Me3Si?SiMe2[oCON(iPr)2?C6H4]: An Unsymmetrical Disilane Reagent for Regio- and Stereoselective Bis-Silylation of Alkynes

Article abstract of DOI:10.1002/anie.201800513

The air-stable unsymmetrical disilane Me₃Si?SiMe₂[oCON(iPr)₂C₆H₄] has been developed for bis-silylation of alkynes. This reagent tolerates a range of functional groups, providing Z-vinyl disilanes in

Full text of DOI:10.1002/anie.201800513

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Accepted Article
Title: Me3Si−SiMe2[o-CON(i-Pr)2-C6H4]: A New Unsymmetrical
Disilane Reagent for Regio- and Stereoselective Bis-silylation of
Alkynes
Authors: Zhen Lei Song, Peihong Xiao, Yanjun Cao, Yingying Gui, and
Lu Gao
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10.1002/anie.201800513
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Organosilanes
Me Si−SiMe [o-CON(i-Pr) -C H ]: A New Unsymmetrical Disilane
3
2
2
6
4
Reagent for Regio- and Stereoselective Bis-silylation of Alkynes**
Peihong Xiao, Yanjun Cao, Yingying Gui, Lu Gao, and Zhenlei Song*
Abstract: The air-stable unsymmetrical disilane Me Si−SiMe [o-
3
2
CON(i-Pr) C H ] has been developed for bis-silylation of alkynes.
2
6
4
This reagent tolerates a range of functional groups, providing Z-
vinyl disilanes in high yields. The phenyl ring-tethered amide group
is proposed to direct oxidative addition of Pd(0) into the Si-Si bond,
that might facilitate formation of a six-membered Pd cycle, leading
to the observed good to excellent regioselectivity.
D
isilane is one of the most important organosilane reagents in both
[
1]
industry and academia. It has been widely used in the formation of
[
2]
[3]
[4]
[5]
[6]
vinyl, allyl, aryl and acyl silanes, silyl halides, and
[
7]
protection of alcohols to silyl ethers. A particularly attractive use
is transition metal-catalyzed bis-silylation of unsaturated C−C bonds,
since two Si−C bonds can be constructed in one step. Generally,
intermolecular bis-silylation of alkynes is conducted using
[
2a-2l]
symmetric disilanes with two identical silyl groups.
While this
approach avoids regioselectivity problems, it limits downstream
transformations because the two silyl groups are difficult to
differentiate chemoselectively. Using unsymmetrical disilanes can
potentially solve this problem. However, the regio- and
stereoselective control during bis-silylation is a notoriously difficult
Scheme 1. (a) Bis-silylation of alkynes using traditional unsymmetrical
disilanes activated by electron-negative groups. (b) Bis-silylation of alkynes
[
8]
task. The challenge can be evident from prior studies (Scheme 1a).
3 2 2 6 4
using Me Si−SiMe [o-CON(i-Pr) -C H ] (1e).
Moderate regio- or stereoselectivity were frequently obtained using
unsymmetrical disilanes, in which one of the two silicons is
activated by electro-negative substituents such as halide or methoxy
group. In addition, the strong Lewis acidity of the halogenated
silicon center renders these unsymmetrical reagents toxic, moisture-
sensitive and intolerant of various Lewis base functional groups.
This severely limits the use of these bis-silylation reagents in
organic synthesis. Therefore, new disilane reagents are highly
demanded that are easy to handle, tolerant of a broad range of
functional groups, and able to provide high regio- and
stereochemical control.
develop a new type of unsymmetrical disilane reagents. Amide-
directed Si-Si bond activation would be distinct from the traditional
approach of activating disilanes by introducing an electron-negative
substituent, and it might improve regio- and stereoselectivity in
alkyne bis-silylation. Here we report the new unsymmetrical
disilane Me Si−SiMe [o-CON(i-Pr) C H ] (1e, Scheme 1b), in
3
2
2
6
4
which the disilane and amide moieties are tethered by a phenyl ring.
This reagent is an air-stable white solid and can be prepared
practically on gram scale. It tolerates a range of functionalities
including Lewis basic groups in bis-silylation of terminal alkynes,
giving Z-vinyl disilanes in high yields with good regio- and
stereoselectivity.
