Regio- and stereoselective hydrosilylation of alkynes catalyzed by three-coordinate cobalt(I) alkyl and silyl complexes

Article abstract of DOI:10.1021/ja510924v

A three-coordinate cobalt(I) complex exhibits high catalytic efficiency and selectivity as well as good functional group compatibility in alkyne hydrosilylation. [Co(IAd)(PPh₃)(CH₂TMS)] (1) (IAd = 1,3-diadamantylimidazol-2-ylidene) facilitates regio- and stereoselective hydrosilylation of terminal, symmetrical internal, and trimethylsilyl-substituted unsymmetrical internal alkynes to produce single hydrosilylation products in the forms of β-(E)-silylalkenes, (E)-silylalkenes, and (Z)-α,α-disilylalkenes, respectively, in high yields. The comparable catalytic efficiency and selectivity of the Co(I) silyl complex [Co(IAd)(PPh₃)(SiHPh₂)] that was prepared from the reaction of 1 with H₂SiPh₂, and the isolation of an alkyne Co(I) complex [Co(IAd)(η²-PhC≡CPh)(CH₂TMS)] from the reaction of 1 with the acetylene, point out a modified Chalk-Harrod catalytic cycle for these hydrosilylation reactions. The high selectivity is thought to be governed by steric factors.

Full text of DOI:10.1021/ja510924v

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Communication
Regio- and Stereoselective Hydrosilylation of Alkynes Catalyzed
by Three-Coordinate Cobalt(I) Alkyl and Silyl Complexes
Zhenbo Mo, Jie Xiao, Yafei Gao, and Liang Deng
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/ja510924v • Publication Date (Web): 26 Nov 2014
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Regio- and Stereoselective Hydrosilylation of Alkynes Cata-
lyzed by Three-Coordinate Cobalt(I) Alkyl and Silyl Complexes
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Zhenbo Mo, Jie Xiao, Yafei Gao, and Liang Deng*
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
345 Lingling Road, Shanghai 200032, P. R. China
Supporting Information Placeholder
oselective hydrosilylation of alkynes with a structurally wellꢀ
defined cobalt(I) complex [Co(IAd)(PPh3)(CH2TMS)] (IAd = 1,3ꢀ
ABSTRACT: A threeꢀcoordinate cobalt(I) complex exhibits
high catalytic efficiency and selectivity as well as good
diadamantylimidazolꢀ2ꢀylidene) as precatalyst.
functionalꢀgroup compatibility in alkyne hydrosilylation. The
Scheme 1. Preparation Route for the Cobalt Catalyst
threeꢀcoordinate cobalt(I) complex [Co(IAd)(PPh3)(CH2TMS)] (1)
(IAd = 1,3ꢀdiadamantylimidazolꢀ2ꢀylidene) facilitates regioꢀ and
stereoselective hydrosilylation of terminal, symmetrical internal,
and trimethylsilylꢀsubstituted unsymmetrical internal alkynes to
produce single hydrosilylation products in the forms of ßꢀ(E)ꢀ
silylalkenes,
respectively, in high yields. The comparable catalytic efficiency
and selectivity of the cobalt(I) silyl complex
(E)ꢀsilylalkenes,
and
(Z)ꢀα,αꢀdisilylalkenes,
[Co(IAd)(PPh3)(SiHPh2)] (2) that was prepared from the reaction
of 1 with H2SiPh2, and the isolation of an alkyneꢀcobalt(I)
complex [Co(IAd)(η2ꢀPhC≡CPh)(CH2TMS)] (3) from the reaction
of 1 with the acetylene point out a modified ChalkꢀHarrod
catalytic cycle for these hydrosilylation reactions. The high
selectivity is thought to be governed by steric factors.
The quest for a new generation of economic and environmenꢀ
tally benign catalysts for important chemical transformations has
intrigued intensive exploration on nonꢀprecious metal catalysis.1
Transitionꢀmetalꢀcatalyzed hydrosilylation of alkyne2 is a useful
synthetic method for vinylsilanes that are versatile and valuable
Figure 1. Molecular structures of [Co(IAd)(PPh3)(CH2TMS)] (1)
showing 30% probability ellipsoids and the partial atom numberꢀ
ing scheme.
