Reductive transformation of CO2 with hydrosilanes catalyzed by simple fluoride and carbonate salts

Article abstract of DOI:10.1246/cl.150510

Hydrosilylation of CO₂ catalyzed by simple fluoride and carbonate salts, such as CsF and K₂CO₃, is described. Total yields up to 87% for the formylated product were achieved. Mechanistic investigations indicate the reaction proceeds via the formation of an active formate species. This catalytic system was also found to be applicable to formamide synthesis from amines, CO₂, and hydrosilane.

Full text of DOI:10.1246/cl.150510

Received: May 25, 2015 | Accepted: June 7, 2015 | Web Released: June 19, 2015
CL-150510
Reductive Transformation of CO2 with Hydrosilanes
Catalyzed by Simple Fluoride and Carbonate Salts
Ken Motokura, Masaki Naijo, Sho Yamaguchi, Akimitsu Miyaji, and Toshihide Baba*
Department of Environmental Chemistry and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502
(E-mail: tbaba@chemenv.titech.ac.jp)
Hydrosilylation of CO2 catalyzed by simple fluoride and
Table 1. Hydrosilylation of CO2 using fluoridesa
carbonate salts, such as CsF and K2CO3, is described. Total
yields up to 87% for the formylated product were achieved.
Mechanistic investigations indicate the reaction proceeds via the
formation of an active formate species. This catalytic system
was also found to be applicable to formamide synthesis from
amines, CO2, and hydrosilane.
fluoride salt
(0.10 mmol)
O
CO2
(1 atm)
Si
+
Si
+
HCOOH
Ph
H
DMSO (2 mL)
O
Ph
H
60 °C, 24 h
Catalyst
Conv. of silane/%
Yield/%b
CsF
90
96
87
86
50
41
28
7
87
87
69
62
38
26
1
TBAF¢3H2O
KHF2
KF
The reductive transformation of CO2 into valuable chem-
icals has received a great deal of research attention.1 The
hydrosilylation of CO2 to silyl formate, shown in Scheme 1, is
one of the most important of these reactions, as the silyl formate
can be further transformed into a range of useful chemicals, such
as formic acid, amides, and other carbonyl compounds.2 Various
transition-metal complexes,3 metal nanoparticles,4 and organo-
catalysts5 have been reported as effective catalysts for the
hydrosilylation of CO2.
NaF
K3AlF6
AlF3
MgF2
1
aReaction conditions: dimethylphenylsilane (2.9 mmol), carbon diox-
ide (1 atm, balloon), fluoride catalyst (0.10 mmol), DMSO (2 mL),
60 °C, 24 h. bTotal yield of silyl formate and formic acid is reported.
1
Determined by H NMR using mesitylene as an internal standard.
Other possible catalysts for the hydrosilylation of CO2 are
fluoride salts, which provide fluoride anions to strongly interact
with the Si atom, affording penta- or hexacoordinate silicon
intermediates.6 In such intermediates, the nucleophilicity of
hydride ion in the hydrosilane is increased, promoting more
efficient hydride transfer from the hydrosilane to the carbon
atom of CO2. Actually, fluoride-catalyzed hydrosilylation and
silylation of organic compounds, such as aldehydes and
alcohols, with hydrosilanes have been reported.6,7
Another class of compounds that may act as catalysts for the
hydrosilylation of CO2 is carbonate salts. K2CO3 loaded on
alumina was reported as an active catalyst for the hydrosilylation
of benzaldehyde.7e,7f Cui and co-workers also reported the
Cs2CO3-catalyzed reduction of amides, aldehydes, and ketones
with hydrosilane.8,9 A possible mechanism for the activation of
Cs2CO3 with hydrosilane involves the formation of pentacoor-
dinate silicon species to afford formate anions.8 If the formate
anion reacts with hydrosilane, a silyl formate product is
generated.
examined, and the results are presented in Table 1. The
conversion of the hydrosilane and formation of the formylated
products were monitored using 1H NMR analysis of the reaction
mixture (Figure S1, Supporting Information). Hydrolysis of a
part of silyl formate to formic acid occurred due to the presence
of a small amount of water in the NMR sample (Figure S1).10 To
compare the formylation activity of each catalyst, the total yields
of formylated products are shown.
Cesium fluoride and tetrabutylammonium fluoride trihydrate
(TBAF¢3H2O) acted as excellent catalysts for the reaction,
affording the corresponding silyl formate in 87% total yield.11
Other fluoride salts with monovalent cations, such as K and Na,
afford the product in moderate yields. AlF3 and MgF2 were
almost inactive in the reaction. These results indicate that
fluoride salts with large, monovalent counter cations enhance the
hydrosilylation.
To clarify the effect of the halide anion on the hydro-
silylation, reactions using sodium halides were conducted,
as summarized in Table S1 (Supporting Information). Only
sodium fluoride afforded silyl formate product, and other sodium
halides, such as NaCl, were almost inactive in the hydro-
silylation.
Reactions employing carbonate salts with a monovalent
counter cation for the hydrosilylation of CO2 were performed,
as summarized in Table 2. Cesium carbonate and potassium
carbonate showed good catalytic activity, giving maximum
yields of 80 and 83%, respectively. However, product yield was
significantly lower with sodium carbonate. This is due to the
lower solubility of Na2CO3 compared with other carbonate
salts.12
In this paper, the reaction of CO2 with hydrosilane using
simple fluoride and carbonate salts, such as CsF and K2CO3, is
investigated. This strategy is more practical than hydrosilylation
systems using transition-metal complexes, because simple
fluoride and carbonate salts are readily available, cost effective,
and stable.
The hydrosilylation of 1 atm of CO2 with dimethylphenyl-
silane in DMSO using different fluoride salts as the catalyst was
O
catalyst
+
R3Si-H
CO2
R3Si
O
H
Scheme 1. The catalytic hydrosilylation of CO2 to silyl formate.
Chem. Lett. 2015, 44, 1217–1219 | doi:10.1246/cl.150510
© 2015 The Chemical Society of Japan | 1217

