Sweet chiral porphyrins as singlet oxygen sensitizers for asymmetric Type II photooxygenation

Article abstract of DOI:10.1039/c1pp05052d

Carbohydrate-decorated meso-tetraarylporphyrins P-G and P-C were synthesized via Lewis-acid catalyzed condensation of acetylated carbohydrate-substituted benzaldehydes and pyrrole. Their efficiency of singlet oxygen production was compared with the corresponding non-substituted porphyrin. The oxidation of the spin trap molecule TEMP (2,2,6,6-tetramethyl-4-piperidone) by singlet oxygen to TEMPO was measured by ESR spectroscopy, showing higher reaction rates for the sugar porphyrins. These results were corroborate by laser flash photolysis measurements that resulted in higher triplet lifetimes of glucosyl- and cellobiosyl porphyrins in comparison with tetrakis(4- hydroxyphenyl)porphyrin. Low ee was detected in the photooxygenation of ethyl tiglate. The Royal Society of Chemistry and Owner Societies.

Full text of DOI:10.1039/c1pp05052d

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Communications
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S. Mariappan, M. T. Mwangi, M. C. Tiedman and L. R.
MacGillivray, Photochem. Photobiol. Sci., 2011, 10, 1384
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Kobaru, A. Unosawa, T. Mino and T. Fujita, Photochem.
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photocycloaddition reactions of cyanonaphthalenes
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1393
Chirogenic [3 + 2]-photocycloaddition reactions of 2-
substituted naphthoquinones with cyclic alkenes, C.
Müller, A. Bauer and T. Bach, Photochem. Photobiol. Sci.,
2011, 10, 1463
Hydroxy-group effect on the regioselectivity in a
photochemical oxetane formation reaction (the
Paternò-Büchi Reaction) of geraniol derivatives, K.
Hisamoto, Y. Hiraga and M. Abe, Photochem. Photobiol.
Sci., 2011, 10, 1469
Microphotochemistry: a reactor comparison study
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furanones as a model reaction, O. Shvydkiv, A.
Yavorskyy, S. B. Tan, K. Nolan, N. Hoffmann, A. Youssef
and M. Oelgemöller, Photochem. Photobiol. Sci., 2011, 10,
1399
Photochemical and photophysical properties of
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Competitive photocyclization/rearrangement of 4-aryl-
1,1-dicyanobutenes controlled by intramolecular
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The synthesis and stereospecific solid-state
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Garibay, Photochem. Photobiol. Sci., 2011, 10, 1480
A new photo-switchable “on-off” host–guest system,
Y. Kim, Y. H. Ko, M. Jung, N. Selvapalam and K. Kim,
Photochem. Photobiol. Sci., 2011, 10, 1415
Kinetic studies on visible-light-switchable
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Seefeldt, K. Altenhöner, O. Tosic, T. Geisler, M. Sauer and
J. Mattay, Photochem. Photobiol. Sci., 2011, 10, 1488
Effect of sodium chloride on the binding of
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aggregates, D. Fuentealba, K. Thurber, E. Bovero, T. C.
S. Pace and C. Bohne, Photochem. Photobiol. Sci., 2011,
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Regiospecific [2 + 2] photocyclodimerization of trans-
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Nakamura, H. Irie, S. Hara, M. Sugawara and S. Yamada,
Photochem. Photobiol. Sci., 2011, 10, 1496
Sweet chiral porphyrins as singlet oxygen sensitizers
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Photobiol. Sci., 2011, 10, 1431

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PAPER
Sweet chiral porphyrins as singlet oxygen sensitizers for asymmetric Type II
photooxygenation†
Axel G. Griesbeck,*a Miguel A. Mirandab and Johannes Uhliga
Received 31st January 2011, Accepted 8th March 2011
DOI: 10.1039/c1pp05052d
Carbohydrate-decorated meso-tetraarylporphyrins P-G and P-C were synthesized via Lewis-acid
catalyzed condensation of acetylated carbohydrate-substituted benzaldehydes and pyrrole. Their
efficiency of singlet oxygen production was compared with the corresponding non-substituted
porphyrin. The oxidation of the spin trap molecule TEMP (2,2,6,6-tetramethyl-4-piperidone) by singlet
oxygen to TEMPO was measured by ESR spectroscopy, showing higher reaction rates for the sugar
porphyrins. These results were corroborate by laser flash photolysis measurements that resulted in
higher triplet lifetimes of glucosyl- and cellobiosyl porphyrins in comparison with
tetrakis(4-hydroxyphenyl)porphyrin. Low ee was detected in the photooxygenation of ethyl tiglate.
