Homo- and heterogeneous α-pinene photooxidation using a protoporphyrin-derived amide

Article abstract of DOI:10.1002/ejoc.201201304

6,7-Bis[3-(N^Iμ-tert-butyloxycarbonyllysine methyl ester)]-1,3,5,8-tetramethyl-2,4-divinylporphyrin (3) was synthesized and successfully immobilized in a silica matrix by a sol-gel method. Protoporphyrin (PP)-derived amide 3 showed much higher photostability than its parent PP-IX. Its UV/Vis absorption, excitation, and fluorescence spectra as well as its ability to generate ¹O₂ were measured both in solution and in the matrix. Subsequently, free and immobilized porphyrins 3 were used as sensitizers in the photooxidation of α-pinene, and their photocatalytic properties were compared. Newly synthesized protoporphyrin (PP)-derived amide 3 is more photostable than PP-IX. Because it is able to generate ¹O₂, it could be used as a biomimetic catalyst for the oxidation of α-pinene under visible light. After successful immobilization in a silica matrix, a transparent organic-inorganic hybrid was obtained that shows intensive luminescence. Copyright

Full text of DOI:10.1002/ejoc.201201304

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201304
Homo- and Heterogeneous α-Pinene Photooxidation Using a
Protoporphyrin-Derived Amide
Mariusz Trytek,*[a] Agnieszka Lipke,[b] Marek Majdan,[b] Sabina Pisarek,[c] and
Dorota Gryko*[c]
Keywords: Photocatalysis / Porphyrinoids / Singlet oxygen / Sol-gel processes / Organic–inorganic hybrid composites
6,7-Bis[3-(Nε-tert-butyloxycarbonyllysine methyl ester)]-
1,3,5,8-tetramethyl-2,4-divinylporphyrin (3) was synthesized
and successfully immobilized in a silica matrix by a sol–gel
method. Protoporphyrin (PP)-derived amide 3 showed much
higher photostability than its parent PP-IX. Its UV/Vis ab-
sorption, excitation, and fluorescence spectra as well as its
ability to generate 1O2 were measured both in solution and
in the matrix. Subsequently, free and immobilized porphyrins
3 were used as sensitizers in the photooxidation of α-pinene,
and their photocatalytic properties were compared.
prosthetic groups) with the potential to be efficiently used
as photocatalysts include: hematoporphyrin and protopor-
phyrin IX (PP-IX). These compounds are poorly soluble in
the common organic solvents that are used in photocataly-
sis; therefore, introduction of substituents that are capable
of enhancing their solubilities in hydrophobic solvents is
desirable.
Introduction
Oxidation of α-pinene, (R)-limonene, and other terpenes,
as well as terpenols, has led to a broad range of oxygenated
derivatives present in nature. As a result of their strong fra-
grance and anticancer properties, they have recently at-
tracted much interest.[1] In nature, these compounds are ox-
idized by metalloenzymes by using molecular oxygen. How-
ever, in the laboratory, the oxygenation of, for example, β-
cytronellol to rose oxide is achieved using singlet oxygen.[2]
Photooxidation reactions generally consist of generating
singlet oxygen (1O2) through photosynthetic methods, even
by utilizing visible light, provided that a suitable dye is used.
