyridyl)porphyrin and iron(III) meso-tetrakis-(2,6-dichlorophenyl)porphyrin for O2-mediated oxidations of alkenes

Article abstract of DOI:10.1002/1521-3765(20010817)7:16<3564::AID-CHEM3564>3.0.CO;2-2

Immobilisation of both palladium(II) meso-tetrakis(N-methyl-4-pyridyl)porphyrin (PdTMPyP⁴⁺) and iron(III) meso-tetrakis(2,6-dichlorophenyl)-porphyrin (FeTDCPP⁺) in the same membrane of Nation creates a new composite system, in which the photo-excited palladium complex induces the O₂-mediated oxidation of cyclohexene to the corresponding allylic hydroperoxide and the iron porphyrin works as a catalyst for specific oxygenations of cyclohexene and cyclooctene. The role of PdTMPyP⁴⁺ is to induce the photo-activation of O₂ with visible light (λ > 500 nm) to generate singlet oxygen (¹O₂) by means of energy transfer from the excited triplet state. Consequently, the ¹O₂-mediated oxidation of cyclohexene to cyclohexenyl hydroperoxide can be realised with a selectivity greater than 90%. Spectroscopic and photophysical investigations show that the tetracationic palladium porphyrin is mainly fixed to the external part of the Nafion membrane, it is characterised by a triplet-state lifetime significantly higher than that in the solution phase. The monocationic FeTDCPP⁺ is able to diffuse into the anionic cavities of Nafion, where it works as a catalyst for O₂-mediated autooxidation processes that are initiated by the photogenerated hydroperoxides. These processes continue in the dark for many hours giving cyclohex-2-en-1-ol and trans-cyclohexane-1,2-diol monoethyl ether as main oxidation products. The presence of this ether, indirectly, reveals the formation of cyclohexene epoxide which undergoes a nucleophilic attack by ethanol and epoxide opening because of the strong acidic environment inside Nafion. The good photocatalytic efficiency of the oxidation process is demonstrated by an overall quantum yield of 1.1, as well as by a turnover value of 4.7 × 10³ with respect to the iron porphyrin. When cyclooctene is present as co-substrate, it also undergoes oxygenation. In contrast to what was observed for cyclohexene, cyclooctene epoxide can be accumulated in a significant amount. As far as the stability of the system is concerned, FeTDCPP⁺ undergoes about 1% degradation during the process, while the Nafion matrix can be utilised several times without observable modification.

Full text of DOI:10.1002/1521-3765(20010817)7:16<3564::AID-CHEM3564>3.0.CO;2-2

FULL PAPER
Photoinitiated Catalysis in Nafion Membranes Containing Palladium(ii)
meso-Tetrakis(N-methyl-4-pyridyl)porphyrin and Iron(iii) meso-Tetrakis-
(
2,6-dichlorophenyl)porphyrin for O -Mediated Oxidations of Alkenes
2
[a]
[a]
[a]
[b]
Andrea Maldotti,* Leonardo Andreotti, Alessandra Molinari, Sergey Borisov, and
Victor Vasilꢀev[
b]
Abstract: Immobilisation of both palla-
dium(ii) meso-tetrakis(N-methyl-4-pyri-
dyl)porphyrin (PdTMPyP ) and iron-
investigations show that the tetracation-
ic palladium porphyrin is mainly fixed to
the external part of the Nafion mem-
brane, it is characterised by a triplet-
state lifetime significantly higher than
that in the solution phase. The mono-
ether, indirectly, reveals the formation
of cyclohexene epoxide which under-
goes a nucleophilic attack by ethanol
and epoxide opening because of the
strong acidic environment inside Nafion.
The good photocatalytic efficiency of
the oxidation process is demonstrated
by an overall quantum yield of 1.1, as
4‡
(
iii) meso-tetrakis(2,6-dichlorophenyl)-
‡
porphyrin (FeTDCPP ) in the same
membrane of Nafion creates a new
composite system, in which the photo-
excited palladium complex induces the
‡
cationic FeTDCPP is able to diffuse
into the anionic cavities of Nafion,
where it works as a catalyst for O2-
mediated autooxidation processes that
are initiated by the photogenerated
hydroperoxides. These processes contin-
ue in the dark for many hours giving
cyclohex-2-en-1-ol and trans-cyclohex-
ane-1,2-diol monoethyl ether as main
oxidation products. The presence of this
3
O -mediated oxidation of cyclohexene
well as by a turnover value of 4.7 Â 10
2
to the corresponding allylic hydroper-
oxide and the iron porphyrin works as a
catalyst for specific oxygenations of
cyclohexene and cyclooctene. The role
of PdTMPyP4 is to induce the photo-
with respect to the iron porphyrin. When
cyclooctene is present as co-substrate, it
also undergoes oxygenation. In contrast
to what was observed for cyclohexene,
cyclooctene epoxide can be accumulat-
ed in a significant amount. As far as the
stability of the system is concerned,
‡
activation of O with visible light (l >
2
1
5
00 nm) to generate singlet oxygen ( O )
2
‡
by means of energy transfer from the
FeTDCPP undergoes about 1% degra-
excited triplet state. Consequently, the
dation during the process, while the
Nafion matrix can be utilised several
times without observable modification.
