Polyisobutylene-supported N-heterocyclic carbene palladium catalysts

Article abstract of DOI:10.1016/j.jorganchem.2010.10.058

This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable when such solvent systems are perturbed to be biphas

Full text of DOI:10.1016/j.jorganchem.2010.10.058

Journal of Organometallic Chemistry 696 (2011) 1272e1279
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Polyisobutylene-supported N-heterocyclic carbene palladium catalysts
*
David E. Bergbreiter , Haw-Lih Su, Hisao Koizumi, Jianhua Tian
Department of Chemistry, Texas A&M University, College Station, TX, 77842-3012, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 September 2010
Received in revised form
This paper describes how the nonpolar polymer polyisobutylene (PIB) can be used as a handle to prepare
PIB-bound NHC ligands that are soluble in monophasic mixtures of mixed solvents but phase separable
when such solvent systems are perturbed to be biphasic. The results here show that such PIB-bound NHC
ligands can be used to synthesize useful palladium catalysts. In this paper, both PIB-bound analogs of an
N,N -bis(2,6-diisopropylphenyl) heterocyclic carbene and simpler N,N -dialkyl heterocyclic carbene
ligand were prepared and were successfully used to form palladium cross-coupling catalysts. The
reactivity, recycling and reusability of these catalysts has been examined.
Ó 2010 Elsevier B.V. All rights reserved.
22 October 2010
Accepted 26 October 2010
0
0
Keywords:
Supported catalysts
N-Heterocyclic carbene (NHC)
Palladium
Catalyst recycling
Cross-coupling
1. Introduction
leads to the formation of carbonecarbon and carbonenitrogen
bonds.
Since Arduengo isolated the first stable N-heterocyclic carbene
NHC) in 1991 [1], NHCs have become of increasing interest in
organometallic chemistry, catalysis, and materials science [2]. Their
use as stabilizing ligands for highly oxidized transition metals [3]
and late transition metals has been widespread [4,5]. As neutral
Many palladium catalysts have been designed for cross-coupling
chemistry using phosphine ligands. However, while these catalysts
can be exceptionally active, such catalysts are often sensitive to air
and moisture and require air-free conditions to minimize ligand
oxidation. These problems and/or catalyst instability can lead to
problems in the design of recyclable catalysts [6,10,11] NHC ligated
metal complexes used in this chemistry have been suggested to be
more stable yet are also highly active. This has led us to explore
ways to use these sorts of ligands to prepare recyclable NHCepal-
ladium complexes. In planning this approach, we had many
potential cross-coupling NHC-complexed Pd catalysts we could
(
excellent two-electron s-donors with little p-accepting character,
NHCs are a class of ligands that are often compared to phosphine
ligands [6]. In 1995, Herrmann first noted the similarity between
NHCs and electron-rich trialkylphosphines in metal coordination
chemistry [6h]. These properties led to many other studies that
demonstrated that NHC-metal complexes have reactivities similar
to those of phosphineemetal complexes, often with better air and
moisture stability . NHC ligands can form stable bonds with metals
and phosphineemetal complexes are sometimes used as precur-
sors for NHC-metal complexes [7,8]. These properties of NHC
ligands have led to many examples of metal complexes of these
electron-rich ligands, NHC ligated complexes that are often used in
homogeneous catalysis using transition metals like palladium,
ruthenium, rhodium, gold, copper, and nickel [2].
0
study [12]. These included Pd complexes with simpler N,N -dialkyl
NHC ligands or more elaborate Pd complexes prepared from
0
sterically encumbered N,N -diaryl NHC ligands. For example, Pd
complexes with NHC ligands with alkyl groups were first described
in 1995 by Herrmann who showed that the NHCepalladium
complex 1 (Fig. 1) was effective as a catalyst for Heck chemistry [6
(h)]. Later, palladium complexes with sterically hindered N-aryl
substituents were designed and studied by Nolan’s group. Their
work showed that the Pd complex 2 (Fig. 1), (IPr)PdCl(allyl), was
able to catalyze aryl animation [13,14], Suzuki-Miyaura cross-
We were interested in using polymer-bound NHC ligands for
catalyst reuse, recycling and separation. While we have already
begun to explore these questions in connection with Ru-metathesis
catalysts [9], we were intrigued by reports that show that NHCePd
complexes are useful in Pd-catalyzed cross-coupling chemistry that
couplings [14], and a-arylation of ketones [15]. Below we describe
our efforts to prepare polymeric versions of these NHC-complexed
Pd catalysts and our results in attempted recycling of these cata-
lysts in cross-coupling chemistry.
While the organometallic chemistry of NHC-metal complexes
and the studies of NHCePd complexes in cross-coupling chemistry
*
Corresponding author.
E-mail address: Bergbreiter@tamu.edu (D.E. Bergbreiter).
0
022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.10.058

D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
1273
of protons present on the polymer and because these protons appear
in the 2.00e1.00 ppm region of the H NMR spectrum. a region that
i
i
Pr
Pr
1
N
N
N
N
N
does not obscure signals from most functional groups. The
commercially available PIB derivatives used in this work have a vinyl
terminus [19] that can be modified by simple chemistry to prepare
many sorts of soluble polymeric reagents and ligands including
PIB-bound phosphine ligands, olefin metathesis catalysts, salen
catalysts, organocatalysts, Cu(I) catalysts for atom transfer radical
polymerizations, azideealkyne cyclizations, Rh(II) and Cu(I) cyclo-
propanation catalysts, and vinyl ether sequestring agents [9,20].
Because PIB is a noncrystalline polymer, it is soluble even at low
temperatures in nonpolar and weakly polar solvents like alkanes,
chlorinated alkanes, arenes, and ethers. However, it is generally not
soluble in polar solvents like acetonitrile, DMF, ethanol, and water
though it does dissolve in many solvent mixtures containing these
solvents at elevated temperature. This phase selective solubility
makes PIB and its derivatives attractive because the polymer and
species attached to it can be easily separated from reaction mixtures
by extraction or by a gravity based separation of an immiscible
mixture of polar and nonpolar liquid phases of different density.
