Reversible N-Heterocyclic Carbene-Induced α-H Abstraction in Tungsten(VI) Imido Dialkyl Dialkoxide Complexes

Article abstract of DOI:10.1002/chem.202000840

The first reversible N-heterocyclic carbene (NHC) induced α-H abstraction in tungsten(VI) imido-dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH₂Ph)₂(OtBu)₂ (Ar=2,6-dichlorophenyl, 2,6-dimethylphenyl, 2,6-diisopropylphenyl) with different NHCs leads to the formation of complexes of the type W(NAr)(CHPh)(NHC)(CH₂Ph)(OtBu) in excellent isolated yields of up to 96 %. The highly unusual release of the tert-butoxide ligand as tBuOH in the course of the reaction was observed. The formed alkylidene complexes and tBuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by ¹H NMR spectroscopy. A correlation between the steric and electronic properties of the NHC and the reaction rates was observed. Kinetics of a deuterium-labeled complex in comparison to its non-deuterated counterpart revealed the presence of a strong primary kinetic isotope effect (KIE) of 4.2, indicating that α-H abstraction is the rate-determining step (RDS) of the reaction.

Full text of DOI:10.1002/chem.202000840

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Accepted Article
Title: Reversible N-Heterocyclic Carbene-Induced α-H Abstraction In
Tungsten (VI) Imido Dialkyl Dialkoxide Complexes
Authors: Janis V. Musso, Mathis Benedikter, Dongren Wang, Wolfgang
Frey, Hagen Altmann, and Michael Rudolf Buchmeiser
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10.1002/chem.202000840
Chemistry - A European Journal
Reversible N-Heterocyclic Carbene-Induced α-H Abstraction In
Tungsten (VI) Imido Dialkyl Dialkoxide Complexes
Janis V. Musso1, Mathis J. Benedikter1, Dongren Wang1, Wolfgang Frey3, Hagen J. Altmann1, Michael R. Buchmeiser*1,2
1Institute of Polymer Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
2German Institutes of Textile and Fiber Research (DITF) Denkendorf, Körschtalstr. 26, D-73770 Denken-
dorf, Germany.
3Institute of Organic Chemistry, University of Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany.
ABSTRACT:
The first reversible N-heterocyclic carbene (NHC) induced α-H abstraction in tungsten (VI) imido-
dialkyl dialkoxide complexes is reported. Treatment of W(NAr)(CH2Ph)2(OtBu)2 (Ar = 2,6-dichlorophenyl, 2,6-
dimethylphenyl, 2,6-diisopropylphenyl) with different NHCs leads to the formation of complexes of the type
W(NAr)(CHPh)(NHC)(CH2Ph)(OtBu) in excellent isolated yields of up to 96%. The highly unusual release of the
tert-butoxide ligand astBuOH in the course of the reaction was observed. The formed alkylidene complexes and
tBuOH are in an equilibrium with the NHC and the dialkyl complexes. Reaction kinetics were monitored by 1H
NMR. A correlation between the steric and electronic properties of the NHC and the reaction rates was observed.
Kinetics of a deuterium-labeled complex in comparison to its non-deuterated counterpart revealed the presence
of a strong primary kinetic isotope effect (KIE) of 4.2, indicating that α-H abstraction is the rate-determining step
(RDS) of the reaction.
t
2
Since the characterization of the first tungsten-car-
bene complexes by Fischer in 19641 the number of
group 6 carbene complexes has steeply increased and
yielded a variety of highly applicable developments2-
3, one of the most prominent being the discovery of
olefin metathesis-active homogeneous group 6 cata-
lysts by Schrock.4 The key step in the synthesis of
those catalysts is the formation of an alkylidene com-
plex which is ‒ among alternative routes5-7 ‒ usually
achieved by the intramolecular abstraction of an α-
hydrogen atom of an alkyl ligand and elimination of
the hydrogen-accepting ligand.8-9 This process is be-
lieved to be induced by a sterically crowded coordi-
nation sphere around the metal atom.10-11 Nomura and
Zhang reported the successful use of NHCs for the α-
H abstraction of vanadium (V) complexes of the type
V(NR)(CH2SiMe3)3 to the corresponding alkylidene
complexes V(NR)(CHSiMe3)(NHC)(CH2SiMe3) [R = 1-
adamantyl, 2,6-Me2C6H5, NHC = 1,3-bis(2,6-diiso-
Cl2C6H5)(CHPh)(IMes)(CH2Ph)(OBu) ( ) in benzene at
room temperature, which was obtained as a 1:1 mix-
ture of the corresponding syn- and anti-isomer
(1JCH(syn) = 118.9 Hz, JCH(anti) = 140.1 Hz, Scheme
1
2).
