Covalently immobilized porphyrins as photooxidation catalysts

Article abstract of DOI:10.1016/j.tet.2007.05.084

Porphyrins covalently linked to aminomethylated Merrifield polymers, by chlorosulfonation activation of the porphyrin nucleus, are able to generate singlet oxygen with an efficiency which is related to the spacer between porphyrin and the polymer backbone. Juglone and ascaridole are efficiently produced in the presence of these supported catalysts.

Full text of DOI:10.1016/j.tet.2007.05.084

Tetrahedron 63 (2007) 7885–7891
Covalently immobilized porphyrins as photooxidation catalysts
*
ꢀ
Sonia M. Ribeiro, Armenio C. Serra and A. M. d’A. Rocha Gonsalves
Departamento de Quꢀımica, Faculdade de Cie^ncias e Tecnologia, Universidade de Coimbra, Rua Larga, P-3000 Coimbra, Portugal
Received 31 March 2007; revised 21 May 2007; accepted 22 May 2007
Available online 26 May 2007
Abstract—Porphyrins covalently linked to aminomethylated Merrifield polymers, by chlorosulfonation activation of the porphyrin nucleus,
are able to generate singlet oxygen with an efficiency which is related to the spacer between porphyrin and the polymer backbone. Juglone and
ascaridole are efficiently produced in the presence of these supported catalysts.
Ó 2007 Elsevier Ltd. All rights reserved.
1. Introduction
Other recent and interesting proposals have appeared, such
as immobilization of porphyrins in polymer micro-
channels,12 linking fullerenes to amino functionalized silica
gel,13 benzophenone copolymer,14 polymer sphere-
supported seco-porphyrazines,15 porphyrins loaded in poly-
styrene beads,16 porphyrin dendrimer structures,17 and
polystyrene nanocontainers doped with porphyrins.18
Light promoted singlet oxygen reactions are attractive and
clean oxidation processes.1–3 The oxidant is environmen-
tally friendly, allowing for a process following the basic
principles of the so-called green chemistry.4,5
Requiring light activation, the key element in this process is
the presence of a sensitizer molecule that has the role of cap-
turing the radiant energy and passing the energetic surplus to
molecular oxygen via the sensitizer triplet state. Sensitizers
are organic molecules with good photochemical characteris-
tics, such as conjugated double bonds and aromatic rings,
and so they also have some structural weakness relatively
to singlet oxygen reactivity and are frequently inactivated
in the process. An attempt to circumvent this quenching
process is through the immobilization of the sensitizer on
a macromolecular framework in order to minimize destruc-
tive bimolecular processes.6 One more advantage of immo-
bilization is to have an heterogeneous catalyst, which allows
for an easier separation of products and sensitizer, particu-
larly useful if several cycles are wanted. Another possibility
for photooxygenation catalysis is the use of fluorinated sol-
vents in biphasic conditions.7 In this case the sensitizers are
in homogeneous form in the catalytic process and are con-
verted into a heterogeneous form, recovered by extraction
at the end of the reaction.
2. Results and discussion
2.1. Synthesis of polymer supported sensitizers
Polystyrene is an attractive and stable structure to graph sen-
sitizers,19 being supported rose Bengal an example of a com-
mercial product.20 Also important is the structure of the
porphyrin, which can be decisive in the activity of the pho-
tosensitizer. Tetra (2,6-dichlorophenyl) porphyrin (TDCPP)
4 (Scheme 2) is one of the most active sensitizers for use in
photooxidations.21,22 The presence of chlorine atoms at the
ortho positions of the meso phenyl groups confers a great
resistance to degradation. As far as we know, there has been
no attempt to study the behavior of this kind of porphyrin
macrocycle covalently linked to polystyrene based polymers
as heterogeneous sensitizers.
The strategy for covalently linking porphyrins to the Merri-
field polymer involves modification of the polymer structure
by reaction with an excess of a,u-diamines (1 and 2)23 and
diamine 3 to obtain the aminoalkylated polymers AAP1 to
AAP3 (Scheme 1).
