Reductive Amidation without an External Hydrogen Source Using Rhodium on Carbon Matrix as a Catalyst

Article abstract of DOI:10.1002/cctc.201901465

An efficient method for preparation of secondary amides from primary amides and aldehydes using rhodium on carbon matrix as catalyst was developed. The method does not require any external hydrogen source and carbon monoxide is used as a reducing agent. The most active rhodium catalysts were characterized by BET, TEM and XPS techniques. Unexpectedly, it was found that heterogeneous rhodium on carbon matrix works as precatalyst for homogenous active species due to leaching of rhodium to the solution. Various secondary amides were synthesized and checked for antifungal activity. 4-Methoxy-N-(4-methoxybenzyl)benzamide demonstrated promising activity against Rhizoctonia Solani.

Full text of DOI:10.1002/cctc.201901465

Accepted Article
Title: Reductive Amidation without an External Hydrogen Source Using
Rhodium on Carbon Matrix as a Catalyst
Authors: Alexey A. Tsygankov, Maria Makarova, Oleg I Afanasyev,
Alexey S Kashin, Alexander V Naumkin, Dmitry A Loginov,
and Denis Chusov
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10.1002/cctc.201901465
ChemCatChem
COMMUNICATION
Reductive Amidation without an External Hydrogen Source Using
Rhodium on Carbon Matrix as a Catalyst
Alexey A. Tsygankov,[a] Maria Makarova,[a,b] Oleg I. Afanasyev,[a] Alexey S. Kashin,[c] Alexander V.
Naumkin,[a,d] Dmitry A. Loginov,[a] and Denis Chusov*[a,e]
Abstract: An efficient method for preparation of secondary amides
from primary amides and aldehydes using rhodium on carbon matrix
as catalyst was developed. The method does not require any external
hydrogen source and carbon monoxide is used as a reducing agent.
The most active rhodium catalysts were characterized by BET, TEM
and XPS techniques. Unexpectedly, it was found that heterogeneous
rhodium on carbon matrix works as precatalyst for homogenous active
species due to leaching of rhodium to the solution. Various secondary
amides were synthesized and checked for antifungal activity. 4-
Methoxy-N-(4-methoxybenzyl)benzamide demonstrated promising
activity against Rhizoctonia Solani.
these systems like low step economy (e.g. production of silanes)
and disposal of stoichiometric wastes. In case of potential
industrial applications, it is highly desirable to use inexpensive
and accessible components.
Carbon monoxide is a multimillion ton byproduct of steel
making industry. Moreover, the use of CO is attractive from
environmental point of view. It can be obtained by gasification of
biomass.[8]. Previously in our group we demonstrated the unique
deoxygenative potential of carbon monoxide.[9a] The use of
carbon monoxide proved to be effective in reductive alkylation[9b]
amination[9c] and esterification[9d] reactions of carbonyl
,
,
compounds. The key advantage of this reducing agent is the lack
of external hydrogen source, which leads to unique selectivity
often surpassing more traditional, hydride-based reducing
agents.[9e] Previously, we developed a method for reductive
amidation with carbon monoxide as the reducing agent using
homogeneous rhodium and ruthenium catalysts.[7f,7g] We decided
to develop a method of reductive amidation using heterogeneous
catalysis which might be more suitable for potential industrial
application of this method. Herein we report reductive amidation
on rhodium on carbon matrix.
We started with investigation of catalytic activity of various
commercially available rhodium and ruthenium catalysts on
different supports. We have chosen the reaction between p-
anisaldehyde and acetamide as a model reaction due to clear and
precise analysis of its reaction mixture using 1H NMR
spectroscopy (Table 1). In general, rhodium catalysts
demonstrated higher activity than ruthenium catalysts (Entries 1-
5 vs. entries 6-9). Alumina supports are less effective than carbon
supports (Entries 1-3 vs. entries 4-5). Among the rhodium on
carbon supports rhodium on carbon matrix demonstrated superior
activity and afforded desired product 1a with highest yield (Entry
1). Therefore, we have chosen rhodium on carbon matrix as
catalyst for further optimization of reaction conditions.
