Highly selective ruthenium metathesis catalysts for ethenolysis

Article abstract of DOI:10.1021/ja200246e

N-Aryl,N-alkyl N-heterocyclic carbene (NHC) ruthenium metathesis catalysts are highly selective toward the ethenolysis of methyl oleate, giving selectivity as high as 95% for the kinetic ethenolysis products over the thermodynamic self-metathesis products. The examples described herein represent some of the most selective NHC-based ruthenium catalysts for ethenolysis reactions to date. Furthermore, many of these catalysts show unusual preference and stability toward propagation as a methylidene species and provide good yields and turnover numbers at relatively low catalyst loading (<500 ppm). A catalyst comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and stability and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a steady state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher conversion to cross-metathesis products.

Full text of DOI:10.1021/ja200246e

ARTICLE
pubs.acs.org/JACS
Highly Selective Ruthenium Metathesis Catalysts for Ethenolysis
Renee M. Thomas,† Benjamin K. Keitz,† Timothy M. Champagne,‡ and Robert H. Grubbs*,†
†The Arnold and Mabel Beckman Laboratories of Chemical Synthesis, Division of Chemistry and Chemical Engineering, California
Institute of Technology, Pasadena, California 91125, United States
‡Materia, Inc., 60 North San Gabriel Boulevard, Pasadena, California 91107, United States
S Supporting Information
b
ABSTRACT: N-Aryl,N-alkyl N-heterocyclic carbene (NHC)
ruthenium metathesis catalysts are highly selective toward the
ethenolysis of methyl oleate, giving selectivity as high as 95% for
the kinetic ethenolysis products over the thermodynamic self-
metathesis products. The examples described herein represent
some of the most selective NHC-based ruthenium catalysts for
ethenolysis reactions to date. Furthermore, many of these catalysts
show unusual preference and stability toward propagation as a
methylidene species and provide good yields and turnover numbers at relatively low catalyst loading (<500 ppm). A catalyst
comparison showed that ruthenium complexes bearing sterically hindered NHC substituents afforded greater selectivity and
stability and exhibited longer catalyst lifetime during reactions. Comparative analysis of the catalyst preference for kinetic versus
thermodynamic product formation was achieved via evaluation of their steady-state conversion in the cross-metathesis reaction of
terminal olefins. These results coincided with the observed ethenolysis selectivities, in which the more selective catalysts reach a
steady state characterized by lower conversion to cross-metathesis products compared to less selective catalysts, which show higher
conversion to cross-metathesis products.
’ INTRODUCTION
renders them easy to handle and does not require extensive
purification of starting material.11 High selectivities and yields for
the ethenolysis of methyl oleate and cyclooctene have been
disclosed by Schrock and co-workers using molybdenum
systems.12 Molybdenum metathesis catalysts gave up to 99%
selectivity for the ethenolysis of methyl oleate in up to 95% yield,
with turnover numbers (TONs) as high as 4750. Ideally, selective
ethenolysis would be carried out by robust catalysts exhibiting
high TONs for an efficient process. Accordingly, our research
efforts are directed toward the development of ruthenium-based
catalysts exhibiting these attributes for selective ethenolysis.
