Sustainable photochemistry: solvent-free singlet
oxygen-photooxygenation of organic substrates embedded in
porphyrin-loaded polystyrene beads†
Axel G. Griesbeck* and Anna Bartoschek
Institute of Organic Chemistry, University of Cologne, Greinstr. 4, D-50939 Köln, Germany.
E-mail: griesbeck@uni-koeln.de
Received (in Cambridge, UK) 26th April 2002, Accepted 10th June 2002
First published as an Advance Article on the web 24th June 2002
A solvent-free photooxygenation process that uses organic
substrates embedded in porphyrin-loaded polystyrene beads
as solid support is described and applied for ene- and
[4+2]-cycloaddition reactions involving singlet oxygen
(1Dg).
media where loading and unloading of substrate and product,
respectively, occurs simply by washing. This approach is
suggested by the zeolite photooxygenation protocol as elabo-
rated by Ramamurthy et al.10 Here, however, the confined
media is highly polar and protic which is disadvantageous for
the singlet oxygen lifetime and the use of nonpolar sensitizers
with high singlet oxygen quantum yields (FD).
Among all processes that are discussed as suitable for
sustainable chemistry, photooxygenation is one of the most
appealing because this reaction uses only visible light, oxygen,
dye and the substrate. Like its natural oxygen-producing
antagonist, photosynthesis, photooxygenation is an archetype of
a green chemical process.1 The type II process involves singlet
oxygen (1Dg), formed by energy-transfer from an electronically
excited dye molecule.2 This process is the cleanest way to
oxygenated products with excellent chemo-, regio- and ster-
eoselectivity pattern and (as one of few photochemical
reactions) is used for industrial applications.3–5 Severe prob-
lems, however, stem from the chemical setup: (a) the dye
contaminates the product and has to be removed after reaction
by chromatography or distillation; (b) solvents show conflicting
properties, i.e. singlet oxygen is short-lived in environmentally
friendly solvents (4 ms in water) and long-lived in problematic
solvents like halogenated hydrocarbons (0.06 s in tetrachloro-
methane);6 (c) pure oxygen is not applicable for industrial
applications and also air-purging is elaborate. Here we show
experimentally that these disadvantages can be circumvented by
using a routine protocol by which porphyrin dyes are embedded
in polystyrene beads, substrate and product are loaded and
extracted by use of environmentally friendly solvents and
visible irradiation is performed under an atmosphere of air.
From a synthetic point of view, the ene-3 and the [4 +
2]-cycloaddition4 reaction are the most important chemical
reactions of organic substrates with singlet oxygen resulting in
the formation of allylic hydroperoxides and endoperoxides,
respectively, important intermediates in the synthesis of
oxyfunctionalized products such as allylic alcohols, epoxy
alcohols, vicinal diols, 1,4-diols, saturated polyols and many
more4,5 The fundamental rules for green chemistry are nicely
achieved with respect to the theoretical framework and have
been exemplarily demonstrated decades ago. In reality, photo-
oxygenation is much less ‘green’ than possible due to solvent
effects, product stability, safety precautions, and sensitizer
recovery and separation, respectively. One solution to the long-
standing problem of sensitizer dye recovery is the use of
covalently polymer-bound singlet oxygen sensitizers such as
the commercially available polystyrene-immobilized rose ben-
gal (Sensitox®)7 and many more. Most sensitizer systems are,
however, not very useful in nonpolar solvents due to severe dye-
bleaching and/or bleeding. A series of classical dyes and also
C60 have been immobilized and used for photooxidation in
aqueous suspensions.8,9 For organic synthesis, these systems
have not yet been applied successfully. An attractive alternative
would be the use of dyes non-covalently embedded in confined
Therefore, we followed another route and used commercially
available‡ polystyrene beads (60 15 mm diameter) crosslinked
with divinylbenzene as solid support. These beads are known to
have easily modifiable space structures which can be controlled
by polymer swelling with an appropriate nonpolar solvent.11 By
this swelling process, the nonpolar sensitizer (meso-arylated
porphyrins) is introduced into the polymer beads (120 25 mm
diameter, after swelling with ethyl acetate) and, due to its
extreme low solubility in more polar solvents, is fixed in the
polymer network. The polystyrene beads were loaded with the
sensitizer by swelling with a solution of catalytic amounts
meso-tetraphenylporphyrin (TPP) or tetratolylporphyrin (TTP)
in EtOAc with subsequent evaporation of the excess solvent.
Subsequently, the beads were treated with a solution of the
substrate dissolved in a minimum amount of ethyl acetate and
by evaporation of the excess transfer solvent, a layer of sandy
solid is obtained that was irradiated in a loosely covered petri
dish by means of a sodium street lamp or a halide lamp,
respectively (Fig. 1). It is worth mentioning, that the dye is
nearly insoluble in most substrates investigated herein.
By repeated washing with ethanol, the product was extracted
from the polymer beads. The dyestuff stayed nearly completely
in the solid support and substrate loading could be repeated. We
investigated two modes of singlet oxygen reactions, ene- and [4
+ 2]-cycloadditions and checked whether the reactivity and
selectivity behaviour is comparable to solution photochemistry.
The highly reactive substrates a-pinene (1) and the alcohol from
sorbic acid (3) gave, in excellent yields, the singlet oxygen
products 2 and 4, respectively. The regiochemical sensitive
probes 1-methylcyclohexene (5) and citronellol (7) gave the
hydroperoxide mixtures 6a–c and 8a,b in the same composition
(by NMR of the crude reaction mixture) as in nonpolar solvents
† Dedicated to Professor Waldemar Adam on the occasion of his 65th
birthday and his retirement from the stage of photooxygenation chem-
istry.
Fig. 1 Porphyrin- and citronellol loaded polystyrene beads irradiated by a
halide lamp.
1594
CHEM. COMMUN., 2002, 1594–1595
This journal is © The Royal Society of Chemistry 2002