Polymer-Supported Palladium(II) Carbene Complexes: Catalytic Activity, Recyclability, and Selectivity in C?H Acetoxylation of Arenes

Article abstract of DOI:10.1002/chem.201700777

Heterogeneous catalysts for selective oxidation of C?H bonds were synthesized by co-polymerization of new N-heterocyclic carbene-palladium(II) (NHC-Pd^II) monomers with divinylbenzene. The polymer-supported NHC-Pd^II-catalysed undirect

Full text of DOI:10.1002/chem.201700777

A Journal of
Accepted Article
Title: Polymer-supported palladium (II) carbene complexes: catalytic
activity, recyclability and selectivity in C-H acetoxylation of arenes
Authors: Maitham H. Majeed, Payam Shayesteh, Reine Wallenberg,
Axel R. Persson, Niclas Johansson, Lei Ye, Joachim Schnadt,
and Ola F. Wendt
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Polymer-supported palladium (II) carbene complexes: catalytic
activity, recyclability and selectivity in C–H acetoxylation of
arenes
Maitham H. Majeed,[a] Payam Shayesteh,[b] L. Reine Wallenberg,[a,c] Axel R. Persson,[a,c] Niclas
Johansson,[b] Lei Ye,[d] Joachim Schnadt,[b] and Ola F. Wendt*[a]
Abstract: Heterogeneous catalysts for selective oxidation of C–H
bonds were synthesized by co-polymerization of new N-heterocyclic
carbene-palladium(II) (NHC-Pd(II)) monomers with divinylbenzene.
The polymer-supported NHC-Pd(II) catalyses undirected C–H
Pd catalysts for the undirected C−H acetoxylation of arenes
using different oxidants.[6-8] One successful strategy has been
the use of Pd(OAc)2 as a catalyst, PhI(OAc)2 as an oxidant, and
pyridine derivatives as supporting ligands,[9] a system that
acetoxylation of simple and methylated arenes as well as polyarenes, catalyses the acetoxylation of undirected arenes via non-radical
with similar or superior efficiency compared to their homogeneous
analogues. In particular, the regioselectivity has been improved in
the acetoxylation of biphenyl and naphthalene compared to the best
homogeneous catalysts. The new polymer-supported catalysts
maintain the original oxidation state of Pd(II) after repeated catalytic
pathways.[10] As a ligated alternative to the Pd acetate catalyst in
C–H activation NHC-Pd complexes have been reported
recently.[11-18] The ligand adds stability to the system due to the
strong NHC-M interaction, and it also enables functionalization
and ligand control.[15,19,20] Moreover, to further catalyst
reactions, and exhibit no significant leaching of palladium. In addition, separation and recovery the NHC-M complex can be
the new catalysts have been successfully recovered and reused
without loss of activity over several cycles of reactions.
heterogenized.[21] Indeed, the development of stable
heterogeneous catalysts would substantially facilitate any
industrial application.[22] There are many examples of supported
Pd(II) catalysts,[23-26,27] and polymer-supported NHC-Pd
complexes have been successfully used in many applications
and for different chemical transformations, and advances in
support strategies have been made recently.[26,28-35] However,
metal leaching, reduction of the Pd(II) species during
preparation steps and catalysis and aggregation of Pd(0)
nanoparticles inside the polymer matrixes remains an unsolved
problem in many cases.[36,37] However, although directed C–H
activation over supported Pd complexes has been reported,
there are no examples of the application of such catalysts for
undirected C–H activation reactions.[38-41]
Here we report on the synthesis and characterization of a Pd(II)
containing polymeric material based on the co-polymerization of
polymerizable arms in the structure of an N-heterocyclic carbene
ligand and divinylbenzene (Scheme 1). Our concept gave a
material which is stable towards reduction and recyclable with
no or little loss in activity. Its reactivity and regioselectivity in the
C–H oxidation of arenes is reported together with the effect of
various stabilizing pyridine ligands. Productivity is on par with
the corresponding homogeneous catalysts and selectivity is in
many cases substantially better.
