Ruthenium Catalysts Supported by Amino-Substituted N-Heterocyclic Carbene Ligands for Olefin Metathesis of Challenging Substrates

Article abstract of DOI:10.1002/chem.201604934

N-Heterocyclic carbene (NHC) ligands IMes (Formula presented.) and IMes (Formula presented.) derived from the well-known IMes ligand by substituting the carbenic heterocycle with one and two dimethylamino groups, respectively, were employed for the synthesis of second-generation Grubbs- and Grubbs–Hoveyda-type ruthenium metathesis precatalysts. Whereas the stability of the complexes was found to depend on the degree of dimethylamino-substitution and on the type of complex, the backbone-substitution was shown to have a positive impact on their catalytic activity in ring-closing metathesis, with a more pronounced effect in the second-generation Grubbs-type series. The new complexes were successfully implemented in a number of challenging olefin metathesis reactions leading to the formation of tetra-substituted C=C double bonds and/or functionalized compounds.

Full text of DOI:10.1002/chem.201604934

A Journal of
Accepted Article
Title: Ruthenium Catalysts Supported by Amino-Substituted NHC
Ligands for Olefin Metathesis of Challenging Substrates
Authors: Vincent César, Yin Zhang, Wioletta Kośnik, Adam Zieliński,
Adam A. Rajkiewicz, Mirko Ruamps, Stéphanie Bastin, Noël
Lugan, Guy Lavigne, and Karol Grela
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To be cited as: Chem. Eur. J. 10.1002/chem.201604934
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10.1002/chem.201604934
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Ruthenium Catalysts Supported by Amino-Substituted NHC
Ligands for Olefin Metathesis of Challenging Substrates
Vincent César,*[a] Yin Zhang,[a]§ Wioletta Koꢀnik,[b]§ Adam Zieliꢁski,[b]§ Adam A. Rajkiewicz,[b]§ Mirko
Ruamps,[a] Stéphanie Bastin,[a] Noël Lugan,*[a] Guy Lavigne,[a] Karol Grela*[b]
Abstract: N-Heterocyclic Carbene (NHC) ligands IMesNMe and
2
IMes(NMe ) derived from the well-known IMes ligand by substituting the
carbenic heterocycle for one and two dimethylamino groups,
respectively, were employed for the synthesis of 2nd generation
2
2
NHC ligands
N
L
Cl
Ru
N
Cl
Grubbs-
and
Grubbs-Hoveyda-type
ruthenium
metathesis
SIMes =
PCy3
precatalysts. Whereas the stability of the complexes was found to be
function of the degree of dimethylamino-substitution and of the type
of complex, the backbone-substitution was shown to have a positive
impact on their catalytic activity in RCM, with a more pronounced
effect in the 2nd generation Grubbs-type series. The new complexes
were successfully implemented in a number of challenging olefin
metathesis reactions leading to the formation of tetrasubstituted C=C
double bonds and/or functionalized compounds.
2a
: L = SIMes (GII)
2b: L = IMes(NMe )
2c: L = IMes(NMe )
2
NMe2
2
2
IMes(NMe )
=
2
N
N
L
Cl
1b
Ru
Cl
O
Me2N
N
NMe2
N
IMes(NMe )
=
2 2
3a: L = SIMes (HGII)
1c
(NMe )
2
3b
: L = IMes
Introduction
3c: L = IMes(NMe )
2
2
Olefin metathesis has gained considerable significance during the
last decades as a powerful tool for the formation of carbon-carbon
double bonds, Grubbs' complex (PCy3)2Cl2Ru=C(H)Ph (GI)
representing the first archetype of user-friendly and versatile Ru-
based precatalysts.[1,2] The advent of the ‘2nd generation’ Ru-
based metathesis precatalysts RuCl2(PCy3)(NHC)(=CHPh) (GII),
featuring a non-labile, strongly electron-donating and sterically
demanding N-heterocyclic carbene (NHC) ligand in place of one
of the PCy3 ligands,[3,4] brought a more efficient and robust
precatalysts family exhibiting a broad application scope, ranging
from fine-chemical synthesis to material science.[5] Parallel
development of the '2nd generation' Hoveyda-Grubbs complexes
RuCl2(NHC)(=CH(o-iPrO-C6H4)) (HGII) led to a more robust Ru-
based precatalysts series, sometimes to the expense of a lower
initiation rate.[6]
Figure 1. Standard Grubbs II (2a) and Hoveyda-Grubbs II (3a) complexes and
targeted Ru-based complexes bearing amino-substituted NHCs 2b-c and 3b-c.
