Palladium on Polydopamine: Its True Potential in Catalytic Transfer Hydrogenations and Heck Coupling Reactions

Article abstract of DOI:10.1002/cctc.201700609

The application of Pd–polydopamine and magnetic Fe₃O₄@Pd–polydopamine catalysts in catalytic transfer hydrogenation reactions and the Heck arylation is reported. The reduction of a wide range of aromatic nitro-compounds bearing both electron-donating and -withdrawing substituents to the corresponding anilines could be efficiently performed, although the reduction of carbonyl compounds was found to be less general. In the latter case, only aromatic ketones could be reduced to the corresponding alcohols, whereas aldehyde substrates were unaffected, which may be owing to their reaction with the catalyst support leading to catalyst deactivation. By using magnetic Fe₃O₄@Pd–polydopamine system, facilitated catalyst recovery and reuse for five consecutive cycles without considerable loss of activity in nitro-group reduction. The efficiency of the catalyst in Heck reactions was comparable to that in transfer hydrogenation, however, no catalytic activity was observed upon reuse in this case, likely as a result of metal leaching. We also explored tandem Heck reaction/catalytic transfer hydrogenation sequences, however, the two reactions showed limited compatibility under the applied conditions.

Full text of DOI:10.1002/cctc.201700609

Accepted Article
Title: Palladium on polydopamine: its true potential in catalytic transfer
hydrogenations and Heck coupling reactions
Authors: Attila Kunfi, Vivien Szabó, Ágnes Mastalir, Imre Bucsi, Miklós
Mohai, Péter Németh, Imre Bertóti, and Gabor London
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Palladium on polydopamine: its true potential in catalytic transfer
hydrogenations and Heck coupling reactions
Attila Kunfi,[a,b] Vivien Szabó,[a] Ágnes Mastalir,[a] Imre Bucsi,[a] Miklós Mohai,[c] Péter Németh,[c] Imre
Bertóti[c] and Gábor London*[b]
Abstract: The application of Pd-polydopamine and magnetic
Fe3O4@Pd-polydopamine
catalysts
in
catalytic
transfer
hydrogenation reactions and Heck-arylation is reported. The
reduction of a wide range of aromatic nitro-compounds bearing both
electron donating and withdrawing substituents to the corresponding
anilines could be efficiently performed, while the reduction of
carbonyl compounds found to be less general. In the latter case,
only aromatic ketones could be reduced to the corresponding
alcohols, while aldehyde substrates were unaffected, which may be
due to their reaction with the catalyst support leading to catalyst
deactivation. Using magnetic Fe3O4@Pd-polydopamine system,
facilitated catalyst recovery and reuse for five consecutive cycles
without considerable loss of activity in nitro-group reduction. The
efficiency of the catalyst in Heck reactions was comparable to that in
transfer hydrogenation, however, no catalytic activity was observed
upon reuse in this case, likely due to metal leaching. We also
explored tandem Heck reaction/catalytic transfer hydrogenation
sequences, however, the two reactions showed limited compatibility
under the applied conditions.
Figure 1. Representative model of PDA structural components.
There are several reasons for its popularity: easy to prepare
through dopamine (DA) self-polymerization under slightly basic
aqueous conditions, adheres to virtually any solid material,[3] can
be functionalized further after formation;[4] redox-active and
biocompatible.[5] These properties led to a broad spectrum of
applications including surface coatings,[6,7] solar energy
conversion,[8]
water
purification,[9]
microrobots[10]
or
nanomedicine.[11]
It also meets several current environmental and economic
requirements in the field of catalysis.[12] PDA is an
environmentally benign material, its preparation is facile and
does not require excess of hazardous reagents and it is non-
toxic, bench-stable and cheap. In contrast to these desirable
properties, its application is limited in catalytic organic
transformations. As an organocatalyst, PDA promotes the aldol
reaction between 4-nitrobenzaldehyde and cyclohexanone due
to the presence of both acidic phenol groups and basic amino-
groups.[13] It has been used as a catalyst in the oxidative
coupling of thiols, where its redox activity (transformation of
phenolic OH-groups to quinoid form) was beneficial.[14] This
latter property, the capability of PDA to induce chemical
reduction has been exploited in metal catalysis as well. The
catalytically active metal component (Au, Ag, Cu, Pd) can be
reduced onto the polymer surface without any additional
reducing agent. Due to the multiple coordinating O and N atoms
present in the material low particle sizes, uniform particle
distributions and high particle stability have been described.
However, only a few types of reactions have been reported
regarding the use of metal/PDA systems in heterogeneous
catalytic transformations. These include Suzuki couplings and
catalytic transfer hydrogenation (CTH) reactions. While the
Suzuki chemistry is well described in terms of reaction
conditions, scope and limitations,[15,16] in the CTH field the
reports follow a clear pattern: the assembled metal/PDA (or
PDA-based) material containing Au,[17-26] Ag,[26-28] Pd[29,30] or
Pt/Au[31] as metal component is mostly used in a single
transformation involving 4-nitrophenol, 100-2000 equivalent of
NaBH4 as hydrogen source and the progress of the reaction is
followed by UV-vis spectroscopy. Although nitrophenol reduction
is considered as a benchmark model reaction in nanoparticle
catalysis,[32] the increasing number of publications where the
Introduction
New materials are continuously in demand. Better performance
in diverse applications, environmental and economic benefit and
biocompatibility are some of the main driving forces for
development. One of the materials attracting major attention in
recent years is polydopamine (PDA), an organic polymer that is
inspired by mussel adhesives (Figure 1).[1,2,3]
[a]
[b]
A. Kunfi, V. Szabó, Dr. Á. Mastalir, Dr. I. Bucsi
Department of Organic Chemistry
University of Szeged
Dóm tér 8, 6720 Szeged, Hungary
A. Kunfi, Dr. G. London
Institute of Organic Chemistry
Research Centre for Natural Sciences, Hungarian Academy of
Sciences
Magyar tudósok körútja 2., 1117, Budapest, Hungary.
