Cobalt-Catalyzed Regio- and Enantioselective Markovnikov 1,2-Hydrosilylation of Conjugated Dienes

Article abstract of DOI:10.1021/acscatal.8b04481

The asymmetric 1,2-Markovnikov hydrosilylation of conjugated dienes with primary silanes catalyzed by a quinoline-oxazoline cobalt complex has been described. This protocol provides an efficient approach to chiral allyl dihydrosilanes with high regioselectivity and enantioselectivity (up to 96% ee). The catalyst system is effective for a wide array of conjugated dienes, including mono- and 1,2-disubstituted dienes with aryl and/or alkyl substituents. Further, the products are applied to the synthesis of polyorganosiloxanes (organo-silicone copolymers) containing side chains of enantioenriched allylic functionalities using a pyridine-oxazoline cobalt-catalyzed step-growth polymerization with terephthalaldehyde. The result of a deuterium-labeling experiment involving the reaction of PhSiD₃ and 1,3-pentadiene suggests that the hydrosilylation most likely proceeds through a modified Chalk-Harrod mechanism involving the 1,2-insertion of the terminal double bond of the diene into the Co-Si bond.

Full text of DOI:10.1021/acscatal.8b04481

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Article
Cobalt-Catalyzed Regio- and Enantioselective
Markovnikov 1,2-Hydrosilylation of Conjugated Dienes
Huanan Wen, Kuan Wang, Yanlu Zhang, Guixia Liu, and Zheng Huang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04481 • Publication Date (Web): 14 Jan 2019
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ACS Catalysis
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Cobalt-Catalyzed Regio- and Enantioselective Markovnikov
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†
†
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†
,†,‡
Huanan Wen , Kuan Wang , Yanlu Zhang , Guixia Liu and Zheng Huang*
†State Key Laboratory of Organometallic Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of
Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China.
‡Chang-Kung Chuang Institute, East China Normal University, Shanghai 200062, China
Supporting Information Placeholder
ABSTRACT: The asymmetric 1,2-Markovnikov hydrosilylation of conjugated dienes with primary silane catalyzed by a
quinoline-oxazoline cobalt complex has been described. This protocol provides an efficient approach to chiral allyl dihydrosilanes with
high regioselectivity and enantioselectivity (up to 96% ee). The catalyst system is effective for a wide array of conjugated dienes, including
mono- and 1,2-disubstituted dienes with aryl and/or alkyl substituents. Further, the products are applied to the synthesis of
polyorganosiloxanes (organo-silicone copolymers) containing side chains of enantioenriched allylic functionalities using
pyridine-oxazoline cobalt-catalyzed step-growth polymerization with terephthalaldehyde. The result of a deuterium-labelling experiment
involving the reaction of PhSiD and 1,3-pentadiene suggests the hydrosilylation most likely proceeds through the modified Chalk-Harrod
a
3
mechanism involving the 1,2-insertion of the terminal double bond of the diene into the Co−Si bond.
KEYWORDS: cobalt, conjugated dienes, allylsilane, polyorganosiloxane, hydrosilylation, asymmetric catalysis
INTRODUCTION
Scheme 1. Catalytic hydrosilylation of conjugated dienes
Transition metal-catalyzed hydrosilylation reactions of conjugated
a) Classical 1,4-hydrosilylation
1
,2
dienes have been extensively studied,
methods for 1,2-Markovnikov hydrosilylation
but enantioselective
remain
H
[Si]
[
Si]H
+
[
M]
[Si]
R
H
proposed
R
or
underdeveloped. Such a transformation encounters both regio-
and enantioselectivity challenges. Among the reported catalytic
methods of diene hydrosilylation, 1,4-addition reactions prevail in
part because of the propensity to form -allyl transition metal
R
-allyl intermediates Ref. 1
Ref. 2
b) This work: asymmetric 1,2-Markovnikov hydrosilylation
PhSiH3
+
SiH2Ph
(QuinOx)Co
SiH Ph
2
[CoI]
1,2
H
R
intermediates (Scheme 1a). Although a handful of catalysts are
R
R
3
R'
proposed
,2-insertion intermediate
now known for diene 1,2-hydrosilylation with anti-Markovnikov
R'
R'
>99:1 rr
up to 96% ee
4
R = Ar or Alk
or Markovnikov selectivity, there has been only one example for
1
R' = H, Ar or Alk
asymmetric version of 1,2-Markovnikov hydrosilylation with
limited success:
a
difluorphos Co complex catalyzed
RESULTS AND DISCUSSION
Based on our earlier observations of Markovnikov-selective
hydrosilylations of alkenes or alkynes using Co complexes of
tridentate ligands, which was attributed to inherent 1,2-insertion
1
,2-hydrosilylation of three 1-aryl-substituted dienes with
4
b
moderate enantioselectivity (overall range of ee: 76−80%),
whereas known catalysts cannot effect asymmetric
8
9
1,2-Markovnikov hydrosilylation of alkyl-substituted dienes.
