Cationic nickel(II)-catalyzed hydrosilylation of alkenes: Role of p, n?type ligand scaffold on selectivity and reactivity

Article abstract of DOI:10.1021/acs.organomet.0c00551

Seven structurally similar cationic nickel(II)?alkyl complexes were synthesized by using a series of P, N ligands, and their reactivity was explored in the hydrosilylation of alkenes. More electron-rich phosphines enhanced the overall reactivity of the transformation; in contrast, groups on the imine donor had little impact. Overall, these catalysts displayed reactivity and selectivity that was previously unknown or very rare in nickel-catalyzed hydrosilylation. In reactions with Ph₂SiH₂, 1,2-disubstituted vinylarenes showed complete benzylic selectivity for silane addition, whereas terminal selectivity was observed for 1,1-disubstituted alkenes. The related PhSiH₃ led to exclusively Markovnikov selectivity for monosubstituted vinylarenes with no competing double addition observed. Mechanistic investigations supported the hypothesis that a Ni?H functions as the active species in this catalytic hydrosilylation, which in turn also showed catalytic competence for the silane redistribution reaction, especially with sterically unhindered silanes.

Full text of DOI:10.1021/acs.organomet.0c00551

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Article
Cationic Nickel(II)-Catalyzed Hydrosilylation of Alkenes: Role of P,
N‑Type Ligand Scaffold on Selectivity and Reactivity
Istiak Hossain and Joseph A. R. Schmidt*
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ABSTRACT: Seven structurally similar cationic nickel(II)−alkyl complexes
were synthesized by using a series of P, N ligands, and their reactivity was
explored in the hydrosilylation of alkenes. More electron-rich phosphines
enhanced the overall reactivity of the transformation; in contrast, groups on
the imine donor had little impact. Overall, these catalysts displayed reactivity
and selectivity that was previously unknown or very rare in nickel-catalyzed
hydrosilylation. In reactions with Ph2SiH2, 1,2-disubstituted vinylarenes
showed complete benzylic selectivity for silane addition, whereas terminal selectivity was observed for 1,1-disubstituted alkenes.
The related PhSiH3 led to exclusively Markovnikov selectivity for monosubstituted vinylarenes with no competing double addition
observed. Mechanistic investigations supported the hypothesis that a Ni−H functions as the active species in this catalytic
hydrosilylation, which in turn also showed catalytic competence for the silane redistribution reaction, especially with sterically
unhindered silanes.
idine (BPI), iminopyridine oxazoline (IPO),12 and bis-
INTRODUCTION
Hydrosilylation of alkenes is one of the most important
■
(oxazolinylphenyl)amide.13 Additionally, several bidentate
ligands12,14 were also employed in base-metal catalysis;
transformations in homogeneous catalysis, performed in large
scale to produce organosilicon feedstocks.1 These compounds
however, to date, the utilization of bidentate P, N-type ligands
in alkene hydrosilylation is rare. This class of nonsymmetrical
ligand is composed of one soft (P) and one hard (N) donor
atom and has been previously employed for many successful
catalytic transformations.15 Having two different donor sites in
the scaffold opens up several opportunities for tuning of the
sterics and electronics with suitable substituents. Figure 1
shows the general structure of the P, N-type scaffold and
indicates the various sites that can be modified in an effort to
are then broadly applied as monomers in the production of
silicone rubbers, oils, and resins.1 Synthetically, the production
of carbon−silicon bonds is also valuable as the resultant
products can be utilized as transmetalating reagents in coupling
reactions2 as well as stereospecific oxidation.3 Traditionally,
two platinum-based complexes, known as Speier’s4 and
Karstedt’s5 catalysts, are widely used in industry due to their
stability and selectivity. However, their use is not sustainable,
as platinum is expensive and has a low natural abundance, and
the low recovery from the organosilicon products hampers its
recyclability.6 To find viable replacements, in the past two
decades, research in this field has targeted the development of
catalysts using earth-abundant, environmentally friendly base
metals.7 One of the key challenges in utilizing base-metal
catalysts is to control selectivity. For many of these systems, in
addition to regioisomers, products are often also contaminated
by olefin isomers, polymers, and dehydrogenative silylation
products, leading to poor overall yield.7 Furthermore, products
resulting from double or triple hydrosilylation reactions can
also compete when primary or secondary silanes are used.7 To
achieve high selectivity, careful design and tuning of a ligand
scaffold is a critical step for the development of base-metal
catalysts.8 In 2004, the Chirik group reported a remarkably
efficient iron complex supported by a bis(imino)pyridine
(MePDI) ligand.9 The report triggered many studies using
analogous N3-type scaffolds to support alkene hydrosilylation
catalysts, including an alternate bis(imino)pyridine (EtPDI),10
terpyridine,11 bis(oxazolinyl)pyridine (PyBox),11b iminobipyr-
Figure 1. General structure of the P, N-scaffold depicting sites of
variation.