Amides are an important class of organic compounds because of
their wide applications in various fields. Nearly 25% of all
pharmaceutical drugs currently on the market contain an amide
bond.[1] Peptidomimetics, pseudopeptides, β-peptidoids comprise
an important class of drug molecules.[2] Moreover, amide bond
coupling is one of the most frequent transformations in medicinal
chemistry.[3] Conventional methods for synthesis of amides
require the usage of stoichiometric amounts of additives which
causes low atom efficiency and formation of stoichiometric
amounts of wastes. Therefore, development of new and efficient
methods of amide synthesis is important for pharmaceutical
chemistry.[4]
For the last decade there was a growing interest in
nonclassical routes for amide synthesis.[5,6] These methods
include direct catalytic condensation of carboxylic acids with
amines using boron-based catalysts, catalytic amidation of esters,
oxidative amidation of primary alcohols or aldehydes, and
carbonylative amidation of aryl halides. Among them reductive
amidation of aldehydes is a potentially powerful method for
modification of amides because of its high atom efficiency.[7]
Hydrogen or silanes are usually used as reducing agents for this
transformation. Since both reducing systems have a hydride
source, they can have low tolerance to potentially reducible
functional groups. Moreover, there are other disadvantages of
Solvent screening demonstrated that Et2O is the optimal
solvent for this transformation (Table 2, Entry 2), although toluene,
i
tBuOH, and PrOH were also efficient (Entries 1, 6, and 7).
Presence of water significantly decreased selectivity of the
formation of the target amide 1a and lead to the formation of
symmetrical tertiary amine 2a. The amine 2a could be formed
through hydrolysis of acetamide or target amide 1a and
consequential reductive amination. Removal of water from the
catalyst under vacuum allowed to increase selectivity towards
formation of alkylated amide 1a. It also improved reproducibility of
the reaction results. Primary alcohols such as MeOH, EtOH and
nBuOH favoured formation of the tertiary amine 2a (Entries 8-10).
Higher pressure did not influence the reaction outcome, while
decrease of pressure to 20 bar significantly reduced conversion
of the aldehyde and the product yield. Dilution of reaction mixture
led to further increase of product yield (Entry 12). Overall, amide
1a can be obtained in 93% yield under optimized conditions.
[a]
A. A Tsygankov, Dr. O. I. Afanasyev, Dr. A.V. Naumkin, Prof. Dr.
D.A. Loginov, Dr. D. Chusov
Nesmeyanov Institute of Organoelement Compounds
Russian Academy of Sciences
28 Vavilova str., 119991 Moscow (Russia)
E-mail: denis.chusov@gmail.com
M. Makarova
[b]
[c]
Higher Chemical College
Dmitry Mendeleev University of Chemical Technology of Russia
Miusskaya sq. 9, Moscow 125047 (Russia)
Dr A. S. Kashin
Zelinsky Institute of Organic Chemistry
Russian Academy of Sciences
Leninsky Prospekt, 47, Moscow, 119991 (Russia)
Dr. A.V. Naumkin
Moscow Institute of Physics and Technology (State University)
Dolgoprudny, Russia.
[d]
[e]
Dr. D. Chusov
G.V. Plekhanov Russian University of Economics, 36 Stremyanny
Per., Moscow 117997, Russia
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10.1002/cctc.201901465
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Table 1. Catalysts screening.
We then examined scope and limitations of the reaction
(Scheme 2). Different aromatic aldehydes can be used in this
transformation. Aldehydes bearing electron-donating functional
groups show higher activity than aldehydes with electron-
withdrawing groups (1a, 1b, 1f, 1g vs. 1h, and 1k vs. 1m). The
system is also sensitive to sterical hindrance. The use of
aldehydes with ortho substituents led to lower yields than those
with meta and para substituents (1a, 1d vs. 1e). Ketones react
poorly under these conditions due to their lower electrophilicity
and higher sterical hindrance (1q, 1r). Another limitation of the
method is inability of using aliphatic aldehydes. In the reaction
conditions they form complex mixtures of self-aldol condensation
products due to high reaction temperature which results in low
yield of target amide (1s). Different types of amides can be used.
Aliphatic and aromatic amides afford products in similar yields (1a,
1j, 1k, 1n, and 1p). Electron-donating groups in benzamide
derivatives increase product yield (1p vs. 1n vs. 1k). Catalytic
system demonstrated tolerance to different functional groups
such as -OBn and -CN, which can be reduced in amidation
reactions involving external hydrogen source such as H2 or
hydrides.
