Nanopalladium on amino-functionalized mesocellular foam as an efficient and recyclable catalyst for the selective transfer hydrogenation of nitroarenes to anilines

Article abstract of DOI:10.1002/cctc.201300769

Herein, we report on the use of nanopalladium on amino-functionalized siliceous mesocellular foam as an efficient heterogeneous catalyst for the transfer hydrogenation of nitroarenes to anilines. In all cases, the protocol proved to be highly selective and favored the formation of the desired aniline as the single product in high yields with short reaction times if naturally occurring and renewable γ-terpinene was employed as the hydrogen donor. Furthermore, the catalyst displayed excellent recyclability over five cycles and negligible leaching of metal into solution, which makes it an eco-friendly and economic catalyst to perform this transformation. The scalability of the protocol was demonstrated with the reduction of 4-nitroanisole on a 2 g scale, in which p-anisidine was isolated in 98 % yield. Copyright

Full text of DOI:10.1002/cctc.201300769

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DOI: 10.1002/cctc.201300769
Nanopalladium on Amino-Functionalized Mesocellular
Foam as an Efficient and Recyclable Catalyst for the
Selective Transfer Hydrogenation of Nitroarenes to
Anilines
Oscar Verho,[a] Anuja Nagendiran,[a] Cheuk-Wai Tai,[b] Eric V. Johnston,*[a] and Jan-
E. Bꢀckvall*[a]
Herein, we report on the use of nanopalladium on amino-func-
tionalized siliceous mesocellular foam as an efficient heteroge-
neous catalyst for the transfer hydrogenation of nitroarenes to
anilines. In all cases, the protocol proved to be highly selective
and favored the formation of the desired aniline as the single
product in high yields with short reaction times if naturally oc-
curring and renewable g-terpinene was employed as the hy-
drogen donor. Furthermore, the catalyst displayed excellent re-
cyclability over five cycles and negligible leaching of metal
into solution, which makes it an eco-friendly and economic
catalyst to perform this transformation. The scalability of the
protocol was demonstrated with the reduction of 4-nitroani-
sole on a 2 g scale, in which p-anisidine was isolated in 98%
yield.
Introduction
The selective reduction of nitroarenes to anilines is an impor-
tant transformation in both academic and industrial-scale or-
ganic chemistry given that anilines constitute key intermedi-
ates or targets in the synthesis of agrochemicals, dyes, phar-
maceuticals, pigments, and polymers.[1] This transformation has
traditionally been performed with stoichiometric amounts of
metallic Fe-[2] or Zn-based[3] reagents in the presence of
a proton source by various Raney Ni-based protocols[4] or by
other catalytic systems that employ toxic reductants such as
NaBH4,[5] H2S,[6] or N2H4.[7] Consequently, a substantial amount
of research has been dedicated into the development of new
and efficient catalytic protocols that use environmentally
friendly reductants. Many catalytic systems for the transition-
metal-catalyzed hydrogenation of nitroarenes to anilines have
shown excellent efficiency and synthetic utility,[8] but the han-
dling of H2 on an industrial scale is associated with safety
issues. Therefore, recent attention has been directed towards
transfer hydrogenation methods that avoid the use of H2 by
the application of safer and more convenient hydrogen
donors. These studies have resulted in a range of catalytic
methods for the reduction of nitroarenes based on both ho-
mogeneous[9] and heterogeneous[10] transition-metal catalysts
that employ green hydrogen donors, such as formic acid, for-
mate salts, and isopropanol. Heterogeneous compared to ho-
mogeneous catalysis holds greater promise for use in large-
scale industrial applications because of the simple separation
and recycling of the catalyst as well as the minimization of
metal contamination in the final product.
