The synthesis and characterisation of immobilised palladium carbene complexes and their application to heterogeneous catalysis

Article abstract of DOI:10.1016/j.jorganchem.2011.06.042

Silica supported palladium NHC complexes have been prepared by two different routes: one involving the reaction of silica-supported imidazolium salts with palladium acetate and a direct immobilisation of a pre-formed complex by reacting a (trimethoxysilylpropyl)-N-aryl-imidazolylidene palladium complex with surface hydroxyl groups. A small range of catalysts of varying steric bulk were prepared in order to evaluate the effect on catalytic conversion. The activity of the palladium catalysts in Suzuki cross-coupling reactions has been established. The catalysts prepared by immobilising pre-formed palladium complexes gave superior results for the conversion of aryl bromides and aryl chlorides. In addition, use of sterically bulky NHCs (such as the N-2,6-(diisopropyl)phenyl-substituted ligand) resulted in increased catalytic activity, which is analogous to the trends noted in homogeneous catalysis.

Full text of DOI:10.1016/j.jorganchem.2011.06.042

Journal of Organometallic Chemistry 696 (2011) 3465e3472
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The synthesis and characterisation of immobilised palladium carbene complexes
and their application to heterogeneous catalysis
*
*
Elizabeth Tyrrell , Leon Whiteman, Neil Williams
School of Pharmacy and Chemistry, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey KT1 2EE, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Silica supported palladium NHC complexes have been prepared by two different routes: one involving
the reaction of silica-supported imidazolium salts with palladium acetate and a direct immobilisation of
a pre-formed complex by reacting a (trimethoxysilylpropyl)-N-aryl-imidazolylidene palladium complex
with surface hydroxyl groups. A small range of catalysts of varying steric bulk were prepared in order to
evaluate the effect on catalytic conversion. The activity of the palladium catalysts in Suzuki cross-
coupling reactions has been established. The catalysts prepared by immobilising pre-formed palla-
dium complexes gave superior results for the conversion of aryl bromides and aryl chlorides. In addition,
use of sterically bulky NHCs (such as the N-2,6-(diisopropyl)phenyl-substituted ligand) resulted in
increased catalytic activity, which is analogous to the trends noted in homogeneous catalysis.
Ó 2011 Elsevier B.V. All rights reserved.
Received 13 September 2010
Received in revised form
17 June 2011
Accepted 30 June 2011
Keywords:
N-Heterocyclic carbene complexes
Silica support medium
Palladium catalysis
Suzuki coupling reactions
Sonogashira coupling reactions
Immobilised catalyst
1. Introduction
advantages of metal-NHC catalysts are that the ligands are less
prone to dissociation from palladium than phosphine ligands and
The immobilisation of metal catalysts onto insoluble supports is
of great commercial interest because of the sustainable green
chemistry applications it offers [1]. Although current homogeneous
metal catalysts may have sufficient activity, heterogeneous
analogues offer potential practical advantages of ease of product
separation and the possibility of recycling as the catalysts can be
filtered off from the product solution, washed and recycled.
Designing immobilised catalysts, in which dissociation or
decomposition of immobilised ligands from the metal is mini-
mised, is a significant challenge. Numerous reports of supported
homogeneous catalysts have appeared in the literature, many of
which involve phosphine donor groups [2]. We have previously
reported the use of a novel silica-supported palladium phosphine
catalyst and its use in a number of copper-free Sonogashira
coupling reaction between aryl halides and alkynes [3].
that the complexes are more thermally stable.
Of particular interest are the recent attempts at immobilising
various NHC complexes of palladium for use in heterogeneous
Suzuki couplings. Lee et al. [5] and Weck et al. [6] have synthesised
polymer-bound palladium NHC catalysts which can catalyse the
coupling of relatively unreactive aryl chlorides. Herrmann et al. [7]
prepared an N-heterocyclic dicarbene complex tethered to Wang
resin that catalysed Heck reactions of both activated and non-
activated aryl bromides. In contrast to polymer-based tethers, the
use of a silica-support requires no pre-swelling in organic solvents
which thus reduces solvent usage and may reduce the propensity
for catalyst leaching. As a result Sen et al. [8] and Lin et al. [9] have
prepared silica-supported palladium NHC hybrid catalysts by
reacting a metal salt with a tethered ligand. The disadvantage of
this particular methodology is the difficulty in confirming the co-
ordination around the metal centre. A more elegant route would be
to synthesise a well-defined metalecarbene complex that may then
be grafted directly to the silica surface. There have been a number
of reports of trialkoxysilylpropyl functionalised palladium NHC
complexes being immobilised on silica surfaces [10e12], however
Lin et al. found that immobilisation of a pre-formed complexes was
unsuccessful [9].
More recently, attention has shifted to the use of N-heterocyclic
carbene (NHC) ligands in binding transition metals to an insoluble
support, because of their high activity for palladium catalysed
cross-coupling reactions and ruthenium catalysed olefin metath-
esis [4a,b]. We have previously reported the use of novel NHCs in
palladium and copper catalysed homogeneous reactions [4c,d]. The
When designing palladium NHC complexes for tethering to
a support medium via an organic linker, it is somewhat surprising
* Corresponding authors. Tel.: þ44 208 547 8810.
E-mail address: N.A.Williams@kingston.ac.uk (N. Williams).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.06.042

3466
E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
Table 1
2. Results and discussion
Synthesis of imidazolium salts 1.
2.1. Synthesis of ligand precursors
N-Aryl imidazoles were prepared according to standard proce-
dures [13]. The trialkoxysilyl group allows facile tethering to silica
via condensation with surface silanol groups [14]. Imidazolium
salts 1aed were prepared by heating a molten mixture of the
N-substituted imidazole and 3-(bromo/iodopropyl)trimethox-
ysilane at 80e100 ꢀC (Table 1). It was found that the diisopropyl-
phenyl (dipp) derivative 1d could be prepared in a more pure form
by using acetonitrile as a solvent. Reactions were monitored by TLC
and in all cases the imidazolium salt was detected on the baseline
within 20 h accompanied by the complete disappearance of the
imidazole starting material. The imidazolium salts were isolated in
high yields after trituration of the reaction mixture with diethyl
ether. The N-alkyl imidazolium bromides were isolated as viscous
oils whilst the N-aryl derivatives were isolated as white solids
which were not appreciably hygroscopic. A downfield signal
Entry (name)
X
R
Temperature (ꢀC)
% Yield
1a
1b
1c
1d
Br
Br
Br
I
Benzyl-
Mesityl-
2,6-(Diisopropyl)phenyl-
2,6-(Diisopropyl)phenyl-
80
80
100
100
79
86
76a
96
a
Reaction carried out in acetonitrile.
between
d
9.5e10.5 ppm in the 1H NMR spectra, characteristic of
the C2 proton of an imidazolium salt, confirmed the successful
formation of the imidazolium salts 1aed (Table 1).
