Cobalt(0) and Iron(0) Isocyanides as Catalysts for Alkene Hydrosilylation with Hydrosiloxanes

Article abstract of DOI:10.1021/acs.organomet.8b00389

Iron and cobalt isocyanides, Fe(CNR)₅ (1) and Co₂(CNR)₈ (2), where R = t-butyl (^tBu), adamantyl (Ad), and mesityl (Mes), were prepared by reduction of FeBr₂ or CoI₂ in the presence of CNR by C₈K or silica-Na. These complexes were subjected to catalytic hydrosilylation of alkenes with hydrosiloxanes, and the results are compared with those obtained by previously reported Fe(OPiv)₂/CNAd or Co(OPiv)₂/CNAd catalyst systems. Hydrosilylation of allylic ethers with 1,1,1,3,3-pentamethyldisiloxane (PMDS) catalyzed by 1 and the reaction of several alkenes with PMDS or 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD′M) catalyzed by 2 exhibited greater catalytic activity than that observed for the Fe(OPiv)₂ or Co(OPiv)₂/CNR catalyst system. Complexes 1 and 2 were effective for catalytic chemical modification of silicone fluids containing Si-H groups and for two-component silicone curing. In all cases, selectivity of the reaction in terms of formation of the desired product by hydrosilylation and of byproducts due to dehydrogenative silylation did not differ between the metal isocyanide complexes and the corresponding M(OPiv)₂/CNR catalyst system. Catalytically active species generated from 1, 2, and the M(OPiv)₂/CNR catalyst system were also investigated.

Full text of DOI:10.1021/acs.organomet.8b00389

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Cobalt(0) and Iron(0) Isocyanides as Catalysts for Alkene
Hydrosilylation with Hydrosiloxanes
Atsushi Sanagawa and Hideo Nagashima*
Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga, Fukuoka 816-8580, Japan
S
* Supporting Information
ABSTRACT: Iron and cobalt isocyanides, Fe(CNR)5 (1) and
Co2(CNR)8 (2), where R = t-butyl (tBu), adamantyl (Ad), and
mesityl (Mes), were prepared by reduction of FeBr2 or CoI2 in
the presence of CNR by C8K or silica-Na. These complexes
were subjected to catalytic hydrosilylation of alkenes with
hydrosiloxanes, and the results are compared with those
obtained by previously reported Fe(OPiv)2/CNAd or Co-
(OPiv)2/CNAd catalyst systems. Hydrosilylation of allylic
ethers with 1,1,1,3,3-pentamethyldisiloxane (PMDS) catalyzed
by 1 and the reaction of several alkenes with PMDS or
1,1,1,3,5,5,5-heptamethyltrisiloxane (MD′M) catalyzed by 2
exhibited greater catalytic activity than that observed for the
Fe(OPiv)2 or Co(OPiv)2/CNR catalyst system. Complexes 1 and 2 were effective for catalytic chemical modification of silicone
fluids containing Si−H groups and for two-component silicone curing. In all cases, selectivity of the reaction in terms of
formation of the desired product by hydrosilylation and of byproducts due to dehydrogenative silylation did not differ between
the metal isocyanide complexes and the corresponding M(OPiv)2/CNR catalyst system. Catalytically active species generated
from 1, 2, and the M(OPiv)2/CNR catalyst system were also investigated.
the base metal catalysts investigated,10−12 few are active for
INTRODUCTION
■
hydroalkoxysilanes11 and hydrosiloxanes.11d−f,12
Hydrosilylation of alkenes is a key technology for the
Base metals, such as iron and cobalt, prefer one- to two-
production of organosilicon compounds, including industrially
electron redox reactions, often seen in precious-metal-catalyzed
important silicones.1 Platinum compounds have significant
reactions.1 Similar to hydrogenation of alkenes, alkene
advantages over other transition metal complexes or Lewis
hydrosilylation involves a sequence of two-electron redox
acids as practical catalysts due to their high catalytic activity
and ease of handling. Several platinum catalysts,2 such as
processes. Appropriate selection of ligands which make the
Speier’s catalysts2a,c and Karstedt’s catalysts,2b have been used
metal center suitable for two-electron chemistry is key to
achieving efficient hydrosilylation of alkenes by base metal
catalysts. One typical example involves bis(imino)pyridines
in the silicone industry, which utilizes 5.6 ton of platinum per
year. Most of this platinum is not recovered.3 Since platinum is
ligated to iron and cobalt, which behave as noninnocent redox-
an expensive precious metal and may become increasingly
active ligands.13 Another good example involves the use of
scarce in the future,4 research is important5 to find new
strong π-acids such as CO as the ligand.6 Several iron14 and
hydrosilylation catalysts containing base metals such as iron,6
cobalt carbonyls15 have been reported as catalysts for
hydrosilylation of alkenes.
cobalt,7 and nickel.8
Base metal catalysts that are considered as substitutes for
platinum catalysts in industrial production of silicone
materials1,2,5 need to possess high catalytic efficiency toward
alkene hydrosilylation with hydroalkoxysilanes or hydro-
siloxanes. New catalysts must demonstrate high enough
catalytic activity to promote hydrosilylation at a catalyst
loading on the order of a ppm [catalyst turnover numbers
(TON) > 103−106].3,9 Hydroalkoxysilanes, such as HSi(OR)3,
HMeSi(OR)2, and HMe2Si(OR), where R = Me or Et, are
useful resources for the production of silane coupling reagents,
whereas polysiloxanes containing Si−H groups are starting
materials for silicone fluids and resins. Hydrosilylation
reactions of various alkenes with HSiMe2OSiMe3 (PMDS)
and HSiMe(OSiMe3)2 (MD′M) are model reactions. Among
Isocyanide ligands, CNR, are of interest because they are
isoelectronic to the CO ligand and effective as a π-acid
ligand to achieve two-electron chemistry in base metal
catalysis.16 A 2016 report described efficient catalyst systems
consisting of cobalt or iron pivalates [Fe(OPiv)2 or Co-
(OPiv)2] and isocyanide ligands for hydrosilylation of alkenes
with hydrosiloxanes.17 One advantage of this catalyst system is
that both metal pivalates and isocyanide ligands are relatively
easy to handle. Catalytically active species are generated from
this mixture in contact with hydrosilanes. Model reactions
Received: June 7, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00389
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using PMDS suggested good catalytic activity of these catalyst
systems that achieved TON of 103−104. The catalysts could be
applied to the modification of silicone fluids using no more
than 70 ppm catalyst loading. Silicone curing, i.e., cross-linking
of Si−H groups containing polysiloxanes with vinyl terminated
polysiloxanes, was achieved preliminary to the production of
silicone rubbers and resins.
changed to dark orange, and the color of the insoluble
materials turned to black. After 12 h, the solid materials were
removed by filtration, and recrystallization of the residue from
pentane afforded Fe(CNR)5 (1) in 23−70% isolated yield
(Scheme 2). Similar reactions of CoI2 with KC8 (2 equiv to 1
Scheme 2. Synthesis of Fe(CNR)5 and Co2(CNR)8
Of keen interest is catalytic species generated from a metal
pivalate, isocyanide, and hydrosilane. A previous report
indicated possible involvement of “Fe(0)(CNR)2” and “Co-
(I)(CNR)3” species in the reaction mechanisms.17 In the
literature,14,15 Fe(CO)5 and Co2(CO)8 are conventional base
metal catalysts for alkene hydrosilylation and dehydrogenative
silylation, and Fe(CO)3, R3SiCo(CO)3, and HCo(CO)3 have
been discussed as plausible catalytic intermediates. Analogous
electronic structures of CNR with CO provide possible
hydrosilylation mechanisms involving “Fe(0)(CNR)3” and
“Co(I)(CNR)3” species. This assumption regarding Fe(0)-
(CNR)3 and Co(I)(CNR)3 as possible intermediates could be
verified by preparing isocyanide homologues of Fe(CO)5 and
Co2(CO)8 and comparing their catalytic alkene hydrosilylation
performance with that of a Fe(OPiv)2/CNR or Co(OPiv)2/
CNR catalyst system activated by hydrosilanes. The present
report describes the hydrosilylation of several alkenes catalyzed
by Fe(CNR)5 (1) and Co2(CNR)8 (2) (Scheme 1). Catalytic
equiv of CoI2) in the presence of CNR (4 equiv to 1 equiv of
CoI2) gave the corresponding Co2(CNR)8 (2) in 47−66%
isolated yield. Commercially available sodium silica gel (stage
I) could be used as a reducing reagent instead of KC8. A typical
example involved the preparation of 1 from FeBr2 (1 mmol),
CNtBu (5 mmol), and sodium silica gel (ca. 5 mmol) in 42%
yield, whereas 2 was synthesized from CoI2 (1 mmol), CNtBu
(4 mmol), and silica-Na including 35 wt % Na (340 mg) in
Scheme 1. Hydrosilylation of Alkenes
1
48% yield. The products were characterized by H and 13C
NMR, IR, crystallography, and elemental analysis.
