Chemoselective hydrogenation of the olefinic bonds using a palladium/magnesium-lanthanum mixed oxide catalyst

Article abstract of DOI:10.1002/adsc.201100569

A palladium/magnesium-lanthanum mixed oxide catalyst is found to be an efficient heterogeneous catalyst for the chemoselective hydrogenation of olefinic double bonds in the presence of various functional groups. The catalyst was recovered by centrifugation and reused for several cycles with consistent activity and selectivity. Copyright

Full text of DOI:10.1002/adsc.201100569

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DOI: 10.1002/adsc.201100569
Chemoselective Hydrogenation of the Olefinic Bonds Using a Pal-
ladium/Magnesium-Lanthanum Mixed Oxide Catalyst
Mannepalli Lakshmi Kantam,a,* Ramineni Kishore,a Jagjit Yadav,a
Medak Sudhakar,a and Akula Venugopala
a
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad – 500007, India
Fax : (+91)-40-2716-0921; phone: (+91)-40-2719-3510; e-mail: mlakshmi@iict.res.in
Received: July 18, 2011; Revised: November 24, 2011; Published online: February 23, 2012
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100569.
Abstract: A palladium/magnesium-lanthanum mixed trifugation and reused for several cycles with consis-
oxide catalyst is found to be an efficient heterogene- tent activity and selectivity.
ous catalyst for the chemoselective hydrogenation of
olefinic double bonds in the presence of various Keywords: chemoselectivity; hydrogenation; magne-
functional groups. The catalyst was recovered by cen- sium-lanthanum mixed oxide; palladium; recyclabili-
ty
Introduction
Wai and co-workers explored palladium nanoparti-
cles, dispersed by a water-in-oil and water-in-CO2
In synthetic organic chemistry, one of the most impor- micro-emulsion, and showed important catalytic activ-
tant synthetic tools is the chemoselective hydrogena- ity for the hydrogenation of olefins in the presence of
tion of olefinic double bonds in the presence of other organic solvents.[9] Both the advantages of homogene-
functional groups.[1] The commercially available Pd/C ous and heterogeneous catalysts were achieved when
catalysts generally lead to poor selectivities in olefin ligand-stabilized nanosized catalysts in ionic liquids
hydrogenation reactions. However, when Pd/C is asso- were used in the olefin hydrogenation reactions.[10]
ciated with additives such as amines, pyridine, ammo- Moreover, silica-supported[11] and polymer-supported
nia or ammonium acetate, it mediates the chemose- palladium catalysts are also reported in the liquid
lective reduction of olefins without concomitant re- phase selective hydrogenation reactions of olefinic
duction of aromatic carbonyls,[2] aromatic halogens,[2] double bonds.[12] Ihm and co-workers reported the hy-
benzyl groups[3] and epoxides.[4] Misiti et al. reported drogenation of olefinic double bonds by using gel-like
the selective hydrogenation of substituted g-amino and macroporous polymer-supported palladium(II)
a,b-unsaturated esters in the presence of benzyl ether complexes[13a] while Zecca et al. used the ion-ex-
or benzyl ester functionality over commercial Pd/C as change resins-supported palladium catalyst for the
a heterogeneous catalyst in EtOAc.[5]
same reaction.[13b] Manorama and co-workers report-
The palladium-catalyzed chemoselective transfer ed the Pd immobilized on amine-terminated ferrite
hydrogenation of olefinic double bonds has been re- nanoparticles for hydrogenation of aromatic nitro and
ported using different hydride sources such as formic azide compounds.[14] The chemoselective conjugate re-
acid,[6a] ammonium formate,[6b] limonene,[6c] and 1,4- duction of a,b-unsaturated carbonyl compounds over
cyclohexadiene.[6d] Hirota and co-workers reported polymer-supported palladium-N-heterocyclic carbene
palladium supported on silk-fibroin as an effective was studied by Bhanage and co-workers.[15] Crooks
catalyst for the chemoselective hydrogenation of ole- and co-workers used the dendrimer-encapsulated pal-
fins.[7] Recently, Liu et al. reported Hantzsch 1,4-dihy- ladium nanoparticles for the selective hydrogenation
