Visible-Light-Induced Catalyst-Free Carboxylation of Acylsilanes with Carbon Dioxide

Article abstract of DOI:10.1021/acs.orglett.1c00435

Intermolecular carbon-carbon bond formation between acylsilanes and carbon dioxide (CO2) was achieved by photoirradiation under catalyst-free conditions. In this reaction, siloxycarbenes generated by photoisomerization of the acylsilanes added to the C═O bond of CO2 to give α-ketocarboxylates, which underwent hydrolysis to afford α-ketocarboxylic derivatives in good yields. Control experiments suggest that the generated siloxycarbene is likely to be from the singlet state (S1) of the acylsilane and the addition to CO2 is not in a concerted manner.

Full text of DOI:10.1021/acs.orglett.1c00435

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Letter
Visible-Light-Induced Catalyst-Free Carboxylation of Acylsilanes
with Carbon Dioxide
Zhengning Fan, Yaping Yi, Shenhao Chen, and Chanjuan Xi*
Cite This: Org. Lett. 2021, 23, 2303−2307
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ABSTRACT: Intermolecular carbon−carbon bond formation between acylsilanes and carbon dioxide (CO2) was achieved by
photoirradiation under catalyst-free conditions. In this reaction, siloxycarbenes generated by photoisomerization of the acylsilanes
added to the CO bond of CO2 to give α-ketocarboxylates, which underwent hydrolysis to afford α-ketocarboxylic derivatives in
good yields. Control experiments suggest that the generated siloxycarbene is likely to be from the singlet state (S1) of the acylsilane
and the addition to CO2 is not in a concerted manner.
cylsilane derivatives, as first synthesized by Brook,1
the siloxycarbene to alkynes or alkenes is reported by Bolm
(Scheme 1b) to give silylated acrylic ketones.7 In spite of that,
Aexhibit distinctive photochemical reactivity due to the
only limited reports have appeared, to date, utilizing the
photochemically generated siloxycarbenes for C−C bond
formation.8 To the best of our knowledge, the reaction of
light-derived nucleophilic siloxycarbenes with CO2 has not
been reported (Scheme 1c).
abnormal inductive effect of the silicon atom and their long
n−π* absorption (380−420 nm).2 The acylsilanes could
undergo a 1,2-silyl shift, which is similar to the well-known
Brook rearrangement, to generate nucleophilic siloxycarbenes
under photolysis3 or high temperature (>250 °C).4 The light-
derived nucleophilic siloxycarbenes are capable of insertion
into a wide range of X−H bonds, including O−H, halogen−H,
B−H, and Si−H bonds (Scheme 1a).3a,5 In addition, Kusama
and co-workers reported the cross-coupling between the
acylsilanes and organoboronic esters through a formal B−C
bond insertion intermediate and following a rearrangement to
afford ketones.6 Recently, intra- and intermolecular addition of
CO2 is an ideal one-carbon source with a cheap, nontoxic,
and abundant nature.9 Chemists have been challenged to
create catalytic C−C bond-forming avenues to carboxylic
acids, which are privileged motifs in a mass of molecules
displaying significant biological properties.10 Although consid-
erable advances have been realized, the catalytic synthesis of
valuable carboxylic acids from CO2 remains limited to the use
of either metal catalysts with stoichiometric reductants11 or the
use of photoredox catalysts.12,13 To the best of our knowledge,
there are very few reports of photoinduced catalyst/reductant-
free carboxylation with CO2.14 Recently, Murakami reported a
photoinduced direct carboxylation of o-alkylphenyl ketones
with CO2, which undergoes a photoenolization step and [4 +
2] cycloaddition.15 However, the employment of energetic UV
light is almost compulsory. With our continuous interest in C−
C bond construction with CO2, we envisaged that the visible-
Scheme 1. Photochemical Reaction Behavior of Acylsilanes
Received: February 5, 2021
Published: March 8, 2021
https://dx.doi.org/10.1021/acs.orglett.1c00435
© 2021 American Chemical Society
Org. Lett. 2021, 23, 2303−2307
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light-generated siloxycarbene could capture CO2, which is
more challenging than reacting with ketone and aldehyde to
afford carboxylic acids.8b,c Herein, we would like to report the
first visible-light-induced catalyst-free carboxylation of acylsi-
lanes under atmospheric CO2. A series of α-ketoesters can be
obtained under simple conditions in moderate to high yields
after esterification.
