C-H Activation and Proton Transfer Initiate Alkene Metathesis Activity of the Tungsten(IV)-Oxo Complex

Article abstract of DOI:10.1021/jacs.8b06603

In alkene metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation state can serve as precatalysts. However, the activation mechanism for these latter species - generating alkylidenes - is still an open question. Here, we report the syntheses of tungsten(IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands, WO(OR)₂py₃ (R = CMe(CF₃)₂ (2a), R = Si(OtBu)₃ (2b), and R = C(CF₃)₃ (2c); py = pyridine), and show that upon activation with B(C₆F₅)₃ they display alkene metathesis activities comparable to W(VI)-oxo alkylidenes. The initiation mechanism is examined by kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the alkene reactant is crucial for initiating alkene metathesis. Deuterium labeling of the allylic C-H group shows a primary kinetic isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the allylic C-H bond and show that formation of the metallacyclobutane from the allyl "hydride" involves a proton transfer facilitated by the coordination of a Lewis acid (B(C₆F₅)₃) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining and yields the metathesis active species.

Full text of DOI:10.1021/jacs.8b06603

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Article
C-H Activation and Proton Transfer Initiate Alkene
Metathesis Activity of Tungsten (IV)-Oxo Complex
Ka Wing Chan, Erwin Lam, Vincenza D'Anna, Florian Allouche, Carine
Michel, Olga Safonova, Philippe Sautet, and Christophe Copéret
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 15 Aug 2018
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Ka Wing Chan†, Erwin Lam†, Vincenza D'Anna‡, Florian Allouche†, Carine Michel‡, Olga V. Safono-
va§, Philippe Sautet‡,&,ç, Christophe Copéret*,†
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† ETH Zürich, Department of Chemistry and Applied Biosciences, Vladimir Prelog Weg 1-5, ETH Zürich, CH-8093 Zurich,
Switzerland
‡
i L , de L , N M , i e i Claude Bernard Lyon 1, Laboratoire de Chimie, F-69342, Lyon,
France
§ Paul Scherrer Institute, CH-5232 Villigen, Switzerland
& Department of Chemical and Biomolecular Engineering, University of California, Los Angeles, Los Angeles, California
90095, United States
ç Department of Chemistry and Biochemistry, University of California, Los Angeles, Los Angeles, California 90095, United
States
ABSTRACT: In alkene metathesis, while group 6 (Mo or W) high-oxidation state alkylidenes are accepted to be key reaction
intermediates for both homogeneous and heterogeneous catalysts, it has been proposed that low valent species in their +4 oxidation
state can serve as pre-catalysts. However, the activation mechanism for these latter species – generating alkylidenes – is still an
open question. Here, we report the syntheses of tungsten (IV)-oxo bisalkoxide molecular complexes stabilized by pyridine ligands,
WO(OR)2py3 (R = CMe(CF3)2 (2a), R = Si(OtBu)3 (2b) and R = C(CF3)3 (2c); py = pyridine) and show that upon activation with
B(C6F5)3, they display comparable alkene metathesis activities as W(VI)-oxo alkylidenes. The initiation mechanism is examined by
kinetic, isotope labeling and computational studies. Experimental evidence reveals that the presence of an allylic CH group in the
alkene reactant is crucial for initiating alkene metathesis. Deuterium-labeling of the allylic C-H group shows a primary kinetic
isotope effect on the rate of initiation. DFT calculations support the formation of an allyl hydride intermediate via activation of the
allylic C-H bond and show that formation of the metallacyclobutane from the allyl ―hydride‖ involves a proton transfer facilitated
by the coordination of a Lewis acid (B(C6F5)3) and assisted by a Lewis base (pyridine). This proton transfer step is rate determining
and yields the metathesis active species.
INTRODUCTION
Alkene metathesis is increasingly impacting different fields
of chemistry, ranging from petrochemical to pharmaceutical
industries.1-13 Today, the largest industrial alkene metathesis
process is based on WO3/SiO2, a heterogeneous catalyst,
which only operates at high temperatures (> 400 °C).14 Pre-
treating this catalyst under reducing conditions (e.g. alkene, H2
etc.) allows the process to be operated at lower temperatures,
thus indicating that the high temperature conditions are mostly
needed for generating the active sites.15-19 Similar pre-
treatment effect has also been reported for supported Mo
based catalysts.20-22 Recently, our group has reported a low
temperature activation method based on organosilicon reduct-
ants, which allow MO3/SiO2 (M = Mo or W) and the related
well-defined silica-supported metal-oxo species to catalyze
alkene metathesis at 70 °C. We have proposed that M(IV)
species bearing an oxo ligand, which are formed upon reduc-
tion, can transform into alkylidene species in the presence of
alkenes.23-24 Previous works in homogeneous catalysis have
shown that Mo(IV)(NAr)(X)(Y)(alkene) and dimeric
{W(NA ’)[OCMe2(CF3)]2}2 complexes (Scheme 1), which can
be formed from the alkylidenes during metathesis, can also re-
initiate metathesis, albeit with a very low efficiency.25-26 In
fact, the numbers of active species generated under these con-
ditions have been reported to be very small (< 3%).