Ethenolysis reactions require catalyst stability as a propagating
methylidene species for high product selectivity and TON.6,13
The desirable ethenolysis catalytic cycle involves crossing an
internal olefin onto the active metal complex to generate an
alkylidene species, followed by reaction with ethylene to form a
1,2-metallacycle (Scheme 1). Breakdown of this metallacycle
then yields the desired terminal olefin and a ruthenium methy-
lidene species. The methylidene complex can react with the
substrate to release a terminal olefin and afford a ruthenium
alkylidene species, which can subsequently react with ethylene
and repeat the cycle. Most olefin metathesis catalysts are unstable
as methylidene complexes and possibly as the corresponding
unsubstituted metallacycle, and undergo rapid decomposition.13
This catalyst degradation significantly limits both product
Olefin metathesis is widely used in both organic and polymer
synthesis and has become a standard methodology for construct-
ing carbonꢀcarbon double bonds.1 Metathesis catalysts have
been successfully designed for stability,2 functional group
tolerance,3 activity,4 and selectivity,5 enabling metathesis meth-
odology to be broadly applied. The development of catalysts
exhibiting preference for kinetically versus thermodynamically
controlled product ratios continues to be a challenging area in
olefin metathesis.6 An example of a metathesis reaction that
requires kinetic selectivity is ethenolysis, the reaction of an
internal olefin with ethylene to generate thermodynamically
disfavored terminal olefin products. There is significant interest
for selective formation of terminal olefins due to the potential
conversion of fatty acids derived from renewable biomass to
valuable commercial products.7 Such a process would enable the
“green” synthesis of commercial commodities from renewable
source materials such as natural seed oils and their derivatives
instead of petroleum products.8 Natural seed oils are particularly
attractive due to their built-in functionality, widespread avail-
ability, and relatively low cost. Specifically, ethenolysis of methyl
oleate (MO) affords chemically desirable products with extensive
applications, including use in cosmetics, detergents, soaps,7 and
polymer additives,9 as well as potential applications as a renew-
able biofuel source.10
Most reported studies have focused on ruthenium complexes
in the development of an efficient ethenolysis catalyst due to their
functional group tolerance and stability to air and water, which
Received: January 21, 2011
Published: April 21, 2011
r
2011 American Chemical Society
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Scheme 1. Cross-Metathesis Reactions during Ethenolysis
Scheme 2. Ethenolysis of Methyl Oleate
desired olefin products 8 and 9, for the ethenolysis of methyl
oleate (Scheme 2). However, these complexes decompose due to
the instability of the propagating methylidene species, resulting
in a limited catalyst lifetime. Complex 1 catalyzes the ethenolysis
of methyl oleate (7), with 93% selectivity for ethenolysis
products 8 and 9 over self-metathesis products 10 and 11. The
yield (54%) is moderate, although the TON (5400) is good. The
first-generation chelate catalyst 2 improves selectivity slightly to
94%, but the yield (48%) and TON (4800) are lower.14 Catalyst
inhibition by ethenolysis products is reported for the first-
generation ruthenium catalysts, and instability of the methyli-
dene undermines the use of these catalysts.6,7 Phoban ruthenium
catalysts are reported to have some increased stability relative to
first-generation catalysts, while maintaining comparable selectiv-
ities and TONs.15
NHC ruthenium catalysts are known to be very active for self-
metathesis and cross-metathesis of methyl oleate with 2-butene
(TON of up to 470 000).14,16 These complexes propagate as an
alkylidene and are known to be unstable as a methylidene,
leading to their inability to viably produce metathesis products
requiring steps involving propagation as a methylidene. Accord-
ingly, the selectivity of these complexes (3ꢀ5, Figure 1) for the
production of terminal olefins 8 and 9 is poor. It has been
reported that complex 3 exhibits only 44% selectivity for
ethenolysis products 8 and 9 with 28% yield at a TON of
2800. Catalyst 4 was shown to display even lower selectivity at
33% and only 20% yield with a TON of 2000. However,
increasing the temperature from 40 to 60 °C improved the
selectivity to 47% and the yield to 32% with a TON of 3200.
More sterically hindered NHC ligands also improved selectivity.
Complex 5 afforded ethenolysis products 8 and 9 in 55%
selectivity over 10 and 11, with 38% yield and a TON of
3800.16 While N-aryl,N-aryl NHC-based ruthenium catalysts
are generally more active and stable than first-generation cata-
lysts, they are significantly less selective for ethenolysis due to
their propensity to undergo self-metathesis reactions.
Cyclic (alkyl)(amino)carbene (CAAC) ruthenium catalysts,
such as 6, have been found to be more selective for ethenolysis
products over self-metathesis products. However, improvements
in selectivity, activity, and catalyst stability are still necessary for
the reaction to be viable.6 With complex 6, selectivities as high as
92% have been achieved at 100 ppm loading, with 56% yield and
a TON of 5600. Changing the isopropyl aryl substituents to ethyl
substituents improved the TON to 35 000 at 10 ppm loading,
although the selectivity was reduced to 83% and the yield to 35%.