Keywords: biomimetic synthesis
heterogeneous catalysis ´ immobili-
sation O-O activation ´ photo-
oxidations
´
1
O -mediated oxidation of cyclohexene
2
to cyclohexenyl hydroperoxide can be
realised with a selectivity greater than
´
9
0%. Spectroscopic and photophysical
Introduction
the establishment of composite oxidation photocatalysts,
whose peculiar efficiencies and selectivities in oxidation
reactions are controlled by the specific functions of their
The oxo-functionalization of hydrocarbons is an interesting
reaction from the synthetic and industrial points of view.[1]
Recent research has focused on the employment of molecular
oxygen as oxidizing species in order to realise innovative and
economically advantageous processes, and, at the same time,
move toward a ªsustainable chemistryº.[2] We are interested in
[3]
constituents. Within this framework, we investigate here a
new system based on the use of palladium(ii) meso-tetrakis(N-
4‡
methyl-4-pyridyl)porphyrin (PdTMPyP ) and iron(iii) meso-
‡
tetrakis(2,6-dichlorophenyl)porphyrin (FeTDCPP ) support-
[4]
ed on Nafion membranes. Cyclohexene and cyclooctene
have been chosen as representative substrates of unsaturated
hydrocarbons, and their selective oxidation by molecular
oxygen is one of the current challenges in fine and industrial
chemistry.[
On the basis of pure thermodynamic considerations, most
of the organic compounds are not stable with respect to
[
a] A. Maldotti, L. Andreotti, A. Molinari
Dipartimento di Chimica e Centro di Studio su Fotoreattivit aÁ e Catalisi
del C.N.R.
University of Ferrara, Via L. Borsari 46, 44100 Ferrara (Italy)
E-mail: mla@unife.it.
1]
[
b] S. Borisov, V. Vasilꢀev
oxidation by O . There are, however, kinetic limitations in this
2
Department of Chemistry
Russian State Pedagogical University
Moyka, 48 191186 St. Petersburg (Russia)
process mainly imposed by the triplet ground state of the O2
molecule, which is not consistent with the singlet states of
3564
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Chem. Eur. J. 2001, 7, No. 16

3564±3571
PdTMPyP4 (T
‡
)‡O
)‡O
! PdTMPyP4‡‡ O
1
(1)
(2)
many organic substrates. O2 activation can be efficiently
1
1
2
2
2
2
4
‡ [5]
achieved by photosensitizers such as PdTMPyP .
In
PdTMPyP4 (T
‡
! PdTMPyP5‡.‡O
�
particular, this complex is able to transfer energy from its
1
excited triplet state to O to generate singlet oxygen ( O )
2
2
according to Equation (1). Alternatively, the excited state of
the palladium porphyrin may activate O through its reduc-
2
�
.
tion to the superoxide radical anion (O2 ), as schematised by
Equation (2). Part of this paper is concerned with the effects
of the Nafion support on the photosensitising properties of
‡
[7]
4‡
including FeTDCPP , and their presence in the composite
system investigated might have, therefore, significant effects
on the overall product distribution.
PdTMPyP . Particular attention is devoted to the verification
1
that the photogenerated O can react with cycloalkenes to
2
give allylic hydroperoxides, in agreement with the well-known
behaviour in the homogeneous phase[6] [Eq. (3)]. These allylic
hydroperoxides have been widely used as oxidants in hydro-
carbon monooxygenations catalysed by iron porphyrins
Increasingly, interest is devoted to heterogenisation of
metalloporphyrin catalysts since the support makes them
easier to handle and recycle than in homogeneous solution. In
addition, the support may control some main parameters
affecting the reactivity of the reaction intermediates, such as
Abstract in Italian: L'immobilizzazione di palladio(II) meso-
their local concentration, their mobility and the polarity of the
reaction environment.[
4
‡
tetrakis(N-metil-4-piridil)porfirina (PdTMPyP ) e di fer-
7a, 8]
Several cationic porphyrins have
ro(III) meso-tetrakis(2,6-diclorofenil)porfirina (FeTDCP-
been successfully immobilised on negatively charged ion-
exchange resins.[ In this study the matrix employed is Nafion,
a polymeric support comprising a perfluorinated backbone
connected to sulfonic groups. Its structure resembles that of a
reverse micelle, where the sulfonic head groups are clustered
together in polar pockets that are interconnected by short
channels within the perfluorocarbon matrix.[
Nafion has been extensively employed for photophysical
and photochemical studies.[ In particular, it is a promising
support for oxidative photocatalytic purposes, because it
presents a very high affinity for O2[ and it is totally
transparent to the excitation wavelengths of interest. Some
of our preliminary results indicate that this polymeric matrix
‡
P ) sulla stessa membrana di Nafion d aÁ luogo ad un nuovo
9]
sistema composito, in cui l'eccitazione fotochimica del com-
plesso di palladio induce l'ossidazione del cicloesene, mediata
dall'O , al corrispondente allil idroperossido e la ferro
2
porfirina agisce da catalizzatore per specifiche ossigenazioni
del cicloesene e del cicloottene.
10]
4‡
Il ruolo della PdTMPyP eÁ quello di indurre la fotoattivazio-
ne dell'O con luce visibile l > 500 nm, generando ossigeno di
2
11]
1
singoletto ( O ) attraverso un meccanismo di trasferimento di
2
energia dal suo stato di tripletto eccitato. Una diretta conse-
12]
guenza di questo processo eÁ l'ossidazione del cicloesene a
1
cicloesenil idroperossido, mediata dall' O e realizzata con
2
una selettivit aÁ maggiore del 90%. Studi spettroscopici e
fotofisici dimostrano che la palladio porfirina tetracationica eÁ
localizzata nella parte esterna della membrana di Nafion, dove
eÁ caratterizzata da un tempo di vita del suo stato di tripletto
nettamente pi uà alto di quello ottenuto in soluzione omogenea.
