Protonolysis of a ruthenium-carbene bond and applications in olefin metathesis

Article abstract of DOI:10.1021/ja203070r

The synthesis of a ruthenium complex containing an N-heterocylic carbene (NHC) and a mesoionic carbene (MIC) is described wherein addition of a Br?nsted acid results in protonolysis of the Ru-MIC bond to generate an extremely active metathesis catalyst. Mechanistic studies implicated a rate-determining protonation step in the generation of the metathesis-active species. The activity of the NHC/MIC catalyst was found to exceed those of current commercial ruthenium catalysts.

Full text of DOI:10.1021/ja203070r

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pubs.acs.org/JACS
Protonolysis of a RutheniumꢀCarbene Bond and Applications in
Olefin Metathesis
Benjamin K. Keitz,† Jean Bouffard,‡ Guy Bertrand,‡ and 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
‡UCRꢀCNRS Joint Research Chemical Laboratory, Department of Chemistry, University of California, Riverside, California 92521,
United States
S Supporting Information
b
Scheme 1. Initial Discovery of Acid-Induced Dissociation of
MIC 2 from 3 (Dipp = 2,6-Diisopropylphenyl)
ABSTRACT: The synthesis of a ruthenium complex con-
taining an N-heterocylic carbene (NHC) and a mesoionic
carbene (MIC) is described wherein addition of a Brønsted
acid results in protonolysis of the RuꢀMIC bond to
generate an extremely active metathesis catalyst. Mecha-
nistic studies implicated a rate-determining protonation
step in the generation of the metathesis-active species.
The activity of the NHC/MIC catalyst was found to exceed
those of current commercial ruthenium catalysts.
lefin metathesis has gained widespread use as a robust
O
method for the formation of CꢀC double bonds, largely
as a result of the development of increasingly powerful catalysts.1
Key to a catalyst’s efficiency are its activity and stability, which
can be tuned through a judicious choice of ligands. Specifically,
the stability of a ruthenium-based catalyst can be improved by
preventing decomposition pathways that rely on nucleophilic
attack by a dissociated ligand.2 For instance, replacing a dis-
sociating phosphine ligand by a chelating ether moiety results in a
catalyst that is more stable under metathesis reaction conditions.3
A second N-heterocyclic carbene (NHC) may be used in place of
a phosphine, and in fact, complexes such as this were among the
first metathesis catalysts to incorporate NHCs.4 However, be-
cause of the low dissociation rate of NHCs on ruthenium, all bis-
NHC complexes require thermal activation at temperatures well
above room temperature (RT).4 Nevertheless, these catalysts are
still effective in a variety of metathesis transformations and have
the added benefit of initiating only in response to an external
stimulus (latent catalysis), a behavior which is critical in materials
science applications.5,6 We report herein that ruthenium com-
plexes incorporating a traditional NHC and a mesoionic carbene
(MIC)7 may be activated by the addition of a Brønsted acid.
The resulting catalyst combines the stability and latency of bis-
NHC complexes while maintaining low activation temperatures.
Furthermore, we demonstrate that in some reactions, the per-
formance of this catalyst surpasses that of the best commercially
available catalysts.
Figure 1. (left) Synthesis of 5 and (right) its solid-state structure with
50% probability ellipsoids. H atoms have been omitted for clarity.
Selected bond lengths (Å) and angle (deg): C13ꢀRu, 2.086; C5ꢀRu,
2.097; C13ꢀRuꢀC5, 169.34.
presence of a solvent containing acidic impurities, the transfor-
mation of 3 to 1 occurred. Although relatively rare, protonolysis
reactions of metalꢀNHC bonds have been observed for
ruthenium9 and other late metals.10 Given these precedents,
we concluded that MIC 2 is acid-labile and imagined that it could
be incorporated into a metathesis catalyst as a dissociating ligand.
Combining free MIC 2 with 4 in C6H6 resulted in the new
complex 5, which was isolated in excellent yield after washing
with cold pentane (Figure 1). The solid-state structure of 5 is
consistent with those of analogous bis-NHC complexes.4c
Initial metathesis screens revealed that 5 is completely inactive
at RT. For instance, 1 mol % 5 in C6H6 was unable to polymerize
1,5-cyclooctadiene (COD) to any detectable extent within a
period of 12 h at RT.11 Some minimal conversion was observed
after extended periods, presumably as a result of very slow
catalyst initiation due to the acidic glassware or acid impurities.
