Tailor-made N-heterocyclic carbenes for nanoparticle stabilization

Article abstract of DOI:10.1039/c4cc00654b

N-heterocyclic carbenes (NHCs) represent a leading class of ligands in organometallic chemistry, but have been rarely exploited as stabilizers for metal nanoparticles (NPs). We report the first example of NHC stabilized Pd-NPs that demonstrate long term stability. These NHC Pd-NPs were synthesized by a facile ligand exchange protocol using rationally designed long chained NHCs (LC-NHCs). Furthermore, we demonstrate that the surface modification of Pd-NPs results in significant chemoselectivity in a model reaction. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4cc00654b

View Article Online
View Journal
ChemComm
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: C. Richter, K.
Schaepe, F. Glorius and B. J. Ravoo, Chem. Commun., 2014, DOI: 10.1039/C4CC00654B.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/chemcomm

ChemComm
Page 1 of 4Journal Name
Dynamic Article Links ►
View Article Online
DOI: 10.1039/C4CC00654B
Cite this: DOI: 10.1039/c0xx00000x
www.rsc.org/xxxxxx
ARTICLE TYPE
Tailor-made N-Heterocyclic Carbenes for Nanoparticle Stabilization
Christian Richter‡, Kira Schaepe‡, Frank Glorius*a, Bart Jan Ravoo*b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
5
N-heterocyclic carbenes (NHCs) represent a leading class of
Ligand
exchange
NHC complex
decomposition
ligands in organometallic chemistry, but have been rarely
exploited as stabilizers for metal nanoparticles (NPs). We
report the first example of NHC stabilized Pd-NPs that
demonstrate long term stability. These NHC Pd-NPs were
NHC
NHC
NHC
NHC
L
L
L
L
H- or H2
NHC
M
M
L
M NHC
10 synthesized by a facile ligand exchange protocol using
rationally designed long chained NHCs (LC-NHCs).
Furthermore, we demonstrate that the surface modification
of Pd-NPs results in significant chemoselectivity in a model
reaction.
Chaudret et al. (Ru)
Tilley et al. (Au)
Chechik et al. (Au, Pd)
This work
(Pd, Au)
50
Scheme 1. Approaches for the synthesis of NHC modified NPs. L:
Ligand. M: Metal
15 In recent years nanoparticles (NPs) have become increasingly
important in medicine, material chemistry and catalysis.[1] They
are particularly important in catalysis due to their high surface
area and the metalꢀmetalꢀinteractions within the NP, which can
open up new reaction pathways. The major challenge in the
20 application of NPs is stabilization to avoid agglomeration which
is typically achieved by ligands such as thiols, amines, disulfides,
thioethers or phosphines.[2] During the last 20 years Nꢀ
heterocyclic carbenes (NHCs) established themselves as a leading
class of ligands in organometallic chemistry.[3] NHCs are neutral
25 electronꢀrich ligands, which often form very strong metal ligand
bonds. These properties would also be very attractive for the
stabilization and activation of NPs, but it is unclear if these
characteristics can be extrapolated from molecular complexes to
NHCꢀNP adducts as NHCs have rarely been exploited in
30 nanoparticle chemistry.[4] There are three seminal examples of
structurally defined NHCꢀstabilized metal NPs with limited
utility: Chaudret et al.[4a] and Tilley et al.[4b] could prepare NHCꢀ
stabilized Ruꢀ and AuꢀNPs by the selective degradation of the
corresponding metal complexes, whilst Chechik et al.
35 investigated a ligand exchange approach (Scheme 1).[4c] Chechik
et al. could demonstrate that the exchange of thioethers by NHCs
on the surface of Auꢀ and PdꢀNPs is possible but their resulting
NP systems lacked stability and aggregated within hours. Very
recent work introduced NHC stabilized Agꢀ[4e] and PtꢀNP,[4f] and
40 the application of NHC stabilized Ruꢀ and PtꢀNP in the
hydrogenation of aromatics[4d] and nitroaromatics[4f] was
demonstrated. Herein we report the first example of NHC PdꢀNPs
that demonstrate long term stability by rational design of novel
micelle inspired long chained NHCs (LCꢀNHCs). Furthermore,
45 we demonstrate that the surface modification results in significant
chemoselectivity in a model reaction. Our data suggest that this is
not merely an effect of catalyst deactivation.
