A gemini amphophilic phase transfer catalyst for dark singlet oxygenation

Article abstract of DOI:10.1002/poc.1344

A new gemini surfactant phase transfer catalyst, namely diethyl-ether-a,w-bis-(dimethyldodecylammonium molybdate) codified as 12-EO-12-Mo, was prepared by anion exchange from the analogous gemini dichloride (12-EO-12-Cl₂). The physico-chemical properties of these compounds such as Krafft temperature, critical micelle concentration, surface activity and binary water-surfactant behavior were compared and the influence of the molybdate counterion was examined. Though both compounds are highly hydrophilic, the cmc of 12-EO-12-Mo (0.4 mmol L^-1) is about five times lower than of its dichloride analogue (2.2 mmol L^-1). Moreover, 12-EO-12-Mo exhibits an additional cubic liquid crystal phase between 53 and 64 wt%. The usefulness of 12-EO-12-Mo as an amphiphilic phase transfer catalyst for the dark singlet oxygenation was demonstrated with the peroxidation of two typical organic substrates: α-terpinene which reacts with ¹O ₂ according to a [4 + 2] cycloaddition and the less reactive β-citronellol, which provides two hydroperoxides according to the ene-reaction. 12-EO-12-Mo provides a simple reaction medium with only three components for the preparative peroxidation of hydrophobic substrates by chemically generated singlet oxygen. Copyright

Full text of DOI:10.1002/poc.1344

Special Issue
Received: 29 October 2007,
Revised: 14 January 2008,
Accepted: 15 January 2008,
Published online in Wiley InterScience: 22 May 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1344
A gemini amphiphilic phase transfer
catalyst for dark singlet oxygenation
a
a
b
*
´
´
Cedric Borde , Veronique Nardello , Laurent Wattebled ,
Andre Laschewsky and Jean-Marie Aubrya
b,c
´
A new gemini surfactant phase transfer catalyst, namely diethyl-ether-a,v-bis-(dimethyldodecylammonium molyb-
date) codified as 12-EO-12-Mo, was prepared by anion exchange from the analogous gemini dichloride (12-EO-12-Cl2).
The physico-chemical properties of these compounds such as Krafft temperature, critical micelle concentration,
surface activity and binary water-surfactant behavior were compared and the influence of the molybdate counterion
was examined. Though both compounds are highly hydrophilic, the cmc of 12-EO-12-Mo (0.4 mmol LS1) is about five
times lower than of its dichloride analogue (2.2 mmol LS1). Moreover, 12-EO-12-Mo exhibits an additional cubic liquid
crystal phase between 53 and 64 wt%. The usefulness of 12-EO-12-Mo as an amphiphilic phase transfer catalyst for the
dark singlet oxygenation was demonstrated with the peroxidation of two typical organic substrates: a-terpinene
which reacts with 1O2 according to a [4 R 2] cycloaddition and the less reactive b-citronellol, which provides two
hydroperoxides according to the ene-reaction. 12-EO-12-Mo provides a simple reaction medium with only three
components for the preparative peroxidation of hydrophobic substrates by chemically generated singlet oxygen.
Copyright ß 2008 John Wiley & Sons, Ltd.