Hydrosilylation of Three Alkenes Catalyzed by 1B
and 2B. Three alkenes were selected to examine the catalytic
activity of Fe(CNAd)5 (1B) and Co2(CNAd)8 (2B) and the
selectivity of reactions involving them. Previous studies17 on a
catalyst system composed of Fe(OPiv)2 and CNAd (1:2) had
the following features: (a) The catalyst system promoted
hydrosilylation of styrene with PMDS to give
PhCH2CH2SiMe2OSiMe3 (3a) as a single product (Scheme
3, eq 1). (b) No branched silane, PhCH(Me)SiMe2OSiMe3,
was formed. (c) At temperatures higher than 50 °C, a 1:1
mixture of PhCHCHSiMe2OSiMe3 (3b) and PhCH2CH3
activity of 1 or 2 was often greater than that of the
corresponding metal carboxylates/isocyanide catalyst systems,
while maintaining a similar selectivity. The results suggested
that the catalytically active species generated from 1 or 2 with
hydrosilanes was similar to those formed from a Fe(OPiv)2/
CNR or Co(OPiv)2/CNR catalyst system activated by
hydrosilanes.
RESULTS AND DISCUSSION
Scheme 3. Hydrosilylation of Alkenes with PMDS
■
Convenient Synthesis of Fe(CNR)5 (1) and Co2(CNR)8
(2). Prior to hydrosilylation studies, the synthetic methods for
1 and 2 were improved. Two isocyanide complexes,
Fe(CNtBu)5 (1a) and Co2(CNtBu)8 (2a), were synthesized
by reduction of Fe(II) or Co(II) salts with Na/Hg or sodium
naphthalenide in the presence of CNR as described
previously.18 However, both of these reducing reagents have
problems: Na/Hg contains toxic Hg, and sodium naphthale-
nide produces large amounts of naphthalene, which is difficult
to separate from the metal isocyanides. Two solid materials,
KC8 and commercially available sodium silica gel, were
examined as reducing reagents. An advantage of these solid-
state reducing reagents over conventional Na/Hg and sodium
naphthalenide is the easy removal of solid wastes by filtration
after the reaction. Metal isocyanides 1 and 2 could be isolated
by recrystallization of the corresponding crude products.
Treatment of FeBr2 with KC8 (2 equiv to 1 equiv of FeBr2)
in the presence of RNC (5 equiv to 1 equiv of FeBr2), where R
= tert-butyl (tBu), adamantyl (Ad), or mesityl (Mes), at room
temperature gave a suspension of bronze KC8 in a brown
solution of FeBr2. The color of the solution immediately
B
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Table 1. Iron- or Cobalt-Catalyzed Reactions of Three Alkenes with PMDS
a
a
entry
cat. (mol % per metal)
temp (°C)
time (h)
conv. (%)
products (% yield)
alkene = styrene (Scheme 3, eq 1)
3a
81
>99
85
>99
24
83
4a
<1
<1
>99
>99
5a
16
25
3b
9
<1
7
3c
10
<1
11
<1
38
7
4b′
<1
<1
<1
<1
5c
21
18
<1
<1
1
2
3
4
5
6
1B (1)
1B (1)
80
50
80
50
50
50
24
24
24
24
24
24
>99
>99
>99
>99
>99
>99
Fe(OPiv)2 (1)/CNAd(2)/(EtO)3SiH (4)
Fe(OPiv)2 (1)/CNAd(2)/(EtO)3SiH (4)
2B (1)
<1
38
10
4b
<1
<1
<1
<1
5b
6
Co(OPiv)2 (1)/CNAd(3)/(EtO)3SiH (4)
alkene = α-methylstyrene (Scheme 3, eq 2)
4c
<1
<1
<1
<1
7
1B (1)
80
80
50
50
24
24
24
24
3
5
>99
>99
8
9
Fe(OPiv)2 (1)/CNAd(2)/(EtO)3SiH (4)
2B (1)
10
Co(OPiv)2 (1)/CNAd(3)/(EtO)3SiH (4)
alkene = 1-octene (Scheme 3, eq 3)
11
12
13
14
1B (1)
r.t.
r.t.
50
24
24
24
24
49
73
>99
>99
Fe(OPiv)2 (1)/CNAd(2)/(EtO)3SiH (4)
2B (1)
Co(OPiv)2 (1)/CNAd(3)/(EtO)3SiH (4)
15
<1
<1
>99
>99
80
a
1
Conversion of the alkene and product yields were determined by H NMR in the presence of internal standard.
(3c), due to dehydrogenative silylation, was formed as a
byproduct. (d) Preactivation of the catalyst system by
HSi(OEt)3 accelerated the reaction, when the reaction was
performed relatively high catalyst loadings (0.1−1 mol %).19
(e) No reaction occurred under the same conditions when α-
methylstyrene was used as the olefin (Scheme 3, eq 2). (f)
Reaction of 1-octene with PMDS resulted in both hydro-
silylation and dehydrogenative silylation concomitantly, which
gave a mixture of octyl silane 5a, allysilane 5b, and octane 5c
(Scheme 3, eq 3).
afford 3a as a single product. As shown in entries 5 and 6,
significant amounts of 3b and 3c were obtained.
Entries 7−10 show the results of hydrosilylation of α-
methylstyrene. Essentially no reaction occurred with 1B or the
Fe(OPiv)2/CNAd catalyst system at 80 °C. In sharp contrast,
both 2B and the Co(OPiv)2/CNAd catalyst system promoted
hydrosilylation at 50 °C to give 4a as a single product.
Reactions of 1-octene with PMDS (entries 11−14) revealed:
(a) When 1B or the Fe(OPiv)2/CNAd/(EtO)3SiH catalyst
system was used as the catalyst, the reactions were faster than
the hydrosilylation of styrene and occurred at room temper-
ature to give a mixture of 5a, 5b, and 5c. (b) As shown in
Table 1, entries of 11 and 12, the yields of 5a, 5b, and 5c were
16, 6, and 21%, respectively, in the reaction catalyzed by 1B,
whereas they were 25, 15, and 18% in the reaction catalyzed by
Fe(OPiv)2/CNAd/(EtO)3SiH catalyst system. The 5a/5b/5c
ratio was 37/14/49 in the former reaction, while the latter was
43/26/31. They were not significantly different. (c) Isomer-
ization of a CC bond of 1-octene to internal octenes
occurred minimally in these iron-catalyzed reactions. (d) Both
2B and the Co(OPiv)2/CNAd catalyst system catalyzed the
reaction at 50 °C to give 5a as a single product in high yield.
(e) In both cobalt-catalyzed reactions, migration of the CC
bond from terminal to internal was observed at the initial stage.
(f) The resulting internal CC bond underwent hydro-
silylation involving isomerization of alkenes to form 5a as a
single product.
The selectivity of the reaction is an important outcome of
this study. The desired hydrosilylation product 3a was
selectively formed by 1B and the Fe(OPiv)2/CNAd catalyst
system, whereas 4a and 5a were obtained as a single product
by 2B and the Co(OPiv)2/CNAd catalyst system. With other
olefin/catalyst combinations, hydrosilylation was competitive
with dehydrogenative silylation. No difference was found in
selectivity of the reaction between the metal isocyanide catalyst
and the M(OPiv)2/CNAd catalyst system, regardless of the
metal used (iron or cobalt). This selectivity outcome clearly
indicates that the catalytically active species generated from
Fe(CNAd)5 (1B) should be identical to that from Fe(OPiv)2/
CNAd, and that the catalytically active species formed from
The following results were obtained with the catalyst system
composed of Co(OPiv)2 and CNAd (1:3) preactivated by
HSi(OEt)3:19 (a) Hydrosilylation of α-methylstyrene with
PMDS gave a single product, 4a (Scheme 3, eq 1). No
byproducts, such as those due to dehydrogenative silylation
and hydrogenation, were detected. (b) Reaction of styrene
with PMDS under the same conditions gave a mixture of 3a,
3b, and 3c derived from hydrosilylation and concomitantly
occurring dehydrogenative silylation. (c) Reaction of 1-octene
with PMDS resulted in rapid olefin isomerization to internal
octenes, followed by hydrosilylation to give 5a as a single
product (Scheme 3, eq 3). (d) Similar to the Fe(OPiv)2/CNR
catalyst system, preactivation of the catalyst system by
HSi(OEt)3 accelerated the reaction, when the reaction was
performed relatively high catalyst loadings (0.1−1 mol %).19
Table 1 summarizes the results of the hydrosilylation of
styrene, α-methylstyrene, and 1-octene, with PMDS, with 1B
or 2B (1 mol %) as the catalyst. Catalyst concentration and
reaction time were fixed to 1 mol % and 24 h, respectively. The
results are compared with those from the corresponding
M(OPiv)2/CNR system preactivated by (EtO)3SiH. Treat-
ment of styrene (1 mmol) with PMDS (1.3 mmol) was
conducted at 80 °C for 24 h in the presence of 1B (1 mol %)
to give 3a in 81% yield. Dehydrogenative silylation occurred
concomitantly to give 3b and 3c in 9 and 10% yield,
respectively (Table 1, entry 1). Dehydrogenative silylation was
suppressed at 50 °C to afford 3a as a single product (Table 1,
entry 2). A similar temperature effect was observed in reactions
with the Fe(OPiv)2/CNAd catalyst preactivated by (EtO)3SiH
(Table 1, entries 3 and 4). Reaction of styrene with PMDS
catalyzed by 2B and the Co(OPiv)2/CNAd system did not
C
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Co2(CNAd)8 (2B) should be similar to that from Co(OPiv)2/
CNAd.