Scheme 1. Nomuras NHC-Induced -H Abstraction
α
in Vanadium(V) Complexes.
R
R
Ar
Ar
N
N
N
V
Ar
N
V
N
CHSiMe3
Me3SiCH2
CH2SiMe3
benzene, 80 °C, 18 h
15-60%
CH2SiMe3
Me3SiCH2
N
Ar
R = 2,6-dimethylphenyl, adamantyl
Ar = 2,6-iPr2C6H3
Scheme 2. Reaction of 1 With IMes to 2 Under Con-
comitant Elimination of tBuOH.
propylphenyl)imidazole-2-ylidene]
(Scheme
1).12
Those results encouraged us to initiate studies to-
wards the NHC-induced α-H abstraction in tung-
sten(VI) imidodialkyl complexes in order to find a
more direct route to highly metathesis-active neutral
and cationic tungsten imido alkylidene NHC com-
plexes similar to those that have previously been syn-
At a first glance, the involvement of one of the tBuO
ligands in the α-H abstraction instead of the alkyl
group and its eliminated as tBuOH appears surprising.
However, the rather shielded a-carbon in the dialkyl
species (d = 66.1 ppm) point towards a low alkylidene
character of the M-C bond and a full involvement of
the p-orbital in the M-C, C-H and C-C bonds. In turn,
this leads to a lower basicity compared to the fully
developed lone pair of the alkoxide.14 Notably, for a
thesized by our group.13 To our delight, the reaction
t
1
of W(N-2,6-Cl2C6H5)(CH2Ph)2(OBu)2 ( ) with 1,3-
dimesitylimidazol-2-ylidene (IMes) led to the rapid
formation of the alkylidene complex W(N-2,6-
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10.1002/chem.202000840
Chemistry - A European Journal
quantitative description, the values for d11, d22 and d33
would have to be determined. Although the elimina-
tion of alkoxides in silica-supported metathesis cata-
lysts has been proposed as a possible deactivation
pathway in computational studies,15-16 to the best of
our knowledge, this is the first case in organometallic
chemistry in which the elimination of an alkoxide lig-
and is exploited synthetically while the benzyl ligand
does not act as an acceptor for the eliminated hydro-
gen as observed in α-H abstraction reactions of other
tungsten (VI) imido dialkyl dialkoxide complexes.9
Scheme 3. Substrate Scope of NHC-Induced -H abstraction.
α
1
initial concentration of 37 mM of IMes and , the re-
action reached its equilibrium state at 40% conversion
after 90 min. We hypothesize that the eliminated
tBuOH is evaporated to a large extent in course of the
work-up procedure, thereby shifting the equilibrium
towards the product side, which serves as an expla-
nation for the isolated yield of 78%. To prove this as-
sumption, tBuOH was added to the isolated alkylidene
2
complex , which indeed resulted again in the de-
scribed equilibrium.