oxygen solvents such as chlorinated or fluorinated hydrocarbons
Introduction
(where singlet oxygen lifetimes can reach the 100 ms time regime);7
The lowest electronically excited singlet state of oxygen (1O2) is
(b) the selectivity problem, especially non substrate-induced
a versatile reagent in organic oxidation chemistry. This molecule
enantioselectivity of alkenes in singlet oxygen ene reactions. Two
can be generated by numerous methods, photochemical as well as
possible solutions are the use of non-traditional chiral reaction
thermal, and thus singlet oxygen chemistry is often not considered
environments and the use of chiral singlet oxygen sensitizers.
as a pure photochemical topic. In order to generate singlet oxygen
In the last decade, the area of polymer-supported organic
in solution, energy transfer sensitization from triplet dye molecules
reactions and polymer-supported catalysts has impressively in-
is the most convenient method. Apparently, singlet oxygen is
creased. Photooxygenation in solution using insoluble polymer-
an electronically excited molecule and thus decays to its ground
bound sensitizers facilitates the problem of dye recovery. A
state by radiationless or radiative processes in competition with
commercial polymer-bound sensitizer is the polystyrene-bound
Rose Bengal developed by Schaap and Neckers,8 followed by a
series of immobilized sensitizers, e.g. the immobilized fullerene
C60,9 ionic porphyrins immobilized on cationically functionalized
polystyrene,10 tetrakis-(4-hydroxyphenyl)porphyrin supported to
polyethylene glycol,11 polystyrene-bound benzophenones,12 im-
mobilized pyrylium salts on Merrifield resins,13 sensitizer-
incorporated Nafion membranes,14 or ion-exchange resins ioni-
cally bound to photosensitizers.15 Other heterogeneous catalysts
using clay,16 silica,17 and zeolites18,19 as support materials were also
recently developed. From the viewpoint of solution photochem-
istry, the use of non-polar solvents (especially chlorinated hydro-
carbons) enhances dye oxidation and bleeding if long reaction
times are needed, which decreases the singlet oxygen quantum
yield and hence the reaction efficiency. On the other hand,
photooxygenation reactions carried out in aqueous solutions are
not favored due to low solubility of most organic substrates,
the low singlet oxygen lifetime, and hydrophobic aggregations
of nonpolar sensitizers (leading to self quenching) which as
a consequence reduces the triplet lifetime.20 We have recently
reported a solution to circumvent some of the mentioned technical
problems: under solvent-free reaction conditions, in which the
substrates are embedded in a porphyrin-loaded polystyrene ma-
trix, irradiation and product isolation accompanied by complete
chemical reactions which makes the use of this oxygenation
reagent often cumbersome because of the limited lifetime of 1O2,
especially in protic solvents.1–3 Concerning the “atom economy”
of photooxygenation, the process where both oxygen atoms are
transferred into the substrate, 100% from molecular oxygen is
an obvious advantage over all other oxidants, e.g. hydrogen
peroxide.4 The primary peroxidic products from this “Type II
photooxygenation” can be reduced to numerous polyoxygenated
derivatives, e.g. to epoxyalcohols from allylic hydroperoxides by
means of titanium(IV)-catalysis.5
Taking into account the relatively low singlet oxygen lifetimes
in protic and especially aqueous (biological or non-biological)6
solution phase in the region of a few ms, as well as the high diffusion
rate of singlet oxygen, two major problems for modern synthetic
applications are apparent: (a) the reactivity problem of substrates
with low nucleophilicity and low solubility in “typical” singlet
aDepartment of Chemistry, University of Cologne, Greinstr. 4, 50939, Ko¨ln,
Germany. E-mail: griesbeck@uni-koeln.de
bDepartamento de Quimica/Instituto de Tecnolog´ıa Qu´ımica UPV-CSIC,
Universidad Politecnica de Valencia, Camino de Vera s/n, Apdo. 22012,
46022, Valencia, Spain. E-mail: mmiranda@qim.upv.es
† This article is published as part of a themed issue in honour of Yoshihisa
Inoue’s research accomplishments on the occasion of his 60th birthday.
