Rhodium-Catalyzed Reaction of Silacyclobutanes with Unactivated Alkynes to Afford Silacyclohexenes

Article abstract of DOI:10.1002/anie.201814143

A Rh-catalyzed reaction of silacyclobutanes (SCBs) with unactivated alkynes has been developed to form silacyclohexenes with high chemoselectivity. Good enantioselectivity at the stereogenic silicon center was achieved using a chiral phosphoramidite ligand. The resulting silacyclohexenes are useful scaffolds for synthesizing structurally attractive silacyclic compounds.

Full text of DOI:10.1002/anie.201814143

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Accepted Article
Title: Rhodium-catalyzed Reaction of Silacyclobutanes with
Unactivated Alkynes to Afford Silacyclohexenes
Authors: Zhen Lei Song, Hua Chen, Yi Chen, Xiaoxiao Tang, Shunfa
Liu, Runping Wang, Tianbao Hu, and Lu Gao
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201814143
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10.1002/anie.201814143
Angewandte Chemie International Edition
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Silacycles
Rhodium-catalyzed Reaction of Silacyclobutanes with Unactivated
Alkynes to Afford Silacyclohexenes**
Hua Chen, Yi Chen, Xiaoxiao Tang, Shunfa Liu, Runping Wang, Tianbao Hu, Lu Gao, and Zhenlei
Song*
Abstract: A Rh-catalyzed reaction of silacyclobutanes (SCBs) with
unactivated alkynes has been developed to form silacyclohexenes
with high chemoselectivity. Good enantioselectivity at the
stereogenic silicon center was achieved using
a
chiral
phosphoramidite ligand. The resulting silacyclohexenes are useful
scaffolds for synthesizing structurally attractive silacyclic
compounds.
Silacycles[1] are attractive organosilanes because they are widely
used in developing π-conjugated organic materials with unique
optical and electronic properties.[2] Lately they have become the
focus of efforts to develop silicon-containing bioactive molecules of
interest.[3] Silacycles are typically synthesized by cyclization of
chain-like substrates,[4] but some desirable products cannot be
produced in this way. Instead, they can be obtained using an
alternative approach in which small, strained silacycles such as
silacyclobutane (SCB) are expanded into larger rings. Ever since
Kipping pioneered the synthesis of SCB,[5] a series of SCB ring-
expansion reactions have been developed that exploit its high ring
strain and Lewis acidity.[6]
In 1975, Sakurai and Imai reported the first Pd-catalyzed reaction
of SCB 1 with alkynes substituted with electron-withdrawing groups,
giving silacyclohexenes 3.[7] Revisiting this reaction in 1991,
Oshima and Utimoto observed formation of a large amount of
allylsilane 4 as a by-product.[8] They proposed that 3 formed via
reductive elimination of pallada-silacycloheptene 2, while β-H
elimination of 2 would generate 4. Recently, Shintani and Hayashi
made an important breakthrough at developing an asymmetric
version of this approach based on a Pd-catalyzed desymmetrization
of SCB.[9] Using a sterically demanding chiral phosphoramidite
Scheme 1. (a) Si-organic materials and Si-biological molecules containing
silacycles. (b) Pd-catalyzed reaction of SCB with unactivated alkyne leading to
non-cyclic allylsilane. (c) Rh-catalyzed reaction of SCB with unactivated alkyne
leading to silacyclohexenes.
ligand facilitated the reductive elimination step, leading to 3 with
high chemoselectivity and ultimately to high enantioselectivity at
the stereogenic silicon center[10]. Despite these progresses, this
reaction requires electron-deficient alkynes to generate
silacyclohexenes effectively. Using unactivated alkynes such as
phenylacetylene leads in many intermolecular reactions to severe β-
H elimination with SCBs, giving allylsilanes 4 as single or major
8]
products.[7,
To the best of our knowledge, only Shintani and
Hayashi have succeeded in forming silacyclohexenes, for which
they used an intramolecular approach involving an aryl-substituted
alkyne and SCB tethered by a phenyl ring.[11] Therefore, it would be
beneficial to develop general processes that generate
silacyclohexenes intermolecularly from unactivated alkyne and
SCBs. Such approaches would largely expand the structural
diversity of the resulting silacyclohexenes. Here we report such a
reaction in the presence of Rh catalyst that generates
silacyclohexenes with high chemoselectivity. The good
enantioselectivity at the stereogenic silicon center was also achieved
using a newly synthesized chiral phosphoramidite ligand.
