Insights into the oxidative dehydrogenation of amines with nanoparticulate iridium oxide

Article abstract of DOI:10.1002/chem.201301596

The aerobic oxidation of amines offers a promising route towards many versatile chemical compounds. Within this contribution, we extend our previous investigations of iridium oxide-catalyzed alcohol oxidation to amine substrates. In addition to demonstrat

Full text of DOI:10.1002/chem.201301596

FULL PAPER
DOI: 10.1002/chem.201301596
Insights into the Oxidative Dehydrogenation of Amines with Nanoparticulate
Iridium Oxide
Ceri Hammond, Martin T. Schꢀmperli, and Ive Hermans*[a]
Abstract: The aerobic oxidation of
amines offers a promising route to-
wards many versatile chemical com-
pounds. Within this contribution, we
extend our previous investigations of
iridium oxide-catalyzed alcohol oxida-
tion to amine substrates. In addition to
demonstrating the versatility of this
catalyst, particular attention is focused
on the mechanisms of the reaction.
Herein, we demonstrate that although
amines are oxidized slower than the
corresponding alcohols, the catalyst has
a preference for amine substrates, and
oxidizes various amines at turnover fre-
quencies greater than other systems
found in the open literature. Further-
more, the competition between double
amine dehydrogenation, to yield the
corresponding nitrile, and amine–imine
coupling, to yield the corresponding
coupled imine, has been found to arise
from a competitive reaction pathway,
and stems from an effect of substrate-
to-metal ratio. Finally, the mechanism
responsible for the formation of N-
benzACTHNUTRGENUGyN lidene-1-phenylmethanamine was
examined, and attributed to the cou-
pling of free benzyl amine substrate
and benzACHTNUTRGNEUNGaldehyde, formed in situ
through hydrolysis of the primary reac-
tion product, benzyl imine.
Keywords: amines · heterogeneous
catalysis · kinetics · oxidation · reac-
tion mechanisms
Introduction
surprising, considering that amines can act as versatile plat-
form molecules for the synthesis of a number of important
chemical species, such as oximes, imines, nitriles and azo-
compounds (Scheme 1).[5] The development of catalysts that
are active for the selective oxidation of amines, in addition
to alcohols, is thus a key challenge for the future.
Selective oxidations are some of the most important pro-
ACHTUNGTRENNUNGcesses across all levels of the chemical industry, and they
play a vital role in the functionalization of molecules. Never-
theless, in spite of their importance, they remain problemat-
ic transformations, from both a mechanistic and environ-
mental point of view.[1] Typically, selective oxidations are
performed with inorganic salts of high-valent transition
metals, such as CrVI or MnVII, or organic metal-free oxidants,
such as the Dess–Martin periodinane or various hydroperox-
ides. Although allowing for high levels of selectivity, the use
of these oxidants is often accompanied by the coproduction
of large amounts of organic and/or toxic waste, leaving each
oxidant rather undesirable from a sustainable chemistry
standpoint.[2] In recent times, research has, therefore, fo-
cused on the development of catalysts and catalytic systems
that are capable of selectively oxidizing hydrocarbons with
green and atom-efficient oxidants, such as dioxygen and hy-
drogen peroxide.[1,3] Nevertheless, achieving sufficient space-
time-yield with these more sustainable oxidants, while main-
taining high levels of selectivity, remains a challenge.
Scheme 1. Some important target molecules that can be obtained through
selective oxidation of amines.
Recently, we have shown that a heterogeneous catalyst,
comprised of ceria-supported nanoparticulate iridium oxide,
is an active catalyst for the selective oxidation of alcohols
with molecular oxygen.[6] The formation of iridium
ACHTUNGTRENNUNG(III)
oxide was found to be critical to the function of this catalyst,
which was able to oxidize alcohols to aldehydes through
À
cleavage of C H bonds, in a b-hydride elimination mecha-
In spite of the successes achieved in recent years with re-
gards to the aerobic oxidation of alcohols,[4] considerably
less attention has focused upon the aerobic oxidation of
analogous amine substrates. This lack of attention is rather
nism. Rapid reoxidation of the so-formed metal-hydroxide
species was achieved through the efficient transport of O2
by the collaborative support, CeO2, which allowed a catalyt-
ic cycle to be achieved. Herein, we demonstrate the applica-
bility of this catalyst for the selective oxidation of amines,
and pay particular attention to the numerous competing
mechanisms possible in this unique case.