We initiated our studies by irradiating 4-methylbenzoyl-
trimethylsilane (1a) and CO2 under irradiation with blue
LEDs (Table 1). After systematic exploration of reaction
detection of 2a or 3a without illumination proves the
indispensable role of light (entry 12). A decreased yield was
observed without Cs2CO3 (entry 13), and no 2a was obtained
when the reaction was carried out without CO2, ruling out the
possibility of Cs2CO3 as additional C1 source (entry 14). The
reaction was also viable in the presence of CsF without
illumination (entry 15) and removal of Cs2CO3 hardly affected
the yield, which suggests that the phototransformation of
acylsilane undergoes a different route from F-mediated thermal
protocol (entry 16).16
With the optimal conditions in hand, we subsequently
commenced the evaluation of substrate generality. As is shown
in Scheme 2, a number of para-substituted electron-rich and
a
Table 1. Optimization of Reaction Conditions
a
Scheme 2. Substrate Exploration of Acylsilanes
b
b
yield of 2a
yield of 3a
entry
deviation
(%)
(%)
1
2
3
4
5
None
75 (69)
49
30
59
52
6
7
4
9
10
6
MeCN instead of DMSO
DCE instead of DMSO
DMF instead of DMSO
DMA instead of DMSO
0.1 M instead of 0.05 M
c
6
50
7
8
3.0 equiv of Cs2CO3 instead of
2.0 equiv
1.0 equiv of Cs2CO3 instead of
2.0 equiv
69
3
43
10
9
1.0 equiv of CsF
68
48
57
ND
49
ND
58
52
6
4
12
ND
8
48
6
5
10
11
12
13
14
15
16
12 h instead of 24 h
40 °C instead of rt
without light
without Cs2CO3
without CO2
1.0 equiv of CsF, without light
1.0 equiv of CsF, without Cs2CO3
and light
a
Reaction conditions: 1a (0.1 mmol), DMSO (2.0 mL), 1 atm of
CO2, 5 W blue LED, rt, 24 h, workup, then TMSCHN2 (2.0 equiv in
b
0.5 mL of MeOH/Et2O, v/v = 1:1). Yields determined by crude 1H
NMR using CH2Br2 as internal standard, isolated yield in parentheses.
c
Diphenylethanedione was observed. ND = not detected.
a
Reaction conditions: 1a (0.2 mmol), Cs2CO3 (2.0 equiv), DMSO
(4.0 mL), 1 atm of CO2, 5 W blue LED, rt, 24 h, quenched by HCl (2
N), workup, then TMSCHN2 (2.0 equiv in 1.0 mL of MeOH/Et2O,
v/v = 1:1). Isolated yield. −SiPh2Me instead of −TMS in acylsilane.
conditions, compound 2a was detected in 75% yield in the
presence of 2 equiv of Cs2CO3 within 0.05 M DMSO
(dimethyl sulfoxide) at room temperature for 24 h (entry 1).
Screening of other solvents such as acetonitrile (MeCN), 1,2-
dichloroethane (DCE), N,N-dimethylformamide (DMF), and
N,N-dimethylacetamide (DMA) suggested that DMSO is the
optimal solvent (entries 1−5). Increasing the concentration to
0.1 M resulted in a decreased yield of 2a. In this case,
diphenylethanedione was observed (entry 6). The amount of
additive also has a non-negligible effect on the reaction. The
yield of 2a was not further increased with the addition of 3
equiv of Cs2CO3 (entry 7), while reducing it to 1 equiv also led
to inferior yield (entry 8). Considering that fluoride may
facilitate the removal of the trimethylsilyl group (−TMS), CsF
was chosen as a coadditive, but the yield of 2a was not
improved (entry 9). Although the fade of solution color was
detectable after the reaction proceeding in 30 min, shortening
the reaction time to 12 h led to 48% yield (entry 10),
indicating that the addition of the siloxycarbene to CO2 might
be the rate-determining step. Elevating the temperature to 40
°C gave the desired product 2a in 57% yield (entry 11). No
b
electron-neutral benzoyl trimethylsilanes gave the desired
products (2a−2g) in good yields. When −TMS is replaced
by more sterically hindered −SiPh2Me in the acylsilane 1b, the
yield of 2b drops significantly, which indicates that the steric
effect is obvious in the 1,2-silyl shift process. Then examination
of substrates bearing halides (1h, 1i, and 1j) proved that
fluorine and chlorine are well-tolerated in the protocol,
indicating their potential in late-stage functionalization.
However, strong electron-withdrawing groups, such as
−CO2Me and −CF3, failed to afford the desired products, in
which the corresponding benzaldehydes and benzoates were
detected as byproducts. This observation manifests that the
reactivity of substrates is strongly influenced by the
nucleophilicity of the in situ generated siloxycarbenes. We
then turned our attention to other sites of the aryl motif.
