Ruthenium-indenylidene olefin metathesis catalyst with enhanced thermal stability

Article abstract of DOI:10.1002/chem.201001659

Two new ruthenium complexes bearing a bidentate (κ²O,C)- isopropoxy-indenylidene ligand and a PPh₃ (9) or PCy₃ (10, Cy=cyclohexyl) ligand have been synthesized and fully characterized by ¹H and ¹³C NMR spectroscopy and X-ray crystallography. Complex 10 displays a very high thermal stability with a half life of six days at 110°C in [D₈]toluene. Complex 10 was evaluated in various ring-closing metathesis reactions and ring-opening metathesis polymerization of dicyclopentadiene, in which it showed a latent behavior with low activity at room temperature and high activity upon thermal activation. Hot catalysts! Two new ruthenium complexes bearing a bidentate (κ²O,C)-isopropoxy- indenylidene ligand and a PPh₃ or PCy₃ (1, Cy=cyclohexyl; see picture) ligand are thermally stable catalysts. Complex 1 displays a latent behavior in olefin-metathesis transformations and possesses a very high thermal stability with a half life of six days at 110°C in [D₈]toluene. This complex features very low activity at room temperature and higher activity upon thermal activation.

Full text of DOI:10.1002/chem.201001659

FULL PAPER
DOI: 10.1002/chem.201001659
Ruthenium–Indenylidene Olefin Metathesis Catalyst with Enhanced Thermal
Stability
Anzhelika Kabro,[a] Thierry Roisnel,[b] Cꢀdric Fischmeister,*[a] and Christian Bruneau*[a]
Abstract: Two new ruthenium com-
plexes bearing a bidentate (k2O,C)-iso-
thermal stability with a half life of six
days at 1108C in [D8]toluene. Complex
10 was evaluated in various ring-closing
metathesis reactions and ring-opening
metathesis polymerization of dicyclo-
pentadiene, in which it showed a latent
behavior with low activity at room tem-
perature and high activity upon ther-
mal activation.
propoxy–indenylidene ligand and
a
PPh3 (9) or PCy3 (10, Cy=cyclohexyl)
ligand have been synthesized and fully
characterized by 1H and 13C NMR
spectroscopy and X-ray crystallogra-
phy. Complex 10 displays a very high
Keywords: indenylidene
·
latent
catalysts · metathesis · olefins ·
ruthenium
Introduction
heterocyclic carbene (NHC) ligands[9] and the development
of a series of new complexes based on the Grubbs benzyli-
dene architecture (i.e., 1–3; Scheme 1) and the more stable
Hoveyda-type complexes (i.e., 4 and 5; Scheme 1).[10]
Beside these catalysts, several structurally different com-
plexes based on vinylidene,[11] allenylidene,[12] and indenyli-
dene[13] compounds have been reported. Ruthenium–indeny-
lidene complexes are particularly interesting because they
can be easily prepared without the requirement of a diazo
compound as the benzylidene source and also because of
their better stability under harsh conditions.[13e] Prominent
examples are 6,[14] 7,[15] and 8,[16] which are now commercial-
ly available (Scheme 2).
