Synthesis and Activity of Six-Membered Cyclic Alkyl Amino Carbene-Ruthenium Olefin Metathesis Catalysts

Article abstract of DOI:10.1021/acs.organomet.9b00862

Ru-cyclic alkyl amino carbene (Ru-CAAC) olefin metathesis catalysts perform extraordinarily in metathesis macrocyclization and ethenolysis, but previous studies have been limited to the use of five-membered CAAC (CAAC-5) ligands. In this work, we synthesized a different group of ruthenium catalysts with more σ-donating and π-accepting six-membered CAAC (CAAC-6) ligands, and their metathesis activity was probed through initiation studies, ring-closing metathesis (RCM), cross-metathesis, and ethenolysis. These catalysts display higher initiation rates than analogous Ru-CAAC-5 complexes but demonstrate lower activity in RCM and ethenolysis.

Full text of DOI:10.1021/acs.organomet.9b00862

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Synthesis and Activity of Six-Membered Cyclic Alkyl Amino
Carbene−Ruthenium Olefin Metathesis Catalysts
Adrian E. Samkian,† Yan Xu,† Scott C. Virgil, Ki-Young Yoon, and Robert H. Grubbs*
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ABSTRACT: Ru−cyclic alkyl amino carbene (Ru−CAAC) olefin
metathesis catalysts perform extraordinarily in metathesis macro-
cyclization and ethenolysis, but previous studies have been limited to
the use of five-membered CAAC (CAAC-5) ligands. In this work, we
synthesized a different group of ruthenium catalysts with more σ-
donating and π-accepting six-membered CAAC (CAAC-6) ligands,
and their metathesis activity was probed through initiation studies,
ring-closing metathesis (RCM), cross-metathesis, and ethenolysis.
These catalysts display higher initiation rates than analogous Ru−
CAAC-5 complexes but demonstrate lower activity in RCM and ethenolysis.
he design and development of new catalysts is pivotal in
commercializing new processes and improving existing
activity, and (2) a stabilized, unsubstituted ruthenacyclobutane
(e.g., against β-hydride elimination) that retards catalyst
decomposition in the presence of ethylene.21
T
ones. Ruthenium-based olefin metathesis has enjoyed wide-
spread research and application in the past 30 years, and its
usefulness continues to expand as cheaper, more active, and
more robust catalysts are discovered.1,2 Ligand variation
arguably contributes the most to catalyst improvement, a
notable example being the switch from the PCy3-flanked first-
generation catalysts to the N-heterocyclic carbene (NHC)-
bearing second-generation catalysts (Figure 1).3−5 Further
While receiving ever-increasing attention, the current study
of Ru−CAAC catalysts is mostly limited to the use of five-
membered CAAC (CAAC-5) ligands.13,18,19,22 Recently,
Bertrand and co-workers reported a new type of ligand, the
CAAC-6 ligand, that demonstrated even stronger σ-donating
and π-accepting abilities as well as a larger steric shielding
effect.23 It was thus reasonable to hypothesize that these
ligands would further stabilize reactive ruthenium intermedi-
ates and lead to more robust olefin metathesis catalysts. The
synthetic difficulty of these unprecedented Ru−CAAC-6
complexes can be traced back to a challenging metalation
step. Compared with identically substituted CAAC-5 ligands,
CAAC-6 ligands are sterically less accessible to metal species
and more prone to decomposition as a result of their higher
nucleophilicity/electrophilicity, thus complicating their in-
stallation onto a metal.23 In this work, a group of Ru−
CAAC-6 complexes were targeted to compare their structure
and activity to those of their Ru−CAAC-5 analogues. Under
optimal metalation conditions, these complexes were prepared
in good to moderate yields. Their reactivities were further
examined in the context of initiation studies, RCM, cross-
metathesis, and ethenolysis.
