Covalently immobilized porphyrins on silica modified structures as photooxidation catalysts

Article abstract of DOI:10.1016/j.molcata.2010.05.001

Heterogeneous photooxidation catalysts were prepared by the covalent linkage of a porphyrin to aminoalkylated silica particles by chlorosulphonation activation of the porphyrin nucleus. These catalysts were tested in [4+2] cycloaddition and ene reactions promoted by singlet oxygen. Photooxidation of substrates originates high yields of products but with longer reaction times compared to the free porphyrin catalysts. Quenching of singlet oxygen by the silica structure can be the plausible explanation. In the case of the ene reaction with citronellol the supported catalysts originate a distribution of the regioisomeric products which is opposite to that of the homogeneous catalyst.

Full text of DOI:10.1016/j.molcata.2010.05.001

Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Covalently immobilized porphyrins on silica modified structures as
photooxidation catalysts
∗
Sónia M. Ribeiro, Arménio C. Serra, A.M.d’A. Rocha Gonsalves
Departamento de Química, Faculdade de Ciências e Tecnologia, Universidade de Coimbra, Rua Larga, P-3000 Coimbra, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
Heterogeneous photooxidation catalysts were prepared by the covalent linkage of a porphyrin to
aminoalkylated silica particles by chlorosulphonation activation of the porphyrin nucleus. These cat-
alysts were tested in [4+2] cycloaddition and ene reactions promoted by singlet oxygen. Photooxidation
of substrates originates high yields of products but with longer reaction times compared to the free por-
phyrin catalysts. Quenching of singlet oxygen by the silica structure can be the plausible explanation.
In the case of the ene reaction with citronellol the supported catalysts originate a distribution of the
regioisomeric products which is opposite to that of the homogeneous catalyst.
Received 27 December 2009
Received in revised form 8 April 2010
Accepted 1 May 2010
Available online 10 May 2010
Keywords:
Photooxidation
Singlet oxygen
Porphyrin
©
2010 Elsevier B.V. All rights reserved.
Heterogeneous catalysis
1
. Introduction
arated from a Merrifield polymer structure by spacers of different
lengths. This proved to be a helpful approach since fast kinetics is
observed with longer spacers. In this work we extended the same
principles of immobilization, using silica as the support and sepa-
rating the porphyrin from the silica surface by spacers of different
length. The effects of different kinds of silica gel and different spac-
ers on photocatalyst activity are evaluated.
Singlet oxygen mediated oxidations are very attractive reactions
which make use of clean, accessible and economical reaction con-
ditions [1]. Also very important is that they can give access to very
interesting synthetic products, which are very hard to obtain by
other methods, as illustrated by the synthesis of artemisin com-
pounds [2], inositols derivatives [3] and natural furan products [4].
Porphyrins are easily accessible compounds which have shown
good photosensitizer properties and stability in reaction media,
particularly if halogen atoms are present [5]. Their incorporation in
heterogeneous matrices is a useful and practical approach to carry
out the reaction, since it greatly simplifies the isolation process [6].
However, the structure of the solid matrices can also influence the
performance of the photoactive molecule since it is located close
to the solid surface. Light, oxygen, and substrate accessibility or
quenching effects can also affect the rate of the oxidation reaction
2. Experimental
2.1. General
All solvents were purified before use according to lit-
erature procedures. ␣-Terpinene (85% purity), citronellol
95% purity), ␣-pinene (98% purity), (−)-linalool (95%
purity), (3-aminopropyl)-trimethoxysilane (97% purity),
3-glycidyloxypropyl)-trimethoxysilane (98% purity), 1,12-
(
(
[
7] and these factors can be influenced by the solid matrix. Silica gel
diaminododecane (98% purity), 1,6-diaminohexane, were used
as purchased from Aldrich. Silica gel type 60, 35–70 mesh, with
particle size of 0.2–0.5 mm was purchased from Fluka; silica
is an inexpensive, available and resistant material for the immobi-
lization of catalysts [8] and recent examples of immobilization of
photosensitizers on silica gel have been described [9]. However,
one particular aspect of the use of silica as a support for photosen-
sitizers is the possibility of singlet oxygen quenching effects by the
silica surface [10]. In previous works we reported the development
®
gel Davisil , grade 643, 200–425 mesh, with particle size of
3
5–70 ␮m and silica gel, type 9385, 230–400 mesh, with particle
size 40–63 ␮m were purchased from Aldrich. Porphyrin 1 [12] and
Porphyrin 2 [6a] were prepared as previously described.