[]
H. Chen, Y. Chen, X. X. Tang, S. F. Liu, R. P. Wang, T. B. Hu, Dr. L.
Gao, Prof. Dr. Z. L. Song
Sichuan Engineering Laboratory for Plant-Sourced Drug and Research
Center for Drug Industrial Technology, Key Laboratory of Drug-Targeting
and Drug Delivery System of the Education Ministry
West China School of Pharmacy, Sichuan University
Chengdu, 610041 (China)
E-mail: zhenleisong@scu.edu.cn
Prof. Dr. Z. L. Song
First we revisited the Pd-catalyzed reaction of SCB 1a[12] with
phenylacetylene.[13] In most cases, we obtained a mixture of 3a and
4a, with 4a as the major product, similar to Sakurai and Imai[7] as
well as Oshima and Utimoto.[8] Shintani and Hayashi’s conditions[9]
generated predominantly 3a but in only 17% yield. These failures
led us to focus on other transition metals such as monomeric and
dimeric Rh catalysts possessing CO or alkene ligands (Table 1,
entries 1-8), but all reactions provided complex mixtures without
desired 3a. We were delighted to find that Wilkinson’s catalyst
[Rh(PPh3)3Cl][14] significantly improved efficiency, affording 3a in
State Key Laboratory of Elemento-organic Chemistry, Nankai
University, Tianjin, 300071 (China)
[]
We are grateful for financial support from the NSFC (21622202,
21502125). The authors thank Prof. Shuyu Zhang in Shanghai Jiao Tong
University and Prof. Qingwei Zhang in University of Science and
Technology of China for their invaluable discussion. The authors also
thank Liying Huang for invaluable help with performing NMR
experiments.
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/anie.201xxxxxx.
1
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10.1002/anie.201814143
Angewandte Chemie International Edition
Table 2. Scope of Alkynes [a]
83% yield without formation of the by-product 4a (entry 9). This
result implies that the phosphine ligand is crucial for catalysis,
consistent with the different results obtained in entries 8 and 10.
Premixing [Rh(CH2=CH2)2Cl]2 with PPh3 for 1.0 h facilitated ligand
transfer from alkene to PPh3, giving 3a in 91% yield (entry 10).
Decreasing Rh(PPh3)3Cl loading from 0.1 to 0.02 equiv. improved
the yield from 83% to 97% (entry 11). Further decreasing the
loading of Rh(PPh3)3Cl to 0.01 equiv lowered the yield (entry 12).
Increasing the temperature from 25 to 60 oC accelerated the reaction
(entry 13). The reaction also proceeded well in non-anhydrous
toluene under air, giving 3a in 98% yield (entry 13).
Table 1. Screening of Reaction Conditions[a]
Entry
[Rh] (mol %)
equiv.
T (°C)
t (h)
Yield (%)[c]
1
2
3
4
5
6
7
8
9
10
Rh(CO)2(acac)
[Rh(CO)2Cl]2
Rh(cod)2BF4
Rh(cod)2OTf
[Rh(cod)Cl]2
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
60
25
60
25
25
60
25
60
25
60
3 h
3 h
3 h
3 h
3 h
3 h
3 h
3 h
4 h
4 h
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
N.D.
83
Rh(nbd)2BF4
[Rh(nbd)Cl]2
[Rh(CH2=CH2)2Cl] 2
Rh(PPh3)3Cl
[Rh(CH2=CH2)2Cl]2/
PPh3 (0.2)
91
11
12
13
Rh(PPh3)3Cl
Rh(PPh3)3Cl
Rh(PPh3)3Cl
0.02
0.01
0.02
25
25
60
4 h
4 h
2 h
97
83
98(98[c]
)
[a] Reaction conditions: 1a (0.20 mmol), phenylacetylene (0.24 mmol), 1.0 mL
of toluene. [b] Isolated yields. [c] The yield was obtained using non-anhydrous
toluene under air.