Received: August 18, 2020
https://dx.doi.org/10.1021/acs.organomet.0c00551
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Figure 2. List of cationic Ni complexes examined in this study.
Figure 3. X-ray crystal structures of 1a, 1b, 1d, and 1g with 50% probability thermal ellipsoids. Selected bond distances (Å) and angles (deg) are
given. Hydrogen atoms and triflate counterions are omitted for clarity.
obtain the desired reactivity. Over the past decade, our group
has published numerous catalytic transformations using Pd and
Ni supported by a P, N-type ligand known as 3-
iminophosphine (3IP).16
Utilizing a range of 3IP ligands, a strong correlation was
observed between catalytic activity and ligand scaffold/
substituents in related hydroamination reactions.17 These 3IP
ligands have also proven successful in the highly efficient
hydrosilylation of allenes and ketones using palladium18 and
nickel centers,19 respectively. These previous findings set the
stage for this study, where we sought to attain several goals,
including application of nickel complexes of P, N ligands in the
hydrosilylation of alkenes and investigation of how the
variation in the P, N scaffold affects the reactivity and/or
selectivity with different silanes. Additionally, since most
aforementioned studies were limited to the use of mono-
substituted alkenes, the current effort also aimed to employ
nickel complexes in the hydrosilylation of more highly
substituted alkenes.
Results showed some promising catalytic activities with rare
regiocontrol in product formation. We observed terminal
selectivity when employing 1,1-disubstituted alkenes and
benzylic selectivity for 1,2-disubstituted vinylarenes. This
interchangeable regioselectivity based on the substitution
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pattern of the alkenes using the same base-metal catalyst was
previously unknown. These catalysts can also hydrosilylate
vinylarenes with PhSiH3, giving the internal products with high
selectivity.
Table 1. Catalyst Testing: Hydrosilylation of α-
Methylstyrene with Ph2SiH2
RESULTS AND DISCUSSION
■
In this study, a series of cationic nickel−alkyl complexes 1a−
1g, supported by various P, N ligand frameworks (Figure 2),
were examined as hydrosilylation catalysts. Ligands were
prepared by following literature procedures,17a,20 with minor
modifications in some cases. For synthesis of the catalysts, we
used the previously developed protocol from our group.16c As
shown in Figure 2, the chosen complexes displayed steric and
electronic diversity with various modifications throughout the
ligand scaffold. Substituents on the phosphine donor atom
were varied from the standard phenyl group (1a−1e) to more
electron-donating tert-butyl (1f) and cyclohexyl (1g) groups.
An sp3 dimethylamino group (1a) and two sp2 imino groups
(tert-butyl and xylyl) were tested as the N donors of the P, N
ligands. Other variables included aromatic (1a and 1b) versus
nonaromatic backbone units (1c−1g) and benzyl versus allyl
groups as the catalyst initiating ligand on the metal center.
