Thermally stable, latent olefin metathesis catalysts

Article abstract of DOI:10.1021/om200911e

Highly thermally stable N-aryl, N-alkyl N-heterocyclic carbene (NHC) ruthenium catalysts were designed and synthesized for latent olefin metathesis. These catalysts showed excellent latent behavior toward metathesis reactions, whereby the complexes were inactive at ambient temperature and initiated at elevated temperatures, a challenging property to achieve with second-generation catalysts. A sterically hindered N-tert-butyl substituent on the NHC ligand of the ruthenium complex was found to induce latent behavior toward cross-metathesis reactions, and exchange of the chloride ligands for iodide ligands was necessary to attain latent behavior during ring-opening metathesis polymerization (ROMP). Iodide-based catalysts showed no reactivity toward ROMP of norbornene-derived monomers at 25 °C and upon heating to 85 °C gave complete conversion of monomer to polymer in less than 2 h. All of the complexes were very stable to air, moisture, and elevated temperatures up to at least 90 °C and exhibited a long catalyst lifetime in solution at elevated temperatures.

Full text of DOI:10.1021/om200911e

Article
pubs.acs.org/Organometallics
Thermally Stable, Latent Olefin Metathesis Catalysts
Renee M. Thomas, Alexey Fedorov, Benjamin K. Keitz, and Robert H. Grubbs*
The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
S
* Supporting Information
ABSTRACT: Highly thermally stable N-aryl, N-alkyl N-heterocyclic
carbene (NHC) ruthenium catalysts were designed and synthesized for
latent olefin metathesis. These catalysts showed excellent latent behavior
toward metathesis reactions, whereby the complexes were inactive at
ambient temperature and initiated at elevated temperatures, a challenging
property to achieve with second-generation catalysts. A sterically hindered
N-tert-butyl substituent on the NHC ligand of the ruthenium complex was found to induce latent behavior toward cross-
metathesis reactions, and exchange of the chloride ligands for iodide ligands was necessary to attain latent behavior during ring-
opening metathesis polymerization (ROMP). Iodide-based catalysts showed no reactivity toward ROMP of norbornene-derived
monomers at 25 °C and upon heating to 85 °C gave complete conversion of monomer to polymer in less than 2 h. All of the
complexes were very stable to air, moisture, and elevated temperatures up to at least 90 °C and exhibited a long catalyst lifetime
in solution at elevated temperatures.
butyl substituent on the NHC were synthesized and screened
for latent behavior during cross-metathesis (CM) and ring-
opening metathesis polymerization (ROMP) reactions (Figure 1).21
INTRODUCTION
■
Olefin metathesis is widely used as a method of constructing
carbon−carbon double bonds.1 Toward this end, highly
efficient metathesis catalysts have been designed through
improvement of activity,2 stability,3 and selectivity of the
catalysts.4 Recently, efforts have been directed toward the
development of latent metathesis catalysts.5,6 Latent catalysts
are defined as complexes that show little or no activity at a
particular (usually ambient) temperature and initiate only upon
activation. This activation can be caused by a variety of different
stimuli, including heat,7,8 acid,9 light,10−13 and chemical
activation.14 Latent metathesis catalysts primarily have
applications in polymer chemistry.14 One such application is
the advantage of preparing monomer solutions in a mold with a
catalyst that is unreactive at ambient temperature, thus allowing
for good mixing and even distribution of monomeric solution
before initiating polymerization.7 Previous literature reports
describe latent ruthenium catalysts whereby the initiators and
organic ligand structure were altered to induce latency.15−17
The structure of initiators, such as variations of the Hoveyda-
type chelating ligand, has been particularly well explored and
documented.18−20 Another approach toward tuning the latency
of a catalyst involves manipulation of the N-heterocyclic
carbene (NHC) ligand. This method of inducing latent
behavior is attractive in that it enables a straightforward
catalyst design and synthesis while maintaining the functional
group tolerance and stability of second-generation ruthenium
catalysts. Reported herein is the investigation of four new
ruthenium-based latent catalysts for cross-metathesis and
ROMP that were prepared by adopting this strategy.
Figure 1. N-aryl, N-alkyl NHC ruthenium catalysts for latent olefin
metathesis reactions.