2
Single crystals of were grown from a pentane so-
lution at -35 °C. A thermal ellipsoid drawing of the
Figure 1. Single crystal X-ray structure of
2
displayed as
2
structure is depicted in Figure 1. crystallizes in the
a thermal ellipsoid plot (50% probability). Relevant bond
lengths (pm) and angles (°): W(1)-N(3) 1.704(4), W(1)-
O(1) 1.920(4), W(1)-C(32) 1.911(5), W(1)-C(39) 2.250(7),
W(1)-C(1) 2.312(6), C(1)-N(1) 1.373(9), C(1)-N(2)
1.371(9); N(3)-W(1)-O(1) 148.9(3), N(3)-W(1)-C(32)
104.9(4), O(1)-W(1)-C(32) 106.1(3), N(3)-W(1)-C(39)
91.7(2), O(1)-W(1)-C(39) 87.3(2), C(32)-W(1)-C(39)
90.0(4), N(3)-W(1)-C(1) 94.3(2), O(1)-W(1)-C(1) 83.1(2),
C(32)-W(1)-C(1) 96.8(3), C(39)-W(1)-C(1) 169.5(2), N(1)-
C(1)-N(2) 102.5(5).
monoclinic space group, P21/n, a = 1031.21(3) pm, b
= 2104.93(8) pm, c =1896.91(5) pm, α = γ = 90°, β
2
= 91.105(2)°, Z = 4. In the solid state,
adopts a
square pyramidal (SP) configuration with the alkyli-
dene ligand occupying the apical position and all
other ligands laying in the equatorial plane. The ste-
rical bulk of the mesityl groups leads to a high degree
of distortion of the SP configuration, which is indi-
cated by a calculated τ5 value17 of 0.34. The NHC
takes the position trans to the benzyl ligand, which
shows a rather long bond distance for a W-C single
bond of 231.2(6) pm compared to other tungsten alkyl
1
The reaction of with IMes was conducted again in
1
C6D6 and monitored by H-NMR spectroscopy. At an
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10.1002/chem.202000840
Chemistry - A European Journal
complexes.9 This can be explained by a structural
trans effect induced by the strong σ-donor properties
of the NHC.
Similar to , in the solid state,
2
11
adopts a SP config-
uration and crystallizes in the monoclinic space
group, P21/c, a = 2053.71(9) pm, b = 961.56(4) pm, c
= 2024.87(9) pm, α = γ = 90°, β = 112.431(2)°, Z =
4 (Figure 2). The τ5 value was 0.016, indicating an
almost perfect SP configuration. It can be assumed
that the lower degree of distortion originates from the
lower steric demand of the isopropyl groups of the
A screening of different NHCs and tungsten com-
plexes with a variation of the imido ligands is depicted
12
in Scheme 3. With the exception of , all complexes
were obtained in good to excellent isolated yields be-
13
10
tween 69% ( ) and 96% ( ). Reactions were con-
ducted in benzene at room temperature and gave ac-
cess to the desired products in short reaction times of
‒ in most cases - only several minutes to a few hours.
We assume that a 2,6-substitiution pattern of the im-
ido-ligand contributes to a crowded coordination
sphere and is therefore required for an efficient α-H
abstraction, although we have no experimental evi-
dence, since the preparation of the analogous W(N-
3,5-dichlorophenyl)(CH2Ph)2(OtBu)2 and W(N-3,5-di-
2
NHC compared to . The lower steric bulk of IPr is
reflected by higher reaction rates compared to IMes
for R2 = Me (Table 1).
Table 1. Reaction Kinetics of NHC-Induced -H Ab-
α
straction.
k25 °C
R1 = Cy
R1 = iPr
R1 = Mes
(L.mol-1.s-1)
methylphenyl)(CH2Ph)2(OtBu)2
complexes
failed.
R2 = Cl
R2 = Me
R2 = iPr
2.72
9.24
6.31
± 0.06.10-1
± 0.19.10-2
± 1.3.10-3
While the reaction of the analogous W (VI) imido
dineophyl dialkoxide complexes with various NHCs
also resulted in the desired α-H abstraction, the prod-
ucts turned out to be unstable at room temperature
and readily decomposed before full characterization
could be completed.