Recently, we reported the preparation of Pd nanoparticles
on aminopropyl-functionalized siliceous mesocellular foam
(Pd0-AmP-MCF).[11] This powerful catalyst has thus far been suc-
cessfully employed as an efficient and recyclable catalyst for
the aerobic oxidation of alcohols,[11a] racemization of ami-
nes,[11b] Suzuki cross-couplings,[11c] and for the transfer hydroge-
nation of alkenes that uses 1-methyl-1,4-cyclohexadiene as the
hydrogen donor.[11c]
This mesoporous silica material possesses many desirable
properties, which are reflected by its frequent use as a support
for both chemical and biochemical catalysts.[11,12] The material
has an adjustable 3D network of pores that confers a high sur-
face area and shields the catalytic species from mechanical
grinding, which thus reduces catalyst leaching. Furthermore,
the material has a high surface concentration of silanol groups
that can serve as handles for the immobilization of various cat-
alytic species, such as enzymes, nanoparticles, and defined
transition-metal complexes.
[a] O. Verho, A. Nagendiran, Dr. E. V. Johnston, Prof. Dr. J.-E. Bꢀckvall
Department of Organic Chemistry
Arrhenius Laboratory, Stockholm University
SE-106 91, Stockholm (Sweden)
If we consider our successful results in the transfer hydroge-
nation of alkenes with the nanopalladium catalyst on AmP-
MCF, we envisioned that it would be possible to extend this
methodology to also include the selective reduction of nitro
compounds to the corresponding amines. The protocol report-
ed herein can be applied to the reduction of a broad range of
nitro compounds to amines in an economical and practical
fashion.
E-mail: eric@organ.su.se
jeb@organ.su.se
[b] Dr. C.-W. Tai
Department of Materials and Environmental Chemistry
Arrhenius Laboratory, Stockholm University
SE-106 91 Stockholm (Sweden)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300769.
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Results and Discussion
Table 1. Hydrogen donor screening of the Pd0-AmP-MCF-catalyzed
transfer hydrogenation of 1a.[a]
Screening of reaction conditions
4-Nitroanisole (1a) was chosen as a model substrate for reac-
tion optimization, in which the effects of the hydrogen donor,
solvent, and reaction temperature were investigated. From pre-
vious studies in our laboratory on related hydrogen-transfer re-
actions, the best solvent was found to be EtOH, which worked
well at 908C.[11c] Therefore, we used this as a starting point and
screened the most commonly used hydrogen donors. These
experiments were performed with 0.40 mmol of substrate 1a
and 1 mol% Pd nanocatalyst in 1 mL EtOH at 908C. All experi-
ments were performed under identical conditions in pressure-
sealed microwave vessels under air atmosphere and were
Entry
H donor
Equiv.
Conv.[b]
[%]
1
3.0
3.0
73
91
2
3
3.0
>99
14
1
probed by H NMR spectroscopy.
Initial experiments performed with our Pd nanocatalyst and
formate salts showed promising results, in which sodium for-
mate (2), potassium formate (3), and cesium formate (4) gave
73, 91, and >99% conversion to 4-methoxyaniline (1b), re-
spectively (Table 1, Entries 1–3). The observed cation effect for
these hydrogen donors was expected as an increase in cation
size increases the solubility of the formate salt in EtOH and
allows for a more efficient reaction. Unfortunately, the use of
formate salts (2–4) as hydrogen donors resulted in substantial
amounts of precipitated ethoxide salts during the course of
the reaction. Not only did the ethoxide salt formation hinder
the stirring of the reaction but it also increased the pH of the
reaction significantly, which had a negative effect on the silica-
based support. Consequently, the recyclability of the catalyst
for these reactions was poor, and TEM images of the collected
catalyst indicated agglomeration of the Pd nanoparticles:
a marker of support degradation. Moreover, we cannot rule
out the possibility that the ethoxide salt blocks the pores of
the support, which thereby restricts access to the nanoparticles
on the inside. Attempts to improve the recyclability of the cat-
alyst by decreasing the extent of ethoxide salt precipitation
were performed by diluting the reaction or by the addition of
a phosphate buffer; however, neither method proved to be
successful.