Keywords: singlet oxygen; oxidation; catalytic amphiphile; gemini surfactant; phase transfer catalyst; lyotropic liquidcrystal
INTRODUCTION
In contrast to ground state molecular oxygen, singlet oxygen is
a selective and powerful oxidizing species that has found
considerable synthetic utility since it can undergo reactions with
In organic synthesis, hydrophobic molecules often have to react
with hydrophilic species. Despite the incompatibility between
a wide variety of electron-rich molecules such as olefins,
antagonist reactants, interactions and reactions can be favored
sulfides, and heterocycles.[16–18] However, some drawbacks,
by several ways using: (i) monophasic media based on dipolar
aprotic solvents (DMSO, DMF, NMP. . .) or homogenous mixtures
of water and a miscible organic solvent (alcohols, THF. . .), (ii)
conjugated dienes, polycyclic aromatic hydrocarbons, phenols,
inherent to its transitory nature and to its mode of formation,
have to be taken into account: (i) owing to its short lifetime (about
a few microseconds in water), 1O2 must be generated close to the
biphasic media of two immiscible solvents, generally associated
with
a phase transfer catalyst (PTC) typically quaternary
organic substrate, (ii) the disproportionation of the intermediate
triperoxomolybdate, MoO(O2)23ꢀ, which is the main precursor of
1O2, is efficient solely in an aqueous or highly polar environment,
and (iii) poorly reactive substrates require important amounts of
hydrogen peroxide that induces an important dilution of the
reaction medium resulting in a significant competition between
the chemical reaction of 1O2 with the substrate S (rate constant kr)
and the physical deactivation by the solvent molecules (rate
ammonium compounds, and (iii) submicronic and thermodyna-
mically stable dispersions of two immiscible solvents stabilized by
an appropriate amphiphile, that is, microemulsions.[1–4] Among
these strategies, phase transfer catalysis is probably the most
widely used in industry. This process is applicable to a large
number of reactions involving (in)organic anions and organic
reactants.[5–8] Therefore, it has known a remarkable development
during these last decades, owing to its numerous advantages.
Besides its simplicity, reaction rates can considerably be
increased compared to a simple biphasic system.[9,10] Still, as
most PTC do not exhibit amphiphilic properties, the unstable
emulsions formed under stirring are coarse. Moreover, at the end
of the reaction, the PTC is partitioned between the two phases,
thus complicating the recovery of the products. Furthermore,
phase transfer catalysis is unsuitable when short lifetime species
have to react with hydrophobic substrates. In particular, it cannot
*
LCOM, Equipe Oxydation & Formulation, UMR CNRS 8009, Ecole Nationale
´
Superieure de Chimie de Lille, BP 90108, F-59652 Villeneuve d’Ascq Cedex,
France.
E-mail: veronique.rataj@univ-lille1.fr
a
C. Borde, V. Nardello, J.-M. Aubry
LCOM, Equipe Oxydation & Formulation, UMR CNRS 8009, Ecole Nationale
´
Superieure de Chimie de Lille, BP 90108, F-59652 Villeneuve d’Ascq Cedex,
France
1
be simply applied to the use of singlet molecular oxygen, O2
(1Dg). This reagent is efficiently chemically generated in aqueous
phase by disproportionation of hydrogen peroxide catalyzed by
molybdates anions (Eqn 1):[11–15]
b
c
L. Wattebled, A. Laschewsky
Fraunhofer Institut fu¨r Angewandte Polymerforschung FhG-IAP, Geisel-
bergstr.69, 14476 Potsdam-Golm, Germany
MoO24ꢀ=water
1
A. Laschewsky
¨
Universitat Potsdam, Karl-Liebknechtstr.24–25, 14476 Potsdam, Germany
2H2O2
ꢀ!
2H2O þ O2ð100%Þ
(1)
pH 9ꢀ11
J. Phys. Org. Chem. 2008, 21 652–658
Copyright ß 2008 John Wiley & Sons, Ltd.

GEMINI AMPHIPHILIC PHASE TRANSFER CATALYST
constants kd) (Eqns 2–4).[19]
from Carlo Erba. Hydrogen peroxide 50% (17.5 M) was from
Prolabo. Milli-Q water (18.2 MV cm) was used.
kd
1O2 ꢀ! 3 O2
solvent
(2)
Methods
kq
3
1O2 þ S ꢀ! O2 þ S
(3)
(4)
Elemental analysis was performed by the University College of
London. NMR spectra were taken with an Avance 300 apparatus
(Bruker). Thermogravimetric experiments were performed under
nitrogen with a TGA/SDTA851 apparatus (Mettler Toledo), heating
rate 10 8C/min. Surface tensions were measured at 25 8C for
different concentrations of geminis using a Tracker Tensiometer
supplied by IT Concept. These measurements have been
performed by analysis of the profile of an air bubble formed
in the surfactant solutions. All measurements were made as a
function of time until stabilization of the surface tension.