Email: london.gabor@ttk.mta.hu
[c]
Dr. M. Mohai, Dr. P. Németh, Dr. I. Bertóti
Institute of Materials and Environmental Chemistry
Research Centre for Natural Sciences, Hungarian Academy of
Sciences
Magyar tudósok körútja 2., 1117, Budapest, Hungary.
Supporting information for this article is given via a link at the end of
the document.
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CTH is limited to the single substrate 4-nitrophenol is also likely
due to the rush to exploit PDA materials in several fields of
chemistry and not driven by the deeper examination of its
potential as catalyst in organic transformations.
In this report we prepare and study Pd-PDA and Fe3O4@Pd-
PDA catalysts in CTH of nitro and carbonyl compounds to
anilines and alcohols, respectively, and in Heck couplings to
explore the scope and limitation of the systems regarding
functional group tolerance, side reactions, reusability and
possibilities of integrating the two processes. Furthermore, we
examine the surface composition and chemical states of the
active Pd metal on the surface of applied catalysts by XPS
technique.
presence of redox active catechol moieties in the preparation of
Pd-PDA composites. Without any additional reducing agent,
such as NaBH4,[30] Pd(OAc)2 was stirred in methanol in the
presence of PDA for 16 h resulting in the deposition of Pd
particles on the surface of the polymer particles (see
Experimental for further details). The imaging of the obtained
material was performed by TEM technique (Figure 3).
Results and Discussion
Preparation of PDA and Pd-PDA
The polymerization of DA has been investigated extensively
regarding mechanism, conditions, additives and polymer
structure.[4,33-39] Generally, the polymeric material forms under
basic aqueous conditions in the presence of O2 in water or
water/alcohol solvent mixtures.[40] We chose to use only distilled
water with no additional co-solvents as a medium for the
Figure 3. TEM images of Pd-coated PDA.
The size of the aggregates, as well as the size and shape of the
round shape PDA units do not seem to alter during the
deposition of Pd onto the PDA as demonstrated in Figure 3.a. At
higher magnification the presence of the Pd particles is revealed
as the highly absorbing small dark spots of 1–3 nm size, densely
covering the surface of the PDA particles (Figure 3.b).
Elemental analysis (ICP) revealed that the Pd content of the
catalyst is 3.03 wt.%, which is somewhat less than the
calculated amount (5 wt.%) based on the added Pd(OAc)2 metal
source.
polymerization.
A pH level of 8.5 was maintained with
tris(hydroxymethyl)aminomethane (Tris) base, while vigorous
stirring ensured the dissolution of enough oxygen for the
polymerization (see Experimental and SI for detailed
information) which was performed in an open Erlenmeyer flask.
With this system, after 48 h stirring 53 % of DA was converted
into PDA. It is noted, that Tris is the most widely used base in
the polymerization process and has been shown to facilitate
lower PDA particle size than phosphate or bicarbonate buffers,
and a three-dimensional particle morphology.[38] On the other
hand, well-dispersed PDA nanospheres obtained in the
presence of NH4OH as base in 20 % methanol-water and 30 %
ethanol-water solvent mixtures with 50 % and 40 % yields,
respectively. However, a decreased yield of 15.5 % was found in
pure water.[40] The imaging of the resulting polymeric material
was performed with TEM technique (Figure 2).
CTH reaction – optimization and substrate scope
Ethyl 4-nitrobenzoate (1a), was chosen as a model substrate for
the optimization of reaction conditions (Table 1). This compound
is also of interest as the corresponding aniline product, ethyl 4-
aminobenzoate (1b), also known as benzocaine, is a local
anesthetic used to treat oral, pharyngeal, and otic pain.[41]
We found that the conversion of compound 1a was complete
within 60 min at 85°C in 96 % EtOH using 4 eq. HCOONa as a
hydrogen source in the presence of Pd-PDA (0.57 mol% Pd)
catalyst (Table 1, entry 1).
Interestingly, by changing the solvent to absolute EtOH, the
conversion of 1a decreased significantly to 30 % while only
starting material was detected next to the aniline product (Table
1, entry 2). The lower solubility of HCOONa in abs. EtOH is likely
the reason for this difference. Conducting the reaction in pure
water resulted in no conversion of 1a (Table 1, entry 3). Upon
changing the solvent from EtOH to MeOH we observed low
conversion (23 %) and a substantial amount of methyl 4-
aminobenzoate, a nitro reduction/transesterification product, was
detected in the product mixture (Table 1, entry 4). Pd-PDA
showed no activity in iPrOH, MeCN and toluene solvents (Table
Figure 2. TEM images of PDA.
TEM micrographs of the as prepared PDA show aggregates of
round shaped particles of 150–250 nm size. We took advantage
of the capability of PDA to induce chemical reduction due to the
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1, entries 5, 6 and 9). Changing the temperature to 50°C and
70°C led to decreased conversions even after prolonged
reaction times (Table 1, entries 10 and 11), however increasing
it to 100°C led to full conversions in 30 min (Table 1, entry 12). It
is important to note, that the hydrogenation of the ester moiety of
1a did not occur under any of the described conditions, only the
nitro group was reduced to the corresponding amine.
Decreasing the amount of formate to 2 eq. led to weaker results
(Table 1, entry 13), however using higher excess (6 eq.) was
found unnecessary as it was not affecting the outcome of the
reaction.
conversion, comparable to the pre-prepared Pd-PDA catalyst
(Table 1, entry 15). In this case, Pd(II) reduction on PDA surface
as an initial step is suggested, which provides in situ generated
Pd-PDA particles which is comparable to the pre-prepared Pd-
PDA in activity.
Table 2 Substrate scope of the Pd-PDA catalysed transfer hydrogenation
reaction of nitro compounds[a]
Entry
1
Reactant
Product
Yield (%)[b]
81
Table 1 Optimisation of the Pd-PDA catalysed CTH reaction conditions[a]
.