This journal is
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©

sensitizer separation by extraction with ethanol offers a shortcut
to green photo-oxygenation reactions.21
shell needs to be favoured in comparison with the achiral reaction
medium. This might result from a situation where the singlet
oxygen lifetime is considerably higher in the chiral shell than in
the surrounding solvent.
In contrast to the high diastereoselectivity control of the singlet
oxygen ene reaction with chiral allylic alcohols,22 enantioselective
reactions are still rare. Several attempts to control the enantiose-
lectivity were reported. The use of cyclodextrins covalently bound
to porphyrin sensitizers in the photooxygenation of linoleic acid
resulted in low ee values (10–20%).23,24 The photooxygenation of
2-methyl-4-phenyl-2-butene in NaY zeolite in presence of (+)-
ephedrine as chiral inductor also resulted in 15% ee hydroperoxide
formation.25 Recently, Co´rdova et al. published a fundamental
report on the amino acid-catalyzed asymmetric incorporation of
molecular oxygen into the a position of a series of aldehydes and
ketones.26,27
The idea behind the concept of triplet dye sensitization with
singlet oxygenation in the vicinity of the sensitizer is depicted
in Scheme 1: in order to compensate the fast diffusion of
singlet oxygen from the area of the collision-induced energy
transfer, a relatively unpolar large core-shell structure is necessary
that allows substrate molecule binding as well as higher singlet
oxygen lifetimes that in the polar protic solvent environments.
In a first approach, the chiral modification of porphyrins by
anionic polymerisation with glycidol led to remarkable changes
of diastereoselectivity in the photooxygenation of allylic alcohol
mesitylol (Scheme 2).28
In order to study the dependence of singlet oxygen gen-
eration on the shell size, singlet oxygen quantum yields of
carbohydrate-substituted porphyrins were compared to those of
a non-functionalized porphyrin in methanol solution. Several
carbohydrate-containing shell structures were designed that could
build up a series of spatially increasing structures for singlet oxygen
diffusion (Scheme 3). The substitution of tetraphenylporphyrin
derivatives with mono- and disaccharides leads to core-shell
systems with similar properties as polyglycerol porphyrins, but
with exactly defined stereogenic centres and molecular weights.
Sugars are cheap and widely available from the chiral pool,
exhibiting stereochemically defined hydroxy functions. Their in-
corporation as porphyrin substituents was described by Maillard
and coworkers.29,30 In order to compare shell sizes only and not
configurational differences, glucose and cellobiose substituents
were synthesized in this study. The TEMPO-formation was
measured by ESR spectroscopy and compared for three core-
shell systems of different shell sizes. Sensitizers used for this
technique were glycosyl- and cellobiosyl-substituted porphyrins
and the meso-tetrakis-(4-hydroxyphenyl)-porphyrin (4HPP).
Scheme 3 Core-shell structured polyglycerol porphyrin.
Scheme 1 Core-shell structured sensitizer concept.
The detection of singlet oxygen by the spin trap 2,2,6,6-
tetramethyl-4-piperidone (TEMP) is a technique that uses ESR
spectroscopy detecting the oxidized radical 2,2,6,6-tetramethyl-
4-piperidone-N-oxyl (TEMPO).32 Integrals of the triplet signal
of TEMPO correlate to the amount of TEMP oxidized by singlet
oxygen and are used in order to compare quantum yields of singlet
oxygen generation by plotting the time-dependent TEMPO-
formation.