As the NHC moiety present in both GII and HGII complexes
constitutes the supporting ligand of the actual catalytic species,
namely [(NHC)RuCl2(=CHR)], during the whole catalytic process,
modifications of its stereoelectronic properties is expected to have
an impact on the stability of the catalyst and/or on the outcome of
metathesis reaction in terms of activity and selectivity. Such an
approach has been particularly effective for the conversion of
challenging hindered substrates for which the standard GII and
HGII complexes containing the 1,3-dimesitylimidazolidin-2-
ylidene (SIMes) ligand (2a and 3a, Figure 1) found their limitations
because of stability issues and lack of reactivity. In this regard,
better activities for the RCM of sterically hindered olefins, and
especially for the formation of tetra-substituted olefins, were
obtained acting on the substituents attached to the two nitrogen
atoms of the supporting NHC ligands, replacing both mesityl
groups for more hindered aryl groups or for ortho-unsubstituted
aryl groups, or by replacing one Mes group for a bulky alkyl
group.[7,8] Comparatively, only a few studies were focused on the
incidence of a modification of the backbone on the activity of the
derived metathesis catalysts. Fürstner et al. first reported in 2001
that the substitution of the backbone H atoms in IMes by two
chlorine atoms had only a little effect on the activity of the resulting
GII-type complex in a series of RCM reactions.[9] More recently, it
was shown that slightly improved catalytic efficiencies in RCM
[a]
[b]
§
Dr V. César, Dr. Y. Zhang, M. Ruamps, Dr. S. Bastin, Dr N. Lugan,
Dr. G. Lavigne
LCC-CNRS, Université de Toulouse, INPT, UPS, Toulouse, France.
Fax: (+)33 (0) 5 61 55 30 03
E-mail: vincent.cesar@lcc-toulouse.fr, noel.lugan@lcc-toulouse.fr
Dr W. Koꢀnik, A. Zieliꢁski, A. Rajkiewicz, Prof. K. Grela
Biological and Chemical Research Centre, Faculty of Chemistry
University of Warsaw
Zwirki i Wigury 101, 02-089 Warsaw (Poland)
E-mail: karol.grela@gmail.com
Authors contributed equally to this work
Supporting information for this article is given via a link at the end of
the document
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10.1002/chem.201604934
Chemistry - A European Journal
FULL PAPER
Me2N
X
could be obtained in the formation of tetra-substituted olefins
using chlorine or methyl backbone-substituted IMes ligands.[10] In
parallel, backbone substitution in saturated NHCs was shown to
greatly improve the catalyst stability by restricting rotations
around the N-Caryl axes, hence slowing down the catalyst
decomposition through C-H activation.[11,12] However, despite
intense research efforts, no ruthenium catalyst was found to be
general and highly efficient enough for all metathesis
transformations, thus catalyst design remains of particular
importance for new advancements.
