Three phase microemulsion/sol-gel system for aqueous C-C coupling of hydrophobic substrates

Article abstract of DOI:10.1002/ejoc.200800028

Heck, Stille, Suzuki and three-component coupling reactions with hydrophobic substrates have been carried out in water. The substrates are initially transformed by a general procedure into a microemulsion, which consists of nearly 90% water with the aid of sodium dodecyl sulfate and either PrOH or BuOH. The surfactant carries the molecules of the substrates to Pd(OAc)₂ entrapped within a hydrophobicitized silica sol-gel matrix where the coupling between the substrates is assumed to take place. The products are then returned by the surfactant into the microemulsion from which it can be released. The immobilized palladium catalyst is leach proof and recyclable. It can be used in various coupling processes at least six times without loss of activity. Experiments with D₂O have revealed that the water does not take part in the coupling process, but it has an effect on the pore size of the sol-gel matrix, which hosts the palladium catalyst. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800028

FULL PAPER
DOI: 10.1002/ejoc.200800028
Three Phase Microemulsion/Sol–Gel System for Aqueous C–C Coupling of
Hydrophobic Substrates
Dmitry Tsvelikhovsky[a] and Jochanan Blum*[a]
Keywords: C–C coupling / Green chemistry / Sustainable chemistry / Microemulsion / Palladium / Sol–gel processes
Heck, Stille, Suzuki and three-component coupling reactions
with hydrophobic substrates have been carried out in water.
the surfactant into the microemulsion from which it can be
released. The immobilized palladium catalyst is leach proof
The substrates are initially transformed by a general pro- and recyclable. It can be used in various coupling processes
cedure into a microemulsion, which consists of nearly 90%
water with the aid of sodium dodecyl sulfate and either PrOH
or BuOH. The surfactant carries the molecules of the sub-
strates to Pd(OAc)2 entrapped within a hydrophobicitized sil-
ica sol–gel matrix where the coupling between the substrates
is assumed to take place. The products are then returned by
at least six times without loss of activity. Experiments with
D2O have revealed that the water does not take part in the
coupling process, but it has an effect on the pore size of the
sol–gel matrix, which hosts the palladium catalyst.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
face of the sol–gel matrix and may spill the load of sub-
strates into the porous medium that contains the catalyst.
Consequently, a catalytic reaction takes place within the
sol–gel material to form the products. The latter are then
extracted by the desorbing surfactant, which carries them
back, in their emulsified form, into the aqueous medium
from where the free products can be separated.[11] This cata-
lytic system, which relies on a three-phase emulsion/sol–gel
transport model (EST), proved already applicable to vari-
ous hydrogenations of hydrophobic substrates.[11] The EST
system could be further developed and improved by re-
placement of the traditional emulsions by microemul-
sions.[12] This change in medium enables further selective
hydrogenations, as well as catalytic hydroformylations, to be
carried out in aqueous media.[13] The use of microemulsion
(which are formed by titration of regular emulsions to clar-
ity with an alcoholic cosurfactant) face, however, a major
drawback associated with the fact that the component of
the microemulsions are usually different in nature and
quantity for each substrate and for each catalytic reaction.
Thus, we found it imperative to seek in the framework of
this work a general formula for the composition of stable
microemulsions, suitable for the various hydrophobic sub-
strates. Positive results led us to further investigation of the
ability to utilize the microemulsion/sol–gel system to ad-
ditional catalytic processes, apart from the aforementioned
hydrogenations and hydroformylations. In this paper, we de-
scribe the use of this catalyst system in the Heck, Stille and
Suzuki reactions, including the three-component coupling
of phenylboronic acid with a diarylalkyne and an iodoar-
ene. In all these processes the catalyst could be recovered at
the end of the reactions and recycled.
