Poly(norbornene)-supported N-heterocyclic carbenes as ligands in catalysis

Article abstract of DOI:10.1002/adsc.200606135

In this contribution, we present the synthesis of norbornene-supported N-heterocyclic (NHC) carbenes. These functionalized norbornenes were polymerized via ring-opening metathesis polymerization in a controlled fashion either before or after metalation with a variety of palladium and ruthenium precursors resulting in the formation of polymer-supported NHC-based metal catalysts. The activities of the palladium-based catalysts in the Suzuki-Miyaura, Sonogashira and Heck coupling reactions were studied in detail. In all cases, the polymeric catalysts demonstrated the same activity as their small molecule analogues. Furthermore, we carried out preliminary investigations into the stability of these catalysts using poisoning studies. A clear dependence of the stability of the polymer-supported catalysts on their palladium precursor was observed with palladium acetate-based polymeric NHC catalysts being the most stable. Finally, we have studied the reactivity of our supported NHC ruthenium complexes as catalysts for ring-closing metathesis. Again, in all cases good conversions were observed with comparable activities to other supported NHC-ruthenium catalysts. Lastly, we were able to remove the ruthenium catalysts from the solution quantitatively demonstrating the possibility of metal removal.

Full text of DOI:10.1002/adsc.200606135

FULL PAPERS
DOI: 10.1002/adsc.200606135
Poly(norbornene)-Supported N-Heterocyclic Carbenes as Ligands
in Catalysis
William J. Sommera and Marcus Wecka,*
a
School of Chemistry and Biochemistry, Georgia Institute of Technology, 770 State Street, Atlanta, GA 30332-0400, USA
Fax : (+1)-404-894-7452; e-mail: marcus.weck@chemistry.gatech.edu
Received: March 24, 2006; Revised: June 18, 2006; Accepted: July 13, 2006
Abstract: In this contribution, we present the synthe- of the stability of the polymer-supported catalysts on
sis of norbornene-supported N-heterocyclic (NHC) their palladium precursor was observed with palladi-
carbenes. These functionalized norbornenes were um acetate-based polymeric NHC catalysts being the
polymerized via ring-opening metathesis polymeri- most stable. Finally, we have studied the reactivity of
zation in a controlled fashion either before or after our supported NHC ruthenium complexes as cata-
metalation with a variety of palladium and rutheni- lysts for ring-closing metathesis. Again, in all cases
um precursors resulting in the formation of polymer- good conversions were observed with comparable ac-
supported NHC-based metal catalysts. The activities tivities to other supported NHC-ruthenium catalysts.
of the palladium-based catalysts in the Suzuki– Lastly, we were able to remove the ruthenium cata-
Miyaura, Sonogashira and Heck coupling reactions lysts from the solution quantitatively demonstrating
were studied in detail. In all cases, the polymeric cat- the possibility of metal removal.
alysts demonstrated the same activity as their small
molecule analogues. Furthermore, we carried out
preliminary investigations into the stability of these Keywords: catalysis; coupling; N-heterocyclic car-
catalysts using poisoning studies. A clear dependence bene; palladium; polymer
Introduction
or palladium sources resulting in the formation of
well-defined polymer-supported catalysts for a variety
Transition metal-based catalysis has been a corner of transformations ranging from carbon-carbon bond
stone for the synthesis of specialty chemicals for di- formations such as the Heck reaction or the Suzuki–
verse applications ranging from materials to pharma- Miyaura coupling to ring-closing metathesis
ceuticals.[1–3] The design of transition metal catalysts (RCM).[5,6] The use of these soluble polymer-support-
has to take into account several basic characteristics ed catalysts in the carbon-carbon bond forming trans-
including high efficiencies and selectivities as well as formations resulted in comparable activities and se-
economic and environmental considerations. An im- lectivities to their small molecule analogues.[7–13] Fur-
portant prerequisite for the commercialization of thermore, through the use of a highly controlled poly-
most metal-containing catalysts in the pharmaceutical merization technique, ring-opening metathesis poly-
and fine chemical industries is the easy removal of merization (ROMP),[14–19] we are able to tune the
any metal species from the product as well as, in solubility of the catalysts as well as the metal loading
some cases, the recyclability of these generally expen- via the formation of copolymers. This is the first re-
sive metal-containing catalysts. One strategy to ach- ported supported NHC carbene system where the
ieve these goals is the use of supported catalysts.[4]
Highly desirable would be to support a versatile port can be tuned in a systematic manner.
ligand that has been employed extensively in catalysis Stable N-heterocyclic carbenes, first synthesized by
metal type as well as the metal loading along the sup-
and can be metalated with a wide variety of metal Arduengo, have been studied extensively over the
precursor thereby allowing for the formation of a li- past decade.[6,20–24] This class of ligands has several ad-
brary of catalysts for diverse applications. Moving to- vantages over the closely related phosphine ligands
wards this goal, we report the synthesis of norbor- including their increased stability to high tempera-
nene-based monomers containing N-heterocyclic car- tures and air.[6,24–29] Over the past decade, metal com-
benes (NHC). These monomers can be metalated plexes containing NHC ligands have been utilized as
before or after polymerization with either ruthenium catalysts for a variety of transformations.[6,9,10,16] The
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majority of reports employ NHC-containing rutheni- Results and Discussion
um complexes as catalysts in olefin metathesis, with
RCM attracting the most attention.[7,16,20,30–33] Rutheni- To demonstrate the versatility of our strategy to pre-
um-based olefin metathesis catalysts containing NHC pare supported catalysts, we investigated the catalytic
ligands have the highest activities reported to activity of our novel polymer-supported catalysts in i)
date.[16,31] Furthermore, they are able to catalyze the Suzuki–Miyaura coupling of a library of aryl hal-
RCM reactions of sterically demanding compounds ides with phenylboronic acid, ii) the Sonogashira cou-
and are tolerant to a wide variety of functional pling of ethynyl(trimethyl)silane or 1-phenyl(trime-
groups resulting in the transformation of adducts that thylsilyl)acetylene with bromobenzenes, iii) the Heck
were unreactive to earlier olefin metathesis cata- reaction of n-butyl acrylate with benzyl halides and
lysts.[5,16]
iv) the ring-closing metathesis of diethyl diallylmalo-
The second carbon-carbon bond forming reaction nate.
