Indenylidene complexes of ruthenium bearing NHC ligands - structure elucidation and performance as catalysts for olefin metathesis

Article abstract of DOI:10.1002/ejoc.200800973

Second-generation catalysts of the general formula Cl₂Ru-(SIMes) (L)(3-phenylinden-1-ylidene), 3a (L = PCy₃), 3b (L =PPh₃), 3c (L = py), and Cl2Ru(SIMe)(L)(3-phenylinden-1-yl-idene), 4a (L = PCy ₃), 4b (L = PPh<

Full text of DOI:10.1002/ejoc.200800973

FULL PAPER
DOI: 10.1002/ejoc.200800973
Indenylidene Complexes of Ruthenium Bearing NHC Ligands – Structure
Elucidation and Performance as Catalysts for Olefin Metathesis
Stijn Monsaert,[a] Els De Canck,[a] Renata Drozdzak,[a] Pascal Van Der Voort,[a]
Francis Verpoort,*[a] José C. Martins,*[b] and Pieter M. S. Hendrickx[b]
Keywords: Ruthenium / Olefin metathesis / Conformation analysis / Homogeneous catalysis / Indenylidene
Second-generation catalysts of the general formula Cl2Ru-
(SIMes)(L)(3-phenylinden-1-ylidene), 3a (L = PCy3), 3b (L =
PPh3), 3c (L = py), and Cl2Ru(SIMe)(L)(3-phenylinden-1-yl-
idene), 4a (L = PCy3), 4b (L = PPh3), 4c (L = py) were found
to be of interest in various metathesis transformations. The
catalysts containing SIMe ligands showed improved initia-
tion compared to the more robust SIMes substituted cata-
lysts. A strong temperature effect was noted on all of the re-
actions tested. Interestingly, complex 3a, showing the lowest
initiation rate at room temperature, emerged as the most pro-
ductive of all systems examined at elevated temperature. It
is shown that complexes containing the SIMe ligand display
higher initiation efficiency than their corresponding SIMes
analogues. Since the higher initiation is related to the ease
of phosphane dissociation while phosphane dissociation also
promotes catalyst decomposition, complexes bearing the
resonance assignment and the procedure applied to obtain
these from a combination of 1D and 2D NMR techniques, is
also reported. Combined with the ROESY technique, these
enabled to investigate several conformational processes in-
volving rotations around N–phenyl and C–Ru bonds on the
millisecond to second timescale. A clear correlation is dem-
onstrated between the bulkiness of the axial ligand (L) and
the rotational freedom of the SIMe(s) ligand. A qualitative
analysis also suggests that the extra para-methyl of SIMes
leads to additional steric interactions with the 3-phenylin-
den-1-ylidene ligand. The data reported in this paper dem-
onstrates that substitution patterns of the N-aryl have a sig-
nificant influence on the activity of the second-generation in-
denylidene catalysts for a given metathesis reaction.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
SIMe ligand decompose faster. The complete 1H, 13C and 31
P
Blechert,[5a–5c] leading to catalysts that display excellent re-
activities toward electron-deficient substrates.
Introduction
The appearance of well-defined ruthenium-based precat-
alysts for metathesis has prompted an unusual growth of
interest in this transformation both from the fine chemical
and polymer industry.[1a–1d] In particular, the neutral ruthe-
nium carbene complex 1a developed by Grubbs and co-
workers turned out to be exceedingly useful, combining a
high catalytic activity with a good to excellent tolerance
towards polar functional groups.[2a,2b] Among the numer-
ous variations on the ligand sphere of 1a, the replacement
of one phosphane ligand by a N-heterocyclic carbene moi-
ety was found to impart a significant increase in activity as
well as stability in solution.[2c–2i] Another important mile-
stone on the metathesis road of success was the introduc-
tion of chelating ligands by Hoveyda,[3a,3b] Grela[4a,4b] and
Although the rapidly increasing demand for metathesis
catalysts has led to the development of highly active ruthe-
nium precatalysts for advanced synthetic tasks, a search for
commercially relevant, alternative metathesis initiators of
comparable performance to Grubbs’ catalysts and im-
proved accessibility remains challenging. In this context, the
ruthenium indenylidene complex 2a, has been introduced
and described as a particularly well suited precatalyst for
cyclization of medium-sized rings by Ring-Closing Metath-
esis (RCM) reactions (Figure 1).[6–8]
The incorporation of NHC ligands led to complexes 2c
and 3a in which there is a pronounced decrease in the initia-
tion rate compared to 2a. This drawback is offset, however,
by their increased thermal stability, which is beneficial for
many Ring-Closing Metathesis reactions performed at ele-
vated temperatures.[6,8] Even though the indenylidene cata-
lysts appeared to be very attractive from a practical point of
view, as they can be very easily prepared from commercially
available precursors,[7] the structure and substantial varia-
tions of its basic structural motif have not been well ex-
plored. In the following we summarize our investigations in
this field. In an attempt to expand the application profile
[a] Department of Inorganic and Physical Chemistry, Ghent Uni-
versity,
Krijgslaan 281 (S3), 9000 Ghent, Belgium
Fax: +32-9-264-4183
E-mail: Francis.Verpoort@UGent.be
[b] Department of Organic Chemistry, Ghent University,
Krijgslaan 281 (S4), 9000 Ghent, Belgium
Fax: +32-9-264-4972
E-mail: Jose.Martins@UGent.be
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F. Verpoort, J. C. Martins et al.
FULL PAPER
of the NMR spectroscopic data. Here, a general route to
and complete NMR characterization of the complexes 3a–
1
c, 4a–c is presented, with full H, 13C and 31P assignment
and an investigation of the dynamics of rotameric processes
occurring from hindered rotation along a variety of bonds.
The activity of this new class of indenylidene catalysts
4a–c with respect to the analogous family 3a–c and Grubbs’
catalysts 1c, was determined by establishing their perform-
ance in catalysing different reaction types. More specifically,
Ring-Closing Metathesis (RCM) of dienes, alkene Cross-
Metathesis (CM) and Ring-Opening Metathesis Polymeri-
zation (ROMP) have been evaluated.