At ambient temperature, solutions of complexes 1 and 2 in
CDCl3 showed no decomposition by NMR spectroscopy over
14 days and were stable in the solid state in air for over four
months. Additionally, complexes 1 and 2 could be heated in
chloroform at 60 °C for 5 days without any signs of
decomposition, thus indicating good thermal stability. At 90 °C in
toluene, catalysts 1 and 2 began showing slight decomposition
after 30 h. This observed catalyst stability was promising for
applications in latent metathesis chemistry, where the catalyst
must remain active at high temperature for the duration of the
reaction.
Catalysts 1 and 2 were initially screened and compared for
latency for the homodimerization of 1-hexene to 5-decene
(Scheme 1). Both catalysts showed less than 5% conversion of
1-hexene after 24 h at 25 °C while maintaining catalyst
RESULTS AND DISCUSSION
Since N-aryl, N-alkyl NHC ruthenium catalysts display good
stability, complexes 1 and 2 bearing a sterically hindered N-tert-
■
Received: September 29, 2011
Published: November 18, 2011
© 2011 American Chemical Society
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Scheme 1. Latent Olefin Cross-Metathesis of 1-Hexene
Table 2. Temperature Optimization and Substrate Scope of
Catalyst 1
1
structural integrity, as confirmed by H NMR spectroscopy
(Table 1, entries 1 and 2). The reactions were subsequently
Table 1. Comparison of Catalysts for Latent Homo-
dimerization of 1-Hexene
a
b
entry
cat.
temp (°C)
time (h)
conv. (%)
1
2
3
4
1
2
1
2
25
25
85
85
24
24
24
24
<5
<5
90
41
a
The catalyst loading was 2 mol %. The concentration of 1-hexene in
benzene was 0.5 M, and the reactions were carried out sealed under a
b
1
nitrogen atmosphere. The conversion was determined by H NMR
spectroscopy. 1-Hexene was cleanly converted to 5-decene.
heated at 85 °C for 24 h, and the conversions were determined
1
by H NMR spectroscopy (Table 1, entries 3 and 4). Both
catalysts gave clean conversion of 1-hexene to 5-decene,
1
without detectable side products by H NMR spectroscopy.
Since catalyst 1 achieved 90% conversion of 1-hexene to 5-
decene compared to 41% afforded by catalyst 2, it was
considered optimal for further latent cross-metathesis studies.
Catalyst 2 was still active with no signs of decomposition after
24 h at 85 °C. Presumably the better activity of catalyst 1 in
relation to catalyst 2 is due to less steric hindrance of the
former on the N-aryl ring (mono-tert-butyl versus diisopropyl).
Since both catalysts are latent for cross-metathesis of 1-hexene,
the better activity of catalyst 1 is preferential for these reactions.
The crystal structures of complexes 1 and 2 show the expected
geometry (Figure 2).21
a
The loading of catalyst 1 was 2 mol %. The concentration of substrate
in benzene was 0.5 M, and the reactions were carried out sealed under
b
1
a nitrogen atmosphere. The conversion was determined by H NMR
spectroscopy. The reactions went cleanly to the target homodimeriza-
tion products.
be latent for these substrates even at 40 °C, showing no
appreciable reactivity until 50 °C. Moderate conversion to
product was obtained for all substrates at 60 °C; however,
85 °C was considered optimal for attaining good conversion
of the starting material. The reactions gave the desired
homocoupled product of these substrates as a mixture of cis
and trans isomers.
Following temperature optimization, substrate scope was
subsequently explored to determine the general applicability of
catalyst 1 (Table 2). For simple, longer chain olefins, catalyst 1
efficiently catalyzed conversion of the terminal olefin to dimer
product (Table 2, entries 5, 10, 13, and 16). Catalyst 1 showed
lower reactivity toward more sterically demanding olefins
(Table 2, entries 18 and 20), as would be expected given the
hindrance of the N-aryl, N-tert-butyl NHC ligand. Functional
groups were tolerated well when attached several carbon atoms
away from the double bond. However, substrates with func-
tional groups allylic to the olefin, particularly those containing
oxygen, underwent significant olefin isomerization upon
heating at 85 °C. While catalyst 1 was latent for allyl alcohol,
allyl acetate, allyl ethyl ether, and allylbenzene at ambient
temperature, subsequent heating of the reaction mixtures pro-
duced multiple isomerization products in addition to desired
product, affording inseparable mixtures of olefinic cross-
products. In contrast, catalyst 1 completely isomerized
allyloxytrimethylsilane (3) to cis- and trans-propenyl trimethyl-
silyl ether (4) after 18 h at 85 °C and showed no subsequent
cross-metathesis conversion of the internal olefin product (4)
(Scheme 2). Since 1,4-benzoquinone has been reported to
Figure 2. Crystal structures of complexes 1 (left) and 2 (right).