Under similar reaction conditions, <5% conversion of the ring-
We previously reported the synthesis and activity of ruthe-
nium olefin metathesis catalysts of type A bearing MICs in place
of more traditional NHCs (Scheme 1).8 In our attempts to
prepare analogues bearing the unhindered H-substituted MIC 2
from 1, we observed the formation of 3. We noted that in the
Received: April 4, 2011
Published: May 16, 2011
r
2011 American Chemical Society
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Table 1. RCM of 7 with 5 (1 mol %) and Acid (20 mol %) in
C6D6 (0.1 M)
entry
acid
time (h)
conv. (%)a
1
2
none
18þ
0.3
4
<5
>95
73
HCl (1 M in Et2O)
perchloric (70%)
trifluoroacetic
acetic
3
4
0.3
18
18
4
>95
20
5
6
formic (88%)
hydrobromic (48%)
hydroiodic (57%)
HBF4 (Et2O)
BH3 (THF)
B(C6F5)3
91
7
>95
>95
16
8
4
Figure 3. RCM of 9 with 5 and TFA (blue) or HCl (black) and RCM of
9 with NHC complex 6 (white). Conditions: 9 (0.08 mmol), 5 or 6
(0.0008 mmol), and HCl (1 M in Et2O, 31 equiv., 0.025 mmol) or TFA
(16 equiv., 0.013 mmol) in C6D6 (0.8 mL) at 30 °C. Conversion was
measured by 1H NMR spectroscopy.
9
1
10
11
12
13
18
17
1
19
33
ZnCl2
>95
<5
SnCl4
18
a Measured by 1H NMR spectroscopy.
Because of their proficiency in activating 5, HCl and trifluoro-
acetic acid (TFA) were chosen to investigate the RCM of 7 to 8
more closely. Under standard RCM screening conditions, a
mixture of 5 and either HCl or TFA showed complete conversion
of 7 to 8 within 10 min at 30 °C (Figure 2). The reaction with
TFA was particularly fast, reaching 100% conversion within only
a few minutes. Catalyst 5 also excelled at the RCM of trisub-
stituted substrate 9 (Figure 3). Notably, under the above RCM
reactions, catalyst 5 was found to be superior to commercial
catalysts such as (H2IMes)Cl2Ru(dCHPhOiPr) (6; H2IMes =
1,3-dimesitylimidazolidin-2-ylidene).12 As expected on the basis
of these results, 5 also performed well at ring-opening metathesis
polymerization (ROMP) with both HCl and TFA [see the
Supporting Information (SI)].
After the activation of 5 with acid had been established,
additional experiments were performed with the two best acid
activators, TFA and HCl, to study the mechanism of activation
in greater detail. The benzylidene proton resonance of 5 was
monitored by 1H NMR spectroscopy following addition of vary-
Figure 2. RCM of 7 with 5 and TFA (blue) or HCl (black) and RCM of
7 with NHC complex 6 (white). Conditions: 7 (0.08 mmol), 5 or 6
(0.0008 mmol), and HCl (1 M in Et2O, 31 equiv., 0.025 mmol) or TFA
(16 equiv., 0.013 mmol) in C6D6 (0.8 mL) at 30 °C. Conversion was
measured by 1H NMR spectroscopy.
ing amounts of TFA. A plot of the observed rate constant (kobs
)
versus concentration of TFA in C6D6 displayed a second-order
dependence on TFA (Figure S10 in the SI). This behavior is
consistent with protonation of 5 by an acid dimer instead of an
acid monomer.13 In order for the above situation to be plausible,
however, protonation must be involved in the rate-determining
step of the reaction. To probe this possibility and also to simplify
the acidꢀbase chemistry of the system, we decided to monitor
the initiation of 5 in CD3CN rather than in C6D6.
