Catalytic transfer hydrogenation of hydrophobic substrates by water-insoluble hydrogen donors in aqueous microemulsions

Article abstract of DOI:10.1016/j.molcata.2012.09.025

In the course of our attempts to replace harmful organic solvents used in organic processes by environmentally favored media, we investigated the transfer hydrogenation of various, unsaturated substrates by cyclohexene and similar water-insoluble hydrogen donors. The catalyst in these reactions was Pd(0) in form of nanoparticles, stabilized by hydrophobic silica sol-gel which can be reused at least 4-6 times without significant loss of activity. The process was shown to depend on the electronic nature of the surfactants. Marlipal 24/70 gave in most experiments the best results. Except for the hydrolysable 1-chloromethylnaphthalene, where chlorine can be substituted by a hydroxyl group, the aqueous medium does not take part in the catalytic process.

Full text of DOI:10.1016/j.molcata.2012.09.025

Journal of Molecular Catalysis A: Chemical 366 (2013) 210–214
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Catalytic transfer hydrogenation of hydrophobic substrates by water-insoluble
hydrogen donors in aqueous microemulsions
Charlie Batarseha , Zackaria Nairoukha , Iryna Volovychb , Michael Schwarzeb , Reinhard Schomäckerb ,
Monzer Fanunc, Jochanan Bluma,∗
a Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel
b Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, 10623 Berlin, Germany
c Center for Colloid and Surface Research, Al-Quds University, East Jerusalem 51000, Palestinian Authority
a r t i c l e i n f o
a b s t r a c t
Article history:
In the course of our attempts to replace harmful organic solvents used in organic processes by environ-
mentally favored media, we investigated the transfer hydrogenation of various, unsaturated substrates
by cyclohexene and similar water-insoluble hydrogen donors. The catalyst in these reactions was Pd(0)
in form of nanoparticles, stabilized by hydrophobic silica sol–gel which can be reused at least 4–6 times
without significant loss of activity. The process was shown to depend on the electronic nature of the
surfactants. Marlipal® 24/70 gave in most experiments the best results. Except for the hydrolysable 1-
chloromethylnaphthalene, where chlorine can be substituted by a hydroxyl group, the aqueous medium
does not take part in the catalytic process.
Received 13 August 2012
Received in revised form
19 September 2012
Accepted 21 September 2012
Available online 23 October 2012
Keywords:
Heterogeneous catalysis
Transfer hydrogenation
Aqueous microemulsion
Palladium
© 2012 Elsevier B.V. All rights reserved.
Sustainable chemistry
1. Introduction
of the palladium acetate pre-catalyst the EST system is also appli-
cable to catalytic transfer hydrogenation reactions of a variety of
Transfer hydrogenation of unsaturated compounds is a “green”
version of the conventional hydrogenation. It may avoid the danger
associated with the transport of hydrogen in cylinders [1]. When
the hydrogen donors (such as alcohols or formic acid derivatives) as
well as the unsaturated acceptors are water-soluble, the reactions
can be conducted in aqueous media [2] (see also the comprehensive
monographs on organic synthesis in water [3,4]). When how-
ever, the reaction components are water-insoluble, most industrial
plants prefer the application of organic media. In the course of our
attempts to replace as much as possible of the large quantities of
the harmful solvents by environmentally favored aqueous media,
we have already shown that the use of either aqueous emulsions [5]
or microemulsions [6] obtained in the presence of micelle-forming
surfactants, enables the performance of a variety of organic catal-
yses in aqueous media even if all the reaction components are
water insoluble [7–10]. The use of sol–gel entrapped pre-catalysts
gives emulsion/sol–gel transport systems (EST) [6] applicable to
some different hydrogen transfer processes and to C–C couplings
of water-insoluble substrates. We now find that in the presence
lipophilic acceptors from water insoluble donors. The reactions
shown in Schemes 1-6a were usually very efficient. Except for a
few exceptions, the turnover frequencies (TOF) were between 100
and 5200 h−1. The turnover numbers (TON) extended under our
conditions to 1300 and the catalyst could be recycled easily.