Amide has been used widely as a good directing group in many
[
9]
transition metal-catalyzed transformations. The reactions generally
take advantage of the strong ability of amide to coordinate with
transition metals, thereby facilitating activation of the neighboring
bond to form a metal cycle species. We envisioned that introducing
an amide group into one of the silicon in disilane might allow us to
[]
P. H. Xiao, Y. J. Cao, Y. Y. Gui, Dr. L. Gao, Prof. Dr. Z. L. Song
Sichuan Engineering Laboratory for Plant-Sourced Drug and Research
Center for Drug Industrial Technology
West China School of Pharmacy, Sichuan University
Chengdu, 610041 (China)
E-mail: zhenleisong@scu.edu.cn
Scheme 2. Syntheses of disilanes 1a-1e.
Prof. Dr. Z. L. Song
Our studies began by synthesizing the amide-substituted disilane
State Key Laboratory of Elemento-organic Chemistry, Nankai
University, Tianjin, 300071 (China)
reagents 1a-1e from Me Si-SiMe Cl. These reagents were prepared
3
2
via nucleophilic addition with the corresponding organolithium
[
10]
reagents (Scheme 2). The resulting disilanes are much more air-
stable than Me Si-SiMe Cl. The disilanes can be categorized
[]
We are grateful for financial support from the NSFC (21622202,
21502125).
3
2
according to the position of the amide moiety attached to the
internal silicon: (a) 1a is a unique acylsilane, with the amide moiety
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/anie.201xxxxxx.
1
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Angewandte Chemie International Edition
10.1002/anie.201800513
phenylacetylene carrying an electron-donating group. Changing the
position of the substitution on the phenyl ring did not affect the high
regioselectivity (2g-2i), although it did affect yield to some extent.
The approach also proved efficient at synthesizing 2o and 2p from a
heterocycle-substituted alkyne, or 2q from an ester group-activated
alkyne. However, the internal alkyne, which is sterically more
demanding than terminal alkyne, is currently not suitable to
synthesize 2r by our approach with 1e.
attached directly to silicon; (b) 1b and 1c are α-silyl amides, in
which the disilane and amide groups are tethered by a methylene
[
11]
group; (c) 1d and 1e are β-silyl amides, in which the disilane and
amide moieties are tethered using a phenyl ring as a linkage.
[
a]
Table 1. Screening of Disilanes 1a-1e for Bis-silylation of Phenylacetylene
[
a]
Table 2. Scope of Alkynes to Synthesize Vinyl Disilanes 2
1
2
[c]
[d]
Entry
Si -Si Catalyst
Additive
Yield
2:3
1
2
3
4
5
6
7
8
9
1a
1a
1b
1c
1d
1e
1e
1e
1e
1e
1e
Pd(acac)
Pd(acac)
Pd(acac)
Pd(acac)
Pd(acac)
Pd(acac)
Pd(acac)
Pd(acac)
Pd(acac)
2
2
2
2
2
2
2
2
2
w/o
N.R.
65%
32%
N.R.
59%
95%
<5%
91%
<5%
45%
N.R.
N.D.
62:38
75:25
N.D.
87:13
92:8
t-BuOK (10 mol %)
t-BuOK (10 mol %)
t-BuOK (10 mol %)
t-BuOK (10 mol %)
t-BuOK (10 mol %)
w/o
N.D.
90:10
N.D
NaOMe (10 mol %)
2 3
K CO (10 mol %)
10
1
Pd(OAc)
2
t-BuOK (10 mol %)
t-BuOK (10 mol %)
87:13
N.D.
1
Pd(PPh )
3 4
[
a] Reaction conditions: disilane 1 (0.20 mmol), phenylacetylene (0.60 mmol), t-
BuNC (0.12 mmol), palladium catalyst (4 mol %), 2.0 mL of toluene, 120 °C, 3 h.
[
b] Regio- and stereochemistry was assigned based on NOE experiments of 2a
1
generated from 1e. [c] Isolated yields. [d] Ratios were determined from crude
H
NMR.
