N-Trifluoromethyl NHC Ligands Provide Selective Ruthenium Metathesis Catalysts

Article abstract of DOI:10.1021/acs.organomet.6b00028

We report the synthesis of ruthenium metathesis catalysts containing unsymmetrical N-trifluoromethyl NHC ligands. These complexes have been fully characterized, and a Ru-F interaction has been identified in the solid state by X-ray crystallographic analysis for three catalysts with Ru-F distances between 2.629(2) and 2.652(2) ?. The influence of the N-trifluoromethyl NHC ligands on the initiation rates and activation parameters was studied. The activity of these catalysts was evaluated in benchmark olefin metathesis reactions and compared to the standard second-generation Grubbs catalyst. Remarkably, N-trifluoromethyl catalysts display an unusually high selectivity for the formation of terminal olefins (up to 90%) in the ethenolysis of ethyl oleate. Much improved selectivity is demonstrated for alternating copolymerization of cyclooctene and norbornene as well. These results underline the importance of electronic effects exerted by the NHC ligand.

Full text of DOI:10.1021/acs.organomet.6b00028

Article
pubs.acs.org/Organometallics
N‑Trifluoromethyl NHC Ligands Provide Selective Ruthenium
Metathesis Catalysts
Pascal S. Engl, Alexey Fedorov, Christophe Coperet, and Antonio Togni*
́
Department of Chemistry and Applied Biosciences, ETH Zurich, Vladimir-Prelog-Weg 2, CH-8093 Zurich, Switzerland
̈
̈
S
* Supporting Information
ABSTRACT: We report the synthesis of ruthenium meta-
thesis catalysts containing unsymmetrical N-trifluoromethyl
NHC ligands. These complexes have been fully characterized,
and a Ru−F interaction has been identified in the solid state by
X-ray crystallographic analysis for three catalysts with Ru−F
distances between 2.629(2) and 2.652(2) Å. The influence of
the N-trifluoromethyl NHC ligands on the initiation rates and
activation parameters was studied. The activity of these
catalysts was evaluated in benchmark olefin metathesis
reactions and compared to the standard second-generation Grubbs catalyst. Remarkably, N-trifluoromethyl catalysts display
an unusually high selectivity for the formation of terminal olefins (up to 90%) in the ethenolysis of ethyl oleate. Much improved
selectivity is demonstrated for alternating copolymerization of cyclooctene and norbornene as well. These results underline the
importance of electronic effects exerted by the NHC ligand.
relevant intermediates.9 Metathesis catalysts such as 1 contain
INTRODUCTION
■
the phosphine ligand trans to the NHC at a latent substrate
binding site. While H2IMes is a poor π acceptor, its
replacement by NHC ligands with greater π acceptor capacity
has been proposed to attenuate competing Ru−PCy3 π-back-
donation, thereby labilizing the Ru−PCy3 bond and facilitating
catalyst initiation.10 Currently, the development of selective
catalysts is one of the most challenging aspects of olefin
metathesis. Most recent research efforts led to the discovery of
Z-selective catalysts.6b,11 Another, perhaps less recognized yet
important aspect is the development of chemoselective
catalysts. One of the early examples was the discovery of
selective alternating copolymerization catalysts based on
bidentate P,O ligands.12,13 Ruthenium catalysts supported by
unsymmetrical NHC ligands also show improved selectivities in
the ring-opening−ring-closing metathesis reactions compared
to 1.14 In recent developments cyclic alkyl amino carbene
(CAAC) ligands gave Ru metathesis catalysts such as 2 (Figure
1) with exceptional activity and improved selectivity in
ethenolysis reactions.8a,15
Over the last decades, transition-metal-catalyzed olefin meta-
thesis has emerged as a powerful synthetic tool for the
construction of carbon−carbon double bonds.1,2 Research has
focused on finding highly active and stable well-defined
catalysts. In the specific case of Ru, the first breakthroughs
were the identification of Cl2Ru(CHR)(PR3)2 as active
metathesis catalysts.3 The second one was the introduction of
N-heterocyclic carbene (NHC) supporting ligands,4 such as
1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene
(H2IMes) in the so-called second-generation Grubbs catalyst
(1, Figure 1).5 Such catalysts combine high olefin metathesis
NHC ligands can be readily fine-tuned via substitution in the
backbone or at the nitrogen atoms to tailor catalyst
properties.16 For instance, asymmetry can be installed in the
metathesis catalyst through a Ru−F interaction, which was
proposed to occur in complex 3 after PCy3 dissociation.17 The
superior catalytic efficiency of 3 in ring-closing metathesis as
compared to 1 was explained via such Ru−F interaction, which
reduced the activation energy of the rate-limiting PCy3
dissociation required to initiate the catalytic cycle.17 Unex-
Figure 1. Ruthenium olefin metathesis catalysts 1−3. Cy, cyclohexyl;
Mes, mesityl.