‡
strongly affects the activity of the photoexcited FeTDCPP in
the oxidation of cyclohexene, and the formation of cyclo-
hexene epoxide as reaction intermediate is favoured.[
3a]
‡
La FeTDCPP monocationica eÁ capace di diffondere nelle
cavit aÁ anioniche del Nafion, dove catalizza processi di auto-
Results and Discussion
ossidazione mediati dall'O , che sono iniziati dagli idro-
2
Heterogenisation of PdTMPyP4 and FeTDCPP with
‡
‡
perossidi fotogenerati. Questi processi continuano al buio per
molte ore, dando luogo alla formazione di cicloes-2en-1olo e di
trans-cicloesan-1,2-diol monoetiletere come prodotti principali
di ossidazione. La presenza di questo etere eÁ un'evidenza
indiretta della formazione di cicloesene ossido, il quale, a causa
della forte acidit aÁ all'interno del Nafion, subisce attacco
nucleofilo da parte dell'etanolo con conseguente apertura
dell'anello epossidico.
Nafion membranes: Either PdTMPyP4 or FeTDCPP can
be successfully immobilised on Nafion membranes as a
consequence of their interaction with the anionic SO3 groups
of the polymeric matrix. According to the procedure descri-
bed in the Experimental Section, the uptake of 0.3 ± 0.6 mmol
of each complex in 1 g of resin, typically, occurred in a few
minutes.
‡
‡
�
Optical microscopy allows us to establish where the metal-
loporphyrins are mainly located. The edge view of the cross
La buona efficienza fotocatalitica del processo di ossidazione eÁ
dimostrata da un rendimento quantico complessivo di 1.1, e da
4‡
3
section of the membrane containing PdTMPyP (PdTM-
un valore di turnover di 4.7 Â 10 rispetto alla ferro porfirina.
4‡
PyP /Nafion system) reported in Figure 1A shows that the
clustering of the complex inside the support is unlikely and
that it is mainly fixed in close proximity to the surface of the
solid matrix. This is even more evident upon comparison of
Figure 1C and Figure 1D, which show that the fluorescence
emission of PdTMPyP4 is clearly visible on the external part
of the Nafion support. These observations are not surprising
in view of the strong coulombic attractions between the
tetracationic ring of PdTMPyP4 and the negatively charged
Quando il cicloottene eÁ presente come co-substrato, anch'esso
viene ossidato; tuttavia, al contrario di quanto osservato con il
cicloesene, il cicloottene epossido eÁ relativamente stabile e pu oÁ
essere accumulato in quantit aÁ significative. Per quanto riguar-
‡
da la stabilit aÁ del sistema, FeTDCPP si degrada solamente di
‡
circa l'1% durante l'intero processo, mentre la membrana di
Nafion pu oÁ essere riutilizzata pi uà volte senza subire apprez-
zabili modifiche.
‡
Chem. Eur. J. 2001, 7, No. 16
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3565

Maldotti et al.
FULL PAPER
observed by other authors in the case of cationic iron or
[
7a]
manganese porphyrins supported on anionic polymers.
Both the metalloporphyrins PdTMPyP4 and FeTDCPP
‡
‡
can be immobilised simultaneously in Nafion membranes
4
‡
‡
(
PdTMPyP /FeTDCPP /Nafion system). The formation of
‡
aggregates is unlikely in the case of FeTDCPP because of the
presence of the bulky chlorine atoms in the ortho positions of
the phenyl groups.[ Accordingly, the UV/Vis spectrum of the
immobilised iron porphyrin is typical of its monomeric form.
In comparison with PdTMPyP , FeTDCPP is character-
ised by a reduced positive charge distributed in a different
way. Therefore, its coulombic attraction to the negatively
charged sulfonic groups of the membrane is weaker and the
iron complex is able to diffuse into the depth of the polymer,
where it is expected to be caged inside the large anionic
cavities. In line with this statement, Figure 1B shows that the
15]
4‡
‡
Figure 1. Optical microscopy of Nafion containing PdTMPyP4‡ or
‡
‡
red-brown FeTDCPP is immobilised, at least in part, inside
FeTDCPP . A) and B): edge views of the cross section of membranes
4
‡
‡
contacted with PdTMPyP and FeTDCPP , respectively (white light); C)
and D): cross sections of an unmodified bead and of a bead contacted with
PdTMPyP4 , respectively (UV light).
the depth of the membrane.
‡
4‡
Photosensitised activation of O with the PdTMPyP /Nafion
2
system: The absorption spectrum of the lowest triplet of the
4
‡
palladium porphyrin (PdTMPyP (T )) can be observed after
sulfonic groups of the membrane, and this may prevent the
diffusion of the complex from the bulk solution to the depth of
the polymer.
1
excitation with a laser pulse at 532 nm under anaerobic
conditions and is reported in Figure 2 (insert). Since this
spectrum is not noticeably affected by immobilisation of the
palladium porphyrin in Nafion, we can compare the decay of
the transient absorption at 460 nm of PdTMPyP (T ) dis-
solved in ethanol with those obtained in membranes of Nafion
either immersed in the same alcohol or in the dry state. In all
Figure 2 provides evidence that heterogenisation with
Nafion membranes strongly affects the UV/Vis spectrum of
4‡
4‡
PdTMPyP . A significant hypsochromic shift of the Soret
1
4
‡
band of PdTMPyP (curve B) compared with the spectrum in
4‡
cases, PdTMPyP (T ) decays by first-order kinetics, as
1
exemplified in Figure 3A for the ethanol swollen membrane.