Olefin metathesis[1] is a powerful tool for the transformation
of C=C bonds that has found many applications in organic
synthesis[2] and polymer chemistry.[3] Since the first well-de-
fined ruthenium-based catalyst isolated by Grubbs and co-
workers in 1992,[4] the continuous development of new cata-
lysts with enhanced performances[5] now offers new perspec-
tives for the transformation of functionalized olefins that
arise from renewable resources[6] and in enantioselective
transformations.[7] Thus, over the last 15 years the portfolio
of ruthenium catalysts has considerably increased, driven by
several families of complexes. Since the neutral 16-electron
complex [Ru
(=CHPh)Cl
E
To date, most transformations of ruthenium–indenylidene
catalysts have focused on modification of the NHC ligand[17]
and substitution of tricyclohexylphosphine by other ligands,
such as pyridine[18] or Schiff bases.[19] So far, a complex that
combines
a
(k2O,C)-isopropoxy-indenylidene bidentate
ligand has never been reported. We anticipated that such a
complex should display very high stability that results in
some degree of latency, which is a feature of increasing in-
terest in particular for the preparation of polymers.[20]
Herein, we present the synthesis and characterization of a
new family of olefin metathesis catalysts that feature a
(k2O,C)-isopropoxy–indenylidene bidentate ligand. The
first-generation complex 10 displays a very high thermal sta-
bility that surpasses those of the related complexes 4b and
6. This complex has been evaluated in various test reactions,
such as ring-closing metathesis (RCM), cross-metathesis
(CM), and ring-opening metathesis polymerization (ROMP)
of dicyclopentadiene (DCPD), in which it showed a latent
[a] Dr. A. Kabro, Dr. C. Fischmeister, Dr. C. Bruneau
Sciences Chimiques de Rennes, Catalyse et Organomꢀtalliques
UMR 6226 CNRS Universitꢀ de Rennes 1
Campus de Beaulieu, 263 Avenue du Gꢀnꢀral Leclerc
35042 Rennes (France)
Fax : (+33)223236939
E-mail: cedric.fischmeister@univ-rennes1.fr
christian.bruneau@univ-rennes1.fr
[b] Dr. T. Roisnel
Centre de diffractomꢀtrie X
UMR 6226 CNRS Universitꢀ de Rennes 1
Campus de Beaulieu, 263 Avenue du Gꢀnꢀral Leclerc
5042 Rennes (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201001659.
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Thus, the synthesis of com-
plexes 9 and 10 was envisioned
via the intermediate complex
11, which results from an initial
reaction of the propargylic alco-
hol 12 with [RuCl2ACHTUNGTRENNUNG(PPh3)3]
[Eq. (2)]. The symmetrical alco-
hol 12 was chosen to prevent
Scheme 1. Ruthenium-based olefin metathesis catalysts.
any regioselectivity problems
during the allenylidene-to-in-
denylidene rearrangement. Note that 11 is represented as a
16-electron complex but a 18-electron structure with a
(k2O,C)-isopropoxy–indenylidene bidentate ligand could
also be postulated at this stage.
With this synthetic pathway in mind, the synthesis of
propargylic alcohol 12 was performed in five steps by modi-
fication of a reported protocol (Scheme 3).[22]
Scheme 2. Ruthenium–indenylidene complexes.
Next, the synthesis of the diphosphine–indenylidene com-
plex 11 was attempted by reaction of the propargylic alcohol
behavior with low activity at room temperature and higher
activity upon thermal activation.
12 with [RuCl2ACHTNGUTERNNU(G PPh3)3] in THF heated to reflux. The reac-
tion progress was monitored by 31P NMR spectroscopy,
which showed two signals at d=65 and 55 ppm. Surprisingly,
none of these signals corresponded to the chemical shift ex-
pected for a complex such as 11,[23] but were close to the
chemical shift of a Hoveyda-type architecture that compris-
es a bidentate ligand (i.e., d=61.1 ppm for 4a).[10b] The pro-
cedure was modified by using CuCl as a phosphine scaveng-
er, thus resulting in a cleaner reaction mixture that consisted
of a major and minor compound (d=65 and 55 ppm, respec-
tively). Purification by column chromatography on silica gel
furnished the complex, which displayed a 31P NMR chemical
shift of d=65 ppm and was isolated in 67% yield as a
brown-red powder (Scheme 4). Crystallization of this com-
pound by slow diffusion of hexane in THF provided crystals
suitable for X-ray characteriza-
Results and Discussion
Synthesis of the catalyst: The most straightforward synthesis
of the targeted complexes certainly involves the reaction of
the Grubbs catalysts with an indenyl fragment [Eq. (1)].
However, because the synthesis of such an indenyl ligand
did not look straightforward we turned our attention to the
classical synthesis of ruthenium–indenylidene complexes
that involves the thermal-[14] or acid-promoted[13b,21] rear-
rangement of a ruthenium–allenylidene transient complex
synthesized from [RuCl2ACHTNUTRGNE(NUG PPh3)3] and a propargylic alcohol.
tion. The molecular structure
unambiguously revealed the ex-
pected (k2O,C)-isopropoxy-in-
denylidene bidentate complex 9
(Figure 1).
