Distinct Catalytic Performance of Cobalt(I)- N -Heterocyclic Carbene Complexes in Promoting the Reaction of Alkene with Diphenylsilane: Selective 2,1-Hydrosilylation, 1,2-Hydrosilylation, and Hydrogenation of Alkene

Article abstract of DOI:10.1021/acscatal.8b02513

Selectivity control on the reaction of alkene with hydrosilane is a challenging task in the development of non-precious-metal-based hydrosilylation catalysts. While the traditional way of selectivity control relies on the use of different ligand type and/or different metals, we report herein that cobalt(I) complexes bearing different N-heterocyclic carbene ligands (NHCs) exhibit distinct selectivity in catalyzing the reaction of alkene with Ph₂SiH₂. [(IAd)(PPh₃)CoCl] (IAd = 1,3-diadamantylimidazol-2-ylidene) is an efficient catalyst for anti-Markovnikov hydrosilylation of monosubstituted alkenes. [(IMes)₂CoCl] (IMes = 1,3-dimesitylimidazol-2-ylidene) shows Markovnikov-addition selectivity in promoting the hydrosilylation of aryl-substituted alkenes. [(IMe₂Me₂)₄Co][BPh₄] (IMe₂Me₂ = 1,3-dimethyl-4,5-dimethylimidazol-2-ylidene) can catalyze hydrogenation of alkenes with Ph₂SiH₂ as the terminal hydrogen source. Mechanistic studies in combination with the knowledge on the steric nature of cobalt-NHC species suggest that (NHC)cobalt(I) silyl species and bis(NHC)cobalt(I) hydride species are the probable key intermediates for these hydrosilylation and hydrogenation reactions, respectively. The different steric nature of IAd versus IMes and the potential of IMes incurring π···π interaction with aryl-substituted alkenes are thought to be the causes of the observed 1,2- and 2,1-addition selectivity.

Full text of DOI:10.1021/acscatal.8b02513

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Article
Distinct Catalytic Performance of Cobalt(I)-NHC Complexes in
Promoting the Reaction of Alkene with Diphenylsilane: Selective 2,1-
Hydrosilylation, 1,2-Hydrosilylation, and Hydrogenation of Alkene
Yafei Gao, Lijun Wang, and Liang Deng
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02513 • Publication Date (Web): 07 Sep 2018
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ACS Catalysis
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Distinct Catalytic Performance of Cobalt(I)-NHC Complexes in
Promoting the Reaction of Alkene with Diphenylsilane: Selective 2,1-
Hydrosilylation, 1,2-Hydrosilylation, and Hydrogenation of Alkene
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*
Yafei Gao, Lijun Wang, and Liang Deng
State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Orꢀ
ganic Chemistry, University of Chinese Academy of Sciences, Chinese Academy of Sciences, 345 Lingling Road, Shanghai,
2
00032, P. R. China
ABSTRACT: Selectivity control on the reaction of alkene with hydrosilane is a challenging task in the development of nonꢀ
precious metalꢀbased hydrosilylation catalysts. While the traditional way of selectivity control relies on the use of different ligandꢀ
type and/or different metals, we report herein that cobalt(I) complexes bearing different Nꢀheterocyclic carbene ligands (NHCs)
exhibit distinct selectivity in catalyzing the reaction of alkene with Ph SiH . [(IAd)(PPh )CoCl] (IAd = 1,3ꢀdiadamantylimidazolꢀ2ꢀ
2
2
3
ylidene) is an efficient catalyst for antiꢀMarkovnikov hydrosilylation of monoꢀsubstituted alkenes. [(IMes) CoCl] (IMes = 1,3ꢀ
2
dimesitylimidazolꢀ2ꢀylidene) shows Markovnikovꢀaddition selectivity in promoting the hydrosilylation of arylꢀsubstituted alkenes.
[
(IMe Me ) Co][BPh ] (IMe Me = 1,3ꢀdimethylꢀ4,5ꢀdimethylimidazolꢀ2ꢀylidene) can catalyze hydrogenation of alkenes with
2 2 4 4 2 2
Ph SiH as the terminal hydrogen source. Mechanistic studies in combination with the knowledge on the steric nature of cobaltꢀ
2
2
NHC species suggest that (NHC)cobalt(I) silyl species and bis(NHC)cobalt(I) hydride species are the probable key intermediates
for these hydrosilylation and hydrogenation reactions, respectively. The different steric nature of IAd versus IMes and the potential
of IMes incurring π···π interaction with arylꢀsubstituted alkenes are thought the causes for the observed 1,2ꢀ and 2,1ꢀadditionꢀ
selectivity.
KEYWORDS: Nꢀheterocyclic carbene, cobalt, alkenes, hydrosilylation, hydrogenation
23
Introduction
tion of hydrosilanes with alkenes in the 1960s. Since then, a
12, 22
plenty of cobalt complexes featuring phosphine,
cyclopenꢀ
Transitionꢀmetalꢀcatalyzed hydrosilylation of alkene is a useꢀ
ful method for the production of organoꢀsilicon compounds,
and noble metal complexes of platinum, rhodium, and palladiꢀ
2
4
9, 15, 19
17
tadienyl, Nꢀheterocyclic carbene,
βꢀdiketiminato, 2,6ꢀ
and isocyanide ligands are found
1
0, 12, 21
20
diiminopyridine,
1, 2
effective in promoting alkene hydrosilylation. In terms of seꢀ
lectivity, the majority of these known cobalt catalysts facilitate
the addition reaction in an antiꢀMarkovnikov addition manner.
