Recyclable glucose-derived palladium(0) nanoparticles as in situ-formed catalysts for cross-coupling reactions in aqueous media

Article abstract of DOI:10.1039/c5ra25712c

In situ-generated, glucose-derived palladium(0) nanoparticles were shown to be convenient and effective catalysts for aqueous Mizoroki-Heck, Sonogashira and Suzuki-Miyaura cross-coupling reactions. The addition of only 4-10 mol% glucose to the reaction mixture lead to a significant increase in yield of the desired products in comparison to processes that omitted the renewable sugar. Interestingly, the Mizoroki-Heck reaction was observed to proceed in good yield even as the reaction reached acidic pH levels. Extensive analysis of the size and morphology of the in situ-formed palladium nanoparticles using advanced analytical techniques showed that the zero valent metal was surrounded by hydrophilic hydroxyl groups. The increased aqueous phase affinity afforded by these groups allowed for facile recycling of the catalyst.

Full text of DOI:10.1039/c5ra25712c

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Recyclable glucose-derived palladium(0)
nanoparticles as in situ-formed catalysts for cross-
coupling reactions in aqueous media†
ac
Cite this: RSC Adv., 2016, 6, 16115
Jay J. Dunsford,a Oliver S. G. Dacosta,a Rebecca K. Blundell,a
*
Jason E. Camp,‡
James Adams,a Joshua Britton,a Robert J. Smith,a Thomas W. Bousfieldc
and Michael W. Fayb
In situ-generated, glucose-derived palladium(0) nanoparticles were shown to be convenient and effective
catalysts for aqueous Mizoroki–Heck, Sonogashira and Suzuki–Miyaura cross-coupling reactions. The
addition of only 4–10 mol% glucose to the reaction mixture lead to a significant increase in yield of the
desired products in comparison to processes that omitted the renewable sugar. Interestingly, the
Mizoroki–Heck reaction was observed to proceed in good yield even as the reaction reached acidic pH
levels. Extensive analysis of the size and morphology of the in situ-formed palladium nanoparticles using
advanced analytical techniques showed that the zero valent metal was surrounded by hydrophilic
hydroxyl groups. The increased aqueous phase affinity afforded by these groups allowed for facile
recycling of the catalyst.
Received 2nd December 2015
Accepted 29th January 2016
DOI: 10.1039/c5ra25712c
www.rsc.org/advances
support and capping agents.6 Once isolated, Pd0NPs can then be
used as efficient catalysts for bond formation under standard
cross-coupling conditions.7 Two key drawbacks to the use of
Introduction
Palladium-catalysed cross-coupling reactions are some of the
most powerful methods for the creation of new carbon–carbon
bonds due to their high selectivity, functional group tolerance
and regio-/stereoselectivity.1 Many industries, including the
pharmaceutical and agrochemical industries, have made exten-
sive use of palladium mediated cross-couplings for the synthesis
of added-value compounds.2 In order to work effectively these
processes oen require toxic and expensive additives, which
leads to unnecessary waste, expense and cost to the environ-
ment. One area of catalysis that addresses some of the limita-
tions of traditional palladium-mediated bond formation is the
use of palladium nanoparticles (PdNPs).3 The rapid increase in
the use of metal nanoparticles for catalysis is due to the fact that
they have a good reactivity/selectivity prole,4 require low cata-
lyst loadings and are recoverable and recyclable.5 Traditionally,
palladium(0) nanoparticles (Pd0NP) are formed via reduction of
a palladium(II) precatalyst in the presence of a nanoparticle
nanoparticles in palladium catalysis are the cost associated with
the synthesis, isolation and purication of the nanoparticles8
and the use of toxic additives, capping reagents and excess
reagents that lead to a decrease in the activity and recyclability of
the catalyst.9 The in situ formation of catalytically active PdNPs
using a renewable reductant without the addition of capping
agents would overcome many of these issues. Whilst it has been
established that simple monosaccharides such as glucose,10
fructose,11 sucrose,11 other biomass (cellulose/starch/beet juice/
lignan)12,13 and even whole plants14 can reduce metal salts and
form metal nanoparticles (MNPs),15,16 very little research has
been conducted on the ability of in situ formed nanoparticles to
catalyse carbon–carbon bond forming reactions (Fig. 1).17,18
Recently Nacci et al. showed that the addition of a reducing
sugars to palladium-catalysed Ullmann couplings in the pres-
ence of TBAOH resulted in the formation of the desired
symmetric biaryl products.17a In addition, we have shown that
sugar derived palladium nanoparticles are viable catalysts for
Suzuki–Miyaura cross-coupling reactions of aryl iodides and
boronic acids in isopropanol.17c Despite the important role of
monosaccharides in transition metal catalysis, where they have
been mainly used as ligands,19 their effect on cross-coupling
processes is poorly understood. Herein, we harness renewable
sugars for the in situ formation and stabilisation of palladium(0)
nanoparticles in aqueous solutions, which allowed for the
development of a variety of palladium-mediated cross-coupling
reactions as well as facial recycling of the catalyst.
