Mechanical activation of a latent olefin metathesis catalyst and persistence of its active species in ROMP

Article abstract of DOI:10.1021/om300161z

Preparation, substrate scope, and activity of a previously reported mechanically activated metathesis catalyst were investigated. Scission of the catalyst under ultrasound irradiation was followed by GPC, which showed a first-order scission rate constant

Full text of DOI:10.1021/om300161z

Article
pubs.acs.org/Organometallics
Mechanical Activation of a Latent Olefin Metathesis Catalyst and
Persistence of its Active Species in ROMP
Robert T. M. Jakobs and Rint P. Sijbesma*
Laboratory for Macromolecular and Organic Chemistry, and Institute for Complex Molecular Systems, Eindhoven University of
Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands
S
* Supporting Information
ABSTRACT: Preparation, substrate scope, and activity of a previously reported
mechanically activated metathesis catalyst were investigated. Scission of the
catalyst under ultrasound irradiation was followed by GPC, which showed a
first-order scission rate constant of 0.011 min−1. The resulting active species
was shown to have catalytic reactivity in ring closing metathesis (RCM) of
various (un)hindered substrates. Further investigations of the active species
showed that it was not influenced by radicals formed during ultrasound and
that the most effective method to increase its lifetime was an increase in
the substrate concentration. In ring-opening metathesis polymerization
(ROMP), the lifetime of the active species was shown to be several hours, in contrast to the short lifetimes found in RCM.
ROMP experiments also showed that all of the latent precatalyst broken leads to active catalyst species.
Chart 1. Structure of Polymeric Catalyst 1a
INTRODUCTION
■
Controlling the activity of a catalyst by an external trigger
creates opportunities in applications ranging from alternative
production methods of polymeric materials (e.g., reaction injec-
tion molding) to self-healing materials. Latent catalysts are the
subject of numerous studies because the inactive state, most
often, is also a stable state, making it possible to create a catalyst
that lives forever.1 Ideally, a latent catalyst can be stored together
with the substrate for a prolonged period, only initiating the re-
action after the appropriate stimulus.2 Traditionally, this stimulus
consists of an increase in temperature,3−5 addition of a chemical
agent,6,7 or irradiation with light.8 Activation is often based on
breakage of a metal−ligand bond; the resulting coordinatively
unsaturated species can bind to a substrate.
An alternative way to break metal−ligand bonds is the
application of mechanical force.9,10 Mechanochemical scission
of covalent bonds in polymers is well-studied; in these large
molecules, forces on the molecule accumulate in the center of
the chain to result in preferential scission midchain.11−13 However,
when one of the bonds in the chain is weaker, it selectively breaks
at this weaker bond; examples found in literature include azo14 and
peroxide15 bonds. Recently, we demonstrated that this principle
also holds for coordination bonds.16,17
Combining the work of latent catalysts with mechanical
scission of coordination bonds recently led us to develop latent
catalysts that can be activated by mechanical force, i.e.,
mechanocatalysts.17−20 The newly defined field of mechanoca-
talysis opens ways to signal high stresses in materials or to
reinforce materials that tend to fail (e.g., self-healing materials).21
One of the catalysts reported by us (complex 1a, Chart 1) is a
modification of the Grubbs-type olefin metathesis catalyst18 with
two carbene ligands.22
Grubbs catalysts catalyze various olefin metathesis reactions,
including ring-closing metathesis (RCM), ring-opening meta-
thesis polymerization (ROMP), cross metathesis (CM), and
ene-yne metathesis.23 These ruthenium alkylidene complexes are
known for their tolerance to most functional groups.24,25 Their
versatility and functional group tolerance makes them into an
increasingly important tool for organic and polymer chemists.
