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the temperature can be used to adjust reversibly the stability
region of the microemulsion and the phase separation can be
induced, also reversibly, just by temperature change after the
reaction process.
phases were separated and mixed again in the desired pro-
portions. Finally, a-terpinene was added to the mixtures
(0.1 mol kgꢀ1). Oxidations were performed at 25 ꢁC under
vigorous stirring, by adding hydrogen peroxide in two
batches (1 and 0.5 mol kgꢀ1 after 30 min). The substrate
conversion was followed by gas chromatography and the
peroxidation products were characterized by 1H NMR.
4. Conclusions
Single-phase or multiphase microemulsion systems are ideal
media to carry out ‘dark’singlet oxygenation of hydrophobic
and labile substrates since they offer several benefits: (i)
cosolubilization of great amounts of hydrophilic and hydro-
phobic compounds; (ii) compartmentalization of hydro-
philic and hydrophobic species avoiding side reactions;
(iii) nanometric size (10–100 nm) of the droplets. This latter
feature is of utmost importance for an uncharged short-live
species such as 1O2 since, once generated in aqueous micro-
droplets, it diffuses freely through the interfacial film to
the organic compartment where it reacts with the organic
substrate.
A certain amount of C10E4 (see Table 3) is dissolved into 1 g
of octane and 1 g of water containing 10ꢀ2 mol Lꢀ1 of
sodium molybdate. To this microemulsion system, placed
in a thermostated bath at a given temperature, was added
a-terpinene (40 mL, 0.1 mol Lꢀ1) followed by a batch of
30 mL H2O2 (50%) at zero time leading to an orange-red
mixture. The reaction was monitored by HPLC. The values
of x and of the temperature which define the Winsor-type
system are given in Table 3.
5.3. Instrumentation
Gas chromatography (GC) analyses were performed on
a Agilent 6890 N chromatograph equipped with an apolar
Single-phase microemulsions are easy to formulate and to
handle but they require large amounts of surfactants that
impede recovery of oxidized products. Multiphase micro-
emulsion systems must be prepared under well-defined
physicochemical conditions of salinity, temperature or
hydrophilic–lipophilic balance of the surfactant. They
require much lower amounts of surfactants and they allow
simple recovery of the products localized in the oil excess
phase. They can sustain effective dark singlet oxygenation
provided that the possible excess water phase is removed
beforehand.
1
HP-1 (60 mꢃ0.32 mmꢀ0.25 mm) column. H NMR of the
peroxidation products was carried out on a AC 200 Brucker
spectrometer. Molybdate concentrations were determined by
UV spectrometry on a Varian spectrometer at l¼204 nm.
High-performance liquid chromatography analyses were
carried out with a reversed-phase column (Nova-pack C18,
4 mm, 4.6ꢃ250 mm) using a 600 controller pump from
Waters, a mixture of CH3CN/H2O (90:10) as the eluent,
and UV detection with a Waters 490E programmable multi-
wavelength detector.
5. Experimental
5.1. Chemicals
Acknowledgements
We gratefully appreciate financial support by the DSM com-
pany and by the EU Growth Program (G1RD-CT-2000-
thanks the Alexander von Humboldt-Stiftung and the Fonds
der Chemischen Industrie for generous support.
Sodium molybdate dihydrate (99%), n-propanol (99%),
sodium hydroxide (98%) and 1-isopropyl-4-methyl-1,3-
cyclohexadiene (a-terpinene, 85%) were purchased from
Aldrich and used without further purification. Sodium do-
decyl sulfate (SDS) (98%), toluene (99%), ethyl acetate
(99%), dichloromethane (99%), n-butanol (98.5%) and
hydrogen peroxide (50%) were obtained from Prolabo.
Milli-Q water (18.2 MU cm) was used.
References and notes
1. Prein, M.; Adam, W. Angew. Chem., Int. Ed. 1996, 35, 477–
494; Clennan, E. L. Tetrahedron 2000, 56, 9151–9179;
Wahlen, J.; De Vos, D. E.; Jacobs, P. A.; Alsters, P. L. Adv.
Synth. Catal. 2004, 346, 152–164.
2. Aubry, J.-M. J. Am. Chem. Soc. 1985, 107, 5844–5849; Aubry,
J. M. New Chemical Sources of Singlet Oxygen. In Membrane
Lipid Oxidation; Vigo-Pelfrey, Ed.; CRC: Boca Raton, 1991;
Vol. II, pp 65–102.
3. Foote, C. S.; Wexler, S. J. Am. Chem. Soc. 1964, 86, 3879–
3880.
4. Aubry, J. M.; Pierlot, C.; Rigaudy, J.; Schmidt, R. Acc. Chem.
Res. 2003, 36, 668–675.
5. Aubry, J. M.; Cazin, B. Inorg. Chem. 1988, 27, 2013–2014.
6. Nardello, V.; Marko, J.; Vermeersch, G.; Aubry, J. M. Inorg.
Chem. 1995, 34, 4950–4957.
7. Nardello, V.; Bogaert, S.; Alsters, P. L.; Aubry, J. M.
Tetrahedron Lett. 2002, 43, 8731–8734.
5.2. Procedures
Salinity scans: samples (5 ml) were prepared in SVL tubes
by mixing appropriate amounts of oil, co-surfactant, water,
catalyst and surfactant. Mixtures were gently stirred and
maintained at a constant temperature (25ꢄ0.1 ꢁC) for a suf-
ficient time in order to get thermodynamically stable sys-
tems. To further identify each phase according to Winsor
label, water was coloured in blue and oil in yellow so that
the microemulsion phase was green. Comparative oxida-
tions: 40 g of mem were prepared by predissolving SDS
and sodium molybdate in water+n-PrOH and by adding
toluene to the mixture. After shaking, the systems were
allowed to stabilize overnight at 25ꢄ0.1 ꢁC. For modified
W II and W III systems, greater volumes were prepared.
After sufficient time to reach thermodynamic stability, the