N
N
Cl
1) KHMDS, toluene
Cy3P
Mes
Mes
2)
50°C, 2h
Cl
Ru
Ru
Cl
PCy3
TfO
Cl
PCy3
(GI)
Me2N
X
2b
2c
: X = H (73%)
: X = NMe2 (61%)
N
N
Mes
Mes
Me2N
X
1b
1c
HOTf (X = H)
HOTf (X = NMe2)
A strong σ-donation associated to a well-balanced steric
protection/accessibility of the metal center were shown to be
decisive features of the NHC ligand for giving efficient metathesis
N
N
Mes
Mes
Cl
1) KHMDS, toluene
Ru
O
catalysts.[13] Following
a general study on the backbone-
Cl
Cy3P
2)
60°C, 3h
substitution of NHCs,[14] some of us recently disclosed the
synthesis of IMes and IPr derivatives bearing one or two
dimethylamino groups on the 4/5 positions of the carbenic
heterocycle.[15] These NHCs were found to display sequentially
enhanced electronic donation properties compared to their non-
substituted analogues, as illustrated by their respective Tolman
Electronic Parameter (TEP) values [IMes (TEP = 2050.8 cm-1),
Cl
Ru
Cl
(HGI)
3b
3c
: X = H (76%)
: X = NMe2 (56%)
O
3) CuCl, 60°C, 2h
Scheme 1. Synthesis of ruthenium complexes 2b-c and 3b-c.
IMesNMe (TEP = 2048.6 cm-1), IMes(NMe ) (TEP = 2046.6 cm-1)]
Beyond this, it was also demonstrated that such a “simple”
introduction of amino groups into the backbone of NHCs had a
highly positive impact onto the Pd-catalyzed Buchwald-Hartwig
amination, giving rise to one of the most efficient and general
NHC-based catalytic systems reported to date for this reaction.[16]
This naturally prompted us to investigate the incidence of such a
NHC backbone decoration in Ru-catalyzed olefin metathesis
reactions. We report herein the synthesis and characterization of
2
2 2
In the solid state, complex 3c exhibits a distorted square-
based pyramidal geometry around the ruthenium center, the Cl
ligands being trans to each other and the benzylidene moiety
occupying the apical position, as typically found in HGII-like
complexes.[18] As expected, the imidazolyl ring is almost coplanar
with the benzylidene-containing ruthenacycle [{N1-C1-Ru1-C8} =
- 3.07°], placing one mesityl group facing the H atom of the
the GII- and HGII-type complexes supported by the IMesNMe and
2
Comparison of the structure of 3c to the one
benzylidene moiety.
IMes(NMe ) ligands, respectively, and their evaluation as catalysts
2
2
i
of the parent (IXyl)Cl Ru(
) complex[18c,19] didn't reveal
o-PrO-C6H4
2
in a set of challenging metathesis reactions (Figure 1).
any significant metrical differences neither around Ru nor within
the o-isopropoxybenzylidene ligand. Noticeably, both NMe2
groups are orthogonal to the heterocyclic system and are
Results and Discussion
considerably pyramidalized (
347.0° (
342°),
ΣN3b = ΣN4
ΣN3a
=
=
342°). Such a specific conformation has previously been pointed
out by Weiss and coll. in parent imidazolium salts and can be
ascribed to the anomeric donation of the NMe2 lone pairs into the
adjacent C−N σ* orbitals of the imidazolyl ring.[20] Consequently,
the N1-C2 [1.412(4) Å] and N2-C3 [1.406(4) Å] bonds are slightly
Synthesis and characterization of the complexes
RuCl2(PCy3)(IMesNMe )(=CHPh)
(2b)
and
and
and
2
Complexes
RuCl2(PCy3)(IMes(NMe ) )(=CHPh)
(2c),
(3b)
were
(1b)
2
2
RuCl2(IMesNMe )(=CH(o-PrO-C6H4))
i
2
RuCl2(IMes(NMe ) )(=CH(o-PrO-C6H4))
(3c)
readily
i
2
2
elongated compared to the corresponding bonds in the reference
)
(NMe2 2
synthetized upon reaction of the free IMesNMe
or IMes
2
i
(IXyl)Cl Ru(
) complex reported by Merino et al.
o-PrO-C6H4
2
[1.379-1.392 Å].[18c,21] In addition, the C3-N2-C21 [125.0(3)°] and
C2-N1-C31 [122.5(2)°] angles are wider than the corresponding
angles in the IXyl-derived complex [122.3-122.5° and 120.4-
121.05° respectively], reflecting an increased steric pressure
exerted by the NMe2 groups onto the N-aryl arms in complex 3c.
