Stereoselective synthesis of Z-vinylsilanes via palladium-catalyzed direct intermolecular silylation of C(sp2)-H bonds

Article abstract of DOI:10.1021/acs.orglett.7b02486

An efficient and convenient palladium-catalyzed direct intermolecular silylation of C(sp²)-H bonds by using disilanes as the silicon source with the assistance of a readily removable bidentate directing group is reported. This strategy provided a regio- and stereoselective protocol for exclusive synthesis of Z-vinylsilanes with reasonable to excellent yields and good functional group compatibility. Silylation of the isolated palladacycle intermediate revealed the Z-stereoselective pathway. Moreover, the practicality and effectiveness of this method were illustrated by a gram-scale experiment and further functionalization of the silylation product.

Full text of DOI:10.1021/acs.orglett.7b02486

Letter
pubs.acs.org/OrgLett
Stereoselective Synthesis of Z‑Vinylsilanes via Palladium-Catalyzed
Direct Intermolecular Silylation of C(sp2)−H Bonds
Jin-Long Pan,† Chao Chen,† Zhi-Gang Ma,† Jia Zhou,† Li-Ren Wang,§ and Shu-Yu Zhang*,†,‡
†School of Chemistry and Chemical Engineering, ‡Key Laboratory for Thin Film and Microfabrication of Ministry of Education, and
§Zhiyuan College, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China
S
* Supporting Information
ABSTRACT: An efficient and convenient palladium-catalyzed
direct intermolecular silylation of C(sp2)−H bonds by using
disilanes as the silicon source with the assistance of a readily
removable bidentate directing group is reported. This strategy
provided a regio- and stereoselective protocol for exclusive
synthesis of Z-vinylsilanes with reasonable to excellent yields and
good functional group compatibility. Silylation of the isolated
palladacycle intermediate revealed the Z-stereoselective pathway.
Moreover, the practicality and effectiveness of this method were
illustrated by a gram-scale experiment and further functionaliza-
tion of the silylation product.
inylsilanes,1 an important class of valuable building blocks
in organic chemistry, are widely applied in organic
Scheme 1. Synthetic Approaches to Z-Vinylsilanes via Direct
C(sp2)−H Silylation of Alkenes
V
synthesis (Hiyama−Denmark coupling, Tamao−Fleming oxi-
dation, etc.), polymer chemistry, and medicines.2,3 Numerous
classical transformations have been developed for the formation
of the vinylsilane compounds, such as hydrosilylation,4a−d
silylmetalation,4e,f metathesis,4g,h silyl-Heck coupling,4i,j silyla-
tive coupling,4k,l dehydrogenative silylation,4m,n etc.4o−r Among
these, the transition-metal-catalyzed direct selective conversion
of the C(sp2)−H bond of alkenes to the corresponding
vinylsilanes is one of the most attractive synthetic approaches
due to its high efficiency and directness.5−7 However, one of
the main drawbacks of the C−H silylation reaction for the
formation of vinylsilanes is that stereoselectivity-related issues
must always be addressed (Z/E mixture) in this reaction.8
In past decades, significant advances have been made in the
direct silylation or hydrosilylation of C(sp2)−H bonds to E-
selective vinylsilanes.9 Nonetheless, direct regio- and stereo-
selective silylation of terminal alkene C(sp2)−H bonds to
prepare the Z-vinylsilanes remains a great challenge in synthetic
chemistry.10 In 2010, Falck reported that [Ir(OMe)(cod)]2/
dtbpy catalyzed the Z-selective, dehydrative silylation of
terminal alkenes to give 7:1−10:1 Z/E vinylsilanes (Scheme
1, eq 1).11a In 2013, Hartwig reported that the use of Ir-
catalyzed, 3,4,7,8-tetramethyl-1,10-phenathroline (Me4phen) as
ligand is better for the highly Z-selective (Z/E up to 92:8)
dehydrogenative silylation of terminal alkenes (Scheme 1, eq
2).11b,c Obviously, the development of more efficient and novel
Z-vinylsilane formation methods is an intensively investigated
field that is still a focus of synthetic chemistry research. Our
continuous interest and effort has been directed toward the
development of efficient and selective unactivated C−H
functionalizations and their applications in the synthesis of
natural biological products.12 We describe herein an efficient
palladium-catalyzed C(sp2)−H silylation for a stereoselective
synthesis of Z-vinylsilanes under mild conditions (Scheme 1, eq
3). This method offers a practical and environmentally friendly
strategy for the rapid synthesis of Z-stereoselective vinylsilanes
from simple starting materials.
