Pd embedded in chitosan microspheres as tunable soft-materials for Sonogashira cross-coupling in water-ethanol mixture

Article abstract of DOI:10.1039/c4gc02175d

Easy shaping of chitosan (CS) as porous self-standing nanofibrillar microspheres allows their use as a palladium carrier. Amino-groups on CS enable the modulation of Pd coordination, giving rise to three different support-catalyst interactions: weakly-coo

Full text of DOI:10.1039/c4gc02175d

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Pd Embedded in Chitosan Microspheres as Tunable Soft-Materials for
Sonogashira Cross-Coupling in Water-Ethanol Mixture
Sana Frindy,a,b Ana Primo,b Mohamed Lahcini,a Mosto Bousmina,c,d Hermenegildo Garcia,b Abdelkrim El Kadib.c,*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Easy shaping of chitosan (CS) as porous self-standing
nanofibrillar microspheres allows their use as palladium
carrier. Amino-groups on CS enables Pd coordination to be
unambiguously established instability of the porous system in
aqueous and basic conditions, necessary for cross-coupling
50 catalysis, constitutes serious setbacks of these supports.8
Additionally, the preparation of most mesoporous metal oxides
10 modulated, given rise to three different support-catalyst
interactions: weakly-coordinated Pd-CS in native CS,
incarcerated Pd-CS-Glu in cross-linked CS and strongly-
ligated Pd-CS-SH, attained by introduction of thiol arms in
CS. These catalysts efficiently promote under mild and
15 sustainable conditions (water-ethanol as solvent at 65°C)
Sonogashira cross-coupling of a large library of functional
substrates and stand as recyclable, metal-scavenging
catalytic systems.
is
energy-consuming
and
time-demanding
making
investigations on easily accessible and/or available renewable
materials appealing as possible alternative supports. In this
55 context, polysaccharides are the most important family of
bioresources with enormous potential to provide directly or
upon functionalisation, a remarkable variety of well-designed,
macromolecular materials. Among them, chitosan (CS, a by
product of fishing industry) displays the dual feature of being
60 easily physically moldable (to provide porous open framework
hydrogels) and chemically modulable (owing to the presence of
The attribution in 2010 of the prestigeous Nobel prize to the
20 laureates of cross-coupling catalysis reflects the importance of
these pionnering works as landmark in organic chemistry and
their strong impact on material science.1 Since their discovery,
these reactions have been widely used in organic synthesis,
palladium being recognised as the most active metal and
amino groups amenable to undergo
a set of chemical
modifications).9 CS hydrogels were recently reported to be
efficient medium for the controlling-growth of the transient sol-
65 gel species allowing to generate porous hybrid materials10,11
and an ideal fibrillar confined microspace to stabilize smaller
nano-objects and metallic particles.12 Moreover, CO2
supercritical drying of these microspheres has been found to be
a reliable procedure to ensure drying without structural or
70 textural alteration from the solvated state (dispersed hydrogel
and alcogel) to light-weight aerogel.10,13 Beside, CS
graphitization offers sustainable access to N-doped graphene-
based materials.14
25 applicable catalyst for
a
broad range of organic
transformations.2 The high cost of palladium complexes has
stimulated the search for alternative more affordable transition
metal catalysts including copper and iron, to name a few3 and
even, lastly, to the more sustainable metal-free cross-coupling
30 catalysis.4 While this approach in homogeneous catalysis seems
promising, another strategy to reduce these limitations consists
on the entrapment or strong coordination of palladium
complexes (and their reactive nanosized particles) in
heterogeneous porous supports.5 This enables an easy recovery
35 from the reaction mixture and palladium recycling. Although
the “true” heterogeneous nature of these Pd-coupling catalysts
is still questionable, as many of them have proven to act as a
“release and catch catalytic system”,6 this immobilization
principally minimizes the amount of residual toxic metal in the
40 final products, which is another key issue complicating
homogeneous Pd catalysis applied to the synthesis of
pharmaceuticals.7 Indeed, a large library of inorganic and
organic supports have been already evaluated, including
charcoal, inorganic silica, alumina and synthetic polystyrene.
