Synthesis of Functionalized Vinylsilanes via Metal-Free Dehydrogenative Silylation of Enamides

Article abstract of DOI:10.1021/acs.orglett.9b04645

A novel method of metal-free dehydrogenative silylation of enamides has been developed. The desired functionalized vinylsilane products were obtained in moderate to good yield and with high stereoselectivities. This protocol displays good tolerance of various functionalities. Furthermore, the high chemoselectivity of this reaction enables us to introduce different unsaturated C-C moieties to the products. The ease of further derivatization of the products to other useful compounds also demonstrates the highly synthetic utility of the current methodology.

Full text of DOI:10.1021/acs.orglett.9b04645

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Letter
Synthesis of Functionalized Vinylsilanes via Metal-Free
Dehydrogenative Silylation of Enamides
Xi-Hao Chang, Zi-Lu Wang, Meng Zhao, Chao Yang, Jie-Jun Li, Wei-Wei Ma, and Yun-He Xu*
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ABSTRACT: A novel method of metal-free dehydrogenative
silylation of enamides has been developed. The desired function-
alized vinylsilane products were obtained in moderate to good yield
and with high stereoselectivities. This protocol displays good
tolerance of various functionalities. Furthermore, the high chemo-
selectivity of this reaction enables us to introduce different
unsaturated C−C moieties to the products. The ease of further
derivatization of the products to other useful compounds also
demonstrates the highly synthetic utility of the current methodology.
inylsilanes are very useful synthetic intermediates in
organic synthesis and building blocks in organic material
works and combined with our continuous efforts on
developing the C−Si bond formation and the functionalization
of enamides,14,2n herein we would communicate the results of
the metal-free direct silylation of enamides. The trisubstituted
functionalized alkenes were obtained in good yield and with
high stereoselectivities.
V
science.1 Accordingly, a variety of methods are known for the
preparation of vinylsilanes, such as the transition-metal-
catalyzed hydrosilylation of alkynes or allenes,2 cross-coupling
reactions,3 Wittig/Peterson olefination,4 the addition of vinyl
Grinard reagents or vinyllithiums to silyl electrophiles,5 and so
on. However, the difficulty in controlling the regioselectivity
and stereoselectivity of products as well as the limitations of
the harsh reaction conditions still spur chemists to discover
new and practical methods. In recent years, the direct silylation
of alkenes catalyzed by Co, Fe, Pd, Rh, Ru, Ir, Re, Zr, and Ni
metal catalysts has emerged as a powerful tool for the synthesis
of vinylsilanes.6 To access the desired products with high
efficiency, careful tuning of the catalytic system and fine design
of specific substrates are necessary. Therefore, the develop-
ment of a simple and convenient synthetic method is highly
desirable to prepare the functionalized vinylsilanes. It is
noteworthy that the C−Si bond formation via a radical
process has been developed as an alternative pathway for the
synthesis of functionalized organosilicon compounds by
Studer,7 Yu,8 Grubbs,9 and Liu10 et al. Very recently, Liu
and coworkers elegantly reported a copper-catalyzed decar-
boxylative silylation of α,β-unsaturated acids to synthesize the
vinylsilanes with trans configuration.10d A silyl radical addition
process was suggested to be involved in this reaction.
Following this, Cai and colleagues demonstrated an elegant
copper-catalyzed and iron-catalyzed direct silylation reaction of
alkenes.11 Transition-metal-catalyzed silyl radical addition to
C−C triple bonds for the synthesis of vinylsilanes was also
discovered. In 2016, a copper-catalyzed ipso-annulation of
active alkynes and silanes was described by Gao et al.12
Consequently, a similar spirocyclization of N-arylpropiola-
mides with silanes catalyzed by an iron catalyst was
investigated by Li and coworkers.13 Inspired by these fruitful
At the beginning, enamide 1a and triphenylsilane were
chosen as the model reaction to optimize the reaction
conditions. To our delight, the desired product, vinylsilane
3a, could be obtained in 45% yield in the presence of 10 mol %
CuCl and 3 equiv of dicumyl peroxide (DCP) in toluene
solution at 110 °C (Table 1, entry 1). The configuration of
1
product vinylsilane 3a was confirmed by H nuclear magnetic
resonance (NMR) spectroscopy using nuclear Overhauser
effect spectroscopy. (See the Supporting Information.) Next,
we found that a higher product yield was obtained when tert-
butanol was used as a solvent in this reaction (entry 2). Next,
different peroxides were screened to improve the reaction
efficiency (entries 3−5). When switching the peroxide to tert-
butyl peroxybenzoate (TBPB), the yield of 3a was increased to
93% (entry 5). Additionally, other copper salts, such as CuCl2,
were found to be more efficient in this reaction (entry 6).
However, when the controlled experiment was performed in
the absence of a copper catalyst, it still resulted in a good yield
of product 3a (entry 8), whereas the peroxide is indispensable
to obtain the desired product 3a (entry 7). Different
temperatures were also investigated (entries 8−14), and the
Received: December 26, 2019
https://dx.doi.org/10.1021/acs.orglett.9b04645
© XXXX American Chemical Society
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A