1.12
4.93
4.93
± 0.01.10-2
± 0.09.10-3
± 0.15.10-4
7.51
4.93
n.d.1
± 0.07.10-4
± 0.15.10-4
It was observed that both the reaction rates and yields
1no conversion.
highly depend on the combination of the imido ligand
6
the NHC used. Thus, the reaction of IMes and re-
It was anticipated that the reaction kinetics correlate
with the Tolman electronic parameter (TEP)18-19 and
the percent Buried volume (%Vbur)20-21 of the NHC. The
kinetics of several reactions were therefore monitored
by 1H NMR and compared to the steric and electronic
properties of the NHCs used as summarized in the lit-
erature (Table 2).18, 21 The combination of the compa-
rably small and electron-withdrawing 2,6-dichloro-
phenylimido ligand with 1,3-dicyclohexylimidazol-2-
ylidene (ICy) and IPr, both having a %Vbur of 27.5 and
a TEP of 2049.5 cm-1 and 2050.3 cm-1, respectively,
quired several days to reach equilibrium and resulted
only in a moderate yield of 58% whereas no conver-
sion was observed for the analogous reaction with the
more sterically demanding 1,3-diisopropylimidazol-2-
ylidene (IPr).
.
resulted in fast reaction rates with k25 °C = 2.27 10-1
.
.
.
L.mol-1 s-1 and k25 °C = 9.24 10-1 L.mol-1 s-1 that were push-
ing the limits of reaction monitoring by 1H NMR. The
rate constant for the reaction of the same complex
with the more sterically demanding IMes (36.5 %Vbur)
was determined as k25 °C = 6.31 10-3 L.mol-1 s-1. This
.
.
3
trend continues in the reactions of . With ICy, the
highest reaction rate of k25 °C = 1.12 10-2 L.mol-1 s-1 was
.
.
observed, while the reaction rates with IPr and IMes
.
were determined as k25 °C = 4.93 10-3 L×mol-1×s-1and
Figure 2. Single crystal X-ray structure of 11 displayed
as a thermal ellipsoid plot (50% probability). Relevant
bond lengths (pm) and angles (°): W(1)-N(3) 1.7756(19),
W(1)-O(1) 1.9049(16), W(1)-C(22) 1.912(2), W(1)-C(29)
2.225(2), W(1)-C(1) 2.279(2), C(1)-N(1) 1.359(3), C(1)-
k25 °C = 4.93 10-4 L.mol-1 s-1. Lastly, the reactions of
.
.
4
with IPr and ICy, respectively, revealed smaller rate
3
constants compared to the analogous reactions of ,
which can be explained by the higher steric bulk of
the isopropyl groups. This can also be considered the
N(2),
N(3)-W(1)-O(1)
154.72(8),
N(3)-W(1)-C(22)
101.39(10), O(1)-W(1)-C(22) 103.70(9), N(3)-W(1)-C(29)
94.02(9), O(1)-W(1)-C(29) 86.79(8), C(22)-W(1)-C(29)
95.31(10), N(3)-W(1)-C(1) 87.05(8), O(1)-W(1)-C(1)
82.20(8), C(22)-W(1)-C(1) 108.30(9), C(29)-W(1)-C(1)
155.70(9), N(1)-C(1)-N(2) 103.7(2).
4
reason for the poor reactivity of and IMes, for which
no conversion was observed. In all the monitored re-
actions, no intermediates were observed by NMR;
however, their formation could not be fully ruled out
3
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10.1002/chem.202000840
Chemistry - A European Journal
1
at that stage. Therefore, two reaction pathways
seemed plausible (Scheme 4).
the non-deuterated complex
1d
and the deuterated
complex
revealed a kinetic isotope effect (KIE) of
kH/kD = 4.2. This can be considered a strong primary
KIE25 and is an indication that C-H bond cleavage is
involved in the RDS.26 Consequently, coordination of
the NHC in pathway B cannot be rate determining.
Together with the mechanistic considerations and the
Table 2. Buried Volumes (%Vbur) and Tolman Elec-
tronic Parameters (TEP) of the NHCs Used.18, 21
%VBur (@ 2 Å, AuCl(NHC))
TEP (cm-1)
1
observed absence of intermediates in H NMR, we
R1 = iPr
R1 = Cy
27.5
27.5
36.5
2050.3
Pathway A
conclude that a concerted mechanism (
) is
operative.