Figure 1. Comparison of different NHC designs
It is striking that currently stable NHCꢀNPs have only been
55 prepared by a complex decomposition approach (Scheme 1).
Preparation using a simple ligand exchange protocol would be
highly desirable because this method can utilize NPs prepared
using varied and established protocols providing access to various
metal NPs and NP shapes.[5] We hypothesized that suitable NHCs
60 should bear small or flexible substituents on the nitrogen atoms,
to minimize steric repulsion between the NHC and the surface.[4c]
Therefore, we chose small Nꢀmethyl substituents, or alternatively,
flexible benzyl substituents which may provide additional
coordination via πꢀsurface interactions. Furthermore, we designed
65 our NHCs with long aliphatic chains on positions 4 and 5 of the
heterocycle to benefit from the steric repulsion between the NPs
and provide the possibility to form protective monolayers on the
surface.[2] The NHC design is illustrated in Figure 1. Notably,
since we proposed that the long aliphatic chains would provide
70 the required stabilization of the NPs, the Nꢀsubstituents are still
available for further modification. This would provide the
possibility to fix a small functional group pointing directly to the
metal surface.[4k] This characteristic is difficult to realize with
conventional stabilizers such as thiols or amines.
This journal is © The Royal Society of Chemistry [year]
[journal], [year], [vol], 00–00 | 1

ChemComm
Page 2 of 4
View Article Online
DOI: 10.1039/C4CC00654B
9
9
1•HI: R=Me, X=I
2•HBr: R=Bn, X=Br
N
N
R
R
H
X
R
NaOtBu
N
R
R
9
Pd
S
Pd
N
MeCN / hexane
overnight
n
9
R
m
thioether
O
1
3
@Pd-NP: R=Me
@Pd-NP: R=
O
3
2@Pd-NP: R=Bn
4@Pd-NP: R= C12H25
Scheme 2. Ligand exchange reaction on thioether stabilized PdꢀNP with
Figure 3. TEM images of 3@PdꢀNP (left) and 1@PdꢀNPs (right) (for
2@PdꢀNPs see SI).
LCꢀNHCs.
obtained identical results to those in the exchange reaction of IPr,
40 strongly supporting our concept.[6] It should be pointed out that
sterically demanding NHCs were successfully applied to stabilize
smaller NPs (1.2 ꢀ2.2 nm) synthesized via the decomposition
approach.[4a,b] It seems feasible that smaller NP with a bigger
curvature tolerate bulky ligands, whilst with increasing size of the
45 NP and thus a smaller curvature, the substitution becomes more
critical. Therefore, we propose that small substituents would be
preferable to enable the development of a general exchange
protocol.
NMR analysis of 1@PdꢀNP and 2@PdꢀNP proved that the
50 thioether 3 was completely exchanged by the LCꢀNHCs, as no
signals of the thioether 3 were detected and only signals of the
alkyl chains of the NHCs were observed (Figure 2). The signals
of the protons located close to the NP surface are not visible due
to the broadening of the signals.[4a,8] When the unpolar PdꢀNPs
55 (4@PdꢀNP) were used to perform the exchange the thioether 4
could not be completely removed leading to a mixed ligand
environment (1•4@PdꢀNP; ratio of 1/4 = 1:1 according to 1H
NMR). Intriguingly, in this case sharp NMR signals were
observed for the NHC ligands and the binding of the NHCs to the
60 NP surface was verified by PFGꢀNMR measurements.[6]
Furthermore, a 13C labelled analogue of 1•HI was synthesized to
investigate the binding mode of the NHC. After the exchange
reaction with the 4@PdꢀNP,[9] the C1 atom of 13C1@PdꢀNPs
showed a downfield shift compared to the imidazolium salt in the
65 13C NMR spectrum (168.2 ppm (13C1) / 136.5 ppm (13C1•HI)).
Thus, NMR indicates that the carbene is indeed the binding
species.
The size of the LCꢀNHC PdꢀNPs were analyzed by transmission
electron microscopy (TEM) and dynamic light scattering (DLS).
