2-Arylsilacyclobutane as a Latent Carbanion Reacting with CO2

Article abstract of DOI:10.1002/anie.201805333

An electronically neutral 2-arylsilacyclobutane generates a nucleophilic carbanion at room temperature through cleavage of the benzylic C?Si bond when simply dissolved in polar aprotic solvents such as N,N-dimethylformamide (DMF). The nucleophilic species is capable of capturing carbon dioxide to furnish a silalactone. The carboxylation reaction is unique in that no additional activating agents are required.

Full text of DOI:10.1002/anie.201805333

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Accepted Article
Title: 2-Arylsilacyclobutane as a Latent Carbanion Reacting with CO2
Authors: Naoki Ishida, Shintaro Okumura, Tairin Kawasaki, and
Masahiro Murakami
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805333
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10.1002/anie.201805333
Angewandte Chemie International Edition
COMMUNICATION
2-Arylsilacyclobutane as a Latent Carbanion Reacting with CO2
Naoki Ishida, Shintaro Okumura, Tairin Kawasaki, and Masahiro Murakami*
Dedication ((optional))
Abstract: An electronically neutral 2-arylsilacyclobutane generates a
nucleophilic carbanion at room temperature through cleavage of the
benzylic carbon–Si bond when simply dissolved in polar aprotic
solvents such as N,N-dimethylformamide (DMF). The nucleophilic
species is capable of capturing carbon dioxide to furnish a silalactone.
The carboxylation reaction is unique in that no additional activating
agents are required.
only transiently, it is then capable of capturing carbon dioxide to
furnish a silalactone.
Our investigation began with the observation of the
unexpected behavior of 2-phenylsilacyclobutane cis-1a in DMF-
d7 (Scheme 1). cis-Enriched 1a (cis:trans = 98:2) was dissolved
in DMF-d7 at room temperature, and left for 5 days. Much to our
surprise, cis-to-trans isomerization took place to give a 3:1
mixture of cis- and trans-isomers. This isomerization proceeded
faster in the presence of molecular sieves 4Å (MS4Å), and cis-1a
Generation of carbanionic nucleophiles has been a subject of
fundamental interest in organic chemistry.[1] Mostly employed
methods of generating carbanions include (a) metallation of
organic (pseudo)halides with metallic agents like magnesium(0)
metal and n-butyllithium and (b) deprotonation of an acidic
hydrogen by a base. A milder pathway is available with
organosilicon compounds which are activated by a fluoride
anion.[2] Formation of a strong Si–F bond provides a driving force
for generation of carbanionic species. A carbanionic species is
also assumed in the carboxylation reaction of organic iodides with
carbon dioxide using a strained disilane and cesium fluoride as
the activating agents.[3]
was completely converted to trans-1a in
4
days.[9] No
isomerization occurred when less polar aprotic solvents such as
CDCl3 and C6D6 were used, even in the presence of MS4Å. In
contrast to cis-1a, trans-1a failed to undergo trans-to-cis
isomerization when dissolved in DMF in the presence of MS4Å.
Me Me
Si
Me Me
Si
DMF-d7, RT
Me
Me
trans-1a
cis-1a
The strain energy inherent in three- and four-membered ring
structures often brings forth unique reactivities that are not
available with the corresponding acyclic structures. This applies
Scheme 1. cis-to-trans Isomerization of 1a monitored by 1H NMR.
also
to
organosilicon
compounds.[4,5]
For
example,
The cis-to-trans isomerization is explained by assuming that
the ring opening/closing process is induced by the action of the
polar DMF molecule upon cis-1a as the Lewis base (Scheme
2).[10] The Lewis acidity of the silicon atom of 1a is enhanced by
the four-membered ring structure.[11] Although no generation of an
appreciable amount of the corresponding silicate is observed in
the 29Si NMR spectra in DMF-d7, the polar solvent DMF could act
as the Lewis base to transiently coordinate to the silicon atom of
1a.[12] This coordination would in turn induce opening of the
strained four-membered ring to generate benzylic carbanion A,
which is configurationally labile. Upon displacement of the DMF
molecule on silicon again, A reverts back to a four-membered ring,
thereby forming cis-1a and trans-1a as well. The trans-1a is
thermodynamically more stable than cis-1a because of the sterics
(calculated DG is -4.2 kcal/mol), and the equilibrium eventually
shifts to the side of the trans-isomer. Thus, the strain energy
contained in the four-membered ring can drive the generation of
silacyclopropane has a strain energy of 150 kJ/mol, which is
larger than that of cyclopropane (117 kJ/mol).[6] Whereas ordinary
tetraorganosilicon compounds generally fail to add across the
carbonyl group of aldehydes, silacyclopropanes act through ring
opening when either heated at 100 °C,[4b] photolyzed,[4a] treated
with a t-BuOK/18-crown-6,[4b] or treated with a copper salt.[4d]
carbonyl insertion reaction can also take place with
silacyclopropanes and formamides upon
heating.[4c]
A
Silacyclobutanes react with aldehydes in an analogous manner
with the assistance of t-BuOK.[5d] Transition metals, like palladium
and nickel, catalytically induce cleavage of the C–Si bond of the
silacyclopropane or silacyclobutane rings,[7] and a variety of their
ring-opening/-expansion reactions have been developed.[8] We
now report that a carbanion is transiently generated from an
electronically neutral 2-arylsilacyclobutane in a remarkably simple
way. When a 2-arylsilacyclobutane is dissolved in an aprotic polar
solvent such as DMF at room temperature, the four-membered
ring is opened at the benzylic carbon–Si bond, through equilibrium,
to generate a benzylic carbanion. Although the carbanion forms
a
carbanionic species stabilized by a phenyl group with
assistance of a solvent molecule, although transiently.
