A collaborative research presented by groups of
Prof. Ferenc Fülöp, Prof. István Pálinkó (University of
Szeged, Hungary) and Dr. Sándor B. Ötvös (University of
Graz, Austria).
Exploiting a silver–bismuth hybrid material as
heterogeneous noble metal catalyst for decarboxylations
and decarboxylative deuterations of carboxylic acids under
batch and continuous flow conditions
A silver-containing hybrid material with structurally-bound
catalytic centers has been exploited as an efficiently
recyclable and highly active heterogenous noble metal
catalyst for protodecarboxylations and decarboxylative
deuterations of carboxylic acids. After an initial batch
method development, a chemically intensified continuous
flow process was established in a simple packed-bed system
which enabled gram-scale protodecarboxylations without
detectable structural degradation of the catalyst.
As featured in:
Green
Chemistry
Volume 23
Number 13
7 July 2021
Pages 4623-4904
Cutting-edge research for a greener sustainable future
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ISSN 1463-9262
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Exploiting a silver–bismuth hybrid material as
heterogeneous noble metal catalyst for
decarboxylations and decarboxylative deuterations
of carboxylic acids under batch and continuous
flow conditions†‡
Rebeka Mészáros,a András Márton,b Márton Szabados, b,c Gábor Varga,
Zoltán Kónya, e,f Ákos Kukovecz, e Ferenc Fülöp, *a,g István Pálinkó
Sándor B. Ötvös *g,h
*c,d
§b,c and
Herein, we report novel catalytic methodologies for protodecarboxylations and decarboxylative deuterations of carboxylic acids utilizing a silver-containing hybrid material as a heterogeneous noble metal catalyst. After an initial batch method development, a chemically intensified continuous flow process was
established in a simple packed-bed system which enabled gram-scale protodecarboxlyations without
Received 14th March 2021,
Accepted 17th May 2021
detectable structural degradation of the catalyst. The scope and applicability of the batch and flow pro-
DOI: 10.1039/d1gc00924a
cesses were demonstrated through decarboxylations of a diverse set of aromatic carboxylic acids.
Catalytic decarboxylative deuterations were achieved on the basis of the reaction conditions developed
rsc.li/greenchem
for the protodecarboxylations using D2O as a readily available deuterium source.
1.
Introduction
Carboxylic acids are of outstanding importance as inexpensive
and easily accessible intermediates for the synthesis of an
array of value-added products.1 Among carboxylic acid transformations, protodecarboxylations and related decarboxylative
a
Institute of Pharmaceutical Chemistry, University of Szeged, Eötvös u. 6, Szeged,
H-6720 Hungary
b
Department of Organic Chemistry, University of Szeged, Dóm tér 8, Szeged, H-6720
Hungary
c
Material and Solution Structure Research Group and Interdisciplinary Excellence
Centre, Institute of Chemistry, University of Szeged, Aradi Vértanúk tere 1, Szeged,
H-6720 Hungary
d
Department of Physical Chemistry and Materials Science, University of Szeged,
Rerrich Béla tér 1, Szeged, H-6720 Hungary.
E-mail: gabor.varga5@chem.u-szeged.hu
e
Department of Applied and Environmental Chemistry, University of Szeged,
Rerrich Béla tér 1, Szeged, H-6720 Hungary
f
MTA-SZTE Reaction Kinetics and Surface Chemistry Research Group, Hungarian
Academy of Sciences, Rerrich Béla tér 1, Szeged, H-6720 Hungary
g
MTA-SZTE Stereochemistry Research Group, Hungarian Academy of Sciences,
Eötvös u. 6, Szeged, H-6720 Hungary. E-mail: fulop@pharm.u-szeged.hu
h
Institute of Chemistry, University of Graz, NAWI Graz, Heinrichstrasse 28, Graz,
A-8010 Austria. E-mail: sandor.oetvoes@uni-graz.at
† Dedicated to the memory of our friend and colleague Prof. István Pálinkó.
‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1gc00924a
§ Deceased.