Substrates with methyl (1k) and methoxy (1l) group on ortho-
position of the benzene ring exhibited moderate reactivity. The
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lower yield of 2l agreed with the conclusion that a larger
substituted group may impede the rearrangement of the silyl
group, despite its stronger electron-donating ability. Mean-
while, we found that acylsilanes containing functional groups at
meta-position (1m and 1n) also proceed smoothly under the
protocol. Afterward, we examined disubstituted acylsilanes,
such as (3,5-dimethylphenyl)(trimethylsilyl)methanone (1o)
and (3-fluoro-4-methylphenyl)(trimethylsilyl)methanone
(1p), and the corresponding carboxylic esters were isolated
in 80% (for 2o) and 76% yields (for 2p), respectively. What’s
more, α- and β-substituted naphthalene derivatives (1q and
1r) were well-accommodated where no discrepancy in site-
activity was observed. Lastly, we replaced the benzene ring
with other electron-rich heterocycles, such as thiophene and
furan, and the desired carboxylated product (2s) was acquired
in 38% yield while methyl 2-(furan-2-yl)-2-oxoacetate (2t) was
obtained in 18% yield. When an alkyl acylsilane was applied,
such as 3-phenylpropanoyltrimethylsilane, a complex mixture
was observed, which may due to the less stability of the formed
alkylsiloxycarbene than arylsiloxycarbene.
that the reaction is not likely to proceed in a radical pathway.
To clarify from which state of the excited acylsilane does the
siloxycarbene originate, the reaction was carried out with the
addition of a triplet-sensitizer, trans-stilbene (E-5). It is
noteworthy that 1 equiv of E-5 (ET = 49.3 kcal/mol) does
not inhibit the reaction and only a small amount of the olefin is
converted to its Z-isomer (E: Z = 12.5:1) (Scheme 3d). These
results indicate that the siloxycarbene is more likely to be
generated from S1 state of 1a,18 although minor formation of
the Z-isomer suggests that the T1 state is not totally forbidden.
UV−vis spectroscopy (see the Supporting Information) shows
that Cs2CO3 does not affect the absorption spectra of 1a. As a
result, the additive mainly functions to stabilize the formed
siloxycarbene and enhances its nucleophilicity. In addition, the
formed cesium carboxylate also precipitates, which will push
the reaction forward.
Based on the above results and previous reports, a plausible
mechanism is depicted in Scheme 4. Initially, the acylsilane 1 is
Scheme 4. Proposed Mechanism
To gain more insight into the mechanism, control
experiments were carried out to elucidate the reaction pathway
(Scheme 3). According to literature, addition of the formed
Scheme 3. Mechanistic Studies
irradiated to its singlet excited state 1*, which undergoes an
1,2-silyl shift to generate the singlet siloxycarbene intermediate
I and/or its resonance form I′.3a,8a Subsequent nucleophilic
attack to the CO bond of CO2 generates the cesium
carboxylate 4 in the presence of Cs2CO3. The presence of
cesium carboxylate 4 has been confirmed by MALDI-TOF MS
analysis. A cation exchange between Cs+ and the −TMS group
exists in the system. Hydrolysis of the intermediate 4 affords
the desired α-keto carboxylic acid 2′ and trimethylsilanol 6.
The existence of 6 is verified by its condensation on GC−MS.
Additionally, the formation of byproduct benzoic acid 3′ can
be rationalized by the insertion of residual I to water after
workup.6a Aldehyde 7 is generated from protonation of I′ after
hydrolysis, which is confirmed by deuterium-labeling experi-
ments (50% yield, 92% D incorporation).16c
In conclusion, we have developed a novel visible-light-
induced carboxylation of acylsilanes with CO2. The reaction
provides a new reaction mode for siloxycarbene, where
addition to more inert carbonyls is realized, providing 1,2-
dicarbonyl compounds under mild conditions. In the 1,2-silyl
shift process, the siloxycarbene is more likely to occur from the
S1 state of the acylsilane. Further efforts on developing intrigue
photoinduced catalyst-free carboxylation with CO2 as a C1
source are currently in progress.