Due to its photochemical characteristics and resistance to
degradation, porphyrins are good sensitizers for singlet
oxygen generation that are currently used in the non-
metallated form under homogeneous conditions.8 Notewor-
thy results for immobilized porphyrins were obtained with
porphyrins anchored to a soluble PEG matrix9 and with
porphyrins copolymerized with polystyrene polymer.10
The immobilization of sensitizers was recently reviewed.11
Reaction of 1,6-diaminohexane (1) in THF at 50 ꢀC with
Merrifield polymer (1% cross linked) did not originate sig-
nificant substitution. Changing the solvent to DMF and the
temperature to 70 ꢀC we obtained a light yellow material
with 0.35 mmol/g of amine incorporated as shown by ele-
mental analysis. The same strategy was followed with
* Corresponding author. Fax: +351 239 826068; e-mail: arg@qui.uc.pt
0040–4020/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2007.05.084

7886
S. M. Ribeiro et al. / Tetrahedron 63 (2007) 7885–7891
AAP1
Merrifield
resin
R=C6H12
Merrifield
resin
CH2Cl
CH2Cl
NH2RNH2/DMF
CH2NHRNH2
CH2NHRNH2
AAP2
R=C12H24
(1)- NH2C6H12NH2
(2)- NH2C12H24NH2
AAP3
R=
(3)- H2N
NH2
Scheme 1. Sequence of reactions to originate aminoalkylated polymers AAP1 to AAP3.
1,12-diaminododecane (2) but with 1,4-diaminobenzene (3)
reaction at room temperature gives a better material with
higher incorporation (Table 1).
band at 1262 cmꢁ1 and the appearance of new bands due to
the presence of NH2 groups at 1630 and 1115 cmꢁ1
.
For linking the porphyrin macrocycle to the modified poly-
styrene derivatives (AAP1 to AAP3) we used the non-
symmetric porphyrin 5 obtained from the one-pot mixed
aldehyde pyrrol condensation with nitrobenzene as oxi-
dant.24 Controlled chlorosulfonation of 5 allows the intro-
duction of one single chlorosulfonyl group on the phenyl
ring. Differences in reactivity between the phenyl ring and
the 2,6-dichlorophenyl ring allow the chlorosulfonation reac-
tion of the mixture of porphyrins 4 and 5 obtained from the
one-pot reaction. As only the unsubstituted phenyl ring is
chlorosulfonated, any 4 which is present does not react with
aminopolymers and can be easily separated from polymer
products and recovered at the end. This chlorosulfonation
strategy to link the porphyrin macrocycle greatly simplifies
the work because it avoids the troublesome isolation of 4
from 5 and allows a new straightforward procedure. Reaction
of the chlorosulfonyl derivative 6 with aminopolymers
AAP1 to AAP3 gives porphyrin attached polymers PS1 to
PS3 with different spacers between the photosensitive part
and the polymer backbone (Scheme 2).
Different amines originate different incorporation values:
1,6-diaminohexane the lowest and 1,12-diaminododecane
the highest value. When we tried higher temperatures for
this reaction, darker materials with lower values of incorpo-
ration resulted. Taking into account that Merrifield resin
presents 1.0–1.5 mmol of chlorine atoms (labeled value)
we concluded that about 1/3–1/4 of the groups have been
substituted. Infrared spectra of the modified Merrifield resin
AAP1 showed significant differences relatively to the origi-
nal Merrifield polymer, such as the reduction of the CH2–Cl
Table 1. Values for amine incorporation (mmol/g) in Merrifield polymer
Amine
Reaction
conditions
Amino- Values of
alkylated amine
polymer incorporation
(mmol/g)
1,6-Diaminohexane (1)
DMF/70 ꢀC/24 h AAP1
0.35
0.52
0.23
1,12-Diaminododecane (2) DMF/70 ꢀC/24 h AAP2
1,4-Diaminobenzene (3)
DMF/rt/24 h
AAP3
R1
CHO
Cl
Cl
Cl
R1=R2=
4
NH
N
N
Cl
AcOH/PhNO2
+
R2
R1
Cl
N
H
CHO
R1=
R2=
HN
Cl
5
R1
HClSO3
R1
Merrifield
resin
Cl
Cl
R1=
R2=
6
NH
N
N
R2-Porphyrin
CH2NHRSO2
R2
R1
SO2Cl
PS1 to PS3
AAP1 to AAP3
HN
R1
Cl
R1=
NH
N
N
R1
Porphyrin=
R2
Cl
R1
HN
R2
=
R1
Scheme 2. Sequence of reactions originating supported photosensitizers PS1 to PS3.