using NHC ligands is based on palladium complexes
The synthesis of the four supported catalysts
that are able to catalyze coupling reactions such as (Scheme 1) commences with the formation of 2 that
the Heck or the Suzuki–Miyaura coupling reac- was synthesized by reacting 1 with N-mesitylimida-
tion.[34–38] The NHC ligands in these palladium cata- zole. The poly(norbornene)-supported Pd-NHC cata-
lysts are outstanding s-donors facilitating the oxida- lysts (6 and 7) were synthesized by treating 2 with
tive reactions of aryl halides to the palladium silver oxide yielding 3, followed by the addition of
center.[6,39–41] Furthermore, these catalysts do not re- either Pd
(OAc)2, Pd2dba3, or Pd2allyl2chloride2 to
G
quire the addition of excess ligand during the reac- yield 5a–c. Monomers 5a–c and 4 were then co-poly-
tion. Finally, the NHC ligands enhance the elimina- merized in ratios of 1:4 and 1:0, respectively, using the
tion of the final product from the palladium center as first generation Grubbs catalyst 10 to yield copoly-
a result of their bulky nature.[6,11,23,29,42,43]
mers 6a–c and 7a–c, respectively. For the synthesis of
The above described versatility, stability and unique the poly(norbornene)-supported catalyst 9, 2 and 4
catalytic properties of NHC-containing transition were co-polymerized in ratios of 1:9 with 10. A ruthe-
metal catalysts make them interesting candidates for nium monomer precursor for 9 could not be synthe-
supported catalysis. Over the past five years, NHCs sized since this monomer would be polymerize via
have been grafted onto different supports ranging ROMP during the metallation step.
from mesoporous silica to soluble poly
A
All catalytic reactions (Scheme 2) were carried out
While often successful, metal leaching and low metal under inert atmospheres using screw-cap vials and
loadings remain a major shortcoming for most sup- were repeated at least three times. The products were
1
ported catalysts. In particular the use of a soluble po- characterized by GC-MS and H and 13C NMR spec-
lymer support to immobilize N-heterocyclic carbene troscopy.
metal complexes has often been limited to one cata-
First, we investigated the activity of our palladium-
lytic moiety per polymer chain.[50,54–57] One exception supported catalysts in the Suzuki–Miyaura transfor-
to this is the work by Buchmeiser et al., whose group mation. The Suzuki–Miyaura transformation is an im-
reported the functionalization of insoluble monolithic portant tool for the synthesis of complex molecules
polymer discs with a variety of ruthenium catalysts with applications ranging from supramolecular
using elegant post-polymerization functionaliza- chemistry[64] to natural product synthesis.[65,66] To eval-
tions.[58] In this contribution, we report the synthesis uate the generality of our supported catalysts, aryl
of supported NHCs using poly(norbornene) as soluble chlorides with electron-donating or electron-with-
polymer support. Poly(norbornene) supports have the drawing groups as well as sterically hindered aryl
unique advantage that a) the support is often soluble chlorides were employed as reactants and coupled to
during the catalytic reaction but can be removed from aromatic phenylboronic acid derivatives. We carried
the reaction media and reused by simple precipitation out the Suzuki–Miyaura coupling reactions with all
methods, b) poly(norbornene)s can be synthesized via six different supported palladium catalysts (6a–c and
ROMP, a highly controlled, functional group-tolerant 7a–c) using a variety of reaction conditions to evalu-
and often living polymerization method that allows ate the different catalysts and to optimize reaction
for the formation of controlled architectures such as conditions.
random and block copolymers thereby allowing us to
The first system investigated consisted of a mixture
control catalyst density,[14–19] and c) as a result of its of Pd2dba3 and 8, with the carbene and ultimately the
functional group tolerance, ROMP can be carried out catalyst generated in situ. Cs2CO3 was used as the
on fully functional and characterized monomers base and the reaction was carried out in dioxane at
thereby eliminating low-yielding post-polymerization 808C using 4-chlorotoluene and phenylboronic acid as
reactions.[59–63]
reactants. For all substrates, the Suzuki–Miyaura cat-
alysis was complete within three hours with above
85% of products isolated for all transformations,
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Poly(norbornene)-Supported N-Heterocyclic Carbenes
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Scheme 1. Synthesis of the polymer-supported NHC-based catalysts 6, 7 and 9 utilized in this study.
demonstrating the catalytic activity of our in situ gen- Initially, potassium tert-butoxide was used as the base
erated catalyst. Control experiments using the same and isopropyl alcohol as the solvent. When the reac-
reaction conditions in the absence of either the palla- tions were carried out at room temperature, 70%
dium precursor or 8 did not result in the formation of conversions were observed after 24 h. Switching to
any product.
cesium carbonate as the base and dioxane as the sol-
The second Suzuki–Miyaura system studied em- vent and increasing the reaction temperature to 808C
ployed the fully palladated and characterized poly- allowed us to optimize the isolated yields. The catalyt-
mers 6 and 7. We investigated these supported cata- ic results of the reactions under these conditions are
lysts in the coupling of phenylboronic acid to a small outlined in Table 1. For all substrates, isolated yields
library of chloroaryl compounds. The chloroaryl com- of 80–99%, with the vast majority of reactions giving
pounds were chosen to investigate the influence of above 90%, were obtained within hours.
electron-donating/withdrawing groups as well as
The different functional groups on the phenyl chlor-
bulky substrates on the catalytic activity of 6 and 7. ides affect the conversions only slightly. Substrates
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Scheme 2. The catalytic reactions that have been employed to evaluate catalysts 6, 7 and 9.
Table 1. Catalytic results for the Suzuki–Miyaura coupling reaction.