Catalytic Performance of Fe(CNR)5. Table 2 summarizes
Fe(OPiv)2/CNR catalyst system is better than Fe(CNR)5.
Fe(CO)5 possessed inferior catalytic activity and selectivity
compared to Fe(CNR)5 toward the product.
the catalytic performance of 1A, 1B, and 1C for hydrosilylation
The above results indicate that 1B is a catalyst that can
convert styrene and PMDS to 3a with good selectivity, but had
a lower catalytic activity than that of the Fe(OPiv)2/CNAd
catalyst system, especially at low catalyst loadings. An opposite
trend was observed for hydrosilylation of allyl ethers with
PMDS. The Fe(OPiv)2/CNAd catalyst system was effective for
reaction of allyl glycidyl ether, allyl benzyl ether, and allyl 2-
methoxyethyl ether when 3 mol % Fe(OPiv)2 was used. As
shown in Scheme 4, eq 1, hydrosilylation of allyl glycidyl ether
Table 2. Hydrosilylation of Styrene with PMDS Catalyzed
by Fe(CNR)5 and the Fe(OPiv)2/CNAd Catalyst System
e
yield (%)
Scheme 4. Hydrosilylation of Allylic Ethers by Iron
Catalysts [Si− = Me3SiOMe2Si−]
temp.
time
(h)
conv.
(%)
entry cat. (mol %)
(°C)
3a
3b
3c
1
1B (1)
50
r.t.
r.t.
r.t.
50
50
50
50
50
50
50
50
50
50
24
24
24
24
24
3
>99
7
>99
2
>99
73
>99
19
83
8
>99
7
>99
2
>99
73
>99
19
5
3
4
3
91
14
<1
<1
<1
<1
<1
<1
<1
<1
39
<1
<1
<1
<1
17
<1
<1
<1
<1
<1
<1
<1
<1
38
<1
<1
<1
<1
17
2
3
4
5
6
7
8
1B (1)
a
b
Fe-1 (1)
Fe-2 (1)
1B (0.1)
1B (0.1)
1A (0.1)
1A (0.1)
24
3
9
1C (0.1)
24
24
24
24
24
24
10
11
12
13
14
1B (0.01)
1A (0.01)
Fe-3 (0.01)
Fe-4 (0.01)
11
8
91
46
c
d
Fe(CO)5 (1)
a
Fe-1: Fe(OPiv)2 (1 mol %)/CNAd (2 mol %) /(EtO)3SiH (4 mol
b
c
%). Fe-2: Fe(OPiv)2 (1 mol %)/CNAd (2 mol %). Fe-3:
Fe(CNAd)5 (0.01 mol %)/CNAd (0.03 mol %). Fe-4: Fe(OPiv)2
d
e
(0.01 mol %)/CNAd (0.08 mol %). Conversion of the alkene and
product yields were determined by 1H NMR in the presence of
internal standard.
of styrene with PMDS. The reaction catalyzed by 1B
proceeded at 50 °C (entry 1), resulting in 3a as a single
product after 24 h. Reaction at room temperature resulted in
low conversion of styrene (entry 2), which is in sharp contrast
to the results from hydrosilylation of styrene catalyzed by the
Fe(OPiv)2 (1 mol %)/CNAd system in the presence of
(EtO)3SiH, which was complete under the same conditions
(entry 3). Entry 4 shows the reaction in the absence of
(EtO)3SiH for comparison, which was slower than the result
shown in entry 3. The reaction with a lower catalyst loading
(0.1 mol %) at 50 °C converted >99% styrene after 24 h and
73% after 3 h (entries 5 and 6, respectively). The results shown
for entries 7 and 8 indicate that the catalytic activity of
Fe(CNtBu)5 (1A) was lower than that for 1B. The reaction
catalyzed by Fe(CNMes)5 (1C) gave a mixture of 3a, 3b, and
3c (entry 9). Competition between hydrosilylation and
dehydrogenative silylation also was seen when the Fe-
(OPiv)2/CNMes system was used as the catalyst. A previous
study revealed that a mixture of Fe(OPiv)2 and CNAd
catalyzed the hydrosilylation of styrene to give 3a with 0.01
mol % catalyst loading. With such low catalyst concentration,
additional CNAd resulted in an increase in the yield of 3a, and
(EtO)3SiH was not necessarily useful to accelerate the
reaction. The TON reached 104 under optimal conditions
shown in entry 13. In contrast, addition of CNAd (3 equiv to
1B) to the complex 1B did not improve the catalytic activity
(entry 12). For the experiments to lower the catalyst loadings,
was catalyzed by 1A, 1B, or the Fe(OPiv)2/CNAd/(EtO)3SiH
catalyst system with 0.1 mol % catalyst loading at room
temperature for 24 h. Catalytic activity increased in the order
Fe(OPiv)2/CNAd/(EtO)3SiH ≪ 1B < 1A. Reaction of allyl
benzyl ether under similar conditions indicated 1B possessed
greater catalytic activity than did Fe(OPiv)2/CNAd/
(EtO)3SiH (Scheme 4, eq 2).20 The catalytic activity of 1A
was higher than 1B. In both cases, the corresponding
hydrosilylated product, 6a or 7a, formed as a single product
in high yield. Hydrosilylation of allyl 2-methoxyethyl ether
with PMDS is also shown in Scheme 4, eq 3. With 1 mol % 1A,
8a was obtained as a single product in 87% yield. The activity
of 1B was lower than 1A but greater than that of the
Fe(OPiv)2/CNAd catalyst system preactivated by (EtO)3SiH;
yield of 8a was as low as 45%, even when using 3 mol %
catalyst.
As seen for hydrosilylation of alkenes with the Fe(OPiv)2/
CNR catalyst system, a limited number of hydrosilanes and
alkenes were effective for the Fe(CNR)5-catalyzed hydro-
silylation. As shown in the Table S2, hydrosilylation of styrene
D
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catalyzed by 1B selectively afforded the desired β-phenethylsi-
lane in good yield, when PhMe2SiH, (EtO)Me2SiH, or
(EtO)2MeSiH was used as the hydrosilane. In contrast,
reactions with (EtO)3SiH and Ph2SiH2 were slower, and no
reaction occurred with PhSiH3. Reaction with MD′M afforded
a mixture of products due to hydrosilylation and dehydrogen-
ative silylation. In all cases, no α-phenethylsilane was formed.
Among the attempts of hydrosilylation of other alkenes with
PMDS, norbornene was hydrosilylated to afford 2-silylnorbor-
nane product 9a in 68% isolated yield. A small amount of
norbornane was also formed. Essentially no reaction took place
with α-methylstyrene and 2-octene.
h, while 2C was deactivated and lost activity during reaction at
50 and 80 °C (entries 8−10). Catalytic activity of Co2(CO)8
was somewhat lower than that of 2A, 2B, and 2C at 80 °C for
3 h (entry 11). As reported previously, the greatest TON value
achieved using the Co(OPiv)2/CNAd catalyst system was
1850, although the catalyst was deactivated before conversion
of α-methylstyrene reached >99%. Entries 12 and 13
demonstrate TON values of 3500−3600, achieved at 50 °C
with 2B and at room temperature with 2C. As shown in Table
S3, Co2(CNR)8 generally acted as a good catalyst for
hydrosilylation of α-methylstyrene with tertiary hydrosilanes,
which included MD′M, Men(EtO)3−nSiH (n = 0−2),
PhMe2SiH, and Et3SiH. The Co(OPiv)2/CNAd catalyst
system could be applied to the hydrosilylation of various
olefins with hydrosiloxanes. Table 4 shows representative
Catalytic Performance of Co2(CNR)8. Table 3 summa-
rizes cobalt-catalyzed hydrosilylation of α-methylstyrene with
Table 3. Hydrosilylation of α-Methylstyrene with PMDS
Catalyzed by Co2(CNR)8 and Co(OPiv)2/CNAd Catalyst
Systems
Table 4. Hydrosilylation of Olefins with PMDS or MD′M
Catalyzed by 2A
yield (%)
cat.
(mol % Co)
temp.
(°C)
time
(h)
conv.
(%)
entry
4a
a
1
2
3
4
5
6
7
8
Co-1 (1)
50
r.t.
r.t.
50
80
50
80
r.t.
50
80
80
24
24
24
24
3
24
3
24
24
3
>99
16
>99
16
a
Co-1 (1)
2B (1)
>99
>99
>99
>99
>99
>99
79
>99
>99
>99
>99
>99
>99
79
2B (0.1)
2B (0.1)
2A (0.1)
2A (0.1)
2C (0.1)
2C (0.1)
2C (0.1)
9
10
11
50
88
50
88
Co2(CO)8
(0.1)
3
12
13
14
2B (0.02)
2C (0.01)
Co-1 (0.02)
80
r.t.