Entry[a]
Catalyst
Aldehyde
conversion
[%]
Yield
[%]
1
2
3
4
5
6
7
Rh on carbon matrix
97
48
43
24
45
10
14
58
18
21
7
Rh on activated charcoal
Rh on carbon
Rh on activated alumina
Rh on alumina (Degussa-type)
Ru on activated charcoal
7
0
Ru on activated carbon (reduced, 50%
water)
2
8
9
Ru on alumina
47
45
5
7
Ru on activated alumina
[a] In all cases 5% wt of the metal on support were used. 0.2 mmol of acetamide,
1:1 ratio of acetamide and p-anisaldehyde, yield was determined by NMR with
mesitylene as internal standard.
Table 2. Solvent screening
Entry[a]
Solvent
Aldehyde
Yield of
Yield of
conversion [%]
1a [%]
2a [%]
1
toluene
Et2O
>99
>99
97
70
79
58
24
24
77
73
22
35
5
5
2
6
3
THF
17
traces
8
4
CH2Cl2
CH3CN
tBuOH
iPrOH
nBuOH
EtOH
MeOH
H2O
43
5
53
6
>99
>99
>99
>99
>99
>99
>99
7
7
9
8
42
35
28
47
3
9
10
11
12[b]
Scheme 1. Investigation of substrate scope. 1 mmol of amide, 1:1 ratio of amide
and aldehyde, yield was determined by 1H NMR with mesitylene as internal
standard. Isolated yield is placed in parenthesis.
17
93
Et2O
[a] 5% wt of the rhodium on carbon matrix were used. 0.2 mmol of acetamide,
1:1 ratio of acetamide and p-anisaldehyde, 1M, yield was determined by NMR
with mesitylene as internal standard. [b] 0.5M.
Study of biological activities of new compounds is important
for drug development and agricultural chemistry. Herein, we
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10.1002/cctc.201901465
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COMMUNICATION
investigated the activity of synthesized amides in vitro against
three phytopathogenic fungi Fusarium sambucinum (F.s.),
Fusarium oxysporum (F.o.) and Rhizoctonia Solani (R.s.) (Table
3). The effect of the tested compounds on the mycelium radial
growth in the potato-saccharose agar media was measured in a
concentration 30 mg/L. The tested amides showed low to
moderate activity against F. sambucinum and F. oxysporum.
However, they showed moderate to high activity in case of R.
solani. The results showed that the synthesized amides possess
a selective antifungal activity against one of the three tested fungi,
which can be very valuable since it allows target plants treatment
against particular fungi. In general, benzamide (1k-1p) derivatives
demonstrated higher antifungal activity in comparison to
acetamide (1a-1h) and isovaleramide (1i-1j) derivatives. Although
amide 1f, containing p-heptoxybenzyl substituent, also
demonstrated high antifungal activity against R. solani.
Furthermore, activities of 1f, 1l, 1n, 1o and 1p against R. solani
exceeded those of triadimefon, which was used as a reference
fungicide and showed only 36% inhibition under these conditions.
Amide 1p appeared to be the most active among all tested
compounds and showed 79% inhibition of mycelium growth.
Interestingly, amide 1n, which contains two more methoxy groups
was less active (51%). Therefore, these compounds showed
promising antifungal activity and require further studies.
in Table 1. Firstly, the results showed great difference in activities
depending on the type of support, even between different carbon
supports. It can be related to pore size which can have significant
impact on catalytic activity. In order to check the influence of
porosity on catalytic activity, we measured specific surface area
by adsorption using BET isotherm. We measured the surface area
for rhodium on carbon matrix, rhodium on activated charcoal and
rhodium on carbon. However, differences in surface area
between the catalysts appeared to be insignificant (see
supporting information) pointing on another origin of difference in
catalytic activity. We then studied morphology of these three
catalysts using TEM (Figure 1). For rhodium on carbon matrix
(Figures 1a and 1b), regardless of the presence of catalyst drying
step, the mean particles size was about 2.0 nm (approximate size
range – 1.5-3.0 nm). Morphology of the support did not depend
on the catalyst pretreatment. In the case of rhodium on carbon
and rhodium on activated charcoal the size of the metal particles
was somewhat higher and the particles agglomeration was more
pronounced (Figures 1c and 1d). The mean size of metal particles
was about 3.0 nm (approximate size range – 2.0-4.0 nm) in the
case of carbon support and about 2.5 nm (approximate size range
– 1.5-3.5 nm) in the case of activated charcoal support. The
morphology of carbon materials had no notable features in both
cases. It is important to mention that for all samples under study
the spatial distribution of the metal on carbon-based material was
non-uniform and particles sizes varied from site to site.
Nevertheless, on the basis of electron microscopy data one can
conclude that there is no significant difference in the catalyst
nano-structure, which could have such a notable impact on the
catalytic reaction outcome.