Entry
Substrates
Product
Catalyst
Time [min]
Yield [%]
6a
6b
6c
7a
7b
7c
3099
3097
3099
45
45
45
1
99
94
97
6a 0
6b
6c
7a
7b
7c
6a
6b
6c
7a
7b
7c
6a
6b
6c
7a
7b
7c
3010
3099
3099
3095
3097
3098
13093
13088
13084
13090
13088
13081
12092
12088
12085
12092
12086
12085
2
3
4
5
6b
18084
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Poly(norbornene)-Supported N-Heterocyclic Carbenes
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containing electron-withdrawing groups such as CN during the catalysis.[48,61–63,67,68] We and others have
react slightly faster. In contrast, electron-donating shown for several of these cases that the supported
groups such as methoxy on the substrates slow down palladium species do not catalyze any carbon-carbon
the conversions. Nevertheless, even with substrates bond formations but that the leached palladium spe-
containing electron-donating groups, quantitative re- cies are the sole catalytically active species.[61–63,67] To
actions were still obtained within two to three hours. identify if the same restrictions are true for our poly-
The sterically bulky substrates such as dimethylbro- (norbornene)-supported NHC palladium complexes,
mobenzene slowed down the reaction and quantita- we investigated whether or not palladium leaches
tive conversions could not be obtained with any of during the reaction and if the polymer-supported spe-
the catalytic species.
cies are active during the catalysis. To identify the
Reactions using copolymers 7 yielded very similar nature of the catalytic species, we employed three cat-
results to the homopolymers suggesting that the spac- alyst poisons: a) highly cross-linked insoluble poly(vi-
ing of the metal complex does not affect its activity. nylpyridine) (PVPy), b) Quadra-Pureꢁ, a microporous
Overall the different polymer-supported catalysts resin metal scavenger that is especially sensitive to
showed very similar conversions compared to their palladium, and c) mercury(0).[61–63] PVPy is known to
small molecule analogues with catalysts 6a and 6b interact only with molecular palladium(0) by coordi-
being the most active ones.
nating to the metal center thereby removing the palla-
To investigate our catalytic system further, we per- dium from the reaction phase into the solid phase
formed kinetic studies on the most active polymer while Hg(0) has been reported to interact mainly with
supported catalyst, 6a, using chlorotoluene and phe- palladium nanoparticles by amalgamating the Pd par-
nylboronic acid as substrates. Samples of the reaction ticles thereby creating a catalytically inactive Hg/Pd
mixtures were taken every five minutes until com- amalgamate.[61–63,69–71] For both poisons, leached palla-
plete conversion. The kinetic data are outlined in dium species are the only metal species affected, that
Figure 1. The data clearly show that no induction is, if the catalysis is due to a leached Pd(0) species,
period is present.
both poisons will shut down any catalytic activity. In
Over the past two years, a variety of supported pal- contrast, ligand-protected Pd(0) species such as our
ladium catalysts have been shown to leach palladium supported NHC palladium complexes are untouched
by both poisons and catalysis should be possible.[62]
We carried out leaching test for the Suzuki cou-
plings using the same reaction conditions as outlined
above. When carrying out the PVPy poisoning test (a
ratio of 1:500 of Pd to PVPy was used) using catalyst
6a, we observed a decrease in activity with only 15%
conversions after 24 h (Table 2, Entry 1). Neverthe-
less, the catalyst stayed active during the whole ex-
periment. To test if this decrease was due to palladi-
um leaching off the supported NHC ligands or due to
the lack of accessibility of the reactants to the catalyst
sites, we carried out the same reaction using cross-
linked poly
N
N
not able to coordinate to leached palladium and
should therefore not inhibit the catalysis from any
leached metal species. When carrying out the catalysis
in the presence of 500 equivalents of poly(styrene) for
N
each catalytic moiety, we observed again a dramatic
drop in catalyst activity with a conversion of approxi-
mately 15% after 24 h (Table 2, Entry 2), that is, the
same drop in activity was observed as described
above for the poly(vinylpyridine) poisoning experi-
ment. This result suggests that the reduced activity in
the poly(vinylpyridine) leaching test is most likely not
due to metal leaching during the catalysis but reduced
accessibility of the active sites in this case.
When carrying out the mercury test with our poly-
mer-supported palladium NHC catalyst 6a, we ob-
served 90% conversions of the phenylboronic acid to
the corresponding biphenyl, the homocoupling prod-
Figure 1. Kinetic study for a) the Suzuki–Miyaura reaction
with 6a (”), b) the Sonogashira reaction with 6a ( ), c) the
~
Suzuki–Miyaura reaction with 6a in the presence of Quadra-
Pureꢁ ( ), d) the Sonogashira reaction with 6a in the pres-
&
ence of QuadraPureꢁ ( ), and e) the Suzuki–Miyaura reac-
^
tion with 6a with the addition of QuadraPureꢁ after 20min
(+).
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William J. Sommer and Marcus Weck
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Table 2. Leaching tests results.
Entry
Substrates
Products
Catalyst
6a
Poison
PVPy (500 mol%)
Poly(styrene) (500 mol%)
Yield [%]
1
2
3
4
5
6
7
8
9
<15
<15
95
0
6a
ACHTREUNG
6a
Mercury (500 mol%)
Mercury (500 mol%)
Mercury (500 mol%)
Mercury (500 mol%)
QuadraPureꢁ (2 equivs.)
QuadraPureꢁ (2 equivs.)
QuadraPureꢁ (2 equivs.)
None
Pd(0)
6a
0
98
89
79
0
6a
6b
6c
uct (Table 2, Entry 3). To investigate this unexpected no further PVPy or Hg(0) leaching tests were carried
result we carried out a series of control experiments. out on any other supported catalysts 6b, c or 7a–c.
First, we carried out the reaction without the support-
The next leaching tests that were carried out used
ed palladium complex, that is, only the reactants and QuadraPureꢁ to trap leached palladium. Quadra-
the mercury (Table 2, Entry 4) were added to the re- Pureꢁ is a microporous resin that scavenges different
action flask. No conversions were observed. When metals and is especially efficient for trapping palladi-
using a non-supported Pd(0) source as catalyst, either um.[76] When carrying out the Suzuki–Miyaura cou-
Pd on carbon or Pd2dba3, and mercury (Table 2, pling reaction in the presence of QuadraPureꢁ, 89%
Entry 5) we again observed no conversions. This and 79% conversions were observed after 120min (in
proves that Pd(0) metal is not active in the presence comparison, the reactions without the addition of
of mercury. In the literature a variety of reductants in- QuadraPureꢁ gave 99% yields after 30min) with 6a
cluding mercury(0) are described to catalyze the ho- and 6b, respectively, while no conversions were ob-
mocouplings of arylboronic acids as well as aryl io- tained when using 6c as catalyst. Kinetic studies using
dides or bromides.[72–75] To investigate if the mercury 6a and QuadraPureꢁ (Figure 1 squares and crosses)
acts as a reductant in our poisoning tests, we carried showed that the presence of QuadraPureꢁ slows the
out the catalytic reaction without the addition of reaction but does not inhibit conversion. The conver-
chlorotoluene, that is, only phenylboronic acid, the sion obtained was 86% within 75 min. The presence
supported catalyst and mercury were present during of QuadraPureꢁ slowed down the conversion but the
the reaction. This experiment resulted in 95% conver- kinetic curve looks very similar to the one without
sion (Table 2, Entry 6) of the phenylboronic acid to the poison present. By comparing the kinetic data
the corresponding biphenyl. These results suggest that with and without QuadraPureꢁ present during the cat-
the mercury acts as a reductant in our poisoning tests alysis, one can clearly see that the poison, while slow-
but also that the supported palladium catalyst seems ing down the catalysis, does not prevent the catalytic
to be stable during the poisoning test and that no pal- transformation, suggesting that the active species for
ladium leaches out. If leaching would have occurred the catalysis is, at least in part, the polymer-supported
and the catalysis (homocoupling) would have been palladium complex.