This journal is © The Royal Society of Chemistry 2021
couplings play a crucial role in the formation of C–C, C–X and
C–H bonds, and hence they are appealing for the generation
of molecular diversity.2,3 The most common catalysts for protodecarboxylations contain copper, silver, gold, palladium or
rhodium metals, typically as homogeneous sources in combination with various bases or ligands.4–6 For example, in
copper- and rhodium-catalyzed examples, well-defined
complexes are predominant over reusable heterogeneous
sources.7–10 Palladium-catalyzed protodecarboxylations generally require high catalyst loading which severely limits their
practical applicability.11–13 In addition, various hexaaluminate
catalysts proved useful for decarboxylation of biomassderived carboxylic acids.14–17 Due to the high costs involved,
only a few studies have been reported for gold-catalyzed
protodecarboxylations.18–20 Silver-catalyzed reactions have also
emerged in the field of protodecarboxylations and decarboxylative transformations, such as decarboxylative allylations
and azidations, and exhibited a highly beneficial reactivity
trend, comparable to that of the more costly gold-catalyzed
protocols.21–24 However, with a few exceptions,25 such reactions are promoted by soluble silver salts as non-reusable catalytic sources,26,27 typically in the presence of various ligands,
which can be regarded as a considerable drawback from an
environmental point of view.28–30
Due to economic and environmental reasons, there is a
continuously growing need for heterogeneous noble metal cat-
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alysts.31 However, immobilization of metal catalysts on various
prefabricated supports is often accompanied by reduced
selectivity or loss of activity, and in the case of inadequate
catalyst-support interactions, leaching of the metal component
may lead to substantial environmental concerns.32,33
Nowadays, in organic synthesis silver catalysis is considered as
a significant methodology, which is due to its wide applicability, environmentally-benign nature and its lower costs compared with other precious noble metals such as gold, platinum
or palladium.34,35 Typical synthetic applications of silver catalysis rely on Ag(I) salts or complexes as homogeneous sources
for the catalytically active metal.36–38 As concerns heterogeneous silver sources, supported nanoparticles (nanosilver)
are the most widely applied.33,39,40 Such heterogeneous
materials are easily obtained via immobilization on various
surfaces, however their main limitation is weak catalystsupport interactions which give rise to unsatisfactory stability
and limits their practical synthetic utilities, especially under
demanding reaction conditions, such as high-temperature continuous flow conditions or in the presence of coordinating
ligands. On the basis of a naturally occurring mineral, called
beyerite, we recently developed a heterogeneous silver–
bismuth hybrid material (AgBi-HM) with structurally-bound
silver catalytic centers.41 The material exhibited a layered structure and contained Ag(I) and Bi(III) cationic and carbonate
anionic components with silver ion as the minor cationic component. As compared with traditionally immobilized catalysts,
structurally-bound catalytic centres imply increased thermodynamic stability and robustness, and exhibit an increased tolerance against challenging reaction conditions and improved
compatibility with various reactants and solvents.42
Continuous flow reaction technology in combination with
heterogeneous catalysis have attracted significant attention in
recent years,43–48 and now comprise a powerful methodology for
the synthesis of an array of useful products.49–56 Heterogeneous
catalysts can easily be handled, recycled and reused in packedbed reactors, moreover, unlike in traditional batch processes,
separation from the reaction products is really straightforward.57 Due to the enhanced control over the most important
reaction conditions (e.g. residence time and temperature),58–60
reaction selectivity can easily be improved while less waste is
generated.61,62 Moreover, in loaded catalyst columns, the continuous stream of reactants interacts with a superstoichiometric
amount of catalyst species, which improves reaction rates
significantly.63–65 On the downside, with increasing reactor
dimensions scale-up may involve difficulties, such as insufficient intraparticle heat transfer rates, intraparticle diffusion
limitations as well as susceptibility to liquid maldistribution.66
However, if catalyst deactivation and leaching can be eliminated, the scale of production becomes a direct function of the
process time without modifying the reactor geometry (i.e. scaleout).67–69 In spite of these obvious benefits, there are very few
precedents for heterogeneous silver-catalysts being utilized in
continuous flow processes,70,71 which may be explained by the
fact that stable and robust heterogeneous silver catalyst are at
scarce.41,42,72,73
4686 | Green Chem., 2021, 23, 4685–4696
Green Chemistry
To the best of our knowledge, protodecarboxylations promoted by heterogeneous noble metal catalysts have not yet
been achieved under efficient continuous flow conditions. We
speculated that our silver-containing hybrid material may act
as a ligand-free heterogeneous silver catalyst for protodecarboxylations, and because of its stability and robustness, not
only under batch but also under more demanding flow conditions. We intended to investigate the flow reactions in
a high-temperature packed-bed reactor system to exploit
extended parameter spaces, and to study the possibility of
chemical intensification as compared with the batch process.
Considering the outstanding significance of deuterated compounds in chemistry, biochemistry, environmental sciences
and also in pharmacological research,74,75 we were intrigued to
explore not only protodecarboxylations but also decarboxylative deuterations as facile and site-specific access to valuable
deuterium-labelled compounds.76,77 Our results are presented
herein.
2. Experimental
2.1.
General information
All chemicals used were analytical grade and were applied
without further purification. Reaction products were characterized by NMR spectroscopy and mass spectrometry. 1H NMR
and 13C NMR spectra were recorded on a Bruker Avance NEO
500 spectrometer, in CDCl3 as solvent, with tetramethylsilane
as internal standard at 500.1 and 125 MHz, respectively.
GC-MS analyses were performed on a Thermo Scientific Trace
1310 Gas Chromatograph coupled with a Thermo Scientific
ISQ QD Single Quadrupole Mass Spectrometer using a Thermo
Scientific TG-SQC column (15 m × 0.25 mm ID × 0.25 μm
film). Measurement parameters were as follows. Column oven
temperature: from 50 to 300 °C at 15 °C min−1; injection temperature: 240 °C; ion source temperature: 200 °C; electrospray
ionization: 70 eV; carrier gas: He at 1.5 mL min−1 injection
volume: 2 μL; split ratio: 1 : 33.3; and mass range: 25–500 m/z.