Results and Discussion
Figure 1. Benzylidene and indenylidene ruthenium complexes.
of indenylidene catalysts, we have investigated the effects of
Synthesis of Indenylidene-Ru Complexes 3a–c and 4a–c
A most attractive feature of neutral indenylidene com-
changing the substitution patterns in the aryl groups of the plexes such as 2a and 2b stems from the ease of formation
NHC on the catalytic activity of the corresponding ruthe- of the Ru=C bond of the indenylidene moiety, which is
nium complexes. Recently Schrodi et al. described that re- achieved by reaction of Ru(Cl)2(PPh3)3–4 with a propargyl
moving the methyl substituents in ortho- and para-position alcohol.[7] It has been recently described by our group, that
of the N-aryl ring led to a large increase in reactivity of the most suitable method for converting 2a into its 2nd-gen-
Grubbs’ catalyst 1c.[9] Following this lead, catalysts in eration analogues, appears to be the “one-pot” thermal de-
which the NHC ligands only bear ortho-methyl substituents composition of specific adducts[6] such as the chloroform
(SIMe) 4a–c were prepared and compared to 3a–c bearing adduct 5a or pentafluorobenzene adduct 5b (Figure 2).
both ortho- and para-methyl substituents.
These methods give access to a set of structurally diverse
The series of indenylidene complexes prepared in this indenylidene ruthenium complexes by variation of the labile
study were fully characterised using NMR spectroscopy. ligands as well as the NHC ligands.
This was motivated by the fact that little information is
Reaction progress was easily followed by 31P NMR
available on the solution behaviour of such complexes. In through the appearance of a new, upfield peak (δ =
addition, the chemical shift is very sensitive to both confor- 27.0 ppm and 26.1 ppm for 3a and 4a, respectively, vs. δ =
mational and subtle stereo electronic effects. Therefore, pro- 33.5 ppm for the starting complex 2a). Complete conversion
vided such NMR spectroscopic data is obtained from a suf- was observed within 1.5 h. Complexes 3a and 4a, red pow-
ficiently large collection of complexes, correlations with the ders after precipitation in methanol, were isolated in high
catalytic activity may ultimately be inferred from analysis yields. The synthesis of 3c and 4c proceeded easily by treat-
Figure 2. Synthetic pathways to 2nd- and 3rd-generation indenylidene-Ru metathesis catalysts.
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Eur. J. Org. Chem. 2009, 655–665

Indenylidene Ruthenium Catalysts for Olefin Metathesis
ment with an excess of pyridine.[2e,10] The indenylidene signment of the common ligands is collected in Table 1
complexes 3c and 4c, precipitated from the reaction mixture while Tables 2, 3, and 4 hold the assignments of the variable
with hexane at –40 °C, were washed several times with small ligand.
portions of hexane, and recrystallized from CH2Cl2/hexane
to yield orange-brown solids (η = 70% for 3c and 60% for
1
Table 2. H, 13C and 31P assignments of the PCy3 ligand in 3a and
4a.[a]
4c) that were fully characterized spectroscopically. Com-
plexes 3b and 4b were obtained from 3c and 4c by simple
ligand exchange and isolated as clear red powders. In ad-
dition, they were straightforwardly obtained from reaction
of 2b with 5.
Atom
label
3a
4a
1Hax
1Heq
13C
1Hax
1Heq
13C
35
2.45
1.22
1.18
1.10
1.09
1.06
27.0
–
33.0
29.5
29.4
27.8
27.7
26.2
2.45
1.20
1.14
1.11
1.09
1.09
26.1
–
33.0
29.4
29.3
27.9
27.8
26.3
36/40
36/40
37/39
37/39
38
1.84
1.72
1.59
1.53
1.52
1.82
1.67
1.60
1.53
1.51
Structural Analysis of Complexes 3a–c and 4a–c
31P
In order to fully characterize this set of compounds, a
series of 1D 1H and 13C spectra complemented by 2D spec-
tra consisting of homonuclear 1H-{1H} COSY, TOCSY,
[a] Arbitrary numbering is depicted in Figure 3. Chemical shifts are
quoted in ppm with respect to residual C6D5H as secondary in-
ternal reference. Assignments are partly based on chemical shift
predictions. The measurement conditions are the same as in
1
NOESY and heteronuclear H-{13C} HSQC and HMBC
1
spectra, was used. The complete H and 13C resonance as- Table 1.
1
Table 1. H and 13C assignments of the NHC and indenylidene moieties common to compounds 3a–c and 4a–c at 298 K in [D6]benzene
solution.[a]
Atom
label
3a
3b
3c
4a
4b
4c
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
1H
13C
1
2
3
4
5
6
Me7
Me8
Me9
10a
10b
11a
11b
12
13
14
15
16
–
–
6.96
–
6.96
–
2.85
2.87
2.23
3.37
3.37
3.26
3.17
–
–
6.47
–
6.02
–
2.22
2.38
1.80
–
135.6
139.0
130.0
136.6
130.0
139.0
20.3
20.3
20.9
52.1
52.1
–
–
6.93
–
6.91
–
2.82
2.84
2.26
6.42
6.42
6.25
3.17
–
–
6.43
–
6.07
–
2.15
2.31
1.82
–
136.7
140.2
131.0
138.8
131.0
140.4
21.6
21.5
22.0
52.8
52.8
–
–
6.96
–
7.03
–
2.75
3.02
2.16
3.45
3.58
3.22
3.37
–
–
6.41
–
6.28
–
2.02
2.50
1.73
–
134.3
140.8
129.6
138.5
129.2
140.4
21.0
21.2
20.7
50.2
50.2
–
–
138.0
139.4
128.9
128.9
128.9
139.6
20.5
20.5
–
51.7
51.7
51.5
51.5
139.0
137.0
128.2
128.2
127.7
136.8
18.9
18.7
–
–
–
138.0
139.6
129.0
128.9
128.9
139.7
20.5
20.5
–
51.4
51.4
51.3
51.3
138.8
137.1
128.3
128.3
128.0
136.9
18.6
18.7
–
–
–
136.9
141.1
128.9
128.4
128.4
140.5
20.9
21.2
–
50.0
50.0
51.5
51.5
138.3
137.5
128.4
128.4
127.9
137.5
18.4
18.5
–
7.14
7.10
7.14
–
2.82
2.87
–
3.28
3.30
3.08
3.17
–
≈ 7.15
≈ 7.15
