Reactivation of a ruthenium-based olefin metathesis catalyst

Article abstract of DOI:10.1021/om301042a

First-generation Hoveyda-Grubbs olefin metathesis catalyst was purposely decomposed in the presence of ethylene, yielding inorganic species that are inactive in the ring-closing metathesis (RCM) of the benchmark substrate diethyl diallylmalonate (DEDAM).

Full text of DOI:10.1021/om301042a

Communication
pubs.acs.org/Organometallics
Reactivation of a Ruthenium-Based Olefin Metathesis Catalyst
†
†
Daniel S. Tabari, Daniel R. Tolentino, and Yann Schrodi*
Department of Chemistry and Biochemistry, California State University Northridge, Northridge, California 91330, United States
S
* Supporting Information
ABSTRACT: First-generation Hoveyda−Grubbs olefin meta-
thesis catalyst was purposely decomposed in the presence of
ethylene, yielding inorganic species that are inactive in the
ring-closing metathesis (RCM) of the benchmark substrate
diethyl diallylmalonate (DEDAM). The decomposed catalyst
was treated with 1-(3,5-diisopropoxyphenyl)-1-phenylprop-2-
yn-1-ol (3) to generate an olefin metathesis active ruthenium
indenylidene-ether complex in 43% yield. This complex was also prepared independently by reacting RuCl2(p-cymene)(PCy3)
with the organic precursor 3. The activity of the isolated reactivated catalyst in the RCM of DEDAM is similar to that of the
independently prepared complex.
turn the products of decomposition of the first-generation
Hoveyda−Grubbs catalyst into active catalytic species.
lefin metathesis is a powerful reaction that finds an
Oimpressive array of applications in organic and polymer
synthesis.1 Among the different complexes capable of
promoting olefin metathesis, the ruthenium-based catalysts
are the most attractive, because they combine high activity with
excellent tolerance to functional groups.2 The most popular
ruthenium-based olefin metathesis promoters are the seminal
systems discovered by Grubbs and the highly robust catalysts
introduced by Hoveyda.3 Despite tremendous advances in
catalyst design in recent years,4 there remain some limitations
to this technology, including catalyst decomposition, high
catalyst costs, and ruthenium contamination of the organic
products. Much understanding regarding catalyst decomposi-
tion5 has been gained and used to develop more stable
systems,6 while the issue of high catalyst costs has motivated
studies to substantially decrease catalyst loadings.7 Other recent
efforts have been directed at developing supported catalysts in
order to limit catalyst decomposition and product contami-
nation.8 Nevertheless, deactivated homogeneous or supported
catalysts cannot be reused, as there is no direct method to
reactivate them. Therefore, the only ruthenium recycling option
to date is to recover the precious metal from the inorganic
residue of metathesis processes. As the number and scale of
industrial processes utilizing Grubbs-type catalysts continue to
grow, a simple and direct method for catalyst reactivation
becomes increasingly attractive.
Grubbs benzylidene complexes are known to decompose
primarily by a bimolecular process, giving stilbenes as organic
byproducts.5a,11 Grubbs methylidene complexesthe resting
states in ring-closing metatheses (RCM) and cross-metatheses
(CM) of terminal olefinsdegrade predominantly by a
unimolecular pathway involving the attack of a dissociated
tricyclohexylphosphine on the methylidene fragment, ulti-
mately liberating methyltricyclohexylphosphonium chlori-
de.5a,d,e,11,12 Notwithstanding, a secondary bimolecular decom-
position pathway for methylidene complexes is conceivable.5a,13
The inorganic species produced from catalyst decomposition
have been characterized in the case of the second-generation
Grubbs methylidene5d but not for the first-generation
methylidene or alkylidene complexes. Conversely, very little is
known regarding the decomposition of Hoveyda−Grubbs
catalysts. While second-generation Hoveyda−Grubbs com-
plexes were shown to produce intractable ruthenium hydride
species upon degradation in the presence of ethylene,5e the
decomposition of their first-generation analogues is very poorly
understood. Considering that the absence of a dissociating
phosphine ligand on Hoveyda−Grubbs catalysts rules out the
aforementioned unimolecular decomposition pathway, we
hypothesized that the decomposition of first-generation
Hoveyda−Grubbs complexes in the presence of ethylene may
involve a bimolecular pathway and may generate oligomers
14
We have recently reported a convenient one-step method for
the in situ generation of an olefin metathesis catalyst involving
the reaction of RuCl2(p-cymene)(PCy3) and 1-(3,5-dimethox-
yphenyl)-1-phenylprop-2-yn-1-ol.9 A similar method based on
reacting an analogous organic precursor with RuCl2(PPh3)3 was
independently developed by Bruneau and co-workers.10 We
became interested in testing whether these types of 1,1-
diphenylprop-2-yn-1-ol derivatives could also be used to
reactivate decomposed olefin metathesis catalysts. Herein, we
describe that a related organic precursor can indeed be used to
such as [RuCl2(PCy3)]n and [RuCl2(PCy3)(CH2CH2)m]n.