With disilanes 1a-1e in hand, we examined the bis-silylation of
phenylacetylene (Table 1). The reaction was initially carried out
[
12]
with acylsilane 1a and under the conditions developed by Ito:
4
mol % of Pd(acac) as catalyst, 60 mol % of t-BuNC as ligand in
2
toluene solvent at 120 °C. Unfortunately, no desired bis-silylation
was detected (entry 1). We added 10 mol % of t-BuOK as additive
with the expectation that base might be capable of activating the Si-
Si bond to facilitate bis-silylation. The efficiency was dramatically
improved to give Z-vinyl disilanes in 65% yield as a 62:38 mixture
of regioisomers 2 and 3 (entry 2). The methylene-tethered α-silyl
amides 1b and 1c appeared to retard or even inhibit bis-silylation
(
entries 4 and 5). In sharp contrast, reaction of disilanes 1d and 1e,
in which the phenyl ring is a linker, provided a remarkable
improvement of regioselectivity than that of acylsilane 1a (entries 5
and 6). Screening different substituents on the amide group revealed
the bulkier i-Pr group to be superior to the Et group, providing the
optimal yield of 95% and regioselectivity of 92:8 (entry 6). Similar
to the result shown in entry 1, bis-silylation of phenylacetylene with
[
a] Reaction conditions: 1e (0.20 mmol), alkyne (0.60 mmol), t-BuNC (0.12
1
e was barely detected without addition of t-BuOK (entry 7).
mmol), t-BuOK (0.02 mmol), Pd(acac)
2
(4 mol %), 2.0 mL of toluene, 120 °C, 3
Replacing t-BuOK with NaOMe led to comparable yield and
1
h. [b] Isolated yields. [c] Ratios were determined from crude H NMR.
regioselectivity (entry 8), while the additive K CO was ineffective
2
3
for bis-silylation (entry 9). Palladium catalyst also showed a distinct
impact on bis-silylation. Using Pd(OAc) as catalyst decreased yield
2
and regioselectivity (entry 10), while Pd(PPh ) did not catalyze bis-
3
4
silylation at all (entry 11).
A range of alkynes was further tested in bis-silylation reactions
with disilane 1e (Table 2). The reaction proved useful for
synthesizing styrene-type vinyl disilanes 2b-2e, in which the phenyl
ring is substituted with various electron-donating groups. It is
noteworthy that the Lewis basic aniline group, which may be
incompatible with halogenated disilanes such as Me Si-SiMe Cl,
3
2
showed a good tolerance to 1e, giving 2e in 77% yield with 92:8
regioselectivity. The reaction also afforded 2f-2n containing a
phenyl ring substituted with a range of electron-withdrawing groups
such as halide, CF , NO and carbonyl groups. These substitutions
Scheme 3. A sequential one-pot bis-silylation/Suzuki cross-coupling process to
synthesize 5a-5c.
3
2
generally resulted in higher regioselectivity than that obtained with
2
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Angewandte Chemie International Edition
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The two silyl groups in Z-vinyl disilane 2a can easily be
differentiated. As shown in Scheme 6a, the SiMe [o-CON(i-
2
The synthetic potential of our approach was further illustrated
when we achieved the bis-silylation of alkynes 4 containing a p-
Bpin-substituted phenyl ring. Disilane 1e showed good
compatibility with the Bpin group, giving 2s in 91% yield with
9
0:10 regioselectivity (Scheme 3). This clean bis-silylation allowed
us to realize a sequential one-pot bis-silylation/Suzuki cross-
[
13]
coupling process. Once the bis-silylation was complete and had
formed 2s, a solution of aryl iodide, Pd(PPh ) and K CO in DMF
3
4
2
3
were added to the same reaction tube. In this way, biaryl-substituted
vinyl disilanes 5a-5c were generated in 68-78% yields.
Scheme 5. Proposed catalytic cycle of bis-silylation of alkyne with 1e.
Pr) C H ] moiety appeared being more reactive to fluoride anion
2
6
4
than SiMe . 2a underwent chemoselective desilylation with TBAF
3
to give vinyl silane 10 in 97% yield. On the other hand, acid-
promoted desilylation of 2a led exclusively to elimination of the
SiMe group, affording 11 in 90% yield. In addition, the benzamide
3
moiety is not only a directing group, it can also be used to transform
vinyl disilane into structurally interesting cyclic organosilanes
(
2
Scheme 6b). For example, aldehyde 12, generated by reduction of
[
20]
Scheme 4. Control experiments using disilanes 1e, 1f, 1g and 1h.
a,
underwent BF •OEt -promoted cyclization/oxidation to give
3 2
sila-flavone 13 in 72% yield. This compound mimics biologically
To gain a deeper understanding of the unique reactivity of
disilane 1e, we performed a control experiment using 1f containing
a phenyl ring, and 1g containing a highly electronegative C F group
[
21]
important flavone,
bioactivities.
so it may have interesting reactivities or
6
5
(Scheme 4). Bis-silylation of phenylacetylene with 1f and 1g led,
respectively, to 2t and 2u with regioselectivity of 75:25 and 55:45.