In our previous study,12d we found that environmentally
benign 1,4-benzoquinone (BQ) can act as a nonmetal oxidant,
13a,b
replacing the stoichiometric metal oxidants Ag2CO3
and
AgOAc.13c Encouraged by these results, we began our
investigation of silylation of vinylic C(sp2)−H bonds by
examining the reactivity of 2-phenyl-N-(quinolin-8-yl)-
Received: August 10, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.7b02486
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Letter
acrylamide (1a)14 with hexamethyldisilane (HMDS, 2a) as the
silylation partner. After an initial screening of the catalysts and
oxidants, optimization studies employing Pd(OAc)2 as the
catalyst with nonmetal oxidant BQ were performed, and the
results are summarized in Table 1. A detailed screening of
With the optimal conditions in hand, the scope and
limitations of our silylation protocol were investigated (Scheme
2). α,β-Unsubstituted acrylamide 1b showed a poor reactivity
a b
,
Scheme 2. Substrate Scope
a
Table 1. Optimization of Reaction Conditions
b
entry
reagents (equiv), temp (°C), and solvent
BQ (2), 80, 1,4-dioxane
yield (%)
1
2
3
4
5
14
19
8
BQ (2), 80, toluene
BQ (2), 80, MeCN
BQ (2), 80, DMF
83
79
77
84
33
62
74
BQ (2), Na2CO3 (0.2), 80, DMF
BQ (2), 4 Å MS, 80, DMF
c
6
7
BQ (2), PivOH (0.2), 80, DMF
BQ (2), MesCO2H (0.2), 80, DMF
BQ (2), L-proline (0.2), 80, DMF
BQ (2), s-BINA-PO2H (0.2), 80, DMF
BQ (2), (BnO)2PO2H (0.2), 80, DMF
BQ (2), (PhO)2PO2H (0.2), 80, DMF
BQ (2), (BnO)2PO2H (0.2), 60, DMF
BQ (2), (BnO)2PO2H (0.2), 100, DMF
BQ (2), (BnO)2PO2H (0.2), 80, DMF
Ag2CO3 (2), CaSO4 (2), 130, 1,4-dioxane
8
9
10
11
12
13
14
d
90 (82)
84
66
89
64
20
32
e
15
f
16
f
17
Ag2CO3 (2), NaHCO3 (2), LiOAc (0.5),s-BINA-PO2H
(0.3), 125, toluene
a
Reaction conditions: 1a (0.2 mmol), 2a (1.0 mmol), Pd(OAc)2 (5
a
mol %), and additive (20 mol %) in solvent (1.0 mL) at the indicated
Reaction conditions: 1 (0.2 mmol), 2 (1.0 mmol), Pd(OAc)2 (5 mol
b
temperature for 12 h. Yields were determined by 1H NMR analysis of
%), BQ (0.4 mmol), (BnO)2PO2H (20 mol %), DMF (1.0 mL), at 80
°C for 12 h. Isolated yield. Reaction conditions: 1b (0.2 mmol), 2a
(1.0 mmol), Pd(OAc)2 (5 mol %), BQ (0.4 mmol), 3 Å MS (30 mg),
c
d
b
c
the crude reaction mixture. 4 Å MS (30 mg) was used. Isolated yield.
e
f
Pd(OAc)2 (2 mol %) was used. Pd(OAc)2 (10 mol %) was used.