Figure 2. Absorption spectra of PdTMPyP4‡; A) in aqueous solution; B) in
dry membranes of Nafion. Insert: transient absorption spectrum of
PdTMPyP4 (T
‡
) after laser pulse at 532 nm.
1
4
‡
Figure 3. Absorption changes at 460 nm of PdTMPyP (T
1
) after pulse
irradiation at 532 nm of PdTMPyP4 immobilized in ethanol swollen
‡
water (curve A) is clearly observed. Similar effects of the solid
2
Nafion; A) in the absence of oxygen; B) upon O equilibrated conditions.
4‡
matrix on the UV/Vis spectrum of PdTMPyP were also
observed when the modified membrane was immersed in
ethanol, which is the solvent employed in the photocatalytic
experiments described below. The spectral changes observed
in Figure 2 are well known to accompany the conversion of
The respective lifetimes (t ) are collected in Table 1.
A
PdTMPyP4 supported on Nafion undergoes an enhancement
of its excited triplet lifetime compared with the solution-
phase, and this indicates that the inhomogeneity of the solid
support may reduce the efficiency of thermal-deactivation
processes of the excited state.
‡
[
13]
monomeric porphyrins to face-to-face aggregates. Aggre-
gate formation should be favoured because the sulfonic
groups of the Nafion surface are expected to screen the
positive charges at the periphery of the porphyrin ring, and
hence reduce the electrostatic repulsion between the com-
Table 1 also reports the triplet lifetimes in O -equilibrated
2
media (t ), which indicate that the quenching of the excited
B
[
14]
4‡
plexes and promote p ± p interactions. The formation of
state of PdTMPyP by O2 occurs both in homogeneous
aggregates along the surface of sulfonated polymers was
solution and in the supported systems. Figure 3B shows, as an
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Chem. Eur. J. 2001, 7, No. 16

Porphyrin-Catalyzed O -Mediated Oxidations of Alkenes
3564±3571
2
Table 1. Photophysical properties of PdTMPyP4‡
.
1
oxidation of RNO by O . Also in this case, we did not have
2
t [
a]
[b]
any experimental evidence of formation of peroxides derived
Reaction media
A
(ms)
t
B
(ms)
by the reductive activation of O2.
EtOH[c]
Nafion
Nafion/EtOH[
10
700
400
0.5
40
5
4‡
[d]
The ability of the photoexcited PdTMPyP /Nafion system
d]
1
to induce the O mediated oxidation of alkenes has been
2
a] Lifetime of PdTMPyP4‡(T
Æ5%. [b] Lifetime of PdTMPyP (T
assessed by using cyclohexene as a representative substrate.
Photochemical excitation (l > 500 nm) of the PdTMPyP /
Nafion system in oxygenated mixtures of cyclohexene/ethanol
[
1 2
) in the absence of O . Reported values are
4‡
4‡
1
) in the presence of O
2
(760 Torr).
4
‡
� 5
Reported values are Æ5%. [c] PdTMPyP concentration: 1.7 Â 10
�
3
A
moldm . t is in good agreement with the value reported in a previous
(
1:3), results in the oxidation of the alkene to the correspond-
work.[ [d] Amount of PdTMPyP in Nafion: 8 Â 10 molg
19]
4‡
� 7
� 1
.
ing allyl hydroperoxide (C H OOH) as the main product
6
9
(
>90%), with only minor amounts of cyclohex-2-en-1-ol and
cyclohex-2-en-1-one. The good selectivity for the production
of the hydroperoxide is in agreement with the formation of a
peroxirane intermediate as schematised in Equation (3).
Irradiation with monochromatic light allowed us to calculate
an overall quantum yield for the oxidation of cyclohexene of
example, the decay of the transition absorption in the ethanol
swollen membrane. The effect of O on the triplet lifetime of the
2
4‡
heterogenised PdTMPyP is more significant in the presence
of ethanol than with membranes in the dry state, this is
because O diffusivity is known to increase remarkably (about
2
0.025 at 525 nm.
16]
2
0 times) in wet Nafion with respect to the dry polymer.[
As far as the stability of the palladium porphyrin is
As schematised by Equations (1) and (2), the quenching of
concerned, it is noteworthy that the modified membranes
can be used at least for three subsequent experiments with
almost negligible loss of their photochemical activity. In
particular, at the end of every photooxidation process, the
amount of palladium porphyrin in Nafion is about 5% lower
than the initial one. Since there is no evidence of leaching of
the catalyst from the resin into the solution, this means that
the porphyrin ring undergoes a slow degradation with the
formation of small colourless fragments.
the palladium porphyrin triplet state by O might give both
2
1
.�
O and O . To establish that the energy transfer pathway to
2
1
2
4‡
O2 is prevalent during photoexcitation of the PdTMPyP /
Nafion system under aerobic conditions the following experi-
ments were carried out.
Triethanolamine (TEA) is known to capture the porphyrin
radical cation eventually formed as a consequence of the
electron transfer reaction 2, and this favours the diffusion of
�
.
O2 and leads to the accumulation of detectable H O in the
2
2
A comparison between the described results and those
obtained in analogous experiments carried out with dissolved
PdTMPyP4 in homogeneous solutions indicates that hetero-
genisation with Nafion membranes does not affect signifi-
cantly the photosensitising properties of PdTMPyP4 in terms
[
11e]
solution bulk.