80
24
48
24
70
36
10
70 (TON = 3500)
36 (TON = 3600)
10 (TON = 500)
b
a
Co-1: Co(OPiv)2 (1 mol %)/CNAd (3 mol %)/(EtO)3SiH (4 mol
b
%). Co-2: Co(OPiv)2 (0.02 mol %)/CNAd (0.06 mol %).
PMDS (additional results in Table S3). Apparently,
Co2(CNtBu)8 (2A) and Co2(CNAd)8 (2B) possessed greater
catalytic efficiency than the Co(OCOR)2/CNR catalyst
system. As reported earlier,17 the same reaction was catalyzed
by the corresponding Co(OCOR)2 (1 mol %)/CNR catalyst
system to produce desired 4a quantitatively with 100%
selectivity when the reaction was performed at 50 °C for 24
h (entry 1). However, when the reaction was performed at
room temperature, the product yield decreased to 16% (entry
2). In contrast, hydrosilylation catalyzed by 2B (1 mol %)
proceeded at room temperature to give 4a as a single product
in quantitative yield after 24 h (entry 3). Experiments to
reduce catalyst concentration from 1 to 0.1 mol % provided the
optimum conditions at 50 °C for 24 h or 80 °C for 3 h for both
2A and 2B as the catalyst (entries 4−7). Interestingly, the
catalytic performance of Co2(CNMes)8 (2C) differed from
that of 2A and 2B. The catalytic activity of 2C at room
temperature was good enough to complete the reaction in 24
a
b
Isolated yield. exo/endo ratio = 9:1.
examples catalyzed by 2A. All reactions were performed using
1 mol % 2A at 50 °C for 24 h. As described above,
hydrosilylation of α-methylstyrene with PMDS gave 4a in good
yield. Reaction of α-methylstyrene with MD′M
(Me3SiOSiMeHOSiMe3) also proceeded smoothly to give
10a in high yield (entry 1). Reaction of 1-octene with PMDS
gave n-octylsilane 5a as a single product (entry 2). The same
product, 5a, was obtained by reaction of 2-octene with PMDS
(entry 3), indicating that hydrosilylation was accompanied by
CC double bond migration. Norbornene was hydrosilylated
with PMDS in good yield as shown in entry 4. Reactivity of
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alicyclic alkenes was lower than that for linear terminal alkenes.
In particular, cyclohexene is minimally hydrosilylated using a
platinum catalyst.2 The use of 2A as the catalyst permitted the
conversion of both cyclopentene and cyclohexene to the
corresponding product, 11a and 12a, respectively, in 86 and
75% yield. Allyl glycidyl ether was hydrosilylated using 2A as
the catalyst (entry 7).
Modification of Silicone Fluids. To help develop
industrially important modification processes of silicone fluids,
the three reactions shown in Scheme 5 were investigated. The
The reaction shown in eq 2 was complete with 0.5 mol % (per
Co) of 2B at 50 °C for 24 h, and 14a was formed in 99% yield.
In both cases, reaction using the Co(OPiv)2/CNAd catalyst
system activated by (EtO)3SiH provided the corresponding
product in low yields. The second example involved hydro-
silylation of polydimethylsiloxane end-capped with two
Me2SiH groups containing a glycidyl ether (Scheme 5, eq 3).
Both Fe(CNtBu)5 (1A) and Co2(CNAd)8 (2B) catalyzed the
reaction to afford 15a. Thus, the iron catalyst showed greater
catalytic activity than the cobalt catalyst; reaction with 1A was
accomplished with 0.25 mol % catalyst at room temperature in
24 h.
Scheme 5. Iron- and Cobalt-Catalyzed Modification of
Silicone Fluids with Alkenes
Two-Component Silicone Curing. Two-component
silicone curing is a process used to produce silicone resins
by hydrosilylation of a polydimethylsiloxane containing Si−H
groups or vinyl groups.3,11f,21 A typical example involved
treatment of (CH2CH)(Me2SiO)(Me2SiO)47SiMe2(CH
CH2) (Si-1) and Me3SiO(MeHSiO)9SiMe3 (Si-2) with 2B or
(EtO)3SiCo(CNAd)4 (17B) (vide infra). For comparison, a
1:3 mixture of Co(OPiv)2 and CNAd also was examined as the
catalyst. The molar ratio of Si-1 and Si-2 was adjusted to a 1:1
ratio of the CH2CH group of Si-1 to the Si−H moiety in Si-
2. Table 5 summarizes the results of curing performed at 120
°C for 3−24 h. Hydrosilylation of the vinyl groups in Si-1 with
Si−H groups in Si-2 resulted in cross-linking of two silicone
chains to give insoluble silicone gel. The degree of cross-
linking (DCL) was estimated by IR and solid-state NMR
spectroscopy of silicone gel 16 and a 1:1 mixture of Si-1 and
Si-2 (the standard). The IR spectra of these two samples
showed νSi−H and νSi−C vibrations at 2150 and 1258 cm−1,
respectively. The peak integrals I(νSi−H) and I(νSi−C) were
determined from the IR spectra of 16, whereas I0(νSi−H) and
I0(νSi−C) were estimated from the standard. DCL(IR) was
defined as {1 − I(IR)/I0(IR)} × 100, where I(IR) =
[I(νSi−H)]/[I(νSi−C)] and I0(IR) = [I0(νSi−H)]/[I0(νSi−C)].
1
Solid-state H NMR of 16 and the standard sample showed
signals due to Si−H, vinyl, and methyl groups around 4.7 ppm,
5.6−6.3 ppm, and 1.0 to −2.0 ppm, respectively. The 29Si
NMR spectrum contained signals at 4 to 5 ppm and −22 ppm,
assigned to a Si nucleus bonded with the vinyl group and that
of SiMe2 moieties, respectively. As described in the Supporting
Information in detail, the integral of these signals provided the
basis of I and I0 determined by NMR, from which DCL(1H
NMR)-1, DCL(1H NMR)-2, and DCL(29Si NMR) were
estimated.
Table 5 shows that the time required for curing and the
DCL were dependent on the catalyst and catalyst concen-
tration. As shown in entries 1−3, curing occurred when 98−
195 ppm of 2B was used as the catalyst. With higher catalyst
loadings, a shorter period was needed for gelation to occur.
While only a viscous gel was obtained using 49 ppm 2B, a solid
gel was formed with 17B (entry 5), which is more soluble in
silicones than 2B. The Co(OPiv)2/CNAd catalyst system (Co-
2) was not useful for curing at a lower catalyst loading (98
ppm). Catalytic activity for silicone curing was 17B > 2b > Co-
2. The DCL was 50−60% for the experiments shown in entries
2 and 5, which produced soft gels. Gels close to a solid state
were obtained from the experiments shown in entries 1 and 4,
which resulted in a DCL of 66−71% and 75−93%,
respectively. The color of the gel after curing was brown,
which turned pale blue upon exposure to air. A similar color
change was observed for the samples produced from reactions
first two reactions involved cobalt-catalyzed hydrosilylation
with α-methylstyrene, in which polydimethylsiloxane end-
capped with two Me2SiH groups and polydimethylsiloxane
having two Me3Si terminal and two −MeHSiO− units were
used as the hydrosilane. The former reaction was achieved with
0.1 mol % 2B at 50 °C to give 13a quantitatively after 24 h
(Scheme 5, eq 1). The catalyst loading of 0.1 mol % (per Co)
of 2B corresponded to 70 ppm of Co per the starting material.
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Table 5. Cobalt-Catalyzed Silicone Curing
a
degree of cross-linking (DCL)
ppm/
Co
time required for the
curing
total reaction
time
DCL(IR)
(%)
DCL(1H NMR)-1
DCL(1H NMR)-2
DCL(29Si NMR)
(%)
c
d
entry catalyst
(%)
(%)
1
2
3
4
5
6
7
2B
2B
2B
17B
17B
Co-2
Co-2
195
98
49
195
49
195
98
3 min
30 min
24 h (gelatinous)
3 min
3 h
3 h
3 h
24 h
3 h
24 h
3 h
66
50
39
79
50
69
68
62
b
75
62
59
71
59
b
89
64
73
66
46
b
93
51
60
e
e
11 min
no curing
24 h
a
Degree of cross-liking (DCL) was determined by IR and NMR spectra according to the methods described in the text and the Supporting
b
c
Information. Since the sample was viscous gel, we cannot measure the solid state NMR. DCL(1H NMR)-1 is determined by the 1H signals due to
d
e
the Si−H and Si−Me groups. DCL(1H NMR)-2 is determined by the 1H signals due to the Si−vinyl and Si−Me groups. Co-2 = 1:3 mixture of
Co(OPiv)2 and CNAd.
using 2B or 17b as the catalyst. The gel formed using 49 ppm
of 17b was nearly colorless.
Scheme 6. (a) Chalk−Harrod Cycle, (b) Modified Chalk−
Harrod Cycle for Cobalt-Catalyzed Hydrosilylation of
Alkenes, (c) Possible Mechanism for Alkene Isomerization
Mechanistic Considerations. The results presented here
suggest catalytically active species generated from the Fe-
(OPiv)2/CNR and Co(OPiv)2/CNR catalyst systems were
similar to those from Fe(CNR)5 and Co2(CNR)8, respectively.