d
DMF (1.0 mL), at 110 °C for 12 h. Th = thiophene-2-yl.
with a yield of 31% under the optimal conditions. An improved
yield of 63% for 3ba could be achieved when 3 Å MS was used
instead of (BnO)2PO2H at 110 °C. As depicted in Scheme 2, α-
substituted acrylamides bearing methyl, benzyl, and 3-phenyl-
propyl groups were readily silylated in 58−71% yields (3ca−
ea). Exceptionally, itaconic acid derivative 3fa was afforded in a
low yield. Irrespective of electronic properties or substitution
patterns in the aryl ring, a variety of α-aryl acrylamides with
different substituents were able to smoothly undergo the
silylation reaction and provided the corresponding silylation
products in good to excellent yields (3ga−wa). In addition, β-
substituted E-acrylamides could be utilized to afford the
silylation products (3xa−za) in low to moderate yields.
Obviously, the conversion is sensitive to the β-substituted
groups since it is difficult for the higher steric hindrance to form
the stabilized palladacycle intermediates. We then sought to
investigate the scope of the organosilicon sources. Compared to
HMDS (2a), (Me2PhSi)2 (2b) and (Me2ThSi)2 (2c) were less
reactive, affording 3ab and 3ac, respectively. When (Ph3Si)2
was used as the organosilicon source, no desired product (3ad)
was isolated because of steric effects. Notably, stereoselectivity
in favor of Z silylation of acrylamides was excellent throughout,
and no E-silylation products were observed.
various solvents revealed that the solvent was a very important
parameter and that the silylation reaction was sluggish in 1,4-
dioxane, toluene, and MeCN (Table 1, entries 1−3), while it
could be notably improved to give the desired product 3aa in
good yield when DMF was used as the solvent at 80 °C (Table
1, entry 4). To promote the circulation of this reaction system,
further studies surveyed a series of additives (Table 1, entries
5−12). Significantly, the use of a catalytic amount of
(BnO)2PO2H12a in the reaction system gave a slightly
enhanced yield (Table 1, entry 11). We speculate that
phosphate may facilitate the dissociation of palladium from
Pd−Si intermediate to accelerate catalyst turnover in this
context.12c Variation of the temperature was not beneficial for
the silylation reaction (Table 1, entries 13 and 14). In addition,
reducing catalyst loading from 5 to 2 mol % resulted in a
significant decrease of the yield (Table 1, entry 15).
Stoichiometric Ag2CO3 as oxidant was not applied successfully
to 1a (Table 1, entries 1613a and 1713b). Finally, the
combination of HMDS (5 equiv), Pd(OAc)2 (5 mol %), BQ
(2 equiv), and (BnO)2PO2H (20 mol %) in DMF at 80 °C for
12 h was found to be the optimal system for the palladium-
catalyzed silylation of 1a to exclusively afford the product 3aa
with 82% isolated yield (the stereochemistry of 3aa was
unambiguously confirmed by X-ray crystallography).
B
DOI: 10.1021/acs.orglett.7b02486
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Letter
To gain preliminary insight into the mechanism, a primary
kinetic isotope effect (∼1.0) was obtained (see the Supporting
Information), which indicated that the palladation was not the
rate-limiting step in this C−H silylation reaction. We next
examined the effect of other directing groups carefully (Scheme
3a). The corresponding silylation products containing an N,N-
afforded by initial chelation of 2-phenylacrylamide 1a with
Pd(II) catalyst followed by ligand exchange. The key
palladacycle intermediate B was produced via a C−H
concerned metalation−deprotonation (CMD) process. Sub-
sequently, intermediate C was formed by insertion of the Si−Si
bond and simultaneous transfer of trimethylsilyl group. Finally,
a reductive elimination and a final protonation of the amide
gave the desired silylation product 3aa with the generation of
Pd(0) species, which will be reoxidized to Pd(II) with the
assistance of 1,4-benzoquinone to continue the catalytic cycle.