Table 2. Hydrosilylation of CO2 using carbonatesa
(A)
carbonate salt
(0.10 mmol)
O
Ph3Si-F (0.10 mmol)
CO2
(1 atm)
O
Si
+
Si
+
HCOOH
CO2
(1 atm)
Si
+
Ph
H
O
Ph
H
DMSO (2 mL)
Si
+
HCOOH
Ph
H
DMSO (2 mL)
O
Ph
H
(2.9 mmol)
60 °C, 24 h
<1% Yield
60 °C, 24 h
(B)
Catalyst
Conv. of silane/%
Yield/%b
Cesium formate
(0.10 mmol)
O
c
Cs2CO3
Cs2CO3
K2CO3
Na2CO3
83
96
84
43
80
72
83
35
+
Si
HCOOH
CO2
(1 atm)
Si
+
Ph
(2.9 mmol)
H
O
Ph
H
DMSO (2 mL)
81% Total Yield
60 °C, 24 h
Scheme 2. Hydrosilylation of CO2 with dimethylphenylsilane using
(A) fluorotriphenylsilane and (B) cesium formate.
aReaction conditions: dimethylphenylsilane (2.9 mmol), carbon diox-
ide (1 atm, balloon), carbonate catalyst (0.10 mmol), DMSO (2 mL),
60 °C, 24 h. bTotal yield of silyl formate and formic acid is reported.
Determined by 1H NMR using mesitylene as an internal standard. c12 h
25 min.
R
R
Si
R
(A)
CO2
R
CO2
H
R
Si
R
R
Si
R
R
R
Si
Table 3. Hydrosilylation of CO2 with silanes using CsFa
O
F
O
H
H
C
O
R
O
H
H
R
Si
CsF (0.10 mmol)
O
O
C
R
Cs
F
+
R3Si-H
CO2
R
R
Cs
F
R3Si
+
HCOOH
O
Cs
O
DMSO (2 mL)
H
(1 atm)
Cs
H
O
60 °C
O
R
Conv. of
silane/%
R
Entry
Hydrosilane
Yield/%b
Si
R
R
O
R
O
Si
1
2
3
PhMe3Si-H
Ph2MeSi-H
Et3Si-H
90
98
28
87
74
12
H
O
R
(B)
R
R
R
R
Cs
R
H
R
Si
Si
O
Si
O
R
aReaction conditions: hydrosilane (Si-H: 2.9 mmol), carbon dioxide
(1 atm, balloon), CsF (0.10 mmol), DMSO (2 mL), 60 °C. Total yield
of silyl formate and formic acid is reported. Determined by H NMR
using mesitylene as an internal standard.
H
R
R
H
b
O
O
1
O
O
O
O
O
Since CsF showed the highest performance among the
simple fluoride and carbonate salts examined, it was used to
investigate the substrate scope of hydrosilanes for the reaction of
CO2, as shown in Table 3. Hydrosilanes with phenyl substitution
showed high reactivity to give the corresponding formylated
product in 74-87% yields. The sterically bulky substrate
diphenylsilane showed high reactivity (Table 3, Entry 2). Since
the catalysts investigated in this study, CsF in particular, are
much smaller than transition-metal complexes with bulky
ligands, these simple fluoride- and carbonate salt-catalyzed
hydrosilylation systems have potential for application in
reactions with bulky substrates. Unfortunately, the reaction
of alkylsilanes, such as triethylsilane, did not proceed well
(Entry 3). This reactivity of silanes is opposite to the reaction
with Pt nanoparticle catalyst,13 suggesting the formation of a
highly polar active species during the reaction.
Because of the similar results observed in the catalytic
hydrosilylation of CO2 using CsF and Cs2CO3, we hypothesized
the formation of the same active species in both systems. In the
case of the fluoride salts, the formation of a quantitative amount
of fluorosilane was detected before the formation of the
formylated products (Supporting Information, Figure S3). To
eliminate the possibility that the fluorosilane acts as an active
species or intermediate, the reaction of CO2 with dimethyl-
phenylsilane using a catalytic amount of fluorotriphenylsilane
(Ph3Si-F) was conducted. As shown in Scheme 2A, the hydro-
silylation reaction barely occurred. This result indicates that
catalytically active species is generated by the reaction between
the fluoride salt and hydrosilane along with fluorosilane.
Scheme 3. Possible reaction pathway for hydrosilylation of CO2
using (A) CsF and (B) Cs2CO3.
1
During the reaction using CsF, a broad H NMR signal was
observed around 8.6 ppm. A similar signal was observed in a
DMSO solution of cesium formate dissolved in CDCl3 (Sup-
porting Information, Figure S4). These observations suggest the
formation of cesium formate during the reaction of CO2 with
hydrosilanes using CsF. We also examined the hydrosilylation
of CO2 with dimethylphenylsilane using a catalytic amount of
cesium formate: the corresponding formylated products were
obtained in 81% yield (Scheme 2B). NMR measurements of the
reaction mixture with 13CO2 revealed incorporation of 13C atom
into the formylated product (1H NMR: ¤ 8.0 ppm, d, J1H-13C
=
214 Hz; 13C NMR: ¤ 163.0 ppm). Overall, the possible activation
route and catalytic cycle using CsF is shown in Scheme 3A.
The proposed pentacoordinate Si species with fluoride anion6
readily reacts with CO2 to afford cesium formate, which further
reacts with the hydrosilane and CO2 to form silyl formate.14
In the case of Cs2CO3, cesium formate can also form through
the formation of a pentacoordinate silicon species,8 before
incorporation into the catalytic cycle (Scheme 3B).
Our simple fluoride salt-catalyzed reaction system can also
be applied to the one-pot synthesis of formamides from amines,
CO2, and hydrosilane.15 Formylation of piperidine using CsF
proceeded smoothly to afford the N-formylpiperidine in 90%
yield (Scheme 4A). N-Methylaniline also performed well as the
substrate to give the corresponding aromatic formamide in 60%
yield (Scheme 4B).
1218 | Chem. Lett. 2015, 44, 1217–1219 | doi:10.1246/cl.150510
© 2015 The Chemical Society of Japan

