Nickel-Catalyzed Hydrosilylation of Terminal Alkenes with Primary Silanes via Electrophilic Silicon-Hydrogen Bond Activation

Article abstract of DOI:10.1021/acs.orglett.1c00111

We report a simple and effective nickel-based catalytic system, NiCl2·6H2O/tBuOK, for the electrophilically activated hydrosilylation of terminal alkenes with primary silanes. This protocol provides excellent performance under mild reaction conditions: ex

Full text of DOI:10.1021/acs.orglett.1c00111

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Letter
Nickel-Catalyzed Hydrosilylation of Terminal Alkenes with Primary
Silanes via Electrophilic Silicon−Hydrogen Bond Activation
Xiaoyu Wu, Guangni Ding, Wenkui Lu, Liqun Yang, Jingyang Wang, Yuxuan Zhang, Xiaomin Xie,*
and Zhaoguo Zhang*
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ABSTRACT: We report a simple and effective nickel-based
catalytic system, NiCl2·6H2O/tBuOK, for the electrophilically
activated hydrosilylation of terminal alkenes with primary silanes.
This protocol provides excellent performance under mild reaction
conditions: exclusive anti-Markovnikov selectivity, broad functional
group tolerance (36 examples), and good scalability (TON =
5500). However, the secondary and tertiary silanes are not suitable.
Mechanistic studies revealed that this homogeneous catalytic
hydrosilylation includes an electrophilically activated Si−H bond
process without the generation of nickel hydrides.
he transition metal-catalyzed hydrosilylation of unsatu-
rated groups, especially olefins, is an important source of
hydrosilylation of alkenes with primary and secondary silanes
utilizing cationic Ru or Ir complexes as the catalysts (Scheme
1a).10 The 16e cation metal centers of the catalysts are
reluctant to undergo oxidative addition, and the electrophilic
η3-complexes or silylenes were confirmed to be the actual
active species to catalyze the direct and fast hydrosilylation.
Although similar η2-(Si−H)M complexes have been synthe-
T
commodity organosilicon compounds.1 Among all hydro-
silylation reactions, the activation of the Si−H bond is the
most crucial for controlling the reactivity and selectivity.2
Metal hydrides are the common catalytic species in the
classical Chalk−Harrod mechanism, which are generated via
various activations mediated by a transition metal, such as
oxidative addition, σ-bond metathesis, and [2+2] addition.3
The highly active metal hydrides also result in many limitations
for the application of hydrosilylation, like complex side
reactions, the narrow scope of substrates, and the high cost
of ligands.4 Recently, these barriers are gradually overcome
with the development of ligand-controlled5 or heterogeneous6
protocols based on earth-abundant transition metals. Another
solution is to develop new processes avoiding the Chalk−
Harrod frame, which has inspired great interest, such as radical
hydrosilylation7 and electrophilic activated hydrosilylation.8
Electrophilic activation modes for Si−H bonds have been
proposed to explain the reactivity of the main group Lewis acid
catalysts in the hydrosilylation of alcohols, carbonyl derivatives,
alkenes, CO2, and pyridines.9 In these processes, the Si−H
bond preferentially undergoes η1-coordination with the strong
Lewis acid center, which can render the Si center more
electrophilic through electron-withdrawing induction effects
and allow attacks from nucleophile. Nonetheless, it is
intrinsically challenging for electrophilic Si−H bond activation
to occur with transition metal catalysts,8 because the abundant
d orbital electrons of the central transition metal tend to back-
donate to the σ* orbital of the Si−H bond and facilitate
oxidative addition. Thus, the transition metal center of the
catalysts must be extremely electron-deficient in electrophilic
hydrosilylation. Recently, Tilley’s group demonstrated the
Scheme 1. ηx-(Si-H)M Complexes (x = 2 or 3)
Received: January 12, 2021
Published: January 30, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00111
© 2021 American Chemical Society
Org. Lett. 2021, 23, 1434−1439
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Letter
sized from various silanes and transition metals,8b,11 their
practical reactivity in the hydrosilylation reactions has not been
well explored, especially for the first-row transition metals.
