Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in Olefin Metathesis Reactions

Article abstract of DOI:10.1002/anie.201914667

The development of selective olefin metathesis catalysts is crucial to achieving new synthetic pathways. Herein, we show that cis-diiodo/sulfur-chelated ruthenium benzylidenes do not react with strained cycloalkenes and internal olefins, but can effectively catalyze metathesis reactions of terminal dienes. Surprisingly, internal olefins may partake in olefin metathesis reactions once the ruthenium methylidene intermediate has been generated. This unexpected behavior allows the facile formation of strained cis-cyclooctene by the RCM reaction of 1,9-undecadiene. Moreover, cis-1,4-polybutadiene may be transformed into small cyclic molecules, including its smallest precursor, 1,5-cyclooctadiene, by the use of this novel sequence. Norbornenes, including the reactive dicyclopentadiene (DCPD), remain unscathed even in the presence of terminal olefin substrates as they are too bulky to approach the diiodo ruthenium methylidene. The experimental results are accompanied by thorough DFT calculations.

Full text of DOI:10.1002/anie.201914667

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Accepted Article
Title: Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in
Olefin Metathesis Reactions
Authors: Noy B. Nechmad, Ravindra Phatake, Elisa Ivry, Albert Poater,
and N. Gabriel Lemcoff
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201914667
Angew. Chem. 10.1002/ange.201914667
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Unprecedented Selectivity of Ruthenium Iodide Benzylidenes in
Olefin Metathesis Reactions
Noy B. Nechmad,a Ravindra Phatake,a Elisa Ivry,a Albert Poater,b and N. Gabriel Lemcoff*a,c
Abstract: The development of selective olefin metathesis catalysts is
crucial to achieve new synthetic pathways. Here we show that cis-
diiodo sulfur chelated ruthenium benzylidenes do not react with
strained cycloalkenes and internal olefins, but can effectively catalyze
metathesis reactions of terminal dienes. Surprisingly, internal olefins
may partake in olefin metathesis reactions once the ruthenium
methylidene intermediate is generated. This unexpected behavior
allows the facile formation of strained cis-cyclooctene by the RCM
reaction of 1,9-undecadiene. Moreover, cis-1,4-polybutadiene may be
transformed to small cyclic molecules, including its smallest precursor,
1,5-cyclooctadiene, by the use of this novel sequence. Norbornenes,
including the reactive DCPD, remain unscathed even in the presence
of terminal olefin substrates; being too bulky to approach the diiodo
ruthenium methylidene. The impressively latent catalyst was activated
by addition of an external chloride source, unveiling a novel method
for controlled polymerization of DCPD. The experimental results are
accompanied by thorough DFT calculations that provide insights
towards further developments.
The remarkable versatility of ruthenium olefin metathesis
catalysts has led to valuable and efficient reactions on a great
Figure 1. Halide ligand exchange in chelated Ru benzylidenes.
variety of alkene substrates.1 Indeed, thoughtful modifications in
the ligand sphere have expanded the catalytic properties of
ruthenium alkylidenes. In this manner, asymmetric and
stereoselective reactions were put forth and even alternating
copolymers could be made.2 An important contribution has been
the development of latent precatalysts,3 mainly achieved by sulfur
chelation.4 Latency is important in the field of polymerization, and
it can be used for 3-D printing and to selectively guide orthogonal
photochemical pathways.5 Undoubtedly, tinkering with the ligand
sphere has been the prevalent approach towards tuning olefin
metathesis catalysts’ properties. As such, the anionic ligands
have had a great influence on the efficiency and selectivity of
ruthenium catalysts for olefin metathesis reactions.6 However,
Slugovc et al. have shown that full exchange of the chloride
ligands in HG-2nd-Cl can be challenging.7 On the other hand, the
Figure 2. ROMP and RCM substrates.
trans effect in cis-dichloro ruthenium precatalysts greatly
facilitates the substitution of the anionic ligands, disclosing a
unique pathway for halide exchange, e.g. Ru-S-I, (Figure 1).8 Ru-
S-I was shown to be an efficient latent catalyst for ring-closing
metathesis (RCM) reactions; whereas it was completely inert for
Table 1 summarizes the preliminary scope conducted following
the typical reaction protocols (for latent complexes), i.e. 0.1M
monomer with 1 mol% of Ru-S-I in toluene-d8 at 80°C for 1h.