Table 3. Study of antifungal activity of synthesized amides by calclulation of
growth inhibition rates for three different fungi Fusarium sambucinum (F.s.),
Fusarium oxysporum (F.o.) and Rhizoctonia Solani (R.s.).[a]
Entry
Amide
F.s.
F.o.
R.s.
1
1a
1b
1c
1d
1f
13%
5%
5%
28%
19%
6%
2
4%
3
10%
11%
10%
5%
3%
4
6%
24%
60%
19%
40%
14%
29%
55%
28%
51%
42%
79%
5
11%
16%
16%
12%
4%
6
1g
1i
7
9%
8
1j
3%
9
1k
1l
11%
3%
10
11
12
13
14
9%
1m
1n
1o
1p
8%
0%
14%
14%
13%
17%
0%
Figure 1. TEM images of rhodium on carbon matrix (a), rhodium on carbon
matrix after drying in vacuo (b), rhodium on carbon (c) and rhodium on activated
charcoal (d).
27%
We then decided to investigate catalyst stability and turnover
(see supporting information). We found that on the second cycle
the activity of the catalyst significantly decreased, and product
yield dropped below 30%. On the third cycle yield of the product
was almost the same. These results can be explained by
[a] Plates were incubated at 24 ℃ and the growth inhibition rates (I) were
calculated after 72h. The effect of the tested compounds was measured in
a concentration 30 mg/L
Afterwards we conducted a comparative study of the tested
catalysts in order to explain the differences in their activity shown
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10.1002/cctc.201901465
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passivation of active centers or by leaching of rhodium to solution
and deactivation of these rhodium particles during recycling.
We tested these possibilities by analysis of the catalyst after
reaction by TEM (Figure 2). No metal particles or agglomerates
can be seen on the sample, which supports theory of rhodium
leaching during the reaction.
Scheme 2 another portion of starting material was added. Target
amid was formed in 35% yield (Scheme 2, (b) which is
considerably lower than in standard conditions. ICP-AES also
confirmed leaching of rhodium (see Supporting information).
In order to determine oxidation states of rhodium, the catalysts
were analysed with XPS (Figure 3). The spectra of rhodium on
carbon matrix 1 and 2 practically coincide in shape and position.
The Rh 3d5/2 and Rh 3d3/2 peaks at 309.8 and 314.7 eV
respectively can be assigned to Rh2O3•5H2O (309.6 eV) [10]. In
contrast to the spectrum of rhodium on carbon matrix, three states
giving Rh 3d5/2 peaks at 307.5/308.7/309.8 and 307.3/308.7/309.8
eV, which can be assigned to Rh, Rh2O3 and Rh2O35H2O, were
deconvoluted in those of rhodium on carbon 3 and rhodium on
activated charcoal 4, respectively. Their corresponding intensity
ratios are 0.44/0.37/0.19 and 0.43/0.45/0.12.
Figure 2. TEM image of rhodium on carbon matrix after reaction.
To confirm the leaching of the catalyst into solution we
conducted an experiment similar to hot-filtration experiment. First,
we packed the catalyst in a piece of filter paper and placed it into
an autoclave with starting materials. After one hour of heating the
catalyst was removed and reaction mixture was heated further for
21 hours. Analysis of reaction mixture after one hour with catalyst
and after 21h without heterogeneous particles proved that
leaching occurs since formation of the product continued after
removal of the catalyst (Scheme 2, a).
Figure 3. Rh 3d photoelectron spectra of rhodium on carbon matrix (a), rhodium
on carbon matrix after drying in vacuo (b), rhodium on carbon (c) and rhodium
on activated charcoal (d).
From this data it seems that only rhodium (III) is active in
reductive amidation since both rhodium on carbon and activated
charcoal showed at least twice less activity than rhodium on
carbon matrix. Therefore, rhodium on carbon matrix works more
like homogeneous catalyst than heterogeneous, which was
confirmed by leaching of catalyst from the support. To confirm that
rhodium (III) can catalyse the reaction, we tested the activity of
RhCl3 in the model reaction (Scheme 3, a). Under these
conditions RhCl3 demonstrated some catalytic activity although it
was less active than rhodium on carbon matrix. We also tested
activity of Rh6(CO)16 as a homogeneous source of rhodium (0).
Interestingly it demonstrated catalytic activity comparable with
rhodium on carbon matrix (Scheme 3, b). So rhodium on carbon
matrix is giving the “cocktail” of catalysts[11] in which
homogeneous species seem to be much more active.