In particular, product ratios between n-alkylsilane from
hydrosilylation and allylsilane and alkane from dehydrogen-
ative silylation are good supporting evidence. The geometric
and electronic structures of CNR are similar to those of CO,
which is useful to consider when investigating the reaction
mechanism and determining the net catalyst species in the
catalytic cycle. Thus, the mechanism for reactions catalyzed by
Fe(CNR)5 and Co2(CNR)8 would not differ from those
proposed for reactions catalyzed by Fe(CO)5 and Co2(CO)8.
It was reported that treatment of R3SiH with Co2(CO)8
resulted in formation of R3SiCo(CO)4 and HCo(CO)4.
Further reaction of HCo(CO)4 with HSiR3 afforded R3SiCo-
(CO)4 and H2.15a For the Co2(CO)8-catalyzed hydrosilylation
of alkenes, a Chalk−Harrod cycle was proposed in the 1960s,
which involves insertion of a CC bond into a Co−H species
generated from Co2(CO)8 (Scheme 6a). Chalk and Harrod
obtained no evidence for the participation of R3SiCo(CO)4 in
thermal catalytic hydrosilylation of alkenes.15b In contrast,
Wrighton and co-workers observed efficient hydrosilylation of
HSiEt3 with 1-pentene under photoirradiation. Detailed
mechanistic studies on the photoassisted hydrosilylation of
15c,d
alkenes with Co2(CO)8
supported a modified Chalk−
Harrod cycle (Scheme 6b), which involved insertion of a C
15e
C bond into a Co−Si species generated from Co2(CO)8
Thus, the hydrosilylation proceeds through a Chalk−Harrod
cycle thermally, possibly via a photochemically modified
Chalk−Harrod cycle.
The approach to investigate net catalytic species began with
experiments to elucidate the species generated by reaction of
Co(OPiv)2 with HSi(OEt)3 in the presence of CNR. Catalytic
activity was enhanced when this treatment was performed prior
to hydrosilylation of alkenes with hydrosiloxanes. Reaction of a
1:6 mixture of Co(OPiv)2 and CNAd or CNtBu with excess
HSi(OEt)3 resulted in a color change from violet to brown and
produced the corresponding silylcobalt complexes, {(EtO)3Si}-
Co(CNR)4 [R = Bu (17A) or Ad (17B)], which were
obtained as stable complexes in greater than 70% yields
(Scheme 7, eq 1). The reaction was accompanied by formation
of hydrogen.
t
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Scheme 7. Synthesis of (EtO)3SiCo(CNR)4
Scheme 8. Reaction Profiles for Hydrosilylation of α-
Methylstyrene with PMDS Using Co2(CNAd)8 (2B) or
a
(EtO)3SiCo(CNAd)4 (17B) as the Catalyst
These new silyl-cobalt complexes were characterized by
NMR and IR spectroscopy, and X-ray structural determination
of {(EtO)3Si}Co(CNAd)4 supported the spectral assignments
(Figure 1). The same silyl-cobalt complexes were also formed
a
1
The yield was determined by H NMR.
The resulting mixture of 2-, 3-, and 4-octenes also are
hydrosilylated to form the 1-silyloctane, which is the same
product as that obtained from the hydrosilylation of 1-octene.
These results can be explained by HCo(CNR)3-mediated
hydrosilylation. Addition of an H−Co bond to a CC moiety
followed by β-hydrogen atom elimination resulted in alkene
isomerization to give internal octenes (Scheme 6c), while
sequential addition−elimination reactions of a H−Co bond to
a CC moiety in internal octenes formed an n-octyl-Co
moiety, which is susceptible to reaction with Si−Co species to
form 1-silyloctane.
A recent report from the research group of Chirik suggested
the involvement of a modified Chalk−Harrod mechanism in
the hydrosilylation of alkenes catalyzed by bis(imino)pyridine
cobalt complexes.25 As shown in Scheme 6, the modified
Chalk−Harrod cycle promoted both hydrosilylation and
dehydrogenative silylation in the reaction of terminal alkenes,
while dehydrogenative silylation was difficult to achieve
through a Chalk−Harrod mechanism. Unlike Chirik’s bis-
(imino)pyridine cobalt catalysts, products from dehydrogen-
ative silylation in the reactions of terminal alkenes with
hydrosilanes catalyzed by Co2(CNR)8 or Co(OCOR)2/CNR
have not been reported. This also supports a Chalk−Harrod
mechanism for cobalt-isocyanide catalysts.
The mechanism for the hydrosilylation of alkenes catalyzed
by Fe(CNR)5 can be examined through analogy with
Fe(CO)5. However, the mechanism of Fe(CO)5-catalyzed
reactions has not been as thoroughly investigated as
Co2(CO)8-catalyzed hydrosilylation. Hydrosilylation of al-
kenes catalyzed by Fe(CO)5 and other iron carbonyls is
competitive with dehydrogenative silylation of alkenes.14a,26 In
many cases, dehydrogenative silylation predominated over
hydrosilylation. Wrighton and co-workers proposed oxidative
addition of an H−Si bond to coordinatively unsaturated iron
carbonyls, which was generated photochemically from Fe-
(CO)5, an alkene, and a hydrosilane.14b Two reaction pathways
for the resulting oxidative adduct “H−Fe−Si” were proposed:
insertion of a CC bond into the H−Fe bond that initiated a
Chalk−Harrod cycle to form the hydrosilylated product or
insertion into the Si−Fe bond to promote a modified Chalk−
Harrod cycle that provided a mixture of the three products due
to both hydrosilylation and dehydrogenative silylation.
Simplified schemes for these processes are illustrated in
Scheme 9. Recent DFT calculations14c for the possible catalytic
cycles for Fe(CO)5-catalyzed hydrosilylation of alkenes
support the mechanisms involving (CO)4Fe(H)(SiR3) and
Figure 1. Molecular structure of 17B. Hydrogen atoms and ether as a
solvent molecule have been omitted for clarity.
from Co2(CNR)8 and HSi(OEt)3 (Scheme 7, eq 2). For
example, 2B was treated with a large excess of HSi(OEt)3 at
room temperature for 12 h without solvent. Excess HSi(OEt)3
was removed in vacuo, and the residue recrystallized from Et2O
to give 17B in 50% yield. Evolution of hydrogen was observed.
Similar to the reaction of Co2(CO)8 with R3SiH, two reactions,
formation of HCo(CNR)4 and (EtO)3SiCo(CNR)4 and
reaction of the resulting HCo(CNR)4 with HSi(OEt)3 to
produce (EtO)3SiCo(CNR)4 and H2, occurred successively.
The results confirmed that the Si(OEt)3 group in 17B could be
exchanged with a Me3SiOMe2Si group in contact with PMDS.
When a 1:3 mixture of 17B and PMDS was heated in C6D6 at
50 °C for 5 h, signals due to 17B were decreased, and two new
singlets appeared at δ 1.19 and 0.69 ppm, which were assigned
to SiMe2 and SiMe3 moieties of (Me3SiOMe2Si)Co(CNAd)4
(18), respectively. The ratio of 17B to 18 was 3:7 (Scheme 7,
eq 3). These experiments suggest that (Me3SiOMe2Si)Co-
(CNR)4 was a precursor of net catalytic species, which was
generated from either (EtO)3SiCo(CNR)4 or Co2(CNR)8 in
contact with PMDS during catalysis. Compound 17B actually
acted as the catalyst for hydrosilylation of α-methylstyrene with
PMDS. Comparison of two reaction profiles obtained by
hydrosilylation of α-methylstyrene with PMDS catalyzed by
17B and 2B revealed a significantly longer induction period for
the reaction with 17B, as shown in Scheme 8. This is
consistent with observations reported in the literature, in which
HCo(CO)3 exists inside the catalytic cycle.22 A Chalk−Harrod
cycle mediated by HCo(CNR)3 is highly likely and could
explain the present results from thermal hydrosilylation.23,24
H
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dehydrogenative silylation became a major reaction pathway
with excess 1-octene.
Scheme 9. Chalk−Harrod and Modified Chalk−Harrod
Cycles for Iron-Catalyzed Hydrosilylation of Alkenes
An interesting feature of the Fe(CNR)5 catalyst and the
Fe(OPiv)2/CNR catalyst system is the selectivity of the
reactions involving styrene. Dehydrogenative silylation is a
major reaction pathway when iron carbonyl complexes are
used as the catalyst.26 Marciniec and co-workers reported that
direction of the reaction toward hydrosilylation or dehydrogen-
ative silylation was affected not only by the hydrosilane/
styrene ratio but also by the reaction temperature. Selective
hydrosilylation of styrene catalyzed by (divinylsiloxane)Fe-
(CO)3 catalysts was achieved only when using a 1:1 ratio of
styrene and hydrosilane and a temperature carefully controlled
at 0−10 °C.26d The Fe(CNR)5 catalyst and the Fe(OPiv)2/
CNR catalyst system generally achieved selective hydro-
silylation at temperatures below 50 °C. The difference in the
electronic nature of CNR and CO facilitated selective
hydrosilylation at relatively high temperatures.