Previous work from our group has shown that PIB is a useful
support for ligands, that ligands attached to PIB behave like their
low molecular weight counterparts in metal coordination, and that
such ligands are useful in catalysis [9,20]. Here we describe the
synthesis of two types of imidazolium salt precursors of NHC
ligands that contain PIB polymers. These heptane-soluble NHC
ligands can be used to form active Pd catalysts for cross-coupling
chemistry. The utility and limitations of the resulting PIB-bound
NHCePd catalysts are described.
Cl
i
Pd
Cl
i
Pr
Pr
Pd
N
Cl
2
1
Fig. 1. Prior examples of NHCePd catalysts useful in cross-coupling chemistry.
in particular has made significant progress, strategies that facilitate
separation, recovery and reuse of these catalysts have received less
attention. NHC ligands and Pd catalysts have been immobilized in
various ways including in ionic liquids [16a,16b], in water, and in
fluorous phases [16c]. Various sorts of solid supports including
polymer supports have also been used. These examples mostly
used insoluble cross-linked polymers [17aeg] and nanoparticles
17h,17i]. Studies using soluble polymers have largely been
restricted to the use of polar polymers like poly(ethylene glycol)
18].
The reports of immobilized NHC ligands and of their metal
complexes as noted above have mainly focused on insoluble
supports or always biphasic systems. An alternative approach that
we have emphasized is the use of soluble polymers as catalyst
supports. Polyisobutylene (PIB) is of particular interest since
ligands and transition metal complexes formed with this polymer
as a phase handle can partition into nonpolar solutions with
products remaining in a more polar phase after catalyst recovery in
a post-reaction separation step. Thus, catalysts and products can be
separated by a simple biphasic liquid/liquid separation after
a monophasic reaction.
[
[
2. Results and discussion
PIB oligomers are readily available commercial materials [19]
that are used in various applications including as oil or fuel addi-
tives. These PIB derivatives have the advantage that they have
a structurally simple and chemically inert alkane backbone. Func-
tional groups attached to such polymers can be readily analyzed by
2.1. Synthesis of PIB-bound imidazolium salts
Synthesis of the PIB supported imidazolium salt 5 is straight-
forward and proceeds via a S
N
2 reaction of an N-methylimidazole
on an appropriately substituted PIB derivative (Scheme 1).The
1
solution state spectroscopy. For example, H NMR spectroscopic
analyses of PIB are simplified because there are only two major types
necessary PIB substrate was obtained by hydroborationeoxidation
1
) BH -SMe , hexane
3 2
PIB
PIB
N
OH
PIB =
2
) NaOH(aq), H O
H
2
2
2
2
>
99 %
3
N
,
NaI
I-
N
MsCl, NEt3, DCM
PIB
N+
PIB
OMs
toluene, DMF, reflux
98%
6
5%
4
5
PIB
+
O
H
O
H
AlCl3
NH2
2
00 oC,
2 h. PIB
5%
N
N
NH2
cat. HCO H
hexane, PrOH
PIB
PIB
7
2
i
6
9
4%
6
7
Cl
O
o
THF, 40 C, 8 h.
N+
N
PIB
PIB
Cl-
6
5%
8
Scheme 1. Synthesis of the PIB-bound imidazolium salts 5 and 8.

1274
D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
Scheme 2. Synthesis of the PIB-bound (IPr)PdCl(allyl) catalyst 10.
reaction of the alkene-terminated PIB and the 2,6-disubstituted
aniline was successful leading to a 65% yield [20f] of the desired
-polyisobutyl-2,6-diisopropylaniline. Two more synthetic steps
4
were carried out using the chemistry described previously. This
chemistry could be carried out to afford the desired imidazolium salt
8
on a two-gram scale.
2.2. PIB-bound NHCePd catalyst synthesis
Scheme 3. Synthesis of the PIB-bound palladium catalyst 11.
We first opted to examine the activity, separability, and recy-
reaction of an alkene-terminated polyisobutylene. While the
ligands herein were prepared with PIB whose average molecular
weight was 1300 Da, the same chemistry works with higher
molecular weight PIB derivatives. The product hydroxy-terminated
polyisobutylene was then converted into a mesylate [20b]. This PIB
mesylate was then allowed to react with N-methylimidazole in the
presence of sodium iodide to form the desired imidazolium salt 5.
clability of catalyst 10. In this case, the desired PIB-bound (IPr)PdCl
(allyl) catalyst was prepared via a PIB-bound AgeNHC intermediate
9 (Scheme 2). This silver complex was prepared in quantitative
yield from 8 using silver oxide following procedures reported in our
earlier work [9(a)]. The silver complex was then used in a trans-
metallation reaction with the allylpalladium chloride dimer to form
the PIB-bound (IPr)PdCl(allyl) catalyst 10.
1
13
This product was characterized by H and C NMR spectroscopy.
A second sort of PIB-bound NHCePd complex was prepared by
treating imidazolium salt 5 first with potassium t-butoxide and
then with bis(benzonitrile)dichloropalladium(II) (Scheme 3). This
deprotonationemetalation reaction was carried out in the presence
of NaI and led to a 58% yield of the product 11. This reaction was
1
The H NMR spectrum was diagnostic in that it contained a peak at
10.59 ppm which was assigned as the acidic proton at the 2-posi-
tion of the imidazolium ring.
The synthesis of the imidazolium salt 8 followed a general
0
1
procedure used in our prior work where we prepared an N,N -bis(4-
monitored by H NMR spectroscopy. The disappearance of the peak
polyisobutyl-2,6-dimethyl)imidazolium chloride NHC ligand
precursor [9(a)]. In this case, the initial step involved the reaction of
polyisobutylenewith 2,6-diisopropylaniline at 200 C in the presence
at 10.59 ppm (which was assigned to the proton at the 2-position of
the imidazolium ring) was followed to determine when this reac-
tion was complete. Complex 11 was then isolated and characterized
spectroscopically.