Herein we disclose the development of bidentate oxazoline-based
Co complexes, one of which catalyzes 1,2-Markovnikov
hydrosilylation of various mono- and 1,2-disubstituted conjugate
dienes bearing aryl and/or alkyl substituents with unprecedented
levels of regio- and enantiocontrol (Scheme 1b).
I
preference of C−C multiple bond into a Co−Si bond of Co silyl
8
,10
intermediates,
we envisioned that Co complexes supported by
suitable chiral ligands might engender regio- and enantioselective
1,2-Markovnikov diene hydrosilylation. The less hindered
terminal C=C bond of 1,3-dienes may undergo facile 1,2-insertion
into the Co−Si bond (see the proposed intermediate in Scheme 1b),
thus avoiding the formation of a -allyl intermediate that would
lead to the common 1,4-hydrosilylation.
Products of enantioselective 1,2-Markovnikov diene
hydrosilylation, chiral allylsilanes, are highly valuable building
blocks, as demonstrated by their wide applications in asymmetric
5
Given the superior performance of oxazoline-containing
synthesis of natural products. Moreover, allyl dihydrosilanes
tridentate
ligands
in
Co-catalyzed
asymmetric
derived from primary silane can potentially act as coupling
reagents by hydrosilylating unsaturated compounds. Such a
strategy may be further extended to the synthesis of functional
organo-silicone copolymers that contain chiral allylic branchings
by reacting allyl dihydrosilane with suitable bifunctional
monomers. It is worth noting that such chiral allyl dihydrosilanes
are difficult to access by the existing methods for asymmetric
1
1
12
hydrofunctionalizations of alkenes and alkynes, we initially
tested the iminopyridine-oxazoline (IPO) Co complexes (Table
11a,b
13
1
).
3 2
Upon activation with NaBEt H, (IPO)CoCl complex 1a
(2.0 mol %) with an iPr substituent at the oxazoline proved
1,2-Markovnikov selective for the hydrosilylation of
1
3
-phenyl-1,3-butadiene 3a with PhSiH . The reaction in THF at
6
25 °C furnished allyl dihydrosilane (S)-4a in 72% NMR yield
with 95:5 rr without any detectable 1,4-hydrosilylation product.
allylsilane synthesis, such as catalytic cross-coupling and allylic
substitution reactions.
7
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Unfortunately, the enantioselectivity was unsatisfactory, with
50% ee favoring the (S)-enantiomer (entry 1). The tBu analog 1b
was more enantioselective than 1a, giving (S)-4a in 68% ee (entry
for the hydrosilylation of the Z-1-phenyl-1,3-butadiene (Z)-3a,
1
6
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 >95% (Z)-3a remaining intact (entry 14).
Table 1. Evaluation of cobalt catalysts for hydrosilylation of
2
).
a
3a with PhSiH
3
Next we turned to bidentate oxazoline-containing ligands. A
precat. (2.0 mol %)
NaBEt3H (6.0 mol %)
THF, 25 oC, 12 h
SiH2Ph
(S/R)-4a
II
series of new pyridine/quinoline-oxazoline chiral Co dichloride
complexes were prepared (Figure 1). The incorporation of a Me
substituent at the C6-position of the pyridine moiety is key to
+
PhSiH3
Ph
Ph
(E)-3a
Entry
1
Precat.
Yield [%]
rr
ee [%]
50 (S)
form
a mononuclear complex, whereas the unsubstituted
H
iPr
pyridine-oxazoline ( PyOx ) (L1, Table 1) was previously
reported to form a dinuclear species [L1
reaction of the Me-substituted ligands ( PyOx ) with CoCl in
1a
72
95:5
94:6
1
4
2
Co][CoCl
4
]. The
2
3
1b
2a
88
85
68 (S)
9 (S)
Me
R
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
>99:1
>99:1
>99:1
>99:1
90:10
THF afforded four-coordinate complexes 2a-2d in high yields.
4
2b
2c
2d
91
89
96
71
6 (S)
2 (S)
R
Two quinoline-oxazoline (QuinOx ) complexes 2e, 2f were
5
obtained in a similar manner. The mononuclear structures were
confirmed by X-ray diffraction analysis of 2b and 2f, which
revealed a tetrahedral coordination geometry (see Figure 1).
6
7b
28 (R)
15 (R)
[L1 Co][CoCl ]
2 4
8c
[L2 Co][CoCl ]
90
93:7
19 (R)
2
4
O
O
R = iBu, 2a, MePyOxiBu, 94%
iPr, 2b, MePyOxiPr, 97%
9
2e
2f
2f
95
93
45
>99:1
>99:1
>99:1
4 (R)
94 (R)
87 (R)
N
N
PyOx
or
N
N
R
R
sBu, 2c, MePyOxsBu, 93%
10
11d
12e
Me
Co
CoCl2
THF
tBu, 2d, MePyOxtBu, 97%
Cl
Cl or
O
O
N
2f
2f
2f
61
NR
>99:1
83 (R)
1 f
3
--
--
--
--
N
N
N
R
R
4g
R = iPr, 2e, QuinOxiPr, 96%
tBu, 2f, QuinOxtBu, 95%
1
<5%
Co
QuinOx
Cl
Cl
O
O
iPr
N
N
N
R
HPyOx
N
N
(IPO)CoCl2
Co
R = iPr, HPyOxiPr L1
Cl Cl
iPr
R
R = iPr, 1a
tBu, 1b
tBu, HPyOxtBuL2
2b
2f
aConditions: 3a (0.3 mmol), PhSiH
(0.6 mmol) in THF (3 mL). Yields
3
and rr values (1,2-Markovnikov product:1,2-anti-Markovnikov product)
were determined by H NMR with dibromomethane as an internal
standard, and ee values were determined by chiral HPLC. Formed In situ
Figure 1. Synthesis of pyridine/quinoline-oxazoline Co
complexes 2a-2f and the solid-state structures of 2b and 2f.
