Nickel-Catalyzed C-H Silylation of Arenes with Vinylsilanes: Rapid and Reversible β-Si Elimination

Article abstract of DOI:10.1021/jacs.7b05574

The reaction of C₆F₅H and H₂C=CHSiMe₃ with catalytic [ⁱPr₂Im]Ni(²-H₂C=CHSiMe₃)₂ (1b) exclusively forms the C-H silylation product C₆F₅SiMe₃ with ethylene as a byproduct ([ⁱPr₂Im] = 1,3-di(isopropyl)imidazole-2-ylidene). Catalytic C-H bond silylation is facile with partially fluorinated aromatic substrates containing two ortho fluorine substituents adjacent to the C-H bond and 1,2,3,4-tetrafluorobenzene. Less fluorinated substrates react slower. Under the same reaction conditions, catalytic [IPr]Ni(η²-H₂C=CHSiMe₃)₂ (1a) ([IPr] = 1,3-bis[2,6-diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene) provided only the alkene hydroarylation product C₆F₅CH₂CH₂SiMe₃. Mechanistic studies reveal that the C-H activation and β-Si elimination steps are reversible under catalytic conditions with both catalysts 1a and 1b. With catalytic 1a, reversible ethylene loss after β-Si elimination was also observed despite its inability to catalyze C-H silylation; the reductive elimination step to form the silylation product is much slower than reductive elimination to form the alkene hydroarylation product. Reversible ethylene loss was not observed with 1b, which suggests that the rate-limiting step in the reaction is neither C-H activation nor β-Si elimination but either ethylene loss or reductive elimination of cis-disposed aryl and SiMe₃ moieties.

Full text of DOI:10.1021/jacs.7b05574

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Article
Nickel-Catalyzed C–H Silylation of Arenes with
Vinylsilanes: Rapid and Reversible #-Si Elimination
Matthew Ryan Elsby, and Samuel Alan Johnson
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 14 Jun 2017
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NickelꢀCatalyzed C–H Silylation of Arenes with Vinylsilanes:
Rapid and Reversible βꢀSi Elimination
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Matthew R. Elsby and Samuel A. Johnson*
Department of Chemistry and Biochemistry, University of Windsor, Sunset Avenue 401, Windsor, ON, N9B 3P4,
Canada
ABSTRACT: The reaction of C6F5H and H2C=CHSiMe3 with catalytic [iPr2Im]Ni(η2-H2C=CHSiMe3)2 (1b) forms the C–H
silylation product C6F5SiMe3 exclusively, with ethylene as a byproduct ([iPr2Im] = 1,3-di(isopropyl)imidazole-2-ylidene).
Catalytic C–H bond silylation is facile with partially fluorinated aromatic substrates containing two ortho fluorine sub-
stituents adjacent to the C–H bond and 1,2,3,4-tetrafluorobenzene, with the less fluorinated substrates reacting slower.
Under the same reaction conditions, catalytic [IPr]Ni(η2-H2C=CHSiMe3)2 (1a) ([IPr] = 1,3-bis[2,6-diisopropylphenyl]-1,3-
dihydro-2H-imidazol-2-ylidene), provided only the alkene hydroarylation product C6F5CH2CH2SiMe3. Mechanistic studies
reveal that the C–H activation and β-Si elimination steps are reversible under catalytic conditions with both catalysts 1a
and 1b. With catalytic 1a, reversible ethylene loss after β-Si elimination was also observed, despite its inability to catalyze
C–H silylation; the reductive elimination step to form the silylation product is much slower than reductive elimination to
form the alkene hydroarylation product. Reversible ethylene loss was not reversible with 1b, which suggests that the rate
limiting step in the reaction is neither C–H activation nor β-Si elimination, but either ethylene loss, or reductive elimina-
tion of cis-disposed aryl and SiMe3 moieties.
In C–H bond stannylation, competition was observed be-
Introduction
tween the two mechanistic pathways, C and D, that interme-
i
The transition metal catalyzed functionalization of C–H
bonds1 has extensive applications for organic synthesis.2 The
silylation of aryl C–H bonds is an atom economical route to
organosilicon compounds with numerous applications, such
as Hiyama coupling.3 Advances in C–H bond silylation have
been the subject of several reviews;4 however, the majority of
examples require the use of noble metal complexes. Recent
efforts have focused on eliminating the need for expensive
heavy metals in these reactions.5
diate 4 can undergo. With E = SnBu3 and L = Pr3P or {NQA,
catalysis yielded almost exclusively the stannylation product
i
C6F5SnBu3. Using SnPh3 with Pr3P also led to stannylation
products; however, using the {NQA} ligand with SnPh3 re-
sulted in a mixture of stannylation product and hydroaryla-
tion product, C6F5CH2CH2SnPh3, with the latter being fa-
vored (95 %). Furthermore, using the IPr carbene as the an-
cillary ligand (IPr
=
1,3-bis[2,6-diisopropylphenyl]-1,3-
dihydro-2H-imidazol-2-ylidene) resulted in similar product
distributions as the {NQA} ligand.9
Our group has reported the nickel catalyzed C–H stannyla-
tion of fluorinated aromatics, as shown on the top of Scheme
1 where ER3 = SnBu3.6 This transformation uses readily avail-
able H2C=CHSnBu3 to convert a plethora of partially fluori-
nated aromatics into organotin compounds suitable for Stille
coupling,7 with only ethylene as a byproduct. A proposed
mechanistic pathway for catalysis using 1, which is a resting
state for the catalyst, is shown in Scheme 1. Step A features a
reversible dissociation of the vinyl moiety to give 2. This is
followed by C–H bond activation in step B, which occurs via
oxidative addition coupled with insertion through the pro-
posed transition state 3, alternatively viewed as a ligand to
ligand hydrogen transfer.8 The β-agostic Ni intermediate 4
can undergo two possible reaction pathways that yield differ-
ent products. Reductive elimination from 4, shown as step C,
provides the unwanted alkene hydroarylation product
C6F5CH2CH2ER3. Alternatively, 4 can undergo β-ER3 elimina-
tion to form Ni(L)(C6F5)(ER3)(η2-C2H4) (5), which could lose
ethylene gas to give Ni(L)(C6F5)(ER3) (6), as shown in step D.