ꢀ
of aluminum chloride. Under these conditions, the FriedeleCrafts
Fig. 2. The phase selective solubility of 10 and 11 in heptane vs. polar solvents at room temperature: (a) 10 in heptane vs. acetonitrile; (b) 10 in heptane vs. DMF; (c) 11 in heptane
vs. acetonitrile; and (d) 11 in heptane vs. DMF.

D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
1275
1
mol % 10
Br
+
HN
O
N
O
t
KO Bu, heptane or DME
o
2
0 min, 80 C
Scheme 4. The use of the PIB-bound palladium complex 10 as a catalyst for a BuchwaldeHartwig reaction.
O
O
2
.5 mol % 10
n
O Bu
+
NEt , decane, DMF
3
o
O
65 C, 10 h
I
n
CO2 Bu
Scheme 5. The use of the PIB-bound palladium complex 10 as a catalyst for a Heck reaction.
2
.3. Separability of PIB-bound NHCePd complexes
was dissolved in decane and the solution was placed in capillary
tubes that were examined under a microscope. In the absence of
base, no discoloration and no precipitation was apparent after
heating at 100 C for 60 h. However, when another solution of 10
3
was prepared containing 10 equivalents of Et N, the solution
The PIB-bound NHC complexes 10 and 11 were found to be
ꢀ
stable for up to a year when stored on a bench top and were visually
phase selectively soluble in heptane versus polar solvents. This
phase selective solubility is illustrated in Fig. 2. This figure shows
that both 10 and 11 are phase selectively soluble in heptane versus
acetonitrile or DMF.
formed some black precipitate after several days at room temper-
ꢀ
ature or after heating at 60 C for 1 h. Fig. 3 shows the black
precipitate that was observed. We presume this precipitate is a Pd
colloid. We believe this decomposition of 10 is why solutions of 10
darken and become increasingly inactive in both the Buch-
waldeHartwig and Heck reactions.
2
.4. Studies of the reactivity and recyclability of PIB-bound
NHCePd complexes in Pd-catalyzed cross-coupling reactions
Given the problems in recycling 10, we next turned to studies
0
using the simpler N,N -dialkyl NHCePd complex 11 since this
The PIB-bound catalyst 10 was first examined in a Buch-
waldeHartwig aryl amination reaction (Scheme 4). With 1 mol%
catalyst, 4-bromotoluene underwent complete conversion to N-(4-
NHCePd complex could be recycled (cf. Fig. 2) if the catalyst formed
using 11 were stable. This premise was tested by examining the use
of 11 in the Heck coupling of 4-iodoacetophenone and n-butyl
ꢀ
methylphenyl)morpholine within 20 min at 80 C in heptane or 1,
acrylate in the presence of Et N in a thermomorphic mixture of
3
ꢀ
2
-dimethoxyethane. However, we also observed that the solution
heptane and DMF at 75 C. Recycling was examined with various
discolored during the reaction forming a dark brown solution. In
the case of the heptane reaction, we separated the active catalyst by
using acetonitrile to extract the reaction product from this dark
brown solution. This led to a biphasic mixture of a solution of the
N-(4-methylphenyl)morpholine product and a heptane phase that
we presume contained the PIB-NHC complexed Pd catalyst. This
heptane solution was successfully used in a second reaction cycle.
However, complete conversion of 4-bromotoluene to the product
required that the reaction time be extended to 8 h. Attempts to
separate product from the catalyst phase and reuse the catalyst in
a third cycle were even less successful with the reaction not
catalyst loading levels. While the reactivity of the catalyst is
unexceptional, the results in Scheme 6 show that recycling was
successful. These recycling experiments used a thermomorphic
mixture of heptane and DMF that is miscible hot but immiscible at
room temperature. In this case, when the reaction mixture was
cooled to room temperature, a biphasic mixture forms. In this
ꢀ
proceeding to completion after 16 h at 80 C.
We also examined the use of 10 as a catalyst for a Heck coupling
reaction (Scheme 5). Recycling of 10 in this Heck reaction led to
similar results as seen above for recycling 10 in amination chem-
istry. The coupling between 4-iodoacetophenone and n-butyl
acrylate in the presence of Et
at 65 C required 10 h in the first cycle. However, the catalyst
3
N in the mixture of decane and DMF
ꢀ
reactivity decreased in a second and third cycle with complete
ꢀ
formation of a Heck product only occurring after 36 h at 100 C in
the third cycle.
To understand why recycling of 10 was ineffective, we examined
the stability of 10 under the reaction conditions. First, complex 10
1
was dissolved in benzene-d
6
and its H NMR spectrum was
measured. Then potassium tert-butoxide (50 equivalents) was
ꢀ
1
added. After 3 h at 60 C, the solution became brown. H NMR
spectroscopic analysis of this solution showed that the peaks in the
4
.90e2.50 ppm region of 10 (which were assigned to the allylic
protons of 10) became broad. This suggested some decomposition
of 10 had occurred. If an acrylate ester and an aryl iodide were
added at this point, no conversion to a Heck product was seen in
Fig. 3. Complex 10 was dissolved in decane in a capillary tube and examined micro-
scopically after the addition of Et
observed is believed to be palladium colloid formed in decomposition of 10.