2.2.
Synthesis and characterization of the AgBi-HM
AgBi-HM was synthesized by using the urea hydrolysis method
according to a modified version of our procedure reported previously.42 AgNO3 (3.73 g) and Bi(NO3)3·5H2O (5.36 g) were dissolved in 50–50 mL 5 wt% nitric acid and the solutions were
combined. Urea (7.05 g) dissolved in 100 mL of deionized
water was next added to the mixture which was then placed
into an oven for 24 h at 105 °C. The obtained material was
next filtrated, washed with aqueous thiosulfate solution, water
and ethanol four times, and dried at 60 °C to obtain the final
product.
The as-prepared material was fully characterized by means
of diverse instrumental techniques as detailed earlier.40,41 The
X-ray diffraction (XRD) patterns were recorded on a Rigaku
XRD-MiniFlex II instrument applying CuKα radiation (λ =
0.15418 nm), 40 kV accelerating voltage at 30 mA. The morphology of the as-prepared and treated samples were studied
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by scanning electron microscopy (SEM). The SEM images were
registered on an S-4700 scanning electron microscope
(Hitachi, Japan) with accelerating voltage of 10–18 kV. The
actual Ag/Bi metal ratios in the samples were determined with
energy dispersive X-ray analysis (EDX) measurements (Röntec
QX2 spectrometer equipped with Be window coupled to the
microscope). More detailed images, both of the as-prepared
and the used samples, were taken by transmission electron
microscopy (TEM). For these measurements, an FEI Tecnai™
G2 20 X-Twin type instrument was applied, operating at an
acceleration voltage of 200 kV. The thermal behaviour of the
catalyst samples were investigated by thermogravimetry (TG)
and differential thermogravimetry (DTG) using a Setaram
Labsys derivatograph operating in air at 5 °C min−1
heating rate. For the measurements, 20–30 mg of the samples
were applied. The amount of metal ions was measured by ICP–
AES on a Thermo Jarell Ash ICAP 61E instrument. Before
measurements, a few milligrams of the samples measured
with analytical accuracy were digested in 1 mL cc. nitric acid;
then, they were diluted with distilled water to 50 mL and
filtered.
2.3.
General procedure for the batch reactions
A typical procedure for the decarboxylation and decarboxylative
deuteration reactions is as follows. N,N-Dimethylformamide
(DMF, 3 mL), the appropriate carboxylic acid (0.45 mmol,
0.15 M, 1 equiv.), KOH (6 mg, 15 mol%) and AgBi-HM as catalyst (60 mg, corresponding to 5 mol% Ag loading) were combined in an oven-dried Schlenk tube equipped with a magnetic
stir bar. In case of decarboxylative deuteration, 10 equiv. of
D2O (90 µL) was also added to the reaction mixture. After stirring for 24 h at 110 °C, the reaction mixture was cooled to
room temperature, and the catalyst was filtered off. The crude
products were diluted with diethyl ether and were washed with
aqueous NaHCO3 and brine. The combined organic layers
were dried over Na2SO4, and concentrated under reduced
pressure. The crude products were checked by NMR spectroscopy to determine conversion and selectivity. The products
of the batch reactions were characterized by NMR and GC-MS
techniques. In case deuterondecarboxylations, deuterium contents were determined from the relative intensities of the 1H
NMR indicator signals. Characterization data can be found in
the ESI.‡
Paper
section 2.3. The removed catalyst was washed with DMF (four
times) and was dried in nitrogen flow before the next reaction
cycle. Conversion and selectivity were determined after each
cycle by using 1H NMR.
2.5.
General procedure for the flow reactions
To carry out the decarboxylation and decarboxylative deuteration reactions under flow conditions, a simple continuous flow
set-up was assembled as shown in Fig. 1. The system consisted
of an HPLC pump (JASCO PU-2085), a stainless steel HPLC
column with internal dimensions of 4.6 × 100 mm as catalyst
bed and a 5-bar backpressure regulator (BPR) from IDEX to
prevent solvent boil over. The column encompassed 2 g of
AgBi-HM as catalyst. For each reaction, the corresponding carboxylic acid (c = 0.1 M) and 15 mol% KOH were dissolved in
acetonitrile (MeCN) or DMF. In order to achieve a clear solution, 20 equiv. of H2O was also added to the reaction mixture.
In case of deuterodecarboxylation reactions, 20 equiv. D2O was
added to the reaction mixture as deuterium source. In each
run, 4 mL of product solution was collected under steady-state
conditions. Between two experiments, the system was washed
for 20 min by pumping the appropriate solvent at a flow rate of
0.5 mL min−1. When DMF was used as solvent, the crude
product was worked-up similarly as detailed in section 2.3. In
case of MeCN as solvent, the reaction mixture was simply evaporated. Samples were checked by NMR spectroscopy to determine conversion and selectivity. For scale-out, 2-nitrobenzoic
acid (c = 0.1 M) and 15 mol% KOH was dissolved in MeCN
together with 20 equiv. of H2O to achieve a clear solution. The
reaction mixture was pumped continuously at 100 µL min−1
through the heated catalyst bed at 170 °C. The product solution was collected for 20 h under steady state conditions, and
samples were taken in every hour to determine conversion and
selectivity. The products of the flow reactions were characterized by NMR and GC-MS techniques. In case deuterondecarboxylations, deuterium contents were determined from
the relative intensities of the 1H NMR indicator signals.