Cobalt complexes promoting Markovnikov hydrosilylation are
um are the most widely used catalysts. Hypothetically, the
addition of a hydrosilane to a terminal alkene can give two
types of alkylsilanes, antiꢀMarkovnikov addition (2,1ꢀaddition)
and Markovnikov addition (1,2ꢀaddition) products (Chart 1).
In some cases, side reactions of dehydrogenative silylation and
alkene hydrogenation, which afford vinylsilane and allylsilane,
and alkanes, respectively, also exist (Chart 1). Hence, selectivꢀ
ity control is a great challenge in the development of useful
hydrosilylation catalysts.
iꢀPr
Me
13
iꢀ
rarely known. The pincer system ( PNN )CoCl
(
2
Pr
Me
i
PNN
and Co(acac)2/ PDI
N=CMe) C H N), which are used in the Markovnikov hyꢀ
= 2ꢀ( Pr PCH )ꢀ6ꢀ(2’,6’ꢀMe C H ꢀN=CMe)C H N)
2 2 2 6 3 5 3
Mes 12 Mes
(
PDI = 2,6ꢀ(2’,4’,6’ꢀMe C H ꢀ
3 6 2
2
5
3
drosilylation of alkylꢀsubstituted alkenes with PhSiH , the
Co(acac) /Xantphos system that catalyzes the reactions of
3
12
2
arylꢀsubstituted
alkenes
with
PhSiH3,
and
the
t
18
(OIP)CoCl /KO Bu (OIP = chiral oxazoline iminopyridine
2
ligand) system that promotes the Markovnikov addition of
ArSiH3 with both arylꢀ and alkylꢀsubstituted alkenes, are
among the rare examples. Interestingly, all these Markovnikov
hydrosilylation systems utilize the primary hydrosilane PhSiH3
as the silane source. Cobalt complexes that can catalyze selecꢀ
tive dehydrogenative silylation of alkene are also scarce.
Co (CO) was known to catalyze the reactions of acrylates
2
8
Chart 1. Transition-Metal-Catalyzed Reactions of Alkenes
with Hydrosilanes
25
with tertiary hydrosilanes to form (E)ꢀ3ꢀsilyl acrylates. The
Mes
cobalt methyl complex ( PDI)Co(CH ) proved an efficient
3
catalyst for the reaction of alkylꢀsubstituted alkenes with terꢀ
In the pursuit of new economical and environmentally beꢀ
nign catalysts for alkene hydrosilylation reaction, great reꢀ
2
6
3ꢀ8
tiary silanes to produce allyl silanes in good yields. As the
only known example of cobaltꢀcatalyzed hydrogenation of
alkene using hydrosilane as hydrogen source, Rajanbabu and
cent efforts have been made to develop cobaltꢀbased
9ꢀ22
catalysts.
The capability of cobalt in promoting alkene hyꢀ
Dipp
Dipp
his coworkers showed that (
PDI)CoCl2
(
PDI = 2,6ꢀ
drosilylation has long been recognized since Chalk and Harꢀ
i
(
2’,6’ꢀ Pr C H ꢀN=CMe) C H N) in combination with Naꢀ
2
6
3
2
5
3
rod’s report on Co (CO) ꢀcatalyzed antiꢀMarkovnikov addiꢀ
2
8
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BEt H catalyzes the reaction of alkenes with (EtO) SiMeH to along with small amount of the Markovnikov hydrosilylation
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2
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form alkane. In this reaction, polymeric silicon compounds
were observed as byproduct and hydrosilylation product was
hardly observed.
product 3a (5%) and the hydrogenation product ethyl benzene
1+ [BPh4]
N
N
N
Mes
CoI Cl
Mes
N
N
N
Ad
N
Mes
CoI Cl
Mes
Mes
Mes
Mes
N
Mes
Mes
Ad
CoI
As the aforementioned results point out the viability of selecꢀ
tivity control upon selection of suitable catalysts, we are curiꢀ
ous whether the use of different cobaltꢀNHC complexes could
render selectivity control on the reaction of alkene with hyꢀ
drosilane. Previously, we found lowꢀcoordinate cobalt(II)
complex (NHC)Co(N(SiMe ) ) can catalyze the hydrosilylaꢀ
CoI Cl
Mes
Mes
N
N
N
N
Ph3P
N
N
Mes
[(IAd)(PPh3)CoCl]
[(IMes)2CoCl]
[(sIMes)2CoCl]
[(IMes)2Co][BPh4]
BPh4]
1+
[
1
+
[BF4]
R
N
Cl
NR
Cy
N
Cy
N
Cy
Cy
Cy
N
CoI
N
N
Cy
Cy
N
N
R
CoI
R
3
2 2
CoI
N
Cy
N
N
Cy
R
R
N
N
R = Me,
Et, iPr
tion of alkylꢀsubstituted alkenes with HSi(OEt) , affording
Cy
N
3
N
N
Cy
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
9
N
Cy
NR
R N
selectively antiꢀMarkovnikov addition products. The catalytꢀ
[(ICy)3Co][BF4]
[(ICy)3CoCl]
[(IR2Me2)4Co][BPh4]
ic competence of the cobalt(I) complex [(IPr)Co(N(SiMe ) )]
3
2
(
(
IPr = 1,3ꢀdi(2’,6’ꢀdiisopropylphenyl)imidazolꢀ2ꢀylidene) as
IPr)Co(N(SiMe ) ) indicated that cobalt(I) species could be
Chart 2. Cobalt(I)-NHC Complexes Examined in This
Study
3
2 2
the genuine catalytic species for the alkene hydrosilylation
reaction. In addition, cobalt(I)ꢀNHC complexes