ꢀ
10 h at 60 C. Darkening of Pd catalyst solutions is often associated
ꢀ
3
N and heating at 60 C for 1 h. The black precipitate
with formation of Pd colloids. To determine if this was the case, 10

1276
D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
I
O
O
catalyst 11
n
O Bu
+
R
n
O Bu Et3N, heptane, DMF
R
>
97% conversion based
O
1
3
R = CH , OCH3
on H NMR spectroscopy
O
catalyst loading
Temperature
reaction time
cycle tested
o
5
mol %
75 C
14 h
46 h
0.5 h
1
10
0
o
1
1
mol %
mol %
75 C
o
130 C (MW)
6
Scheme 6. The use of 11 as a reusable catalyst for a Heck reaction.
mixture, the upper heptane-rich phase contained the PIB-bound
catalyst, some DMF, and some product. No starting material was
present. The lower DMF phase contained most of the product. The
absence of starting material in this phase and in the upper phase
allowed us to estimate that the conversion was >97%. As is true in
other themomorphic systems, some extractable product remains in
the catalyst-containing upper phase. While we did not demonstrate
this here, prior work has shown that the product concentration in
the upper phase rapidly becomes constant because the this
heptane-rich phase becomes saturated in product [21]. Thus, high
yields of product can be isolated from the polar phase after several
cycles of such separations. Just as was done previously, we sepa-
rated the lower polar product-containing phase by a gravity sepa-
ration. Recycling simply involved the addition of fresh substrates,
base and DMF to the heptane-rich phase. This procedure was used
for 10 cycles without any change in measured conversion of
counterpart. However, the instability of this complex in the reac-
tion mixture made recycling this catalyst more than two cycles
problematic in aryl amination or even in reactions with very
reactive Heck substrates. Catalyst recyclability was more successful
with simpler PIB-bound NHC-complexed Pd catalyst 11.
4. Experimental section
All reactions were carried out under an inert atmosphere unless
1
13
otherwise noted. H and C NMR spectra spectra were recorded on
Inova 499.9 MHz or 299.9 MHz spectrometers. Chemical shifts were
reported in parts per million (d) relative to residual proton reso-
nances in the deuterated chloroform (CDCl
3
). Integrations of the
chemically different protons on pendant groups on PIB-bound
intermediates reflect the normal accuracy associated with integra-
substrate to product.
1
1H NMR spectroscopy was used to determine the extent of
tion in H NMR spectroscopy. The large signal due to the PIB protons
1
in the 1e2
d
region of the H NMR spectrum of these PIB-bound
conversion of 4-iodoacetophenone to product. We also showed that
this catalyst could be used with a different aryl iodide substrate. In
this experiment, methyl 4-iodobenzoate was substituted as
a substrate after four cycles of a Heck coupling of n-butyl acrylate
and 4-iodoacetophenone. The subsequent catalytic reaction led to
products can also be integrated. A typical value for the polymers we
use is 180/1 since the PIB used has a M of ca. 1300. However, since
samples of PIB vary in M and since the normal ca. 5e10% errors in
n
n
integration are exaggerated in NMR analyses of a very large (e.g.
a 180 proton signal in this case) and a very small signal (e.g. a 1
1
00% conversion of methyl 4-iodobenzoate to the expected Heck
1
1
proton signal), we have not listed these integrations in the H NMR
product based on H NMR spectroscopy. We also briefly examined
microwave conditions. At 130 C in a microwave reactor, the reac-
ꢀ
spectra below. ICP-MS (inductivelycoupled mass spectroscopy) data
were obtained using a Perkin Elmer DRC II instrument. Acetonitrile,
heptane, DMF, ethanol, CH Cl , THF, and toluene were purchased
2 2
from EMD and used as received. Polyisobutylene was a gift from
BASF [19]. All other chemicals were purchased from SigmaeAldrich
and used as received.
tion time was considerably shortened. Catalyst recycling was still
equally effective. These results are summarized in Scheme 6.
While we have not tried to determine whether 11 is a pre-
catalyst leading to an active Pd(0) catalyst or if Pd colloids are
formed in these experiments using 11, we did carry out an ICP-MS
(
inductively coupled mass spectroscopy) analysis of the DMF phase
from a Heck coupling of 4-iodoacetophenone and n-butyl acrylate
with 1 mol% of 11. That analysis showed that a modest amount of
metal leaching occurred with ca. 1% of the charged Pd metal loading
being lost in cycle 2 and 3 and 0.2% in cycle 10.
4
.1. Hydroxy-terminated polyisobutylene (3)
In a 250-mL round-bottomed flask, equipped with a magnetic
stir bar and a rubber septum, the vinyl-terminated PIB (39 g,
39 mmol) was dissolved in 50 mL of hexanes. Then a 2.0 M solution
3
. Conclusion
of BH
3
$SMe
2
(7.7 mL, 15.4 mmol) in THF was slowly added. After
ꢀ
Two imidazolium salts bound to a nonpolar solvent-soluble
18 h, the reaction mixture was cooled to 0 C and 12 mL of a 4 N
aqueous NaOH solution and 100 mL of water were added to the
polymer, polyisobutylene, were prepared. On deprotonation, either
can bind to palladium(II) to form NHCePd(II) complexes that can be
phase isolated in the heptane phase of a mixture of heptane and an
immiscible polar solvent. We have examined these complexes’
catalytic properties and the abilities of the PIB support for recycling
and reuse of these Pd catalysts. Using a PIB-bound NHCePd aryl
amination catalyst, we were able to successfully carry out an aryl
amination even with a relatively unreactive aryl bromide and were
able to show that the PIB-bound allylpalladium complex 10 had
activity in a heptane solution like that of its low molecular weight
2 2
hydroboration product. Then 7 mL of a 30% H O solution was
slowly added and the mixture was stirred for overnight at room
temperature. At this point, the two phases were separated and the
organic phase was washed with H
2
O (3 ꢁ 50 mL) and brine (50 mL),
and then dried over MgSO . After filtration, the solvents were
4
removed under reduced pressure and the product was dried under
1
vacuum for 24 h, a total yield of 40.0 g of product was obtained. H
NMR (299.9 MHz, CDCl
J ¼ 7.5, 10.2 Hz, 1H), 0.75e1.46 (PIB protons).
3
),
d
: 3.48 (dd, J ¼ 5.4, 10.2 Hz, 1H), 3.31 (dd,

D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
1277
1
4.2. Polyisobutyl mesylate (4)
yield). H NMR (299.9 MHz, CDCl
4
3
),
d
: 8.10 (s, 2H), 7.17 (s, 4H), 2.97 (m,
13
H), 1.83 (s, 2H), 1.60e0.80 (PIB protons). C NMR (125.72 MHz,
In a 250-mL round-bottomed flask, equipped with a magnetic
CDCl ), : 163.88, 146.44, 145.55, 136.00, 121.55, multiple poorly
3
d
stir bar a rubber septum, the hydroxy-terminated polyisobutylene
resolved peaks between 60.00e58.00 and 40.00e30.00, 18.90.