We aimed to develop an efficient and low-cost electrophilic
hydrosilylation of alkenes catalyzed by earth-abundant
transition metal-based catalysts. Nickel has exhibited a special
electrophilic Si−H bond activation ability.12 Typically, in the
work of Iwasawa and Sun,13 the nickel(0) center has an
obviously lower energy barrier to undergo η2-coordination with
Si−H bonds in [PSiP]-pincer complexes, whereas other metals,
like Fe and Co, tend to undergo oxidative addition (Scheme
1b). In addition, strong electron-withdrawing ligands are not
required for the construction of η2-(Si-H)Ni(0) complexes.14
Hence, a nickel-based ligand-free catalytic protocol for the
electrophilically activated hydrosilylation is practically possible.
Herein, we report a simple and effective Ni-catalyzed
electrophilic hydrosilylation of alkenes with primary silanes
in the absence of additional ligands.
We began our studies with the reaction between 1-octene
(1a) and PhSiH3 (2a) in THF as the model reaction. We
assumed that substrate alkenes serve as the weak electron-
withdrawing ligands to provide sufficient stability for the
putatively active η2-(Si-H)Ni(0) complexes, because of the
strong coordinating interaction between the Ni(0) center and
CC bonds.15 Thus, without the addition of any extra ligands,
Ni(COD)2, a commercially available nickel(0) precursor, was
first examined as a catalyst at room temperature (Table 1,
entry 1). However, rapid accumulation of Ni0 black was
observed, and none of the desired product 3a was detected; 1a
was recovered almost quantitatively (Table 1, entry 1). We
next attempted to generate the Ni0 species in situ, and
(DME)NiCl2 was added to the model reaction mixture in the
t
presence of BuOK, which can activate silane in the reduction
and silylation.16 Surprisingly, although nickel black still
precipitated, hydrosilylation product 3a was obtained in 9%
yield with double hydrosilylation byproduct 4a being obtained
in 11% yield (Table 1, entry 2). Moreover, nickel no longer
precipitated when the reaction temperature was decreased to 0
°C, and the yield of 3a was increased to 35% together with a
sharp decrease in the level of generation of 4a to 5% (Table 1,
entry 3). This implied that the low temperature is beneficial for
the stability of the Ni0 species and electrophilic interactions
with the Si−H bond. Gratifyingly, when the reaction
temperature was further decreased to −30 °C, 1a was
converted completely into target product 3a in quantitative
NMR yield without the formation of 4a [92% isolated yield
(Table 1, entry 4)].
Other nickel precatalysts, except (DME)NiBr2, exhibited
excellent activity and selectivity for the hydrosilylation (Table
1, entries 5 and 6, and Table S2). Even with NiCl2·6H2O as a
precatalyst, the reaction gave 3a in 99% yield (Table 1, entry
6). Upon replacement of the catalytic system, NiCl2·
6H2O/tBuOK, with the reported nanocatalyst Ni(OtBu)2·
xKCl,6e the conversion was decreased to 55%, and the
selectivity was maintained (Table 1, entry 7). Moreover, 10%
conversion of 1-octene was observed in the hydrosilylation
catalyzed by nano Ni(OtBu)2·xKCl at room temperature
(Table S2, entry 8). It is worth noting that other first-row
metals, such as Fe, Mn, Cu, Co, and Zn, are not suitable for
this reaction (Table 1, entries 8−12, respectively). Indeed,
nickel shows excellent suitability for the electrophilic hydro-
silylation as it allows for η2-coordination with the Si−H bond
to activate Si−H bond electrophilically as we hypothesized.