The reductive activation of O during irradi-
2
4
‡
ation of the PdTMPyP /Nafion system can be ruled out since
the iodometric analysis after two hours irradiation in the
‡
presence of TEA does not reveal the presence of peroxides.
‡
1
The photogenerated O can be detected by the method of
2
of both the mechanism for O activation and the oxidation
2
[
17]
Kraljic and Moshni.
This species is able to react with
quantum yield. On the other hand, the solid matrix has a
imidazole and the imidazole-O adduct oxidises and bleaches
2
4‡
positive effect on the stability of PdTMPyP . In fact the
N,N-dimethyl-4-nitrosoaniline (RNO). Figure 4 shows the
equivalents of produced hydroperoxide per equivalent of
consumed porphyrin in the heterogeneous system are about
3
4
 10 , and about four times higher than those obtained in the
homogeneous phase.
Photocatalytic properties of the PdTMPyP /FeTDCPP /
4
‡
‡
Nafion system: The photocatalytic properties of the
4‡
‡
PdTMPyP /FeTDCPP /Nafion system have been assessed
by irradiation with light of wavelength higher than 500 nm.
Under these conditions PdTMPyP4 is the photochemically
active species of the composite system, since the well-known
‡
‡
redox photoactivity of FeTDCPP can be induced only with
wavelengths lower than 400 nm.[
3]
The results so far discussed provide evidence that irradi-
4‡
ation of the PdTMPyP /Nafion system in the presence of
cyclohexene and O leads to the formation of C H OOH.
Figure 4. Spectral variations of the solution phase upon irradiation at l ˆ
25 nm of PdTMPyP4 immobilized in Nafion membranes (1.5 cm )
‡
2
2
6
9
5
Interestingly, curve A of Figure 5 shows that the formation of
this hydroperoxide continues in the dark for many hours after
irradiation of Nafion membranes containing both
immersed in oxygenated water solutions (3 mL) of RNO (4 Â
�
5
moldm ), and imidazole (5 Â 10� 3 moldm ). Irradiation times:
� 3
� 3
1
1
0
0 min.
PdTMPyP4 and FeTDCPP for 240 min. Curves B and C
‡
‡
spectral variations of RNO upon continuous irradiation of the
PdTMPyP /Nafion system in contact with ethanol containing
indicate that this thermal process does not occur in the
4‡
‡
absence of FeTDCPP or upon irradiation of a mixture of
PdTMPyP4 and FeTDCPP in homogeneous solution.
‡
‡
RNO, imidazole and O . Since RNO is not stable in acidic
2
media, we were forced to use membranes in the sodium form.
The absorbance decay at 420 nm can be ascribed to the
These findings are a clear indication that the iron ± por-
phyrin complex inside Nafion works as a catalyst for O
3567
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2
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Maldotti et al.
FULL PAPER
autooxidation processes that would account for the accumu-
lation of C H OOH (steps c, d, e).
6
9
Table 2 reports the nature and yields of all the oxidation
products of cyclohexene after 240 min irradiation followed by
9
5 h in the dark (Table 2, entry 2). It is evident that, in
‡
addition to C H OOH, the catalytic activity of FeTDCPP
6
9
yields other products, whose formation already begins during
the first 240 minutes irradiation (Table 2, entry 1). In partic-
ular, cyclohex-2-en-1-ol and trans-cyclohexane-1,2-diol mon-
oethyl ether represented about the 80% of the overall
oxidised cyclohexene. It has been possible to verify that the
thermal oxidation process carried on for at least another three
days, and induced a further increase (70%) of the oxidation
products.
6 9
Figure 5. Variation of the C H OOH concentration as during 240 min
4‡
‡
irradiation (l > 500 nm) and 46 h in the dark; A) PdTMPyP /FeTDCPP /
2
Nafion system (membranes of 1.5 cm ) in a ethanol/cyclohexene (3:1;
The presence of trans-cyclohexane-1,2-diol monoethyl
ether among the products indirectly reveals the formation of
cyclohexene epoxide which, in the strong acidic environment
inside Nafion, undergoes a nucleophilic attack by ethanol, and
leads to the epoxide opening. This is confirmed by the
observation that the epoxide in acidic solution undergoes the
same nucleophilic attack with the formation of the same
product during the photochemical oxidation.
4
‡
3
mL) mixed solvent; B) PdTMPyP /Nafion system in the same mixed
4
‡
‡
solvent; C) PdTMPyP and FeTDCPP dissolved in the same mixed
�
2
� 3
solvent acidified with CF
3
SO
3
H (2 Â 10 moldm ).
mediated autooxidation processes, which are initiated by the
photochemically obtained C H OOH. On the basis of liter-
6
9
ature data, it is plausible that inhomogeneity effects and high
oxygen concentration inside the polymeric matrix may favour
the reaction of the allyl hydroperoxide with the porphyrin
complex both in its ferric and ferrous form according to
4‡
The good photocatalytic efficiency of the PdTMPyP /
‡
FeTDCPP /Nafion system is demonstrated by an overall
quantum yield for the oxidation of cyclohexene of 1.1 at
5
25 nm, that is, about forty times higher than that obtained
[
7h]
Scheme 1 (steps a,b). The so obtained radical intermediates
4‡
during irradiation of PdTMPyP /Nafion (see before) and a
turnover value (moles of oxidised cyclohexene per mole of
.