[
6a,11] of a strategy for covalently immobilizing porphyrins, sep-
2.2. Instrumentation
∗ Corresponding author. Fax: +351 239 826069.
E-mail address: arg@qui.uc.pt (A.M.d’A. Rocha Gonsalves).
1H NMR spectra were recorded on a 400 MHz Bruker spectrom-
eter. J values are given in Hertz. Mass spectra were obtained on a HP
1
381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.05.001

1
22
S.M. Ribeiro et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
5
973 MSD apparatus by electron impact at 70 eV. Elemental anal-
tions were irradiated during the time described with a stream of
ysis was carried out using a Fisons Instruments EA1108-CHNS-0
apparatus. Absorption spectra were measured on a Hitachi U-2001
spectrometer. Gas chromatography was carried out using a Supel-
cowax (30 m × 0.25 mm) capillary column on a Hewlett-Packard
air continuously flowing into the flask.
2.5.2. ˛-Terpinene photooxidation
␣-Terpinene (4.9 mmol) in 65 mL of chloroform was mixed with
−
4
5
890A instrument with a Hewlett-Packard 3396A integrator. GC
the appropriate amount of photosensitizer (9.8 × 10 mmol of
porphyrin or the equivalent molar amount of supported porphyrin)
in order to produce a 1/5000 sensitizer to substrate molar ratio and
◦
◦
−1
◦
analysis was run at 80 C (5 min)/20 C min /200 C (20 min);
◦
◦
detector temperature 250 C, injector temperature 220 C.
2
03 mg of sodium hydrogenocarbonate were added.
2.3. Synthesis of the aminoalkylated silica derivatives
The reaction was monitored by analysing the disappearance of
AAS1–AAS5
the reagent by GC. The reaction can also be followed by UV–vis spec-
troscopy at 268 nm. When the reagent was consumed, the reaction
mixture was filtered to recover the photosensitizer. The solvent
was evaporated, dried by nitrogen flow and the products analysed
by NMR spectroscopy.
2.3.1. Synthesis of AAS1 [13]
To a stirred solution of 70 ml dry toluene containing 10% of
3
-aminopropylltrimethoxysilane was added 45 g of SiO2 (silica
◦
gel type 60, 35–70 mesh) activated as described [13] at 120 C.
The resulting mixture was refluxed overnight. The amino func-
tionalized silica AAS1 was isolated by filtration and washed with
tetrahydrofuran and methanol, then dried under vacuum for sev-
eral days. Elemental analysis showed nitrogen content of 0.95%.
2.5.2.1. Ascaridole. 1H NMR (CDCl ): ı 0.97 (d, 3H, J = 6.9 Hz, CH ),
3
3
0.98 (d, 3H, d, J = 6.9 Hz, CH ), 1.31 (s, 3H, CH ), 1.51–1.56 (m, 2H),
3
3
1.85 (sept, 1H, J = 6.9 Hz, isopropyl), 1.97–1.92 (m, 2H), 6.42 (d, 1H,
J = 8.6, olefinic CH), 6.53 (d, 1H, J = 8.6 Hz, olefinic CH). MS (EI): m/z
+
M = 168. Spectral data were identical to reported [15].
2
.3.2. Synthesis of AAS2 and AAS3 [14]
To a stirred solution of 80 ml dry toluene containing 3.0 mmol
2.5.2.2. p-Cymene. 1H NMR (CDCl3): ı 1.22 (d, 3H, J = 1.68 Hz, CH3),
1.24 (d, 3H, J = 1.68 Hz, CH3), 2.31 (s, 3H, CH3), 2.87 (sept, H, iso-
propyl), 7.11 (s, 4H, Ar–H); MS (EI): m/z M = 134. NMR data were
of 1,12-diaminohexane, 1.0 mmol of (3-glycidyloxypropyl)-
trimethoxysilane was added. The resulting mixture was allowed
to react at 80 C for 24 h. To this solution, 1.5 g of activated SiO2
+
◦
identical to those reported [16].