45 Mesoporous silicas for instance present some advantages, such
as high surface area, modulable pore diameter and broad
possibilities for surface functionalization. However, the
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In this contribution, three differents types of CS-based hydrogel
75 microspheres were designed (native CS one, their cross-linked
analogues CS-Glu and the thiolated chitosan CS-SH) and used
as porous carrier for supporting homogeneous Pd(OAc)2. The
rationale behind this selection was to increase the thermal and
mechanical stability of CS microspheres by cross-linking (case
80 of CS-Glu) and to increase the affinity of CS for Pd2+ (case of
CS-SH) compared to the native CS. The resulting supported
Pd-containing CS microspheres were then evaluated for
Sonogashira cross-coupling catalysis including the synthesis of
some relevant pharmaceutical precursors. The catalytic
85 behavior of hydrogels versus CO2 dried aerogels was studied in
terms of their kinetics, recovery, recycling and catalytic
stability against metal leaching.
Briefly, CS microspheres were prepared by introducing single
droplets of soluble CS acidic solution - via a 0.8 mm needle -
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into a 0.1 N NaOH aqueous solution. Self-standing hydrogel
microspheres in which 2% of the polymer is dispersed in 98%
water were immediatly formed. Extensive washing until pH ~ 7
affords the targeted neutral hydrogels. Their immersion in
ethanol:water solutions of increased ethanol ratio up to 100%
ethanol provides alcogels and their CO2 supercritical drying
enables access to chitosan aerogels. From these CS soft alcogel
microspheres, two other functional materials have been
prepared (Figure 1). Indeed, the amino groups belonging to the
mercaptopropyltrimethoxysilane-tethered silica.17
Gratifyingly, CO2 supercritical drying allows avoiding the gel
shrinkage that may occur during solvent evaporation due to the
capillary forces exerted on the soft-material walls.
60 Consequently, Pd-CS and Pd-CS-Glu aerogels exhibit specific
surface area of 239 m2.g-1 as mesured by nitrogen sorption
while the area of Pd-CS-SH aerogel is slightly larger reaching
302 m2.g-1 (Table 1). This specific surface area enhancement
can be tentatively attributed to the weaker hydrogen-bonding
65 forces among the fibrils of the biopolymer upon thiol grafting.
The hydrogen-bonding network stores polar water and ethanol
within the microspheres and are responsible for the slight
shrinkage (~5%) that routinely occurs during the gel drying.
Expectedly, examination of the cross-section microspheres by
70 scanning electron microscopy reveals the presence of entangled
fibrillar network with a void space exeeding 500 nm, typical of
the macroporosity generated during CS gelation (Figure 2c-d
and S5, SI). Nanosized palladium particles have been observed
by TEM analysis (average diameter ~ 5 nm) for Pd-CS
75 pointing to the efficacy of CS to limit the Ostwald-ripening
growth of palladium (Figure 2f and S6, SI). ICP analysis
enables to estimate the amount of palladium content within the
three porous microspheres (Table 1).
5
10 chitosan backbone react with glutaraldehyde to yield cross-
linked chitosan microspheres CS-Glu.15 Additionally, another
fonctionalised CS containing thiol has been prepared by the
ring opening of ɤ-thiobutyrolactone induced by nucleophilic
addition of the amino groups16 given rise to novel 4-mercapto-
15 N-butanamide functionalized chitosan (CS-SH). The success of
these modifications has been ascertained by DRIFT
spectroscopy, 13C CP MAS NMR analyses and comparison
with literature data of similar fragments.15,16 In CS-Glu, the
microspheres turned yellow upon reaction with glutaradehyde
20 n(NH2/Glu) = 1 indicating the spontaneous imine (C=N)
formation (S2a, SI). This has been confirmed by DRIFT where
the typical signal of primary amine at 1592 cm-1 significantly
decreases at the expense of new band at 1638 cm-1 due to
(C=N) vibration (S3, SI). 13C MAS NMR spectroscopy reveals
25 the existence of additional peaks with respect to those of native
chitosan (signals around 27.9, 43.6 and 96.7 ppm can be
respectively attributed to central CH2-CH2-CH2, the CH2-CH2-
HC=N and the carbon directly attached to nitrogen atom
(C=N)) (S4, SI). In the case of CS-SH material, carbon signals
30 of the grafted thiol overlaps with those of native chitosan.