(
10 g, 9.8 mmol) and methanesulfonyl chloride (2.3 mL, 29 mmol)
ꢀ
were dissolved in 100 mL of CH
2
Cl
2
and cooled to 0 C. Then Et
3
N
4.6. 1,3-Bis(2,6-diisopropyl-4-(polyisobutyl)phenyl)imidazolium
(
4.3 mL, 31 mmol) was added dropwise. The reaction mixture was
chloride (8)
allowed to stir for 3 h after warming to room temperature. The
solvent was removed under reduced pressure to yield a crude
viscous oil. This oil was dissolved in 300 mL of hexane and this
hexane solution was washed with water (3 ꢁ 100 mL), dried over
In a 100-mL round-bottomed flask, equipped with a magnetic stir
0
bar and
a
rubber septum, N,N -bis(2,6-diisopropyl-4-poly-
isobutylphenyl)ethylenediimine (6.3 g, 2.6 mmol) was dissolved in
25 mL of THF. Chloromethyl ethyl ether (0.25 g, 2.6 mmol) was added
MgSO
4
and filtered. The solvent was removed under reduced
1
ꢀ
pressure to yield 10.5 g (98%) of product. H NMR (299.9 MHz,
CDCl ),
and the mixture was heated to 40 C for 20 h. The solvent was
3
d
: 4.06 (dd, J ¼ 5.4, 9.3 Hz, 1H), 3.88 (dd, J ¼ 7.5, 9.3 Hz, 1H),
removed under reduced pressure and the residue was purified by
2
.96 (s, 3H), 1.95 (m, 2H), 1.39e0.88 (PIB protons).
column chromatography (eluted first with CH
solution of CH Cl and methanol). Solvent removal afforded the
product as a light brown viscous liquid (2.0 g, 31% yield). H NMR
299.9 MHz, CDCl ), : 8.63 (s, 1H), 8.41 (s, 2H), 7.33 (s, 4H), 2.46 (m,
4H), 1.90 (s, 6H), 1.60e0.80 (PIB protons). C NMR (125.72 MHz,
CDCl ), : 154.44, 144.00, 137.88, 127.77, 127.22, 122.22, multiple
2 2
Cl and then with 9:1
2
2
0
1
4
.3. N-Methyl-N -polyisobutylimidazolium iodide (5)
(
3
d
13
In a 50-mL round-bottomed flask, equipped with a magnetic stir
barand a rubber septum, polyisobutyl mesylate(1.21 g,1.1 mmol), N-
3
d
methylimidazole (0.49 g, 5.9 mmol) and sodium iodide (0.32 g,
poorlyresolvedpeaks between60.00e58.00and40.00e30.00,18.90.
2
.2 mmol) was mixed with 15 mL of toluene and 10 mL of DMF. After
refluxing for 2 d, the reaction mixture was cooled to room temper-
ature. At this point, the upper phase of the two-phase mixture that
formed was separated and washed with water (3 ꢁ 50 mL) and dried
4.7. 1,3-Bis(2,6-diisopropyl-4-(polyisobutyl)phenyl)imidazo-2-
ylidenesilver(I) chloride (9)
over MgSO
4
. After filtration, the solvents were removed under
In a 25-mL round-bottomed flask, equipped with a magnetic stir
bar and a rubber septum, 1,3-bis(2,6-diisopropyl-4-(polyisobutyl)
phenyl)imidazolium chloride 8 (0.86 g, 0.40 mmol) was dissolved
in 6 mL of hexane. Silver oxide (0.058 g, 0.25 mmol) was added and
the mixture was fluxed for 24 h. After cooling to room temperature,
the reaction mixture was filtered through celite to remove excess
reduced pressure to yield a crude product which contained the
excess N-methylimidazole and the desired product. This mixture
was further purified by column chromatography (elution first with
CH
.7 g (59%) of product. H NMR (299.9 MHz, CDCl
.18 (s, 2H), 4.30e4.00 (br, 5H), 2.40e0.80 (PIB protons). C NMR
75.43 MHz, CDCl ), : 138.10, 123.13, 121.71, multiple poorly
2
Cl
2
and then with 9:1 solution of CH
2
Cl
2
and methanol) to yield
1
0
7
3
d
), : 10.59 (s, 1H),
13
2
Ag O and the celite was washed with hexane. The solvent was
(
3
d
removed from the combined hexane solution under reduced
1
resolved peaks between 60.00e58.00, 49.68, multiple poorly
resolved peaks between 40.00e30.00.
pressure, affording 0.85 g (95%) of a viscous oilproduct. H NMR
(499.9 MHz, CDCl
3
), d: 7.26 (s, 4H), 7.11 (s, 2H), 2.58e2.46 (m, 4H),
13
1
.84 (s, 6H), 1.60e0.80 (PIB protons). C NMR (125.72 MHz, CDCl
3
),
: 184.74 (dd, J Ce Ag ¼ 236 Hz, J Cꢂ Ag ¼ 271 Hz), 152.22, 144.20,
31.88, 123.63, 122.14, multiple poorly resolved peaks between
13107
13109
4.4. 2,6-Diisopropyl-4-(polyisobutyl)aniline (6)
d
1
In a 100-mL pressure vessel, a mixture of 2,6-diisopropylaniline
60.00e58.00 and 40.00e30.00, 28.72, 24.84, 24.03.