3
starting materials in organic synthesis.2, Consequently, great
The
lowꢀcoordinate
cobalt(I)
alkyl
complex
[Co(IAd)(PPh3)(CH2TMS)] (1) was prepared by alkylation of
efforts have been excised on the development of nonꢀprecious
metal alternatives for classic noble metal catalysts. So far, a handꢀ
[Co(IAd)(PPh3)Cl] with LiCH2TMS (Scheme 1). It has been fully
1
6
ful of Fe,4 Co, 5 and Ni complexes have been found effective to
characterized by H NMR, UVꢀvisꢀNIR, elemental analyses, as
well as singleꢀcrystal Xꢀray diffraction studies.13 Complex 1 is
paramagnetic and has a solution magnetic moment of 3.7(1) ꢁB in
C6D6.14 Its molecular structure established by an Xꢀray diffraction
study shows a distorted trigonal planar CoPCC core with a large
facilitate alkyne hydrosilylation, but these known systems generꢀ
ally exhibit low catalytic efficiency and poor regioꢀ and stereoseꢀ
lectivity. For examples, the hydrosilylation reactions of terminal
5b
alkynes
using
Co2(CO)6(HC≡CCMe2OH)
and
o
C(carbene)ꢀCoꢀC(alkyl) angle (137.32(7) ) (Figure 1).13 The Coꢀ
5c
(But2PCH2CH2C5H4)Co(CH2=CH2)
as catalysts produce mixꢀ
C(alkyl) bond distance of 2.041(2) Å is longer than that in the
cobalt(II) complex [(nacnac)Co(CH2TMS)] (1.999(15) Å).15 The
steric demanding nature of IAd ligand renders a large dihedral
tures of the regioꢀisomers, synꢀα and synꢀß adducts. The
[FeH(CO)(NO)(PPh3)2]ꢀcatalyzed hydrosilylation of disubstituted
alkynes gives mixtures of stereoꢀisomers, synꢀ and antiꢀadducts.4b
Aside from regioꢀ and stereoselectivity issue, the concurrence of
alkyne oligmerization,6 and alkyne hydrogenation7 also compliꢀ
cates late 3d metalꢀcatalyzed hydrosilylation of alkynes.
o
angle (79.5 ) between its imidazole plane and the CoPCC coorꢀ
dination plane. Short contact between the cobalt center and one of
the CꢀH bonds of an adamantyl group has also been noticed in the
structure.
9
Recently, Chirik,8 Hanson, Holland,10 and we11 found that
Complex 1 proved an efficient catalyst for hydrosilylation of
alkynes. As shown in Table 1, the reaction of 1ꢀoctyne with
H2SiPh2 (1.2 equiv.) and 2 mol% of 1 in 3 h at room temperature
lowꢀcoordinate cobalt alkyl complexes can serve as effective cataꢀ
lysts for hydrogenation, hydrosilylation, hydroboration, dehydroꢀ
genative silylation, and isomerization of olefins as well as CꢀH
bond borylation. Inspired by these, we target lowꢀcoordinate coꢀ
balt alkyl complexes bearing Nꢀheterocyclic carbene ligands12 as
catalysts for selective hydrosilylation of alkynes with the aim to
exploit the steric bulk of NHC ligands to induce selectivity. In
this regard, we report the achievement of highly regioꢀ and stereꢀ
can
yield,
exclusively,
the
ßꢀ(E)
adduct
(E)ꢀnꢀ
C6H13CH=CHSiHPh2 in 96% yield (entry 1). The identity of the
ßꢀ(E) adduct has been authenticated by comparing its NMR data
and GC retention time with those of the sample prepared by the
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(entry 3). The high efficiency and selectivity of catalyst 1 form a
Table 1. Catalytic Performance of Some Metal Complexes in
Hydrosilylation of 1ꢀOctyne a
1
2
3
4
5
6
7
8
sharp contrast to other potential cobaltꢀNHC catalysts and the
traditional alkyne hydrosilylation catalysts. For examples, under
paralleled reaction conditions all the reactions with
Co(IMes’)(IMes)(N2),12a Co2(CO)8, Rh(PPh3)3Cl, and Pt2(dvtms)3
afforded mixtures of the vinylhydrosilanes (entries 4, 6ꢀ8); and
the reactions with the threeꢀcoordinate cobalt(II) complex
12b
(IPr)Co(CH2TMS)2
(IPr
=
1,3ꢀbisꢀ2,6ꢀ
En-
try
Catalyst
Hy-
drosilane
GC Yield (%) b
diisopropylphenylimidazolꢀ2ꢀylidene) gave the ßꢀ(E) adduct in
low yield (entry 5).