4
11.0
neat
3.0
5
0
6[c]
85
7[d]
3.0
92
8
9
3.0
3.0
29
23
10
11
12
13
3.0
4.0
5.0
3.0
73
91
>99
0
14
15[e]
iPrOH
iPrOH
neat
neat
0
0
In contrast to the formate salts, the use of formic acid (5) as
the hydrogen donor proved to be significantly less efficient,
with only a low conversion to 1b observed if a 1:1 mixture of
formic acid/ethanol was used (Table 1, Entry 4). If the reaction
was performed in neat formic acid (Table 1, Entry 5), it resulted
in only the formation of traces of product with significant by-
16[f]
3.0
32
17[g]
18[h]
3.0
–
0
0
1
product formation as observed by H NMR spectroscopy. The
–
addition of triethylamine (NEt3) to the reactions that employed
5 as the hydrogen donor enhanced the rate and selectivity
greatly to result in 85% conversion with 1.2 equivalents of
NEt3 (Table 1, Entry 6) and 92% conversion with three equiva-
lents of NEt3 (Table 1, Entry 7). Unfortunately, a slight loss in
catalyst activity was observed over multiple cycles under the
employed reaction conditions, most likely because of the basic
nature of NEt3, therefore, this donor system was not investigat-
ed further.
[a] Unless otherwise noted the reaction conditions were: 1 mol% of Pd,
0.40 mmol of 1a, hydrogen donor (see table), in 1 mL of EtOH at 908C
for 30 min in an 8 mL microwave vial. [b] Conversion determined by
1H NMR spectroscopy. [c] Performed with 1.2 equiv. of NEt3 (commercial
formic acid/NEt3 complex 5:2 was used). [d] Performed with 3.0 equiv. of
NEt3. [e] Performed with 1 equiv. of Cs2CO3. [f] Performed with 1 mol% of
Pd/C. [g] Performed in the absence of Pd0-AmP-MCF. [h] Performed in the
absence of hydrogen donor.
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We next turned our attention towards 1-methyl-1,4-cyclo-
hexadiene (6), which was used as the hydrogen donor in our
previous study.[11c] The smooth transfer of two hydrogen atoms
with such dienes makes the overall reaction pH stay neutral,
and under these conditions no catalyst degradation occurs.
Diene 6 was rather slow as a hydrogen donor, and only 29%
conversion to aniline 1b was obtained from 1a after 30 min of
reaction (Table 1, Entry 8). Attempts to increase the rate by the
use of the corresponding unsubstituted 1,4-cyclohexadiene (7)
were unsuccessful and led to a lower rate with 23% conver-
sion after 30 min (Table 1, Entry 9). Therefore, we decided to
try the more substituted diene g-terpinene (8). Diene 8 is
a readily available and inexpensive natural product extracted
from plants and trees;[13] its cost is comparable to the formate
salts 2 and 3. The byproduct from 8, p-cymene, is not to be
considered only as waste as it is used in the chemical industry
for the production of p-cresol, fragrances, pharmaceuticals,
herbicides, and other fine chemicals.[14] There are few reports
on the successful use of 8 in the literature.[15]
Table 2. Solvent and temperature screening of the Pd0-AmP-MCF-
catalyzed transfer hydrogenation of 1a.[a]
Entry
Solvent
T
[8C]
t
Conv.[b]
[%]
[min]
1
2
3
4
5
6
7
8
EtOH
90
90
90
90
90
90
90
80
80
80
80
10
10
10
10
10
10
10
30
60
120
30
68
73
69
32
26
0
MeOH
acetone
EtOAc
THF
DMSO
toluene
EtOH
EtOH
EtOH
EtOH
0
49
63
82
95
9
10
11c
[a] Reaction conditions: 1 mol% Pd, 0.40 mmol 1a, 4 equiv. 8 in 1 mL
total volume (hydrogen donor+EtOH) at the temperature and time given
in the table in an 8 mL microwave vial. [b] Conversion determined by
1H NMR spectroscopy. [c] 1 mol% Pd, 1.20 mmol 8 (4 equiv.) in 3 mL total
volume (hydrogen donor+EtOH) at the temperature and time given in
the table in a 4 mL capped vial.