If protonation is involved in the rate-determining step of the
initiation reaction, a plot of kobs versus acid concentration should
be linear at constant pH.14 This would parallel the behavior of
general acid-catalyzed reactions, although in this case, kinetic
runs were conducted under pseudo-first-order conditions. In-
deed, when an initiation study was performed with TFA in
CD3CN using potassium trifluoroacetate to maintain an approxi-
mately constant pH, a linear plot was obtained (Figure S12).
Further evidence of the involvement of acid in the rate-determin-
ing step was provided by a Brønsted plot (Figure 4), which
displays a linear relationship between the pKa of the acid in
CD3CN and the logarithm of the initiation rate of 5.15 Finally,
a plot of log(kobs) versus the pH of the solution exhibited
behavior characteristic of the involvement of acid in the rate-
determining step (Figure S15). When HCl was used in place of
closing metathesis (RCM) substrate diethyl diallylmalonate (7)
was observed over a period of several weeks at RT. In contrast,
addition of HCl (1 M in Et2O) resulted in complete and
immediate conversion of 7 to the RCM product 8 within 20 min
(Table 1, entry 2). Having established the feasibility of our initial
hypothesis, we set about studying the protonolysis reaction in
greater detail.
Our initial efforts focused on the effect of different acids on
the RCM of 7 (Table 1). Strong acids (entries 2ꢀ4, 7, and 8)
were found to be the most effective and were capable of initiating
the reaction even when added as aqueous solutions. However,
the identity of the conjugate base was also important, as HBF4
performed poorly (entry 9) in comparison with acids with similar
pKa’s. Weaker acids (entries 5 and 6) were less efficient and
reached full conversion only after several hours or not at all.
Interestingly, some Lewis acids were also capable of affecting
the transformation. For instance, addition of ZnCl2 resulted
in complete conversion within 2 h at RT, while addition of
B(C6F5)3 resulted in only 33% conversion after several hours.
Other Lewis acids such as SnCl4 were found to be even less
effective. In general, Brønsted acids significantly outperformed
Lewis acids.
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Scheme 3. Initiation study of 15a
a Conditions: 15 (0.0032 mmol), 14 (0.032 mmol), and HCl (0.05
mmol) in C6D6.
negative ΔSq is inconsistent with a rate-limiting dissociative step.
Therefore, a simple interpretation of the above saturation
kinetics as a fast protonation equilibrium followed by slow ligand
dissociation is inaccurate. However, any conclusions based on
ΔSq alone are complicated by the likely formation of charged
transition states in solvents that are largely incapable of stabiliz-
ing them (e.g., C6D6).18 Nevertheless, the observed initiation
Figure 4. Brønsted plot for initiation of 5 at RT in CD3CN. Conditions:
5 (0.003 mmol) and acid (0.045 mmol) in CD3CN (0.6 mL). Acids were
acetic acid, Cl2HCCO2H, F3CCO2H, and CH3SO3H.
Scheme 2. Proposed Mechanism for Initiation of 5
rate of 5 in C6D6 under saturation conditions at RT (0.0011 sꢀ1
)
is slightly higher than that of (H2IMes)(PCy3)Cl2Ru(dCHPh)
(0.00046 sꢀ1 at 35 °C),17 which explains the superior perfor-
mance of 5 in RCM.19
A complete proposed mechanism for the initiation event of 5
is shown in Scheme 2. Although our mechanistic studies could
not definitively establish the nature of the protonation event, the
fact that some Lewis acids also activated the catalyst strongly
suggests that the unsubstituted nitrogen (N2) on the MIC ligand
(2) plays an important role. Previously reported density func-
tional theory calculations on free MICs (e.g., 2) suggest that N2
has the second-highest proton affinity after the carbene itself,
meaning that protonation at this position is plausible.8 Thus, it is
likely that initiation entails protonation at the MIC N2 in 5 to
give 11, followed by dissociation with a concomitant 1,3-proton
shift to give 13 and 12, both of which were observable by mass
spectrometry (see SI). This mechanism is consistent with our
experimental results to date, but at this time we cannot defini-
tively rule out other possibilities.