activity with functional group tolerance as well as good air and
moisture stability. This enabled numerous applications
including the synthesis of natural products and bioactive
compounds,6 advanced materials,7 and industrial-scale produc-
tion of olefins from biomass-derived olefinic feedstocks.8 The
high stability and remarkable performance of 1 are attributed to
the enhanced σ-donor ability of the supporting NHC ligand,
which provides strong metal−carbon bonds and stabilizes both
the precatalyst and coordinatively unsaturated, catalytically
Received: January 13, 2016
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of Ru Benzylidenes 6a−d and Structure of 7
plored in catalysis are unsymmetrical NHC ligands with one N-
trifluoromethyl substituent,18 which could provide beneficial
effects because the strongly electron withdrawing CF3 group
imposes electronic and steric bias on the NHC ligand. The
fluorine−metal interaction17,19 in such complexes is predeter-
mined by the ligand design. Here we describe the synthesis and
the catalytic activity of a series of Ru olefin metathesis catalysts
containing N-trifluoromethyl benzimidazolidene NHC ligands
with the goal to develop chemoselective catalysts. We focused
on alternating copolymerization of cyclic olefins and selective
synthesis of terminal olefins in ethenolysis of ethyl oleate. From
the catalysis standpoint, the important result is significantly
improved chemoselectivity displayed by our catalysts in these
reactions.
of hexane. In the solid state both 6a and 6d (see the SI for the
related structure of 6b) exhibit a square pyramidal geometry
that is distorted toward octahedral structure with the
benzimidazolium ring bisecting the Cl−Ru−Cl angle and the
N-aryl (6a,b) and the N-n-butyl (6d) groups almost in a plane
that is parallel to the benzylidene ligand (Figures 2 and
S24−26). The C2−F1 bonds of the N-trifluoromethyl groups
are nearly in plane with the heterocycle (the torsion angle C1−
N1−C2−F1 is 18.9(5)° for 6a and 11.0(3)° for 6d), which
RESULTS AND DISCUSSION
■
Synthesis of Ru NHC Benzylidene Complexes.
Benzimidazolium salts 5a−d (Scheme 1) were synthesized via
electrophilic arylation or alkylation of N-trifluoromethyl
benzimidazole 4. Complexes 6a−d were then obtained in
moderate to fair yields by in situ deprotonation of the salts 5a−
d with NaHMDS in toluene in the presence of stoichiometric
[RuCl2(CHPh)(PCy3)2] (Scheme 1). Benzylidene 7 was
synthesized analogously and used as a N-CF3-free benchmark
catalyst with a benzimidazolidene NHC framework (see the SI
for details). The majority of Ru metathesis catalysts with NHC
ligands contain ortho-substituents on the N-aryl groups,
necessary to avoid catalyst decomposition through ortho C−
H activation.19a,20,21 We initially tested several diaryliodonium
salts with ortho-substituted aryl groups in arylation of 4 but
observed no formation of the respective NHC salts. As
diaryliodonium salts that are devoid of ortho aryl substitution
proved competent arylation reagents, we resorted to using
electronic rather then steric effects to suppress the afore-
mentioned ortho C−H insertion pathways deactivating Ru
complexes. Toward this end we installed electron-neutral or
electron-depleted aryl groups on the benzimidazolium ligand,
providing stable Ru benzylidenes 6a−c and 7. In addition, the
alkyl analogue 6d was prepared to evaluate the influence of aryl
vs alkyl substitution. Complexes 6a−d and 7 are air stable in
the solid state but slowly decompose in solution at room
temperature. All complexes were characterized by NMR
spectroscopy, IR, elemental analysis, and HRMS.