The generally accepted activation mechanism for Grubbs type
of catalysts is a dissociative one, where one of the ligands
decoordinates to induce catalyst activation.26,27
Ruthenium alkylidene complexes containing two N-hetero-
cyclic carbene (NHC) ligands are known latent catalysts.22,28,29
The lack of activity at room temperature is ascribed to the
strong NHC−Ru bond, which dissociates only at elevated
temperatures. Modification of the facile one-step synthesis of
bis NHC complexes reported by Verpoort et al.22 by attaching
polymer chains onto the NHC ligands (Chart 1) led to a latent
Received: February 27, 2012
Published: March 13, 2012
© 2012 American Chemical Society
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metathesis catalyst that was activated by application of
ultrasound.18
RESULTS AND DISCUSSION
■
Synthesis and Characterization. In order to build a
Ultrasound is one of the most efficient ways to apply me-
chanical forces in solution. Upon sonication, strong shear
gradients arise around collapsing cavitation bubbles. In these
gradients, polymers of sufficient length are stretched, and they
eventually break midchain due to accumulation of stress.30 For
ultrasonic activation of complex 1a, the rate of activation was
dependent on the molecular weight of the polymer. As ex-
pected, lower molecular weight polymer chains led to lower
activity, and an analogue containing only butyl chains was not
activated at all. Switching off the ultrasound led to an imme-
diate stop of the catalytic reaction; this was ascribed to fast
decomposition of the active species.
catalyst that can be activated by mechanical force, a latent
catalyst should be incorporated into a polymer chain which
transfers macroscopic forces to the individual molecular bonds.
More specifically, polymer chains should be attached to the
ligands that have to dissociate in order to activate the latent
catalyst. Complex 1a was synthesized (Scheme 1) following the
original report by Piermattei et al.18 and analyzed by gel
permeation chromatography (GPC) in dimethylformamide
(DMF) as the eluent. The GPC trace shows a shoulder at the
molecular weight of the uncoordinated pTHF chain (Figure 1,
red line). Attempts to minimize the amount of unfunctionalized
pTHF chains were not successful (Supporting Information).
However, thermal activity at 75 °C of complex 1a and model
complex 1b were exactly the same when the concentration of
complex 1a was corrected for the presence of unfunctionalized
pTHF chains (Supporting Information). This suggests that
these chains do not influence the catalytic activity. From the
absence of a signal of an imidazolium proton around 10 ppm in
1H NMR spectroscopy, we concluded that product 1a did not
contain original 2a, and analysis using hexafluoroacetone31−33
showed that the amount of OH end groups is <1% (Supporting
Information). Therefore, we speculate that the shoulder in
GPC is caused by pTHF chains with decomposed NHC
ligands.34 The fraction of these unfunctionalized pTHF chains
varies from batch to batch and was estimated using 1H NMR in
order to allow determination of catalyst concentration.
In the article by Piermattei et al.,18 the mechanochemical
activation of complex 1a was established; however, fundamental
questions still remained. Here, we describe a thorough
investigation of the earlier described mechanically activated
metathesis catalyst. We show that mechanochemical scission of
the complex follows first-order reaction kinetics and that the
activated catalyst performs ring-closing metathesis of various
(un)hindered substrates. The limited lifetime of the active
catalyst in RCM is extended at higher substrate concentration.
In ring-opening metathesis polymerization, the lifetime of the
active catalyst lifetime is increased to at least several hours. The
concentration of active catalyst is estimated using a ROMP
substrate, which indicates that all broken complex leads to an
active catalyst.
Scission by Sonication in Solution. Further investiga-
tions focused on scission and activation of complex 1a. Upon
sonication of a toluene solution of 1a, the weak NHC−Ru
bond is broken, leading to products 5a and 6a (Scheme 2).
Previously, this was demonstrated18 in an experiment in which
the adduct of 5a with tricyclohexylphosphine was obtained when
sonication was performed in the presence of PCy3. Since 1a has
twice the molecular weight of 5a and 6a and rebinding of the
ligand was shown not to occur, the extent of scission could be
followed by GPC.
Scheme 1. Synthesis of Complexes 1a and 1b
During sonication, the higher molecular weight peak de-
creased and the lower molecular weight peak increased (Figure 1).
By fitting two Gaussian peaks with similar peak width to the
data, scission of complex 1a could be followed quantitatively.
Figure 1. (a) GPC traces in DMF of solution containing 0.2 mM 1a (batch 2), 200 mM diethyldiallyl malonate 7, and 136 mM hexadecane in
toluene before and during sonication. GPC trace of ligand 2a. (b) First-order kinetic plot of relative area of fitted Gaussian curve belonging to
complex 1a. Data points and error bars based on average and standard deviation of experiments in triplicate (2 experiments with 200 mM 7, 1
experiment with 200 mM 9).
