N-Heterocyclic carbenes: Application of ruthenium-alkylidene complexes in ring-opening metathesis polymerization

Article abstract of DOI:10.1016/S0022-328X(99)00275-2

Novel ruthenium-alkylidene catalysts bearing N-heterocyclic carbenes were applied in ring-opening metathesic polymerization (ROMP) reactions of various norbornene derivatives. High tolerance towards polar functional groups as well as high catalytic activity is demonstrated. The combination of N-heterocyclic carbenes and coordinatively labile ligands (phosphanes or chloro-bridged transition metals) on the ruthenium center proves not only successful regarding catalytic performance but also promising with respect to polymer properties.

Full text of DOI:10.1016/S0022-328X(99)00275-2

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Journal of Organometallic Chemistry 586 (1999) 263–265
Note
N-Heterocyclic carbenes: application of ruthenium–alkylidene
complexes in ring-opening metathesis polymerization
Ulrich Frenzel a, Thomas Weskamp b, Florian J. Kohl b, Wolfgang C. Schattenmann b,
Oskar Nuyken a, Wolfgang A. Herrmann b,*
a Lehrstuhl fu¨r Makromolekulare Stoffe, Technische Uni6ersita¨t Mu¨nchen, Lichtenbergstraße 4, D-85747 Garching bei Mu¨nchen, Germany
b Anorganisch-chemisches Institut der Technischen Uni6ersita¨t Mu¨nchen, Lichtenbergstraße 4, D-85747 Garching bei Mu¨nchen, Germany
Received 2 May 1999
Abstract
Novel ruthenium–alkylidene catalysts bearing N-heterocyclic carbenes were applied in ring-opening metathesic polymerization
(ROMP) reactions of various norbornene derivatives. High tolerance towards polar functional groups as well as high catalytic
activity is demonstrated. The combination of N-heterocyclic carbenes and coordinatively labile ligands (phosphanes or chloro-
bridged transition metals) on the ruthenium center proves not only successful regarding catalytic performance but also promising
with respect to polymer properties. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: N-Heterocyclic carbene; Homogeneous catalysis; Olefin metathesis; Ring-opening metathesis polymerization; Ruthenium
1. Introduction
2. Results and discussion
Ring-opening metathesis polymerization (ROMP) is
a thoroughly investigated olefin metathesis reaction [1].
The combination of high catalytic activity and toler-
ance towards various polar functionalities is an impor-
tant goal for the development of well-defined
metathesis catalysts. The discovery of the ruthenium–
alkylidene complex 1 by Grubbs and co-workers was a
milestone in this respect [2,3].
The tolerance of the NHC catalysts towards polar
functional groups is demonstrated by ROMP reactions
of various norbornene derivatives 5 using the dicarbene
complex 2a (Table 1).
As shown in Table 1, a strong dependence regarding
catalyst activity and lifetime on the polar groups of the
monomer is found: quantitative conversion is observed
in the presence of carboxyl moieties (acid, esters) at
room temperature, whereas carbonyls (aldehydes, ke-
tones) afford lower yields. At elevated temperatures
and prolonged reaction times, however, nearly quanti-
tative conversion can be achieved. In contrast, the
However, substitution of phosphane ligands by N-hete-
rocyclic carbenes (NHC) [4–6] yielded quite an im-
provement. Herein, we report on the application of our
catalysts 2, 3 and 4 in ROMP reactions.
* Corresponding author. Tel.: +49-89-28913080; fax: +49-89-
28913473.
E-mail address: lit@arthur.anorg.chemie.tu-muenchen.de (W.A.
Herrmann)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00275-2

264
U. Frenzel et al. / Journal of Organometallic Chemistry 586 (1999) 263–265
Table 1
ROMP of norbornene derivatives 5 with catalyst 2a a
b
R
t (min)
Yield (%)
Mn (kg mol−1
)
PDI b
Tg (°C)
CO2H
OCOCH3
CHO
COCH3
OH
60
5
240
45
120
150
240
98 c
96
61
42
4
91 c
256
53
1.5
2.0
2.3
1.4
–
177
70
95
99
–
162
Insoluble
Insoluble
–
CH2OH
CN
18
0
–
–
–
–
a Reaction conditions: T/25°C; CH2Cl2; [monomer]/[catalyst]=500:1.
b GPC in CHCl3 vs. polystyrene standards.
c GPC in THF vs. polystyrene standards.
Table 3
ROMP of 7 with bimetallic catalysts 4a, 4b and 4c a
Catalyst
t (min)
Yield (%)
TOF (h−1
)
4a
4b
4c
3
3
1
95
95
80
1.0×105
1.0×105
2.4×105
presence of alcohols and nitriles results in the loss of
catalytic performance. These tendencies also apply to
our second- and third-generation catalysts 3 and 4, but
activities are increased, especially in the case of 4.
The evolution of NHC type catalysts is clearly
demonstrated by ROMP of 2,3-dicarbomethoxynorbor-
nadiene 6 (Table 2) by means of a considerably in-
creased activity as well as by the properties of the
resulting polymers. The low PDI of 1.1 in the case of 4a
indicates rapid initiation and a living polymerization
reaction. A [monomer]/[catalyst] ratio of 500:1 has been
chosen to ensure solubility of the polymers and low
viscosity of the solutions for NMR and GPC
investigations.
a Reaction conditions: T/25°C; CH2Cl2; [monomer]/[catalyst]=
5000:1.
3. Conclusions
In view of high catalytic activity, stability against air
andmoisture, andtolerancetowardspolarfunctionalities,
ruthenium–alkylidene complexes with NHC represent a
valuable alternative to literature-established systems in
ring-opening metathesis polymerization. Beyond that, the
combination of NHC with coordinatively more labile
ligands on the ruthenium center is not only successful
regarding catalytic performance but also promising with
respect to the influence on the properties of the resulting
ROM polymers.
To illustrate the potential of the bimetallic catalysts
4a–c regarding catalytic performance, ROMP of 5-nor-
bornenyl-2-acetate (7) was performed at a [monomer]/
[catalyst] ratio of 5000:1 (Table 3).
Acknowledgements
This work received generous support from the Fonds
der Chemischen Industrie (Ph.D. fellowship to T.W.), the
Bayerische Forschungsstiftung (Bayerischer Forschungs-
verbund Katalyse, FORKAT), the Deutsche Forschungs-
gemeinschaft, Aventis R&T, and Degussa AG (loans of
RuCl3).
Table 2
ROMP of 6 with catalysts 2b, 3 and 4a a
c
Catalyst
t (h)
Yield (%)
|
(%) b
Mn (kg mol−1
)
PDI c
cis
2b
3
4a
14
2
0.75
51
59
93
36
40
41
206
367
187
1.8
1.8
1.1
a Reaction conditions: T/25°C; CH2Cl2; [monomer]/[catalyst]/500:1.
b Fraction of double bonds in the polymer backbone possessing cis configuration.
c GPC in CHCl3 vs. polystyrene standards.

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265
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