.
C H O and C H OO , in turn, are able to initiate chain
6
9
6
9
‡
3
FeTDCPP immobilised) of 4.7 Â 10 , which corresponds to a
�
1
reaction rate of 0.8 turnovermin . Another important issue is
the catalyst stability. In this respect, we note that the Nafion
matrix can be utilised at least three times without any
‡
appreciable modification and that FeTDCPP undergoes only
1
% degradation to colourless fragments during the process.
4‡
‡
PdTMPyP is less stable than FeTDCPP and about 30%
PdTMPyP4 was lost during the entire catalytic experiment.
The described processes take place because the optimum
conditions have been found, the conditions are related to the
peculiar reactivity of both the photogenerated intermediates
and the organic substrates inside the solid matrix. For
example, with tert-butyl hydroperoxide as initiator of the
catalytic oxidation of cyclohexene instead of the photogen-
erated allyl hydroperoxide the reaction yields are reduced by
ten times. We also showed that the effect of the Nafion matrix
is not just of providing an acidic environment. In fact, entries 3
and 4 of Table 2 show that although irradiation of mixtures of
‡
PdTMPyP4 and FeTDCPP dissolved in EtOH acidified with
trifluoromethanesulfonic acid give the same oxidation prod-
ucts as the heterogeneous system, no subsequent oxidation
process in the dark is initiated.
‡
‡
4‡
‡
The PdTMPyP /FeTDCPP /Nafion system significantly
improves the photocatalytic properties of the previously
‡
[3a]
investigated FeTDCPP /Nafion photocatalyst since 1) it is
activated by visible light instead of ultraviolet wavelengths; 2)
as a consequence of its significant activity in the dark, it
presents a quantum yield value for the oxygenation of
cyclohexene about two orders of magnitude higher; 3) its
�
1
‡
catalytic rate in terms of turnovermin is about four times
higher.
Scheme 1. Thermal catalysis by FeTDCPP in Nafion initiated by allyl
hydroperoxides.
3568
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Chem. Eur. J. 2001, 7, No. 16

Porphyrin-Catalyzed O -Mediated Oxidations of Alkenes
3564±3571
2
Table 2. Photoinduced catalytic oxygenation of cyclohexene and cyclooctene by the PdTMPyP4‡/FeTDCPP /Nafion system.[a]
‡
Entry
1
System
Substrate
Product distribution [%]
Total turnover[b]
340
4‡
‡
PdTMPyP /FeTDCPP /
Nafion[ ‡ hn
c]
7
1
6
1
4
5
13
36
7
9
7
2
3
4
4‡
‡
4700
370
PdTMPyP /FeTDCPP /
Nafion[ ‡ hn ‡ dark
c]
4
46
17
45
4‡
‡
‡
PdTMPyP /FeTDCPP
solution[ ‡ hn
d]
11
4‡
360
PdTMPyP /FeTDCPP /
solution[ ‡ hn ‡ dark
d]
3
1
8
5
11
76
6
9
4‡
‡
5
6
PdTMPyP /FeTDCPP /
210
Nafion[ ‡ hn ‡ dark
c]
4‡
‡
6100
PdTMPyP /FeTDCPP /
ROOH
7
Nafion[ ‡ hn ‡ dark
c]
52
11
15
9
6
[
a] The composite system (Nafion membranes of 1.5 cm2 containing both the porphyrins immersed in mixtures of alkene/ethanol 1/3) has been
photoirradiated for four hours at l > 500 nm in the presence of 760 Torr of O
2
. In some cases irradiation was followed by 95 h of thermal catalysis in the dark.
‡
[
1
b] Moles of oxidized alkenes per mole of FeTDCPP which was present at the beginning of the process. [c] 0.06 Æ 0.01 g of Nafion contained 3.0 Æ 0.3 Â
�
7
� 1
4‡
� 7
� 1
‡
4‡
� 6
� 3
0
molg of PdTMPyP and 6.0 Æ 0.6 Â 10 molg of FeTDCPP . [d] Initial concentration of PdTMPyP
� 3
:
6.5 Æ 0.5 Â 10 moldm . Initial
‡
� 6
� 3
� 2
concentration of FeTDCPP : 1.3 Æ 0.1 Â 10 moldm . The solutions were acidified with CF
3
SO
3
H (2 Â 10 moldm ).
The metalloporphyrin-catalysed oxygenation of alkenes by
alkyl hydroperoxides has been extensively investigated in the
Conclusion
last two decades.[7a,b] The factors that control the competition
between the proposed mechanisms are strongly connected to
the specific properties of the reaction environment and are
still the object of much-discussed investigation by different
The composite photocatalyst investigated shows a peculiar
activity that is the result of specific roles of all of its
4‡
‡
components: PdTMPyP , FeTDCPP , and the Nafion matrix.
4‡
The catalytic process is photoinitiated by PdTMPyP . This
complex is mainly fixed near the surface of the polymer
matrix, where it undergoes a pronounced enhancement of the
lifetime of its triplet excited state compared with that of the
[
7c±h]
research groups.
