Oxidation in three-liquid-phase microemulsion systems using "balanced catalytic surfactants"

Article abstract of DOI:10.1021/ja805220p

A series of "Balanced Catalytic Surfactants" (BCS) [(C_n)₂N(C₁)₂]₂MoO₄ (n = 8, 9, 10, 12) based on amphiphilic double-tailed quaternary ammonium with molybdate as a counterion has been developed for the dark singlet [4 + 2] cyclooxygenation of organic substrates in three-liquid-phase microemulsion systems. These cationic surfactants form three-liquid-phase microemulsion systems at room temperature in the presence of an appropriate organic solvent and water without addition of any cosurfactant or electrolyte. Comparative peroxidation of rubrene points out the specific advantages of these three-phase media over phase transfer catalysis in two phase systems and on conventional one-phase microemulsions based on sodium molybdate: (i) only three constituents, (ii) low amounts of surfactants, (iii) insensitivity to water dilution, (iv) fast separation of the three phases, (v) straightforward recovery of the product and the surfactant from the oil and microemulsion phases, respectively. The preparative peroxidation of α-terpinene and 1,4,5-trimethylnaphtalene was performed in the ternary systems [(C₈)₂N(C₁)₂]₂MoO₄/water/tert-butyl acetate or benzene. The reusability of the catalyst, the catalytic nature of the BCS, and the ability of the systems to oxidize poorly reactive substrates were demonstrated showing the broadness of the applicability of such systems. Copyright