Newly synthesized catalysts 1a, 1b, 1d, and 1g were
characterized by single-crystal X-ray diffraction along with
various standard spectroscopic techniques (see the Supporting
Information). As shown in Figure 3, each of the nickel
complexes exhibited pseudo-square-planar geometry. The allyl
ligands in 1a, 1b, and 1g are bound via the η3 coordination
mode, and the Ni−C bonds oriented trans to the stronger
donor phosphines are longer than those trans to the N donor
due to the greater trans influence of phosphorus. Interestingly,
for catalyst 1d, the benzyl group was found to be severely bent
(Ni1−C27−C28 = 73.06°), indicating coordination by more
than one carbon atom. An examination of the C−C bond
lengths in the phenyl ring and the NMR spectroscopic data
confirmed an η2 coordination mode. This η2-coordinated
benzyl ligand serves as a three-electron donor and is similar to
that observed in a previously reported nickel complex.21
For an initial comparison of the reactivity of the seven nickel
catalysts, α-methylstyrene (2a) was chosen as a test substrate,
primarily because substituted alkenes are recognized as
inherently difficult substrates for hydrosilylation.22 Reactions
were performed in deuterated benzene for ease of monitoring
with 1H NMR spectroscopy. All nickel complexes 1a−1g were
tested with 5 mol % catalytic loading at room temperature. As
shown in Table 1, having the sp3 dimethylamino group on the
P, N scaffold completely inhibits the reaction, and no product
was detected by using 1a (entry 1). Changing the amino group
to a weaker σ-donor imino group resulted in modest catalytic
activity, producing the terminal product 3a in 30% yield after
24 h (Table 1, entry 2). No other hydrosilylation products
a
b
entry
[Ni] cat.
time (h)
NMR conv (%)
1
2
3
4
5
6
7
1a
1b
1c
1d
1e
1f
24
24
24
24
24
24
16
NR
30
33
27
29
94
93
1g
a
Conditions: alkene (0.25 mmol), Ph2SiH2 (1.1 equiv), and [Ni] cat.
(5 mol %) in 0.6 mL of C6D6. Conversion was calculated by using in
situ H NMR spectroscopy. Trace amounts (<5%) of Ph3SiH were
detected.
b
1
catalyst 1g, with a dicyclohexylphosphine group, furnished the
best rate (Table 1, entry 7).
With the most reactive catalyst identified, we explored a
wide range of 1,1-disubstituted alkenes (Table 2). Initially,
different substituents on the phenyl ring of the α-
methylstyrene were tested including methyl (2b), methoxy
(2c), fluoro (2f), and chloro (2g, 2h) groups. Each provided
the corresponding products in excellent yield with terminal
selectivity. No significant steric inhibition of the catalysis was
observed for the substrates having ortho substituents (2d, 2h).
A substrate with a fused ring, 5-vinylbenzo[d][1,3]dioxole
(2e), also smoothly converted to the corresponding product
(3e) with excellent yield. We also examined aliphatic alkenes,
and they were found to be more reactive than the
corresponding vinylarenes. Limonene (2k) underwent hydro-
silylation efficiently to produce 3k as a 1:1 diastereomeric
mixture. Unlike previous hydrosilylation of limonene,23 our
catalysis was entirely selective for the exocyclic 1,1-
disubstituted double bond with no competing reactions at
the trisubstituted internal alkene, even at higher temperatures
and with excess silane. We attribute this to the higher steric
hindrance of the internal alkene. The steric effect is also
evident for the substrates 2l and 2m, as they reacted very
slowly giving only modest yields. Overall, the catalytic activity
of 1g outperformed the previously reported nickel catalysts,24
either displaying improved reactivity or broader substrate
scope with 1,1-disubstituted alkenes. For instance, Trovitch25
recently reported a wide substrate scope for α-methylstyrenes
using an α-diimine−nickel complex, but it required higher
temperature and longer reaction time (7 days, 70 °C).
1
were detected by H NMR spectroscopy, but rather leftover
Internal alkenes were also hydrosilylated under the same
catalytic conditions utilizing 1g (Table 3). Unsymmetrical
internal alkenes 4a and 4b displayed complete benzylic
selectivity (>99%) to produce 5a and 5b in excellent yields.