[(IAd)(PPh )Co(CH SiMe )] and [(IAd)(PPh )Co(SiHPh )]
were also found effective in catalyzing the hydrosilylation of
alkynes with Ph SiH to give alkenylsilanes. The success
thus prompted exploration on different cobalt(I)ꢀNHC comꢀ
plexes with the aim of achieving selectivity control on the
reaction of alkene with hydrosilane. Pertinent to this goal, we
report herein cobalt(I) complexes featuring different NHC
ligands can indeed promote the reaction of alkene with
Ph SiH in different manner: [(IAd)(PPh )CoCl] is an effective
Table 1. Hydrosilylation of Styrene Using Cobalt Com-
2
plexes with Ph SiH2
3
2
3
3
2
2
8
2
2
2
2
3
catalyst for antiꢀMarkovnikov hydrosilylation of monoꢀ
substituted alkenes, [(IMes) CoCl] catalyzes Markovnikov
hydrosilylation
[
2
of
arylꢀsubstituted
alkenes,
and
(IMe Me ) Co][BPh ] shows selectivity in catalyzing the
2
2 4
4
hydrogenation of alkenes with Ph SiH as the terminal hydroꢀ
2
2
gen source. Mechanistic study in combination with the
knowledge on the steric nature of cobaltꢀNHC species led to
the proposals that cobalt(I) silyl and cobalt(I) hydride species
with NHC ligation, presumably (NHC)Co(SiHPh2) and
(NHC) CoH, are the key active species for the hydrosilylation
2
and hydrogenation reactions, respectively, and that the differꢀ
ent reaction outcome is controlled by the electroꢀsteric nature
of the lowꢀcoordinate cobalt(I)ꢀNHC species.
a
b
Concentration of styrene: 1.0 mol/L in toluene or THF; GC
c
yield based on styrene with nꢀdodecane as an internal standard; 5
mol% catalyst loading; Solvent: THF; 1.5 equiv. of Ph
were used; 1 mol% of [(IAd)Co(SiHPh )] inꢀsitu generated from
d
e
Results and Discussion
2
SiH
2
f
2
2
Catalytic performance of the Cobalt(I)-NHC Complexes.
Our previous synthetic study showed that monodentate NHCs
can support cobalt(I) species in diverse coordination geomeꢀ
tries, including trigonal planar, diagonal, tetrahedral, and
square planar. Chart 2 lists some examples of isolable coꢀ
balt(I)ꢀNHC complexes. The ratio of NHCꢀtoꢀcobalt in these
complexes varies from 4 to 3 to 2 and to 1 as the NHC ligand
changes from IR Me (1,3ꢀdialkylꢀ4,5ꢀdimethylimidazolꢀ2ꢀ
ylidene) to ICy (1,3ꢀdicyclohexylimidazolꢀ2ꢀylidene) to
IMes
demanding nature of these NHC ligands. Examining the cataꢀ
lytic performance of these cobalt complexes in the reaction of
styrene with Ph SiH revealed their different catalytic perforꢀ
the reaction of [(IAd)Co(vtms) ] with Ph SiH based on the reꢀ
2
2
2
ported procedure of ref. 41.
4a (2%) (entry 1). In stark contrast, the reactions using
bis(NHC)cobalt complexes [(IMes) CoCl] and [(sIMes) CoCl]
2
2
as catalysts showed different regioꢀselectivity, wherein the
Markovnikov hydrosilylation product 3a was formed as the
major product (66% and 58% GC yields, respectively, entries
2
2
29
30
2 and 3). When increasing the catalyst loading of
31, 32
28
and IAd , which parallels with the increased steric
[(IMes) CoCl] to 5 mol%, the yield of 3a could reach 93% in
2
16 h (entry 4). The ionic complex [(IMes) Co][BPh ] shows
2
4
similar catalytic activity and selectivity as [(IMes) CoCl] (enꢀ
2
try 5). On the other hand, reactions employing
tris(NHC)cobalt complexes [(ICy) CoCl] and [(ICy) Co][BF ]
2
2
mance.
3
3
4
as catalysts gave mixtures of 2a (23% and 15%), 4a (44% and
8%), and 5a (30% and 22%) (entries 6 and 7). Testing the
performance of tetra(NHC)cobalt(I) complexes
[(IR Me ) Co][BPh ] (R = Pr, Et, Me) revealed that the reacꢀ
As shown in Table 1, using the mono(NHC)cobalt complex
4
[
(IAd)(PPh )CoCl] (2 mol%) as catalyst, the reaction of styꢀ
3
o
rene with Ph SiH (1.2 equiv.) at 70 C for 10 hours gave the
antiꢀMarkovnikov hydrosilylation product 2a in 92% GC yield
2
2
i
2
2 4
4
tions using [(IPr Me ) Co][BPh ] and [(IEt Me ) Co][BPh ]
2
2 4
4
2
2 4
4
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as catalysts also afforded a mixture of 2a, 4a, and 5a (entries 8
and 9). However, the one using [(IMe Me ) Co][BPh ] could
of 9:1 to 27:1 (entries 1ꢀ9). The reaction of orthoꢀsubstituted
1
2
3
4
5
6
7
8
9
styrene 1ꢀmethylꢀ2ꢀvinylbenzene didn’t produce product even
2
2 4
4
o
produce the hydrogenation product 4a selectively (entry 10).