In the absence of a surfactant the formation of the microemul-
sions did not take place (checked by the laser light scattering
analysis) and consequently the heterogenized catalyst failed to pro-
mote any transfer hydrogenation.
2. Experimental
2.1. Instruments
NMR spectra were recorded with either a Bruker DRX-400 or a
Bruker Avance II 500 in CDCl3. Infrared spectra were obtained with
a Perkin-Elmer 65 FTIR spectrometer. Mass spectral measurements
were performed with a Q-TOF-II spectrometer. ICP-MS analyses
were carried out with a Perkin-Elmer model Elan DRC II instrument.
Gas chromatographic separations and analyses were carried out on
a Hewlett Packard model Agilent 4890D equipped with either a
30 m long column packed with Carbowax 20 M-poly(ethylene gly-
col) in fused silica (Supelco 25301-U) or with a 15 m long column
∗
Corresponding author. Tel.: +972 2 6585329; fax: +972 2 6513832.
E-mail address: jochanan.blum@mail.huji.ac.il (J. Blum).
1381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2012.09.025

C. Batarseh et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 210–214
211
O
O
+
+
2
2
Scheme 1. Transfer hydrogenation of benzylidene acetone.
O
O
O
O
2
2
+
+
Scheme 2. Transfer hydrogenation of ethyl cinnamate.
+
2
+
2
Scheme 3. Transfer hydrogenation of (Z)-stilbene.
2
+
+
2
Scheme 4. Transfer hydrogenation of diphenylacetylene.
NH2
+
NO2
R
+
4H2O
3
2
3
+
2
R
R: H, CH3, Cl
Scheme 5. Transfer hydrogenation of some nitrobenzene derivatives.
packed with a bonded crosslinked (5% phenyl) methylpolysilox-
ane [HP-5]. XPS measurements were performed with a Kratos Axis
ultra X-ray photoelectron spectrometer. The spectra were acquired
with monochromated AlK␣ (1486.7 eV) X-ray source with 0◦ take
off angle. The pressure in the test chamber was maintained at
1.5 × 10−9 Torr during the acquisition process. High resolution XPS
scans were collected for Pd 3d and C 1s peaks with pass energy
of 20 eV. The XPS binding energy was calibrated with respect to
the peak position of C 1s as 285.0 eV. Data analysis was per-
formed with Vision processing data reduction software (Kratos
Analytica Ltd. and Casa SPX Software Ltd.). Transmission electron
microscopy (TEM) was done with a Scanning Transmission Electron
Microscope Technai G2 F20 (FEI Company, USA) operated at 200 kV
and equipped with an EDAX-EDS device for identification of the
elemental composition. Initial powders were dispersed in ethanol
and dropped onto a standard 400 mesh carbon coated copper TEM
grid.
ethers (7-EO-Marlipal 24/70) was obtained from the Sasol Com-
pany.
2.3. Preparation of the heterogenized palladium catalyst
Typically, the alkoxysilane RSi(OR1)3, where R = n-C3H7 or n-
C8H17 and R1 = Et or Me (6.68 mmol), was hydrolyzed by stirring
with a mixture of EtOH (73 mmol) and triply distilled water
(TDW, 22 mmol) for 24 h. Separately TMOS (3.6 ml, 24.2 mmol) was
hydrolyzed with TDW (2.0 ml) by stirring for 10 min. The solu-
tions of the two hydrolyzed silicon compounds were united and
stirred magnetically for 10 min at 25 ◦C and added to a solution
of Pd(OAc)2 (30 mg, 0.134 mmol in 4 ml CH2Cl2). The stirring was
CH2Cl
+
CH3
2HCl
2
2
+
+
+
(a)
2.2. Chemicals
All the substrates and reference compounds, as well as pal-
ladium acetate, cetyltrimethylammonium bromide (CTAB) and
dodecyltrimethylammonium bromide (DTAB) were purchased
from the Sigma–Aldrich Company. Tetramethoxysilane (TMOS),
propyltrimethoxysilane (PTMOS), octyltriethoxysilane (OTEOS)
and phenyltriethoxysilane (PhTEOS) were obtained from ABCR
(Glest, Inc.). Sodium dodecyl sulfate (SDS) was purchased
from Riedel de Haën. C12–C14 alcohols polyoxyethyleneglycol
CH2OH
CH2Cl
HCl
+
H2O
(b)
Scheme 6. (a) Palladium catalyzed transformations of 1-chloromethylnaphthalene
in aqueous microemulsion by cyclohexene. (b) Hydrolysis of 1-
chloromethylnaphthalene in the aqueous medium.

