Green photochemistry: The use of microemulsions as green media in photooxygenation reactions

Article abstract of DOI:10.1039/c004869k

The use of 'green' microemulsions of ethyl acetate and water, using sodium dodecyl sulfate and alcoholic co-surfactants as a reaction medium, in photooxygenation reactions was investigated. This work looked at the optimisation of the microemulsion (the optimum ratio of components), an investigation of the effect of changing co-surfactant and optimisation of the reaction work-up for the synthesis of 5-hydroxy-1,4-naphthoquinone, Juglone. Isolated yields of 36-88% were achieved in just 4 h of irradiation. Microemulsions also allowed the usage of the sensitiser tetraphenylporphyrin (TPP) in a benign environment. The optimised procedure was furthermore applied to the synthesis of 5-amido-1,4-naphthoquinones, and moderate yields were achieved.

Full text of DOI:10.1039/c004869k

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www.rsc.org/greenchem | Green Chemistry
Green photochemistry: the use of microemulsions as green media in
photooxygenation reactions
a
a
a
b
Emma E. Coyle, Kieran Joyce, Kieran Nolan and Michael Oelgem o¨ ller*
Received 1st April 2010, Accepted 8th July 2010
DOI: 10.1039/c004869k
4
a–c
10
11
The use of ‘green’ microemulsions of ethyl acetate and
water, using sodium dodecyl sulfate and alcoholic co-
surfactants as a reaction medium, in photooxygenation
reactions was investigated. This work looked at the op-
timisation of the microemulsion (the optimum ratio of
components), an investigation of the effect of changing
co-surfactant and optimisation of the reaction work-up
for the synthesis of 5-hydroxy-1,4-naphthoquinone, Ju-
glone. Isolated yields of 36–88% were achieved in just
use of alcohols,
ionic liquids, solvent-free synthesis and
12
supercritical carbon dioxide, we chose to use microemulsions
as green reaction media.
13
Microemulsions are thermodynamically stable mixtures of
oil (organic solvents), water, surfactants and co-surfactants,
in which the surfactant surrounds microdroplets of water in
the organic phase. The mixtures appear homogeneous but
when examined microscopically are seen to consist of nano-
sized aqueous droplets in the oil phase. Holmberg has reviewed
organic reactions in microemulsions, and outlined the advan-
14
4
h of irradiation. Microemulsions also allowed the usage
14a
tages and disadvantages of this reaction medium. Advantages
include increased solubilising ability (to overcome incompatibil-
ity problems), increased reaction rate compared to traditional
synthesis and the ability to induce regioselectivity. A down-side
to these mixtures is the use of surfactant in large quantities,
which may be overcome through its recovery and recycling
during purification. The use of microemulsions in oxygenation
of the sensitiser tetraphenylporphyrin (TPP) in a benign
environment. The optimised procedure was furthermore
applied to the synthesis of 5-amido-1,4-naphthoquinones,
and moderate yields were achieved.
Introduction
15
reactions has been demonstrated previously by Nardello et al.,
Over the past few years, there has been a significant growth
in interest in green chemistry, and its principles in academia
where singlet oxygen was generated in the “dark” from hydrogen
peroxide using a sodium molybdate catalyst. The procedure has
1
and industry. One of these approaches is use of light as a
15f
been scaled-up for the synthesis of a rose oxide precursor.
Photooxygenation reactions of the ‘Schenck’ ene-type have also
2
clean reagent. We have been active in this research area and
have demonstrated the feasibility of photochemical syntheses
for a number of commodity chemicals. Our work has looked
15b
been demonstrated using a series of allylic alcohols, and
were found to give different chemo- and diastereoselectivity to
the photochemically-generated singlet oxygen reactions. Green
microemulsions have also been reported, in which ethyl acetate
is used as the organic component, instead of a halogenated
3
at green photoacylation reactions, as well as the synthesis of
4
naphthoquinones through photooxygenation reactions. In par-
ticular, we have examined the synthesis of 1,4-naphthoquinones,
which are important natural products that serve as valuable
building blocks in synthesis and are key moieties in biologically-
15a
organic solvent. These have demonstrated good singlet oxygen
lifetimes and a suitability for use in oxygenation reactions. In
addition, Griesbeck and co-workers have studied the selectivity
of the Schenck-ene photooxygenation of an g,d-unsaturated
5
active compounds. A well established route to these compounds
6,7
is through the oxidation of the corresponding 1-naphthols,
but the known thermal pathways demonstrate disadvantages
16
ketone in microemulsion compared to acetonitrile. This study
shows that the use of microemulsions provide an environmental
influence that may affect the selectivity of reactions.