These complexes are unusual in that they exhibit a higher
preference for propagation as a methylidene relative to pre-
viously reported NHC-based complexes.17
Figure 1. Example ruthenium catalysts previously studied for etheno-
lysis reactions.
selectivity and TON during ethenolysis reactions. Side reactions
that would reduce product selectivity include self-metathesis and
secondary metathesis (Scheme 1).6 Self-metathesis occurs when
the substrate-bound catalyst reacts with another substrate mole-
cule rather than ethylene, thereby yielding another internal olefin
and ruthenium alkylidene species. Secondary metathesis involves
further cross-metathesis of two desired terminal olefins to
generate an internal olefin and release ethylene. Because the
key steps involve propagation via a ruthenium methylidene,
catalyst stability as a propagating methylidene is essential for
viable ethenolysis reactions.
A variety of ruthenium metathesis catalysts have been
screened for kinetic selectivity for the ethenolysis of internal
olefins (Figure 1).14 Phosphine-based ruthenium catalysts (1and 2,
Figure 1) show high initial selectivity, where selectivity is
defined as the percentage of the product mixture that is the
Previous work in our group studying degenerate metathesis
reactions has demonstrated that greater catalyst preference for a
methylidene species appears to be related to selectivity for
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degenerate metathesis over productive metathesis.17 Therefore,
degenerate metathesis studies can be used as a means of identifying
promising catalysts for ethenolysis reactions. For instance, CAAC
catalysts, such as 6, exhibit higher degenerate turnovers than 1ꢀ5.
Interestingly, unsymmetrical N-aryl,N-alkyl NHC catalysts feature
even higher preference for unproductive turnovers.17 This led us to
believe that these catalysts would show promising ethenolysis
selectivity, with the propensity to propagate as a methylidene species
providing the desired kinetic selectivity for terminal olefin products
over thermodynamically favored internal olefins.
Herein, we describe the unusual stability of unsymmetrical
N-aryl,N-alkyl NHC catalysts toward propagation as a methylidene
species and their application as catalysts for highly selective ethenolysis.
Most of the catalysts also display good thermal stability, and all are
stable to air and moisture. In comparison to standard NHC and
phosphine-derived ruthenium catalysts (1ꢀ5), these complexes ex-
hibit longer lifetimes in cross-metathesis reactions, presumably as a
result of their stability to existing as a methylidene.
sized (Figure 2). Complex 12 was targeted first to enhance the
ethenolysis selectivity through increased steric bulk of the N-aryl
and, primarily, the N-alkyl substituent. Initial screening of
catalyst 12 for the ethenolysis of methyl oleate afforded promis-
ing results (Table 1, entry 1). At 150 psi of ethylene and 40 °C,
86% selectivity for ethenolysis products 8 and 9 over cross-
metathesis products 10 and 11 was achieved, with 46% yield of 8
and 9 in 6 h and a TON of 4620 at a low catalyst loading of 100
ppm. The yield increased to 68% of ethenolysis products at a
loading of 500 ppm, although the TON was reduced to 1370.
Lowering the loading of 12 to 10 ppm gave a significantly higher
TON of 8340, although the yield of 8 and 9 was only 8% after 4 h.
Since good kinetic selectivity and TONs were obtained with 12,
further efforts were directed toward synthesizing new complexes
to determine the effect of the NHC ligand substituents on
catalyst behavior and toward identifying a catalyst with excellent
kinetic selectivity (Figure 2, 12ꢀ21). Crystal structures of
complexes 12 and 15 confirmed the expected geometry and that
their bond lengths are consistent with those of previously
reported NHC ruthenium complexes (Figures 3 and 4).
’ RESULTS AND DISCUSSION
Complexes 12ꢀ21 were all compared for catalytic activity
for the ethenolysis of methyl oleate at 100 ppm catalyst loading
and 150 psi of ethylene (Table 1). Complex 13 exhibited
lower kinetic selectivity compared to 12 (Table 1, entry 1), with
77% selectivity for 8 and 9 over 10 and 11 (Table 1, entry 2),
presumably due to the decreased sterics of the N-aryl substituent
With the goal of improving both selectivity and TON during
ethenolysis reactions, a variety of N-aryl,N-alkyl NHC complexes
bearing different ligand substituents were designed and synthe-
Table 1. Catalyst Comparison for the Ethenolysis of Methyl
Oleate
entrya
catalyst
conv (%)b
selectivity (%)c
yield (%)d
TONe
1
2
3
4
5
6
7
8
9
12
13
15
16
17
18
19
20
21
54
11
52
42
59
52
15
17
40
86
77
86
86
87
89
95
69
79
46
9
4620
845
45
36
51
46
15
11
31
4450
3600
5070
4604
1460
1120
3080
a The reactions were run neat for 6 h at 40 °C and 150 psi of ethylene.