212
C. Batarseh et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 210–214
continued until the gelation was completed (usually 1–3 days).
The resulting gel was dried for 24 h at 100 ◦C and 0.1–0.5 Torr.
Then it was washed and sonicated with warm CH2Cl2 (3× 10 ml)
and redried to obtain a constant weight. For the entrapment of
the palladium compound within phenylated sol–gel, the TMOS
and PhTEOS could be hydrolyzed together in the same vessel for
24 h. The organic solvent used to wash the sol–gel and the dried
sol–gel material was subjected to ICP-MS analyses. The exact pal-
ladium content of the sol–gel matrix was calculated from these
analyses.
3. Results and discussion
We chose benzylidene acetone, ethyl cinnamate, (Z)-stilbene,
nitrobenzene, diphenylacetylene and 1-chloromethylnaphthalene
as representative hydrogen acceptors and cyclohexene,
S(−)-limonene, rac-␣-phellandrene and tetralin as typical
lipophilic hydrogen donors. Palladium acetate which is known
to be converted into recyclable Pd(0) nanoparticles during its
entrapment within sol–gel materials [10], has been used as our
standard pre-catalyst. For the solubilization of the water-insoluble
reaction components, we used differently charged, micelle-
forming surfactants: Anionic (SDS), cationic (CTAB, DTAB) as well
as the non-ionic Marlipal 24/70. Suitable reaction conditions
that led usually to quantitative transfer hydrogenation of the
aforementioned hydrogen acceptors were achieved by heating
the microemulsified substrates for 0.25–6 h together with the
cyclohexene at 140 ◦C (the ratio of acceptor/donor was ∼1/4).
Under standard conditions the molar ratio catalyst/acceptor was
7.5/92.5 and the surfactant, together with the co-surfactant were
3.3 wt.% of the microemulsion. A detailed study with the different
types of surfactants is summarized in Table 1.
2.4. General procedure for the transfer hydrogenation process
Typically, to a stirred mixture (stirring rate 400 rpm) of the
unsaturated hydrogen acceptor (0.893 mmol) and the hydrogen
donor (3.572 mmol), calculated to make the combination of the
acceptor and the donor 1.65 wt.% of the emulsion, was added in
a preheated pressure tube (or a mini-autoclave) with a mixture of
a surfactant (3.27 wt.%), a co-surfactant (usually 1-PrOH, 6.54 wt.%)
and TDW (88.54 wt.%). In most cases, this operation formed trans-
parent microemulsions that dispersed laser beams. To this mixture
was added the immobilized palladium pre-catalyst (containing e.g.,
0.067 mmol Pd). Following this addition the reaction vessel was
heated up to the desired reaction temperature. After the desired
reaction period the reaction mixture was cooled to 20 ◦C and the
sol–gel material was filtered off. The filtrate was separated into
two phases by addition of NaCl (2 g). The solid material was son-
icated with hexane (8 ml) and the aqueous layer of the filtrate
was extracted with hexane (2× 8 ml). The combined organic lay-
ers were dried (MgSO4) concentrated and analyzed by GC and 1H
NMR. Some of the products were separated by column chromatog-
raphy. The used catalyst was dried over P2O5 and reused in further
runs.