H
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(A)
CO2
(1 atm)
Si
+
+
Ph
H
(1.0 mmol)
(2.9 mmol)
O
CsF (0.10 mmol)
N
H
+
disiloxane
DMSO (2 mL)
80 °C, 24 h
90% Yield
(B)
H
N
CO2
(1 atm)
Si
+
+
Ph
H
Ph
(1.0 mmol)
(2.9 mmol)
O
CsF (0.10 mmol)
Ph
+ disiloxane
N
H
DMSO (2 mL)
80 °C, 39 h
60% Yield
4
5
Scheme 4. Formamide synthesis from (A) piperidine and (B) N-
methylaniline and CO2 with hydrosilane using CsF.
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In summary, we have found that a catalytic amount of a
simple fluoride and carbonate salts, such as CsF and K2CO3, can
act as an effective catalyst in the hydrosilylation of 1 atm of
CO2. Since such simple fluoride and carbonate salts are readily
available, cost effective, and stable, this finding facilitates the
preparation of silyl formate, formamide, and other related
carbonyl compounds from CO2. A possible reaction route
involves the formation of a pentacoordinate silicon species,
which activates CO2 during the reaction. Detailed mechanistic
investigations as well as the scope and limitations of the simple
fluoride- and carbonate salt-catalyzed reductive transformation
of CO2 are now under investigation.
7
This study was supported by JSPS KAKENHI (grant
Nos. 24686092 and 25630362) and JSPS Grant-in-Aid for
Scientific Research on Innovative Areas “3D Active-Site
Science” (grant No. 26105003).
Supporting Information is available electronically on J-STAGE.
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Chem. Lett. 2015, 44, 1217–1219 | doi:10.1246/cl.150510
© 2015 The Chemical Society of Japan | 1219