Figure 2. Ortep drawings of 6a and 6d. Hydrogen atoms are omitted
for clarity, and thermal ellipsoids are set to 50% probability. Selected
bond lengths [Å], bond angles [deg], and torsion angles [deg] for 6a:
Ru1−F1 2.652(2), C2−F1 1.335(4), C2−F2 1.324(5), N1−C1−Ru1
123.1(2), N2−C1−Ru1 133.5(3), Cl1−Ru1−Cl2 168.28(2), C1−
N1−C2-F1−18.9(5). For 6d: Ru1−F1 2.629(2), C2−F1 1.332(3),
C2−F2 1.328(3), N1−C1−Ru1 121.44(15), N2−C1−Ru1
134.32(16), Cl1−Ru1−Cl2 166.56(2), C1−N1−C2−F1 11.0(3).
Single crystals of 6a,b,d suitable for X-ray analysis were
obtained from the respective CH2Cl2 solution by slow diffusion
B
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places the fluorine atom close to the ruthenium center. The
Ru−F distances in 6a (2.652(2) Å) and 6d (2.629(2) Å) are
significantly shorter than the sum of the van der Waals radii of
ruthenium and fluorine (3.52 Å). In addition, the C2−F1 bond
(1.335(4) Å for 6a and 1.332(3) Å for 6d) is elongated when
compared to the C2−F2 bond (1.324(5) Å for 6a and 1.324(5)
Å for 6d), while the NHC ligand is slightly tilted toward the
side occupied by the N-trifluoromethyl group (N1−C1−Ru1
123.1(2)° vs N2−C1−Ru1 133.5(3)° for 6a and N1−C1−Ru1
123.1(2)° vs N2−C1−Ru1 133.5(3)° for 6d). The latter could
be aided by electrostatic interactions in the case of the N-
phenyl complex 6a. However, the closely related molecular
structure of the N-n-butyl complex 6d suggests that it is the
Ru−F interaction that determines such geometry.
donation and hence weaker trans influence of the benzimida-
zolidene NHC ligand compared to the imidazolinium one.18
The most striking difference can be noted between the entropy
of activation ΔS⧧ of N−CF3 catalyst 6b and its N−iPr analogue
7 (Table 1, entries 4 and 7). This may reflect the influence of
the alkyl vs aryl N-group, as catalyst 6d, containing N−CF3,
features low ΔS⧧ as well (Table 1, entry 6). While catalysts 6a−
c demonstrate activation entropies ΔS⧧ typical for the generally
accepted dissociative mechanism, the significantly lower ΔS⧧
values of catalysts 6d and 7 might hint toward a change of the
catalyst activation pathway. However, as was mentioned earlier,
the initiation rate constants of 6d and 7 are independent of
olefin concentration, which excludes an associative or
interchange mechanism.23 The lower values for ΔS⧧ can
therefore be tentatively ascribed to a reduced degree of solvent
reorganization around the N-alkyl rather than N-aryl groups in
the transition state leading to phosphine dissociation. In
addition, the enthalpies of activation ΔH⧧ are also lower for
catalysts 6d and 7 (Table 1, entries 6, 7). This is likely due to
the increased electron-donating ability of NHC ligands bearing
N-alkyl groups (6d and 7) and therefore a stronger trans effect,
which weakens the ruthenium phosphine bond.24
Benchmarking of Activity in Ring-Closing Metathesis
(RCM) and Cross-Metathesis (CM) Reactions. We initially
tested 6a−d and 7 in the model ring-closing reaction of diethyl
diallylmalonate (8) and compared their catalytic activity to the
benchmark second-generation Grubbs catalyst 1.25 The
reactions were carried out in CD2Cl2 with 1 mol % catalyst
loading, and conversion to cycloalkene 9 was monitored by 1H
NMR spectroscopy. Although 6a−d were competent catalysts
in the RCM of 8, longer reaction times were required when
compared to 1 (Figure 3). Catalysts 6a−c exhibit an identical
1H NMR spectroscopy indicated that for all complexes 6a−d
only a single rotational isomer is present in solution. 2D 1H/1H
NOESY NMR spectra contain a cross-peak between the
benzylidene proton and the N-aryl ortho-hydrogens in 6a−c
and the N-n-butyl methylene hydrogens in 6d (Figures S20−
23, SI), suggesting that the solid-state geometry is also
preferred in solution.