dropyridine as an efficient reducing agent in the Pd/ of olefins.[16] However, due to poor solvent compati-
C-catalyzed hydrogenation of inactivated olefins. The bility, the application of polymer-supported catalysts
a,b-unsaturated ketones also underwent hydrogena- was limited. Thus, the development of more improved
tion, and selectively produced the corresponding satu- synthetic methods for the chemoselective hydrogena-
rated ketones.[8]
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tion of olefinic double bonds remain an active re- lic Pd phases. Diffraction lines appeared at 2q=29.55,
search area. 22.84, 13.1, 30.82, 31.328 [ICSD # 23-0435] and their
Recently, Figueras and co-workers examined the corresponding ꢁdꢂ values of 0.302, 0.389, 0.675, 0.289
direct condensation of alcohols[17] and Michael addi- and 0.285 nm are attributed to the lanthanum oxide
tion reactions[18] over highly basic Mg-La mixed oxide carbonate phase. The diffraction peaks observed at
catalyst. Earlier we had also demonstrated the catalyt- 2q=40.21, 46.778 [ICSD # 87-0639] and the ꢁdꢂ values
ic activity of the Mg-La mixed oxide in the condensa- of 0.224, 0.194 nm are indicative of a metallic Pd
tion of aldehydes and imines with ethyl diazoacetate phase. The transmission electron microscopic (TEM)
using water as a solvent.[19] Palladium impregnated on pictures of the fresh and used 10%Pd/Mg-La mixed
Mg-La mixed oxide catalyst (Pd/Mg-La) was used in oxide catalysts are reported in Figure 2 [a] and [b].
the catalytic transfer hydrogenation of cinnamic acid The Pd particle size was measured from TEM images
and its derivatives in ionic liquid medium. However, (Figure 2) and found to be 28.5 and 32.0 nm for fresh
a dehalogenated product was obtained in the case of and used catalysts respectively. The XPS analyses of
chloro-substituted cinnamic acid and the yields were the fresh (in situ reduced using hydrazine hydrate)
poor with substrates containing other reducible and used 10%Pd/Mg-La mixed oxide catalyst are re-
groups such as cyano.[20] Hell and co-workers reported ported in Figure 3. The metallic Pd species are pres-
an air- and moisture-stable Pd/Mg-La mixed oxide as ent on the surface which was confirmed by XPS
an efficient heterogeneous catalyst in the Heck and signal appearing at binding energies of 335.65 eV and
the Suzuki–Miyaura reactions.[21] In the search for 340.15 eV for Pd 3d5/2 and Pd 3d3/2 over fresh catalyst.
a suitable catalyst for the chemoselective hydrogena- However, the reduction of Na2PdCl4 was not com-
tion of olefinic double bonds in the presence of other plete during the in-situ reduction as there was some
functional groups, we anticipated that Pd/Mg-La amount of Pd in the form of halide as observed at
mixed oxide should exhibit interesting catalytic activi- BE=337.6 eV. The XPS analysis of the used catalyst
ties. In this investigation, we have explored the activi- showed a signal at BE=335.59 eV which suggests that
ty of this Pd/Mg-La mixed oxide catalyst towards the the Pd(0) species are intact after the reaction.[22]
chemoselective hydrogenation of olefinic double There is not much shift in binding energies attributed
bonds and a comparative study with Pd/C catalyst has to metallic Pd, similarly the state of Pd halide, as was
been undertaken.
evidenced from the XPS spectra of fresh and used
catalysts. The BET surface areas were measured by
N2 adsorption at 77 K and revealed values of
35.9 m2 gÀ1 for Mg-La mixed oxide and 31.5 m2 gÀ1 in
the case of Pd-loaded Mg-La mixed oxide. The Pd
Results and Discussion
A preliminary study on the catalytic activity of Pd/ metal content of the fresh catalyst is about 9.62% and
Mg-La mixed oxide for the hydrogenation of chalcone in the case of the used catalyst it was found to be
was performed using ethanol as a solvent at room 9.58% as measured by atomic absorption spectrosco-
temperature under one atmosphere hydrogen pres- py (AAS), which suggested that there was no leaching
sure (Scheme 1). It is very interesting to note that the of Pd.
reaction proceeded with complete chemoselectivity
and only the olefinic bond was reduced.