This benzylic selectivity for internal alkenes is very rare in
metal-catalyzed hydrosilylation using secondary silanes. Pre-
viously reported metal catalysts using platinum,4 cobalt,12 and
nickel26 instead underwent tandem isomerization−hydro-
silylation to yield products with terminal silyl groups. To
date, there are a very few reports27 of analogous benzylic
selectivity, in those cases using the primary silane, PhSiH3.
unreacted alkene and a trace amount of Ph3SiH, resulting from
the redistribution of Ph2SiH2, were observed in addition to
product 3a. The comparison of 1a and 1b implied that the
presence of an imino donor group is critical to achieve
catalysis; however, changing other variants in the scaffold such
as the ligand backbone, imine substituents, or alkyl group on
the nickel had little effect on the catalytic performance (Table
1, entries 3−5). On the other hand, it was then subsequently
found that catalytic effectiveness can be greatly improved by
using electron-rich dialkylphosphines on the scaffold, and the
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Table 2. Hydrosilylation of 1,1-Disubstituted Alkenes Using the Nickel Complex (1g) with Diphenylsilane
a
b
Conditions: alkene (0.25 mmol), Ph2SiH2 (1.1 equiv), and 1g (5 mol %) in 0.6 mL of C6D6. Isolated yields are reported after purification by
c
d
e
column chromatography. 2-Phenyl-2-butene (35%) was observed as a byproduct. Reactions were run at 40 °C. Catalyst 1f was used.
Symmetrical cyclic alkenes 4c and 4d also reacted smoothly
under the reaction conditions, producing excellent yields.
Additionally, hydrosilylation of a cyclic enol ether, 2,3-
dihydrofuran (4e), yielded the corresponding product 5e,
with complete alpha selectivity. Such selectivity is comple-
mentary to that reported by Yao,23 whose system operates
under a radical mechanism.
Nickel catalysts were also evaluated for the hydrosilylation of
monosubstituted alkenes, including vinylarenes. Unsurpris-
ingly, they displayed enhanced reactivity compared to the
disubstituted alkenes, proceeding with lower catalyst loading.
As shown in Table 4, the selectivity for hydrosilylation versus
silane redistribution depended strongly on the steric properties
of the alkene substrates as well as the silane used. Sterically
encumbered, electron-donating tert-butyl- and cyclohexyl-
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yields. In contrast, catalytic silane redistribution reactions led
to product mixtures when using less hindered alkenes under
these reaction conditions. For example, 1-octene reacted with
Ph2SiH2 and Et2SiH2 to produce the expected hydrosilylation
products (7c and 7d) but also formed significant amounts of
7c′ and 7d′ as reaction products due to silane redistribution
reactions (Table 4, entries 3 and 4).
Table 3. Hydrosilylation of Internal Alkenes Using the
Nickel Complex (1g) with Diphenylsilane
The chemical preference for formation of the observed
redistribution products remains unclear, but since the
redistribution reaction is dependent on the steric bulk of the
substrates and the silanes, we reasoned that a more sterically
crowded ligand scaffold could potentially furnish better
selectivity for hydrosilylation without redistribution. Unfortu-
nately, our attempts to synthesize a well-defined Ni complex
analogous to 1g with sterically demanding mesityl groups on
the phosphine instead of cyclohexyl groups were not
successful. Though such a complex was likely formed, its
highly nonpolar nature thwarted the isolation of a clean
product, while its paramagnetism precluded our ability to
characterize the crude material. Moving on to styrene,
hydrosilylation led to a mixture of both linear and branched
isomers with Ph2SiH2, regardless of which nickel catalyst (1b−
1g) was employed (see the Supporting Information, Table S2).
The best branched to linear ratio was observed with the least
sterically hindered catalyst (1b) (Table 4, entry 5). On the
other hand, Et2SiH2 furnished excellent regioselectivity for the
a
Conditions: alkene (0.25 mmol), Ph2SiH2 (1.1 equiv), and 1g (5
1
branched isomer 7f (96%); in situ H NMR spectroscopy
b
mol %) in 0.6 mL of C6D6. Isolated yields are reported after
purification by column chromatography. Reaction was performed at
room temperature. NMR yield is reported.