(1c) carbenes,
generated in situ by deprotonation of the
corresponding imidazolium triflate salts 1bꢀHOTf or 1cꢀHOTf with
KHMDS in toluene at 0°C,[15] with commercially available
RuCl2(PCy3)2(=CHPh)
complexes, respectively (Scheme 1). The new complexes were
fully characterized by spectroscopic and analytical methods,
supplemented for 3c by a single-crystal X-Ray diffraction
analysis[17] (Figure 2).
and
RuCl2(PCy3)(=CH(o-iPrO-C6H4))
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10.1002/chem.201604934
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[75 kJ.mol-1 < ∆G≠ < 91 kJ.mol-1],[22,24] due to the absence of the
stabilizing interaction between the N-aryl substituents and the
benzylidene moiety in HGII complexes.[23] This rotation barrier
was found to be even lower than the barrier measured on a HGII-
type complex bearing a saturated meso-1,3-bis(o-tolyl)-4,5-
diphenylimidazolidin-2-ylidene ligand [∆G≠ = 72 kJ.mol-1].[25] This
lowering can be ascribed to a reduced π-backdonation from the
ruthenium center to the NHC ligand, since the unsaturated
imidazol-2-ylidenes are known to be less π-acidic than their
corresponding saturated imidazolidin-2-ylidene analogues.[26]
Figure 2. Molecular structure of 3c (ellipsoids drawn at the 30% probability level,
H atoms omitted for clarity, only main component of the disordered groups
shown). Selected bond distances (Å) and angles (°) (esd): Ru1-C1 1.991(3),
Ru1-C8 1.825(3), Ru1-O1a 2.269(2), C1-N1: 1.356(4), C1-N2: 1.371(4), N1-C2:
1.412(4), N2-C3: 1.406(4), C2-C3: 1.348(5); C1-Ru1-O1a = 178.3(2), C3-N2-
C21: 125.0(3), C2-N1-C31: 122.5(2).
NMR data of complexes 2b-c and 3b-c are in agreement with
the proposed structures. Of note, the NMR spectra of 2b recorded
at 25°C shows that the complex exists in solution as a 55:45
mixture of two isomers, as illustrated by the presence of two
singlet signals for the benzylidene proton at δ 19.43 and 19.36
ppm in the 1H NMR spectrum, and by two singlet resonances for
the coordinated phosphorus at δ 30.6 and 30.3 ppm in the 31P{1H}
NMR spectrum. Heating a solution of 2b in toluene-d8 up to 110°C
did not allow to reach the coalescence temperature of those
signals. Yet, a 31P-31P EXSY NMR experiment carried out at 75°C
clearly evidenced cross-peaks in testimony of a polarization
exchange between the two isomers. The existence of pairs of
atropisomers in unsymmetrical GII-type complexes has been
previously documented and was shown to arise from the hindered
rotation around the Ru-CNHC bond,[22] through a stabilizing
transannular interaction between the mesityl group and the
benzylidene carbon atom.[23] On the other hand, the 1H NMR
spectrum for 2c showed two distinct signals for the NMe2 groups
Figure 3. Variable temperature 1H NMR spectra of 3b in the region 16.0-17.0
ppm (CD2Cl2, 500 MHz).
The relative stabilities of the complexes 2b, 2c, 3b and 3c in
CD2Cl2 were investigated in sealed NMR tubes under Ar
atmosphere and compared to the ones of GII (2a) and HGII (3a)
(Figure 4). It was observed that the introduction of the
dimethylamino groups in the GII-type complexes has
a
deleterious effect on the stability of the complexes, with about
10% of decomposition measured for 2b, and about 20% for 2c
after 35 days at room temperature. Conversely, the HGII-type
complexes 3b and 3c were shown to be more stable than the
standard HGII complex 3a with only 2-5% decomposition after 35
days. It thus appears that the amino groups have divergent
influence on the stability of the ruthenium complexes depending
on the coordination sphere.