S. M. Ribeiro et al. / Tetrahedron 63 (2007) 7885–7891
7887
We also tried the chlorosulfonation reaction described above
over the mixture of 4 and 5 and reacted the product with
commercial aminomethylated polystyrene divinylbenzene
copolymer (PSDV-NH2), obtaining the corresponding por-
phyrin linked polymer PS4. Due to the absence of a spacer,
this structure corresponds to a greater proximity of the por-
phyrin to the polymer backbone. Values for the amount of
porphyrin bonding to aminomethylated polymers were cal-
culated from the total nitrogen content of polymers PS1 to
PS4 and discounting the value of nitrogen corresponding
to the initial aminomethylated polymers (Table 2). Polymer
PS2, with the larger spacer, presents the lowest value for
porphyrin incorporation and this can be related to the possi-
bility already suggested23 that, due to the greater size of the
alkylated chain, the two amino groups can react with chloro-
methyl groups of the polymer, leaving fewer free amino
groups to react with chlorosulfonated porphyrins.
The major problem with the photochemical route is the de-
activation of the photosensitizer. In homogeneous medium
we have obtained good results for the synthesis of juglone
using TDCPP (4) as sensitizer.27 The catalyst stability has
been found to be particularly relevant.
We started our studies with the supported photosensitizers
PS1 to PS4 by trying the juglone photosynthesis in acetoni-
trile at 30 mM concentration with a catalyst/substrate ratio
of 1:100 using air flow and three 50 W halogen lamps. The
reactions were followed by UV–visible spectroscopy, by an-
alyzing the increase in the 450 nm absorption band. When an
increase was no longer observed, the reaction was stopped
and the juglone isolated by chromatography. The blank ex-
periment corresponds to the reaction without photosensitizer
(Fig. 1).
Results of Figure 1 show that the activity of supported pho-
tosensitizers is dependent on the structure of the spacer be-
tween porphyrin and polymer. PS2 with a C12 chain spacer
has the greatest activity of the photosensitizers tested, com-
parable to that of the free porphyrin TDCPP (4). PS1 with
a smaller chain spacer gives similar juglone yields, but re-
quiring longer reaction time. PS3 and PS4 present much
smaller activities. The effects of the length of the spacer
on the activity of supported catalysts are documented for
thermal oxidations23,31 and may be related to the character-
istics of the catalyst environment. Long spacers allow for an
environment more similar to the free catalyst in solution.23,32
With catalyst PS2 we tried an experiment with a substrate/
catalyst ratio of 600:1 and obtained 58% of product in 15 h.