≈ 7.15
–
2.79
2.87
–
3.30
3.30
3.09
3.16
–
7.16
6.53
7.20
–
2.74
3.04
–
3.38
3.50
3.13
3.31
–
51.6
51.6
52.6
52.6
51.8
51.8
136.8
136.8
129.2
136.6
128.6
137.6
18.7
18.5
20.7
217.3
137.9
137.9
130.2
137.7
130.0
137.6
19.8
19.6
21.8
216.8
136.3
136.9
129.2
137.3
129.3
136.8
18.2
18.5
20.7
215.2
–
–
–
6.62
6.54
6.28
–
2.23
2.39
–
6.56
6.56
6.30
–
2.13
2.33
–
7.20
6.53
7.16
–
2.04
2.51
–
17
Me18
Me19
Me20
21
–
≈ 215
–
≈ 215
–
214.8
22
23
24
25
26
27
28
29
30
31
32
33
34
–
7.84
–
292.2
137.8
138.0
145.0
116.0
126.8
128.2
129.5
141.2
136.0
126.2
128.9
127.3
–
7.10
–
300.9
139.1
141.2
144.5
117.2
128.8
128.4
130.2
142.4
137.9
127.5
129.9
128.7
–
7.22
–
300.4
139.3
139.2
143.4
116.5
128.5
128.6
128.4
141.2
137.2
126.4
128.9
127.5
–
7.76
–
293.1
138.0
137.3
145.2
116.5
127.2
128.1
129.6
141.4
136.7
126.5
129.0
127.3
–
7.01
–
300.7
138.1
140.6
143.4
116.4
127.4
127.8
129.2
141.6
136.9
126.5
128.7
127.6
–
7.11
–
301.3
139.6
139.6
143.2
116.7
128.7
129.2
128.3
141.3
137.2
126.5
128.7
127.5
–
–
–
–
–
–
7.08
7.11
7.18
9.15
–
6.96
7.04
6.90
8.39
–
7.11
7.07
7.03
9.08
–
7.07
7.12
7.19
9.20
–
6.96
6.88
7.05
8.35
–
7.07
7.03
7.00
9.06
–
–
–
–
–
–
–
7.89
7.25
7.32
7.63
7.20
7.33
7.82
7.14
7.30
7.86
7.23
7.30
7.62
7.19
7.32
7.80
7.15
7.31
[a] The arbitrary numbering is depicted in Figure 3. Chemical shifts are quoted in ppm with respect to residual C6D5H as secondary
internal reference. The dividing horizontal line separates the assignments of the SIMe(s) N-heterocyclic carbene ligand C from those of
the indenylidene group.
Eur. J. Org. Chem. 2009, 655–665
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F. Verpoort, J. C. Martins et al.
FULL PAPER
Table 3. 1H and 13C assignments of the pyridine ligand in 3c and correlations involving H7–H18 and H8–H19 allowed to de-
4c.[a]
termine the relative position of these methyl groups and
their associated aromatic ring with respect to each other, as
shown in Figure 3.
Atom
label
3c
4c
1H
13C
1H
13C
Furthermore, strong nOe contacts, which correlate the
ortho-methyl groups to four protons in the 3.10–3.40 ppm
range, were also present. As these latter protons are pair
wise correlated to carbon atoms resonating at δ = 52.1 and
51.6 ppm in the HSQC spectrum and show strong mutual
nOe and TOCSY contacts, they could be unambiguously
assigned to the SIMes bridgehead protons (H10 and H11).
HMBC correlations to a carbon resonance at δ =
217.3 ppm allowed identifying the carbene carbon atom
(C21) which is expected to resonate at high frequency.
As for the assignment of the indenylidene ligand, the C22
carbene at δ = 292.7 ppm is chosen as a starting point be-
cause of its conspicuous low-field frequency. As only two
nJCH correlations to the carbene were found in the HMBC
spectrum, H23 and H29 could be identified; they could be
35
36
37
8.18
6.09
6.37
152.9
123.7
136.8
8.51
6.07
6.35
150.8
123.8
136.8
[a] Arbitrary numbering is depicted in Figure 3. Chemical shifts are
quoted in ppm with respect to residual C6D5H as secondary in-
ternal reference. The measurement conditions are the same as in
Table 1.
Table 4. H, 13C and 31P assignments of the PPh3 ligand in 3b and
1
4b.[a]
Atom
label
3b
4b
1H
13C
1H
13C
35
36
37
38
31P
–
135.5
135.5
130.8
129.8
–
132.9
135.6
129.8
129.9
7.45
6.91
6.97
27.3
7.45
6.92
6.96
27.5
1
differentiated on the basis of their H multiplicity (singlet
vs. doublet). Further correlations in TOCSY, NOESY and
HMBC spectra lead to the complete assignment of the H26–
H29 spin system as well as the quaternary carbon atoms of
the main aromatic ring system. The H32–H34 spin system
Arbitrary numbering is depicted in Figure 3. Chemical shifts are
quoted in ppm with respect to residual C6D5H as secondary in-
ternal reference. The measurement conditions are the same as in
Table 1.
In the following, only the spectral assignment of 3a will from the phenyl moiety was identified using both TOCSY
be described in detail, all other compounds being charac- and HSQC spectra and was linked to the main ring by nOe
n
terized in a similar fashion. The arbitrary numbering, as and JCH correlations. As the ortho- and meta-protons in
depicted in Figure 3, will be used throughout.
the phenyl ring are isochronous, it can be concluded that
As an unambiguous starting point of the assignment, the the rotation around the C24–C31 bond is fast on the NMR
six methyl 1H resonances of the SIMes ligand, which occur time scale.
as sharp singlets in the aliphatic spectral region, were cho-
Since the six methyl groups of the SIMes phenyls experi-
sen. The resonances of their attached carbon atoms were ence different magnetic environments, interpretation of
obtained from the HSQC spectrum. The aromatic protons their nOe contacts is greatly facilitated. Multiple nOe con-
of the SIMes ligand (H3, H5, H14 and H16) could then be tacts connecting the SIMes methyl groups Me18–Me20 with
n
identified using a combination of nOe and JCH corre- many indenylidene protons together with the absence of
lations to the methyl groups. This also afforded to group such contacts involving Me7–Me9, allows to position the
1
individual methyl and aromatic H and 13C resonances to phenyl ring with the higher numbering over the indenyl-
each of the mesitylene ring moieties. Methyl to methyl nOe idene ligand (Figure 3).