Organic Letters
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Table 1. Optimization of Reaction Conditions of
Dehydrogenative Silylation of Enamide 1a
Scheme 1. Reaction Scope of Dehydrogenative Silylation of
a b
a b
,
,
Enamides
catalyst
Ph3SiH
(x equiv)
peroxide
(y equiv)
temp yield
entry (10 mol %) solvent
(°C)
(%)
c
1
2
CuCl
CuCl
CuCl
CuCl
CuCl
CuCl2
CuCl2
toluene
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
tBuOH
10
10
10
10
10
10
10
10
10
10
10
5
DCP (3)
DCP (3)
DTBP (3)
TBHP (3)
TBPB (3)
TBPB (3)
110
110
110
110
110
110
110
110
90
45
59
c
3
83
c
4
36
5
93
99
6
7
8
TBPB (3)
TBPB (3)
TBPB (3)
TBPB (3)
TBPB (2)
TBPB (2)
TBPB (2)
89
88
95
9
10
11
12
13
14
70
50
70
92
84
28
5
60
3
60
a
Unless otherwise noted, typical reaction conditions: Enamide 1 (0.3
mmol), Ph3SiH (x equiv), catalyst (10 mol %), and peroxide (y equiv)
b
in solvent (1.5 mL) were heated for 24 h under N2. Isolated yield.
c
1H NMR yield.
a
Unless otherwise noted, typical reaction conditions: Enamide 1 (0.3
mmol), silane (1.5 mmol, 5.0 equiv), and TBPB (0.6 mmol, 2.0
t
equiv) in BuOH (1.5 mL) were heated to 70 °C for 24 h under N2.
desired product could be obtained in 95% yield at 70 °C (entry
10). Finally, the amounts of triphenylsilane and peroxide were
examined; then, the optimized reaction conditions were
determined as follows: Enamide, silane (5 equiv), and tert-
butyl peroxybenzoate (TBPB, 2 equiv) were stirred in tert-
butanol for 24 h at 70 °C (entry 12).
b
c
d
Isolated yields. Reaction temperature was 110 °C. Reaction time
e
was 28 h. Enamide 1 (0.3 mmol), silane (2.1 mmol, 7.0 equiv), and
f
TBPB (0.9 mmol, 3.0 equiv) were heated for 36 h. Gram-scaled
synthesis of 3a product: Enamide 1 (1.0 mmol), triphenylsilane (5.0
mmol, 5.0 equiv), and TBPB (2.0 mmol, 2.0 equiv) were heated in
tBuOH (5 mL) to 70 °C for 36 h.
With the optimized reaction conditions at hand, the scopes
of substituted enamides and silanes were tested, and the results
were shown in Scheme 1. The trisubstituted functionalized
vinylsilanes were obtained with high stereoselectivities. First,
different silanes were examined to react with enamide 1a. The
corresponding vinylsilane products were isolated in good yield
when the triethylsilane and dimethylphenylsilane were used for
this reaction (Scheme 1, 3b, 3c). However, the chloro- and
alkoxyl-silanes failed to afford the desired products under the
standard reaction conditions. Subsequently, the enamides with
various substituents on the aryl ring were investigated. Both
electron-donating and electron-withdrawing groups such as
ortho-Me, meta-OMe, para-CN, para-F, and para-Cl were all
tolerated to produce the desired products 3d−3h in good
yield. When the aryl substituent was changed to a methyl
group, the product 3k was isolated in 63% yield. Additionally,
the thienyl- and pyridyl-substituted enamides also could afford
the desired products 3l and 3m in moderate yield.
Unfortunately, the tetrasubstituted vinylsilane 3n was obtained
in only 31% yield when the trisubstituted enamide with an (E)
configuration was used in the reaction. The stereoselectivity of
products in these cases possibly resulted from the steric
repulsion between the silyl and protected amide groups.
Finally, it was found that replacing the acetyl group with the
propionyl functionality did not affect the desired yield of the
product (3o). Unfortunately, when the free enamide was
subjected to this reaction, no reaction happened (3p).
On the basis of above results, various substituents tethering
at the N atom were also examined (Scheme 2). When the
benzyl-group-protected enamides were subjected to this
reaction, good compatibility of the substituents on the phenyl
ring was observed under the optimized reaction conditions
(4b, 4e−j). In addition, changing the substituent from the
benzyl group to an ether, acetyl, ester, or aliphatic group could
afford the desired products in good to high yield (4a, 4c, 4d,
4l, 4m, and 4p). When replacing the benzyl group with an allyl
or propargyl group, the corresponding products 4k and 4n also
were obtained, albeit in low yield. It was noteworthy that the
dienyl-substituted vinylsilane 4o was also formed in moderate
yield and with excellent stereoselectivity.
Additionally, the transformation of vinylsilane product was