Figure 1. Representative ruthenium catalysts with various ligands.
modification of these ligands improved their metathesis activity
or tailored them to specific applications.6−12 In particular, the
incorporation of more σ-donating and π-accepting cyclic alkyl
amino carbene (CAAC) ligands13,14 led to the discovery of a
panel of new Ru−CAAC complexes, which have been shown
to outperform their NHC analogues (e.g., Ru−H2IMes
complexes) by orders of magnitude in the context of
metathesis macrocyclization and ethenolysis.15−19 For in-
stance, a highly active Ru−CAAC catalyst was developed
that attained a turnover number (TON) of 340 000 in the
ethenolysis of a renewable oil.20 The exceptional performance
of the Ru−CAAC complexes can be mainly attributed to (1)
strong ligand donating ability, which ensures high metathesis
Several CAAC-6 salts were prepared in four steps using an
adapted version of a procedure from the literature,23 and 1a
was used to test ligand installation onto a first-generation
Received: December 23, 2019
https://dx.doi.org/10.1021/acs.organomet.9b00862
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ruthenium catalyst (HG1) (Scheme 1a). The original
conditions used for installing CAAC-5 ligands (KHMDS/
Scheme 1. Synthesis of Ru−CAAC-6 Catalysts 3a−d
Figure 2. Solid-state structures of 3c, 3d, and 4a with thermal
ellipsoids drawn at 50% probability. For clarity, hydrogen atoms have
been omitted. Selected bond lengths (in Å): for 3c: C1−Ru1
1.963(5), C23−Ru1 1.839(4), O1−Ru1 2.354(4), Cl1−Ru1
2.353(2), Cl2−Ru1 2.344(1); for 3d: C1−Ru1 1.960(2), C22−Ru1
1.836(2), O1−Ru1 2.370(1), Cl1−Ru1 2.3408(9), Cl2−Ru1
2.3634(8); for 4a: C1−Ru1 1.942(6), C19−Ru1 1.839(6), O1−
Ru1 2.294(5), Cl1−Ru1 2.330(2), Cl2−Ru1 2.335(2).
increased significantly (2.354 Å for 3c and 2.370 Å for 3d)
compared with an analogous Ru−CAAC-5 complex (4a, 2.294
Å),20 which is consistent with the greater σ-donating ability of
the CAAC-6 ligands. The lengths of the Ru−C bonds between
the CAAC ligand and the metal center remains similar (1.963
Å for 3c and 1.960 Å for 3d vs 1.942 Å for 4a), which can be
rationalized by an offsetting steric and electronic effect. The
CAAC−Ru−O angles for the synthesized Ru−CAAC-6
complexes (175.1° for 3c and 172.4° for 3d) were also
observed to be smaller than those in a five-membered analogue
(178.5° for 4a).
Catalyst initiation rates were then probed by reacting 3a−d
with 30 equivalents of butyl vinyl ether in benzene at 30 °C
(Table 1). Catalysts bearing an N-diisopropylphenyl (N-Dipp)
a
1
a
Determined by H NMR analysis.
Table 1. Initiation Study for Ru−CAAC-6 Catalysts 3a−d
THF) led to clean generation of free carbene 2a through
deprotonation.20 However, complete decomposition of 2a was
observed over several hours after HG1 was added, and only a
5% yield of the desired complex 3a was observed (Scheme 1a,
entry 1). As free CAAC-6 ligands (e.g., 2a) are more prone to
decompose than CAAC-5 ligands, conditions that do not
rapidly decompose the free carbenes are crucial for the
preparation of 3a. Accordingly, solvents, bases, and other
reaction parameters were evaluated. Using benzene in place of
THF successfully furnished 3a in 42% yield (entry 2);
however, the conversion could not be further improved due
to decomposition of 2a (entries 3 and 4). While the use of an
alkoxide base led to instant carbene decomposition (entry 5),
we were delighted to find that the deprotonation of 1a with
LiHMDS followed by the removal of LiBF4 by filtration greatly
increased the carbene’s stability (entry 6). Finally, when the
number of equivalents of 1a and the reaction temperature were
slightly increased, 3a was formed in 78% yield (entry 8).
Interestingly, the use of KHMDS instead of LiHMDS led to a
much lower yield under otherwise identical conditions, despite
the fact that both KBF4 and LiBF4 salts were removed.