The reductive elimination step E regenerates the Ni(0) cata-
lyst and forms the desired C–H bond functionalization prod-
uct, C6F5ER3. 6b, c, 9
Scheme 1. Proposed C–H Bond Functionalization Mecha-
nism
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iPr
N
iPr
Ni
iPr
SiMe3
SiMe3
N
iPr
5
1a
n = 2 + IPr
6
7
8
9
N
iPr
N
SiMe3
+ iPr2Im
n = 1
iPr
iPr
Ni(COD)2
+
n Me3SiCH=CH2
Ni
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N
N
iPr
9
+ iPr2Im
n = 10
iPr
N
SiMe3
Ni
N
iPr
SiMe3
SiMe3
+
iPr
1b
N
Ni
N
iPr
SiMe3
Figure 1. Syntheses and ORTEP depictions of 1a, 9, and 1b.
Hydrogen atoms are omitted for clarity.
The use of carbene ligands in lieu of phosphines often pro-
vides more thermally robust complexes for transition metal
catalysis.12 The reaction of Ni(COD)2 (COD
=
1,5-
A study of nickel catalyzed alkene hydroarylation reactions
with IPr as the ancillary ligand provided a detailed computa-
tional mechanism, and found experimentally that the reac-
tion of 1,3-bis(trifluoromethyl)benzene and H2C=CHSiEt3
provided conversion to the hydroarylation product exclusive-
ly.10 The absence of silylation product in this reaction sug-
gests that β-Si elimination does not occur under these condi-
tions, possibly because it is both kinetically and thermody-
namically more difficult than β-Sn elimination. Herein we
report the Ni–catalyzed C–H silylation of partially fluorinated
aromatics, and reexamine this assumption regarding the ease
of β-Si elimination and its importance on the selectivity of
these systems towards C–H silylation vs hydroarylation.
cyclooctadiene) with IPr and two equivalents of
H2C=CHSiMe3 forms the expected10, complex [IPr]Ni(η2-
13
H2C=CHSiMe3)2 (1a). The crystal structure is shown in Figure
1. A catalytic amount of 1a was reacted with H2C=CHSiMe3
and pentafluorobenzene at 90 °C for 24 h, but only the al-
kene hydroarylation product, C6F5CH2CH2SiMe3 (7), was
formed, with no observable silylation product 8. This is con-
sistent with a related previous study of hydroarylation.10
The potential influence14 of carbene steric bulk on catalysis
led us to examine if a smaller carbene could promote selec-
tive C–H silylation instead of alkene hydroarylation. The
reaction of the iPr2Im carbene ligand (iPr2Im
= 1,3-
di(isopropyl)imidazole-2-ylidene) with Ni(COD)2 and two
equivalents of H2C=CHSiMe3 resulted in the isolation of the
unanticipated bis-carbene Ni complex, [iPr2Im]2Ni(η2-
H2C=CHSiMe3) (9). The reactivity of 9 towards silylation was
tested with H2C=CHSiMe3 and a series of fluorinated sub-
strates. Reaction with pentafluorobenzene resulted in stoi-
chiometric conversion to the known C–F bond activation
product (iPr2Im)2NiF(C6F4H).15 More encouragingly, C–H
silylation products were observed with the substrates 1,2,4,5-
Results and Discussion
Synthesis of Nickel Complexes. To determine if si-
lylation could be achieved under similar conditions to
stannylation,6 a 5 % loading of the previously reported11 com-
plex (iPr3P)Ni(η2- H2C=CHSiMe3)2 was reacted with
H2C=CHSiMe3 and pentafluorobenzene at 80 °C for 24 h. The
crude 19F{1H} NMR spectrum showed 3 % conversion to the
C–H silylation product, C6F5SiMe3 (8), along with unreacted
starting material, but no hydroarylation product. Decompo-
sition of the Ni catalyst was indicated by nickel metal precip-
tetrafluorobenzene,
1,3,5-trifluorobenzene,
and
1,3-
difluorobenzene, along with C–F activation products and
FSiMe3. However, examination of the kinetics of these reac-
tions revealed an incubation period, which suggested that 9
is not the active catalyst for silylation. During these reac-
tions, two new broad peaks were observed in the H NMR
spectrum, consistent with the bis-vinyl species,
[iPr2Im]Ni(η2-H2C=CHSiMe3)2 (1b).
i
itate and the observation of only Pr3P in the 31P{1H} NMR.