The catalyst loading was 100 ppm. b Conversion =100 ꢀ [(final moles
c
of 7) ꢁ 100/(initial moles of 7)]. Selectivity = (moles of ethenolysis
products 8 and 9) ꢁ 100/(moles of total products 8 þ 9 þ 10 þ 11).
d Yield = (moles of ethenolysis products 8 þ 9) ꢁ 100/(initial moles of 7).
e TON = yield ꢁ [(moles of 7)/(moles of catalyst)].
Figure 3. Crystal structure of complex 12 shown at the 50% ellipsoid
probability level. Selected bond lengths: RuꢀC3 = 1.83 Å, RuꢀC8 =
1.98 Å, RuꢀO = 2.26 Å.
Figure 2. N-Aryl,N-alkyl NHC ruthenium complexes synthesized to selectively catalyze ethenolysis.
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(mesityl in complex 13 versus 2,6-di-isopropyl phenyl in complex
12). The TON was lower for 13 in comparison to 12 as well,
likely a result of greater instability of 13 as a propagating
methylidene species. Catalyst 14 was very unstable and degraded
early during the reaction, affording low conversion. Since the
reaction equilibrium was not reached due to the catalyst’s fast
decomposition, the selectivity most likely does not represent the
inherent selectivity of 14 and is therefore not reported.
The kinetic selectivities of 12, 15, and16 were identical (86% for 8
and 9 over 10 and 11), suggesting that small changes in the sterics of
the alkyl substituents do not have a significant impact on catalyst
selectivity (Table 1, entries 1, 3, and 4). The more sterically
demanding ligand substituents of 17 and 18 did slightly improve
kinetic selectivity (Table 1, entries 5 and 6). High selectivities of 87%
and 89% for 17 and 18, respectively, were obtained, and both 17 and
18 displayed good TONs of 5070 and 4600, respectively. Catalyst 19
showed excellent kinetic selectivity at 95%, markedly higher than
those of other reported ruthenium NHC catalysts and comparable to
those of first-generation ruthenium catalysts (Table 1, entry 7).
Catalysts 20 and 21 gave lower selectivity compared to the
catalysts with a di-isopropyl N-aryl group on the NHC, as
expected from the results with 13 and 14. The selectivities of
20 and 21 were 69% and 79%, respectively (Table 1, entries 8 and
9). The yield (40%) and TON (3080) of 21 were significantly
better than those of 20 and 13. Comparison of the various
complexes screened shows a consistent trend that both N-aryl
and N-alkyl groups with more sterically hindering substituents
improve the desired selectivity. Di-isopropyl N-aryl groups
enhance catalyst stability, leading to better product yields. Our
next efforts were directed toward exploring catalyst loadings for
the more promising catalysts for the ethenolysis of methyl oleate
(Table 2).
Raising the catalyst loading to 500 ppm showed significant
improvement in yield, more than doubling it in many cases, for
the same given amount of time. Specifically, going from a loading of
100 ppm to 500 ppm of 19 increased the ethenolysis product yield
from 15% to 46% (Table 2, entries 1 and 2). Alternatively, lowering
the catalyst loading to 50 ppm decreased the yield from 15% to 5%
(Table 2, entry 3). Similar results were obtained for the other
catalysts upon varying catalyst loading. Complex 17, at 500
ppm loading, generated an ethenolysis product yield of 78%
(Table 2, entry 4), compared to 51% at 100 ppm (Table 2, entries
5). Analogously, 16 gave an ethenolysis yield of 72% at 500 ppm -
(Table 2, entry 7), compared to 36% yield at 100 ppm (Table 2,
entry 8). Conversion of methyl oleate increases with higher catalyst
loading, as demonstrated with 21 (Table 2, entries 10ꢀ13). Com-
plexes 14 and 18 have markedly higher ethenolysis product yields at
500 ppm as well (Table 2, entries 14 and 15). While the selectivities
remain constant at variable catalyst loading for most complexes,
catalyst 14 shows increased selectivity at a 500 ppm loading
compared to a 100 ppm loading (58% versus 19%). This is believed
to be due to the fast degradation of 14 during the reaction. Higher
loading of 14 enables the catalyst to come closer to its inherent steady
state for the reaction before all of the complex has decomposed.