Characterization data can be found in the ESI.‡ The residence
time on the catalyst bed was determined experimentally by
pumping a dye solution. The elapsed time between the first
2.4. Investigation of the catalyst reusability under batch
conditions
For investigation of catalyst reusability, the decarboxylation of
2-nitrobenzoic acid was carried out multiple times utilizing a
single portion of catalyst. DMF (3 mL), 2-nitrobenzoic acid
(0.45 mmol, 0.15 M, 1 equiv.), KOH (6 mg, 15 mol%) and AgBiHM as catalyst (60 mg, corresponding to 5 mol% Ag loading)
were combined in an oven-dried Schlenk tube equipped with a
magnetic stir bar. The reaction mixture was stirred for 24 h at
110 °C. The mixture was next cooled to room temperature, and
the solid material was removed by centrifugation. The liquid
phase was extracted, dried and evaporated as detailed in
This journal is © The Royal Society of Chemistry 2021
Fig. 1
Experimental setup for the continuous flow experiments.
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contact of the dye with the column and the moment when
the coloured solution appeared at column the outlet was
measured.
3. Results and discussion
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3.1. Decarboxylation of carboxylic acids under batch
conditions
In order to achieve an initial picture on the catalytic activity of
the silver-containing hybrid material in decarboxylation of carboxylic acids, batch reactions were explored first. The decarboxylation of 2-nitrobenzoic acid was chosen as model reaction to demonstrate the performance of the AgBi-HM in comparison with various commercially available silver and copper
salts as the most typical homogeneous catalytic sources for
this reaction type (Fig. 2). Based on literature data,25,78 DMF
was selected as solvent, and the reaction mixture containing
the substrate (0.15 M) together with 5 mol% of the appropriate
catalyst and 15 mol% of KOH as base was stirred for 24 h at
110 °C.
It was corroborated, that product formation was not occurring without any catalyst present. Gratifyingly, the application
of the hybrid material as catalyst resulted quantitative
and selective decarboxylation to nitrobenzene. AgOAc, Ag2O,
Ag2CO3 and AgNO3 as catalyst gave slightly lower conversions
(95–97%) and 100% selectivity in each cases. In contrast to
silver catalysts, copper salts performed poorer. In the presence
of CuOAc and Cu(NO3)2, conversion was 68% and 70%,
respectively, whereas CuBr2 was proven even less effective with
a conversion of merely 39%. In all the copper-catalyzed reactions, potassium 2-nitrobenzoate appeared in the reaction
mixture. Considering that the reaction is initiated by deprotonation of the carboxylic acid, the presence of the corresponding potassium salt as side product therefore indicates the
Green Chemistry
incompleteness of the reaction.25 As corroborated by a
test reaction carried out in the presence of 5 mol% of
Bi(NO3)3·5H2O, the Bi(III) component of the hybrid material is
inactive in decarboxylation of 2-nitrobenzoic acid.
After achieving promising preliminary results, the effects of
the major reaction conditions were next explored. Upon investigation of solvent effects (Table 1), the best results were
achieved by using DMF (entry 1). MeCN and N,N-dimethylacetamide (DMA) also gave acceptable conversions (85% and
62%, respectively) and high selectivities (100% and 85%,
respectively; entries 2 and 3). In EtOAc and toluene only
trace amounts of nitrobenzene formation was detected (entries
4 and 5), whereas in N-methyl-2-pyrrolidone (NMP) and
dimethyl sulfoxide (DMSO), no decarboxylation occurred
(entries 6 and 7).
As concerns reaction time, 24 h was required for completion, lower reaction times gave incomplete transformations
(Fig. S1‡). As was expected, decarboxylation was not taking
place at temperatures ≤50 °C, however conversion started to
increase at 80 °C and reached completion at 110 °C (Fig. S1‡).
The reaction gave the best results with substrate concentrations of 0.1 or 0.15 M (Table S1‡) The optimum value of the
catalyst loading was 5 mol% as lower amounts resulted in
decrease of the conversion (Table 2, entries 1–4). Upon investigation of the effects of the amount of the extraneous KOH
(Table 2, entries 5–9), it was observed that without base the
reaction gives only traces of the decarboxylated product;
however only catalytic amounts are required for completion
(e.g. 100% conversion was achieved with 15 mol% KOH). This
is in accordance with the mechanistic proposal of Jaenicke
and co-workers suggesting a negatively charged aryl–silver
intermediate upon decarboxylation which is responsible for
deprotonation after the base-promoted initiation of the reaction.25 In our study, KOH was selected as base as it involved no
precipitation and ensured a pumpable clear solution when
Table 1 Investigation of various solvents in the AgBi-HM-catalyzed decarboxylation of 2-nitrobenzoic acid under batch conditions
Selectivitya (%)
Fig. 2 Investigation of various catalysts in the decarboxylation of
2-nitrobenzoic acid.