Table 1. Optimization of the Hydrosilylation Reaction of 1-
Octene with PhSiH3
t
t
t
Switching BuOK to BuONa or BuOLi resulted in a marked
decrease in reactivity (Table 1, entry 13 or 14, respectively),
which is in agreement with literature reports.16b,17 This is
caused by the quite different ionizations of the alkoxides in an
organic solvent. The activities can be improved partly by
adding appropriate crown ethers (Table S3, entries 3 and 5).
Solvent and reaction time are also screened (Tables S4 and
S5). In addition, control experiments showed that the nickel
yield (%)
a
entry
catalyst
activator
conversion (%)
3a
4a
b
1
Ni(COD)2
(DME)NiCl2
(DME)NiCl2
(DME)NiCl2
(DME)NiBr2
NiCl2·6H2O
Ni(OtBu)2·xKCl
FeCl2·4H2O
MnCl2·4H2O
CuCl2·2H2O
CoCl2·6H2O
ZnCl2
−
<1
31
47
99
40
99
55
2
0
9
0
11
5
b
t
2
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
−
precatalyst and BuOK are both required for the reaction
(Table 1, entries 15 and 16, respectively). Thus, the optimum
c
3
35
99
33
99
54
0
reaction conditions were determined with 2.0 mol % NiCl2·
4
0
t
6H2O as the catalyst and 4.0 mol % BuOK as the activator in
5
0
THF at −30 °C within 1 h.
6
0
With the optimal conditions in hand, we examined the scope
of this anti-Markovnikov hydrosilylation of terminal olefins and
various secondary silanes 3 were obtained in good yields with
excellent selectivity. The hydrosilylation of alkyl and aryl
alkenes (Scheme 2, 1a−1d, 1f, and 1g) proceeded with
complete substrate conversion and afforded products in >90%
isolated yields. In the case of dienes, 1h and 1i gave the
selective hydrosilylation products of the terminal alkenes in
87% and 91% yields, respectively, and the internal alkene and
1,1-disubstituted alkene moieties remained unaffected. To our
delight, the hydrosilylation of olefins bearing a C−X bond (X =
halide, oxygen, sulfur, or nitrogen) proceeded smoothly, allyl
ethers (1j−1p) and halogenated substrates (1v and 1w) being
completely converted into the corresponding secondary silanes
in good yields ranging from 82% to 92%. These phenomena
strongly support the idea that this reaction proceeds through
an electrophilic Si−H bond activation process; in traditional
7
8
0
0
tBuOK
tBuOK
tBuOK
tBuOK
tBuOK
tBuONa
tBuOLi
tBuOK

9
4
0
0
10
11
12
13
14
15
16
8
0
0
2
0
0
4
0
0
NiCl2·6H2O
NiCl2·6H2O
−
40
5
40
0
0
0
7
0
0
NiCl2·6H2O
8
0
0
a
General reaction conditions: 1-octene (1a, 224 mg, 2.0 mmol),
PhSiH3 (2a, 260 mg, 2.4 mmol), catalyst (2.0 mol %), activator (4.0
mol %), THF (3.0 mL), −30 °C, 12 h. Conversions and yields were
1
determined by H NMR analysis (internal standard, benzyl ether).
b
c
The reaction temperature was 25 °C. The reaction temperature was
0 °C.
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Letter
corresponding product in a comparatively lower yield because
of the double hydrosilylation. With a further decrease in the
reaction temperature to −40 °C, 3gg was obtained in 92%
yield. Conversely, secondary and tertiary silanes are not
amenable to this protocol (Table S7).
Scheme 2. Scope of Substrates in the Hydrosilylation
Reaction
a
To demonstrate the practical utility of this Ni-catalyzed
hydrosilylation reaction, the model reaction was conducted on
a gram scale with a lower catalyst loading. The hydrosilylation
of 1-octene (1a) and PhSiH3 (2a) worked well and afforded
the corresponding product in 97% isolated yield (99% NMR
yield), as illustrated in Scheme 3. So far, the highest turnover
number (TON) for the reaction on an 11 g scale is up to 5500
over 12 h with a 0.01 mol % loading of NiCl2·6H2O.
