Silicone wastes as reducing agents for carbon dioxide transformation: Fluoride-catalyzed formic acid synthesis from CO2, H2O, and disilanes

Article abstract of DOI:10.1246/cl.150654

Disilanes were found to be reactive reducing agents for the transformation of carbon dioxide to formic acid in the presence of H₂O. The reaction is catalyzed by fluoride salts such as tetrabutylammonium fluoride. Isotopic experiments revealed that the proposed reaction pathway includes Si-Si bond cleavage to afford hydrosilane followed by the hydrosilylation of CO₂, and, finally, the hydrolysis of silyl formate.

Full text of DOI:10.1246/cl.150654

CL-150654
Received: July 9, 2015 | Accepted: July 23, 2015 | Web Released: July 31, 2015
Silicone Wastes as Reducing Agents for Carbon Dioxide Transformation:
Fluoride-catalyzed Formic Acid Synthesis from CO2, H2O, and Disilanes
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)
Disilanes were found to be reactive reducing agents for the
the Si-Si bond was investigated by examining the use of fluoride
compounds as homogeneous catalysts.10 Several fluoride com-
pounds also demonstrate activity in the second step, the
hydrosilylation of CO2.11
transformation of carbon dioxide to formic acid in the presence
of H2O. The reaction is catalyzed by fluoride salts such as
tetrabutylammonium fluoride. Isotopic experiments revealed that
the proposed reaction pathway includes Si-Si bond cleavage to
afford hydrosilane followed by the hydrosilylation of CO2, and,
finally, the hydrolysis of silyl formate.
The reaction of 0.5 mmol of tetraphenyldimethyldisilane
with H2O under 1 atm CO2 was examined using catalytic
amounts of fluoride salts such as tetrabutylammonium fluoride
trihydrate (TBAF¢3H2O), as shown in Table 1. When the
amount of H2O was 0.4 mmol, formation of 0.06 mmol of formic
acid was detected (Entry 1). On increasing the amount of H2O
to 2.5 mmol, the amount of formic acid increased to 0.17 mmol
(Entry 4) (Figure S1, SI). The yields of formic acid and
disiloxane were calculated to be 34% and 71%, respectively,
based on the disilane (Entry 4). However, a further increase in
the amount of H2O reduced the amount of formic acid formed
due to the decreasing solubility of disilane (Entries 5 and 6).
Formic acid formation was also detected with other fluoride
salts, such as CsF and KF (Entries 7-9). This is the first report
on the synthesis of formic acid from disilane, CO2, and H2O.
Formic acid was not produced in the absence of disilane, CO2,
or fluoride catalyst (Entries 10-12).
Transformation of CO2 to valuable chemicals is one of the
most important technologies to ensure a sustainable society.1 As
the reductive transformation of CO2 to formates is a straightfor-
ward process for the production of a variety of chemicals, the
selection of an appropriate reducing agent, which is affordable,
abundant, and highly reactive with CO2, is the key to the success
of the transformation system.
Nowadays, most silicone products are synthesized through
direct reaction between metallic Si and organochlorides, the so-
called Müller-Rochow process.2 In this process, the high-boiling
fraction, which is mainly composed of disilanes, is co-produced
and disposed of by incineration.2 However, the electrons of the
Si-Si bond of such “silicone wastes” are potentially useful for
the reduction of CO2 accompanying the production of siloxane.
Recently, Lescot et al. reported a pioneering work, the produc-
tion of CO by the reaction between CO2 and disilanes.3 Another
possible method for the reduction of CO2 with disilanes includes
the formation of the hydrosilane. The reaction of a proton source
such as H2O, with the Si-Si bond would lead to the formation
of the hydrosilane,4 which could react with CO2 to afford silyl
formate (Scheme 1). Lately, metal complexes,5,6 metal nano-
particles,7 and organocatalysts8 have been reported as being
capable of promoting the second step, namely the hydrosilyla-
tion of CO2. The silyl formate then would react with H2O to give
formic acid (Scheme 1).9
Table 1. Formic acid synthesis from CO2 and H2O using
tetraphenyldimethyldisilanea
CO2
H2O
+
Ph Si Si Ph
Ph Ph
+
(1 atm)
(0.50 mmol)
O
C
catalyst (0.05 mmol)
DMSO
+
Ph Si O Si Ph
H
OH
Ph
Ph
Conv
disilane
/%
Yield
disiloxane
/%
H2O
/mmolb
HCOOH
/mmol
Entry Catalyst
Herein, we wish to demonstrate the catalytic synthesis of
formic acid from CO2, H2O, and several disilanes as model
compounds of the silicone wastes. The nucleophilic cleavage of
1
2
3
4
5
6
7
8
9
TBAF¢3H2O
0.4
0.7
1.5
2.5
5.0
10
0.5
0.5
0.6
1.5
0.5
1.5
>99
>99
>99
>99
>99
68
93
95
>99
®
>99e
4
0.06
0.10
0.14
0.17
0.16
0.12
0.04
0.03
0.07
93
85
78
71
60
43
82
79
79
®
TBAF¢3H2O
TBAF¢3H2O
TBAF¢3H2O
TBAF¢3H2O
TBAF¢3H2O
CsF
KF
KHF2
TBAF¢3H2O
TBAF¢3H2O
None
R
HO Si R
R
HOH
R
R
R
R
cat.
R
H
Si
R Si Si R
R
1. Si-Si bond cleavage
R
10c
11d
12
<0.01
<0.01
<0.01
<1
<1
CO2
O
C
O
C
, cat.
H2O
3. Hydrolysis
SiR3
aReaction conditions: disilane (0.50 mmol), CO2 (1 atm, balloon), H2O,
catalyst (5.0 © 10¹2 mmol), DMSO (1 mL), 80 °C, 24 h. bThis value
contains hydrated water in TBAF¢3H2O. cWithout disilane. dUnder Ar
H
O
H
OH
2. Hydrosilylation
Scheme 1. Strategy toward formic acid from CO2, H2O, and
disilane.
e
atmosphere. Polymerization product was detected.
1464 | Chem. Lett. 2015, 44, 1464–1466 | doi:10.1246/cl.150654
© 2015 The Chemical Society of Japan

