Selective reductive coupling of nitro compounds with aldehydes to nitrones in H2 using carbon-supported and -decorated platinum nanoparticles

Article abstract of DOI:10.1002/anie.201402878

Nitrones were synthesized in high yields directly from nitro compounds, aldehydes, and H₂ using carbon-decorated platinum nanoparticles. The high selectivity for nitrone synthesis contrasts that of common supported metal catalysts and correspon

Full text of DOI:10.1002/anie.201402878

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Angewandte
Communications
DOI: 10.1002/anie.201402878
Selective Reductions
Selective Reductive Coupling of Nitro Compounds with Aldehydes to
Nitrones in H2 Using Carbon-Supported and -Decorated Platinum
Nanoparticles**
Larisha Cisneros, Pedro Serna,* and Avelino Corma*
Abstract: Nitrones were synthesized in high yields directly
from nitro compounds, aldehydes, and H2 using carbon-
decorated platinum nanoparticles. The high selectivity for
nitrone synthesis contrasts that of common supported metal
catalysts and corresponds to an increase from roughly 6 to
97%. The catalytic performance is tuned by precise control of
the structure of the active sites and the characteristics of the
support.
N
itrones are important organic building blocks with
applications as radical spin-traps,[1] antioxidants,[2] enzymes
inhibitors,[3] and precursors of a number of specialty nitrogen-
containing derivatives such as isoxazolidines, isoxazolines,
and b-amino alcohols.[4] b-Lactams, which are among the most
commonly prescribed drugs in the world, can be obtained, for
example, from nitrones through 1,3-dipolar cycloaddition
with alkynes,[5] and important g-amino ester derivatives can
be synthesized by the addition of nitrones to a,b-unsaturated
esters.[6] The synthetic value of nitrones is prominent and new
reactions are continuously emerging, for example, for the
preparation of novel molecules that incorporate chiral centers
by means of stereocontrolled nucleophilic additions.[7a–g]
Typically, nitrones are synthesized by one of the following
routes (Scheme 1A):[8] a) oxidation of secondary amines,[9]
hydroxylamines,[10] or imines;[11] b) condensation of carbonyl
compounds with hydroxylamines;[12] and c) N-alkylation of
oximes.[13] Hydroxylamines can be synthesized by partial
hydrogenation of nitro compounds (Scheme 1A, route b).
The low stability of the NHOH group limits, however, the
class of reducing agents that can be employed. Whereas
stoichiometric reducing agents such as Zn dust afford the
hydroxylamine intermediate in high yields,[14] solid metal
catalysts used with H2, industrially preferable, are inefficient
Scheme 1. A) Some representative routes for the synthesis of nitrones.
Common protocols require several reaction steps and the use of
stoichiometric reducing agents. Carbon-supported Pt nanoparticles
form the nitrone selectively directly from an aldehyde, a nitro com-
pound, and H2. B) Typical reactions in mixtures of H2 and nitro and
carbonyl compounds.
due to complete reduction of the NO2 group, and/or the
formation of azo, azoxy, or hydrazo intermediates.[15] Further
inconveniences associated with hydroxylamines are the fact
that cannot be stored, are potentially explosive,[15] and have
only limited commercial availability.
Oximes, on the other hand, are often synthesized by
condensation of hydroxylamine and carbonyl compounds.
Alternatively, oximes can be obtained by hydrogenation of
a,b-unsaturated nitro compounds, which requires a catalyst
[*] L. Cisneros, Dr. P. Serna, Dr. A. Corma
Instituto de Tecnologꢀa Quꢀmica
Universidad Politꢁcnica de Valencia-Consejo Superior de Investi-
gaciones Cientꢀficas
=
that prevents the reduction of the C C bond. Gold nano-
particles supported on TiO2 exemplify such a catalyst, facil-
itating the formation of the oxime with high selectivity in
H2.[16] The overall sequence of reactions to obtain nitrones
from oximes is, however, complex, as the a,b-unsaturated
nitro compound must be first prepared from a nitroalkane
and an aldehyde (or ketone) by the Henry reaction followed
by two additional reaction steps (Scheme 1A, route c).