Introduction
The selective oxidation of undirected C–H bonds can simplify
the functionalization of hydrocarbons and is therefore potentially
very useful in the synthesis of both fine and bulk chemicals.[1]
Thus, it can help in reducing waste in large scale
transformations, and in the synthesis of more complex
molecules it can enable late stage functionalization. So far, the
most successful approach to the selective oxidation of C–H
bonds has been ligand-directed C–H functionalization where a
heteroatom controls the regioselectivity.[2,3] However, although
some progress has been made,[4,5] there are still no general,
regioselective methods for the functionalization of simple
alkanes or arenes neither by homogeneous nor heterogeneous
catalysts. To address this shortcoming substantial research
efforts have been devoted during several decades to developing
[a]
M. H. Majeed, Prof. L. R. Wallenberg, A. R. Persson, Prof. O. F.
Wendt
Centre for Analysis snd Synthesis, Department of Chemistry, Lund
University
Box 124, SE-221 00, Lund (Sweden)
E-mail: ola.wendt@chem.lu.se
[b]
[c]
[d]
P. Shayesteh, N. Johansson, Prof. J. Schnadt
Division of Synchrotron Radiation Research, Department of Physics,
Lund University
Box 118, SE-221 00, Lund (Sweden)
Prof. L. R. Wallenberg, A. R. Persson
National Center for High Resolution Electron Microscopy, Lund
University
Results and Discussion
Monomers: Synthesis and Characterization. Imidazolium
chloride precursors [bmim]Cl 1a and [bvbim]Cl 1b were
synthesized according to previously reported procedures.[42-45]
We chose these precursors based on that they either have one
(1a) or two (1b) polymerizable arms to investigate whether
Box 124, SE-221 00, Lund (Sweden)
Prof. L. Ye
Centre for Applied Life Sciences, Department of Chemistry, Lund
University
Box 124, SE-221 00, Lund (Sweden)
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Scheme 1. Polymerized NHC-Pd(II) monomer can catalyse C-H acetoxylation of undirected arene.
this leads to a difference in stability or selectivity. In particular,
the degree of cross-linking is expected to vary with the number
Ag2O/ DCM
PdCl2(PhCN)2
DCM, 24h
N
N
N
1b
of vinyl groups in the monomer. Direct metallation using
N
Cl
r.t, 48h
literature procedures afforded [Ag(bmim)2]2[Ag2Cl4] 2a[46] and
Ag
Cl
Cl
Pd
Pd
Ag[bvbim]Cl complex 2b in good yield (Figure 1). 13C NMR
spectra showed the characteristic carbene signals at 180.7 ppm
and 180.9 ppm for 2a and 2b, respectively.[46-50] The use of silver
intermediates precluded any radical polymerization of the vinyl
moiety at an earlier stage; some other protocols use harsh
Cl
Cl
N
[PdCl2(bvbim)]2
AgCl [bvbim]
N
3b
2b
2 eq. of pyridine derivatives, DCM, 6h, r.t
N
N
reaction conditions where such unwanted side reactions could
occur.[51] Transmetallation with PdCl2(PhCN)2 gave NHC-Pd(II)
dimers 3a and 3b (Ref. 47 and Figure 1), which showed broad
signals in the 1H NMR spectra due to the dynamic equilibrium
between monomeric and dimeric species as previously
shown.[16] Adding pyridine derivatives cleaved the dimer and
afforded NHC-Pd complexes 4a, 4b, 5a, 5b and 6b in excellent
yield with one (a) or two-vinyl (b) moieties and 3-phenylpyridine
(4), 4-phenylpyridine (5), and 9-chloroacridine (6) as stabilizing
ligands, respectively (cf. Figure 1 and Scheme 2). They were
N
Cl
N
N
N
N
N
N
Cl
Pd
N
Cl
Pd
N
Cl
Cl
Pd
N
Cl
Cl
Cl
1
characterized by H and 13C NMR spectroscopy and elemental
4b
6b
5b
analysis. All signals were sharp and, again, characteristic
carbene signals around 150 ppm were observed in the 13C NMR
spectra in CDCl3.[51,52] Slow diffusion of n-hexane into a DCM
solution of the complexes gave X-ray quality crystals and an X-
ray diffraction (XRD) analysis confirmed the molecular structures
of 4a, 5a,[47] 4b and 6b. They are given in Figures 1 and S1; all
details including selected bond distances and angles are,
likewise, found in the supporting information.