2.2. Immobilisation of imidazolium salts and complexation with
palladium
Scheme 1. Formation of a palladium complex. R ¼ benzyl (3a), mesityl (3b).
Imidazolium salts 1aeb were immobilised by heating under
reflux in toluene in the presence of silica according to the literature
procedure [8]. The tethered imidazolium salts 2 were treated with
palladium acetate to yield immobilised bis NHC palladium(II)
complexes 3 (Scheme 1). The formation of a mono-NHC complex is
also a possibility.
Elemental analysis of catalyst 3b (R ¼ mesityl) shows an excess of
palladium (theory ¼ 4.77%; found 6.88%), suggesting the presence of
other palladium species. Formation of palladium black (palladium
nanoparticles) at some stage in the synthesis is a strong possibility
and would explain the dark colour of the catalyst. It appears the
excess palladium (used to ensure complete reaction of all imidazo-
lium salt moieties on the catalyst precursor) cannot be removed from
the support material by simple washing steps alone (Fig. 1).
that the examples in the literature focus upon the use of N-alkyl
substituted NHCs despite results from homogeneous catalysis that
suggest that NHCs bearing bulky N-aryl substituents are more
active towards CeC and CeN coupling reactions [12]. Artok et al.
[10] did report the synthesis of an N-aryl substituted NHC palla-
dium catalyst and its successful application in the Heck reaction.
Our aim was to synthesise palladium complexes of NHCs
bearing bulky N-aryl or N-alkyl substituents and an alkoxysilylalkyl
chain to enable tethering to a silica surface. This process would
enable characterisation of the co-ordination geometry around the
palladium metal prior to immobilisation of the catalyst onto the
silica surface. It was anticipated that catalysts bearing bulky N-aryl
substituents would have enhanced activity compared to those with
simpler substituents. In this work we report the synthesis of bis (N-
(trimethoxysilylpropyl)-N0-aryl)imidazolylidenepalladium compl-
exes, their immobilisation onto a silica support and an evaluation of
their activity for Suzuki coupling reactions. In addition, a similar
catalyst was prepared by an alternative route involving the teth-
ering of the imidazolium salt 2 (NHC ligand precursor) onto the
silica support followed by reaction with palladium acetate, to afford
3. This would enable a direct comparison of the two methodologies
upon the catalytic activity.
2.3. Screening of 3aeb for Suzuki cross-coupling
Catalysts 3aeb were screened for activity in a standard
Suzuki coupling reaction between phenylboronic acid and
4-haloacetophenones (Table 2).
The catalytic activity of these complexes with aryl bromides is
satisfactory, affecting the Suzuki coupling reaction at a low catalyst
loading of 0.2 mol% and is comparable with contemporary silica-
Fig. 1. SEM images of (a) Silica support medium, (b) Silica supported imidazolium salt 1b (c) Silica supported NHC catalyst 3b illustrating the presence of surface debris.

E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
3467
Table 2
Palladium complexes 5aec were soluble in polar organic
solvents and were characterised by mass spectrometry and 1H & 13
Screening for activity in Suzuki reaction of 4-haloacetophenones and phenylboronic
acid.
C
NMR spectroscopy. Chemical ionisation methods allowed the
molecular ion of the bis-NHC palladium complexes 5aec to be
measured. Satisfyingly the near-perfect match between the
observed data and the theoretical isotope profiles (Fig. 2) and the
high resolution mass spectral data provide overwhelming evidence
for the presence of the bis-NHC complexes, as opposed to a variant
such as a mono-NHC or dimeric species.
Entry
Catalyst (loading)
X
Temperature (ꢀC)
Conversion (%)a
1
2
3
4
3a (0.2 mol%)
3b (0.2 mol%)
3a (2 mol%)
3b (2 mol%)
Br
Br
Cl
Cl
80
80
100
100
79
84
4b
7b
In the 1H and 13C NMR spectra most of the expected signals of
a bis-NHC complex 5b are duplicated. This duplication of reso-
nances is often observed for two closely related species that are in
equilibrium and such an observation has been previously reported
with N-methyl imidazol-2-ylidene complexes where an explana-
tion based upon the presence of cis- and trans-isomers was offered
[11]. However, this seems less likely for bulkier NHC complexes
such as 5b due to steric crowding around the metal centre. A recent
paper, however, described the synthesis and characterisation of
a series of bis-NHC complexes of palladium also reported that the
NMR resonances were duplicated [16]. The authors attributed this
phenomenon to the presence of trans-anti and trans-syn rotameric
isomers resulting from hindered rotation around the PdeC bond.
Support of this attribution was forthcoming from variable
temperature NMR studies where the duplicated resonances were
seen to coalesce as the temperature of the NMR study was
increased (Fig. 3).
a
% Conversion determined by GC using dodecane as an internal standard.
Reaction time increased to 60 min.
b
supported NHC-Palladium catalysts. Significantly the use of
a bulkier mesityl-derived NHC catalyst 3b provides enhanced
conversions compared to the corresponding benzyl-NHC complex
3a. This provides some supporting evidence to the assertion that
increased steric bulk around the metal centre has a positive effect
upon the catalytic activity [20a]. However, the aryl chloride
substrate is much less tolerated under these conditions. Conver-
sions were low, even with increased catalyst loadings and
temperature. This is not surprising as most silica-supported NHC
catalysts tend to exhibit low conversions with aryl chloride
substrates. As the activity of the catalysts was fairly modest and also
because the catalysts did not appear to be composed of a pure
immobilised complex (i.e. a mixture of mono- and bis-NHC species
as well as uncoordinated Pd was likely to be present), an alternative
approach to their synthesis was employed.