aSchool of Chemistry, University of Nottingham, Nottingham, UK
bNottingham Nanotechnology and Nanoscience Centre, University of Nottingham,
Nottingham, UK
cDepartment of Chemical Sciences, University of Hudderseld, Queensgate,
Hudderseld, UK. E-mail: j.e.camp@hud.ac.uk
† Electronic supplementary information (ESI) available: Nanoparticle
characterisation (TEM, EDS-TEM, XPS, DLS, q-Nano and NanoSight data),
experimental procedures, and 1H/13C{1H} NMR data for all compounds. See
DOI: 10.1039/c5ra25712c
‡ Present address: Department of Chemical Sciences, University of Hudderseld,
Hudderseld, HD1 3DH, UK. Tel: +44 (0)14 8447 3180.
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Table 1 Sugar-derived palladium(0) nanoparticles as catalysts for the
Mizoroki–Heck reaction
Entry
Sugar
Pd/sugar ratio
Yielda (%)
1
2
3
4
Fructose
Cellulose
Sucrose
Glucose
—
1 : 2
1 : 10b
1 : 2
1 : 2
1 : 0
1 : 0
1 : 2
—
1 : 1
1 : 3.5
1 : 4
1 : 2
2
21
58
97
18
12
6
5
6c
7c
8c,d
9
—
Glucose
Glucosee
Glucose
Glucose
Glucose
Glucose
0
70
50
42
20
Fig. 1 Use of renewable sugars in palladium-catalysed coupling
reactions.
10
11
12f
a
Isolated yield. b A 1 : 10 weight to weight ratio of palladium acetate to
cellulose was used. c No Et3N was added. d No palladium was added. e
mol% glucose was added. f PdNPs were preformed and isolated.
4
Results and discussion
Initial investigations were aimed at establishing the catalytic
viability of the sugar-derived palladium nanoparticles under
aqueous conditions20 and to gain an increased understanding
of the role of the reducing sugar in the overall process. Due to its
extensively studied mechanism and synthetic importance, the
Mizoroki–Heck reaction was chosen as an archetypal trans-
formation.21 Importantly, aqueous conditions were investigated
that should allow for increased recyclability of the catalysts and
overall greener processes (vide infra).22 Two key variables that
needed to be examined were the choice of reducing sugar and
the ratio of sugar to palladium. Thus, the coupling of iodo-
benzene (1a) with methyl acrylate (2a) in the presence of
accessible for catalysis whilst preventing catalyst poisoning via
aggregation (vide supra). Having established the benets of
glucose in the Mizoroki–Heck reaction, the ratio of sugar to
palladium was examined (Table 1, entries 9–11). A 1 : 2 ratio of
palladium to sugar was found to give the highest yield of 3a.
Excess sugar may prevent the surface of the catalyst from being
solvent exposed, whilst too little results in increased aggrega-
tion of the palladium(0) and deactivation of the catalyst. It was
also found that a reaction time of 16 h was required for
completion of the process at 100 ꢀC.29 Additionally, experiments
showed that the in situ formed palladium nanoparticles gave
a signicantly better yield of 3a than ones that were preformed
and isolated prior to addition to the reaction (Table 1, entry 4 vs.
12).11,30 This is most likely due to the surface of the nanoparticle
being coated with a layer of organic material, which can block
many of the catalytic sites.
