Catalytic transfer hydrogenation and hydrogenolysis in ionic liquids with Pd/MgLa mixed oxide and Pd/MgAl hydrotalcite as recyclable catalysts

Article abstract of DOI:10.1080/00397910801929523

A new Pd/MgLa mixed oxide and the known Pd/MgAl hydrotalcite catalysts were applied and recycled successfully in catalytic transfer hydrogenation reactions in ionic liquids. Some α,β-unsaturated carboxylic acid derivatives were hydrogenated in excellent yields. The catalysts were recycled without significant loss of activity. Besides that, a number of halogenated aromatic compounds were dehalogenated under similar catalytic transfer conditions. Copyright Taylor & Francis Group, LLC.

Full text of DOI:10.1080/00397910801929523

Synthetic Communicationsw, 38: 1601–1609, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801929523
Catalytic Transfer Hydrogenation
and Hydrogenolysis in Ionic Liquids with
Pd/MgLa Mixed Oxide and Pd/MgAl
Hydrotalcite as Recyclable Catalysts
1
Zoltan Baan, Attila Potor, Agnieszka Cwik, Zoltan Hell,
1
1
1
´
´
Gyorgy Keglevich, Zoltan Finta, and Istvan Hermecz
´
1
2
2
¨
´
´
1Department of Organic Chemical Technology, Budapest University of
Technology and Economics, Budapest, Hungary
2Chinoin Pharmaceutical and Chemical Works Ltd., Budapest, Hungary
Abstract: A new Pd/MgLa mixed oxide and the known Pd/MgAl hydrotalcite
catalysts were applied and recycled successfully in catalytic transfer hydrogenation
reactions in ionic liquids. Some a,b-unsaturated carboxylic acid derivatives were
hydrogenated in excellent yields. The catalysts were recycled without significant
loss of activity. Besides that, a number of halogenated aromatic compounds were deha-
logenated under similar catalytic transfer conditions.
Keywords: Hydrotalcite, ionic liquids, palladium, transfer hydrogenation
As a new class of solvents for organic synthesis, ionic liquids have recently
attracted considerable attention.[1] From the aspect of application, the
important advantages of ionic liquids include negligible vapor pressure,
high thermal stability, easy handling, and compatibility with various organic
compounds and organometallic catalysts. Ionic liquids have recently been
applied for many types of reactions, for example, the Diels–Alder
reaction,[2] Heck reaction,[3] dimerization,[4] hydroformylation,[5]
Received October 25, 2007
Address correspondence to Istvan Hermecz, Chinoin Pharmaceutical and Chemical
Works Ltd., Budapest, Hungary. E-mail: istvan.hermecz@sanofi-aventis.com
´
1601

´
Z. Baan et al.
1602
oxidation,[6] and hydrogenation,[7] include catalytic transfer hydrogenation
(CTH).[8–10]
CTH comprises useful and safe alternative methods for catalytic hydro-
genation with molecular hydrogen.[11] In combination with ionic liquids, the
transfer hydrogenation turns into an environmentally safe and green
process, thanks to its beneficial properties. Up to now, only a few articles
have reported about CTH in ionic liquids. Bertold et al. reported a
microwave-assisted CTH of unsaturated compounds in [bmim][PF6] incorpor-
ating Pd/C catalyst.[8] The ruthenium-catalyzed asymmetric CTH of ketones
was also investigated, where the ionic liquid had a role in the stabilization of
the catalyst.[9] We have recently reported the Pd(OAc)2- and PdCl2-catalyzed
homogenous CTH of a,b-unsaturated compounds in ionic liquids.[10]
In the present article, we report our success in finding a recyclable Pd
catalyst, which is useful in the selective transfer hydrogenation of unsaturated
compounds in ionic liquids. Because the most common heterogeneous Pd/C
was found to be ineffective in ionic liquids, we turned to a special
palladium carrier. A number of heterogeneous palladium supports are
available, but most of them are neutral or solid acids. Our selected hetero-
geneous palladium carrier, the magnesium-lanthanum mixed oxide (MgLa
mixed oxide), has a special feature: it is basic because of the hydroxides on
its surface. It appears to be a stronger base than hydrotalcites and fluorides.
We expected that the basic support may have an influence on the reaction
rate, and the palladium on this could be an active catalyst in CTH. We have
previously prepared this Pd/MgLa mixed oxide and successfully applied it
in Heck, Sonogashira, and Suzuki–Miyaura cross-coupling reactions.[12,13]
The reaction parameters for the initial experiments were based on our
previous results,[10] and the Pd/MgLa mixed oxide was prepared according
to the procedure reported previously.[13] In the CTH of cinnamic acid,
[bmim][BF4] was used as ionic liquid, HCO2NH4 as hydrogen source, and
Pd/MgLa mixed oxide as catalyst (Scheme 1).
The data in Table 1 reveal the effects of the various reaction parameters
on the CTH of cinnamic acid. When 1 mol% of PdII/MgLa mixed oxide was
used, after 5 h the reactions resulted in moderate yields at all of the investi-
gated reaction temperatures (entries 1–3). With 10 mol% of catalyst, even
at 65 8C, the conversion was increased to 99% (entry 4). At 100 8C, with
this amount of catalyst, the reaction took place quantitatively within 20 min
(entry 6). Finally, 2 mol% of PdII/MgLa mixed oxide was found to be
Scheme 1.