1
b
H
iPr
c
H
tBu
These new Co complexes are bench-stable, and can be used for
from CoCl
2
and PyOx L1. Formed In situ from CoCl
2
and PyOx L2.
d
e
catalysis
upon
PyOx )CoCl 2a-d were found to be even more regioselective
than (IPO)CoCl , furnishing 4a with >99:1 rr (entries 3-6).
simple
activation
with
NaBEt
3
H.
MeLi, instead of NaBEt
NaBEt H, was used as activator. Without an activator. (Z)-3a was used as
3
H, was used as activator. EtMgBr, instead of
f
g
Me
R
3
(
2
the substrate.
2
However, all reations resulted in low enantioselectivity.
Introducing a larger substituent at the oxazoline led to declined
selectivity for the (S)-enantiomer (9% ee for iBu (2a), 6% ee for
iPr (2b), and 2% ee for sBu (2c)). The run with the tBu variant
The scope of conjugated 1,3-dienes for asymmetric
1,2-Markonikov hydrosilylation was then explored by using 2f as
the precatalyst (Table 2). Despite the structural diversity of the
substrates, all reactions of 1- and 1,2-substituted conjugated
dienes offered exclusive regioselectivity for 1,2-Markonikov
(
2d) resulted in enrichment of the (R) isomer (28% ee). The
inversion of the stereochemistry implies that the selectivity
springs from the synergistic interaction of the oxazoline and
pyridine components.
products when using PhSiH
3
(one exception was the case with
Ph SiH as the reagent, vide infra), and most reactions gave high
2
2
ee values (90%). 1-Aryl-substituted dienes (see Table 2a) with
both electron- donating and -withdrawing groups on the arene
were suitable substrates. The position of the substituents had little
impact on the hydrosilylation. For example, three substrates with
a Me-substituent at the para, meta, or ortho position respectively
yielded the corresponding allylsilanes (4b-d) with comparable
selectivity. Halogen (F, Cl, and Br) (4i-m), amine (4n), CF O (4o),
3
ether (4p, q), thioether (4r), and acetal (4u) functionalities were
tolerated. The ester group in 3s is subject to reduction with
To probe the effect of the pyridine component on the selectivity,
two truncated ligands (L1, L2) lacking the Me substituent at the
pyridine moiety were tested. Submission of the precatalysts
2
formed in situ from CoCl and L1/L2 to the hydrosilylation led to
a modest drop in regioselectivity, and the (R)-enantiomer was
H
iPr
formed preferentially (15% ee for PyOx L1 and 19% ee for
H
tBu
PyOx L2) (entries 7, 8). Motivated by the observed change of
enantioselectivity with modified pyridine components, the
17
silanes. Thus it is relatively impressive that using Ph
2 2
SiH (a
quinoline-oxazoline complexes (2e, f) were further examined.
iPr
less reactive silane compared to PhSiH ) afforded the desired
3
(
2
QuinOx )CoCl 1e with an iPr-substituted oxazoline yielded 4a
product 4s in 42% yield, 91:9 rr and 83% ee. The dienes with
heterocyclic (4t, 89% ee) and 1-naphthyl (4v, 89% ee)
substituents performed well. To demonstrate scalability, the
hydrosilylation of 3a was performed on a 10 mmol scale using
with high rr, but low ee (4%, entry 9). Delightfully, the
tBu
tBu-substituted analog (QuinOx )CoCl
2
2f is highly regio- and
enantioselective, furnishing (R)-4a in 93% yield with >99:1 rr and
1
5
9
4% ee (entry 10). The hydrosilylations of 3a with other
1
.0% of 2f, delivering 2.3 g of 4a in 95% yield, >99:1 rr and 93%
activators, such as MeLi and EtMgBr, have also been carried out,
which were active for the silylation, albeit with a drop of the yield
entries 11, 12). The control reaction without an activator afforded
no hydrosilylation products (entry 13). The catalytic activity is
sensitive to the geometry of the diene; 2f was found to be inactive
ee (Table 2a).