Krafft temperatures were estimated by observing the
dissolution of a small crystal of the gemini compounds in water
by a microscope equipped with a hot stage. The binary phase
diagrams were built by preparing 12-EO-12-Xn/water samples by
weight in glass tubes, which were homogenized by repeated
heating and centrifugation back and forth. No results are
presented for concentrations higher than 85 wt%, because the
solubility temperature increases rapidly with concentration,
while excessive heating induces a decomposition of 12-EO-12-Xn.
Polarized optical microscopy (POM) was used to characterize the
mesophases formed in 12-EO-12-Xn -water mixtures. POM texture
was observed with an Olympus BX60 (ꢃ 100 magnification)
equipped with a LTS120 Analysa Peltier temperature controlling
stage (Linkam). In penetration experiments, pieces of broken
lamella were placed between the microscopic slide and coverslip
to form a gradient of thickness. The compounds were deposited
on the thinner corner and a drop of water was carefully placed on
the thicker side, and allowed to diffuse toward the solid
compound, creating a concentration gradient throughout the
slide. High-performance liquid chromatography (HPLC) analyses
were carried out with a Waters 600 chromatograph equipped
with Novapak C18 (4 mm) column. A mixture of Milli-Q water and
HPLC grade CH3CN was used as eluent, and UV detection was
performed with a variable-wavelength monitor Waters 490E
multi-wavelengths. UV/visible spectrophotometry analyses were
carried out with a Varian Cary 50 spectrometer.
kr
1O2 þ S ꢀ! SO2
The cumulative amount of 1O2 generated in the medium
([1O2]1) is thus related to the final concentration of the substrate
[S] by the following relation:[19]
1
kr
kd
½Sꢁ0
ln
½1O2ꢁ
¼
þ ½Sꢁ0 ꢀ ½Sꢁ1
(5)
1
kr þ kq
kr þ kq ½Sꢁ1
The ratio g ¼ kr/(kr þ kq) expresses the contribution of the
chemical quenching in the overall quenching whereas b ¼ kd/
(kr þ kq) is the Foote reactivity index that corresponds to the
minimum concentration of substrate required so that the
interaction of O2 with S (Eqns 3 and 4) becomes predominant
over the deactivation of O2 by the solvent (Eqn 2).
1
1
Several solutions have been brought up in order to apply the
chemical source of 1O2 to organic synthesis involving the use of
dipolar aprotic solvents such as DMF,[20] of polar protic solvents
such as methanol,[20,21] and of w/o microemulsions.[22–29] These
latter systems are particularly well suited to the peroxidation of
hydrophobic substrates because the size of the aqueous
microdomains (10–100 nm) is smaller than the mean travel
distance of 1O2 in water (ꢂ200 nm). However, several drawbacks
such as the use of high amounts of surfactants and cosurfactants
to obtain a microemulsion, the difficulty to recover the catalyst
and the oxidation products from the complex final reaction
medium, and the instability of the system toward dilution
generated by the addition of hydrogen peroxide have limited the
industrial applications of these media.
In the present work, we developed a new PTC exhibiting
interfacial properties, which enables efficient dark singlet
oxygenation of organic substrates by chemically generated
1O2. Because of the divalent nature of the molybdate anion, we
have chosen a gemini amphiphilic PTC based on two quaternary
ammonium functions to which one molybdate anion has been
associated as a counterion (Fig. 1).
The behavior of 12-EO-12-Mo in aqueous solution was
investigated and compared with one of the known dichloride
analogue 12-EO-12-Cl2, and its usefulness for dark singlet
oxygenation in biphasic media was demonstrated.
Synthesis of
diethyl-ether-a,v-bis(dimethyldodecylammonium
molybdate) (12-EO-12-Mo)
The dimeric surfactant 12-EO-12-Mo was prepared by exchan-
ging the chloride counterions (Clꢀ) of diethyl-ether-a,v-bis
(dimethyldodecylammonium chloride (12-EO-12-Cl2) against
hydroxide counterions and making the latter species react with
molybdenum trioxide to form the molybdate (MoO24ꢀ). The
parent dimeric surfactant dichloride was synthesized by alkylat-
ing an excess of N,N-dimethyldodecylamine by 2,20-dichloro-
diethylether[30] as shown in Scheme 1.