However, a new signal observed at 36 ppm has been
unambiguously assigned to the CH2 neighboring SH group
(CH2-SH)16 (S4, SI). Notably, neither textural, morphological
alteration nor visible macroscopic damage of the microspheres
35 have been observed after functionalization.
e
a
c
80
b
d
f
85 Figure 2. Representative Pd-CS images at different magnification : a.
digital photos of native CS chitosan aerogels. b. black Pd-CS
microspheres (scale bar: each small square is equal to 1 mm2). c. SEM
of native CS showing fibrillar opened porous network (scale bar : 901
nm). d. SEM of Pd-CS evidencing that the initial network is preserved
90 (scale bar: 6 µm). e. palladium distribution mapped by EDX analysis
along the cross-section microsphere. f. TEM analysis evidencing the
presence of palladium nanoparticles inside of the biopolymer.
40
Figure 1. Multi-step preparation of the three chitosan-based catalysts
Pd-CS, Pd-CS-Glu and Pd-CS-SH studied in the present work.
Table 1. Specific surface areas, palladium content and leached palladium during
catalysis.
45 Lastly, the ability of these porous hydrogels to scavenge Pd2+
complexes has been assessed. Upon immersion in Pd(OAc)2
ethanol solution (ratio NH2:Pd = 4), the initial orange solution
turned clear while the microspheres become black (in the case
of CS and CS-Glu) or orange (in the case of CS-SH) (see
50 Figure 2a-b and S2b, SI). This indicates that within native CS
and the cross-linked one, Pd nanoparticles (naked Pd0) are
formed from Pd2+ in the presence of ethanol and both
amine/imines as gentle reducing agents, whereas the strong
coordination of thiol-to-Pd in CS-SH enables stabilization of
55 Pd2+, reminiscent of Pd2+ ligation by the ubiquitously used
aSBET
b Pd
(wt%)
13.89
c Leached Pd
Material
%
0.014
239
239
302
Pd-CS
Pd-CS-Glu
Pd-CS-SH
13.10
17.49
0.029
0.022
95 a measured by nitrogen sorption analysis. measured by ICP analysis. residual
palladium in solution after Sonogashira cross-coupling of 4-iodo-acetophenone to
phenylacetylene.
b
c
The highest Pd content (17.49 wt%) is obtained for CS-SH in
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consistency with the higher affinity of thiol groups for
palladium. Native chitosan CS contains 13.89 wt% Pd whereas
the cross-linked CS-Glu has slightly lower Pd content of 13.10
wt%, being the lowest value uptake of palladium. To accuratly
establish the chemical structure of the palladium hosted within
the microspheres, the three grinded aerogels were analysed by
X-ray photo-electron spectroscopy (XPS: (setting reference
C1s= 284.5 eV)). Pd 3d3/2,5/2 binding energy indicates that the
metal ligated to CS-SH corresponds to Pd+2 (340 eV for 3d5/2
10 and 345 eV for 3d3/2) with virtually no Pd0 observed (S7, SI). In
Pd-CS microspheres, similar binding energies are observed
with an additional minor peak at 338 eV that typically suggests
the presence of a fraction of reduced Pd0 (S7, SI). In sharp
contrast, Pd-CS-Glu microspheres features two palladium
15 species; the most abundant one being Pd0 with its binding
energy at 337 eV and 343 eV and those of Pd+2 at 340 and 346
eV for 3d5/2 and 3d3/2 respectively (S7, SI).18,19
At this point, three Pd-hosted alcogel- and aerogel-based
materials with similarities in their intrinsic textural properties
20 of the support (macroporous fibrillar network and millimetric
dimension of the microspheres), but with different metal-
support coordination strength (weakly coordinated Pd in CS,
incarcerated Pd nanoparticles in CS-Glu and strongly
coordinated Pd2+ in CS-SH) were prepared. Beside our
25 motivation to elucidate - for the first time - the difference on
the performance of support in catalysis between CS alcogels
and aerogels, a second important point is to determine how CS
functionalization influences the catalytic activity and if there is
a borderline between efficient scavenger and efficient catalyst.
30 To address thes issues, Sonogashira cross-coupling of aryl
iodide and acetylene derivatives was selected as a prototypical
reaction. With some guidelines of sustainable chemistry in
mind, solvents like dimethylformamide and toluene, organic
bases like NEt3, harsh temperatures and the presence of copper
35 as a second metal were all discarded. Under milder conditions
(EtOH-water as solvent, K2CO3 as base, T = 65 °C) and
regardless of the alcogel used, 4-iodoacetophenone coupled to
phenylacetylene affording selectively, after 6 h, a quantitative
yield of the targeted product 1.