ßꢀ(E) ßꢀ(Z)
a
0
1
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H2SiPh2
H3SiPh
HSiPh3
96
95
12
39
16
12
65
32
94
45
0
0
0
0
0
0
5
8
0
0
15
0
1
9
Encouraged by the high activity of catalyst 1 in hydrosilylation
of 1ꢀoctyne, the reactions with other terminal alkynes were examꢀ
ined, which revealed that all the reactions could selectively proꢀ
duce ßꢀ(E) adducts in high yields (Table 2). Hydrosilylation of
phenyl and alkyl substituted alkynes can afford Eꢀvinylsilanes in
yields high than 88% (entries 1ꢀ6, Table 2). Alkynes bearing chloꢀ
ro, protected amine, olefin, and ester functionalities can all be
successfully employed (entries 5, 6ꢀ9), which demonstrates the
good functional groupꢀcompatibility of the catalytic system. Intriꢀ
guingly, the hydrosilylation of 1,6ꢀdiynes can also be achieved
with the formation of the silylated 1,2ꢀdialkylidenecyclopentanes
in good yields with high selectivity (entries 8 and 9).
2 c
3 d
4 e
5 e
6
0
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
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1
0
1
9
Co(IMes’)(IMes)(N2)
0
(IPr)Co(CH2SiMe3)2
5
Co2(CO)8
7
0
Rh(PPh3)3Cl
8
6
Pt2(dvtms)3
9
0
2
1
1
Table 3. Hydrosilylation of Internal Alkynes a
10 f
11 g
30
0
Alkynes
Yield b
Product
Entry
1
a 1ꢀOctyne (0.20 mmol) and H2SiPh2 (0.24 mmol) at room temꢀ
perature in C6H6 (0.2 mL). b Yields based on 1ꢀoctyne. c 0.1 mol%
SiHPh2
96% (83%)
Ph
Et
Ph
d
e
catalyst loading, 12 h. 0.01 mol% catalyst loading, 48 h.
Ph
Et
Ph
[Co(IMes’)(IMes)(N2)], and [(IPr)Co(CH2TMS)2] were prepared
SiHPh2
according to ref. 12a and 12b, respectively. f 8 h. g At 70 oC.
2
3
4
92% (85%)
94% (89%)
90% (75%)
Et
Et
SiHPh2
Table 2. Hydrosilylation of Terminal Alkynes with Catalyst 1 a
n-C4H9
C4H9-n
n-C4H9
C4H9-n
SiHPh2
Entry
Product
SiHPh2
Yield b
Alkynes
Ph
1
2
96% (88%)
88% (78%)
Ph2HSi
Me
Ph2HSi
Me
SiHPh2
Ph
Ph
Me
Me
Ph
96% (70%)
96% (95%)
5
6
SiHPh2
SiHPh2
Ph
1:1
Me
Me-Ph
4-
4-Me-Ph
SiHPh2
tBu
tBu
n-C4H9
3
4
99% (84%)
96% (85%)
92% (70%)
tBu
Me
n-C4H9
1:1.4
SiHPh2
SiHPh2
SiHPh2
n-C6H13
7
Me
TMS
n-C6H13
94% (90%)
96% (90%)
96% (90%)
Me
TMS
Me
N
Me
5
6
Me N
SiHPh2
Me
n-C4H9
TMS
TMS
8
9
SiHPh2
n-C4H9
TMS
Cl
94% (82%)
86% (78%)
Cl
SiHPh2
4
Ph
Ph
TMS
7 c
SiHPh2
SiHPh2
4-F-C6H4
10
11
TMS
TMS
95% (92%)
98% (92%)
SiHPh2
4-F-C6H4
TMS
MeOOC
MeOOC
MeOOC
8 c,d
62%
56%
SiHPh2
MeOOC
4-MeO-C6H4
4-MeO-C6H4
H
TMS
SiHPh2
9 c,d
SiHPh2
TsN
TsN
TMS
12
88% (86%)
TMS
S
S
a
Alkynes (1.0 mmol) and Ph2SiH2 (1.2 mmol) with 2 mol% 1 at
b
room temperature in 0.5 mL C6H6. GC yield (isolated yield)
a
Alkynes (1.0 mmol) and Ph2SiH2 (1.2 mmol) with 2 mol% 1 at
based on alkynes. c At 70oC. d Isolated yield.