Figure 3. Arbitrary numbering of the common ligands of compounds 3a–c (top, left) and 4a–c (top, right) and of the variable ligand of
compounds 3a and 4a (bottom, left), 3c and 4c (bottom middle) and 3b and 4b (bottom right).
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Indenylidene Ruthenium Catalysts for Olefin Metathesis
In both complexes 3a and 4a, the tricyclohexylphos-
phane ligand shows one set of broad resonances in the 1D
proton spectrum that sharpen somewhat as temperature is
increased. This is explained by assuming non-equivalent en-
vironments for the cyclohexyl moieties in the ligand that
interconvert through rotation around the Ru–P and P–C35
bonds in an intermediate to fast exchange regime. The as-
1
signment of all H and 13C resonances (Table 2) was vali-
dated by means of characteristic nOe contacts or through
confrontation with predicted chemical shift values (Chem-
Draw 7.0, Cambridge Soft).
In the complexes bearing triphenylphosphane ligands (3b
and 4b) only one set of resonances is observed for this li-
gand, thus the rotation around the Ru–P bond must be fast
on the NMR timescale. Furthermore, ortho- and meta-pro-
tons are again isochronous due to fast rotations around all
P–C35 bonds. Resonance frequencies were obtained from
HSQC and TOCSY spectra. Assignments were based on
COSY correlations, signal intensity and multiplicity.
The pyridine ligand in complex 3c and 4c shows sharp
resonances in the aromatic region. Its resonance frequencies
were obtained from the HSQC and TOCSY spectra. As-
signments were based on COSY correlations, signal inten-
sity and multiplicity. As ortho- and meta-protons are yet
again isochronous, it can be concluded that the rotation
around the Ru–N bond is fast on the NMR time scale. The
1H and 13C NMR spectra clearly indicate coordination
of only one pyridine,[6] in contrast to Grubbs’s[2e,10a] and
Nolan’s[10b] complexes incorporating two pyridines.
A further characterization of the dynamical properties of
complexes 3a–c and 4a–c was obtained by recording
ROESY spectra using an off-resonance spin-locking
scheme,[11] allowing to unambiguously distinguish nOe and
exchange cross-peaks (Figure 4). Such exchange cross-
peaks, due to rotations around the Ru–C21, N–C1 and N–
C12 bonds in the SIMes and SIMe ligand, could be ob-
served for some complexes and are summarized in Table 5.
It should be noticed that individual ring flips, i.e. rotations
around the N–C1/12 bonds, are only observed in complexes
bearing pyridine. This can be explained as a steric effect
when considering the size of the ligands. As PPh3 and PCy3
are bulky ligands, the indenylidene and chlorine ligand will
be in closer contact with the NHC ligands and considerably
hamper the phenyl ring flipping. Thus, a larger axial ligand
opposite Ru will inhibit ring flipping. A hindered rotational
motion around the Ru–C21 bond, in which the phenyl rings
swap places, is observed in all complexes except for 3a. The
exchange peaks in 3b being close to the detection limit al-
ready, we assume that the kinetics for this process in 3a
is too slow to yield detectable exchange peaks. While no
quantitative processing was attempted, two trends are note-
worthy.
Figure 4. Detail of the 200 ms off-resonance ROESY spectrum
demonstrating exchange cross-peaks (grey) indicative of hindered
rotations that are slow on the NMR time scale around all possible
SIMes bonds in compound 3c (top) while a hindered rotation
around the Ru–N(SIMe) bond only is observed in compound 4c
(bottom). Here, black cross-peaks indicate genuine nOe corre-
lations (Me7–Me18; Me8–Me19) between methyl groups on both
sides of SIMe.
Table 5. Qualitative overview of hindered rotations in compounds
3a–c and 4a–c.[a]
Bond
3a
3b
3c
4a
4b
4c
Ring flip
N–C1/12
–
–
+
–
–
+
First, under identical measuring conditions larger ex-
change cross-peaks are observed for the SIMe swapping
than for the SIMes swapping process. This is tentatively in-
terpreted as the result of an additional steric hindrance be-
tween the indenylidene phenyl ring and the para-meth-
Ring swap
–
+
+
+
+
+
Ru–C21
[a] Hindered rotations which are observed in off-resonance ROESY
spectra are indicated with a (+) sign, unobserved rotations are indi-
ylgroups in SIMes. Also, exchange cross-peaks that can cated using a (–) sign.
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659

F. Verpoort, J. C. Martins et al.
FULL PAPER
only arise through a consecutive ring flip and swapping op- importance in synthetic chemistry[8] and the fact that they
eration appear weaker than those resulting from a single hold as general benchmark reactions for RCM reactions
flip or swap exchange peak, indicating that they occur inde- (Scheme 1 and Figures 5, 6, and 7).
pendently from one another.
The indenylidene moiety is the only ligand that is consist-
1
ently present in all complexes; therefore analysis of it’s H
and 13C chemical shifts can reveal similarities and differ-
ences as a function of the various ligands. For most protons
and carbons, the chemical shift is not significantly affected
by the presence or absence of the para-methyl in the SIMes
and SIMe ligands, the chemical shifts of 3a, 3b and 3c being
almost identical to those seen in 4a, 4b and 4c respectively.
Keeping the NHC ligand fixed, some difference can be seen
depending on the type of ligand L. Most notably, the chem-
ical shift of H23 and the nearby carbene C22 of the inden-
ylidene in 3a and 3b, differ by more than 0.5 and 8 ppm
respectively, while these are quite similar in the other com-
plexes. On the other hand, H29 is shifted 0.6 ppm to higher
field in 3b and 4b, with respect to the other complexes. At
the level of the SIMe/SIMes ligand, changes in the 1H
chemical shifts are clearly apparent when L is a pyridine as
opposed to a PCy3 or PPh3. Since the 1H chemical shifts of
the PCy3 and PPh3 bearing complexes are quite similar, this
is believed to reflect changes in the relative orientation of
the various ligands in 3c and 4c concomitant with the con-
siderably lower steric bulk of the pyridine ligand.