Furthermore, we theorized that these decomposition species
may react with a 1-(3,5-dialkoxyphenyl)-1-phenylprop-2-yn-1-
ol to yield an active olefin metathesis catalyst in much the same
way that RuCl2(p-cymene)(PCy3) does.
First-generation Hoveyda−Grubbs catalyst (1) was pur-
posely decomposed by subjecting it to an atmosphere of
Received: November 1, 2012
© XXXX American Chemical Society
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dx.doi.org/10.1021/om301042a | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
ethylene (120 psi) in dichloromethane at 55 °C for 4 h
1
(Scheme 1). H NMR spectroscopy suggests that >97% of
Scheme 1. Decomposition of First-Generation Hoveyda−
Grubbs Catalyst 1 under Ethylene
complex 1 was consumed, as no alkylidene signal can be
detected around 17 ppm.15 In order to confirm the
decomposition of complex 1, the residual inorganic species
(2) was isolated and tested in the RCM of diethyl
1
diallylmalonate (DEDAM). According to H NMR spectros-
copy, a 0.1 M solution of DEDAM in CD2Cl2 treated with the
inorganic residue 2 (1 mol % Ru, assuming that the
decomposition species are [RuCl2(PCy3)]n) showed less than
3% conversion after 1 h at 40 °C (compared to 96% conversion
for the RCM of DEDAM catalyzed by 1 after 45 min at 30 °C
under similar conditions16). The fact that there is some RCM
activity is likely due to a very small amount of remaining
Hoveyda−Grubbs catalyst.
Decomposition species 2 were then treated with 1-(3,5-
diisopropoxyphenyl)-1-phenylprop-2-yn-1-ol (3) in THF-d8.
The dark dirt brown mixture of 2 and 3 was heated in a sealed
NMR tube at 70 °C for 18 h to give a bright orange-brown
solution (Scheme 2a). The 31P NMR spectrum of this solution
Figure 1. Representation of complex 4 with thermal ellipsoids drawn
at the 50% probability level. Hydrogen atoms have been omitted for
clarity.
and 2.4349(18) Å, respectively) are very close to those found in
other ruthenium indenylidene-ether complexes.
Scheme 2. Preparation of Ruthenium Indenylidene-Ether
Complex 4 from Decomposition Species 2 (a) and from
RuCl2(p-cymene)(PCy3) (b)
The solution of in situ reactivated catalystgenerated by
treating the inactive decomposition species 2 with organic
precursor 3 as shown in Scheme 2awas found to promote the
RCM of DEDAM (Figure 2). Indeed, the in situ reactivated
catalyst (∼1 mol % total Ru) converted 90% of DEDAM to its
RCM product in 2 h under standard conditions. Complex 4
prepared in situ from RuCl2(p-cymene)(PCy3) achieves ∼94%
conversion within 1 h under the same RCM conditions, while
the isolated complex 4 reaches ∼97% conversion (the same as
catalyst 1) at the 1 h time point (Figure 2). The in situ
reactivated catalyst is clearly slower than isolated complex 4. In
order to shed some light on this activity difference and
determine the yield of catalyst reactivation, the reactivation
procedure was repeated on a preparative scale and the
reactivated catalyst was isolated by silica gel chromatography.
features a singlet resonance at 67.5 ppm as the most prominent
peak, and mass spectrometry (HRMS-FAB) shows a peak at m/
z 758.2397, consistent with the presence of ruthenium
indenylidene-ether complex 4.