CONCLUSIONS
■
(CO)nFe(H)(SiR3) (alkene), which can accommodate both
the thermal and photochemical processes.
For the hydrosilylation experiments, Fe(CNR)5 and
Co2(CNR)8 acted as good catalysts for hydrosilylation of
alkenes with hydrosiloxanes. Similar to the corresponding
M(OPiv)2/CNR catalyst system, Fe(CNR)5 catalyzed the
hydrosilylation of styrene and allylic ethers with high
selectivity, whereas Co2(CNR)8 acted as a good catalyst for
hydrosilylation of α-methylstyrene, 1-octene, cycloalkenes, and
allylic ethers, providing the desired siloxane as a single product.
The catalyst efficiency of Fe(CNR)5 was higher than that of
the Fe(OPiv)2/CNR catalyst system for hydrosilylation of
allylic ethers, whereas Co2(CNR)8 generally had greater
catalytic activity than the Co(OPiv)2/CNR catalyst system.
The discovery of catalysts with greater catalytic activity
improved the processes of silicone modification and curing,
which proceeded at a lower catalyst concentration than
processes using the M(OPiv)2/CNR catalyst system.
In platinum-catalyzed hydrosilylation of alkenes, Spier’s
catalyst, H2PtCl6 in iPrOH, is easy to handle and
inexpensive.2a,c In contrast, Karstedt’s catalyst,
(divinylsiloxane)3Pt(0)2, is costly but possesses greater
catalytic performance than Spier’s catalyst.2b Spier’s catalyst
requires reduction of H2PtCl6 for generation of catalytically
active Pt(0) species, while Karstedt’s catalyst containing a
Pt(0) center easily forms the active species by dissociation of
divinylsiloxane. The relationship between the M(II)(OPiv)2/
CNR catalyst system and metal isocyanide (0 or I) catalysts
was comparable to that of Spier’s catalyst with Karstedt’s
catalyst. Thus, base-metal-catalyzed hydrosilylation of alkenes
can be accomplished with the M(OPiv)2/CNR catalyst system
and metal isocyanide catalysts depending on the situation. The
former conveniently generates the catalytically active species
from stable catalyst precursors and reducing reagents, whereas
the latter requires preparation of the Fe(0) or Co(I) complexes
but possesses good catalytic performance. It is important from
the production of silicones that discovery of new catalysts,
Fe(CNR)5 and Co2(CNR)8 made possible efficient modifica-
tion of silicone fluids and two-component silicone curing.
These are key technologies in silicone industry1−3 and are
achieved by using platinum catalysts. Lately, achievement of
these processes by base-metal catalysts attracted attention, and
several papers regarding two-component silicone curing
appeared.11f,21 The present report provided one of the
successful examples.
Involvement of coordinatively unsaturated Fe(CNR)n
species was indicated from the significant acceleration of
Fe(CNR)5-catalyzed hydrosilylation by photoirradiation
(Scheme S2). Similar to the photoassisted CO dissociation
from Fe(CO)5, photolysis induced elimination of CNR from
Fe(CNR)5. The mechanistic analogy to Fe(CO)5 suggested
that the net catalytic species would be (RNC)4Fe(H)(SiR3)
and (RNC)3Fe(H)(SiR3)(alkene). However, attempted NMR
analysis of the oxidative adduct, (RNC)4Fe(H)(SiR3), by the
reaction of 1A with PMDS was prevented due to the formation
of paramagnetic species. Involvement of (RNC)4Fe(H)(SiR3)
and (RNC)3Fe(H)(SiR3)(alkene) was inconsistent with the
results that the greatest catalytic activity was achieved in the
Fe(OPiv)2/CNR catalyst system, when the Fe:CNR ratio was
adjusted to 1:2 (see, Scheme S3). The number of CNR ligands
in the catalytically active species is likely to be two.
Hydrosilylation and dehydrogenative silylation catalyzed by
Fe(CO)5 required application of high temperatures or
photoirradiation.14b In contrast, Fe(CNR)5 catalyzed the
reaction even at room temperature. Nevertheless, the catalytic
activity of Fe(CNR)5 was inferior to that of the Fe(OPiv)2/
CNR (Fe/CNR = 1:2) system when catalyst loadings were
low, and photolysis accelerated the reaction. These results
suggest that a significant energy barrier existed for dissociation
of CNR from Fe(CNR)5 to generate a catalytically active
species, although the barrier was lower than that for liberation
of CO from Fe(CO)5. However, the better catalytic perform-
ance of Fe(CNR)5 compared to that of the Fe(OPiv)2/CNR
system for the hydrosilylation of allylic ethers was interesting.
Bidentate coordination of the oxygen atom and CC bond in
the allylic ether to the iron center would facilitate dissociation
of CNR.20
If the (RNC)nFe(H)(SiR3)(alkene) species is assumed to be
present in the reaction medium, then the next step would be
insertion of the coordinated alkene between either the Fe−H
or Fe−Si bond. Similar to the Fe(CO)5-catalyzed hydro-
silylation, these two processes are sensitive to the ratio of
charged alkene and H-SiR3. As shown in Table S4, reaction of
1-octene with PMDS in the presence of Fe(CNR)5 and the
Fe(OPiv)2/CNR catalyst system resulted in hydrosilylation,
especially in the presence of excess PMDS. In contrast,
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56.4, 45.4, 36.3, 30.0. IR(ATR): ν = 2106 (CNR) cm−1. Anal.
Calcd for C55H75N5Fe: C 76.63; H 8.77; N 8.12. Found: C 77.10; H
8.85; N 8.34.
In addition to these synthetic benefits, the experimental
results provided clues for understanding the net catalyst
species. In a previous communication, possible mechanisms
involving coordinately unsaturated Fe(0) and Co(I) isocyanide
species were suggested. The results presented here clearly
support this hypothesis that the same catalyst species would be
generated from Fe(CNR)5 or from Co2(CNR)8. In particular,
no difference was found in the selectivity of the two reaction
pathways, hydrosilylation or dehydrogenative silylation,
between Fe(CNR)5 and the Fe(OPiv)2/CNR system and
between Co2(CNR)5 and the Co(OPiv)2/CNR system.
Similar to the hydrosilylation mechanism catalyzed by
Co2(CO)8, experiments using (EtO)3SiCo(CNR)4 indicate
that a Chalk−Harrod cycle involving insertion of a CC bond
between Co−H bond of HCo(CNR)3(alkene) is reasonable.
Although some mechanistic studies could not be accom-
plished, mechanisms analogous to those for the Fe(CO)5-
catalyzed hydrosilylation and dehydrogenative silylation
indicate that a Chalk−Harrod pathway from the intermediary
(RNC)nFe(H)(SiR3) was competitive with modified Chalk−
Harrod pathway.
Fe(CNMes)5 (1C). Isolated yield: 23%. Mp. 140 (decomp). 1H
NMR (396 MHz, C6D6) δ: 6.60 (s, 10H), 2.48 (s, 30H), 2.05 (s,
15H). 13C NMR (100 MHz, C6D6) δ: 196.5, 134.4, 133.0, 129.9,
128.7, 44.5, 36.1, 21.0, 19.3. IR(ATR): ν = 1971, 1940 (CNR)
cm−1. Anal. Calcd for C50H55N5Fe: C 76.81; H 7.09; N 8.96. Found:
C 76.45; H 6.95; N 8.62.
General Procedure for Synthesis of Co2(CNR)8. To a solution
of CoI2(1.0 mmol) in THF (15 mL) was added RNC (4.0 mmol) and
KC8 (270 mg, 2.0 mmol) in a globe box. The solution was stirred at
room temperature for 12 h, then the resulting mixture was passed
through a short pad of Celite. The filtrate was concentrated under
t
reduced pressure. When BuNC was used as the isocyanide, the
residue obtained was extracted with pentane. Complex 1A was
obtained by recrystallization from pentane. In the cases where R of
RNC was Ad or Mes, toluene was used for the extraction, and
recrystallization was performed from toluene/pentane. The desired
product was obtained as orange crystals.
Co2(CNtBu)8 (2A). Isolated yield: 57%. Orange crystals. Mp. 140 °C
1
(decomp). H NMR (600 MHz, C6D6) δ: 1.44 (s, 72H). 13C NMR
(100 MHz, C6D6) δ: 54.8, 31.7. IR(ATR): ν = 1666 [CNR
(bridge)], 2093, 1977, 1942 [CNR (terminal)] cm−1. Anal. Calcd
for C40H72N8Co2: C 61.36; H 9.27; N 14.31. Found: C 61.06; H 9.52;
N 14.05.
EXPERIMENTAL SECTION
■
Co2(CNAd)8 (2B). Isolated yield: 57%. Orange crystals. Mp. 200 °C
General. Manipulation of air- and moisture-sensitive compounds
was carried out under a dry nitrogen atmosphere using Schlenk tube
techniques associated with a high-vacuum line or in the glovebox,
which was filled with dry nitrogen. All solvents were distilled over
appropriate drying reagents prior to use. 1H, 13C, and 29Si NMR
spectra were recorded on a JEOL Lambda 400 or a Lambda 600
1
(decomp). H NMR (396 MHz, C6D6) δ: 2.32 (s, 48H), 2.06 (s,
24H), 1.71 (d, J = 10.3, 24H), 1.58(d, J = 10.3, 24H). 13C NMR (100
MHz, C6D6) δ: 55.6, 45.0, 36.7, 30.2. IR(ATR): ν = 1647 [CNR
(bridge)], 2101, 2000, 1954 [CNR (terminal)] cm−1. Anal. Calcd
for C88H120N8Co2: C 75.08; H 8.59; N 7.96. Found: C 75.16; H 8.62;
N 7.46.