Initiation Studies. Detailed studies by Grubbs have
revealed a complex relationship between phosphine dissocia-
tion rates and the catalyst activity.14d,22 In order to further
evaluate the influence of the N-trifluoromethyl group on the
properties of 6a−d and 7, we determined the activation
parameters of phosphine dissociation via quantitative and
irreversible reaction of these complexes with butyl vinyl ether
(BVE) in benzene-d6 and compared the obtained values with
those of 1 and 3. The disappearance of the starting benzylidene
signal of 6a−d and 7 in the presence of 30 equiv of BVE was
1
monitored by H NMR at different temperatures as a function
of time and showed clean first-order kinetics over a period of at
least three half-lives. The initiation rate constants were found to
be independent of the concentration of BVE, thereby indicating
that phosphine dissociation is rate limiting in the initiation step.
The activation parameters were extracted via analysis of
respective Eyring plots and are summarized in Table 1. The
decrease of the free energy of activation ΔG⧧ for the initiation
step of 3 with respect to 1 has previously been associated with a
Ru−F interaction (Table 1, entries 1, 2).17
In contrast, catalysts 6a−d and 7 displayed free energies of
activation indistinguishable from that of 1 within experimental
error. This indicates that the labilization of the Ru−PCy3 bond
due to the Ru−F interaction and greater π acceptor capacity of
the unsaturated benzimidazolium NHC framework is offset by
another effect, such as the substantially weaker σ electron
a
Table 1. Activation Parameters for 1, 3, 6a−d, and 7
b
ΔH⧧
ΔS⧧
ΔG⧧
303K
entry [Ru]
[kcal mol−1
]
[cal K−1 mol−1
]
[kcal mol−1
]
1
2
3
4
5
6
7
1
27
27
27
30
28
24
24
2
2
1
1
2
2
2
13
19
16
22
18
4
6
9
2
3
4
3
4
23.0 0.422
3
21.8 0.114d
Figure 3. RCM reaction of 8 with catalysts 1, 6a−d, and 7 monitored
1
6a
6b
6c
6d
7
22
23
22
22
23
1
1
2
2
2
by H NMR spectroscopy.
kinetic profile in the first 50 min; however, while 6b and 6c
reach >97% conversion, the activity of 6a drops with time.
Catalysts bearing small alkyl N-substituents on the NHC ligand
are known to be highly reactive; however their stability in
solution is also decreased.26 Interestingly, while the N-
trifluoromethyl and N-isopropyl groups are sterically similar,27
the replacement of CF3 in 6b by i-Pr in 7 results in significantly
0
a
Reaction conditions: [Ru] = 0.0106 mmol, BVE = 0.318 mmol,
ferrocene (3 μmol as an internal standard) in 0.6 mL of C6D6 at
temperatures from 303 to 330 K. Extrapolated from the Eyring
equation.
b
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diminished RCM activity (Figure 3). The superior activity of
catalyst 6b is possibly due to greater catalyst stability toward
deactivation through C−H bond insertion pathways.19a,20,28
Unexpectedly, catalyst 6d, with an N-n-butyl group, outper-
forms analogous catalysts 6a−c, exhibiting activity on par with
the benchmark catalyst 1.
Table 2. Alternating Copolymerization of COE (13) and
NBE (14)
a b
,
We next assessed our catalysts in the cross-metathesis
reaction of allylbenzene 10 and cis-1,4-diacetoxy-2-butene 11.
Catalysts 6a−d display activities lower than both 1 and 7
(>87% and >80% conversion, respectively, Figure 4). While
conversion
c
[mmol]
poly-13
d
poly-14
d
15
d
entry
[Ru]
6a
COE
NBE
[%]
[%]
[%]
e
1
0.131
0.289
0.099
0.068
0.186
0.199
0.131
0.262
0.086
0.061
0.149
0.129
3
5
−
−
−
8
97 (64)
95 (62)
96 (65)
85 (61)
79 (64)
47 (61)
2
3
4
5
6
6a
6b
6c
6d
7
4
7
8
13
−
53
a
Reaction conditions: NBE = 0.42 mmol, COE = 21 mmol, [Ru] =
b
2.1× 10−4 mmol in 7 mL of CH2Cl2 at 30 °C for 15 min. Low
solubility of polymers did not allow collecting the GPC data.
c
Conversions calculated from the consumed moles of norbornene
d
e
and cyclooctene. Determined by 13C NMR spectroscopy. Reaction
time was 5 min.