Extensive NMR analysis of complex 5b enabled the resonances
of both rotamers to be assigned unambiguously. The protons of the
N-propyl chain experience some degree of shielding from the
aromatic ring in the trans-anti rotamer, as a result the resonances
attributed to this rotamer appear slightly upfield with respect to
the corresponding trans-syn rotamer. In addition, cis- and trans-bis-
2.4. Synthesis of palladium NHC complexes
NHC palladium complexes can be distinguished by the 13C
d values
for the carbene carbon resonances. Trans-carbene complexes
usually have values in the range 168e172 ppm whilst cis-carbene
complexes exhibit more high field shifts, typically 160e165 ppm
[17]. In the 13C NMR spectrum of 5b two resonances for the carbene
In order to synthesise the desired bis-NHC palladium dichloride
complexes the imidazolium salts 1aed were deprotonated, using
Ag2O, to yield silver imidazol-2-ylidene complexes 4aed (not iso-
lated), which were employed as carbene transfer reagents (Scheme
2).
carbon atom are evident at a chemical shift
d 170.3 and 170.9 ppm
respectively. These data coupled with the variable temperature
NMR studies suggest the presence of two rotameric forms with
a trans-arrangement consistent with trans-anti and trans-syn
rotamers respectively.
Heteronuclear correlation experiments on 5b revealed the
presence of the alkyl eCH2 groups which are obscured in the 1H
NMR spectrum by the ortho-methyl groups of the mesityl substit-
uents. The 1H NMR signals for 5b (both rotamers) are assigned
(Table 3).
Bis-(benzonitrile)palladium (II) dichloride was added to a solu-
tion of the silver NHC species 4aed and stirred at room temperature
for 16 h. After filtration through celite the (N-(triethoxysilylpropyl)-
N0-aryl)imidazolylidenepalladium complexes 5aed were isolated in
moderate yields (35e57%). Interestingly, the imidazolium iodide 1d
did not furnish the corresponding N-(triethoxysilylpropyl)-N0-aryl)
imidazolylidenepalladium dichloride complex 5d (formed in 35%
yield) as cleanly as the analogous bromide 1c [15]. In our hands we
found that all palladium species, prepared from imidazolium
iodides, had significantly more complex NMR spectra, compared to
the derivatives obtained from the corresponding bromides, and are
not reported further here. It is possible that the iodide undergoes
halide exchange with the palladium chloride ligands to afford
palladium iodide complexes.
2.5. Synthesis of silica-supported palladium NHC catalysts
Tethering of the palladium complexes 5aec to silica to form the
supported palladium complexes 6aec was efficiently accomplished
Scheme 2. Synthesis of palladium complexes prior to immobilisation.

3468
E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
Fig. 2. Low-resolution mass spectrum of bis-[1-(mesityl)-3-(trimethoxysilyl)propylimidazole]palladium dichloride 5b showing theoretical and observed isotope profiles.
by refluxing in chloroform overnight. The noteworthy feature of
this reaction was that no trace of the bright yellow coloured
palladium complex remained in the reaction mixture after filtra-
tion. The product, in the form of a pale yellow powder, was simply
filtered off and washed with chloroform and dichloromethane. No
trace of the complex was detected by NMR analysis of the washings,
again indicating a highly efficient immobilisation process.
coupling, consistent with couplings performed in homogenously
catalysed reactions. Early work by Nolan et al. [18] demonstrated
that both N-mesityl and N-diisopropyl(phenyl) substituted imida-
zolylidenes were significantly more active (99% and 53% conversion
respectively) than N-alkyl substituted imidazolylidenes (14e16%
conversion) for the palladium catalysed Suzuki cross-coupling of
aryl chlorides with an arylboronic acid. This work also indicated
that when using Pd2(dba)3, as a palladium source, the diisopro-
pyl(phenyl) substituted imidazolylidene ligand gave a slightly
higher conversion (95%) than the N-mesityl substituted imidazo-
lylidene (90%). Similar behaviour has been seen for other palladium
catalysed cross-coupling reactions such as Negishi Coupling,
Buchwald-Hartwig coupling and the arylation of ketones [19].
2.6. Suzuki coupling reactions
Initially catalysts 6aec were screened for activity in a series of
Suzuki cross-coupling reactions (Table 4).
Our attempts to optimise the coupling reaction included an
investigation into both the choice of base (Na2CO3, Cs2CO3 and
morpholine) and solvent systems (DMF:H2O [1:1], acetonitrile,
DMF and DMF/H2O [10:1]). We concluded that the use of Na2CO3
and DMF:H2O (1:1) gave the best results and were used for the rest
of the testing. At the 1 mol% level all three catalysts 6aec provided
rapid and quantitative conversion of the substrates (entries 1e3).
At a lower loading of 0.2 mol% catalyst 6c, containing the most
bulky N-substituent, readily outperformed 6a and 6b to again
provide a rapid and quantitative conversion (Compare entries 4 & 5
with entry 6 as well as entry 3 with entry 6). This observation is
consistent with our initial thesis that a more bulky N-substituent
would result in a more active immobilised catalyst for Suzuki
Table 3
1H NMR assignments for NHC complex 5b.
Proton(s)
Resonance(s) (
d, ppm)
Integral, Multiplicity
Si-OCH3
SiCH2
CH2CH2CH2
N-CH2
CH]CH
o-CH3
Ar-H
3.58
18H, s
4H, m
4H, m
4H, m
4H, m
12H, s
4H, s
0.49, 0.78
1.90, 2.21
4.18, 4.62
6.64, 6.70, 6.92, 6.97
1.87, 2.19
6.84, 6.98
p-CH3
2.36, 2.46
6H, s
Fig. 3. Rotamers of palladium complexes.

E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
3469
Table 4
Table 5
Optimisation of Suzuki coupling reactions.
Testing of catalyst 6c on a range of substrates.
Ar1
X
R1
Conversion (%)a
Entry
Catalysta
Loading (mol%)
Temperature (ꢀC)
Conversion (%)b
Entry
1
2
3
4
5
6
7
8
9
6a
6b
6c
6a
6b
6c
6c
6c
6c
6c
1
1
1
0.2
0.2
0.2
0.2
0.1
1
80
80
80
80
80
80
60
80
RT
RT
>99
>99
>99
81
90
>99
28
1
2
3
4
5
6
7
8
C6H5
2-(Me)C6H5
3-C5H4N
C6H5
4-(MeCO)C6H4
C6H5
I
I
I
C6H5
C6H5
C6H5
C6H5
>99 (89)
67
48
>99
98 (90)
<1
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
C6H5
3-C5H4N
4-C5H4N
C6H5
C6H5
2-C4H3S
C6H5
C10H7
C6H5
C6H5
C6H5
<1
69
80
4-(O2N)C6H5
4-(MeO)C6H4
4-(MeCO)C6H5
C6H5
trace
9
35 (29)
<1
10
0.2
12c
10
11
12
13
14
15
52b
a
Catalyst 6a ¼ N-benzyl, 6b ¼ N-mesityl and 6c ¼ N-2,6-(diisopropyl)phenyl.