Polymers: Synthesis and Characterization. Palladium
monomers 4a, 4b, 5a, 5b, and 6b were co-polymerized with
divinylbenzene (DVB) using precipitation polymerization which
has been previously shown to give microspheres.[53-55] The
polymerization conditions did not influence similar palladium
complexes without polymerizable arms, i.e. the metal centre is
not influenced by e.g. the free radical initiator AIBN (see SI for
details). The polymerization gave polymers 7a, 7b, 8a, 8b and
9b, respectively (Scheme 2).
Figure 1. Synthesis of NHC-Pd(II) monomers and crystal structure of 4b with
thermal ellipsoids displayed at 50% probability. Hydrogen atoms have been
omitted for clarity.
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Scheme 2. Precipitation polymerization of NHC-Pd(II) monomers and pyridine derivatives removing step from the microspheres.
Neat acetonitrile or a mixture of acetonitrile and toluene, which
are near θ-solvents for DVB type polymers, were used. These
solvents are known to form a clean surface of microspheres of
2-5 µm and to create porous polymer beads, which is very
important for catalysis.[56-59] The Pd:DVB ratio was 1:4 and the
total monomer concentration used in this study was ~ 2% (w/v)
of the solvent; all preparation details are given in the supporting
information, Table S3. In order to study the impact of the
pyridine ligands on the catalytic performance they were removed
through protonation in one series of catalysts, giving the
acetonitrile adducts 7aAN, 7bAN, 8aAN, 8bAN, and 9bAN
(Scheme 2). This procedure in principle renders the metal
centres of the a and b series identical. However, the different
catalysts within each series (a and b) could still display different
activities and selectivities, since the use of the different pyridines
could have induced differences in the shape of the polymer
cavity.
All polymers were characterized by an Inductively coupled
plasma (ICP) analysis. For selected polymers we also performed
Solid state NMR spectroscopy (SS-NMR), High resolution
transmission electron microscopy (HRTEM), X-ray energy-
dispersive spectroscopy (XEDS), and X-ray photoelectron
spectroscopy (XPS). The solid-state 13C-Cross polarization
magic angle spinning (CP-MAS) spectra are almost identical for
the different polymers except a shoulder peak which is present
near 150 ppm for the pyridine polymers, but absent in the
acetonitrile series. This is shown in Figure S2 for 7a and 7aAN
and provides good evidence for the success of the washing
process. Figure S3 in the supporting information show similar
details for polymers 8b and 8bAN, respectively. Peaks near 30,
41, and 55 ppm are assigned to the aliphatic CH3-, CH2-, and
CH- carbons for the NHC-Pd(II) and poly-DVB skeleton.
Unreacted vinyl linkers in the poly-DVB skeleton exhibit two
peaks around 113 and 138 ppm assigned to the terminal and
internal carbons respectively.[60] Aromatic phenyl, aromatic
phenylpyridine, aromatic poly-DVB skeleton and C10, C11 of the
imidazolyl show resonances near 129, 138 and 146 ppm,
respectively. HRTEM images were recorded for polymer 7bAN.
EDX data show a low, but detectable level of Pd and Cl inside
the particles of 7bAN (Figure S4a and S4b), whereas the
concentrations of those two elements were below the detection
limit outside the particles. Furthermore, HRTEM images showed
no metallic palladium particles in the polymer (Figure 2a and 2b).
This was confirmed by Fast Fourier transform (FFT) analysis of
HRTEM images which revealed no spots corresponding to
lattice fringes. The palladium found is thus homogeneously
dispersed and presumably molecular, although no information
on the oxidation state can be inferred. Using bright-field STEM
mode, an EDX mapping of the edge of the particles of 7bAN
was performed. Again, Pd and C are clearly visible (as brighter
areas) in the particles, while the area outside the particles
contains very little Pd and C (supporting information Figure S6).