Full text of DOI:10.1021/ja805220p

Published on Web 10/15/2008
Oxidation in Three-Liquid-Phase Microemulsion Systems Using “Balanced
Catalytic Surfactants”
V e´ ronique Nardello-Rataj, Laurent Caron, C e´ dric Borde, and Jean-Marie Aubry*
LCOM, Equipe Oxydation et Physico-Chimie de la Formulation, UMR CNRS 8009, Cit e´ Scientifique,
ENSCL, BP 90108, F-59652 VilleneuVe d’Ascq, France
Received July 7, 2008; E-mail: jean-marie.aubry@univ-lille1.fr
Microemulsions (µem) are thermodynamically stable submicron
dispersions of two immiscible liquids stabilized by a surfactant
1
monolayer. They are used as reaction media in organic synthesis
to overcome the incompatibility between hydrophilic reagents and
2
5
hydrophobic substrates. The considerable increase (∼10 times)
of the oil-water interfacial area and the compartmentalization of
reactants often provide a significant rate enhancement and better
selectivity compared to the routine Phase-Transfer Catalysis (PTC)
3
,4
or to cosolubilization in polar solvents. Single-phase water-in-
oil microemulsions have been applied to the peroxidation of fragile
Figure 1. Dark singlet oxygenation of a substrate S in a three-liquid-phase
microemulsion (µem) system based on [(C ) N(C ) ] MoO .
8
2
1 2 2
4
1
1
organic compounds by singlet oxygen, O
2 g
( ∆ ), chemically gen-
4
Table 1. Microemulsion Systems as a Function of Catalytic
erated by the H /Na MoO catalytic system. These microemul-
O
2 2
2
4
Surfactants, [(Cn)2N(C1)2]2MoO4, and Oil Hydrophobicity at 25 °C
a
sions suffer two main drawbacks: (i) lengthy recovery of products
(
Composition is Water/Oil ) 1/1 (v/v) and 5 wt % CS)
because of the high concentration of amphiphiles (≈ 20%); (ii)
2 2
demixing after addition of a certain amount of H O . Two-phase
microemulsion systems with an excess oil phase require lower
amounts of surfactant (∼5%) and allow easier workups, but they
are still sensitive to dilution by water arising from H
portionation.
2
O
2
dispro-
5
a
WI, WII, WIII refer to the Winsor systems, i.e. to microemulsions
5
,12
in equilibrium with oil, aqueous, or both excess phases respectively.
This work describes the concept of “Balanced Catalytic Sur-
factants” (BCSs) giving three-liquid-phase systems that overcome
2
-
the reagents and the products. Under stirring, oxidation products
are continuously extracted from the microemulsion to the organic
phase and the water arising from disproportionated H O is expelled
2 2
in the aqueous excess phase without destabilizing the microemulsion
(Figure 1). At completion of the reaction, stirring is stopped and
the three phases separate within a few seconds.
all these drawbacks. The catalyst (MoO
4
) is electrostatically
bound to a cationic surfactant moiety leading to a “catalytic
surfactant” (CS). Cetyltrimethylammonium bicarbonate may be
considered as a CS since it catalyzes the micellar oxidation of aryl
6
sulfides by H
2
O
2
. Furthermore, a CS is named “balanced” if it
spontaneously forms a three-liquid-phase system when mixed with
an appropriate organic solvent and water. Several types of nonionic
A balanced three-phase microemulsion system is formed when
the interfacial film between oil and water is flexible and has a zero
7
surfactants possess this uncommon feature, whereas most ionic
1
0
surfactants require a cosurfactant and an electrolyte to give a three-
phase system. Only a few double-tailed ionic surfactants behave
mean curvature. To fulfill this condition, the interfacial packing
parameter p ) V/al, where V, a, l are the alkyl chain volume swollen
with oil, the headgroup area, and the alkyl chain length respectively,
must be equal to 1. p is related to the structure of the surfactant in
8
,9
as balanced surfactants.
A series of CSs suitable for dark singlet oxygenation has been
11
2
-
+
solution and to the ability of the oil to penetrate the interfacial film.
Single-chain ammonium molybdates [C N(C MoO (n ) 12,
4, 16) do not behave as BCSs (p , 1) since they do not form
prepared by coupling MoO
4
with single-chain [C
n 1 3
N(C ) ] and
+
n
1
)
3
]
2
4
double-tailed [(C ) N(C ) ] ammonium groups. The latter provide
n 2 1 2
1
BCSs which play the role of surfactant, cosurfactant, and catalyst
simultaneously, leading to a three-liquid-phase system with only
three-phase systems without electrolytes, the anion of which would
exchange with molybdate ions. To increase p and to obtain a flexible
2-
three constituents (Figure 1). In this medium, MoO
4
is specifically
interfacial film without cosurfactant, double-tailed CSs [(C
n 2
) -
localized at the water-oil interface of the middle-phase micro-
emulsion. The hydrophobic substrate partitions between the mi-
N(C MoO (n ) 8, 9, 10, 12) were prepared and turned out to
1
)
2
]
2
4
be balanced in the presence of water and an appropriate oil. The
range of experimentally effective chain lengths, i.e. the values of
n providing three-phase systems at room temperature, lies between
croemulsion and the excess oil phase, whereas H
2 2
O partitions
between the microemulsion and the excess water phase. The
1
2
generation of O exclusively takes place in the aqueous micro-
8
and 12 carbons. The ternary systems based on these CSs have
domains of the middle-phase microemulsion. As the typical size
been investigated with various oils of increasing hydrophobicity
of these microdomains (ca. 10 nm) is much smaller than the mean
1
(
Table 1).
travel distance of O
2
in water (ca. 200 nm), this small uncharged
and rather hydrophobic excited molecule diffuses freely through
the charged interfacial film to the organic compartments where it
reacts with the substrate S. In this process, the upper oil and the
lower aqueous excess phases can be regarded as reservoirs for
The three-phase microemulsion system based on one of the BCSs
(n ) 8) was applied to the peroxidation of three substrates reacting
according to a [4 + 2] cycloaddition, the most specific reaction of
2
O . Rubrene, 1, which is very hydrophobic, bulky, and poorly
1
1
4914 9 J. AM. CHEM. SOC. 2008, 130, 14914–14915
10.1021/ja805220p CCC: $40.75  2008 American Chemical Society