C6H5
40b
b
% Conversion based on the halide substrate, determined by GC using dodecane
4-(MeCO)C6H4
4-(O2N)C6H5
4-(MeO)C6H4
30b
as an internal standard.
13b
c
Reaction time extended to 24 h.
C6H5
4b
a
% conversion of the halide substrate was established by GCeMS using dodecane
as an internal standard. Numbers in parentheses represent % yield isolated by
column chromatography.
The established mechanism of Suzuki coupling involves three
major steps: oxidative addition of the aryl halide, transmetallation
followed by reductive elimination which yields the product and
regenerates the active catalyst. It is well established that bulky,
electron rich NHCs give the most active catalysts for cross-coupling
reactions. The development of several different mono-NHC palla-
dium catalysts also highlights that the palladium to ligand ratio is
very important [20]. A number of mechanistic studies indicate that
the active catalyst is in fact a mono-NHC species, similar observa-
tions have been made with phosphine ligands [21]. Mechanistic
work by Cloke et al. [22] indicates that oxidative addition of an aryl
halide is the rate determining step, an observation also supported
by work by Nolan [18]. The greater activity of the sterically
demanding ligands can then be attributed to their ability to stabi-
lize such low co-ordinate palladium species which are then much
more reactive towards oxidative addition due to their low valence
electron count. Increased steric bulk is also likely to increase the
rate of reductive elimination of the product as this step reduces
steric crowding around the palladium centre. In our work we
expect the prevalent species to be a bis-NHC species, this might
explain its inability to catalyse coupling with the more demanding
aryl chloride substrates. Nonetheless, it is possible that some ligand
dissociation occurs in catalyst 6c to give a low concentration of
a more active mono-carbene species. The greater activity could then
be attributed to this process being more likely for the more steri-
cally demanding ligand. Another possible hypothesis is that with
the use of aryl bromides the actual reductive elimination is the rate
determining step. In this case the greater activity of the (diiso-
propyl)phenyl-substituted imidazolylidene catalyst 6c may be
explained by its steric bulk favouring reductive elimination.
b
Reaction temperature increased to 100 ꢀC, catalyst loading increased to 2 mol %,
reaction time increased to 60 min.
lead to co-ordination to palladium and inhibition of the active
catalyst species [23,24]. Both Buchwald et al. [25] and Fu et al. [26]
have reported some success with homogenous catalysts for these
substrates and some success has also been reported recently with
a hybrid catalyst [27]. Both electron-withdrawing and electron-
donating substituents resulted in lower conversion rates. These
data suggest that for the aryl bromides the rate determining step is
unlikely to be the oxidative addition of the aryl halide.
Suzuki coupling of aryl chlorides continues to be a challenge for
most existing hybrid catalysts [7,8,27], although exceptions to this
rule do exist [28]. There are numerous examples of homogeneous
catalysts, such as ‘PEPPSI’ [21a,b] and ‘IBiox’ [29] which are capable
of coupling even sterically hindered and electronically deactivated
substrates at room temperature. Catalyst 6c does however,
compare well with most hybrid NHC-palladium catalysts in the
literature, in that it does convert aryl chloride substrates, but only
slowly (entries 11e15) [7,8,27].
2.7. Recycling studies
Catalyst 6c was tested for recyclability using the Suzuki coupling
of 4-bromoacetophenone and phenylboronic acid. Although the
catalyst is recyclable, the amount of conversion after 30 min drops
off quickly. A similar reduction in catalytic activity was also
reported by Artok et al. [10]. Catalyst 6c exhibited activity over 4
Having identified the optimum grafted catalyst as well as the
reaction conditions for effecting the Suzuki reaction we then
studied the tolerance of catalyst 6c to a range of electronically
activated and deactivated substrates as well as functionally bulkier
substrates. The data for these experiments, using 0.2 mol% of the
catalyst, are summarised in Table 5 (Ar1 is derived from the aryl
halide component and R1 is the boronic acid component). Several
interesting conclusions may be drawn from these results. Firstly, it
is apparent that sterically hindered substrates such as 2-
iodotoluene (entry 2) experience lower conversion rates.
Although heterocyclic aromatic halides exhibit a lower conversion
(entry 3), heterocyclic boronic acids were not tolerated in these
studies (entries 6, 7 and 10). This observation is perhaps not so
surprising as nitrogen-containing heterocycles are difficult
substrates for Suzuki coupling reactions as their basic nature can
Fig. 4. The performance of the various NHC catalysts in the Suzuki reaction of
4-chloroacetophenone. Conditions: 100 ꢀC, 60 min %Age conversion of the aryl chlo-
ride was determined by GC analysis.

3470
E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
Fig. 5. SEM images of catalyst 3b (left) and of catalyst 6b (right). The consistent morphology demonstrated by the SEM of 6b may correlate to the enhanced activity of this catalyst
compared to 3b.
consecutive cycles and, as we observed with the corresponding
phosphine catalyst [3b] a steady reduction in activity was observed,
In cycle 1, quantitative conversion was observed. This was reduced
to 55% for the second cycle, 20% for the 3rd and 15% for the 4th
cycle. These observations may be explained in terms of palladium
leaching [30]; the changing colour of the catalyst suggests the
formation of palladium metal particles with concomitant degra-
dation of the tethered catalyst.
imidazolium salt bearing the bulkiest group suggesting that steric
bulk plays an important role in controlling the catalytic activity of
tethered palladium NHC catalysts. Although this has been docu-
mented for homogeneous reactions and reactions employing ‘non-
tethered’ solid supported catalysts [2,31,32], this is the first time
that the correlation between ligand size and activity has been
reported for organically tethered NHC complexes. This is an
important development as previous studies have been limited to
methyl and benzyl derivatives. Our best prototype catalyst, 6c,
demonstrated an encouraging range of activities in the Suzuki
coupling reactions of aryl bromides and chlorides. Further studies
are currently underway in our laboratories and results will be
disseminated in due course.
2.8. Comparison of catalysts 3b and 6b
Catalyst 3b, prepared by the immobilisation of the imidazolium
salt 2b to silica before the formation of the NHC complex and
catalyst 6b, prepared by tethering the NHC complex 5b directly to
silica should in theory be reasonably similar in both structure and
activity. A comparison of the data from Tables 2 and 4 (Suzuki
reaction) reveals that catalyst 3b provided an 84% yield of the
coupled product at the 0.2 mol% loading of catalyst. In contrast NHC
6b gave a 90% yield under the same conditions. These data suggest
that 6b is slightly more active in the Suzuki coupling of aryl
bromides than 3b. Furthermore the catalyst containing the bulkiest
ligands, 6c, demonstrated enhanced activity with conversions of
>99% under the same reaction conditions.