In the case with a higher catalyst loading of 5 mol%, the yield
of 4a could reach 70% (entry 11), and the amount of the hyꢀ
drosilylation and dehydrogenative silylation products are trace.
In contrast, the cobalt(I) phosphine complex [(PPh ) CoCl]
at 80 C for 12 hours, indicating the sensitivity of the catalytic
system to steric nature of C=C double bond (entry 10).
[(IAd)(PPh )CoCl] catalyzes the hydrosilylation of alkylꢀ
3
substituted alkenes with Ph SiH2 to give exclusively antiꢀ
2
Markovnikov hydrosilylation products in high yields (entries
11ꢀ14). The exclusive hydrosilylation of the terminal C=C
double bond of 4ꢀvinylꢀcyclohexene instead of the internal one
hints the siteꢀselectivity of the catalytic system (entry 12). This
selectivity should come from the different steric nature of the
two C=C double bonds in 4ꢀvinylꢀcyclohexene. Accordingly,
the catalytic trials using diꢀsubstituted alkenes 1,1ꢀ
diphenylethylene and (E)ꢀ3ꢀoctene did not produce any hyꢀ
drosilylation product (entries 15 and 16).
3
3
was found a poor catalyst in promoting the reaction of styrene
o
with Ph SiH as the reaction at 70 C for 48 hours only proꢀ
2
2
duced 2a, 3a and 4a in 10%, 2%, and 4%, respectively (entry
2).
Table
IAd)(PPh )CoCl as the Catalyst
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2.
Hydrosilylation
of
Alkenes
Using
(
3
2
mol% (IAd)(PPh3)CoCl
toluene, 70 oC, t
SiHPh2
R
+
Ph2SiH2
SiHPh2
+
R
R
1
1.2 equiv
2
(l)
major product
SiHPh2
3 (b)
Table 3. Hydrosilylation of Alkenes Using (IMes) CoCl as
2
entry a
conv.b (%)
selectivity b (l:b) yield c (%)
the Catalyst
Substrate
t (h)
10
1
>99
19:1
92
1a
1b
1c
2a
SiHPh2
2
3
10
10
>99
>99
2b
27:1
27:1
95
95
F
F
F
F
SiHPh2
SiHPh2
2c
2d
4
10
>99
20:1 d
94
1
1
1
1
d
e
f
Cl
Cl
SiHPh2
5
6
7
10
10
10
>99
>99
>99
9:1
85
92
90
2e
2f
Ph
Ph
SiHPh2
SiHPh2
16:1
13:1
g
2g
SiHPh2
SiHPh2
8
9
10
10
>99
13:1
11:1
91
85
1h
2h
2i
MeO
MeO
But
>99
But
1i
1j
0 e
-
1
12
10
NR
-
-
Ph
Ph
SiHPh2
SiHPh2
11
>99
84:1
89
1k
2k
2l
1
1
2
3
30
>99
>99:1
87
89
1l
SiHPh2
SiHPh2
30
12
>99
>99
>99:1
>99:1
1m
2m
2n
1
1
4
5
n-C6H13
n-C6H13
95
-
1n
Ph
48
48
NR
NR
-
-
-
-
Ph
1
6
-
a
b
Concentration of alkenes: 1.0 mol/L in toluene; Determined
c
by GC based on alkenes with nꢀdodecane as an internal standard;
Isolated yield; Determined by NMR; Temperature: 80 C.
d
e
o
Encouraged by the distinct selectivity of [(IAd)(PPh )CoCl],
3
[
(IMes) CoCl], and [(IMe Me ) Co][BPh ] in promoting the
2 2 2 4 4
reaction of styrene with Ph SiH , we further examined the
2
2
scope of alkene for these catalytic systems. [(IAd)(PPh )CoCl]
3
proved an efficient catalyst in catalyzing antiꢀMarkovnikov
hydrosilylation of Ph SiH with both alkylꢀ and arylꢀ
2
2
substituted alkenes in good yields and high selectivity. As
shown in Table 2, styrenes bearing either electronꢀ
withdrawing or electronꢀdonating groups (fluoro, chloro, meꢀ
thyl, phenyl, tertꢀbutyl, methoxy) at paraꢀ or metaꢀposition
could undergo hydrosilylation to produce alkyl diphenꢀ
ylsilanes in the yields of 85ꢀ95% with high linear/branch ratio
a
b
Concentration of alkenes: 1.0 mol/L in toluene; Determinedc
by GC based on alkenes with nꢀdodecane as an internal standard;
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Page 4 of 9
d
o
e
f
Isolated yield; Temperature: 80 C; 10 mol% catalyst loading;
GC yield.
cyclooctane was obtained merely in 15% yield (entry 10).
Prompted by the success with Ph SiH as the silane and hyꢀ
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
2
2
drogen source in the cobalt(I)ꢀNHCꢀcatalyzed reaction, we
Examining the reactions of alkenes with Ph SiH (1.2 equiv.)