2
3
4
79
82
89
Entry[a]
Solvent
EtOH
T (°C)
85
t (min)
60
60
60
60
60
60
60
60
60
90
90
30
60
60
60
conv. (%)[b]
1
100
30
0
2[c]
3
EtOH
H2O
85
85
5
6
81
87
4
MeOH
iPrOH
MeCN
DMF
85
23[d]
<5
0
5
85
6
85
7
85
10
24
0
7
8
9
89
89
81
8[e]
9
DMF
85
toluene
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
85
10
11
12
13[f]
14[g]
15[h]
50
8
70
73
100
63
40
100
100
85
No reaction
No reaction
10
85
11
12
13
14
85
[a] General reaction conditions: 10 mg Pd-PDA (0.57 mol% Pd), 98 mg (0.5
mmol) ethyl 4-nitrobenzoate, 136 mg (2.0 mmol) HCOONa, 2 ml solvent. [b]
Conversion of the nitro compound; determined by GC-MS, with undecane as
an internal standard. Selectivity of the aniline was within 5 % of the conversion
value in each case. [c] Absolute solvent was used. [d] Methyl 4-
aminobenzoate was formed in 75 % selectivity. [e] 4 % water was added. [f] 2
eqiv. of HCOONa was used. [g] 1 mol% Pd(OAc)2 was used as catalyst. [h] 1
mol% Pd(OAc)2 + 10 mg PDA was used as catalyst.
68[c]
No reaction[d]
No reaction[d]
Furthermore, the absence of HCOONa, or catalyst, or using
PDA without Pd loading led to no reaction. Pd(OAc)2 itself,
although catalysed the CTH reaction, only 40 % conversion was
detected after 1 h (Table 1, entry 14). On the other hand, mixing
pristine PDA with Pd(OAc)2 in the reaction mixture led to full
[a] General reaction conditions: 10 mg Pd-PDA (0.57 mol% Pd), 0.5 mmol
reactant, 136 mg (2.0 mmol) HCOONa, 2 ml 96 % EtOH, 85°C, 60 min. [b]
Isolated yield. [c] 32 % nitrobenzene was also formed. [d] <5 % nitrobenzene
was detected.
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Based on the above results, we continued our investigations
using the reaction conditions described in Table 1, entry 1.
A range of nitroarenes with diverse substitution patterns were
submitted to the optimized reaction conditions. Several electron
donating and withdrawing groups were tolerated including -CH3,
-OH, -NH2, -OMe, amide, ester and acetyl, where full
conversions and high isolated yields of the corresponding
anilines were obtained (Table 2, entries 1-9). Interestingly, in the
presence of a nitrile or aldehyde group at para-position no
product could be isolated (Table 2, entries 10 and 11). Further
control experiments showed that the nitrile-compound is
poisoning the catalyst and prevents the hydrogenation process.
Submitting mixtures of nitrobenzene and 4-nitrobenzonitrile (1 :1
and 1 : 2 ratios) to the reaction conditions no reduction of the
nitrobenzene to aniline occurred, which otherwise was complete
within 1 h. Interestingly, while the reaction of 1-chloro-4-
nitrobenzene led to dechlorination, 1-bromo-4-nitrobenzene and
1-iodo-4-nitrobenzene remained essentially untouched (Table 2,
entries 13 and 14). These latter two compounds, similarly to the
nitrile-derivative, acted as catalyst poisons and prevented the
reduction of nitrobenzene to aniline as well. Such preference for
the oxidative insertion of Pd into the Ar-halogen bond over the
reduction has been recently reported in tandem Suzuki-
coupling/transfer hydrogenation sequences.[42]
Table 3 Substrate scope of the Pd-PDA catalysed transfer hydrogenation
reaction of carbonyl compounds[a]
Entry
1
Reactant
Product
Yield (%)[b]
88
2[c]
18[d]
3
85
4[c]
90[d]
It is worth mentioning that in the case of 3-nitroacetophenone
the nitro reduction occurred selectively after 1 h while the oxo-
group reduction occurred only in traces (Table 2, entry 9).
However, upon prolonged reaction time (2 h) and increased
amount of HCOONa (6 eq.) the ratio of the fully reduced
compound (4-aminobenzyl alcohol) was increased to 32 %. This
result shows that the reduction of the nitro group takes place
more rapidly than the carbonyl hydrogenation.
On the other hand, acetophenone itself was a reactive substrate
under the reaction conditions and phenylethanol could be
obtained in high yield (Table 3, entry 1). In case of other
substituted acetophenones the catalyst showed low or no
activity in the reduction process.
4-Methoxyacetophenone could be converted to the
corresponding alcohol in poor conversion, while for 4-
chloroacetophenone dechlorination occurred mostly (Table 3,
entries 2 and 3). The conversion of benzophenone was
comparable to that of acetophenone, however only after
prolonged reaction time (5 h) (Table 3, entry 4) while
cyclopentanone did not undergo reaction (Table 3, entry 5).
Although the carbonyl group of acetophenone and
benzophenone was possible to reduce, molecules containing
aldehydes remained unaffected under the reaction conditions
(Table 2 entry 10, and Table 3 entry 6). We speculated that an
imine formation of the aldehydes with the amino groups of
PDA[13] could block the catalytic sites or reduce the catalytic
activity in other ways.
No reaction
No reaction
5
6
[a] General reaction conditions: 10 mg Pd-PDA (0.57 mol% Pd), 0.5 mmol
reactant, 136 mg (2.0 mmol) HCOONa, 2 ml 96 % EtOH, 85°C, 60 min. [b]
Isolated yield. [c] Performed for 5 h. [d] Yield determined by 1H NMR
spectroscopy of the crude product mixture.
To support this possibility, we performed a control experiment
and stirred 50 mg PDA and 50 mg 4-nitrobenzaldehyde for 1 h
at 85°C in EtOH. Only 32 mg aldehyde could be recovered,
which indicates reaction between the aldehyde and PDA which
could be a potential catalyst deactivation pathway.