6
regarding selectivity, sustainability and scale-up. Dye-sensitised
photooxygenations represent a useful alternative, and various
4,8
examples have been reported in the literature. Our research
has looked at the development of an entirely green procedure for
dye-sensitised photooxygenation through the use of the solvent
9
Results and discussion
selection criteria developed by Pfizer for the photochemical key-
step and subsequent purification. The original irradiation proce-
dures require the use of the hazardous solvents dichloromethane,
acetonitrile or methanol (or their mixtures). After an extensive
To demonstrate the use of microemulsions as green me-
dia for photooxygenation reactions, the conversion of 1,5-
dihydroxynaphthalene (1) to Juglone (2, 5-hydroxy-1,4-
naphthoquinone) was selected as a model reaction (Scheme 1).
Under conventional conditions, i.e. using alcohols as reaction
solvents, 2 has been obtained in yields up to 58% using artificial
8
investigation of alternate media, in particular looking at the
a
4c
Dublin City University, School of Chemical Sciences and NCSR, Dublin
light illumination.
9
, Ireland
Green microemulsions of water in ethyl acetate, using sodium
dodecyl sulfate and an alcoholic co-surfactant, were investigated
and shown to be suitable solvents for the dye-sensitised pho-
tooxygenation. These exhibited an excellent solubilising ability
b
James Cook University, School of Pharmacy and Molecular Sciences,
Townsville, QLD 4811, Australia.
E-mail: michael.oelgemoeller@jcu.edu.au; Fax: +61 7 4781 6078;
Tel: +61 7 4781 4543
1
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being more volatile and therefore offering ease of removal when
compared to high boiling n-butanol or t-amyl alcohol. This is
consistent with previous reports by Aubry, stating that the use of
isopropyl alcohol offers similar advantages over n-butanol. The
optimum ratio of sodium dodecyl sulfate–ethanol–ethyl acetate–
water was found to be 3.1/7.9/73.3/15.7% by weight (Table 1,
entry 10). Microscopy (Leica TCS, 40¥ and 63¥ dry lenses) of the
solutions showed that less than 1% of the solution consisted of
aggregates greater than 1 mm, indicating that nanosized droplets
were indeed formed.
Scheme 1 Synthesis of Juglone 2.
for all the reagents, which is attributed to the presence of both
hydrophilic and hydrophobic regions in the medium, as depicted
in Fig. 1.
Following the identification of stable microemulsions, their
stability in the presence of starting materials and sensitisers was
assessed. This was achieved by the addition of 1 (1 mmol) and a
sensitiser (rose bengal, methylene blue or tetraphenylporphyrin,
25 mmol) to the microemulsions (20 ml). The addition of these
substances did not impact on the stability of the microemulsions.
Despite the ionic nature of both rose bengal and methylene
blue, the microemulsions remained stable, thus indicating their
suitability for photooxygenation experiments. To confirm that
the photooxygenation of 1 could be achieved in microemulsions,
these solutions were irradiated for 4 h using a 500 W halogen
lamp and the presence of 2 was determined by TLC.
As microemulsions are complex systems, optimisation of the
work-up was necessary. For t-amyl alcohol-based microemul-
sions, 1 was irradiated for 4 h in a Schlenk flask with a 500 W
halogen lamp in the presence of a sensitiser while the solution
was being purged with air. The progress of the reaction was
monitored by TLC analysis.