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To confirm the previous results (and address the issue of
oxidation state), 7b was treated with ascorbic acid to reduce any
Pd(II) to Pd(0). Powder X-ray diffraction (PXRD) patterns were
recorded for the native polymers and compared with that of the
reduced polymer 7b(red). The PXRD patterns of the native
polymers are featureless, but the PXRD curve of 7b(red) shows
peaks at 40°, 47° and 67° which can be assigned to the (111),
(200), and (220) crystal planes of Pd(0) particles in the polymer
matrix (Figure S5a). These results demonstrate the presence of
Pd(II) in the native polymer which only upon reduction gives
Pd(0) particles.[61] To further characterize and investigate the
elemental composition, oxidation state, and coordination
environment of the Pd species in the monomer and fresh
polymer XPS was employed. Pd 3d core level spectra of the
monomer 4b and polymers 7b and 7bAN show the Pd 3d5/2 and
Pd 3d3/2 peaks at approximately 340 and 345.5 eV binding
energy in agreement with the energies expected for a Pd(II)
species (Figure 2c).[62] A small shift to lower binding energy
between the lines of 4b and 7b is seen as the result of the
polymerization and concomitant change in Pd environment,
while the shift to somewhat higher binding energy between 7b
and 7bAN likely is related to the replacement of phenylpyridine
by acetonitrile. The occurrence of a small shoulder at ca. 337.5
eV, which is still within the Pd(II) region, can probably be
explained by a variation of the chemical environment of the Pd
ion.[63,64] More importantly, there are no peaks which correspond
to the binding energies of Pd in the metallic state. This confirms
that no reduction to Pd(0) takes place during the polymer
Figure 2. (a) and (b) TEM-images of 7bAN. (c) and (d) Pd 3d and Cl 2p XP spectra of the indicated samples.
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Table 1. NHC-Pd(II) polymer-catalysed C-H acetoxylation of
biphenyl.
particles within a size range of several hundred nm to 4 µm
according to Scanning electron microscopy (SEM) (Figures S5b,
S5c, S5d, and S5e). In general, the polymer beads were found
to have a more uniform size distribution after removing the
phenylpyridine, probably due to loss of the nanospheres (less
than 1 µm) during the washing process. Details of the N2
adsorption experiment and ICP analyses of all polymers are
given in the supporting information, Figure S16 and Table S3.
There were no considerable differences in the palladium loading,
which is between 6-8 %. Thermogravimetric analysis (TGA)
shows that all polymers possess a high thermal stability up to
200 °C (see Figure S8a and S8b for details).
[Catalyst]
[mol. catalyst/ mol. oxidant]= 4%
OAc
1 equiv. [PhI(OAc)2]
AcOH:Ac2O (9:1)
104oC, 24h
5 equiv.
Selectivity (%)
Temp.
Yield[b]
(%)
Entry
[Cat.][a]
(oC)
o-
m-
p-
1
8bAN
70
Trace
18
-
-
-
2
8bAN
92
-
39
43
32
32
30
-
61
45
46
46
43
-
3
8bAN
104
115
125
135
104
104
104
104
104
104
104
104
104
104
104
104
45
12
22
22
27
-
4
8bAN
28
Activity in C–H activation. Homogeneous Pd(II) catalysts are
active in both ligand-directed[3,75-77] and non-directed C–H bond
acetoxylation,[4,10,78-80] mainly using Pd(OAc)2 as a catalyst,
pyridines as ancillary ligands, and PhI(OAc)2 as an oxidant. As a
benchmark for the activity and selectivity of our palladium-
containing polymers we chose the non-directed C–H
acetoxylation of arenes, and for the optimization of reaction
conditions we used biphenyl as the substrate, so that there
would also be a selectivity issue. A PEPPSI-iPr[52] catalyst was
employed as a reference homogeneous catalyst. Although
monomers 4-6 would have been preferred these are not stable
towards polymerization under the reaction conditions. It is
expected that the electronic properties of PEPPSI-iPr are not
substantially different than those of 4-6.[81,82] The initial screening
of oxidants using polymer 8bAN as catalyst revealed that only
PhI(OAc)2 and MesI(OAc)2 gave any turnover in the reaction
with 45% and 49% yields, respectively (Table S4). We clearly
observed that the catalysis reaction proceeded without formation
of chlorinated biphenyl products. Traces of phenyl iodide were,
however, detected in the Gas chromatography (GC) analysis.