)
revealing again a hindered rotation of the IMes(NMe2 2 ligand around
the Ru-CNHC bond. By contrast, the 1H NMR spectra of complexes
3b and 3c recorded at 25°C, showing for 3b one set of signals
only, and for 3c one signal only for the two NMe2 groups, clearly
indicate that the NHC ligands are here freely rotating at the NMR
timescale at this temperature. Interestingly, cooling a solution of
3b in CD2Cl2 down to − 90°C allowed the observation of two
rotamers. Focusing on the benzylidene proton, the sharp signal
observed at δ 16.64 ppm in the 1H NMR spectrum at 25°C starts
decoalescing at around – 30°C to finally give two sharp signals at
δ 16.46 and 16.29 ppm at – 80°C, in a 62/38 ratio (Figure 3). A
line shape analysis led to an activation energy barrier ∆G≠ of ca.
46.8 kJ.mol-1 at – 30°C for the associated exchange process
corresponding to the rotation of the NHC around the Ru-CNHC
bond. As expected, this value is much lower than the activation
energies determined for the NHC rotation in GII-type complexes
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10.1002/chem.201604934
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Figure 4. Stability profiles of catalysts 2a,b and 3a,
b compared with
commercially available SIMes complexes 2a and 3a (in CD2Cl2).
To show the effect of NHC backbone substitution on the
efficiency of the catalysts in the metathesis of challenging targets,
we investigated the activity of the new complexes in model RCM
reaction of sterically demanding olefins,[7,11,12] and the results
were compared with those obtained with standard, commercially
available, SIMes supported GII and HGII precatalysts 2a and 3a
(Figure 1). The RCM reaction of sterically very demanding olefin
4 was studied first (Scheme 2 and Table 1) and the time-
conversion profiles of these experiments are presented in Figure
5. The complexes 2a and 3a used in 1 mol% loading showed, as
expected, only limited longevity and activity.[25] Gratifyingly, the
NMe2-substituted GII-type complexes 2b, and 2c exhibited a
much higher activity, leading to 75%, and 66% of conversion,
respectively. However, the HGII-type complexes 3b, and 3c
provided poorer conversions, 42%, and 32%, respectively. In
order to observe a clearer difference between selected catalysts,
a low catalyst loading of 0.1 mol% was employed in the same
challenging test reaction. Again, both complexes 2b and 2c were
found to be more efficient than the classical SIMes supported
precatalyst 2a (Figure 5, Table 1).
Figure 5. Reactivity profiles in RCM of 4 using investigated catalysts 2a,b and
3a,b compared with commercially available complexes 2a and 3a.
In order to test the reactivity limits of the new catalysts and to
demonstrate their application scope, other RCM reactions of
hindered dienes were attempted (Figure 6). The catalytic system
allowed the formation, in usually good conversions (60-97%), of
tetra-substituted olefins 6 and 7 at a [Ru] loading of only 0.1 mol%
(1000 ppm) and the more challenging 8 at loading of 1 mol%. Only
the latter RCM reaction differentiated the tested catalysts, and
GII-type catalysts 2b-c were found to be superior over the HGII
systems 3b-c in formation of tetra substituted C-C double bond.
Considering the growing interest for catalysts able to mediate
metathesis of functionalized substrates at low catalyst loadings,
the newly obtained catalysts were tested with a set of model
dienes (Figure 6). RCM formation of cyclopentene derivatives 8-
10 were possible at loading from 1000 to 500 ppm (with
observable lowering of conversion to one half at 250 ppm of [Ru]).