the ring opening metathesis polymerization (ROMP) of 1,5-
Indeed, several strained ROMP monomers, including the usually
cyclooctadiene (COD).8c This unprecedented behavior led us to
highly reactive dicyclopentadiene (DCPD), did not react (Table 1,
investigate the perplexing paradox of a complex that can only
entries 2-6) even when heated. Previous literature reports with
catalyze olefin metathesis of seemingly less reactive substrates9
ruthenium diiodo olefin metathesis catalysts, revealed that ring-
opening at room temperature occurred only when the reaction
by screening the reactivity of Ru-S-I towards other ROMP
was carried out in a chlorinated solvent,10 hinting that a partial
halogen exchange was in effect.7 Moreover, ROMP initiated by
another diiodo derivative had to be carried out at high
temperatures, but RCM in this case could not be achieved.11
Notably, when norbornenes and DEDAM were mixed together, all
the norbornene derivatives (DCPD, NDC and NAn) remained
unaffected, even though RCM was efficiently achieved (Table 1,
entries 7-9). Thus, to the best of our knowledge, Ru-S-I is the first
catalyst that can efficiently promote RCM while being completely
inert to ROMP of very reactive norbornenes in situ, even under
trying conditions (high temperature and relatively high catalyst
monomers (Figure 2). Herein, we disclose the unprecedented
behavior displayed by Ru-S-I, which shows excellent reactivity for
terminal alkenes, promotes metathesis of cyclooctenes only after
producing the methylidene derivative and is completely inactive
towards norbornene substrates, unless external chloride is
added. By taking advantage of this selectivity, strained
cycloolefins that are usually polymerized by other ruthenium
catalysts could be efficiently synthesized and unusually reactive
substrates could be obtained from the controlled
depolymerization of polybutadiene.
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loading). The uniqueness of this sulfur-chelated system was
further emphasized by a control experiment designed to compare
Ru-S-I with HG-2nd-I. Indeed, also in our hands HG-2nd-I
polymerized COE as previously reported;10 perhaps due to the
partial presence of a monochloro-monoiodo impurity that could
not be removed, even after several attempts. So, although the
actual catalytic active species for Ru-S-I and pure HG-2nd-I are
probably identical (because the initial benzylidene is lost after the
first catalytic cycle of the RCM), the fact that we could not isolate
pure Ru-S-I does not allow for a direct comparison between these
catalysts. Another intriguing outcome of these exploratory studies
was obtained in entries 10 and 11. Thus, mixing equimolar
amounts of COE or COD with DEDAM and Ru-S-I resulted in the
complete consumption of both metathesis substrates. Given that
norbornene derivatives are highly strained and are usually more
reactive than cyclooctenes for ROMP (especially DCPD, one of
the most reactive ROMP monomers known), these results were
quite puzzling. NMR of entry 10 clearly revealed the successful
RCM of DEDAM and the appearance of new terminal vinylic
hydrogens (SI, Fig. S10a). The observed broadness of these
peaks hinted at the presence of oligomers. This assumption was
Scheme 1 Proposed methylene transfer mechanism.
readily confirmed by MALDI-TOF-MS analysis, where
a
distribution of oligomers with mass intervals of 110 daltons was
observed (SI, Fig. S10d). Indeed, the resulting mixture was
reminiscent of the product obtained from a typical ADMET
reaction of 1,9-decadiene,12 even though the monomer used in
this case was cis-cyclooctene.
The presence of terminal alkenes in the final product led us to
envision a mechanism where the terminal methylene of the
DEDAM was being transferred to COE by
a
cross-
metathesis/ring-opening type mechanism (Scheme 1). The RCM
reaction of DEDAM produces a methylidene complex, which is
the least hindered diiodo ruthenium species. We propose that this
basic intermediate can coordinate also non-terminal olefins, such
as COE, and insert them into the metathesis catalytic cycle.