(
Kieselgel 60, 35–70 mesh) and 5 ml of EtOH were added and the
◦
solution was stirred at 80 C for 24 h. The amino functionalized
silica AAS2 was isolated by filtration and washed with MeOH
and EtOH. Then, it was refluxed with EtOH during 1 h, filtered
2.5.3. Citronellol photooxidation
Citronellol (7) (11.5 mmol) in 154 mL of chloroform was mixed
−
3
with the appropriate amount of photosensitizer (2.3 × 10 mmol
of porphyrin or supported porphyrin) to originate a sensitizer to
substrate molar ratio of 1/5000. GC was used to monitor the reac-
tion. After filtration of the photosensitizer and evaporation of the
solvent, the product was analysed by 1H NMR. The proportion
of the two regioisomers thus obtained, (E)-7-hydroperoxy-3,7-
dimethyloct-5-en-1-ol (8) and 6-hydroperoxy-3,7-dimethyloct-7-
en-1-ol (9), was estimated by 1H NMR ([6b,17]).
◦
again and dried at 40 C for several days. The amino functionalized
silica AAS3 was prepared in a similar way to AAS2, but using
1
,12-diaminododecane instead of 1,6-diaminohexane.
2.3.3. Synthesis of AAS4 and AAS5
AAS4 and AAS5 were prepared as described for AAS3, but using
®
two different samples of silica gel: silica gel Davisil (200–425
mesh, pore size 150 Å) for AAS4 and silica gel Kieselgel (230–400
mesh, pore size 60 Å) for AAS5.
2.5.3.1. (E)-7-Hydroperoxy-3,7-dimethyloct-5-en-1-ol (8). 1H NMR
(
CDCl ): ı 0.87 (d, 3H, J = 6.4 Hz, CHCH ), 1.25–1.71 (m, 5H,
3
3
CH CHCH ), 3.64 (m, 2H, CH OH); 1.29 (s, 6H, COOH(CH ) ), 5.55
2.4. Synthesis of photosensitizers PAAS1–PAAS5
2
2
2
3 2
(d, 1H, J = 15.8 Hz, COOHCH CH), 5.63 (m, 1H, COOHCH CH).
The procedure carried out was similar for all five photosensi-
tizers (PAAS1–PAAS5) [6a]. The procedure for PAAS1 is described:
2.5.3.2. 6-Hydroperoxy-3,7-dimethyloct-7-en-1-ol (9). 1
H
NMR
at room temperature 15 mL of chlorosulphonic acid was added to
(CDCl3): ı 0.85 (d, 3H, J = 6.5 Hz, CHCH3), 1.25–1.71 (m, 5H,
CH2CHCH2), 3.64 (m, 2H, CH2OH), 1.65 (s, 3H, CqCH3), 1.95 (m,
2H, COOHCH2), 4.24 (m, 1H, CHOOH), 4.95 (s, 2H, Cq CH2). The
proportion of the two regioisomers (8) and (9) was estimated by
2
13 mg of porphyrin 2. The solution was stirred for 2 h and then
carefully poured over ice in order to precipitate the porphyrin. The
precipitate was filtered, dried, dissolved with dichloromethane,
and the solution dried with sodium sulphate. The solution was con-
centrated to 20 mL and 10 mL of pyridine was added followed by
1
H NMR as follows:
[
(area ıH5.55 + area ıH5.68)/2]
3
80 mg of the aminoalkylated polymer AAS1 (the same procedure
% for (8) = [
× 100
(area ıH5.55 + area ıH5.68)/2 + (area ıH = 4.95)/2]
with AAS2, AAS3, AAS4, AAS5). The mixture was stirred overnight
◦
at 30 C, filtered and washed sequentially with dichloromethane,
tetrahydrofuran, ethanol, and dichloromethane. Non-bonded por-
phyrin was eliminated with these washings. After drying the solid
at 40 C, elemental analysis was carried out in order to determine
2.5.4. Linalool photooxidation
The substrate (11.5 mmol) in 154 mL of chloroform was mixed
◦
−3
with the appropriate amount of photosensitizer (2.3 × 10 mmol
value of porphyrin incorporation.
of porphyrin or supported porphyrin) to originate a sensitizer
to substrate molar ratio of 1/5000. GC was used to monitor the
reaction until disappearance of the reagent. After filtration of the
photosensitizer and evaporation of the solvent, the isolated product
was analysed by 1H NMR [18].