CTH – Catalyst recycling
One of the major promises of heterogeneous catalysis is the
possibility of recycling and reusing the catalyst after the desired
reaction takes place. Pd-PDA, although heterogeneous in nature,
is a fine carbon-like powder which represents a disadvantage in
the recovery procedure. One option is to use a centrifuge to
separate the solid catalyst from the reaction mixture, however,
taking advantage of the universal adhesiveness of the forming
PDA a more attractive option is its deposition on magnetic
nanoparticles prior to Pd deposition. Magnetic nanoparticles
(MNPs) are currently among the most used materials aiding the
recycling process of heterogeneous nanoscale catalysts[43-45]
and have been used in combination with PDA-based materials
also.[18,19] Therefore, we encapsulated MNPs with a PDA layer,
followed by the deposition of Pd on the surface of the resulting
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using undecane as an internal standard. Product 1b was isolated after the 5th
run in 88 % yield.
Fe3O4@PDA particle (see ESI for further details). The TEM
imaging of the resulting catalyst was carried out in this case as
well (Figure 4).
In line with our expectations the activity of the catalyst was
unaffected by the core/shell structure and the corresponding
aniline 1b could be obtained in excellent conversion using the
same catalyst in five consecutive cycles (Figure 5).
The activity of the catalyst is remarkable, considering that the
actual Pd amount is only 0.35 mol% based on the ICP
measurement of the Fe3O4@Pd-PDA. Furthermore, the Pd
content of the catalyst was only slightly decreased to 1.55 wt.%
from the original 1.86 wt.% after five consecutive reactions,
which is clearly not affecting the catalytic activity. It is important
to note, that the multiple types of functional groups present in
PDA are likely to stabilize the particles on the surface of the
support, preventing aggregation or any form of metal loss during
the several reuses, which is a problem in similar reactions using
different polymer-based supports.[46,47]
Figure 4. TEM image of core/shell particles. The ring ED pattern (inset)
corresponds to magnetite (M) and Pd (Pd) nanocrystals. The area marked with
a white rectangle on (a) magnified on (b). Black arrows point to Pd particles.
We performed control experiments in order to compare the
activity of Pd-PDA and Fe3O4@Pd-PDA. As a substrate we used
ethyl 4-nitrobenzoate and the catalyst amounts were set to
contain equivalent amount of Pd in both reactions (0.57 mol%
Pd; 10 mg Pd-PDA and 16 mg Fe3O4@Pd-PDA); the reaction
conditions were otherwise unchanged. Monitoring the
conversion of ethyl 4-nitrobenzoate by time revealed that the
conversion was complete after 40 min with both catalysts
(Figure 6). Compared to the magnetic system, Pd-PDA showed
slightly lower activity in the first 30 min of the reaction which
could be due to the reduction period of higher oxidation state Pd
species to active Pd(0) in this case. (Note that Pd-PDA contains
mainly higher oxidation state Pd, likely due to the very small size
of the Pd deposits, while Fe3O4@Pd-PDA contains larger
amount of Pd(0). See also Figure 10.)
In the case of the magnetite core/Pd-PDA shell type samples,
the high absorbance Fe3O4 particles of irregular shape are
clearly overgrown by PDA layers as demonstrated in Figure 4.a
and b. The assemblies of the magnetite particles not only
covered by PDA but also strongly cemented together by the
polymer layer. At the edges of the less absorbing PDA coating,
the deposited Pd appears as dark, 5–10 nm size particles
(Figure 4.b). The inset in Figure 4.a shows the ED pattern taken
from this composite sample. Rings representing diffractions from
different crystal planes of the magnetite and the metallic Pd are
indicated. Only weak diffraction ring could be detected on the
Pd-PDA sample due, most probably, to the very small particle
size of the Pd deposits in that case (see also Figure S1, SI).
Elemental analysis (ICP) of the Fe3O4@Pd-PDA system showed
a decreased amount of Pd (1.86 wt.%) compared to both the
theoretical amount (5 wt.%) and the Pd-PDA system (3.03 wt.%).
We expected that the presence of the magnetic core will not
interfere with the catalytic activity of Pd-PDA while greatly
facilitate the catalyst recovery process. We tested the magnetic
catalyst Fe3O4@Pd-PDA in the reduction of compound 1a under
the optimized reaction conditions described above (Table 1,
entry 1).
100
90
80
70
60
Pd-PDA
50
40
30
20
10
0
MNP@Pd-PDA
100
80
60
40
20
0
0
10
20
30
40
50
60
time (min)
Figure 6. The progress of the reduction of 1a to the corresponding aniline over
time using Pd-PDA and Fe3O4@Pd-PDA as catalyst. (Reaction conditions: 98
mg (0.5 mmol) 1a, 0.57 mol% Pd (10 mg Pd-PDA or 16 mg Fe3O4@Pd-PDA),
136 mg (4 eq., 2.0 mmol) HCOONa, 2 ml 96 % EtOH, 85°C).
This is also supported by the post-reaction TEM images of both
catalysts. Aggregation of Pd to larger size particles is apparent
in case of Pd-PDA (Figure 7a) while no pronounced change
occurred in the size and morphology of the Pd particles in the
Fe3O4@Pd-PDA system (Figure 7b).
1
2
3
4
5
catalytic cycles
Figure 5. Recycling of Fe3O4@Pd-PDA catalyst in the CTH reaction of
compound 1a. (Reaction conditions: 10 mg Fe3O4@Pd-PDA (0.35 mol% Pd),
98 mg (0.5 mmol) 1a, 136 mg (4 eq., 2.0 mmol) HCOONa, 2 ml 96 % EtOH,
85°C, 60 min.) Conversion of the nitro compound was determined by GC-MS
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the bromide, a significant increase in the conversion and yield
was obtained.
Similarly to the observations in CTH reactions, application of the
magnetic Fe3O4@Pd-PDA catalyst system instead of Pd-PDA
did not affect catalytic activity and resulted in full conversion and
high isolated yield in the reaction of 4-bromoacetophenone with
ethyl acrylate. The isolation of the catalyst was facilitated by its
magnetic property, however, after the second run in the same
reaction no product could be isolated.
Figure 7. Post-CTH reaction TEM images of (a) Pd-PDA and (b) Fe3O4@Pd-
PDA.