Interestingly, diene 8 (Table 1, Entry 10) was a significantly
more efficient hydrogen donor than 1-methyl-1,4-cyclohexa-
diene (6) and 1,4-cyclohexadiene (7) and gave a conversion of
73% after 30 min. If we increased the amount of 8 to four and
five equivalents, the conversion was enhanced to 91 and
>99%, respectively, after 30 min (Table 1, Entries 11 and 12). If
we switched to the related natural product (R)-limonene (9),
no formation of product was observed (Table 1, Entry 13),
which suggests that a higher reaction temperature is most
likely required for this donor to efficiently aromatize through
hydrogen transfer.
It was also investigated whether it was possible to run the
reaction at a lower temperature and retain acceptable yields
and reaction times. Unfortunately, initial experiments in sealed
microwave vessels at 808C proved to be quite disappointing
and gave only 82% after 2 h (Table 2, Entries 8–10). A pro-
longed reaction time only marginally improved the conversion,
which suggests that it was not possible to obtain complete
conversion within reasonable times under these reaction con-
ditions. These results cannot be explained by the decrease in
temperature alone, and we suspected that the pressure of the
reaction was also important for the outcome of the reaction.
To evaluate this hypothesis we decided to change from a mi-
crowave vial (8 mL total volume) to a smaller capped vial
(4 mL). By doing so, the free volume of gas in the reaction was
substantially decreased, which allowed for an increased con-
centration of in situ formed H2 in the solution. Gratifyingly, this
had a significant positive influence on the rate of the reaction,
which allowed for 95% conversion at 808C (Table 2, Entry 11)
after only 30 min.
The reduction of 1a was also tested in neat isopropanol,
with or without base (1 equiv. Cs2CO3), but no formation of 1b
could be observed in either case (Table 1, Entries 14 and 15).
At this stage, control experiments were performed to compare
the efficiency of the Pd nanocatalyst to the commercially avail-
able and standard Pd/C (10 wt%). A conversion of only 32%
was obtained with Pd/C, which demonstrates that our Pd
nanocatalyst is indeed more efficient for this transformation
(Table 1, Entry 16). Control experiments were performed in the
absence of either the Pd nanocatalyst or the hydrogen donor
6, and neither of these experiments gave any detectable
amounts of 1b (Table 1, Entries 17 and 18).
Substrate scope
We next studied the solvent effect on the reaction with the
use of four equivalents of hydrogen donor 8. To clearly visual-
ize the outcome of the reaction, the product formation was
A major strength of this reaction protocol is its ability to toler-
ate substrates that contain a wide range of functional groups.
General reaction trends related to the electronic properties of
the substrate were difficult to establish, and we speculate that
this is the result of several parameters that govern the overall
rate of the reaction. One could argue that electron-withdraw-
ing groups should facilitate the reaction as they would de-
crease the electron density of the nitro group and make it
more prone to undergo reduction. On the other hand, efficient
substrate coordination to the metal surface is a prerequisite
for the reaction to occur and this process would be favored for
substrates that contain electron-rich p systems. An electron-
rich p system could also impede the reaction as the resulting
aniline could compete for the free coordination sites on the
1
measured by H NMR spectroscopy after only 10 min as the re-
action reaches >90% conversion after 30 min. Acetone, EtOH,
and MeOH were shown to be the best solvents, which gave
conversions of around 70% after 10 min (Table 2, Entries 1–3),
whereas EtOAc and THF were poor solvents for this reaction
(Table 2, Entries 4 and 5), which gave 32 and 26% conversion,
respectively. DMSO and toluene were incompatible with this
protocol and resulted in no formation of product 1b after
30 min (Table 2, Entry 6 and 7). Subsequent experiments were
performed by using EtOH as the solvent as it is a greener alter-
native than MeOH and acetone.
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Pd. The introduction of a strongly
electron-donating substituent on
the substrate could, therefore,
result in an increased coordina-
tion ability of the amine group
of the product, which could pre-
vent access of the substrate and
hydrogen donor to the catalyst
surface. The steric properties of
the substrate are also expected
to play an important role in its
coordination ability.
Table 3. Transfer hydrogenation of various nitroarenes catalyzed by Pd0-AmP-MCF.[a]
Entry
Substrate
Product
Catalyst
[mol%]
Donor
[Equiv.]
t
Yield[b]
[%]
[min]
1.0
0.25[c]
4.0
6.0[c]
30
95
98[c]
300[c]
1
The variation of substrate sub-
stitution was observed to affect
the extent of the nonproductive
Pd-catalyzed dehydrogenation of
8 to produce p-cymene and H2.