Introduction
The public concern over the massive use of organic sol-
vents in industrial processes[1] leads to much effort to re-
place the harmful media by water, which is both the cheap-
est and the least risky solvent.[2] The main problem with the
application of water in organic synthesis is the insolubility
of many reagents and catalysts in this medium. In specific
cases, this shortcoming can be overcome by modification of
the reaction components (substrates and/or catalysts) with
hydrophilic auxiliaries.[2,3] Other noteworthy strategies that
enable the formation of certain reactions of hydrophobic
reagents in water are the application of the “on water” pro-
cess,[2,4] the application of hydrophilic cosolvents,[2,5] the use
of phase-transfer catalysis,[6] pH regulation of the reaction
mixture,[2,7] the use of super-heated water[8] or the applica-
tion of ultrasound or microwave irradiation.[1,8b] Sometimes
it is sufficient to employ a suitable surfactant or a micelle-
producing agent to “solubilized” the reactants as emul-
sions.[3f,9] However, the recovery of the catalysts from such
systems is usually difficult. In order to solve this problem
we have found it useful to combine two domains, the emul-
sifications and the sol–gel technology.[10] For example, when
an emulsion of hydrophobic substrates is subjected to an
organometallic catalyst that has been entrapped within a
hydrophobicitized silica sol–gel, the surfactant molecules
that carry the reactants absorb/desorb reversibly on the sur-
[a] Institute of Chemistry, Hebrew University,
Jerusalem 91904, Israel
Fax: +972-2-6513832
jblum@chem.ch.huji.ac.il
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D. Tsvelikhovsky, J. Blum
FULL PAPER
ducted for short time periods it became clear that under
EST conditions the coupling is affected by both electronic
and steric effects. Electron-attracting substituents (located
either on the styrene or on the bromide) enhance the coup-
ling and vice versa for electron-releasing groups. When one
of the components of the Heck reaction (either the styrene
or the bromide) is electron rich and the other one is electron
poor, the effect of the electron releasing substituents pre-
vails and causes the process to slow down (Table 1, En-
tries 7 and 8). This observation suggests that neither the
oxidative addition nor the final reductive elimination step
are rate limiting. Bulky substituents have a strong inhibiting
effect on the reaction. Thus, 1-bromonaphthalene, as well
as 2-bromotoluene, that couple in homogeneous systems,
hardly react under our standard conditions. It is also no-
table that nonsubstituted bromobenzene and styrene react
as fast as the chlorinated reagents probably owing to their
smaller size, which allows the substrate to penetrate more
easily into the small pores of the sol–gel matrix (vide in-
fra).[16]
Results and Discussion
Reinvestigation of our previous work in which we hydro-
genated olefinic and aromatic carbon–carbon bonds under
EST conditions[13a] revealed that by lowering the concentra-
tion of the hydrophobic substrate, highly stable microemul-
sions are formed. These microemulsions could be employed
to all unsaturated substrates studied so far. A typical com-
position of such a microemulsion is 89.3 wt.-% of water,
3.3 wt.-% of the surfactant sodium dodecyl sulfate (SDS),
6.6 wt.-% of the cosurfactant (usually nBuOH or nPrOH)
and 0.8 wt.-% of the hydrophobic substrates. Microemul-
sions having this composition proved suitable not only for
hydrogen-transfer processes but also for a variety of palla-
dium-catalyzed C–C coupling processes.
A series of Heck coupling reactions of aryl bromides
with styrene derivatives are shown in Scheme 1, and they
were carried out under comparable EST conditions. The
results are presented in Table 1. The catalyst in these reac-
tions, as well as in those described below, was phosphane-
free Pd(OAc)2 entrapped within a silica sol–gel matrix
hydrophobicitized with n-octyltri(ethoxy)silane (OTEOS).
The optimal molar ratio of the palladium compound and
the substrates is 1:100. Smaller ratios can be used but much
lower ones cause a decrease in the reaction rate. For exam-
ple, a ratio of 1:200 in the experiment described in Entry 1
Scheme 1.
In all reactions listed in Table 1, the catalyst proved to
affords only 84% of (E)-stilbene under the conditions of be perfectly recyclable. For example, the reaction listed as
Table 1. Attempts to use Pd(OAc)2 entrapped within regu- Entry 1 in Table 1 (coupling of bromobenzene with styrene)
lar, nonhydrophobicitized sol–gel resulted in exceedingly gave a quantitative yield of pure (E)-stilbene in all six runs
low yields. The coupling reactions of chlorine-free sub- studied. In order to examine whether the water takes part
strates (Table 1, Entries 1–4) afforded the corresponding in the coupling under EST conditions, we carried out a
stereochemically pure (E)-stilbene derivatives. Chlorine- series of experiments in D2O instead in H2O. In none of
containing substrates (either bromides or styrenes) gave, these experiments did we observe any incorporation of deu-
however, small amounts of the (Z) products (Table 1, En- terium in the products (or in the recovered starting materi-
tries 5–8), and the reaction of 4-ClC6H4Br with 4- als).
