Use of an electron-reservoir complex together with air to generate N-heterocyclic carbenes

Article abstract of DOI:10.1021/ja058421+

Imidazolium salts with suitable substituents are readily deprotonated to stable NHC carbenes using the electron-reservoir complex [Fe^ICp(η⁶-C₆Me₆)], 1, in the presence of air (via a superoxide radical anion acting as a base). Less stable NHC carbenes can be conveniently obtained from imidazolium salts using the neutral base [Fe^IICp(η⁵-C₆Me₅CH₂)] obtained from 1 and air. Copyright

Full text of DOI:10.1021/ja058421+

Published on Web 03/18/2006
Use of an Electron-Reservoir Complex Together with Air to Generate
N-Heterocyclic Carbenes
Denise M e´ ry, Jaime Ruiz Aranzaes, and Didier Astruc*
LCOO, UMR CNRS No. 5802, UniVersit e´ Bordeaux 1, 351, Cours de la Lib e´ ration, 33405 Talence Cedex, France
Received December 12, 2005; E-mail: d.astruc@lcoo.u-bordeaux1.fr
I
We report that the standard electron-reservoir complex [Fe Cp-
the arene in the presence of aluminum chloride. Upon contact with
η6-C
Me
)], 1, can be conveniently used with air to deprotonate
weak acids, via the intermediacy of superoxide radical anion O
This is exemplified here with the deprotonation of imidazolium
salts, generating N-heterocyclic (NHC) carbenes. Imidazoles are
universally known for their key roles in metalloenzymes, and their
alkylated derivatives, imidazolium salts, have now become popular
1
a stoichiometric amount of 1 under ambient conditions in THF in
(
6
6
•
-
+
-
+
-
2
.
the presence of air, 2H Cl and 3H Cl cleanly give high (virtually
quantitative) yields of the soluble carbenes within a few seconds,
visible by the quick color change from deep-green to yellow. In
+
-
the meantime, the yellow salt 1 Cl precipitates. The carbenes
1
13
formed in this way are characterized by H and C NMR when
these reactions are carried out in THF-d (eq 1).
We know that 1 reacts with O to give the deep-red complex
2
ionic liquids used for “Green Chemistry” conditions. Deprotonation
8
of these salts has been shown by Arduengo to yield isolable and
relatively stable NHCs (2 and 3) when the heterocycle nitrogen
atoms bear suitable substituents.3
2
II
5
17
+
-
[Fe Cp(η -C
6 5 2 6
Me CH )], 5, a base whose conjugated acid 1 PF
18a
has a pK value of 28.2 in DMSO, close to that of t-BuOK.
a
+
-
+
-
A renaissance of carbene chemistry has resulted from this dis-
However, the reaction of 2H Cl and 3H Cl with 1 in the
presence of air does not give the deep-red color of 5, even
transiently, but an off-white solution of the carbene. We also know
2
4
covery with that of Bertrand of the stabilization of non-NHC car-
5
benes, the first isolated carbenes. In particular, NHC carbenes were
introduced in catalysis in the late 90s with olefin metathesis and
that the mechanism of the reaction of 1 with O proceeds by electron
6
I
hetero-cross-carbon-carbon-coupling reactions. The property of
transfer from Fe to O to give the transient intermediate contact
2
II
6
•- 17
•-
18b
excellent σ-donors, non-π-acceptors of these ligands has thus
brought them to the very forefront of organometallic chemistry and
catalysis, especially because they can replace phosphines that are
not environmentally benign. In addition to the key exceptional elec-
tronic properties and very favorable Green Chemistry aspects of
NHC carbene ligands, others7 have shown that they provide re-
markable Grubbs-type, so-called second-generation ruthenium olefin
6 6 2
ion pair [Fe Cp(η -C Me ), O ], in which the O2 anion
deprotonates a methyl group, yielding the double-bonded exocyclic
methylene. The stoichiometry is 0.25 mol O yielding 0.5 mol H O
at room temperature and 0.5 mol O yielding 0.5 mol H O at -78
°C. Thus, the base that deprotonates 2H Cl and 3H Cl in THF
can be O2 (whose basic properties are enhanced by the fast
dismutation of HO ), after fast metathetic ion exchange between
2
2
2
2
2
+
-
+
-
-9
•-
2
2
metathesis catalysts [RuCl (dCHPh)(bis-N-mesityl-NHC)] and their
the two contact ion pairs (eq 2):
multiple efficient derivatives.10 More recently, Nolan has consider-
ably developed the efficiency of a variety of C-C and C-N cross-
coupling reactions (Heck, Suzuki, Stille, Sonogashira, Corriu-
Kumada, Niyama, amination) of Pd complexes bearing a single
NHC carbene 2 or 3.11
Usually, the imidazolium salt is utilized in situ, but this does
not allow for good control of the metal-carbene stoichiometry,
especially because of the duality of complexation modes. Indeed,
Crabtree has shown that the “normal” symmetrical coordination
mode can switch to another nonsymmetrical carbene coordination.1
Moreover, Nolan reported in 2004 that these two complexation
modes can selectively form depending on the reaction conditions
and that their reactivity behaviors in catalysis are different.15
Therefore, it is essential to control and optimize the conditions of
deprotonation of imidazolium salts to NHC carbenes viewing the
central role of these ligands in catalysis.
+
-
+
-
Alternatively, the deprotonation of the salts 2H Cl and 3H Cl