Catalytic Transfer Hydrogenation and Hydrogenolysis
1603
Table 1. Effects of the reaction parameters on the
Pd/MgLa mixed-oxide-catalyzed CTH of cinnamic acida
Catalyst Temperature
(8C)
Conversionb
(%)
Entry (mol%)
Time (h)
1
2
3
4
5
6
7
8
1
1
65
80
5
5
38
51
1
100
65
5
58
10
10
10
2
3
99
99
80
2
100
80
0,3
5
99 (99, 99)c
99
2
80
5
99d (99, 99)c
a0.2 mmol of cinnamic acid, 0.8 mmol of HCO2NH4,
PdII/MgLa mixed oxide catalyst in 1 ml of [bmim][BF4]
at the indicated temperature.
bDetermined by HPLC.
cWith recycled Pd0/MgLa mixed oxide catalyst.
dWith prepared Pd0/MgLa mixed oxide catalyst.
sufficient for complete conversion within 5 h at 80 8C (entry 7). The PdII/
MgLa mixed oxide was converted to the Pd0/MgLa mixed oxide, and this
was also applied for the CTH in excellent conversion (entry 8). These
results indicate that the catalyst activity is independent of the starting state
of the Pd. The Pd0/MgLa mixed oxide formed from the PdII/MgLa mixed
oxide in situ during the reaction or the previously prepared Pd0/MgLa
mixed oxide could be recovered (entry 6,8). The catalyst was removed by
simple filtration from the reaction mixture and then washed with EtOH.
After drying in vacuum, the Pd0/MgLa mixed oxide was reused twice
without any loss of activity.
Besides the [bmim][BF4] used in the model reaction, certain other imida-
zolium and phosphonium-based ionic liquids were also examined. (The ionic
liquids were purchased from Fluka AG and Solvent-Innovation GmbH.)
Although [bmim][BF4] and [bmim][PF6] are the most popular ionic liquids
in organic synthesis, it appeared worthwhile to investigate the reaction in
other ionic media, and especially in halogen-free ionic liquids. Table 2
summarizes the results of the reaction in different ionic liquids.
No major differences were observed between the hydrophilic imidazo-
lium-based ionic liquids except for [bmim][AcO] and [emim][EtSO4]. The
use of [bmim]Cl (entry 2); the imidazolium alkylsulfates [emim][BuSO4]
(entry 7) and [emim][HeSO4] (entry 8); and the hydrophilic [bmim][Tf2N]
(entry 5) led to results similar to those in [bmim][BF4] (entry 1). Unexpect-
edly, the conversion obtained in [emim][EtSO4] (entry 6) was somewhat
lower than in previous cases. When moderate conversions were achieved,
Pd0/MgLa mixed oxide was also examined as the starting catalyst instead