Table 2. Values for porphyrin incorporation (mmol/g) in the Merrifield
polymer
Polymer-spacer-porphyrin
Values of
porphyrin
incorporation
mmol/g
R1
NH
N
N
S
R1
HN
R1
Cl
Cl
R1=
S]NH(CH2)6NHSO2 (PS1)
S]NH(CH2)12NHSO2 (PS2)
S]NHC6H4NHSO2 (PS3)
0.21
0.06
S
S
S
The nature of the solvent can influence the photooxygena-
tion reactions either due to different oxygen solubility or dif-
ferent lifetimes of singlet oxygen.33 Changing the solvent
from acetonitrile to chloroform, where oxygen singlet has
a longer lifetime, originates faster reactions for PS1 and
PS2 giving about the same juglone yields (Fig. 2). For
PS2 after 3 h 77% isolated yield of juglone is obtained,
0.11
0.06
(PS4)
SO2
NH
2.2. Photooxygenation experiments
100
Naphthoquinones are important structural blocks present in
several natural products. One of them, juglone (11), is the
starting material for several synthetic pathways. Although
juglone can be obtained by thermal conditions,25 photooxy-
genation of 1,5-dihydroxynaphthalene (8) is an attractive
route for its synthesis.26 The mechanism of this reaction
seems to involve 1,4-cycloaddition of singlet oxygen, forma-
tion of the corresponding 1,4-endo-peroxide (9), and decom-
position into hydroperoxide (10), which oxidizes to juglone
(11) (Scheme 3).27,28 Recently described photooxidative ap-
proaches use homogeneous sensitizers in conditions of green
photochemistry.29,30
80
60
75
75
70
39
40
20
0
31
20
3
5
OH
OH
OH
O
24
24
24
O
O
Photosensitizer
air,h
24
OH
(8)
OH
(9)
OH
OH
O
OOH
(10)
(11)
Figure 1. Total reaction time and juglone (11) yields for different photo-
sensitizers.
Scheme 3. Photooxygenation of 1,5-dihydroxynaphthalene (8).

7888
S. M. Ribeiro et al. / Tetrahedron 63 (2007) 7885–7891
Photooxygenation of a-terpinene (12) to originate ascaridole
(13) was also studied (Scheme 4).34
100
72
77
70
80
60
40
20
75
Photosensitizer
air,h
O
O
(12)
(13)
Scheme 4. Photooxygenation of a-terpinene (12).
We used the same photooxidative system as beforewith chlo-
roform as solvent, but with a sensitizer/substrate ratio of
1:600. The reaction was followed by analyzing the disappear-
ance of 12 by GC. For supported catalysts the polymer was
filtered at the end of the reaction and the solvent evaporated.
NMR analysis showed that the main product is the endoper-
oxide 13, which is confirmed by GC/MS analysis. The results
with PS1, PS2, and TDCPP (4) are shown in Figure 4.
24
6
0
5
PS2
PS1
chloroform
PS2
3
PS1
acetonitrile
Figure 2. Comparative values in chloroform and acetonitrile of juglone (11)
yield for photosensitizers PS1 and PS2.
99
100
87
84
which shows that this supported sensitizer is superior to
supported rose Bengal.26
80
60
40
20
The possibility of easy recycling is the major advantage of
supported catalysts. Using chloroform as solvent we tried
catalyst PS2 in three cycles of photooxygenations (Fig. 3).
The catalyst is simply filtered, washed, dried, and used
with a new substrate batch. We observed a gradual increase
of time required to finish the reaction and a decrease in the
final yield of isolated juglone. As we never observed leach-
ing of the catalyst, this inactivation means that the link to the
polymer structure was not affected. One possibility is the de-
struction of the porphyrin and consequent loss to solution in
a form not detectable by visible spectroscopy. However, el-
emental analysis for PS2 after recycling shows only a 10%
decrease of the nitrogen content, which shows that the ob-
served inactivation is not related to a massive degradation
of the macrocycle.
0
4
3
2
Figure 4. Isolated yield of ascaridole (13) and total reaction time for a-ter-
pinene (12) photooxidation with PS1 and PS2.
100
From Figure 4 we conclude that PS2 is again more active
than PS1 and less active than the free sensitizer 4, but with
the advantage of the easy procedure to isolate the product.
For PS2 about 10% of another compound is detected by
GC/MS and corresponds to the same percentage for small
signals seen in the NMR spectrum. This compound is al-
ready present in the reagent and was identified as p-cymene.
In the case of PS1 the amount of this product increases to
30% (by NMR), which may be explained by the competitive
oxidation of a-terpinene by oxygen when oxygenation by
singlet oxygen is a slower process.