O
Me Me
Me Me
Si
Me Me
Si
Si
O
-
DMF
DMF
H
NMe2
Ph
[a]
Dr. N. Ishida, Dr. S. Okumura, T. Kawasaki, Prof. Dr. M. Murakami
Department of Synthetic Chemistry and Biological Chemistry
Kyoto University
Ph
Ph
H
NMe2
Me
Me
Me
cis-1a
trans-1a
A
Katsura, Kyoto 615-8510 (Japan)
E-mail: murakami@sbchem.kyoto-u.ac.jp
Scheme 2. Proposed Mechanism for the cis-to-trans Isomerization of 1a.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201805333
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COMMUNICATION
The carbon within carbon dioxide is far less electrophilic than
that contained within an aldehyde. Addition of organosilicon
compounds onto carbon dioxide at the C–Si bond is limited to
choice of the solvent was crucial, as was the case with a cis-to-
trans isomerization of cis-1a. Whereas aprotic polar solvents like
N,N-dimethylacetamide (DMA) and dimethyl sulfoxide (DMSO)
effectively promoted the carboxylation reaction (Entries 2 and
3),[20] less polar aprotic solvents like toluene, chloroform,
acetonitrile and acetone failed to give 3b even upon heating
(Entries 4-7).
When the reaction was performed in the presence of MS4Å
(solvent: DMA), the carboxylation reaction was completed within
10 min. After purification by silica gel column chromatography, 3b
was isolated in 68% yield. 1,1-Dimethylsilacyclobutane, lacking a
phenyl group at the 2-position, and acyclic benzyltrimethylsilane
failed to incorporate carbon dioxide, indicating that the benzylic
carbon–Si bond contained in the strained four-membered cyclic
skeleton was a structural requisite for the carboxylation reaction.
strongly nucleophilic derivatives such as
a
lithiated
(2-
(phosphacyclopentadienyl)silane,[13]
a
zincated
pyridyl)methylsilane.[14] An N-heterocyclic carbene–trimethylsilyl
cation adduct also attacks carbon dioxide at the carbene carbon–
Si bond.[15,16] The addition of neutral organosilicon compounds
onto carbon dioxide often requires assistance of an excess
amount of an aluminum Lewis acid[17] and fluoride.[2,18] Direct
incorporation of carbon dioxide into small ring silacycles had
never been reported until our recent investigation on a nickel-
catalyzed reaction of benzosilacyclobutenes.[19] The ring opening
behavior of the silacyclobutane 1a mentioned above led us to
examine a reaction of 2-phenylsilacyclobutane 1b with carbon
dioxide (Table 1). A DMF-d7 solution of 1b was simply left with
carbon dioxide under an atmospheric pressure at room
temperature, and it was monitored by 1H NMR spectroscopy
(Entry 1). After 3 h, the resonances ascribed to 1b disappeared
and the quantitative formation of silalactone 2b was observed.
The formation of 2b is explained by assuming that the anionic
nucleophilic species, generated by the ring opening of 1b,
undergoes addition onto carbon dioxide and that the resulting
carboxylate anion displaces the DMF molecule on silicon to then
undergo annulation into a six-membered ring. The subsequent
treatment of the reaction mixture with water afforded the siloxane
dimer of the carboxylic acid 3b in 97% NMR yield. An intense
infrared absorption band was observed at 1069 cm-1, which was
ascribed to the siloxane linkage. The data of HRMS and an
elemental analysis also supported the dimeric structure. The
Of note is that no additional activating agents are required for
the carboxylation reaction. It is a significant challenge for organic
chemists to capture gaseous carbon dioxide into organic
molecules by C–C bond formation. Since carbon dioxide is
thermodynamically stable, C–C bond forming carboxylation
reactions of organic molecules with carbon dioxide typically
require the use of a stoichiometric amount of a base such as t-
BuOK and Me3Al, or a metallic reductant such as zinc and
manganese to drive the reaction forward.[21] Carboxylation
reactions that exploit light as the energy source have recently
appeared.[22] The present carboxylation reaction dispenses with
the use of such activating agents because the release of the ring
strain of the four-membered ring of 1b serves as the key driving
force.