provides a copolymer with 96% alternating diads, its fluorine-
free analogue 7 furnishes only 47% alternating copolymeriza-
tion (Table 2, entry 6). This is possibly due to the increased
rotation barrier around the NHC−Ru axis30 in 6b due to the
Ru−F interaction and/or due to the increasing COE:NBE ratio
and is in accordance with the observation of the kinetic
selectivity at lower conversion and time (Table 2).13b,28 The cis-
selectivity lies within a 60−65% range for all initiators 6−7,
which is typical for the NBE-alt-COE polymers prepared with
Ru initiators supported by NHC ligands.29,31 Interestingly,
poly-COE formed in copolymerization experiments along with
15 contains an unusually high amount of cis double bonds,
amounting to 67% in the case of 7 (Figure 5). Such cis content
is difficult to achieve with monocyclic olefins even using a new
generation of powerful Z-selective Ru metathesis catalysts,32 as
these monomers are prone to chain transfer reactions and
secondary metathesis events.33 The observed cis selectivity is
not inherent to 7 and is also found in the polymerization of
pure COE using N−CF3 initiator 6b, which gives 68% cis
double bonds when used at 500 ppm loading (Table S2).
However, when the catalyst loading was increased to 1 mol %,
the cis-selectivity dropped significantly to 23%, reflecting
double-bond isomerization via secondary metathesis events.
Lowering the reaction temperature from 30 °C to 0 °C
improved the cis content to 45%, although at the expense of
polymer yield (Table S2).
Ethenolysis of Ethyl Oleate. Cross-metathesis of internal
olefins with ethylene, also called ethenolysis, has been receiving
increased attention since it allows the synthesis of terminal
olefins from sustainable bioderived olefinic feedstock.8,34 Such
terminal olefins are highly valuable for the chemical industry35
and as a source of renewable biofuels.36 Recently, extremely
high turnover numbers up to 340 000 with 1 ppm catalyst
loadings were reported for ethenolysis of ethyl oleate using Ru
catalysts with advanced cyclic alkyl amino carbene ligands.8a We
tested catalysts 6a−d and 7 in ethenolysis of ethyl oleate 16
and compared their activity to that of catalyst 1. All catalytic
tests were performed by injecting 100 ppm catalyst in toluene
to neat 16 pressurized to 10 bar ethylene (isobaric conditions)
Figure 4. Cross-metathesis of 10 and 11 with catalysts 1, 6a−d, and 7.
For selectivity data, see Table S1 and Figure S6 in the SI.
catalyst 6d showed the highest activity in the RCM reaction, it
is a rather poor CM catalyst, reaching only 37% conversion to
the cross-product 12. Catalyst 6a is more efficient in the CM
reaction, leading to 68% conversion to 12.
Alternating Ring-Opening Metathesis Polymerization
(ROMP). In 2005, Chen and co-workers reported the sequence-
selective ROMP of cis-cyclooctene 13 (COE) and norbornene
14 (NBE) by a mechanistically designed ruthenium carbene
complex.12,13 Buchmeiser and co-workers used Grubbs-type
complexes bearing unsymmetrical NHC ligands for the
synthesis of alternating copolymers as well.29 In both cases,
the control over alternating copolymerization was aided by
optimizing the ratio of comonomers 13 and 14. Catalyst 1
readily but unselectively polymerizes a mixture of 13 and 14 to
give respective homopolymers. We evaluated the catalytic
activity and sequence selectivity of catalysts 6a−d and 7 in the
ROMP of a 50:1 mixture of COE and NBE. The polymers
obtained were characterized by 1H and 13C NMR spectroscopy
(Table 2). The nature of the N-substituents of the NHC ligand
has a profound influence on the chemoselectivity of these
catalysts. Catalysts 6a−c with N-aryl substituents gave
copolymers with 85−96% alternating diads after 15 min of
the reaction (Table 2, entries 2−4). The alternating copolymer
can be obtained with 97% chemoselectivity by reducing the
reaction time from 15 min to 5 min, as shown for catalyst 6a
(Table 2, entry 1 and Figure 5). The copolymer obtained with
6d (Table 2, entry 5) contained a greater amount of the
homopolymers, suggesting that the N-aromatic ring of the
NHC ligand influences chemoselectivity. Remarkably, while 6b
D
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Figure 5. 13C NMR spectra for the polymers presented in Table 2.