To account for this decomposi-
tion, we found it necessary to
optimize the amount of hydro-
gen donor in reactions of certain
substrates.
2
3
1.0
1.0
4.0
5.0
60
45
>99
>99 (96)
4
5
1.0
1.0
6.0
6.0
180
150
97 (93)
Substrate 1a (Entry 1) reacted
rapidly under the optimized
standard conditions (1 mol% Pd,
4 equiv. g-terpinene, 808C) to
afford 1b in 95% yield after
30 min (Table 3). The less elec-
tron-rich substrates 4-nitroto-
luene (10a) and 1-tert-butyl-4-ni-
>99
6
7
1.0
2.0
5.0
4.0
60
60
90 (87)
91 (88)
trobenzene
(11 a)
required
longer reaction times than 1a to
reach completion (Table 3, En-
tries 2 and 3). The reduction of
10a afforded the corresponding
aniline (10b) in quantitative
yield after 1 h under the stan-
dard conditions (Entry 2), where-
as substrate 11 a required five
equivalents of 8 and a reaction
time of 45 min to give compara-
ble results (Entry 3). Interestingly,
nitrobenzene (12a) reacted sig-
nificantly slower than 1a, 10a,
and 11 a and required six equiva-
lents of 8 and a reaction time of
3 h to give aniline 12b in 97%
yield (Table 3, Entry 4). 1-Fluoro-
4-nitrobenzene (13a) displayed
a similar reactivity as 12a and
required six equivalents of 8 and
a longer reaction time to give
8
9
1.0
1.0
4.0
4.0
60
30
96 (93)
97 (95)
1.0
1.0
6.0
4.0
60
60
90
61
10
11
12
1.5
1.0
6.0
4.0
120
60
96
33
a
quantitative yield of 13b
(Table 3, Entry 5). Gratifyingly,
the reaction protocol was com-
patible with substrates that con-
tained trifluoromethyl, cyano,
1.0
5.0
90
>99 (99)
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ity trend of meta>ortho>para
based on electronic effects. We
believe the reason for the high
reactivity of the ortho isomer
20a is because of a directing co-
ordinative effect of the hydroxyl
group, which steers the nitro
group towards the Pd surface
and facilitates the reaction.
Table 3. (Continued)
Entry
Substrate
Product
Catalyst
[mol%]
Donor
[Equiv.]
t
Yield[b]
[%]
[min]
13
1.0
5.0
90
85
2.0
1.0
6.0
5.0
150
90
>99
39
We also demonstrated that
the protocol could be optimized
to work for 2-methyl-5-nitroani-
line (23a) and bulky 2-nitrobi-
phenyl (24a) to give the corre-
sponding anilines 23b and 24b
in yields of 89 and 97%, respec-
tively (Table 3, Entries 15 and
16). Moreover, it was possible to
achieve quantitative yields of
bulky 1-nitronaphthalene (25a)
and the heteroaromatic 3-nitro-
pyridine (26a), but in both cases
elevated catalyst loadings and
14
15
2.0
5.0
150
89
16
17
1.0
2.0
5.0
120
120
97 (96)
10.0
>99 (95)
10 equivalents of
8 were re-
18[d]
19
3.0
2.0
10.0
6.0
120
120
>99
quired (Table 3, Entries 17 and
18). The protocol was also suc-
cessful for the reduction of an
aliphatic substrate, in which 1-
hexylamine (27b) was obtained
in 92% yield from 1-nitrohexane
(27a) (Table 3, Entry 19). Notably,
in all of the reactions described
in Table 3, the desired aniline
was formed as the exclusive
product and the remaining ma-
92
[a] Unless otherwise noted the reactions were performed in a 4 mL capped vial with nitroarenes (1.20 mmol), 8
(see table), and Pd nanocatalyst (see table) in EtOH at 808C for the time given in the table. The total volume of
all the reactions (EtOH+8) was always kept at 3 mL. [b] Yield determined by H NMR spectroscopy with the in-
ternal standard 1,3,5-trimethoxybenzene. Isolated yield in parenthesis. [c] Large-scale experiment (13 mmol).