CH3C6H4CH=CH2 yielded, in addition to the (E)- and (Z)-
In analogy to the Heck reaction, we performed Suzuki
stilbenes, trace amounts of 1-chloro-4-[1-(4Ј-methylphenyl) cross-coupling experiments with hydrophobic bromoarenes
ethenyl]benzene. We recall that the formation of these iso- in water (Scheme 2). Several representative examples are
mers of (E)-stilbenes are common in the Heck coupling un- summarized in Table 2. The table indicates that under the
der conventional homogeneous and heterogeneous condi- EST conditions the coupling is highly selective. Except for
tions.[14,15] When the reactions listed in Table 1 were con- Entry 1 (where the formation of biphenyl was expected), no
Table 1. Heck coupling of styrene derivatives and bromoarenes by sol–gel encaged Pd(OAc)2 under comparable microemulsion–EST
conditions.[a]
Entry
Ar in the bromide
Ar1 in the alkene
Major product
% Yield after 4 h in the 1st run[b]
1
2
3
4
5
6
7
8
C6H5
4-C6H4CH3
C6H5
C6H5
4-C6H4Cl
C6H5
C6H5
C6H5
E-C6H5CH=CHC6H5
Ͼ99
33[c]
47
E-C6H5CH=CH-4-C6H4CH3
E-C6H5CH=CH-4-C6H4CH3
E-C6H5CH=CH-4-C6H4F
E-C6H5CH=CH-4-C6H4Cl
E-C6H5CH=CH-4-C6H4Cl
E-4-ClC6H4CH=CH-4-C6H4CH3
E-4-ClC6H4CH=CH-4-C6H4CH3
4-C6H4CH3
4-C6H4F
C6H5
4-C6H4Cl
4-C6H4CH3
4-C6H4Cl
82
Ͼ99[d]
Ͼ99[d]
20[e]
21[f]
4-C6H4Cl
4-C6H4CH3
[a] Reaction conditions: Pd(OAc)2 (3 mg, 0.0134 mmol) entrapped within a hydrophobicitized sol–gel matrix prepared from n-octyl(tri-
ethoxy)silane and tetramethoxysilane (TMOS) as described in the Experimental Section; composition of the microemulsion 0.8 wt.-% of
the substrates (1.34 mmol of each of them), 3.3 wt.-% of sodium dodecyl sulfate (SDS), 6.6 wt.-% of nBuOH and 89.3 wt.-% H2O; K2CO3
1
(2 mmol); 80 °C. [b] The yields were determined by both GC and H NMR spectroscopic analysis and are the average of at least two
experiments that did not differ by more than Ϯ3%. [c] A quantitative yield was obtained only after 48 h. [d] Contains 4% of the (Z)
isomer. [e] Contaminated with 3% of the (Z) isomer and 1% of 1-chloro-4-[1-(4Ј-methylphenyl)ethenyl]benzene. [f] Contains 3% of the
(Z) isomer.