Subsequently, we tested different temperatures (Table 1, entries
1-6). The results show traces of acetoxybiphenyl derivatives at
5
8bAN
28
6
8bAN
30
7
None
PE-iPr[c]
0
8
44
56[d]
13
13
13
13
12
14
14
13
13
15
10
41
41
29
40
42
41
41
41
41
38
46
46
46
58
47
46
45
45
46
45
47
44
9
8aAN
10
11
12
13
14
15
16
17
18
7aAN
7aAN[e]
45
53
7bAN
50
7bAN[e]
7bAN[f]
7bAN[g]
9bAN
56
51
46
53
9bAN[e]
47
58[h]
Pd(OAc)2
[a] Pd loading described in table S3. [b] Yield and selectivity were
determined by GC using a calibration curve based on decane as an
internal standard. [c] PE-ipr= PEPPSI-iPr (5 mol%). [d] TON≈ 14. [e]
[Oxidant]= MesI(OAc)2. [f] Reaction time=48h, [g] Reaction time= 72h,
[h] 2 mol% of Pd(OAc)2 and 2 mol% of pyridine (1:1 ratio of [Pd] to
[pyr]; catalyst loading relative to oxidant) Ref. [79].
synthesis and washing steps.[65,66] For comparison, the XP
spectrum of 7b(red) shows peaks at ca. 335 and 340.3 eV
binding energy corresponding to the binding energy of Pd(0)
(Figure S14). The Cl 2p3/2 and Cl 2p1/2 peaks at 199.8 and 201.4
eV are clearly visible for monomer 4b (Figure 2d). In addition,
the Cl 2p X-ray photoelectron (XP) spectrum of 7b exhibits an
additional component to lower energy. The presence of several
components can probably be attributed to the larger variation of
chemical environments upon polymerization. A corresponding
line broadening due to polydispersity is observed in the Pd 3d
spectrum of 7b. In the Cl 2p spectrum of polymer 7bAN a high
binding energy species at ca. 201.9 eV is found. Such high Cl 2p
energies are typical for C-Cl bonds and thus are an indication
that H-Cl addition to the vinyl groups occurs to some extent
during washing with triflic acid.[67-69] The N 1s spectra for 4b, 7b,
and 7bAN are shown in Figure S7. The high energy peaks (ca.
402.4 eV) are related to the imidazole nitrogen atoms,[62,70] and
the lower energy peaks (ca. 401.7 eV) are associated with the
phenylpyridine nitrogen. The ratio between the imidazole and
phenylpyridine peaks is 2:1 as expected. Peak broadening
towards lower and higher binding energies is observed in the N
1s spectra of 7b and 7bAN; as above it can be assigned to the
variation of the N chemical environment by polymerization and
washing steps.[71-73] Polymers 7b and 7bAN are spherical
o
o
70 C and 18% yield at 92 C with a ca 61:39 regioselectivity in
favour of the para-substituted over the meta-substituted. The
yield of acetoxybiphenyl derivatives increased to 45% (Table 1,
o
entry 3) at 104 C with a decrease in the regioselectivity to ca.
45:43 for para- and meta-substituted positions, while ortho-
substituted biphenyl was formed as a minor product due to the
steric hindrance in the biphenyl substrate. At higher
temperatures the yield decreased (entries 4-6); this decrease
could be attributed to the decomposition of PhI(OAc)2 in the
reaction mixture in accordance with a previous study.[83] There is
no reaction in the absence of catalyst (entry 7).
The homogeneous PEPPSI-iPr (5 mol %) catalyst shows very
similar productivity and selectivity as 8bAN (entry 8) and the
same is true for Pd(OAc)2/pyridine (1:1), a known homogeneous
Pd(II) system with high activity in C–H oxygenation of arenes
(entry 18).[79] Comparing the different polymeric catalyst all NHC-
Pd(II)(MeCN) polymers in general afforded higher yield of
acetoxybiphenyl products compared to the NHC-Pd(II)(pyridine
derivative) polymers (Table S5, entries 1-9). However, 7bAN
and 8bAN show comparable activity to 7b and 8b respectively in
the presence of PhI(OAc)2. No prominent differences were
observed in the regioselectivities among the polymeric catalysts
except for 7aAN (Table 1, entry 10), which exhibited
a
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regioselectivity of ca. 58:29 for para- and meta-substituted
positions as opposed to the ca 1:1 selectivity for the other types
of catalysts including the homogeneous one. MesI(OAc)2
increased the yields compared to PhI(OAc)2 except for 9bAN
(Table 1, entry 17) when used as an oxidant with the acetonitrile
series of polymers. However, with the pyridine containing
this gave the best turnover in the catalysis (Table S5).