Scheme 1. Transformation of M(IV) pre-catalysts to active
M(VI) alkene metathesis catalysts
Thus, understanding how alkylidenes can be regenerated in-
situ from M(IV) molecular species would be valuable as it
could help to design methods for reactivation and ways to
improve overall catalysts performance.8 In view of the report-
ed metathesis activities of reduced group 6 metal sites in ho-
mogeneous systems and the high activities of the putative
W(IV)-oxo sites in supported systems, we reason that molecu-
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lar W(IV) complexes bearing oxo and alkoxide ligands would
be an ideal system to probe the initiation step and a source of
potentially efficient catalyst precursors.
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2), similar to the reaction of pyrrole complexes with various
alcohols.30 We have so far not been able to prepare the corre-
sponding OtBu or OC(CH3)2(CF3) complex by the approaches
described above. The three six-coordinated octahedral
WO(OR)2py3 complexes described above are diamagnetic and
share the same geometry with one of the OR groups on the
equatorial plane and the other one at the axial position trans to
the oxo ligand (see SI, Figure S2-4). Attempts to use other
weakly coordinating ligands instead of pyridine, such as THF
or dimethoxyethane, did not yield any desirable product.
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There are only few W(IV)-oxo complexes known: i)
WOCl2(PX3)3 (PX3 = P(OMe)3, PMe2Ph, PMePh2)27-28, which
can react with strained cyclopropenes to form metathesis ac-
tive vinylalkylidenes and ii) WO(OR)2(alkene) (OR = 2,6-
dimesitylphenoxide or 2,6-diadamantyl-4-methylphenoxide),
which can be formed upon decomposition of the correspond-
ing alkylidenes, but which has not been reported to engage in
alkene metathesis.29 Here, we report the preparation of mono-
meric d2 W(IV)-oxo complexes bearing electron-withdrawing
alkoxide and stabilizing pyridine ligands; they show high
activities in metathesis upon activation with B(C6F5)3. We
show by using a combined experimental and computational
approach that the activation mechanism for initiating alkene
metathesis and formation of alkylidene involves two key steps:
i) the C-H bond activation of an allylic C-H group that must be
present in the alkene reactant, and ii) a proton transfer process
facilitated by pyridine and B(C6F5)3.
9
Reaction of the complex 2a with cis-4-nonene (300
equiv/[W]) shows no activity in metathesis at 70 °C, even after
24 h. This can be ascribed to the absence of an empty coordi-
nation site due to the relative strong binding of pyridine in
these complexes.31 On the other hand, in the presence of a
Lewis acid, such as ZnCl2(dioxane) or B(C6F5)3, metathesis
activity is observed, presumably due to the removal of pyri-
dine. While equilibrium conversion is not reached with
ZnCl2(dioxane) (3 equiv/[W]) after 24 h (Figure S7), 3 equiv
of B(C6F5)3 leads to equilibrium conversion (51%) of cis-4-
nonene (300 equiv/[W]) within 12 h at 70 °C. The catalytic
activities of complexes 2a-2c in the presence of B(C6F5)3 are
shown in Table 1.
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RESULTS AND DISCUSSION
W(VI)-oxo complexes WOCl2(OR)2L (1a: R = CMe(CF3)2
= tBuF6 and L = pyridine = py, 1b: R = Si(OtBu)3 and L =
Et2O) were synthesized by salt metathesis of WOCl4 and 2
equiv of LiOR in diethyl ether (Scheme 2). The pure products
were obtained as colorless crystals in ca. 70% yield upon
recrystallization in pentane at –40 °C. The alkoxide groups are
trans to each other according to NMR spectroscopies and
single crystal X-ray diffraction studies (see SI).
Table 1. Catalytic activities of complexes 2a-2c.a
Catalyst
precursor /mol %
2a
Loading B(C6F5)
Substrate
TOFc
Equil.
