Copolymer-incarcerated nickel nanoparticles with N-heterocyclic carbene precursors as active cross-linking agents for Corriu-Kumada-Tamao reaction

Article abstract of DOI:10.1021/ja404006w

We have developed heterogeneous polymer-incarcerated nickel nanoparticles (NPs), which catalyze cross-coupling reactions. The matrix structure of these catalysts incorporates both N-heterocyclic carbenes (NHCs) as ligands and Ni-NPs, thanks to a new design of cross-linking agents in polymer supports. These embedded NHCs were detected by field gradient swollen-resin magic angle spinning NMR analysis. They were successfully applied to Corriu-Kumada-Tamao reactions with a broad substrate scope including functional group tolerance, and the catalyst could be recovered and reused several times without loss of activity.

Full text of DOI:10.1021/ja404006w

Communication
pubs.acs.org/JACS
Copolymer-Incarcerated Nickel Nanoparticles with N‑Heterocyclic
Carbene Precursors as Active Cross-Linking Agents for Corriu−
Kumada−Tamao Reaction
Jean-Franco̧ is Soule, Hiroyuki Miyamura, and Shu Kobayashi*
́
̅
Department of Chemistry, School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
S
* Supporting Information
ABSTRACT: We have developed heterogeneous poly-
mer-incarcerated nickel nanoparticles (NPs), which
catalyze cross-coupling reactions. The matrix structure of
these catalysts incorporates both N-heterocyclic carbenes
(NHCs) as ligands and Ni-NPs, thanks to a new design of
cross-linking agents in polymer supports. These embedded
NHCs were detected by field gradient swollen-resin magic
angle spinning NMR analysis. They were successfully
applied to Corriu−Kumada−Tamao reactions with a
broad substrate scope including functional group toler-
ance, and the catalyst could be recovered and reused
Figure 1. Concept of PI M-NPs using active cross-linking agents.
several times without loss of activity.
(Figure 1A).9 Copolymer 1 has been successfully applied to
the stabilization and incarceration of different metal NPs,10 such
as Au-NPs,11 the combination of Au-NPs with Fe, Co, or Ni (Au/
M-NPs),12 and Pt-NPs.13 These catalysts exhibit high activity for
organic transformations, and they can be recovered and reused
several times without leaching of metals or loss of activity.
Recently, we reported PICB-Rh/Ag-catalyzed 1,4-additions of
boronic acids and found that a combination of NPs with external
ligands (e.g., diene or phosphine) could tune the catalyst activity
and/or selectivity.14 One of the most successful recent classes of
ligands in organometallic chemistry are NHCs.15 There are,
however, few reports on the use of NHC ligands for stabilizing or
modifying NPs (Au-NPs,16 Ru-NPs,17 and Pd-NPs18). Although
some homogeneous, efficient NHC-Ni systems for CKT reaction
have been disclosed,19 there are no reports on combinations of
NHC and Ni-NPs. Turning our attention to this reaction
catalyzed by Ni-NPswith which it is probably necessary to use
ligands to increase the activitywe decided to develop new
copolymers with active cross-link moieties consisting of an
imidazole part (2) and an alkyl bromide part (3) (Figure 1B). We
envisioned that the cross-linking between these two copolymers
could lead to the formation of embedded NHCs in one matrix
and, by this protocol, a directly usable heterogeneous Ni-NPs
catalyst without external addition of ligands could be afforded.