Figure 5. RCM of diethyl diallylmalonate (6), catalysts 3a–c, 4a–c
and 2a. Catalyst/substrate ratio 1:200; solvent: CDCl3; tempera-
ture: 20 °C; conversion determined by 1H NMR; lines are intended
as visual aids only.
Metathesis Activity
A standard set of metathesis reactions[11] (see Schemes 1,
2, Table 7) and a more challenging one involving diphenyl
diallylsilane (10) were used to depict the performance of
complexes 3a–c and 4a–c.
Figure 6. RCM of diethyl diallylmalonate (6), catalysts 1c, 3a and
4a. Catalyst/substrate ratio 1:200; solvent: CDCl3; temperature:
20 °C; conversion determined by 1H NMR; lines are intended as
visual aids only.
Scheme 1. Representative Ring-Closing Metathesis reactions.
Figure 7. RCM of N,N-diallyltosylamide (8), catalysts 3a–c, 4a–c.
Catalyst/substrate ratio 1:200; solvent: CDCl3; temperature: 20 °C;
conversion determined by 1H NMR; lines are intended as visual
aids only.
The results for the RCM of 6 using catalysts 2a, 3a–c
and 4a–c are depicted in Figure 5. The first-generation cata-
lyst 2a affords higher conversions at shorter reaction times
compared to its second-generation counterparts 3a–b and
4a–b. Complexes 3c and 4c, containing a pyridine ligand,
Scheme 2. ROM polymerization of 1,5-cyclooctadiene (COD) (18).
The RCM reaction of diethyl diallylmalonate 6 and N,N- show high initial activity but activities drop dramatically
diallyltosylamide 8 were selected for a first assay given their after 0.5 h, pointing to their fast decomposition. The pyr-
660
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Indenylidene Ruthenium Catalysts for Olefin Metathesis
idine ligand proves to stabilize the propagating species in- Table 6. Performance of Ru-indenylidene catalysts 2a, 3a–c and 4a–
c in the RCM of 8.[a]
sufficiently during the RCM reaction. Surprisingly, the ac-
tivity of 3c reaches 40% compared to only 10% for catalyst
4c, suggesting that the propagating species of 4c is much
more vulnerable to decomposition. In case of a PPh3 ligand
trans to the NHC ligand, high conversions are obtained af- 3b
ter 3 h for 3b and 4b, respectively 96% and 85%.
Catalyst
TON at 20 °C
TON at 60 °C
2a
3a
813
170
325
110
113
191
110
787
970
641
114
793
546
110
3c
4a
In contrast to the complexes with a PPh3 ligand, com-
plexes with a stronger coordinating PCy3 ligand exhibit a
4b
4c
slower initiation. 3a shows a conversion of 50% after 5 h
for the RCM of 6. However, the conversion proceeds to
89% after 24 h, which indicates a long lifetime of the com-
plex. Full conversion could not be attained. 4a exhibits the
same behaviour but exceeds the activity of 3a; after 24 h 4a
[a] Conditions: Catalyst/substrate ratio 1:1000, solvent: CDCl3,
conversion determined by H NMR spectroscopy.
1
Where results for PCy3-containing catalysts 3a and 4a
reaches quantitative conversion (Ͼ98%). At elevated tem- are lower than their PPh3-containing analogues 3b and 4b
peratures, catalyst 4a exceeds the activity of both catalyst at 20 °C, they prove to be able to excel the activity of the
1c and 3a, the latter of which is the least reactive of the latter at 60 °C. Upon heating to 60 °C, the activity of the
catalysts examined at higher temperatures (Figure 6). At first-generation catalyst 2a drops slightly, due to a higher
lower temperatures, the need for ligand dissociation again degree of decomposition. The pyridine-containing catalysts
proves to aggrieve PCy3-ligand-containing catalysts, but 3c and 4c again fail to deliver high TON’s in RCM reac-
gives rise to improved catalyst lifetimes of the catalysts.
tions, consistent with the results previously reported.
The results for the RCM reaction of N,N-diallyltosyl-
Schmidt and co-workers have proposed diallylsilane de-
amide (8) using catalysts 2a, 3a–c and 4a–c are given in rivative 10 as a challenging substrate for RCM, since the
Figure 7. Reaction proceeds smoothly using first-generation large silicon atom disfavors the transition state geometry
type catalyst 2a, affording quantitative conversion within for cyclization reactions.[12] Table 7 (entries 1–7) show that
4 min, a result that goes beyond the scope of all other cata- PCy3-ligand-containing second-generation catalysts (3a, 4a,
lysts reported. Catalyst 4a, containing the SIMe ligand, and 1c) exhibit better performance towards the RCM of 10,
shows a much better initial performance compared to 3a, but high catalyst loadings (5 mol-%) remain vital. A litera-
containing the SIMes ligand. The reactivity of the latter in ture report for 1c (5 mol-%) describes only 70% formation
RCM of 8 is very low with only 10% of conversion ob- of 11 after 16 h in CCl4 at 65 °C,[12] compared to 76–95%
served after 24 h at room temperature. Such strong effect of for indenylidene catalysts 4a and 3a respectively. Catalyst
the SIMe ligand on performance has not been observed for 3a excels all other catalysts, with nearly quantitative forma-
catalysts 3b–c and 4b–c. The relatively small difference in tion of 11 within 16 h in refluxing CDCl3. PPh3- and pyr-
substitution pattern between the discussed NHC ligands idine-containing catalysts 3b, 4b, 3c and 4c are less interest-
has a definite effect on the ligand dissociation of the PCy3- ing catalysts regarding to this substrate. In addition, it
ligand-containing catalysts.
should be noted that SIMes-containing catalysts are pre-
Catalysts 3b and 4b are highly efficient toward the RCM ferred to SIMe-containing catalysts for RCM of 10. The
of 8; the former catalyzing the reaction much more ef- reduced steric bulk of the phosphane ligand in 4a compared
ficiently than other second-generation-type catalysts, af- to the SIMes ligand in 1c and 3a again plays a distinctive
fording quantitative yields within 1 h. The activity of 4b role in the higher reactivity of 3a towards this substrate,
stagnates at 60% after 2 h. We presume that the lack of and favors metathesis over deactivation.
stability of the propagating species of the SIMe-ligand-con-
taining catalyst impedes full conversion of the substrate.