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Three Phase Microemulsion/Sol–Gel System for C–C Coupling
Table 2. Suzuki coupling of bromoarenes with arylboronic acids under microemulsion–EST conditions catalyzed by sol–gel entrapped
Pd(OAc)2.[a]
Entry
Ar in the bromide
Ar1 in the boronic acid
Product
% Yield after 7 h in the 1st run[b]
1
2
3
4
5
6
7
phenyl
phenyl
phenyl
mesityl
phenyl
phenyl
phenyl
phenyl
biphenyl
Ͼ 99
87
4-methylphenyl
4-methylphenyl
4-methoxyphenyl
4-chlorophenyl
1-naphthyl
4-methylbiphenyl
4-methylbiphenyl
4-methoxybiphenyl
4-chlorobiphenyl
1-phenylnaphthalene
9-phenylanthracene
33[c]
64
96
92
anthracen-9-yl[d]
80[e]
[a] Reaction conditions: Pd(OAc)2 (0.0134 mmol) entrapped within hydrophobicitized sol–gel as described in the Experimental Section;
microemulsion of the bromoarene (1.34 mmol) and the boronic acid (1.34 mmol). The composition of the microemulsion was identical
with that described in Table 1 except that it contained phenylboronic acid instead of styrene; K2CO3 (2 mmol); 80 °C. [b] The yields are
1
the average of at least two experiments that did not differ by more than Ϯ3% and were determined both by GC and H NMR spectro-
scopic analysis. [c] Yield after 15 h. [d] Dissolved in CH2Cl2 (3 mL). [e] When the octyltri(ethoxy)silane in the hydrophobicitized matrix
was replaced by trimethyl(methoxy)silane the yield rose to 99%.
formation of biphenyl was observed. Thus, unlike the situa- as 40 Å. A similar increase in the pore size was observed in
tion in many cases of the Suzuki coupling under conven- the catalyst used for the Heck coupling of bromobenzene
tional conditions,[17] no homocoupling takes place.
with styrene. One significant difference between the Heck
and Suzuki coupling under the EST condition is, however,
remarkable. Although bulky bromoarenes couple either
very slowly or not at all with styrene, they react quite
smoothly with phenylboronic acid.
Scheme 2.
The palladium-catalyzed three-component coupling of
iodoarenes, internal alkynes and arylboronic acid shown in
Scheme 3 may be regarded as a modification of the Suzuki
reaction.[19] Thus, we were able to couple phenylboronic
acid with diphenylacetylene and several substituted iodo-
benzenes (Scheme 3). Representative results are summa-
rized in Table 3. We noticed that the reaction proceeds
faster under the EST conditions than that in a homogen-
eous system with the same [palladium]/[substrate] ratio.
This rate acceleration may be associated with the pro-
motion of the three-component coupling reaction by
water.[20] In the EST system, the three-component coupling
seems to follow a different mechanism than that under the
homogeneous conditions. In the presence of the non-en-
trapped palladium catalyst, electron-donating substituents
enhance the process and electron attracting groups cause it
to slow down (the nitro group stalls the reaction com-
pletely[19]). In our reaction system, the opposite trend was
observed. Whereas Entries 1, 3 and 4 (Table 3) were com-
plete within less than 2 h to form Ͼ97% of analytically pure
tetraarylethylenes, the reaction of 1-iodo-4-methylbenzene,
diphenylacetylene and phenylboronic acid (Entry 2) was
complete only after 10 h. Recycling of the heterogenized
catalyst in the reactions listed in Table 3 did not decrease
the activity at all for at least six runs.
Owing to the hydrophilic nature of boronic acids it is not
uncommon that Suzuki couplings of partially water-soluble
halides can be performed also in water (see, for example,
the reactions reported by Byun and Lee[18]). When, how-
ever, the halide is highly hydrophobic, as for example 9-
bromoanthracene, no coupling takes place in this medium.
Table 2 indicates that by application of the EST technique
even such water-insoluble substrates undergo smooth coup-
ling in high yields.
Except for Entry 3, the coupling of the halides listed in
Table 2 with phenylboronic acid give within 20 h nearly a
quantitative yield of the expected biaryls. Only when the
unsubstituted acid was replaced by the sterically hindered
mesitylboronic acid was the yield substantially reduced. Ex-
periments conducted for shorter reaction periods (e.g., 4 h)
revealed the existence of a clear electronic effect similar to
that observed in the Heck reaction. The Suzuki cross-coup-
ling reaction under the EST conditions resembles the Heck
reaction also with respect to the recyclability of the immo-
bilized catalyst. In a typical series of six runs in which 4-
bromotoluene was coupled with phenylboronic acid at 80
°C for 20 h the yields of 4-phenyltoluene were nearly quan-
titative. In contrast to many other catalyses by sol–gel en-
trapped metallic compounds,[10,16] the heterogenized cata-
lyst seems not to suffer from clogging of the pores by the
substrate and product molecules. BET/BJH (Brunner,
Emmet, Teller/Barett, Joyner, Halenda) measurements indi-
cated that within the aqueous medium, the average pore
diameters of the matrix do not decrease with time. On the
contrary, owing to some Si–O bond cleavage by water, they
even increase in size after each run. For example, in the
Scheme 3.