Previous ruthenium metathesis catalyst studies have shown that
having an ortho-H on the N-aryl ring increases the rate of catalyst
decomposition.
Temperature-dependent studies were conducted using cata-
lysts 17 and 16 to consider the effect of temperature on selectivity
and TON (Table 3). Ethenolysis of methyl oleate was carried out
Figure 4. Crystal structure of complex 15 at the 50% ellipsoid prob-
ability level. Selected bond lengths: RuꢀC2 = 1.83 Å, RuꢀC1 = 1.97 Å,
RuꢀO = 2.28 Å.
Table 2. Ethenolysis of Methyl Oleate Using Different Catalyst Loadings
entrya
catalyst
cat./MO (ppm)
time (h)
conv (%)b
selectivity (%)c
yield (%)d
TONe
1
19
19
19
17
17
17
16
16
16
21
21
21
21
14
18
500
100
50
6
6
6
6
6
2
2
6
6
4
6
6
2
6
6
48
15
5
95
95
96
88
87
86
86
86
87
82
79
79
80
58
88
46
15
5
913
1460
1010
1570
5070
2050
1440
3600
2010
1060
3080
2880
1680
817
2
3
4
500
100
50
89
59
12
83
42
12
65
40
19
4
78
51
10
72
36
10
53
31
14
3
5
6
7
500
100
50
8
9
10
11
12
13
14
15
500
100
50
20
500
70
41
500
86
76
1520
a The reactions were run neat at 40 °C and 150 psi of ethylene. b Conversion =100 ꢀ [(final moles of 7) ꢁ 100/(initial moles of 7)]. c Selectivity = (moles
of ethenolysis products 8 and 9) ꢁ 100/(moles of total products 8 þ 9 þ 10 þ 11). d Yield = (moles of ethenolysis products 8 þ 9) ꢁ 100/(initial moles
of 7). e TON = yield ꢁ [(moles of 7)/(moles of catalyst)].
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Table 3. Temperature Effects on the Ethenolysis of Methyl
Oleate
temp
time
(h)
conv
(%)b
selectivity
(%)c
yield
(%)d
entrya catalyst
(°C)
TONe
1
2
3
4
5
6
17
17
17
16
16
16
40
50
60
40
50
60
6
4
4
6
6
6
59
67
68
42
51
47
87
81
81
86
81
79
51
55
55
36
41
37
5070
5460
5470
3600
4090
3680
a The reactions were run neat at 150 psi of ethylene. The catalyst loading
was 100 ppm. b Conversion =100 ꢀ [(final moles of 7) ꢁ 100/(initial
moles of 7)]. c Selectivity = (moles of ethenolysis products 8
and 9) ꢁ 100/(moles of total products 8 þ 9 þ 10 þ 11). d Yield =
(moles of ethenolysis products 8 þ 9) ꢁ 100/(initial moles of 7). e TON
= yield ꢁ [(moles of 7)/(moles of catalyst)].
Figure 5. Steady state between CM of 1-hexene and ethenolysis of
5-decene.
Scheme 3. Steady State between Cross-Metathesis and
Ethenolysis
preferences of different catalysts for terminal olefin versus
internal olefin distributions can be determined by identifying
the point at which the forward cross-metathesis reaction is equal
to the reverse ethenolysis reaction. This will be observed when
the conversion to internal olefin product no longer increases (the
steady state has been reached) and requires that the catalyst is
still active and undergoing metathesis turnovers.
For ease of measurement, homodimerization cross-metathesis
was chosen as the model reaction, since the only possible product
is the internal olefin dimer of the substrate. The reactions were
carried out in a sealed NMR tube, preventing loss of generated
ethylene, and the steady state between the forward (cross-
metathesis, CM) and reverse (ethenolysis) reactions for each
catalyst was measured. Although this setup does not yield
absolute steady-state values, as the ethylene generated will be
partitioned between the solution and the NMR tube head space,
it does enable qualitative evaluation of relative steady states for
catalysts screened. The degree of conversion to CM product was
evaluated for catalysts 17, 19, and second-generation catalyst 4 in
order to assess their relative propensities to undergo CM as
compared to ethenolysis. Phosphine-based ruthenium catalyst 1
was also screened during these experiments; however, 1 decom-
posed prior to reaching steady state between CM and etheno-
lysis, and the data obtained were therefore not included.
