Asymmetric Synthesis of Silicon-Stereogenic Vinylhydrosilanes by Cobalt-Catalyzed Regio- and Enantioselective Alkyne Hydrosilylation with Dihydrosilanes

Article abstract of DOI:10.1002/anie.201802806

The strategic carbon-to-silicon substitution at a stereogenic center can produce chiral silanes with significantly improved properties relative to their carbon congeners. We herein report an unprecedented cobalt-catalyzed asymmetric hydrosilylation of unsymmetric alkynes with dihydrosilanes that furnishes silicon-stereogenic vinylhydrosilanes with high regio- and enantioselectivity. The absolute configurations of the products were determined by chiroptical methods in combination with DFT calculations. The synthetic versatility of the vinylhydrosilanes as chiral building blocks was further demonstrated by asymmetric Si?H insertion and catalytic hydroboration reactions.

Full text of DOI:10.1002/anie.201802806

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Accepted Article
Title: Asymmetric Synthesis of Silicon-Stereogenic Vinylhydrosilanes
via Cobalt-Catalyzed Regio- and Enantioselective Alkyne
Hydrosilylation with Dihydrosilanes
Authors: Huanan Wen, Xiaolong Wan, and Zheng Huang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802806
Angew. Chem. 10.1002/ange.201802806
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10.1002/anie.201802806
Angewandte Chemie International Edition
COMMUNICATION
Asymmetric Synthesis of Silicon-Stereogenic Vinylhydrosilanes
via Cobalt-Catalyzed Regio- and Enantioselective Alkyne
Hydrosilylation with Dihydrosilanes
Huanan Wen, Xiaolong Wan and Zheng Huang*
Abstract: The strategic carbon-to-silicon substitution at
a
stereogenic center can produce chiral silanes with significantly
improved properties relative to their carbon-congeners. We report
here an unprecedented cobalt-catalyzed asymmetric hydrosilylation
of unsymmetrical alkynes with dihydrosilane, furnishing Si-
stereogenic vinylhydrosilanes with high regio- and enantioselectivity.
The absolute configurations of the products were determined by
chiroptical methods in combination with DFT calculations. The
synthetic versatility of vinylhydrosilanes as chiral building blocks
were further demonstrated by asymmetric Si–H insertion and
catalytic hydroboration reactions.
Scheme 1. Asymmetric alkyne hydrosilylation with dihydrosilane.
Optically pure organosilanes containing a Si-stereocenter are
not naturally occurring, but have attracted increasing attention in
medicinal chemistry[1] and material sciences[2] because sila-
substitution (i.e., C/Si switch of the stereocenter) can cause
hydrosilylation of internal alkynes to form vinylsilanes with up to
86% ee.[9j] The substrates disclosed therein were symmetrical
internal alkyl alkynes, and thus the issue of regioselectivity did not
arise (Scheme 1a).[9j]
significant
difference
regarding
their
biological
and
physicochemical properties.[3] Additionally, they are vital
intermediates for stereoselective transformations in organic
synthesis.[4] While methods for the synthesis of relevant α-carbon-
stereogenic chiral silanes have been well known,[5] the asymmetric
synthesis of Si-stereogenic compounds is much more challenging.
Due to the low stability of the sp2-hybridized Si species,[6] addition
reactions to Si-containing double bonds are not viable to construct
Si-stereocenters. The traditional methods for synthesis of such
compounds require stoichiometric amounts of chiral reagents.[4a-c,7]
To this end, the development of catalytic approaches using readily
accessible reagent(s) is desired.