Recently, a new method for the preparation of cyclohexanone
oxime directly from nitrobenzene has been reported.[17]
Otherwise, the synthesis of nitrones by oxidation of imines
or secondary amines involves a multistep process, which often
Avda. de los Naranjos s/n, 46022 Valencia (Spain)
E-mail: psername@itq.upv.es
acorma@itq.upv.es
Homepage: http://itq.upv-csic.es/
[**] The research was supported by Project CONSOLIDER INGENIO
(MULTICAT), PROMETEO, the SEVERO OCHOA Program for
centers of excellence, the “Subprograma Ramon y Cajal” (contract
RYC-2012-10662 to P.S.), and the CONACyT Program (fellowship to
L.C.).
Supporting information for this article (including experimental
details and additional characterization) is available on the WWW
under http://dx.doi.org/10.1002/anie.201402878.
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starts with the reduction of a nitro compound at the earliest
reaction steps (Scheme 1A, route a).
caused a notable loss of selectivity due to reduction of 5-
methyl furfural to alcohol 6.
Alternatively, the production of nitrones from a nitro
compound reduced in the presence of aldehydes is a more
direct and atom-efficient route, whereby hydroxylamine
generated in situ reacts with the carbonyl compound in
a cascade-type reaction. This approach circumvents hydroxyl-
amine storage issues and minimizes the number of exper-
imental steps, but requires a catalyst that prevents the
complete reduction of the NO2 group, the hydrogenation of
the aldehyde, and the degradation of the nitrone via
secondary reaction paths (Scheme 1B). The approach has
been proved to work with stoichiometric reagents such as Zn
dust,[18] but then large amounts of waste are generated. The
discovery of a solid catalyst that affords the nitrone in high
yields, directly from nitro compounds, aldehydes, and H2,
would be of interest.
Herein we describe the design of a selective solid metal
catalyst by controlling the characteristics of the support and
the metal particle architecture to avoid other competing
reactions. Specifically, this catalyst consists of platinum
nanoparticles that exclude atoms of high coordination
for interaction with the reactants, as well as supports that
activate the nitroaromatic compound and its reduced deriv-
atives. This control of the catalytic selectivity in complex fine-
chemistry processes by manipulation of a solid catalyst is
unusual.[19]
Remarkably, the nitrone selectivity could be increased
after controlling the structure of the metal active sites and the
characteristics of the support. For example, at 308 K and
5 bar, platinum nanoparticles supported on TiO2 (0.2 wt% Pt)
activated at 573 K in H2 provide 72% selectivity to nitrone 3
at almost complete conversion of the aldehyde, along with the
imine 4 as the main by-product. When the same catalyst was
activated in H2 at a higher temperature (723 K), the nitrone
selectivity dropped to 57% under identical experimental
conditions and similar conversion (the imine was, again, the
most significant by-product). No loss of selectivity due to
hydrogenation of the carbonyl function of 5-methyl furfural
was observed in these cases.
The decay in selectivity as the Pt/TiO2 catalyst is activated
in H2 at the highest temperature (723 K) takes place with the
concomitant appearance of TiOx species on top of the Pt
nanoparticles, as evidenced by HRTEM, EDS analysis, and
IR spectroscopy (Figures S1 and S2). This decoration effect
indicates the occurrence of strong metal–support interactions,
typical of supports that are reducible metal oxides,[23] which
increases the contact area between Pt and TiO2. The
detrimental effect of the metal/support boundary on nitrone
selectivity is consistent with the role attributed to TiO2 in
minimizing the formation of hydroxylamine during the
hydrogenation of nitro compounds[24] and, thus, in minimizing
the hydroxylamine–aldehyde condensation path (Scheme 1).
Table 1 summarizes the catalysts that we tested using the
coupling of nitrobenzene with 5-methylfurfural as
a model reaction. We investigated Au/TiO2, Pt/TiO2,
and Pt/C catalysts, which, properly prepared,[20a–c] are
Table 1: Catalytic performance[a] of various catalysts for the reductive coupling of
nitrobenzene and 5-methyl furfural to nitrone 3.
highly selective for the hydrogenation of NO2
groups, together with other common supported
metal catalysts. The percentage value that accom-
panies each catalyst in this paper refers to a nominal
weight percent metal content. Selectivities and
conversions mentioned in the text are calculated
based on gas chromatography data. Yields of isolated
products are also reported in Table 2.
Catalyst[b]
Metal
[%][g]
t
[h]
Conv.