1
Using H NMR spectroscopy and 9-methylanthracene as an
internal standard, it was determined that 43% of the initial
Hoveyda−Grubbs catalyst was reactivated. The RCM activity of
this isolated reactivated catalyst is comparable to that of isolated
complex 4 (Figure 2). Interestingly, the RCM kinetics of the in
situ reactivated catalyst (∼1 mol % total Ru) are even slower
than those using 0.33 mol % of isolated complex 4 (Figure 2).
Thus, the explanation for the lower activity of the in situ
reactivated catalyst may be 2-fold. Only 43% of the ruthenium
species in the solution of in situ reactivated catalyst are complex
4, and the other products present in the solution may include
catalyst inhibitors.
Complex 4 was independently generated in situ by reacting
RuCl2(p-cymene)(PCy3) with the organic precursor 3 in
refluxing THF for 13 h (Scheme 2b).17 The 31P NMR
spectrum of the reaction mixture features a singlet at 67.5 ppm
for the major product. Complex 4 was isolated by crystallization
from a dichloromethane/hexanes mixture at −15 °C, and single
crystals were obtained by layering a chloroform solution of 4
with heptane. X-ray diffraction analysis shows that complex 4
adopts a distorted-square-pyramidal geometry similar to that of
a related Hoveyda−Grubbs compound3d and essentially
identical with those of different ruthenium indenylidene-ether
complexes (Figure 1).10a,b The ether ligand is positioned trans
to the tricyclohexylphosphine with a P−Ru−O angle of
178.33(5)°. The Ru−Cα and Ru−O bond lengths (1.852(2)
In summary, first-generation Hoveyda−Grubbs catalyst (1)
was degraded by reaction with ethylene. The products of
decomposition, which proved inactive in the ring-closing
metathesis of diethyl diallylmalonate, were transformed into
B
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Communication
Figure 2. RCM of diethyl diallylmalonate.
an olefin-metathesis-active species by treatment with 1-(3,5-
diisopropoxyphenyl)-1-phenylprop-2-yn-1-ol (3) in 43% yield.
These results constitute the first reactivation of a decomposed
ruthenium-based olefin metathesis catalyst and may serve as the
foundation for the development of practical reactivation
methods for industrial olefin metathesis processes. Preliminary
attempts to reactivate decomposed first- and second-generation
Grubbs as well as second-generation Hoveyda−Grubbs
catalysts indicate that this reactivation method may be specific
to first-generation Hoveyda−Grubbs catalysts. Ongoing work
includes further probing of this question and developments
aimed at improving the efficiency of the reactivation method.
ACKNOWLEDGMENTS
■
Acknowledgment is made to the Donors of the American
Chemical Society Petroleum Research Fund (Grant Number
50352-UNI3) and to the National Institute of General Medical
Sciences (Award Number SC3GM098217) for support of this
research. We thank Dr. Kenneth Hardcastle (Emory Uni-
versity) for X-ray structure determination, Dr. Mona Shahgholi
(Caltech) for mass spectrometry data, and Materia, Inc. for
their generous gift of olefin metathesis catalysts and ruthenium
starting materials.
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S
* Supporting Information
Text, figures, and a CIF file giving experimental details for the
decomposition of 1, the reactivation procedure, the preparation
of 4, RCM reactions, and the X-ray structure of 4. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: yann.schrodi@csun.edu.
Author Contributions
†The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript. These authors contributed equally.
Notes
The authors declare no competing financial interest.
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dx.doi.org/10.1021/om301042a | Organometallics XXXX, XXX, XXX−XXX