2049.5
Scheme 5. Reaction of Deuterated Complex 1d With
IMes.
R1 = Mes
2049.6
Scheme 4. Plausible Reaction Pathways for a-H Ab-
straction.
Based on the data outlined above, another reaction
mechanism that comprises the deprotonation of the
dialkyl complex by the NHC (Scheme 6) is also highly
unlikely since signals of intermediary imidazolium
salts could not be observed by 1H NMR.
Scheme 6. Reaction Mechanism Involving an Imid-
azolium Salt.
Mes
Cl
Ph
Mes
N
N
H
Cl
Cl
Mes
Mes
N
tBuO
tBuO
slow
N
N
N
W
OtBu
OtBu
PhCH2
PhCH2
Cl
W
Ph
1
Intermediate 2
- tBuOH
Cl
One feasible pathway proceeds via a concerted
fast
Mes
W
Ph
N
Pathway A
mechanism (
). The NHC binds to the metal
N
and at the same time tBuOH is eliminated without the
formation of intermediates. In this case, C-H bond
cleavage is involved in the rate-determining step
(RDS). On the contrary, a two-step mechanism is plau-
sible, in which the binding of the NHC results in the
N
tBuO
Cl
Mes
Ph
2
In this hypothetical case, the deprotonation step of
1
Intermediate 2
has to be
the dialkyl complex
to
Intermediate 1
formation of
step, undergoes α-H abstraction (
step has to be slow and therefore the RDS since the
Intermediate 1
which, in a consecutive
slow and therefore rate-determining. The subsequent
Pathway B
). The first
protonation of an OtBu-ligand, on the other hand,
Intermediate 2
would have to be fast since
was not
buildup of
was not observed by NMR.
1
observed by H NMR. There are several indications,
however, that this mechanism is not operative. Nomu-
ras investigations on the NHC induced α-H abstrac-
tion of vanadium(V) trialkyl complexes show that, in
some cases, the extremely basic NHCs27-28 with pKb
values in the range of -6.8 to -11.218 are interchange-
able with only mildly basic phosphines such as PMe329
having a pKb of 5.35 (pKb values calculated from the
pKa values of the conjugated acids in aqueous solu-
tion).30-31 In addition, our kinetic measurements sug-
gest that the rate constant is in correlation with the σ-
donor properties of the used NHCs whereas no link to
As shown by Schrock et al., deuterium-labeling can
be an efficient tool for the elucidation of the mecha-
nistic details of the formation of tungsten alkylidene
complexes.22 Therefore, the deuterium-labeled com-
1d
plex
was synthesized from commercially available
toluene-d8 in a radical chlorination reaction using sul-
furyl chloride,23 subsequent synthesis of benzyl-
magnesium chloride-d724 which in the reaction with
W(N-2,6-dichlorolphenyl)Cl2(OtBu)2(THF) gave access
1d
1d
and IMes
to
. The kinetics of the reaction of
were monitored with the aforementioned technique to
obtain a rate constant of k25 °C = 1.49 (± 0.02) 10-3
.
L.mol-1.s-1 (Scheme 5). A comparison of the kinetics of
4
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10.1002/chem.202000840
Chemistry - A European Journal
metal alkylidene complexes. Acc. Chem. Res. 1990, 23, 158-
165.
basicity was found. The combination of those consid-
erations again leads us to conclude that a concerted
mechanism is present.
5.
Johnson, L. K.; Virgil, S. C.; Grubbs, R. H.; Ziller,
J. W., Facile tungsten alkylidene synthesis: alkylidene trans-
fer from a phosphorane to a tungsten imido complex. J. Am.
Chem. Soc. 1990, 112, 5384-5385.
In summary, the first reversible NHC induced α-H
abstraction in tungsten (VI) imido dialkyl complexes
has been accomplished in excellent isolated yields of
up to 96%. The reaction sequence can be considered
highly atom-economic compared to previously pub-
lished routes to W-alkylidene complexes6, 9, 32-33 since
the only byproducts in the 4-step sequence from
6.