Timed
12 h
3 h
12 h
3 /mol%
/min-1
0.3
1
4-noneneb
4-noneneb
4-noneneb
2
4
2
0.3
0.3
1.3
1
2b
2c
0.3
1
4-noneneb 15
0.5 h
0.5 h
Scheme 2. Synthetic routes for complexes 1a-1b, 2a-2c and
X-ray crystal structure of 2ba
0.3
1
1.3
3
4-noneneb 11
1-nonene
7
83%24 h
a1.0 M toluene solution, batch reactor, 70 °C; bcis-4-nonene; cTOF
is expressed as initial TOF after 3 min of reaction; dTime to reach
equilibrium conversion unless otherwise noted by giving the final
conversion after 24 h.
Similar activities are observed for the hexafluoro-tert-
butoxide (2a) and siloxide complexes (2b), but the perfluoro-
tert-butoxide complex (2c) shows a significantly higher activi-
ty (time to equilibrium < 30 min). These activities are compa-
rable to those reported for the corresponding pre-formed mo-
lecular W-oxo alkylidene complexes.32-36 With the terminal
alkene 1-nonene, the initial activity is similar to that of inter-
nal cis-4-nonene, but a significantly higher catalyst loading (1
mol% catalyst) is required to reach 83 % conversion. Increas-
ing the amount of B(C6F5)3 (4 equiv/[W]) further increases the
catalytic activity of complex 2a and reduces the time to reach
equilibrium conversion from 12 h to 3 h, while this approach
has little to no effect on 2c.
Encouraged by the promising catalytic results, we further
studied the activation mechanism involved in transforming
these W(IV)-oxo species into W(VI)-oxo alkylidenes. Several
activation mechanisms have been proposed for the formation
aThermal ellipsoids plot at 50% probability; hydrogen atoms
omitted.
The subsequent reduction of these compounds with KC8 at –
90 °C in the presence of pyridine yields their W(IV) analogues
WO(OR)2py3 (2a and 2b), which can be obtained as dark blue
crystals after recrystallization in pentane at –40 °C. Attempts
of using the same synthetic route to synthesize
WO(OC(CF3)3)2py3 (2c) resulted in a mixture of products.
However, pure 2c could be obtained as pure dark purple solid
from protonolysis of 2b with perfluoro-tert-butanol (Scheme
of active M(VI) (M = Mo or W) alkylidenes from M(IV) pre-
37
catalysts (Scheme 3).26,
The first possibility involves the
formation of a metallacyclopentane, which can subsequently
undergo ring contraction via β-H transfer followed by an in-
sertion to yield a metallacylcobutane that can participate in
metathesis.38-39 Alternatively, the second and third pathways
require a vinyl and an allylic C-H activation, respectively. The
fourth pathway involves a dimeric species, which has been
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proposed to undergo [2+2] cycloaddition with reactive olefins
(norbonene or diallylether).25 Since one of the mechanisms
should be specific to alkene containing the allylic CH group,
we first set to investigate the reactivity of complex 2a towards
alkenes with or without allylic CH groups.
Scheme 4. Reaction of 2a with ethylene in the presence of
a
B(C6F5)3 and X-ray crystal structure of 3a-C2H4
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Scheme 3. Proposed oxidative initiation mechanisms
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aThermal ellipsoids plot at 50% probability; hydrogen atoms
omitted. Yield of 4a is determined by 1H-NMR.
Reaction of 2a with propylene leads to the formation of the
propylene π complex 3a-C3H6 at room temperature. Its struc-
ture is confirmed by X-ray diffraction (Scheme 5). The major
isomer of 3a-C3H6 shows carbon resonances at δ 24.4 (–CH3),
66.0 (=CH2) and 72.4 (=CH–) ppm. In the presence of 3 equiv
of B(C6F5)3, the py-B(C6F5)3 adduct and 3a-C3H6 are formed
Reaction of 2a in C6D6 with ethylene leads to an immediate
color change from dark blue to yellow at room temperature.