70 According to TEM, 1@PdꢀNPs have a narrow size distribution of
4.4 (0.5) nm indicating that no aggregation occurred during the
exchange reaction. The hydrodynamic diameter of the NPs was
determined by DLS. It was found to be to 5.6 nm before and after
the exchange reaction.[6] TGA and elemental analysis were
75 conducted to investigate the composition of the NPs. The NPs
have a metal content of 67ꢀ72% for 1@PdꢀNP and 55ꢀ62% for
2@PdꢀNP, which is consistent with previous reports.[4aꢀc] In
addition to that, the C:H:N ratios were in good agreement with
the calculated values for the NHCs, confirming that the NPs are
80 only stabilized by the carbenes.
5
Figure 2. 1H NMR spectra of 1@PdꢀNP (top) and 3@PdꢀNP. Solvent:
C6D6.
The long chain imidazolium salts 1•HI and 2•HBr, precursor for
the LCꢀNHCs, where synthesized in a 4 step sequence. Starting
from commercially available aldehydes a sequence of benzoin
10 condensation, oxidation, cyclization and alkylation led to the
corresponding imidazolium salts.[6] PdꢀNPs with weak thioether
ligands as leaving group were synthesized according to Obare et
al.[7] Unpolar PdꢀNPs (4.0 (0.5) nm) were stabilized by
didodecylsulfide (4). Polar PdꢀNPs (4.5 (0.4) nm) were
15 functionalized with bis(tetraethylene glycol monoꢀmethyl
ether)sulfide (3). The polar thioether 3 was synthesized in two
steps.[6] We selected this thioether in order to be able to remove
the unbound ligand after the exchange reaction by simple
extraction with a polar solvent. The LCꢀNHC stabilized PdꢀNPs
20 were synthesized by mixing 3@PdꢀNP with the corresponding
LC imidazolium salt (1•HI or 2•HBr) and NaOtBu in a two phase
system of hexane and acetonitrile (Scheme 2). After purification
the LCꢀNHC PdꢀNP could be isolated as brown solid. It should be
noted that the LCꢀNHC PdꢀNPs are stable over four months and
25 can be handled under air. To confirm the crucial role of the
aliphatic chains, the corresponding benzannulated or
unsubstituted NHCs were also tested as stabilizers. Although an
exchange occurred, the resulting systems showed poor stability:
storage for 12 h or exposure to air led to fast aggregation of these
30 NPs.[6] In addition, we crossꢀchecked the ligand exchange by
using the sterically demanding NHC IPr (1,3ꢀbis(2,6ꢀ
diisopropylphenyl)ꢀimidazolydene) and obtained similar results
to the Chechik group. An exchange was observed but the
resulting material quickly aggregated.[6] We also prepared an IPr
35 analogue bearing long alkyl chains on the backbone (LCꢀIPr) and
The strength of the coordination of the LCꢀNHCs to the NP was
investigated by competition experiments by adding either an
excess of a thioether or a thiol, which is known to bind very
strongly to noble metal NPs.[2] After the treatment of 1@PdꢀNP
2
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]

Page 3 of 4
ChemComm
View Article Online
DOI: 10.1039/C4CC00654B
1@Pd-NP
+
+
R
R'
R
R'
10 bar H2, 40 °C
toluene
O
OH
7
5
6
7
8
S: 0%
S: 0%
S: 99%
S: 93%
P
: 97%
P
: 69%
P
P
: 0%
: 0%
Scheme 3. Competition experiment. Conditions: Olefin (0.40 mmol),
1@PdꢀNP (0.5 mg), toluene (1 mL), hydrogen (10 bar), 40 °C, 18 h.
Yields were determined by calibrated GCꢀFID. S: substrate.
P: hydrogenation product
Figure 4. TEM image (left) and DLS measurement (right) of 1@PdꢀNPs
after catalytic reaction (for 2@PdꢀNPs see SI).
5
with excess of thioether 3 and washing, no changes in the NMR
spectra could be observed, indicating no change in the
composition of the ligand sphere occurred. In contrast, treating
55 were applied as catalyst the hydrogenation proceeded with very
low chemoselectivity.[6] These experiments arguably demonstrate
that the NHC influences the reactivity of the Pd surface and that a
strong binding between stabilizer and surface is essential.
