Strategies for cleaner oxidations using photochemically generated singlet oxygen in supercritical carbon dioxide

Article abstract of DOI:10.1039/b914074c

Photochemically generated singlet oxygen, ¹O₂, has been reacted with four different substrates in supercritical carbon dioxide. By using fluorous surfactants and a co-solvent to solubilise more polar photosensitisers and reactants respectively, the applicability of the system is greatly enhanced. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b914074c

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PAPER
www.rsc.org/greenchem | Green Chemistry
Strategies for cleaner oxidations using photochemically generated singlet
oxygen in supercritical carbon dioxide
Xue Han, Richard A. Bourne, Martyn Poliakoff* and Michael W. George*
Received 14th July 2009, Accepted 12th August 2009
First published as an Advance Article on the web 7th September 2009
DOI: 10.1039/b914074c
Photochemically generated singlet oxygen, 1O2, has been reacted with four different substrates in
supercritical carbon dioxide. By using fluorous surfactants and a co-solvent to solubilise more
polar photosensitisers and reactants respectively, the applicability of the system is greatly
enhanced.
We have recently reported the rapid and quantitative con-
Introduction
version of a-terpinene to ascaridole in scCO2 using a CO2-
Photochemistry is a somewhat neglected tool in the green
soluble photosensitiser, 5,10,15,20-tetrakis(pentafluorophenyl)-
chemistry area1 even though the environmental importance
porphyrin (TPFPP), Scheme 1.20 Our studies showed that under
of photochemistry has a long history.2,3 The photochemical
single phase conditions, between 120 and 200 bar, complete
reactions of singlet oxygen, 1O2, are particularly appealing in this
context since they combine the green aspect of photochemistry
conversion of 30 mL of a-terpinene could be achieved in 160 s
with zero-order kinetics at a constant turnover frequency (TOF)
1
1
with the high E-factor of O2 addition reactions. O2 is widely
used in contemporary organic synthesis.4 It can be generated
chemically using sodium hypochlorite and hydrogen peroxide5
or Ph3PO3;6 however by far the most convenient, and cleanest
route, is via photosensitisation using low energy visible light and
an appropriate catalytic photosensitiser such as rose bengal.4
1O2 reacts with 1,3-dienes to form endoperoxides or with
alkene substrates to produce allylic hydroperoxides which can
also be easily reduced to the corresponding allylic alcohol.7
over this pressure range.
Scheme 1 Photo-oxidation of a-terpinene to ascaridole.
1
Although at least one photochemical O2 reaction is carried
out industrially, there are significant problems associated with
the production and use of 1O2 particularly in a large scale
since such reactions are commonly performed in chlorinated
solvents such as CHCl3 or CCl4 which are not industrially
viable and significantly detract from the environmental sus-
tainability of any process using 1O2.8 Chlorinated solvents
are utilised since they are non-flammable and the lifetime of
1O2 in these solvents is long compared to other conventional
solvents.9–11
Supercritical carbon dioxide (scCO2) is fully oxidised and
hence non-flammable and non-toxic. It is completely miscible
with permanent gases such as O2 and possesses lower viscosity
and higher diffusivity than more conventional solvents. This
can enable reactions to proceed without the mass transport
limitations often found in typical multi-phase reaction systems.12
Metal catalysed oxidation reactions using O2 and metal
catalysts13–16 have been successfully performed in scCO2 and
demonstrated these advantages. Photophysical experiments17–19
have shown that 1O2 can be generated in scCO2 with an extended
lifetime compared to many conventional solvents.
Following the initial batch studies, an efficient LED illumi-
nated sapphire tube reactor has been developed for continuous
operation with a significantly enhanced space-time yield when
compared to conventional 1O2 reactor systems.21
TPFPP is a fluorinated analogue of meso-tetraphenylpor-
phyrin (TPP), a common 1O2 photosensitiser, Scheme 2. It was
chosen for these systems because of its high solubility in scCO2
due to the fluorinated phenyl-substituted groups. Thus both
the reagents and the photosensitiser were soluble in the scCO2.