C O M M U N I C A T I O N S
Table 3. Peroxidation (>98%) of Substrates 2 and 3 in
-1
Table 2. Peroxidation of Rubrene 1 (0.02 mol L ) by the H2O2/
+
+ a
X2MoO4 Systems (X ) Na or (C8)nN(C1)m )
Three-Phase Systems Based on Water/Oil ) 1/1 (v/v) and
[((C8)2N(C1)2)2MoO4] ) 75 mmol/kg (see Supporting Information)
b
Ref
b
1
b
2
c
d
PTC
PTC
µem
BCS
H2O2] mol L-
t (h)
yield (%)
1
13
60
10
13
60
13
0.61
15
>98
6.5
0.080
0.5
>98
50
0.092
0.5
>98
45
[
∆
e
f
trapping (%)
0.3
0.4
a
For all experiments, T ) 25 °C, water/benzene ) 1/1 (v/v), [MoO42-]
-1 b
)
0.014 mol L (see Supporting Information). Biphasic systems: PTC/
Na2MoO4/C6H6/H2O with no PTC (entry ref), with (C4)4NBr (entry PTC1)
c
or with (C8)4NBr (entry PTC2). Single-phase microemulsion: sodium dode-
d
a
cyl sulfate (SDS)/BuOH/ Na2MoO4/C6H6/H2O. Three-phase system: [(C8)2-
Overall rate constants as determined by laser flash photolysis in
e
N(C1)2]2MoO4/C6H6/H2O. Yield (%) ) 100 × [oxidation product]/[rub-
microemulsion with sodium tetraphenylporphine sulfonate (TPPS) as
sensitizer. T ) 25 °C. T ) 10 °C.
f
b c
rene]. Trapping (%) ) 100 × [oxidation product]/0.5 × [H2O2]o.
soluble in most organic solvents, was first used as a model for the
comparison with PTC and single-phase microemulsion (Table 2).
Benzene was used as the oil since it is the sole solvent which
solubilizes 1 while providing the Winsor III system.
peroxidized at 10 °C in order to limit the thermolysis of the
endoperoxide (t1/2 ) 290 h at 25 °C).
1
4
Balanced three-phase microemulsion systems based on water,
an appropriate organic solvent, and a balanced catalytic surfactant
combine both distinct advantages of PTC (simplicity, fast phase
separation, easy workup) and microemulsions (huge water-oil
interface, compartmentalization of reactants in nanometric reactors).
The catalytic surfactants developed in the present work are suitable
for dark singlet oxygenation of organic substrates, but the concept
can be extended to many other reactions involving both hydrophilic
reactants and hydrophobic substrates.
When the reaction is performed in a sodium molybdate/water/
1
benzene system (entry Ref), most (>99%) of the generated O
2
is
wasted through deactivation by water molecules before reaching
the organic phase since the aqueous droplets formed during stirring
are millimetric, i.e. much larger than the mean travel distance of
1
2
O in water. No improvement is observed in the presence of the
+
2-
hydrophilic PTC
aqueous phase (entry PTC
used (entry PTC ), the complete conversion of rubrene can be
reached but with a large excess of H
(∼15 times). Actually,
NBr is mainly localized in the organic phase and is able to
1
since (C
4
)
4
N
and MoO
4
mainly lie in the
1
). When the hydrophobic (C
8 4
)
NBr is
Supporting Information Available: Preparation of catalytic sur-
factants; characterization of the three-liquid-phase systems; peroxida-
tions of rubrene 1, R-terpinene 2, and 1,4,5-trimethylnaphtalene 3;
procedure for the recovery of products; recyclability of the microemul-
sion. This material is available free of charge via the Internet at http://
pubs.acs.org.
2
2
O
2
8 4
(C )
convey ∼56% of the intermediate peroxomolybdates into the
1
3
benzene phase where the peroxidation reaction takes place. As
4
reported previously, the single-phase microemulsion based on SDS
(
entry µem) is very efficient but has the drawbacks discussed above.
References
The three-phase system obtained with the “balanced catalytic
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convenient since only 1 wt % of amphiphile (BCS) was used instead
of 30 wt % of (SDS + BuOH) in the single-phase microemulsion.
Moreover, the product is readily recovered almost completely from
the excess oil phase, whereas the BCS remains in the very small
middle-phase µem which can be reused.
Benzene can readily be replaced by more environmentally ac-
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three-phase µem when all reactants are present in the reaction
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JA805220P
J. AM. CHEM. SOC. 9 VOL. 130, NO. 45, 2008 14915