In an effort to develop a better catalytic system,
various reaction parameters were studied for the hy-
The Mg-La mixed oxide was synthesized by co-pre- drogenation of chalcone using Pd/Mg-La mixed oxide
cipitation of Mg and La nitrates and the Pd-doped as a catalyst and molecular hydrogen as the reducing
Mg-La mixed oxide catalyst was prepared by impreg- agent at room temperature and the results are sum-
nation method (see Experimental Section) to obtain marized in Table 1. The reduced product was obtained
Pd(II)/Mg-La mixed oxide. The in-situ reduction of in excellent yield and selectivity by using Pd/Mg-La
Pd(II)/Mg-La mixed oxide was performed using hy- mixed oxide as a catalyst and ethanol as a solvent
drazine hydrate in ethanol at room temperature for (Table 1, entry 1).
3 h to give an air-stable black Pd/Mg-La mixed oxide
However longer reaction time was needed for full
powder. The XRD patterns of the fresh and used Pd/ conversion of the starting material using ethyl acetate,
Mg-La mixed oxide catalysts (Figure 1, [A] and [B]) methanol or toluene as solvents (Table 1, entries 2–4).
revealed the presence of both La2O2ACTHNUTRGNEUN(G CO3) and metal- It is interesting to note that in all the cases the olefin-
ic double bond is reduced selectively and there is no
formation of any by-product. The activity of Pd(II)/
Mg-La mixed oxide catalyst in the hydrogenation re-
action of chalcone was less compared to that of the
Pd/Mg-La mixed oxide with a longer reaction time
for the completion of the reaction and complete che-
moselectivity was also observed (Table 1, entry 5). It
Scheme 1. Hydrogenation of olefinic double bond over Pd/
Mg-La mixed oxide catalyst.
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Chemoselective Hydrogenation of the Olefinic Bonds
Figure 1. XRD patterns of the 10%Pd/Mg-La mixed oxide: [A] fresh and [B] used catalysts.
is remarkable to note that the carbonyl group of the moiety of the chalcone was also reduced and 1,3-di-
chalcone was not effected and not even a trace phenylpropan-1-ol was formed as a by-product with
amount of any by-product was formed when the reac- 30% yield. After the completion of the reaction, the
tion was continued for 10 h (Table 1, entry 6). In the Pd/Mg-La catalyst was removed quantitatively by
comparative study, commercially available 5% Pd/C simple centrifugation and the filtrate was concentrat-
(same equivalents of Pd as in Pd/Mg-La mixed oxide) ed under reduced pressure. It is very important to
showed poor selectivities and yields under similar ex- note that this process does not require column chro-
perimental conditions (Table 1, entry 7). The carbonyl matographic purification of the product. After remov-
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Figure 2. TEM images of the 10%Pd/Mg-La mixed oxide: [a] fresh and [b] used catalysts.
Table 1. Screening of reaction parameters for the hydroge-
nation of chalcone.[a]
[a]
Reaction conditions: trans-chalcone (2 mmol), 15 mg of
Pd/Mg-La mixed oxide catalyst (0.677 mmol% of palladi-
um), solvent (10 mL), stirring under one atmosphere of
hydrogen pressure at room temperature.
Isolated yields.
Commercial 5% Pd/C catalyst was used.
[b]
[c]
Figure 3. The XPS spectra of the (Pd 3d5/2, 3d3/2) fresh and
used 10 wt%Pd/Mg-La mixed oxide catalysts.
sponding alkanes with 94–96% isolated yield. Re-
markable chemoselectivity was observed in the hydro-
ing the solvent under reduced pressure, the pure genation of aromatic halides such as 4-chlorostyrene
product was obtained in all the cases. All these ex- or 4-bromostyrene (Table 2, entries 4 and 5). In these
periments were performed at room temperature and cases, only the olefinic double bonds were reduced
this addresses the safety issues involved using highly and the formation of dehalogenated products was not
flammable hydrogen gas in the laboratory.