Porphyrins are photoactive pigments, and thus, they are
very efficient photocatalysts.[3,4] The ability of these com-
pounds to mimic enzymes with monooxygenase activity
supports the biooxidation of organic substrates by using
molecular oxygen as the terminal oxidant and visible light
as the energy source for biocatalysis. We have already
shown that α-pinene could be oxidized to pinocarvone by
singlet oxygen generated by partially protonated octaethyl-
porphyrin.[5] Other examples of these extraordinary com-
pounds (which play a crucial role in biological systems as
Furthermore, photoexcited porphyrins in solution, in ge-
neral, undergo easy deactivation, and together with their
photodecomposition derivatives can contribute to the con-
tamination of the reaction mixture, which makes the purifi-
cation step troublesome.[6] The use of heterogeneous cata-
lytic systems based on porphyrins immobilized in inorganic
solid supports avoids these difficulties, which facilitates the
reusability of the active catalyst. The sol–gel route is one
the most commonly employed methods for the preparation
of organic–inorganic hybrids at the microscale and even at
a molecular level under mild conditions.[7,8] This process is
an ideal way for the synthesis of materials having controlled
pore sizes. The procedure consists of the preparation of the
proper alkoxide sol, transformation to a gel, and then dry-
ing the gel to a xerogel. The most popular precursors are
metal alkoxides that react readily with water, such as tetra-
methoxysilane (TMOS) and tetraethoxysilane (TEOS), un-
der acid–base conditions. Aluminates, titanates, and borates
are also applied.[9,10] During gelation, different substances
can be inserted into the solution to give the gel unique
properties, such as mechanical or chemical resistance.[4,11]
Silica sol–gels have been reported to improve the proper-
ties of organic pigments effectively,[8,11–13] and with respect
to photocatalysis, they have many advantages, including op-
tical transparency, chemical inertness, and superior durabil-
ity.[14–16] Moreover, negligible swelling of these materials in
organic solvents was noticed relative to that observed for
organic polymers. Silica aerogels also exhibit many intri-
[a] Department of Industrial Microbiology, Faculty of Biology and
Biotechnology, Maria Curie-Skłodowska University,
Akademicka 19, 20-033 Lublin, Poland
Fax: +48-815375959
E-mail: mtrytek1@o2.pl
Homepage: www.umcs.lublin.pl
[b] Faculty of Chemistry, Maria Curie-Skłodowska University,
M. Curie-Skłodowskiej Sq. 2, 20-031, Lublin, Poland
[c] Institute of Organic Chemistry, Polish Academy of Sciences,
Kasprzaka 44/52, 01-224 Warsaw, Poland
Fax: +22-3432051
E-mail: dorota.gryko@icho.edu.pl
Homepage: www.icho.edu.pl
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201304
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M. Trytek, A. Lipke, M. Majdan, S. Pisarek, D. Gryko
SHORT COMMUNICATION
guing properties, which include very low density (0.2–
It is well known that light has a negative effect on
0.01 gcm–3), high porosity (80–98%), high inner surface porphyrins, as it leads to their irreversible degradation.
area (500–1000 m2 g–1), and extremely low thermal conduc- Therefore, the stabilities of porphyrin 3 and precursor 1 in
tivity (0.02 WmK–1).[17,18]
DMSO solution was examined. Their absorption and fluo-
The close proximity of immobilized porphyrins within rescence spectra revealed that porphyrin 3 was significantly
the pores of silica gel allows easy energy transfer between more stable than 1 (Figures 1 and 2). A substantial decrease
the pigment molecules, which is important in catalytic pro- in the Soret and Q bands of 1 was observed after exposure
cesses. Porphyrins in silica gel, in general, are known to re- to light for 10 min, whereas the decomposition of photo-
tain their luminescence properties; however, their catalytic catalyst 3 occurred only after 2 h of irradiation. Further-
activity depends on the porphyrin encapsulated. Recently, a more, the absorption and luminescence spectra of
substantial difference in the biomimetic photooxidation of porphyrin 3 showed no changes in shape, for example, peak
α-pinene in the presence of octaethylporphine (OEP) and position, during illumination, which suggested a lack of a
hematoporphyrin (HmP) was found. When immobilized in transient form of this pigment during degradation. Simi-
porous silica gel, both showed good fluorescence intensity, larly, protonation of the central nitrogen atoms was not ob-
whereas the biocatalytic activity was only preserved for served. The lack of photobleaching suggested that por-
OEP/SiO2. Moreover, the immobilization of porphyrins phyrin 3 could be a potential photocatalyst for the biotrans-
usually enhances their photostability and, thus, their resis- formation of α-pinene and of other electron-rich hydro-
tance towards reactive oxygen species released during pho- phobic compounds.
1
tocatalysis, for example, O2 and free radicals.[19]
The main objective of the present work was to obtain an
amino acid derivative of PP-IX for the purpose of photo-
catalysis and to examine its photostability relative to that
of PP-IX. Also, the effect of its encapsulation in the sol–gel
matrix on its photosensitizing properties was determined.