Additionally, we also tested 7bAN for recyclability to shed some
light on the issue of pyridine vs. acetonitrile ligand. Thus, the
recovered 7b was shaken with 5 mL of 3-phenylpyridine/DCM
solutions for 30 min after each cycle and then washed with
MeCN to remove the remaining non-bonded phenylpyridines, a
procedure which, however, was not entirely effective for all
cycles as can be seen from the shifts in the N 1s spectra in
Figure S7. 7bAN was shaken only with MeCN solvent. The so-
treated catalysts were dried and used directly for the next
reaction cycle. Both 7b and 7bAN were found to be effective in
four cycles (probably in more, but this was not tested) of the
acetoxylation reaction. No significant change in the productivity
and selectivity of the recovered catalysts was observed (Figure
3c and supporting information Figure S9), and importantly the
kinetic profile for the four runs was more or less identical (Figure
3d). The slight increase of activity that is observed could
possibly mean that the polymer material opens up during the
reaction giving a better access to the palladium and therefore a
slightly higher activity. The catalysis stops almost completely
after eight hours and 1H NMR spectroscopy showed no traces of
PhI(OAc)2 in the catalysis mixture. However, in terms of mass
balance what has not been converted to acetoxylated product is
almost exclusively unreacted starting material. Thus, upon
addition of 1 additional equivalent of PhI(OAc)2 into the reaction
vessel after eight hours, the reaction continues and the yield of
polymers
a decrease of the yields was observed using
MesI(OAc)2 as an oxidant (Table S5, entries 1-9). The origin of
this observation is likely connected to an increased steric
congestion in the pyridine derivatives. The use of various
pyridine ligands was introduced to probe if there is any effect on
the polymer cavity induced by the different pyridines. An
increase in yields was identified when 7a and 7b were used in
the reaction compared to 8a and 8b (Table S5, Entries 1, 3-7).
This is possibly due to the creation of a larger cavity when using
the 3-phenylpyridine during polymerization. There was no
significant influence of increase of the reaction time on the yield
of acetoxybiphenyl derivatives (Table 1, entries 14 and 15).
The fact that the polymer bound catalysts behave similarly as
the molecular catalysts in terms of oxidation state and reactivity
suggests a similar reaction mechanism as previously proposed
for molecular Pd(II) systems.[16]
Reusability and Heterogeneity. All the polymer-based
catalysts 7-9 were evaluated for recyclability and they can all be
separated easily from the reaction mixture by
a simple
centrifugation process. We chose to investigate 7b further since
Figure 3. (a) C-H acetoxylation reaction of biphenyl over 7b and 7bAN catalysts using PhI(OAc)2 as an oxidant. (b) Pd 3d XP spectra of 4b, 7b fresh and 7b
after each reuse. (c) Reusability and productivity of 7b and 7bAN; the results are compared to those for a [PEPPSI-iPr] catalyst at 5 mol%. (d) Time profile
and hot filtration study of 7b-catalyzed C-H acetoxylation of biphenyl.
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Table 2. Polymer catalysed C–H acetoxylation of naphthalene.