Two reaction pathways are commonly
proposed, neither of which can be ruled out in the photo-
catalytic system investigated here. 1) A free-radical mecha-
nism initiated by the homolytic cleavage of the peroxidic
O�O bond by the iron ± porphyrin catalyst generates iron-
4‡
porphyrin in solution-phase. The role of PdTMPyP is to
induce the photoactivation of O2 with visible light (l >
1
(iv) ± oxo porphyrin and alkoxyl radicals. 2) The formation of
500 nm), to generate singlet oxygen ( O ) by energy transfer
2
1
a monooxygenating iron(v) ± oxo porphyrin as a consequence
of the heterolytic cleavage of the O�O bond.
from its triplet state. As a consequence, the O -mediated
2
oxidation of cyclohexene to cyclohexenyl hydroperoxide can
be realised with a selectivity higher than 90%.
4‡
‡
Entry 5 of Table 2 shows that the PdTMPyP /FeTDCPP /
Nafion system in the presence of O and light was also able to
The catalytic oxidation of alkenes by alkyl hydroperoxides
generated in situ from molecular oxygen at room temperature
and atmospheric pressure is very attractive from the synthetic
2
induce the oxygenation of cyclooctene. In contrast to what is
observed for cyclohexene, cyclooctene epoxide is relatively
stable and can be accumulated in a significant amount.
However, the catalytic oxidation of cyclooctene is twenty
times lower than that of cyclohexene, probably because of its
low reactivity especially towards allylic hydrogen-atom ab-
straction, which is a key step in the overall oxidation process
as shown in Scheme 1. The fundamental role of cyclohexene
in the oxidation mechanism is demonstrated by entry 6 of
Table 2, this shows that when the experiments are carried out
in the presence of cyclohexene as a cosubstrate, cyclooctene is
oxidised to a greater extent and the total turnover increases
‡
and industrial points of view. This is achieved by FeTDCPP
which works as a catalyst for O -mediated autooxidation
2
processes of cyclohexene and cyclooctene with turnover
3
values higher than 4 Â 10 . Nafion is a very versatile support
for applied synthetic purposes. The isolation of the iron
porphyrins within the solid support causes a significant
thermal activity that continues in the dark for many hours
after the initial photoactivation.
Further experiments are in progress in our laboratory to
investigate the possibility that other unsaturated hydrocar-
bons can be successfully oxidised with good selectivity by the
3
up to 6.1 Â 10 .
4
‡
‡
photoexcited PdTMPyP /FeTDCPP /Nafion photocatalyst.
Moreover, the possibility that this system may be employed in
Chem. Eur. J. 2001, 7, No. 16
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3569

Maldotti et al.
FULL PAPER
large-scale experiments on the use of membrane photo-
reactors should be urgently investigated in view of further
developments in applied organic synthesis.
membranes only with the palladium porphyrin were prepared. The
amounts of metalloporphyrins immobilised in the membrane were
determined following their absorbance decrease in the solution phase.
2
� 7
Typically, a membrane of 1.5 cm contained about 4 ± 8 Â 10 moles of each
complex per gram of Nafion.
2
Photosensitisation experiments: Nafion of 1.5 cm in the sodium form
containing PdTMPyP4 were immersed in solutions of N,N-dimethyl-4-
‡
Experimental Section
� 5
� 3
� 3
� 3
nitrosoaniline (RNO, 4 Â 10 moldm ) and imidazole (5 Â 10 moldm
)
each in ethanol (3 mL) inside a fluorimetric cell. After reaching equilibra-
tion (six hours), the whole system was irradiated at l > 500 nm. The
oxidation of RNO was followed by its typical absorbance decrease at
Materials: The palladium complex [PdTMPyP]Cl
4
was synthesized by the
modified method proposed by Pasternack and co-workers.[ The initial
18]
�
1
3
� 1 [17, 21]
metal-free porphyrin [H
2
TMPyP](C
7
H
7
SO
3
)
4
(Aldrich) was transformed to
420 nm (e ˆ 28100 mol dm cm ).
The absorption of the solution
the chloride form by flow of an aqueous solutions through a column with
phase was read at 908 to avoid interference of the soaked membrane. A
�
4‡
Dowex 1 Â 8 ± 100 anionite (the Cl form). A mixture of [H
2
TMPyP]Cl
4
comparison with the photoreactivity of PdTMPyP in homogeneous
and Na
2
PdCl
4
was refluxed for one hour. The resulting cationic metal-
solutions was carried out by irradiation of the palladium complex (about
�
5
� 3
loporphyrin was precipitated by sodium tetrafluoroborate and then trans-
formed to the chloride form using an anion-exchange resin.[ The purity of
the obtained metalloporphyrin was checked by thin-layer chromatography
and by NMR and optical spectra. Nafion 117 is a DuPont product (about
78 mm thick and 1100 of equivalent weight). Cyclohexene and cyclooctene
Fluka) were distilled before use. All other chemicals were spectroscopic
1 Â 10 moldm ) dissolved in ethanol in the presence of RNO (4 Â
19]
� 5 � 3 � 3 � 3
10 moldm ) and imidazole (5 Â 10 moldm ). To dissolve the complex
4‡
completely, PdTMPyP was diluted in ethanol from a concentrated water
solution. Oxidation of RNO was never observed when blank experiments
were run in the dark. Moreover no photochemical process occurred in the
absence of palladium porphyrin.
1
(
grade products (Aldrich and Fluka) and were used without further
purification.
4‡
In analogous experiments, membranes of Nafion containing PdTMPyP
were immersed in aqueous solutions of triethanolamine (TEA,
.1 moldm ) and HCl (0.1 moldm ) and irradiated at l > 500 nm. After
[
FeTDCPP]Cl was kindly supplied by Drs. D. Mansuy and P. Battioni of the
� 3
� 3
0
University Ren e Descartes of Paris.