Results and discussion
Scheme 2 Core-shell structured polyglycerol porphyrin.27
The method reported by Maillard et al. yielded small amounts of
glycosylated porphyrins. The linear synthesis started with bromi-
nation of per-acetylated sugars at the anomeric center under phase
transfer conditions yielding the corresponding per-acetylated,
carbohydrate-conjugated benzaldehydes in moderate yields of 30–
75%. Their subsequent reaction with pyrrole and Lewis acid gave
low to moderate yields of protected sugar porphyrins, catalyzed
by the “Lindsey method” with Lewis acid boron trifluoride
etherate. Deprotection of the acetylated sugar porphyrins was
Core-shell structured concepts of photooxygenations are based
on a sensitizing core that produces the reactant singlet oxygen
which diffuses into a chiral environment where asymmetric induc-
tion is expected to occur. Although this catalysis concept has been
evaluated in few reactions already, mechanistic and photophysical
properties of core-shell triplet sensitizers are still rare. For efficient
(enantioselective) steering effects, photo-oxygenation in the chiral
1432 | Photochem. Photobiol. Sci., 2011, 10, 1431–1435 This journal is
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achieved with full conversion by addition of sodium methanolate
to dry methanol solutions, giving highly polar porphyrins P-G29,31
(glucose-substituted porphyrin) and P-C (cellobiose-substituted
porphyrin) that are unsoluble in most organic solvents (Scheme 4).
For comparison, the synthetic precursor, unfunctionalized 4HPP
was used.
of the porphyrin sensitizer. The observation of a more efficient
TEMPO conversion still needs to be interpreted, as these results
could either point at a higher lifetime of singlet oxygen in the sugar
shells, leading to a higher effective concentration of reactive oxygen
in the reaction with TEMP; a second possible mechanism would
be the higher efficiency of sugar porphyrins reaching their excited
triplet state compared to the non-substituted porphyrin. A higher
concentration of triplet state porphyrin also leads to a higher
concentration of singlet oxygen and thus a faster conversion.
In order to differentiate between these two mechanisms, the
triplet state formation of porphyrins was measured by laser flash
photolysis.
The laser flash photolysis experiments (Scheme 7) under air
showed the same tendencies as the spin trapping with TEMP.
Sugar porphyrins exhibited a 10–20% higher triplet state for-
mation upon 308 nm laser excitation than non-substituted 4-
HPP. Although the sugar shell of glycoconjugated porphyrin has
the smaller substituent, its triplet formation exceeded cellobiosyl
porphyrin by 9%. The triplet lifetimes that were determined by
LFP from first order exponential decay for the three sensitizers in
this report were: 0.24, 0.28, and 0.30 ms for 4HPP, P-G, and P-C,
respectively, and are in good agreement with the reference value
of 0.32 ms for tetraphenylporphyrin.33
Scheme 4 Synthesis of the meso-tetraphenyl porphyrin P-C.
A methanol solution of TEMP was added to a methanol
solution of the corresponding porphyrin and irradiated with a
halogen lamp under exactly identical conditions. The intensity of
the nitroxyl radical ESR signal that formed upon singlet oxygen
formation (Scheme 5) was determined every five minutes and
plotted against the time. There was no dye bleaching observable
over the whole irradiation time. Scheme 6 shows that TEMPO
(singlet oxygen) formation for 4-hydroxyphenylporphyrin (4HPP)
in methanol is slower than for the sugar porphyrins, although
the large sugar substituent cellobiose does not exhibit the highest
values.
Scheme 7 Triplet decay from laser flash photolyses of 4HPP and the
porphyrins P-G (glucose) and P-C (cellobiose).
With these chiral carbohydrate-decorated porphyrins we per-
formed preliminary photooxygenations experiments. The ene
reaction of ethyl tiglate was used as the standard procedure
and the enantiomeric excesses determined after reduction of the
hydroperoxide by chiral HPLC (Scheme 8). The reactions were
performed in polystyrene matrices,21 with the porphyrins 4HPP, P-
G, and P-C embedded (0.5 mol%). From the substrate conversion
under identical reaction conditions, a profile of the sensitizer
activity resulted that was in good agreement with the results of
the TEMP oxidation P-G ~ P-C > 4HPP. At room temperature,
we could not find any enantioselection, however, at -5 ◦C for
the system P-G in polystyrene, 1% ee was determined. This result
was also obtained in methanol/D2O solution mixture at -10 ◦C.