Heating above 80 °C resulted in rapid decomposition of
(iPr3P)Ni(η2-H2C=CHSiMe3)2. Similar temperature limitations
of the catalyst were noted in our previous work with C–H
stannylation.6
1
Complex 1b was synthesized in 90 % yield by the reaction
of Ni(COD)2 with 10 equivalents of H2C=CHSiMe3 in toluene,
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followed by the slow addition of a dilute solution of Pr2Im.
previous report of Ni-catalyzed reactions of heterocycles with
1
2
3
4
5
6
7
8
As shown in Figure 1, the solid-state structure of 1b features
SiMe3 substituents that are on the same side of the trigonal
Ni coordination plane, with one of the substituents central,
H2C=CHSiEt3, provided solely hydroarylation products;17a in
contrast, the reaction of H2C=CHSiMe3 and 1b with the het-
erocycle benzofuran resulted in selective silylation,19 but a
mixture of silylation and hydroarylation products with the
substrates benzoxazole and benzothiazole. The latter two
substrates feature very activated C–H bonds, and catalysis
was observed at temperatures as low as 60 °C. Further de-
tails are provided in the Supporting Information.
i
and the other adjacent to the Pr2Im ligand, unlike the C2
symmetric 1a.6c At the fast exchange limit, the 1H NMR spec-
trum features resonances for two isomers, shown at the bot-
tom of Figure 1, in a 5:1 ratio, where rotation around the Ni-
η2-alkene bonds is rapid. At low temperature, these peaks
decoalesce to give further rotational isomers. The presence
of multiple similar energy isomers for 1b is presumably the
result of a ligand with less steric bulk.
9
Chart 1. C–H Silylation of Fluorinated Aromatics
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Catalysis with 1b. To investigate the catalytic ability of
1b for C–H silylation, experiments were carried out on a
broad spectrum of fluorinated aromatics. The results are
summarized in Chart 1. In initial NMR scale experiments, the
C–H silylation of pentafluorobenzene was facilitated with a 5
% catalyst loading and performed at two different tempera-
tures. Heating at 100 °C for 7 h resulted in 24 % conversion,
while heating at 120 °C led to conversions of 65 %, 87 % and
98 % after 3 h, 5 h and 7 h, respectively. The reaction was
also successful on larger scales, and using 1 g of pentafluoro-
benzene under similar conditions, the silylation product was
obtained in a 70 % yield after chromatographic purification.
Substrates with a C–H bond ortho to two fluorine substitu-
ents were most reactive towards silylation. The monosilyla-
tion products 8, 10, 12, 14, 15, 16, and 17 were made selectively
using an excess of fluorinated substrate. The only impurities
in these reactions were the disilylated products 11 and 13,
which could be prepared by reacting the substrate with 2.5
equivalents of H2C=CHSiMe3.
Kinetics modelling16 of the rate of formation of mono- and
di-silylated compounds with substrates with two equally
activated sites, such as 1,2,4,5-tetrafluorobenzene, revealed
that the monosilylation product 10 undergoes silylation with
a rate constant about one-half of its precursor, which corre-
lates with the number of C–H bonds in each substrate and is
consistent with a minimal electronic effect of the para-SiMe3
substituent in 10. A similar approximately 2:1 ratio of silyla-
tion rate constants was found for 1,2,3,5-tetrafluorobenzene
and its monosilylation product 12, suggestive that meta-
SiMe3 substituents also have only a minor electronic influ-
ence. In contrast, no silylation next to an ortho-SiMe3 group
was observed in 1,2,3,4-tetrafluorobenzene, which can be
attributed to the steric bulk of this group.
a
Performed with 0.498 mmol of H2C=CHSiMe3 and arene.
bPerformed with 0.498 mmol of H2C=CHSiMe3 and 10 equivalents
(4.98 mmol) of arene to obtain monosilylation products. cPerformed
with 2.5 equivalents (1.25 mmol) H2C=CHSiMe3 relative to arene
(0.498 mmol) to form disilylation product. dPerformed with 3-5
equivalents (1.50-2.50 mmol) of arene relative to H2C=CHSiMe3
e
f
(0.498 mmol). Reaction was carried out with 5 mol % 1b. Reaction
was carried out with 20 mol % 1b.
Yields of product were
determined by 19F NMR and are relative to 0.062 mmol of the
internal standard FSiPh3.
Due to the limited utility of SiMe3 groups in Hiyama cross
coupling reactions, additional silyl groups were
investigated.3b, 20 The reaction of H2C=CHSi(OEt)3 and pen-
tafluorobenzene with a catalytic amount of Ni(COD)2 and
iPr2Im did not result in the silylation product. To investigate
if C–H bond activation was occurring, C6F5D and
H2C=CHSi(OEt)3 was reacted with a catalytic amount of
Substrates with a lesser degree of fluorination required more
time to reach completion. Aryl C–H bonds with only one
ortho fluorine proved to be less efficiently silylated, and re-
quired a higher catalyst loading. The silylation product of
1,2,3,4-tetrafluorobenzene was obtained in only a 30 % yield
when using a 5 % loading of 1b. When performed with a 20 %
loading of 1b, the silylation product of 1,2,3,4-
tetrafluorobenzene was obtained in a 96 % yield, by integra-
tion of 19F NMR spectra using an internal standard. Multinu-
clear NMR revealed 1b to be the resting state of the catalyst
with all these substrates. The fluoroarenes 1,2,3-
trifluorobenzene, 1,2-difluorobenzene, 1,4-difluorobenzene,
and fluorobenzene did not undergo efficient C–H silylation.