Table 1 indicates that in most cases, the non-ionic surfactant is
the most efficient one. Generally, though not always, SDS is second
in its efficiency. The cationic surfactants have shown usually the
lowest activity. Among the two cationic surfactants investigated,
the shorter C12-based one, DTAB, proved more efficient than the
C16-CTAB compound. This order of efficiency resembles the order
observed in the disproportionation of dihydroarenes by a rhodium
(but not by a palladium) catalyst [10], and differs from the order
in some other hydrogen transfer reactions [9] and in catalytic C–C
couplings in microemulsions [7]. Since the substrates in the EST sys-
tem have to penetrate the pores of the sol–gel in which the catalyst
is located, the bulkiness of the hydrogen acceptor plays an impor-
tant role on the efficiency of the reaction (cf. e.g., entries 13, 17
Table 1
Dependence of the transfer hydrogenation of hydrophobic acceptors in aqueous microemulsion on the nature of the surfactant.a
Entry substrate (H-acceptor)
Surfactantb
Reaction times, h
Products (yield, %)c
1
2
3
4
5
6
7
8
Benzylideneacetone
Benzylideneacetone
Benzylideneacetone
Benzylideneacetone
Ethyl cinnamate
Ethyl cinnamate
Ethyl cinnamate
Ethyl cinnamate
(Z)-Stilbene
(Z)-Stilbene
(Z)-Stilbene
(Z)-Stilbene
Nitrobenzene
CTAB
DTAB
SDS
Marlipal
CTAB
DTAB
SDS
Marlipal
CTAB
DTAB
SDS
Marlipal
CTAB
DTAB
SDS
1
1
1
1
0.5
0.5
0.25
0.25
0.5
0.5
0.5
0.5
3
3
1
1
3
3
4
1-Phenyl-3-butanone (42)
1-Phenyl-3-butanone (60)
1-Phenyl-3-butanone (40)
1-Phenyl-3-butanone (100)
Ethyl 3-phenylpropionate (51)
Ethyl 3-phenylpropionate (55)
Ethyl 3-phenylpropionate (37)
Ethyl 3-phenylpropionate (100)
Bibenzyl (59)
Bibenzyl (51)
Bibenzyl (80)
Bibenzyl (74)
Aniline (49)
Aniline (92)
Aniline (72)
Aniline (75)
1-Chloroaniline (23)
4-Toluidine (5)
1-Methylnapthalene (45)
1-Napthalenemethanol (36)
1-Methylnapthalene (39)
1-Napthalenemethanol (49)
1-Methylnapthalene (67)
1-Napthalenemethanol (16)
1-Methylnapthalene (86)
1-Napthalenemethanol (12)
9
10
11
12
13
14
15
16
17
18
19
Nitrobenzene
Nitrobenzene
Nitrobenzene
4-Chloronitrobenzene
4-Nitrotoluene
Marlipal
CTAB
CTAB
CTAB
1-Chloromethylnaphthalene
20
21
22
1-Chloromethylnaphthalene
1-Chloromethylnaphthalene
1-Chloromethylnaphthalene
DTAB
SDS
4
4
4
Marlipal
a
Reaction conditions: a microemulsion of the unsaturated substrate (0.893 mmol), cyclohexene (3.572 mmol), the triply distilled water (adjusted to 88.54 wt.% while
the acceptor + the donor were 1.65 wt.%), surfactant (adjusted to 3.27 wt.%), 1-propanol (6.54 wt.%), Pd(OAc)2 (15 mg, 0.067 mmol) entrapped within octylated silica sol–gel
[prepared from 3.6 TMOS and 2.1 ml OTEOS], 140 ◦C.
b
For the meaning of the abbreviations see Section 2.2. “Marlipal” refers to Marlipal 24/70.
c
The yields were deduced from GC and 1H NMR analyses and were average of at least two experiments that did not differ by more than 3%.

C. Batarseh et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 210–214
213
For this reason the Pd(0) could be traced, e.g., when Pd(OAc)2
was used as catalyst under the EST conditions in the regioselec-
tive hydroformylation of styrene derivatives at 50 ◦C [12]. The part
of the sol–gel-entrapped palladium salt that had not been con-
verted into metallic particles during its entrapment is reduced
at the beginning of the first catalytic run. Consequently, a short
induction period is usually observed in the initial stage of the first
catalytic process. The metallic particles formed during the entrap-
ment in the sol–gel matrix and those produced during the transfer
hydrogenation in the first run differ in their sizes [13]. While the
aggregates formed before the catalytic reaction are mostly between
40 and 50 nm, the diameters of the particles that are added dur-
ing the transfer hydrogenation [of (Z)-stilbene] are of the order of
5–8 nm (see Fig. 1, which exhibits the images of both kinds of the
dark metallic palladium particles).