77
80
61
60
50
40
20
0
The problem of inactivation of the sensitizers in reutilization
experiments was once again studied with this substrate.
Oelgemoller suggested26 that the production of acid from
photooxidation reactions in chloroform may be one ex-
planation for sensitizer inactivation. In our case even though
not promoting sensitizer destruction, acid may originate
protonation of the porphyrins, which may present lower
8
7
1st
2nd
3
3rd
Figure 3. Total reaction time and juglone (11) yields for three consecutive
reactions with PS2.

S. M. Ribeiro et al. / Tetrahedron 63 (2007) 7885–7891
7889
PS2
PS1
PS2+NaHCO3
86
PS1+NaHCO3
81
94
87
84
100
80
60
40
20
0
77
2
3
ascaridole (13)
4
2
3
juglone (11)
time (h)
Figure 5. Comparative yields of ascaridole (13) and juglone (11) for reactions with photosensitizers PS1 and PS2 in the presence and absence of NaHCO3.
values for singlet oxygen quantum yields as positively
charged dyes.35 To circumvent this possibility we repeated
the photooxidation of a-terpinene (12) and 1,5-dihydroxy-
naphthalene (8) in the presence of solid sodium hydrogen
carbonate (Fig. 5).
oxygen singlet oxidations. The presence of sodium hydrogen
carbonate in the reaction medium increases the efficiency of
the system, possibly by avoiding the formation of cationic
porphyrinic species.
From Figure 5 we conclude that the presence of sodium
hydrogen carbonate decreases reaction time of the photooxy-
genations, possibly by avoiding the formation of porphyrin
cations and this may be the major deactivation pathway for
these supported catalysts. The effect of the base is more
pronounced in the case of PS2 than in PS1. This beneficial
effect of the base addition is more pronounced in reutiliza-
tion experiments. Sensitizer PS2 in a-terpinene photooxy-
genations shows small values of activity loss after three
consecutive reactions (Fig. 6), which is a promising charac-
teristic to be explored in future studies.
4. Experimental
4.1. Materials and methods
All solvents were purified before use according to the liter-
ature procedures. 1,5-Dihydroxynaphthalene, a-terpinene,
and aminomethylated polystyrene divinylbenzene copoly-
mer were used as purchased from Aldrich. Porphyrin 4
was obtained as described in the literature.24
4.2. Instrumentation
1H NMR spectra were recorded on a 300 MHz Bruker-AMX
spectrometer. J values are given in Hertz. Mass spectra were
obtained on a HP 5973 MSD apparatus by electronic impact
at 70 eV. Elemental analysis was carried out using a Fisons
Instruments EA1108-CHNS-0 apparatus. Absorption spectra
were measured on a Hitachi U-2001 spectrometer. Gas chro-
matography was carried out using a Supelcowax (30 mꢂ
0.25 mm) capillary column on a Hewlett-Packard 5890A in-
strument with a Hewlett-Packard 3396Aꢁin1tegrator. GC anal-
ysis was run at 80 ꢀC (5 min)/20 ꢀC min /200 ꢀC (20 min);
detector temperature 250 ꢀC, injector temperature 220 ꢀC.
94
100
80
60
40
20
0
86
81
4.3. Synthesis of porphyrin 524
1st
3
2nd
To a solution of 7.0 g (40 mmol) of 2,6-dichlorobenzal-
dehyde, 0.85 g (8.0 mmol) of benzaldehyde in 100 mL
of acetic acid, 8 mL of acetic anhydride and 25 mL of nitro-
benzene at 120 ꢀC, and 3.5 mL (50 mmol) of pyrrole were
slowly added. The reaction was maintained at this tempera-
ture for 2 h. After cooling, 275 mg of a mixture of porphy-
rins 4 and 5 was obtained. By NMR the mixture contains
about 13% of porphyrin 4 and 72% of porphyrin 5.
2
3rd
Figure 6. Total reaction time and ascaridole (13) yields for three consecu-
tive reactions with PS2 as photosensitizer in the presence of sodium hydro-
gen carbonate.