2
2
also examined the reactions of other silanes, e.g. Et SiH,
3
using the bis(NHC)cobalt complex [(IMes) CoCl] (5 mol%) as
2
Ph SiH, (EtO) SiH, and PhSiH3 with styrene using
3
3
catalyst revealed the capability of this complex in catalyzing
Markovnikov hydrosilylation of arylꢀsubstituted alkenes. As
shown in entries 1ꢀ8 in Table 3, under the catalytic conditions,
vinyl arenes containing either electronꢀwithdrawing or elecꢀ
tronꢀdonating groups on aryl rings can undergo selective Marꢀ
kovnikov hydrosilylation reaction, giving alkyl diphenꢀ
ylsilanes with the linear/branched ratio of 1:10 to 1:57. In adꢀ
dition to substituted vinylbenzenes, 2ꢀvinylthiophene and viꢀ
nylnaphthalene can also undergo selective Markovnikov hyꢀ
drosilylation (entries 14ꢀ16). In contrast to the high selectivity
and high yields of the Markovnikov hydrosilylation of arylꢀ
substituted alkenes, the reactions of alkylꢀsubstituted alkenes
with Ph SiH using [(IMes) CoCl] as catalyst produced hyꢀ
[
(IAd)(PPh )CoCl], [(IMes) CoCl], and [(IMe Me ) Co][BPh ]
3 2 2 2 4 4
as catalyst. Unfortunately, neither hydrosilylation product nor
hydrogenation product were detected in appreciable yields
(
Table S1).
Table 4.
Hydrogenation
of
Alkenes
Using
[
(IMe Me ) Co][BPh ] as the Catalyst
2
2 4
4
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
2
2
drosilylation products in low yields (20ꢀ26%), among which
antiꢀMarkovnikov addition products are dominating (entries
1ꢀ13). An exception is the reaction of allylbenzene, which
1
produced hydrosilylation products in 40% yield with a lineꢀ
ar/branch ratio of 1:1.4 (entry 10). The major product (1ꢀ
phenylpropyl)diphenylsilane (3k’) might come from the seꢀ
26
quential alkene isomerizationꢀhydrosilylation processes. The
divergent selectivity in the hydrosilylation reactions of arylꢀ
and alkylꢀsubstituted alkenes using [(IMes) CoCl] as catalyst
2
indicates the importance of the aryl substituent on alkene for
the observed Markovnikov addition selectivity. Similar to
[(IAd)(PPh )CoCl],
the
bis(NHC)cobalt
complex
3
[(IMes) CoCl] also found ineffective in catalyzing hydrosiꢀ
2
lylation reactions of Ph SiH with the diꢀsubstituted alkenes 1ꢀ
2
2
methylꢀ2ꢀvinylbenzene, 1,1ꢀdiphenylethylene, and (E)ꢀ3ꢀ
octene (entries 9, 17 and 18), probably due to steric reason.
Examining the scope of alkene of the catalytic hydrogenation
reaction then revealed that the tetra(NHC)cobalt(I) complex
[
(IMe Me ) Co][BPh ] can catalyze the hydrogenation of both
2 2 4 4
alkylꢀ and arylꢀsubstituted alkenes with Ph SiH as the termiꢀ
2
2
nal hydrogen source (entries 1ꢀ9, Table 4). These reactions
generally gave hydrogenation products in moderate yields (52ꢀ
8%), and alkene hydrosilylation products (1ꢀ13%) were also
detected as byproducts. Different from the formation of polyꢀ
meric silicon compounds in Rajanbabu’s cobaltꢀcatalyzed
7
hydrogenation reactions using (EtO) SiMeH as hydrogen
a
b
2
Concentration of alkenes: 1.0 mol/L in THF; Determined byc
27
source, a disilane, Ph HSiSiHPh , was observed as a byprodꢀ
2
2
GC based on alkenes with nꢀdodecane as an internal standard;
uct in the current catalytic system. Noting that these reactions
generally have quantitative conversion of alkenes, but the
combined yield for the hydrogenation and hydrosilylation
products are less than 100%, there should be other unknown
side reactions that consume alkenes. Considering the probable
formation of cobalt hydride species in these reactions, we
speculate that oligomerization or polymerization of alkenes
Alkene isomerization was observed.
Mechanistic Consideration. The cobalt(I)ꢀNHCꢀcatalyzed
reactions disclosed above represent a unique example of selecꢀ
tivity control of a reaction upon tuning the steric nature of
ligands, which, in return, posed questions on their mechanism
and causes for the observed selectivity. Though their exact
mechanisms are far from clear at this stage, certain experiꢀ
mental evidences in combination with the established
knowledge on cobaltꢀNHC complexes suggest that lowꢀ
coordinate cobalt silyl and cobalt hydride species could be the
genuine catalysts for the hydrosilylation and hydrogenation
reactions, respectively.
3
3ꢀ38
might be the side reaction.