due to a leached Pd species, the Hg should have amal-
In summary, the poisoning studies show that 6c
gamized the leached Pd species resulting in no cataly- leaches under the Suzuki–Miyaura reaction conditions
sis. Based on the PVPy and Hg(0) poisoning tests, no and that the metal complex 6c alone might not be a
definite conclusions can be drawn regarding the sta- catalytically active species. More likely, the catalytic
bility and the potential palladium leaching of the po- species is a leached palladium species off 6c whereby
lymer-supported catalyst 6a. Following these results 6c serves only as a palladium reservoir. In contrast,
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Poly(norbornene)-Supported N-Heterocyclic Carbenes
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our studies suggest that the metal complexes support- and redissolved in DMF. A 1H NMR was taken.
ed on 6a and b are catalytically active. While our Again, no sign of polymer degradation was noticed.
leaching studies cannot exclude small amounts of While we cannot exclude a small percentage of back-
leached species for 6a and b, they show that Quadra- bone degradation or cross-linking, these studies dem-
Pureꢁ does not shut down the catalysis, proving that onstrate that the polymer backbone does not decom-
both polymers are catalytically active species in the pose significantly during the catalysis and/or the
Suzuki–Miyaura coupling.
work-up. However, a weak coordination of leached
We also carried out a recycling experiment for the metal species onto the polymer resulting in metal-
Suzuki–Miyaura coupling of chlorobenzene with phe- based cross-linking cannot be detected with this
nylboronic acid using similar conditions as described method.
above. After complete reaction (confirmed by GC),
To expand the scope of our catalysts, we investigat-
the reactants and products were distilled off and the ed the Sonogashira coupling using the most active
resulting polymeric residue dried. The polymeric resi- polymeric catalysts 6a (Table 3). Because of the im-
due was then reused for the same catalytic transfor- portance of alkyne functionalities for a wide range of
mation using exactly the same reaction conditions. natural compounds as well as in the synthesis of
The polymer became less soluble after the first cycle highly conducting materials,[78–82] the Sonogashira re-
yielding 80% conversions after 90min. Following the
action between an alkyl halide and a terminal alkyne
same isolation procedure after the second cycle, the is the method of choice to incorporate alkyne func-
polymer became only slightly soluble in DMF for the tionalities in aromatic systems. The reaction was de-
third reuse and yielded only 44% conversions after veloped in 1975 by Sonogashira using a mixture of
90min. We have observed the same solubility prob-
lems with other poly(norbornene)-based catalyst sup- been improved steadily over the past 30years.
palladium and copper iodide as the catalyst and has
[83–86]
In
ports.[77] To investigate if any changes of the polymer this work, we investigate two types of Sonogashira re-
backbone during or after catalysis are the reason for actions. The first one consisted of the coupling of a
this reduced solubility, we carried out a series of in silyl-terminated acetylene to an aryl bromide. As re-
situ 1H and 13C NMR experiments. The Suzuki– actants, we employed a silane-protected acetylene and
Miyaura transformation of phenylboronic acid with a small library of aryl bromides. Diisopropylamine
chlorotoluene was chosen for the in situ NMR studies. (ten equivalents) was used as the base and tetrahydro-
A preliminary spectrum of 6b was taken in deuterated furan as the solvent. The base, solvent and CuI (10
DMF at 858C. The reagents were then added using a mol%) were added to the reaction vessel at room
catalyst loading of 50mol% and a cesium carbonate
loading of 150mol%. The reaction was carried out
temperature. When subjected to these conditions, all
aryl halides were converted to the corresponding
for 24 h and monitored via NMR. No sign of polymer products in 83–100% isolated yields in 120–150 min
backbone degradation was noticed, that is, no changes (Table 3). As expected, the reaction using bromoben-
in the olefin signals were observed which would be a zaldehyde as reactant was the fastest with 100% con-
sign of backbone degradation. The polymer was then versions in 120min. The more sterically hindered 2-
recovered through precipitation into cold methanol bromotoluene reacted in 83% conversion within
Table 3. Catalytic results for the Sonogashira coupling reaction.
Entry
Substrates
Products
Catalyst
Time [min]
15010
Yield [%]
1
2
6a
6a
0
0
12010
3
6a
15083
4
5
6a
6a
45
25
96
99
6
6a
6095
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William J. Sommer and Marcus Weck
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150min compared to the non-hindered bromoben-
ing, we wanted to investigate if the polymer-support-
zene which had quantitative conversion within the ed catalyst is catalytically active or if all activity stems
same time. from the leached palladium species. We carried out a
The second Sonogashira reaction consisted of the in Heck catalysis experiment in the presence of the
situ deprotection of a silane-protected acetylene and QuadraPureꢁ poison. The reaction conditions were:
the coupling of the resulting acetylene with a different iodobenzene and n-butyl acrylate as reactants, tri-
aryl bromide.[13] Cesium carbonate (two equivalents) ethylamine (two equivalents) as the base and DMF as
which was employed as the base and deprotection the solvent. The reaction yielded 44% conversions
agent, dimethylacetamide (DMAc), and CuI (2 after 24 h. While this result is inconclusive, the ob-
mol%) were added to a screw-cap vial and stirred at served palladium black formation during the catalysis
808C. Similar yields but faster reaction rates com- in combination with the lower conversions suggests
pared to method A were obtained. The 4-bromoben- that the vast majority of catalytic activity stems from
zaldehyde showed quantitative conversions within leached palladium species and not from 6a.