1
29
spectrometer at ambient temperature. H, 13C, 19F, and Si NMR
chemical shifts (δ values) were given in ppm relative to the solvent
signal (1H and 13C) or standard resonances (19F, external trifluoro-
acetic acid; 29Si, external tetramethylsilane). Elemental analyses were
performed by a PerkinElmer 2400II/CHN analyzer. IR spectra were
recorded on a JASCO FT/IR-550 spectrometer. Isocyanides [1-
isocyanoadamantane (AdNC), t-butylisocyanide (tBuNC)] were
purchased from Tokyo Chemical Industries Co., Ltd., or Sigma-
Aldrich, and were used without further purification. All of the alkenes
described in the text and hydrosilanes, PhMe2SiH, Et3SiH,
(EtO)3SiH, (EtO)2MeSiH, (MeO)3SiH, (MeO)2MeSiH, Ph2SiH2,
and PhSiH3 were purchased from Tokyo Chemical Industries Co.,
Ltd. Hydrosiloxanes, 1,1,3,3,3-pentamethyldisiloxane (PMDS), 1,1,1-
3-5,5,5-heptamethyltrisiloxane (MD′M), a vinylsiloxane polymer
[CH2CHSiMe2O(SiMe2O)nSiMe2CHCH2], and hydrosiloxane
polymers [Me3SiO(SiMe2O)nSi(H)Me2 and Me3SiO{Si(H)-
MeO}mSiMe3] were donated by Shin-Etsu Chemical Co., Ltd. For
the preparation of iron and cobalt isocyanides, KC8, was prepared
according to the literature,27 and sodium silica gel was purchased from
Ardrich.28
Co2(CNMes)8 (2C). Isolated yield, 66%. Orange crystals. Mp. 180
1
°C (decomp). H NMR (396 MHz, C6D6) δ: 6.60 (s, 12H), 6.58 (s,
4H), 2.46 (s, 36H), 2.42 (s, 12H), 2.05 (s, 18H), 2.03(s, 6H). 13C
NMR (100 MHz, C6D6) δ: 132.5, 129.3, 128.4, 125.7 21.0, 19.3.
IR(ATR): ν = 1669 [CNR (bridge)], 2063, 2026, 1954 [CNR
(terminal)] cm−1. Anal. Calcd for C80H88N8Co2: C 75.10; H 6.93; N
8.60. Found: C 75.21; H 6.90; N 8.60.
Preparation of 1A and 2A by Silica-Na. To a solution of
t
FeBr2(216 mg, 1.0 mmol) and THF (20 mL) were added BuNC
(415 mg, 5.0 mmol) and Na silica gel (Stage I) (340 mg, 35 wt % Na)
in a globe box. The solution was stirred at room temperature for 12 h,
and the resulting mixture was passed through a short pad of Celite.
After removal of the solvent, the residue was dissolved in pentane (ca.
20 mL), then stored at −35 °C to give 1A as yellow crystals (199 mg,
42%). With a similar procedure, 2A was obtained from CoI2 (1
t
mmol) and BuNC (4 mmol) as orange crystals (190 mg, 48%).
Procedures for Hydrosilylation of Alkenes Catalyzed by
Fe(CNR)5. In general, PMDS was treated with alkene and a catalytic
amount of Fe(CNR)5 without solvent at r.t. to 50 °C for 3−24 h.
After the reaction, the mixture was passed through a short pad of
Al2O3 by eluting with ether to remove iron residues. After
concentration, the product was isolated by distillation. Alternatively,
anisole as an internal standard was added to the reaction mixture, and
General Procedure for Synthesis of Fe(CNR)5. To a solution of
FeBr2 (1.0 mmol) in THF (20 mL) were added RNC (5.0 mmol) and
KC8 (2.0 mmol) in a globe box. The solution was stirred at room
temperature for 12 h, then the resulting mixture was passed through a
short pad of Celite. The filtrate was concentrated in vacuo. When
tBuNC was used as the isocyanide, the residue obtained was extracted
with pentane. Complex 1A was obtained by recrystallization from
pentane. In the cases where R of RNC was Ad or Mes, toluene was
used for the extraction, and recrystallization was performed from
toluene/pentane. The desired product was obtained as yellow crystals.
1
the product yields were determined by H NMR. For instance, the
reaction described in Scheme 3, eq 1 was performed as follows: 1A
(8.6 mg, 0.01 mmol), allyl glycidyl ether (1.14 g, 10.0 mmol), and
PMDS (2.54 mL, 13 mmol) were placed in a glass vial. The resulting
mixture was stirred at room temperature for 24 h. Product 6a was
isolated in 95% yield (2.45 g) by bulb-to-bulb distillation (8 Pa, 60−
70 °C). In the experiment shown in Table 1, entry 2, a mixture of 1B
(8.6 mg, 0.01 mmol), styrene (115 μL, 1.0 mmol), and PMDS (254
μL, 1.3 mmol) was placed in a glass vial. The resulting solution was
stirred at 50 °C for 24 h. After the reaction, anisole (108 mg, 1.0
mmol) was added to the mixture, and the product yield was
determined to be >99% by comparison in the integral value of the
signals due to 6a and the peaks due to anisole as the internal standard.
Detailed experimental procedures for the iron-catalyzed hydro-
1
Fe(CNtBu)5 (1A). Isolated yield: 57%. Mp. 140 °C (decomp). H
NMR (600 MHz, C6D6) δ: 1.29 (s, 45H). 13C NMR (100 MHz,
C6D6) δ: 199.0, 55.6, 31.6. IR(ATR): ν = 2119, 2000, 1943, 1826
(CNR) cm−1. Anal. Calcd for C25H45N5Fe: C 63.68; H 9.62; N
14.85. Found: C 63.21; H 9.56; N 15.11.
Fe(CNAd)5 (1B). Isolated yield: 70%. Mp. 200 (decomp). 1H NMR
(396 MHz, C6D6) δ: 2.15 (s, 30H), 1.88 (s, 15H), 1.50 (d, J = 11.5,
15H), 1.42 (d, J = 11.5, 15H). 13C NMR (100 MHz, C6D6) δ: 199.9,
J
DOI: 10.1021/acs.organomet.8b00389
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
silylation of styrene, 1-octene, and allylic ethers are described in the
Supporting Information. Characterization of the products were
performed in comparison their spectral data with those reported in
our previous paper.16,17
catalysts without solvent at 120 °C for 3−24 h. Meanwhile, time of
gelation was measured. The cross-linked silicone gel was obtained and
characterized by solid-state 1H NMR and 29Si DD NMR. For
instance, the reaction shown in Table 4, entry 1, was performed by
treatment of a mixture of Si-1 (2.87 g, ca. 1.56 mmol for Si-CH
CH2) and Si-2 (0.13 g, ca. 1.62 mmol for Si−H) with Co2(CNAd)8
(6.4 mg, 0.005 mmol). The resulting mixture was stirred at 120 °C
under nitrogen atmosphere. The solution set to gel after 3 min. The
resulting mixture was kept at 120 °C for 3 h. After cooling, the
resulting cross-linked silicone gel (16). Exposure to air gave 16 as a
slightly green transparent solid, which was characterized by solid-state
1H NMR, 29Si DD NMR, IR spectroscopy. Other examples are
Procedures for Hydrosilylation of Alkenes Catalyzed by
Co2(CNR)8. In general, PMDS or MD′M was treated with alkene and
a catalytic amount of Co2(CNR)8 without solvent at r.t. to 80 °C for
3−24 h. After the reaction, the mixture was passed through a short
pad of Al2O3 by eluting with ether to remove iron residues. After
concentration, the product was isolated by distillation. Alternatively,
anisole as an internal standard was added to the reaction mixture, and
the product yields were determined by 1H NMR. For instance, in the
reaction shown in Table 1, entry 4, 2B (6.4 mg, 0.0005 mmol), α-
methylstyrene (1.29 mL, 10.0 mmol), and PMDS (2.54 mL, 13.0
mmol) were placed in a glass vial. The resulting mixture was stirred at
80 °C for 3 h. After the reaction, the mixture was cooled to room
temperature, the product was isolated by distillation (8 Pa, 60−70
°C). Isolated yield; 2.56 g (96%). The experiment shown in Table 1,
entry 13 was performed with 2B (6.4 mg, 0.005 mmol), 1-octene (129
μL, 1.0 mmol), and PMDS (254 μL, 1.3 mmol) at 50 °C for 24 h.