Another cross-coupling of hydrophobic reagents that
coupling of bromobenzene and phenylboronic acid, the proved to take place in an aqueous medium is the Stille
BJH-N2 adsorption average pore diameter (4V/A) of the reaction (Scheme 4).[21] Some representative results are
immobilized catalyst increased after the second run from listed in Table 4. At 80 °C, the reaction seems not to depend
31.5 to 34.4 Å and after six further runs it became as large on the electronic nature of the substrate, as in the other
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2419

D. Tsvelikhovsky, J. Blum
FULL PAPER
yield of the coupling product of bromobenzene and styrene ex-
tended from 0 to 100% depending on the nature of the hydrophobic
part of the sol–gel (these experiments will be announced in a sepa-
rate publication). It was already mentioned that Pd(OAc)2 encaged
within a nonhydrophobicitized matrix is catalytically inactive. If the
reactions were promoted by leached palladium, the yields (obtained
under identical conditions) should not have been dependent on the
nature of the sol–gel.
Table 3. Three component coupling of iodoarenes, diphenylacety-
lene and phenylboronic acid.[a]
Entry Iodoarene
Time Product
[h]
%
Yield[b]
1
2
C6H5I
4-IC6H4CH3
2
10
(C6H5)2C=C(C6H5)2
(4-C6H4CH3)(C6H5)-
C=C(C6H5)2
98
97
3
4
4-IC6H4Br
2
2
(4-C6H4Br)(C6H5)-
C=C(C6H5)2
(4-C6H4NO2)(C6H5)-
C=C(C6H5)2
98
98
4-IC6H4NO2
Conclusions
[a] Reaction conditions: catalyst and composition of the micro-
emulsion as described in the Experimental Section. The amounts
of the iodoarene, the diphenylacetylene and the phenylboronic acid
were 1.34 mol each; K2CO3 (2 mmol); 80 °C. [b] Yield of isolated
product.
Although conventional conditions for the catalytic Heck,
Stille, Suzuki and three-component coupling reactions usu-
ally involve organic solvents, by using a three-phase micro-
emulsion/sol–gel transport system it is possible to perform
these processes under “greener” conditions, in aqueous
solutions, even with water-insoluble hydrophobic substrates.
All the reactants are solubilized by the formation of micro-
emulsions of the same general composition that contain al-
most 90% water. The catalyst in these reactions is phos-
phane-free palladium acetate entrapped within a hydro-
phobicitized silica sol–gel matrix. This enables facile recov-
ery and recycling of the used palladium compound. Palla-
dium acetate that is encaged in regular hydrophilic sol–gel
material fails to react with the emulsified substrates and
consequently hardly any coupling reactions take place in
such a system.
coupling processes. The catalyst proved completely recycla-
ble in further runs without any loss in catalytic activity for
at least six runs. For example, the reaction of bromobenzene
and phenyltributylstannane under the conditions of Table 4
gave quantitatively pure biphenyl in each run, and the coup-
ling of 4-bromotoluene formed 70–75% of 4-methyl-1,1Ј-
biphenyl in each of the first six cycles.
Scheme 4.