observed. Importantly, a wide range of functional
Performing the reaction of different unsaturated groups, such as aldehydic, ketonic, alcoholic, carboxyl-
compounds under the optimized reaction conditions ic, amide and ester moieties remain intact in the se-
led to high selectivity with yields ranging from 80– lective hydrogenation of olefinic double bonds under
98% (Table 2, entries 1–28). In the case of aromatic the reaction conditions (Table 2, entries 7–28). In the
or aliphatic non-functionalized substrates, the catalyst hydrogenation of cyclic and aromatic a,b-unsaturated
exhibits high activity (Table 2, entries 1–3 and 6). ketones, the olefinic bond was selectively hydrogenat-
Thus trans-stilbene, styrene, 4-methylstyrene and 1- ed without reducing the carbonyl moiety (Table 2, en-
octene were successfully converted to their corre- tries 7–12). Significantly, sterically more hindered tri-
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Chemoselective Hydrogenation of the Olefinic Bonds
Table 2. Selective hydrogenation of different unsaturated compounds.[a]
[a] Reaction conditions: substrate (2 mmol), 15 mg Pd/Mg-La mixed oxide catalyst (0.67 mmol% of Pd), ethanol (10 mL), stir-
ring under one atmosphere hydrogen pressure at room temperature.
substituted olefinic ketones such as isophorone their corresponding saturated alcohols in good yields
(Table 2, entry 8), ionone (Table 2, entry 11) and pule- (Table 2, entries 15, 16 and 17). Moreover, a wide
gone (Table 2, entry 12) afforded the reduced prod- range of functional groups, carboxylic, amide and
ucts with high yields and selectivity. Remarkably, in ester moieties remain intact in the chemoselective hy-
the hydrogenation of a,b-unsaturated aldehydes such drogenation of olefinic double bonds under the reac-
as cinnamaldehyde and a-methyl cinnamaldehyde tion conditions (Table 2, entries 18–22 and 25). Thus,
high yields of the desired products were obtained and maleic acid (Table 2, entry 21) was hydrogenated
no partial reduction of the carbonyl functionality oc- without any further hydrogenation of the carboxylic
curred to produce the alcoholic by-product (Table 2, group. The reduction of diphenylacetylene produces
entries 13 and 14). The aromatic as well as aliphatic dibenzyl as the sole product with 96% yield
alkenols with terminal double bonds such as oct-1-en- (entry 23).
3-ol and 3-methylbut-3-en-1-ol were hydrogenated to
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Since this catalytic system affords high yields for Conclusions
terminal and disubstituted alkenes, it is a useful alter-
native when metal-catalyzed hydrogenations are prob- In conclusion, we have developed a simple and effi-
lematic. cient method for the chemoselective hydrogenation of
Next, we have investigated the selective hydrogena- olefinic double bond in the presence of other func-
tion between olefinic and nitro or cyano groups. Mix- tional groups using Pd/Mg-La mixed oxide as a hetero-
tures of products were obtained when 3-nitrostyrene geneous catalyst at room temperature. The catalyst
was used as a substrate (Table 2, entry 26). The nitro can be readily recovered and reused. We hope that
group was also slightly reduced under the reaction this methodology may find widespread use in organic
conditions and 3-ethylamine was obtained as a by- synthesis.
product in 20% yield. Cyano groups are known as
a reducible functionality under Pd/C-catalyzed hydro-
genation conditions. However, complete chemoselec-
tivity in favor of olefinic bond reduction was observed
using Pd/Mg-La mixed oxide catalyst with E-cinnamo-
Experimental Section
nitrile as a substrate (Table 2, entry 27). Finally, as ex-
pected the azide group was reduced to the amine
group in a short reaction time with high yields when
p-toluenesulfonyl azide was used as a substrate
(Table 2, entry 28).
