Activity of a new class of ruthenium based ring-closing metathesis and ring-opening metathesis polymerization catalysts coordinated with a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and a Schiff base ligand

Article abstract of DOI:10.1016/S0040-4039(02)02247-5

A new class of ruthenium based catalysts coordinated with a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and a Schiff base ligand have been tested in ring-closing metathesis and ring-opening metathesis polymerization reactions. The results obtained point out that the synergy of Schiff base ligands with a strong Lewis basic N-heterocyclic carbene ligand lead to mixed ligand catalytic systems that combine very high activity with excellent stability. Furthermore, the catalytic activity of these catalysts is very dependent on the steric and electronic environment of the Schiff base.

Full text of DOI:10.1016/S0040-4039(02)02247-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9101–9104
Activity of a new class of ruthenium based ring-closing
metathesis and ring-opening metathesis polymerization catalysts
coordinated with a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene
and a Schiff base ligand
Bob De Clercq and Francis Verpoort*
Ghent University, Department of Inorganic and Physical Chemistry, Krijgslaan 281 (S-3), 9000 Gent, Belgium
Received 30 September 2002; accepted 10 October 2002
Abstract—A new class of ruthenium based catalysts coordinated with a 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene and a Schiff
base ligand have been tested in ring-closing metathesis and ring-opening metathesis polymerization reactions. The results obtained
point out that the synergy of Schiff base ligands with a strong Lewis basic N-heterocyclic carbene ligand lead to mixed ligand
catalytic systems that combine very high activity with excellent stability. Furthermore, the catalytic activity of these catalysts is
very dependent on the steric and electronic environment of the Schiff base. © 2002 Elsevier Science Ltd. All rights reserved.
During recent years ring-closing metathesis (RCM) and
ring-opening metathesis polymerization (ROMP) have
become important tools in both fine chemical and
macromolecular synthetic chemistry.1
Concerning the activity, the best results were obtained
when one of the phosphine ligands was substituted by a
so-called N-heterocyclic carbene (NHC) ligand like 1,3-
dimesityl-4,5-dihydroimidazol-2-ylidene (e.g. catalyst
2).3c–3k With respect to the thermal stability enhance-
ment, the most spectacular success was obtained when
one chloride and one phosphine ligand of type 1 sys-
tems were replaced by a chelating Schiff base ligand
(e.g. catalysts 4).5 Recently, we tested catalytic com-
plexes of type 4 in RCM and ROMP reactions and
suggested that for these type of catalytic complexes the
olefin metathesis mechanism involves the decoordina-
tion and coordination of ‘one-arm’ of the bidentate
Schiff base ligand instead of the usual PCy3 dissociation
as for the systems 1 and 2.6 As NHC-ligands are very
Olefin metathesis is a catalytic reaction in which alkenes
are converted into new products via the rupture and
reformation of CꢀC double bonds. Depending on the
starting material (cyclic or acyclic alkenes) and the
reaction condition parameters, RCM, acyclic diene
metathesis (ADMET) or ROMP proceed (Scheme 1).2
Mainly because of the development of Grubbs’ ruthe-
nium catalysts 1 and 23 and Schrock’s molybdenum
complex 3 (Fig. 1),4 olefin metathesis has bridged the
gulf from an academic curiosum to a mild, efficient,
reliable and industrially applicable carbonꢀcarbon
forming technique.
Since the groundbreaking discovery that the well-
defined unicomponent organometallic species of type 1
could promote olefin metathesis reactions, much effort
has been directed towards modification of the ligand
sphere around the metal center in order to improve the
performing characteristics of these catalytic systems.
Keywords: ring-closing metathesis; ring-opening metathesis polymer-
ization; N-heterocyclic carbene ligand; homogeneous catalysis; ruthe-
nium and compounds; olefins.
* Corresponding author. Tel.: +32-9-264-44-36; fax: +32-9-264-49-83;
e-mail: francis.verpoort@rug.ac.be
Scheme 1. Schematic representation of the olefin metathesis
reaction.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02247-5