Table 4. Stille cross-coupling of some bromoarenes with tributyl-
phenylstannane under microemulsion–EST conditions by sol–gel
entrapped Pd(OAc)2.[a]
Experimental Section
General: 1H and 13C NMR spectra were recorded with either a
Bruker AMX-300 or a Bruker AMX-400 instrument in CDCl3. In-
frared spectra were taken with a Bruker Vector 22-FRIT spectrom-
eter. Mass-spectral measurements were performed with a Hewlett–
Packard model 4989 A mass spectrometer equipped with an HP
gas chromatograph. ICP analyses were performed with the aid of
a Perkin–Elmer model Optima 3000 instrument. A Micrometrics
ASAP 2020 instrument was used for BET-N2 and BJH-N2 surface
area and pore diameter measurements of the sol–gel matrices. The
following starting materials and reference compounds were ob-
tained from commercial sources: tetramethoxysilane, octyltriethox-
ysilane, trimethylmethoxysilane, palladium acetate, sodium dodecyl
sulfate, bromobenzene, iodobenzene, 1-bromo-4-methylbenzene, 1-
bromo-4-chlorobenzene, 4-bromobenzaldehyde, 1-bromo-4-me-
thoxybenzene, 1-bromo-4-iodobenzene, 1-iodo-4-methylbenzene, 1-
iodo-4-nitrobenzene, 1-bromonaphthalene, 9-bromoanthracene,
ethenylbenzene, 1-ethenyl-4-methylbenzene, 1-chloro-4-ethenylben-
zene, 1-ethenyl-4-fluorobenzene, B-phenylboronic acid, B-(2,4,6-
trimethylphenyl)boronic acid, tributylphenylstannane, 1,1Ј-(1,2-
ethynediyl)bis(benzene), 1,1Ј-(1Z)- and 1,1Ј-(1E)-(1,2-ethenediyl)-
bis(benzene), 1-methyl-4-[(1E)-2-phenylethenyl]benzene, 1-meth-
oxy-4-[(1E)-2-phenylethenyl]benzene, 1-chloro-4-[(1E)-2-phenyle-
thenyl]benzene, 1-fluoro-4-[(1E)-2-phenylethenyl]benzene, 9-[(1E)-
phenylethenyl]anthracene, biphenyl, 4-methyl-1,1Ј-biphenyl, 4-
Entry Bromoarene
Product
%
Yield[b]
1
2
3
4
C6H5Br
4-BrC6H4CH3
4-BrC6H4OCH3 4-methoxy-1,1Ј-biphenyl
4-BrC6H4CHO 1,1Ј-biphenyl-4-carboxaldehyde
biphenyl
4-methyl-1,1Ј-biphenyl
98
73
81
86
[a] Reaction conditions: immobilized catalyst containing
0.0134 mmol Pd(OAc)2 as described in the Experimental Section;
bromoarene (1.34 mmol) and PhBu3Sn (1.34 mmol); the micro-
emulsion as described in the Experimental Section; 80 °C, 7 h. [b]
The yields are the average of isolated products obtained at least in
two experiments that did not differ by more than Ϯ3%.
Because the Heck and other C–C coupling reactions by supported
palladium compounds have sometimes been attributed to the ac-
tion of soluble nanoclusters derived from the original precatalyst
(see for example, refs.[15,22,23]), we assume that in our system, palla-
dium nanoparticles may also be involved. Pd0 can result from the
PdII acetate by transfer hydrogenation from the carbinols released
during the preparation of the sol–gel matrix, as well as from the
alcoholic cosurfactant. In any event, if such nanoparticles were
formed they must be entrapped with the sol–gel matrix rather than
dispersed in the solution. Inductivity coupled plasma (ICP) analy-
sis indicated that the concentration of palladium in the reaction
solution after removal of the sol–gel material by filtration is less
than 1 ppm. In addition, we found that the filtrate has absolutely
no catalytic activity (even after concentration). Further support of
the assumption that the coupling process takes place within the
matrix was found in a set of experiments in which the Pd(OAc)2
had been entrapped in differently hydrophobicitized sol–gel. The
methoxy-1,1Ј-biphenyl,
[1,1Ј-biphenyl]-4-carboxaldehyde
and
1,1Ј,1ЈЈ,1"Ј-(1,2-ethenediylidene)tetrakisbenzene. The following
chemicals were prepared according to literature procedures: 1-
chloro-4-[(1E)-2-(methylphenylethenyl)]benzene,[24]
1-methyl-4-
1-bromo-4-(1,2,2-triphenyl-
1-nitro-4-(1,2,2-triphenylethenyl)ben-
zene.[19] The structures of 1-chloro-4-[(1Z)-2-phenylethenyl]ben-
(1,2,2-triphenylethenyl)benzene,[25]
ethenyl)benzene[26]
and
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Three Phase Microemulsion/Sol–Gel System for C–C Coupling
zene[27] and 1-chloro-4-[1-(4Ј-methylphenyl)ethenyl]benzene,[28]
Acknowledgments
which were obtained in this study in minute amounts as mixtures,
1
were deduced from the H NMR and MS spectra of the mixture
We gratefully acknowledge the support of this study by the Israel
Science Foundation (ISF) through grant no. 269/06.
of products.