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Increasing the steric demand of the substrate by using
diethylallylmethallyl malonate 8 leads to a significant decrease
in activity, with only 15% conversion after 75 min. RCM of 8 is
known to be more challenging, and the observed decrease in
reaction rate by a factor of 2.8 is well in agreement with the
reduction observed in literature by a factor of 2 to 3.36,37 The
even more challenging substrate methyl-N,N-bis(methylallyl)-
benzenesulfonamide 9 is only 6% converted after 75 min of
sonication.
Scheme 2. Scission of 1a under Ultrasound
Increasing the Lifetime of the Active Species. In our
earlier report,18 it was shown that upon discontinuation of
ultrasound, conversion via ring-closing metathesis immediately
stopped. The most likely cause for this effect was a limited
lifetime of the active species. After confirming this on/off cycle
(Supporting Information), we attempted to increase the
lifetime of the active catalyst, in order to increase the con-
version during ultrasound.
The reaction was shown to follow first-order reaction kinetics,
with a rate constant of (1.12 0.03) × 10−2 min−1 (Figure 1).
The rate constant was found to be independent of substrate
(7, 8, and 9, vide infra) and substrate concentration (20 and
200 mM, vide infra), indicating that the measurement indeed
probes the primary scission process independent of rebinding
or substrate (Supporting Information). When low molecular
One of the known effects of ultrasound is the formation of
radicals due to the high temperatures and pressures reached in
the cavitation bubble.38 Sonication experiments in the presence
of known radical scavengers (2,6-bis(1,1-dimethylethyl)-4-
methylphenol and 1,4-benzoquinone), however, showed that
that radical species do not limit the lifetime of the active species
under the current conditions (Supporting Information).
Decomposition is thought to occur mainly through a 14-
electron species generated upon activation.39 The addition of
pyridine and isopropoxystyrene, both coordinating species,
however, led to a decrease in activity in the RCM of DEDAM 7
(Supporting Information). Possibly, competition between
substrate and these species plays a role. When triphenylphos-
phine was added, a shape change in the time conversion plot
suggests initial coordination of PPh3 toward the catalyst,
followed by activation of the resulting species by dissociation of
PPh3. This, however, did not lead to persistent catalytic activity
in an on−off experiment with added PPh3. For ruthenium−
methylidenes, which are the propagating species in RCM, it is
known39 that adding coordinating phosphine ligands does not
influence their decomposition, in contrast to other ruthenium−
alkylidene complexes.
1
weight analogue 1b was used, H NMR showed no visible
decrease in the alkylidene signal (19.2 ppm) over 80 min of
sonication under similar conditions (Supporting Information),
confirming the mechanical origin of the scission process.
Compound 5 is believed to be the active species in the disso-
ciative mechanism of ruthenium-type olefin metathesis catalysts.26,35
Therefore, we tested catalytic activity against different sub-
strates for ring-closing metathesis. As can be seen in Figure 2,
In a final attempt to increase the lifetime of the active species,
we varied the concentration of substrate. Figure 3 shows that
decreasing the concentration of 7 from 200 mM to 20 mM led
Figure 2. (a) Structures of substrates 7−9. (b) Time−conversion of
■
ring-closing metathesis of diethyldiallyl malonate 7 ( ), diethylallyl-
●
methallyl malonate 8 ( ), or methyl-N,N-bis(methylallyl)-
▲
benzenesulfonamide 9 ( ) using 0.2 mM 1a (batch 2) and 200
mM substrate. Conversion determined by GC-FID, using hexadecane
as an internal standard; data points and error bars based on average
and standard deviation of experiments in duplicate.
1000 equivalents of diethyldiallyl malonate (DEDAM) 7 is
converted by more than 40% after 75 min of sonication in the
presence of polymeric complex 1a (batch 2), resulting in a turnover
number (TON) of 440. Sonication with low molecular weight ana-
logue 1b results in conversion below 1% (Supporting Information),
proving the necessity of polymer chains and the mechanical nature
of activation.
Figure 3. Time−conversion plots of RCM of DEDAM 7 upon sonica-
■
tion of a 0.2 mM solution of 1a (batch 2) and 20 mM ( ), 200 mM
●
▲
( ), or 1 M ( ) of 7 in toluene. Conversions determined by GC-FID,
using hexadecane as an internal standard; data points and error bars
based on average and standard deviation of experiments in duplicate.