9102
B. De Clercq, F. Verpoort / Tetrahedron Letters 43 (2002) 9101–9104
Figure 1. Depiction of some well-defined unicomponent olefin metathesis catalysts.
strong and bulky Lewis bases, we came to the idea that
the combination of a Schiff base and an NHC-ligand in
one catalyst structure could lead to improved catalyst
performance. Indeed, the presence of the bulky carbene
ligand trans to the decoordinating part of the Schiff
base should stabilize the reactive catalytic intermediate
and/or prevent the decomposition of the carbene.
Moreover, thermal stability should also be satisfying
because these phosphine-free complexes do not suffer
from the well-known P–C degradation at higher
temperatures.
monomers tested, although significant differences in
their behavior were noticed. The pronounced observed
conversion sequence 5.a>5.b>5.c>5.d>5.e>5.f clearly
shows that the bulkiness of the Schiff base and the
electron-withdrawing properties of the Schiff base sub-
stituents exert a profound influence on the ROMP
activity. The results gathered in Table 1 reveal that the
introduction of more bulkiness in the Schiff base is
detrimental for catalyst performance in ROMP reac-
tions. For instance, cyclohexenylnorbornene is con-
verted in 100, 83 and 77% yield for 5.a, 5.c and 5.e,
respectively. The influence of the electronic environ-
ment of the Schiff base is best illustrated by comparing
the catalytic performance of system 5.a with 5.b, 5.c
with 5.d and 5.e with 5.f. The data in Table 1 point out
that the complex bearing the electron-withdrawing
nitro substituent (5.b, 5.d and 5.f) reaches systemati-
cally lower conversions. Furthermore, it is clear that the
bulkiness of the Schiff base has a greater impact on
catalytic performance in ROMP reactions than the
electronic influence exerted by the Schiff base sub-
stituents. When the ROMP activity of the best catalyst
5.a is compared with that of 4.b6 (the most performing
system for this class of catalysts in ROMP reactions, R
and R% for 4.b are the same as for 5.b) it is obvious that
type 5 catalysts are more performing. For example,
hydroxymethyl- and cyanonorbornene are converted in
43 and 38% yield with 4.b (reaction conditions: 70°C, 4
h, catalyst/substrate=1/800),6 whereas 5.a converts
these substrates in 76 and 68% yield, respectively. The
characteristics of the polymers obtained with the
highest performing initiator 5.a are also depicted in
Table 1. As can be expected by the living nature of the
ROMP reaction, the polydispersity index (PDI) of the
formed polymers is quite narrow. It also deserves men-
tion that the measured number average molecular
weights (Mn) are close to the theoretical values (assum-
ing the absence of backbiting or other chain-breaking/-
terminating reactions) leading to initiation efficiencies
close to unity. In accordance with the general observa-
tion for ruthenium catalysts in ROMP reactions a trans
configuration of the polynorbornene derivatives and
polycyclooctene is predominate.
Recently, we succeeded in synthesizing and characteriz-
ing the new class of ruthenium-based catalysts 5.a–f
(Fig. 2).7
We now report on the excellent activity of these readily
available and robust complexes for ROMP and RCM
with some representative monomers. Moreover, we also
checked the thermal stability of these systems in RCM
reactions with diethyldiallylmalonate. Catalytic systems
5.a–f were fully characterized by 1H, 13C NMR,
Raman, IR and elemental analysis.7
In a first set of experiments, the catalytic activity of the
Schiff base substituted ruthenium benzylidenes 5.a–f is
checked for ROMP with some representative
monomers. The yields (%) and characteristics of the
formed polymers are depicted in Table 1.
It is obvious from Table 1 that all catalytic systems 5.a
to 5.f succeed in performing ROMP reactions with the
In a second set of experiments, the performance of
catalytic systems 5.a–f were tested in RCM reactions.
Table 2 summarizes the RCM results obtained with
some representative substrates.
Figure 2.