Table 2. Scope of disilane substratea
(A)
13CO2
+
Ph Si Si Ph
Ph Ph
H2O
+
TBAF-3H2O
(0.05 mmol)
O
(ca. 1 mmol)
(1.3 mmol)
CO2
+ disiloxane
H
+ disilane
C
2O
+
(0.25 mmol)
H
OH
DMSO
(1 atm)
(0.50 mmol)
TBAF•3H2O
O
13C
(0.05 mmol)
H2O
HCOOH
/mmol
Yield
/%
Ph Si O Si Ph
Entry
1
Disilane
+
/mmolb
H
OH
>99% 13C
24% Yield
DMSO (1 mL)
80 oC, 3 h
Ph
Ph
Ph Si Si Ph
Ph Ph
2.5
2.5
0.17
0.08
34
(B)
2c
52c
Ph Si Si Ph
CO2
D
+
Si Si
2O
+
Ph
Ph
Ph
Ph
(1 atm)
(2.4 mmol)
(0.50 mmol)
Si Si
Ph
3
4
3.1
2.9
0.22
44
Ph
TBAF•3H2O
(0.05 mmol)
O
C
<0.01
<1
Si Si
+
Si O Si
Sii D
D
OH(D)
DMSO (1 mL)
80 oC, 24 h
Ph
Ph
Ph
aReaction conditions: disilane (0.50 mmol), CO2 (1 atm, balloon), H2O,
catalyst (5.0 © 10¹2 mmol), DMSO (1 mL), 80 °C, 24 h. bThis value
contains hydrated water in TBAF¢3H2O. Disilane (0.15 mmol).
no deuterium
incorporation
80% D
39% yield
detected
c
(C)
TBAF•3H2O
O
Ph
O
H
C
Other polar solvents were examined for the reaction of
tetraphenyldimethyldisilane with CO2. The yield of formic acid
decreased compared with DMSO: 1,4-dioxane (14%); CH3CN
(9%); THF (6%); 1-propanol (trace).
(0.10 mmol)
CO2
(1 atm)
+
Si H
Ph
+
O Si
C
H
OH
DMSO (2 mL)
80 oC, 7 h
0.72 mmol
0.73 mmol
(2.9 mmol)
The scope of the disilanes is summarized in Table 2. After
the optimization of reaction conditions, yield of formic acid
increased up to 52%. Apart from tetraphenyldimethyldisilane,
tetramethyldiphenyldisilane also showed good reactivity to
afford formic acid in 44% yield. In contrast, the reactivity of
hexamethydisilane was lower than that of disilanes with phenyl
substituents.
H2O addition
O
C
H
OH
1.4 mmol
Scheme 2. Formic acid synthesis using (A) 13CO2, (B) D2O,
and (C) consecutive hydrosilylation of CO2-hydrolysis.
To confirm the transformation of CO2 toward formic acid,
the reaction of 13C-enriched CO2 (99 atom % 13C) was conducted
(Scheme 2A): 1H and 13C NMR measurements indicated the
incorporation of 13C atom into the formic acid product
(Figures S2 and S3, SI). The proton source of the formic acid
product was confirmed by conducting an isotopic experiment
using D2O (>99.8 atom % D). When D2O was used instead of
H2O, the mass analysis revealed that 80% of the available
deuterium was incorporated into the formyl group of formic acid
(Scheme 2B) (Figure S4, SI).12 During the reaction, the for-
mation of a small amount of dimethylphenylsilane-d was also
detected (Figure S5, SI).13 It was also confirmed that fluoride-
catalyzed hydrosilylation of CO2 proceeded (Scheme 2C).11,14
After the addition of H2O to the hydrosilylation reaction
mixture, the obtained silyl formate was quantitatively trans-
formed to formic acid (Figure S6, SI). These results shown in
Schemes 2A-2C support the hypothetical reaction step shown in
Scheme 1, including (i) Si-Si bond cleavage giving hydrosilane,
(ii) hydrosilylation of CO2 with the hydrosilane, and (iii)
hydrolysis of silyl formate affording formic acid.
HOH
HOH
R3Si SiR3
R3Si SiR3
R3 Si
F
R3Si
F
F
R3Si OH
OH
O
C
F
cat.
H
R3Si
H
R3Si
O
CO2
O
1/2 H2O
+ 1/2 (R3Si)2O
C
H
OH
Scheme 3. A possible path for the formation of formic acid
from disilane, H2O, and CO2.
During the reaction of tetraphenyldimethyldisilane, the
formation of diphenylmethylfluorosilane was detected. Based
on the formation of fluorosilane and experiments shown in
Scheme 2, the reaction pathway shown in Scheme 3 is pro-
posed. The initial step involves Si-Si bond cleavage as a result
of a nucleophilic attack by the fluoride anion, affording
fluorosilane and hydrosilane. The hydrosilane then reacts with
CO2 in the presence of the fluoride anion to afford silyl
formate,11 followed by hydrolysis to produce formic acid and
disiloxane. Due to the side reaction between hydrosilane and
H2O to silanol (and disiloxane), the yield of formic acid was not
increased more than 52% (Table 2). As shown in Table 1, the
reaction proceeded with a catalytic amount of fluoride salts;
fluoride anion may regenerate by the reaction between fluoro-
Chem. Lett. 2015, 44, 1464–1466 | doi:10.1246/cl.150654
© 2015 The Chemical Society of Japan | 1465