[a] Dr. C. Hammond, M. T. Schꢀmperli, Prof. Dr. I. Hermans
Department of Chemical and Bio-engineering, ETH Zurich
Wolfgang-Pauli-Str. 10, 8093, Zꢀrich (Switzerland)
Fax : (+41)44-633-4258
E-mail: ive.hermans@chem.ethz.ch
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Results and Discussion
Initial amine oxidation studies:
Our preliminary studies focused
on the aerobic oxidation of
benzyl amine, under conditions
analogous to those previously
employed for the aerobic oxida-
tion of benzyl alcohol (Figure 1).
Figure 1 compares the tem-
poral evolution of conversion
for benzyl alcohol and benzyl
amine. As can be seen, the se-
Scheme 2. General mechanism accounting for the formation of N-benzylidene-1-phenylmethanamine and ben-
zonitrile from benzyl amine.
dene-1-phenylmethanamine (Scheme 2), although small
amounts (ca. 5–10% selectivity) of benzonitrile are also ob-
served, regardless of the conversion level. The presence of
benzonitrile can be rationalized by the consecutive oxidative
dehydrogenation of in situ formed benzyl imine, with two
oxidative steps resulting in the doubly dehydrogenated ni-
trile. In contrast, the formation of N-benzylidene-1-phenyl-
methanamine can most likely be attributed to a self-cou-
pling reaction between an unconverted molecule of benzyl
amine and the in situ formed imine.[5] Given that we failed
to detect any benzyl imine through conventional chromato-
graphic or mass-spectrometric techniques, we presume that
this self-coupling step is much faster than the initial oxida-
tive dehydrogenation process. To date, the mechanism of
formation of N-benzylidene-1-phenylmethanamine has not
yet been elucidated; this topic is discussed in the second
part of the publication.
The observation of N-benzylidene-1-phenylmethanamine
as the major product further confirms that 0.5Ir/CeO2(DP400R)
is much more active for the selective oxidation of alcohols
as opposed to amines. Although the initial substrate conver-
sion rate is higher for benzyl alcohol than for benzyl amine
(9.1 vs. 6.0 mmminÀ1), the conversion rate for benzyl amine
is exaggerated by the (homogeneous) removal of one equiv-
alent of benzyl amine through the self-coupling reaction. To
gain a better understanding of the activity of this catalyst
toward amine oxidative dehydrogenation, we investigated
the oxidation of dibenzyl amine, an frequently investigated
substrate in the open literature.
Figure 1. Temporal evolution of conversion for (A, hollow triangles)
benzyl alcohol and (B, filled triangles) benzyl amine. The inset is an ex-
pansion of the conversion vs. time profile for the first 30 min of reaction.
Reaction conditions: substrate (0.2m), catalyst (0.5 mol% relative to sub-
strate), O2 (1 bar), 908C.
lective oxidation of benzyl amine proceeds smoothly under
the influence of 0.5 wt% Ir/CeO2, prepared by deposition-
precipitation and reduced at 4008C prior to use (henceforth
0.5Ir/CeO2(DP400R)). In contrast to the selective oxidation of
benzyl alcohol,[6] a typical kinetic profile is observed, which
is free from both an induction period and a deactivation
mechanism. Therefore, despite being oxidized at a lower ini-
tial rate (6.0 mmminÀ1 benzyl amine converted vs.
9.1 mmminÀ1 benzyl alcohol), benzyl amine is quantitatively
converted within six hours reaction at 908C. In contrast,
benzyl alcohol rapidly reaches approximately 50% conver-
sion within one hour, before a severe deactivation process is
observed, thereby limiting further conversion. We have pre-
viously attributed this deactivation process in the case of
aerobic alcohol oxidation to the radical-based by-production
of benzoic acid,[6] and thus do not expect a similar event to
poison the catalyst in the case of benzyl amine oxidation.
Nevertheless, the lack of deactivation suggests that none of
the products formed during the reaction lead to deactivation
of the catalyst in this case.
As can be seen (Table 1), 0.5Ir/CeO2(DP400R) is a remark-
AHCTUNGTREGaNNNU bly active catalyst for this transformation, and performs
Table 1. Comparison of the catalytic activity of 0.5Ir/CeO2(DP400R) and var-
ious reported catalysts for the selective oxidative dehydrogenation of di-
benzyl amine.
[a]
Catalyst
Total TOF [hÀ1
]
Reference
0.5Ir/CeO2(DP400R)
HAuCl4·3H2O/CeO2
Au/C (activated)
Ru/Al2O3
88
82
99
1.6
this work
[7]
[8]
[9]
The major product formed (more than 90% selectivity)
during the aerobic oxidation of benzyl amine is N-benzyli-
[a] Turnover frequency calculated as moles (N-benzylidene-1-phyenylme-
thanamine) produced per mole (metal) per hour.