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together with ethylene and 2-butene as shown by in-situ H-
By in-situ H-NMR, free pyridine and new proton signals at δ
3.04 and 2.84 ppm are observed. In the 13C-NMR spectrum, a
NMR studies (Figure S13). This result indicates that self-
metathesis of propylene (ca. 6 equiv/[W]) occurs readily and
reaches equilibrium in less than 15 min at room temperature.
characteristic singlet peak is observed at δ 65.3 ppm (JWC
=
22.9 Hz) indicating the formation of ethylene π complex, 3a-
C2H4 (Scheme 4).26, 40 3a-C2H4 can be isolated by recrystalli-
zation in toluene at –40 °C and its structure is confirmed by
single crystal X-ray diffraction studies. The C1=C2 bond
distance of the coordinated ethylene molecule is 1.41 Å, indi-
cating a significant back donation from the metal center to the
π* orbital of ethylene.41 In the presence of 3 equiv of B(C6F5)3,
the reaction of 2a and 13C-dilabeled ethylene yields the py-
B(C6F5)3 adduct, 3a-C2H4 along with a small amount of the
metallacyclopentane 4a (Scheme 4) at room temperature.
Heating the reaction mixture to 50 °C leads to complete con-
version of 3a-C2H4, removal of most of the coordinated pyri-
dine from W centers to give py-B(C6F5)3 (ca. 3 equiv/[W]) and
increases the amount of 4a. The complex 4a gives rise to two
Scheme 5. Reaction of 2a with propylene in the presence of
a
B(C6F5)3 and X-ray crystal structure of 3a-C3H6
1
broad peaks at ca. δ 3.0 ppm and 2.8 ppm in the H-NMR
spectrum. These signals correlate with the carbon resonances
at δ 74.8 (α-C) and 36.4 ppm (β-C), respectively, in the 13C-
NMR spectrum according to 1H-13C HSQC experiment (Figure
S10-S12). These resonances are similar to those reported for
other metallacyclopentane complexes of Mo and W.26, 29, 40-43
No peak associated with the metallacyclobutane or alkylidene
aThermal ellipsoids plot at 50% probability; hydrogen atoms
omitted.
Notably, the reaction of 2a with styrene in the presence of 3
equiv of B(C6F5)3 does not lead to the formation of self-
metathesis products, even upon heating at 70 °C for 13 h, as
shown by in-situ 1H-NMR and GC-MS. In contrast, reaction of
2a, B(C6F5)3 and β-methylstyrene results in the formation of 2-
butene and stilbene under the same conditions. The complex
2a also reacts with allylbenzene in the presence of B(C6F5)3 to
1
is observed in H and 13C NMR, even upon further heating at
70 °C. While a small amount of 1-butene is observed, likely
resulting from the decomposition of 4a after prolonged heating
(Figure S10), this process seems to be relatively slow. The
rearrangement of metallacyclopentane to 1-butene has also
form
ethylene
and
1,4-diphenyl-2-butene
been reported in several high valent metallacyclopentane
40, 42, 44
complexes.26,
The complex 4a slowly decomposes
PhCH2(CH=CH)CH2Ph, in less than 45 min at room tempera-
ture (Scheme 6).
under high vacuum (10-5 mbar) or in the absence of ethylene
making its isolation unsuccessful so far. The absence of metal-
lacyclobutane may also suggest that 4a does not undergo ring
contraction readily (Scheme 3; first pathway).
All these results suggest that the presence of an allylic C-H
group is probably crucial for the conversion of the W(IV)-oxo
pre-catalyst into the active species. In order to gain more in-
sights into the initiation mechanism, we decided to conduct a
detailed investigation of the reaction between 2a and trans-β-
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methylstyrene, in particular by monitoring the organic prod-
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ucts formed during the activation period.
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Scheme 6. Reactions of 2a with styrene, β-methylstyrene
and allylbenzene in the presence of B(C6F5)3
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2
Figure 2. Left: H-NMR spectrum at =CH region (for full spec-
trum, see Figure S17). Right: Concentration of styrene (normal-
ized by the no. of mole of [W]) vs time plot with trans-β-
methylstyrene (red diamond) and trans-β-(methyl-d3)styrene (dark
blue circle) as substrates.
When monitoring the self-metathesis reaction of trans-β-
methylstyrene (6 equiv/[W]) catalyzed by 2a in the presence
of B(C6F5)3 (3 equiv/[W]), a small amount of styrene (0.05
equiv/[W]) is observed as a primary product according to GC-
MS and no further formation is observed after ca. 30 min. In
contrast, the formation of self-metathesis products is observed
after a short induction period (Figure 1).