Next the scope of alkynes was examined using SCB 1a as the
model scaffold. The reaction worked well with aryl alkynes in
which the 4-position of the phenyl ring was substituted with
electron-donating or -withdrawing groups (Table 2), giving 3b-3u in
high yields. Moderate yields were observed only for 3h containing a
-CN group and 3j containing a -COMe group. Changing the position
of the phenyl ring substitution did not reduce efficiency, providing
3m-3o in yields exceeding 80%. The reaction also afforded 3p-3r
possessing naphthaline or the heterocycles. In contrast, 2-
ethynylpyridine did not react to give 3s, probably because the
pyridine's strong Lewis basicity inactivated the catalyst. Reaction of
2,4-diacetylide benzene with 0.5 equiv. of 1a delivered mono-
cyclized 3t in 68% yield, in which the alkyne left can be further
functionalized. Repeating this reaction with 2.0 equiv. of 1a
afforded 3u, in which two silacyclohexenes are linked
symmetrically to a phenyl ring. The reaction proved unsuitable for
preparing 3v from more sterically demanding internal alkynes. The
reactivity difference between internal and terminal alkynes allowed
us to synthesize enyne-type silacyclohexene 3w from the
corresponding diynes. The reaction was suitable for functionalized
alkyl alkynes. The yield differences between 3y and 3z or between
3aa and 3ab indicate that the free hydroxyl and amide groups
interfere with the reaction, probably by interacting with the catalyst
or by partially opening the SCB ring. The cyclopropyl group did not
interfere with the catalytic process, giving 3ad in 81% yield. The
bulky t-Bu group inhibited the reaction to give 3af. Surprisingly,
methyl propiolate, which functions well in Pd-catalyzed
silacyclohexene formation, was ineffective in our Rh-catalyzed
[a] Reaction conditions: 1a (0.20 mmol), alkyne (0.24 mmol), Rh(PPh3)3Cl (2
mol %), 1.0 mL of toluene, 60 °C. [b] Isolated yields. [c] 1a (1.0 mmol),
cyclopropyl acetylene (2.4 mmol).
Table 3. Scope of Silacyclobutanes [a]
[a] Reaction conditions:
1 (0.20 mmol), phenylacetylene (0.24 mmol),
Rh(PPh3)3Cl (2 mol %), 1.0 mL of toluene, 60 °C. [b] Isolated yields.
2
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10.1002/anie.201814143
Angewandte Chemie International Edition
Oshima and Utimoto[8] proposed that oxidative addition of Pd to
SCB initiates silacyclohexene formation from activated alkynes.
Subsequent control experiments by Shintani and Hayashi[9] led them
to favor a mechanism starting with Pd coordination of alkynes.
When we mixed SCB 1a with 1.0 equiv. of Rh(PPh3)3Cl in CDCl3
for 1 h at 60 oC, most 1a remained unchanged. But the signal of the
alkynyl proton in phenylacetylene shifted up-field from 3.0 to 2.0
ppm by mixing with 1.0 equiv. of Rh(PPh3)3Cl.[17] These results
suggest that our reaction may proceed via a mechanism similar to
Shintani and Hayashi’s model. We propose the pathway in Scheme
2: the coordination of Rh with alkyne (5) promotes transmetalation
with SCB to give Rh-silacycloheptene 6. Reductive elimination of 6
proceeds preferentially to β-H elimination, giving silacyclohexene 3.
Formation of the Rh-silacycloheptene 7, which cannot undergo β-H
elimination, could be also possible because we did not observe the
by-product 4.
approach. However, the vinylogous propargyl methyl ester was
effective to form 3ah in 60% yield. The process also provided 3ai
and 3aj containing a similar diene moiety.
The scope of substitution on silicon was examined (Table 3).
Suitable substituents included n-butyl, benzyl and C3H6OTBS
groups, giving 3ak-3am in good yields. The reaction proceeded well
in the presence of allyl and homoallyl groups, affording 3an-3ao.
However, introducing a vinyl group onto the silicon inhibited the
reaction to give 3ap. The silicon can be mono- or diaryl substituted,
leading to 3aq-3aw in high yields. The electron-donating and -
withdrawing groups on the phenyl ring were tolerated. A methyl
group at the 2-position was sterically disfavored, such that 3au was
generated in only 37% yield.