A final question we wished to answer was whether the
behavior of 5 was due to the unique nature of the MIC ligand
or if other conventional NHCs (e.g., H2IMes) would act in
a similar manner. In order to determine this, (H2IMes)2
Cl2Ru(dCHPh) (15) was added to 7, and no RCM activity was
observed at RT.20 Upon addition of HCl (10 equiv), no
immediate activity was detected either. However, after a period
of ∼12 h at RT, ∼70% conversion to 8 was observed by NMR
spectroscopy. When HCl was added to a mixture of 15 and 14 in
order to approximate the extent of catalyst initiation, only ∼12%
conversion to catalyst 6 was achieved after a period of 24 h
(Scheme 3). This result is in contrast to that observed for 5, which
was able to achieve complete conversion to 6 within a matter of
minutes. Thus, although 15 is capable of being activated by acid,
this occurs much less efficiently than for 5.
TFA in CD3CN, a first-order dependence on HCl concentration
was observed (Figure S17). All of the above results are strong
indications that a protonation event rather than dissociation is
the rate-determining step in catalyst activation.
The initiation kinetics of 5 in the presence of inorganic acids in
solvents of lower polarity (C6D6, toluene-d8) are more intricate
and likely involve poorly understood solvation and/or counter-
ion effects, as suggested from the screening of acid initiators. For
instance, the reaction of 5 in C6D6 following the addition of
excess HCl (>15 equiv) revealed a decrease in the benzylidene
proton signal intensity that followed clean first-order kinetics. A
plot of kobs versus HCl concentration displayed saturation
kinetics, which is inconsistent with a protonation event being
rate-determining under these conditions and may be indicative of
a pre-equilibrium step (Scheme 2 and Figure S3). Monitoring the
growth of product 6 after treatment of 5 with acid in the presence
of varying amounts of 14 showed no dependence on the
chelating olefin concentration (Scheme 2 and Figure S5).16
Therefore, any reaction with an olefin must take place after the
rate-determining step. The above experiment with 14 also allowed
us to identify 13, which precipitated from solution. Taken together,
the formation of 6 and 13 suggest that protonation of 5 generates
catalytic intermediate 12, which is the same active species that is
postulated to follow thermally induced ligand dissociation in
common ruthenium metathesis catalysts.17 An Eyring plot of the
activation reaction with HCl in toluene-d8 under saturation condi-
tions (Figure S5) yielded the values ΔHq = 11.9 ( 0.2 kcal/mol and
ΔSq = ꢀ33.3 ( 0.7 eu. The value of ΔHq for 5 is ∼10 kcal/mol
less than those for comparable phosphine-based catalysts, while
the value of ΔSq is much larger in magnitude and negative.17 The
In summary, we have demonstrated that in the presence of
acid, a MIC ligand may act as a leaving group, allowing an other-
wise inactive metathesis complex (5) to enter the metathesis
catalytic cycle. Furthermore, under standard metathesis reactivity
screening conditions, 5 is superior to the latest commercial cata-
lysts and can complete RCM reactions within a matter of minutes
at RT. A mechanistic study of the initiation mechanism con-
cluded that protonation is rate-determining with the most efficient
initiator, TFA, but that the activation step is strongly influenced by
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COMMUNICATION
the identity of the acid and solvent. With strong-acid initiators, 5 is
able to quickly and efficiently access thesamereactive intermediate
as other catalysts (e.g., 12) and thus combines latency with
exceptional reactivity at RT. Finally, we have established that the
observed protonolysis behavior of 5 can also occur, but only to a
limited extent, in other bis-NHC complexes, enabling the incor-
poration of these activation mechanisms in future generations of
metathesis catalysts.
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S
Supporting Information. NMR spectra, kinetic data, me-
b
chanistic analysis, and crystallographic data (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
rhg@caltech.edu
’ ACKNOWLEDGMENT
Lawrence Henling and Dr. Michael Day are acknowledged
for X-ray crystallographic analysis. Prof. Dennis Dougherty and
Dr. Jay Labinger are thanked for helpful discussions. We are
grateful to Dr. Alexey Fedorov for assisting with mass spectrom-
etry studies and instrumentation. Financial support by NDSEG
(fellowship to B.K.K.), FQNRT (fellowship to J.B.), NIH
(R01GM68825, 5R01GM031332), and NSF (CHE-1048404) is
acknowledged. The instrumentation facilities where this work
was carried out were supported by NSF CHE-063094 and NIH
RR027690.
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