In order to understand better how styrene is formed during
the reaction and whether or not C-H activation is indeed an
important elementary step in the initiation process, the reactiv-
ity of trans-β-(methyl-d3)styrene with 2a was also investigat-
ed. According to in-situ 2H-NMR studies (Figure 2), β-
deuteriostyrene is observed upon reaction of trans-β-(methyl-
d3)styrene and 2a in the presence of 3 equiv of B(C6F5)3. This
observation further supports the proposed allyl hydride initia-
tion mechanism (eq. 1). The initial rates of formation of initia-
tion products with trans-β-(methyl-d3)styrene and trans-β-
methylstyrene show a kH/kD of 3.6 (Figure 2; see SI for de-
tails), a primary kinetic isotopic effect (KIE), which suggests
the involvement of the allylic hydrogen atom in the rate-
determining step.45 Notably, the amount of styrene formed
(0.01 equiv/[W]) upon using trans-β-(methyl-d3)styrene is
much lower than upon using trans-β-methylstyrene. The
smaller amount of detected deuterated initiation product could
be due to the slower rate of initiation for the deuterated alkene
while decomposition could occur at similar rates for both
deuterated and non-deuterated alkene during initiation. Such
decomposition is likely one of the reasons for having intrinsi-
cally small amounts of active species. Even though the number
of active species formed by using trans-β-(methyl-d3)styrene
is lower, the conversion of self-metathesis is slightly higher
than that of trans-β-methylstyrene (Figure S19). This may be
due to the advantageous isotope effect that can reduce deacti-
vation via β-H elimination of the metathesis active metallacy-
clobutane intermediates.40, 46-48
Figure 1. Formation of styrene (dark blue circle), trans-stilbene
(red diamond) and conversion (orange cross) vs time plot at 70
°C.
These results support that styrene is an initiation product
formed during the formation of the active alkylidene species.
In addition, a small amount of propylene is observed in the gas
phase by GC-FID implying the formation of a W-methylidene
and its subsequent reaction with trans-β-methylstyrene or 2-
butene. The observation of styrene as an initiation product
points to the allyl hydride mechanism as the initiation step for
alkylidene formation (Scheme 3). No other heavier organic
product is observed suggesting that other initiation mecha-
nisms are unlikely (Scheme S1). The observation of only a
substoichiometric amount of styrene (0.05 equiv/[W]) sug-
gests that only small amounts of active species are formed
upon activation.
Similarly, the reaction of 2a with allylbenzene (6
equiv/[W]) in the presence of B(C6F5)3 (3 equiv/[W]) also
yields styrene which further supports the same allyl hydride
intermediate. Meanwhile, isomerization of allylbenzene into
the thermodynamically more stable trans-β-methylstyrene is
also observed during this reaction. The observed double bond
isomerization is known to possibly involve a π-allyl metal
hydride intermediate.49 Monitoring the reaction by in-situ H-
1
NMR indicates that the isomerization process occurs prior to
metathesis (Figure S22). This result suggests that the for-
mation of a metallacyclobutane from an allyl hydride is slower
than the isomerization process.
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Figure 3. Computed Gibbs free energies and enthalpies in parentheses at 298.15 K (in kcal mol-1) for initiation pathways via allyl hydride
intermediates (Red: non-assisted; Green: assisted) involved in the reaction between 2a and trans-β-methylstyrene (B3LYP-D3/SDD-
TZVP-SMD(Benzene)).
To further probe the allylic C-H activation mechanism, DFT
calculations were performed using 2a and trans-β-
methylstyrene in light of the kinetic findings mentioned above.
The calculations show that formation of the pyridine free π
complex (IM-1) is thermodynamically favorable in the pres-
ence of B(C6F5)3 (Figure S42). The free energy barrier re-
quired for activating the allylic C-H bond (TS-1) to form
W(VI) η1-allyl hydride intermediate (IM-2) from the π com-
plex (IM-1) is 22.7 kcal/mol (Figure 3). IM-2 further converts
into its isomer IM-3 (ΔG = -8.4 kcal/mol). However, transfer-
ring the hydrogen atom to the β-carbon on the allyl group to
form a metallacyclobutane requires a very high free energy
barrier (TS-3A: ΔG‡ = 36.0 kcal/mol with an overall barrier of
ΔG‡ = 47.1 kcal/mol), making this direct process unlikely.