Base-metal-catalyzed hydrosilylation of multiple C–C bonds
have garnered much attention over the past decade, not only
because of the earth abundance and environmentally benign nature
of the metals, but also because they may be complementary to, or
even supplant the traditional precious-metal catalysis in terms of
selectivity.[10,11] In 2016, Lu’s group and our group independently
reported a Co-catalyzed Markovnikov-selective hydrosilylation of
terminal alkynes with Ph2SiH2 by employing iminopyridine-
oxazoline (IPO)[11g] and pyridine-bis(oxazoline) (PyBox)[11f]
ligands, respectively. Later, Lu described (IPO)Co-catalyzed
asymmetric synthesis of α-carbon-stereogenic silanes via
sequential Markovnikov hydrosilylation/hydrogenation of terminal
alkynes[5d] or Markovnikov hydrosilylation of terminal alkenes.[5c]
Herein, we describe a new cobalt catalyst of PyBox for regio- and
enantioselective hydrosilylation of terminal and internal alkynes
with diaryl dihydrosilane (Ph(Ar)SiH2, Ar ≠ Ph), yielding a novel
class of Si-stereogenic vinylhydrosilanes (Scheme 1b).
Two catalytic strategies involving the desymmetrization of
prochiral tetraorganosilanes and dihydrosilanes have been reported
for the synthesis of Si-stereogenic silanes. In the context of
tetraorganosilanes,[8] Shintani and other colleagues have made a
series of contributions, including asymmetric Si–C bond activation
of silacyclobutanes.[8a,8b] The desymmetrization of dihydrosilanes
is of particular interest because the resulting chiral
monohydrosilanes can find various synthetic applications, such as
diastereoselective dehydrogenative Si–O coupling with racemic
alcohols[7c] and synthesis of optically active Si-containing
polymers through intermolecular alkene hydrosilylation.[2a] Among
We commenced the study by examining Co catalysts of chiral
PyBox ligands for asymmetric hydrosilylation of phenylacetylene
1a with
a prochiral dihydrosilane Ph(Ar)SiH2 (Ar = 2,6-
(Me)2C6H3) 2 (Table 1). The Co catalysts were generated in situ
from Co(acac)2 and PyBox, and NaOtBu was selected as the
catalyst activator.[12] In the presence of Co(acac)2 (2.0 mol%) and a
iBu-substituted PyBox L1 (4.0 mol%), the reaction of 1a with 2 (1
equiv) in THF at 25 °C after 12 h formed the branched
vinylhydrosilane 3a in high yield and high regioselectivity (97%
yield, b:l = 98:2), but with low enantioselectivity (22% ee) (entry
1). The run using a iPr-substituted ligand L2 led to similar regio-
and enantioselectivity (entry 2). The enantioselectivity (51% ee)
was substantially enhanced by using L3 with a sBu substituent, but
at the expense of reduced yield (58% yield, entry 3). The use of a
tBu-substituted PyBox L4 gave a poor yield (<10%, entry 4). To
our satisfaction, two PyBox ligands with a hydroxyethyl group
protected by tert-butyldiphenylsilyl L5 and tert-butyldimethylsilyl
L6, respectively, offered significantly improved results in terms of
a
few known approaches for the desymmetrization of
dihydrosilanes,[9] the asymmetric hydrosilylation of alkynes with
dihydrosilanes is an attractive protocol considering that the
products contain two functionalities, vinyl and Si–H, toward
further transformations. In 2012, Tomooka reported a Pt-catalyzed
[*]
H. Wen, X. Wang, Prof. Dr. Z. Huang
State Key laboratory of Organometallic Chemistry, Center for
Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, University of Chinese Academy of Sciences, Chinese
Academy of Sciences, 345 Lingling Rd, Shanghai 200032, China
E-mail: huangzh@sioc.ac.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201802806
Angewandte Chemie International Edition
COMMUNICATION
Table 1. Cobalt catalysts for regio- and enantioselective
simulated spectrum is in good agreement with the experimental
data, leading to the assignment of a R configuration to 3a (see
Supporting Information).
hydrosilylation of phenylacetylene with PhArSiH2.[a]
To explore the scope of the Co-catalyzed asymmetric
hydrosilylation, L5·CoCl2 was applied to the reactions of various
terminal and internal alkynes with 2 as the silane source. The
results are summarized in Table 2. Unless otherwise noted, the
regioselective ratios for all the reactions of terminal aryl alkynes
were greater than 99:1, in favor of the Markovnikov products.