´
Z. Baan et al.
1604
Table 2. Pd/MgLa mixed-oxide-catalyzed CTH of
cinnamic acid in different ionic liquidsa
Conversionb
(%)
Entry
Ionic liquid
1
2
[bmim][BF4]
[bmim]Cl
99
99
3
[bmim][AcO]
[bmim][PF6]
10 (17)c
4
51 (50)c
99
53 (85)c
5
[bmim][Tf2N]
[emim][EtSO4]
[emim][BuSO4]
[emim][HeSO4]
AMMOENG-100TM
[P6,6,6,14]Cl
6
7
99
99
78
9
8
9
10
11
12
13
[P6,6,6,14][BF4]
[P6,6,6,14][PF6]
[P4,4,4,1][TsO]
99
0
0
a0.2 mmol of cinnamic acid, 0.8 mmol of
HCO2NH4, 0.004 mmol of PdII/MgLa mixed oxide
catalyst in 1 ml of ionic liquid at 80 8C for 5 h.
bDetermined by HPLC.
cWith Pd0/MgLa catalyst.
of PdII/MgLa mixed oxide. In the ionic liquids [emim][EtSO4] and
[bmim][AcO], the conversion was slightly increased. Such a dependence on
the starting state of the Pd catalyst has not been reported earlier.
AMMOENG-100TM (entry 9) was also found to be a good medium for the
reaction. The reaction in hydrophobic imidazolium-based ionic liquid (entry
4) resulted in moderate conversions, even with Pd0/MgLa mixed oxide
catalyst. As concerns the phosphonium-based ionic liquids, the CTH took
place in [P6,6,6,14][BF4] with an excellent result (entry 11), but in
[P6,6,6,14]Cl with a poor result (entry 10). No reaction was observed in
[P6,6,6,14][PF6] (entry 12) or in [P4,4,4,1][TsO] (entry 13). At present we have
no explanation for this surprising difference.
A number of cinnamic acid derivatives (Scheme 2) were also hydrogen-
ated in [bmim][BF4] under the CTH reaction conditions (Table 3). Methyl
Scheme 2.

Catalytic Transfer Hydrogenation and Hydrogenolysis
1605
Table 3. Pd/MgLa mixed-oxide-catalyzed CTH of
cinnamic acid derivatives in [bmim][BF4]a
Conversionb
(%)
Entry
R
R1
R2
1
2
3
4
5
6
7
H
H
CH3
Ph
H
H
99
99
42
46
99
99c
99
H
NHAc
CN
H
H
OH
OMe
Cl
H
H
H
H
Me
H
H
a0.2 mmol of cinnamic acid derivative, 0.8 mmol of
HCO2NH4, 0.004 mmol of Pd/MgLa mixed oxide
catalyst in 1 ml of [bmim][BF4] at 80 8C for 5 h.
bDetermined by HPLC.
cDehalogenated product.
cinnamate (entry 7) was reduced with excellent conversion. Of the a-substi-
tuted cinnamic acids, the a-methyl (entry 1) and the a-phenyl (entry 2) deriva-
tives afforded the hydrogenated product quantitatively. The CTH of the
a-acetamido (entry 3) and the a-cyano-4-hydroxy derivatives proceeded
with moderate conversions. From among the compounds with a substituted
aromatic ring, 4-methoxycinnamic acid (entry 5) was converted to the
saturated product with excellent yield. In the case of 4-chlorocinnamic acid
(entry 6), dehalogenation was observed besides hydrogenation of the double
bond.
Although the Pd/MgLa mixed oxide was an effective catalyst in ionic
liquids for the CTH reactions, we intended to increase the avaliablility of
the process by simplifying the catalyst carrier. MgAl 2:1 hydrotalcite is a
commercially available carrier with the same properties and could be an
alternative Pd support to the MgLa mixed oxide.
The catalyst was tested under the conditions found to be optimum in the
CTH of the cinnamic acids in [bmim][BF4] ionic liquid. Cinnamic acid and its
a-methyl and 4-methoxy derivatives were all hydrogenated quantitatively (not
shown). This catalyst was also reused three times in the CTH of cinnamic acid
without any loss of activity. These results show that Pd/MgAl hydrotalcite is a
viable alternative to the Pd/MgLa mixed oxide catalysts for CTH in ionic
liquids.
These results and the observed dehalogenation of 4-chlorocinnamic acid
(enty 6) led us to examine the ability of this CTH system to achieve the
catalytic transfer hydrogenolysis of halogenated aromatic compounds
(Scheme 3). Our earlier studies on catalytic transfer hydrogenolysis and
reaction optimization with 4-chlorobenzonitrile had revealed that the azeo-
tropic mixture of Et3N and HCO2H (2:5) is a better hydrogen source than

