Asymmetric nanocatalysis: N-heterocyclic carbenes as chiral modifiers of Fe3O4/Pd nanoparticles

Article abstract of DOI:10.1002/anie.201002782

Superficial success: A chiral N-heterocyclic carbene (NHC ^*) is used to modify Fe₃O₄/Pd nanoparticles, which then catalyze asymmetric α-arylations. This successful synthesis of a heterogeneous catalyst and its appliation i

Full text of DOI:10.1002/anie.201002782

Communications
DOI: 10.1002/anie.201002782
Nanocatalysis
Asymmetric Nanocatalysis: N-Heterocyclic Carbenes as Chiral
Modifiers of Fe3O4/Pd nanoparticles**
Kalluri V. S. Ranganath, Johannes Kloesges, Andreas H. Schꢀfer, and Frank Glorius*
Although asymmetric homogeneous catalysis has been a
major focus of academic research over the past decades, only
very few catalytic systems have proven suitable for large-scale
industrial production.[1] Heterogeneous catalytic systems are
favored by industry because of the ease of handling, workup,
and purification of products, in addition to the robustness and
reusability of the catalyst itself. In order to overcome
problems of homogeneous catalytic reactions, many different
concepts have been developed to generate chiral heteroge-
neous catalysts.[2]
Herein, we report the formation of Pd NPs on magnetite
(Fe3O4) and subsequent surface modification of the resulting
bimetallic NPs by chiral, enantiomerically pure NHCs. This
readily accessible catalyst was both successfully applied in
asymmetric heterogeneous catalysis and also recycled several
times.
The Fe3O4/Pd NPs were prepared by the so-called wet
impregnation method as reported in the literature.[11] In order
to explore the possibility of achieving asymmetric catalytic
reactions on active NPs, the surface was modified with chiral,
enantiomerically pure imidazolinium salts L1–L5 (Figure 1,
Nanoparticles (NPs)[3] can be considered a semi-hetero-
geneous support,[4] as they are readily dispersed in the
reaction medium, exhibit an intrinsically high surface area,
and display highly accessible surface-bound catalytic sites.
Fe3O4 NPs[5] have been increasingly recognized as an attrac-
tive support for applications in (asymmetric)[2g,6] catalysis;[7]
these NPs are readily available, robust, and magnetically
recoverable (thereby obviating tedious catalyst filtration after
the reaction).
N-Heterocyclic carbenes (NHCs) are known to form
exceptionally stable complexes with many metals. Conse-
quently, they have emerged as versatile donor ligands in
transition-metal catalysis[8] and several highly selective appli-
cations in asymmetric catalysis have been reported.[9] How-
ever, to the best of our knowledge, the successful use of
enantiomerically pure NHCs as chiral modifiers[2,10] for
heterogeneous catalysts has not yet been reported.
Figure 1. Preparation of Fe3O4/Pd NPs modified by chiral NHC; for
clarity, the sizes are not represented proportionally.
Scheme 2) under basic conditions. These modifiers were
selected because they are easily prepared from the corre-
sponding amino alcohols (thus allowing ample structural
diversity) and because they can be readily converted to the
free NHCs using base.[12]
[*] Dr. K. V. S. Ranganath, Dr. J. Kloesges, Prof. Dr. F. Glorius
Westfꢀlische Wilhelms-Universitꢀt Mꢁnster
Organisch-Chemisches Institut
Initially, the Fe3O4/Pd/L1 nanocatalyst[13] was character-
ized by X-ray photoelectron spectroscopy (XPS): the spec-
trum showed binding energies for the Pd3d electrons of 335.7
and 340.7 eV, which corresponds to Pd in the 0 oxidation state.
Further signals at 400 and 285 eV were attributed to the N1s
and C1s energy levels, respectively.[13] ATR-IR spectroscopy
further supported the presence of an NHC-modified surface,
since marked differences between the spectra of the free salt
L1 and of Fe3O4/Pd/L1 are evident.[13] Similarly, different
BET surfaces were obtained for Fe3O4, Fe3O4/Pd and Fe3O4/
Pd/L1 (43, 21, and 35 m2 gÀ1, respectively).[13]
The scanning electron microscopy/energy-dispersive X-
ray analysis (SEM-EDX) spectrum of the NHC-modified
surface also indicated the presence of all expected elements
(Fe, Pd, O, and C) and more precisely a Pd content of
0.9 wt%. Inductively coupled plasma optical emission spec-
troscopy (ICP-OES) also showed a Pd loading of 0.92 wt%.