showed complete consumption of the styrene within 3 h with 2
mol % of the catalyst, 1g. The efficiency and selectivity for
styrene are very comparable to those obtained with
(salicylaldiminato)Ni(II) catalysts.28
c
d
substituted ethylenes (6a and 6b) underwent rapid hydro-
silylation to yield the terminal silane products in excellent
Unlike Ph2SiH2, excellent Markovnikov selectivity (>97%)
was observed when PhSiH3 was reacted with styrene under
Table 4. Hydrosilylation of Monosubstituted Alkenes with Secondary Silanes
a
b
Conditions: alkene (0.5 mmol), R2SiH2 (1.1 equiv), and 1g (2 mol %) in 0.6 mL of C6D6. Unless otherwise specified, isolated yields are reported
c
d
after purification by column chromatography and product ratios were determined by GC-MS. NMR yield of 7c reported. No yield was
determined. Catalyst 1b was used.
e
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Table 5. Hydrosilylation of Vinylarenes with PhSiH3
a
b
Conditions: alkene (0.25 mmol), PhSiH3 (1.1 equiv), and Ni cat. 1g (5 mol %) in 0.6 mL of C6D6. Isolated yields are reported after purification
by column chromatography. In all cases excellent Markovnikov selectivity (>97%) was observed.
hydrosilylation catalysis conditions. Screening of the seven
nickel catalysts was performed for this reaction (see the
Supporting Information, Table S3). Although nickel complex
1a with an amino donor was inactive for hydrosilylation,
complexes 1b−1g were all found to be effective catalysts. In
terms of reactivity, 1f and 1g showed the best catalytic
performance; thus, catalyst 1g was chosen for screening of
substrate scope, as above. The various different ligand scaffolds
had no impact on the Markovnikov selectivity (b:l > 97%),
yielding benzyl(phenyl)silane in all cases. Notably, the
benzyl(phenyl)silane product did not compete with PhSiH3
as a hydrosilylation substrate under these reaction conditions.
Even in the presence of 2 equiv of styrene, very low conversion
(<10%) was detected for the double addition product
(dibenzylphenylsilane). Table 5 shows the substrate scope
for hydrosilylation of vinylarenes with PhSiH3. Electron-
donating and electron-withdrawing groups including tert-
butyl (8c), methoxy (8d, 8e), acetoxy (8f), fluoro (8g),
chloro (8h), bromo (8i, 8j), and cyano (8l) were well
tolerated. As shown in Table 5, the position and nature of the
substituents on the vinylarenes had little impact on the
selectivity. However, unlike Ph2SiH2, ortho-substituted and α-
and β-substituted styrenes were not tolerated, producing no
significant conversion.
In previous work, Markonikov selectivity has been reported
with cobalt29 and iron27a catalysts, but for nickel there are few
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reports;30 these studies suffer from poor selectivity or very
limited substrate scope. Herein, we have demonstrated the
superiority of the P, N ligands, yielding at least 97% internal
selectivity for a wide range of vinylarenes.
To demonstrate the practical utility of this catalysis,
preparative scale reactions (2.5 mmol) were performed in
toluene for hydrosilylation of α-methylstyrene and styrene by
using Ph2SiH2 and PhSiH3, respectively (Scheme 1). Toluene
activated nickel complex (Scheme 2). Formation of CH2Br2
was observed when bromoform was added to a mixture of
catalyst 1d and Ph2SiH2 (see Figure S6). This reaction is
characteristic of the presence of Ni−H and has been observed
for several other nickel−hydride complexes.32 Further evidence
for the presence of a Ni−H as the active species includes the
isomerization of terminal alkenes to internal alkenes as
byproducts through a chain-walking mechanism, as will be
discussed in detail later. On the basis of our previous
studies16c,18 and the published report of a similar nickel
catalyst,31 we postulate that catalyst activation takes place
through a four-centered transition state as shown in Scheme 2.