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Oxidative Dehydrogenation of Amines
FULL PAPER
this reaction at TOFs equal to, or indeed better than, the
most active catalysts reported to date. Surprisingly, we ob-
serve that the initial TOF (at 15 min) obtained for dibenzyl
amine (468 moles dibenzyl amine converted per mole of Ir
per hour) is, in fact, higher than those obtained for benzyl
amine (360 hÀ1) under analogous conditions, despite the ho-
mogeneous coupling reaction overestimating the TOFs by a
factor of two in the second case. We propose that the avail-
À
ACHTUNGTRENNUNGability of two b-hydrides, that is, two N H bonds, accounts
for this increased activity. Nevertheless, even compared to
this easily oxidized substrate, the TOFs obtained for alcohol
oxidation remain higher (significantly greater than
500 hÀ1).[6]
Mechanistic studies: Having benchmarked the activity of
the catalyst for the aerobic oxidation of amines, we subse-
quently turned our focus to the elementary reaction mecha-
nism and the overall mechanistic pathway. In particular, sev-
eral questions prevail: 1) Is the reaction mechanism the
same as previously observed for the aerobic oxidation of al-
cohols? 2) What is the mechanism of formation of N-benzy-
lidene-1-phenylmethanamine? 3) Why is it that some cata-
lysts reported in the literature are selective to nitriles,
whereas others preferentially form the self-coupled imine?
We previously observed that the aerobic oxidation of al-
cohols with this catalyst proceeded through a b-hydride
Figure 3. Effect of metal molar loading (relative to substrate) on the ini-
tial rate of benzyl amine oxidation. Reaction conditions: Substrate
(0.2m), catalyst (various mol% relative to substrate), O2 (1 bar), 908C,
15 min.
elimination mechanism, in which the cleavage of the benzyl-
[6]
À
ic C H bond was rate determining. Rapid re-oxidation of
the metal-hydroxide species closed the catalytic cycle and al-
lowed the relevant aldehyde to be obtained with high selec-
tivity. Key catalytic evidence, namely a zero-order depen-
ACHTUNGTRENNUNGdence on O2 concentration (Figure 2), and a first-order de-
Figure 4. Effect of substrate concentration on the initial rate of oxidation
for (filled triangles) benzyl alcohol, and (hollow triangles) benzyl amine.
Reaction conditions: Substrate, O2 (1 bar), catalyst (0.5 mol% relative to
substrate), 908C, 15 min.
amine concentration and reaction rate is also observed in
this case (Figure 4). From these experiments, we conclude
that the aerobic oxidation of amines proceeds analogously
to the aerobic oxidation of alcohols, and a b-hydride elimi-
nation mechanism is also prevalent for the reaction investi-
gated herein.
As expected from the initial experiments (Figure 1), it is
clear that the aerobic oxidation of benzyl amine proceeds at
a lower rate than that of benzyl alcohol, and this is observed
at all substrate concentrations investigated. This further con-
firms that the catalyst is more reactive towards the alcohol
substrate. Curiously however, the maximum achievable rate
of oxidation (Vmax) for benzyl amine, albeit lower than the
Vmax for benzyl alcohol, is obtained at a substrate concentra-
tion significantly lower than that required for the alcohol
(ca. 100 vs. 200 mm, for benzyl amine and benzyl alcohol, re-
Figure 2. Effect of O2 partial pressure on the initial rate of oxidation of
benzyl amine. Reaction conditions: Substrate (0.2m), catalyst (0.5 mol%
relative to substrate), O2 (1 bar), 908C, 15 min.
pendence on catalyst concentration (Figure 3) indicates that
the same elementary reaction mechanism is at play in this
case. Furthermore, as we previously observed for alcohol
substrates, a Langmuir–Hinshelwood relationship between
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I. Hermans et al.
spectively, see Figure 4). This demonstrates that the equili-
brium between adsorbed, activated substrate, and the free
substrate in solution is more readily achieved with the
amine, and suggests that the amine substrate would be more
readily coordinated in the case of a competitive oxidation
experiment.[10]
If added amine was poisoning the catalyst, then the initial
conversion rate would be expected to decrease again at
higher concentrations, that is, a volcano-type plot should be
observed. To investigate whether the amine was preferen-
tially oxidized over the alcohol substrate, we subsequently
investigated the effect of adding benzyl amine to a reaction
solution primarily containing benzyl alcohol.