can be carried out using 1 and air under ambient conditions in
various solvents, such as pentane, benzene, toluene, and ether, in
which these salts are not soluble. Then, the deep-red color of 5
immediately appears upon contact with air, and the heterogeneous
deprotonation takes place in about 5-30 min at ambient temper-
ature, depending on the solvent (Scheme 1).
2-14
+
-
Indeed, a stoichiometric amount of 5 also deprotonates 2H Cl
and 3H Cl quantitatively in any of these solvents or instanta-
+
-
The use of 1 with O
is easily available in high yields as green-black crystals upon
monoelectronic reduction with Na/Hg in THF or glyme at ambient
2
provides a clean method for this purpose.
neously in THF under ambient conditions, yielding the carbene and
the yellow precipitate of 1 Cl .
The functionalization of NHC carbenes is useful for the deriva-
tization of NHC carbene-containing catalysts with organic or
inorganic polymers en route to supported recoverable catalysts; thus
+
-
1
II
+
- 6
temperature of its Fe 18-electron monocationic form (1 PF
6
)
that can be synthesized in large scales upon heating ferrocene with
5602
9
J. AM. CHEM. SOC. 2006, 128, 5602-5603
10.1021/ja058421+ CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
Scheme 2
In conclusion, this study shows the first use of an electron-
reservoir complex and air for clean deprotonation. Efficient
the new imidazolium salt 4H Cl- was synthesized (Supporting
+
+
-
Information). Reaction of t-BuOK with 4H Cl does not provide
fast deprotonation, but development of a green color corresponding
to a green charge-transfer complex (eq 3)19 whose H NMR peaks
are close to those of the starting material, and the UV-vis spectrum
is shown in Figure 1 in the Supporting Information.
+
-
+
-
deprotonation of the imidazolium salts 2H Cl and 3H Cl to the
stable carbenes 2 and 3 best proceeds under ambient conditions
using the electron-reservoir complex 1 and air in a variety of
solvents, whereas the reaction of 1 with air to give the base 5 should
1
+
-
be carried out in THF before deprotonation of 4H Cl in order to
take into account the sensitivity to air of the functional carbene 4.
Thus, this study is the first one demonstrating that 5 is a very useful
neutral base whose acidic form is removed from the reaction
medium (thus recyclable) after deprotonation by precipitation. This
use of 5 is essential, when the deprotonation yields a fragile species,
and can potentially be generalized to many other systems.
Note Added after ASAP Publication. Production error published
previous revision of Supporting Information March 18, 2006; Editor
We notice that other electron-rich reagents, such as cobaltocene,
approved at acceptance Supporting Information published April 4, 2006.
Supporting Information Available: Syntheses and NMR spectra
of 2-4 and 6, and UV-vis spectra of the colored charge-transfer products
of donors with 4H Cl . This material is available free of charge via the
Internet at http://pubs.acs.org.
also give a blue-green charge-transfer complex upon reaction with
+
-
+
-
2
H Cl or 4H Cl (eq 4; see the UV-vis spectrum in the
+
-
Supporting Information).
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(
(
(
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20
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+
-
the imidazolium salts 2H Cl provides the carbene. The reduction
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carbenes have a rich chemistry,4 thus a deprotonation method
removing the chemical used for deprotonation is advisable.
(
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(
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(
(
(
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+
-
Deprotonation of 4H Cl is best carried out using 5 at -20 °C
in ether and yields 4. The reaction is now very convenient because
it proceeds with precipitation of the salt 1 Cl of the protonated
11) Nolan S. P. J. Organomet. Chem. 2002, 653, 69.
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+
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+
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acidic form 1 of the base 5 at ambient or sub-ambient conditions.
(
14) Crabtree, R. H. Pure Appl. Chem. 2003, 75, 435.
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double bond in 4. The H NMR spectrum of 4 shows the shielding
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(
1
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of all the aromatic and NHC protons and disappearance of the
(
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NCHN proton as expected, and reaction of 4 with PdCl
2
gives the
(
new biscarbene Pd complex 6 (analogous to Nolan’s biscarbene
15
Pd complex obtained with 2 ), whose MALDI TOF mass spectrum
(20) Gorodetsky, B.; Ramnial, T.; Branda, N. R.; Clyburne, J. A. C. Chem.
Commun. 2004, 1972.
+
presents the molecular peak corresponding to [M] at 851 mu
(Scheme 2).
JA058421+
J. AM. CHEM. SOC.
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