Table 4 Substrate scope of the Pd-PDA catalysed Heck reaction of aryl
halides with ethyl acrylate[a]
Although the results of the repeated catalyst recycling pointed
towards the heterogeneous nature of the reaction, we also
performed a hot filtration test. After 15 min reaction time (at
85°C) the catalyst was filtered and the filtrate was stirred at 85°C
for another 45 minutes. The conversion of ethyl 4-nitrobenzoate
to the corresponding aniline was 22 % and 24 % after 15 min
and 60 min, respectively.
Yield
Entry
1
Reactant
Product
(%)[b]
Pd-PDA catalysed Heck reaction
93
Due to our interest in heterogeneous catalytic Heck couplings[48]
and multistep heterogeneous catalytic reactions[49] we examined
Pd-PDA catalysed Heck reactions and Heck reactions in
conjugation with CTH processes. Although Heck reactions
catalysed by a Pd/dopamine system have been reported,[50] the
catalyst was described to be based on dopamine coordination to
the supporting Fe3O4 particles through its phenolic OH-groups,
while the pendant amines of dopamine had the role to
coordinate the catalytically active Pd-species. The Heck
coupling of different aryl halides with ethyl acrylate were tested
using Pd-PDA catalyst, Na2CO3 as base, nBu4NCl (TBAC) salt in
N-methyl-2-pyrrolidone (NMP) as solvent (Table 4). Full
conversions and high isolated yields were obtained in the
presence of electron withdrawing groups at the para-position of
aryl halides (Table 4, entries 1–3), while decreased reactivities
were observed in the case of electron donating para-
substituents (Table 4, entries 4–5), which is consistent with the
reactivity patterns reported for Heck couplings.[51] Using the more
reactive aryl iodide (4-iodoanisole, Table 4, entry 5) instead of
2
3
4
5
88
87
28
74
[a] Reaction conditions: 10 mg Pd-PDA (0.57 mol% Pd), 0.5 mmol aryl halide,
65 µl (0.6 mmol) ethyl acrylate, 74 mg (0.7 mmol) Na2CO3, 35 mg (0.125
mmol) TBAC, and 2 ml NMP, 85°C, 24h. [b] Isolated yield.
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Figure 8. Pd-PDA catalysed sequential Heck-CTH reaction of 4-bromo nitrobenzene.
This may be due to the irreversible leaching of Pd from the
support surface. This is also supported by the ICP analysis of
the spent catalyst showing a considerable decrease of the Pd
content from the original 1.86 wt.% in Fe3O4@Pd-PDA to 0.50
wt.% after the Heck reaction. Furthermore, hot filtration test also
supported the homogeneous mechanism of the reaction. After 2
h reaction time (at 85°C) the catalyst was filtered and the filtrate
was stirred at 85°C for another 22 h. The conversion of 4-
bromoacetophenone to the Heck product was 27 % and 85 %
after 2 h and 24 h, respectively.
time only 2a was detected, with no hydrogenated compound 2b
or 2c in the reaction mixture. Upon the addition of not only the
formate salt, but also additional Pd-PDA catalyst to the reaction
mixture following the Heck step, the reduction product aniline 2b
was produced in 38 % (GC yield) along with traces of 2c after 4
h reaction (Table 5, entry 8). This observation indicates that the
Heck reaction is accompanied with metal leaching, as solvated
Pd species are needed for the efficient process. It also points
out that Pd leaves the support surface without re-deposition after
the reaction which is preventing efficient CTH reaction after the
Heck step.
We also tested the reduction of isolated 2a using the optimized
CTH reaction procedure. This reaction led to the saturated
aniline 2c in 89 % yield, with traces of 2b present in the product
mixture.
Large scale reactions
Increasing the scale of a reaction that works efficiently in
milligram quantities is an important step towards practical
applicability. Increasing the 100 mg to 1 g scale in the reduction
of compound 1a using the same catalytic conditions (0.57 mol%
Pd-PDA, 4 eq. HCOONa, EtOH, 85°C), 1a was consumed after
3 h reaction time and 744 mg (88 % yield) benzocaine product
was isolated.
Table
5
Pd-PDA catalysed sequential Heck-CTH reaction of 4-
bromoacetophenone and ethyl acrylate[a]
Entry
1
Solvent
toluene
Yield (2a)[b]
Yield (2b)[b]
0
0
Moreover, the same increase in scale was performed in the case
of the Heck reaction of 4-bromoacetophenone with ethyl acrylate
where the product was isolated in 81 % yield using 1 g of aryl
halide.
2[c]
3[d]
4
toluene
toluene
EtOH
EtOH
EtOH
EtOH
EtOH
0
0
>99
0
0
0
5[c]
6[d]
7[d,e]
8[d,e,f]
0
0
Tandem Heck-arylation/CTH reaction sequence
>99
>99
>99
0
Heterogeneous catalytic one-pot and tandem reactions are
among the recent focus areas of sustainable chemistry.[52,53] In
line with these efforts we examined the compatibility of Pd-PDA
catalysed Heck coupling and CTH reactions. The first problem
that had to be overcome is the incompatibility of the solvents
that can be used in the reactions. Apolar and polar aprotic
solvents (MeCN, DMF and toluene) were found inapplicable in
CTH reaction earlier (see Table 1 entry 6, 7 and 9), while polar
protic solvent, such as EtOH, was not efficient in the Heck
reaction (Table 5, entry 4, 5). Using 1 eq. TBAC in EtOH
facilitated the Heck coupling between 4-bromonitrobenzene and
ethyl acrylate (Figure 8) resulting in full conversion to compound
2a after 24h reaction (Table 5, entry 6). Subsequently, after 24 h
reaction time, 4 eq. HCOONa was added and the reaction
mixture was stirred for another 1h. However, after this reaction
0
38
[a] Reaction conditions: 10 mg (0.57 mol%) Pd-PDA, 0.5 mmol aryl halide, 65
µl (0.6 mmol) ethyl acrylate, 74 mg (0.7 mmol) Na2CO3, 2 ml solvent. After 24
h at 85°C, 136 mg (2 mmol) HCOONa was added and the reaction was stirred
for additional 1 h. [b] GC yield. [c] 0.25 eq. TBAC was used. [d] 1 eq. TBAC
was used. [e] 10 mg Pd-PDA was added after the Heck reaction. [f] The CTH
reaction time was 4 h.