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Figure 4. (a) Ring-opening metathesis polymerization of 15 (100 mM) using 1a (batch 3) (0.2 mM) under ultrasonic activation in toluene and
■
subsequent quenching with ethyl vinyl ether. (b) Time−conversion plot for ROMP of 15 under continuous sonication ( ) and 10 min sonication
1
▲
on, then sonication off ( ); gray area indicates sonication period for on−off experiment. Conversion determined by H NMR for continuous
experiment and GPC peak area for on−off experiment (for the on−off measurement, points at t = 0, 10, 20, and 33 min are in triplicate; t = 5, 12, 15,
26, and 45 min are in duplicate; t = 60, 90, 120, 180, and 240 min are single experiments; error bars indicate standard deviation). (c) Typical
resulting GPC traces (in THF, 254 nm UV detector) of an on−off experiment taking 100 μL aliquots at different time intervals. Molecular weights
were corrected to correspond to actual molecular weights for this polymer by multiplication by a factor of 1.32.
to a decrease in conversion after 75 min of 44% to 36%. The
calculated TON after the same time period decreased from
440 to 36. On the other hand, increasing DEDAM con-
centration to 1 M led to a higher conversion of 53% and a
TON of 2657 after 75 min. Apparently, the higher substrate
concentration stabilizes the active species and leads to increased
conversion, similar to what was observed for ring-closing
metathesis with Ru−alkylidene catalysts when bulk substrates
were used.40 Since olefin binding to the activated species is rate
determining,26,27 it is likely that the reaction rate can be
enhanced by increasing the substrate concentration further. In
an attempt to push the catalyst to its limit, sonication in pure
DEDAM was attempted. However, no conversion was ob-
served, most likely due to the absence of cavitation bubbles in
DEDAM, due to its high viscosity and low vapor pressure.41 It
should be noted that at all concentrations conversion using
catalyst 1b was found to be below 1% (GC-FID or GC-MS)
upon applying ultrasound, providing additional evidence for the
mechanical nature of the activation step (Supporting
Information).
Persistence of Active Species in ROMP. Conversion and
lifetime of the active species were further investigated in ring-
opening metathesis polymerization. In this reaction, the catalyst
acts as initiator, and one active species leads to a single growing
chain under the conditions that the polymerization can be
considered “living”.23 Therefore, product molecular weight,
combined with conversion, provides information on the
concentration and lifetime of the active species.
Polymerizations were performed with bis-biphenyl-function-
alized norbornene monomer 15, with a UV absorption band at
250 nm (ε = 3.4 × 104 L mol−1 cm−1), which allows distin-
guishing its ROMP product from the polymeric complex 1a
(ε = 1.7 × 104 L mol−1 cm−1 at 250 nm) by GPC using a UV
detector. Continuous sonication of monomer 15 led to a
conversion of 20% after 75 min (Figure 4). Sonication of
control catalyst 1b resulted in a conversion below 0.2%
(Supporting Information). However, the molecular weight of
the product could not be used to analyze the polymerization
rate, because it was established in a separate experiment that
sonication of a solution of high molecular weight poly-15 results
in a reduction of its molecular weight (Supporting Information).
In further experiments, ultrasound was therefore stopped
after 10 min, and conversion and Mn were then followed by
GPC or 1H NMR. Figure 4 shows that conversion continued to
increase for hours after sonication was stopped, albeit at an
approximately 4 times lower rate than under continuous
sonication. A significant part of the reduced rate can be ascribed
to the decrease in temperature from ∼35 to 14 °C when sonica-
tion is stopped. When the data were fitted to a model assuming
that the active catalyst decomposed following first-order
kinetics, a half-life of 4
1 h and a polymerization rate of
1.1 × 10−4 M min−1 were found (Supporting Information). The
long lifetime is in strong contrast with the behavior of the latent
catalyst with DEDAM.18 It is known from literature that
the ruthenium−methylidene intermediate formed in RCM
(Chart 2) is a slow propagating, fast decomposing species.27,39,42
The propagating species in ROMP is much more stable, with a
typical half-life of several hours.39
The concentration of growing chains, and hence the con-
centration of active catalyst, was calculated from the ratio of
increase in degree of polymerization (DP) and conversion
(Supporting Information). During the first 50 min after sonica-
tion is stopped, the calculated concentration is approximately
3 × 10−2 mM, or 15% of the initial catalyst concentration.