b
70oC in 0.5 mL C6H6. GC yield (isolated yield) based on alꢀ
kynes.
reported method. Deuterium labeling experiment using D2SiPh2
as the hydrosilane further confirmed the synꢀß nature of the viꢀ
nylsilane.13 Reducing the catalyst loading to 0.1 mol%, the cataꢀ
lytic system can still produce the vinylsilane in 95% yield in 12 h
without losing selectivity (entry 2). With a 0.01 mol% catalyst
loading, the reaction afforded the vinylsilane in 12% yield in 48 h
In addition to the reactions with terminal alkynes, selective synꢀ
addition of internal alkynes with H2SiPh2 could also be achieved
o
with 1 as catalyst at elevated temperature (70 C). As shown in
Table 3, hydrosilylation of symmetrical alkynes with phenyl or
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o
alkyl substituents proceeded at 70 C for 4 hours and afforded
(E)ꢀvinylsilanes in more than 90% yields (entries 1ꢀ4). Dicycloꢀ
propylacetylene was compatible without ring opening (entry 4).
As for unsymmetrical internal alkynes, the reaction with 1ꢀphenylꢀ
1ꢀpropyne afforded the two Eꢀisomers in nearly 1:1 ratio (entry 5);
the one with 4,4ꢀdimethylꢀ2ꢀpentyne gave (E)ꢀ(4,4ꢀdimethylpentꢀ
2ꢀenꢀ2ꢀyl)diphenylsilane as the major product (entry 6); and to
our delight, the reactions of the unsymmetrical internal alkynes
that bear a trimethylsilyl group could give, exclusively, synꢀ
adducts in the form of (Z)ꢀα,αꢀdisilylalkenes in very high yields
(entries 7ꢀ12). No significant influence on selectivity and yield
was observed in cases of the arylꢀsubstituted alkynes with electroꢀ
withdrawing or electroꢀdonating groups attached on aryl (entries
9ꢀ11). This observation, in addition with the stepwisely increased
selectivity in entries 5ꢀ7, suggests that the high regioꢀselectivity
achieved in the reactions with trimethylsilylꢀsubstituted alkynes is
controlled by steric factors.16 Regioꢀ and stereoselectivity control
is a challenging task in transitionꢀmetalꢀcatalyzed hydrosilylation
of alkynes. While a plenty of noble metal catalysts have been
developed, few of them could promote high regioꢀ and stereoseꢀ
lective hydrosilylation for both terminal and internal alkynes.2 In
this aspect, the high selectivity achieved in the reactions with both
terminal and internal alkynes in the current study signifies the
uniqueness of the lowꢀcoordinate cobalt catalyst.