Increasing the temperature to 140 °C resulted in the decom-
position of 1b, with the formation of a black precipitate.
There are several examples of nickel catalyzed alkene hy-
droarylation of heterocycles,10, 17 however, instances of C–H
i
Ni(COD)2 and Pr2Im, and heated at 120 °C for 12 h. The
19F{1H} NMR showed deuterium exchange into the arene,
indicating that C–H activation still readily occurs, and so it is
likely that the β-Si elimination step is not viable with the
Si(OEt)3 substituent. Although limited information is known
about the propensity of silyl groups to undergo β-Si elimina-
tion, this result is consistent with previous studies on Ru
complexes.21 The SiBnMe2 substituent, where Bn = benzyl,
has also found use in coupling reactions, and seemed more
likely to be capable of β-Si elimination.22 The reaction of
pentafluorobenzene and H2C=CHSiBnMe2 with a 5% catalyst
silylation of heterocycles with any metal are limited.5a, 18
A
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loading of Ni(COD)2 and Pr2Im provided the silylation prod-
uct C6F5SiBnMe2 in poorer yield than with H2C=CHSiMe3
(70%), but without significant byproducts. Increased catalyst
loadings improved yields.
1
2
3
4
5
6
7
8
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Labeling Studies with 1b. Unexpected mechanistic
insights regarding β-Si elimination were obtained from a
series of isotope labeling studies. The reaction of C6F5D and
H2C=CHSiMe3 in the presence of catalytic 1b at 80 °C yielded
scrambling of the D label into all the sp2 C–H bonds of the
vinyl moiety, as shown in Scheme 2. This temperature is
below that at which catalytic silylation is observed, and
scrambling suggests that the C–H bond activation (step B in
Scheme 1) is reversible and not rate limiting, in contrast to
catalytic stannylation.6c Monitoring the reaction by 2H NMR
spectroscopy found that the initial ratio of deuterium incor-
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poration into the two 2-sites and single
1 site of
H2C=CHSiMe3 was 1:1:4. Two possible mechanistic explana-
tions were considered for incorporation of D into the 2-
sites:23 The first is reversible β-Si elimination (step D in
Scheme 1), where the ethylene moiety in 5 reinserts, scram-
bling deuterium into either the 1 or 2 sites of 4. The second
explanation is if the C–H bond activation (step B in Scheme
1) occurs with hydrogen transfer to both the 1 and 2 sites of
the bound H2C=CHSiMe3 moiety.
Carbon-13 labeling studies were performed as a test for
reversible β-Si elimination. The reaction of pentafluoroben-
zene and H213C=CHSiMe3 with a 5 % loading of 1b was moni-
tored using variable-temperature 13C{1H} NMR. At 110 °C the
scrambling of labels to give H2C=13CHSiMe3 was observed. A
mechanism is proposed in Scheme 2. Complete 13C label
scrambling occurred before any silylation product was ob-
served. This result indicates that β-Si elimination is reversi-
ble, and that alkene loss or reductive elimination is the rate
determining step in the silylation reaction.
To test if alkene loss from 5 is the rate limiting step, dou-
bly labeled 13C2H4 was added to a solution containing pen-
tafluorobenzene, H2C=CHSiMe3 and a catalytic amount of 1b.
After undergoing 20 % conversion to silylation product, there
was no observable incorporation of the 13C label to give
H213C=13CHSiMe3. This suggests two possibilities for the rate
determining step of C–H silylation: Either i) rate determining
reductive elimination prior to ethylene loss from an isomer
of 5 with cis-disposed aryl and SiMe3 moieties; or ii) rate de-
termining alkene loss from 5 before reductive elimination.
Both possibilities are shown in the bottom left of Scheme 2.
Labeling Studies with 1a. Insight into why the bulkier
IPr carbene complex 1a gives hydroarylation instead of silyla-
tion products could aid in the design of catalysts for carbon–
heteroatom bond forming reactions as well as other related
processes. The labeling studies using 1b suggested that ca-
talysis with 1a could also feature rapid β-Si elimination, but
still not give the silylation product if the reductive elimina-
tion step E (in Scheme 1) is relatively slow compared to step
C.
To investigate if reversible β-Si elimination is occurring in
this system, C6F5D and H2C=CHSiMe3 was reacted with a 5 %
catalyst loading of 1a. After heating at 90 °C for 5 minutes the
2H NMR showed deuterium scrambling in both the two 2-
sites and single 1-site of H2C=CHSiMe3 in a 1:1:3 ratio, as
2
shown in Scheme 3. After heating the sample overnight, H
NMR showed statistical scrambling of deuterium into all the
sp2 C–H bonds of the alkene. Like the previous experiments
conducted with 1b, full scrambling into both the arene and
Scheme 2. Isotope Labeling Studies with 1b
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H2C=CHSiMe3 suggested C–H activation and β-Si elimination
vided more thermally stable complexes than Pr3P, which
afforded only trace C–H silylation, but the choice of carbene
substituents plays a dramatic role in the selectivity of the
reaction. The nickel complex [iPr2Im]Ni(η2-H2C=CHSiMe3)2
(1b) performs catalytic C–H silylation of partially fluorinated
aromatics with low catalyst loadings. The analogous complex
1a using the IPr carbene gave no trace of C–H silylation, and
instead gives alkene hydroarylation, as previously reported.10
Investigations into the mechanism of the C–H bond func-
tionalization reaction led to several key insights, the most
surprising being that the β-Si elimination is rapid and re-
versible using both catalysts 1a and 1b; the IPr supported
catalyst 1a was even seen to undergo alkene exchange after β-
Si elimination under catalytic conditions, despite the fact
that it does not mediate C–H silylation. The possible rate
determining steps for C–H silylation using 1b are either al-
kene loss from 5, or direct reductive elimination from 5 with
cis disposed aryl and SiMe3 groups, before ethylene loss.