2
2
.5. Photosynthetic oxidation experiments
.5.1. General procedure
Photooxidation experiments were carried out at room tempera-
ture using a laboratory-built photoreactor consisting of three 50 W
halogen lamps. The reactions were carried out in a 100 mL flask
equipped with a water condenser and an inlet for air. The solu-
2.5.4.1. 7-Hydroperoxy-3,7-dimethyloct-2,5-dien-1-ol
(12). 1
H
NMR (CDCl ): ı 1.28 (s, 3H), 1.32 (s, 6H), 2.26 (dd, 1H, J = 13.6,
3
7.1 Hz); 2.31 (dd, 1H, J = 13.6, 6.3 Hz), 5.06 (dd, 1H, J = 10.6, 1.2 Hz),

S.M. Ribeiro et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
123
Scheme 1. Synthesis of the aminoalquilated silicas (AAS1–AAS5) and the photooxidation catalysts (PAAS1–PAAS5).
5
.19 (dd, 1H, J = 17.2, 1.2 Hz), 5.61 (d, 1H, J = 16.0 Hz), 5.68 (ddd, 1H,
3. Results and discussion
.1. Synthesis of silica supported photosensitizers PAAS1–PAAS5
The preparation of supported photosensitizers is indicated in
Scheme 1. The first step is the modification of the silica surface by
two reaction types for introducing different spacers. AAS1 was pre-
pared following the reaction with 3-(aminopropyl)trimetoxysilane
J = 15.8, 7.2, 6.3 Hz), 5.91 (dd, 1H, J = 17.2, 10.6 Hz)
3
2.5.4.2. 6-Hydroperoxy-3,7-dimethyloct-1,7-dien-1-ol
(13). 1
H
NMR (CDCl ): ı 1.27 (bs, 3H), 1.66–1.49 (m, 4H), 1.70 (bs, 3H), 4.28
3
(
m, 1H), 4.99 (m, 2H), 5.05 (dd, 1H, J = 10.8, 1.1 Hz), 5.19 (dd, 1H,
J = 17.4, 1.1 Hz), 5.20 (dd, 1H, J = 17.4, 1.1 Hz), 5.86 (dd, 1H, J = 17.3,
0.6 Hz), 5.85 (dd, 1H, J = 17.3, 10.8 Hz). The proportion of the two
1
(A) [13]. In order to prepare the modified silica supports
regioisomers obtained, (E)-7-hydroperoxy-3,7-dimethyloct-2,5-
dien-1-ol (12) and 6-hydroperoxy-3,7-dimethyloct-1,7-dien-1-ol
AAS2–AAS5 we adapted the procedure described by Louloudi and
co-workers [14] using 3-(glycidyloxypropyl)-trimethoxysilane and
(
13) was estimated by 1H NMR as follows [18]
1
,6-hexanediamine or 1,12-dodecanediamine (B). We selected two
particle sizes and two pore volumes in order to examine if these
structural parameters influence the porphyrin loading and the pho-
tooxidation process.
[
(area ıH2.31 + area ıH2.26)/2]
× 100
(area ıH4.28) + (area ıH2.31 + area ıH2.26)/2]
%
for (12) = [

1
24
S.M. Ribeiro et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
Fig. 1. Comparative FTIR spectra of silica, AAS2 and PAAS2.
The incorporation of the organic parts into the silica structure
groups of modified silica supports (AAS1–AAS5) (C, Scheme 1).