The structural data of 9 were
compared to those of the Hov-
eyda complex 4a.[10b] The main
difference between the two
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Thermally Stable Ru Catalyst for Olefin Metathesis
FULL PAPER
lar to 4b with an naphthalenyli-
dene instead of a benzylidene
ligand.[10b,24] As observed with
the PPh3 complexes, the main
À
variation was the Ru O bond
length, which was again longer
in 10 than in the naphthalenyli-
dene Hoveyda-type complex
(2.43 versus 2.26 ꢂ, respective-
ly). The Ru=C1 bond length
(1.85 ꢂ) falls into the range of
bond lengths observed for ben-
zylidene or indenylidene com-
plexes.[10b,15] With this new com-
plex in hand, we turned our at-
tention to its stability in solu-
tion and catalytic activity in
several RCM, CM, and ROMP
reactions.
Scheme 3. Synthesis of propargylic alcohol 12: a) tBuLi, À788C, Et2O, 3,5-dimethoxybenzaldehyde; b) MnO2,
CH2Cl2, room temperature; c) BBr3, CH2Cl2, room temperature; d) iPrBr, K2CO3, acetone, reflux; e) C2H2,
nBuLi, À788C, THF.
Thermal stability of complex
10: The thermal stability of a
precatalyst is a key property of
a latent catalyst. We studied the
relative stability of the Hovey-
da complex 4b, indenylidene
complex 6, and new complex 10
in solution (23 mm in CD2Cl2 or
[D8]toluene) at room tempera-
ture, 80, and 1108C, thereby
monitoring their rate of decom-
position by 1H NMR spectros-
copy with 1,3,5-trimethoxyben-
zene as an internal standard
25]
(12 mm).[20p,
The results
reveal significant thermal stabil-
ity of the new complex 10,
which only showed 50% de-
composition after nearly 6 days
at 80 and 1108C (Figure 3). In
contrast, 4b had decomposed
by 50% after 2 days at 808C
Scheme 4. Synthesis of complexes 9 and 10.
À
structures concerns the length of the Ru O bond, which is
Table 1. Selected bond lengths [ꢂ] and angles [8] for complexes 9 and
4a.
longer in 9 than in 4a, thus indicating a weaker bond in the
newly prepared complex 9. Other important structural data
matched perfectly with complex 4a (Table 1).
Structural feature
9
4a
1.84
2.24
2.31
2.31
2.32
À
Ru C1
1.85
2.23
2.42
2.32
2.31
The PPh3 ligand was efficiently substituted by the more
electron-donating phosphine PCy3 in dichloromethane at
room temperature to furnish complex 10 as a brown powder
in 70% yield (Scheme 4). This two-step procedure thus af-
forded 10 in about 50% yield. It must be noted that a one-
pot procedure without isolating complex 9 could also be per-
formed and furnished 10 in a better yield (70%).
À
Ru P
À
Ru O
À
Ru Cl1
À
Ru Cl2
Cl-Ru-Cl
P-Ru-O
147.11
178.38
145.17
172.29
Crystals suitable for X-ray analysis were obtained by slow
diffusion of hexane in dichloromethane (Figure 2). The
structural data of 10 were compared to a complex very simi-
and 1.5 days at 1108C. Of the three complexes, 6 showed the
lowest stability and had totally decomposed after 2 days at
808C.
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C. Fischmeister, C. Bruneau et al.
Of note, 10 showed only 20% decomposition at room
temperature in CD2Cl2 after 1 month, whereas 4b had de-
composed by 32% in the same amount of time and 6 had
fully decomposed after 18 days. These results all together
show that, as anticipated, 10 bearing a chelating indenyli-
dene–isopropoxy ligand is extremely stable in solution, even
after long periods at high temperature.