Moreover, the Ru diiodo alkylidenes (not methylidenes) can only
react with olefin substrates that are unobstructed (terminal
alkenes or ethylene). Naturally, the 1,9-decadiene produced by
this manner is itself a terminal alkene which can continue to react
with the alkylidenes formed and produce the oligomers observed
in the reaction mixture. The reaction was then repeated with
different amounts of DEDAM. As expected, excess DEDAM led
mainly to the selective production of 1,9-decadiene (SI, Fig. S12).
Conversely, a 10:1 ratio of COE to DEDAM produced small
amounts of “methylene initiator” that led to much larger oligomers
(Figure 3). COD was also probed as the monomer and also
produced the expected oligomeric species (SI, Fig. S21). As
previously stated, the ADMET polymerization reaction, pioneered
by Wagener et al.,12a allows for the precise synthesis of well-
defined polymers; however, as it is a condensation process, part
of the monomer is lost during the reaction. However, in this
special case (where a catalytic amount of DEDAM is added), the
oligomerization proceeds through cross-metathesis reactions, as
an ADMET does; but no material is lost during the process.
Table 1. Selective RCM and ROMP reactions.
entry
ROMP/RCM
Substrate
ROMP
Conv.
RCM
Conv.
1
2
none/DEDAM
DCPD/none
NDC/none
-
>99%
0%
-
-
3
0%
4
5
NAn/none
0%
-
COE/none
0%
-
6
7
COD/none
0%
-
DCPD/DEDAM
NDC/DEDAM
NAn/DEDAM
COE/DEDAM
COD/DEDAM
0%
75%
90%
93%
>99%
>99%
8
9
10
11
0%
0%
>98%
>99%
[a] Reaction conditions: 0.1M substrates, 1 mol% Ru-S-I, 1h at 80°C in toluene-
d8. Conversions were monitored by 1H NMR. No conversions were observed
for entries 2-6 also after 24h.
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Figure 3. MALDI-TOF MS analysis for the reaction of 10eq COE and 1eq
DEDAM in the presence of Ru-S-I after 24h.
Figure 5. a) 1HNMR of 1,9-undecadiene. b) 1HNMR of reaction of 1,9-
undecadiene (4 mM) with Ru-S-I after 24h. The olefin signals for COE are
observed at 5.63 ppm and for the dimer byproduct at 5.40-5.50 ppm.
The ability of Ru-S-I to promote “ethenolysis” with DEDAM was
also tested with jojoba oil. The oil was mixed with Ru-S-I in
toluene for 1 hour at 80°C without any visible reaction. Then,
addition of excess DEDAM led to efficient cleavage of the jojoba
molecule; proving that also non-strained internal olefins may react
by this mechanism (SI, Fig. S22). Previous literature reports have
shown that specialized complex catalytic systems are needed to
produce strained cycloalkenes by olefin metathesis.13 Having
established that internal olefins cannot coordinate to Ru-S-I
alkylidenes, we imagined that inhibition of methylidene formation
could be achieved by having a terminal olefin only on one side of
an RCM substrate. Then, the formed cycloalkenes should not be
able to react further (see supporting information for proposed
mechanistic cycles, Fig. S30). To test this hypothesis, a mixture
of E,Z-1,9-undecadiene was synthesized as a precursor to cis-
cyclooctene. To our great satisfaction, COE was efficiently
produced (together with small amounts of the expected dimer
byproduct) even at relatively high concentrations (Figure 5 and
SI). Control experiments with HG-2nd-Cl showed that, under the
same reaction conditions, oligomers are the main products and
only traces of cis-cyclooctene can be observed (SI, Fig. S31).
Further experiments were conducted with two additional
substrates, 1,9-decadiene and 2-methyl-1,9-undecadiene. While
the former expectedly afforded oligomers; the latter was
completely unreactive, highlighting the susceptibility of the
catalyst to steric hindrance (SI, Fig. S32 and 33 respectively).
Depolymerization of the widely used industrial polymer, cis-1,4-
polybutadiene (PBD) by olefin metathesis is a well-studied
reaction that usually produces cyclic alkenes.14 As expected for a
non-terminal olefin, no reaction occurred when Ru-S-I was added
to a PBD solution in hot toluene. But, if a small amount of DEDAM
was also added, depolymerization readily took place.