25 min while the bromobenzene and the 2-bromoto-
luene had 96% and 95% conversions within 45 and (norbornene) NHC polymers was ruthenium. The re-
60min respectively. These results compare favorably sulting polymer-supported ruthenium complex (9) was
with those reported by Nolan et al. using the small investigated as active catalysts for olefin metathesis,
molecule analogue to 6a.[13] in particular ring-closing metathesis (RCM).[89] To
The second metal that was supported on our poly-
To verify the robustness of our catalysts in the So- study the activity of these supported catalysts, the
nogashira coupling reactions, leaching tests were car- RCM of diethyl diallylmalonate in dichloromethane
ried out using 6a and QuadraPureꢁ as the poison. We at 458C was investigated.[45,90,91] Under these reaction
employed benzyl bromide and silane-protected acety- conditions, diethyl diallylmalonate was converted in
lene as reactants, CuI (10mol%), diisopropylamine
95% yield to its corresponding RCM product, cyclo-
(ten equivalents) as the base and THF as the solvent. pent-3-ene-1,1-dicarboxylic acid diethyl ester, within
After 24 h, 100% conversions were observed. The 20min using 5.0mol% of 9 (Table 1, Entry 15). This
second Sonogashira reaction tested consisted of bro- activity is comparable to other supported Grubbs cat-
mobenzene and trimethyl(phenylethynyl)silane as re- alyst analogues of 10 that convert diethyl diallylmalo-
actants, cesium carbonate (2 equivalents) as the base, nate using similar catalyst loadings with the same con-
CuI (2 mol%) and dimethylacetamide. Again, the re- versions in the same time frame,[45,90,91] demonstrating
action yielded 100% conversions after 24 h. To deter- that our supported catalyst is an active olefin meta-
mine the activity of 6a in the presence of Quadra- thesis catalyst.
Pureꢁ in more detail, we carried out kinetic studies
The polymeric catalyst 9 can be removed from the
with and without the poison. The results are shown in reaction mixture after complete catalysis using basic
Figure 1. As was the case for the Suzuki coupling poi- precipitation methods into cold methanol. To eluci-
soning studies, the Sonogashira reaction slows down date whether any ruthenium leached into the reaction
slightly in the presence of QuadraPureꢁ. The transfor- solution and if the polymeric catalysts can be re-
mation takes 35 min to reach 80% conversions with-
out the poison while in the presence of the poison the ses (ICP) of the reaction solutions after the removal
same conversions are obtained after 50min. There- of the polymeric catalysts. The elemental analyses
moved quantitatively, we carried out elemental analy-
fore, in analogy to the Suzuki studies outlined above, showed no traces of ruthenium in the reaction solu-
the Sonogashira poisoning studies demonstrate the tion demonstrating the quantitative recovery of the
stability of our catalyst under Sonogashira reaction polymer and thereby the quantitative removal of the
and suggest that the active species for the catalysis is metal species from the reaction mixture. Blechert and
at least partially the polymer-tethered palladium com- co-workers using poly(oxanorbornene) as a support,
plex.
reported similar results using a Grubbs-Hoveyda cata-
The third palladium-catalyzed transformation stud- lyst.[92]
ied was the Heck coupling reaction. This reaction has
been widely studied since its first report in the early
1970s.[87,88] Again, we employed our best catalyst, 6a Conclusions
(2 mol%), for all studies. We used triethylamine (two
equivalents) as the base and iodobenzene (one equiv- In this contribution, we have reported the synthesis of
alent) and n-butyl acrylate (1.5 equivalents) as sub- a new class of polymer-supported N-heterocyclic car-
strates. At 1208C, the catalysis proceeded in 30min
bene ligands, their metallation, before and after poly-
with 99% conversions. However, after the reaction, merization, and their use as supported catalysts for a
some palladium black was observed at the bottom of variety of carbon-carbon bond formations. We dem-
the flask indicating leaching of palladium from the onstrated the versatility of our supported catalysts by
complex. As these results clearly showed metal leach- investigating the catalytic activity of all complexes in
2108
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2101 – 2113

Poly(norbornene)-Supported N-Heterocyclic Carbenes
FULL PAPERS
1
yield: 1.6 g (68%). H NMR (CDCl3): d=10.57 (s, 1H), 7.47
(s, 1H), 7.12 (s, 1H), 6.99 (s, 2H), 6.10(m, 2H), 4.07 (t, J=
6.6 Hz, 2H), 3.03 (m, 1H), 2.33 (s, 5H), 2.07 (s, 9H), 1.91
(m, 1H), 1.75–1.58 (m, 5H), 1.54 (m, 1H), 1.51 (m, 1H),
1.46–1.19 (m, 14H); 13C NMR (CDCl3): d=176.2, 139.4,
138.6, 135.2, 134.5, 129.2, 64.5, 51.7, 46.2, 43.0, 41.9, 31.2,
30.3, 29.5, 28.8, 26.0, 25.5, 21.2, 17.0; MS (ESI): m/z=477.53
(M+, calcd.: 477.35); anal. calcd. for C31H45BrN2O2: C 66.77,
H 8.13, N 5.02; found: C 66.69, H 8.15, N 5.11.
a wide array of reactions ranging from RCM to
Suzuki couplings. For all transformations studied, our
catalysts show high activities that are comparable to
their small molecule analogues. We have shown by
using poisoning studies that the stability of our palla-
dium-based polymeric catalysts depends on the li-
gands around the palladium center. While for the po-
lymer-supported palladium acetate-based NHC com-
plexes the catalytic activity in the Suzuki and Sonoga-
shira couplings stems mainly from the polymer-teth-
ered complexes, palladium dba-based NHC
complexes decompose under these reaction condi-
tions. Finally, for the ring-closing metathesis we dem-
onstrated the ability to remove the polymeric cata-
lysts from the reaction mixture thereby ensuring the
removal of any undesirable metal species from the
product obviating extensive purification steps.
Synthesis of exo-Ag-di-bicyclo[2.2.1]hept-5-ene-2-
carboxylic Acid 11-[3-(2,4,6-Trimethylphenyl)-2,3-
dihydroimidazol-1-yl]undecyl Ester Silver Dibromide
(3)
G
In a screw-cap vial, 2 (306 mg, 0.54 mmol), silver oxide
(64 mg, 0.27 mmol) and CH2Cl2 (10mL) were combined.