4688 | Green Chem., 2021, 23, 4685–4696
Entry
Solvent
Conversiona (%)
A
B
1
2
3
4
5
6
7
DMF
MeCN
DMA
EtOAc
Toluene
NMP
DMSO
100
85
62
11
3
7
8
100
100
85
100
100
0
0
0
0
15
0
0
100
100
a
Determined by 1H NMR analysis of the crude product.
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Paper
Table 2 Investigation of the effects various catalyst and base amounts
in the AgBi-HM-catalyzed decarboxylation of 2-nitrobenzoic acid under
batch conditions
Table 3 Exploring the AgBi-HM-catalyzed decarboxylation of various
aromatic carboxylic acids under batch conditions
Conversiona,b (%)
Selectivitya (%)
1
100 (98)
100
2
100
100
3
100 (97)
100
4
80
100
5
74
100
6
65
100
7
Traces
—
8
Traces
—
9
92
100
10
49
100
11
100
100
12
86
100
13
100 (97)
100
14
100
100
15
97
100
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Entry
a
Entry
Catalyst
(mol%)
KOH
(mol%)
Conv.
(%)
1
2
3
4
5
6
7
8
9
1
3
5
10
5
5
5
5
5
15
15
15
15
—
5
10
15
20
25
69
100
100
8
80
85
100
100
a
Selectivitya (%)
A
B
100
93
100
100
100
100
100
100
100
0
7
0
0
0
0
0
0
0
Determined by 1H NMR analysis of the crude product.
being combined with the substrate which is crucial when considering the upcoming continuous flow experiments.
Having established an optimal set of conditions for the decarboxylation of the model compound (5 mol% catalyst
loading, 15 mol% KOH as base, DMF as solvent, 0.15 M substrate concentration, 110 °C temperature and 24 h reaction
time), we set out to investigate the scope and applicability of
the batch process (Table 3). Besides 2-nitrobenzoic acid (entry
1), its 5-methoxy-substituted derivative as well as 3,5-dinitrobenzoic acid underwent quantitative and selective protodecarboxylations (entries 2 and 3). The reaction tolerated well the replacement of the ortho-nitro substituent with bromine or
methoxy groups, and gave good conversions (80% and 74%,
respectively) and 100% selectivities in reactions of the corresponding benzoic acid derivatives (entries 4 and 5). Despite the
higher steric hindrance, decarboxylation of 2,6-dimethoxybenzoic acid was also successful, although conversion was somewhat lower (65%) than in the case of the mono-substituted
derivative (entry 6 vs. entry 5). Interestingly, decarboxylation of
2-chlorobenzoic acid and 2-hydroxybenzoic acid (salicylic acid)
were not successful (entries 7 and 8), however 2,4-dichlorobenzoic acid proved as an excellent substrate and gave the corresponding dichlorobenzene with 92% conversion and 100%
selectivity (entry 9). Selective decarboxylation of 1-naphtolic
acid to naphthalene was also possible, however only with a
moderate conversion of 49% (entry 10). To our delight, selective decarboxylation of heteroaromatic carboxylic acids, such
as thiophene-2-carboxylic acid and nicotinic acid, proceeded
with excellent conversions (100% and 86%, respectively;
entries 11 and 12). Similarly high conversions (97–100%) and
selectivities were achieved in decarboxylations of fused heteroaromatic substrates, such as indole-3-carboxylic acid, coumarin-3-carboxylic acid and chromone-3-carboxylic (entries
13–15). Decarboxylations of metha- and para-monosubstituted
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Substrate
Determined by 1H NMR analysis of the crude product. b For representative examples, isolated yields are shown in parentheses.
a
benzoic acid derivates, such as 3- and 4-nitrobenzoic acid,
were also attempted, however in these cases no reaction
occurred. These results are in accordance with earlier literature
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Table 4 Investigation of various solvents in the AgBi-HM-catalyzed decarboxylation of 2-nitrobenzoic acid under flow conditions
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Selectivitya
(%)
Fig. 3 Testing the reusability of the AgBi-HM catalyst in the decarboxylation of 2-nitrobenzoic acid. Selectivity was 100% in all reactions.
(Reaction conditions: 0.15 M substrate concentration, 5 mol% catalyst,
15 mol% of KOH as base, DMF as solvent, 110 °C, 24 h reaction time.).
findings suggesting the formation of a metal-centered carboxylate intermediate which is stabilized by the electronic effects
of the substituent(s) on the aromatic rings.79,80 Moreover, decarboxylation of aliphatic carboxylic acids, such as hexanoic
acid and levulinic acid, was proven unsuccessful using this
methodology. Note that isolated yield was determined in some
representative instances.