(
The catalytic system is efficient for diene hydrosilylation with
primary arylsilane other than PhSiH . The reactions using silanes
3
containing Cl or MeO substituent at the para position of aryl ring
afforded the 1,2-Markonikov products in high yields with high
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ACS Catalysis
enantioselectivities (4w, 87% ee; 4x, 94% ee). A sterically.
hindered arylsilane with two ortho-Me substituents furnished the
Table 2. Cobalt-catalyzed asymmetric hydrosilylation of various conjugated dienes with PhSiH
desired product 4y with high ee value (93%), albeit in moderate
1
2
3
4
5
6
7
8
9
a
3
SiH2Ar
2
f (2.0 mol %), NaBEt3H (6.0 mol %)
R
+
ArSiH3
R
THF, RT, 12 h
R'
3
4
R'
a) 1-monosubstituted dienes
SiH2Ph
SiH Ph
SiH Ph
iPr
SiH Ph
SiH2Ph
SiH Ph
2
2
2
2
Ph
4
a, 95% (2.3 g)
4b, 85%
92% ee
4c, 79%
94% ee
4d, 83%
91% ee
4e, 86%
92% ee
4f, 94%
94% ee
b
tBu
93% ee
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
Ph
Br
SiH2Ph
SiH2Ph
SiH2Ph
SiH2Ph
SiH2Ph
SiH2Ph
F3C
F
4g, 81%
4% ee
4h, 80%
93% ee
4i, 81%
95% ee
4j, 75%
93% ee
4k, 79%
93% ee
4l, 77%
94% ee
9
F
Cl
Br
SiH Ph
SiH Ph
SiH Ph
SiH Ph
OMe
SiH Ph
SiH Ph
2
2
2
2
2
2
4
9
m, 76%
5% ee
4n, 85%
87% ee
4o, 53%
90% ee
4p, 92%
93% ee
4q, 85%
94% ee
4r, 87%
96% ee
Me N
2
CF O
MeO
MeS
3
SiHPh2
*
SiH2Ph
SiH Ph
2
2
SiH Ph
Ar = p-ClC6H4, 4w, 83%, 87% ee
p-MeOC H , 4x, 96%, 94% ee
SiH2Ar
O
O
O
6
4
4
s,c 42%,
4t, 80%
89% ee
4u, 96%
88% ee
4v,d 88%
92% ee
Ph
,6-Me2C6H3, 4y,d,e 56%, 93% ee
2
EtO2C
91:9 rr, 83% ee
SiH2Ph
SiH2Ph
SiH2Ph
SiH Ph
2
SiH2Ph
*
SiH2Ph
*
(
)6
*
( )7
*
( )8
*
*
TBSO
f
f
4z, 88%, 92% ee
4aa, 89%, 90% ee
4ab, 82%, 89% ee
4ac, 79%, 90% ee
4ad, 76%
4ae, 95%
b) multisubstituented dienes
SiH2Ph
SiH2Ph
SiH2Ph
SiH Ph
Ph SiH2Ph
SiH2Ph
2
Ph
*
Ph
*
Ph
*
*
Ph
4aj,g 33%, 0% ee
Ph
nC6H13
4ah,d 51%, 81% ee
Ph
4ai,d,e 42%, 93% ee
4
af,d 72%, 96:4 dr
4ag,d 67%, 91% ee
4ak, <5%
aConditions: on 0.3 mmol scale in THF (3 mL) for 12 h. Isolated yields and ee values were determined by HPLC or SFC. The absolute configuration of 4a
was assigned by comparison with reported optical rotation. The absolute configuration of other products derived from 1-aryl-substituted dienes was assigned
based on that of 4a. Unless otherwise noted, the rr values are greater than 99:1. 10 mmol scale, with 1.0% 2f. With Ph SiH (1 equiv). With 5.0% 2f. 24 h.
2 2
The ee value could not be determined. With 5.0% 2b, 24 h.
b
c
d
e
f
g
yield. The tertiary silanes, Et
unreactive toward the diene hydrosilylation.
3
SiH and (EtO)
2
MeSiH, however, are
yield and 93% ee after a prolonged reaction time of 24 h.
1,1-Diphenyl 1,3-butadiene (3aj) was presumably too sterically
bulky to undergo the hydrosilylation. In line with this assertion,
switching 2f to the less hindered precatalyst 2b gave 4aj in 33%
yield, but without any enantiocontrol. 1,4-disubstituted diene (3ak,
however, failed to react.
Significantly, 1-alkyl-substituted conjugated dienes were also
suitable to the hydrosilylation. Linear dienes delivered high yields
of allyl dihydrosilanes (4z-4ab) with excellent 1,2-Markovnikov
regioselectivity (>99:1 rr) and high enantioselectivity (89-92%
ee). The reaction of a triene with PhSiH
without isomerization or hydrosilylation of the isolated internal
C=C bond (4ac, 90% ee). The diene containing
TBSO-functionalized alkyl chain underwent regioselectively
4ad), and simple 1,3-pentadiene was hydrosilylated to the
corresponding allylsilane (4ae) in high yield with exclusive
,2-Markovnikov regioselectivity (the ee values for these two
Scheme 2. Derivatizations of chiral allyl dihydrosilanea
3
proceeded smoothly
OH
a
Ph
5, 86%, 93% ee
H
H
Ph
Ph
Ph
(
H O , KHCO
Si
Bn
2
2
3
Si
a)
(R)-4a
93% ee)
b)
Ph
6
9
Ph
7, 71%, 2.1:1 d.r.