Additionally, the NPs were characterized by transmission
Corrensstrasse 40, 48149 Mꢁnster (Germany)
Fax: (+49)251-833-3202
E-mail: glorius@uni-muenster.de
Homepage: http://www.uni-muenster.de/Chemie.oc/glorius/
index.html
Dr. A. H. Schꢀfer
Nano Analytics GmbH
Heisenberg Strasse 11, 48149 Mꢁnster (Germany)
[**] Generous financial support by the Alexander von Humboldt’Foun-
dation (K.V.S.R.), the Deutsche Forschungsgemeinschaft (IRTG
Mꢁnster-Nagoya; J.K.), and AstraZeneca is gratefully acknowl-
edged. The research of F.G. has been supported by the Alfried Krupp
Prize for Young University Teachers of the Alfried Krupp von Bohlen
und Halbach Foundation. We thank Prof. Dr. Gerhard Wilde and Dr.
Harald Rꢂsner for TEM analyses, Prof. Dr. Uwe Karst and Michael
Holtkamp for ICP-OES analyses, and Dr. Hubert Koller and Mark
Weiß for determination of the BET surfaces.
Supporting information for this article (experimental procedures
and full spectroscopic data for all new compounds) is available on
the WWW under http://dx.doi.org/10.1002/anie.201002782.
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electron microscopy (TEM) after the surface had been
modified with L1. The TEM images show iron oxide NPs
25–35 nm in diameter (Figure 2).
NPs with 2.5 mol% of L1 were found to be optimal in terms
of selectivity and reactivity. Increasing the amount of L1 to
10 mol% led to a dramatically decreased activity, and low-
ering to 1.25 mol% resulted in a significantly lower enantio-
selectivity.[13] Thus, when we used 2.5 mol% L1 and followed
the indicated in situ protocol, the a-arylated product was
isolated in 74% yield with 53% ee (Scheme 2). Alternatively,
Figure 2. TEM image of Fe3O4/Pd NPs after modification with L1.
The catalytic properties of the surface-modified Fe3O4/Pd
NPs were initially evaluated in the a-arylation reaction of 2-
methyl-1-tetralone and various phenyl halides in toluene at
808C using NaOtBu as the base (Scheme 1). This Pd-
catalyzed asymmetric transformation provides valuable inter-
mediates for the pharmaceutical industry.[14–16]
Scheme 2. Effect of various chiral modifiers on the yield and enantio-
selectivity of the a-arylation of 2-methyl-1-tetralone with bromoben-
zene. Reaction conditions: Fe3O4/Pd (50 mg), ligand (2.5 mol%), 2-
methyl-1-tetralone (0.3 mmol), PhBr (0.6 mmol), NaOtBu (0.6 mmol),
toluene (3.0 mL).
prior mixing of the imidazolium salt L1 with the base and
stirring for 1 h, followed by addition of the Fe3O4/Pd NP
resulted in an equally active and selective (48% ee) catalyst.
The heterogeneous nature of this catalyst system was
demonstrated by a number of experiments:
Scheme 1. The asymmetric a-arylation of 2-methyl-1-tetralone using
chloro- and bromobenzene.
Firstly, the Fe3O4/Pd/L1 catalyst was used five times
without any significant decrease in either activity or selectiv-
ity.[13] After completion of the first a-arylation reaction, the
paramagnetic catalyst was removed with a magnet, washed
sequentially with EtOH and CH2Cl2, and finally dried under
high vacuum for 30 min. A new reaction was then performed
with fresh reactants and NaOtBu under the same condi-
tions.[13] It is important to note that the residual solution (after
the magnetic removal of the Fe3O4/Pd/L1 NP) was no longer
catalytically active (filtration test).[13]
Secondly, the leaching of the catalytically active Pd from
the Fe3O4 support was studied by ICP-OES analysis of the
catalyst before and after the second reaction cycle (Scheme 1,
X = Br): the Pd concentration in the NP was found to be
0.92 wt% before and 0.88% after the reaction. The liquid
phase of the reaction mixture had a low Pd content of
0.232 ppm (ICP-OES). This negligible difference in the
catalystsꢀ Pd content shows the stabilizing effect of L1. For
comparison, the unmodified catalyst (Fe3O4/Pd only) dis-
We found that the Fe3O4/Pd/L1 NPs performed as a
versatile chiral nanocatalyst in the reaction with bromoben-
zene, furnishing the corresponding a-arylated product in 72%
yield and with pronounced selectivity of 48% ee; the reaction
with chlorobenzene proceeded in 56% yield and 60% ee.
There was no reaction in the absence of Pd when only L1-
modified Fe3O4 was employed. To better understand the
effect of modifier on the surface, we also conducted the
reaction without any chiral modifier (Fe3O4/Pd only); this
reaction furnished the expected a-arylated product in 22%
yield (naturally, in racemic form) along with a significant
number of by-products, one of which was identified (GC–MS)
as biphenyl.