Another interesting result was obtained when 1•4@PdꢀNPs were
60 applied, which are stabilized by a mixture of thioether 4 and LCꢀ
NHC 1: these NPs give the same chemoselectivity as the pure
1@PdꢀNPs.[6] Keeping in mind that in the case of the mixed
stabilized NPs we could prove by NMR experiments that the
NHC is binding via the carbene carbon atom to the PdꢀNPs and
65 that the thioether 4 could not induce chemoselectivity, this
experiment strongly indicates that the observed chemoselectivity
is caused by the surface modification with the NHC ligands. The
addition of a drop of mercury to a running reaction immediately
suppressed any reactivity, suggesting that the mode of action is
70 heterogeneous and no in situ formed homogeneous species is
responsible for the observed chemoselectivity.[6,10]
1@PdꢀNPs
with
the
corresponding
thiol (2,5,8,11ꢀ
10 tetraoxatridecaneꢀ13ꢀthiol 15)[6] the resulting PdꢀNPs were
extracted into the polar phase indicating that the NHCs were
replaced by the polar thiol. After washing with hexane the NMR
spectra showed that the PdꢀNPs were functionalized with the thiol
and no signals corresponding to the NHC could be observed.
15 These experiments show that the NHC is not irreversibly bound
to the NP surface and can be exchanged by a stronger ligand such
as a thiol. On the other hand, weaker ligands like thioethers are
not able to replace the NHCs from the surface. In terms of the
generality of the developed protocol we were delighted to see that
20 the exchange reaction was not limited to PdꢀNPs. When thioether
protected AuꢀNPs were utilized, the exchange proceeded
smoothly under standard conditions yielding stable LCꢀNHC Auꢀ
NPs.[6]
Regarding the stability of the LCꢀNHC NP system, DLS and
TEM analysis before and after the catalytic runs revealed that the
size of the NPs remained unchanged (4.5 (0.3) nm) (Figure 4).
75 Furthermore, when a two months old batch of PdꢀNPs was used
as catalyst, or the reaction was run under harsher conditions
(100 °C, 20 bar) the same chemoselectivity was observed,
indicating a strong interaction between ligand and surface.[9]
It is important to note that the chemoselectivity is not caused by a
80 simple reduction of the general activity of the PdꢀNP after surface
modification, and that it is not necessary to stop the reaction after
a certain time to prevent conversion of the higher substituted
olefins. The same result for the competition experiment was
obtained when the reaction was stopped after 1 h, which means
85 that the conversion of styrene (5) and 1ꢀdecene (6) is completed
rapidly and in the residual 17 h the catalyst is unable to
hydrogenate the remaining olefins. In addition to this,
citronellol (8) alone was submitted to the reaction conditions and
no hydrogenation product was detected after 18 h.[6] To rule out a
90 catalyst poisoning by starting materials, products or side
products, a second portion of starting materials was added at the
end of the reaction and styrene (5) and 1ꢀdecene (6) were again
completely converted. The observed chemoselectivity may be
caused by the site selective binding of the NHC to corners, edges
95 and faces exhibited on the surface of the NPs. Chaudret et al.
have proven in their systems that NHCs preferentially bind to low
valent sites such as corners.[4a] As different sites offer different
activities, a site specific binding leading to the observed
chemoselectivity is feasible.
After the successful synthesis of the LCꢀNHC PdꢀNPs, we were
25 interested to see whether these new NP systems are still
catalytically active or the long chains of the ligand shield the
active surface. Therefore, we chose the chemoselective
hydrogenation of olefins as a model reaction to test if the NHC
influences the activity or selectivity of the PdꢀNPs.[9] Styrene (5),
30 1ꢀdecene (6), 3ꢀmethylꢀ2ꢀcyclohexenon (7) and citronellol (8)
were selected as representative substrates, because of the different
reactivity of the double bonds (different degree of substitution,
conjugated or nonꢀconjugated). All substrates were submitted
together to different hydrogenation condition in competition
35 experiments, which revealed that the 1@PdꢀNPs could
differentiate between the substrates by their degree of substitution
(Scheme 3).[9] While styrene (5) and 1ꢀdecene (6) were fully
converted, 3ꢀmethylꢀ2ꢀcyclohexenon (7) and citronellol (8)
remained unchanged in the reaction mixture. Interestingly, 1ꢀ
40 decene (6) was not fully hydrogenated but the partial
isomerization of the double bond in internal positions was
observed. These disubstituted double bonds were not
hydrogenated so that 1ꢀdecene (6) was converted to a mixture of
decane and internal decenes. When 2@PdꢀNPs were applied as
45 catalyst, the same selectivity was observed.[6]
Control experiments were conducted to ensure that the observed
chemoselectivity is caused by the NHC surface modification.[6]
First the competition experiment was repeated with commercial
Pd/C as the catalyst. Under identical conditions no
50 chemoselectivity was observed and all substrates were fully
hydrogenated. When thioether stabilized PdꢀNPs (4@PdꢀNP)
100 Finally the selectivity of our NP system was demonstrated by the
This journal is © The Royal Society of Chemistry [year]
Journal Name, [year], [vol], 00–00 | 3

ChemComm
Page 4 of 4
View Article Online
DOI: 10.1039/C4CC00654B
3612; g) T. Dröge, F. Glorius, Angew. Chem. Int. Ed., 2010, 49,
6940; h) NꢀHeterocyclic Carbenes in Transition Metal Catalysis (Ed.:
C. S. J. Cazin), Springer, Berlin, 2011.
4
a) P. Lara, O. RivadaꢀWheelaghan, S. Conejero, R. Poteau, K.
Philippot, B. Chaudret, Angew. Chem. Int. Ed., 2011, 50, 12080; b) J.
Vignolle, T. D. Tilley, Chem. Commun., 2009, 7230; c) E. C. Hurst,
K. Wilson, I. J. S. Fairlamb, V. Chechik, New J. Chem., 2009, 33,
1837; d) D. GonzalezꢀGalvez, P. Lara, O. RivadaꢀWheelaghan, S.
Conejero, B. Chaudret, K. Philippot, P. W. N. M. van Leeuwen,
Catal. Sci. Technol., 2013, 3, 99; e) X. Ling, N. Schaeffer, S. Roland,
M. Pileni Langmuir, 2013, 29, 12647; f) P. Lara, A. Suarez, V.
Colliere, K. Philippot, B. Chaudret ChemCatChem DOI:
10.1002/cctc.201300821. It was suggested that NHCs could be an
intermediate species, when NP are stabilized in ionic liquids. The
following two references deal with this topic: g) L. S. Ott, M. L.
Cline, M. Deetlefs, K. R. Seddon, R. G. Finke, J. Am. Chem. Soc.,
2005, 127, 5758; h) C. J. Serpell, J. Cookson, A. L. Thompson, C. M.
Brown and P. D. Beer Dalton Trans., 2013, 42, 1385. For two
successful applications of supported chiral NHCꢀmodified PdꢀNP in
asymmetric catalysis, see: i) K. V. S. Ranganath, J. Kloesges, A. H.
Schäfer, F. Glorius, Angew. Chem. Int. Ed., 2010, 49, 7786; j) K. V.
S. Ranganath, A. Schäfer, F. Glorius, ChemCatChem, 2011, 3, 1889.
For NHCꢀmodified gold surfaces, see: k) A. V. Zhukhovitskiy, M. G.
Mavros, T. V. Voorhis, J. A. Johnson, J. Am. Chem. Soc., 2013, 135,
7418.
60
65
70
75
80
85
90
95
Scheme 4. Selective hydrogenation of 3,7ꢀdimethyloctꢀ6ꢀenꢀ1ꢀyl acrylate
(10).
hydrogenation of 3,7ꢀdimethyloctꢀ6ꢀenꢀ1ꢀyl acrylate (9), which
contains two double bonds (Scheme 4). Under standard
conditions the hydrogenation processed smoothly and selectively
to 3,7ꢀdimethyloctꢀ6ꢀenꢀ1ꢀyl propionate (10), which was isolated
in 98% yield. Again, Pd/C and the thioethersꢀstabilized 4@PdꢀNP
gave markedly different results.[6]
5
10 Conclusions
In conclusion, we report a novel class of NHC ligands bearing
long alkyl chains at their backbone, which allow for the first time
the synthesis of stable NHCꢀmodified NPs via a ligand exchange
approach. Furthermore, we could demonstrate that the
15 modification by NHCs influences the activity and selectivity of
PdꢀNPs in the hydrogenations of olefins. Utilizing our protocol
we intend to synthesize novel NHC modified nanomaterials of
different metals and shapes. The modification of the LCꢀNHCs
will be investigated with a view to addressing challenges such as
20 asymmetric heterogeneous catalysis.
5
6
7
C.ꢀJ. Jia, F. Schüth, Phys. Chem. Chem. Phys., 2011, 13, 2457.
See supporting information for more details.