Remarkably, GC-MS analysis of the mixture of cyclic olefins
revealed the presence of reactive COD, together with other
macrocycles (SI, Fig. S37). A control experiment with HG-2nd-Cl
under the same conditions also induced the depolymerization of
PBD; however, a much more complex mixture of oligomers was
obtained, and no COD was seen in the mixture (SI, Fig. S38).
Even though these first results only produced a minor amount of
COD, having a catalyst that has the ability to depolymerize a
polymer to its pristine monomer is a highly desirable goal to
achieve unspoiled recycling.15
We then turned our attention to the norbornenes. The lack of
reactivity observed for norbornene molecules is most likely due to
a more congested coordination site caused by the bicyclic
framework. Indeed, when mixing neat DCPD with 0.1% Ru-S-I,
the liquid mixture remained completely fluid for periods longer
than ten days; highlighting the impressive latency for this
important industrial monomer. Knowing that Ru-S-Cl is not latent
for DCPD, we decided to attempt to induce ROMP by an in situ
exchange of the anionic ligands. To our great satisfaction, a
mixture of DCPD and Ru-S-I could be readily polymerized
following addition of (Bu)4NCl (SI, Fig. S41). Given the need to
discover new strong materials for use in added manufacturing
applications,16 the fact that this reactive monomer can be mixed
with its precatalyst for very long periods without reacting and can
be quickly polymerized by the addition of chloride is of great
interest. Density functional theory calculations (DFT) were
conducted to answer several experimental insights that were not
obvious to rationalize. First, we wished to describe why the
methylidene can react with a non-terminal alkene and an
alkylidene cannot. Thus, the energy profiles for methylidene and
ethylidene with COE were calculated (Figure 6). The energy
profile is thermodynamically worse for ethylidene by more than 10
kcal/mol, and more interestingly, the kinetics for the ethylidene
system are also worse for the opening of the metallacycle (9.3
kcal/mol) than for its closure (3.9 kcal/mol), which supports the
extraordinary results observed in Table 1. Overall the upper
energy barriers are 14.6 and 22.1 kcal/mol from the methylidene
and ethylidene species, respectively. On the other hand, with
chlorides, the energy values are more favorable (the upper energy
barriers drop to 5.0 and 15.0 kcal/mol, respectively) and probably
justify why Cl is usually preferred over I in typical Ru catalysts.
Moreover, the latter energy barriers are not for the precatalysts,
but for the 14e species. Thus, there is an additional energy cost
(14.1 kcal/mol, see SI for further details) that needs to be
accounted for. Next, we also wanted to show that the diiodo
alkylidene can react with a terminal alkene. Overall, DFT showed
that metathesis of terminal alkenes catalyzed by Ru-S-I is valid
not only with the methylidene species, but also with the ethylidene
species, which keeps the catalytic cycle alive.
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Keywords: Olefin Metathesis • Latent Catalyst • ROMP • DFT • RCM
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and SI). In conclusion, the exchange of chlorides by iodides in
sulfur chelated benzylidenes brings about an impressive change
in olefin metathesis activity and selectivity. Both catalytic
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unprecedented
efficiency.
Related
to
this
behavior,
depolymerization of PBD afforded cyclic oligomers, with the
singular result that the mixture included COD. Finally, the extreme
reluctance of Ru-S-I to react with norbornenes, provides the
possibility to activate the latent catalyst by the addition of chloride
ions even after prolonged periods of time. We are continuing to
explore novel applications for this surprising new catalyst, such
as its activation by photoinduced chloride release and studying
the generality of the iodo effect on diverse latent Ru alkylidenes.
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Acknowledgements
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The ISF is gratefully acknowledged for financial support. A. P. is
a Serra Húnter Fellow, and thanks the Spanish MINECO for a
project PGC2018-097722-B-I00.
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Angewandte Chemie International Edition
COMMUNICATION
This article is protected by copyright. All rights reserved.

10.1002/anie.201914667
Angewandte Chemie International Edition
COMMUNICATION
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COMMUNICATION
I, the latent.
Noy B. Nechmad,a Ravindra
Phatake,a Elisa Ivry,a Albert
Poater,b and N. Gabriel Lemcoff*a,c
Page No. – Page No.
Unprecedented Selectivity of
Ruthenium Iodide Benzylidenes
in Olefin Metathesis Reactions
This article is protected by copyright. All rights reserved.