EXPERIMENTAL
Chemicals
a-terpinene (1) (85%) was purchased from Acros. b-citronellol (2)
(95%), molybdenum trioxide (95%) and Amberlite Ira-400(Cl)
resin were from Aldrich. Ethyl acetate (Puro) was used as received
140 mL of an Amberlite Ira-400(Cl) resin were allowed to swell
in 300 mL Milli-Q water for 10 min. The resin was then introduced
in a glass column over a height of ca 30 cm. Precautions were
taken to keep the resin wet and to obtain a homogenous
repartition of the resin beads within the column. The resin was
rinsed in the column with 300 mL Milli-Q water. Subsequently, it
was activated by a slow passage of 400 mL of a 10 wt% solution of
sodium hydroxide through the column. It was then washed with
Figure 1. Chemical structure of gemini surfactants diethy-
l-ether-a,-bis(dimethyldodecylammonium) salts (12-EO-12-Xn). For
X ¼Clꢀ, Brꢀ, salicylate: n ¼ 2; for X ¼ MoO24ꢀ: n ¼ 1
J. Phys. Org. Chem. 2008, 21 652–658
Copyright ß 2008 John Wiley & Sons, Ltd.
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C. BORDE ET AL.
Scheme 1. Synthetic route to 12-EO-12-Cl2 and the ion-exchanging route toward 12-EO-12-Mo
Milli-Q water until a pH value of 7 was obtained. After that, a
solution of 12-EO-12-Cl2 (5 g, 8.8 mmol, 100 ml H2O) was
progressively passed through the resin and the resulting solution
was collected in a double-necked flask under a stream of argon.
Two fractions of 100 mL of Milli-Q water were finally added to
recover a maximum amount of ion-exchanged product. Then, the
collected solution was reacted at room temperature with a
stoichiometric amount (1.26 g, 8.8 mmol) of molybdenum
trioxide MoO3 under stirring over 12 h in an argon atmosphere.
After reaction, the possible excess of MoO3 was filtered off on a
Buchner funnel with a small pore size. The filtrate was finally
freeze dried to recover a white hygroscopic powder correspond-
ing to 12-EO-12-Mo.
in the reaction medium and detected at 210 nm. Thus, after 3 h,
98% of (1) was oxidized. The organic phase was separated from
the aqueous phase and evaporated under vacuum. Ascaridole
was recovered as a pure oxidation product in a quantitative yield.
The cumulated amounts of H2O2 consumed, the reaction times,
and the yields are reported in Table 2.
Oxidation of b-citronellol (2)
155 mg of b-citronellol (95%) (2) (0.95 mmol, 0.47 mol Lꢀ1) in the
biphasic system were treated with 30 mL (0.5 mmol) of H2O2 50%.
The dark orange solution was stirred at room temperature for
30 min until the color faded to pale yellow. Ten other fractions of
30 mL of H2O2 50% were allowed to react in the same way, and the
reaction was monitored by HPLC. An internal standard, that is
p-xylene 0.4 mol Lꢀ1, was introduced in the reaction medium and
detected at 210 nm. Thus, after 5 h, 95% of (2) was oxidized. The
organic phase was separated from the aqueous phase and
evaporated under vacuum. The two expected hydroperoxides
were recovered as pure oxidation products with a yield of 80%.
The cumulated amounts of H2O2 consumed, the reaction times,
and the yields are reported in Table 2.
Elemental analysis
(C32H70MoN2O5, Mw ¼ 658.85). Calculated for dihydrate (%): C
55.31; H 10.73; N 4.03; Mo 13.81. Found (%): C 55.24; H 10.49; N
4.08; Mo 13.00; Cl 0.70.
1H-NMR (300 MHz, CDCl3, d in ppm): 0.88 (t, 6H, CH3—);
1.20-1.40 (m, 36H, —(CH2)9—); 1.70 (m, 4H, —CH2—C—Nþ); 3.37
(s, 12H, CH3—Nþ); 3.49 (m, 4H, —CH2—Nþ); 3.93 (m, 4H,
—O—C—CH2—Nþ); 4.25 (m, 4H, —CH2—O—).