In addition it was also observed that 6c was able to affect the
Suzuki coupling of aryl chlorides (30%) (Table 5, entry 13). In
contrast NHC 3b was only able to catalyse the same substrates with
a yield of 7% performed at a higher reaction temperature. The data
summarised (Fig. 4) serves as a comparison of activity between the
2 catalyst types; 3aeb, prepared by formation of the NHC catalyst
on the silica support and catalysts 6aec, produced in an alternative
way that involved the immobilisation of the pre-formed NHC
complex directly onto the silica support.
4. Experimental
4.1. General procedures
All NMR spectra were recorded at 400 MHz (1H) and 100 MHz
(
13C) on a Jeol Eclipseþ 400 NMR spectrometer using Jeol Delta
version 4.3.6 control and processing software. Chemical shifts are
reported in ppm, referenced to residual solvent peaks (acetone or
chloroform). MS were recorded using a Varian CP-3800 Gas Chro-
matograph with Varian 1200L Quadrupole Mass Spectrometer
controlled using Varian Saturn GC/MS System Control Version 6.41,
Copyright 1989e2004 Varian Inc. Melting point determinations
were recorded using a Stuart Scientific SMP3 digital melting point
apparatus and are uncorrected.
4.2. Experimental procedures
The corresponding SEM of the N-mesityl substituted NHC
catalysts produced by the two routes are shown (Fig. 5) and serve to
confirm the appearance of morphological differences between the
two catalysts which may account for the difference in their catalytic
activities.
4.2.1. 1-(Trimethoxysilyl)propyl-3-benzylimidazolium bromide (1a)
To a flame dried round-bottomed flask, containing a magnetic
follower, was added N-benzylimidazole (316 mg, 2 mmol) and 3-
(bromopropyl)trimethoxysilane (482 mg,
2
mmol) under
a nitrogen atmosphere. The mixture was heated, with stirring, at
80 ꢀC for about 6 h. The mixture was cooled to an ambient
temperature and triturated with diethyl ether (3 ꢁ 50 mL) to afford
the title compound 1a as a viscous, colourless oil (630 mg, 79%). 1H
NMR (400 MHz, CHCl3): 10.44e10.47 (s, 1H, Ar-H), 7.44e7.49 (m,
3H, Ar-H), 7.37e7.39 (m, 1H, Ar-H), 7.29e7.33 (m, 2H, Ar-H),
5.56e5.59 (s, 2H, benzyl-H), 4.22e4.27 (t, J ¼ 7.32 Hz, 2H, alkyl-
CH2), 3.47e3.50 (s, 9H, eOCH3), 1.89e1.99 (p, J ¼ 7.87 Hz, 2H, alkyl-
CH2), 0.53e0.59 (t, J ¼ 7.87 Hz, 2H, alkyl-CH2). 13C NMR (100 MHz,
CHCl3): 136.9, 133.2, 129.5, 129.1, 122.2, 65.9, 53.2, 51.9, 50.7, 24.1,
15.3, 5.9. MS (ES): 321 m/z (Mþ ꢂ Br).
3. Conclusion
Our investigations demonstrate that the immobilisation of
a pre-formed palladium N-heterocyclic carbene complex to a silica
support produces a well defined and more active catalyst for Suzuki
cross-coupling reactions than the corresponding catalyst prepared
by the reaction of palladium acetate with the tethered imidazolium
salt. A range of catalysts with varying steric bulk were synthesised.
The most active catalyst was the one prepared from the

E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
3471
4.2.2. 1-(Trimethoxysilyl)propyl-3-mesitylimidazolium bromide
(1b)
was accomplished by trituration with anhydrous diethyl ether
(3 ꢁ 20 mL), yielding the title compound 6b as a yellow solid
(0.530 g, 47%, as a mixture of trans-syn- and trans-anti-rotamers).
1H NMR (400 MHz, CHCl3): 6.95e7.00 (m, 3H, Ar-H), 6.91e6.93 (m,
1H, Ar-H imid.), 6.81e6.86 (m, 2H, Ar-H), 6.69e6.72 (m, 1H, Ar-H),
6.62e6.66 (m, 1H, Ar-H), 4.54e4.70 (m, 2H, alkyl-CH2, syn-isomer),
4.11e4.26 (m, 2H, alkyl-CH2, anti-isomer), 3.56e3.60 (s, 18H, OCH3),
2.41e2.48 (s, 3H, p-CH3, syn-isomer), 2.34e2.38 (s, 3H, p-CH3, anti-
isomer), 2.15e2.30 (m, 8H, alkyl-CH2, syn-isomer and o-CH3, syn-
isomer overlapping), 1.85e1.94 (m, 8H, alkyl-CH2, anti-isomer and
o-CH3, anti-isomer overlapping), 0.79e0.95 (m, 2H, alkyl-CH2, syn-
isomer), 0.45e0.53 (m, 2H, alkyl-CH2, anti-isomer). 13C NMR
(100 MHz, CHCl3): 170.9, 170.5, 138.4, 137.5, 137.4, 136.9, 136.8,
136.5, 136.2, 136.0, 135.7, 135.6, 128.88, 128.83, 128.79, 128.75,
122.65, 122.55, 122.4, 122.25, 121.0, 120.9, 120.85, 120.75, 53.28,
53.1, 53.03, 52.87, 50.7, 24.45, 24.3, 24.1, 24.0, 21.35, 21.08, 19.65,
19.23, 19.13, 19.0, 18.95, 18.88, 18.7, 18.55, 6.44, 6.39, 6.1, 6.07. Note:
duplication of all signals observed as a result of syn- and anti-
isomerism. MS (EI): m/z 835 (Mþ ꢂ Cl). HRMS (EI): calculated for
C36H56O6N4Si2Pd; 870.2161, found: 870.2161.