Scheme 2. Analysis of catalyst leaching.
Moreover, colour of the reaction mixture after one hour was
deep red which indicates the presence of rhodium complexes.
After 21 hours colour of the reaction mixture changed to pale-
yellow. To the catalyst that was removed from reaction (a) on
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Scheme 3. Activity of rhodium (III) and rhodium (0) in reductive amidation.
As it was shown previously in our group, carbon monoxide
possesses unique selectivity because of the lack of external
hydrogen source. Here we decided to compare the selectivity of
carbon monoxide with hydrogen (Scheme 4). While the usage of
carbon monoxide allowed to synthesise the product in nearly
quantitative yield, the use of hydrogen mainly resulted in reduction
of aldehyde to the corresponding alcohol and further
hydrogenolysis to the substituted toluene 3a with some formation
of the desired product.
Scheme 5. A plausible mechanism of reductive amidation with rhodium on
carbon matrix.
To conclude, we developed a new catalytic system for
reductive amidation of aldehydes without external hydrogen
source using rhodium on carbon matrix as a catalyst. Substrate
scope of the reaction was evaluated, and it was shown that
various amides and substituted aromatic aldehydes can be
utilized in this reaction. Less active carbonyl compounds such as
ketones poorly undergo this transformation. Aliphatic aldehydes
also can not be used because of various side reactions. Moreover,
antifungal activity of the synthesized amides was evaluated.
Scheme 4. Activity of rhodium (III) and rhodium (0) in reductive amidation.
Particularly,
4-methoxy-N-(4-methoxybenzyl)benzamide
demonstrated high activity against Rhizoctonia Solani which
surpasses the activity of commercially used Triadimefon.
Catalysts were studied using BET, TEM and XPS techniques.
According to XPS data the most active catalyst (rhodium on
carbon matrix) does not contain Rh(0) species and contains only
Rh(III) species, which provides its higher catalytic activity.
Plausible mechanism is shown on Scheme 5, based on our
results herein and our previous studies. At first rhodium species
are leached from carbon matrix into the solution either by
coordination with carbon monoxide and aldehyde or by itself with
the following coordination of carbon monoxide and aldehyde.
Addition of the amide to the carbonyl group results in the
formation of intermediate B. Intramolecular hydroxylation of Rh-
bound CO ligand leads to intermediate C, with following
decarboxylation and formation of intermediate D. Finally,
reductive elimination leads to formation of the target amide and
the regenerated catalyst.
Experimental Section
General procedure for reductive amidation
A 10 mL stainless steel autoclave was charged with 1 mol% of catalyst,
the corresponding solvent, 1 eq. of the aldehyde and 1 eq. of the amide,
and magnetic stirring bar. The autoclave was sealed, flushed with 5 bar of
CO, and then charged with the indicated pressure of CO. The reactor was
placed into a preheated oil bath. After the indicated time, the reactor was
cooled to room temperature and depressurized. The reaction mixture was
filtrated from catalyst and solvents were removed on a rotary evaporator.
TEM
Before measurements the samples were deposited on the 3 mm carbon-
coated copper grids from isopropanol suspension. Samples morphology
was studied using Hitachi HT7700 transmission electron microscope.
Images were acquired in bright-field TEM mode at 100 kV accelerating
voltage.
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Acknowledgements
This work was supported by the Russian Science Foundation
(Grant No. 19-73-20212). The NMR studies were performed with
the financial support from Ministry of Science and Higher
Education of the Russian Federation using the equipment of
Center for Molecular Composition Studies of INEOS RAS.
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Keywords: Amidation • Homogeneous catalysis •
Heterogeneous catalysis • Reduction • Carbon monoxide
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10.1002/cctc.201901465
ChemCatChem
COMMUNICATION
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COMMUNICATION
Alexey A. Tsygankov, Maria Makarova,
Oleg I. Afanasyev, Alexey S. Kashin,
Alexander V. Naumkin, Dmitry A.
Loginov, Denis Chusov*
Page No. – Page No.
Reductive Amidation without an
External Hydrogen Source Using
Rhodium on Carbon Matrix as a
Catalyst
Leaching forever. Rhodium on carbon matrix was successfully applied as a
catalyst for reductive amidation without an external hydrogen source. It was found
that the reaction proceeds in homogeneous mode. The products were tested for
antifungal activity.
This article is protected by copyright. All rights reserved.