13C NMR (50 MHz, CDCl3, d in ppm): 14.1 (CH3—); 22.7, 23.0
(CH3—CH2—; —CH2—C—Nþ); 26.6 (—CH2—C—C—Nþ); 29.4,
29.6, 29.8, 29.8 (—(CH2)6—); 32.0 (CH3—C—CH2—); 51.4
(CH3—Nþ); 63.7, 64.8 (—CH2—Nþ—CH2—); 65.5 (—CH2—O—).
The ion exchange was effective beyond 97% according to the
amount of molybdate in the product determined by elemental
analysis and by UV spectroscopy (the molybdate anion shows a
UV-band at 208 nm with a molar extinction coefficient e ¼
10430 L cmꢀ1 molꢀ1 as determined from aqueous solution of
sodium molybdate), in agreement with the small residual amount
of chloride found.
RESULTS AND DISCUSSION
Physico-chemical properties of the catalytic
gemini amphiphile
Gemini amphiphiles have two hydrophobic tails and two
hydrophilic heads linked by a spacer. They are known to exhibit
enhanced surface-active properties.[31–35] The greater efficiency
and effectiveness of geminis over comparable conventional
surfactants make them more cost-effective as well as envir-
onmentally desirable. The first studied gemini surfactants were
the bis(quaternary ammonium halides) which exhibit superior
performances, compared to their monomeric counterpart, as
catalysts in organic reactions.[36] This class of cationic surfactants
has been studied for over two decades[37] and detailed
structure-performance relationship has been published by Zana
et al.[38–48] and more recently by Laschewsky et al.[49–52] and Devi
et al.[53]
12-EO-12-Cl2 decomposes according to the thermogravimetric
analysis at about 220 8C.[30] The thermogravimetric analysis of
12-EO-12-Mo shows an amount of 5 wt% water contained in the
sample which is consistent with the dihydrate form found by
elemental analysis. The thermal decomposition starts above
160 8C. Both dimeric surfactants are soluble in water at room
temperature, as their Krafft temperatures are below 0 8C.
Equilibrium surface tension g versus surfactant concentration is
plotted in Fig. 2. The inflection on the curves defines the critical
micelle concentration (cmc). Both dimeric surfactants form micelles
at room temperature, with values for the cmc of 2.2 mmol Lꢀ1
Elaboration of the biphasic reaction medium
The biphasic system was prepared at room temperature by
adding 1 mL of ethyl acetate to a magnetically stirred aqueous
solution of 12-EO-12-Mo (93 mg in 1 mL, 7.10ꢀ2 mol Lꢀ1). A
biphasic system was obtained with a lower aqueous phase,
containing the catalytic amphiphile and some ethyl acetate. The
upper phase was ethyl acetate free of 12-EO-12-Mo.
Oxidation of a-terpinene (1)
160 mg of a-terpinene (85%) (1) (1.0 mmol, 0.5 mol Lꢀ1) were
added to the biphasic system and were treated with 30 mL
(0.5 mmol) of H2O2 50%. The dark orange solution was stirred at
room temperature for 30 min until the color faded to pale yellow.
Four other fractions of 30 mL of H2O2 50% were allowed to react in
the same way, and the reaction was monitored by HPLC. An
internal standard, that is p-xylene 0.4 mol Lꢀ1, was introduced
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J. Phys. Org. Chem. 2008, 21 652–658

GEMINI AMPHIPHILIC PHASE TRANSFER CATALYST
Partial phase diagrams of the water-12-EO-12-Xn binary sys-
tems drawn as a function of temperature and gemini concen-
tration are shown in Fig. 4. 12-EO-12-Mo shows successively, as its
concentration is increased, a micellar phase L1 (until 53 wt%), a
cubic phase I1 (from 53 to 64 wt%), and a hexagonal phase H1
(from 64 to 85 wt%). All these phases are still observed at 80 8C.
The phase behavior of 12-EO-12-Cl2 is different since only L1
and H1 phases are observed until 85 wt%. The H1 hexagonal
phase is observed between 53 and 85 wt%. However, at high
concentration (>80 wt%) of 12-EO-12-Cl2 and above 45 8C, a new
isotropic viscous phase appears.