The solution was stirred for three hours during which a
white precipitate formed. The solution was then filtered
through celite and the solvent removed under vacuum to
afford a brown oil; yield: 320mg (89%). 1H (CDCl3): d=
7.12 (m, 2H), 6.99 (s, 2H), 6.10(m, 2H), 4.07 (t, J=6.6 Hz,
2H), 3.03 (m, 1H), 2.33 (s, 5H), 2.07 (s, 9H), 1.91 (m, 1H),
1.75–1.58 (m, 5H), 1.54 (m, 1H), 1.51 (m, 1H), 1.46–1.19
(m, 14H); 13C NMR (CDCl3): d=177.2, 139.4, 138.6, 135.5,
134.4, 129.2, 64.2, 51.1, 46.9, 43.5, 41.7, 31.4, 30.4, 29.0, 28.3,
26.6, 25.0, 23.5, 21.1, 17.1; MS (ESI): m/z=1061.92 (M+,
calcd.: 1061.65); anal. calcd. for C62H90Ag2Br2N4O4: C 55.95,
H 6.82, N 4.21; found: C 56.03, H 6.85, N 4.42.
Experimental Section
General Experimental Conditions
All reactions with air- and moisture-sensitive compounds
were carried out under a dry nitrogen/argon atmosphere
using an MBraun UniLab 2000 dry box and/or standard
Schlenk line techniques. THF, CH2Cl2, toluene, 1,4-dioxane
and hexanes were distilled from sodium and benzophenone.
Benzyl alcohol and methyl acetate were distilled from calci-
um hydride. Pd(OAc)2, first generation Grubbs catalyst, and
N
Synthesis of exo-Bicyclo[2.2.1]hept-5-ene-2-carboxylic
N
all bases were obtained from commercial sources and gener-
ally used without further purification. The syntheses of mesi-
tylimidazole and 1 were carried out following published pro-
cedures.[93,94] Gas-chromatographic analyses were performed
on a Hewlett Packard G1800 A GCD system GC-MS. 1H
(300 MHz) and 13C NMR (75 MHz) spectra were recorded
on a Varian Mercury VX instrument. All spectra were refer-
enced to residual proton solvent. Mass spectral analyses
were provided by the Georgia Tech Mass Spectrometry Fa-
cility using a VG-70se spectrometer. Gel-permeation chro-
matography (GPC) analyses were carried out using a Waters
1525 binary pump coupled to a Waters 2414 refractive index
Acid Octyl Ester (4)
In a round-bottom flask equipped with a condenser, exo-
bicyclo[2.2.1]hept-5-ene-2-carboxylic acid (2.44 g,
AHCTREUNG
17.6 mmol), 1-octanol (2.8 mL, 17.6 mmol), dicyclohexyldi-
amine (3.6 g, 17.6 mmol), a catalytic amount of diaminopyri-
dine (100 mg) and CH2Cl2 (20mL) were combined and re-
fluxed overnight. The solution was then filtered through
celite and the solvent removed under vacuum to yield a
yellow solution which was further purified using column
chromatography (eluent: hexanes) to afford a colorless oil;
yield: 3.74 g (85%). 1H NMR (CDCl3): d=6.12 (m, 2H),
4.07 (t, 2H, J=6.6 Hz), 3.03 (m, 1H), 2.92 (m, 1H), 2.21 (m,
1H), 1.68–1.49 (m, 3H), 1.41–1.24 (m, 12H), 0.88 (t, 3H, J=
7.1 Hz); 13C NMR (CDCl3): d=176.2, 137.8, 135.6, 46.8,
46.2, 43.2, 41.5, 33.9, 29.2, 28.9, 28.5. MS (ESI): m/z=250.20
(M+, calcd.: 250.38); anal. calcd. for C16H26O2: C 76.75, H
10.47; found: C 76.69, H 10.51.
detector. The GPC was calibrated using poly(styrene) stand-
A
ards on a Styragelꢁ HR 4 and HR 5E column set with
CH2Cl2 as an eluent. All GPC experiments were carried out
with a flow rate of 1 mLminꢀ1. Elemental analyses were car-
ried out by either Atlantic Microlabs, Norcross GA (CHN
analyses) or Galbraith Laboratories, Inc., TN (determina-
tion of the metal content).
Synthesis of exo-Pd
N
N
carboxylic Acid 11-[3-(2,4,6-Trimethylphenyl)-2,3-
dihydroimidazol-1-yl]undecyl Ester (5a)
Synthesis of exo-1-{11-(Bicyclo[2.2.1]hept-5-ene-2-
carbonyloxy)undecyl}-3-(2,4,6-trimethylphenyl)-3H-
imidazol-1-ium (2)
G
Under an inert atmosphere, 3 (200 mg, 0.4 mmol), Pd(OAc)2
A
(141 mg, 0.6 mmol) and CH2Cl2 (10mL) were combined in a
screw-cap vial. The solution was stirred overnight yielding a
silver solid. The reaction mixture was filtered through celite
to yield a yellow solution. The solvent was removed and the
resulting yellow oil was further purified via column chroma-
In a round-bottom flask equipped with a condenser, N-mesi-
tylimidazole (950mg, 5.1 mmol), 1 (1.9 g, 5.1 mmol) and tol-
uene (50mL) were combined. The reaction mixture was re-
fluxed for 72 h. The solvent was removed yielding a brown
oil which was purified using column chromatography
(eluent: 1:20!1:1 ethanol:hexanes) to afford a yellow oil;
tography (eluent 1:1 hexanes:ethylacetate) to afford
a
yellow solid; yield: 190mg (60%). 1H NMR (CDCl3): d=
Adv. Synth. Catal. 2006, 348, 2101 – 2113
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www.asc.wiley-vch.de
2109

William J. Sommer and Marcus Weck
FULL PAPERS
6.88 (s, 2H), 6.10(m, 2H), 4.07 (t, J=6.6 Hz, 2H), 3.03 (m,
1H), 2.33 (s, 5H), 2.07 (s, 9H), 1.91 (m, 1H), 1.75–1.58 (m,
5H), 1.54 (m, 1H), 1.51 (m, 1H), 1.46–1.19 (m, 20H);
13C NMR (CDCl3): d=220.2, 176.2, 139.7, 137.9, 135.1,
134.4, 129.6, 64.1, 51.2, 46.7, 43.4, 41.5, 31.6, 30.5, 29.9, 28.2,
26.4, 25.6, 21.5, 17.0. MS (ESI): m/z=701.3 (M+, calcd.:
700.27); anal. calcd. for C35H50N2O6Pd: C 59.95, H 7.19, N
4.00; found: C 60.21, H 7.23, N 3.92.