This was synthesised as described for 1a using N-mesitylimi-
dazole (0.930 g, 5 mmol). Trituration in diethyl ether (3 ꢁ 50 mL)
provided the title compound as an off-white solid (MP: 120e123 ꢀC,
1.846 g, 86%). 1H NMR (400 MHz, CHCl3): 10.05e10.09 (s, 1H, Ar-H),
7.88e7.91 (m, 1H, Ar-H), 7.19e7.21 (m, 1H, Ar-H), 6.82e6.85 (s, 2H,
Ar-H), 4.50e4.56 (t, J ¼ 6.96 Hz, 2H, alkyl-CH2), 3.38e3.42 (s, 9H,
OeCH3), 2.16e2.19 (s, 3H, eCH3), 1.88e1.99 (m, 8H, alkyl-CH2 and
Ar-CH3 overlapping), 0.48e0.55 (t, J ¼ 8.06 Hz, 2H, alkyl-CH2). 13
C
NMR (100 MHz, CHCl3): 141.1, 137.6, 134.1, 130.7, 129.8, 123.6, 123.5,
51.9, 50.7, 24.3, 21.1, 17.6, 5.6. MS (ES): 349 m/z (Mþ ꢂ Br). HRMS
(EI): calculated for C18H29O3N2Si; 349.1942, found: 349.1939.
4.2.3. 1-(Trimethoxysilyl)propyl-3-(20,60-diisopropylphenyl)
imidazolium bromide (1c)
N-(2,6-diisopropylphenyl)imidazole (764 mg, 3.2 mmol) in
acetonitrile (20 mL) was added to a dry, round-bottomed flask
under an atmosphere of nitrogen. 3-(bromopropyl)trimethox-
ysilane (684 mg, 3.2 mmol) was added and the mixture heated to
100 ꢀC for 7 h. After cooling to room temperature, the solvent was
removed in vacuo and the crude product (now solid) was triturated
in diethyl ether (3 ꢁ 50 mL) to yield the title compound 1c as an off-
white solid (MP: 116e118 ꢀC, 1.17 g, 76%). 1H NMR (400 MHz,
CHCl3): 10.30e10.36 (s, 1H, Ar-H), 7.91e7.94 (s, 1H, Ar-H), 7.47e7.55
(m, 1H, Ar-H), 7.25e7.30 (m, 2H, Ar-H), 7.19e7.22 (m, 1H, Ar-H),
4.74e4.81 (t, J ¼ 6.96 Hz, 2H, alkyl-CH2), 3.52e3.57 (s, 9H,
OeCH3), 2.19e2.30 (septet, J ¼ 6.96 Hz, 2H, alkyl-CH), 2.04e2.14
(quintet, J ¼ 8.06 Hz, 2H, alkyl-CH2), 0.63e0.69 (t, J ¼ 8.06 Hz, 2H,
alkyl-CH2). 13C NMR (100 MHz, CHCl3): 145.4, 138.4, 132.0, 130.2,
124.8, 124.2, 123.2, 52.1, 50.8, 28.8, 24.5, 24.2, 5.6. MS (ES): 391 m/z
(Mþ ꢂ Br). HRMS (EI): calculated for C19H35O3N2Si; 391.2411, found:
391.2401.
4.2.6. Bis-1-(trimethoxysilyl)-3-benzylimidazol-2-ylidene
palladium dichloride (5a)
As
described
using
1-(trimethoxysilyl)propyl-3-
benzylimidazolium bromide 1a (0.399g, 1 mmol) was used as the
imidazolium salt (adjusting the quantities of the other reagents
according to this stoichiometry). The title compound was obtained
as a bright yellow solid (0.205 g, 56%). 1H NMR (400 MHz, CHCl3):
7.15e7.50 (m, 14H, Ar-H), 5.49e5.82 (m, 4H, benzyl-CH2, syn- and
anti-isomers), 4.32e4.49 (m, 4H, alkyl-CH2, syn- and anti-isomers),
3.39e3.55 (m, 18H, eOCH3, several overlapping signals), 2.02e2.35
(m, 4H, alkyl-CH2, syn- and anti-isomers), 0.63e0.89 (m, 4H, alkyl-
CH2, syn- and anti-isomers). 13C NMR (100 MHz, CHCl3):Not
reported due to overlapping signals originating from syn- and anti-
isomers. HRMS (EI): calculated for C32H48O6N4Si2Pd; 814.1535,
found: 814.1537.
4.2.4. 1-(Trimethoxysilyl)propyl-3-(20,60-diisopropylphenyl)
imidazolium iodide (1d)
The procedure was carried out as described for 1c using N-(2,6-
diisopropylphenyl)imidazole (764 mg, 3.2 mmol) and 3-
iodopropyltrimethoxysilane (3.2 mmol). The title compound 1d
was isolated as an orange oil (1.38 g, 96%). 1H NMR (400 MHz,
CHCl3): 9.87e9.91 (s, 1H, Ar-H), 8.01e8.05 (s, 1H, Ar-H), 7.43e7.51
(m, 1H, Ar-H), 7.20e7.27 (m, 3H, Ar-H), 4.64e4.71 (t, J ¼ 6.96 Hz, 2H,
alkyl-CH2), 3.46e3.54 (s, 9H, OeCH3), 2.14e2.26 (septet, J ¼ 6.77 Hz,
2H, alkyl-CH), 1.99e2.09 (quintet, J ¼ 8.06 Hz, 2H, alkyl-CH2),
1.05e1.17 (dd, J ¼ 6.77 Hz, 12H, eCH3), 0.57e0.65 (t, J ¼ 8.06 Hz, 2H,
alkyl-CH2). 13C NMR (100 MHz, CHCl3): 145.1, 137.0, 131.7, 129.7,
124.4, 124.3, 123.6, 51.8, 50.6, 28.4, 24.3, 24.2, 24.0, 5.2. MS (ES):
391 m/z (Mþ ꢂ I).
4.2.7. Bis-1-(trimethoxysilyl)-3-(20,60-diisopropylphenyl)imidazol-
2-ylidene palladium dichloride (5c)
As described using 1-(trimethoxysilyl)propyl-3-(20,60-diisopro-
pylphenyl) imidazolium bromide (0.235 g, 0.5 mmol) was used as
the imidazolium salt (adjusting the quantities of the other reagents
according to this stoichiometry). The title compound was obtained
as a dull-yellow solid (0.112 g, 47%). 1H NMR (400 MHz, CHCl3):
7.26e7.36 (m, 6H, Ar-H), 6.89e7.08 (m, 2H, Ar-H), 6.65e6.81 (m, 2H,
Ar-H), 4.67e4.89 (m, 2H, alkyl-CH2, syn-isomer), 4.05e4.39 (m, 2H,
alkyl-CH2, anti-isomer), 3.43e3.63 (m, 18H, eOCH3), 2.80e3.01 (m,
2H, alkyl-CH2, syn-isomer), 2.52e2.73 (m, 2H, alkyl-CH2, anti-
isomer), 2.20e2.38 (m, 2H, alkyl-CH, syn-isomer), 1.73e1.84 (m,
2H, alkyl-CH, syn-isomer), 0.75e1.41 (m, 28H, alkyleCH3 and alkyl-
CH2). 13C NMR (100 MHz, CHCl3) Not reported due to overlapping
signals originating from syn- and anti-isomers. HRMS (EI): calcu-
lated for C42H68O6N4Si2Pd; 954.3089, found: 954.3090.