The cubic phase of 12-EO-12-Mo could be determined pre-
cisely by simple visual observation since the samples obtained in
the range of concentrations of 53–64 wt% are very viscous gels
with no birefringent behavior under crossed polarizers. The
presence of a cubic phase is often found for surfactant systems
with bulky head groups, which disfavors the formation of
rod-shaped aggregates on account of the large area needed per
polar headgroup. It is the case in particular for dodecyl-1,3-
propylene-bis(ammonium chloride)[58] and its monovalent
homologue.[59] On the contrary, 12-EO-12-Cl2 does not form
any cubic phase in this region. This can be explained by the fact
that 12-EO-12-Xn is much more dissociated when X ¼ MoO42ꢀ
than when X ¼Clꢀ as explained by Gordon and co.[60]
Figure 2. Surface tension curves of cationic dimers 12-EO-12-Cl2 (o)
and 12-EO-12-Mo (ꢄ). This figure is available in color online at www.
interscience.wiley.com/journal/poc
and 0.4 mmol Lꢀ1 for 12-EO-12-Cl2 and for 12-EO-12-Mo,
respectively. Also, the surface tension is somewhat reduced by
ca 3 mN mꢀ1
.
The evolution of the amphiphilic properties according to the
nature of the counterion is well known for classical, that is,
monomeric surfactants.[54–56] This is also true for the gemini
surfactants. For instance, the bromide analogue 12-EO-12-Br2
exhibits a cmc of ca 0.5 mmol Lꢀ1 and gives a surface tension at
the cmc of ca 40 mN mꢀ1, that is notably reduced compared to
12-EO-12-Cl2.[30,57] Thus, the surface active behavior of
12-EO-12-Mo with a divalent molybdate counterion compares
well to its bromide analogue. Although the MoO24ꢀ lowers the
hydrophilicity of the gemini surfactant, it must be still considered
as a relatively hydrophilic counterion, taken the much more
pronounced effect of organic counterions such as salicylate into
account (Table 1).[51]
Penetration scans provide a glimpse of all possible meso-
phases formed in a full concentration range. The samples were
prepared at room temperature. Phase sequences of both gemini
surfactants are presented in Fig. 3. The 12-EO-12-Cl2 penetration
scan indicates the existence of only one type of liquid crystal
phase. With regards to 12-EO-12-Mo, the penetration scan reveals
the existence of two different isotropic phases at low
concentration, the frontier of which being specified on Fig. 3
by a white dotted line, and at upper concentration a birefringent
region. For both gemini compounds, the birefringent region are
hexagonal phase H1. On progressive heating until 95 8C, the
birefringent region does not vanish.
Dark singlet oxygenation in biphasic system based
on 12-EO-12-Mo
Because of the very short lifetime of 1O2 in water (ꢂ4 ms),
water-organic solvent biphasic systems are totally inefficient for
the peroxidation of hydrophobic substrates by this excited
species chemically generated by disproportionation of hydrogen
peroxide catalyzed by molybdate ions. Actually, H2O2 and MoO42ꢀ
are localized in the aqueous phase where they react to form
several peroxomolybdates MoO4ꢀn(O2)2nꢀ among which the
triperoxo, and to a lesser extent the diperoxo, decompose into
1O2 (Scheme 2).[15]
1
However, O2 cannot diffuse into the organic phase to reach
3
the organic substrate because it is deactivated into O2 in the
aqueous phase by transfer of its energy to the vibrational levels to
the water molecules. Addition of typical PTCs such as alkylam-
monium chlorides does not improve the process significantly. To
solve the problem, a first alternative is to resort to microemul-
sions, particularly efficient for the dark singlet oxygenation of
highly hydrophobic substrates but these media are intricate and
unstable through water addition (arising from H2O2 decompo-
sition). The second alternative, described in the present work,
consists in using an amphiphilic PTC based on molybdate, the
diethyl-ether-a,v-bis(dimethyldodecylammonium
molybdate
(12-EO-12-Mo)) that allows the generation of 1O2 near
the substrate in order to favor the chemical reaction instead
of the deactivation by water molecules.