Various 2-arylsilacyclobutanes reacted with carbon dioxide
when their DMA solution containing MS4Å was stirred at room
temperature under an atmospheric pressure of carbon dioxide
(Table 2). An ethyl group was tolerated on silicon in addition to a
methyl group (Entry 1). o-Methyl (Entry 2), p-methoxy (Entry 3)
and dioxolane (Entry 4) groups were tolerated on the aromatic
ring. The reaction of silacyclobutane trans-1a was accompanied
by stereochemical isomerization of the benzylic carbon to give a
diastereomeric mixture of dicarboxylic acid 3a in 63% yield (Entry
5). The reaction of cis-1a also afforded a diastereomeric mixture
of 3a in 65% yield (Entry 6).
Table 1. Addition of 1b to CO2.
Me
Si Me
Me Me
Si
O
O
CO2
(1 atm)
+
solvent, RT, 3 h
1b
2b
O
OH
H2O
O
Si
Me Me
2
3b
Next we examined the reaction of naphthosilacyclobutene 4a
(Scheme 3). When a DMA solution of 4a was stirred at room
temperature under an atmospheric pressure of carbon dioxide, a
nucleophilic attack onto carbon dioxide occurred at the benzylic
carbon rather than at the aromatic carbon to furnish a six-
membered silalactone 5a. Unlike the case with the silalactone 2b,
it was possible to isolate 5a in a form of six-membered silalactone
by column chromatography on silica gel (68% yield). The
silalactone 5a also remained intact even after the treatment with
2N aq HCl. As we previously reported,[19] the silalactone 6a was
produced when 4a was heated at 160 °C in mesitylene under an
atmospheric pressure of carbon dioxide in the presence of a
catalytic amount of a nickel–N-heterocyclic carbene complex.
Carbon dioxide was inserted into the C(arene)–Si bond. Thus, the
two protocols for carboxylation have a contrasting site-selectivity.
Entry[a]
Solvent
NMR yield of 3b / %
1
DMF-d7
DMA
97
86
86
0
2
3
DMSO
4[b]
5[b]
6[b]
7[b]
toluene
chloroform
acetonitrile
acetone
0
0
0
[a] Reaction conditions: 1b (0.10 mmol), solvent (0.5 mL), CO2 (1 atm), RT, 3 h.
[b] 60 °C, 12 h.
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10.1002/anie.201805333
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COMMUNICATION
Table 2. Addition of 2-Arylsilacyclobutanes 1 to CO2.
Table 3. Addition of Benzosilacyclobutenes 6 to CO2.
Entry[a]
1
Substrate 1
Product 3, isolated yield
Entry[a]
1[b]
Substrate 4
Product 7, isolated yield
Et
Et
O
OH
Si
Ph
Ph
Si
O
O
Me
O
Me Si
Si
Et Et
2
1c
3c, 80%
4b
Me Me
Si
5b, 92%
O
OH
2
Me
Me Me
O
2
Si
Si
Me Me
Si Me
O
2
Me
1d
Me
F
F
O
4c
3d, 69%
5c, 79%
Me Me
Si
O
OH
Me Ph
Me
3
4
5
6
3
Si
MeO
Si Ph
O
O
Si
Me Me
MeO
2
1e
MeO
O
MeO
4d
3e, 67%
OH
5d, 80%
O
O
Me
O
Me
4[c]
Me Me
Si
Me Me
Me
Si
Si Me
O
O
O
O
Si
Me Me
O
2
1f
4e
5e, 55%
3f, 75%
Me Me
Si
O
OH
[a] Reaction conditions: benzosilacyclobutene 4 (0.20 mmol), CO2 (1 atm),
DMA (1 mL), MS4Å (50 mg), RT, 3 h; then 2N aq HCl. [b] DMF instead of
DMA, 100 °C, 3 h. [c] DMF instead of DMA, 100 °C, 16 h.