Next the pH of the reaction was monitored as a function of
time (Fig. 2). To accomplish this, a series of reaction between
iodobenzene (1a) and methyl acrylate (2a) under the standard
condition were run for set amounts of time and the pH was
determined. Aer the pH measurement was made, the crude
reaction mixture was worked-up and puried according to the
general procedure to obtain the isolated yields. A plot of pH vs.
time vs. yield revealed that the reaction proceeds in good yield
even at acidic pH (increasing from 30–97% as the pH decrease
from 4.52–2.66). Unfortunately, complete removal of base from
the system gave only a small amount of the desired product
(Table 1, entry 7). These results are in contrast to other pH
dependency studies that established a pH range of 9–11.5 to be
optimal for palladium-catalysed cross-coupling reactions.31 It is
likely that the role of the sugar at basic pH levels is to act as
a ligand for the palladium catalyst, as proposed by Jain et al. for
the use of mannose,19g and that the reaction proceeds through
Pd(OAc)2,
a reducing sugar and triethylamine in water-
: acetonitrile (3 : 1) at 100 ꢀC was used to probe the feasibility of
the process (Table 1).23 Whilst fructose,24 cellulose,25 sucrose26
and glucose27 all have the ability to reduce palladium(II) pre-
catalysts to palladium(0), the yield of product obtained from the
in situ generated nanoparticles varied signicantly (Table 1,
entries 1–4). Of the four sugars tested, fructose has the highest
reduction potential and gives the smallest average particle size
when used to form metal nanoparticles.11 Despite these facts,
fructose gave the lowest yield of product and actually inhibited
the reaction when compared with control experiments (Table 1,
entry 1 vs. 5). The PdNPs formed employing cellulose as the
reductant also gave a low yield (Table 1, entry 2). Sucrose, which
is hydrolysed under the reaction conditions to form a 1 : 1
mixture of fructose and glucose,28 gave a moderate yield of the
ꢀ
desired product aer stirring for 16 h at 100 C (Table 1, entry
3). Nearly quantitative yield of (E)-methyl-cinnamate (3a) was
obtained when glucose was used as the reducing sugar (Table 1,
entry 4). Control experiments in which the sugar, base and
palladium were omitted all showed signicantly lower product
production than the glucose system (Table 1, entry 4 vs. 5–8).
The observed benet of glucose to the process is most likely due
to the fact that it allows enough of the palladium surface to be
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Table 2 Substrate and reaction scope
Fig. 2 Comparison of pH vs. time vs. yield of the PdNP mediated
reaction.
Sodium erythorbate (4 mol%) was used in place of glucose. b Ar–I (1.0
equiv.), (HO)2B–Ph (1.5 equiv.), glucose (10 mol%), Pd(OAc)2 (2 mol%),
Cs2CO3 (2.0 equiv.), DMF : H2O (10 : 1), 100 ꢀC, 16 h. c Ph–Br (1.0 equiv.)
was used.
a
a standard Mizoroki–Heck mechanism.32 As the solution
becomes acid, it is possible that the sugar is then responsible
for reducing the palladium(II) species to the catalytically active
palladium(0) complex.27 These observations could have impor-
tant implications for the development of Pd(0) mediated, base-
free carbon–carbon bond forming reactions under acidic
conditions.
In order to assess the catalytic ability of the in situ formed
PdNPs with respect to both reaction type and substrate scope
a variety of substituted aryl iodides were interrogated. Three
palladium mediated carbon–carbon bond forming reactions,
the Mizoroki–Heck, Sonogashira and Suzuki–Miyaura were
investigated (Table 2). Unprotected arenes with both electron
donating and withdrawing substituents were examined in an
attempt to mitigate the use of protecting groups.33 Several
features of the sugar derived palladium nanoparticle method-
ology are noteworthy. It was shown that both electron with-
drawing groups, such as carboxylic acid and nitro moieties, as
well as basic functionality, such as anilines were well tolerated
in the Mizoroki–Heck reaction between aryl iodides 1 and
various alkenes 2 to give styrene derivatives 3a–i. Substitution at
both the 2- and 4-positions of the iodobenzene ring was found
to be compatible with the cross-coupling procedure. Interest-
ingly, it was also shown that sodium erythorbate, a common
arylbromides and aryl chlorides in the Mizoroki–Heck and
Sonogashira cross-coupling reactions only gave the desired
products in 2–8% yield.29 In contrast to a recent related study,
the nitro moiety was not reduced under the reaction conditions
and cleanly afford the cross-coupled products 3f, 6b, and 7b.19g
Building upon our substrate scope study, we turned our
attention to the synthesis of the important agrochemical
Boscalid (8).34 Thus, reaction of 4-chlorophenylboronic acid (5b)
with 2-iodoaniline (1d) under the standard conditions gave the
desired biaryl 7d. Amidation of aniline 7d afforded the fungi-
cide in good overall yield. Boscalid has been the target of
a number of synthetic approaches.35 Due to the fact that
unprotected anilines are well tolerated under the reaction
conditions, this method provides a nearly two-fold increase in
molar efficiency36,37 compared to
a
standard protocol38
(Scheme 1).