Surface composition and chemical states of palladium in
the catalysts
The surface composition of the freshly prepared PDA, composite
Fe3O4@Pd-PDA and Pd-PDA samples and of the Pd-PDA
sample after CTH reaction are shown in Table 6, in atomic and
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10.1002/cctc.201700609
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also in mass %, as evaluated by the integral intensity ratio of the
C 1s, O 1s, N 1s and Pd 3d signals. Since no signal of Fe
appeared in the XP spectra, Fe content could not be measured
by XPS. This means that the magnetite particles are completely
covered by PDA layer thicker than 15 nm, what is the sampling
depth of the most intense Fe 2p signal.
tabulated data collected in ref. [58]. The relative proportions of
the oxidation states are shown in Figure 10.
Several conclusions can be made based on Figures 9 and 10.
First is that in the as-prepared Pd-PDA the palladium is in
oxidised PdO and PdO2 states, no elemental Pd is present. In
addition, a minor amount of ionic state was also found and
assigned to be the acetate residue of the palladium precursor.
After the CTH reaction, however, the majority of the Pd is found
in the metallic state on the surface of the used Pd-PDA sample,
indicating to the reducing environment present. Differently to this
sample, on the core-shell Fe3O4@Pd-PDA sample the deposited
Pd is predominantly in metallic state in combination with some
amount of PdO and PdO2.
Table 6 Surface composition of PDA, Fe3O4@Pd-PDA and Pd-PDA freshly
prepared samples and of the Pd-PDA sample after CTH reaction.
Entry
Sample
C
O
N
Pd
atomic %
1
PDA
74.3
69.8
71.1
63.9
18.7
19.3
18.6
26.3
7.1
6.9
–
2
Fe3O4@Pd-PDA fresh
Pd-PDA fresh
Pd-PDA used
4.0
4.9
4.5
3
5.4
4[a]
5.3
mass %
7.7
Pd 3d
5
PDA
69.2
50.2
48.8
44.0
23.2
18.5
17.0
24.1
–
6
Fe3O4@Pd-PDA fresh
Pd-PDA fresh
Pd-PDA used
5.8
25.4
29.9
27.7
7
4.3
8[a]
4.2
[a] Small fraction of excess O content originated from the cellulose substrate.
a
As seen from the data, the composition of PDA (Table 6, entries
1
and 5) does not differ significantly from the nominal
composition of the dopamine monomer, except the somewhat
lower N content. Similar statement could be made for the
polymeric constituents of the freshly prepared core-shell sample
(Table 6, entries 2 and 6). Compositions close to ours have
been reported recently for PDA and PDA films deposited on
oxidized Si.[54-57]
During the catalyst preparations 5 wt.% Pd was added to the
PDA samples, however, more than five times higher values were
measured by XPS (Table 6, entries 6-8). This fact clearly
indicates that the Pd is situated on the outer surface of all
samples. For the comparison of the freshly prepared and used
Pd-PDA (in CTH) (Table 6, entries 3 vs. 4 and 7 vs. 8) it turned
out that the quantity of Pd is preserved.
From the point of view of catalytic activity the chemical
environment of Pd is of prime interest. For this purpose the
oxidation state of palladium was accurately evaluated from the
high resolution spectra of the Pd 3d region. Figure 9 illustrates
the Pd 3d doublet for the Fe3O4@Pd-PDA and Pd-PDA freshly
prepared samples and that of the Pd-PDA sample used in the
CTH reaction. The complex shape of the peaks indicates
presence of different chemical environments. For determining
the contribution of each chemical environment, the recorded
peak envelopes were fitted by doublets with the theoretical
constant integral intensity ratio of 3:2 and the constant energy
separation of the Pd 3d5/2 and 3d3/2 lines of 5.4 eV. The results,
the peak-components representing the different chemical states,
are depicted in Figure 9.
b
c
Figure 9. The Pd 3d photoelectron lines of the Fe3O4@Pd-PDA (a) and Pd-
PDA (b) freshly prepared samples and the Pd-PDA sample after CTH reaction
(c).
The assignment of the components to the palladium in elemental
Pd, in PdO and in PdO2 is based on the well-established
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10.1002/cctc.201700609
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suspension was filtered, washed with deionized water and dried under
ambient conditions.
100
80
60
40
20
0
General procedure for the catalytic transfer hydrogenation (CTH)
reaction
Pd
PdO
Pd-PDA (10 mg, 0.57 mol% Pd), reactant (0.5 mmol), HCOONa (136 mg,
2.0 mmol) and 96 % EtOH (2 mL) were placed in a sealed vial and the
mixture was stirred at 85°C for 60 min. The reaction mixture was diluted
with EtOAc, filtered through a pad of silica and concentrated under
vacuum. The product was purified by column chromatography and
characterized by NMR spectroscopy.
PdO2
PdO2
PdOAc
Fe3O4@Pd-PDA
Fe3O4@Pd-PDA
Pd-PDA
fresh
Pd-PDA
reacted
General procedure for the Heck coupling reaction
fresh
fresh
Pd-PDA (10 mg, 0.57 mol% Pd), aryl halide (0.5 mmol), ethyl acrylate (65
µl, 0.6 mmol), Na2CO3 (74 mg, 0.7 mmol), nBu4NCl (35 mg, 0.125 mmol),
and N-methyl pyrrolidone (2 mL) were placed in a sealed vial and the
mixture was stirred at 85°C for 24 h. The reaction mixture was diluted
with EtOAc, filtered through a pad of silica and concentrated under
vacuum. The product was purified by column chromatography, and
characterized by NMR spectroscopy.
Figure 10. The surface composition (atomic %) of the various chemical states
of palladium in the Fe3O4@Pd-PDA and Pd-PDA freshly prepared samples
and in the Pd-PDA sample after the CTH reaction.