These experiments are currently followed with core-shell sensitizer
systems with more extended carbohydrate shells.
Scheme 5 Oxidation of 2,2,6,6-tetramethyl-4-piperidone (TEMP) to
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by singlet oxygen.
Scheme 6 ESR measurements of TEMPO formation for 4HPP and the
porphyrins P-G (glucose) and P-C (cellobiose).
Spin trapping of TEMP is an indirect method and does not nec-
essarily measure the quantum yields of singlet oxygen production
Scheme 8 Singlet oxygen ene reaction with ethyltiglate.
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4-(2,3,4,6-2¢,3¢,6¢-Heptaacetyl-b-D-cellobiosyl)benzaldehyde:
1H-NMR: (300 MHz, CDCl3), d (ppm) = 9.89 (s, 1H, CHO), 7.83
(d, 2H, o-phenyl, J = 8 Hz), 7. 06 (d, 2H, m-phenyl, J = 8 Hz),
5. 65 (d, 1H, “ose”), 5.40–4.97 (m, 7H “ose”), 4.58–3.78 (m, 7H,
“ose”), 2.13–1.93 (m, 21H, COOCH3).
5,10,15,20-Tetrakis[4-(2,3,4,6-2¢,3¢,6¢-heptaacety-D-cello-
biosyl)-phenyl]porphyrin: 1H-NMR: (300 MHz, CDCl3), d (ppm)
= 8.84 (s, 8H, pyrr), 8.13 (d, 8H, ortho, J = 8 Hz), 7.37 (d, 8H, meta,
J = 8 Hz), 5.41 (m, 4H, C1 “ose”, J = 8 Hz), 5.23–5.09 (m, 8H, C6
“ose”), 4.68–4.41 (m, 4H, C2 “ose”), 4.30–4.24 (m, 4H, C3 “ose”),
4.13–4.09 (4H, C4 “ose”), 3.75 (m, 4H, C6 “ose”), 2.21–2.00 (m,
21H, COOCH3), -2.83 (s, 2H, NH).
Experimental
Materials
Chemicals were purchased from Sigma Aldrich and used without
further purification. Solvents for ESR and laser measurements
were of HPLC grade and purchased from the Sigma Aldrich
company.
Porphyrin syntheses
General procedure for the synthesis of sugar-substituted benzalde-
hydes. A solution of 4-hydroxybenzaldehyde (42 mmol, 1.5 eq.)
in 50 ml of methylene chloride was stirred at room temperature
with 70 ml of a 5% aqueous solution of sodium hydroxide and
tetrabutylammonium bromide (7 mmol, 0.25 eq). A solution of
the acyl-protected a-D-carbohydrate bromide (28 mmol, 1.0 eq.) in
20 ml of methylene chloride was subsequently added. The solution
was stirred for 3 days. After separation, the organic phase was
washed twice with 5% aqueous sodium hydroxide solution and
water, then dried over sodium sulfate. The solvent was evaporated
after filtration, yielding slightly yellowish solids.
5,10,15,20-Tetrakis(D-cellobiosyl-phenyl)porphyrine (P-C): 1H-
NMR: (300 MHz, pyridine-d6), d (ppm) = 9.01 (s, 8H, pyrr), 8.25
(d, 8H, ortho, J = 8 Hz), 7.78 (d, 8H, meta, J = 8 Hz), 7.94 broad
(4H, OH “ose”), 7.48 broad (4H, OH “ose”), 6.87 broad (4H, OH
“ose”), 6.00 (m, 4H, C1 “ose”, J = 8 Hz), 4.78–4.52 (m, 8H, C6
“ose”), 4.50 (m, 4H, C2 “ose”, 4H, C3 “ose”, 4H, C4 “ose”), 4.31
(m, 4H, C6 “ose”), 2.39 (s, 2H, NH).