Catalytic performance of 10: Ideally, a latent catalyst should
be inactive at room temperature and switched on by exter-
nal stimuli, such as temperature, light, or chemical activa-
tion. So far most of the reported latent catalysts for olefin
metathesis showed very low activity at room temperature,
and the catalyst efficiency increased with temperature. First,
we focused on testing the catalytic activity of 10 in the
RCM reaction of diethyl diallylmalonate (DEDAM;
Figure 1. Molecular structure of complex 9 represented at 50% ellipsoid
probability. The H atoms and solvent are omitted for clarity.
Scheme 5)
and
diethyl
allyl(2-methylallyl)malonate
(Scheme 6).[26]
Scheme 5. RCM of diethyl diallylmalonate.
Scheme 6. RCM of diethyl allyl(2-methylallyl)malonate.
Catalyst 10 was evaluated at room temperature for the
RCM of DEDAM. Complex 10 showed a very low activity
at this temperature, which allowed less than 10% conver-
sion after 2 h and reached almost complete conversion after
10 h (Figure 4).
Figure 2. Molecular structure of complex 10 represented at 50% ellipsoid
probability. The H atoms and solvent are omitted for clarity.
The reaction was repeated at 308C with 4b, 6, and 10 and
1
the conversions were monitored by H NMR spectroscopy.
All three complexes allowed complete conversion within
1 h, but with very different reaction profiles (Figure 5).
Figure 3. Decomposition of complexes 4b, 6, and 10 as a function of time
~
and temperature. 4b: (~) 808C, ( ) 1108C; 6: (&) RT, (&) 808C; 10:
(*) 808C, (*) 1108C.
Figure 4. RCM conversion of DEDAM with complex 10 at room temper-
ature. The conversion was determined by H NMR spectroscopy.
1
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Thermally Stable Ru Catalyst for Olefin Metathesis
FULL PAPER
10 showed again the lowest initial activity, but almost com-
plete conversion was obtained within 6 h. This important im-
provement is likely because of the combined high thermal
stability and low initiation rate, which resulted in the capa-
bility of 10 to deliver active catalytic species throughout the
extended reaction time.
We extended the scope of 10 to various RCM and enyne
metathesis reactions. Complex 10 showed very good catalyt-
ic activity for the RCM of N,N-diallyltosylamide and 3,3-di-
AHCTUNGTREGaNNUN llyl-2,4-pentanedione by providing very high conversions in
both cases (Table 2, entries 1 and 2). However, 10 failed to
promote the RCM reaction of the sterically demanding di(2-
Figure 5. RCM conversion of DEDAM into 16 with complexes 4b (~), 6
(&), and 10 (*) at 308C.
Table 2. RCM and enyne metathesis catalyzed by 10.
Whereas 6 showed high initial
efficiency with 80% conversion
reached after 10 min, 4b and
especially 10 displayed a much
different behavior that allowed
only 40 and 1% conversion, re-
spectively, after 10 min. The re-
action profile obtained with 4b
and 10 can be attributed to the
higher stability provided by the
chelating ligands, which result-
ed in a slower initiation step.
This result contrasts with the
structural data obtained for 10,
Entry
Substrate
Product[a]
Solvent
CD2Cl2
T [8C]
Time
Conv. [%][b]
100
1
RT
15 min
2
CD2Cl2
408C
11 h
96
C6D6
[D8]toluene
808C
1108C
72 h
5 h
0
0
3[c]
4
CD2Cl2
408C
72 h
68
CD2Cl2
C6D6
C6D6
RT
608C
808C
16 days
24 h
64
64
85
À
which showed a weaker Ru O
5
8 h
bond than in a related Hoveyda
catalyst.[27] It can be postulated
that the extra stability brought
by the indenyl fragment in 10
1
[a] Catalyst loading: 1 mol%. [b] Determined by H NMR spectroscopy. [c] Catalyst loading: 5 mol%.
counterbalances this weaker metal-oxygen bond.