Catalysts 4, 17, and 19 did not undergo any decomposition
during the course of the reaction, as confirmed by monitoring
them by proton NMR spectroscopy. Catalysts 4, 17, and 19 were
chosen to represent a range of selectivities for the ethenolysis of
methyl oleate, with 4 showing a reported 33% selectivity, 17
showing 87% selectivity, and 19 showing 95% selectivity. In
accordance with these data, catalyst 19 was predicted to reach
steady state between the forward and reverse reactions at the
lowest conversion to CM product (higher preference for yielding
ethenolysis products), and catalyst 4 was predicted to reach
steady state at the highest conversion to CM product. Catalyst 17
was expected to have a steady-state point between those of the
other two complexes. Two substrates were employed for these
experiments. First, the experiment was carried out using 1-hex-
ene (Figure 5), and then a duplicate set of experiments were run
with allyl chloride (Figure 6) to ascertain that the observed
results were not substrate specific.
at 40, 50, and 60 °C for each catalyst. The TON for both 17 and
16 increased at higher reaction temperatures, as did the product
yield. A reaction temperature of 60 °C likely induces earlier
catalyst decomposition and may account for the lower TON and
yield for 16 at 60 °C compared to 50 °C (Table 3, entry 6). For
both catalysts, the more significant increase in TON and yield
occurred on going from 40 to 50 °C, indicating that further
increase in temperature produces only minimal benefits and may
in fact initiate catalyst decomposition. The selectivity was
reduced at higher temperatures, dropping from 87% to 81% for
17 and from 86% to 81% for 16 on going from 40 to 50 °C. The
reduction in selectivity between 50 and 60 °C, however, was
minimal. Ethenolysis reactions were not run below 40 °C, as this
would decrease both the yield and TON, undermining the
catalysts’ utility. Accordingly, 40 °C was determined to be the
optimal temperature for the ethenolysis of methyl oleate cata-
lyzed by these N-aryl,N-alkyl NHC complexes.
In order to evaluate catalyst propensity toward ethenolysis, we
conducted a qualitative steady-state study to complement the
ethenolysis results obtained. Observed selectivity in ethenolysis
reactions is believed to arise from a catalyst’s preference for a
product distribution favoring terminal olefins, manifested in its
lack of cross-metathesis reactivity. This preference can be
reflected in cross-metathesis reactions as well, where if ethylene
generated by cross-metathesis of two terminal olefins is trapped
in the reaction vessel, the forward cross-metathesis reaction will
eventually reach a steady state with ethenolysis of the internal
olefin products with the generated ethylene, which affords the
original terminal olefins (Scheme 3). Accordingly, relative
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’ EXPERIMENTAL SECTION
General Considerations. All manipulations of air- or water-
sensitive compounds were carried out under dry nitrogen using a
glovebox or under dry argon utilizing standard Schlenk line techniques.
NMRspectrawererecordedonaVarianMercury(1H, 300 MHz),aVarian
Inova 400 (1H, 400 MHz), or a Varian Inova 500 (1H, 500 MHz; 13C,
125 MHz) spectrometer and referenced to residual protio solvent.
Materials. Deuterated methylene chloride was dried over calcium
hydride and vacuum distilled, followed by three freezeꢀpumpꢀthaw
cycles. Methyl oleate (>99%) was obtained from Nu-Chek-Prep
(Elysian, MN) and stored over activated alumina. 1-Hexene was dried
over calcium hydride, vacuum distilled, and freezeꢀpumpꢀthawed
prior to use. Allyl chloride (99%) was purchased from Aldrich and used
as received.
Procedure for the Ethenolysis of Methyl Oleate. Ethenolysis
reactions were carried out using research-grade methyl oleate (>99%)
that was purified by storage over actived alumina followed by filtration.