Results and Discussion
Our new catalyst was prepared by the coupling reaction
of PP-IX (1) with Nε-Boc-protected lysine methyl ester (2)
by using O-(benzotriazol-1-yl)-N,N,NЈ,NЈ-tetramethyluron-
ium hexafluorophosphate (HBTU) as the coupling reagent
and N,N-diisopropylethylamine (DIPEA) as the base
(Scheme 1). After purification, desired product 3 was ob-
tained in 97% yield.
Figure 1. Absorption spectra of (a) PP-IX (1) and (b) its amino
acid derivative (3) in DMSO over a 2 h period of illumination with
Scheme 1. Synthesis of amide 3.
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visible light showing differences in their photostabilities.
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Homo- and Heterogeneous α-Pinene Photooxidation
1
Figure 3. The emission spectrum of O2 generated at various con-
centrations of 3 in chloroform with (bottom) or without (top) α-
pinene by light excitation at 410 nm.
Figure 2. Emission spectra of (a) PP-IX (1) and (b) its amino acid
derivative (3) in DMSO over a 2 h period of illumination with
visible light showing differences in their photostabilities. The exci-
tation wavelength was 408 nm.
experiments. To the best of our knowledge, the most favor-
able amount of a pigment in a photooxidation reaction has
not been determined by near-IR techniques.
Subsequently, the immobilization of catalyst 3 in silica
The catalytic properties of porphyrin 3 were tested in the gel by the sol–gel method was attempted. Because the spec-
light-promoted oxidation of α-pinene. Initially, the ability tral characteristics of porphyrins are extremely sensitive to
of 3 to generate 1O2 in chloroform was evaluated at various processes such as protonation, metalation, ring oxidation,
concentrations both in the absence and in the presence of α- and aggregation, the absorption and emission spectra of
pinene. The results were monitored by infrared fluorescence porphyrin 3 in chloroform, silica sol, alcogel, and xerogel
spectroscopy (at 1280 nm) and the molar concentration of were registered and compared (Figure 4). In chloroform
3 was varied from 1ϫ10–6 to 5ϫ10–4 m (Figure 3). The and in silica sol, similar characteristic absorption bands in
maximum intensity of the fluorescence band of 1O2 in a the blue (Soret band) and red (Q bands) spectral regions
solution without α-pinene was reached at a concentration were observed. A difference in the absorption spectrum of
of 8.5ϫ10–6 m for 3. A slightly lower concentration of 3 xerogel was observed relative to the spectra registered for
(7.5ϫ10–6 m) was the best for the solution containing α- the sol and alcogel samples. The Soret band was shifted
pinene. However, upon the addition of this monoterpene from 402 to 408 nm, whereas in the Q band region, a new,
to the photocatalytic system, a significant decrease in the very intense band at 555 nm was observed. We assumed
intensities of the fluorescence bands of singlet oxygen for that these changes were a result of the protonation of the
the solution was observed, and this is unambiguous evi- porphyrin inside the acidic SiO2 network with consequent
1
dence for the interaction of O2 with α-pinene.
formation of a dication. This was also confirmed by a re-
In both cases, an increase in the concentration of duction in the number of bands in the Q region from four
1
porphyrin 3 resulted in a decrease in the generation of O2. to two, which is directly related to the well-known change
Presumably, agglomeration of the pigment molecules was in the symmetry of porphyrins from D2h for the free-base
responsible for the decrease in their photosensitizing abili- H2P porphyrin to D4h for the H4P2+ dication. Probably,
ties.[20] This study allowed us to select the optimal concen- xerogel contains a ratio of both the protonated and neutral
tration of the porphyrin (7.5ϫ10–6 m) for further catalytic forms of porphyrin 3. This ratio could have been altered
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M. Trytek, A. Lipke, M. Majdan, S. Pisarek, D. Gryko
SHORT COMMUNICATION
during the drying process in air or influenced by the nature overlapped with those derived from the dications. In the
of the solvent. The concentration of the free-base form was emission spectra of porphyrin 3, a shift in λmax from 630 to
low, and thus, the Q bands originating from these species 602 nm was observed during the transformation of alcogel
to xerogel, and this confirms the formation of a dication in
the dried silica gel. We assume that the appearance of a new
band near 660 nm was a result of porphyrin aggregates.