1H NMR, a few drops of ethyl vinyl ether were added to ter-
minate the polymerization. The polymer was purified by re-
peated precipitations into cold methanol followed by centri-
fugation at 400 rpm for ten minutes. The purification proce-
dure was repeated until the resulting methanol solution
became colorless. The methanol was then decanted and the
resulting polymer dried under vacuum.
1
Polymer 6a: H NMR (CDCl3): d=6.89–6.85 (br s, 2H),
5.53–5.21 (br m, 2H), 4.07–3.99 (br m, 2H), 3.03 (m, 1H),
2.33–2.30(br s, 5H), 2.70–1.99 (br s, 9H), 1.91 (m, 1H),
1.75–1.58 (br m, 5H), 1.54 (m, 1H), 1.51 (m, 1H), 1.46–1.19
(br m, 20H); 13C NMR (CDCl3): d=184.0, 175.2, 175.0,
169.2, 145.5, 143.9, 137.4, 136.2, 128.4, 126.3, 126.0, 122.2,
114.4, 104.5, 50.2, 47.1, 43.1, 40.9, 36.4, 30.8, 29.7, 29.4, 25.2,
14.4, 9.2.
Synthesis of exo-Pd
2-carboxylic Acid 11-[3-(2,4,6-Trimethylphenyl)-2,3-
dihydroimidazol-1-yl]undecyl Ester (5b)
A
G
Under inert atmosphere, 3 (57 mg, 0.04 mmol), Pd2(allyl)2Cl2
A
(16 mg, 0.04 mmol) and THF (5 mL) were combined in a
screw-cap vial. The solution was stirred for five hours form-
ing a silver precipitate. The reaction mixture was filtered
through celite to yield a yellow solution. The solvent was re-
moved under vacuum to yield yellow oil. The oil was
washed three times with hexanes and dried under vacuum
forming a yellow foam; yield: 16 mg (64%). 1H NMR
(CDCl3): d=7.06 (d, 2H), 6.85 (m, 2H), 6.12 (m, 2H), 4.52
(m, 4H), 4.06 (t, J=6.5 Hz, 2H), 3.71 (m, 1H), 3.06 (d, 1H),
3.02 (m, 1H), 2.72 (d, 1H), 2.33 (s, 5H), 2.07 (s, 9H), 1.91
(m, 1H), 1.75–1.55 (m, 6H), 1.54 (m, 1H), 1.51 (m, 1H),
1.46–1.16 (m, 14H); 13C NMR (CDCl3): d=180.1, 174.5,
139.8, 136.7, 135.2, 132.6, 122.4, 121.5, 114.8, 68.5, 64.3, 49.5,
43.2, 42.4, 30.9, 29.4, 29.1, 28.5, 26.3, 25.9, 25.8, 25.5, 21.0;
MS (ESI): m/z = 660.35 (M+, calcd.: 660.27); anal. calcd.
for C34H51ClN2O2Pd: C 61.72, H 7.77, N 4.23; found: C
61.03, H 7.74, N 4.95.
1
Polymer 6b: H NMR (CDCl3): d=7.06–7.03 (br m, 2H),
6.88–6.83 (br m, 2H) 5.55–5.20(br m, 2H), 4.52–4.50(br m,
4H), 4.07–4.02 (m, 2H), 3.71 (m, 1H), 3.07–3.05 (m, 1H),
3.02 (m, 1H), 2.74 (m, 1H), 2.33–2.28 (br s, 5H), 2.05 (br s,
9H), 1.91 (m, 1H), 1.75–1.53 (m, 6H), 1.52 (m, 1H), 1.50–
1.48 (br m, 1H), 1.44–1.13 (br m, 14H); 13C NMR (CDCl3):
d=180.1, 139.0, 136.4, 135.8, 135.5, 132.1, 128.9, 122.0, 121.1,
114.4, 73.2, 50.3, 47.4, 43.4, 40.2, 36.4, 30.2, 29.8, 29.4, 25.4,
14.3, 8.9.
1
Polymer 6c: H NMR (CDCl3): d=7.67–7.64 (br m, 4H),
7.34–7.30(br m, 6H), 7.12–7.10(br m, 2H), 6.95 (m, 2H),
6.86 (m, 2H), 6.84–6.81 (br m, 2H), 5.61–5.54 (br m, 2H)
4.10 (m, 2H), 3.05 (m, 1H), 2.33–2.29 (br s, 5H), 2.09–2.06
(br s, 9H), 1.95–1.93 (br m, 1H), 1.75–1.58 (m, 5H), 1.55–
1.53 (m, 1H), 1.51–1.49 (m, 1H), 1.45–1.16 (m, 14H);
13C NMR (CDCl3): d=189.1, 179.8, 145.8, 139.2, 136.8,
135.8, 135.3, 132.1, 129.5, 128.9, 125.3, 122.1, 114.4, 71.2,
50.8, 46.9, 43.2, 39.8, 36.3, 30.8, 30.0, 29.6, 25.3, 13.5, 8.9.
Synthesis of exo-Pddba-bicyclo[2.2.1]hept-5-ene-2-
carboxylic Acid 11-[3-(2,4,6-Trimethylphenyl)-2,3-
dihydroimidazol-1-yl]undecyl Ester (5c)
N
1
Polymer 7a: H NMR (CDCl3): d=6.89–6.85 (br s, 2H),
5.53–5.21 (br m, 10H), 4.07–3.99 (br m, 10H), 3.03 (m, 5H),
2.33–2.26 (br s, 25H), 2.07–1.92 (br s, 45H), 1.91 (m, 4H),
1.75–1.58 (br m, 5H), 1.56–1.52 (m, 5H), 1.51–1.49 (m, 5H),
1.46–0.63 (br m, 100H); 13C NMR (CDCl3): d=184.0, 175.2,
175.0, 169.2, 151.8, 145.5, 143.9, 137.4, 136.2, 128.4, 126.5,
126.2, 122.2, 114.4, 104.8, 113.7, 64.8, 50.0, 48.7, 47.1, 43.1,
40.9, 36.4, 30.8, 29.7, 29.4, 26.3, 25.2, 14.4, 9.1.
