Transition-metal-catalyzed reactions involving imidazolium salt/N-heterocyclic carbene couples as substrates

Article abstract of DOI:10.1002/anie.200454166

Reactive Couples: Imidazolium salts react with alkenes under mild conditions in the presence of a Ni⁰-based catalyst to produce 2-alkyl imidazolium salts (see scheme, cod = 1,5-cyclooctadiene). The mechanism involves oxidative addition of an im

Full text of DOI:10.1002/anie.200454166

Angewandte
Chemie
Ionic Liquids
application in a wide range of catalytic reactions (such as the
Heck, Kumada, Stille, Suzuki-Miyaura, and Sonogashira
coupling reactions, amination reactions, hydrosilylation,
olefin metathesis, hydroformylation, polymerization, etc.),[4]
and metal hydrides are implicated in a number of these
catalytic processes.
Transition-Metal-Catalyzed Reactions Involving
Imidazolium Salt/N-Heterocyclic Carbene
Couples as Substrates**
Furthermore, 1,3-dialkyl imidazolium salts have been
extensively employed as ionic-liquid solvents, potentially
valuable alternatives to non-aqueous solvents for biphasic
catalysis.[5] Although commonly considered as inert solvents, a
number of reports have suggested their possible participation
during catalysis, forming strong s-donor carbene ligands with
the metal center under basic conditions, and the generation of
carbene complexes from imidazolium salts has frequently
been observed in the presence of base.[6] Our studies
demonstrate that basic conditions are unnecessary and that
imidazolium-based ionic liquids can react with low-valent
Nicolas D. Clement and Kingsley J. Cavell*
In previous studies we demonstrated that hydrocarbyl–M–
carbene complexes (M = Group 10 metal) readily undergo
reductive elimination to give M0 and C2-substituted imida-
zolium salts (Scheme 1, Equation (1)).[1] As this presents a
À
metal complexes of Group 10 through C2 H oxidative
addition.[2,3]
Herein we report on the continuation of our studies in
which the unusual chemical reactivity of imidazolium salts/
NHC couples with transition metals is examined, and the
potential applications of this chemistry are explored. We
À
describe a C C coupling reaction in which imidazolium salts
act as substrates in a novel catalytic reaction that proceeds
through a redox process involving a carbene intermediate.
Combining the two “half-reactions” described in Equa-
tions (1) and (2) (Scheme 1) in the presence of an alkene
establishes a unique catalytic process.
On stirring a solution of imidazolium salts 1a,b in
acetone/thf with various alkenes in the presence of a Ni0
catalyst, the salts are progressively converted into the C2-
alkylated products 2a,b (Scheme 2; Table 1).
Scheme 1. Redox processes undergone by the carbene/imidazolium
couple.
potential impediment for the use of N-heterocyclic carbene
(NHC) complexes in catalysis, we investigated methods for
controlling or restricting the reaction. Subsequently, we were
able to show that this reductive elimination reaction could be
“reversed”. Reaction of imidazolium salts with Pt0 species
gave carbene–Pt–hydride complexes,[2] and very recently we
reported that the reaction of imidazolium salts (ionic liquids)
with low-valent M0 (M = Pd, Ni) bearing strong s-donor
ligands can be a facile process for the generation of surpris-
ingly stable carbene–M–hydride complexes (Scheme 1, Equa-
tion (2)).[3] One notable feature of this study is that direct
formation of a carbene-—M–H provides an atom-efficient,
in situ route to an active catalytic species. NHCs have found
À
Scheme 2. Imidazolium C H/alkene coupling reaction.
Compounds 2a,b were isolated and characterized by
À
NMR spectroscopy. The imidazolium C H/alkene coupling
reaction occurs under very mild conditions in the presence of
the commonly employed Ni(cod)2/PPh3 (cod = 1,5-cycloocta-
diene) catalyst system. The addition of 2 equivalents of PPh3
is essential for the effective catalytic performance of the
system. Use of Ni(cod)2 on its own quickly leads to decom-
position of the catalyst, resulting in poor reaction yields
(Table 1, entry 1). Increasing or decreasing the PPh3/Ni ratio
gives lower conversions. We found that the addition of
2.1 equivalents of PPh3/Ni gives the best overall catalytic
results. The reactions could also be conducted in the ionic
liquid itself, although conversions were considerably lower
[*] N. D. Clement, Prof. K. J. Cavell
Department of Chemistry
Cardiff University
PO Box 912, Cardiff CF10 3TB, Wales (UK)
Fax: (+44)29-2087-5899
E-mail: cavellkj@cf.ac.uk
[**] We are grateful to the Engineering and Physical Sciences Research
Council (EPSRC) for financial support and a stipend for N.D.C.
Supporting information for this article (catalytic procedure, sepa-
ration method and spectroscopic data for compounds 2a,b) is
available on the WWW under http://www.angewandte.org or from
the author.
Angew. Chem. Int. Ed. 2004, 43, 3845 –3847
DOI: 10.1002/anie.200454166
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3845