Under the optimized conditions, four representative Ru−
CAAC-6 catalysts, 3a−d, were successfully prepared (Scheme
1b). We were unable to further reduce the sterics on the
nitrogen (e.g., with an N-mesityl group) or the α-carbon (e.g.,
with gem-dimethyls) of the carbenes because of difficulty in
synthesizing the CAAC salts through HCl-promoted cycliza-
tion.23 Single crystals were obtained for 3c and 3d, leading to
their unambiguous characterization through X-ray diffraction
(XRD) analysis (Figure 2). The Ru−O bond lengths were
a
Initiation rates were determined by measuring the decrease in the
b
benzylidene signal using 1H NMR spectroscopy. Data adopted from
ref 20.
substituent initiated slower than those with an N-methyl-
isopropylphenyl (N-Mipp) substituent by an order of
magnitude (e.g., 3a vs 3c), indicating the significance of the
N substituents in catalyst performance. In contrast, sub-
stituents on the α-carbon of the ligands appear to have less
influence on the catalyst initiation rate (e.g., 3a vs 3b). In
addition, some Ru complexes with CAAC-6 ligands initiate
faster than those with CAAC-5 ligands. For example, 4a−c,
which are among the most active Ru−CAAC-5 catalysts for
ethenolysis in the previous report, initiate at lower rates than
3c and 3d even at elevated temperature.20 We believe that the
observed faster initiation of many Ru−CAAC-6 catalysts likely
can be traced to a weakened Ru−O bond compared with that
in Ru−CAAC-5 complexes, as suggested by XRD analysis.24
B
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Ring-closing metathesis (RCM) of diethyl diallylmalonate
was tested next using 2 mol % Ru−CAAC-6 catalyst in
benzene at 38 °C (Scheme 2). The N-Mipp-containing
ultimately an equilibrium E/Z ratio of 4.7:1 was observed. It is
worth noting that allylbenzene, which is more commonly
applied for E/Z selectivity experiments,25 underwent signifi-
cant olefin migration in the presence of 3d and thus could not
be used herein.
The metathesis activities of the synthesized Ru−CAAC-6
catalysts were also probed in the ethenolysis of renewable oils
(Scheme 4). In a model study, methyl oleate was treated with
Scheme 2. RCM of Diethyl Diallylmalonate with Ru−
CAAC-6 Catalysts 3a−d
Scheme 4. Ethenolysis of Methyl Oleate with Ru−CAAC-6
Catalysts 3a−d and Ru−CAAC-5 Catalyst 4c
a
1
Yield determined by H NMR analysis.
complexes (3c and 3d) showed the highest activities, which
is in line with their higher initiation rates. With gem-diethyl
substitution at the α-carbon, 3d led to full conversion within 3
h, while 3c bearing a cyclohexyl group was notably slower.
When N-Dipp-containing catalysts (3a and 3b) were
employed, a marked decrease in metathesis activity was
observed, with the cyclohexyl-bearing complex again being
significantly less active. In general, the RCM reactions with
Ru−CAAC-6 complexes were much slower than those with the
Ru−CAAC-5 complexes, including 4a and 4b.20 Since the
initiation rates were shown to be higher for the Ru−CAAC-6
catalysts as a result of increased σ donation to the Ru center,
the decrease in catalytic reactivity (lower turnover frequency
(TOF)) is likely linked to the increased steric bulk of the
CAAC-6 ligands.
a
b
Determined by GC using dodecane as an internal standard. Data
adopted from ref 20.
20 ppm catalyst (3a−d) and ethylene (150 psi, 99.99% purity)
under solvent-free conditions to form 6a and 6b. A control
reaction was also conducted under otherwise identical
conditions with CAAC-5 complex 4c, which is known to be
highly active in ethenolysis (entry 6).20 While 4c rapidly
reached equilibrium (48% yield, TON = 24180) within 1.3 h,
the Ru−CAAC-6 complexes were generally much less active.
Despite rapid initiation at 40 °C, TONs of less than 2000 was
observed in all cases after 4.6 h (entries 1−4). Among them,
catalysts 3c and 3d were found to be better than 3a and 3b,
attaining TONs of approximately 1700 (entries 3 and 4). This
is consistent with our previous report on CAAC-5 complexes,
where N-Mipp catalysts were also superior to N-Dipp catalysts,
and in both cases this is likely a result of more facile substrate
binding due to reduced sterics.20 It is worth noting that despite
the lower activity, some Ru−CAAC-6 complexes showed
higher selectivity for ethenolysis over self-metathesis than their
CAAC-5 analogues. In particular, when 3c was used as the
catalyst, the generation of 6c and 6d nearly ceased once the
vessel was pressurized with ethylene. With regard to catalyst
stability, no significant decrease in the catalytic activity of 3c
was observed after 4.6 h, whereas 3a, 3b, and 3d had
decomposed. In the hope of achieving a higher TOF, the
reaction mixture was heated to 60 °C with 3c (entry 5).