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Chart 2. Structures of Ruthenium−Methylidene (left) and
Ruthenium−Alkylidene (right) as the Propagating Species in
RCM and ROMP, Respectively
ACKNOWLEDGMENTS
■
The authors thank R. Groote for fruitful discussions. This work
was supported by grants from Netherlands Organization for
Scientific Research (NWO) and Dutch IOP Self Healing
Materials (project SHM08748)
REFERENCES
(1) Grubbs, R. H. Chem. Sci. 2007, Aug 20.
■
(2) Monsaert, S.; Vila, A. L.; Drozdzak, R.; Van der Voort, P.;
Verpoort, F. Chem. Soc. Rev. 2009, 38, 3360−3372.
(3) Ung, T.; Hejl, A.; Grubbs, R. H.; Schrodi, Y. Organometallics
2004, 23, 5399−5401.
The batch of 1a used for ROMP has a slightly higher molecular
weight than the batch that was used in scission rate deter-
mination. Therefore, 15% activation is nicely in line with the
11% scission after 10 min of sonication calculated from scission
rate data and indicates that many, if not all, chain scission
events give rise to active catalyst species.
(4) Slugovc, C.; Perner, B.; Stelzer, F.; Mereiter, K. Organometallics
2004, 23, 3622−3626.
(5) Szadkowska, A.; Makal, A.; Wozniak, K.; Kadyrov, R.; Grela, K.
Organometallics 2009, 28, 2693−2700.
(6) Monsaert, S.; Ledoux, N.; Drozdzak, R.; Verpoort, F. J. Polym.
Sci., Part A: Polym. Chem. 2010, 48, 302−310.
Finally, the concentration of active species was used to
calculate the second-order rate constant of its reaction with
monomer. The value obtained is 35 min−1 M−1, approximately
20 times lower than for third-generation Grubbs catalyst
((H2IMes)(py)2(Cl)2RuCHPh) at 14 °C (Supporting
Information). The lower activity is most likely due to a com-
bination of steric and electronic differences between the active
species.27 These differences include the presence of a polymer
chain and one instead of two aryl groups on the NHC ligand of
catalyst 1a.
(7) Keitz, B. K.; Grubbs, R. H. J. Am. Chem. Soc. 2009, 131, 2038−
2039.
(8) Ben-Asuly, A.; Aharoni, A.; Diesendruck, C. E.; Vidavsky, Y.;
Goldberg, I.; Straub, B. F.; Lemcoff, N. G. Organometallics 2009, 28,
4652−4655.
(9) Beyer, M. K.; Clausen-Schaumann, H. Chem. Rev. 2005, 105,
2921−2948.
(10) Kaupp, G. CrystEngComm 2009, 11, 388−403.
(11) Nguyen, T.; Kausch, H.-H. Adv. Polym. Sci. 1992, 100, 73−182.
(12) Hickenboth, C. R.; Moore, J. S.; White, S. R.; Sottos, N. R.;
Baudry, J.; Wilson, S. R. Nature 2007, 446, 423−427.
(13) Caruso, M. M.; Davis, D. A.; Shen, Q.; Odom, S. A.; Sottos, N. R.;
White, S. R.; Moore, J. S. Chem. Rev. 2009, 109, 5755−5798.
(14) Berkowski, K. L.; Potisek, S. L.; Hickenboth, C. R.; Moore, J. S.
Macromolecules 2005, 38, 8975−8978.
CONCLUSION
■
The work presented here establishes polymeric complex 1a as a
useful latent catalyst that is activated by mechanical force, as
opposed to a low molecular weight analogue 1b. GPC experi-
ments demonstrated that scission of 1a by ultrasound follows
first-order reaction kinetics, whereas for low molecular weight
analogue 1b, no scission was observed. The activated complex
is active in ring-closing metathesis for various substrates. In
these reactions, the lifetime of the active species is limited, but
it is not affected by radicals formed by ultrasound. The addition
of triphenylphosphine leads to delayed initiation, but increases
the lifetime of the active species. Lifetime is increased more
effectively by increasing substrate concentration. In contrast to
RCM, the active species in ring-opening metathesis polymer-
ization has a lifetime of several hours, opening opportunities for
catalysis in the solid state, where diffusion is low and, hence,
reaction times are longer. Data obtained with ROMP also make
it plausible that most scission events lead to an active catalyst.
However, its activity is estimated to be more than an order of
magnitude lower than for the active species formed from a
third-generation Grubbs catalyst.
(15) Encina, M. V.; Lissi, E.; Sarasua, M.; Gargallo, L.; Radic, D.