We began our studies by preparing various Ni-NP catalysts
using the PI method, following our previously reported
procedures with small adjustments (Table 1). As a modification,
lithium triethylborohydride (LiEt3BH) was used as a stronger
reducing agent to give full reduction of Ni.20 NiBr2(PPh3)2 was
selected as a nickel source, and different copolymers were tried as
arbon−carbon bond cross-coupling reactions catalyzed by
C
transition metals are among the most fundamental organic
reactions and have appeared in recent years as a powerful
synthetic tool for the elaboration of complex organic molecules
(e.g., natural products, pharmaceutical and agricultural com-
pounds, electronic devices).1 Surface-catalyzing C−C bond
formation on supported transition metal nanoparticles (NPs)
has emerged as a suitable chemical method because of the
decreased metal contamination of products and the development
of “greener” routes in organic synthesis by simplifying workup
and using recyclable catalysts.2 However, the development of
heterogeneous metallic NPs effective in cross-coupling reactions
has been limited mostly to Pd chemistry.3 Since the pioneering
work on the Corriu−Kumada−Tamao (CKT) reaction,4 nickel
has appeared as an appropriate alternative. Among the many
contributions on homogeneous Ni systems dedicated to cross-
coupling reactions,5 only a few have focused on Ni immobil-
ization6 or on the development of surface chemistry by using Ni-
NPs. In fact, Ni-NPs have been limited to hydrogen-transfer
reactions;7 and only Ni immobilized on charcoal (Ni/C)
developed by Lipshutz’s group has been applied to C−C bond
formation.6b−e,8 Despite the high activity of Ni/C, limitations
were observed, such as pregeneration of active species by adding
external ligands (phosphine) and in situ reduction of Ni(II), and
in some cases leaching of metal was observed. Here we survey the
development of highly active and useful heterogeneous Ni-NPs
by incarcerating them with embedded N-heterocyclic carbenes
(NHCs) that have the dual roles of ligand and cross-linking
agent, and we look at their application in the CKT reaction.
We have developed polymer-incarcerated (PI) metal catalysts
with polystyrene-based copolymers containing alcohol and
epoxide moieties as cross-linking agents like copolymer 1
Received: April 22, 2013
© XXXX American Chemical Society
A
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Journal of the American Chemical Society
Communication
the corresponding NHC after the base treatment to form a new
composite material that has incarcerated Ni-NPs with embedded
NHC ligands. The same procedure could be applied to the
preparation of PICB-NHC-Ni (6b) containing CB as a second
support (entry 4).
Table 1. Catalyst Preparations
Next, we conducted high-resolution transmission electron
microscopy (HRTEM) and scanning transmission electron
microscopy (STEM) to characterize the newly prepared Ni-
NPs catalysts (Figure 3 and SI). These analyses have shown that
all these catalysts have similar morphology with formation of Ni-
NPs (∼1−4 nm) with the exception of PICB-NHC-Ni (6c)
(prepared without reducing agent), which showed only
dispersed metals.21
a
b
Determined by ICP-AES analysis. Evaluated from Brunauer−
c
Emmett−Teller analysis. Catalyst prepared without reducing agent.
supports. First, we prepared PICB-Ni (4) using the previous
copolymer 1 and carbon black (CB) as a second support to
expanded the surface area (entry 1). Next, we synthesized two
new polymers: (i) copolymer 2 was synthesized by copoly-
merization between styrene and 1-(4-vinylbenzyl)imidazole, and
(ii) copolymer 3 was also synthesized by copolymerization
between styrene and a monomer with an alkyl bromide
function.21 Unfortunately, the copolymerization of three
monomers gave only insoluble materials, probably because of
in situ cross-linking. Nevertheless, using a molar equivalent of
mixed 2 and 3 instead of 1, we synthesized PI-IL-Ni (5a) with the
expected weight and Ni loading (entry 2).
Figure 3. (a) HRTEM of catalyst 6b. (b) STEM of catalyst 6b.
We tried the CKT reaction with these different catalysts
(Table 2). 3-Bromotoluene (7a) and phenylmagnesium chloride
(8a) were selected as model substrates. A slight excess of
Grignard reagent (GR) (1.25 equiv) was necessary because of
the homocoupling of 8a as a side reaction. We initially
investigated the activity of 4. With only 1 mol% loading, high
temperature (65 °C) was required to obtain 41% yield of the
coupling product 9aa because of poor aryl bromide conversion
(entry 1). In addition, inductively coupled plasma atomic
emission spectrometry (ICP-AES) measurements detected
leaching of Ni to the solution phase following the reaction.