The experiments were set up in a glovebox under an atmosphere of
argon. Methyl oleate was charged in a Fisher-Porter bottle equipped
with a stir bar. A solution of ruthenium catalyst of an appropriate
concentration was prepared in dry dichloromethane, and the desired
volume of this solution was added to the methyl oleate. The head of the
Fisher-Porter bottle was equipped with a pressure gauge, and a dip-tube
was adapted on the bottle. The system was sealed and taken out of
the glovebox to the ethylene line. The vessel was then purged with
ethylene (polymer purity 99.9% from Matheson Tri Gas) for 5 min,
pressurized to 150 psi, and placed in an oil bath at 40 °C. The reaction
was monitored by collecting samples via the dip-tube at different
reaction times. Prior to gas chromatography (GC) analysis, the reaction
aliquots were quenched by adding a 1.0 M isopropanol solution of tris-
hydroxymethylphopshine to each vial over the course of 2ꢀ3 h. The
samples were then heated for over 1 h at 60 °C, diluted with distilled
water, extracted with hexanes, and analyzed by GC. The GC analyses
were run using a flame ionization detector. Column: Rtx-5 from Restek,
30 m ꢁ 0.25 mm i.d. ꢁ 0.25 μm film thickness. GC and column
conditions: injection temperature, 250 °C; detector temperature,
280 °C; oven temperature, starting temperature, 100 °C; hold time, 1
min. The ramp rate was 10 °C/min to 250 °C, and the temperature was
then held at 250 °C for 12 min. Carrier gas: helium.
Figure 6. Steady state between CM of allyl chloride and ethenolysis of
1,4-dichloro-2-butene.
For the CM of 1-hexene and corresponding ethenolysis of
5-decene (Figure 5), the resulting relative steady-state values
were as expected, with catalyst 19 showing the highest
selectivity for 1-hexene (only 7% conversion to 5-decene),
relative to the other catalysts, and catalyst 4 showing the
lowest selectivity for 1-hexene, indicated by it producing the
greatest conversion to 5-decene (38% conversion) at its
steady-state point. Catalyst 17 reached steady state at 22%
conversion to 1-hexene, between the values found of 19
and 4.
When allyl chloride was used as the substrate (Figure 6), the
same relative order of steady-state points was obtained for the
catalysts studied. The data from both experiments corroborate
the results found in the ethenolysis of methyl oleate, with 19
exhibiting the greatest preference for kinetic over thermody-
namic products, and this class of N-aryl,N-alkyl catalysts showing
greater preference for kinetic products than previous NHC-
based ruthenium catalysts.
Cross-Metathesis of 1-Hexene/Ethenolysis of 5-Decene
Steady-State Experiments. In a glovebox under a nitrogen atmo-
sphere, 0.5 mL of dry CD2Cl2 was added to an 8 in. NMR tube.
1-Hexene (18.9 μL, 0.149 mmol) was added via a 25 μL syringe, and the
NMR tube was sealed with a septum cap. The appropriate amount of
ruthenium catalyst (3 mol %) was added to a GC vial and dissolved in
0.25 mL of CD2Cl2. The GC vial was capped and brought out of the
glovebox along with the NMR tube. A 1H NMR spectrum (Varian 500
MHz spectrometer) of the 1-hexene solution was taken for time point t = 0,
and the catalyst solution was subsequently injected into the NMR tube
via syringe through the septum cap. The septum cap was wrapped with
’ CONCLUSION
We have developed highly selective N-aryl,N-alkyl NHC
ruthenium catalysts for ethenolysis, with 19 exhibiting the
highest selectivity for an NHC-based ruthenium metathesis
catalyst to date. Catalyst loadings of 500 ppm afforded good
yields of the ethenolysis products 8 and 9. The TONs were
modest for most of the catalysts screened, and future studies
will be directed toward improving those numbers further.
These catalysts show unusual preference for generating
kinetic products over thermodynamic products, which we
believe to be controlled primarily through the NHC ligand
sterics. Increasing the sterics of the NHC substituents en-
hances selectivity and, in general, improves stability as well,
although a limit is reached where NHC ligands bearing
extremely bulky substituents inhibit reactivity. The catalysts
maintained good stability toward existing as a propagating
methylidene species, making them attractive as catalysts for
ethenolysis reactions. High selectivities, a challenging feature
of ethenolysis reactions, were obtained for many of the
complexes of this class of N-aryl,N-alkyl NHC catalysts.