Photooxidation of α-pinene was carried out in a homo-
geneous and heterogeneous system by using the same total
equimolar amounts of porphyrin 3. The oxidation reaction
was monitored periodically by using GC–MS after 1, 2, 4,
10, 22, and 30 h (Table 1).
The free-base amide derivative of PP-IX, that is, 3, in
chloroform was shown to be an efficient photocatalyst for
the production of commercially important compounds by
using light in the visible region. A major terpenoid pro-
duced during the reaction was pinocarveyl hydroperoxide,
whereas trans-pinocarveol, pinocarvone, and myrtenal were
obtained in substantially lower amounts (Table 1). The total
number of turnovers (the total number of mol of products
produced per mol of catalyst used in the phototransforma-
tion) was reached after 30 h with 60% conversion of α-
pinene. Unexpectedly, porphyrin 3 intercalated in the sol–
gel matrix (despite good fluorescence intensity) showed
only slight activity, approximately 36-fold lower than that
observed under the homogeneous conditions. This fact
might be connected to the protonation of porphyrin 3 in
silica, which is known to decrease the catalytic efficacy of
many photoexcited porphyrins.[21] In contrast, the poor
catalytic activity of organic–inorganic hybrid 3/SiO2 might
result from its hindered ability to generate the reactive oxy-
gen species, probably because of strong interactions be-
tween catalyst 3 and the silica matrix. Indeed, only a weak
fluorescence signal of 1O2 was observed for samples of xero-
gel dotted with porphyrin 3. Gratifyingly, during the course
of photocatalysis, no leaking from the silica was observed
(the reaction solution remained colorless). Recently, hema-
Figure 4. Comparison of the (a) UV/Vis absorption and (b) emis-
toporphyrin was found to preserve luminescence in silica
sion spectra of porphyrin 3 in chloroform, sol, alcogel, and silica
pores, whereas its biocatalytic activity was reduced, and this
xerogel.
Table 1. Photooxidation of α-pinene with amide derivative 3 in homogeneous system and heterogeneous system based on the porphyrin
intercalated in the silica matrix.[a]
Time of catalysis [h]
Side products [TN][b]
Pinocarveyl hydroperoxide [TN]
TTN[c]
α-Pinene conversion [%]
1
2
4
10
22
30
110.6 (10.9)
190.2 (51.0)
282.1 (20.7)
486.7 (35.4)
738.8 (46.0)
944.1 (56.8)
428.6 (0)
683.1 (0.8)
1273.2 (3.1)
2872.3 (22.4)
3676.5 (60.3)
4025.0 (79.2)
539.2 (10.9)
873.3 (51.8)
1555.3 (23.8)
3359.0 (57.8)
4415.3 (106.3)
4969.1 (136)
4.5 (0.2)
8.6 (0.7)
15.6 (0.8)
37.2 (0.9)
59.7 (1.5)
60.1 (1.8)
[a] Yields are given as turnover numbers [TN]. Results for the heterogeneous conditions are given in parentheses. [b] Defined as the total
turnover number of side products (e.g. pinocarveol, trans-pinocarvone and myrtenal). [c] Calculated as the total of the turnover numbers
(TTN). Concentration of 3 used in the homogeneous system was 7.5ϫ10–6; the heterogeneous reactions were performed with an equiva-
lent molar amount (2.25ϫ10–8 mol) of encapsulated 3 in powder form (550 mg) under identical conditions; α-pinene concentration in
chloroform (3 mL): 50 mm.