Conclusions
Pd-polydopamine and magnetic Fe3O4@Pd-polydopamine
catalysts were found to be efficient in catalytic transfer
hydrogenation and Heck-arylation reactions. In the reduction of
aromatic nitro compounds a wide range of substrates, bearing
both electron donating and withdrawing substituents, were
tolerated and converted to the corresponding anilines in high
yields. Catalyst recovery and reuse was facilitated by the
magnetic core/shell type structure of the catalyst. No significant
loss of activity was observed in five consecutive catalytic cycles
in nitro-group reduction. The substrate scope in carbonyl
reductions was found to be much more limited, only aromatic
ketones were active in the transformation. Aldehydes were
inactive, moreover, it is likely that they contribute to catalyst
deactivation via reaction with the amino-groups of the
polydopamine support. Heck arylations were similarly efficient as
nitro-reductions, however in their case the catalyst was inactive
in a second reuse, which may be due to metal leaching. We also
explored tandem Heck-arylation/transfer hydrogenation reaction
sequences, however, the two reactions are of limited
compatibility under the reaction conditions.
Acknowledgements
Financial support from the National Research, Development and
Innovation Office, Hungary (NKFIH OTKA Grants K 109278 (AM,
IB) and PD 115436 (GL)) is gratefully acknowledged. GL
acknowledges the János Bolyai Research Scholarship from the
Hungarian Academy of Sciences. Koppány Levente Juhász
(University of Szeged, Department of Applied and Environmental
Chemistry) is acknowledged for TEM imaging of Pd-PDA. The
Chemical Biology Research Group and the Organocatalysis
Research Group (both at the Institute of Organic Chemistry
Research Centre for Natural Sciences, Hungarian Academy of
Sciences) are acknowledged for support. We thank the
anonymous reviewers for their insightful comments, which
improved the quality of the manuscript.
Keywords: palladium • polydopamine • transfer hydrogenation •
Heck reaction • magnetic nanoparticles
[1]
[2]
J. H. Waite, M. L. Tanzer, Science 1981, 212, 1038‒1040.
H. Lee, N. F. Scherer, P. B. Messersmith, Proc. Natl. Acad. Sci. USA
2006, 103, 12999–13003.
Experimental Section
[3]
[4]
H. Lee, S. M. Dellatore, W. M. Miller, P. B. Messersmith, Science 2007,
318, 426–430.
Polymerization of dopamine
R. Mrówczyński, R. Markiewicz, J. Liebscher, Polym. Int. 2016, 65,
1288–1299.
Tris base (484 mg, 4.0 mmol) was dissolved in deionised water (350 mL)
and stirred for 30 min (pH = 8.5). A solution of dopamine hydrochloride
(1.0 g, 5.27 mmol) in water (50 mL) was added and the solution was
stirred at room temperature for 30 hours. The black suspension of
polydopamine (PDA) was filtered, the residue was washed with deionised
water and dried under air overnight.
[5]
[6]
[7]
Y. Liu, K. Ai, L. Lu, Chem. Rev. 2014, 114, 5057−5115.
Q. Ye, F. Zhou, W. Liu, Chem. Soc. Rev. 2011, 40, 4244–4258.
M. Liu, G. Zeng, K. Wang, Q. Wan, L. Tao, X. Zhang, Y. Wei,
Nanoscale 2016, 8, 16819–16840.
[8]
[9]
M. Lee, J. U. Kim, J. S. Lee, B. I. Lee, J. Shin, C. B. Park, Adv. Mater.
2014, 26, 4463–4468.
B. D. McCloskey, H. B. Park, H. Jua, B. W. Rowe, D. J. Miller, B. D.
Freeman, J. Membrane Sci. 2012, 413–414, 82–90.
Preparation of Pd-PDA
[10] J. Mu, C. Hou, H. Wang, Y. Li, Q. Zhang, M. Zhu, Sci. Adv. 2015, 1,
PDA (200 mg) was dispersed in a solution of Pd(OAc)2 (21 mg, 0.094
mmol) in MeOH (50 mL) and stirred vigorously overnight. The
e1500533.
This article is protected by copyright. All rights reserved.

10.1002/cctc.201700609
ChemCatChem
FULL PAPER
[11] B. Wang, G. Wang, B. Zhao, J. Chen, X. Zhange, R. Tang, Chem. Sci.
2014, 5, 3463–3468.
[36] C.-T. Chen, F. J. Martin-Martinez, G. S. Jung, M. J. Buehler, Chem. Sci.
2017, DOI: 10.1039/C6SC04692D
[12] R. A. Sheldon, I. Arends, U. Hanefeld, Green Chemistry and Catalysis,
Wiley-VCH, Weinheim, 2007.
[37] N. F. Della Vecchia, R. Avolio, M. Alfè, M. E. Errico, A. Napolitano, M.
d’Ischia, Adv. Funct. Mater. 2013, 23, 1331–1340.
[13] R. Mrówczyński, A. Bunge, J. Liebscher, Chem. Eur. J. 2014, 20,
8647–8653.
[38] N. . Della Vecchia, A. Luchini, A. Napolitano, G. D’Errico, G. Vitiello, N.
Szekely, M. d’Ischia, L. Paduano, Langmuir, 2014, 30, 9811−9818.
[39] Y. Ding, L.-T. Weng, M. Yang, Z. Yang, X. Lu, N. Huang, Y. Leng,
Langmuir 2014, 30, 12258−12269.
[14] Y. Du, H.-C. Yang, X.-L. Xu, J. Wu, Z.-K. Xu, ChemCatChem 2015, 7,
3822–3825.
[15] A. Xie, K. Zhang, F. Wu, N. Wang, Y. Wang, M. Wang, Catal. Sci.
Technol. 2016, 6, 1764–1771.
[40] X. Jiang, Y.Wang, M. Li, Sci. Rep., 2014, 4, 6070.
[41] G. T. Kluemper, D. G. Hiser, M. K. Rayens, M. J. Jay, Am. J. Orthod.
Dentofacial Orthop. 2002, 122, 359−365.