92% ee & 87% ee
1
(
, 90%, 2.4:1 d.r. Ph
3% ee & 92% ee
Ph
products could not be determined by HPLC). The hydrosilylation
of isoprene and 1,3-butadiene, however, gave mixtures of 1,4- and
,2-addition products, with the former as the major products (see
a
1
2
Catalysts used for the hydrosilylation reactions: a) (S)-(IPO)FeBr (5.0%),
NaBEt H (15%); b) Co(acac) (5.0%), XantPhos (5.0%), NaOtBu (10%).
3 2
Supporting Information for product distributions).
The Co-catalyzed asymmetric hydrosilylation was not limited
to terminally monosubstituted 1,3-dienes. 1,2-Disubstitued dienes
With 4a readily produced on gram-scale by our method (vide
supra), the synthetic utility of chiral allyl dihydrosilane was
investigated (Scheme 2). The Fleming-Tamao oxidation of 4a
gave a chiral allyl alcohol 5 in 86% yield without racemization
were hydrosilylated with PhSiH
3
effectively using 5.0% of 2f
Table 2b). 1,2-Dialkyl-substituted diene derived from
-)-perillaldehyde underwent the hydrosilylation to form 4af in
2% yield and 96:4 d.r.. 2-Methyl-1-phenyl-substituted diene
(
(
7
1
8
(93% ee). Using (S)-(IPO)FeBr
for its structure) as the precatalyst,
anti-Markovnikov hydrosilylation of styrene with 4a proceeded at
RT to give tertiary silane with an enantioenriched
2
(see Supporting Information (SI)
19
the asymmetric
gave 4ag in 67% yield with 91% ee, while the substitution of
n-hexyl for methyl led to a reduced yield and enantioselectivity
4ah, 81% ee). 1,2-Diphenyl 1,3-butadiene (3ai) was less reactive
a
6
2
0
(
Si-stereocenter in 2.4:1 d.r. and 90% yield. Furthermore, the
asymmetric hydrosilylation of phenylacetylene with a cobalt
than 3ag and 3ah, but did form the desired product 4ai in 42%
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Page 4 of 7
catalyst of XantPhos11d, 21 formed allyl vinylsilane 7 with 2.1:1
d.r..
Next, we explored the possibility of synthesizing
regioselectivity, the Chalk-Harrod mechanism must proceed via a
-allyl cobalt intermediate (C) formed by hydrometalation,
whereas the modified Chalk-Harrod mechanism must occur via an
intermediate (F) formed by 1,2-insertion of the terminal C=C
bond of 1,3-dienes into the Co−Si bond (see Figure 2a and 2b for
these two simplified catalytic cycles). The reaction of
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
polyorganosiloxanes by polyhydrosilylation using chiral allyl
dihydrosilane and dicarbonyl compound as the monomers. The
alternating organo-silicone copolymers, which are conventionally
2
2
1,3-pentadiene 3ae with PhSiD
3
yielded a single product
synthesized by condensation of (,-bis)silanol
polyhydrosilylation with ,-dihydrooligosiloxanes,
or
have
2
3
CH CH=CHCH(SiPhD )CH D 11 with the D- and Si-atom added
3
2
2
to the C1 and C2 positions of 1,3-pentadiene, respectively (Figure
2c). This result argues strongly against the Chalk-Harrod
mechanism because silylation of the putative allyl intermediate
garnered widespread attention because these materials combine
the mechanical properties of organic polymers with the surface
24
properties of silicones. We first investigated the reactivity of the
[
Co](CH
3
3
CHCHCHCH
CH=CHCH(SiPhD
CH(SiPhD )CH=CHCH
2
D) C would otherwise form two products
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
secondary allylsilane toward sequential dual-hydrosilylation by
running a model reaction between 4a and benzaldehyde (2 equiv).
Initial efforts were focused on classical precious metal
hydrosilylation catalysts. However, Wilkinson catalyst
CH
CH
2
)CH
2
D
(11)
and
3
2
2
D (11’) in an approximate 1:1 ratio.
Thus, the 1,2-insertion of the terminal double bond of 1,3-dienes
into the Co−Si bond (E F) is likely to be the
→
Rh(PPh
Ir(cod)OMe]
Si−H bond of 4a involved) (see SI). Excitingly, the Co complex
3
)
3
Cl was inactive for the hydrosilylation, whereas
enantio-determining step (the plausible enantio-determining
transition state that accounts for the observed selectivity is
depicted in the Supporting Information).
[
2
gave the monohydrosilylation product (only one
Me
iPr
(
PyOx )CoCl
2
(2b) was found be highly efficient for the
dual-hydrosilylation: the reaction catalyzed by (rac)-2b/NaBEt
3
H
a) Chalk-Harrod mechanism
[
Co] H
formed allyl bis(oxo)silane 8 in 93% yield with complete
retention of the configuration at the carbon stereocenter (93% ee)
R
R
(Scheme 3a).