siloxycarbene to acetaldehyde generates an oxirane, which
reacted with MeMgBr to afford the corresponding carbinol.8b
Based on this result, we treated a reaction mixture generated
from the reaction of 1a with CO2 under blue LED for 12 h
with 1 equiv of PhMgBr. Product 2a and 3b were obtained in
40% and 100% yield, respectively (Scheme 3a). The
observation rules out the possibility of direct siloxycarbene
addition to CO2 to form a 3-membered lactone, which is not
consistent with previous work.14 To further gain the
intermediate of the reaction, we treated 1a with CO2 in
acetonitrile under the standard conditions (Scheme 3b). After
completion of the reaction, the supernatant of the crude
mixture was characterized by NMR and GC−MS, and the
target carboxylic acid was not observed. The residual insoluble
precipitate, meanwhile, was tested by MALDI-TOF MS. The
analytic results exhibit molecular weight of mono- and
dicesium salt 4a as shown in Scheme 3b. Moreover, treating
the precipitate with dilute HCl gave 2a′, proving its role as the
key intermediate. Being aware that homolytic cleavage of the
C−Si bond may potentially result in an acyl radical,17 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO) was incorporated as a
radical scavenger in the reaction and the product 2a was
obtained in 72% yield (Scheme 3c). This outcome manifests
ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.1c00435.
All experimental procedures, compound characterization
data, copies of spectra, UV−visible spectroscopy of 1a,
and reaction setup (PDF)
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Letter
́
̀
acylsilane, le benzoyltrimethyl silane, et a son homologue carbone, la
AUTHOR INFORMATION
Corresponding Author
■
́
pivalophenone. J. Organomet. Chem. 1973, 60, 49−54. (d) Ye, J.-H.;
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Carbene Insertion into B−H Bonds between Acylsilanes and
Pinacolborane. J. Am. Chem. Soc. 2019, 141, 16227−16231.
Chanjuan Xi − MOE Key Laboratory of Bioorganic
Phosphorus Chemistry & Chemical Biology, Department of
Chemistry, Tsinghua University, Beijing 100084, People’s
Republic of China; State Key Laboratory of Elemento-
Organic Chemistry, Nankai University, Tianjin 300071,
People’s Republic of China; orcid.org/0000-0002-9602-
7309; Email: cjxi@tsinghua.edu.cn
(6) (a) Ito, K.; Tamashima, H.; Iwasawa, N.; Kusama, H.
Photochemically Promoted Transition Metal-Free Cross-Coupling
of Acylsilanes with Organoboronic Esters. J. Am. Chem. Soc. 2011,
133, 3716−3719. (b) Ishida, K.; Yamazaki, H.; Hagiwara, C.; Abe, M.;
Kusama, H. Efficient Generation and Synthetic Applications of Alkyl-
Substituted Siloxycarbenes: Suppression of Norrish-Type Fragmenta-
tions of Alkanoylsilanes by Triplet Energy Transfer. Chem. - Eur. J.
2020, 26, 1249−1253.
Authors
Zhengning Fan − MOE Key Laboratory of Bioorganic
Phosphorus Chemistry & Chemical Biology, Department of
Chemistry, Tsinghua University, Beijing 100084, People’s
Republic of China
Yaping Yi − MOE Key Laboratory of Bioorganic Phosphorus
Chemistry & Chemical Biology, Department of Chemistry,
Tsinghua University, Beijing 100084, People’s Republic of
China
Shenhao Chen − MOE Key Laboratory of Bioorganic
Phosphorus Chemistry & Chemical Biology, Department of
Chemistry, Tsinghua University, Beijing 100084, People’s
Republic of China
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F.; Bolm, C. Photochemically Induced Silylacylations of Alkynes with
Acylsilanes. Adv. Synth. Catal. 2012, 354, 2157−2161. (b) Becker, P.;
Priebbenow, D. L.; Zhang, H.-J.; Pirwerdjan, R.; Bolm, C. Photo-
chemical Intermolecular Silylacylations of Electron-Deficient Internal
Alkynes. J. Org. Chem. 2014, 79, 814−817. (c) Becker, P.; Pirwerdjan,
R.; Bolm, C. Acylsilanes in Iridium-Catalyzed Directed Amidation
Reactions and Formation of Heterocycles via Siloxycarbenes. Angew.
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(8) (a) Brook, A. G.; Kucera, H. W.; Pearce, R. Photolysis of 1,1-
Diphenylsilacyclohexanone in Diethyl Fumarate. The Trapping of a
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49, 1618−1621. (b) Brook, A. G.; Pearce, R.; Pierce, J. B.
Nucleophilic Attack of Siloxycarbenes on Carbonyl Groups. The
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Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c00435
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos 21871163, 22071134, and 21911530097). We also thank
Dr. Prof. Wei He (School of Pharmaceutical Sciences,
Tsinghua University) for helpful discussions.
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