2
h irradiation, the eventual formation of H
2
O
2
was tested iodometrically,
�
Apparatus: The employed photomicroscope was Zeiss, model Axiophot,
equipped with a reflected fluorescence condenser and a mercury vapour
lamp, HBO 50W, with BP 436/10 exciter and LP 470 barrier filters.
by measurement of the absorbance value owing to I at 351 nm (e ˆ
3
�
1
3
� 1
[11e]
26400mol dm cm ), and comparison with
a blank experiment.
Similar irradiations were also carried out in the homogeneous phase,
dissolving the palladium complex (about 1 Â 10 moldm ) in aqueous
�
5
� 3
Gas chromatographic analyses were carried out on a DANI 8521 gas
chromatograph, equipped with a flame ionization detector, using columns
packed with Carbowax 20M 5% on Chromosorb W-AW. UV/Vis absorp-
tion spectra were recorded by a Kontron, model Uvikon 940, spectropho-
tometer, and emission spectra by a Fluoro Max-2 fluorimeter from ISA
�
3
�
3
solutions containing TEA (0.1 moldm ) and HCl (0.1 moldm ).
Photocatalytic experiments: Photooxidation of cyclohexene and cyclo-
octene was performed by irradiation at l > 500 nm of Nafion membranes of
2
4‡
4‡
‡
1.5 cm containing PdTMPyP alone or both PdTMPyP and FeTDCPP
instruments. Continuous irradiations were performed with
a
xenon
immersed in mixtures of alkene/ethanol 1/3. The experiments were carried
out under static conditions in a 3 mL fluorimetric cell. Analogous
experiments were carried out by irradiation of homogeneous solutions of
the metalloporphyrins in the same mixed solvent acidified with CF SO H
mercury lamp by Oriel, with a thermostable cell holder (26 Æ 18C). A
glass cut-off filter selected excitation wavelengths over 500 nm.
Photoexcitation at 525 nm, where the palladium porphyrin presents an
absorption maximum was performed using a monochromator. The light
intensity was measured by the ferrioxalate actinometric method.
irradiations were carried out in a spectrophotometric quartz cell of one cm
path length. Deaerated samples were prepared by a vacuum line, equipped
with diffusive pump. When necessary irradiations were carried out under
an atmosphere of oxygen.
3
3
�
2
� 3
(2 Â 10 moldm ). UV absorption spectra were measured before and
after the irradiation to estimate the amount of porphyrin destroyed. A
quantitative comparison between the photoinduced catalytic properties of
the metalloporphyrins in homogeneous solution and in the heterogeneous
system was possible because the amounts of PdTMPyP4 were chosen so its
absorption of the incident light was the same for both the systems. Blank
[
20]
All
‡
Nanosecond flash photolysis transient absorption spectra were measured
by irradiation of the sample with 6 ± 8 ns (fwhm) of a Continuum Surelight
Nd:YAG laser (10 Hz repetition rate) and using a pulsed Xe-lamp as probe
light perpendicular to the laser beam. The excitation wavelength was
obtained by frequency doubling (532 nm). The 150 W Xe lamp was
equipped with an Applied Photophysics Model 408 power supply and an
Applied Photophysics Model 410 pulsing unit (giving pulses of 0.5 ms). The
Nafion membranes were sandwiched between two glass slides and the
orientation of the films was 458 with respect to the laser and probe light.
The scatter light was reflected to the probe light. Using suitable pre- and
post- cutoff and bandpass filters we were able to measure kinetic traces
without any artifacts from scattering. The light was collected in a LDC
Analytical monochromator, detected by a R928 PMT (Hamamatsu), and
recorded on LeCroy 9360 (600 MHz) oscilloscope. The absorption
experiments were carried out in order to verify that no oxidation of the
alkenes was observed without photochemical excitation of PdTMPyP4
‡
.
The determination of hydroperoxides was performed by a spectrophoto-
[22]
metric standard method reported in the literature. The analysis of the
other products was carried out by gas-chromatographic techniques follow-
[21]
ing the procedure previously reported.
Acknowledgement
This research was supported by MURST (PRIN) and C.N.R. (project 95/
95 ± 5%).
transients were plotted as DA ˆ log(I
0
/I
t
) versus time, where I
0
was the
monitoring light intensity prior to the laser pulse and I
signal at delay time t.
t
was the observed
[
1] a) R. A. Sheldon, J. K. Kochi, Metal-catalyzed Oxidation of Organic
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Immobilisation of PdTMPyP4 and of FeTDCPP in Nafion: The
‡
‡
membrane of Nafion was cleaned by boiling it for about 30 min in nitric
�
3
acid (3 moldm ) washed in water and finally dried. Nafion in sodium form
was obtained by placing the purified membrane in contact with a NaOH
�
3
aqueous solution (1 moldm
)
for 30 min. The composite system
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for 1 ± 2 min. Once the polymer had
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‡
3
2
2
�
4
� 3
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)
‡
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2
O/CH
3
OH (3:1 v/v) with
PdTMPyP4 (1 Â 10 moldm ) for about 20 min. When necessary, Nafion
‡
� 4
� 3
3570
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Chem. Eur. J. 2001, 7, No. 16

Porphyrin-Catalyzed O -Mediated Oxidations of Alkenes
3564±3571
2
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[
1
Received: February 12, 2001 [F3069]
Chem. Eur. J. 2001, 7, No. 16
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0947-6539/01/0716-3571 $ 17.50+.50/0
3571