(
23 g, 130 mmol), polyisobutylene (10 g, 10 mmol), and aluminum
ꢀ
trichloride (4.0 g, 30 mmol) was stirredin a sand bath at 200 C. After
3
4.8. (1,3-Bis(2,6-diisopropyl-4-(polyisobutyl)phenyl)imidazo-2-
yliidene)(allyl)palladium(II) chloride (10)
ꢀ
days, the deep purple solution was cooled to approximately 100 C
and added to 200 mL of hexane. After cooling to room temperature,
the mixture was filtered and washed with water (3 ꢁ 150 mL) and
To a 25-mL round-bottomed flask, equipped with a magnetic stir
bar and a rubber septum was added (allylPdCl) (12.4 mg,
2
0.34 mmol) along with a solution of 1,3-bis(2,6-diisopropyl-4-
(polyisobutyl)phenyl)imidazol-2-ylidene silver(I) chloride 9 (0.85 g,
CH
3
CN (3 ꢁ 150 mL). The solvent was removed under reduced
pressure to yield a crude product which contained the desired
product and unreacted starting materials. This mixture was purified
by column chromatography (eluted first with hexane and then with
2 2
0.38 mmol) in 5 mL of CH Cl . After 12 h, the reaction mixture was
CH
2
Cl
2
). Solvent removal afforded the product as a light yellow
filtered through celite and the celite was washed with hexane. The
solvent was removed from the combined hexane solutions under
1
viscous liquid (7.5 g, 63% yield). H NMR (299.9 MHz, CDCl
3
),
d
: 7.03
(
s, 2H), 3.60(s, 2H), 2.96(m, 2H),1.77 (s, 2H),1.60e0.80(PIB protons).
reduced pressure to yield 0.85 g (96%) of a brown residue of product
1
as a viscous oil. H NMR (499.9 MHz, CDCl
3
),
d: 7.23 (s, 4H), 7.06 (s,
0
4
.5. N,N -Bis(2,6-diisopropyl-4-polyisobutylphenyl)
2H), 4.85e4.7 (m, 1H), 3.87 (d, J ¼ 6.8 Hz, 1H), 3.13e3.00 (m, 4H),
ethylenediimine (7)
2.90e2.78 (m, 2H), 2.69 (d, J ¼ 13.3 Hz,1H),1.84 (s, 6H),1.75e0.7 (PIB
13
3
protons). C NMR (125.72 MHz, CDCl ), d: 186.17, 151.09, 144.94,
In a 100-mL round-bottomed flask, equipped with a magnetic stir
bar and a rubber septum, a mixture of 2,6-diisopropyl-4-(poly-
isobutyl)aniline(6.6g, 5.6mmol)andacatalyticamountofformicacid
in20mLofhexanewasprepared. Tothis stirringsolution, a solution of
glyoxal (0.41 g of a 40% aqueous solution, 2.8 mmol) in 5 mL of iso-
propanol was added. The reaction mixture initially turned cloudy for
roughly 5 min and then became a clear yellow solution. The reaction
mixture was allowed to stir for 20 h. At this point, the bright yellow
144.67,133.14,124.31,121.69,114.08, 72.31, multiple poorly resolved
peaks between 60.00e58.00, 49.18, multiple poorly resolved peaks
between 40.00e30.00, 28.51, 28.41, 26.66, 25.91, 22.97, 22.91.
0
4.9. Bis(N-methyl-N -polyisobutylimidazo-2-ylidene)palladium(II)
iodide (11)
0
N-Methyl-N -polyisobutylimidazolium iodide5(0.52g, 0.48mmol)
solution was dried with Na
SO
2 4
and the solvent was removed under
and sodium iodide (0.47 g, 3.1 mmol) were dissolved in 10 mL of THF in
reduced pressure, affording a dark yellow viscous liquid (6.5 g, 99%
a 50-mL round-bottomed flask, equipped with a magnetic stir bar and

1278
D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
ꢀ
a rubber septum. This THF solution was cooled to ꢂ78 C and potas-
reaction mixture became biphasic. The two phases were separated
and fresh substrate solution and Et N were added to the top phase
sium tert-butoxide (62.3 mg, 0.55 mmol) was added. After stirring at
3
ꢀ
ꢂ78 C for 40 min, a solution of bis(benzonitrile)dichloropalladium(II)
to start the second cycle and the bottom polar phase was analyzed
1
(
82.4 mg, 0.22 mmol) in 3 mL of THF was added. The mixture was
for product by H NMR spectroscopy.
ꢀ
allowed to stir at ꢂ78 C for 30 min and then warmed to room
temperature. After stirring at room temperature for another 14 h, the
solvent was removed under reduced pressure. The residue was dis-
solved in 20 mL of hexane and washed with 90% ethanol (3 ꢁ 15 mL).
The hexane phase was concentrated and purified by column chro-
) to yield 0.35 g (58%) of an
): 6.88 (s, 2H), 6.85 (s,
H), 4.6e4.3 (br, 2H), 4.3e4.1 (br, 6H), 4.1e3.9 (br, 2H), 2.8e2.4 (br, 2H),
4.14. Sample digestion and sample preparation for ICP-MS analysis
All solvent was removed from the product-containing DMF-rich
phase of a catalytic reaction. Then 2 mL of concentrated nitric acid
ꢀ
matography (hexane first and then CH
orange viscous liquid. H NMR (299.9 MHz, CDCl
2
Cl
2
was added to this residue at 25 C and the resulting mixture was
1
ꢀ
3
heated in a sand bath at 132 C for 48 h. Another 2 mL of concen-
ꢀ
2
2
trated nitric acid was added and the mixture was heated at 132 C
.0e0.80 (PIB protons).
for a further 55 h. At this point, 2 mL of concentrated sulfuric acid
ꢀ
was added and the sample was heated at 132 C a further 48 h. At
4.10. Aryl amination with catalyst 10 and recycling
this point, the solution was cooled to room temperature and diluted
3
with 1% aqueous HNO and the diluted solution was analyzed by
In a 10-mL Schlenk flask, equipped with a magnetic stir bar and
ICP-MS.
a rubber septum, a mixture of 4-bromotoluene (87 mg, 0.50 mmol),
morpholine (61 mg, 0.70 mmol), potassium tert-butoxide (84 mg,
Acknowledgements
0
.75 mmol), and the catalyst 10 (13 mg, 0.005 mmol) were dis-
ꢀ
solved in 2 mL of heptane. After 20 min at 80 C, the reaction was
Support from the Robert A. Welch Foundation (A-0639) and the
National Science Foundation (CHE-0952134) is gratefully
acknowledged.
complete in the first cycle of an aryl amination using 10. At this
point, 3 mL of CH
vigorous stirring, the mixture was allowed to settle and the two
phases separated. The CH CN layer containing the product was
3
CN was added to the reaction mixture. After
3
References
removed and fresh substrates and base were added to the heptane
phase to start a new catalytic cycle.