The FTIR spectrum of the supported photosensitizer PAAS2 (Fig. 1)
can be monitored by analysing the FTIR spectra of the differ-
ent compounds (Fig. 1). The infrared spectrum of the silica gel
shows the typical Si–O lattice vibrations: a strong and broad band
−
1
−1
shows new bands at 1557 cm , and 715 cm and changes in the
−
1
band at 801 cm , which already appears in the spectrum of the free
−
1
−1
at 1094 cm
with a shoulder at 1200 cm ; a strong band at
−
1
−1
4
70 cm and a medium intensity band at 801 cm . Furthermore,
a very broad band centred a 3450 cm
−1
with medium intensity
due to the O–H stretching vibrations assigned to isolated surface
−1
SiO–H groups is observed. A band at 1641 cm
is attributed to
the H–O–H bending vibrations of adsorbed water [13,19]. Upon
amine functionalization (AAS2) a decrease in the intensity of the
−
1
very broad band at 3450 cm
reaction between the isolated silanol groups and the aminosilane
derivate took place. Additionally the presence of characteristic CH
is observed, indicating that the
2
−1
−1
stretching bands at 2940 cm and 2860 cm confirm the reac-
−
1
tion. A weak band at 1428 cm in the spectrum of AAS2 can also
be correlated with the presence of CH2 groups [20]. Similar trends
are observed in the case of the aminoalkylated silica derivatives
AAS1 and AAS3–AAS5. The UV–vis spectra of meso-tetra(2,6-
dichlorophenyl)porphyrin in solution, PAAS2 and PAAS3 as
suspensions in dichloromethane are presented in Fig. 2. Spectra of
supported photosensitizes shows the Soret band at 418 nm almost
at the same position as the free porphyrin but have a much broader
bands with lower relative intensity. The characteristic and less
intense Q bands of free porphyrin at 514 and 589 nm are red shifted
to 520 and 595 nm respectively. Curiously the very small Q band of
the free porphyrin at 645 nm occurs at 667 nm and is relatively
much more intense in the case of the supported photosensitizers.
Elemental analysis of the nitrogen content gives values of
loadings for the amino groups for the different aminoalkylated
silicas (Table 1). The values for the incorporation of amines depend
on the aminosilane derivative, (3-aminopropyl)-trimethoxysilane
incorporating more effectively than (3-glycidyloxypropyl)-
trimethoxysilane, which is expected due to the easier reaction
process (A versus B, Scheme 1).
The synthesis of the supported photosensitizers PAAS1–PAAS5
was carried out by using conditions for the selective chloro-
sulphonation of the non-symmetrical porphyrin 2 and the reaction
of the chlorosulphonyl derivative 3 [6a] with the terminal amino
Fig. 2. Comparative UV–vis spectra of a solution of the free porphyrin H2TDCPP (a)
and dichloromethane suspensions of PAAS2 (b) and PAAS3 (c). Scales adjusted to
give approximately equivalent absorptions of the Soret peak at 418 nm.

S.M. Ribeiro et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
125
Table 1
Values for amino groups and porphyrin incorporation (mmol/g) for photosensitizers PAAS1–PAAS5.
Aminoalkylated silica
Values of amino incorporation
mmol/g)
Supported photosensitizer
Values of porphyrin
(
incorporation (mmol/g)a
AAS1
AAS2
0.68
0.38
PAAS1
PAAS2
–(CH2)3
0.040
0.142
AAS3
AAS4
0.31
0.29
0.33
PAAS3
PAAS4
PAAS5
0.115
0.158
0.151
AAS5
a
Value obtained by subtracting the value for the corresponding amino modified silicas (AAS1–AAS5).
porphyrin. Elemental analysis of the products indicates the values
for porphyrin incorporation into the solid support (Table 1). For
PAAS2–PAAS5 the values for incorporation have the same mag-
nitude. No significant differences were observed, in spite of the
different particle size of the silica structures. Comparatively, these
values are greater than the ones we observed when the same
porphyrin was linked to the Merrifield modified polymers [11].
However, from the values of the amino incorporation and por-
phyrin incorporation, we can envisage that only 50% or less of the
terminal amino groups reacted with the porphyrin. In the case of
PAAS1 we observed a great decrease in the value of porphyrin
incorporation in spite of the high value for the amino incorpora-
tion. Our explanation is that the proximity of the silica structure
to the terminal amino group somehow hinders the access of the
chlorosulphonated derivative and originates less incorporation of
the porphyrin. A similar situation was already observed by us with
Merrifield structures [6a].