A dissociative mechanism in which catalyst initiation de-
pends upon phosphane dissociation is the most preferred
As in the case of RCM of 6, the pyridine-containing cata- for the olefin metathesis reactions catalyzed by
lysts 3c and 4c exhibit a high initial activity ensued by an Grubbs’[13,14] complexes 1a–c and this also holds for inden-
abrupt drop in activity. Again, the SIMes-containing cata- ylidene complexes. Therefore, it is undeniable that com-
lysts show a higher conversion, supporting the conclusion plexes containing SIMe ligand poses higher initiation effi-
that catalysts containing the SIMe ligand decompose more ciency than their mesityl analogues. As phosphane dissoci-
easily.
ation promotes catalyst decomposition, it is not surprising
A better understanding of the different activities of the then that complexes bearing SIMe ligand decompose faster.
catalysts discussed can be achieved by determining the turn- The pyridine complexes 3c and 4c do not show a good cata-
over numbers (TON’s) at low catalyst loadings. Therefore, lytic activity when they are compared with the other cata-
we tested the six catalysts in the RCM of N,N-diallyltosyl- lysts. The pyridine ligand has weak electron donating prop-
amide 8 at 0.1 mol-% (Table 6). Under these conditions, the erties and is not capable of stabilizing the active species
catalyst lifetime becomes extremely important, such that the formed in the metathesis reaction. This results in decompo-
indenylidene catalyst 3a, which was almost completely im- sition of the catalyst.
potent at room temperature, emerges as the most productive
Cross Metathesis (CM) between two olefinic partners is
of the systems examined at elevated temperature.
an excellent way for the preparation of functionalized
Eur. J. Org. Chem. 2009, 655–665
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661

F. Verpoort, J. C. Martins et al.
FULL PAPER
Table 7. Representative Ring-Closing and Cross-Metathesis reactions.
alkenes and important building blocks for organic synthe-
Figure 8 displays the catalytic performance for ROMP of
sis.[15] Results for the CM reaction between 1,4-diacetoxy- 1,5-cyclooctadiene (COD) (18) (Scheme 2) with a catalyst
2-butene (12) and allylbenzene (13) are listed in Table 7, to monomer ratio of 1:3000.
entries 8–14. Nearly quantitative transformation to 14 can
only be achieved in the case of 3a. The results of reactions
conducted in CD2Cl2 at 40 °C, as compiled in Table 7, indi-
cate that all other complexes tested were almost indistin-
guishable in terms of their activity. Most notably, however,
SIMes-based precatalysts yield higher conversions for the
hetero coupled product with higher E/Z selectivities; 7.4–
13.5 for 1c, 3a–c and 4.5–5.8 for 4a–c. The difference be-
tween the two catalyst classes can be rationalized on the
basis of the greater stability of SIMes-containing catalysts
and their ability to promote secondary metathesis, leading
to the thermodynamically favored E isomer.[11]
Figure 8. ROMP of 1,5-cyclooctadiene (18), catalysts 3a–b and 4a–
Methyl acrylate (16) is a challenging substrate in olefin
b. Catalyst/monomer ratio 1:3000; solvent: CDCl3; temperature:
metathesis, and the Cross-Metathesis reaction with 5-hex-
enyl acetate (15) is a rather demanding reaction and there-
fore a better indicator for catalyst performance towards
electron-deficient olefins. Results are shown in Table 7, en-
20 °C; conversion determined by 1H NMR; lines are intended as
visual aids.
Third-generation indenylidene-Ru complexes 3c and 4c
tries 15–21. The difference between results obtained for yield full monomer conversion within two minutes at a
SIMes- and SIMe-based precatalysts is not as discernible monomer-to-catalyst ratio of 1:3000, a performance far be-
compared to the difference in results for the CM reaction yond that of the 2nd-generation indenylidene-Ru catalysts
of 1,4-diacetoxy-2-butene (12) and allylbenzene (13). Cata- like 3a and 4a. Even though our 2nd-generation complexes
lysts 1c and 3a, however, exhibit conversions excelling those with PPh3 ligands 3b and 4b initiate ROMP slower than 3c
obtained with other catalysts. In general, SIMes-containing and 4c, they still manage 100% conversion within 20 min.
catalysts give rise to higher conversions compared to their Quite rewardingly, at much lower catalyst loadings
SIMe-containing analogues. The same trends in activity, as (10,000 equiv. of COD), 3c and 4c afford total monomer
in the CM reactions described above, were observed. The conversion within 15 min.
third-generation indenylidene catalysts yield less than 10%
It has previously been described, that the polymerization
of product. The only CM product observed is the E isomer, of cyclooctadiene (COD) is initially not stereoselective.[16a]
which is in agreement with the data presented in litera- Since only one double bond of cis,cis-COD is opened, a
ture.[11]
75:25 cis/trans ratio represents the theoretically predicted
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Indenylidene Ruthenium Catalysts for Olefin Metathesis
non-selective polymerization. Although ofefin metathesis of metathesis reactions and described them in terms of their
catalysts show no preference for the trans-orientation in the performance, selectivity, and stability. It was evidenced that
initial stage of the COD polymerization, a secondary me- a small modification of the substituents on the NHC ligand
tathesis event transforms the polymer into a polymer with influences the catalyst initiation rate. Nevertheless, as facile
higher trans content.[16b] Moreover, when sufficient trans- ligand (phosphane, pyridine) dissociation often preceeds
polymer has been produced by secondary metathesis, a ter- catalyst decomposition, complexes bearing a SIMe ligand
tiary metathesis event occurs, which transforms trans-1,4- decompose faster. Although there is no single best catalyst
polybutadiene into t,t,t-1,5,9-cyclododecatriene, 20 (CDT) available, for all metathesis transformation, the reactivity
(Scheme 3, Table 8).[16b]
trends in CM were found to be similar to those observed
in RCM. Furthermore, higher selectivities were found for
SIMes-containing catalysts in the CM of allylbenzene with
1,4-diacetoxy-2-butene 12. Nearly quantitative transforma-
tion to the reaction product has been observed using 3a in
RCM of N,N-diallyltosylamide 8 at low catalysts loadings
(0.1 mol-%) and the sterically demanding diphenyl diallylsi-
lane 10 and CM of allylbenzene with 1,4-diacetoxy-2-but-
ene 12. However, its low initiation efficiency at room tem-
perature, 3a frequently excels other catalysts in RCM and
CM reactions. The efficiency of 3c and 4c for ring-opening
metathesis polymerization of 1,5-cyclooctadiene is remark-
able, affording complete conversion at very low catalyst
loadings within 2 min. As a high initiation rate is a requisite
for this type of reaction, the second-generation indenylidene
3a, 4a catalysts are dramatically less active in this transfor-
mation. For catalysts 3a and 4a an induction period was
observed after which the reaction follows pseudo-first-order
kinetics. Additionally, evidence was found for secondary
and tertiary metathesis reactions in case catalysts 3b,c and
4b,c were used for ROMP of COD, different from classical
second-generation catalysts 3a and 4a.