[1] A.J. Arduengo III, R.L. Harlow, M. Kline, J. Am. Chem. Soc. 113 (1991) 361.
[2] (a) S.P. Nolan, N-Heterocyclic Carbenes in Synthesis. Wiley-VCH, Weinheim,
2
006;
4
.11. Heck reaction with catalyst 10 and recycling
(
(
(
b) S.T. Liddle, I.S. Edworthy, P.L. Arnold, Chem. Soc. Rev. 36 (2007) 1732;
c) F. Glorius, Top. Organomet. Chem. 21 (2007) 1;
d) N. Marion, S. Díez-González, S.P. Nolan, Angew. Chem. Int. Ed. 46 (2007)
A substrate solution containing 4-iodoacetophenone (8.19 g,
3.3 mmol) and n-butyl acrylate (4.86 g, 37.9 mmol) dissolved in
2988.
3
[
[
3] W.A. Herrmann, K. Öfele, M. Elison, F.E. Kühn, P.W. Roesky, J. Organomet.
Chem. 480 (1994) C7eC9.
4] (a) M.C. Jahnke, F.E. Hahn, Top. Organomet. Chem. 30 (2010) 95;
5
1
1.4 g of heptane-saturated DMF was prepared and added to a
0-mL Schlenk flask equipped with a magnetic stir bar and a rubber
septum, that contained catalyst 10 (11 mg, 4.2 mol) dissolved in
N (0.1 g, 1 mmol), the resulting
(
(
b) D. Tapu, D.A. Dixou, C. Roe, Chem. Rev. 109 (2009) 3385;
c) P.L. Arnold, I.J. Casely, Chem. Rev. 109 (2009) 3599;
m
1
mL of decane. After addition of Et
3
(d) S. Díez-González, N. Marion, S.P. Nolan, Chem. Rev. 109 (2009) 3612;
(e) F. Boeda, S.P. Nolan, Annu. Rep. Prog. Chem. Sect. B. 104 (2008) 184;
ꢀ
mixture was stirred at 65 C for 10 h. The reaction mixture was
cooled to room temperature and became two phases. The bottom
phase was removed using a syringe. Fresh substrate solution and
Et N (0.05 g, 0.46 mmol) were added to the remaining heptane
3
phase to start the second cycle and the recovered polar phase was
(
(
f) S. Díez-Gonzáalez, S.P. Nolan, Coord. Chem. Rev. 251 (2007) 874;
g) W.A. Herrmann, Angew. Chem. Int. Ed. 41 (2002) 1290.
[5] (a) J.C.Y. Lin, R.T.W. Huang, C.S. Lee, A. Bhattacharyya, W.S. Hwang, I.J.B. Lin,
Chem. Rev. 109 (2009) 3561;
(b) J.C. Garrison, W.J. Youngs, Chem. Rev. 105 (2005) 3978.
[
6] (a) E. Despagnet-Ayoub, T. Ritter, Top. Organomet. Chem. 21 (2007) 193;
1
analyzed for product by H NMR spectroscopy.
(
(
b) E. Peris, J.A. Loch, J. Mata, R.H. Crabtree, Chem. Commun. (2001) 201;
c) A. Briot, M. Bujard, V. Gouverneur, S.P. Nolan, C. Mioskowski, Org. Lett.
2
(
5
(2000) 1517;
d) J. Huang, H.-J. Schanz, E.D. Stevens, S.P. Nolan, Organometallics 18 (1999)
375;
(e) J. Huang, E.D. Stevens, S.P. Nolan, J.L. Petersen, J. Am. Chem. Soc. 121 (1999)
674;
f) T. Weskamp, F.J. Kohl, W. Hieringer, D. Gleich, W.A. Herrmann, Angew.
4
.12. Heck reaction with catalyst 11 and recycling
In a 10-mL Schlenk flask, equipped with a magnetic stir bar and
2
(
a rubber septum, catalyst 11 (12 mg, 4.8
mL of heptane. 4-Iodoacetophenone (0.123 g, 0.5 mmol), n-butyl
acrylate (0.077 g, 0.6 mmol) and 1 mL of heptane-saturated DMF
mmol) was dissolved in
1
Chem. Int. Ed. 38 (1999) 2416;
(g) W.A. Herrmann, C. Köcher, Angew. Chem. Int. Ed. 36 (1997) 2162;
(h) W.A. Herrmann, M. Elison, J. Fischer, C. Kocher, G.R.J. Artus, Angew. Chem.
were added followed by 0.2 mL of Et
3
N. The resulting mixture was
Int. Ed. 34 (1995) 2371.
ꢀ
stirred at 75 C for 2 d. After cooling to room temperature, the
solution became biphasic. The bottom phase was removed using
a syringe. Fresh substrates and base were added to the remaining
heptane solution in the flask to start the second cycle.
[
7] (a) T. Steinke, B.K. Shaw, H. Jong, B.O. Patrick, M.D. Fryzuk, Organometallics
28 (2009) 2830;
(
1
(
2
b) D. Kremzow, G. Seidel, C.W. Lehmann, A. Fürstner, Chem. Eur. J. 11 (2005)
833;
c) A. Fürstner, G. Seidel, D. Kremzow, C.W. Lehmann, Organometallics
2 (2003) 907;
4
.13. Microwave-assisted Heck reaction with catalyst 11 and
(d) J.A.Chamizo, J.Morgado, M.Castro, S.Bernès, Organometallics21(2002)5428.