acetoxybiphenyl derivatives approximately doubles (Table S5,
entry 10). The yields are thus primarily controlled by the access
to oxidant not by the stability of the catalyst. In line with this the
recycled catalysts have (within error) the same activity as the
fresh one. A hot filtration test (Figure 3d) underlines the
heterogeneous nature of the catalyst with a complete inhibition
of the activity after removal of the solid catalyst. These results
were supported with results from an ICP-OES analysis, in which
only a small leaching of Pd from the polymer is detected after
each catalytic cycle (Pd wt% = 6.58, 6.52, 6.41, and 6.30 after
the 1st, 2nd, 3rd, and 4th catalytic cycle, respectively). To probe
the palladium oxidation state after catalysis, 7b was analysed by
XPS. The Pd 3d5/2 and Pd 3d3/2 binding energies of 7b after
each catalytic cycle (ca. 339.3 and 344.6 eV, respectively)
showed only a small shift compared to those of the monomer
and fresh polymer (Figure 3b). This shift is probably related to
ligand exchange between chloride and acetate. It is clear,
however, from the spectra that the palladium remains in
oxidation state (II) for all cycles. Similar results were obtained for
7bAN after the fourth catalytic cycle (Figure S15). Subsequently,
we focused our attention to the Cl 2p core level of 7b after each
catalytic cycle, which showed significant broadening in the
spectral features and shifts by 2 eV toward higher binding
energies compared to the spectra of the monomer and fresh
polymer (supporting information Figure S10). The spectrum can
be convoluted into two peaks, one corresponding to a Pd-Cl
species (with a Cl 2p3/2 binding energy of 199.8 eV) and a
second Cl 2p3/2 peak at 201.9 eV, which can be assigned to a C-
Cl species according to literature.[67-69] In order to understand
this remarkable change in Cl 2p binding energy, we also
characterized 7b before and after catalysis with SS-NMR
spectroscopy. 13C CP-MAS NMR spectra (which were performed
at two different spinning rates to differentiate between peaks and
side bands) revealed a new peak after catalysis at 169.8 ppm,
which corresponds to (C=O) of an acetate group (supporting
information Figure S11 and S12).[84-87] In contrast, the peaks
related to terminal and internal carbons of vinyl linkers at ca. 113
and 138 ppm respectively observed for the fresh catalyst are
absent after catalysis (supporting information Figure S12). For
comparison the biphenyl acetoxylation reaction was carried out
in the presence of poly(divinylbenzene) under the described
reaction conditions. This gave no conversion of biphenyl but,
again, 13C CP-MAS NMR spectra showed a sharp peak from an
acetate group at 169.5 ppm and the intensities of carbon peaks
related to vinyl linkers were reduced (supporting information
Figure S13). These data suggest that PhI(OAc)2 can
functionalize unreacted vinyl groups (but not aromatic groups) in
the absence of a catalyst. In the presence of a catalyst we
therefore suggest that all remaining vinyl groups in the polymer
are acetoxylated or chlorinated during catalysis. These results
were supported by elemental analysis of 7b after catalysis,
which confirmed that the ratio between Pd:Cl is almost 1:2 (85%
of Cl is still present in the catalyst). Thus, the majority of the
chlorine, ca. 75% according to the XPS results, have been
transferred to the polymer matrix, while the rest is still
coordinated to Pd; ca 15 % have been washed out from the
catalyst according to the ICP analysis.
[Catalyst]
[mol. catalyst/ mol. oxidant]= 4%
1 equiv. [PhI(OAc)2]
OAc
OAc
+
AcOH:Ac2O (9:1)
104oC, 24h
(β)
(α)
5 equiv.
Yield[b]
(%)
Selectivity (%)
Entry
[Catalyst][a]
(α : β)
1
None
0
-
2
PEPPSI-iPr[c]
PEPPSI-iPr[c,d]
7b
36
33
18
25
18
26
15
19
9
44:56
33:67
28:72
56:44
17:83
54:46
20:80
47:53
11:89
3
4
5
7bAN
7b[d]
7bAN[d]
8b[d]
8bAN[d]
8bAN[d,e]
6
7
8
9
10
11
12
Pd(OAc)2/Acridine[79]
Pd(OAc)2/FPCA[80]
88
69
20:80
29:71
[a] Pd loading described in Table S3. [b] Yield and selectivity were
determined by GC using a calibration curve and based on decane as
an internal standard. [c] [PEPPSI-iPr]=
5 mol%, [d] [Oxidant]=
MesI(OAc)2, [e] 1 equiv. of 4-Phenylpyridine/Pd added to the reaction
mixture. [80] FPCA= 6-Fluoropicolinic acid.
Oxidation of naphthalene and other arenes. We further
investigated the scope of C–H acetoxylation for other arenes
using the optimized reaction conditions. With naphthalene as a
substrate (Table 2) the polymeric catalysts showed a significant
increase in selectivity compared to homogeneous ones. Thus,
PEPPSI-iPr showed no regioselectivity using PhI(OAc)2 as an
oxidant, while a moderate regioselectivity of ca 33:67 in favour
of the β-substitution was observed with MesI(OAc)2 (Table 2,
entries 2 and 3). This is in line with other homogenous catalysts
such as Pd(OAc)2 and sulfinyl-NHC−Pd complexes reported
previously.[16,79] On the other hand, the polymeric catalysts are
significantly better with
a
regioselectivity in favour of
acetoxylation at the β-position of ca 28:72 using 7b/PhI(OAc)2
(Table 2, entry 4). This improvement in selectivity was
diminished significantly using 7bAN, in slight favour of
acetoxylation at the α-position (α:β = 56:44) (Table 2, entry 5).