Also oxacycles, such as 11-13, or azacycles 14-15 can be formed
with the new catalysts used at 1000-250 ppm loading. Notably, in
the case of relatively undemanding formation of disubstituted C=C
bonds, the differences in activity of catalysts 2b-c and 3b-c were
very small both at higher loadings (0.1 mol%) as well as at very
low ones (up to 100 ppm). Using the example of RCM reactions
leading to 12 and 13 catalyzed by 3c we demonstrated that
increasing the temperature of the reaction to 80 °C can lead to
further increment of conversion, without need of raising the
catalyst loading.[26]
EtO2C CO2Et
EtO2C CO2Et
[Ru]
toluene 0.1M, 80 °C
4
5
Scheme 2. Model RCM reaction of demanding diene 4
Table 1. Screening of 2b, 2c, 3b and 3c in the model RCM reaction
of diene 4
Loading
[mol%]]
Time
[h]
Conversion
[%][a]
Entry
Catalyst
1
2
3
4
5
6
2a
2b
2c
3a
3b
3c
1 (0.1)
28
21
21
28
21
21
31 (12)
75 (22)
66 (22)
26
1 (0.1)
1 (0.1)
1
1
1
49
32
[a] Conversion determined by GC using durene as internal standard. In
parentheses are results for 0.1 mol% of catalyst.
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10.1002/chem.201604934
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Having obtained satisfactory results with the model RCM
reactions, we decided to test the new catalysts in other types of
the metathesis reactions, i.e. in enyne cycloisomerisation, and in
cross-metathesis (CM) (Figure 7). The cycloisomerisation of
substrate 16 proceeded easily, leading to almost quantitative
conversion at only 0.1 mol% of [Ru]. Next, we attempted to
perform a well-known CM of allylbenzene and cis-1,4-diacetoxy-
2-butene 19.[27] This reaction promoted by 1 mol% of [Ru]
proceeded as expected, leading to 90-91% of the expected
product. It shall be noted, that in literature 2.5 mol% of [Ru] is
suggested to test this reaction. Having in hand the conditions
suitable to perform cross-metathesis we turned to the citronellol
derivative 21. This CM reaction between a trisubstituted C=C
bond and 19 showed again that in the case of more challenging
reactions the GII-type catalysts 2b-c are substantially more active
as compared with their HGII counterparts 2b-c (Figure 7).
R
R
Y
Y
[Ru]
toluene (0.1M)
n
n
R
R
R
R
Ts
N
Ts
N
6
7
2b, 24h, 0.1 mol%, 80 °C, 60%
2b, 21h, 0.1 mol%, 80 °C, 90%
2c, 21h, 0.1 mol%, 80 °C, 90%
3b, 21h, 0.1 mol%, 80 °C, 97%
2c
, 24h, 0.1 mol%, 80 °C, 56%
3b
, 24h, 0.1 mol%, 80 °C, 78%
3c, 24h, 0.1 mol%, 80 °C, 83%
3c
, 21h, 0.1 mol%, 80 °C, 90%
O
O
[Ru]
O
toluene (0.1M), 60 °C
O
Ph Ph
17
Ph Ph
16
8
9
2b
2c
3b
2b
2c
3b
, 24h, 1.0 mol%, 80 °C, 93% (81%)
, 24h, 1.0 mol%, 80 °C, 91%
, 24h, 1.0 mol%, 80 °C, 77%
, 30 min, 0.1 mol%, 60 °C, 99%
, 10 min, 0.1 mol%, 60 °C, 99%
, 10 min, 0.1 mol%, 60 °C, 99%
2b
2c
3b
, 10min, 0.1 mol%, 99%
, 10min, 0.1 mol%, 99%
, 30min, 0.1 mol%, 99%
3c, 24h, 1.0 mol%, 80 °C, 74%
3c, 10 min, 0.1 mol%, 60 °C, 99%
MeO2C CO2Me
3c, 30min, 0.1 mol%, 99%
O
AcO
OAc
OAc
11
10
19
1.3 equiv. , [Ru]
toluene (0.1M), 60 °C
2b
2b
2c
, 30min, 0.1 mol%, 60 °C, 96%