Communications
Table 1: Catalytic coupling results.[a]
Entry
Ionic Liquid
Alkene
T [8C]
Yield [%][b]
1
2
3
4
1a,b
1a
55, RT
55
<15[c]
56
1b
55
51
1b
RT
64
5
6
7
8
1b
1a
1b
1b
55
55
55
RT
39[d]
100;[a] 72[f]
100;[a] 95[g]
35
[a] Conditions: Catalyst [Ni(cod)2/PPh3 (1:2.1)] (10 mol%); acetone/
THF (3:1); reaction stopped after 48 h. [b] Determined by 1H NMR
spectroscopy, average of two runs or more. [c] Catalytic [Ni(cod)2] only.
[d] Mixture of linear and branched product (4:1). [e] Pressure: 1 bar.
[f] Catalyst (5 mol%). [g] Catalyst (3 mol%).
Scheme 3. Proposed catalytic cycle for imidazolium/alkene coupling
reaction.
(yields < 10%). The various 1-alkenes (1-hexene, ethylene)
and styrene undergo coupling with the imidazolium salts to
varying extents (Table 1).
Not surprisingly, ethylene (1 bar) couples more readily
with imidazolium salts than does 1-hexene (Table 1,
entries 6,7 and 2,3). This follows the expected reactivity
pattern for alkenes (ethylene > propylene > butene ~ higher
alkenes). Styrene gives lower conversions than either of the
aliphatic alkenes (Table 1, entries 5 and 2–4/6–7); steric
elements may be a factor in this observed behavior. Interest-
ingly, styrene uniquely gives rise to linear and branched
isomers — the branched isomer has a chiral center linking the
imidazolium ring to the benzene ring. Both the tetrafluor-
oborate 1a and bromide 1b salts react with the different
alkenes tested, especially with ethylene for which complete or
nearly complete conversions were observed (Table 1,
entries 6–7). With ionic liquid 1b at 558C, almost complete
conversion was observed even with a lower catalyst loading of
3%. (Table 1, entry 7).
Figure 1. Proposed mechanism for carbene–alkyl reductive elimination.
For the alkenes studied, only the monoinsertion product is
observed (i.e. for ethylene, only the 2-ethyl product is
formed), which suggests that under the conditions employed
the rate of carbene–alkyl reductive elimination is faster than
À
the rate of alkene insertion into the Ni alkyl bond. Consistent
with this proposal, in earlier studies with Ni–carbene com-
plexes as catalysts for alkene dimerization, effective catalysis
was only observed in ionic liquid solvents.[7] In toluene the
predominant reaction was reductive elimination to give
imidazolium salts and Ni0. In studies pertinent to our
investigations, Bergman, Ellman, and co-workers have
recently used a RhI complex to catalyze the conversion of
benzimidazoles, thiazoles, and oxazoles into 2-substituted
azole rings and also into fused azole-based heterocyclic
compounds.[8] Rh–carbene complexes have been isolated
from the reaction mixture, indicating that these may be
intermediates in the catalytic reaction. On first consideration
the reaction appears similar to that reported herein. However,
the mechanism proposed by Bergman, Ullman and co-
workers consists of an initial H-atom shift on the imidazole
substrate to generate a RhI-NHC complex. This is then
followed by the insertion of the coordinated alkene into the
Based on previous detailed theoretical and experimental
studies on the mechanism of individual reaction steps,[1–3] the
following catalytic cycle is proposed (Scheme 3) and descri-
bed below.
Imidazolium salts 1a,b oxidatively add to low-valent,
[Ni(PPh3)n] generated in situ to form a carbene–Ni–hydrido
À
species. C H oxidative addition of imidazolium salts has been
shown to be facile with low-valent Ni complexes, in particular
with coordinatively unsaturated complexes bearing s-donor
ligands.[3] Alkene insertion into the ensuing M H bond
À
À
produces an alkyl Ni intermediate that subsequently under-
goes reductive elimination with the carbene ligand to produce
2-alkyl imidazolium salts 2a,b and re-form M0Ln. Kinetic and
density functional studies have shown that reductive elimi-
nation from NHC–alkyl complexes proceeds in a concerted
fashion.[1] The reductive elimination effectively involves
orbital overlap between the carbene carbon p(p) orbital, the
alkyl sp3 orbital, and the metal d orbital, which occurs with