Scheme 3. Gram Scale Reaction and TON of the
Hydrosilylation
We next turned our attention to mechanistic investigations.
The model reaction proceeded normally with Ni(COD)2
instead of NiCl2·6H2O under the standard conditions (Scheme
4a), but the reaction rate decreased significantly in the absence
t
of BuOK (Scheme 4b), suggesting that Ni(0) is the real
catalytic species and the activator is crucial for the catalytic
cycle. Although the reactions described previously provide a
reasonable insight into the electrophilic Si−H bond activation
(Table 1 and Scheme 2), detailed experiments were still
needed. First, this system was confirmed to be homogeneous
because the presence of Hg did not result in any deleterious
effects on the model reaction (Scheme 4c). Next, a radical
clock experiment with (2-vinylcyclopropyl)benzene (1ii)
afforded normal addition product 3ii in 75% yield with none
of the ring-opened product being detected (Scheme 4d),
showing that a radical process is not possible. Additionally,
standard reactions with deuterated materials, d-2a and d-1f,
were carried out independently, and no scrambling or
dissociation of the deuterium was detected in the correspond-
ing product 3f (Scheme 4e); conversely, scrambling and
dissociation of the deuterium caused by β-H elimination or
allyl metallization are commonly observed in metal hydride-
catalyzed reactions.20 These results indicate that metal
hydrides are not likely involved in this reaction.
Furthermore, we measured the kinetic data of the model
reaction by ReactIR (Table S7). The reaction is first-order
with regard to the catalyst [Ni] and silane [PhSiH3], meaning
that the Ni catalyst and silane are involved in the rate-
determining step (RDS) as reactants. Meanwhile, the reaction
is zero-order with respect to 1-octene concentration, meaning
that olefin is not involved during the RDS and the addition
between the Si−H bond and olefin is not the RDS. It was also
confirmed by the kinetic isotope effect (KIE) experiments that
the value of KIE in the hydrosilylation reaction with PhSiH3/
PhSiD3 was determined to be 1.0 (Scheme 4e). Additionally,
the electronic effect of the Si−H bond exhibited an obvious
effect on the reaction rate (Scheme 4f). The electron-deficient
silane (2gg) is more active to react than electron-donating
silane p-OMeC6H4SiH3 (2hh). Hence, we speculated that the
electrophilic activation of the Si−H bond is probably the RDS.
a
General reaction conditions: olefin (1, 2.0 mmol), silane (2, 2.4
t
mmol), NiCl2·6H2O (9.5 mg, 0.04 mmol, 2.0 mol %), BuOK (9.0
mg, 0.08 mmol, 4.0 mol %), THF (3.0 mL), −30 °C, 1 h. The yields
b
c
are isolated yields. The reaction time was extended to 4 h. PhSiH3
d
(489 mg, 4.4 mmol, 2.2 equiv) was used. The reaction temperature
was set to −40 °C.
metal hydride-catalyzed reactions, retention of the internal
alkenes and C−X bonds is difficult.18 A variety of functional
groups are well-tolerated to generate the corresponding
products bearing pyridine (1q), furan (1r), thiophene (1s),
indole (1t), benzotriazole (1u), epoxy (1x and 1y), cyano
(1z), acetal (1bb), ester (1cc), and amide (1dd) moieties in
good yields (83−93%). Hydrosilylation of a strained imide
substrate (1ee) proceeded in 71% yield, and the ketone
substrate (1aa) was reduced to alcohol, as seen in our previous
work.19 The hydrosilylation of substrates containing N−H and
O−H moieties ran along with intermolecular and intra-
molecular dehydrosilylation (Figure S1). In addition, styrene
(1e) gave a mixture of linear and branched products in a 5:2
ratio and 94% yield.