The validation of this speculaꢀ
tion, however, needs further study. In addition to monoꢀ
substituted alkenes, hydrogenation of diꢀsubstituted alkenes
with [(IMe Me ) Co][BPh ] as catalyst and Ph SiH as the
2
2 4
4
2
2
hydrogen source were also attempted. Unfortunately, limited
success was achieved. The complex [(IMe Me ) Co][BPh ]
2
2 4
4
proved ineffective in catalyzing the hydrogenation of 1,1ꢀ
diphenylethylene, (E)ꢀ3ꢀoctene, and (Z)ꢀ2ꢀoctene (entries 11ꢀ
3). In the case of cyclooctene, the hydrogenation product
The first supporting evidence for the involvement of cobalt
silyl intermediate for the hydrosilylation reactions is the eviꢀ
dence of trace amount of dehydogenative silylation products in
1
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ACS Catalysis
the hydrosilylation reactions of styrene with Ph SiH catalyzed the βꢀhydride elimination of the migratory insertion product
2
2
1
2
3
4
5
6
7
8
9
by [(IAd)(PPh )CoCl] and [(IMes) CoCl] (entries 1 and 4 in
(Scheme 1d). The capability of cobalt(I) species to perform
oxidative addition with hydrosilanes suggests that the proꢀ
posed steps involving cobalt(I) species might also operate in a
4s9equential oxidative additionꢀreductive elimination manner.
3
2
Table 1), which should arise from βꢀhydride elimination of βꢀ
3
9, 40
silylalkyl cobalt species.
Secondly, the reactions of styrene
4
7ꢀ
with Ph SiH2 using 8the cobalt(I) silyl complexes
2
2
41
[
(IAd)(PPh )Co(SiHPh )] and [(IAd)Co(SiHPh )] as cataꢀ
3 2 2 2
lysts also resulted in antiꢀMarkovnikov hydrosilylation (enꢀ
tries 13 and 14 in Table 1). The relatively low catalytic effiꢀ
ciency of [(IAd)Co(SiHPh )]
as compared to
(IAd)(PPh )Co(SiHPh )] indicates the beneficial role of PPh
3
2
2
[
3
2
for improving catalytic efficiency, probably due to its capabilꢀ
ity to stabilize reactive cobalt silyl species in mononuclear
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
manner. Similar effect has been observed
on
[(IAd)(PPh )Co(SiHPh )]ꢀcatalyzed alkyne hydrosilylation
3
2
28
reaction. With the aim to prepare cobalt silyl complexes diꢀ
rectly from cobalt(I)ꢀNHC chlorides, the reactions of
[
(IAd)(PPh )CoCl] and [(IMes) CoCl] with Ph SiH were tried.
3 2 2 2
These reactions did not lead to the isolation of the desired
cobalt silyl complexes, rather revealed the capability of the
two complexes in mediating the conversions of Ph SiH to
2
2
Ph SiH and Ph HSiSiPh H (Scheme 1aꢀ1c). Noting transitionꢀ
3
2
2
metal silyl species are recognized as the key intermediates in
42ꢀ45
transitionꢀmetal catalyzed silane redistribution reactions,
Scheme 2. Simplified Activation Mode of the Cobalt
Precatalysts and Possible Catalytic Cycles for the Cobalt-
Catalyzed Hydrosilylation and Hydrogenation Reactions
the formation of Ph SiH supports the proposal that cobalt silyl
3
species could be formed via the reaction of cobalt(I)ꢀNHC
chlorides with Ph SiH .
2
2
The migratory insertion reaction of B with alkenes could
proceed via either a 2,1ꢀinsertion or 1,2ꢀinsertion way that
could eventually lead to the observed selectivity of antiꢀ
4
Markovnikov addition or Markovnikov addition. Considering
the different selectivity of the hydrosilylation reaction of arylꢀ
substituted alkenes catalyzed by [(IAd)(PPh )CoCl] and
3
[
(IMes) CoCl] and the ineffectiveness of [(IMes) CoCl] in
2
2
facilitating Markovnikov hydrosilylation of alkylꢀsubstituted
alkenes, we thought that the selectivity observed in the reacꢀ
tions catalyzed by [(IAd)(PPh )CoCl] and [(IMes) CoCl]
3
2
might be controlled by the different steric nature of the NHC
ligands and also the capability of incurring π···π interaction
between the substituents.
Scheme 1. Reactions of the Cobalt(I)-NHC Complexes with
Ph SiH or Styrene
2
2
Based on these, we propose that the reactions of the coꢀ
balt(I)ꢀNHC chlorides with Ph SiH might give initially coꢀ
2
2
balt(I) hydride intermediates (A) and silyl chloride (Scheme
46
2). The hydride species then interact with Ph SiH to form
2
2
Figure 1. Spaceꢀfilling models of (IAd)Co (top) and (IMes)Co
cobalt(I) silyl intermediates (B) and H . Once formed, cobalt(I)
2
(
[
bottom) generated from the crystal structures of
silyl intermediates could interact with alkenes via migratory
insertion to form cobalt alkyl species (C or C’) that further
interact with Ph SiH to give hydrosilylation products and
28 32
(IAd)(PPh )CoCl] and [(IMes) CoCl] , respectively. Blue
3
2
ball: Co; grey ball: carbon; white ball, hydrogen.