As can be seen (Figure 5), the addition of even small
amounts of benzyl amine leads to remarkable decreases in
the rate of alcohol oxidation, confirming that the catalyst
preferentially oxidizes the amine substrate in the case of a
competitive oxidation experiment. It is likely that the extra
basicity of the amine functional group enhances its coordi-
nation to the active sites of the catalyst, and thereby pre-
vents those same sites from oxidizing the more weakly coor-
dinating alcohol.
Figure 6. Effect of benzyl amine concentration on the selectivity towards
benzonitrile. Reaction conditions: benzyl amine, catalyst (0.5 mol% rela-
tive to substrate), O2 (1 bar), 908C, 6 h.
further oxidized by the catalyst, thus leading to the corre-
sponding nitrile, or be attacked by a free molecule of benzyl
amine (Scheme 3), eventually yielding N-benzylidene-1-phe-
nylmethanamine. The balance between these two steps ap-
pears to be dependent on the concentration of free benzyl
amine and/or the catalyst. At high amine concentrations (or
low catalyst loadings), coupling is favored over the second
dehydrogenation step. However, in the absence of sufficient
benzyl amine, that is, when the amount of imine and the cat-
alyst surface dominates, consecutive dehydrogenation can
take place. It is notable that despite only a fourfold increase
in amine concentration (i.e., an increase from 50 to 200 mm
benzyl amine concentration), a tenfold decrease in nitrile se-
lectivity is observed. This suggests that the additional amine
substrate actively plays a role in displacing the imine inter-
mediate, and greatly enhances the coupling route by releas-
ing benzyl imine into solution.
In the context of the available literature, these observa-
tions explain why some catalysts—notably based on
Ru[9,11]— are so selective to nitriles, whereas others—such as
Au-based catalysts[7,8,12]— are primarily selective to the cou-
pled product. To date, most investigations based on Ru cata-
lysts have been performed at low substrate/metal ratios of
approximately 20–40, that is, a large amount of catalytic ele-
ment has been utilized. Under such conditions, the reaction
appears to be dominated by the available catalyst surface,
and the relative concentration of free amine substrate is low.
As such, it is unsurprising that a large selectivity for nitrile
products can be achieved. Conversely, most reactions involv-
ing Au have been performed at much higher substrate/metal
ratios (more than 100). Under such conditions, the relative
concentration of free benzyl amine is higher, and thereby
the self-coupling route is favored. We note here that for 200,
100, and 50 mm starting concentrations the substrate/metal
ratios in Figure 4 are 200, 100 and 50, respectively. The clear
increase in nitrile selectivity as the substrate/metal ratio de-
creases correlates well with this competition. We note that it
Figure 5. Influence of benzyl amine addition on the initial rate of benzyl
alcohol oxidation. Reaction conditions: benzyl alcohol (100 mm), benzyl
amine, catalyst (0.5 mol% relative to alcohol), O2 (1 bar), 908C, 15 min.
Investigating the influence of benzyl amine concentration
(Figure 6) also provided some insight regarding the ability
of some catalysts to selectively oxidize amines to nitriles,[9,11]
whereas others, such as 0.5Ir/CeO2(DP400R) lead to the forma-
tion of the coupled imine product.[6–8,12] By performing reac-
tions at various benzyl amine concentrations, we observed a
considerable increase in nitrile selectivity at lower amine
concentration (Figure 6). As can be seen, decreasing the
concentration of benzyl amine by one quarter leads more
selectively to benzonitrile, that is, the doubly dehydrogenat-
ed product, increased by a factor of ten, despite the fourfold
decrease in conversion.
Clearly, kinetic competition occurs during the reaction,
whereby the imine intermediate (Scheme 2) can either be
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Oxidative Dehydrogenation of Amines
FULL PAPER
Scheme 3. Competition between oxidative dehydrogenation and self-coupling during the aerobic oxidation of benzyl amine.
is only by investigating this Ir-based catalyst, which produces
both benzonitrile and N-benzylidene-1-phenylmethanamine,
that this insight has become apparent.
yields the coupled product. Indeed, by stirring a solution of
benzaldehyde and benzyl amine at room temperature, N-
benzylidene-1-phenylmethanamine was detected in quantita-
tive yield within 5 min, clearly demonstrating the efficiency
and rate of this reaction.