[%]
Selectivity [mol%][h]
nitrone imine amine alcohol
Despite their expected tolerance of the carbonyl
function,[20a] gold nanoparticles supported on TiO2
showed low selectivity to nitrone formation (15%)
and, instead, the imine 4 was obtained preferentially
(Table 1), in agreement with previous reports.[21] The
reaction temperature required with the Au/TiO2
catalyst is relatively high (373 K) due to the limited
capacity of gold for activating H2 in the presence of
the nitroaromatic compound.[22] We noticed that the
use of high temperatures causes the selectivity to
nitrone 3 to decrease (Table S1).
By switching from gold to platinum we were able
to tackle the reaction at lower temperatures (298–
308 K). Unfortunately, attempts to use typical (com-
mercially available) supported metal catalysts to
obtain the nitrone were fruitless. For example, the
reaction with 5 wt% Pt/Al2O3 (Aldrich) at 308 K and
5 bar of H2 gave nitrone 3 with low selectivity (6%;
Table 1) and the secondary amine 5 as the predom-
3
4
5
6
[c,d]
1% Au/TiO2
0.50
2.5
3.0 95
2.0 97
0.7 99
0.5 98
1.7 98
1.3 97
1.1 100
0.9 80
4.3 95
0.7 93
2.0 54
3.0 93
15
6
69
4
25
42
3
5
3
4
14
8
11
57
3
3
15
0
0
6
13
0
6
0
0
5% Pt/Al2O3-CM
[e]
0.2% Pt/TiO2
0.2% Pt/TiO2
0.11
0.20
0.20
1.01
0.18
72
57
78
51
97
74
84
82
0
[f]
1
0.2% Pt/Al2O3
5% Pt/Al2O3
0.2% Pt/C
12
30
0
14
0
8
61
93
0.2% Pt/graphite 0.12
2% Ru/C
0.2% Pd/C
1.95
0.10
5% Pd/Al2O3-CM 4.6
1% Pd/C-CM 0.48
0
0
39
1
6
[a] Reactions performed at 308 K and 5 bar of H2, unless otherwise indicated. Feed
composition (mol%): 90.5% ethanol, 6% nitrobenzene, 3% 5-methyl furfural, 0.5%
o-xylene. [b] Catalysts activated in H2 flow at 723 K, unless otherwise indicated. The
notation CM refers commercially available samples. Metal content given as
a nominal weight percent. [c] Activated in air at 673 K. [d] Tested at 373 K and 8 bar.
[e] Activated in H2 at 473 K. [f] Activated in H2 flow at 723 K, leading TiOx-decorated
Pt nanoparticles. [g] (mol of metal)/(mol of 5-methyl furfural)ꢂ100 [h] Determined
inant reaction product. This catalyst, moreover, by GC analysis. Balance corresponds to small amounts of unidentified products.
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Table 2: Catalytic performance[a] of 0.2 wt% Pt/C reduced in H2 at 723 K
for the reductive coupling of various nitro compounds and aldehydes.
Consistent with our inference, the yield of nitrone could
be increased when a non-reducible metal oxide, Al2O3, served
as the support for the platinum nanoparticles. In the absence
of a strong metal–support interaction, the selectivity to
nitrone was 78% at 98% conversion using a 0.2 wt% Pt/
Al2O3 catalyst. We detected other differences in the product
distribution depending on the support, as the 0.2 wt% Pt/TiO2
sample produces the imine 4 as the main by-product, whereas
0.2 wt% Pt/Al2O3 forms significant amounts of the secondary
Product
t [h] Conv. [%][b] Sel. [%][b] Yield [%][c]
1.1
1.5
99
99
97
89
86
76
=
amine 5 (through further reduction of the N C group). We
stress, moreover, that despite the overall higher nitrone
selectivity of the 0.2 wt% Pt/Al2O3 catalyst, this sample is not
completely selective for reduction of the nitro group of
nitrobenzene over the carbonyl group of 5-methylfurfural.
Indeed, alcohol 6 is produced in 5% yield; it was not
produced with the 0.2 wt% Pt/TiO2 catalyst.