We continued our study by preparing the catalysts in situ,
mixing Fe3O4/Pd NPs with one of the corresponding imida-
zolinium salts L1–L5 in the presence of base. All further
experiments were run using this in situ protocol, obviating the
isolation/purification of the desired NHC-modified NPs. The
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Table 1: Asymmetric a-arylation of ketones with aryl halides catalyzed by
Fe3O4/Pd/L1 (using the in situ protocol).[a]
played a significantly decreased activity in the second cycle,
and in the third cycle no reaction took place. This behavior
can be attributed to the leaching of Pd into the solution
(16.6 ppm, determined by ICP-OES) and depletion of the Pd
content on the magnetite surface.
The mercury-poisoning experiment[17] is well established
for the investigation of the heterogeneous nature of a catalyst
system. While a homogeneous Pd–L1 complex derived from
[{Pd(allyl)Cl}2] and L1 did not lose its catalytic activity upon
addition of elemental Hg, the same addition of Hg completely
deteriorated the catalytic activity of the Fe3O4/Pd/L1 NP
catalyst by poisoning the surface—unequivocal proof of the
heterogeneous nature of the catalyst.[13]
Entry
1
Ketone
Aryl halide
Yield [%][b]
56
ee [%][c]
À
Ph Cl
61
À
2
3
Ph Br
74
70
53
45
À
Ph I
Finally, the heterogeneous nature of the Fe3O4/Pd/L1
catalyst system is confirmed by the dramatically lower ee
obtained with several homogeneous Pd–L1 complexes. As a
typical example, the combination of Pd(OAc)2 and L1 led to
the formation of racemic a-arylation product only.[13] In
addition, under all homogeneous reaction conditions exam-
ined, biphenyl and other products were also observed by GC–
MS. Similarly, when magnetite NPs were added to the
reaction mixture (again using Pd(OAc)2 and L1) only the
racemic product was obtained.[13] These experiments strongly
emphasize the importance of the components of this novel
catalyst system: a formed heterogeneous paramagnetic Fe3O4/
Pd NP catalyst stabilized and activated by chiral NHCs
bearing secondary hydroxy groups.[18] To the best of our
knowledge, this represents the first successful use of NHCs as
chiral modifiers of NP catalysts.[19]
4
58
33
5
90
61
6
7
83
78
60
57
À
Ph Br
8
9
61
66
35
40
Clearly, the success of the asymmetric reaction will
strongly depend on the structure of the ligand. A screening
of various alternative ligands showed pronounced effects not
only with regard to the activity, but also with regard to the
enantioenrichment of the a-arylated product (Scheme 2).
Especially for the latter feature, hydroxy groups on the ligand
seem to be crucial.[20]
10
11
91[d,e]
81[e]
85
68[d]
To test the substrate scope of the Fe3O4/Pd/L1-catalyzed
enantioselective a-arylation reaction, various aryl halides
were examined (Table 1). Notably, unactivated aryl chlorides
also reacted with 2-methyl-1-tetralone, furnishing the desired
product in 56% yield with 61% ee, whereas in the same
reaction with iodobenzene the product was formed with a
slightly reduced enantioselectivity of 45% ee (Table 1,
entries 2 and 3). The meta- and para-substituted aryl bromides
gave good yields and moderate to good ee values. The
reactions of 2-methyl-1-indanone with aryl bromides resulted
in good yields with substantial enantioselectivities (Table 1,
entries 8 and 9). Moreover, in challenging intramolecular a-
arylation reactions (Table 1, entries 10 and 11) the L1-
modified NP catalyst showed good activity and selectivity,
providing the indanone products in up to 85% ee.
In conclusion, we have reported the formation of a
heterogeneous catalyst from Fe3O4/Pd NPs and enantiomer-
ically pure NHCs. The resulting NP catalyst with its new
ensemble of catalytically active entities[21] catalyzed asym-
metric a-arylation reactions with up to 85% ee. In addition,
simple magnetic removal and recycling of the catalyst was
shown to proceed without loss of activity and selectivity. We
are currently working on the elucidation of the underlying
[a] Reaction conditions: Ketone (0.3 mmol), aryl halide (0.6 mmol),
NaOtBu (0.6 mmol), Fe3O4/Pd NP (50 mg), ligand L1 (2.5 mol%).
[b] Yield of isolated product. [c] Determined by HPLC on a chiral
stationary phase. [d] Substrate (0.3 mmol), NaOtBu (0.6 mmol), Fe3O4/
Pd modified with L1 (50.0 mg), PhMe(3.0 mL). [e] Under homogeneous
conditions ([{Pd(allyl)Cl}2] (5.0 mol%), ligand L1 (10 mol%), substrate
(0.3 mmol), NaOtBu (2.0 equiv), PhMe (3.0 mL)); the product was
formed in 32% yield with 24% ee.
active principle of this new class of heterogeneous catalysts
for asymmetric catalysis.
Received: May 7, 2010
Revised: July 1, 2010
Published online: September 15, 2010
Keywords: a-arylation · asymmetric catalysis · chiral modifiers ·
.
nanoparticles · N-heterocyclic carbenes
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