Scheme 3. Proposed Catalytic Cycle
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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59
60
Testing the catalytic performance of the new cobalt(I) comꢀ
plexes revealed that 2 has comparable catalytic activity and selecꢀ
tivity as that of 1 in the hydrosilylation reaction of 1ꢀoctyne with
H2SiPh2 (entry 9 in Table 1), implying the inꢀcycle nature of the
cobalt(I) silyl species. In contrast, the reaction of PhC≡CPh with
H2SiPh2 using 3 (2 mol%) as catalyst only gave (Eꢀ1,2ꢀ
o
diphenylvinyl)diphenylsilane in 25% yield after heated at 70 C
for 4 hours. The lower yield, as compared to that of the reaction
using 2 as catalyst (96% yield, entry 1 in Table 3), indicates the
indispensable role of PPh3 for achieving high catalytic efficiency.
Based on these results, a simplified catalytic cycle shown in
Scheme 3 is proposed for the hydrosilylation reaction.20 The inꢀ
teraction of 1 with H2SiPh2 yields the silyl intermediate 2, which
then undergoes ligandꢀsubstitution reaction with an alkyne moleꢀ
cule to form the alkyneꢀcobalt(I)ꢀsilyl species, Co(IAd)(η2ꢀ
R1C≡CR2)(SiHPh2) (4). Once formed, migratory insertion of the
silyl moiety to the C≡C triple bond in 4, triggered probably by the
coordination of PPh3, will produce the alkenyl metal species 5
that could further react with H2SiPh2 to yield the hydrosilylation
product and regenerate 2 (Scheme 3). While our attempts to preꢀ
pare intermediates 4 and 5 from the reactions of 2 with alkynes
were unsuccessful, the isolation of 1 and 3, which are structurally
relevant to 5 and 4, respectively, hints the capability of the IAd
ligand to support lowꢀcoordiante silylꢀcobalt(I)ꢀalkyne and
alkenylꢀcobalt(I) intermediates. Notably, the conversion of 1 to 2
and that of 5 to 2 might follow an oxidative additionꢀreductive
elimination mechanism when noting the precedents of oxidative
addition of hydrosilanes with cobalt(I) species.18 On the other
hand, considering the steric demanding nature of IAd that might
render the formation of a fiveꢀcoordinate trigonalꢀbipyramidal
Co(III)ꢀspecies difficult, a sigmaꢀbond metathesis mechanism
therefore could not be excluded absolutely.
Scheme 2. Reactions of 1 with H2SiPh2 and PhCCPh
Figure 2. Molecular structures of [Co(IAd)(PPh3)(SiHPh2] (2,
left) and [Co(IAd)(η2ꢀPhC≡CPh)(CH2TMS)] (3, right) showing
30% probability ellipsoids and the partial atom numbering
scheme.
Aiming to throw lights on the reaction mechanism, stoichioꢀ
metric reactions of 1 with alkynes and hydrosilanes were then
studied. Complex 1 proved reactive toward H2SiPh2 to yield a
threeꢀcoordinate cobalt(I) silyl complex, [Co(IAd)(PPh3)(SiHPh2)]
(2).13 On the other hand, a cobalt(I) alkyne complex [Co(IAd)(η2ꢀ
PhC≡CPh)(CH2TMS)] (3) has been isolated from the reaction of
1 with PhC≡CPh (Scheme 2).13 Figure 2 depicts the molecular
structures of both complexes. Complex 2 represents the first exꢀ
ample of threeꢀcoordinate cobalt silyl complex. Its cobalt center
adopts a triganol planar geometry with a large C1ꢀCoꢀSi angle of
148.4(2) o. The CoꢀC(carbene) distance (1.980(6) Å) in 2 is shortꢀ
er than those of the reported lowꢀspin cobalt(I)ꢀNHC complexes,17
whereas, the CoꢀSi distance (2.226(2) Å) is comparable to its
congeners in lowꢀspin cobalt silyl complexes.18 Complex 3 is a
peculiar lowꢀcoordinate complex bearing both an η2ꢀbonded alꢀ
kyne ligand and a σꢀalkyl group. The long C(alkyne)ꢀC(alkyne)
bond distance (1.271(3) Å) and bent C(Ph)ꢀC(alkyne)ꢀC(alkyne)
Chart 1. Newman projections of intermediate 4.