concurrent opening of the angle between the two nonparti-
cipating ligands L (Figure 1).
À
Rh carbene bond to form a zwitterionic intermediate and
À
generate the C C bond. An intramolecular proton transfer,
À
C H reductive elimination, and product displacement com-
plete the catalytic cycle.[8b]
3846
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Angewandte
Chemie
Y. Kou, E. Min, Catal. Today 2002, 74, 157; J. Dupont, R. F.
Our study represents the first example in which the
imidazolium oxidative addition and carbene reductive elim-
ination processes have been combined into the same catalytic
de Souza, P. A. Z. Suarez, Chem. Rev. 2002, 102, 3667.
[6] L. Xu, W. Chen, J. Xiao, Organometallics 2000, 19, 1123; C. J.
Mathews, P. J. Smith, T. Welton, A. J. P. White, D. J. Williams,
Organometallics 2001, 20, 3848.
À
cycle, and the first demonstration that the imidazolium C H
activation step can be used to generate an active catalyst.
These results also illustrate the ease with which interconver-
sion between imidazolium salts and NHC–transition-metal
complexes occur even under very mild conditions, particularly
under conditions where low-valent, unsaturated metal species
are generated.
[7] D. S. McGuinness, W. Mueller, P. Wasserscheid, K. J. Cavell, B. W.
Skelton, A. H. White, U. Englert, Organometallics 2002, 21, 175.
[8] a) K. L. Tan, R. G. Bergman, J. A. Ellman, J. Am. Chem. Soc.
2001, 123, 2685; b) K. L. Tan, R. G. Bergman, J. A. Ellman, J. Am.
Chem. Soc. 2002, 124, 3202; c) K. L. Tan, R. G. Bergman, J. A.
Ellman, J. Am. Chem. Soc. 2002, 124, 13964; d) K. L. Tan, A.
Vasudevan, R. G. Bergman, J. A. Ellman, A. J. Souers, Org. Lett.
2003, 5, 2131; e) J. C. Lewis, S. H. Wiedemann, R. G. Bergman,
J. A. Ellman, Org. Lett. 2004, 6, 35.
These results have important catalytic implications. First,
this reaction represents a unique example of Ni0-catalyzed
À
À
C H activation/C C bond-formation reactions under mild
conditions.[9] Second, the results also provide a catalytic
methodology for producing 2-alkyl imidazolium-type salts
through a coupling reaction. Third and most importantly,
these results have direct ramifications for the use of imida-
zolium-based ionic liquids as solvents in transition-metal
catalysis. It is clear that imidazolium salts can form car-
bene–hydrido species in the presence of a M0 source, or when
M0 species are generated in situ during a reaction. Generation
of carbene—-M–hydrido species would have a major impact
on catalysis reactions in ionic-liquid media. This study, when
considered in conjunction with our earlier investigations on
imidazolium-based ionic-liquid solvents in catalysis,[7] dem-
onstrates the unique potential for employing ionic liquids
concomitantly as solvent and activator, generating long-lived
(stabilized) active catalyst sites for a range of catalytic
processes.
À
À
[9] For reviews on catalytic C H activation/C C bond formation
reactions see: Y. Guari, S. Sabo-Etienne, B. Chaudret, Eur. J.
Inorg. Chem. 1999, 1047; G. Dyker, Angew. Chem. 1999, 111,
1808; Angew. Chem. Int. Ed. 1999, 38, 1698; C. Jia, T. Kitamura, Y.
Fujiwara, Acc. Chem. Res. 2001, 34, 633; V. Ritleng, C. Sirlin, M.
Pfeffer, Chem. Rev. 2002, 102, 1731; F. Kakiuchi, S. Murai, Acc.
Chem. Res. 2002, 35, 826.
In conclusion we have demonstrated a new type of Ni0-
À
À
catalyzed C H activation/C C bond-forming reaction to
produce 2-alkyl imidazolium-type salts under mild conditions.
These results illustrate the facile redox processes associated
with these salts, commonly used as ionic liquids. Of signifi-
cance is the generation of (carbene)metal-hydride species
in situ that may be employed in catalytic processes using the
ionic liquid as both solvent and activator. The optimization
and extension of the methodology described herein is
currently under investigation.
Received: March 2, 2004 [Z54166]
À
À
Keywords: C C coupling · C H activation · carbene ligands ·
ionic liquids · nickel
.
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Angew. Chem. Int. Ed. 2004, 43, 3845 –3847
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