One of the main benefits of heterogeneous catalysis is the
ability to reuse and recycle the catalytic material. In order to
evaluate this sustainable feature of the AgBi-HM, protodecarbxylation of 2-nitrobenzoic acid was performed repeatedly
under optimized conditions utilizing the same portion of catalyst for each reactions. The used hybrid material was removed
between each cycle by centrifugation and after washing and
drying, it was simply reused. Gratifyingly, no decrease in catalytic activity or selectivity occurred during the first 7 consecutive catalytic cycles, and conversion was around 90% even after
the 10th reaction which implies the significant stability and
robustness of the catalytic material (Fig. 3).
Entry
Solvent
c (M)
Conversiona (%)
A
B
1
2
3
4
DMA
DMF
MeCN
MeCN
0.1
0.1
0.1
0.15
81
90
100
96
100
89
100
100
0
11
0
0
a
Determined by 1H NMR analysis of the crude product.
ment considering that MeCN is much more acceptable from
environmental aspects than DMF which performed best under
batch conditions (Table 1, entry 1).81
Upon investigation of the effects of the residence time and
reaction temperature, it was verified that a significant chemical intensification is possible under flow conditions. Due to
the backpressure applied, it was possible to easily overheat the
reaction mixture and to study the effects of temperatures far
above the boiling point of MeCN. As shown in Fig. 4, quantitat-
3.2. Decarboxylation of carboxylic acids under continuous
flow conditions
After achieving convincing batch results, we next turned our
attention to continuous flow operation with the aim to improve
the efficacy and sustainability of the catalytic process. As
detailed in the Experimental, AgBi-HM was employed in a
simple packed-bed reactor setup (see also Fig. 1). Similarly as in
the batch study, the effects of the reaction conditions were
investigated again using the decarboxylation of 2-nitrobenzoic
acid as a model reaction. The effects of solvents which gave
good results in the batch reactions were explored again under
flow conditions. For this, the catalyst bed was heated to 170 °C,
and the solution of the substrate together with 15mol% KOH
was pumped continuously at 50 µL min−1 flow rate. Gratifyingly,
under these conditions, MeCN performed superior compared to
DMF and DMA, and resulted selective decarboxylation with conversions of 100% and 96% at 0.1 M and 0.15 M substrate concentrations, respectively (Table 4). This is a remarkable improve-
4690 | Green Chem., 2021, 23, 4685–4696
Fig. 4 Investigation of the effects of the reaction temperature (a) and
residence time (b) in the AgBi-HM-catalyzed decarboxylation of 2-nitrobenzoic acid in a continuous flow reactor. (Reaction conditions: 0.1 M
substrate concentration, 15 mol% of KOH as base, MeCN as solvent.).
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ive and selective decarboxylation could be achieved at 170 °C
while the reaction mixture (containing the substrate in 0.1 M
concentration together with 15 mol% KOH) was streamed at
50 µL min−1 flow rate. Notably, this corresponded to a residence
time of only 10.5 min which is a significant improvement compared to the batch reaction time of 24 h. When residence time
was decreased to approximately 3.5 min (150 µL min−1 flow
rate), the conversion of the decarboxylation was still 75% at
170 °C. When residence time was kept constant at 10.5 min, a
rapid decrease of conversion was observed with the temperature; for example at 100 °C conversion was only 13%.
A range of aromatic carboxylic acids exhibiting diverse substitution patterns were next submitted to the optimized flow
conditions (Table 5). Similarly as in the batch reactions, quantitative and selective decarboxylation was achieved in cases of
2-nitrobenzoic acid, its 5-methoxy-substituted derivative as
well as 3,5-dinitrobenzoic acid (entries 1–3). To our delight,
the flow protocol proved more effective in numerous reactions
than the batch method. For example, 2-bromo-, 2-methoxy- as
well as 2,6-dimethoxybenzoic acid furnished quantitative conversions (entries 4–6), whereas in batch, conversions were
much lower. Notably, selective decarboxylations of 2-chloroand 2-hydroxybenzoic acid were achieved successfully under
flow conditions (conversions were 100% and 23%, respectively;
entries 7 and 8), whereas these substrates remained inert
under batch conditions. 2,4-Dichlorobenzoic acid and 1-naphtolic acid were also successfully decarboxylated and gave
similar conversions than in the corresponding batch reactions
(entries 9 and 10). Fused heteroaromatic substrates showed
excellent reactivity, and gave quantitative conversion and 100%
selectivity, similarly as under batch conditions (entries 11–13).
Unfortunately, flow reactions of thiophene-2-carboxylic acid
and nicotinic acid could not be evaluated due to possible
deposition of the substrates and/or the products within the
catalyst column. (Isolated yield was also determined for some
representative examples.)
In order to investigate the preparative capabilities of the
AgBi-HM catalyzed protodecarboxylation under flow conditions, the reaction of 2-nitrobenzoic acid was scaled-out
(Fig. 5). With the aim to maximize the productivity of the synthesis, the flow rate was increased to 100 µL min−1 (approx.