Terminal aryl alkynes bearing both electron-donating and -
withdrawing groups on the aryl rings gave the corresponding
products in excellent isolated yields with high enantiopurities. Me
(para, meta, ortho), F (para, ortho), and Cl (para, ortho)
substituents at different positions of the aryl rings were tolerated,
yielding the branched vinylsilanes (3b-d, 3i-l) with 89-91% ee.
Other alkyl- and Ph-substituted aryl alkynes furnished the desired
products (3e-g) in high yields and selectivities. The hydrosilylation
of α-ethynylnaphthalene provided 3h in 95% yield with 90% ee. In
addition to F and Cl, the Br-substituted alkyne (3m) was a suitable
substrate. Moreover, the substrates containing amine (3o), ether
(3p, 3q), thioether (3r), and acetal (3s) functionalities furnished the
products with 82-91% ee. Noteworthily, a range of reactive
functional groups, such as ketone (3t), ester (3u), amide (3v), and
cyano (3w), were compatible with the hydrosilylation conditions
(83-88% ee). The unprotected primary anilines (3x, 3y) were also
tolerated as demonstrated by the isolation of 3x and 3y in
satisfactory yields and selectivities. Furthermore, the S- and N-
containing heteroaromatic alkynes (3z, 3aa) reacted in useful
yields with good ee.
Importantly, the reactions are not limited to the terminal aryl
alkynes. The challenging terminal alkyl alkynes underwent
hydrosilylations smoothly, albeit with slightly reduced
regioselectivity. The reactions of linear 1-hexyne (3ab), 1-heptyne
(3ac) and 1-octyne (3ad), and branched alkyne (3ae) delivered
high yields of chiral vinylsilanes with high regio- (93:7–95:5) and
enantioselectivities (85-88% ee). The chloride functionality in the
alkyl chain was tolerated, as shown by the isolation of 3af with
high yield and selectivity. The reaction of benzyl alkyne gave the
desired product 3ag in 85% yield with 99:1 b:l ratio and 90% ee.
[a] Conditions: 1a (0.3 mmol), 2 (0.3 mmol), and solvent (3 mL). [b]
Yields and b:l ratios were determined by GC with mesitylene as an
internal standard. [c] Determined by chiral HPLC. [d] With 2.0 equiv
2. [e] 24 h. [f] With L5·CoCl2 (2.0 mol%) and NaBEt3H (6.0 mol%).
yield and selectivity (entries 5 and 6). In particular, the run with L5
furnished the highest regio- (b:l = >99:1) and enantioselectivity
(82%) among all PyBox ligands investigated in this work, along
with a useful yield (81%). Further optimization using L5 as the
ligand revealed that MTBE was the optimal solvent (entry 8).
Moreover, decreasing the reaction temperatures led to further
enhanced ee and even higher regioselectivity, but moderate-to-low
yields (entries 9-10). Performing the reaction with two equiv of
Ph(Ar)SiH2 2 and prolonged reaction time (24 h) did afford a high
yield (95%) of 3a with >99:1 b:l ratio and 91% ee (entry 12). The
activator NaOtBu was found to react with Ph(Ar)SiH2 2 to form
Ph(Ar)SiH(OtBu) (S1). Although in low yield (<5%), the polarity
of S1 is rather similar to that of 3a, making the product purification
difficult. To address this issue, we used an isolated, L5-ligated
complex L5·CoCl2 (2.0 mol %) as the precatalyst and NaBEt3H as
the activator. Under such conditions, the reaction gave 3a in
excellent yield (97%) with high regio- (>99:1) and
enantioselectivity (91% ee) (entry 13).