Scheme 3. Formation of CDT (20) during the ROMP of COD (18).
Table 8. Formation of CDT (20) during the ROMP of COD (18).[a]
Catalyst
T [°C]
Time [h] cis [%][b]
CDT [%]
TON
1c
3a
3b
3c
4a
4b
4c
25
25
25
25
25
25
25
24
24
0.3
0.25
24
0.3
0.25
54
75
17
8
75
20
9
0
0
4.7
10
0
1
10
2900
3000
3000
10000
3000
3000
10000
Complete resonance assignment of all complexes studied
is shown to be relatively straightforward, and should be eas-
ily extendable to other such complexes, especially since the
extensive ring currents in one or more ligands confer favor-
[a] Conditions: catalyst concentration 0.453 m, solvent CDCl3, T
= 20 °C, conversion determined by 1H NMR spectroscopy. [b] Per-
centage of olefin with cis configuration in the polymer backbone;
ratio based on data from 1H and 13C NMR spectra (13C NMR
spectroscopy: δ = 32.9 ppm allylic carbon trans; δ = 27.6 ppm al-
lylic carbon cis).
1
able dispersion in the H NMR spectrum. The assignment
affords the characterization of various conformational pro-
cesses and the possible influences of the ligand there upon.
Given the small set of molecules characterized so far, and
the fact that several effects can simultaneously contribute
to the chemical shift, the investigation of correlations with
the catalytic performance are currently beyond reach. Keep-
ing in mind that the chemical shift information is relevant
to the initial state of the catalyst only, the confrontation
of chemical shift and conformational information collected
using NMR with catalytic performance may become of
interest when a sufficiently large number of well considered
complexes have been fully characterized.
Transformation of the 1,4-polybutadiene chain into t,t,t-
CDT is not observed in case of catalysts 3a, 4a and 1c
(Scheme 3). Contrary to the indenylidene-type catalysts 3a
and 4a, catalyst 1c exhibits moderate secondary metathesis
activity as reflected by the higher trans-content.
Catalysts 3b–c and 4b–c yield high conversions in very
short reaction times accompanied by high percentages of
trans-polymer, a result of their excellent initiation and prop-
agation rates. The high performance of these catalysts fur-
ther allowed tertiary metathesis to occur transforming the
trans-1,4-polybutadiene into t,t,t-CDT.
Experimental Section
Conclusions
General: Reactions were performed under inert argon atmosphere
using the Schlenk technique. Argon was dried by passage through
drierite. Solvents like tetrahydrofuran (THF), toluene, dichloro-
methane (DCM), hexane, [D6]benzene, [D]chloroform were dried
by standard methods and degassed by a standard three freeze-
pump-thaw cycles. Methanol was not dried before use. Pyridine
was nor dried nor degassed before use. Diethyl diallylmalonate was
purchased from Aldrich and used as received. Complexes 2a,b[7]
In this article, we presented the synthesis of and catalytic
data for a series of second- and third-generation indenylid-
ene ruthenium catalysts applied to a set of standard olefin
metathesis transformations. The aim of this study was to
reveal the relative efficacies of different catalysts containing
a SIMes or a SIMe ligand. We have compared six of the
indenylidene ruthenium olefin metathesis catalysts in a set and 3a–c[6] were synthesized as described before in the literature.
Eur. J. Org. Chem. 2009, 655–665
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663

F. Verpoort, J. C. Martins et al.
FULL PAPER
1D and 2D H, 13C and 31P NMR spectra were recorded on either
1
Acknowledgments
a Bruker Avance 300 MHz spectrometer (for 31P only using a 5 mm
BBO probe), Bruker DRX 500 MHz spectrometer (5 mm TXI
probe) and a Bruker Avance II 700 MHz NMR spectrometer
(5 mm TXI probe). The latter was mainly used to validate assign-
ments made at 500 MHz that were ambiguous due to insufficient
resolution. ROESY spectra were recorded using the off-resonance
scheme with an angle of 60°.[17] Chemical shift values (δ) are given
in parts per million (ppm) using the residual C6D5H as secondary
internal calibration reference. For 31P, H3PO4 was used as external
reference. All kinetic experiments were performed on a Varian
Unity 300 MHz spectrometer.
R. D. and F. V. thank Ralf Karch and Roland Winde of UM-
ICORE AG for a generous gift of Neolyst M1 and Neolyst M2.
The IWT Flanders is thanked for a PhD grant to P. M. S. H.
J. C. M. and F. V. thank the FWO for financial support and several
NMR equipment grants (G.0036.00N, G.0365.03). The 700 MHz
equipment of the Interuniversitary NMR Facility was financed by
Ghent University, the Free University of Brussels (VUB) and the
University of Antwerp via the “Zware Apparatuur” Incentive of
the Flemish Government. S. M. and F. V. gratefully acknowledge
the Research Fund of Ghent University (BOF) for the financial
support.