However, a marked decrease in activity was observed instead,
which could be attributed to the decreased ethylene
concentration in solution at a higher temperature.
3d was further tested in the homodimerization reaction of 1-
decene in a sealed J. Young tube (Scheme 3). In the presence
of 1 mol % catalyst, the metathesis rapidly reached equilibrium
in 2 h (at 31% conversion). The kinetically controlled initial E/
Z ratio of the products was determined to be around 1:1, while
Scheme 3. Homodimerization of 1-Decene with Ru−CAAC-
6 Catalyst 3d
Aiming to further tune the steric bulk at the CAAC-6 ligand,
we synthesized CAAC-6 salt 1e featuring a methyl group and a
phenyl group at the α-position (Scheme 5). Unfortunately,
metalation of the carbene ligand (2e) with HG1 was
unsuccessful, leading to only trace amounts of the desired
product. Instead, complete decomposition of 2e occurs
through intramolecular carbene insertion into the nearby
methyl C−H bond, furnishing cyclopropane 7 in high yield
a
1
Yield determined by H NMR analysis.
C
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California Institute of Technology, Pasadena, California 91125,
United States; orcid.org/0000-0003-2808-7964
Scott C. Virgil − Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Scheme 5. Rapid Decomposition of 2e through
Intramolecular C−H Insertion
Ki-Young Yoon − Arnold and Mabel Beckman Laboratories of
Chemical Synthesis, Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States; orcid.org/0000-0003-
3257-6103
within 1 h at room temperature. This decomposition pathway
is similar to the one Bertrand observed with a menthol-derived
CAAC-6 ligand, and in both cases it is likely due to a rigid
proximal effect.23 For 2e, we hypothesize that the combination
of the adjacent phenyl group and N-Dipp moiety may
cooperatively force the methyl group into a favorable
conformation for carbene insertion.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.9b00862
Author Contributions
†A.E.S. and Y.X. contributed equally.
To summarize, we have successfully synthesized a new type
of Ru−CAAC complex through the incorporation of more σ-
donating and π-accepting CAAC-6 ligands. These catalysts
were found to initiate faster than the previously reported Ru−
CAAC-5 complexes but demonstrated much lower activity in
RCM and ethenolysis. According to the data presented, it
appears that Ru complexes bearing less bulky CAAC-6 ligands
(e.g., with N-Mipp) are catalytically more active. However,
attempts to synthesize Ru complexes with even smaller CAAC-
6 ligands were hampered by the synthetic difficulty in the
preparation of CAAC-6 salts and ligand installation. Never-
theless, as the first report of Ru−CAAC-6 complexes, the
presented study should provide valuable insight into this type
of catalyst and may further stimulate the design and
development of new Ru−CAAC olefin metathesis catalysts.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge Umicore for financial support and
donation of HG1. Dr. Adam Johns is acknowledged for
insightful suggestions and discussions. Dr. Mona Shahgoli is
acknowledged for assistance with acquiring mass spectra. Dr.
William Wolf, Dr. Jeong Hoon Ko, and Dr. Jianchun Wang are
acknowledged for helpful discussions.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
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■
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AUTHOR INFORMATION
Corresponding Author
■
Robert H. Grubbs − Arnold and Mabel Beckman Laboratories
of Chemical Synthesis, Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States; orcid.org/0000-0002-
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Authors
Adrian E. Samkian − Arnold and Mabel Beckman Laboratories
of Chemical Synthesis, Division of Chemistry and Chemical
Engineering, California Institute of Technology, Pasadena,
California 91125, United States
Yan Xu − Arnold and Mabel Beckman Laboratories of Chemical
Synthesis, Division of Chemistry and Chemical Engineering,
D
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