After the reaction, anisole (108 mg, 1.0 mmol) was added, and the
1
described in the Supporting Information. Solid-state H NMR (400
MHz) δ: −1.0−2.0 (br, SiRMe and SiCH2CH2Si), 4.9 (br, unreacted
SiHMe), 5.8−6.4 (m, unreacted Si(vinyl)Me). Solid-state 29Si DD
NMR (119 MHz) δ: −38 to −33 (m, unreacted SiHMe), −23 to −20
(br, SiMe2), −4.2 (m, unreacted Si(vinyl)Me). IR (KBr): ν = 2150
(unreacted Si−H), 1258 (Si−C) cm−1.
ASSOCIATED CONTENT
* Supporting Information
■
S
1
product yield of >99% was determined by H NMR spectroscopy.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00389.
Detailed experimental procedures for the cobalt-catalyzed hydro-
silylation of other alkenes are described in the Supporting
Information. Characterization of the products were performed in
comparison their spectral data with those reported in our previous
paper.16,17
Experimental details including the data briefly noted in
the text, crystallographic data, and NMR charts of the
products (PDF)
Procedure for the Hydrosilylation of Alkenes with the
M(OPiv)2/CNR (M = Fe, Co) Catalyst System.16 The catalyst
systems composed of M(OPiv)2 and CNR were preactivated by
alkoxyhydrosilanes; the resulting catalyst solution was subjected to the
hydrosilylation studies. In a typical example, Fe(OPiv)2 (2.6 mg 0.01
mmol) and CNAd (3.6 mg, 0.02 mmol) were dissolved in DME (0.10
mL). (EtO)3SiH (11 mg, 0.08 mmol) was added to the solution, and
the mixture was stirred at room temperature for 1 h. A mixture of
styrene (115 μL, 1.0 mmol) and PMDS (254 μL, 1.3 mmol) was
added, and the resulting mixture was stirred at room temperature for
Cartesian coordinates (XYZ)
Accession Codes
CCDC 1840036 contains the supplementary crystallographic
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 Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
24 h. The yield of the product (>99%) was determined by H NMR
spectroscopy in the presence of the internal standard (anisole). The
reaction of PMDS (254 μL, 1.3 mmol) with α-methylstyrene (129 μL,
1.0 mmol) was performed in similar manner as above by using
Co(OPiv)2 (2.6 mg 0.01 mmol) and CNAd (4.8 mg, 0.03 mmol)
dissolved in DME (0.10 mL) as the catalyst and (EtO)3SiH (11 mg,
0.08 mmol) as the activator.
AUTHOR INFORMATION
Corresponding Author
*E-mail: nagasima@cm.kyushu-u.ac.jp.
■
ORCID
Procedure for the Modification of Silicone Polymer. In
general, Me2Si(H)O(SiMe2O)nSi(H)Me2 (n = ca. 18) or Me3SiO-
(SiHMeO)2(SiMe2O)nSiMe3 (n = ca. 27) was treated with alkene and
a catalytic amount of Fe(CNR)5 or Co2(CNR)8 without solvent at r.t.
to 50 °C for 24 h. After the reaction, the reaction mixture was passed
through a short pad of Al2O3 by eluting with ether to remove the iron
catalysts. The filtrate was dried under vacuum to afford the product
was obtained. In a typical example (Scheme 4, eq 1), Co2(CNAd)8
(6.4 mg, 0.005 mmol), α-methylstyrene (1.68 mL, 13.0 mmol), and
Me2Si(H)O(SiMe2O)nSi(H)Me2 (n = ca. 18) (7.26 g, ca. 10.0 mmol
for Si−H) were placed in a glass vial. The resulting mixture was
stirred at 50 °C for 24 h. After the reaction, the mixture was cooled to
room temperature, and the conversion of hydrosilane (>99%) and the
yield of the product (>99%) were determined by 1H NMR
spectroscopy using anisole as the internal standard. The resulting
mixture was passed through a pad of Al2O3. The filtrate was dried
under vacuum. Product 13a was obtained in 99% yield (8.40 g) as
colorless oil. 1H NMR (395 MHz, CDCl3) δ: −0.06 (s, SiMe), −0.03
(s, SiMe), 0.04 (s, SiMe3), 0.07 (s, SiMe2), 0.92−1.02 (m, 4H,
CH2Si), 1.28 (d, J = 6.8 Hz, 6H, CH3), 2.92 (sext, J = 6.8 Hz, 2H,
CH), 7.13−7.21 (m, 6H, ArH), 7.24−7.27 (m, 4H, ArH). Other
results are described in the Supporting Information.
Hideo Nagashima: 0000-0001-9495-9666
Funding
This work was supported by the Core Research Evolutional
Science and Technology (CREST) program of Japan Science
and Technology Agency (JST), Japan, and a Grant-In-Aid for
Scientific Research (B) (No. 18H01980) from Ministry of
Education, Culture, and Sports, Science and Technology,
Japan.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Donation of silanes and silicones and helpful discussion by Dr.
Noda and Mr. Sakuta of Shin-Etsu Chemical Co., Ltd., are
acknowledged. We are grateful for solid-state NMR measure-
ment carried out by Ms. Akane Matsumoto and experimental
assistance by Ms. Yuka Matsuzaki, Ms. Yoshiko Kusumori, and
Ms. Takako Matsunaga.
Procedure for Cross-Linking of Silicone Polymers. In general,
the reaction of CH2CHSiMe2O(SiMe2O)nSiMe2CHCH2 (n =
ca. 47; Si-1) with Me3SiO[Si(H)MeO]mSiMe3 (m = ca. 8; Si-2) (0.13
g, ca. 1.62 mmol for Si−H) was performed in the presence of cobalt
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DOI: 10.1021/acs.organomet.8b00389
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L
DOI: 10.1021/acs.organomet.8b00389
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(17) Noda, D.; Tahara, A.; Sunada, Y.; Nagashima, H. Non-
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Co2(CNR)8, no internal alkenes were observed in the latter catalysis.
We consider that alkene isomerization occurred in the Co2(CNR)8-
catalyzed hydrosilylation of 1-octene, but the resulting internal
alkenes underwent rapid hydrosilylation to form 1-silyloctane.
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(19) A reviewer raised two questions. One is the fate of the OPiv
group after M(OPiv)2 was treated with HSi(OEt)3, and another is the
role of HSi(OEt)3 in the M(OPiv)2/CNR catalyst system, which may
be more than facilitating the generation of active species. As the
answer of the former question, we confirmed two products,
(EtO)3SiOCOtBu and (EtO)4Si by GC-MS and 1H NMR
(Supporting Information, page S5). As for the latter question, we
performed the reactions of α-methylstyrene with PMDS catalyzed by
either 1A or 2A (0.1 mol%) in the presence and absence of
HSi(OEt)3 (8 equiv to the catalyst). No difference was observed in
reactivity and selectivity.
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Generated Cr(CO)52‑. Multivariant Optimization of an Organo-
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E.; Lefenfeld, M. Alkali Metals Plus Silica Gel: Powerful Reducing
Agents and Convenient Hydrogen Sources. J. Am. Chem. Soc. 2005,
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(20) A reviewer pointed out the catalyst efficiency of Fe(CNR)5 is
higher for allylic ethers than that of Fe(OPiv)2/CNR system, and the
difference of the catalytic activity between these two catalyst systems
are particularly great when using allylic ethers. The reason could be
ascribed to coordination of oxygen atoms in the allylic ethers to iron.
In the case of Fe(OPiv)2, coordination of the oxygen atoms would be
disturb the coordination of CNR to the Fe(II) center. Activation of
Fe(OPiv)2 with hydrosilanes does not occur in the absence of CNR.
In contrast, generation of active species from Fe(CNR)5 needs
dissociation of CNR. Coordination of the oxygen atoms in allylic
ethers to the Fe(CNR)5 may help the dissociation of CNR.
(21) (a) Marciniec, B.; Kownacka, A.; Kownacki, I.; Taylor, R.
Hydrosilylation Cross-linking of Silicon Fluids by a Novel Class of
Iron(0) Catalysts. Appl. Catal., A 2014, 486, 230−238. (b) Lee, K. L.
Angew. Chem., Int. Ed. 2017, 56, 3665−3669.
(22) How to generate active species from 17B is not clear at present.
A possibile pathway is the modified Chalk−Harrod pathway to form
allyl or vinyl silanes. This process is slow , but could generate
HCo(CNR)3(alkene) (X in Scheme 6), which rapidly catalyzed
hydrosilylation of alkenes.
(23) It should be noted that photochemical hydrosilylation catalysed
by cobalt-isocyanide catalysts may proceed through the modified
Chalk−Harrod mechamism. In fact, photoirradiation of 17B provided
a better result than that obtained by the thermal process (see Scheme
S1).
(24) Support for the Chalk−Harrod mechanism was provided by the
rapid isomerization of the terminal CC bond of 1-octene to internal
positions during hydrosilylation catalyzed by both Co2 (CO)8 and the
Co(OPiv)2/CNR catalyst system. Alkene isomerization catalyzed by
the Co(OPiv)2/CNR catalyst system occurred through a mechanism
shown in Scheme 6c involving addition and elimination of cobalt-
hydride species. Although the same catalytically active species was
generated from the Co(OPiv)2/CNR catalyst system and
M
DOI: 10.1021/acs.organomet.8b00389
Organometallics XXXX, XXX, XXX−XXX