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CHIMIA 2008, 62, No. 4
C(4)-bound carbenes are stronger electron
donors than their C(2)-bound analogs.
B(OH)2
R
R
X
Scheme 4.
Catalysic Applications
The impact of C(4)-bound carbene li-
cat. 15
gands has been exploited in catalysis, par-
H 2
ticularly in Suzuki cross-coupling and in
Scheme 5.
alkene hydrogenation. Complexes 10 and
13 both catalyze the Suzuki cross-coupling
of aryl bromides with phenyl boronic acid
(Scheme 4). For both complexes yields are
acceptable but high catalyst loadings and
[%]
long reaction times are required. Accord-
ingly TONs and TOFs are very low.
The limited activity of complexes 10
and 13 in cross-coupling reactions may be
rationalized by the fact that Pd0 formation
is required for initiation of the catalytic
Fig. 3. Hydrogenation
of cyclooctene with
cycle. Reduction of the metal in 10 or 13
seems unfavorable because of the strong
the C(4)-bound
donor ability of the dicarbene ligands and
dicarbene complex
because of the rigid 85° bite angle, which
favors a square planar metal geometry.
Given the high electron density at pal-
ladium, oxidative addition reactions may be
more likely to be mediated by complexes
such as 10. Preliminary studies concen-
trated on the hydrogenation of cyclooctene
and indicated appreciable activity for the
solvento complex 15 (Scheme 5). Carbene
C(4)-bonding seems to be essential for high
catalytic activity, since the C(2)-bound ana-
log 16 is a significantly less active hydroge-
nation catalyst (Fig. 3).
Hydrogenation reactions were typi-
cally run under mild conditions, that is, at
room temperature and under atmospheric
H2 pressure. Conversions are fastest when
using EtOH as solvent. Under these con-
ditions hydrogenation of cyclooctene is
complete in less than 5 h, whereas in non-
polar solvents like toluene the conversion
is incomplete even after prolonged reaction
15 and its normal
analog 16 in different
solvents. Conversions
(GC) determined after
8 h (* after 24 h).
pass C(2)-bound carbenes. This electronic
C. W. Kohlpaintner, Angew. Chem. 1993,
105, 1588.
impact has remarkable consequences on the
stability and reactivity of the coordinated
metal center. We have exploited these ef-
fects in catalytic alkene hydrogenation and
have developed an efficient system based
on abnormal carbene bonding. Similar
complexes may become promising candi-
dates for the activation of other less reactive
bonds.
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Acknowledgements
We thank Dr. A. Neels (University of
Neuchâtel) for X-ray analyses, Dr. G. M.
Garnier and Prof. P. Aebi (University of
Neuchâtel) for XPS measurements, and Ms. E.
[7] L. N. Appelhans, D. Zuccaccia, A.
times. Strongly coordinating solvents such Kluser for experimental assistance. This work
Kovacevic, A. R. Chianese, J. R.
was fi nancially supported by the Swiss National
Science Foundation. M. A. is very grateful for
an Alfred Werner Assistant Professorship.
Miecznikowski, A. Macchioni, E. Clot, O.
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as DMF appear to poison the catalytic ac-
tivity of the complex and no conversion is
observed at all.
[8] A. R. Chianese, A. Kovacevic, B. M.
Zeglis, J. W. Faller, R. H. Crabtree,
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P. Nolan, J. Am. Chem. Soc. 2004, 126,
Under the applied conditions, 1 mol%
catalyst loading is required for useful con-
versions. At 0.1 mol% loading, cyclooctene
reductionproceedstoabout70%andateven
lower loadings (0.01 mol%) only traces of
product were detected. Since hydrogena-
tion is supposed to be initiated by oxidative
addition of H2 to the metal center, the high
catalytic activity of 15 may be another con-
sequence of the exceptional donor power of
abnormally C(4)-bound carbenes.
Received: February 25, 2008
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Conclusions
Detailed analyses and reactivity studies
have shown that C(4)-bound carbenes are
exceptionally strong donor ligands that sur-
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