B
Scheme 3. Synthesis of chiral polyorganosiloxane
a)
(rac)-2b (5.0 mol %)
NaBEt3H (15.0 mol %)
Si(OBn)2Ph
2
PhCHO
+
[Co]-H
A
Ph
8, 93%, 93% ee
[D28 = -7.8 (c = 0.27 M, CH2Cl2)
(
R)-4a (93% ee) THF, RT, 8 h
SiH2Ph
Ph
Si
[
Co]
b)
[
(
L12Co][CoCl4]
2.5 mol %)
H2
C
H2
C
H
(
R)-4a (93% ee)
O
O
R
*
H
n
R
+
NaBEt3H (15 mol %)
C
Ph
OHC
CHO THF, RT, 60 h
PhSiH3
10, 74%, Mw = 8,700, PDI = 1.8
D = -5.3 (c = 0.49 M, CH2Cl2)
9
22
[
b) modified Chalk-Harrod mechanism
The high efficiency demonstrated in the model reaction
suggests that Co-catalyzed step-growth polymerization using allyl
dihydrosilanes as the monomers is feasible. Bifunctional
[
Co] SiH2Ph
R
R
E
terephthalaldehyde
rac)-2b/NaBEt H was active for the polyhydrosilylation reaction
between 9 and 4a (1 equiv), but yielded low molecular weight
MW) polymer (see SI for GPC data). A screen of other Co
9
was selected as the comonomer.
(
3
1,2-
(
[
Co] SiH Ph
D
insertion
2
catalysts developed in this work revealed that the use of a less
sterically hindered ligand has a beneficial effect on the generation
of high MW copolymer: the polymerization catalysed by
SiH2Ph
H
iPr
[
L1
created relatively high MW copolymer 10 (M
Scheme 3b). NMR analysis of the reaction mixture revealed the
disappearance of the signals for the CHO group of 9 and for the
SiH group of 4a, and the appearance of the signal for the CH
2
Co][CoCl
4
] with the less congested PyOx ligand L1
H
SiH Ph
R
2
*
w
= 8,700, PDI = 1.8)
[
Co]
R
*
(
F
PhSiH3
c)
2
4
2
units between the aryl rings and O-atoms. Noteworthily, the
allylic moieties remained intact during the polyhydrosilylation
process (see SI for spectra). Although the enantiopurity of 10
cannot be determined, the data obtained by chiroptical methods in
combination with the observed stereochemistry for 8 resulting
from the model reaction strongly support the generation of chiral
polymer. Despite the rigid backbones composed of perfectly
alternating aromatic rings and silicone units, the pale gray solid
material of 10 exhibits good solubility in common organic
solvents, such as THF and DMF.
2f (2.0 mol %)
SiD Ph
2
SiD Ph
2
3
+
1
NaBEt3H (6.0 mol %)
*
CH D
2
2
CH D
THF, RT, 12 h
PhSiD3
11, 82%
11', not formed
Figure 2. a) Simplified Chalk-Harrod and b) modified
Chalk-Harrod mechanism that account for the observed
regioselectivity for Co-catalyzed diene hydrosilylation; c)
deuterium-labling experiment supporting the modified
Chalk-Harrod mechanism.
Although a full understanding of the mechanism for the
cobalt-catalyzed Markovnikov 1,2-hydrosilylation of conjugated
dienes needs additional computational and experimental studies, a
deuterium-labeling experiment provides evidence in support of
the modified Chalk-Harrod mechanism (alkene insertion into a
Co−Si bond) versus the Chalk-Harrod mechanism (alkene
insertion into a Co−H bond). To accommodate the observed
CONCLUSIONS
In conclusion, we report the first general asymmetric 1,2-
Markovnikov hydrosilylation of conjugated dienes with excellent
regio- and stereoselectivity using an easily-accessible
quinoline-oxazoline Co catalyst. The hydrosilylation is applicable
to a wide variety of 1,3-dienes with aromatic and aliphatic
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ACS Catalysis
substituents. In addition, the chiral allyl dihydrosilane product can
be used for Co-catalyzed polyhydrosilylation of dicarbonyls,
Hatanaka, Y.; Goda, K.-i.; Yamashita, F.; Hiyama, T. Catalytic
asymmetric hydrosilylation of conjugated dienes: Effective control of
regio- and enantioselectivities. Tetrahedron Lett. 1994, 35, 7981-7982. (d)
Marinetti, A.; Ricard, L. Phosphetanes as Chiral Ligands for Catalytic
Asymmetric Reactions: Hydrosilylation of Olefins. Organometallics 1994,
13, 3956-3962. (e) Ohmura, H.; Matsuhashi, H.; Tanaka, M.; Kuroboshi,
M.; Hiyama, T.; Hatanaka, Y.; Goda, K.-i. Catalytic asymmetric
hydrosilylation of 1,3-dienes with difluoro(phenyl) silane. J. Organomet.
Chem. 1995, 499, 167-171. (f) Park, H. S.; Han, J. W.; Shintani, R.;
Hayashi, T. Asymmetric hydrosilylation of cyclohexa-1,3-diene with
trichlorosilane by palladium catalysts coordinated with chiral
phosphoramidite ligands. Tetrahedron: Asymmetry 2013, 24, 418-420.