Characterisation of in situ-formed palladium(0) nanoparticles
food additive, could be used to afford a viable catalytic system A number of analytical techniques were used to conrm the
for the synthesis of (E)-3-phenyl-2(E)-propenoic acid (3b), formation of the glucose-derived palladium nanoparticles as
though in a slightly reduced yield. The sugar derived palladium well as to establish their size, morphology, surface character-
nanoparticles were also efficient catalysts for the Sonogashira istics and oxidation state. Transmission Electron Microscopy
and Suzuki–Miyaura reactions under aqueous conditions. (TEM) analysis indicated that the palladium nanoparticles were
Coupling of aryl iodides 1 and phenyl acetylene (4) gave semi-crystalline, approximately 5–20 nm in size and exist in
disubstituted alkynes 6a–c in moderate to good yield. Addi-
a variety of conformations and morphologies, including
tionally, the Suzuki–Miyaura reaction between aryl iodides 1 spheres and prisms (Fig. 3a and b).29 In addition, EDS-TEM was
and phenyl boronic acid (5) gave biaryls 7a–c in good to excel- used to determine that the surface of the nanoparticles is
lent yields under slightly modied conditions. In contrast to our decorated with carbon and oxygen molecules (Fig. 3c). Thus, the
recently reported methods in isopropanol,17c aryl bromides also lighter amorphous material at the periphery most likely
coupled efficiently in a DMF : H2O (10 : 1) solvent system to contains the sugar residues, which served as both the reductant
afford biphenyl (7a) in excellent yield. Unfortunately, the use of and stabiliser of the palladium nanoparticles. This sugar
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Scheme 1 Synthesis of Boscalid (8) using in situ formed palladium nanoparticles.
Fig. 3 Structural and surface characterisation of the sugar–PdNPs. (a and b) typical TEM image of sugar-derived palladium nanoparticles (c) EDS-
TEM map of the nanoparticles showing the nanoparticles surrounded by a hydrophilic hydroxyl shell; palladium (green), oxygen (red), and carbon
(blue) (d) XPS analysis (e) scanning ion occlusion sensing analysis (f) nanoparticle tracking analysis.
coating provides a hydrophilic environment around the palla- probe this hypothesis the recyclability of the in situ formed
dium catalyst. XPS analysis determined that the individual palladium nanoparticles was investigated via a series of reac-
nanoparticles were predominately in the palladium(0) oxidation tions between iodobenzene (1a) and methyl acrylate (2a,
state. Analysis of the sugar-derived nanoparticles suspended in Table 3). Aer the initial reaction was performed under the
water at room temperature by nanoparticle tracking analysis standard conditions, diethylether was added and the solution
(NTA), scanning ion occlusion sensing (SIOS) and dynamic light was subjected to centrifugation. The organic layer was isolated,
scattering (DLS) analysis showed that the nanoparticles aggregate the solvent was removed under reduced pressure and the
into larger clusters of around 100 nm. This average particle size residue was puried via ash chromatography. To the aqueous
in solution is signicantly greater than the small particles iden- layer were added the substrates (1a and 2a), acetonitrile (1 mL)
tiable by TEM analysis. Similar levels of aggregation have been and triethylamine (1.5 equiv.). No additional palladium or
observed in related sugar-derived palladium nanoparticles.39
glucose was added at this stage. Importantly, it was not neces-
sary to wash the aqueous mixture to remove excess salts or
base.40 This study showed that the in situ formed palladium
nanoparticles were recyclable for up to three additional cycles
without signicant loss of catalytic activity. A h reaction
catalysed by the same palladium(0) nanoparticles gave the
product in 61% yield. A similar, though more substantial, drop-
off in reactivity was reported for the recycling of sugar-derived
Recycling of the palladium nanoparticle catalysts
To exploit the hydrophilic nature of the palladium catalysts we
investigated the recyclability of the in situ formed catalyst in an
aqueous solvent. Importantly, the hydrophilic surface of the
renewable sugar derived PdNP should help keep the catalyst in
the aqueous layer during extraction by an organic solvent. To
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Table 3 Recyclability of the in situ-formed palladium nanoparticle
catalysts
for 16 h. The mixture was cooled to rt and water (10 mL) and
diethylether (10 mL) were added. The combined organic
extracts were dried over Na2SO4 and the solvent was removed
under reduced pressure. The crude residue was puried by ash
column chromatography on silica gel (petrol/EtOAc, 15 : 1) to
afford the desired cross-coupled product.