Spin trapping experiments
Spin trapping measurements were performed on a Bruker EMX
10/12 ESR spectrometer. 10 mM methanol solutions of TEMP
were adjusted to an absorption of 0.3 at 300 nm with the
corresponding porphyrins and irradiated with a halogen lamp in
a flat cell under exactly identical conditions. The integral of the
triplet ESR signal that formed upon singlet oxygen formation was
recorded every five minutes and plotted against the time. No dye
bleaching was observed over the whole irradiation time.
General procedure for the synthesis of protected sugar-substituted
porphyrins. 2.2 mg of pyrrole (1.0 eq.) and 2.2 mmol of the acyl-
protected a-D-carbohydrate benzaldehyde (1.0 eq.) in 45 ml of
methylene chloride were added to 200 ml of a mixture of methylene
chloride containing 0.75% ethanol; the solution was purged with
argon for 10 min. A 0.5 M boron trifluoride-etherate solution
(100 ml) in 2 ml of methylene chloride was added. The mixture
was stirred at room temperature for 16 h, subsequently 0.4 g of
p-chloranil (1.63 mmol, 0.74 eq.) was added and the mixture was
refluxed for 1 h. The solvent was evaporated and the resulting
black oil was purified by flash column chromatography.
Laser flash photolysis
Methanol stock solutions of three sugar porphyrins were adjusted
to an absorption of 0.34 in a standard cuvette. Samples were
measured with a 308 nm laser setup and the relative absorption was
plotted against time. Two series of measurement were performed,
first under standard photooxidation conditions (saturated with
air), then under argon atmosphere in order to avoid potential
interference by molecular oxygen.
General procedure for the deprotection of acetylated sugar-
substituted porphyrins. The protected porphyrin (0.045 mmol)
was solved in 10 ml of dry methanol and 100 ml of a solution
of sodium methanolate in dry methanol (0.1 N) was added. The
mixture was stirred for 2 h at room temperature, then the solvent
was evaporated and the product was purified by Sephadex LH20
column chromatography.
4-(2,3,4,6-Tetraacetyl-b-D-glucopyranosyl)benzaldehyde: 1H-
NMR: (300 MHz, CDCl3), d (ppm) = 9.93 (s, 1H, CHO), 7.84
(d, 2H, o-phenyl, J = 8 Hz), 7.10 (d, 2H, m-phenyl, J = 8 Hz), 5.26
(m, 5H, “ose”), 4.21 (m, 2H, “ose”), 2.05 (s, 12H, COOCH3).
5,10,15,20-Tetrakis[4-(2,3,4,6-tetracetyl-b-D-glucosyl)phenyl]-
porphyrin:29 1H-NMR: (300 MHz, CDCl3), d (ppm) = 8.83 (s,
1H, pyrr.), 8.10 (d, 2H, o-phenyl, J = 8 Hz), 7.35 (d, 2H, m-
phenyl, J = 8 Hz), 5.45 (m, 12H, C-1¢, C-2¢, C-3¢, “ose”), 5.29 (m,
4H, C-4¢, “ose”), 4.40 (m, 8H, C-6¢, “ose”), 4.03 (m, 4H, C-5¢,
“ose”), 2.20 (s, 12H, acetyl), 2.12–1.98 (m, 36H, COOCH3), -2.84
(s, 2H, NH).
5,10,15,20-Tetrakis(4-b-D-glucosylphenyl)porphyrin (P-G):29
1H-NMR: (300 MHz, CDCl3), d (ppm) = 9.01 (s, 8H, pyrr), 8.25
(d, 8H, ortho, J = 8 Hz), 7.78 (d, 8H, meta, J = 8 Hz), 7.94 broad
(4H, OH “ose”), 7.48 broad (4H, OH “ose”), 6.87 broad (4H,
OH “ose”), 6.00 (d, 4H, C1 “ose”, J = 8 Hz), 4.78–4.52 (m, 8H,
C6 “ose”), 4.50 (m, 4H, C2 “ose”, 4H, C3 “ose”, 4H, C4 “ose”),
4.31 (m, 4H, C6 “ose”), -2.39 (s, 2H, NH).
Acknowledgements
The financial support by the Deutsche Forschungsgemeinschaft
(DFG) is acknowledged. J.U. thanks the Alder foundation for a
half-year research stay in Valencia.
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