Initial work-up procedures looked at direct evaporation of the
15
liquid components, followed by washing with cyclohexane or
directly introducing the residue onto a silica column to separate
the product and surfactant. The former furnished 2 in a low
yield of 20%, while the latter procedure gave 2 only in trace
amounts. The use of Soxhlet extraction was also investigated,
but 2 was again only obtained in trace amounts. Matrix effects
caused by the surfactant thus appear to prevent or limit isolation
of the product. As an alternative, the use of multiple ethyl
acetate–water extractions was investigated. Preliminary results
showed that this protocol was suitable (Table 2, entry 1), but
complete removal of SDS was not achieved and subsequent
column chromatography was required.
Fig. 1 A schematic of a microemulsion using RB or MB as a sensitiser.
The basic composition of the microemulsions used is water
in ethyl acetate. To form homogeneous solutions, a surfactant is
added, in this case sodium dodecyl sulfate (SDS). In addition, a
co-surfactant is used. The ratio of the components determines
whether or not a microemulsion is obtained. The influence
of solutes is important, and so optimisation of the ratio of
components was initially carried out, as shown in Table 1.
Early investigations looked at the use of n-butanol as a co-
surfactant as this was the co-solvent reported in previous green
17
15a
microemulsions. The substitution of n-butanol with t-amyl
alcohol was studied, as this solvent has previously been used by
To optimise the work-up for ethanolic microemulsions, the
recovery of independently synthesised 2 was assessed using
three procedures, namely extraction using ethyl acetate, washing
with cyclohexane and washing with toluene (Table 3). Toluene
4a
us in photooxygenation reactions. Finally, the use of ethanol
was investigated, which proved to be the most effective co-
surfactant as it resulted in the use of less surfactant, as well as
Table 1 Optimisation of the ratio of components for microemulsions
Entry
% SDS
Co-surfactant
% Co-surfactant
% EtOAc
% H
2
O
Appearance
1
2
3
4
5
6
7
8
9
7.5
7.3
7.1
7.5
7.3
7.1
7.5
7.7
5.1
3.1
n-BuOH
n-BuOH
n-BuOH
t-AmOH
t-AmOH
t-AmOH
EtOH
EtOH
EtOH
EtOH
7.5
9.8
11.9
7.5
9.8
11.9
7.5
5.1
7.7
7.9
70.0
68.3
66.7
70.0
68.3
66.7
70.0
71.8
71.8
73.3
15.0
14.6
14.3
15.0
14.6
14.3
15.0
15.4
15.4
15.7
Clear, homogeneous
Clear, homogeneous
Cloudy, no bilayer
Clear, homogeneous
Clear, homogeneous
Clear, homogeneous
Cloudy, no bilayer
Cloudy, no bilayer
Cloudy, no bilayer
Clear, homogeneous
1
0
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Table 2 Optimisation of the work-up for t-amyl alcohol-based
sensitiser in a mixture of dichloromethane and methanol (9 : 1)
gave an isolated yield of 2 of 79%. Due to their excellent
solubilising properties, microemulsions thus enable the use of
TPP in a benign, non-chlorinated environment. TPP is likely
to bind to the hydrocarbon tail of the surfactant and is thus
microemulsions
Entry
Work-up
Yield of 2 (%)
1
2
Extraction using ethyl acetate
Evaporation, washing with
cyclohexane then filtration
Evaporation, Soxhlet extraction
using cyclohexane
Evaporation, direct column
chromatography
57
20
15b
localized at the interface of the microemulsion. As a result,
TPP gave the highest conversions and isolated yields of 58 and
3
4
trace
trace
8
5% (Table 4, entries 3 and 7).
The optimised procedure was consequently applied to two
other 1-naphthol derivatives (Scheme 2). Photooxygenations of
-acetamido-1-naphthol (3a; R = CH ) and 5-benzoylamido-1-
5
3
Table 3 Optimisation of the work-up for ethanolic microemulsions
Entry Work-up Recovery of 2 (%) % SDS
naphthol (3b; R = Ph) furnished the corresponding 5-amido-1,4-
naphthoquinones 4a and 4b in moderate yields of 36 and 32%,
respectively.
a
1
2
Break emulsion with brine,
93
93
8
2
extraction using ethyl acetate
Break emulsion with brine, dry
organic residue, wash with
cyclohexane
3
Break emulsion with brine, dry
organic residue, wash with toluene
92
3
a
1
Determined by integration of selected signals in the H NMR spectrum
of the product.