Sonogashira. A 5 mL microwave vial was charged with
Pd(OAc)2 (2 mol%) and glucose (4 mol%). Water/acetonitrile
(3 : 1, 0.2 M) was added followed by triethylamine (1.2 equiv.),
phenyl acetylene (1.0 equiv.) and iodobenzene (1.0 equiv.). The
Entry
Pd/sugar ratio
Yieldb (%)
Notes
1
1 : 2
—
—
—
—
97
92
92
82
61
2a
3a
4a
5a
1st recycle
2nd recycle
3rd recycle
4th recycle
ꢀ
vial was sealed and the mixture was heated to 100 C for 16 h.
The mixture was cooled and water (10 mL) and diethylether (10
mL) were added. The combined organic extracts were dried over
Na2SO4 and the solvent was removed under reduced pressure.
The crude residue was puried by ash column chromatog-
raphy on silica gel (petrol/EtOAc, 9 : 1) to afford the desired
cross-coupled product.
a
Et3N (1.5 equiv.) was added aer each cycle, but no additional
palladium or glucose. b Isolated yield.
catalysts in the Ullmann reaction.17a The decrease in yield in our
system is most likely caused by the low level of sugar residue on
the surface of the nanoparticles at this point in the reaction,
which lead to increased aggregation of the palladium(0) and
deactivation. Similar results were observed in our initial stoi-
chiometry study (see, Table 1, entries 4 vs. 9). Thus, it may be
necessary to add glucose at various points of the reaction in
order to improve the recyclability of the catalyst. Efforts are
currently ongoing in our laboratory to study the effect of
continuous glucose addition on the overall yield of the process.
Suzuki–Miyaura. A 5 mL microwave vial was charged with
Pd(OAc)2 (2 mol%) and glucose (10 mol%). Water/DMF (1 : 10,
0.2 M) was added followed by cesium carbonate (2.0 equiv.),
iodobenzene (1.0 equiv.) and phenylboronic acid (1.5 equiv.).
The vial was sealed and the mixture was heated to 100 ꢀC for 16
h. The mixture was cooled and water (10 mL) and diethylether
(10 mL) were added. The combined organic extracts were dried
over Na2SO4 and the solvent was removed under reduced pres-
sure. The crude residue was puried by ash column chroma-
tography on silica gel (petrol/EtOAc, 30 : 1) to afford the desired
cross-coupled product.
Conclusion
Conflict of interest
In summary, the ability of simple sugars to both form and sta-
bilise recyclable, catalytically active palladium(0) nanoparticles
was demonstrated for a variety of synthetically important carbon–
carbon bond forming reactions. Additionally, the sugar coating
on the surface of the palladium not only prevented aggregation
and deactivation via palladium black formation, but also allowed
for facial recycling of the in situ formed catalyst. Only a catalytic
amount of palladium and a small amount of sugar are required in
the C–C bond forming reactions, which can be run under aqueous
conditions. This in situ catalyst formation method compares
favorably to other recyclable bio-derived palladium nanoparticles
catalyst as it mitigates the requirement to perform and isolate the
palladium nanoparticles.29 We believe that our study provides the
groundwork for a simple technology that opens up exciting
opportunities for the development of a variety of catalytic systems
in which the reducing potential of renewable sugars is harnessed
for the generation, stabilisation and turnover of catalytically active
metal nanoparticles – sugar-powered catalysis.
The authors declare no competing nancial interest.
Acknowledgements
The authors thank Prof. Andrei Khlobystov and Dr Graham
Rance (TEM), Drs Wim Thielemans and Graham Rance (DLS),
Dr Christopher Parmenter (Nanosight/qNano) and Dr Emily
Smith (XPS) for their ort of this research. This work was sup-
ported by the School of Chemistry at the University of Not-
tingham, University of Nottingham Early Career Research
Award, The Nottingham Nanotechnology and Nanoscience
Centre Early Career Researcher's fund and the EPSRC (post-
doctoral associateship for J. J. D. through a First-Grant EP/
J003298/1), as well as the UKRC (summer fellowship for R. K.
B.), GlaxoSmithKline (summer fellowship for J. B.) and the
University of Nottingham internship scheme (summer fellow-
ship for J. A.). We also thank Inochem Ltd. for the kind donation
of sodium erythorbate.
Experimental section
General procedures
References
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iodobenzene (1.0 equiv.) and methyl acrylate (1.2 equiv.). The
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