studied with 3b as the model substrate. As shown in Scheme 3,
when the vinylsilane compound 3b reacted with N-
bromosuccinimide (NBS), silicon-based vinylbromo com-
pound 5 was obtained in 76% yield (Scheme 3, eq 1).
Interestingly, when compound 3b was used to react with N-
iodosuccinimide (NIS) in an acetonitrile solution, a mixture of
vinyl iodide compound 6 in Z/E configurations was obtained
in 89% yield (Scheme 3, eq 2).15a Subsequent Sonogashira
coupling using a terminal alkyne with (Z)-6 afforded the enyne
compound 9 in a high yield (Scheme 3, eq 4).15a On the
contrary, we also tested the Suzuki−Miyaura coupling reaction
B
https://dx.doi.org/10.1021/acs.orglett.9b04645
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Letter
a b
,
A plausible mechanistic pathway of the metal-free
dehydrogenative silylation reaction of enamides is depicted
in Scheme 4.8,10a,11−13,14e,16 Despite the fact that the thermal
Scheme 2. Scope of Enamide Substituted at the N Atom
Scheme 4. Proposed Mechanism for the Metal-Free
Dehydrogenative Silylation of Enamides
decomposition of TBPB at the reaction temperature (70 °C)
occurred slowly,17 a trace amount of the tert-butoxy radical II
generated from TBPB after initiation would abstract one
a
Unless otherwise noted, typical reaction conditions: Enamide 1 (0.3
mmol), Ph3SiH (1.5 mmol, 5.0 equiv), and TBPB (0.6 mmol, 2.0
1
t
proton from silane to form a silyl radical. (The H NMR
equiv) in BuOH (1.5 mL) were heated to 70 °C for 24 h under N2.
b
c
d
Isolated yields. Reaction temperature was 90 °C. Reaction
experiment with the mixture of enamide 1a and TBPB (mole
ratio 1:1) indicated that no direct electron transfer from 1a to
the peroxide happened.) Then, the silyl radical would undergo
an addition process toward affording an intermediate III,
which would be oxidized by another TBPB to furnish a cation
intermediate IV or its resonator V along with delivering the
essential tert-butoxy radical for the next cycle. After the
deprotonation of intermediate IV or V with a benzoate anion,
the desired vinylsilane products would be afforded.
temperature was 110 °C.
Scheme 3. Subsequent Transformation of 3b and Its
Derivatives
In conclusion, a novel method of metal-free dehydrogenative
silylation of enamides has been developed under simple
reaction conditions. The desired functionalized vinylsilane
products were obtained in moderate to good yield and with
high stereoselectivities. Notably, a wide substrate scope was
well tolerated. Therefore, this protocol provides a simple and
environmentally friendly route for the synthesis of function-
alized vinylsilanes.
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.9b04645.
Experimental procedures and spectral data (PDF)
AUTHOR INFORMATION
Corresponding Author
■
Yun-He Xu − Department of Chemistry, University of Science
and Technology of China, Hefei 230026, P. R. China;
orcid.org/0000-0001-8817-0626; Email: xyh0709@
ustc.edu.cn
between the compound (Z)-6 and different aryl boronic acids
to form the stereodefined products 7 and 8 in high yield
(Scheme 3, eq 3).15b Therefore, we believe that the vinylsilanes
obtained via this protocol are useful for the construction of
complex molecules.
Authors
Xi-Hao Chang − Department of Chemistry, University of Science
and Technology of China, Hefei 230026, P. R. China
C
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Zi-Lu Wang − Department of Chemistry, University of Science
and Technology of China, Hefei 230026, P. R. China
Meng Zhao − Department of Chemistry, University of Science
and Technology of China, Hefei 230026, P. R. China
Chao Yang − Department of Chemistry, University of Science
and Technology of China, Hefei 230026, P. R. China
Jie-Jun Li − Department of Chemistry, University of Science and
Technology of China, Hefei 230026, P. R. China
Wei-Wei Ma − Department of Chemistry, University of Science
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.9b04645
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We gratefully acknowledge the funding support of the National
Natural Science Foundation of China (21871240, 21672198),
the Fundamental Research Funds for the Central Universities
(WK2060190082), and the State Key Program of National
Natural Science Foundation of China (21432009).
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