For the acetonitrile-supported systems no improvement in
selectivity was obtained using MesI(OAc)2, but for 8b and 7b the
bulky oxidant gave an even better selectivity of α:β = 20:80 and
17:83, respectively. Thus, the presence of phenylpyridines
increase the selectivity substantially and this goes hand in hand
with a decreased activity. To confirm the improved selectivity
one equiv. of 4-phenylpyridine (with respect to Pd) was added to
the reaction mixture using 8bAN. As expected this gave a lower
activity compared to previously reported Pd(OAc)2 systems
(Table 2, entries 11 and 12), but a dramatically increased
selectivity (11:89) (Table 2, entry 10). In general the polymeric
catalysts showed lower activity than the homogeneous ones.
Finally, we tested catalyst 8b under the optimized reaction
conditions in C–H acetoxylation of mono- and polycyclic arene
substrates. It exhibited modest yields but a higher selectivity
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10.1002/chem.201700777
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FULL PAPER
(compared with PEPPSI-iPr) when employed to the
acetoxylation of pyrene. For simple aromatic substrates the
polymeric catalyst offered only a modest improvement in terms
of selectivity but surprisingly enough some improvement in
terms of activity in the acetoxylation of toluene (Table S6).
Recyclability, heterogeneity and time study experiments
Recycling Experiment. The acetoxylation reaction was carried
out under the same reaction conditions as described above. The
reaction mixture was centrifuged after it had reached room
temperature. The remaining solid was washed with a pyridine
derivative/DCM solution (1×5 mL) and MeCN (3×10 mL), dried,
and reused for the following run. Using these conditions, the
recyclability study was performed at designated reaction times
(2 h, 4 h, 6 h, 8 h, 12 h, 20 h, and 24 h).
Hot Filtration test. The acetoxylation reaction was carried out
under identical reaction conditions as described above. After 4 h
(yield 28%) the catalyst was filtered off at the reaction
temperature (104 °C) and the catalyst free filtrate was allowed to
stir under identical reaction conditions. The GC analysis of the
products after 8 h, 18 h, and 24 h revealed no further
acetoxylation of biphenyl (GC final yield 26%).
Conclusions
The development of a catalytically active, reusable and selective
NHC-Pd(II) polymer based on co-polymerization of divinyl
benzene with vinyl moieties in the structure of the N-heterocyclic
carbene ligand is described. Characterization of the NHC-Pd(II)
polymers showed the presence of Pd(II) species in the native
polymer without any trace of Pd(0) nanoparticles. The catalytic
activity and selectivity in direct acetoxylation of unactivated C–H
bonds in arenes was evaluated for all polymers with and without
phenylpyridine as supporting ligands on the palladium. The
presence of phenylpyridines increased the selectivity and
decreased the activity of the polymers, reaching a 9:1 selectivity
in the acetoxylation of naphtalene. While the catalysis stops
almost completely after eight hours due to decomposition of the
oxidant it can be restarted through addition of fresh oxidant and
there is no indication that the catalyst loses its activity. Thus, it
can be recycled four times without any loss of activity, but likely
it will be effective for many more cycles. Moreover, the catalyst
was shown to be truly heterogeneous in a hot filtration test. No
significant leaching of Pd from the polymer after each catalysis
cycle was detected. The supported polymer contains reactive
vinyl moieties in the backbone of the polymer, and these are
acetoxylated or chlorinated during catalysis. The present system
is the first heterogeneous catalyst for acetoxylation of non-
functionalized arenes. The heterogeneous nature of these
catalysts could open up for continuous flow catalysis.
Acknowledgements
Financial support from the Swedish Research Council (contract
no. 621-2012-5815), the Knut and Alice Wallenberg Foundation,
and the Royal Physiographic Society is gratefully acknowledged.
Keywords: N-heterocyclic carbenes • Palladium(II) • Polymer
supported Pd(II) • C–H activation • Undirected acetoxylation
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