However, there are many oxidation reactions of 1O2 where one
or more components may not be sufficiently soluble in scCO2.
This limitation needs to be addressed if the use of scCO2 as a
solvent for 1O2 is to become widespread.
This paper initially focuses on expanding the scope of
reactions that can be performed with photochemically generated
1O2 in scCO2. Then the use of a co-solvent and surfactants in
scCO2 to solubilise reactants and alternative photosensitisers
respectively are described.
Results and discussion
Oxidation of 2,3-dimethyl-2-butene
School of Chemistry, University of Nottingham, University Park,
Nottingham, UK NG7 2RD. E-mail: martyn.poliakoff@nottingham.
ac.uk, mike.george@nottingham.ac.uk; Fax: +44 155 9513508;
Tel: +44 115 953512
1
We have examined the scCO2 reaction of O2 with an alkene
substrate to demonstrate the applicability of scCO2 to a Schenk
“ene” type [2 + 2] reaction. This is a common reaction of
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Scheme 2 Structures of photosensitisers investigated in these studies.
Fig. 1 (a) FTIR monitoring of the reaction of 2,3-dimethyl-2-butene with O2 in scCO2 using TPFPP at 40 ◦C during irradiation (300 W xenon lamp
with a filter to cut out UV irradiation). Spectra were recorded at 20 s intervals showing (b) loss of starting material (2737 cm-1) and (c) formation of
peroxide (3098 cm-1).
1O2 which has been widely used for the synthesis of allylic
hydroperoxides and related compounds such as reduction to
allylic alcohols.7 Fig. 1 shows that quantitative conversion of
2,3-dimethyl-2-butene to 3-hydroperoxy-2,3-dimethylbut-1-ene
(Scheme 3) can be achieved using TPFPP as photosensitiser with
2.63 mol% O2 in scCO2 within 250 s. The yield was confirmed
upon the nature of the solvent.22 Therefore it is an excellent
reaction with which to evaluate scCO2 as a solvent. We have
examined the photo-oxidation of MCH in scCO2 using TPFPP
as the photosensitiser with 2.63 mol% O2 in scCO2 at 140 bar.
Monitoring of the reaction was again by FTIR and plotting
of the band maxima at 3081 cm-1, 3047 cm-1 and 3034 cm-1
versus irradiation time, Fig. 2, again showed zero-order kinetics
reaction with complete conversion after 400 s with a TOF
of 9 s-1.
The reaction of 1-methyl-cyclohexene with 1O2 results in the
production of both secondary and tertiary allylic hydroperox-
ides, see Scheme 4. The yield of each product was calculated from
1H-NMR spectra and is shown in Scheme 4. The reaction in
scCO2 shows similar regioselectivity to reactions that have been
previously performed in non-polar n-alkane solutions.22 This is
consistent with other areas of supercritical chemistry which have
concluded that scCO2 often behaves similarly to n-hexane.23
1
by H-NMR spectroscopy. The reaction was monitored using
FTIR spectroscopy and the reaction kinetics were followed
using the intensity of the 2737 cm-1 and 3098 cm-1 bands,
Fig. 1(b) and 1(c). The reaction was found to proceed with
zero-order kinetics under these conditions. The TOF of this
reaction, 14 s-1 is of similar magnitude to that for the TPFPP
photo-catalysed oxidation of a-terpinene, suggesting that the
key rate-determining step is the photo-generation of 1O2 rather
than the reaction with the organic substrate.
Scheme 3 The Schenk “ene” addition of 1O2 to 2,3-dimethyl-2-butene
to form 3-hydroperoxy-2,3-dimethylbut-1-ene.
Photo-oxidation of 1-methyl-cyclohexene
Scheme 4 Photo-oxidation of 1-methyl-cyclohexene to yield (from
left to right) 3-hydroperoxy-3-methylcyclohex-1-ene, 6-hydroperoxy-1-
methylcyclohex-1-ene and 1-hydroperoxy-2-methylenecyclohexane.