3.2. Photooxygenation experiments
Evaluations of the photoactivitiy of the different photosensi-
tizers were carried out using ␣-terpinene (4) as substrate with
sensitizer to substrate ratios of 1:600 and 1:5000. The major prod-
uct is the [4+2]-cycloadduct ascaridole (5) [15] and p-cymene (6) is
the minor product resulting from the air oxidation of ␣-terpinene.
For comparison a reaction with the free porphyrin (1) was carried
out (Table 2).
Results from Table 1 show that photosensitizers PAAS1–PAAS5
are active catalysts for singlet oxygen generation but with a lower
activity than the free porphyrin 1, which can be partially attributed
to the silica structure. The result of entry 3 shows that silica sig-
nificantly slow down the homogeneous catalytic process; it seems
that the quenching effects by hydroxyl groups at the silica surface
may be operating [10]. For supported photosensitizers, especially
for high substrate to catalyst ratios, the amount of non-singlet oxi-
Table 2
Results for the photooxidation of ␣-terpinene using porphyrin 1 or PAAS1–PAAS5 as sensitizers.
.
Entry
Photosensitizer
R = nph/nterpa
Time (h)
5 (%)b
93 (7)
96 (4)
98 (2)
1
2
3
4
5
6
7
8
1
1
1
1/600
1.5
2.3
5
1/5000
1/5000
1/600
1/5000
1/5000
1/600
1/5000
1/5000
1/5000
c
PAAS1
PAAS1
PAAS2
PAAS3
PAAS3
PAAS4
PAAS5
5
94 (6)
8.5
7.5
5.5
7.5
10.5
13
90 (10)
76 (24)
97 (3)
86 (14)
77 (23)
76 (24)
1
1
1
2
a
Photosensitizer/␣-terpinene ratio.
Isolated yield of 5. In parenthesis, the amount of formed p-cymene (6). Determined by 1H NMR of the reaction mixture after filtration the catalyst and solvent removal
b
taking into account that 5% of p-cymene exist in the initial reagent.
c
8
mg of silica is added to the homogeneous solution.

1
26
S.M. Ribeiro et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
Table 3
reaction (Table 3). For all the PAAS1 and PAAS2 time for reaction
completion increases from first to second reutilization which indi-
cate a progressive deactivation of the catalyst. Also the amount of
p-cymene (not derived from singlet oxygen oxidation) gradually
increases. Photosensitizer PAAS3 which was the most active show
to be the most resistant structure to reutilization as showed by the
small increase of reaction time but maintaining the selectivity for
ascaridole (5).
Reutilization experiments with PAAS1–PAAS3 in the photooxidation of ␣-
terpinene. .
a
Photosensitizer
Time (h)
5 (%)b
PAAS1
PAAS1
PAAS1
PAAS2
PAAS2
PAAS2
PAAS3
PAAS3
PAAS3
8.5
8.5
12.5
7.5
9.5
16
7.5
11.5
11.5
90 (10)
78 (22)
50 (50)
76 (24)
67 (34)
42 (58)
86 (14)
83 (17)
85 (15)
1
2
st Reutilization
nd Reutilization
1
2
st Reutilization
nd Reutilization
The photooxidation of citronellol (7) originates 2 regioisomeric
hidroperoxydes (8 and 9) resulting from the ene reaction [6b]
1
2
st Reutilization
nd Reutilization
(Scheme 2). Compound 8 can be reduced to the corresponding
alcohol and by acid-mediated cyclization furnishes a diastere-
omeric mixture of rose oxide (10), which is a valuable compound
in perfumery [21]. It is also possible that by using acid treatment
compound 9 can be transformed to 10 [22].
a
Photosensitizer/␣-terpinene ratio: 1/5000.
Isolated yield of 5. In parenthesis, the amount of formed p-cymene (6). Deter-
b
1
mined by H NMR of the reaction mixture after filtration the catalyst and solvent
removal taking into account that 5% of p-cymene exist in the initial reagent.
Regioisomer 8 is the preferable compound for large scale process
for rose oxide production [23] therefore it would be very useful if a
process could originate some preference for this isomer. In the case
of the ene reaction of allylic alcohols, hydrogen bonds in the sub-
strate are determinant for the regio- and diastereoselectivity [24].