aerogels were found to be more active than their alcogel
analogues (Figure 3). Δconv (t = 1h) = Convaerogel – Convalcogel
55 calculated for the three materials reveals the highest value of
43.54 for Pd-CS-Glu, 23.34 for Pd-CS and 14.59 for Pd-CS-
SH. A plausible explanation can be the difference in swelling
wetted versus dried microspheres, a phenomenon that may
impact the adsorption and desorption of the reactants and
60 products respectively, to- and from- active catalytic sites. In
this context, several discrepancies have been previsouly
reported between hydrogels and dried aerogels with regard to
their interaction with molecular species (different diffusion
rates during the sol-gel process,20 different selectivities in
65 organic synthesis,21,22 different viscosities, variable storage
capacities for water).
5
100
100
80
60
40
20
0
100
a
c
b
80
60
40
20
0
80
60
40
20
0
70
0
1
2
3
4
5
0
1
2
3
4
5
0
1
2
3
4
5
Time (h)
Time (h)
Time (h)
Figure 3. Catalytic activity of alcogels versus aerogels in cross-
coupling of 4-bromoacetophenone to phenylacetylene. a. Pd-CS. b.
Pd-CS-Glu. c. Pd-CS-SH. Dashed lines correspond to alcogels and
normal lines to aerogels.
75 Plotting the activity with respect to the nature of functional
groups revealed - for both alcogels and aerogels - inferior
catalytic activity for Pd-CS-SH compared to Pd-CS and Pd-
CS-Glu (S8c-d). TOF (h-1) was found to decrease from 25.84
for Pd-CS-Glu to 17.43 for Pd-CS to 14.2 for Pd-CS-SH for
80 aeorgels. Similar trend was observed for alcogels with TOF (h-
1) values decreasing from 9.55 for Pd-CS-Glu to 8.28 for Pd-
CS to 6.66 for Pd-CS-SH. This activity order suggests that Pd
nanoparticles are active sites promoting the coupling, as it is
known in the literature, with higher efficiency than Pd2+
85 ligated. The catalytic activity ranking seems in agrement with
XPS analysis from which it has been concluded that Pd-CS-
Glu features the highest proportion of palladium nanoparticles,
compared to Pd-CS and Pd-CS-SH. At the end of the reaction,
however, XPS investigation on the recovered Pd-CS-SH
90 reveals four binding sites assignable to Pd0 (338.2 and 343.2
eV) and Pd+2 (341.1 and 346.2 eV). This result indicates that
some Pd+2 species escape the strong thiol ligation during
catalysis and are subsequently converted to naked Pd0. ICP
analysis performed on the cross-coupling reactions after
95 cooling of the resulting solutions enabled quantification of
leached palladium catalyst from the three CS-based catalyst.
Whatever the material used, only tiny amount of palladium is
released in the solution : 0.014% for Pd-CS, 0.029% for Pd-
CS-Glu and 0.022% for Pd-CS-SH (Table 1). In the three
40
At the end of the reaction, the catalyst is easily recovered by
simple removal of the microspheres even without the need of
45 filtration making very simple the work-up procedure related to
the catalyst removal.
Kinetic investigations have been performed aiming to get a
better understanding on the catalytic behavior of these
microspheres depending on the nature of the support (wetted
50 gel versus CO2-dried aerogels) and that of the support-metal
interaction (weakly-coordinated, incarcerated nanoparticles or
strongly-chelated). Indeed, for the three materials, dried
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cases, more than 99.97% of the Pd initially present in the
catalyst is retained within the microspheres. The absence of
significant leaching is of particular interest given numerous
precedents on the occurrence of palladium leaching during
Sonogashira cross-coupling. This fact is attributed to the strong
coordination of palladium to the alkyne functionality either of
the product or the starting material.7,23 Herein and given their
low metal contamination, the obtained products do not need
additional scavening to meet regulatory specifications.
functionalized CS microspheres. In the case of CS-Glu,
although hydrogen-bonding network is disrupted, the chemical
cross-linking of the chains considerably improves the
25 microspheres stability allowing their further recycling.