The product distribution following photo-oxidation of 1-methyl-
cyclohexene (MCH) has been reported to be highly dependant
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Fig. 2 (a) FTIR monitoring the reaction of MCH with O2 in scCO2 at 40 ◦C using TPFPP at 140 bar during irradiation (300 W xenon lamp with a
filter to cut out UV irradiation), (Table 1, entry 11). Spectra were recorded at 40 s intervals. The kinetic traces show changes in intensity with time at
band maxima at (b) 3034 cm-1, (c) 3047 cm-1 and (d) 3081 cm-1. All three traces follow zero-order kinetics.
Oxidation of 1-naphthol with dimethyl carbonate co-solvent
will investigate the effect of co-solvents and the potential use
of gas expanded liquids (GXLs27) to optimise these reaction
systems further.
The use of scCO2 is often hampered by the poor solubility of
organic compounds with a polar protic character. The solubility
of polar materials can often be dramatically increased by use
of a co-solvent.24 We have tried using a co-solvent in scCO2
for oxidation using 1O2 of 1-naphthol to naphthalene-1,4-dione
(see Scheme 5). This reaction is of particular interest because
of the biological activity possessed by the quinonoid class of
natural products,25 which are derivatives of Juglone (5-hydroxy-
1,4-naphthoquinone) formed by the photo-oxidation of
1,5-dihydroxynaphthalene.26
Use of fluorous surfactants to solubilise photosensitisers
Porphyrin-based molecules are some of the most widely used
1O2 photosensitisers. Although it is possible to dissolve the
perfluorinated TPFPP in pure scCO2, TPFPP is not ideal,
particularly since its cost may be prohibitive for widespread
application. TPFPP has not been as widely used for 1O2 reactions
as the more common photosensitisers, methylene blue, rose
bengal and TPP, Scheme 2, but these are insoluble in scCO2.
However, there are a variety of strategies for enhancing solubility
in scCO2 including the use of co-solvents (as above). An
alternative approach is to use fluorinated surfactants, which
has been previously used for polymer synthesis,28,29 nanoparticle
formation,30 biocatalysis,31 Lewis acid catalysis,32 and cobalt
catalysed oxidation.33 Here we investigate two highly fluorinated
surfactants, Krytox and Fluorolink (see Scheme 6) for solubilis-
ing scCO2-phobic photosensitisers to promote 1O2 reactions in
scCO2.
Scheme 5 Photo-oxidation of 1-naphthol to form naphthalene-1,4-
dione.
We have previously demonstrated that dimethyl carbonate can
be used as a co-solvent to enhance the solubility of TPFPP in
citronellol in continuous photo-oxidation, allowing the substrate
and photosensitiser to be pumped continually.21
Thus, photo-oxidation of 1-naphthol was performed with
dimethyl carbonate as a co-solvent to liquefy the substrate and
to enhance the solubility of the substrate in scCO2. The volume
of the batch reactor was increased by changing the path length
to 15 mm to enable 500 mL of a solution consisting of dimethyl
carbonate containing 7 mg of 1-naphthol and 0.2 mg of TPFPP
to be injected into the cell and pressurised to 160 bar with a
mixture of O2 and scCO2 (1.31 mol% O2 in CO2).
The yield was calculated using gas chromatography because
absorptions of the dimethyl carbonate co-solvent swamp the
FTIR spectrum and the reaction reached completion within
1 h. The TOF was much lower than for the reactions performed
previously, probably due to the higher dilution used and
quenching effects caused by dimethyl carbonate. Future studies
Scheme 6 Molecular formulae of the fluorinated surfactants.
Fig. 3 shows the UV/visible spectra of a reaction mixture of
a-terpinene, TPP, and O2 in scCO2 with and without the addition
of Krytox. In the absence of the surfactant, TPP was essentially
insoluble in scCO2 and the characteristic UV/visible bands of
TPP were not observed. The addition of 3.2 wt% of Krytox into
the reaction system resulted in both the Soret band and Q bands
of TPP becoming quite intense.