O
Si
Me Me
2
Me
Me
3a, 63%[b]
trans-1a
Me Me
Si
O
OH
The Hiyama coupling reaction of the silalactone 5a[23] was
performed to demonstrate the synthetic utility of the carboxylated
product (Scheme 4). The C–Si bond was successfully converted
in to a C–C bond to furnish biaryl 7 in 70% yield.
O
Si
Me Me
2
Me
Me
3a, 65%[c]
cis-1a
[a] Reaction conditions: 2-arylsilacyclobutane 1 (0.20 mmol), DMA (1 mL), CO2
(1 atm), MS4Å (50 mg), RT, 3 h; then 2N aq HCl. [b] It was difficult to determine
the dr of 3a due to the significant overlap of the 1H NMR spectrum. The cis/trans
ratio of the silalactone 2a derived from 3a was determined to be 4.5:1.0 by 1H
NMR spectroscopy. [c] The cis/trans ratio of the silalactone 2a derived from 3a
was 2.7:1.0.
Me
Me Si
Pd2dba3•CHCl3 (5 mol %)
NaOH (3 equiv)
O
O
Ph CO2H
+
PhI
THF, 60 °C, 19 h
(2.0 equiv)
7 70%
5a
Scheme 4. Hiyama coupling of 5a with iodobenzene.
Me
Me Si
O
O
In summary, we have found 2-arylsilacyclobutanes transiently
generate carbanions by ring opening when simply dissolved in
polar aprotic solvent like DMF. The nucleophilic species are
capable of capturing carbon dioxide to furnish silalactones. The
present ring opening/carboxyation reaction is unique in that no
additional transition metal catalysts, Lewis acids, nor strong
bases are required.
DMA, MS4Å, RT, 3 h
Me
Si
Me
CO2
(1 atm)
5a, 68%
Me
+
O
O
Ni(cod)2 (10 mol %)
IPr (10 mol %)
Si Me
4a
mesitylene, 160 °C
ref. 19
6a
Scheme 3. Carboxylation of Benzosilacyclobutene 4a.
Experimental Section
Incorporation of CO2 into silacyclobutane 1a. A typical procedure:
MS4Å (50 mg) was placed in a Schlenk tube. The reaction vessel was filled
with CO2 using a balloon, and then were added 1b (34.2 mg, 0.19 mmol)
and DMA (1.0 mL). The reaction mixture was stirred at RT for 10 min. Then,
2 N HCl was added to quench the reaction and the resulting mixture was
extracted with Et2O (three times). The combined organic phase was
washed with H2O (twice) and brine, and then was dried over anhydrous
The arene-fused silacyclobutenes 4b-e listed in Table 3
underwent a nucleophilic addition onto carbon dioxide also at their
benzylic carbon to furnish silalactones 5. Fluorine- (Entry 2) and
methoxy (Entry 3) substituents were well tolerated on the aromatic
ring.
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10.1002/anie.201805333
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COMMUNICATION
MgSO4. After removal of the solvent under a reduced pressure, the residue
was purified by flash column chromatography on diol silica gel
(hexane/ethyl acetate = 2/1) to give 3b (30.2 mg, 0.066 mmol, 68%).
Angew. Chem. 2016, 128, 6427; k) S. Okumura, F. Sun, N. Ishida, M.
Murakami, J. Am. Chem. Soc. 2017, 139, 12414.
[9]
MS4Å suppressed undesired hydrolysis of silacyclobutane substrates
induced by a trace amount of water remaining in the solvent. We assume
the acceleration is also attributed to the removal of water, which
coordinates to the Lewis acidic silicon center to encumber the
coordination of DMF. Analogous acceleration by the removal of water is
described in the direct aldol reaction of enoxysilanes with benzaldehydes,
see: Y. Génisson, L. Gorrichon, Tetrahedron Lett. 2000, 41, 4881
Acknowledgements
This work was supported by JSPS KAKENHI (Grant Numbers
JP15H05756 and JP18H04648), JST ACT-C (Grant
Number JPMJCR12Z9), the Sumitomo Foundation, and the
Fukuoka Naohiko Memorial Foundation.
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Keywords: Carboxylation • carbanions • organosilicon
compounds • strain • CO2
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[6]
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Dr. Naoki Ishida, Dr. Shintaro Okumura,
Tairin Kawasaki, Prof. Dr. Masahiro
Murakami*
Page No. – Page No.
2-Arylsilacyclobutane as a Latent
Carbanion Reacting with CO2
An electronically neutral 2-arylsilacyclobutane generates a nucleophilic carbanion at
room temperature through cleavage of the benzylic carbon–Si bond when simply
dissolved in polar aprotic solvents such as DMF. The nucleophilic species is
capable of capturing carbon dioxide to furnish a silalactone. The carboxylation
reaction is unique in that no additional activating agents are required.
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