The oxidation state of the palladium species before and
after the catalyst reactions were studied by X-ray photoelectron
spectroscopy (XPS). The starting non-encaged Pd(OAc)2 revealed
binding energy peaks (BE) centered at 336.80 and 342.11 eV which
are typical for Pd(II) 3d5/2 and Pd(II) 3d3/2 species [14]. During the
entrapment process these BE peaks disappear almost completely
forming new signals centered at 335.55 and 340.81 eV which are
characteristic for palladium 3d5/2 and 3d3/2 of Pd(0) [15]. At this
stage of our research we were unable to provide clear evidence for
the exact kind of the metallic particles which are responsible for
the catalytic transfer hydrogenations and to what extent.
Fig. 1. TEM micrograph of a typical silica sol–gel support section after transfer
hydrogenation of (Z)-stilbene by cyclohexene at 140 ◦C under EST conditions.
Another major factor which has been found to influence
the transfer hydrogenation is the hydrophobicity of the sol–gel
support in which the pre-catalyst had been entrapped. Non-
hydrophobicized silica sol–gel usually caused metal leaching
together with the formation of catalytically inactive metal mir-
rors. We have investigated three types of hydrophobic sol–gel
matrices: one which was doped with n-propyl moieties by co-
polycondensation of TMOS (78%) with PTMOS (22%); the second
one was hydrophobicized with n-octyl groups (using 22% OTEOS),
and the third one was modified with phenyl moieties (by 22%
PhTEOS). As in the case of the isomerization of some allylarenes
by a rhodium pre-catalyst [9] (but opposite to the disproportion
of dihydroarnes, and to the Suzuki and Heck reactions in aque-
ous microemulsions [7,10]), the doping of the support with propyl
groups led to the highest activity and by the phenyl functions to the
lowest. Transfer hydrogenation of nitrobenzene in the presence of
phenylated sol–gel did not take place at all.
and 18 in Table 1). Thus, unsubstituted nitrobenzene reacts faster
than both bulkier electron rich and electron poor derivatives of the
nitro-compound.
Transfer hydrogenation of C–C double bonds (as for example
(Z)-stilbene, Table 1 entries 9–12) leads readily to the formation
of the saturated hydrocarbon, bibenzyl. The analogous alkyne (e.g.,
diphenylacetylene) is however converted initially into (Z)-stilbene
and transferred to bibenzyl only after most of the starting alkyne
has been consumed. Thus, after 6 h the major products were (Z)- and
(E)-stilbenes (36 and 41, respectively). Only after 24 h the bibenzyl
started to accumulate in the reaction mixture.
As mentioned above, a part of the Pd(OAc)2 is transferred under
the EST conditions (above ∼80 ◦C) into metallic palladium particles
(cf. also the behavior of other metal catalysts [11]). We assume that
these particles represent the “true catalyst” in the transfer hydro-
genation process. The formation of Pd(0) does not take place at low
temperature (<60 ◦C) nor in the presence of metal-carbonyl ligands.