The hydrosilylation products in the reactions with terminal alꢀ
kynes and silylꢀsubstituted unsymmetrical internal alkynes exhibit
different regioꢀselectivity, which, we thought, should result from
the intricate balance of steric repulsion among the IAd group, the
silyl group, and the coordinated alkyne in intermediate 4 (Chart
1).21, 22 For terminal alkynes, the alkyl and phenyl substituents are
not bulky and could position toward IAd. Then, the following
migratory insertion of the silyl group to the terminal carbon of the
alkyne, which is less sterically hindranced, will readily happen
and eventually lead to the formation of β synꢀadducts. For the
trimethylsilylꢀsubstituted internal alkynes, in order to avoiding
severe steric repulsion between the trimethylsilyl and IAd groups,
the alkynes have to orient their small substituents above the imidꢀ
o
angle (148 ) suggest strong metalꢀtoꢀligand backdonation.19 The
unique steric nature of the IAd ligand has induced the near coplaꢀ
narity of the C(alkyne)ꢀC(alkyne) bond with the C(carbene)ꢀCoꢀ
C(alkyl) plane with one of the phenyl groups fitting in the valley
between two adamantyl groups.
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(7) Enthaler, S.; Haberberger, M.; Irran, E. Chem. Asian J. 2011, 6,
1613.
azole plane, which, subsequently, will produce α,αꢀdisilylalkenyl
1
2
3
4
5
6
7
8
cobalt intermediate and lead the formation of α,αꢀdisilylalkenes.
Consistent with this explanation, we found that the catalytic sysꢀ
tem is invalid for the hydrosilylation of TMSC≡CTMS, and that
the steric property of the silyl moiety affects the reaction selectiviꢀ
ty. For examples, the hydrosilylation reaction of 1ꢀoctyne with the
primary silane H3SiPh (entry 10 in Table 1) exhibits poor selecꢀ
tivity, and the reaction with HSiPh3 (entry 11 in Table 1) could
not afford the hydrosilylation product.
(8) (a) Monfette, S.; Turner, Z. R.; Semproni, S. P.; Chirik, P. J. J.
Am. Chem. Soc. 2012, 134, 4561. (b) Obligacion, J. V.; Chirik,
P. J. J. Am. Chem. Soc. 2013, 135, 19107. (c) Friedfeld, M. R.;
Shevlin, M.; Hoyt, J. M.; Krska, S. W.; Tudge, M. T.; Chirik, P.
J. Science 2013, 342, 1076. (d) Obligacion, J. V.; Semproni, S.
P.; Chirik, P. J. J. Am. Chem. Soc. 2014, 136, 4133. (e) Atienza,
C. C. H.; Diao, T.; Weller, K. J.; Nye, S. A.; Lewis, K. M.; Delꢀ
is, J. G. P; Boyer, J. L.; Roy, A. K.; Chirik, P. J. J. Am. Chem.
Soc. 2014, 136, 12108.
(9) (a) Zhang, G.; Scott, B. L.; Hanson, S. K. Angew. Chem. Int. Ed.
2012, 51, 12102. (b) Zhang, G.; Vasudevan, K. V.; Scott, B. L.;
Hanson, S. K. J. Am. Chem. Soc. 2013, 135, 8668.
(10) Chen, C.; Dugan, T. R.; Brennessel, W. W.; Weix, D. J.; Holꢀ
land, P. L. J. Am. Chem. Soc. 2014, 136, 945.
In summary, we found a lowꢀcoordinate cobalt(I) alkyl complex
bearing bulky IAd ligand effects hydrosilylation of alkynes to
produce synꢀadduct of vinylsilanes with high efficiency, selectiviꢀ
ty, and good functional group compatibility. Reactivity study on
the precatalyst has led to the isolation of the first example of lowꢀ
coordinate cobalt(I) silyl complex and a novel cobalt(I)ꢀalkyneꢀ
alkyl complex. The silyl complex shows comparable activity and
selectivity in alkyne hydrosilyation as the alkyl complex. These
results collectively point out a modified ChalkꢀHarrod mechanism
for the cobaltꢀcatalyzed reaction. Molecular structure analysis on
the lowꢀcoordinate cobalt complexes implies that the selectivity is
governed by steric factors. Currently, we are exploring the utility
of the vinylhydrosilanes for silicon polymer synthesis.