The feasibility of repeated use of Pd/Mg-La mixed
oxide catalyst was also examined. In Table 3, we pres-
Preparation and Characterization of Pd/Mg-La Mixed
Oxide
The Mg-La mixed oxide was obtained by co-precipitation of
Mg and La nitrates (0.39 mol and 0.13 mol, in water 0.5 L
for an atomic ratio Mg/La=3) at a constant pH 10.0 using
a mixture of KOH (1 mol) and K2CO3 (0.26 mol) in 0.52 L
of distilled water. The gel was washed until the pH reached
neutral followed by oven dried at 1208C for 16 h subse-
quently calcined in static air at 7508C/5 h at a ramping rate
ent the results from the investigation of recycling of
the catalyst. The Pd/Mg-La mixed oxide catalyst
of 108CminÀ1. The Pd supported on Mg-La mixed oxide cat-
alyst was prepared by an impregnation method. In a typical
procedure about 1.5 g of Mg-La (calcined) mixed oxide was
suspended in 150 mL of aqueous Na2PdCl4 (0.44 g;
1.5 mmol) solution and stirred at 258C/12 h under an N2 at-
mosphere. The solid sample was filtered and washed thor-
oughly with 500 mL distilled water and vacuum dried to
obtain Pd(II)/Mg-La mixed oxide. The in-situ reduction of
Pd(II)/Mg-La mixed oxide (~1.0 g) was performed using hy-
drazine hydrate (1 g, 20 mmol) in ethanol (10 mL) at room
temperature for 3 h to give an air-stable black powder, the
Pd/Mg-La mixed oxide. The X-ray diffraction (XRD) pat-
terns of the fresh and used samples were obtained on
a Rigaku Miniflex X-ray diffractometer using Ni filtered Cu
Ka radiation (l=0.15406 nm) from 2q=15 to 708, at a scan
rate of 28minÀ1, with the beam voltage and beam current of
30 kV and 15 mA, respectively. The XPS analysis of the Pd/
Mg-La mixed oxide sample was recorded using a Kratos
Axis Ultra Imaging X-ray photoelectron spectrometer
equipped with Mg anode and a multichannel detector.
Charge referencing was done against adventitious carbon (C
1s, 284.8 eV). Shirley-type background was subtracted from
the signals. The recorded spectra were always fitted using
Gauss–Lorentz curves to determine the binding energies of
the different elements. For the TEM analysis the samples
were dispersed in methanol solution and suspended on
a 400-mesh, 3.5 mm diameter Cu grid and images were
taken using JEOL JEM 2010 high-resolution transmission
electron microscope. The BET surface areas of the fresh
and used catalysts were recorded on a Micromeritics ASAP
2020 Surface Area and Pore Analyzer instrument. Prior to
surface area analysis all the samples were degassed at
shows consistent activity and selectivity up to 5 cycles
with four different substrates. After each cycle, Pd/
Mg-La mixed oxide catalyst was recovered by simple
centrifugation, washed, air-dried and used directly for
the next cycle without further purification. No loss of
catalytic activity was observed for the catalyst in the
hydrogenation reaction. The leaching of the metal
after the first cycle was determined by AAS and was
found to be negligible.
Table 3. Recyclability test of the catalyst with different sub-
strates.[a]
[a]
Reaction conditions: substrate (2 mmol), 15 mg of Pd/
Mg-La mixed oxide catalyst (0.677 mmol% of palladi-
um), ethanol (10 mL), stirring under one atmosphere of
hydrogen pressure at room temperature.
1
1008C. H and 13C NMR spectra were recorded on a Bruker
200, Inova 500 and Avance 300 (300 MHz) spectrometer in
CDCl3 using TMS as internal standard. The elemental analy-
sis of the fresh and used Pd/Mg-La mixed oxide samples
[b]
Isolated yield.
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Chemoselective Hydrogenation of the Olefinic Bonds
were analyzed by atomic absorption spectroscopy (AAS)
Perkin–Elmer, Analyst-300.
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Catalytic Tests
The reactions were performed at room temperature using
2 mmol of substrate and 10 mL of solvent with 15 mg of
solid catalyst. The products were separated at the end of the
reaction and characterized by NMR (1H and 13C). The reac-
tion usually produced single product with no other side reac-
tion.
Supporting Information
Characterization of all compounds with 1H and 13C NMR
spectra are available in the Supporting Information.
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Acknowledgements
R.K. and M.S. thank to CSIR, New Delhi for the award of
Junior Research Fellowships.
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