Both ratios were much lower than the 92:8 obtained using 1e. These
results clearly indicate that the presence of the amide group on the
phenyl ring is crucial for high regioselectivity. A similarly moderate
ratio of 77:23 was also observed using 1h, which contains a CON(i-
Pr) group on the phenyl ring but at the para position. This result
2
suggests that the electron-withdrawing effect of amide should not
account for the high regioselectivity of the reaction with 1e.
2
9
[14]
Contrary to expectations, Si NMR studies of 1e, 1f
and 1h
showed no evidence that the amide group in 1e activates the internal
[
15]
silicon by forming
a
penta-coordinated silicate.
Ortho-
Scheme 6. (a) Chemoselective desilylation of 2a; (b) Synthesis of 13 from 2a.
substitution of the amide in 1e did affect the chemical shift of the
In summary, we have developed the new unsymmetrical disilane
internal silicon, but not in the expected direction: the corresponding
2
9
reagent Me Si−SiMe [o-CON(i-Pr) C H ], which is an air-stable
Si chemical shift (-18.38 ppm) in 1e is downfield 3.3 ppm than
3
2
2
6
4
[
16]
white solid and can be prepared on the gram scale. This reagent
shows good functional compatibility in bis-silylation of terminal
alkynes, giving Z-vinyl disilanes in good yields. The phenyl ring-
tethered amide group is proposed to direct oxidative addition of the
Si-Si bond to Pd(0), facilitating formation of a six-membered Pd
cycle responsible for the observed high regio- and stereoselectivity.
More detailed studies and application of this useful activation mode
in organosilane-involved transformations are underway.
that of 1f (-21.65 ppm) and 3.1 ppm than that of 1h (-21.45 ppm),
in contrast to the upfield shift expected if a penta-coordinated
silicate forms.
[
17]
Based on our experimental results, we propose the catalytic cycle
outlined in Scheme 5. The reaction should be initiated by oxidative
addition of the Si-Si bond to Pd(0). This step is facilitated most
likely by the amide-directed intramolecular approach 6. t-BuOK
might attack the silicon centers to form silicates, thus activating the
[
18]
2
Si-Si bond.
In the resulting complex 7, the Pd-Si bond is
1
embedded in a six-membered Pd cycle, while the Pd-Si bond is an
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
exo-cyclic orientation. Thus, the subsequent alkyne insertion via 8
1
would proceed regioselectively into the Pd-Si bond to give 9, in
Keywords: Disilanes • Bis-silylation • Amide • Alkynes • Vinyl Silanes
[
19]
which the Pd is attached to the internal carbon of the alkyne.
Finally, the vinyl Pd species 9 undergoes reductive elimination to
afford 2 as the major or single regioisomer, thereby regenerating the
Pd catalyst.
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Angewandte Chemie International Edition
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4
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Angewandte Chemie International Edition
10.1002/anie.201800513
Entry for the Table of Contents (Please choose one layout)
Organosilanes
Peihong Xiao, Yanjun Cao, Yingying
Gui, Lu Gao, and Zhenlei Song*
_
_________ Page – Page
3 2 2 6 4
Me Si−SiMe [o-CON(i-Pr) -C H ]: A
New Unsymmetrical Disilane Reagent
for Regio- and Stereoselective Bis-
silylation of Alkynes
3 2 2 6 4
The air-stable unsymmetrical disilane Me Si−SiMe [o-CON(i-Pr) C H ] has been
developed for bis-silylation of alkynes. This reagent tolerates a range of functional
groups, providing Z-vinyl disilanes in high yields. The phenyl ring-tethered amide
group is proposed to direct oxidative addition of Pd(0) into the Si-Si bond, that might
facilitate formation of a six-membered Pd cycle, leading to the observed good to
excellent regioselectivity.
5
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