Synthetic application of photoactive porous monolithic polymers

Article abstract of DOI:10.1016/j.tetlet.2010.04.065

The oxidation of 2-furoic acid to 5-hydroxy-5H-furan-2-one has been accomplished in quantitative yield in chloroform using a novel supported photocatalyst. This material comprises Rose Bengal grafted to the surface of a highly crosslinked polystyrene-divinylbenzene polymer, which was synthesized in a porous monolithic format.

Full text of DOI:10.1016/j.tetlet.2010.04.065

Tetrahedron Letters 51 (2010) 3360–3363
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Tetrahedron Letters
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Synthetic application of photoactive porous monolithic polymers
*
*
M. Isabel Burguete, Raquel Gavara, Francisco Galindo , Santiago V. Luis
Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Av. Sos Baynat, s/n, E-12071 Castellón, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
The oxidation of 2-furoic acid to 5-hydroxy-5H-furan-2-one has been accomplished in quantitative yield
in chloroform using a novel supported photocatalyst. This material comprises Rose Bengal grafted to the
surface of a highly crosslinked polystyrene-divinylbenzene polymer, which was synthesized in a porous
monolithic format.
Received 17 March 2010
Revised 13 April 2010
Accepted 14 April 2010
Available online 28 April 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Photocatalysis
Singlet oxygen
Oxidation
Rose Bengal
Selective oxygenations are key steps in many organic syntheses,
and methods to achieve them in a quantitative, selective, clean,
cheap, and sustainable way are continuously under development.1
Photocatalytic approaches are gaining increasing acceptance pro-
vided they use light to trigger the reaction processes and hence al-
low the possibility to carry out oxidation in a more controllable
manner than conventional thermal methods. In this regard, the uti-
lization of molecular oxygen in combination with a photosensitizer
is currently in commonplace in many synthetic laboratories.2
Among the photocatalytic processes using oxygen, triplet sensiti-
zation of ground-state triplet oxygen to produce excited-state sin-
glet oxygen (1O2 1Dg) is one of the most popular reactions since 1O2
has a well-known reactivity as an electrophile.3
One of the drawbacks of using a photocatalyst is the need for
removing it from the reaction medium once the oxidation is fin-
ished, which is typically done by means of a chromatographic sep-
aration. In order to improve this purification step, several solid-
supported photocatalysts for the generation of 1O2 have been de-
scribed, which allow the removal of the catalyst by means of a sim-
ple filtration or centrifugation.4 One of the most employed
photocatalysts of this type is Rose Bengal (a well-known 1O2 gen-
erator)5 covalently linked to a low crosslinked polystyrene matrix
(Merrifield resin),6 or other supports.7 However, to date, the utili-
zation of the soluble sensitizer (Rose Bengal disodium salt) is pre-
ferred in the majority of photooxygenations described in the
literature.
from the gel-type resins in that it has been synthesized in the for-
mat of a porous monolithic polymer, which implies higher cross-
linking degrees and lower particle sizes than the classical gel-
type support. The studies carried out so far showed higher efficien-
cies for the bleaching of 9,10-diphenylanthracene as well as a po-
sitive photodynamic action against melanoma cancer cells.8 As
continuation of such a project, we were interested in exploring
the synthetic photocatalytic potential of this new class of porous
polymers for the oxidation of a substrate of synthetic interest,
especially in comparison with the classical gel-type photocatalyst.
For such purpose we selected the oxidation of 2-furoic acid 1 to
butenolide 2 (5-hydroxy-5H-furan-2-one) as the benchmark reac-
tion (Scheme 1). Butenolide 2 is an important starting substrate
for the synthesis of insecticides, prostanoids, alkaloids, etc. A long
list of applications of 2 can be found in the literature.9 Several
strategies for the syntheses of 2 have been reported so far, which
demonstrates the importance of this starting material.10 Moreover,
the oxidation of furans to butenolides is the final key step in a great
number of described total syntheses,11 but typically using soluble
photosensitizers.
The oxidation of 1 to 2 was carried out using four different
photosensitizers. The employed photocatalysts were Rose Bengal
We have recently reported a polystyrene-derived photosensi-
tizer that utilizes Rose Bengal as a photoactive agent, which differs
* Corresponding authors. Tel.: +34 964 728239; fax: +34 964 728214.
E-mail addresses: francisco.galindo@uji.es, luiss@uji.es (F. Galindo).
Scheme 1.
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.04.065