To prove both the practicality and effectiveness of this
method in organic synthesis, the silylation reaction was scaled
up to 5 mmol (1.37 g) under the optimal conditions by the
synthesis of 3aa (Scheme 5a). Gratifyingly, the auxiliary can be
Scheme 3. Mechanistic Studies
Scheme 5. Synthetic Applicability
bidentate directing group 2-pyridinyl isopropyl (PIP)15a 4 or 2-
oxazolinyl (Oxa)15b 5 or an N,S-bidentate directing group15c
6
could be isolated with much lower yields. Nevertheless, the
desired product with an N,O-bidentate group could not be
afforded under standard conditions. When the 5-position of 8-
aminoquinoline was substituted with Cl, the modification
would dramatically decrease the reactivity. Interestingly, 5-
OMe-substituted 8-aminoquinoline retained good directing
activity, providing the corresponding silylation product in a
similar yield. Based on these results, we believe that the N,N-
bidentate directing group16 plays a critical role in this Pd-
catalyzed Z-selective C−H silylation reaction. Fortunately, the
5-membered palladium complex 1t-int via vinyl C(sp2)−H
bond activation was isolated, which can be used to react with
HMDS to give the desired silylation product 3ta in 50% yield
(Scheme 3b). The X-ray determined structure of 1t-int can
effectively explain and describe the Z-stereoselectivity of
vinylsilanes.
readily removed by a two-step sequence to afford the
corresponding methyl ester 10 (Scheme 5a). To demonstrate
the application of the Z-vinylsilanes prepared by our C−H
silylation, a high stereoselectivity Z-configured α,β-unsaturated
ester 11 was prepared via ipso-iodination/Suzuki−Miyaura
cross-coupling reaction (Scheme 5b).
In summary, we have developed an efficient and convenient
palladium-catalyzed direct intermolecular silylation reaction of
acrylamides with disilanes, providing a protocol to exclusively
generate Z-vinylsilanes with reasonable to excellent yields. A
broad range of functional groups was proved to be well-
tolerated by this strategy. The silylated products can be
obtained on a preparative scale, and the auxiliary can be readily
removed. Further application of this approach in the synthesis
of natural products such as lythridine17a and vertine17b is
currently under investigation.
On the basis of our experimental results and previous
reports,12,13,15 we proposed a plausible reaction mechanistic
pathway as shown in Scheme 4. First, Pd(II) species A was
Scheme 4. Proposed Catalytic Cycle
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.7b02486.
Experimental procedures, NMR spectra, and X-ray and
analytical data for all new compounds (PDF)
X-ray data for compound 3aa (CIF)
X-ray data for compound 3ba (CIF)
X-ray data for compound 1t-int (CIF)
X-ray data for compound 11 (CIF)
C
DOI: 10.1021/acs.orglett.7b02486
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Hartwig, J. F. Angew. Chem., Int. Ed. 2014, 53, 8471. (c) Toutov, A. A.;
Liu, W.-B.; Betz, K. N.; Fedorov, A.; Stoltz, B. M.; Grubbs, R. H.
Nature 2015, 518, 80. (d) Toutov, A. A.; Liu, W.-B.; Betz, K. N.;
Stoltz, B. M.; Grubbs, R. H. Nat. Protoc. 2015, 10, 1897. (e) Zhang, Q.-
W.; An, K.; Liu, L.-C.; Yue, Y.; He, W. Angew. Chem., Int. Ed. 2015, 54,
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: zhangsy16@sjtu.edu.cn.
ORCID
6918. (f) Wubbolt, S.; Oestreich, M. Angew. Chem., Int. Ed. 2015, 54,
̈
Jin-Long Pan: 0000-0002-1085-4566
Shu-Yu Zhang: 0000-0002-1811-4159
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the “Thousand Youth Talents
Plan”, NSFC (21672145), the Shuguang program from
Shanghai Education Development Foundation and Shanghai
Municipal Education Commission, and startup funds from
Shanghai Jiao Tong University.
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