3. Conclusion
Porphyrins can be easily and selectively chlorosulfonated
and covalently linked to the Merrifield polymer. Selected
spacers can also be easily intercalated between the porphyrin
nucleus and the polymer, with a C12 carbon chain as spacer
being the most convenient to make a good sensitizer for
4.4. Synthesis of aminoalkylated polymers AAP1 to
AAP3
To a mixture of 3.0 g of Merrifield polymer in 25 mL of
DMF, 1.5 g of amine (1–3) was added. The mixture was

7890
S. M. Ribeiro et al. / Tetrahedron 63 (2007) 7885–7891
placed at 70 ꢀC for 24 h (room temperature for amine 3).
After cooling the mixture was poured in 150 mL of water,
filtered, and washed with water, methanol, dichloromethane,
and methanol again. The residue was dried in an oven
under vacuum. Elemental analysis of this product gives the
incorporation of the amino alkyl chain in the polymer
structure.
photosensitizer in order to originate the proper molar ratio
of sensitizer to substrate. The evolution of the reaction was
monitored by analyzing the disappearance of the reagent us-
ing GC. The reaction evolution can also be followed by
UV–vis spectroscopy at 268 nm. When the reagent was
consumed the reaction mixture was filtered to recover the
sensitizer. The solvent was evaporated, dried by nitrogen
flow, and analyzed by NMR spectroscopy. In the experiment
made in the presence of base, 60 mg of sodium hydrogen
carbonate was initially added.
4.5. Synthesis of polymeric photosensitizers PS1 to PS4
At room temperature 15 mL of chlorosulfonic acid was
added to 150 mg of a mixture of porphyrins 4 and 5. The
solution was stirred for 2 h and then carefully poured over
ice in order to precipitate the porphyrins. The precipitate
was filtered, dried, dissolved with dichloromethane, and
the solution dried with sodium sulfate. The solution was
concentrated to 30 mL, 1 mL of pyridine was added fol-
lowed by 300 mg of the aminoalkylated polymers AAP1
(AAP2, AAP3 or PSDV-NH2). The mixture was stirred
overnight at 30 ꢀC, filtered, and washed with dichloro-
methane, tetrahydrofuran, methanol, and dichloromethane
again. Non-bonding porphyrin was eliminated with these
washings. After drying the solid under vacuum elemental
analysis was carried out in order to determine porphyrin
incorporation.
Ascaridole: colorless oil, 1H NMR (300 MHz, CDCl3):
dH¼0.97 (3H, d, J 6.90, CH3), 0.98 (3H, d, J 6.9, CH3),
1.31 (3H, s, CH3), 1.51–1.56 (2H, m), 1.85 (H, sept, J
6.90, isopropyl), 1.97–1.92 (2H, m), 6.42 (H, d, J 8.58, ole-
finic CH), 6.53 ppm (H, d, J 8.58, olefinic CH); MS (EI,
70 eV): m/z¼168 (M+, 1%), 150 (7%), 134 (32%), 119
(100%), 107 (33%), 91 (37%). Spectral data were identical
to those reported.36
p-Cymene: 1H NMR (300 MHz, CDCl3): dH¼1.22 (3H, d, J
1.68, CH3), 1.24 (3H, d, J 1.68, CH3), 2.31 (3H, s, CH3), 2.87
(H, sept, isopropyl), 7.11 (4H, s, Ar-H); MS (EI, 70 eV):
m/z¼134 (M+, 29%), 119 (100%), 115 (6%), 103 (6%), 91
(17%), 77 (5%). NMR data were identical to those re-
ported.37
4.6. Photosynthetic oxidation experiments
4.6.1. General photooxidation procedure. Photooxidation
experiments were carried out at room temperature using
a laboratory-built photoreactor consisting of three 50 W
lamps. The reactions were carried out in a 100 mL flask
equipped with a water condenser (an ice condenser in the
case of the substrate a-terpinene) and an entrance for air.
The solutions were irradiated with a stream of air continu-
ously flowing into the flask.