Thermal ellipsoids were set at the 50% probability level, and hydrogen
atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg) for 1: C1−Ru, 1.961; C2−Ru, 1.796; O−Ru 2.417; Cl1−Ru,
2.348; Cl2−Ru, 2.376; Cl−Ru−Cl, 7.43; C2−Ru−O, 77.95; Selected
bond lengths (Å) and angles (deg) for 2: C1−Ru, 1.982; C2−Ru,
1.837; O−Ru, 2.312; Cl1−Ru, 2.369; Cl2−Ru, 2.348; Cl−Ru−Cl,
8.75; C2−Ru−O, 78.50.
Further studies were conducted with catalyst 1 to determine
the optimal temperature for carrying out cross-metathesis
reactions. In addition to 1-hexene, 1-octene, 5-hexenyl acetate,
and 4-penten-1-ol were used as cross-metathesis substrates for
homodimerization to ensure that the observed results were
general to simple alkyl olefins (Table 2). Catalyst 1 proved to
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Scheme 2. Isomerization of Allyloxytrimethylsilane
Scheme 3. Synthesis of Complexes 9 and 10
prevent olefin isomerization, this additive was tested in the
homodimerization of allylbenzene and allyl ethyl ether to see if
it would eliminate the observed isomerization and improve the
selectivity and yield of the desired product.22 However, the
addition of 0.1 equiv of 1,4-benzoquinone resulted in catalyst
decomposition. Interestingly, 1 showed no reactivity toward
dienes, including 1,3-hexadiene, 1,3-pentadiene, and trans-1-
phenyl-1,3-butadiene, even at an elevated temperature of
100 °C, possibly due to the low activity of the ruthenium
vinylalkylidene intermediate.23 Longer chain olefins were
therefore considered ideal substrates, since their conversions
to desired homodimerization products were clean.
Toward the goal of developing a practical latent metathesis
catalyst for ROMP applications, catalyst 1 was tested for
polymerization of cyclooctadiene (COD) in benzene. In
contrast to cross-metathesis reactions, 2 mol % of 1 initiated
the ROMP of COD (0.7 M in benzene) at ambient
temperature, giving 80% conversion to polymer after 35 min.
Expectedly, higher conversion (90%) of COD to polymer was
achieved by 1 at 85 °C in 35 min. Catalyst 2 was also active for
the ROMP of COD (0.7 M in benzene) at ambient
temperature, affording 39% conversion to polymer in 19 h.
To gain more insight into the reactivity of these catalysts, we
screened complex 2 for ROMP of norbornene-derived
monomer 5, since 2 would presumably show better latency
than 1 due to its increased steric bulk. The increased sterics of 2
was expected to reduce its activity and necessitate higher
temperature for initiation. Unfortunately, catalyst 2 polymer-
ized 5 at 25 °C in tetrahydrofuran (THF), giving 56%
conversion to polymer in 5 h. These results showed that 1 and
2 were not effectively latent for ROMP as they were for cross-
metathesis. Therefore, the complex structure was modified to
develop a catalyst that would display latent behavior toward
ROMP of norbornene-derived monomers (Figure 3).
Figure 4. Crystal structure of catalyst 10. Thermal ellipsoids were set
at the 50% probability level, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg) for 10: C1−Ru,
1.987; C2−Ru, 1.840; O−Ru 2.332; I1−Ru, 2.702; I2−Ru, 2.683; I−
Ru−I, 6.66; C2−Ru−O, 78.09.