Relatively few catalytic systems have taken advantage of β-Si
1
2
3
4
5
6
7
8
are rapidly reversible.
If β-Si elimination is reversible and not the rate limiting
step for silylation with 1a, then once again either alkene loss
or the final C–Si reductive elimination could prevent silyla-
tion in this system. The reaction of pentafluorobenzene,
H2C=CHSiMe3 and doubly labeled 13C2H4 with 5 % of 1a at 90
°C results in intermolecular scrambling of the 13C label to give
H213C=13CHSiMe3 before any hydroarylation product is ob-
served. This result shows that not only is β-Si elimination
reversible, but so is alkene loss from 5, as shown in Scheme 3.
This result is different from that obtained with catalyst 1b.
Remarkably, even though catalyst 1a gives only alkene hy-
droarylation, it is not because the system does not undergo
rapid β-Si elimination and subsequent reversible alkene loss
to form 6; silylation is not observed because the rate of the
C–Si reductive elimination step E is much slower than C–C
reductive elimination step C.
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Scheme 3. Isotope Labeling Studies with 1a
elimination
for
the
synthesis
of
organosilicon
compounds.26,27
Experimental Section
General Considerations. Unless otherwise stated, all
reactions were carried out under an atmosphere of dry oxy-
gen free dinitrogen by means of standard Schlenk or glove-
box techniques. Benzene–d6, and toluene–d8 were degassed
by three freeze-pump-thaw cycles, and subsequently dried by
running through a column of activated alumina. Toluene,
THF, and pentane were purchased anhydrous from Aldrich
or Alfa Aesar. 1H, 13C{1H}, 19F{1H}, 2H and 29Si{1H} NMR spectra
were recorded on a Bruker AMX Spectrometer operating at
either 300 MHz or 500 MHz with respect to proton nuclei. 1H
NMR spectra were referenced to residual protons (C6D6, δ
7.15) or (tol-d8, δ 2.17) with respect to tetramethylsilane at δ
0.00. 13C {1H} NMR spectra were referenced relative to solvent
resonances (C6D6, δ 128.26) or (tol-d8, δ 21.37). 19F {1H} NMR
spectra were referenced to an external sample of 80% CCl3F
in CDCl3 at δ 0.00. Benzene-d6 and toluene-d8 was purchased
from Cambridge Isotope Laboratory. All reagents were pur-
chased from commercial suppliers. The compounds
Ni(COD)2,28 IPr,29 iPr2Im,30 and C6F5D31 were prepared ac-
cording to literature procedures. Elemental analyses were
carried out at the Centre for Catalysis and Materials Re-
search, Windsor, Ontario.
General Procedure for catalytic C–H bond silylation. A
solution of fluorinated arene and trimethyl(vinyl)silane in
0.6 g of toluene was added to a 5 % loading of 1b and an in-
ternal standard, triphenylfluorosilane. The NMR tube was
flame sealed under vacuum and the solution was fully im-
mersed in an oil bath at 120 °C. Equivalents of fluorinated
arene and trimethyl(vinyl)silane, and time of reaction varied
upon desired product. (See Supporting Information).