5 min residence time), all further reaction parameters were
kept at the previously optimized values (0.1 M substrate concentration, 15 mol% of KOH as base, MeCN as solvent, 170 °C
temperature). A 20 h reaction window was explored, with conversion and selectivity being determined in every hour to
obtain a clear view of the actual catalyst activity. Gratifyingly,
the packed-bed system proved highly stable. No decrease in
activity or selectivity occurred in the first 18 h of the experiment: conversion remained steady around 80–85% and no
side product formation occurred. In the last two hours, a
slight loss of catalytic activity was detected, however after 20 h,
at the end of the experiment, a satisfying conversion of 71%
could still be achieved. Finally, as the result of the scale-out,
1.207 g of nitrobenzene was isolated which corresponded to an
overall yield of 82%.
This journal is © The Royal Society of Chemistry 2021
Paper
Table 5 Exploring the AgBi-HM-catalyzed decarboxylation of various
aromatic carboxylic acids under continuous flow conditions
Conversiona,b (%)
Selectivitya (%)
1
100 (99)
100
2
100
100
3
100
100
4
100 (98)
100
5c
100
100
6
100
100
7
100
100
8
23
100
9c
87
100
10
48
100
11c
100
100
12c
100
100
13c
100 (97)
100
Entry
Substrate
Determined by 1H NMR analysis of the crude product. b For representative examples, isolated yields are shown in parentheses. c DMF was
used as solvent due to solubility issues.
a
3.3.
Characterization of used AgBi-HM samples
With the aim to evaluate catalyst stability and robustness,
AgBi-HM samples previously used in batch recycling experiments as well as in flow scale-out were examined extensively by
various instrumental techniques. The materials were character-
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Paper
Fig. 5 Scaling-out of the AgBi-HM-catalyzed decarboxylation of
2-nitrobenzoic acid in a continuous flow reactor. (Selectivity of the reaction was 100% in all points investigated.).
ized by TG, SEM (SEM-EDX), TEM and XRD measurements,
and the structure of the used catalyst samples was compared
to that of the as-prepared material (Fig. 6).
Thermal analysis revealed that the original structure was
kept up to 380 °C, and weight losses occurred in three
endothermic steps which was also observed in both used catalyst samples (Fig. 6a). In the case of the AgBi-HM sample used
in the batch recycling experiment, slightly greater weight
loss could be observed at lower temperatures which may be
explained by trace amounts of organic deposition on the
surface. The X-ray patterns of both used samples seemed to be
the same as was experienced in case of the as-prepared sample
Fig. 6 (a) Thermal behaviour of used AgBi-HM samples: sample used in
flow scale-out (A), sample used in batch recycling experiment (B). (b)
Comparison of the X-ray patterns of various AgBi-HM samples: as-prepared sample (A), sample used in batch recycling experiments (B),
sample used in flow scale-out (C). (c) SEM images: as-prepared AgBiHM sample – micrograph taken from ref. 41 (A), AgBi-HM sample used
in flow scale-out (B).
4692 | Green Chem., 2021, 23, 4685–4696
Green Chemistry
(Fig. 6b), there was no evidence of structural degradation
visible. Identification of the X-ray patterns were accomplished
on the basis of our previous work.42 These results provided
some further information about primer crystallite size of the
composite calculated by using the well-known Scherrer
equation. This resulted in an average primer crystallite size of
20.98 nm, not only for the as-prepared catalyst but also for the
used ones. As shown earlier,41 the SEM image of the freshlymade catalyst displayed a lamellar ( plate-like) morphology
which was also observed in the used material (Fig. 6c).
Additionally, this observation was also strengthened by
TEM images, in which well-aggregated plates with a secondary
particle size of around 100 nm could be seen for the as-prepared as well as for the used catalyst sample (Fig. S2‡). The
SEM images also confirmed that organic contaminants in the
form of larger aggregates (up to 10 µm) remained on the
surface which makes more difficult to identify the original
morphology. The SEM-EDX elemental maps demonstrated that
the silver and bismuth ions are located evenly in the used
sample as well (Fig. S3‡). ICP-AES measurements confirmed
that the quantity of silver and bismuth ions are in arrangement with the as-prepared sample considering errors of
measurements.41
Taking into account all the characterization data, it can be
ascertained that the AgBi-HM is a highly robust heterogeneous
catalyst which proved to be invariable in a structural point of
view after extensive and demanding use under batch or flow
conditions.
3.4.
Decarboxylative deuterations
Due to its relatively good availability and also because of the
considerably large isotope effect, deuterium is of outstanding
importance among stable isotopes used for labelling
studies.82,83 Synthetic protocols that incorporate deuterium
into various organic substances have therefore many applications in medicinal, analytical or pharmaceutical chemistry.74
Deuterium-labelled compounds are typically applied as
analytical standards, for the evaluation of the metabolic pathways or in tracer studies to investigate pharmacokinetics, catalytic cycles and reaction pathways.84–86 As exemplified by
Austedo®, the first deuterated drug marketed, pharmaceutical
ingredients may also be potentiated by deuterium
exchange.87,88 In contrast to deuterations of C–C or C–X (X =
hetero atom) multiple bonds,89–93 synthetic processes for the
site-specific incorporation of a single deuterium into an aromatic ring are more challenging.74,76,77,94,95 In most cases,
these methods involve halogen/D exchange and are commonly
mediated by strong bases which severely limits the functional
group tolerance.96 Furthermore, catalytic and acid- or basemediated H/D exchange reactions are also available, however,
unlike halogen/D exchange reactions, these often involve
selectivity issues.97–99
Inspired by these limitations, we were intrigued to explore
decarboxylative deuterations of benzoic acid derivatives in the
presence of the silver-containing hybrid material as catalyst.