Scheme 2 The photooxygenation of 1-hydroxy-5-amidonaphthalenes
was chosen to wash the residue after evaporation of the liquid
components (Table 3, entry 3) as the partition of SDS into
toluene is zero.
3
a and 3b.
18
In each case, there was residual sodium dodecyl sulfate
present. However, the use of cyclohexane washing gave the
highest recovery with the lowest percentage of contaminant.
This may be removed by further washings or by column
chromatography.
Conclusions
The use of ethanolic microemulsions of water in ethyl acetate
with sodium dodecyl sulfate as a co-surfactant have been
demonstrated as excellent reaction media for the dye-sensitised
photooxygenation reactions of selected 1-naphthols. These offer
excellent solubilising abilities and, in particular, enable the use
of TPP as a sensitiser in a non-chlorinated environment. Hence,
the developed procedure represents another example of green
photochemistry. Isolating the products, however, remains a
key challenge, and isolated yields are currently lower compared
to common organic solvent systems.
Using the photooxygenation of 1, the use of different sen-
sitisers in microemulsions with ethanol (Table 4, entries 1–
4) and t-amyl alcohol (Table 4, entries 5–7) as co-surfactants
was probed. In all cases, the work-up procedure consisted
of multiple ethyl acetate and water extractions, followed by
column chromatography using a mixture of ethyl acetate and
cyclohexane. For reactions using ethanol-based microemulsions,
2,19
2
was isolated in yields of 18–58%, while using t-amyl alcohol
gave 39–85% yields.
Experimental
Due to its poor solubility in polar solvents, photooxy-
genations using tetraphenylporphyrin (TPP) are traditionally
performed in halogenated solvents such a dichloromethane. For
the synthesis of 2, however, alcohols are generally required as
General procedure for the preparation of microemulsions
Sodium dodecyl sulfate, ethyl acetate and the co-surfactant were
stirred together at room temperature (16–25 C) to form a slurry.
4c
◦
a co-solvent to ensure the solubility of starting material 1.
A comparison photooxygenation reaction of 1 using TPP as
Water was added to this and the resulting mixture was stirred
Table 4 Sensitiser study in t-amyl alcohol-based and ethanolic microemulsions
Entry
Sensitiser
Co-surfactant
Conversion of 1 (%)
Isolated yield of 2 (%)
a
1
2
3
4
5
6
7
Rose bengal
EtOH
EtOH
EtOH
EtOH
t-AmOH
t-AmOH
t-AmOH
44
33
70
8
n.d.
n.d.
23 (53 )
a
Methylene blue
Tetraphenylporphyrin
(none)
Rose bengal
Methylene blue
Tetraphenylporphyrin
18 (55 )
a
58 (83 )
a
7 (87 )
b
52
39
85
b
b
n.d.
a
b
Yield based on conversion. Not determined.
1
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vigorously for 15 min. The resulting microemulsions have been
shown to be stable for more than 3 months.
5 (a) R. H. Thomson, Naturally Occurring Quinones IV: Recent
Advances, Blackie Academic & Professional, London, 1997; (b) R. H.
Thomson, Naturally Occurring Quinones III: Recent Advances,
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Bostian and K. L. McNitt, CRC Handbook of Antibiotic Compounds.
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General procedure for photooxygenation reactions
1
,5-Dihydroxynaphthalene 1 (0.5 mmol) and the sensitiser (25
1
980; (d) R. H. Thomson, Naturally Occurring Quinones, 2nd edn,
mmol: 25 mg RB, 8 mg MB or 15 mg TPP) were dissolved in 50 ml
of microemulsion (Table 1, entries 6 or 10). The clear solution
was irradiated using a 500 W halogen lamp in a Pyrex Schlenk
flask equipped with a cold finger and a reflux condenser for 4 h
at ambient temperature while purging with a gentle stream of
air. Following irradiation, the reaction mixture was subjected to
one of the work-up procedures outlined in Table 2 or Table 3,
and the resultant crude product was further purified by column
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Green Chem., 2010, 12, 1544–1547 | 1547