Conclusions
[IPr]Ni(η2-H2C=CHSiMe3)2 (1a). Ni(COD)2 (0.43 g, 1.55
mmol) was dissolved in 10 mL of toluene. Trimethyl(vinyl)
silane (0.31 g, 3.10 mmol, 2 equiv) was added to the reaction
mixture. The solution was added to 1,3-bis[2,6-
diisopropylphenyl]-1,3-dihydro-2H-imidazol-2-ylidene) (0.60
g, 1.55 mmol), stirred for 30 minutes and evaporated in vacuo
to provide a brown solid. Compound 1a was recrystallized
from pentane at –40 °C affording 0.600 g of yellow crystals
While the application of nickel in catalysis continues to
expand,24 to the best of our knowledge, there is only one
previous instance of nickel catalyzed C–H silylation, which
required a reactant with a strained Si–Si bond.25 The C–H
silylation reaction reported here requires higher tempera-
tures than analogous C–H stannylation reactions; this was
expected to be due to an increased barrier to β-Si elimina-
tion, and seemed to be a likely rate-determining step for the-
se reactions. The use of N-heterocyclic carbene donors pro-
1
(60 % yield). H NMR (tol-d8, 25 °C, 500.129 MHz): δ -0.15 (s,
18H, Si(CH3)3); 0.98 (d, 6H, CH(CH3)2, 3JHH = 6.95 Hz); 1.10 (d,
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3
3
6H, CH(CH3)2, JHH = 6.95 Hz); 1.16 (d, 6H, CH(CH3)2, JHH
=
°C, 470.59 MHz): δ –146.4 (AA′MM′ second order m, 2F, 2,6–
3
3
1
2
3
4
6.95 Hz); 1.50 (d, 6H, CH(CH3)2, JHH = 6.95 Hz); 2.35 (d, 2H,
vinyl–H, JHH = 15.9 Hz); 2.51(dd, 2H, vinyl–H, JHH = 12.8 Hz,
2JHH = 15.9 Hz); 2.73 (d, 2H, vinyl–H, 2JHH = 12.8 Hz); 2.95 (sep-
tet, 2H, CH, JHH = 6.95 Hz); 3.31 (septet, 2H, CH, JHH = 6.95
Hz); 6.63 (s, 2H, HC=CH); 7.02 (d, 3,5–Ar–CH, 3JHH = 7.58 Hz
); 7.11 (d, 3,5–Ar–CH, JHH = 7.58 Hz); 7.18 (t, 4–Ar–CH, JHH
Ar–F); -159.6 (t, 1F, 4–Ar–F, JFF = 20.4 Hz); -163.8 (AA′MM′X
2
2
second order m, 2F, 3,5–Ar–F). 13C{1H} NMR (C6D6, 25 °C,
125.75 MHz): δ -1.9 (s, Si(CH3)3); 17.1 (s, SiCH2); 17.5 (s,
3
3
2
SiCH2CH2);119.2 (t, 1–Ar–C, JCF = 19.2 Hz); 137.8 (dm, Ar–C,
1JCF = 248.7 Hz); 145.7(dm, 4–Ar–C, 1JCF = 247.3 Hz); 150.6 (dm,
Ar–C, 1JCF = 247.8 Hz).
5
3
3
=
6
7
8
9
7.58 Hz). 13C{1H} NMR (C6D6, 22 °C, 500.133 MHz): δ 1.2 (s,
6C, Si(CH3)3); 22.3 (s, isopropyl–(CH3)2); 22.7 (s, isopropyl–
(CH3)2); 25.6 (s, isopropyl–(CH3)2); 27.0 (s, isopropyl–(CH3)2);
29.0 (s, isopropyl–CH); 30.8 (s, isopropyl–CH); 50.5 (s, vinyl–
C); 53.5 (s, vinyl–C); 124.2 (s, H2C=CH2); 124.3 (s, H2C=CH2);
129.9 (s, Ph–C); 137.6 (s, Ph–C); 145.8 (s, Ph–C); 146.5 (s, Ph–
C); 206.3 (s, Ni–C). 29Si{1H} NMR (C6D6, 27 °C, 59.647 MHz):
δ -3.9 (s, 2Si, Si(CH3)3). Calcd for C37H60N2NiSi2: % C 68.61; %
H 9.34; % N 4.32. Found: % C 66.47; % H 9.05; % N 4.31. Re-
peated elemental analyses gave variable but consistently low
values for C, possibly due to Ni-carbide formation.
Trimethyl(2,3,4,5,6-pentafluorophenyl)silane (8). Syn-
thesized according to General Procedure for catalytic C–H
bond silylation. Pentafluorobenzene (0.083 g, 0.498 mmol),
trimethyl(vinyl)silane (0.05 g, 0.498 mmol), and 1b (0.010 g,
0.025 mmol, 5 mol %). The NMR tube was flame sealed un-
der vacuum and solution was fully immersed in an oil bath at
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1
120 °C for 7 h. (98 % yield by NMR spectroscopy). H NMR
5
(C6D6, 25 °C, 500.12 MHz): δ 0.21 (t, 9H, Si(CH3)3, JHF = 1.4
Hz). 19F{1H} NMR (C6D6, 25 °C, 470.59 MHz): δ -127.8
(AA′MM′N second order m, 2F, 2,6–Ar–F); -152.2 (tt, 1F, 4–
3
4
Ar–F, JFF = 20.6 Hz, JFF = 3.5 Hz); -161.5 (AA′MM′N second
[iPr2Im]Ni(η2-H2C=CHSiMe3)2 (1b). Ni(COD)2 (1.34 g, 4.87
mmol) was dissolved in 20 mL of toluene and trime-
thyl(vinyl) silane (4.88 g, 48.7 mmol, 10 equiv) was added.