Bryan, J. C.; Mayer, J. M., Oxidative addition of
carbon-oxygen and carbon-nitrogen double bonds to
WCl2(PMePh2)4. Synthesis of tungsten metallaoxirane and
tungsten oxo- and imido-alkylidene complexes. J. Am.
Chem. Soc. 1990, 112, 2298-2308.
7.
Johnson, L. K.; Grubbs, R. H.; Ziller, J. W., Synthe-
t
W(O)Cl4 are CO2, LiCl, MgCl2 and BuOH. We found
sis of tungsten vinyl alkylidene complexes via the reactions
of WCl2(NAr)(PX3)3 (X = R, OMe) precursors with 3,3-disub-
stituted cyclopropenes. J. Am. Chem. Soc. 1993, 115, 8130-
8145.
that the reaction kinetics correlate with the electronic
and steric properties, both of the imido ligand and the
NHC, and the α-H abstraction is the RDS of the reac-
tion. The formed alkylidene complexes are excellent
candidates for the synthesis of highly metathesis ac-
tive cationic complexes similar to those that have
been published previously by our group13, 34-36 since
they already contain the NHC that is required for the
delocalization of the positive charge.37 Currently, we
are working on those cationic complexes; results will
be reported in due course.
8.
Schrock, R. R.; Murdzek, J. S.; Bazan, G. C.; Rob-
bins, J.; DiMare, M.; O'Regan, M., Synthesis of molybdenum
imido alkylidene complexes and some reactions involving
acyclic olefins. J. Am. Chem. Soc. 1990, 112, 3875-3886.
9.
Schrock, R. R.; DePue, R. T.; Feldman, J.; Yap, K.
B.; Yang, D. C.; Davis, W. M.; Park, L.; DiMare, M.;
Schofield, M., Further studies of imido alkylidene complexes
of tungsten, well-characterized olefin metathesis catalysts
with controllable activity. Organometallics 1990, 9, 2262-
2275.
ASSOCIATED CONTENT
10.
Nugent, W. A.; Mayer, J. M.; Mayer, J. M., Metal-
ligand Multiple Bonds: The Chemistry of Transition Metal
Complexes Containing Oxo, Nitrido, Imido, Alkylidene or
Alkylidene Ligands. Wiley New York: 1988.
Experimental details and characterization data. The
Supporting Information is available free of charge
from the publisher.
11.
Feldman, J.; Schrock, R. R., Recent advances in the
2
5
11
CCDC 1981737 ( ), 1981738 ( ) and 1981739 ( )
chemistry of d0 alkylidene and metallacycobutane com-
plexes. Prog. Inorg. Chem. 1991, 39, 1-74.
contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge
from the Cambridge Crystallographic Data Centre.
12.
Zhang, W.; Nomura, K., Facile Synthesis of (Im-
ido)vanadium(V)-Alkyl, Alkylidene Complexes Containing
an N-Heterocyclic Carbene Ligand from Their Trialkyl Ana-
logues. Organometallics 2008, 27, 6400-6402.
AUTHOR INFORMATION
13.
Imbrich, D. A.; Elser, I.; Frey, W.; Buchmeiser, M.
Corresponding Author
R., First Neutral and Cationic Tungsten Imido Alkylidene N-
Heterocyclic Carbene Complexes. ChemCatChem 2017, 9,
2996-3002.
1,2michael.buchmeiser@ipoc.uni-stuttgart.de
Notes
14.
Gordon, C. P.; Yamamoto, K.; Searles, K.; Shirase,
S.; Andersen, R. A.; Eisenstein, O.; Copéret, C., Metal alkyls
programmed to generate metal alkylidenes by α-H abstrac-
tion: prognosis from NMR chemical shift. Chem. Sci. 2018,
9, 1912-1918.
The authors declare no competing financial interest.
ACKNOWLEDGMENT
Financial support by the Deutsche Forschungsgemein-
schaft DFG (grant numbers BU2174/22-1 and 358283783
‒ CRC 1333) is gratefully acknowledged.
15.