1
parafilm, and the reaction progress was monitored over time by H
NMR spectroscopy. Catalyst stability was monitored by following the
ruthenium benzylidene H peak over time, since catalyst decomposition
causes the benzylidene H peak to shift or disappear altogether. Conver-
sion of 1-hexene to 5-decene was determined by relative integration of
the allylic CH2 protons of 5-decene to those of 1-hexene. 1H NMR of
1-hexene (CD2Cl2, 500 MHz): δ 5.83 (ddt, J = 17.0, 10.2, 6.7 Hz, 1H),
5.02 ꢀ 4.96 (m, 1H), 4.92 (ddt, J = 10.2, 2.3, 1.2 Hz, 1H), 2.08ꢀ2.02 (m,
2H) [CH2], 1.40ꢀ1.28 (m, 4H), 0.91 (t, J = 5.0 Hz, 3H) ppm. 1H NMR
of 5-decene (CD2Cl2, 500 MHz): δ 5.43ꢀ5.38 (m, 1H), 2.00ꢀ1.91 (m,
2H) [CH2], 1.34ꢀ1.28 (m, 4H), 0.90 (t, J = 5.0 Hz, 3H) ppm.
Cross-Metathesis of Allyl Chloride/Ethenolysis of 1,4-Di-
chloro-2-butene Steady-State Experiments. In a glovebox
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dx.doi.org/10.1021/ja200246e |J. Am. Chem. Soc. 2011, 133, 7490–7496

Journal of the American Chemical Society
ARTICLE
under a nitrogen atmosphere, 0.5 mL of dry CD2Cl2 was added to an 8
in. NMR tube, and the NMR tube was sealed with a septum cap. The
appropriate amount of ruthenium catalyst (3 mol %) was added to a GC
vial and dissolved in 0.25 mL of CD2Cl2. The GC vial was capped and
brought out of the glovebox along with the NMR tube. Allyl chloride
(12.2 μL, 0.150 mmol) was added via a 25 μL syringe through the
septum cap, which was then wrapped with parafilm. 1H NMR spectrum
(Varian 500 MHz spectrometer) of the allyl chloride solution was taken
for time point t = 0, and the catalyst solution was subsequently injected
into the NMR tube via syringe through the septum cap. The reaction
progress was monitored over time by 1H NMR spectroscopy. Catalyst
stability was monitored by following the ruthenium benzylidene H peak
over time, since catalyst decomposition causes the benzylidene H to shift
or disappear altogether. Conversion of allyl chloride to 1,4-dichloro-2-
butene was determined by relative integration of the vinyl
H2CdCHCH2Cl proton of allyl chloride to the vinyl ClCH2-
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1
CHdCHCH2Cl protons of 1,4-dichloro-2-butene. H NMR of allyl
chloride (CD2Cl2, 500 MHz): δ 5.98 (ddt, J = 10.0, 8.7, 6.6 Hz, 1H),
5.35 (ddd, J = 16.9, 2.5, 1.3 Hz, 1H), 5.21 (ddd, J = 10.1, 2.0, 0.9 Hz, 1H),
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’ ASSOCIATED CONTENT
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S
Supporting Information. Experimental details for the
b
synthesis of the catalysts and X-ray crystallographic data. This
material is available free of charge via the Internet at http://pubs.
acs.org.
’ AUTHOR INFORMATION
Corresponding Author
rhg@caltech.edu
(16) Schrodi, Y.; Ung, T.; Vargas, A.; Mkrtumyan, G.; Lee, C. W.;
Champagne, T. M.; Pederson, R. L.; Hong, S. H. Clean 2008,
36, 669–673.
’ ACKNOWLEDGMENT
(17) Stewart, I. C.; Keitz, B. K.; Kuhn, K. M.; Thomas, R. M.;
Grubbs, R. H. J. Am. Chem. Soc. 2010, 132, 8534–8535.
(18) Vehlow, K.; Maechling, S.; Blechert, S. Organometallics 2006,
25, 25–28.
(19) Vehlow, K.; Wang, D.; Buchmeiser, M. R.; Blechert, S. Angew.
Chem. Int. Ed. 2008, 47, 2615–2618.
This research was supported by the National Science Founda-
tion through a Graduate Research Fellowship to R.M.T. B. K. K.
acknowledges the NDSEG for a graduate fellowship. The authors
acknowledge Drs. Lawrence Henling and Michael Day for
obtaining the X-ray crystallographic structures of complexes
12 and 15. We thank the NSF (CHE-1048404) and NIH
(5R01GM031332-Z7) for funding and Materia, Inc. for the gift
of methyl oleate and catalysts 1, 2, and 4.
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