Additional experiments pointed to differences in product
distribution as a function of the architecture of the platinum
nanoparticles. To obtain Pt nanoparticles with different
average crystallite size on Al2O3, the metal content on the
surface of Al2O3 was varied from 0.2 wt% to 5 wt%, followed
by reduction of the catalyst in H2 at 723 K. Results are shown
in Figure 1. The microscopy images show that the fraction of
species larger than 2 nm is negligible in the 0.2 wt% Pt/Al2O3
catalyst, whereas particles within the range 2–4 nm are more
abundant in the 5 wt% Pt/Al2O3 sample, together with some
aggregates larger than 15 nm (Figure 1A,B). IR spectra of the
1.3
0.5
98
99
96
94
87
85
2.0
0.4
1.0
0.6
98
97
98
99
96
92
90
95
86
88
79
85
Pt/Al2O3 catalysts probed with CO show that the ratio of low-
[25]
coordination Pt atoms (band centered at 2056 cmÀ1
)
to
atoms on terrace positions (band centered at 2070 cmÀ1
)
[26] is
greater in the 0.2 wt% Pt/Al2O3 catalyst, as expected for
metal nanoparticles that are, on average, smaller. Together
with these bands, a less intense signal at about 1825 cmÀ1 is
observed for both samples, which is ascribed to CO species
bridging platinum sites of the nanoparticles.[26]
When the results obtained with the 0.2 wt% and 5 wt%
Pt/Al2O3 samples are compared for the reaction of nitro-
benzene and 5-methyl furfural in H2, 78% and 51%
selectivity to nitrone 3 is obtained, respectively, at high
conversion of the aldehyde (Table 1). Thus, we infer that the
presence of extended platinum surfaces has a negative impact
on the nitrone selectivity. Moreover, the 5 wt% Pt/Al2O3
catalyst produces larger amounts of the alcohol 6, which
agrees well with earlier reports pointing to a loss of chemo-
selectivity in the hydrogenation of nitro groups when the
number of platinum terrace sites is increased.[20a]
4.5
91
90
51
50
n.d.[d]
43
13.0
From all the above, we infer that to avoid products other
than the nitrone the catalyst should contain neither an
activating support nor terrace Pt atoms. As will be shown,
we achieved such a catalyst using a high-surface-area carbon
as the support.
14
96
27
24
Deposition of 0.2 wt% Pt onto porous active carbon
(Darco KB-B, 100 mesh), followed by reduction in H2 at
723 K, results in the formation of small crystallites that range
from 0.6 to 3.5 nm in diameter, as determined by STEM
(Figure 1C). Interestingly, whereas the particle size distribu-
tion of this catalyst and that of the 0.2 wt% Pt/Al2O3 sample
are similar, the IR spectrum of the former (probed with CO)
indicates that the carbon-supported sample consists of more
[a] Typical feed composition (mol): 90.5% ethanol, 6% nitro compound,
3% aldehyde, 0.5% o-xylene (internal standard). 50 mg of catalyst per
900 mg of feed. See the Supporting Information for the exception with
the bromine-substituted nitrone. [b] Conversion and selectivity calcu-
lated based on gas chromatography. [c] Yield of isolated product
determined for scaled-up reactions (16 g of feed). [d] Not determined.
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with H2PtCl6 followed by reduction in H2 at 723 K, the
selectivity to nitrone 3 was only 74% at 80% conversion
(Table 1). This catalyst behaves similar to the 0.2 wt% Pt/
Al2O3 sample, both showing similar particle size distributions,
and the secondary amine and 5-methyl furfuryl alcohol as the
main by-products.
To investigate whether the preceding synthetic approach
could be expanded to prepare selective catalysts with other
metals, porous active carbon was used to support palladium
and ruthenium. Accordingly, 0.2 wt% Pd/C and 0.2 wt% Ru/
C samples led to selectivities to the nitrone of 80 and 84%,
respectively, at almost complete conversion of the aldehyde.
These values are lower than those obtained with the 0.2 wt%
Pt/C catalyst (Table 1), but remarkably better than those
obtained with commercially available Pd catalysts (< 6%,
Table 1).