Acknowledgements
The authors would like to thank Chymiotechnon and FCT-
POCTI/QUI/55931/2004 for financial support.
References and notes
1. Clennan, E. L.; Pace, A. Tetrahedron 2005, 61, 6665.
2. DeRosa, M. C.; Crutchley, R. J. Coord. Chem. Rev. 2002, 233–
234, 351.
3. Prein, M.; Adam, W. Angew. Chem., Int. Ed. 1996, 35, 477.
4. (a) Pure Appl. Chem. 2000, 72, 1207; (b) Clark, J. H. Green
Chem. 1999, 1, 1.
5. Albini, A.; Fagnoni, M. Green Chem. 2004, 6, 1.
6. (a) Yu, X.-Q.; Huang, J.-S.; Yu, W.-Y.; Che, C.-M. J. Am. Chem.
Soc. 2000, 122, 5337; (b) Cooke, P. R.; Lindsay Smith, J. R.
J. Chem. Soc., Perkin Trans. 1 1994, 1913.
7. Pozzi, G.; Mercs, L.; Holczknecht, O.; Martimbianco, F.;
Fabris, F. Adv. Synth. Catal. 2006, 348, 1611.
8. Gollnick, K. Adv. Photochem. 1968, 8, 1.
9. Benaglia, M.; Danelli, T.; Fabris, F.; Sperandio, D.; Pozzi, G.
Org. Lett. 2002, 4, 4229.
4.6.1.1. 1,5-Dihydroxynaphthalene photooxidation.
The substrate (96 mg) in acetonitrile (20 mL) was mixed
with an amount of photosensitizer (porphyrin or sup-
ported porphyrins) in order to originate the proper molar
ratio of sensitizer to substrate. The evolution of the reac-
tion was monitored by UV–vis spectroscopy at 416 nm.
The reaction mixtures were filtered to recover the sensi-
tizer and the solvent evaporated. The residue was chro-
matographed on a silica gel column using CH2Cl2 as
an eluent to give juglone (5-hydroxy-1,4-naphthoquinone)
as the product.
This reaction was also carried out in the presence of CHCl3
(14 mL), using acetonitrile (6 mL) only to dissolve the sub-
strate. In the experiment made in the presence of base, 60 mg
of sodium hydrogen carbonate was added.
10. Griesbeck, A. G.; El-Idreesy, T. T.; Bartoschek, A. Adv. Synth.
Catal. 2004, 346, 245.
11. Wahlen, J.; De Voos, D.; Jacobs, P. A.; Alsters, P. L. Adv. Synth.
Catal. 2004, 346, 152.
1
5-Hydroxy-1,4-naphthoquinone: Yellow brownish solid, H
NMR (300 MHz, CDCl3): dH¼7.28 (1H, d, J 2.50,
H-naph.), 7.30 (1H, d, J 2.50, H-naph.), 7.61–7.68 (3H, m,
H-naph.), 11.9 ppm (1H, s, OH); MS (EI, 70 eV): m/z¼
174 (M+, 100%), 146 (19%), 118 (28%), 92 (24%), 63
(19%). Spectral data were identical to those reported.27
12. Kitamura, N.; Yamada, K.; Ueno, K.; Iwata, S. J. Photochem.
Photobiol. A: Chem. 2006, 184, 170.
13. Hino, T.; Anzai, T.; Kuramoto, N. Tetrahedron Lett. 2006, 47,
1429.
14. Koizumi, H.; Shiraishi, Y.; Tojo, S.; Fujitsuka, M.; Majima, T.;
Harai, T. J. Am. Chem. Soc. 2006, 128, 8751.
15. Fuchter, M. J.; Hoffman, B. M.; Barrett, A. G. M. J. Org. Chem.
2006, 71, 724.
4.6.1.2. a-Terpinene photooxidation. The substrate in
CHCl3 was mixed with the appropriate amount of

S. M. Ribeiro et al. / Tetrahedron 63 (2007) 7885–7891
7891
16. Griesbeck, A. G.; Bartoschek, A.; El-Idreesy, T. T.; Hoinck, O.;
Miara, C. J. Mol. Catal. 2006, 41, 48.