4.2.5. Bis-1-(trimethoxysilyl)-3-mesitylimidazol-2-ylidene
palladium dichloride (5b)
1-(Trimethoxysilyl)propyl-3-mesitylimidazolium bromide 1b
(1.287 g, 3 mmol) was added to a dry, light-protected Schlenk tube
under a nitrogen atmosphere. To this was added silver (I) oxide
(0.348 g, 1.5 mmol) and anhydrous chloroform (7.5 mL) and the
resulting mixture was stirred at room temperature overnight. The
mixture was carefully removed under continuous nitrogen flow, by
pipette and filtered through celite directly into another dry, light-
protected Schlenk tube under nitrogen (an extra 2.5 mL of anhy-
drous chloroform was used to dissolve any product remaining in
the original vessel). The white precipitate was discarded. Bis-
(benzonitrile)palladium (II) dichloride (0.504 g, 1.3 mmol) was
added gradually and the mixture was stirred overnight. The
mixture (bright-yellow) was again filtered through celite and the
solvent removed in vacuo to yield the crude product. Purification
4.2.8. General procedure for immobilisation of imidazolium salts
1aec
Under nitrogen, a dry round-bottomed flask was charged with
imidazolium salt (1 mmol), anhydrous toluene (60 mL) and silica
gel (1.00 g, pre-dried). The mixture was fitted with a DeaneStark
trap and refluxed for 24 h. After cooling to room temperature, the
mixture was filtered and washed with anhydrous dichoromethane
(3 ꢁ 50 mL). The resulting white solid was dried in vacuo at 60 ꢀC
over phosphorous pentoxide (24 h) to yield the modified support
material (>95% by weight). 1H NMR analysis of the dichloro-
methane washings showed no trace of the imidazolium salt starting

3472
E. Tyrrell et al. / Journal of Organometallic Chemistry 696 (2011) 3465e3472
Eur. J. 13 (2007) 520e528;
material, indicating near-quantitative immobilisation. The imida-
zolium salt loading was determined by TGA analysis.
(c) A. Corma, H. Garcia, Adv. Synth. Cat. 348 (2006) 1391e1412;
(d) T.S. Halbach, S. Mix, D. Fischer, S. Maechling, J.O. Krause, C. Sievers,
S. Blechert, O. Nuyken, M.R. Buchmeiser, J. Org. Chem. 70 (2005) 4687e4694.
[3] (a) E. Tyrrell, A. Al-Saardi, J. Millet, Synlett 3 (2005) 487e488;
(b) E. Tyrrell, L. Whiteman, N. Williams, Synthesis 5 (2009) 829e835.
[4] (a) W.J. Sommer, M. Weck, Coord. Chem. Rev. 251 (2007) 860e873;
(b) C.S.J. Cazin, C. R. Chim. 12 (2009) 1173e1180;
4.2.9. Complexation of immobilised imidazolium salt 2b (Synthesis
of 3b)
The immobilised imidazolium salt (1.00 g, 1 mmol) was sus-
pended in dimethylsulfoxide (4.5 mL) and to this was added
palladium (II) acetate (0.11 g, 0.5 mmol). This mixture was stirred at
60 ꢀC for 4 h, then the temperature was increased to 100 ꢀC for
a further 30 min, then the mixture was allowed to cool to room
temperature. The mixture was filtered, washed with dichloro-
methane (4 ꢁ 50 mL) and finally dried in vacuo at 60 ꢀC over
phosphorous pentoxide (48 h) to yield a light-brown solid (99% by
weight). ICP-AES analysis: calculated 4.77% Pd, found 6.88% Pd.
(c) T. Moore, M. Merzouk, N.A. Williams, Synlett (2008) 21e24;
(d) M. Merzouk, T. Moore, N.A. Williams, Tetrahedron Lett. 48 (2007)
8914e8917.
[5] (a) J.W. Kim, J.-H. Kim, D.-H. Lee, Y.-S. Lee, Tetrahedron Lett. 47 (2006)
4745e4748;
(b) D.-H. Lee, J.-H. Kim, B.-H. Jun, H. Kang, J. Park, Y.-S. Lee, Org. Lett. 10 (2008)
1609e1612.
[6] W.J. Sommer, M. Weck, Adv. Synth. Catal. 348 (2006) 2101e2113.
[7] J. Schwarz, V.P.W. Bohm, M.G. Gardiner, M. Grosche, W.A. Herrmann,
W. Hieringer, G. Randaschl-Sieber, Chem. Eur. J. 6 (2000) 1773e1780.
[8] S. Tandukar, A. Sen, J. Mol. Catal. A: Chem. 268 (2007) 112e116.
[9] H. Qiu, S.M. Sarkar, D.-H. Lee, M.-J. Lin, Green Chem. 10 (2008) 37e43.
[10] Ö Aksin, H. Türkmen, L. Artok, B. Çetinkaya, C. Ni, O. Büyükgünor, E. Özkal,
J. Organomet. Chem. 691 (2006) 3027e3036.
[11] B. Karimi, D. Enders, Org. Lett. 8 (2006) 1237e1240.
[12] S.-M. Lee, H.-J. Yoon, J.-H. Kim, W.-J. Chung, Y.-S. Lee, Pure Appl. Chem. 79
(2007) 1553e1559.
4.2.10. Immobilisation of complex 5c (Synthesis of 6c)
A solution of the bis-NHC complex 5c (0.1 mmol) in anhydrous
chloroform (20 mL) was added to a dry Shlenk tube under nitrogen.
Silica gel (1.00 g, pre-dried) was added and the mixture was
refluxed for 20 h. After cooling to room temperature, the mixture
was filtered under a cone of nitrogen and washed with anhydrous
[13] J. Liu, J. Chen, J. Zhao, Y. Zhao, Z. Li, H. Zhang, Synthesis 17 (2003) 2661e2666.
[14] (a) J. Blumel, Coord. Chem. Rev. 252 (2008) 2410e2423;
(b) T. Posset, J. Blumel, J. Am. Chem. Soc. 128 (2006) 8394e8395.
[15] I. Lin, C. Vasam, Coord. Chem. Rev. 251 (2007) 642e670.
[16] H. Vinh Huynh, J. Wu, J. Organomet. Chem. 694 (2009) 323e331.