2
2
regenerate cobalt(I) silyl species B. In supportive to the proꢀ
posed migratory insertion step, the interaction of
The NHC ligands IAd and IMes differ in their steric nature
(Figure 1). Consequently, an alkene ligand on the cobalt center
of cobalt silyl alkene intermediate with IAd ligation, presumaꢀ
bly (IAd)Co(RCH=CH )(SiHPh ), might position its substitute
[
(IAd)(PPh )Co(SiHPh )] that was generated inꢀsitu from the
3 2
reaction of [(IAd)(PPh )Co(CH SiMe )] with Ph SiH with
3
2
3
2
2
2
2
28
styrene (2 equiv.) was found to afford the alkenyl silane (Z)ꢀ
R in the valley between two adamantyl groups (Figure 2a) to
have least steric repulsion among the silyl, R, and Ad groups,
PhCH=CHSiH Ph in 26% GC yield, which should arise from
2
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ACS Catalysis
Page 6 of 9
that will give antiꢀMarkovnikov addition products. Similar
steric model has been applied to explain the selectivity of the
alkyne hydrosilylation reactions catalyzed by
[(IAd)(PPh )Co(CH SiMe )]. As for the catalytic reactions
Table 5. Catalytic Performance of In-Situ-Generated Co-
balt-NHC Species in Catalyzing the Reaction of Styrene
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
with Ph SiH2
2
2
8
3
2
3
using [(IMes) CoCl], the steric nature of IMes might force the
2
coordinated aryl alkene ArCH=CH to position its methylene
2
group being proximal to IMes and its aryl group to mesityl to
5
0, 51
have π···π interaction
(Figure 2b), which then induces
Markovnikov addition selectivity. The proposed π···π interacꢀ
tion could explain the distinct selectivity in the hydrosilylation
reactions of arylꢀ and alkylꢀsubstituted alkenes catalyzed by
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[
(IMes) CoCl]. Considering this, a 1:1.4 mixture of linear and
2
branched addition products in the reaction of allylbenzene
with Ph SiH catalyzed by [(IMes) CoCl] might be due to the
2
2
2
capability of allylbenzene to form π···π interaction with the
mesityl group on IMes. The validation of these proposed modꢀ
els need further theoretical study.
a
Concentration of styrene: 1.0 mol/L in THF, and 1.2 equiv. of
b
KC were used (relative to cobalt); GC yield based on styrene
with nꢀdodecane as an internal standard; 1.5 equiv. of Ph SiH
8
c
2
2
were used.
Another important mechanistic question is the cause for the
selectivity of hydrosilylation versus hydrogenation. Following
the proposed mechanisms in Scheme 2, the selectivity should
come from the preference of the cobalt hydride species
(NHC) CoH (A) or cobalt silyl species (NHC) Co(SiHPh ) (B)
m
m
2
to react selectively with Ph SiH or alkene (Scheme 2). Thus,
2
2
we reason that the number of NHC ligands in these cobalt
species should play a key role. As the synthesis of lowꢀ
coordinate cobalt hydride and silyl species with IMes and
IMe Me ligation proved challenging, we examined the perꢀ
2
2
formance of inꢀsitu generated cobalt(I)ꢀNHC catalysts that
have different NHC/cobalt ratio. As shown in Table 5, with a
NHC/cobalt ratio of 1:1, the catalytic systems using IAd and
IMes could promote selective antiꢀMarkovnikov and Markovꢀ
nikov hydrosilylation reaction, respectively (entries 1 and 2),
whereas, the catalytic systems using IMe Me as ligand with
Figure 2. Proposed steric models rendering different selectiviꢀ
ty of the cobalt(I)ꢀNHC complexꢀcatalyzed hydrosilylation
reactions.
For the alkene hydrogenation reactions catalyzed by
2
2
[
(IMe Me ) Co][BPh ], we propose that cobalt hydride species
2 2 4 4
52ꢀ54
IMe Me/cobalt ratios of 2:1 and 3:1 proved more effective in
2
should be responsible for the alkene insertion step.
Upon
catalyzing hydrogenation of styrene than that with a 1:1 ratio
the interaction of [(IMe Me ) Co][BPh ] with of Ph SiH , the
2
2 4
4
2
2
(entries 3ꢀ5). Based on these, it can be speculated that the acꢀ
cobalt(I) complex might convert into a cobalt(I) hydride speꢀ
tive cobalt silyl species B in the catalytic cycles of the reacꢀ
cies A (Scheme 2). Noting the known reaction of nucleophilic
tions catalyzed by [(IAd)(PPh )CoCl] and [(IMes) CoCl]
5
5
3
2
attack of NHC toward hydrosilanes, this precatalyst activaꢀ
tion step might have 2ꢀsilyl imidazolium salt as the byproduct.
might have the identity of (NHC)Co(SiHPh ), and that of the
2
cobalt hydride species A in the reactions catalyzed by [(IMe
29
Indeed, GC and Si NMR analyses indicated the formation of
a small amount of 2ꢀsilyl imidazolium salt in the reaction mixꢀ
Me ) Co][BPh ] might be (IMe Me ) CoH. The steric nature
2
2 4
4
2
2 2
of (NHC) Co and πꢀπ interactions between (NHC) Co and aryl
n
n
ture
of
Ph SiH
with
[(IMe Me ) Co][BPh ]
or
2
2
2
2 4
4
substituted alkene could induce the observed selectivity.
[
(IAd)(PPh )CoCl]. The hydride species A then reacts with
3
alkene to give an alkyl intermediate D that further interact
with Ph SiH to yield the hydrogenation product and cobalt
Conclusion
2
2
silyl intermediate B. Species B interact with another equivaꢀ
lent of Ph SiH to regenerate the cobalt hydride species A with
We
disclosed
that
cobalt(I)ꢀNHC
complexes
and
2
2
[(IAd)(PPh )CoCl],
[(IMes) CoCl],
3
2
Ph HSiSiHPh as the byproduct that was observed in the cataꢀ
2
2
[(IMe Me ) Co][BPh ] display distinct performance in catalyzꢀ
2 2 4 4
lytic reactions. As a probable alternative mechanism, the hyꢀ
drogenation products might arise directly from cobaltꢀ
catalyzed hydrogenation of alkene with H2 that could be
ing the reaction of alkene with Ph SiH . [(IAd)(PPh )CoCl] is
2
2
3
an effective antiꢀMarkovnikov hydrosilylation catalysts for a
series of monoꢀsubstituted alkenes, [(IMes) CoCl] is an effecꢀ
2
formed from cobaltꢀcatalyzed dehydrogenation of Ph SiH .