1
2
3
4
5
6
7
8
9
thereby
offering
an
efficient
approach
to
novel
polyorganosiloxanes containing chiral, reactive allylic groups as
side chains, which are expected to undergo further alkene
functionalization and hold promise for synthesis of chiral
crosslinked networks.
EXPERIMENTAL SECTION
General Procedure for Cobalt-Catalyzed 1,2-Hydrosilylation
of Conjugated Dienes. In an Ar-filled glovebox, to a solution of
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
f (2.3 mg, 6.0 µmol) in 3.0 mL of THF, conjugated dienes 3 (0.3
mmol, 1.0 equiv) and ArSiH (0.6 mmol, 2.0 equiv) were added.
After the solution was cooled to -30 ºC, a solution (1.0 M in THF)
of NaBEt H (18 µL, 18.0 µmol) was slowly added. The reaction
(3) (a) Parker, S. E.; Börgel, J.; Ritter, T. 1,2-Selective Hydrosilylation of
Conjugated Dienes. J. Am. Chem. Soc. 2014, 136, 4857-4860. (b)
Greenhalgh, M. D.; Frank, D. J.; Thomas, S. P. Iron-Catalysed Chemo-,
Regio-, and Stereoselective Hydrosilylation of Alkenes and Alkynes using
a Bench-Stable Iron(II) Pre-Catalyst. Adv. Synth. Catal. 2014, 356,
3
3
mixture was stirred for 12 h at RT and then was quenched by
exposing the solution to air. The resulting solution was
concentrated in vacuum and the residue was purified by
chromatography on silica gel eluting with ethyl acetate/petroleum
ether to give the product 4.
5
84-590. (c) Ibrahim, A. D.; Entsminger, S. W.; Zhu, L.; Fout, A. R. A
Highly Chemoselective Cobalt Catalyst for the Hydrosilylation of Alkenes
using Tertiary Silanes and Hydrosiloxanes. ACS Catal. 2016, 6,
3
589-3593. (d) Raya, B.; Jing, S.; Balasanthiran, V.; RajanBabu, T. V.
Control of Selectivity through Synergy between Catalysts, Silanes, and
Reaction Conditions in Cobalt-Catalyzed Hydrosilylation of Dienes and
Terminal Alkenes. ACS Catal. 2017, 7, 2275-2283.
ASSOCIATED CONTENT
(4) (a) Hu, M.; He, Q.; Fan, S.; Wang, Z.; Liu, L.; Mu, Y.; Peng, Q.; Zhu,
S. Ligands with 1,10-phenanthroline scaffold for highly regioselective
iron-catalyzed alkene hydrosilylation. Nat. Commun. 2018, 9, 221, DOI:
10.1038/s41467-017-02472-6. (b) Sang, H. L.; Yu, S.; Ge, S.
AUTHOR INFORMATION
Corresponding Author
Cobalt-catalyzed
,2-hydrosilylation of conjugated dienes. Chem. Sci. 2018, 9, 973-978.
(5) (a) Panek, J. S.; Yang, M. Diastereoselective additions of chiral
regioselective
stereoconvergent
Markovnikov
huangzh@sioc.ac.cn
Notes
The authors declare no competing financial interest.
1
(E)-crotylsilanes to -alkoxy and -alkoxy aldehydes. A one-step,
silicon-directed tetrahydrofuran synthesis. J. Am. Chem. Soc. 1991, 113,
9868-9870. (b) Hu, T.; Panek, J. S. Enantioselective Synthesis of the
Protein Phosphatase Inhibitor (−)-Motuporin. J. Am. Chem. Soc. 2002,
124, 11368-11378. (c) Su, Q.; Panek, J. S. Total Synthesis of
Supporting Information
Experimental procedures and characterization data. This material
is available free of charge via the Internet at http://pubs.acs.org.
(
−)-Apicularen A. J. Am. Chem. Soc. 2004, 126, 2425-2430. (d) Tinsley, J.
M.; Roush, W. R. Total Synthesis of Asimicin via Highly Stereoselective
3+2] Annulation Reactions of Substituted Allylsilanes. J. Am. Chem. Soc.
005, 127, 10818-10819. (e) Va, P.; Roush, W. R. Total Synthesis of
ACKNOWLEDGMENTS
[
2
Financial support was provided by the Ministry of Science and
Technology of China (2016YFA0202900, 2015CB856600),
NSFC (21825109, 21732006, 21432011, 21602236), and Chinese
Academy of Sciences (XDB20000000, QYZDB-SSW-SLH016),
and Science and Technology Commission of Shanghai
Municipality (17JC1401200).
Amphidinolide E. J. Am. Chem. Soc. 2006, 128, 15960-15961. (f)
Binanzer, K.; Fang, G. Y.; Aggarwal, V. K. Asymmetric Synthesis of
Allylsilanes by the Borylation of Lithiated Carbamates: Formal Total
Synthesis of (−)-DecarestrictineꢀD. Angew. Chem. Int. Ed. 2010, 49,
4264-4268. (g) Wu, J.; Pu, Y.; Panek, J. S. Divergent Synthesis of
Functionalized Carbocycles through Organosilane-Directed Asymmetric
Alkyne–Alkene Reductive Coupling and Annulation Sequence. J. Am.