Table 1. Values of the cmc, g cmc, for the gemini amphiphiles
for the 12-EO-12-Xn series
In the presence of the same volume of water and an immiscible
organic solvent, 12-EO-12-Mo, as well as 12-EO-12-Cl2, are
preferentially localized into the aqueous phase whatever
the polarity of the solvent is, what attests of their hydrophilic
nature. However, the amphiphilic character of both these
compounds allows the cosolubilization of a small amount of
organic solvent into the aqueous phase because of the formation
of swollen micelles of 12-EO-12-Xn. Such systems are similar to
the two-phase microemulsion systems, called Winsor I, but the
low proportion of cosolubilized solvent into the aqueous phase
cmc
(mM)
gcmc
(mN mꢀ1
Surfactant
)
References
12-EO-12-Mo
12-EO-12-Cl2
12-EO-12-Br2
0.4
2.2
0.52
0.08
42
45
40
32.0
This work
[30]
[57]
[51]
12-EO-12-(salicylate)2
J. Phys. Org. Chem. 2008, 21 652–658
Copyright ß 2008 John Wiley & Sons, Ltd.
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C. BORDE ET AL.
Table 2. Oxidation of 0.5 M of organic substrates in the oxidizing biphasic water/ethyl acetate (1/1 v/v) system in the presence of
12-EO-12-Mo at 25 8C
a
kr
Substrate
log(kr þ kq)a
[H2O2]b (M)
1.3
Dt (h)
Product
Yield (%)c
98
kr þ kq
8.0
1a
2.8
6.0
0.87a
3.2
4.8
37
43
a According to Reference[24]
b Cumulative amount of H2O2 for a 95% substrate conversion.
c Yield in isolated and pure products directly from the organic phase by evaporation of the solvent.
.
Figure 3. Liquid crystal texture by POM in the penetration scan experiment at 25 8C for (a) 12-EO-12-Cl2-water and (b) 12-EO-12-Mo-water. L1 denotes an
isotropic liquid phase, I1 is an isotropic cubic phase, and H1 is an anisotropic hexagonal phase
Figure 4. Partial phase diagrams of the binary (left) 12-EO-12-Cl2-water and (right) 12-EO-12-Mo-water systems. The types of phases were determined
by POM and visual observation
does not allow us to talk about microemulsions in the present
case. The hydrophobic substrate partitions between the organic
phase in excess and the proportion of this phase contained in the
swollen micelles. Thus, during the reaction, 12-EO-12-Mo, which
is localized at the interface, reacts with hydrogen peroxide and
1O2 is produced close to the substrate so that it can react with it
before being deactivated by water molecules (Scheme 3).
Scheme 2. Mechanism of singlet oxygen formation from the system
hydrogen peroxide/molybdate ions[15]
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J. Phys. Org. Chem. 2008, 21 652–658

GEMINI AMPHIPHILIC PHASE TRANSFER CATALYST
With such a biphasic system, the recovery of the oxidation
products is straightforward. First, when stirring is stopped, the
phase separation is very rapid (only a few seconds). Then, as only
a small amount of oil is located in the water phase, almost all the
oxidation products are extracted into the organic phase. Thus,
more than 95% of ascaridole resulting from the oxidation of 1 and
about 80% of the two hydroperoxides of 2 are present in the
organic phase. In that latter case, a significant proportion of
the oxidation products are also partitioned in the aqueous phase
due to their amphiphilic nature providing them a cosurfactant
behavior. Evaporation of the oil phase without any other post
treatment allows a very good recovery of the oxidation products,
as shown in Table 2. When some oxidation products are present
in the aqueous phase, they can easily be recovered and separated
from the catalyst by addition of an excess of H2O2 which leads to
a third middle-phase concentrated in peroxidized catalyst.
In comparison with polyphasic microemulsions of Winsor I type
(i.e., a microemulsion in equilibrium with an oil phase in excess)
based on aqueous Na2MoO4/SDS/PrOH/toluene,[28] this biphasic
system presents two advantages: (1) it allows the addition of
important amounts of hydrogen peroxide without the loss of
efficiency and (2) it can be used with any hydrophobic organic
solvent (e.g., dichloromethane, benzene, cyclohexane. . .) pro-
vided that the substrate is soluble whereas the formation of a
Winsor I based on sodium molybdate is conditioned both by the
nature of the solvent and the concentration of Na2MoO4
otherwise an inefficient Winsor II system, that is, a microemulsion
in equilibrium with a water excess phase, is formed. Under the
same conditions, the system based on sodium molybdate and
SDS requires a cosurfactant such as PrOH to form a microemul-
sion. Otherwise the resulting biphasic system is completely
inefficient since all the catalyst lies in the water phase.