[16] A. V. Dubey, A. V. Kumar, RSC Adv. 2016, 6, 46864–46870.
[17] T. Zeng, X. Zhang, H. Niu, Y. Ma, W. Li, Y. Cai, Appl. Catal. B 2013,
134–135, 26–33.
[42] N. Dehury, N. Maity, S. K. Tripathy, J.-M. Basset, S. Patra, ACS Catal.,
2016, 6, 5535–5540.
[18] M. Álvarez-Paino, G. Marcelo, A. Mu oz-Bonilla, M. ernández-García,
Macromolecules 2013, 46, 2951−2962.
[43] D. Wang, D. Astruc, Chem. Rev 2014, 114, 6949–6985.
[44] M. B. Gawande, R. Luque, R. Zboril, ChemCatChem 2014, 6, 3312–
3313.
[19] R. Liu, Y. Guo, G. Odusote, F. Qu, R. D. Priestley, ACS Appl. Mater.
Interfaces 2013, 5, 9167–9171.
[45] A. Baeza, G. Guillena, D. J. Ramûn, ChemCatChem 2016, 8, 49–67.
[46] S. K. Mahato, R. U. Islam, C. Acharya, M. J. Witcomb, K. Mallick,
ChemCatChem 2014, 6, 1419–1426.
[20] J. Luo, N. Zhang, R. Liu, X. Liu, RSC Adv. 2014, 4, 64816–64824.
[21] J. Cao, S. Mei, H. Jia, A. Ott, M. Ballauff, Y. Lu, Langmuir 2015, 31,
9483−9491.
[47] Y.-M. Lu, H.-Z. Zhu, W.-G. Li, B. Hu, S.-H. Yu, J. Mater. Chem. A 2013,
1, 3783-3788.
[22] W. Gong, L. Su, X. Zhang, Chem. Commun. 2015, 51, 6333−6336.
[23] I. Kim, H. Y. Son, M. Y. Yang, Y. S. Nam, ACS Appl. Mater. Interfaces
2015, 7, 14415−14422.
[48] A. Kunfi, Á. Mastalir, I. Bucsi, G. London, Reac. Kinet. Mech. Cat. 2016,
119, 165–178.
[24] Y. Ni, G. Tong, J. Wang, H. Li, F. Chen, C. Yu, Y. Zhou, RSC Adv.
2016, 6, 40698–40705.
[49] D. Zsolnai, P. Mayer, K. Szőri, G. London, Catal. Sci. Technol. 2016, 6,
3814-3820.
[25] Z. Wu, H. Lin, Y. Wang, X. Yu, J. Li, Z. Xiong, Y. Wang, Y. Huang, T.
Chen, F. Liu, RSC Adv. 2016, 6, 62302‒62309.
[50] R. B. N. Baig, J. Leazer, R. S. Varma, Clean Techn. Environ. Policy
2015, 17, 2073–2077.
[26] X. Yu, G. Cheng, S.-Y. Zheng, Sci. Rep. 2016, 6, 25459.
[27] Q. Zhang, C. Zhoua, A. Huang, RSC Adv. 2015, 5, 91056–91061.
[28] G. H. Choi, D. K. Rhee, A. R. Park, M. J. Oh, S. Hong, J. J. Richardson,
J. Guo, F. Caruso, P. J. Yoo, ACS Appl. Mater. Interfaces 2016, 8,
3250−3257.
[51] I. P. Beletskaya, A. V. Cheprakov, Chem. Rev. 2000, 100, 3009–3066.
[52] F.-X. Felpin, E. Fouquet, ChemSusChem 2008, 1, 718–724.
[53] M. J. Climent, A. Corma, S. Iborra, M. J. Sabater, ACS Catal. 2014, 4,
870–891.
[54] Y. Ding, L-T. Weng, M. Yang,X. Lu, N. Huang, Y. Leng, Langmuir 2014,
30, 12258–12269.
[29] S.-W. Bian, S. Liu, M.-X. Guo, L.-L. Xu, L. Chang, RSC Adv. 2015, 5,
11913–11916.
[55] M. Zhang, X. He, L. Chen, Y. Zhang, J. Mater. Chem. 2010, 20, 10696–
10704.
[30] J.-X. Ma, H. Yang, S. Li, R. Ren, J. Li, X. Zhang, J. Ma, RSC Adv. 2015,
5, 97520–97527.
[56] R. Luo, L. Tang, J. Wang, Y. Zhao, Q. Tu, Y. Weng, R. Shen, N. Huang,
Colloids and Surfaces B: Biointerfaces 2013, 106, 66–73.
[57] X-C. Liu, G-C. Wang, R-P. Liang, L. Shi. J-D. Qiu, J. Mater. Chem. A
2013, 1, 3945–3953.
[31] W. Ye, J. Yu, Y. Zhou, D. Gao, D. Wang, C. Wang, D. Xue, Appl. Catal.
B 2016, 181, 371–378.
[32] T. Aditya, A. Pal, T. Pal, Chem. Commun. 2015, 51, 9410–9431.
[33] S. Hong, Y. S. Na, S. Choi, I. T. Song, W. Y. Kim, H. Lee, Adv. Funct.
Mater. 2012, 22, 4711–4717.
[58] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-
Ray Photoelectron Spectroscopy, Perkin-Elmer Corporation, Physical
Electronics Division, Eden Prairie, MN, 1992.
[34] Q. Wei, F. Zhang, J. Li, B. Li, C. Zhao, Polym. Chem. 2010, 1, 1430–
1433.
[35] J. Liebscher, R. Mrówczyński, H. A. Scheidt, C. ilip, N. D. Hădade, R.
Turcu, A. Bende, S. Beck, Langmuir 2013, 29, 10539–10548.
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Entry for the Table of Contents
FULL PAPER
Attila Kunfi, Vivien Szabó, Ágnes
Mastalir, Imre Bucsi, Miklós Mohai,
Péter Németh, Imre Bertóti, Gábor
London*
Page No. – Page No.
Palladium on polydopamine: its true
potential in catalytic transfer
hydrogenations and Heck coupling
reactions
Text for Table of Contents
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