Mechanism of N-benzylidene-1-phenylmethanamine forma-
tion: At this stage, we are still left with the question of pre-
To further investigate which pathway is responsible and/
or dominant for the formation of N-benzylidene-1-phenyl-
methanamine, we subsequently investigated the influence of
water on the reaction rate and selectivity. By performing ex-
periments in the presence of dehydrated molecular sieves
(3 ꢂ Zeolite), we observed a large decrease in the final
yield of N-benzylidene-1-phenylmethanamine. This suggest-
ed that if the hydrolysis of benzyl imine is inhibited by the
removal of water, then the formation of N-benzylidene-1-
phenylmethanamine proceeds much less smoothly and the
reaction becomes trapped at the first step, that is, the forma-
tion of benzyl imine. We note that although an increase in
benzonitrile selectivity was observed following the addition
of molecular sieves, again confirming the competition be-
tween oxidative dehydrogenation and coupling, we were still
unable to detect any intermediate benzyl imine by conven-
tional chromatographic techniques; however, its absence
from the GC–FID/MS spectra could be due to the poor sta-
bility of the compounds, rendering typical gas chromato-
cisely
how
N-benzylidene-1-phenylmethanamine
is
formed.[5] To date, two competing pathways for the forma-
tion of this product have been put forth (Scheme 4); either
the benzyl imine intermediate is directly attacked by benzyl
amine, leading to the formation of a hemiaminal intermedi-
ate and subsequently the coupled imine, or benzyl imine is
hydrolyzed in situ by traces of water; rapid coupling of the
amine and aldehyde would then produce the coupled imine
product. Nevertheless, although these competing pathways
have been proposed, little effort has been expended to eluci-
date which pathway is dominant. The first indication as to
which reaction pathway prevails was provided by the com-
petitive oxidation experiments (Figure 3). During these ex-
periments, we noted that while benzyl alcohol was converted
by the catalyst, no free benzaldehyde was detected until all
of the amine substrate had also been consumed. This indi-
cates that if benzaldehyde is formed, it is immediately cap-
tured by the remaining amine substrate in situ, and rapidly
Scheme 4. Potential routes for the formation of N-benzylidene-1-phenylmethanamine from the oxidative dehydrogenation of benzyl amine.
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I. Hermans et al.
graphic analysis useless. These observations lead us to con-
clude that pathway A (i.e., hydrolysis of benzyl imine and
subsequent coupling of benzaldehyde and benzyl amine) is
the dominant pathway for the formation of N-benzylidene-
1-phenylmethanamine. We note that this conclusion is in full
agreement with the recent work of Jones and co-workers,
who also observed that the addition of water both accelerat-
ed the rate of reaction and increased the maximal yield ob-
tained during the aerobic oxidation of benzyl amine with
nonpromoted CeO2.[13]
Experimental Section
The aerobic oxidation of benzyl amine or benzyl alcohol was performed
in a 50 mL round-bottomed flask, which was sealed with a balloon of O2
or air as appropriate. The flask was charged with the desired amount of
catalyst (corresponding to 0.5 mol% Ir relative to substrate) and the re-
actant solution (0.2m benzyl amine or benzyl alcohol in toluene, 5 mL
total volume). Reactions were performed at 908C for 6 h. When appro-
priate, a mixed solution of benzyl amine and benzyl alcohol was em-
ployed, again at a total substrate concentration of 0.2m. High-pressure
experiments were performed in a stainless steel Parr Autoclave (100 mL)
equipped with a Teflon liner (working volume 70 mL). Product analysis
and quantification was performed by GC-FID/MS analysis (30 m FFAP
column) against a biphenyl internal standard.
The catalyst investigated, 0.5Ir/CeO2(DP400R), was prepared by a deposi-
tion-precipitation methodology and fully characterized, as described else-
where.[6]
Conclusion
We have demonstrated that ceria-supported iridium oxide
nanoparticles are an efficient, heterogeneous catalyst for the
aerobic oxidation of amines. Although oxidized at a lower
rate than the corresponding alcohol substrates, the greater
coordinating power of the amine substrate ensures their
preferential oxidation in a competitive oxidation experi-
ment. We have noted that the TOFs exhibited by this cata-
lyst for the aerobic oxidation of dibenzyl amine are among
the highest reported in the open literature.
Whereas other catalysts examined for this reaction are
typically highly selective towards a single reaction product,
uniquely, during the aerobic oxidation of benzyl amine, the
catalyst examined herein produces both N-benzylidene-1-
phenylmethanamine and benzonitrile. Due to this peculiari-
ty, we have discovered that two competing pathways act
during the aerobic oxidation of amines. At high substrate-
to-metal ratios the excess of free amine in solution leads to
the formation of an imine, N-benzylidene-1-phenylmethan-
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ACHTUNGTRENNUNGamine, through the coupling of free benzyl amine with in
situ formed benzyl imine. Conversely, at low substrate-to-
metal ratios, the reaction is dominated by the available cata-
lyst surface, and benzyl imine undergoes further oxidative
dehydrogenation to benzonitrile.
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Received: April 25, 2013
Published online: August 12, 2013
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