17. Chavan, S. A.; Maes, W.; Gevers, L. E. M.; Wahlen, J.;
Vankelecom, I. F. J.; Jacobs, P. A.; Dehaen, W.; De Vos,
D. E. Chem.—Eur. J. 2005, 11, 6754.
18. Griesbeck, A. G.; El-Idreesy, T. T. Photochem. Photobiol. Sci.
2005, 4, 205.
19. Inbaraj, J. J.; Vinodu, M. V.; Gandhidasan, R.; Murugesan, R.;
Padmanabhan, M. J. Appl. Polym. Sci. 2003, 89, 3925.
20. Schaap, A. P.; Thayer, A. L.; Blossey, E. C.; Neckers, D. C.
J. Am. Chem. Soc. 1975, 97, 3741.
21. Azenha, E. G.; Serra, A. C.; Pineiro, M.; Pereira, M. M.; de
Melo, J. S.; Arnaut, L. G.; Formosinho, S. J.; Rocha
Gonsalves, A. M. d’A. Chem. Phys. 2002, 280, 177.
22. Quast, H.; Dietz, T.; Witzel, A. Liebigs Ann. 1995, 1495.
23. Evans, S.; Lindsay Smith, J. R. J. Chem. Soc., Perkin Trans. 2
2001, 174.
26. Suchard, O.; Kane, R.; Roe, B. J.; Zimmermann, E.; Jung, C.;
Waske, P. A.; Mattay, J.; Oelgemoller, M. Tetrahedron 2006,
62, 1467.
27. Murtinho, D.; Pineiro, M.; Pereira, M. M.; Rocha Gonsalves,
A. M. d’A.; Arnaut, L. G.; Miguel, M. G.; Burrows, H. D.
J. Chem. Soc., Perkin Trans. 2 2000, 2441.
28. Griffiths, J.; Chu, K.-Y.; Hawkins, C. J. Chem. Soc., Chem.
Commun. 1976, 676.
29. Oelgemoller, M.; Healy, N.; Oliveira, L.; Jung, C.; Mattay, J.
Green Chem. 2006, 8, 831.
30. Oelgemoller, M.; Jung, C.; Mattay, J.; Zimmermann, E. Green
Chem. 2005, 7, 35.
ꢀ
31. Brule, E.; Miguel, Y. R.; Hii, K. K. Tetrahedron 2004, 60, 5913.
32. Molinari, H.; Montanari, F.; Tundo, P. J. Chem. Soc., Chem.
Commun. 1977, 639.
33. Hurst, J. R.; McDonald, J. D.; Schuster, G. B. J. Am. Chem. Soc.
1982, 104, 2065.
24. Johnstone, R. A.; Nunes, M. L. P. G.; Pereira, M. M.; Rocha
Gonsalves, A. M. d’A.; Serra, A. C. Heterocycles 1996, 43,
1423.
25. (a) Crouse, D. J.; Wheeler, M. M.; Goemann, M.; Tobin, P. S.;
Basu, S. K.; Wheeler, D. M. S. J. Org. Chem. 1981, 46, 1814;
(b) Villemin, D.; Hammadi, M.; Hachemi, M. Synth. Commun.
2002, 32, 1501.
34. Schenck, G. O. Angew. Chem. 1975, 69, 579.
35. Dougherty, T. J. Advances in Photochemistry; Volman, D. H.,
Hammond, G. S., Neckers, D. C., Eds.; Wiley-Interscience:
New York, NY, 1992; Vol. 17, pp 275–311.
36. Tokuyama, H.; Nakamura, E. J. Org. Chem. 1994, 59, 113.
37. Pouchert, C. J. The Aldrich Library of NMR Spectra, 2nd ed.;
Aldrich Chemical: Millwaukee, WI, 1983; Vol. 1, p 742.