́
J. Polym. Sci.: Polym. Lett. Ed. 1980, 18, 757−760.
(16) Paulusse, J. M. J.; Sijbesma, R. P. Angew. Chem., Int. Ed. 2004,
43, 4460−4462.
(17) Karthikeyan, S.; Potisek, S. L.; Piermattei, A.; Sijbesma, R. P.
J. Am. Chem. Soc. 2008, 130, 14968−14969.
(18) Piermattei, A.; Karthikeyan, S.; Sijbesma, R. P. Nat. Chem. 2009,
1, 133−137.
(19) Tennyson, A. G.; Wiggins, K. M.; Bielawski, C. W. J. Am. Chem.
Soc. 2010, 132, 16631−16636.
(20) Wiggins, K. M.; Hudnall, T. W.; Tennyson, A. G.; Bielawski, C. W.
J. Mater. Chem. 2011, 21, 8355−8359.
(21) Wu, D. Y.; Meure, S.; Solomon, D. Prog. Polym. Sci. 2008, 33,
479−522.
(22) Ledoux, N.; Allaert, B.; Linden, A.; VanDerVoort, P.; Verpoort, F.
Organometallics 2007, 26, 1052−1056.
(23) Grubbs, R. H. Handbook of Metathesis; Wiley-VCH: Weinheim,
Germany, 2003.
(24) Trnka, T. M.; Grubbs, R. H. Acc. Chem. Res. 2001, 34, 18−29.
(25) Grubbs, R. H. Tetrahedron 2004, 60, 7117−7140.
(26) Sanford, M. S.; Ulman, M.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 749−750.
(27) Sanford, M. S.; Love, J. A.; Grubbs, R. H. J. Am. Chem. Soc.
2001, 123, 6543−6554.
(28) Trnka, T. M.; Morgan, J. P.; Sanford, M. S.; Wilhelm, T. E.;
Scholl, M.; Choi, T. L.; Ding, S.; Day, M. W.; Grubbs, R. H. J. Am.
Chem. Soc. 2003, 125, 2546−2558.
(29) Sashuk, V.; Peeck, L. H.; Plenio, H. Chem.−Eur. J. 2010, 16,
3983−3993.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization data, fitting
procedure, estimation of active catalyst concentration and
lifetime, additional catalytic tests, and control experiments. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(30) Basedow, A.; Ebert, K. Adv. Polym. Sci. 1977, 22, 83−148.
(31) Keizer, H. M.; van Kessel, R.; Sijbesma, R. P.; Meijer, E. W.
Polymer 2003, 44, 5505−5511.
AUTHOR INFORMATION
Corresponding Author
*E-mail: r.p.sijbesma@tue.nl.
■
(32) Ronda, J. C.; Serra, A.; Mantecon
Chem. Phys. 1994, 195, 3459−3468.
(33) L-Ho, F. F. Anal. Chem. 1973, 45, 603−605.
́ ́
, A.; Cadiz, V. Macromol.
Notes
The authors declare no competing financial interest.
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Article
(34) Denk, M. K.; Rodezno, J. M.; Gupta, S.; Lough, A. J.
J. Organomet. Chem. 2001, 617−618, 242−253.
(35) Dias, E. L.; Nguyen, S. T.; Grubbs, R. H. J. Am. Chem. Soc. 1997,
119, 3887−3897.
(36) Bieniek, M.; Michrowska, A.; Usanov, D.; Grela, K. Chem.−Eur.
J. 2008, 14, 806−818.
(37) Vougioukalakis, G. C.; Grubbs, R. H. Chem.−Eur. J. 2008, 14,
7545−7556.
(38) Makino, K.; Mossoba, M. M.; Riesz, P. J. Phys. Chem. 1983, 87,
1369−1377.
(39) Ulman, M.; Grubbs, R. H. J. Org. Chem. 1999, 64, 7202−7207.
(40) Gatti, M.; Vieille-Petit, L.; Luan, X.; Mariz, R.; Drinkel, E.;
Linden, A.; Dorta, R. J. Am. Chem. Soc. 2009, 131, 9498−9499.
(41) Mason, T. J.; Lorimer, J. P. Applied Sonochemistry; Wiley-VCH
Verlag: Weinheim, 2002.
(42) Hong, S. H.; Day, M. W.; Grubbs, R. H. J. Am. Chem. Soc. 2004,
126, 7414−7415.
2481
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