Next, the RCM reaction of the more sterically demanding
diethyl allyl(2-methylallyl)malonate was investigated
(Scheme 6). As observed with DEDAM, 6 showed the high-
est initial activity, but failed to reach high conversion
(Figure 6). In the same manner, 4b provided little improve-
ment and reached 72% conversion within 6 h. In contrast,
methylallyl)malonate, even at a temperature as high as
1108C. These results are in good agreement with those re-
ported for other first-generation complexes, and to the best
of our knowledge only a few second-generation catalysts
can promote the formation of tetrasubstituted olefins with
moderate-to-good efficiency.[28] Two enyne metathesis reac-
tions were also realized with moderate-to good efficiency
still in the range of reactivity of other first-generation cata-
lysts (Table 2, entries 4 and 5).[13d,29]
The activity of 10 was evaluated for the transformation of
methyl oleate by CM with ethylene (ethenolysis) and com-
pared to our previous results obtained with 4b (Table 3).[6b]
Under our standard conditions (i.e., toluene, room tempera-
ture, 3.5 h, 1 bar of ethylene, 2.5 mol% 10), similar results
were displayed relative to 4b with 90% conversion and 1-
decene and methyl 9-decenoate were provided in 89 and
90% yields, shown by GC analysis, without double-bond mi-
gration. Of note: the performance of 10 was slightly im-
proved by using dimethyl carbonate (DMC) as the sol-
vent.[6d,30]
Figure 6. RCM conversion of diethyl allyl(2-methylallyl)malonate into 17
with complexes 4b (~), 6 (&), and 10 (*). The conversion was deter-
mined by H NMR spectroscopy.
Finally, 10 was evaluated in the ROMP of dicyclopenta-
diene (DCPD) (Scheme 7). Bulk polymerization with a mo-
1
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C. Fischmeister, C. Bruneau et al.
Table 3. Ethenolysis of methyl oleate with catalysts 4b and 10.[a]
Acknowledgements
Catalyst
Solvent
Conv. [%][b]
Yield [%]
1-decene[b]
methyl 9-decenoate[b]
The authors are grateful to the French Agence Nationale de la Recher-
che (ANR) for financial support and a post-doctoral grant for A.K.
(ANR-08-EFC-03–01 BIOTANDEM) and to Prof. W. Leitner and Dr. J.
4b
10
toluene
toluene
DMC
93
90
93
93
89
91
93
90
92
Klankermayer (RWTH Aachen) for
a fruitful ANR–Deutsche For-
schungsgemeinschaft (DFG) collaboration.
[a] Room temperature, 2.5 mol% catalyst, 3.5 h, ethylene (1 bar).
[b] Determined by GC analysis with hexadecane as an internal standard.
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Scheme 7. Bulk polymerization of DCPD.
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As previously observed, 10 showed very low activity for
the ROMP of DCPD at room temperature. The reaction
mixture was only slightly viscous after five days at room
temperature, and the polymer was isolated in 7% yield after
precipitation in methanol. Polymerization was repeated at
80, 110, and 1208C, which lead to gels in 100, 20, and
10 min, respectively, and to polymer yields of 40%. Fast gel-
ification of the reaction media is certainly the cause of the
low yields observed due to mass transport limitation Further
optimization of the polymerization is necessary to improve
the polymer yields. However, 10 once again displayed the
main features of a latent catalyst for polymerization, that is,
a very low activity at room temperature that is restored
upon thermal activation.
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Conclusion
We have prepared the first member of a new family of ole-
finmetathesis catalysts bearing a (k2O,C)-isopropoxy–inden-
ylidene bidentate ligand. The latency of this complex was
demonstrated by very high thermal stability combined with
low activity at room temperature and high activity upon
thermal activation. Further development and fine-tuning of
the catalyst activity and stability can be envisaged by the in-
troduction of NHC ligands. The flexibility of the synthetic
route starting from propargylic alcohols should also allow
the synthesis a variety of complexes with various steric and
electronic demands.
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Full experimental details and the structural data for complexes 9 and 10
are given in the Supporting Information.[32]
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Thermally Stable Ru Catalyst for Olefin Metathesis
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[32] CCDC-791241 (9) and CCDC-791240 (10) contain the supplementa-
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free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Received: June 11, 2010
Published online: September 13, 2010
Chem. Eur. J. 2010, 16, 12255 – 12261
ꢁ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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