Under milder conditions at room temperature (RT) using
catalyst 4 with no additive, no reaction occurred (entry 2). Next,
using 1 mol% of catalyst 5a at RT, moderate activity without
metal leaching was observed (entry 3). This higher catalyst
activity of 5a compared with 4 might be explained by the effect of
the ionic liquid-like structure as stabilizer or by the formation of
NHC ligand by deprotonation with basic aryl GRs. Encouraged
by these results, we next tried catalyst 6awith which we have
observed embedded NHC ligands in the polymeric structure by
FGSR-MAS NMR analysisat RT. Under these conditions, an
excellent yield in favor of the cross-coupling product 9aa was
observed (entry 4). Catalyst 6b, which was prepared by adding
CB as a second support, showed even higher activity with 98%
yield due to the larger surface area (27.9 m2/g for 6b; < 0.5 m2/g
for 6a) (entry 5). Catalyst 6c, prepared without the reduction
step, had comparable activity; however, nickel leaching was
observed (entry 6).24 Next, we tried to decrease the catalyst
loading. We found that 0.25 mol% of catalyst 6b was enough to
obtain full conversion with the high turnover number (TON) of
392 (entry 7). This TON can be increased to 950 using only 0.1
mol% of catalyst 6b (entry 8). Catalyst 6d, prepared with higher
Ni loading, showed lower activity, which can be explain by a
lower ratio between NHC and Ni (entry 9). Having succeeded in
the CKT reaction using aryl bromide as the electrophile, we
focused on other aryl halides. Under the same conditions, 3-
iodotoluene showed similar reactivity (entry 10). With 3-
Figure 2. FGSR-MAS cpmg spectra of 5a and 6a in d8-THF.
To probe our conceptembedded imidazolium salt and/or
NHC formation using active cross-linkerswe conducted field
gradient swollen-resin magic angle spinning (FGSR-MAS) NMR
experiments with the catalyst 5a using d8-THF as the swelling
solvent (Figure 2, blue signal).22 The cpmg-NMR spectrum of 5a
shows three broad signals at 7.5, 8.0, and 9.9 ppm attributed
respectively to the CHCH and −N-CH-N groups of the
imidazolium salt. This cpmg-NMR spectrum confirmed that the
formation of the imidazolium salts resulted from the reaction of
copolymers 2 and 3 in the cross-linking step.23 It is also known
that imidazolium salts can easily be converted to NHCs by a base
treatment. Therefore, 5a was treated with large excess (25 equiv)
of 1,8-diazabicycloundec-7-ene (DBU), and after filtration and
washing with organic solvents to remove the excess of the base,
PI-NHC-Ni (6a) was obtained (Table 1, entry 3). Next, FGSR-
MAS cpmg-NMR analysis was conducted (Figure 2, red signal).
As expected, the previous signals at 9.9 ppm, attributed to −N-
CH-N of the imidazolium salt structure, have completely
disappeared, and only the broad signals at 7.5 and 8.0 ppm,
assigned this time to CHCH of the NHC structure, were
observed. Two new sharp signals at 7.9 and 3.2 ppm were
detected and assigned to the protonated DBU (DBUH+), which
might be partially retained in the polymeric structure. At this
stage, these FGSR-MAS NMR spectra provided strong evidence
that copolymer 2 reacted with copolymer 3 with the formation of
B
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Journal of the American Chemical Society
Communication
chlorotoluene the yield dropped to 33% (entry 11). To improve
the reactivity, the temperature was increased to 65 °C (entry 12);
at RT, higher catalyst loading (0.5 mol%) and additional LiBr (1
equiv) were necessary to obtain a similar yield of 9aa (entry
13).25 Even C−F bonds could be activated to give the product
9aa in 71% yield at 65 °C (entry 15).26 Finally, using
dimethoxyethane (DME) as a solvent at 100 °C, 9aa could
also be obtained in excellent yield from aryl fluoride (entry 16).