For experimental details see the Supporting Information. [d] 0.60 mmol nitroarene in 2.2 mL EtOH and 0.8 mL
8 (10 equiv.).
1
ester, ketone, and hydroxyl groups to afford the corresponding
anilines 14b–22b in high to excellent yields in short reaction
times (Table 3, Entries 6–14). Of particular interest is that the
nitro group can be reduced selectively in the presence of
a keto group as shown by the reduction of nitroacetophe-
nones 17a–19a. This selective reduction is considered to be
particularly challenging.[9b,16]
terial was identified as unreacted starting material by quantifi-
cation against an internal standard. This confirms that the for-
mation of other reduction products, such as hydroxylamines,
oximes, nitrones, and azo compounds, can only occur to negli-
gible extent. Moreover, a great portion of the aniline products
were isolated by either extraction or column chromatography
to confirm that the isolated yields were in line with the yields
measured by 1H NMR spectroscopy.
Furthermore, in the reduction of 17a–19a, a reactivity trend
was observed in which the para isomer reacted significantly
faster than the ortho isomer, which in turn reacted faster than
the meta isomer under the standard conditions. The higher re-
activity of the para and ortho isomers can be attributed to an
electronic resonance effect, which withdraws electrons from
the nitro group and makes it more prone to undergo reduc-
tion. This effect does not exist in the meta isomers, and conse-
quently it reacts significantly slower. The difference in reactivity
between the para and ortho isomers can be ascribed to sterics,
in which the rather bulky keto group in the ortho position pre-
vents access to the nitro group in 18a. Interestingly, if we per-
form the same comparison for the three isomers of nitrophe-
nol, we observed the following trend; ortho>meta>para. As
the hydroxyl group is electron-donating, we predicted a reactiv-
Unfortunately, the protocol gave poor results in the reduc-
tion of nitroarenes that contained olefin, acetylene, and halo-
gen (Cl, Br, I) substituents. In the case of 4-nitrophenylacety-
lene and 3-nitrostyrene, no reaction took place and only start-
ing material was recovered after 1 h at 808C, which suggests
that the acetylene and olefin moieties exhibited a poisoning
effect on the catalyst. Most likely the hydrogen donor cannot
compete with these substrates for access to the Pd surface,
and no Pd hydrides are formed that can perform the reduc-
tion. Also, in contrast to our previous study of the transfer hy-
drogenation of alkenes,[11c] no reduction of the C=C bond
could be observed under these reaction conditions, which was
attributed to the lower reaction temperature of the current
protocol.
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A similar effect was observed for the reactions that involved
halogenated substrates, in which no signs of products that
arise from nitro-group reduction were observed. It is suspected
that the cause for the poor conversion of these substrates can
also be ascribed to a poisoning effect, in which we propose
that the aryl halide undergoes an oxidative addition to the Pd
and prevents it from taking part in further reactions. This
1
Scheme 1. Large-scale reduction of 1a.
theory was supported by H NMR spectroscopy of the traces of
reaction products from the reactions of 1-chloro-4-nitroben-
zene, 1-bromo-4-nitrobenzene, and 1-iodo-4-nitrobenzene, in
which peaks that belong to nitrobenzene could be observed,
which arise from a reductive dehalogenation reaction. Howev-
er, this process was slow under the reaction conditions em-
ployed, with less than 5% of the dehalogenated products ob-
served in all cases. The remaining >95% was also identified as
(Scheme 1). At a 2.0 g scale (13 mmol) by using 0.25 mol%
Pd0-AmP-MCF and six equivalents of 8, we were able to isolate
1b in 98% yield after a reaction time of 5 h (at 808C) and
a simple extraction procedure.
Comparison with other systems for the transfer hydrogena-
tion of nitroarenes
1
unreacted starting material by H NMR spectroscopy.