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(11) Mo, Z.; Liu, Y.; Deng, L. Angew. Chem. Int. Ed. 2013, 52,
10845.
(12) (a) Mo, Z.; Chen, D.; Leng, X.; Deng, L. Organometallics 2012,
31, 7040. (b) Przyojski, J. A.; Arman, H. D.; Tonzetich, Z. J.
Organometallics 2013, 32, 723. (c) Danopoulos, A. A.; Braunꢀ
stein, P. Dalton Trans. 2013, 42, 7276. (d) Day, B. M.; Pal, K.;
Pugh, T.; Tuck, J.; Layfield, R. A. Inorg. Chem. 2014, 53,
10578.
(13) For detailed information please, see supporting information.
(14) The large solution magnetic moments suggests the contribution
of orbital angular momentum as observed in other lowꢀ
coordinate cobaltꢀNHC complexes in Ref. 12.
(15) Young, J. F.; Yap, G. P. A.; Theopold, K. H. J. Chem. Cryst.
2009, 39, 846.
(16) Similar selectivity has been observed in [Cp*Ru(NCMe)3][PF6]ꢀ
catalyzed hydrosilylation reactions. See: Ding, S.; Song, L.ꢀJ.;
Chung, L. W.; Zhang, X.; Sun, J. J. Am. Chem. Soc. 2013, 135,
13835.
(17) (a) Vélez, C. L; Markwick, P. R. L.; Holland, R. L.; DiPasquale,
A. G.; Rheingold, A. L.; O’Connor, J. M. Organometallics
2010, 29, 6695. (b) Mo, Z.; Li, Y.; Lee, H. K.; Deng, L. Organꢀ
ometallics 2011, 30, 4687.
(18) (a) Yong, L.; Hofer, E.; Wartchow, R.; Butenschön, H. Organꢀ
ometallics 2003, 22, 5463. (b) Whited, M. T.; Mankad, N. P.;
Lee, Y.; Oblad, P. F.; Peters, J. C. Inorg. Chem. 2009, 48, 2507.
(19) Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L. Inorg.
Chem. 2006, 45, 5742.
(20) Several mechanisms, for examples ChalkꢀHarrod, modified
ChalkꢀHarrod, sigmaꢀbond metathesis, and metalꢀsilylene mechꢀ
anisms, have been proposed for transitionꢀmetalꢀcatalyzed hyꢀ
drosilylation of alkenes and alkynes. For references: see: (a)
Chalk, A. J.; Harrod, J. F. J. Am. Chem. Soc. 1965, 97, 16. (b)
Seitz, F.; Wrighton, M. S. Angew. Chem. Int. Ed. 1988, 27, 289.
(c) Fu, P. F.; Brard, L.; Li, Y. W.; Marks, T. J. J. Am. Chem.
Soc. 1995, 117, 7157. (d) Glaser, P. B.; Tilley, T. D. J. Am.
Chem. Soc. 2003, 125, 13640. The modified ChalkꢀHarrod
mechanism or sigmaꢀbond metathesis mechanism via Coꢀsilyl
intermediates is very likely involved in the current catalytic sysꢀ
tem. However, with limited information in hands at this stage,
we could not exclude one or the other.
ASSOCIATED CONTENT
Supporting Information
Xꢀray crystallographic files in CIF format for the new complexes,
experimental procedures, and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: deng@sioc.ac.cn
ACKNOWLEDGMENT
We thank the financial support from the National Basic Research
Program of China (No. 2011CB808705), the National Natural
Science Foundation of China (Nos. 21121062, 21222208, and
21432001), and the Chinese Academy of Sciences.
REFERENCES
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(21) Markó et al. has developed similar model for his PtꢀNHC cataꢀ
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(22) The possible interaction between the SiꢀH bond and the cobalt
center in intermediate 5 that might prevent the isomerization of
vinyl cobalt intermediates via CrabtreeꢀOjima mechanism to
form transꢀadduct might be another important factor contribꢀ
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