Scheme 3. Schematic representation of the peroxidation of a-terpinene
1 by 1O2 generated from H2O2/MoO24ꢀ in a biphasic system based on the
12-EO-12-Mo amphiphilic PTC
It is noteworthy that, as soon as hydrogen peroxide is added to
the biphasic system, a third middle-phase rich in the red-brown
tetraperoxomolybdate MoO(O2)24ꢀ appears suggesting that the
highest peroxidized derivative of 12-EO-12-Mo catalyst is much
less hydrophilic, loosing its affinity both for water and for the
organic solvent. Some phase transfer catalytic systems with a
third liquid phase have already been described previously in the
literature.[61] This third ‘catalytic phase’ allows a local concen-
tration of the reactants improving the activity of the reaction
during the catalytic process and constitutes a simple mean to
recover the catalyst at the end of the reaction contrary to usual
biphasic systems based on PTC.
The two substrates, namely a-terpinene (1) and b-citronellol
(2), were oxidized on the preparative scale (ca 0.5 M) in a water/
ethyl acetate mixture (50/50) to show the scope of the oxidizing
biphasic system based on the amphiphilic 12-EO-12-Mo PTC.
These substrates represent two standard types of the 1O2
reactions, namely, the ene reaction for b-citronellol and the
[4 þ 2] cycloaddition for a-terpinene. The overall rate constants
(kr þ kq) of b-citronellol and a-terpinene are respectively equal
to 106 and 108 Mꢀ1 sꢀ1 in dichloromethane (Table 2).[24] Although
these values have been measured in CH2Cl2, a similar reactivity is
expected in ethyl acetate since both these solvents are non
hydrogenated and of comparable polarity.[19,62] a-Terpinene
reacts with 1O2 very efficiently according to a pure chemical
process whereas b-citronellol has a fair reactivity with significant
physical quenching (kq). The respective oxidation products were
isolated in high yields as indicated in Table 2. During the process,
hydrogen peroxide must be added in batches to favor the
formation of the triperoxomolybdate MoO(O2)23ꢀ, the main
precursor of 1O2, leading to a characteristic red-brown coloration.
Each fraction of H2O2 corresponds to about 3.5 equivalents
with respect to MoO24ꢀ in order to maximize the reaction rate.
When the medium fades, an additional batch of H2O2 can be
added.
CONCLUSION
A new diammonium gemini surfactant phase-transfer catalyst
with molybdate anion as the counterion was prepared from the
chlorinated analogue by anion exchange. The replacement of
the chloride ion by the molybdate ion notably modifies the
amphiphilic properties of the gemini compound leading to a
lower cmc, reduced surface tension at the cmc, and the formation
of a cubic liquid crystalline phase. By virtue of its interfacial
properties, this compound provides biphasic systems in the
presence of water and an organic solvent, whatever its polarity is,
in which the aqueous phase contains swollen direct micelles.
Hence, formation of singlet oxygen by addition of hydrogen
peroxide occurs at the interface, thus enabling the interaction
with the organic substrate contrary to biphasic systems based
on typical PTCs such as alkylammonium chloride, used in
combination with sodium molybdate, which are much less
inefficient since 1O2 is produced far from the organic droplets and
most of it is deactivated by the water molecules. Furthermore,
with 12-EO-12-Mo, the resulting system constitutes a very simple
reaction medium with only three components in comparison to
known Winsor I microemulsions based on sodium molybdate
which requires addition of surfactant (e.g., SDS) and cosurfactant
(e.g., PrOH). The new compound provides very simple and
efficient biphasic reaction media for the preparative peroxidation
of electron-rich substrates from which the recovery of the
oxidation products and the catalyst is easy.
J. Phys. Org. Chem. 2008, 21 652–658
Copyright ß 2008 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

C. BORDE ET AL.
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 652–658