M. I. Burguete et al. / Tetrahedron Letters 51 (2010) 3360–3363
3361
disodium salt (RB), a conventional low crosslinking (1%) gel-type
polystyrene with attached RB (PGel), and two porous resins differ-
ing in their degree of crosslinking: P20 and P80, with 20% and 80%
crosslinking degrees, respectively. All the photosensitive polymers
were prepared by reaction of the corresponding chloromethylated
matrices with RB in dry DMF (see details in the Supplementary
data).12 They were characterized by means of mercury intrusion
porosimetry, Infrared and Raman spectroscopies,13 scanning
electron and fluorescence microscopies and diffuse reflectance
UV–vis spectroscopy. The content of RB grafted onto the surface
of each polymer was determined by basic hydrolysis of such RB
moieties and UV–vis measurement of the hydrolysates. Figures 1
and 2 show some selected microscopic and spectroscopic features
of photocatalyst P20, as an illustrative example. Table 1 collects
pertinent characterization data.
For the oxidation of 2-furoic acid 1 to (5-hydroxy-5H-furan-2-
one) 2, each photosensitizer was added to a solution of substrate
([1] = 3 Â 10À2 M) in either MeOH or CHCl3, equilibrated with air.
Such solutions were irradiated with a 125 W medium pressure
Hg vapor lamp for 6 h, with continuous agitation (in open reaction
tubes). The reactions were monitored by UV–vis following bleach-
ing of the characteristic absorption band of 1 at 247 nm (note that
2 does not absorb at such wavelength). At the end of the irradiation
period the solutions were filtered, concentrated, and analyzed by
1H NMR confirming the formation of 2 in different yields. Table 2
summarizes the reaction yield of 2 for the different reactions.
As can be seen from Table 1, the main differences between the
photocatalysts are the degree of crosslinking, the RB loading, the
pore diameter of the corresponding monolith, and the particle size
Figure 2. Representative spectroscopic characterization of photocatalyst P20. (A)
Excitation (left) and emission (right) spectra of solid P20 with em: 620 nm and exc:
550 nm, respectively. (B) idem with Rose Bengal (10 lM) in methanol, for
comparative purposes (note the spectral shift between RB supported and in
solution). (C) Diffuse–reflectance UV–vis spectrum of P20.
Table 1
Supported photocatalysts properties
Property
PGel
1
P80
80
P20
20
% Crosslinking
Particle sizea
(
lm)
50–110
1–10
1.16
5–35
10.95
Mean pore diamaterb
Abs. max.c (nm)
(lm)
—
n.d.e
574, 400–550
(broad)
579, 400–550
(broad)
RB loadingd
(l
mol/g)
150
2–3
2–3
a
Microscopic estimation of the aggregates (in the case of porous polymers each
aggregate is composed of several microspheres of 1–2 m).
Determined by mercury intrusion porosimetry from the corresponding mono-
l
b
lithic polymer.
c
d
e
Diffuse reflectance UV–vis.
Hydrolysis and UV–vis determination of RB.
Not determined due to signal saturation.
of the aggregates. Despite attempts to increase the loading of RB
above 3 mol/g (for the porous matrices), it remained elusive. Nev-
l
ertheless, this feature seems to have a positive effect on the photo-
catalytic performance of the photosensitizers, especially for P20, as
it will be shown later. The reason for such difficulty in the attach-
ment of a higher number of photosensitizer molecules onto the
surface of P80 and P20 can be found in the limited accessibility
of solvent and RB to the chloromethylated groups in a highly cross-
linked resin, in striking contrast to the much more swellable gel-
type polymer PGel. The morphology of the porous photocatalysts
(Fig. 1) is in accordance with that described for analogous matrices,
used mainly for chromatographic14 or catalytic applications.15 It is
Figure 1. Representative microscopic characterization of photocatalyst P20. Top
image: optical microscopy (fluorescence). Bottom image: scanning electron
microscopy.

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M. I. Burguete et al. / Tetrahedron Letters 51 (2010) 3360–3363
Table 2
Supplementary data
Photooxidation of 1 to 2 (photosensitized oxidation)a
Entry
Photocatalyst
Solvent
Yield of 2 (%)
Supplementary data associated with this article can be found, in
1
2
3
4
5
6
7
8
RBb
MeOH
MeOH
MeOH
MeOH
CHCl3
CHCl3
CHCl3
CHCl3
79
70
68
71
100
65
the online version, at doi:10.1016/j.tetlet.2010.04.065.
PGelc
P80c
P20c
RBb
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In summary, the utilization of photochemically active porous
matrices for the synthesis of butenolide 2, from furoic acid 1, has
been demonstrated. This kind of materials represents a practical
alternative to both gel-type resins and soluble RB. Current work
is in progress in order to know the mechanistic details of the ob-
served enhanced reactivity and to expand the number of porous
polymers used as supports.
12. Summarized preparation of polystyrene matrices: The monomer/porogens
mixture (4/6 w/w) with 1 wt % of AIBN (respect to monomers) was purged
with N2 and heated at 80 °C for 24 h. The resulting polymer was washed with
THF in a Soxhlet apparatus for 24 h and dried in a vacuum oven. Summarized
procedure for the grafting of RB: The chloromethylated resin (1.00 g) and Rose
Bengal sodium salt (1.07 g) were reacted in DMF (25 ml) at 60 °C for 20 h. Then,
the mixture was filtered through a sintered-glass funnel and the resin was
washed with 100 ml portions of several solvents and finally with methanol in a
Soxhlet apparatus. Finally the polymer was dried in a vacuum oven.
Acknowledgments
Financial support from the Spanish MICINN (CTQ2008-04412/
BQU) and F. Caixa Castelló-UJI (project P1-1B-2009-58) is acknowl-
edged. F.G. and R.G. thank the support from MICINN (Ramón y
Cajal and FPI programs, respectively).

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