Entrapment of Palladium Acetate within a Hydrophobicitized Sol–
gel Matrix: Typically, n-octyl(triethoxy)silane (OTEOS, 2.1 mL,
6.68 mmol) was stirred magnetically at 20 °C for 24 h with a mix-
ture of EtOH (5.6 mL, 729 mmol) and triply distilled water (TDW,
0.4 mL, 22.2 mmol). Separately, tetramethoxysilane (TMOS,
3.6 mL, 24.2 mmol) was stirred for 10 min at 20 °C with MeOH
(2.4 mL, 59.3 mmol) and TDW (2.0 mL, 111 mmol). The two solu-
tions were mixed and stirred for an additional 10 min and added
to a stirred solution of Pd(OAc)2 (30 mg, 0.134 mmol) in CH2Cl2
(4 mL). Gelation was achieved within 10–12 h. The gel was dried
at 80 °C/0.1 Torr for 24 h then washed with CH2Cl2 (3ϫ10 mL)
and redried under the same conditions to yield the immobilized
palladium compound (3.0 g). Analysis of the combined washing
revealed that they rarely contained more than 1 ppm of palladium.
The resulting sol–gel material was then divided into ten equal por-
tions for the catalytic processes.
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Reactions in Water: Principles, Strategies and Applications,
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H. C. Hailes, Org. Process Res. Dev. 2007, 11, 114–120; b) C. J.
Li, Chem. Rev. 2005, 105, 3095–3166; c) B. Cornils, W. A.
Herrmann, Aqueous Phase Organometallic Catalysis: Concept
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For comparison, nonhydrophobicitized sol–gel entrapped Pd(OAc)2
was prepared, which did not include the OTEOS.
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Preparation of the Microemulsions: Typically, the microemulsions
were prepared by mixing the hydrophobic substrates (1.34 mmol of
each) with TDW (89.3 wt.-%), sodium dodecyl sulfate (SDS,
3.3 wt.-%) and the cosurfactant (6.6 wt.-% of either n-propanol or
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(280 mg, 2 mmol) was also added. The reaction mixture was then
heated with stirring at the desired temperature for the required
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added to the reaction mixture. This caused the emulsion to separate
into two phases. (In the Stille coupling 10 mL of a saturated KF
solution was also added). The organic layer was diluted with diethyl
ether (50 mL), dried with MgSO4 and concentrated. The ceramic
catalyst was extracted with ether (usually 3ϫ50 mL), dried and
concentrated under reduced pressure. The residue was then ana-
lyzed for leached palladium by ICP. The products can be either
separated by column chromatography or analyzed by GC, GC–MS
and NMR spectroscopy. The physical data of the products were
compared with those of authentic samples. All the products ob-
tained in this study are known compounds and their physical data,
except the 1H and 13C NMR spectra of 1-bromo-4-(1,2,2-triphenyl-
ethenyl)benzene,[26] have already been published in the literature.
The NMR spectra of this bromo compound are as follows: 1H
NMR (400 MHz, CDCl3): δ = 7.26–7.36 (m, 5 H), 7.46–7.49 (m, 6
H), 7.57–7.60 (m, 8 H) ppm. 13C NMR (100 MHz, CDCl3): δ =
121.5, 122.2, 123.3, 127.0, 128.2, 128.4, 128.8, 128.9, 131.6, 131.9,
133.5, 139.1, 140.0, 140.2 ppm. The filtered sol–gel-entrapped used
catalyst was washed with water (3ϫ40 mL) to remove remains of
the surfactant, sonicated for 10 min with CH2Cl2 (3ϫ30 mL) and
dried at 80 °C/0.1 Torr for 12 h. The washings were combined and
again analyzed for the presence of palladium. The dried catalyst
was ready for use in the next run.
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www.eurjoc.org
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Received: January 10, 2008
Published Online: March 27, 2008
444–445.
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www.eurjoc.org
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2008, 2417–2422