Leduc, A.-M.; Salameh, A.; Soulivong, D.; Chaba-
nas, M.; Basset, J.-M.; Copéret, C.; Solans-Monfort, X.; Clot,
E.; Eisenstein, O.; Böhm, V. P. W.; Röper, M., β-H Transfer
from the Metallacyclobutane: A Key Step in the Deactivation
and Byproduct Formation for the Well-Defined Silica-Sup-
ported Rhenium Alkylidene Alkene Metathesis Catalyst. J.
Am. Chem. Soc. 2008, 130, 6288-6297.
REFERENCES
1.
Fischer, E. O.; Maasböl, A., On the Existence of a
Tungsten Carbonyl Carbene Complex. Angew. Chem. Int.
Ed. 1964, 3, 580-581.
16.
Solans-Monfort, X.; Coperet, C.; Eisenstein, O.,
2.
from
Dötz, K. H., Synthesis of the Naphthol Skeleton
Pentacarbonyl-[methoxy(phenyl)carbene]chromium
Shutting Down Secondary Reaction Pathways: The Essential
Role of the Pyrrolyl Ligand in Improving Silica Supported
d0-ML4 Alkene Metathesis Catalysts from DFT Calculations.
J. Am. Chem. Soc. 2010, 132, 7750-7757.
(O) and Tolan. Angew. Chem. Int. Ed. 1975, 14, 644-645.
3.
Gómez-Gallego, M.; Mancheño, M. J.; Sierra, M.
A., Catalytic Transmetalation from Group 6 Fischer Carbene
Complexes:ꢀ An Emerging Powerful Tool in Organic Synthe-
sis. Acc. Chem. Res. 2005, 38, 44-53.
17.
Addison, A. W.; Rao, T. N.; Reedijk, J.; van Rijn,
J.; Verschoor, G. C., Synthesis, structure, and spectroscopic
properties of copper(II) compounds containing nitrogen‒sul-
phur donor ligands; the crystal and molecular structure of
4.
Schrock, R. R., Living ring-opening metathesis
polymerization catalyzed by well-characterized transition-
5
This article is protected by copyright. All rights reserved.

10.1002/chem.202000840
Chemistry - A European Journal
aqua[1,7-bis(N-methylbenzimidazol-2′-yl)-2,6-dithiahep-
tane]copper(II) perchlorate. J. Chem. Soc., Dalton Trans.
1984, 1349-1356.
34.
Benedikter, M. J.; Schowner, R.; Elser, I.; Werner,
P.; Herz, K.; Stöhr, L.; Imbrich, D. A.; Nagy, G. M.; Wang,
D.; Buchmeiser, M. R., Synthesis of trans-Isotactic Poly(nor-
bornene)s through Living Ring-Opening Metathesis
Polymerization Initiated by Group VI Imido Alkylidene N-
Heterocyclic Carbene Complexes. Macromolecules 2019,
52, 4059-4066.
18.
Nelson, D. J.; Nolan, S. P., Quantifying and under-
standing the electronic properties of N-heterocyclic car-
benes. Chem. Soc. Rev. 2013, 42, 6723-6753.
19.
Dröge, T.; Glorius, F., The Measure of All Rings̶
N-Heterocyclic Carbenes. Angew. Chem. Int. Ed. 2010, 49,
6940-6952.
35.
Buchmeiser, M. R., Molybdenum Imido, Tungsten
Imido and Tungsten Oxo Alkylidene N-Heterocyclic Car-
bene Olefin Metathesis Catalysts. Chemistry ‒ A European
Journal 2018, 24, 14295-14301.
20.
Poater, A.; Cosenza, B.; Correa, A.; Giudice, S.;
Ragone, F.; Scarano, V.; Cavallo, L., SambVca: A Web Ap-
plication for the Calculation of the Buried Volume of N-Het-
erocyclic Carbene Ligands. Eur. J. Inorg. Chem. 2009, 2009,
1759-1766.
36.
Schowner, R.; Frey, W.; Buchmeiser, M. R., Cati-
onic Tungsten-Oxo-Alkylidene-N-Heterocyclic Carbene
Complexes: Highly Active Olefin Metathesis Catalysts. J.