The Co-catalyzed hydrosilylation of arylalkyl-disubstituted
internal alkynes furnished the products (3ah, 3ai) in high yields
with high regioselectivity (91:9–95:5), favoring the products with
the Si group added to the C-atom attached to the aryl group. The
enantioselectivities (78-82% ee) are slightly lower than that
obtained for the terminal alkynes (versus 3a and 3ab). The
reactions of symmetrical diaryl (3aj, 63% ee) and dialkyl (3ak, 3al,
76-79% ee) internal alkynes occurred with moderate-to-good
enantioselectivities. Notably, the unsymmetrical dialkyl alkyne, 4-
methyl-2-pentyne, gave the product 3am with a 77:23 rr ratio and
88% ee, in favor the product with Si group addition to the sterically
more demanding C-atom.
Preliminary investigations of the scope of diaryl dihydrosilanes
revealed that the ortho-substituents in the aryl ring have a large
impact on the enantioselectivity. The incorporation of an additional
Me group at the para position has a negligible effect on the
selectivity (3an, 87% ee), as expected. However, the substitution of
The absolute configuration of 3a was determined by the
chiroptical method.[13] In addition, the ECD spectra for 3a were
calculated by TD-DFT to assign its absolute configuration.[14] The
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10.1002/anie.201802806
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Table 2. Cobalt-catalyzed asymmetric hydrosilylation of various terminal and internal alkynes with PhArSiH2.[a]
R'
L5•CoCl2 (2.0 mol%), NaBEt3H (6.0 mol%)
Ar
Ph(Ar)SiH2
R
R'
+
R
Si
MTBE, -20 oC, 36 h
R' = H or alkyl, 1
2
3
H
Ph
Si
Si
Si
Si
Si
Si
Ph
Ph
H
H
Ph
Ph
Ph
Ph
H
H
H
H
nPr
tBu
3a, 96%, 91% ee
3b, 95%, 90% ee
3c, 90%, 90% ee
3d, 89%, 89% ee
3e, 89%, 90% ee
3f, 99%, 84% ee
F
Cl
Si
Si
Si
Si
Si
Si
Ph
Ph
Ph
H
H
H
Ph
Ph
3j,[b] 83%, 89% ee
Ph
H
H
H
Ph
F
Cl
3g, 88%, 88% ee
3h, 95%, 90% ee
3i, 98%, 91% ee
3k, 85%, 91% ee
3l,[b] 94%, 89% ee
OMe
Si
Si
Si
Si
Si
Ph
Ph
H
H
Ph
Ph
H
H
Ph
H
F3C
Me2N
Br
MeO
3m, 72%, 88% ee
3n, 72%, 89% ee
3o, 96%, 82% ee
3p, 97%, 89% ee
3q, 94%, 86% ee
O
O
Si
Si
Si
Si
O
Si
MeO
N
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
MeS
3s, 98%, 91% ee
3t,[c] 72%, 87% ee
3u,[b] 62%, 87% ee
3v, 82%, 83% ee
3r, 95%, 89% ee
O
O
O
N
H2N
Si
Si
Si
Si
Si
S
Ph
H
Ph
Ph
H
H
Ph
3y,[b] 64%, 89% ee
H
Ph
H
H2N
NC
3w,[c] 64%, 88% ee
3x, 82%, 85% ee
3z, 89%, 86% ee
3aa,[c] 59%, 83% ee
( )2
Si
Si
Si
Si
Cl
Si
Ph
Ph
Ph
Ph
Ph
H
H
H
H
H
3ab, 95%, b:l = 93:7, 87% ee 3ac, 92%, b:l = 95:5, 88% ee 3ad, 96%, b:l = 95:5, 88% ee 3ae, 91%, b:l = 96:4, 85% ee 3af, 88%, b:l = 97:3, 89% ee
Ph
Ph
Si
Ph
Si
Ph
Si
Ph
Si
Si
Si
Ph
Ph
Ph
Ph
3aj, 98%, 63% ee
Ph
3ak, 98%, 79% ee
H
H
H
H
H
Ph
H
3ag, 85%, 90% ee
3ah, 97%, rr = 95:5, 82% ee 3ai, 96%, rr = 91:9, 78% ee
3al, 96%, 76% ee
Et
Ph
Si
Ph
Si
Si
Ph
Si
Ph
Si
Ph
Si
OMe
Ph
Ph
3aq, 93%, 11% ee
Ph
Ph
Ph
3ao, 91%, 6% ee
H
H
H
H
H
Et
Ph
H
3ap, 98%, b:l = 98:2, 9% ee
3ar, <10%
3am, 76%, rr = 77:23, 88% ee 3an, 97%, 87% ee
[a] Conditions: on 0.3 mmol scale in MTBE (3 mL) for 36 h. Isolated yields and ee values determined by HPLC or SFC on a chiral stationary
phase. Unless otherwise noted, b:l >99:1. [b] With 5.0 mol% L5·CoCl2. [c] With 10.0 mol% L5·CoCl2 for 70 h.