B. De Clercq, F. Verpoort / Tetrahedron Letters 43 (2002) 9101–9104
9103
Table 1. Yield (%) for ring-opening metathesis polymerization of some representative monomers using catalysts 5.a–f and
characteristics of the polymers formed with 5.a
Yield (%)a
Characteristics of the polymers formed with 5.a
Mn (×103)b
PDIb
1.53
|
c
fid
c
Substrate
5.a
5.b
5.c
93
5.d
5.e
84
5.f
79
Cyclooctene
100
100
88
109
0.45
0.94
Norbornene derivatives
R=H
R=ethyl
R=butyl
R=hexyl
R=decyl
R=ethylidene
R=phenyl
R=cyclohexenyl
R=ethylnorbornane
R=cyano
R=hydroxymethyl
R=chloromethyl
R=triethoxysilyl
100
100
100
100
100
100
100
100
100
68
100
100
100
100
100
100
100
98
100
58
69
100
100
100
100
96
93
90
87
82
83
98
46
53
100
94
100
100
92
84
83
77
76
79
93
43
51
100
90
100
100
87
88
83
78
62
77
85
29
42
93
81
100
100
82
74
82
73
57
69
78
22
37
84
82
202
272
342
420
579
270
410
430
448
232
290
328
564
1.31
1.35
1.38
1.30
1.34
1.40
1.43
1.39
1.33
1.29
1.35
1.38
1.42
0.23
0.24
0.21
0.22
0.25
0.20
0.26
0.27
0.22
0.24
0.23
0.21
0.25
0.93
0.90
0.88
0.85
0.81
0.89
0.83
0.81
0.84
0.70
0.65
0.87
0.91
76
100
100
a In a typical ROMP experiment 0.005 mmol of the catalyst solution in toluene (0.1016 M) was transferred into a 15 ml vessel followed by the
addition the appropriate amount of monomer solution (800 equiv. for cyclooctene and 2000 equiv. for the norbornene derivatives) in toluene.
The reaction mixture was then kept stirring at 70°C for 4 h. To stop the polymerization reaction, 2–3 ml of an ethylvinylether/BHT solution
was added and the solution was stirred for 0.5 h to make sure that the deactivation of the active species was completed. The solution was poured
into methanol (50 ml containing 0.1% BHT) and the polymers were precipitated and dried in vacuum overnight.
b Mn and the polydispersities (PDI) are determined by size-exclusion chromatography (SEC) with polystyrene calibration.
c Fraction of polymers with cis configuration.
d fi=initiation efficiency=Mn, theor./Mn, exp. with Mn, theor.=([monomer]0/[initiator]0)×MW(monomer)×conversion.
tively) are converted in 21 and 17% with 4.f (reaction
conditions: 70°C, 4 h, 5 mol% catalyst),6 whereas 5.a
converts these substrates in 72 and 41%, respectively.
The outstanding stability of the initiators of type 4 have
already been demonstrated by Grubbs et al. In order to
assess the stability of catalytic systems 5.a–f, they were
stored for 1 week in the solid state under an air
atmosphere after which we tested them in RCM reac-
tions with diethyldiallylmalonate (reaction conditions
identical to those in Table 2). The results of these
experiments indicated no noteworthy loss of perfor-
mance. The same conclusion could be drawn when
these complexes were transferred into a solution of
deuterated benzene under inert atmosphere. Again,
after 1 week, no loss of activity was revealed in RCM
experiments using diethyldiallymalonate as substrate
(reaction conditions identical to those in Table 2). Even
when a solution of 5.a is heated to 85°C in deuterated
benzene under an inert atmosphere for 17 h, no signs of
decomposition appear.
When analyzing the results gathered in Table 2, one
observes the same order in catalyst performance for
RCM and ROMP reactions. For instance, the tetra-
substituted malonatederivative (entry 3, Table 2) is
converted with 41, 33, 19, 11, <5 and <5% with systems
5.a, 5.b, 5.c, 5.d, 5.e and 5.f, respectively. The conclu-
sions about the steric bulk of the Schiff base ligands
and the electronic-withdrawing properties of the Schiff
base substituents that were made for the ROMP reac-
tions can also be drawn here. Catalytic RCM of dienes
diethyldiallylmalonate (entry 1), 1,7-octadiene (entry 4)
and diallylether (entry 5) was performed quantitatively
by all systems. As can further be seen from the data in
Table 2, the reactivity of the complexes is sufficiently
high to allow the preparation of tri- and tetra-
substituted cycloalkenes in good to moderate yields
(entries 2 and 3). Furthermore, diallylphtalate (entry 6)
and linalool (entry 7) are also converted smoothly.
With the best catalytic system 5.a and when heating the
reaction mixture for 4 h at 55°C, conversions of 87 and
73%, respectively, are reached. When the RCM activity
of the best catalyst 5.a is compared with that of 4.f6
(the best performing system for this class of catalysts in
RCM reactions, R and R% for 4.f are the same as for
5.f) it is again clear that type 5 catalysts are better
performing. For example, the trisubstituted and tetra-
substituted malonatederivative (entries 2 and 3, respec-
In conclusion, the catalytic systems 5.a–f are highly
efficient catalysts for promoting ring-closing metathesis
and ring-opening metathesis polymerization reactions
under mild reaction conditions. We succeeded in our
goal to improve catalytic performance of type 4 systems
while
maintaining
their
excellent
stability
characteristics.

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B. De Clercq, F. Verpoort / Tetrahedron Letters 43 (2002) 9101–9104
Table 2. Yield (%) for RCM of some representative substrates using catalytic systems 5.a–fa
Acknowledgements
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B.D.C. is indebted to the IWT (Vlaams instituut voor
de bevordering van het wetenschappelijk-technologisch
onderzoek in de industrie) for a research grant. F.V. is
indebted to the FWO-Flanders (Fonds voor weten-
schappelijk onderzoek-Vlaanderen) for financial sup-
port and to Research funds of Ghent University.
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