´
Z. Baan et al.
1606
Scheme 3.
HCO2NH4. When the hydrogenolysis was carried out in [bmim][BF4] or
[emim][EtSO4], there was no difference between the conversions obtained,
in contrast with the results of the CTH of cinnamic acid. The following
reaction conditions were found to be optimum: 5 mol% of Pd/MgAl hydrotal-
cite catalyst, 2 equiv. of Et3N, 5 equiv. of HCO2H in [emim][EtSO4] at 60 8C
for 4 h.
Table 4 presents the results of the hydrogenolysis of different para-
substituted chloro- and bromobenzenes. During the optimization with 4-chlor-
obenzonitrile, it was found that a 30- to 40-min prestirring of the
halo-aromatic compound with the catalyst in the ionic liquid before addition
of the hydrogen source is crucial. Without prestirring at the reaction tempera-
ture, dehalogenation of the 4-chlorobenzonitrile took place with a conversion
of only 32% instead of 98% (entry 5). In the further studies, this 30- to 40-min
prestirring was employed in each case. As expected, the p-bromobenzenes
Table 4. Pd/MgAl hydrotalcite-catalyzed
transfer hydrogenolysis of halogenated aro-
matic compounds in [emim][EtSO4]a
Conversionb
(%)
Entry
X
R
1
2
Cl
Cl
Cl
Cl
Cl
Br
Br
Br
Br
Br
CN
COOH
CH3
98 (32)c
32
14
3
4
OH
11
30
5
OMe
Cl
6
84
93
7
COOH
CH3
8
44
60
9
10
t-Bu
OMe
80
a0.2 mmol of halo-aromatic compound,
1 mmol of HCO2H, 0.4 mmol of Et3N,
0.01 mmol of Pd/MgAl hydrotalcite catalyst
in 1 ml of [emim][EtSO4] at 60 8C for 4 h.
bDetermined by HPLC.
cWithout the 30–40 min prestirring.

Catalytic Transfer Hydrogenation and Hydrogenolysis
1607
were more active than the p-chlorobenzenes in the hydrogenolysis (see entries
6–10).
In conclusion, we have demonstrated that the new Pd/MgLa mixed oxide
catalyst can be used in the CTH of cinnamic derivatives in an ionic liquid. In
this new, alternative media, the reaction can be performed under mild con-
ditions with comparable results with conventional alcohols. The conversions
in the imidazolium-based ionic liquids were higher than in the examined phos-
phonium-based ionic liquids. The catalyst can be recycled without loss of
activity after a simple filtration. Pd/MgAl hydrotalcite is likewise an appro-
priate catalyst for this type of hydrogenation. This material was also
employed in the catalytic transfer hydrogenolysis of halogenated aromatic
compounds.
EXPERIMENTAL
General Procedure for the CTH
In a typical reaction, HCO2NH4 (0.8 mmol) was added to a stirred mixture of
cinnamic acid derivative (0.2 mmol) and Pd/MgLa mixed oxide catalyst
(2 mol% Pd) in ionic liquid (1 ml). Reaction temperatures and times are
indicated in Tables 1–3. The mixture was then cooled down and the
product was extracted with Et2O (3 Â 4 ml). Removal of the solvent in
vacuum furnished the desired product. Known products were identified by
comparison with standards using HPLC, or the 1H NMR data were
compared with those reported in the literature.
General Procedure for Catalytic Dehalogenation
In a typical reaction, HCO2H (1 mmol) and Et3N (0.4 mmol) were added to a
stirred mixture of halo-aromatic compound (0.2 mmol) and Pd/MgAl hydro-
talcite catalyst (5 mol% Pd) in [emim][EtSO4] (1 ml), and stirring was
continued at 60 8C for 4 h. The mixture was then cooled down, and known
products were identified by comparison with standards, using HPLC.
ACKNOWLEDGMENT
Z.B. is grateful for a PhD fellowship from Sanofi-Aventis (Chinoin) and the
´
Varga Jozsef Foundation.
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