The effect of hydrosilanes was also investigated. Alkyl silane
2ff exhibited excellent reactivity to give 3ff in 94% yield. An
electron-donating aryl silane, such as a methoxyl group [p-
OMeC6H4SiH3 (2hh)], had no obvious impact on selectivity,
and 3hh was obtained in 95% yield; on the contrary, an
electron-deficient aryl silane [p-CF3C6H4SiH3 (2gg)] gave its
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Letter
Scheme 4. Mechanistic Studies
Scheme 5. Proposed Mechanism
undergoes an unconventional electrophilically activated hydro-
silylation, differing from reported first-row metal-catalyzed
hydrosilylation with primary silanes using Fe,16a,21a,22 Co,20a,23
Ni,7b,24 Cu,25 and Mn26 catalysts.
In conclusion, a Ni-catalyzed selective anti-Markovnikov
hydrosilylation of terminal alkenes in the absence of extra
ligands has been developed. This protocol is economical and
operationally simple. A new electrophilic hydrosilylation
mechanism, rather than oxidative addition and radical
mechanisms, has been proposed in which the CC bond is
attacked directly by an electrophilic Si−H bond with the
assistance of a Ni(0) species and activator. Due to this
unconventional mechanism, this reaction exhibits exclusive
anti-Markovnikov selectivity for alkyl alkenes with a high TON
(up to 5500) and broad tolerance for functionalized alkenes
bearing sensitive groups, such as allyl ethers, halides, esters,
amides, and various heterocycles, in good yields (71−94%).
This electrophilic activation mode can be extended to a range
of types of silanes with potential application for the synthesis of
multifunctional silanes.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00111.
Detailed experimental procedures, mechanistic studies,
and characterization data (PDF)
On the basis of the evidence presented above, we have
proposed a feasible reaction mechanism based on electrophilic
Si−H bond activation, as illustrated in Scheme 5. The reaction
begins with the reduction of nickel(II) to nickel(0) by the
primary silane. To avoid deposition, the nickel(0) species are
wrapped in complexes composed of alkene substrates and
solvent molecules at low temperatures.15a It explains why
coordinating solvents are beneficial for the reaction (Table
AUTHOR INFORMATION
Corresponding Authors
■
Zhaoguo Zhang − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China; State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai 200032,
China; orcid.org/0000-0003-3270-6617;
t
S4). The activator, BuOK, and silane are combined into an
energetic pentacoordinate silicate. The RDS is the step I in
which the Si−H bonds of the pentacoordinate silicate displace
the weakly coordinated solvent molecule to form the active η2-
(Si-H)Ni(0) complex. This process completes the electrophilic
Email: zhaoguo@sjtu.edu.cn
1
activation of the Si−H bond, which can be observed by H
Xiaomin Xie − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China; orcid.org/0000-0002-5798-
291X; Email: xiaominxie@sjtu.edu.cn
NMR (Figure S12).21 Next, electrophilically intermolecular or
intramolecular attack from the Si−H bond to alkene occurs
and directly completes the addition. Then the secondary silane
product and activator are released. In summary, this protocol
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Organic Letters
Authors
pubs.acs.org/OrgLett
Letter
M.; Hu, X. Chemoselective Alkene Hydrosilylation Catalyzed by
Nickel Pincer Complexes. Angew. Chem., Int. Ed. 2015, 54, 14523−
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Xiaoyu Wu − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
Guangni Ding − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
Wenkui Lu − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
Liqun Yang − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
Jingyang Wang − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
Yuxuan Zhang − Shanghai Key Laboratory for Molecular
Engineering of Chiral Drugs, School of Chemistry and
Chemical Engineering, Shanghai Jiao Tong University,
Shanghai 200240, China
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ACKNOWLEDGMENTS
■
The authors acknowledge the financial support provided by the
National Natural Science Foundation of China.
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