at 60 °C, and the reaction progress was monitored via ethylene
uptake at constant pressure. Benchmark catalyst 1 achieves
nearly full conversion (>97%) of ethyl oleate 16 in less than 10
min according to the gas uptake curve (Figure 6), but also
Table 3. Ethenolysis of Ethyl Oleate 16 with Catalysts 1, 6a−
a
d, and 7
b
c
d
e
entry [Ru] loading [ppm] C [%]
S [%]
Y [%]
TON
1
2
3
4
5
6
7
8
1
100
20
97
95
44
57
74
56
60
47
46
43
40
80
90
75
89
26
60
64
19
48
69
43
55
17
5998
32 018
1901
1
6a
6b
6c
6c
6d
7
100
100
100
20
4778
6860
21 312
5516
100
100
1694
a
Reaction conditions: ethyl oleate 16 = 2.802 mmol, [Ru] = 0.28
b
μmol, 10 bar of C2H4, 2 mL of toluene. Conversion = 100 − [(final
moles of ethyl oleate) × 100/(initial moles of ethyl oleate)].
c
Figure 6. Gas uptake curves for the ethenolysis of ethyl oleate 16 with
catalysts 1, 6a−d, and 7 at 100 ppm [Ru] loading. The y-axis was back
calibrated from GC data of the reaction mixtures.
Selectivity = (moles of ethenolysis product, terminal olefins)/[2 ×
d
(moles of total product)] × 100. Yield of terminal olefins =
[conversion × selectivity]/100. TON = yield of terminal olefins ×
[(moles of ethyl oleate)/(moles of catalyst)].
e
affords a relatively high amount of secondary metathesis
products 19−21, which substantially lowers the yield of
terminal olefins (60%, Table 3, entry 1). Although only
moderate to fair conversion of 16 was observed for catalysts
6b−d (57−74%), they display a remarkably high selectivity
(80−90%) for the desired ethenolysis products 17 and 18.
The moderate conversion and selectivity of catalyst 6a, on
the other hand, leads to a yield of only 19%. With a selectivity
of 90%, catalyst 6c outperforms catalyst 1 under these
conditions and achieves an overall yield of 69% for the
terminal olefins with a TON of 6860. However, at 20 ppm
catalyst loading the selectivity of catalyst 6c as well as the
conversion of 16 is significantly reduced, while catalyst 1
maintains its high activity and achieves a TON of 32 018,
presumably due to the higher stability of 1. Remarkably, the
replacement of an N-isopropyl group in 7 by the N-
trifluoromethyl group in 6b results in a 54% increase in the
catalyst’s selectivity. This high preference of catalysts 6b−d for
the terminal olefins over the internal ones underlines that
electronic effects exerted by the NHC ligands could be used for
fine-tuning of activity and selectivity, in addition to traditional
approaches utilizing sterically hindered NHC ligands.8
CONCLUSIONS
■
A series of new ruthenium metathesis catalysts bearing sterically
and electronically unsymmetrical NHC ligands have been
synthesized and fully characterized. X-ray crystallographic
analysis of complexes 6a,b,d indicates a Ru−F interaction in
the solid state. The initiation rates and activation parameters
are consistent with a dissociative initiation mechanism, in which
the displacement of the phosphine ligand is rate limiting. The
E
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free energy of activation ΔG⧧ of catalysts 6a−d does not
significantly deviate from that of 1. Although catalysts 6a−d do
not display improved performances in benchmark reactions
compared to original catalysts, they show greatly improved
chemoselectivity in alternating copolymerization of cyclooctene
and norbornene as well as in ethenolysis of ethyl oleate. In
particular, 6a achieves 97% chemoselectivity in alternating
copolymerization, while 6b−d display a remarkably high
selectivity (80−90%) for terminal olefins in ethenolysis of
ethyl oleate by avoiding the competing homodimerization of
primary terminal olefins. Catalyst 6c gave 90% selectivity for
the kinetic over the thermodynamic ethenolysis products in
69% yield, with a TON of 6860. This enhanced selectivity
stems from the electronically unsymmetrical nature of the N-
trifluoromethyl NHCs, as the selectivity is substantially
decreased when N−CF3 is replaced with a sterically equivalent
N−iPr group. We therefore propose that the electronic bias
imposed on the NHC ligand by the strongly electron
withdrawing CF3 group and/or a Ru−F interaction is crucial
for achieving high selectivity in the sequence-selective ROMP
and ethenolysis reactions. It shows that one should be careful in
discarding poor catalysts based on benchmark reactions and
that it is essential (at this stage) to evaluate catalysts in a
broader range of reactions. These encouraging results suggest
that improved chemoselective catalysts can be designed, and we
are currently exploring this field.