The kinetics of the reaction of a-terpinene were monitored by
FTIR using TPP with surfactants or TPFPP. Both surfactant
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Fig. 4 Kinetic traces showing the zero-order kinetics of the reaction
of a-terpinene with O2, in scCO2 at 40 ◦C during irradiation (300 W
xenon lamp with a filter to cut out UV irradiation) with (a) TPFPP, (b)
TPP plus Fluorolink surfactant and (c) TPP plus Krytox surfactant by
plotting the normalised absorbance at 3042 cm-1. See Table 1 for full
experimental details.
Fig. 3 UV spectra of the reaction system composed of a-terpinene,
TPP, and O2 in scCO2 (Table 1, entry 2), with (a) and without (b) the
addition of Krytox surfactant.
systems gave quantitative conversion in <7 min with zero-order
kinetics. Despite differing loadings of the various photosensitis-
ers, the molar TOFs of the TPP–surfactant systems were similar
ranging for Krytox (TOF = 9 s-1), Fluorolink (TOF = 10 s-1)
and TPFPP (TOF = 12 s-1), see Fig. 4 and Table 1, entries 1–3.
A control experiment showed that the addition of Fluorolink
itself had no adverse effect upon the reactivity of TPFPP with
no decrease in the TOF of the reaction, see Table 1, entry 4.
These results demonstrate that even though TPP is essentially
insoluble in scCO2, it can still act as an efficient photosensitiser
in the presence of suitable surfactants to enhance its solubility.
We have also investigated our approach using Krytox and
Fluorolink surfactants with two commonly used ionic photo-
sensitisers, methylene blue and rose bengal, see Scheme 2, which
are again insoluble in pure scCO2.34,35
conditions. However, solubility of a photosensitiser in scCO2
does not in itself ensure efficient photo-oxidation reactions.
1
One major problem with dye-sensitised O2 reactions is the
bleaching of the dye. This is not a problem using the TPP and
TPFPP photosensitisers as indicated by the zero-order kinetics.
By contrast, the oxidation of a-terpinene using methylene blue
in scCO2 with the addition of 2.8 wt% Krytox gave incomplete
conversion (Table 1, entries 7 and 8). The reason for this is
demonstrated by the UV spectra in Fig. 6 which show that
complete photo-bleaching of methylene blue occurs before
conversion of the organic substrate was complete; hence this
reaction did not follow simple zero-order kinetics.
The photo-oxidation of 2,3-dimethyl-2-butene described
above, provided an additional opportunity to compare directly
the TPFPP system with the TPP and surfactants system via
FTIR monitoring of the conversion to 3-hydroperoxy-2,3-
dimethylbut-1-ene. Addition of TPP and Krytox to the reaction
system resulted in quantitative conversion with zero-order
Solubility trials were conducted using a high pressure view
cell. The solubility of methylene blue could be increased using a
surfactant in scCO2 as evident by the striking blue colour of the
solution, shown in Fig. 5. Unfortunately, it was not possible
to dissolve high concentrations of rose bengal under these
Table 1 Experiments with surfactants to solubilise photosensitisers at 40 ◦C during irradiationa
Oxygen
concentration
(mol%)
Substrate
Photosensitiser
concentration/
mol L-1
System
pressure/bar
concentration/
Entry
Substrate
Photosensitiser
Surfactant
mol L-1
TOF/s-1
1
2
3
4
5
6
7
8
9
a-Terpinene
a-Terpinene
a-Terpinene
a-Terpinene
a-Terpinene
a-Terpinene
a-Terpinene
a-Terpinene
DMB
TPFPP
TPP
TPP
TPFPP
Rose bengal
Rose bengal
Methylene blue
Methylene blue
TPFPP
TPP
TPFPP
None
Krytox
Fluorolink
Fluorolink
Krytox
Fluorolink
Krytox
Fluorolink
None
Krytox
None
Krytox
1.31
1.31
1.31
1.31
1.31
1.31
1.31
1.31
2.63
2.63
2.63
2.63
140
140
140
140
140
140
140
140
140
140
140
140
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.24
0.32
0.32
0.32
0.32
0.158
0.079
0.079
0.158
0.81
0.73
0.82
0.76
0.079
0.13
0.089
0.13
12
9
10
12
0
0
N/Ab
N/Ac
14
11
10
3
10
11
12
DMB
MCH
MCH
TPP
a 300 W xenon lamp with a filter to cut out UV irradiation. b Due to photobleaching non zero-order kinetics observed. Reaction yield = 56%. c Due
to photobleaching non zero-order kinetics observed. Reaction yield = 77%.