Am. Chem. Soc. 2015, 137, 6188-6191.
21.
Gómez-Suárez, A.; Nelson, D. J.; Nolan, S. P.,
Quantifying and understanding the steric properties of N-
heterocyclic carbenes. Chem. Commun. 2017, 53, 2650-
2660.
37.
Herz, K.; Podewitz, M.; Stöhr, L.; Wang, D.; Frey,
W.; Liedl, K. R.; Sen, S.; Buchmeiser, M. R., Mechanism of
Olefin Metathesis with Neutral and Cationic Molybdenum
Imido Alkylidene N-Heterocyclic Carbene Complexes. J.
Am. Chem. Soc. 2019, 141, 8264-8276.
22.
Schrock, R. R.; Shih, K.-Y.; Dobbs, D. A.; Davis, W.
M., alpha-Elimination Can Be Faster than beta-Elimination
in d2 Alkyl Complexes of Molybdenum and Tungsten That
Contain the Trimethylsilyl-Substituted Triamidoamine Lig-
and. J. Am. Chem. Soc. 1995, 117, 6609-6610.
23.
Kharasch, M. S.; Brown, H. C., Chlorinations with
Sulfuryl Chloride. I. The Peroxide-Catalyzed Chlorination of
Hydrocarbons. J. Am. Chem. Soc. 1939, 61, 2142-2150.
24.
Griffiths, D. C.; Young, G. B., Mechanisms of ther-
molytic rearrangement of dineophylplatinum(II) complexes
via intramolecular carbon-hydrogen bond activation. Or-
ganometallics 1989, 8, 875-886.
25.
Kwart, H., Temperature dependence of the primary
kinetic hydrogen isotope effect as a mechanistic criterion.
Acc. Chem. Res. 1982, 15, 401-408.
26.
Gómez-Gallego, M.; Sierra, M. A., Kinetic Isotope
Effects in the Study of Organometallic Reaction Mecha-
nisms. Chem. Rev. 2011, 111, 4857-4963.
27.
Wang, N.; Xu, J.; Lee, J. K., The importance of N-
heterocyclic carbene basicity in organocatalysis. Org. Bio-
mol. Chem. 2018, 16, 8230-8244.
28.
Amyes, T. L.; Diver, S. T.; Richard, J. P.; Rivas, F.
M.; Toth, K., Formation and Stability of N-Heterocyclic Car-
benes in Water:ꢀ The Carbon Acid pKa of Imidazolium Cati-
ons in Aqueous Solution. J. Am. Chem. Soc. 2004, 126,
4366-4374.
29.
Nomura, K.; Hou, X., Synthesis of vanadium‒al-
kylidene complexes and their use as catalysts for ring open-
ing metathesis polymerization. Dalton Trans. 2017, 46, 12-
24.
30.
P-Donor Ligands Based in part on the article P-Do-
nor Ligands by Charles A. McAuliffe & Anthony G. Mackie
which appeared in the Encyclopedia of Inorganic Chemis-
try, First Edition. In Encyclopedia of Inorganic Chemistry.
31.
Streuli, C. A., Determination of Basicity of Substi-
tuted Phosphines by Nonaqueous Titrimetry. Anal. Chem.
1960, 32, 985-987.
32.
Tsang, W. C. P.; Hultzsch, K. C.; Alexander, J. B.;
Bonitatebus, P. J.; Schrock, R. R.; Hoveyda, A. H., Alkyli-
dene and Metalacyclic Complexes of Tungsten that Contain
a Chiral Biphenoxide Ligand. Synthesis, Asymmetric Ring-
Closing Metathesis, and Mechanistic Investigations. J. Am.
Chem. Soc. 2003, 125, 2652-2666.
33.
Pedersen, S. F.; Schrock, R. R., Preparation of
Tungsten(VI)Phenylimido Alkyland Alkylidene Complexes.
J. Am. Chem. Soc. 1982, 104, 7483-7491.
6
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Synopsis Graphic
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