M. Ganesan, R. G. Freemantle, S. O. Obare, Chem. Mater., 2007,
129, 13364.
a) C. Pan, K. Pelzer, K. Philippot, B. Chaudret, F. Dassenoy, P.
Lecante, M.ꢀJ. Casanove, J. Am. Chem. Soc., 2001, 123, 7584; b) R.
H. Terrill, T. A. Postlethwaite, C.ꢀH. Chen, C.ꢀD. Poon, A. Terzis, A.
Chen, J. E. Hutchison, M. R. Clark, G. Wignall, J. D. Londono, R.
Superfine, M. Falvo, C. S. Johnson Jr., E. T. Samulski, R. W.
Murray, J. Am. Chem. Soc., 1995, 117, 12537; c) A. Badia, W. Gao,
S. Singh, L. Demers, L. Cuccia, L. Reven, Langmuir, 1996, 12, 1262;
d) I. Favier, S. Massou, E. Teuma, K. Philippot, B. Chaudret, M.
Gómez, Chem. Commun., 2008, 3296.
8
Acknowledgement
Generous
financial
support
by
the
Deutsche
Forschungsgemeinschaft (SFB 858) is gratefully acknowledged.
We thank Dr. Martin Peterlechner from the Institute of Materials
25 Physics (WWU) for TEM measurements and Vassilios Siozios
from MEET (WWU) for TGA analysis. We are grateful to Jonas
Fuchs and Prof. Monika Schönhoff from the Institute of Physical
Chemistry (WWU) for performing the PFG NMR measurements.
Dr. Karl Collins is acknowledged for helpful discussions.
[9] M. V. Vasylyev, G. Maayan, Y. Hovav, A. Haimov, R. Neumann,
Org. Lett., 2006, 8, 5445.
[10] C. M. Hagen, J. A. Widegreen, P. M. Maitlis, R. G. Finke, J. Am.
Chem. Soc., 2005, 127, 4423.
30 Notes and references
a Organic Chemistry Institute, Westfälische Wilhelms-Universität
Münster, Corrensstrasse 40, 48149 Münster, Germany. Fax: +49 251 83
33202; Tel: +49 251 83 33248; E-mail: glorius@uni-muenster.de
b Organic Chemistry Institute, Westfälische Wilhelms-Universität
35 Münster, Corrensstrasse 40, 48149 Münster, Germany. Fax:+49 251 83
36557; Tel:+49 251 83 33287; E-mail: B.J.Ravoo@uni-muenster.de
† Electronic Supplementary Information (ESI) available: Experimental
details and spectral data. See DOI: 10.1039/b000000x/
‡ These authors contributed equally.
40
45
50
55
1
a) A. Roucoux; J. Schulz, H. Patin, Chem. Rev., 2002, 102, 3757; b)
C. Burda, X. Chen, R. Narayanan, M. A. EIꢀSayed, Chem. Rev.,
2005, 105, 1025; c) D. Astruc, F. Lu, , J. R. Aranzaes Angew. Chem.
Int. Ed., 2005, 44, 7852; d) L. D. Pachon, G. Rothenberg Appl.
Organometal. Chem., 2008, 22, 288; e) R. J. White, R. Luque, V. L.
Budarin, J. H. Clark, D. J Macquarrie,. Chem. Soc. Rev., 2009, 38,
481.
2
3
B. L. Cushing, V. L. Kolesnichenko, C. J. O'Connor, Chem. Rev.,
2004, 104, 3893.
a) N-Heterocyclic Carbenes in Synthesis (Ed.: S. P. Nolan), Wileyꢀ
VCH, New York, 2006; b) N-Heterocyclic Carbenes in Transition
Metal Catalysis (Ed.: F. Glorius), Springer, Berlin, 2007; c) E. A. B.
Kantchev, C. J. O’Brien, M. G. Organ, Angew. Chem. Int. Ed., 2007,
46, 2768; d) S. Würtz, F. Glorius, Acc. Chem. Res., 2008, 41, 1523;
e) F. E. Hahn, M. C. Jahnke, Angew. Chem. Int. Ed., 2008, 47, 3122;
f) S. DiezꢀGonzalez, N. Marion, S. P. Nolan, Chem. Rev., 2009, 109,
4
| Journal Name, [year], [vol], 00–00
This journal is © The Royal Society of Chemistry [year]