Initially, reactions were investigated in batch (Table 6), under
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Table 6 Exploring AgBi-HM-catalyzed decarboxylative deuterations
under batch conditions
Conv.a (%)
Selectivitya (%)
Da,b (%)
Entry
1
100
100
98
2
100
100
3
100
4
Entry
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Table 7 Exploring AgBi-HM-catalyzed decarboxylative deuterations
under continuous flow conditions
Product
Conv.a (%)
Selectivitya (%)
Da,b (%)
1c
2
100
100
100
100
31
100
86
3
100
100
71
100
100
4
100
100
100
88
100
100
5
100
100
100
5
79
100
76
6
100
100
100
6
100
100
100
Product
Determined by 1H NMR analysis of the crude product. b Deuterium
content (represent deuterium incorporation rate over incidental hydrogen incorporation). c 10 equiv. of D2O was used as deuterium source.
a
Determined by 1H NMR analysis of the crude product. b Deuterium
content (represent deuterium incorporation rate over incidental hydrogen incorporation).
a
4.
conditions optimized for the protodecarboxylations earlier
(0.15 M substrate concentration, 5 mol% AgBi-HM as catalyst,
15 mol% of KOH as base, DMF as solvent, 110 °C temperature
and 24 h reaction time). As deuterium source, 10 equiv. of D2O
was added to the reaction mixture. We were satisfied to find
that with this simple protocol, deuterodecarboxylations of
various nitrobenzoic acids as well as 2-bromo-, 2,6-dimethoxyand 2,4-dichlorobenzoic acid went smoothly. Excellent conversions (79–100%) and 100% chemoselectivity were achieved in
all cases. In all reactions, deuteration was highly favoured over
incidental hydrogen incorporation as indicated by deuterium
contents of 76–100%.
Continuous flow deuterodecarboxylations were next
attempted in a packed bed reactor charged with AgBi-HM.
Reaction conditions were simply taken from the protodecarboxylation experiments (0.1 M substrate concentration,
15 mol% of KOH as base, MeCN as solvent, 170 °C temperature, 50 µL min−1 flow rate, 10.5 min residence time). In these
cases, 20 equiv. of D2O was used as deuterium source to
achieve high deuterium contents. Gratifyingly, in all reactions
investigated (Table 7), quantitative conversion and 100%
chemoselectivity was achieved, and deuterium incorporation
was also perfect in most cases.
This journal is © The Royal Society of Chemistry 2021
Conclusion
A silver-containing hybrid material with structurally-bound
catalytic centers has been exploited as heterogenous noble
metal catalyst for decarboxylations of carboxylic acids under
batch and continuous flow conditions. It proved to be a
robust, efficiently recyclable and highly active ligand-free catalyst which outperformed the most typical homogeneous catalytic sources in the decarboxylation of 2-nitrobenzoic acid as
model reaction. Although, under batch conditions the catalyst
performed best in DMF as solvent, the application of a simple
packed-bed flow system enabled a solvent switch to the environmentally more acceptable MeCN. After the optimization of
the most important reaction conditions, the selective decarboxylation of diversely substituted aromatic carboxylic acids
were achieved with high conversions either in batch or in continuous flow mode. Importantly, the application of continuous
flow conditions offered a marked chemical intensification as
compared with the batch reactions (10 min residence time vs.
24 h reaction time) and ensured time-efficient syntheses. The
preparative utility of the flow process was verified by a 20 h
scale-out run in which the multigram-scale decarboxylation of
2-nitrobenzoic acid was achieved without notable decrease in
the activity and without detectable degradation of the structure
of catalyst. On the basis of the reaction conditions established
for the protodecarboxylations, heterogeneous catalytic batch as
well as flow methodologies were developed for decarboxylative
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deuterations in the presence of D2O as a readily available deuterium source.
Conflicts of interest
There are no conflicts to declare.
Published on 19 May 2021. Downloaded on 8/10/2021 1:01:46 PM.
Acknowledgements
This research was funded by the Hungarian Ministry of
National Economy, National Research Development and
Innovation Office (GINOP2.3.2-15-2016-00034) and by TKP2020. We are grateful to the Hungarian Research Foundation
(OTKA No. K115731). R. M. was supported by the ÚNKP-19-3
New National Excellence Program of the Ministry for
Innovation and Technology (Hungary). S. B. Ö. acknowledges
the Premium Post Doctorate Research Program of the
Hungarian Academy of Sciences. G. V. thanks for the postdoctoral fellowship under the grant number PD 128189.
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