Finally, the most selective Pt/C catalyst was tested for
reactions of various nitro compounds and aldehydes in H2
(Table 2). Good selectivities to the corresponding nitrones
(> 94%) were obtained for reactions of aromatic aldehydes
such as benzaldehyde, furfural, and 5-(hydroxymethyl) furfu-
ral with nitrobenzene. Nitrobenzenes incorporating electron-
withdrawing group such as Cl and Br also yielded the halide-
substituted nitrones in high yields (94% and 90%, respec-
tively) in the presence of 5-methylfurfural, with no dehaloge-
nation side reactions. Good results were obtained as well with
substrates having electron-donating such as 4-nitroanisole
and 4-nitrotoluene (88 and 93% yield, respectively). Ali-
phatic nitro compounds showed lower nitrone selectivities
(30–50%), as a parallel reduction of the carbonyl function
was then observed. We note that some of the products, such as
N,a-diphenyl nitrone and its derivatives, are very useful
building blocks in organic synthesis,[5,7,10–13] and some nitrones
reported here for the first time may contribute to expand the
scope of applications.
Figure 1. STEM images, particle size histograms, and IR spectra of the
following catalysts: A) 0.2 wt% Pt/Al2O3; B) 5 wt% Pt/Al2O3; and
C) 0.2 wt% Pt/C. IR spectra were recorded after probing the catalysts
with CO at 298 K. Kinetic curves (right hand) correspond to conversion
of 5-methyl furfural (diamonds), and selectivities to nitrone (squares),
imine (crosses), secondary amine (triangles) and 5-methyl furfuryl
alcohol (dash) during the reductive coupling of nitrobenzene and 5-
methyl furfural at 308 K and 5 bar.
While the catalyst provides only moderate selectivity with
nitroalkanes, we would like to highlight the remarkable
selectivity of the 0.2 wt% Pt/C catalyst for the formation of
aromatic nitrones in the presence of alkenes and alkynes. For
example, mixtures of nitrobenzene, 5-methyl furfural, and
styrene (or phenylacetylene) yielded the nitrone in high yields
=
ꢀ
with very little hydrogenation of the C C (or C C) bond
(< 2%; Table 3). In contrast, other catalysts, such as 0.2 wt%
Pt/Al2O3 or 0.2 wt% Pd/C, caused a fast and complete
reduction of styrene (or phenylacetylene) before the nitrone
reaction could be completed. The compatibility of the
0.2 wt% Pt/C catalyst with alkenes and alkynes is relevant,
because the latter and nitrones are susceptible to 1,3-dipole
cycloadditions giving isoxazolidines or isoxazolines, and, from
them, important b-amino alcohols could be synthesized.[4]
In conclusion, we show that platinum nanoparticles can be
tuned to synthesize nitrones selectively directly from nitro
compounds, aldehydes, and H2. This is accomplished by
hindering the accessibility of the reactant to platinum on
terrace positions of the nanoparticles and by avoiding
supports that activate the nitro compound to minimize the
hydroxylamine intermediate.
highly unsaturated Pt species (band at 2040 cmÀ1) and
completely lacks sites attributable to Pt in terrace
(> 2050 cmÀ1) or bridging (< 1850 cmÀ1) positions. HRTEM
images of this sample reveal that the platinum nanoparticles
are embedded in the pores of the support (Figure S2), and we
infer that the lack of terrace sites accessible to CO is due to
the presence of carbon species covering the surface of the
platinum nanoparticles. With only highly unsaturated plati-
num species exposed, almost quantitative yields to the nitrone
is achieved (Table 1, 0.2 wt% Pt/C, 97% selectivity at 100%
conversion). No hydrogenation of 5-methyl furfural is
observed, corresponding to a catalyst that is highly chemo-
selective.[20a]
Consistent with the results above, when small platinum
nanoparticles (1.4 Æ 0.4 nm, Figure S3) were synthesized on
a nonporous graphitic support (0.2 wt% Pt) by impregnation
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Table 3: Reductive coupling of nitrobenzene and 5-methylfurfural in the
presence of styrene or phenylacetylene[a] using various catalysts at 308 K
and 5 bar of H2.
Catalyst[b]
Cosubstrate
t
[h]
C C or C C Nitrone
=
ꢀ
conv. [%][c]
yield [%][d]
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0.2 wt% Pt/Al2O3 phenylacetylene 1.3 100
0.2 wt% Pd/C phenylacetylene 0.3 100
styrene
phenylacetylene 1.1
1.0
1
2
96
95
22
27
[a] 900 mg of a feed with the following molar composition: 87.5%
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=
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Received: February 28, 2014
Revised: May 15, 2014
Published online: July 9, 2014
Keywords: cascade reactions · hydrogenation ·
.
supported catalysts · nitrones
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