one ortho-Me for H-atom led to
a
sharp decline in the
The silicon-stereogenic vinylhydrosilanes could undergo
various transformations through the derivatization of the vinyl and
Si–H moieties (Scheme 2). For example, treatment of 3a with
excess CH2I2 and ZnEt2 in DCE at 25 °C resulted in a Si–H
insertion reaction, giving the Me-substituted tetraorganosilane 4 in
high yield with a retention of the configuration on Si-stereocenter
(76% yield, 90% ee) (Scheme 2a, the R configuration of 4 was
again determined by the combination of chiroptical methods and
TD-DFT calculations, see Supporting Information for details).
Next, we explored the possibility for the formation of chiral 1,2-
borosilane via Co-catalyzed asymmetric anti-Markovnikov
hydroboration of the Si-stereogenic vinylsilane.[11g] Using (S)-
enantiocontrol (3ao, 6% ee). Similarly, the mono ortho-MeO
substituted silane led to low enantioselectivity (3ap, 9% ee), while
the run with 1-naphthalenyl substituted silane gave 3aq with 11%
ee. The silane with Et substituents is too sterically bulky to
undergo the hydrosilylation (3ar, <10%).
The hydrosilylation process proved to be scalable. Using 1
mol% of L5·CoCl2, the reaction of phenylacetylene 1a (6.0
mmol) with 2 equiv of dihydrosilane 2 at −20 °C yielded 1.8 g of
3a (94% isolated yield) with 91% ee.
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COMMUNICATION
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borysilane
5 with excellent diastereoselectivity (94:6) and
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chiral Si-stereogenic center on the hydroboration reaction is
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chiral silane 3a than its S isomer, furnishing 5 with 25:75 dr
(Scheme 2b).
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In conclusion, we have described an efficient and selective
cobalt complex of PyBox for asymmetric formation of synthetic
valuable silicon-stereogenic vinylhydrosilane via alkyne
hydrosilylation with a prochiral dihydrosilane. The method shows
high Markovnikov selectivity in the reactions of terminal
alkynes.To our knowledge, this is the first example of alkyne
hydrosilylations combining both high regioselectivity and
enantioselectivity. Moreover, the results achieved here represent
the highest level of enantiocontrol for alkyne hydrosilylation.
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Acknowledgements
We gratefully acknowledge the financial support from National
Key R&D Program of the MOST of China (2016YFA0202900,
2015CB856600), NSFC (21422209, 21432011, 21732006),
CAS (XDB20000000, QYZDB-SSW-SLH016), and STCSM
(17JC1401200).
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Conflict of interest
The author declare no conflict of interest.
Keywords: asymmetric catalysis • cobalt • hydrosilylation • Si-
stereogenic • vinylsilanes
References:
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Entry for the Table of Contents (Please choose one layout)
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Huanan Wen, Xiaolong Wan, Zheng
Huang*
Page No. – Page No.
Asymmetric Synthesis of Silicon-
Stereogenic Vinylhydrosilanes via
Cobalt-Catalyzed Regio- and
Enantioselective Alkyne
Hydrosilylation with Dihydrosilanes
A chiral cobalt catalyst of PyBox effects regio- and enantioselective hydrosilylation
of unsymmetrical alkynes with a prochiral dihydrosilane, yielding optically pure
vinylhydrosilanes with a silicon-stereogenic center.
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