Table 3. Substrate Generality
Table 2. Optimization of the Reaction Conditions
a
R-X (1 equiv), GR (1.25 equiv), 6b (0.25 mol% as Ni), THF, RT, 12
b
h. R-X (1 equiv), GR (1.25 equiv), LiBr (1 equiv), 6b (0.5 mol% as
Ni), THF, RT, 12 h. The reaction was performed at 65 °C. The
reaction was performed in DME at 100 °C. 2.5 equiv of GRs was
used. GR was slowly added. Isolated with contaminating reductive
dehalogenated (RH) product (∼5−10%). R-X (1 equiv), R-
MgCl·LiCl (0. 1.25 equiv), 6b (2.5 mol% as Ni), THF, 0 °C, 12 h.
c
d
e
f
g
g
a
Determined by gas chromatography analysis using dodecane as an
b
c
internal standard. Determined by ICP-MS analysis. The reaction was
performed at 65 °C. LiBr (1 equiv) was used as an additive. The
reaction was performed in DME at 100 °C. Average yield. The
catalyst was reactivated by treatment with DBU. ND = Below the
detection limit of the ICP equipment (<0.003−0.005 ppm).
d
e
substrates for both aryl halide and/or GRs worked well in this
reaction (entries 1−7). Under these conditions, 1-bromo-3-
chlorobenzene (7e) reacted only via the bromide part to lead
exclusively to product 9ea in high yield (entry 8). 1,3-
Dichlorobenzene was converted under reflux conditions to m-
terphenyl (9fa) in 8% yield (entry 9). Similar reactivity was
observed with p-chloro-fluorobenzene (7g), and the reaction was
performed at 100 °C in DME to give 9gc in 85% yield (entry 10).
The naphthyl group was also tolerated, and a high yield with tert-
butyl-phenylmagnesium bromide (8c) was obtained (entry 11).
Next, sp2−sp3 bond formation was investigated using alkyl GRs.
Cyclohexylmagnesium chloride (8f) reacted with 7i to afford 9if
in 84% yield, and 9he was also obtained with the more difficult
tert-butylmagnesium bromide (8e) (entries 12 and 13).19b We
then turned our attention to heteroaromatic halides. 3-
Bromoquinoline (7j) reacted with 8a to afford 9ja in 79% yield
under standard conditions, while 8-chloroquinoline (7k)
required higher reaction temperatures (reflux conditions) to
form the cross-coupling product 9ka in 68% yield, whereas 2-
chloropyridine (7l) reacted with hexylmagnesium chloride (8g)
at RT to give 9lg in excellent yield (entries 13−16). Next, a
mixture of (E) and (Z) isomers of β-bromostyrene (7m) was
tested with 8f, and the product 9mf was obtained with only (E)-
configuration in excellent yield (entry 17). With alkyl iodine
compounds 7n and 8a, the cross-coupling product 9na was
obtained in excellent yield (entry 18). With sp3−sp3 cross-
coupling, using 7n and benzylmagnesium chloride (8g), the
corresponding coupling product 9ng was obtained in high yield
(entry 19). Next, we investigated the functional group tolerance
using GR prepared from Knochel’s method.28 The coupling
products 9oh and 9ph were obtained in good yields with
complete integrity of ester and cyano groups using lower
f
g
The viability of recycling catalyst 6b was examined in the CKT
reaction of aryl bromide 7a with 8a (entries 17−25). Initially, the
catalysts were easily recovered by simple filtration and could be
reused for the next cycle after washing with THF and CH2Cl2
and then drying; however, significant loss of activity was
observed with only 48% yield for the second use. STEM and
energy-dispersive spectroscopy analysis of the recovered catalyst
showed no change of morphology, but a large amount of
magnesium species derived from the GRs remained. To remove
the magnesium salts, we investigated different washing solvent
systems and finally found that washing with an aqueous ammonia
solution followed by washing with THF and CH2Cl2 could help
to revive the catalytic activity completely.27 For the next three
cycles, using the same washing procedure, no significant loss of
activity was observed. The yield dropped to 76% for the sixth use,
since this catalyst had been stored for 2 months without any
precaution (i.e., in bench/open-air conditions) (entry 21).