5
10 Without any chemical or thermal activation, these soft-
supported catalysts can be reused – after simple washing with
ethanol- five additional runs, while maintaining their activity
(Figure 4). After that, whereas Pd-CS and Pd-CS-Glu preserve
their initial shape allowing more extended reuse in catalysis (to
15 reach 6 runs), Pd-CS-SH dissolves to a slurry form and under
these circumstances, the catalyst recovery and its further reuse
become extremely difficult. The low morphological stability of
Pd-CS-SH microspheres can be explained by the weakening of
hydrogen bonding at the fibrillar level, as a result of amine
20 fuctionalisation (that are normally engaged in NH2---HO
bonding), in contrast to those occurring in the pristine, non-
30
Figure 4. Recycling of the three Pd supported on CS and modified CS
alcogel catalysts in the Sonogahsira coupling of 4-iodoacetophenone
with phenylacetylene.
35
Table 2. Extending the scope of Sonogashira cross-coupling to other alkyne and halide substrates.
Entry
Arylhalide
Alkyne
Catalyst
Pd-CS
3%
Time
6h
Product
Yield
98%
1
Pd-CS-Glu
6h
52%
2
1%
Pd-CS
8h
100%
100%
3
4
0.1%
Pd-CS-Glu
20h
1%
Pd-CS-SH
8h
96%
5
3%
Pd-CS-SH
8h
72%
6
3%
40
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Prompted by these promising results, we subjected these
catalysts to more demanding, challenging and/or prevalent
coupling catalysis. With phenylacetylene as substrate, three
aryl bromides were successfully coupled using tiny amounts of
supported Pd (0.1 to 3% of either Pd-CS, Pd-CS-Glu or Pd-
CS-SH) under the previously described conditions yielding the
targeted alkynes in good to quantitative yield (Entries 2 to 4,
Table 2). Heterocycles with biological activity and terminal
propargylic fragment were also tested for Sonogashira cross-
10 coupling in the presence of 4-iodoacetophenone as starting
halide (Entries 5 and 6, Table 2). The efficacy of these catalysts
enables to obtain, with a simple work-up, the targeted coupling
products in 72% and 96%, respectively.
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70
75
80
To sum up, this work highlights the multifaceted advantages of
15 CS as sustainable material for textural enginnering of
macroporous supports, while chemical modification of its
amino groups allows tuning the metal-support interaction. Mild
and sustainable reaction conditions (copper-free, ethanol-water
as solvent, 65 °C) enable Sonogashira cross-coupling catalysis
20 to be performed with quantitative yields, low metal
contamination, easy work up and good recyclability. This
straightforward and sustainable chemistry affords versatile eco-
efficient catalysts bearing many useful properties: their gentle
reducing conditions to produce metal nanoparticles (without
25 using toxic reducing agents neither hydrogen treatment) and
their easy recovery from the reaction medium whereas
heterogeneous system needs specific filters are among their
most illustrative advantages over the existing Pd-supported
catalysts. The metal scavenging ability of these microspheres
30 can be interesting for the synthesis of active pharmaceutical
ingredients, specifically those for which the metal
contamination has to be reduced to few ppb.
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We are indebted to Dr. Rachid Bouhfid for the donation of the
starting propargylic derivatives. S.F. is thankful to the
35 EMMAG-Erasmus mundus program for funding. UEMF is
warmly acknowledged for the financial support.
19 Similar binding energy was observed for Pd0 grown in MOF,
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Laboratory of Organometallic and Macromolecular Chemistry -
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105 21 D. Kühbeck, G. Saidulu, K. R. Reddy, D. D. Díaz, Green Chem.,
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Composites Materials, Faculty of Sciences and Technologies, Cadi
40 Ayyad University, Avenue Abdelkrim Elkhattabi, B.P. 549, 40000
Marrakech, Morocco
b
Instituto de Tecnologia Quimica CSIC-UPV and Departamento de
22 A. K.-Nezhad, S. Mohammadi, RSC Adv., 2014, 4, 13782-13787.
23 A. El Kadib, K. McEleney, T. Seki, T. K.Wood, C. M. Crudden,
ChemCatChem, 2011, 3, 1281-1285
Quimica. Univ. Politicnica de Valencia, Av. de los Naranjos s/n 46022
Valencia (Spain).
c
45
50
Euro-Med Research Institute. Engineering Division.
Euro-
Mediterranean University of Fes (UEMF), Fès-Shore, Route de Sidi
Hrazem, 30070 Fès (Morocco). E-mail: a.elkadib@ueuromed.org
d
Hassan II Academy of Science and Technology, Avenue Mohammed
VI, 10222 Rabat, Morocco.
† Electronic Supplementary Information (ESI) available: [Synthesis
and characterizations of precursors and materials]. See
DOI: 10.1039/b000000x/
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Green Chemistry
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