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ASSOCIATED CONTENT
■
S
* Supporting Information
(9) (a) Jacobsen, H.; Correa, A.; Poater, A.; Costabile, C.; Cavallo, L.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.6b00028.
́
Coord. Chem. Rev. 2009, 253, 687−703. (b) Díez-Gonzalez, S.; Nolan,
S. P. Coord. Chem. Rev. 2007, 251, 874−883. (c) Occhipinti, G.;
Bjorsvik, H. R.; Jensen, V. R. J. Am. Chem. Soc. 2006, 128, 6952−6964.
(10) Lummiss, J. A. M.; Higman, C. S.; Fyson, D. L.; McDonald, R.;
Fogg, D. E. Chem. Sci. 2015, 6, 6739−6746.
Experimental procedures and characterization of new
compounds; Eyring plots for 6a−d and 7 (PDF)
Crystallographic data for 6a (CIF)
Crystallographic data for 6b (CIF)
Crystallographic data for 6d (CIF)
(11) (a) Herbert, M. B.; Grubbs, R. H. Angew. Chem., Int. Ed. 2015,
54, 5018−5024. (b) Occhipinti, G.; Hansen, F. R.; Tornroos, K. W.;
Jensen, V. R. J. Am. Chem. Soc. 2013, 135, 3331−3334. (c) Furstner, A.
̈
Science 2013, 341, 1357−1363. (d) Keitz, B. K.; Endo, K.; Patel, P. R.;
Myles, H. B.; Grubbs, R. H. J. Am. Chem. Soc. 2012, 134, 693−699.
(12) Bornand, M.; Chen, P. Angew. Chem., Int. Ed. 2005, 44, 7909−
7911.
AUTHOR INFORMATION
■
́
(13) (a) Jovic, M.; Torker, S.; Chen, P. Organometallics 2011, 30,
Corresponding Author
*E-mail: atogni@ethz.ch.
3971−3980. (b) Torker, S.; Muller, A.; Chen, P. Angew. Chem., Int. Ed.
2010, 49, 3762−3766. (c) Bornand, M.; Torker, S.; Chen, P.
Organometallics 2007, 26, 3585−3596.
Notes
(14) (a) Queval, P.; Jahier, C.; Rouen, M.; Artur, I.; Legeay, J. C.;
Falivene, L.; Toupet, L.; Crevisy, C.; Cavallo, L.; Basle, O.; Mauduit,
M. Angew. Chem., Int. Ed. 2013, 52, 14103−14107. (b) Kavitake, S.;
Samantaray, M. K.; Dehn, R.; Deuerlein, S.; Limbach, M.; Schachner, J.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
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A.; Jeanneau, E.; Coper
12443−12446. (c) Karame,
K.; Alauzun, J.; Basset, J. M.; Coper
́
et, C.; Thieuleux, C. Dalton Trans. 2011, 40,
I.; Boualleg, M.; Camus, J. M.; Maishal, T.
et, C.; Corriu, R. J.; Jeanneau, E.;
The authors are grateful to the Scientific Equipment Program
́
of ETH Zurich and the Swiss National Science Foundation
̈
́
(R’Equip grant 206021_150709/1) for financial support. A.F.
Mehdi, A.; Reye, C.; Veyre, L.; Thieuleux, C. Chem. - Eur. J. 2009, 15,
11820−11823. (d) Vougioukalakis, G. C.; Grubbs, R. H. Chem. - Eur. J.
2008, 14, 7545−7556. (e) Van Veldhuizen, J. J.; Garber, S. B.;
Kingsbury, J. S.; Hoveyda, A. H. J. Am. Chem. Soc. 2002, 124, 4954−
4955.
thanks Holcim Stiftung for a Habilitation Fellowship. Lukas
Sigrist and Dr. Deven P. Estes (both ETH Zurich) are greatly
̈
acknowledged for X-ray crystallographic analyses and assistance
with the kinetic analysis, respectively.
(15) Anderson, D. R.; Lavallo, V.; O’Leary, D. J.; Bertrand, G.;
Grubbs, R. H. Angew. Chem., Int. Ed. 2007, 46, 7262−7265.
(16) (a) Samantaray, M. K.; Alauzun, J.; Gajan, D.; Kavitake, S.;
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