In the case of citronellol photooxidation, examples in the literature
show that with different reaction conditions 8 or 9 can predominate
or be produced in near equal amounts [6b,11,23,25]. Our expecta-
tion was that hydroxyl groups of the silica surface could establish
hydrogen interactions with 7 and influence the regioselectivity
of the products. The results of the photooxidation of citronellol
with silica supported photosensitizers PAAS1–PAAS5 are shown
in Table 4.
dation product is increased. Among the photosensitizers we do
not observe a clear relationship between the length of the spacer
(
PAAS1–PAAS3) and the activity of the photocatalysts as observed
for Merrifield based photosensitizers [6a]. Even with the longer
spacer chain, silica deactivating effects are operating. The photo-
sensitizer with the C3 chain spacer presents very similar activity to
photosensitizers with C6 or C12 chain spacers. Another observation
is that changing to a smaller silica particle size originates photo-
sensitizers (PAAS4, PAAS5) with lower activities possibly because
a more efficient quenching effect occurs, due to the relative surface
increase of silica [10c].
From Table 4 we can observe that supported photocatalysts orig-
inate a high level of conversion of citronellol into hydroperoxides
The ability of the different supported catalysts in reusing
experiments was attempted in the photooxidation of terpinene,
photosensitizers were separated by filtration and tried in a new
8
and 9. From the isolated products, 9 was found to have predomi-
Scheme 2. Products from the singlet oxygen oxidation of citronellol (7) and reactions to originate rose oxide (10).
Table 4
Results for the photooxidation of citronellol using porphyrin 1 or PAAS1–PAAS5 as sensitizers.
Entry
Photosensitizer
R = nph/nterpa
Time (h)
Isolated yield (%)
8/9 (%)b
1
2
3
4
5
6
7
8
9
1
1
1
1/600
1.5
4
5
99
97
99
95
99
99
99
99
99
98
99
49/51
48/52
53/47
47/53
34/66
36/64
44/56
35/65
45/55
40/60
39/61
1/5000
1/5000c
1/600
1/5000
1/5000
1/600
1/5000
1/5000c
1/5000
1/5000
PAAS1
PAAS1
PAAS2
PAAS3
PAAS3
PAAS3
PAAS4
PAAS5
9
45
59
28
44
47
47
71
1
1
0
1
a
b
c
Photosensitizer/␣-terpinene ratio.
of 8 and 9 in reaction mixture by 1H NMR after filtration the catalyst and solvent removal.
CCl4 was the reaction solvent.
%

S.M. Ribeiro et al. / Journal of Molecular Catalysis A: Chemical 326 (2010) 121–127
127
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7
(
2
(
Scheme 3. Relative yields of products from the singlet oxygen oxidation of linalool
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4
(
[
1
[
[
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In solution with porphyrin 1 (entries 1 and 2) a slight excess of 9
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Supported photosensitizers, however, show a preference for regioi-
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The preference for 9 was already observed in polystyrene supported
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The photooxidation of 11 with the silica supported photosensitizer
PAAS3 favors the formation of the hydroperoxide 13 relatively to
the photooxidation with the free photosensitizer 1 (Scheme 3).
1
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photooxidation of ␣-terpinene (4) with these heterogeneous cata-
lysts gives the [4+2] cyclic product ascaridole with high yields but
reaction times are much longer than using the non-supported por-
phyrin 1. Photooxidation of citronellol (7) with supported catalysts
also gives high yields of products derived from the ene reaction and
shows a preference for regioisomer 9 which is substantially differ-
ent from the photooxidation with homogeneous catalysts which
favor regioisomer 8.
3
(
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Acknowledgements
(
The authors would like to thank Chymiotechnon, and FCT-
POCTI/QUI/55931/2004, FCT(SFRH/BD/40228/2007) for financial
support. We acknowledge the Nuclear Magnetic Resonance Lab-
oratory of the Coimbra Chemical Centre-University of Coimbra
8
(
2
(
(
Proj/REEQ/481/QUI/2006) for obtaining the NMR data.
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