2
2
tive Markovnikov hydrosilylation catalysts for the hydrosilylaꢀ
tion of arylꢀsubstituted alkenes, and [(IMe Me ) Co][BPh ]
This route, however, seems unlikely, as the cobalt complex
(IMe Me ) Co][BPh ] is a poor catalyst in promoting the deꢀ
2
2 4
4
[
2
2 4
4
catalyzes hydrogenation of alkenes with Ph SiH as hydrogen
2 2
hydrogenation of Ph SiH (Scheme 1a).
2
2
source. For the hydrosilylation reactions, cobalt(I) silyl species
was proposed as the key intermediates. The different steric
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ACS Catalysis
nature of IAd versus IMes and the potential of IMes incurring
(11) Schuster, C. H.; Diao, T.; Pappas, I.; Chirik, P. J. Benchꢀ
Stable, SubstrateꢀActivated Cobalt Carboxylate PreꢀCatalysts for
Alkene Hydrosilylation with Tertiary Silanes. ACS Catal. 2016, 6,
1
2
3
4
5
6
7
8
9
π···π interaction with aryl alkenes are believed the causes for
the 1,2ꢀ and 2,1ꢀadditionꢀselectivity. On the hydrogenation
reaction, bis(NHC)cobalt(I) hydride species was proposed as
the key inꢀcycle intermediates for the cobalt(I)ꢀNHC catalyzed
hydrogenation reaction. The study points out the wide vista of
cobaltꢀNHC complexes as nonꢀprecious metal catalysts for
other hydroꢀfunctionalization reactions which is under exploꢀ
ration in our laboratory.
2
632ꢀ2636.
(12) Wang, C.; Teo, W. J.; Ge, S. CobaltꢀCatalyzed
Regiodivergent Hydrosilylation of Vinylarenes and Aliphatic
Alkenes: Ligandꢀ and SilaneꢀDependent Regioselectivities. ACS
Catal. 2017, 7, 855ꢀ863.
(13) Du, X.; Zhang, Y.; Peng, D.; Huang, Z. BaseꢀMetalꢀ
Catalyzed Regiodivergent Alkene Hydrosilylations. Angew.
Chem., Int. Ed. 2016, 55, 6671ꢀ6675.
ASSOCIATED CONTENT
(14) Lee, K. L. (Aminomethyl)pyridine Complexes for the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Supporting Information. The Supporting Information is
available free of charge on the ACS Publications website at DOI:
xxxxxxxxx
Detailed experimental procedures, characterization data, NMR,
and GCꢀMS spectra (PDF)
CobaltꢀCatalyzed AntiꢀMarkovnikov Hydrosilylation of Alkoxyꢀ
or Siloxy(vinyl)silanes with Alkoxyꢀ or Siloxyhydrosilanes.
Angew. Chem., Int. Ed. 2017, 56, 3665ꢀ3669.
(15) Mo, Z.; Liu, Y.; Deng, L. Anchoring of Silyl Donors on a Nꢀ
heterocyclic Carbene through the CobaltꢀMediated Silylation of
Benzylic CꢀH Bonds. Angew. Chem., Int. Ed. 2013, 52, 10845ꢀ
10849.
AUTHOR INFORMATION
Corresponding Author
(
16) Sang, H.; Yu, S.; Ge, S. CobaltꢀCatalyzed Regioselective
Stereoconvergent Markovnikov 1,2ꢀHydrosilylation of
Conjugated Dienes. Chem. Sci. 2018, 9, 973ꢀ978.
*
Eꢀmail: deng@sioc.ac.cn
Notes
(17) Chen, C.; Hecht, M. B.; Kavara, A.; Brennessel, W. W.;
Mercado, B. Q.; Weix, D. J.; Holland, P. L. Rapid,
Regioconvergent, SolventꢀFree Alkene Hydrosilylation with a
Cobalt Catalyst. J. Am. Chem. Soc. 2015, 137, 13244ꢀ13247.
The authors declare no competing financial interests.
ACKNOWLEDGE
(18) Cheng, B.; Lu, P.; Zhang, H.; Cheng, X.; Lu, Z. Highly
The work was supported by the National Key Research and
Development Program (2016YFA0202900), the National Natural
Science Foundation of China (Nos. 21725104, 21690062, and
Enantioselective CobaltꢀCatalyzed Hydrosilylation of Alkenes. J.
Am. Chem. Soc. 2017, 139, 9439ꢀ9442.
(19) Liu, Y.; Deng, L. Mode of Activation of Cobalt(II) Amides
2
1432001), and the Strategic Priority Research Program of the
for Catalytic Hydrosilylation of Alkenes with Tertiary Silanes. J.
Am. Chem. Soc. 2017, 139, 1798ꢀ1801.
Chinese Academy of Sciences (No. XDB20000000).
(20) Noda, D.; Tahara, A.; Sunada, Y.; Nagashima, H. Nonꢀ
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