[17] (a) J. Schwarz, V.P.W. Bohm, M.G. Gardiner, M. Grosche, W.A. Herriman,
W. Hieringer, G. Randaschl-Sieber, Chem. Eur. J. 6 (2000) 1773e1780;
(b) D.S. Clyne, J. Jin, E. Genest, J.C. Galluci, T.V. Rajan-Babu, Org. Lett. 2 (2000)
1125e1128.
chloroform (3
ꢁ
50 mL) and anhydrous dichloromethane
(4 ꢁ 50 mL). The yellow solid was dried under reduced pressure and
stored under nitrogen at ꢂ4 ꢀC. The washings were concentrated
under reduced pressure and analysed by 1H NMR, which showed no
trace of the complex. ICP-AES analysis: calculated 0.98% Pd, found
0.89% Pd.
[18] G.A. Grasa, M.S. Viciu, J. Huang, C. Zhang, M.L. Trudell, S.P. Nolan, Organo-
metallics 21 (2002) 2866e2873.
[19] (a) A.C. Frisch, F. Rataboul, A. Zapf, M. Beller, J. Organomet. Chem. 687 (2003)
4.2.11. General procedure for Suzuki coupling reactions
403e409;
(b) A.C. Frisch, A. Zapf, O. Briel, B. Kayser, N. Shaikh, M. Beller, J. Mol. Catal. 214
(2004) 231e239;
(c) M. Eckhardt, G. Fu, J. Am. Chem. Soc. 125 (2003) 13642e13643;
(d) R. Singh, M.S. Viciu, N. Kramareva, O. Navarro, S.P. Nolan, J. Org. Lett. 7
(2005) 1829e1832;
(e) O. Navarro, H. Kaur, P. Mahjoor, S.P. Nolan, J. Org. Chem. 69 (2004)
3171e3180;
(f) N. Marion, P. De Fremont, I.M. Puijk, E.C. Ecarnot, D. Amoroso, A. Bell,
S.P. Nolan, Adv. Synth. Catal. 349 (2007) 2380e2384.
The haloarene (1 mmol) and catalyst (0.2 mol%) were added to
a dry Shlenk tube equipped with a magnetic stirring bar under
nitrogen. To this was added DMF/H2O (1:1, 2 mL), sodium carbonate
(2 mmol) and the boronic acid (1.4 mmol). The mixture was heated
at 80 ꢀC for 30 min, then cooled to room temperature, diluted with
acetone (3 mL) and filtered to remove the catalyst, base and
unwanted salts. The % conversion was established using gas chro-
matography with reference to dodecane as an internal standard.
Recycling was accomplished by washing the used catalyst with
water (10 mL) then with DMF/H2O (1:1, 10 mL). The catalyst, usually
somewhat discoloured, may then be used directly in the next
coupling reaction with a substantial decrease in activity being
observed.
[20] (a) C.J. O’Brien, A.B. E-Kantchev, G.A. Chass, N. Hadei, A.C. Hopkinson,
M.G. Organ, D.H. Setiadi, T.-H. Tang, D.-C. Fang, Tetrahedron 61 (2005)
9723e9735;
(b) N. Marion, P. De Fremont, I.M. Puijk, E.C. Ecarnot, D. Amoroso, A. Bell,
S.P. Nolan, Adv. Synth. Catal. 349 (2007) 2380e2384.
[21] (a) C.J. O’Brien, E.-A.B. Kantchev, C. Valente, N. Hadei, G.A. Chass, A. Lough,
A.C. Hopkinson, M.G. Organ, Chem. Eur. 12 (2006) 4743e4748;
(b) E.O. Kantchev, C.J. O’Brien, M. Organ, Angew. Chem. Int. Ed. 46 (2007)
2768e2813;
(c) U. Christmann, R. Vilar, Angew. Chem. Int. Ed. 44 (2005) 366.
[22] A.K. De Lewis, S. Caddick, F.G.N. Cloke, N.C. Billingham, P.B. Hitchcock,
J. Leonard, J. Am. Chem. Soc. 125 (2003) 10066e10073.
[23] M. Feuerstein, H. Doucet, M. Santelli, Tetrahedron Lett. 46 (2005) 1717e1720.
[24] T. Itoh, T. Mase, Tetrahedron Lett. 46 (2005) 3573e3577.
[25] K.L. Billingsley, K.W. Anderson, S.L. Buchwald, Angew. Chem. Int. Ed. 45 (2006)
3484e3488.
[26] N. Kudo, M. Perseghini, G.C. Fu, Angew. Chem. Int. Ed. 45 (2006) 1282e1284.
[27] Y. Zhang, L. Zhao, P. Patra, D. Hu, J. Ying, Nano Today 4 (2009) 13e20.
[28] J. Kim, B. Jun, J. Byun, Y. Lee, Tetrahedron Lett. 45 (2004) 5827e5831.
[29] G. Altenhoff, R. Goddard, C. Lehmann, F. Glorius, Angew. Chem. Int. Ed. 42
(2003) 3690e3693.
Acknowledgements
The authors wish to thank the EPSRC National Mass Spectrom-
etry Service Centre, University of Swansea for High Resolution Mass
Spectra. We are also very grateful to the Biological and Pharma-
ceutical Research Group (BPRSG), at Kingston University, for
financial support with this project in the form of a studentship to
LW.
[30] Our method of determining palladium leaching involved AES analysis of
mother liquors. The results of these suggested minimal leaching of palladium
(below the detection limits of the instrumentation used), although this
method does not eliminate the possibility that leached palladium may be
associated with the support material.
[31] N. Gurbuz, I. Ozdemir, T. Seckin, B. Cetinkaya, J. Inorg. Organomet. Polym. 14
(2004) 149e159.
[32] Y. Zhou, Y. Zhou, D. Ma, J. Liu, L. Li, T. Zhang, H. Zhang, Org. Biomol. Chem. 1
(2003) 1643e1646.
References
[1] (a) C.A. McNamara, M.J. Dixon, M. Bradley, Chem. Rev. 102 (2002) 3275e3300;
(b) N. End, K.U. Schoning, Top. Curr. Chem. 242 (2004) 241e271;
(c) N.E. Leadbetter, M. Marco, Chem. Rev. 102 (2000) 3217e3274.
[2] (a) V. Polshettiwar, C. Lenn, A. Fihri, Coord. Chem. Rev. 253 (2009)
2599e2626;
(b) M. Bortenschlager, N. Schoellhorn, A. Wittmann, R. Weberskirch, Chem.