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CO2
H
+
Ph Si Si Ph
Ph Ph
(0.50 mmol)
2O
+
(1 atm)
(2.5 mmol)
CsOH-H2O (0.06 mmol)
Ph Si F
(0.06 mmol)
Ph
O
C
+ Ph Si O Si Ph
H
OH
Ph
Ph
DMSO (2 mL)
80 oC, 24 h
18% yield
60% yield
Scheme 4. Formic acid synthesis from CO2 and H2O using
CsOH-H2O and fluorosilane.
¹
silane and OH . As shown in Scheme 4, the transformation of
CO2 to formic acid proceeded in the presence of CsOH-H2O
and dimethylphenylfluorosilane, indicating the regeneration of
¹
active F from a fluorosilane.15
In summary, several fluoride salts, especially TBAF, were
found to actively catalyze the synthesis of formic acid from CO2,
H2O, and silanes containing a Si-Si bond. A reaction pathway,
including Si-Si bond cleavage followed by the hydrosilylation of
CO2 and hydrolysis, was proposed. This reaction opens a novel
possibility for developing a chemical process in which abundant
and waste compounds, namely H2O, CO2, and disilanes, are used
as feedstock for obtaining valuable chemical products.
This study was supported by JSPS KAKENHI (grant
Nos. 24686092, 25630362, and 15H04182) and JSPS Grant-
in-Aid for Scientific Research on Innovative Areas “3D Active-
Site Science” (grant No. 26105003).
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6m, 6o, 7.
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10 Fluoride catalyzed silylation using disilanes has been reported,
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14 Without the fluoride salt, formylated products were scarcely
observed.
15 The formic acid formation was scarcely observed with either
CsOH-H2O or diphenylmethylfluorosilane.
3
4
5
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1466 | Chem. Lett. 2015, 44, 1464–1466 | doi:10.1246/cl.150654
© 2015 The Chemical Society of Japan