8] I Özdemir, A. Denizci, H.T. Öztürk, B. Çetinkaya, Appl. Organomet. Chem.
_
[
[
recycling
18 (2004) 318.
9] (a) C. Hongfa, H.-L. Su, H.S. Bazzi, D.E. Bergbreiter, Org. Lett. 11 (2009) 665;
(b) C. Hongfa, J. Tian, H.S. Bazzi, D.E. Bergbreiter, Org. Lett. 9 (2007) 3259.
10] J. Tsuji, Palladium Reagents and Catalysts. John Wiley & Sons, England, 1995.
11] M. Beller, C. Bolm, Transition Metals for Organic Synthesis, second ed.., vol.
The palladium catalyst 11 (29.3 mg, 11.6
mL of heptane in a microwave reaction tube. Then 2 g of
a substrate solution (prepared using 4-iodoacetophenone (8.19 g,
3.3 mmol) and n-butyl acrylate (4.86 g, 37.9 mmol) in 51.4 g of
mmol) was dissolved in
[
[
2
1
e2, Wiley-VCH, Weinheim, 2004.
3
[12] E.A.B. Kantchev, C.J. O’Brien, M.G. Organ, Angew. Chem. Int. Ed. 46 (2007) 2768.
13] N. Marion, O. Navarro, J. Mei, E.D. Stevens, N.M. Scott, S.P. Nolan, J. Am. Chem.
Soc. 128 (2006) 4101.
[
[
heptane-saturated DMF) and 0.4 mL of Et
solution of 11. The reaction mixture was heated at 130 C for 30 min
3
N were added to the
ꢀ
14] O. Navarro, H. Kaur, P. Mahjoor, S.P. Nolan, J. Org. Chem. 69 (2004) 3173.
in a microwave reactor. On cooling to room temperature the
[15] M.S. Viciu, R.F. Germaneau, S.P. Nolan, Org. Lett. 4 (2002) 4053.

D.E. Bergbreiter et al. / Journal of Organometallic Chemistry 696 (2011) 1272e1279
1279
[
16] (a) R. Wang, B. Twamley, J.M. Shreeve, J. Org. Chem. 71 (2006) 426;
[18] (a) M. Meise, R. Haag, ChemSusChem 1 (2008) 637;
(b) S.H. Hong, R.H. Grubbs, J. Am. Chem. Soc. 128 (2006) 3508.
[19] Glissopal is one of family of polyisobutylene derivatives supplied
(
(
b) K. Selvakumar, A. Zapf, M. Beller, Org. Lett. 4 (2002) 3031;
c) M. Skalický, M. Rybá ꢀc ková, O. Kysilka, M. Kví ꢀc alová, J. Cva ꢀc ka, J. Cejka,
ꢀ
a
J. Kví ꢀc ala, J. Fluorine Chem. 130 (2009) 966.
commercially by BASF: http://daile.com.cn/BASF/Glissopal%20550%201000%
201300%202300.pdf.
[
17] (a) M.I. Burguete, E. García-Verdugo, I. Garcia-Villar, F. Gelat, P. Licence,
S.V. Luis, V. Sans, J. Catal. 269 (2010) 150;
[20] (a) D.E. Bergbreiter, J. Li, Chem. Commun. (2004) 42;
(b) J. Li, S. Sung, J. Tian, D.E. Bergbreiter, Tetrahedron 61 (2005) 12081;
(c) D.E. Bergbreiter, J. Tian, Tetrahedron Lett. 48 (2007) 4499;
(d) D.E. Bergbreiter, P.N. Hamilton, N.M. Koshti, J. Am. Chem. Soc. 129 (2007)
10666;
(
b) G. Nan, F. Ren, M. Luo, Beilstein J. Org. Chem. 6 (2010). doi:10.3762/
bjoc.6.70;
c) D.-H. Lee, J.-H. Kim, B.-H. Jun, H. Kang, J. Park, Y.-S. Lee, Org. Lett. 10 (2008)
609;
d) S. Zhang, X. Zeng, Z. Wei, D. Zhao, T. Kang, W. Zhang, M. Yan, M. Luo,
Synlett (2006) 1891;
(
1
(
(e) D.E. Bergbreiter, Y.-C. Yang, J. Org. Chem. 75 (2010) 873;
(f) D.E. Bergbreiter, C. Hobbs, J. Tian, H. Koizumi, H.-L. Su, C. Hongfa, Pure Appl.
Chem. 81 (2009) 1981;
(g) D.E. Bergbreiter, D. Ortiz-Acosta, Tetrahedron Lett. 49 (2008) 5608;
(h) C. Hongfa, J. Tain, J. Andreatta, D.J. Darensbourg, D.E. Bergbreiter, Chem.
Commun. (2008) 975;
(
(
(
e) J.-H. Kim, J.-W. Kim, M. Shokouhimehr, Y.-S. Lee, J. Org. Chem. 70 (2005) 6714;
f) J.-W. Byun, Y.-S. Lee, Tetrahedron Lett. 45 (2004) 1837;
g) J. Schwarz, V.P.W. Böhm, M.G. Gardiner, M. Grosche, W.A. Herrmann,
W. Hieringer, G. Raudaschl-Sieber, Chem. Eur. J. 6 (2000) 1773;
(
(
h) A. Taher, J.-B. Kim, J.-Y. Jung, W.-S. Ahn, M.-J. Jin, Synlett (2009) 2477;
i) S.-M. Lee, H.-J. Yoon, J.-H. Kim, W.-J. Chung, Y.-S. Lee, Pure Appl. Chem.
(i) H.-L. Su, C. Hongfa, H.S. Bazzi and D.E. Bergbreiter, Macromol. Symp. (2010)
in press.
79 (2007) 1553.
[21] D.E. Bergbreiter, Y.-S. Liu, P.L. Osburn, J. Am. Chem. Soc. 120 (1998) 4250.