Since H-transfer is the most energy demanding step, we also
consider additional factors that might play a role in facilitating
this process. However, coordination of B(C6F5)3 to the oxo
ligand or B(C6F5)3 assisted H-transfer are both high in energies
(Figure S43). In particular, the free energy required for ab-
stracting the H by B(C6F5)3 is ΔG‡ = 30.2 kcal/mol with an
e all ba ie f ΔG‡ = 41.3 kcal/mol (TS-3D; Figure S43),
suggesting that the H atom may not have a strong hydridic
character and the subsequent formation of cationic W center is
unfavorable.
the energy required for abstracting H by pyridine to form a
pyridinium (PyH+) intermediate is shown to be quite high
(ΔG‡ = 26.4 kcal/mol) (TS-3E; Figure S44), the energy barrier
is significantly lowered (TS-3B: ΔG‡ = 11.2 kcal/mol) upon
coordination of B(C6F5)3 onto the oxo ligand (Figure 3). This
is likely facilitated by the decrease in electron density at the W
ce e , a d he ce i c ea i g he acidi f he ―h d ide‖ lig-
and. Furthermore, coordination of B(C6F5)3 can also stabilize
the anionic W center formed after H abstraction (IM-4B lies
18.0 kcal/mol above the reference state IM-1). While protona-
tion of the β-carbon by PyH+ (TS-4B) is the highest in energy
for this pathway ( e all e e g ba ie f ΔG‡ = 30.7
kcal/mol), it lies ig ifica l l we (ΔΔG‡ = -16.4 kcal/mol)
than TS-3A that corresponds to the non-assisted formation of
metallacyclobutane. Note that the entropy and resulting free
energy of activation for TS-4B is likely overestimated since it
involves the dissociation and reaction of py-B(C6F5)3 in solu-
tion (for detailed discussion see SI),50 making this proton
transfer step accessible. In fact, this step is similar to the re-
cently reported reverse elementary step involved in the de-
composition pathway of ruthenium metallacyclobutane in-
duced by base.51 Such proton transfer reactions (protonation or
deprotonation) on the β-carbon are consistent with its formally
positively charged nature as recently evidenced by detailed
solid-state NMR analysis of trigonal bipyramidal metallacy-
clobutane intermediates.31 In addition, the calculated KIE on
this rate-determining proton transfer step (TS-4B) ((kH/kD)DFT
= 3.7) matches well with the experimentally observed KIE
((kH/kD) = 3.6). The thus-formed metallacyclobutane (IM-6B)
Alternatively, pyridine can abstract the H from W to form
an anionic π-allyl intermediate along with a pyridinium
(PyH+), which can then transfer the proton to the β-carbon of
the allyl group and thus generate a metallacyclobutane. While
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can then undergo cycloreversion to give alkylidenes. The
combination with Lewis acids and strained surface sites could
play a similar role. We are currently investigating this possi-
bility.
1
2
3
4
5
6
7
8
overall initiation process is endergonic by 10.8 and 13.6
kcal/mol with respect to the π complex IM-1 (Figure S47)
which explains the observation of only a small amount of
initiation product and the difficulty to detect the propagating –
alkylidenes and metallacyclobutanes – species by NMR. The
small amounts of alkylidenes formed will participate in the
catalytic conversion of β-methylstyrene into stilbene and 2-
butene.
Supporting Information. The Supporting Information is availa-
ble free of charge on the ACS Publications website. Full experi-
mental and computational details and catalytic data. (PDF)
All the data above point to an initiation mechanism involv-
ing an allylic C-H activation by the W(IV) pre-catalyst. The
thus-formed allyl hydride intermediate generates the required
metallacyclobutane for metathesis by a proton transfer which
is assisted by both pyridine and B(C6F5)3. From this metal-
lacyclobutane, cycloreversion yields a putative W(VI) alkyli-
dene, which is active in metathesis, along with the olefinic
initiation product. Both the observed primary KIE and the
calculated highest energy transition state indicate that proton
transfer is the rate-determining step in the activation process
yielding metallacyclobutanes and alkylidenes (Scheme 7).
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* ccoperet@ethz.ch
The authors are grateful to the Swiss National Foundation (SNF)
for financial support of this work (grant no. 200021L_157146)
and to the French National Agency (ANR MASCAT, grant no.
ANR-14-CE36-0010-01). This work was granted access to the
HPC resources of TGCC and IDRIS under the allocation 2017-
080609 made by GENCI.
Scheme 7. Proposed initiation mechanisma
Dr. Keith Searles is thanked for fruitful discussions. Christopher
P. Gordon is thanked for providing NMR assistance.
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aW = W containing organometallic fragment
CONCLUSION
We have reported the first example of well-defined mono-
meric d2 W(IV)-oxo bisalkoxide complexes that can initiate
alkene metathesis and show comparable catalytic activities to
W(VI)-oxo alkylidene complexes. Detailed studies of the
initiation step including deuterium-labeling have shown that
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