due to the iodide causing more steric hindrance for the association
of the olefin substrate.25 Ruthenium metathesis catalysts with
iodide ligands are known to be slower initiators.26 Accordingly, we
utilized this property to achieve latent ROMP catalysts.27
Since norbornene-derived polymers have numerous applica-
tions, research toward an efficient latent ROMP catalyst was
focused on these monomers. Therefore, norbornene-derived
compounds 5−8, representing a variety of functional groups and
different degrees of steric hindrance, were explored as monomers
for latent metathesis (Figure 3). Complexes 9 and 10 both proved
to be latent for the ROMP of 5, affording no conversion of
monomer 5 in THF after 4 h at ambient temperature. Upon
heating, >95% conversion to polymer was achieved with both
catalysts 9 and 10 in 2 h. Due to better overall initiation, as well as
superior latency after extended time periods, catalyst 10 was used
for further latent ROMP studies. Catalyst 10 showed excellent
latency at ambient temperature, remaining stable but inactive for
at least 24 h and subsequently initiating on heating to 85 °C in a
sealed reaction vessel to give 99% conversion to polymer 11
(Scheme 4). This observed superior latent behavior of catalysts
In effort to improve the latency of complexes 1 and 2, we
converted them to complexes 9 and 10, respectively, by in situ
reaction with an excess of sodium iodide according to the
literature procedure (Scheme 3).24 While H NMR chemical
1
shifts of new complexes 9 and 10 are very similar to those of
chlorine-based precursors, X-ray analysis unambiguously estab-
lished the structure of 10, showing the typical spacial arrangement
for a second-generation ruthenium catalyst (Figure 4).21
We anticipated that the effect of changing the chloride ligands
to iodide ligands would induce latency for ROMP
Scheme 4. Latent ROMP of 5 with Catalyst 10
bearing iodide ligands compared to chloride ligands is consistent
with previously reported reactivity trends for catalysts with
different halogen ligands.28
Figure 3. ROMP monomers. Fmoc is fluorenylmethyloxycarbonyl.
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Table 3. Latent ROMP of Norbornene-Derived Monomers
with Catalyst 10
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures and X-ray crystallographic data (CIF
files). This material is available free of charge via the Internet at
http://pubs.acs.org.
b
temp
(°C)
time
(h)
conv.
(%)
yield
c
a
d
d
entry
monomer
(%)
Mn
PDI
1
2
3
4
5
6
7
8
5
5
6
6
7
7
8
8
25
85
25
85
25
85
25
85
24
2
0
99
0
81
94
48
68
24 300 1.16
36 900 1.27
2 000 3.25
3 800 1.79
AUTHOR INFORMATION
37
2
■
95
0
Corresponding Author
*E-mail: rhg@caltech.edu.
24
2
99
0
37
2
ACKNOWLEDGMENTS
■
78
This research was supported by the National Science
Foundation through a Graduate Fellowship to R.M.T. and by
the Department of Defense through a Graduate Fellowship to
B.K.K. A.F. thanks the Swiss National Science Foundation for a
postdoctoral fellowship. We thank Dr. Rosemary Conrad for
the synthesis of monomer 6. We thank the NSF and NIH for
providing funding and Materia for gifts of catalyst precursors.
a
The loading of catalyst 10 was 2 mol %. The substrate concentrations
were 0.5 M in THF, and the reactions were carried out sealed under a
b
1
nitrogen atmosphere. The conversion was determined by H NMR
c
d
spectroscopy. Isolated polymer yield. The molecular weight and PDI
were determined by GPC.
The solvent also plays a role in the degree of latency of the
catalysts, as THF proved to result in significantly improved
latency at 25 °C for ROMP in comparison to benzene.
Specifically, complex 10 showed excellent latency toward the
ROMP of COD in THF at 25 °C, giving no polymerization
product after 18 h. However, repeating the same reaction with
complex 10 using benzene as the solvent yields 28% conversion
of COD to polymer after 30 min at 25 °C. THF may increase
the latency of the catalysts by functioning as a coordinating
solvent, thereby potentially slowing olefin association with
ruthenium.
The results of latent ROMP of monomers 5−8 with catalyst
10 are presented in Table 3. Catalyst 10 was latent for the
ROMP of all monomers screened, affording no reaction at
25 °C up to 37 h. Excellent conversion was achieved in 2 h at
85 °C for each of the monomers (Table 3, entries 2, 4, 6, and
8), and the corresponding polymers were isolated in good
yield. The polydispersity index (PDI) was moderately low for
the polymerization of 5 and 6 (Table 3, entries 2 and 4),
indicating good catalyst initiation and propagation. The PDI
for the polymerization of 7 was significantly broader and for 8
was moderately broader (Table 3, entries 6 and 8,
respectively), suggesting poorer catalyst initiation for these
monomers.
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CONCLUSIONS
■
We have developed N-aryl, N-alkyl NHC ruthenium catalysts
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