Table 2
Dependence of the transfer hydrogenation of unsaturated hydrogen acceptors under EST conditions on the hydrophobicity of the sol–gel support.a
Entry
Substrate (H-acceptor)
Sol–gel hydrophobic moiety
Reaction time, h
Products (yield, %)b
1
2
3
4
5
6
7
8
Benzylidene acetone
Benzylidene acetone
Benzylidene acetone
Ethyl cinnamate
Ethyl cinnamate
Ethyl cinnamate
(Z)-Stilbene
n-C3H7
n-C8H17
C6H5
n-C3H7
n-C8H17
C6H5
n-C3H7
n-C8H17
C6H5
1
1
1
0.5
0.5
0.5
4
2
4
1-Pheny-3-butanone (97)
1-Pheny-3-butanone (42)
1-Pheny-3-butanone (19)
Ethyl 3-phenylpropionate (49)
Ethyl 3-phenylpropionate (51)
Ethyl 3-phenylpropionate (21)
Bibenzyl (100)
(Z)-Stilbene
(Z)-Stilbene
Bibenzyl (96)
Bibenzyl (79)
9
10
11
12
13
Nitrobenzene
Nitrobenzene
Nitrobenzene
1-Chloromethylnaphthalene
n-C3H7
n-C8H17
C6H5
3
3
3
4
Aniline (100)
Aniline (49)
Aniline (0)
1-Methylnaphthalene (37)
1-Naphthalenemethanol (40)
1-Methylnaphthalene (32)
1-Naphthalenemethanol (49)
1-Methylnaphthalene (18)
1-Naphthalenemethanol (43)
n-C3H7
14
15
1-Chloromethylnaphthalene
1-Chloromethylnaphthalene
n-C8H17
4
4
C6H5
a
Reaction conditions as in Table 1, except that the sol–gel support was prepared from 78% TMOS and 22% RSi(OR1)3 where R = n-C3H7, n-C8H17 or C6H5 and R1 = CH3 or
C2H5 and the surfactant was CTAB.
b
The yields were the average of at least two experiments that did not differ by more than 3%.

214
C. Batarseh et al. / Journal of Molecular Catalysis A: Chemical 366 (2013) 210–214
Except for the disproportionation of dihydroarenes, where, we
metallic particles as the true catalyst in the “green” hydrogena-
tion process. In comparison to the molecular hydrogenation, a
big advantage of the transfer hydrogenation process is that there
is no need for high hydrogen pressures to overcome the mass-
transport-limitations due to the low hydrogen concentration at
higher temperatures. Furthermore, the concentration of the hydro-
gen donor and the transferred molecules of hydrogen can be easily
adjusted to the concentration of the substrate. Although the trans-
fer hydrogenation in aqueous microemulsions is applicable to many
water-insoluble acceptors, the latter should be inert to water.
Water-sensitive substrates as, for example, benzylic halides form in
addition to the hydrogenated products, also, the hydrolyzed com-
pounds.
noticed very slow hydrogen exchange between a substrate and
the water [10], we have not yet observed any participation of
the aqueous medium of the EST system in the catalytic reactions.
However, now we have found that during the transfer hydrogena-
tion of 1-chloromethylnaphthalene (Scheme 6a) that undergoes
easy hydrolysis (Scheme 6b), part of the substrate reacts with the
aqueous medium to form 1-naphthalenemethanol (Table 2, entries
13–15).
As in all other catalyses under EST conditions studied so far,
the catalyst could be recycled for several runs. The metallic parti-
cles generated during the transfer hydrogenations listed in Table 1,
entries 1–18 proved recyclable. They could be isolated and reused
at least 4–6 times, without significant loss of their catalytic poten-
cies. Thereafter the catalytic activity drops gradually in most cases.
For example, in the transfer hydrogenation of (Z)-stilbene under the
conditions of Table 2 entry 7, the yields of bibenzyl in the presence
of CTAB for 2.5 h were in the first five runs 97, 97, 96, 93 and 58%.
The drop in the activity has often been observed when the pores in
the sol–gel become clogged by the substrates and/or the products
[16].
Acknowledgements
We thank Dr. Inna Popov for performing the TEM measurements
and Dr. Vitaly Gutking for the XPS analyses. We also acknowl-
edge the financial support of the Deutsche Forschungsgemeinschaft
(DFG) for a trilateral grant No. SCHO687/8-2.
Replacement of the cyclohexene by some other well known
lipophilic hydrogen donors proved to be less efficient. The trans-
fer hydrogenation of (Z)-stilbene that yields 96% of bibenzyl after
2 h at 140 ◦C, dropped to 34, 5 and 2%, respectively, under the same
conditions when ␣-phellandrene, tetralin and rac-limonene were
employed.
Finally, it should be noted that transfer hydrogenation is often
substantially enhanced by the addition of a base [17]. Under our
conditions however, the addition of either KHCO3 or K2CO3 has only
a marginal effect. We assume that the reactions in microemulsions
differ in their mechanism from that in other media. Therefore, our
reactions were performed in the absence of an added base.
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