The faster catalysis observed with the electron-rich
phosphines is likely related to their enhanced stabilization of
the Ni−H intermediates. When these stoichiometric reactions
were run for a longer time (16 h), redistribution of the silane
phenyl groups took place, yielding additional silanes. For
example, Ph2SiH2 and PhCH2SiHPh2 were produced in
addition to 10a when PhSiH3 was employed. Silane
redistribution side reactions are often observed for metal-
catalyzed hydrosilylation reactions and a metal−silyl complex
is the commonly accepted key intermediate.33 Treatment of
catalyst 1d or 1g with 20 equiv of PhSiH3 resulted in its very
slow conversion to Ph2SiH2 along with a trace amount of other
unidentified species (Scheme 3a).
Scheme 1. Preparative Scale Reactions (2.5 mmol) with 3
mol % of the Catalyst
is a more environmentally benign solvent, frequently used in
organic synthesis in place of benzene. Utilizing a lower catalyst
loading (3 mol %), we observed excellent results, similar to
benzene without compromising yield or selectivity.
In contrast to other related systems,29d,31 we did not observe
any significant silane dehydrocoupling reactions with PhSiH3
(see Figure S7). Recently, Waterman34 studied the relationship
between ligand sterics and product distribution in an iridium
system. He observed that less sterically hindered ligands
favored silane redistribution reactions, while bulkier catalysts
led to dehydrocoupling of PhSiH3. None of the nickel catalysts
tested herein showed any significant steric effects with tert-
butyl, phenyl, xylyl, or cyclohexyl groups. Thus, based on this
past report,34 it is likely that the catalysts tested herein were
not sterically crowded enough to favor silane dehydrocoupling
reactions. As mentioned earlier, nickel complex 1a with an
amino donor group was catalytically inactive, while variation of
the substituent on the imine did not provide any notable
impact on the catalysis. These findings can be attributed to the
hemilability associated with P,N ligands.35 The amino donor
group likely coordinates strongly to the Ni(II) center, while
the weak imino donor can reversibly coordinate and
decoordinate from the metal. Because of this hemilability,
the imine substituent had little impact on catalytic efficiency.
Decoordination of the imine exposes an additional coordina-
tion site and provides access to the nickel center for sterically
Mechanistic Insights. Metal-catalyzed hydrosilylation
typically proceeds via metal−hydride or metal−silyl inter-
mediates, formed via either oxidative addition or σ-bond
metathesis within the catalytic cycle.7,31 Several stoichiometric
reactions were undertaken to probe the operating mechanism
for our nickel catalysts. We chose to investigate the nickel
complex 1d with a benzyl ligand rather than an allyl substituent
for various reasons. First, if a benzylphenylsilane was formed, it
would be unlikely to undergo further hydrosilylation.
Alternatively, if the alkyl ligand was liberated, it would produce
the less volatile toluene instead of propene gas. Treatment of
the nickel complex 1d with excess PhSiH3 or Ph2SiH2
produced the corresponding benzylsilanes (10a or 10b) and
a paramagnetic nickel species after 3 h (Scheme 2). The newly
formed silanes were detected by GC-MS (see Figures S4 and
S5). The resulting nickel complex proved to be resistant to
isolation, and its paramagnetic nature made in situ NMR
characterization inconclusive. Based on mass balance, the
product is presumably a nickel−hydride complex. In addition,
we investigated a halogen exchange reaction with the silane-
Scheme 2. (a) Proposed Activation Pathway of the Precatalyst 1d and (b) Halogen Exchange Reaction with the Silane-
Activated Nickel Catalyst
G
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Scheme 3. (a) Catalytic Redistribution of PhSiH3 and (b) Proposed Mechanism Based on a Previous Report31
Scheme 4. Proposed General Representation of the Operating Mechanism
Scheme 5. Key Observations for Mechanistic Insights
hindered substituted alkenes. Valerga24b also noted the
importance of hemilability in a similar cationic nickel−allyl
catalyst. Because of the likely hemilability of the P, N ligands,
the effective steric congestion at the metal center is not
significant. Thus, the observed selectivity in these catalytic
reactions is more likely controlled instead by the steric and
electronic features of the alkenes and silanes employed.