, 1h, 500 ppm, 60 °C, 90%
, 1h, 500 ppm, 60 °C, 93%
20
18
2c
, 10min, 0.1 mol%, 60 °C, 99%
3b, 10min, 0.1 mol%, 60 °C, 98%
3b, 1h, 500 ppm, 60 °C, 90%
3c, 1h, 500 ppm, 60 °C, 91%
2b,
2c,
1h, 1 mol%, 90% E:Z 8:1
1h, 1 mol%, 90% E:Z 8:1
2b
2c
2b
2c
, 24h, 250 ppm, 60 °C, 72%
, 24h, 250 ppm, 60 °C, 69%
, 6h, 100 ppm, 60 °C, 51%
, 6h, 100 ppm, 60 °C, 44%
3b, 1h, 1 mol%, 91% E:Z 8:1
3c, 1h, 1 mol%, 91% E:Z 8:1
3b, 24h, 250 ppm, 60 °C, 56%
3c, 24h, 250 ppm, 60 °C, 81%
3b, 6h, 100 ppm, 60 °C, 45%
3c, 6h, 100 ppm, 60 °C, 51%
a
AcO
OAc
O
O
29
1.3 equiv. , [Ru]
OBn
toluene (0.1M), 60 °C
OBn
MeO
13
12
21
OAc
22
2b, 4h, 0.1 mol%, 60 °C, 48%
2b, 4h, 0.1 mol%, 60 °C, 46%
2b,
2c,
24h, 1 mol%, 77% E:Z 6.6:1
24h, 1 mol%, 85% E:Z 6.5:1
2c
3b
3c
2c
3b
3c
, 4h, 0.1 mol%, 60 °C, 47%
, 4h, 0.1 mol%, 60 °C, 45%
, 4h, 0.1 mol%, 60 °C, 33%
, 4h, 0.1 mol%, 60 °C, 52%
, 4h, 0.1 mol%, 60 °C, 62%
, 4h, 0.1 mol%, 60 °C, 68%
3b, 24h, 1 mol%, 25% E:Z 4.7:1
3c, 24h, 1 mol%, 23% E:Z 4.3:1
3b, 0.5h, 0.1 mol%, 80 °C, 69%
3b, 0.5h, 0.1 mol%, 80 °C, 72%
Figure 7. Application in various types of metathesis reaction.
O
O
O
N
N
Conclusions
N
H
F
15
14
To date, optimization of ruthenium olefin metathesis catalysts
has been mainly achieved through the decoration of the
benzylidene moiety, or through the variation of the nitrogen-
substituents of the ancillary NHC ligand. We have now
demonstrated that the direct substitution of the carbenic
heterocycle of the NHC by one or two dimethylamino group(s)
constitutes a viable strategy towards improved metathesis
catalysts. The ruthenium complexes supported by these amino-
2b, 24h, 250 ppm, 60 °C, 43%
, 24h, 250 ppm, 60 °C, 40%
3b
, 24h, 250 ppm, 60 °C, 75%
3c, 24h, 250 ppm, 60 °C, 48%
2b, 4h, 0.1 mol%, 60 °C, 88%
, 4h, 0.1 mol%, 60 °C, 90%
, 4h, 0.1 mol%, 60 °C, 66%
3c, 4h, 0.1 mol%, 60 °C, 21%
2c
2c
3b
Figure 6. Low catalyst loading experiments.
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2
2 2
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A.Z., A.R., W.K. and K.G. are grateful to the National Science
Centre (Poland) for the NCN OPUS Grant No. UMO-
2014/15/B/ST5/02156. V.C., Y.Z., M.R., S.B., N. L. and G. L.
thank the ‘Centre National de la Recherche Scientifique’ (CNRS)
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Keywords: C-C bond formation • metathesis • carbenes •
homogeneous catalysis • heterocycles
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Entry for the Table of Contents
FULL PAPER
Vincent César,* Yin Zhang, Wioletta
Kośnik, Adam Zieliński, Adam A.
Rajkiewicz, Mirko Ruamps, Stéphanie
Bastin, Noël Lugan,* Guy Lavigne, Karol
Grela*
Page No. – Page No.
Ruthenium Catalysts Supported by
Amino-Substituted NHC Ligands for
Olefin Metathesis of Challenging
Substrates
The introduction of N-heterocyclic carbene ligands that incorporate dimethylamino
groups as heterocyclic substituents leads to an increased activity in 2nd generation
ruthenium metathesis catalysts.
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