Chem. Soc. 2012, 134, 18440-18446.
(6) (a) Hayashi, T.; Konishi, M.; Ito, H.; Kumada, M. Optically active
allylsilanes. 1. Preparation by palladium-catalyzed asymmetric Grignard
cross-coupling and anti stereochemistry in electrophilic substitution
reactions. J. Am. Chem. Soc. 1982, 104, 4962-4963. (b) Hayashi, T.;
Konishi, M.; Kumada, M. Optically active allylsilanes. 2. High
stereoselectivity in asymmetric reaction with aldehydes producing
homoallylic alcohols. J. Am. Chem. Soc. 1982, 104, 4963-4965. (c)
Hofstra, J. L.; Cherney, A. H.; Ordner, C. M.; Reisman, S. E. Synthesis of
Enantioenriched Allylic Silanes via Nickel-Catalyzed Reductive
Cross-Coupling. J. Am. Chem. Soc. 2018, 140, 139-142.
(7) (a) Schmidtmann, E. S.; Oestreich, M. Mechanistic insight into
copper-catalysed allylic substitutions with bis(triorganosilyl) zincs.
Enantiospecific preparation of -chiral silanes. Chem. Commun. 2006,
3643-3645. (b) Kacprzynski, M. A.; May, T. L.; Kazane, S. A.; Hoveyda,
A. H. Enantioselective Synthesis of Allylsilanes Bearing Tertiary and
Quaternary Si-Substituted Carbons through Cu-Catalyzed Allylic
Alkylations with Alkylzinc and Arylzinc Reagents. Angew. Chem. Int. Ed.
2007, 46, 4554-4558. (c) Delvos, L. B.; Vyas, D. J.; Oestreich, M.
Asymmetric Synthesis of a-iral Allylic Silanes by Enantioconvergent
g-selective Copper(I)‐Catalyzed Allylic Silylation. Angew. Chem. Int. Ed.
2013, 52, 4650-4653. (d) Takeda, M.; Shintani, R.; Hayashi, T.
Enantioselective Synthesis of a-Tri- and a -Tetrasubstituted Allylsilanes
by Copper-Catalyzed Asymmetric Allylic Substitution of Allyl Phosphates
with Silylboronates. J. Org. Chem, 2013, 78, 5007-5017.
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Regio- and Enantioselective Cobalt-Catalyzed Sequential Hydrosilylation/
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Silicon-Stereogenic Vinylhydrosilanes by Cobalt-Catalyzed Regio- and
Enantioselective Alkyne Hydrosilylation with Dihydrosilanes. Angew.
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with
-Dihydridooligodimethylsiloxanes.
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Linear Oligo-Carbosiloxanes Prepared via Hydrosilylation from
1
3340-13341.
14) Guo, J.; Liu, H.; Bi, J.; Zhang, C.; Zhang, H.; Bai, C.; Hu, Y.; Zhang,
(
X. Pyridine–oxazoline and quinoline–oxazoline ligated cobalt complexes:
Synthesis, characterization, and 1,3-butadiene polymerization behaviors.
Inorg. Chim. Acta 2015, 435, 305-312.
1,3-Divinyl-1,3-Dimethyl-1,3-Diferrocenyldisiloxane.
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SanzꢀMiguel, P. J.; Pérez-Torrente, J. J.; Oro, L. A. Synthesis of Poly(silyl
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versus Heterogeneous Catalysis. ChemCatChem 2013, 5, 1133-1141.
(24) (a) Putzien, S.; Nuyken, O.; Kühn, F. E. Functionalized
(15) The absolute configuration of 4a was determined by oxidizing it to
the corresponding allyl alcohol, which was assigned by comparison with
reported optical rotations. See: Chen, F.; Zhang, Y.; Yu, L.; Zhu, S.
Enantioselective NiH/Pmrox-Catalyzed 1,2-Reduction of -Unsaturated
Ketones. Angew. Chem. Int. Ed. 2017, 56, 2022-2025.
polysilalkylene
siloxanes
(polycarbosiloxanes)
by
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687-713. (b) Yilgör, E.; Yilgör, I. Silicone containing copolymers:
Synthesis, properties and applications. Prog. Polym. Sci. 2014, 39,
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applications of protective silicone coatings: A review of PDMS functional
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(16) The lack of reactivity of Z-1-phenyl-1,3-butadiene is presumably
due to steric effect. The formation of a cobalt diene complex with a s-cis
form, a putative catalytic intermediate (cf Figure 2) would be sterically
unfavorable because of the repulsion between the Ph group of the
coordinated diene and ligand.
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SiH2Ph
PhSiH3
+
2
.0% (QuinOx)-Co
30 examples
avg 91% ee
>99:1 rr
1
1
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R
R
R'
N
N
Cl
R'
R = Ph, R' = H
R = Ar, Alk
Co
Cl
OHC
CHO
R' = H, Ar, Alk
H
2
.5% ( PyOx)-Co
Ph
Si
H
H
C
H
O
C
O
Mw = 8,700, PDI = 1.8
[D22 = -5.3
H
n
Ph
*
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