Table 4. Scope of the Asymmetric Process[a]
Scheme 3. Functionalization of silacyclohexenes.
[a] Reaction conditions: 1 (0.20 mmol), alkyne (0.240 mmol), [Rh(CH2=CH2)2Cl]2
(2 mol %), L18 (4 mol %), 1.0 mL of toluene, 25 °C, 10 h. [b] The configuration
at Si was determined by transformation of 3ax into the known compound. [c]
Isolated yields. [d] Determined by HPLC analysis using a chiral stationary phase.
[e] The yield was obtained on 1.0 mmol scale.
Silacyclohexenes 3 could be further transformed into structurally
attractive silacyclic compounds. For example, 3ay participated in an
endo-type Diels-Alder reaction to construct the silabicyclic 8 in the
presence of EtAlCl2 and silatricyclic 9 under heating. 3az generated
from selective O-desilylation of the corresponding silylether was
subjected to Johnson-Claisen rearrangement to give cyclic
allylsilane 10 in 84% yield, and to epoxidation to give 3-silyl-2,3-
epoxy alcohol 11 in 86% yield. Allylic oxidation of 3av with SeO2
afforded silacyclohexenol 12, which was further oxidized to
silacyclohexenone 13 in 97% yield.
Inspired by the success of using [Rh(CH2=CH2)2Cl]2 and PPh3 to
synthesize racemic 3a (Table 1, entry 10), we screened a range of
chiral phosphine ligands to enantioselectively construct the silicon
stereocenter in silacyclohexenes.[15] The binaphthyl phosphoramidite
L18, containing 3,3’-trifluoromethyl groups,[16] most efficiently
generated 3 with high er (Table 4). Increasing the bulkiness of the 3,
In summary, we have developed a Rh-catalyzed reaction of
SCBs with unactivated alkynes. The reaction chemoselectively
generates a wide range of silacyclohexenes. Using a chiral
binaphthyl phosphoramidite bearing 3,3’-trifluoromethyl- and N-
morpholino-substitutions provided good enantioselectivity at the
stereogenic silicon center. The resulting silacyclohexenes serve as
versatile synthons to generate diverse silacyclic compounds. More
detailed studies and application of this approach for the synthesis of
Si-biological molecules are underway.
3’-groups
in
binaphthyl
ligands
typically
improves
enantioselectivity, but in our case, it severely reduced the er for 3.
This unusual steric bias implies that the bulkier 3, 3’-substituents
might simultaneously increase the steric interaction with both the
aryl and methyl groups on silicon when interacting with two
prochiral Si-CH2 bonds in silacyclobutanes 1, therefore decreasing
the efficiency of desymmetrization. In addition, the morpholine ring
in L18 was better than traditional nitrogen moieties such as NMe2,
N(i-Pr)2 or NBn2.
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: Silacyclobutane • Transition Metal • Catalysis • Alkynes •
Silacyclohexene
Scheme 2. Proposed catalytic cycle.
3
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10.1002/anie.201814143
Angewandte Chemie International Edition
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the procedure reported in this reference.
[13] See Supporting Information (Table S1) for details.
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[17] Reaction of 1a with phenylacetylene in CDCl3 under otherwise
identical conditions provided 3a in 70% yield.
4
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10.1002/anie.201814143
Angewandte Chemie International Edition
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Organosilanes
Hua Chen, Yi Chen, Xiaoxiao Tang,
Shunfa Liu, Runping Wang, Tianbao
Hu, Lu Gao, and Zhenlei Song*
__________ Page – Page
Rhodium-catalyzed Reaction of
Silacyclobutanes with Unactivated
Alkynes to Afford Silacyclohexenes**
A Rh-catalyzed reaction of silacyclobutanes (SCBs) with unactivated alkynes has
been developed to form silacyclohexenes with high chemoselectivity. The approach
tolerates a range of functionalities on the alkynes and silicon of SCBs. Good
enantioselectivity at the stereogenic silicon center was achieved using a newly
synthesized chiral phosphoramidite ligand. The resulting silacyclohexenes are
useful scaffolds for synthesizing structurally attractive silacyclic compounds.
5
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