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Homo- and Heterogeneous α-Pinene Photooxidation
21.0, 12.7, 11.6 ppm. HRMS (ESI): calcd. for C58H78N8O10 [M +
Na]+ 1069.5746; found 1069.5733. UV/Vis (CH2Cl2): λmax (ε,
Lmol–1 cm–1) = 668 (1.45ϫ103), 629 (4.24ϫ103), 574 (7.19ϫ103),
541 (1.06ϫ104), 505 (1.22ϫ104), 406 (1.59ϫ105). C58H78N8O10
(1047.30): calcd. C 66.52, H 7.51, N 10.7; found C 66.50, H 7.50,
N 10.52.
has been assigned to the interaction of its carboxyl groups
with the SiO2 structure.[5]
Conclusions
Reported porphyrin 3 showed better resistance to the re-
active species released during photocatalysis relative to that
Preparation of Transparent Monolithic Silica Gels: Porphyrin 3 was
immobilized in silica gel according to a literature procedure.[22] Ap-
shown by PP-IX. The porphyrin can serve as an enticing propriate amounts of mother solutions of porphyrin 3 in THF were
added (by using Hamilton microsyringes) to the sol in the dispos-
able spectrofluorimetric cells. Two parallel series of doped sols were
prepared in which the concentrations of the porphyrins were varied
from 1ϫ10–6 to 2.5ϫ10–5 m. The prepared hybrid materials were
dried in the absence of light. The name alcogel was used in refer-
ence to samples obtained without a mass change, whereas the term
xerogel was used in reference to the material after shrinking due to
solvent evaporation.
target for potential application in biocatalysis and for the
oxidation of natural terpenes. Although the newly synthe-
sized compound retained good luminescence properties in
silica gel, unfortunately it showed low catalytic activity in
the oxidation of α-pinene, presumably as a result of strong
interactions with the silica network. Therefore, future work
will consider the possibility of modifying the porphyrin
structure by attaching electronegative substituents onto the
macrocyclic core, which should result in a decrease in the
number of porphyrin–silica interactions. This modification
should also increase the resistance of the porphyrin to un-
dergo protonation.
Photocatalytic Studies: The resulting dry monolith samples con-
taining 3 were broken and powdered in a mill to an average particle
size of 0.3 mm. Prior to the catalytic cycle, the powder was washed
extensively with methanol and dried by using a rotary evaporator
with a water bath at 60 °C. The procedure was repeated twice with
chloroform to remove any released porphyrin. In a standard photo-
oxidation procedure, the reaction vessels were regularly irradiated
with four fluorescent visible lamps at 20 °C. After the appropriate
time intervals, and quick sedimentation of dispersed silica particles
(about 1 min) in the case of heterogeneous catalysis, each solution
was sampled. The products were detected by GC–MS. Details of
the biotransformation experiment and chromatographic analyses
experiment can be found elsewhere.[22]
Experimental Section
General Methods: Analytical-grade solvents were used as received.
1H NMR and 13C NMR spectra were recorded at room tempera-
ture with a 500 MHz instrument. DCVC (dry column vacuum
chromatography) was performed by using 200–300 mesh silica gel.
Flash column chromatography was performed by using 60 mesh
silica gel. Thin-layer chromatography (TLC) was performed by
using silica gel GF254, 0.20 mm thickness. Room-temperature UV/
Vis absorption spectra were recorded with a V-660 (JASCO) spec-
trophotometer in the 300–750 nm spectral region.[22] A spectral res-
olution of 1 nm was preserved. The fluorescence spectra were ob-
tained with a FP-6300 spectrofluorometer JASCO. UV/Vis reflec-
tance spectra were registered with a Horizontal Sampling Integrat-
ing Sphere (Model: PIV-756) connected to a V-660 spectrophotom-
eter. Singlet oxygen luminescence spectra were registered by using
Supporting Information (see footnote on the first page of this arti-
1
cle): H NMR and 13C NMR spectra of porphyrin 3.
Acknowledgments
The authors would like to thank Dr. K. ó. Proinsias for helpful
discussions. This work was partially supported by the Polish Minis-
try of Science and Higher Education (grant number
N N204 187139 (to D. G. and S. P.) and grant number empk bs 03-
1101-00 zfin 00000040).
a
Photon Technology International, Inc., spectrofluorometer
equipped with an infrared module (NIR PMT Module with InP/
InGaAs photocathode material, Hamamatsu Photonics K. K.) op-
erating at a spectral response range of 950–1700 nm.
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atmosphere. The mixture was then poured into DCM and washed
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Received: October 5, 2012
Published Online: February 4, 2013
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