The present system compares very well with systems pub-
lished previously for the transfer hydrogenation of nitroarene-
s.[7a,10a–f] It has the advantage that it works under neutral condi-
tions, whereas some of the previous systems require the pres-
ence of base.[10a,b,f] Another advantage of the present method
is that the catalyst allows for simple separation and can be re-
cycled several times without any significant loss of activity.
This has not been possible with some of the previous systems
in which catalyst deactivation and difficulties in the separation
of the catalyst from the reaction mixture have resulted in de-
creased reaction efficiencies over multiple cycles.[10c,e]
Recycling and large-scale experiments
The recyclability of heterogeneous catalysts is of fundamental
importance and an important advantage over homogeneous
counterparts. Consequently, we performed a recycling study, in
which the activity and selectivity of Pd0-AmP-MCF was investi-
gated for the reaction of 4-nitroacetophenone over five cycles.
Gratifyingly, the catalyst exhibited excellent recyclability with
sequential conversions of 97, 97, 97, 95, and 95% with a selec-
tivity of >99% towards 4-aminoacetophenone over all cycles.
In conformity with previous studies in which the recyclability
of this catalyst was exhibited, a sample of the catalyst was re-
covered from the last cycle of this transformation and analyzed
by high-angle annular dark-field scanning transmission elec-
tron microscopy (HAADF-STEM) to evaluate the nanoparticle
size and distribution.[11] As can be seen from the HAADF-STEM
images (Figures 1 and S1), no significant change in either parti-
cle size or distribution occurred after use in multiple cycles,
which confirms the recyclability and robustness of the catalyst.
The high recyclability can also be ascribed to the low leaching
of Pd from the catalyst, which was found to be <0.1 ppm by
inductively coupled plasma optical emission spectroscopy
(ICP-OES).[17]
Conclusions
We have demonstrated that Pd nanoparticles on aminopropyl-
functionalized siliceous mesocellular foam (Pd0-AmP-MCF) is
a highly efficient catalyst for the selective transfer hydrogena-
tion of a wide variety of nitroarenes, which furnishes the corre-
sponding anilines in good to excellent yields after short reac-
tion times on using g-terpinene (8) as the hydrogen donor.
With this hydrogen donor, the overall reaction pH stays neutral
and this feature makes 8 suitable for the transfer hydrogena-
tion of substrates that are acid- or base-sensitive. The Pd nano-
catalyst was highly recyclable and displayed low leaching (<
0.1 ppm) in the reaction of 4-nitroacetophenone. Moreover,
the protocol proved to be scalable, which was demonstrated
by performing the reduction of 4-nitroanisole on a 2 g scale to
give p-anisidine in 98% isolated yield. Future work will be
dedicated towards discovering new synthetic applications of
Pd0-AmP-MCF to enable the construction of tandem reaction
sequences that make use of the many reactivity modes of this
nanocatalyst. This would offer a more eco-friendly and waste-
reducing approach to produce chemicals for both academic
and industrial use.
As a measurement to test the practical utility of this proto-
col, the reduction of 1a was performed on a larger scale
Acknowledgements
Figure 1. HAADF-STEM images of Pd0-AmP-MCF with a 20 nm scale bar,
which show well-dispersed Pd nanoparticles in the size range of 1–2 nm.
a) Unused Pd nanocatalyst, b) Pd nanocatalyst recovered after the fifth cycle.
The Berzelius Center EXSELENT, the European Research Council
(ERC AdG 247014), and the Swedish Research Council are grate-
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 205 – 211 210

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Keywords: heterogeneous catalysis
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[17] A leaching test was performed on the reduction of 4-nitroacetophe-
none. Upon completion of the reaction, the reaction vessel was centri-
fuged, and an aliquot was withdrawn from the liquid phase. The aliquot
was passed through a pad of Celite to remove smaller Pd nanocatalyst
particles that did not sediment during the centrifugation. The filtered
aliquot was analyzed by elemental analysis (ICP-OES by Medac Ltd, UK),
and the Pd content was determined to be <0.1 ppm, which is below
the limit of quantification for this technique.
Received: September 13, 2013
Published online on October 25, 2013
ꢁ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2014, 6, 205 – 211 211