The solution was stirred for 1 h to ensure the Ni(COD)2 was
fully dissolved. A solution of 1,3-di(isopropyl)imidazol-2-
ylidene (0.74 g, 4.87 mmol) diluted in 3 mL of toluene was
added to the reaction mixture dropwise while stirring. Solu-
tion was left to stir for 30 minutes and evaporated in vacuo to
provide a light brown oil. Compound 1b was dissolved in
minimal pentane, and slow evaporation at –40 °C provided
1.54 g of a brown solid (77 % yield). Compound 1b was recrys-
tallized by slow evaporation at room temperature from a
mixture of HMDSO and minimal benzene, affording yellow
crystals. Major isomer: 1H NMR (C6D6, 25 °C, 500.133 MHz): δ
0.21 (s, 18H, Si(CH3)3); 0.93 (d, 6H, CH(CH3)2, 3JHH = 6.75 Hz);
order m, 2F, 3,5–Ar–F). 13C{1H} NMR (C6D6, 25 °C, 470.59
4
MHz): δ 0.3 (t, Si(CH3)3, JCF = 2.9 Hz); 111.2 (t of apparent
2
3
4
quartets, 1–Ar–C, JCF = 33.2 Hz, JCF = 3.7 Hz, JCF = 3.7 Hz);
1
1
138.9 (dm, Ar–C, JCF = 251.3 Hz); 143.5 (dtt, 4–Ar–C, JCF
=
=
253.3 Hz, JCF = 12.9 Hz, JCF = 6.2 Hz); 150.6 (dm, Ar–C, 1JCF
2
3
253.4 Hz). 29Si{1H} NMR (C6D6, 27 °C, 59.64MHz): δ -1.4 (ttd,
1–Ar–Si, 3JSiF = 2.9 Hz, 4JSiF = 1.8 Hz, 5JSiF = 1.1 Hz).
[iPr2Im]2Ni(η2-H2C=CHSiMe3) (9). Ni(COD)2 (0.595 g, 2.16
mmol, 1 equiv) was dissolved in 10 mL of toluene. 1,3-
Di(isopropyl)imidazol-2-ylidene (0.658 g, 4.32 mmol,
2
equiv) and trimethyl(vinyl) silane (0.217 g, 2.16 mmol, 1
equiv) were added and the solution was stirred for 30
minutes. The solution was evaporated in vacuo leaving 0.950
g of a bright yellow solid (95 % yield). Compound 9 was re-
1
crystallized from pentane at –40 °C. H NMR (C6D6, 25 °C,
3
1.01 (d, 6H, CH(CH3)2, JHH = 6.75 Hz); 2.54 (fluxional multi-
500.133 MHz): δ 0.3 (s, 9H, Si(CH3)3); 0.97 (broad fluxional
multiplet, 12H, [CH(CH3)2]2); 1.16 (d, 6H, CH(CH3)2, 3JHH = 6.8
Hz); 1.19 (d, 6H, CH(CH3)2, JHH = 6.8 Hz); 1.37 (dd, 1H, vinyl–
CH, 3JHH = 12.2 Hz, 3JHH = 13.6 Hz); 1.66 (dd, 1H, vinyl–CH, 3JHH
= 2.8 Hz, 3JHH = 13.6 Hz); 2.16 (dd, 1H, vinyl–CH, 3JHH = 2.8 Hz,
3JHH = 12.2 Hz); 5.38 (septet overlapped with broad multiplet,
plet, 2H, vinyl-H); 2.69 (fluxional multiplet, 2H, vinyl-H);
2.87 (fluxional multiplet, 2H, vinyl-H); 4.39 (septet, 2H, CH,
3JHH = 6.75 Hz); 6.40 (s, 2H, CH=CH). 13C{1H} NMR (C6D6, 22
°C, 500.133 MHz): δ 1.1 (s, 6C, Si(CH3)3); 23.0 (s, 4C, isopro-
pyl–CH3); 23.6 (s, 2C, isopropyl–CH); 50.9 (s, vinyl–C); 52.5
(s, vinyl–C); 116.6 (s, H2C=CH2); 198.0 (s, Ni–C). 29Si{1H} NMR
(C6D6, 27 °C, 59.647 MHz): δ -4.4 (s, 2Si, Si(CH3)3). Minor
3
4H, CH, JHH = 6.8 Hz); 6.42 (s, 2H, HC=CH); 6.42 (s, 2H,
HC=CH). 13C{1H} NMR (C6D6, 23 °C, 75.48 MHz): δ 1.8 (s,
Si(CH3)3); 22.7 (s, isopropyl–(CH3)2); 23.5 (s, isopropyl–
(CH3)2); 28.2 (s, vinyl–C); 29.0 (s, vinyl–C); 50.7 (s, isopropyl–
CH); 114.7 (s, H2C=CH2); 202.0 (s, Ni–C); 202.6 (s, Ni–C).
29Si{1H} NMR (C6D6, 27 °C, 59.64MHz): δ 7.39 (s, Si(CH3)3).
Calcd for C23H44N4NiSi: % C 59.61; % H 9.57; % N 12.09.
Found: % C 59.29; % H 9.92; % N 12.07.
1
isomer: H NMR (C6D6, 25 °C, 500.133 MHz): δ 0.14 (s, 18H,
3
Si(CH3)3); 0.96 (d, 6H, CH(CH3)2, JHH = 6.75 Hz); 0.99 (d,
3
3
6H, CH(CH3)2, JHH = 6.75 Hz); 2.25 (dd, 2H, vinyl-H, JHH
=
3
3
12.58 Hz, JHH = 16.20 Hz ); 2.53 (d, 2H, vinyl-H, JHH = 16.20
Hz); 3.19 (d, 2H, vinyl-H, JHH = 12.58 Hz); 4.39 (septet, 2H,
3
CH, 3JHH = 6.75 Hz); 6.41 (s, 2H, CH=CH).13C{1H} NMR (C6D6,
22 °C, 500.133 MHz): δ 0.8 (s, 6C, Si(CH3)3); 23.2 (s, 4C, iso-
propyl–CH3); 23.7 (s, 2C, isopropyl–CH); 50.4 (s, vinyl–C);
50.9 (s, vinyl–C); 116.7 (s, H2C=CH2). 29Si{1H} NMR (C6D6, 27
°C, 59.647 MHz): δ -4.4 (s, 2Si, Si(CH3)3). Calcd for
C19H40N2NiSi2: % C 55.47; % H 9.80; % N 6.81. Found: % C
52.15-54.49; % H 9.76; % N 6.92. Repeated elemental analyses
gave variable but consistently low values for C, possibly due
to Ni-carbide formation.