Nevertheless, the activity could be recovered using a similar
washing method followed by treatment with DBU (in order to
regenerate NHC ligands). The high activity was successfully
restored and remained for a further five uses. These recovery and
reuse experiments demonstrate the recyclability of 6b and in
particular the robustness of the embedded NHC ligands.
With optimized conditions in hand, we next studied the scope
and limitation of the CKT reaction (Table 3). First, we focused
on sp2−sp2 bond formation using aryl bromide at RT and 0.25
mol% of 6b. Both electron-rich and electron-deficient aryl
C
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Journal of the American Chemical Society
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21).29 Finally, even functionalized Knochel-type GR such as 8i
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our best of knowledge, the direct use of Knochel-type GRs with
Ni catalysts is unprecedented in the literature.
In summary, we have developed Ni-NPs catalysts with NHC
precursor moieties, which play dual roles: cross-linking parts in
the PI method and access to NHC ligands to activate Ni-NPs.
These NHCs embedded in the polymer matrix were
characterized by FGSR-MAS NMR analysis. Using this novel
immobilization method, we demonstrate that ligands and Ni-
NPs could be immobilized together to offer a direct, usable
heterogeneous catalyst for which no external ligands are
necessary. This heterogeneous catalyst was successfully applied
to CKT reactions with quite wide substrate generality (e.g., aryl,
vinyl, and alkyl halides with aryl or alkyl, Knochel-type GRs) in
high yields including functional group tolerance. As for
heterogeneous reactions in general, the simplicity in workup is
especially attractive. Importantly, retention of nickel on the solid
support without leaching allows products without metal
contamination and results in recovery and reuse to decrease
waste and to develop greener synthetic methods. We believe that
the direct comparison of catalytic activities between PI Ni-NPs
with embedded NHCs and PI Ni-NPs prepared from copolymer
1 could explain the positive effect of NHC ligands on the Ni-NPs.
Further applications of these Ni-NPs catalysts to other reactions
and more investigations to understand the surface interaction
between Ni-NPs and NHC moieties in polymer are ongoing.
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ASSOCIATED CONTENT
* Supporting Information
Reaction procedure and spectra. This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
(18) Ranganath, K. V. S.; Kloesges, J.; Schafer, A. H.; Glorius, F. Angew.
Chem., Int. Ed. 2010, 49, 7786.
̈
AUTHOR INFORMATION
■
(19) (a) Tennyson, A. G.; Lynch, V. M.; Bielawski, C. W. J. Am. Chem.
Soc. 2010, 132, 9420. (b) Joshi-Pangu, A.; Wang, C.-Y.; Biscoe, M. R. J.
Am. Chem. Soc. 2011, 133, 8478. (c) Mitsudo, K.; Doi, Y.; Sakamoto, S.;
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metallics 2011, 30, 2546. (e) Iglesias, M. J.; Prieto, A.; Nicasio, M. C. Org.
Lett. 2012, 14, 4318. (f) Kremzow, D.; Seidel, G.; Lehmann, C. W.;
Corresponding Author
shu_kobayashi@chem.s.u-tokyo.ac.jp
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Furstner, A. Chem.Eur. J. 2005, 11, 1833.
̈
This work was partially supported by a Grant-in-Aid for Science
Research from JSPS, Global COE Program, The University of
Tokyo, MEXT, Japan, JST, and NEDO. We also thank Mr.
Noriaki Kuramitsu (The University of Tokyo) for STEM analysis
and JEOL Co. Ltd. for help with FGSR-MAS analysis.
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(21) See the SI for more informations.
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(24) Ni-NPs were formed during the reaction from Ni(II) with
PhMgCl as reducing agent.
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NOTE ADDED AFTER ASAP PUBLICATION
Table 1 graphic was replaced on July 10, 2013.
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