(SIMe)(PCy3)Cl2Ru(3-phenylindenylid-1-ene) (4a): A flame-dried
reaction flask is charged with 286.0 mg (0.3098 mmol) of com-
pound 2a and 159.3 mg (0.3568 mmol; 1.15 equiv.) of the penta-
fluorobenzene adduct 5b. The mixture is dissolved in 10 mL of tol-
uene, stirred and heated to 100 °C for 1.5 h. The reaction mixture
is cooled down to room temperature and filtered off. All volatiles
are removed by evaporation and the residue is suspended in 5 mL
of MeOH. After filtration, the residue is washed with another 5 mL
of MeOH and dried in vacuo to afford 160.5 mg (0.1743 mmol;
56%) of 4a as a red powder. The NMR spectroscopic data is avail-
able in Tables 1 and 2 in the main text.
[1] For selected reviews on olefin metathesis, see a) T. M. Trnka,
R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18–29; b) R. H.
Grubbs, Handbook of Metathesis, Wiley-VCH, Weinheim (Ger-
many), 2003; c) S. J. Connon, S. Blechert, Angew. Chem. 2003,
115, 1944–1968; d) D. Astruc, New J. Chem. 2005, 29, 42–56.
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Chem. Int. Ed. Engl. 1995, 34, 2039–2041; b) P. Schwab, R. H.
Grubbs, J. W. Ziller, J. Am. Chem. Soc. 1996, 118, 100–110; c)
M. Scholl, T. M. Trnka, J. P. Morgan, R. H. Grubbs, Tetrahe-
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tenmann, M. Spiegler, W. A. Herrmann, Angew. Chem. Int. Ed.
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Grubbs, Organometallics 2001, 20, 5314–5318; f) J. Huang,
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S. P. Nolan, Organometallics 1999, 18, 5375–5380; h) J. C. Con-
rad, G. P. A. Yap, D. E. Fogg, Organometallics 2003, 22, 1986–
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Wilhelm, M. Scholl, T.-L. Choi, S. Ding, M. W. Day, R. H.
Grubbs, J. Am. Chem. Soc. 2003, 125, 2546–2558.
[3] a) J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus, A. H. Hov-
eyda, J. Am. Chem. Soc. 1999, 121, 791–799; b) S. B. Garber,
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2000, 122, 8168–8179.
[4] a) R. Bujok, M. Bieniek, M. Masnyk, A. Michrowska, A. Sa-
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V. Sashuk, G. Dolgonos, K. Grela, J. Am. Chem. Soc. 2004,
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(SIMe)(PPh3)Cl2Ru(3-phenylindenylid-1-ene) (4b). Method A: Un-
der an inert atmosphere of Ar, 35.1 mg PPh3 (0.134 mmol;
1.10 equiv.) is added to 87.3 mg 4c (0.121 mmol) in dichlorometh-
ane (10 mL) and the mixture is stirred for 30 min at room tempera-
ture. After evaporation of all volatiles, the residue is suspended in
n-hexane and filtered off. Thoroughly washing with 3ϫ5 mL n-
hexane and drying in vacuo yielded 57.7 mg of 4b (0.064 mmol;
53%) as a deep red powder.
Method B: Under an inert atmosphere of Ar, a flame-dried reaction
flask is charged with 275.3 mg (0.3105 mmol) of complex 2b and
159.4 mg (0.3571 mmol; 1.15 equiv.) of the pentafluorobenzene ad-
duct 5b. The mixture is dissolved in 10 mL of toluene, stirred and
heated to 100 °C for 1 h. The reaction mixture is cooled down to
room temperature and filtered off. All volatiles are removed by
evaporation and the residue is suspended in 5 mL of MeOH. After
filtration, the residue is washed with another 5 mL of MeOH and
dried in vacuo to afford 211.7 mg (0.2299 mmol; 74%) of 4b. The
NMR spectroscopic data is available in Table 1 and in the main
text.
[5] a) S. Gessler, S. Randl, S. Blechert, Tetrahedron Lett. 2000, 41,
9973–9976; b) H. Wakamatsu, S. Blechert, Angew. Chem. Int.
Ed. 2002, 41, 2403–2405; c) H. Wakamatsu, S. Blechert, Angew.
Chem. Int. Ed. 2002, 41, 794–796.
(SIMe)(py)Cl2Ru(3-phenylindenylid-1-ene) 4c: 152.0 mg (0.165
mmol) of complex 4a is dissolved in pyridine (2.0 mL) and stirred
at room temperature for 2 h. A brown precipitate is formed upon
addition of n-hexane (10 mL) and subsequent cooling to –40 °C.
Filtration of the precipitate, washing with 3ϫ5 mL n-hexane and
drying in vacuo yielded 87.3 mg (0.121 mmol; 73%) of compound
4c as an orange powder. The NMR spectroscopic data is available
in Tables 1 and 3 in the main text.
[6]
S. Monsaert, R. Drozdzak, V. Dragutan, I. Dragutan, F. Ver-
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atty, E. J. Valente, H.-J. Schanz, J. Organomet. Chem. 2007,
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H. Clavier, S. P. Nolan, Chem. Eur. J. 2007, 13, 8029–8036.
I. C. Stewart, T. Ung, A. A. Pletnev, J. M. Berlin, R. H.
Grubbs, Y. Schrodi, Org. Lett. 2007, 9, 1589–1592.
a) T. M. Trnka, E. L. Dias, M. W. Day, R. H. Grubbs, ARKI-
VOC 2002, 13, 28–41; b) H. Clavier, J. L. Petersen, S. P. Nolan,
J. Organomet. Chem. 2006, 691, 5444–5447.
Monitoring ROMP of cis,cis-Cycloocta-1,5-diene (COD, 18): An
NMR tube is charged with the appropriate amount of catalyst,
dissolved in 0.60 mL of CDCl3. 0.10 mL of cis,cis-cycloocta-1,5-
diene is added, the NMR tube is closed and the conversion is deter-
mined by integration of the olefinic 1H signals of the formed poly-
mer and the consumed monomer.
[8]
[9]
[10]
Monitoring RCM of Diethyl Diallylmalonate (6) and N,N-Diallyl-
tosylamide (8): An NMR tube is charged with the appropriate
amount of catalyst, dissolved in 0.60 mL of CDCl3. Next 0.10 mL
of the substrate is added, the NMR tube is closed and the conver-
sion is determined by integration of the allylic 1H signals of the
formed product and the consumed substrate.
[11]
[12]
[13]
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Received: October 3, 2008
Published Online: December 19, 2008
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