Reaction of C6F5D and 3 equivalents of H2C=CHSiMe3
with 5% [iPr2Im]Ni(η2-H2C=CHSiMe3)2 (1b). A solution of
C6F5D (0.083 g, 0.498 mmol) and trimethyl(vinyl)silane
(0.150 g, 1.49 mmol, 3 equivalents) in 0.4 g of toluene was
added to 1b (0.010 g, 0.024 mmol, 5 mol %). The solution was
put in a J-Young tube, heated in the NMR probe, and tracked
2
by H NMR. Deuterium scrambling was observed into the 1
and 2 sites of free H2C=CHSiMe3 at 90 °C after 5 minutes. The
2H spectrum was modelled, and it was determined that the
ratio of deuterium scrambling into the 1 and 2 sites was 4:1:1
respectively. (See supporting information Figure S1).
C6F5CH2CH2SiMe3 (7). A solution of pentafluorobenzene
(0.167 g, 0.998 mmol) and trimethyl(vinyl)silane (0.10 g,
0.998 mmol) in 0.6 g of toluene was added to 1a (0.039 g,
0.099 mmol, 5 mol %). The solution was added to an NMR
tube and placed in an oil bath at 90 °C and heated for 20 h.
(60 % NMR yield). 1H NMR (C6D6, 25 °C, 500.12 MHz): δ -0.01
(s, 9H, Si(CH3)3); 0.59 (second order m, 2H, CH2SiMe3); 0.23
(second order m, 2H, CH2CH2SiMe3). 19F{1H} NMR (C6D6, 25
Reaction of C6F5H and H213C=CHSiMe3 with 35%
[iPr2Im]Ni(η2-H2C=CHSiMe3)2 (1b). A solution of C6F5H
(0.012 g, 0.007 mmol) and H213C=CHSiMe3 (0.007 g, 0.070
mmol) in 0.6 g of toluene was added to 1b (0.010 g, 0.002
mmol, 35 mol %) in a J-Young tube. The solution was heated
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Journal of the American Chemical Society
in the NMR probe, and after 10 minutes at 110 °C the 13C{1H}
Supporting Information. The Supporting Information is
1
2
3
4
5
6
7
8
NMR showed scrambling of the carbon-13 label into
H2C=CHSiMe, forming H2C=13CHSiMe3. (See supporting in-
formation, Figure S58).
available free of charge on the ACS Publications website at
DOI:
Includes full experimental details, NMR data and spectra for
new compounds, description of all catalytic and mechanistic
studies.
X-ray crystallographic file for 1a.
X-ray crystallographic file for 1b.
X-ray crystallographic file for 9.
Reaction of C6F5D and 3 equivalents of H2C=CHSiMe3
with 5% [IPr]Ni(η2-H2C=CHSiMe3)2 (1a). A solution of
C6F5D (0.083 g, 0.498 mmol) and trimethyl(vinyl)silane
(0.150 g, 1.49 mmol, 3 equivalents) in 0.4 g of toluene was
added to 1b (0.016 g, 0.024 mmol, 5 mol %). The solution was
put in a J-Young tube, heated in the NMR probe, and tracked
9
2
AUTHOR INFORMATION
by H NMR. Deuterium scrambling was observed into the 1
10
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12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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49
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59
60
and 2 sites of free H2C=CHSiMe3 at 80 °C after 5 minutes. The
2H spectrum was modelled and it was determined that the
ratio of deuterium scrambling into the 1 and 2 sites was 3:1:1
respectively. (See supporting information, Figure S2).
Corresponding Author
*sjohnson@uwindsor.ca
Reaction of H213C=13CH2 with C6F5H, H2C=CHSiMe3 and
5% [IPr]Ni(η2-H2C=CHSiMe3)2 (1a). Doubly labeled carbon-
13 ethylene was vacuum transferred to a 5 mL flask (0.208
mmol), and subsequently vacuum transferred to a J-Young
tube charged with pentafluorobenzene (0.083 g, 0.498
mmol), trimethyl(vinyl)silane (0.05 g, 0.498 mmol), and 1b
(0.016 g, 0.025 mmol, 5 mol %). The solution was heated at
90 °C for 6 h and 13C{1H} NMR showed scrambling of the car-
bon-13 label into H2C=CHSiMe3, forming H213C=13CHSiMe3.
(See supporting information, Figure S59).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We acknowledge the Natural Sciences and Engineering Re-
search Council (NSERC) of Canada. Supporting computa-
tional work was made possible by the Shared Hierarchical
Academic Research Computing Network (SHARCNET:
www.sharcnet.ca).
ASSOCIATED CONTENT
Wang, B.; Zhang, L.; Hou, Z., J. Am. Chem. Soc. 2016, 138,
3663-3666.
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23.An alternate mechanism,
a type 1 Ni/Si dyotropic
rearrangement in 4 with L = [iPr2Im] cannot be ruled out
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