Under inert atmosphere, 3 (76 mg, 0.06 mmol), Pd2dba3
(52 mg, 0.06 mmol) and THF (5 mL) were combined in a
screw-cap vial. The solution was stirred for five hours form-
ing a black precipitate. The reaction mixture was filtered
through celite to yield a black solution. The solvent was re-
moved under vacuum to yield a black solid. The solid was
washed three times with hexanes and dried under vacuum
to afford a brown solid; yield: 21 mg (45%). 1H NMR
(CDCl3): d=7.65 (d, 4H), 7.31 (m, 6H), 7.12 (d, 2H), 6.95
(m, 2H), 6.86 (m, 2H), 6.82 (m, 2H), 6.11 (m, 2H), 4.08 (t,
J=6.4 Hz, 2H), 3.05 (m, 1H), 2.33 (s, 5H), 2.07 (s, 9H),
1.93 (m, 1H), 1.75–1.58 (m, 5H), 1.55 (m, 1H), 1.51 (m,
1H), 1.46–1.20(m, 14H); 13C NMR (CDCl3): d=189.1,
179.7, 145.8, 139.3, 136.2, 135.8, 135.3, 132.1, 129.5, 127.9,
125.3, 122.1, 115.4, 71.8, 50.8, 46.9, 43.2, 39.8, 36.3, 30.8, 30.0,
29.6, 25.3, 24.1; MS (ESI): m/z=818.30(M +, calcd.: 818.41);
anal. calcd. for C48H59N2O3Pd: C 70.44, H 7.27, N 3.42;
found C 70.33, H 7.32, N 3.15.
1
Polymer 7b: H NMR (CDCl3): d=7.08–7.03 (br m, 2H),
6.89–6.80(br m, 2H) 5.55–5.22 (br m, 2H), 4.52–4.50(br m,
4H), 4.07–4.02 (m, 10H), 3.71 (m, 1H), 3.07–3.03 (m, 5H),
2.33–2.23 (br m, 25H), 2.05–1.89 (br m, 49H), 1.75–1.49 (br
m, 6H), 1.48–1.43 (br m, 5H), 1.41–0.59 (br m, 100H);
13C NMR (CDCl3): d=180.1, 139.0, 136.4, 135.8, 135.5,
132.1, 128.9, 122.0, 121.1, 114.4, 73.2, 50.3, 48.3, 47.4, 43.4,
40.8, 40.2, 36.4, 31.1, 30.2, 29.8, 29.4, 26.7, 25.4, 14.3, 8.9, 8.6.
1
Polymer 7c: H NMR (CDCl3): d=7.67–7.64 (br m, 4H),
7.35–7.30(br m, 6H), 7.12–7.10(br m, 2H), 6.98–6.94 (br m,
2H), 6.86 (m, 2H), 6.84–6.81 (br m, 2H), 5.61–5.54 (br m,
2H) 4.10–4.01 (br m, 11H), 2.33–2.21 (br s, 25H), 2.09–1.98
(br s, 49H), 1.75–1.52 (m, 7H), 1.45–0.86 (m, 100H);
13C NMR (CDCl3): d=189.1, 179.8, 145.8, 139.2, 136.8,
135.8, 135.3, 132.1, 129.5, 128.9, 125.3, 122.1, 114.4, 71.2,
50.8, 46.9, 42.1, 43.2, 39.8, 37.3, 36.3, 35.2, 32.1, 31.4, 30.8,
30.0, 29.6, 28.4, 27.6, 25.3, 20.1, 13.5, 9.2, 8.7.
General Polymerization Procedure for the Synthesis
of Polymers 6–9
1
Polymer 9: H NMR (CDCl3): d=7.20–7.03 (br m, 4H),
The respective monomer(s) was dissolved in CDCl3 fol-
lowed by the addition of the desired amount of catalyst. The
polymerization was monitored by NMR spectroscopy. After
the polymerization was complete as determined by
6.86–6.82 (br m, 2H) 5.53–5.21 (br m, 2H), 4.55–4.50(br m,
4H), 4.10–4.02 (m, 10H), 3.71 (m, 1H), 3.12–3.05 (m, 5H),
2.45–2.23 (br m, 25H), 2.04–0.59 (br m, 184H); 13C NMR
2110
www.asc.wiley-vch.de
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2006, 348, 2101 – 2113

Poly(norbornene)-Supported N-Heterocyclic Carbenes
FULL PAPERS
(CDCl3): d=190.4, 150.1,139.0, 136.4, 135.8, 135.5, 132.1,
128.9, 126.4, 123.4, 122.2, 121.1, 114.4, 73.1, 52.3, 49.1, 47.3,
42.7, 40.5, 40.1, 31.5, 30.1, 29.9, 29.4, 28.7, 25.1, 11.3, 9.3, 8.2.
equivalents) and DMF. After stirring for 10min, the cesium
carbonate (1.5 equivalents) was added in one portion to the
reaction mixture. The solution was heated to 808C and sam-
ples for GC analysis were taken every 5 min for the first
60min, then every 30min for the next 120min.
General Procedure for the Suzuki–Miyaura Reaction
Under an atmosphere of nitrogen, a screw-cap vial was
loaded with the desired catalyst, phenylboronic acid, chloro-
toluene and DMF. After stirring for ten minutes, the cesium
carbonate was added in one portion. The solution was
heated to 808C until completion. The product was then puri-
fied via column chromatography.
General Procedure for Precipitation Process
The precipitation of the polymer consists on reducing the
amount of solvent to a minimum and adding cold methanol.
The formed precipitate is then centrifuged for 10min and
the remaining solution decanted.
General Procedure (A) for the Sonogashira Reaction
Acknowledgements
Under an inert atmosphere, a screw-cap vial was loaded
with the desired catalyst, bromobenzene, ethynyltrimethylsi-
lane, CuI, diisopropylamine and THF. The solution was stir-
red at room temperature until completion. The product was
purified via column chromatography.
The US DOEOffice of Basic Energy Sciences is acknowl-
edged for financial support through Catalysis Science Con-
tract No. DE-FG02–03ER15459. MW gratefully acknowledg-
es a 3M Untenured Faculty Award, a DuPont Young Profes-
sor Award, an Alfred P. Sloan Fellowship, a Camille Dreyfus
Teacher-Scholar Award and the Blanchard Assistant Profes-
sorship.
General Procedure (B) for the Sonogashira Reaction
Under an inert atmosphere, a screw-cap vial was loaded
with the desired catalyst, bromobenzene, trimethyl(phenyle-
thynyl)silane, CuI, cesium carbonate and dimethylacet-
amide. The solution was stirred at 808C until completion.
The product was purified via column chromatography.
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General Procedure for the Leaching Tests using
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