Utilizing this composite set of observations, we propose a
general mechanism (Scheme 4) for the nickel-catalyzed
hydrosilylation reported herein with two complementary
pathways (A and B). Each pathway involves the migratory
insertion of an alkene CC bond into the Ni−H intermediate
(1-H), followed by subsequent σ-bond metathesis. As shown
in Scheme 4, the 2,1- and 1,2-insertion steps are reversible and
yield a mixture of 2′ and 3′, respectively. Notable supporting
evidence for this reversibility derives from the formation of 2-
phenyl-2-butene (3i′) when 2-phenyl-1-butene (2i) was
subjected to the catalytic conditions (Scheme 5a). The side
product 3i′ was obtained from 2,1-insertion into the Ni−H
followed by β-H elimination, resulting in a chain-walking
reaction, whereas 3i was produced from 1,2-insertion and
subsequent σ-bond metathesis (path B). This result also
suggests that the insertion step does not set the regioselectivity
in these reactions, even for substituted alkenes. Rather, it
would seem that a mixture of 1,2- and 2,1-insertion
intermediates is formed, and their rate of silylation determines
product selectivity. Therefore, we suggest that gem-disub-
stituted alkenes proceed predominantly via path B following
the Curtin−Hammett principle as TS1 is predicted to have
significantly higher energy compared to TS2. The high
Markovnikov selectivity for Et2SiH2 and PhSiH3 toward
styrene was attributed to their reduced steric effects with
such a monosubstituted alkene. Because vinylarenes have an
electronic preference for 2,1-insertion,36 these small silanes
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follow the electronically favored path A to yield benzylsilanes.
Path B becomes competitive for moderately hindered Ph2SiH2,
resulting in a mixture of isomers, as noted earlier in Table 4
(entry 5). This steric effect was further examined by employing
an even more sterically hindered silane (6d′) with styrene
(Scheme 5b). In this case, the reaction was very slow, and we
observed a much higher proportion of linear products (12a)
than we had with Ph2SiH2. Similar silane-dependent
regioselectivity has also been observed recently with a cobalt
catalyst.29b Furthermore, when PhSiH3 was utilized in the
hydrosilylation of allylbenzene 13a, we observed faster β-H
elimination (chain walking) than hydrosilylation, giving the
isomerized internal alkene (14a′) as the major product
(Scheme 5c). Thus, overall in this nickel-catalyzed hydro-
silylation, insertion of alkenes leads to a mixture of 1,2- and
2,1-insertion intermediates that proceed to either undergo β-
hydride elimination or hydrosilylation with product distribu-
tions based primarily on the comparative kinetics of these
reactions.
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
Joseph A. R. Schmidt − Department of Chemistry &
Biochemistry, School of Green Chemistry and Engineering,
College of Natural Sciences and Mathematics, The University of
Toledo, Toledo, Ohio 43606-3390, United States;
orcid.org/0000-0003-3019-0055; Phone: 419-530-1512;
Email: Joseph.Schmidt@utoledo.edu; Fax: 419-530-4033
Author
Istiak Hossain − Department of Chemistry & Biochemistry,
School of Green Chemistry and Engineering, College of Natural
Sciences and Mathematics, The University of Toledo, Toledo,
Ohio 43606-3390, United States
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.organomet.0c00551
We acknowledge that an alternative or competitive
mechanism can be proposed involving nickel−silyl intermedi-
ates for hydrosilylation. However, this is very unlikely as we
have seen the formation of β-methylstyrene (14a′), even when
allylbenzene (13a) was added to the mixture of catalyst 1g and
PhSiH3 after 16 h. Although lower conversion was observed,
the product ratio was almost the same as we had observed
when they were added concurrently, as reported in Scheme 5c.
This result indicates that the nickel hydride intermediate is
fairly stable and formation of the nickel silyl species is not
kinetically competent or catalytically active.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We gratefully acknowledge the support from the Natural
Sciences and Mathematics Instrumentation Center at The
University of Toledo for assistance with NMR and X-ray
crystallography. This work was supported by The University of
Toledo via a Summer Research Award.
REFERENCES
■
CONCLUSIONS
■
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.0c00551.
■
sı
Full experimental procedures; the characterization data
for the nickel complexes and hydrosilylation products
(PDF)
Accession Codes
CCDC 2012290−2012293 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
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