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lower TOF of 3 s-1 compared to 9 s-1 with TPFPP, Table 1, entries
11 and 12.
Conclusions
In summary, the scCO2-soluble photosensitiser TPFPP has been
used to perform 1O2 reactions of a-terpinene, 2,3-dimethyl-
2-butene and 1-methyl-cyclohexene in scCO2. These three re-
actions were all monitored using FTIR, showing zero-order
kinetics and quantitative conversions with reasonable TOFs. The
insoluble photosensitisers, TPP, methylene blue and rose bengal,
were also evaluated with two highly fluorinated surfactants,
1
Krytox or Fluorolink, to generate O2 for reaction. TPP and
methylene blue display considerably enhanced solubility in
scCO2 in the presence of these surfactants. However, methylene
blue undergoes photo-bleaching during irradiation; hence the
reactions do not reach full conversion. TPP plus a fluorinated
surfactant showed very similar results to those achieved with
TPFPP albeit with slightly lower TOFs. The solubility of
rose bengal in scCO2 is still very low even with the addition
of surfactants. Finally photo-oxidation of a non-soluble sub-
strate, 1-naphthol, was also tested using dimethyl carbonate as
co-solvent to liquefy and solubilise the substrate into scCO2.
Quantitative conversion was achieved, but with a lower TOF. It
is clear that the applicability of 1O2 reactions in scCO2 has
been significantly enhanced by the use of these surfactants and
co-solvents.
Fig. 5 Photographs of different photosensitisers in a variable volume
view cell.36 (a) TPFPP (2.7 mmol with no surfactant) showing full
solubility, (b) TPP (1.8 mmol) showing full solubility (345 mg Krytox),
(c) methylene blue (8.4 mmol) showing high solubility (216 mg Krytox),
(d) rose bengal (7.3 mmol) showing negligible solubility (190 mg Krytox).
All photographs taken with 200 bar scCO2 at 40 ◦C and 0.5 g of
a-terpinene.
Experimental
Safety warning: these reactions involve high pressures and should
only be carried out in an apparatus with the appropriate pressure
rating and with due regard to the potentially explosive reaction
between O2 and organic compounds.
All reactions were performed using a multi-purpose photo-
chemical and spectroscopic cell,37 with CaF2 windows and a
path length of ca. 2 mm unless otherwise noted. The cell was
heated to 40 ◦C in all experiments using aluminium heating
blocks. For a schematic of the reactor please see our previous
publication.20
Acknowledgements
We thank EPSRC(grant no. EP/FO15275/1) and Thomas Swan
& Co. Ltd. for support. We also thank M. Dellar, M. Guyler,
D. Litchfield, R. Wilson and P. Fields for technical support at
the University of Nottingham. M. W. G. gratefully acknowledges
receipt of a Wolfson Merit Award from The Royal Society.
Fig. 6 UV/visible spectra of methylene blue in scCO2 with Krytox
system showing steady photo-bleaching during the photo-oxidation of
a-terpinene over a period of 37 min.
Notes and references
kinetics and a high TOF of 11 s-1. As with the a-terpinene
system, a slight drop in the TOF, from 14 s-1 to 11 s-1 was
observed when the TPP–surfactant rather than TPFPP was used
for the oxidation of 2,3-dimethyl-2-butene, see Table 1, entries
9 and 10. However this reaction also clearly demonstrates the
potential applicability of the significantly less expensive TPP
plus surfactant. In addition, the photo-oxidation of MCH was
also performed using the TPP and Krytox surfactant system;
however the TPP in this reaction system exhibited a significantly
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