Copper-catalyzed formic acid synthesis from CO2 with hydrosilanes and H2O

Article abstract of DOI:10.1021/ol301034j

A copper-catalyzed formic acid synthesis from CO₂ with hydrosilanes has been accomplished. The Cu(OAc)₂?H ₂O-1,2-bis(diphenylphosphino)benzene system is highly effective for the formic acid synthesis under 1 atm of CO₂. The TON value approached 8100 in 6 h. The reaction pathway was revealed by in situ NMR analysis and isotopic experiments.

Full text of DOI:10.1021/ol301034j

ORGANIC
LETTERS
2012
Vol. 14, No. 10
2642–2645
Copper-Catalyzed Formic Acid Synthesis
from CO2 with Hydrosilanes and H2O
Ken Motokura, Daiki Kashiwame, Akimitsu Miyaji, and Toshihide Baba*
Interdisciplinary Graduate School of Science and Engineering, Department of
Environmental Chemistry and Engineering, Tokyo Institute of Technology,
4259 Nagatsuta-cho, Midori-ku, Yokohama, 226-8502, Japan
tbaba@chemenv.titech.ac.jp
Received April 19, 2012
ABSTRACT
A copper-catalyzed formic acid synthesis from CO2 with hydrosilanes has been accomplished. The Cu(OAc)2 H2Oꢀ1,2-bis(diphenylphosphino)-
3
benzene system is highly effective for the formic acid synthesis under 1 atm of CO2. The TON value approached 8100 in 6 h. The reaction pathway
was revealed by in situ NMR analysis and isotopic experiments.
The utilization of CO2 as a renewable chemical feed-
stock has received much attention.1 Reduction of CO2
to formic acid is an attractive transformation reaction.
Various highly active metal catalysts have been reported
for the hydrogenation of CO2 with H2.2 Because the
reaction of CO2 and H2 to HCOOH is thermodynamically
unfavorable, addition of a base, such as KOH, is necessary
to promote the reaction. As a result, the hydrogenation
product becomes a stable formate salt.
proceeds exothermically, giving silyl formate as a product
(Scheme 1a). Silyl formate could be easily decomposed to
formic acid by addition of H2O (Scheme 1b).4 This reac-
tion pathway is a simple approach to formic acid from
CO2. Several reaction systems using transition metals, such
as Ir,5 Ru,6 Zr,7 and an organocatalyst8 have been reported
for the hydrosilylation of CO2. However, to the best of our
knowledge, the one-pot synthesis of formic acid from CO2
and hydrosilane has been scarcely reported.
Reduction of CO2 with hydrosilane is an alternative
methodology to synthesize formic acid. Several hydrosilanes,
such as polymethylhydrosiloxane (PMHS), are cheap,
easy to handle, and readily available as byproducts of the
silicone industry.3 The reaction of CO2 and hydrosilane
Scheme 1. (a) Hydrosilylation of CO2 and (b) Hydrolysis of Silyl
Formate
(1) Aresta, M. Carbon Dioxide as Chemical Feedstock; Wiley: Weinheim,
2010; p 1, and references cited therein.
(2) For examples, see: (a) Langer, R.; Diskin-Posner, Y.; Leitus, G.;
Shimon, L. J. W.; Ye., B.-D.; Milstein, D. Angew. Chem., Int. Ed. 2011,
50, 9948. (b) Tanaka, R.; Yamashita, M.; Nozaki, K. J. Am. Chem. Soc.
2009, 131, 14168. (c) Preti, D.; Resta, C.; Squarcialupi, S.; Fachinetti, G.
Angew. Chem., Int. Ed. 2011, 50, 12551. (d) Schaub, T.; Paciello, R. A.
Angew. Chem., Int. Ed. 2011, 50, 7278. (e) Tsai, J.-C.; Nicholas, K. M.
J. Am. Chem. Soc. 1992, 114, 5117.
€
€
(4) Schafer, A.; Saak, W.; Haase, D.; Muller, T. Angew. Chem., Int.
(3) Jacquet, O.; Gomes, C. D. N.; Ephritikhine, M.; Cantat, T. J. Am.
Chem. Soc. 2012, 134, 2934.
Ed. 2012, 51, 2981.
r
10.1021/ol301034j
Published on Web 04/27/2012
2012 American Chemical Society

The hydrolysis of silyl formate affords silanol as a copro-
duct. Recently, significant efforts have been devoted to the
oxidation of hydrosilanes to silanols9 because of the highly
utility of the oxidation products for silicon-based polymers10
and organic donors.11 The above formic acid synthesis
procedure through consecutive hydrosilylation/hydration
enables oxidation of hydrosilanes to the useful silanols.
Herein, we report a copper-catalyzed formic acid synth-
esis from CO2, hydrosilanes, and H2O through the con-
secutive hydrosilylation of CO2 and hydrolysis of silyl
formate. PMHS can be used as a hydrosilane. Further-
more, oxidation of aromatic, aliphatic, and alkoxy hydro-
silanes to silanols usingCO2 as anoxidant isdemonstrated.
A copper hydride complex having a 1,2-bis(diphenyl-
phosphino)benzene (1) ligand has been reported as a
new, active reduction catalyst for organic carbonyl com-
pounds.12,13 First, we examined the reaction of 1 atm of
To confirm the reaction pathway shown in Scheme 1, 1H
NMR analysis was conducted (Figure S5, Supporting
Information). After the reaction of PMHS with CO2 in
the presence of the Cu catalyst, a signal assignable to Si-H
of PMHS at 4.7 ppm disappeared and a broad signal
assignable to Si-OC(O)H appeared at 8.1 ppm. The methyl
group connected to the Si atom was shifted to 0.4 ppm from
0.2 ppm. Then, addition of H2O to the reaction mixture
allowed formation of formic acid (HCOOH, 8.0 ppm) and
the silyl formate signals (8.1 and 0.4 ppm) disappeared.
GCꢀMS analysis also revealed the formation of formic
acid.
Scheme 2. Copper-Catalyzed Reaction of Diphenylmethylsilane
with C18O2
CO2 in the presence of PMHS, Cu(OAc)2 H2O, and ligand
3
1. An excess amount of CO2 compared with the hydro-
silane was used in a balloon apparatus. As shown in
Table 1, entry 1, the hydrosilylation was complete within
4 h. Next, H2O was added, and the reaction mixture was
allowed to further react at room temperature for 1 h,
affording 91% yield of formic acid. Hydrosilylation did not
proceed at all in the absence of Cu(OAc)2 H2O or ligand 1
(entries 2 and 3). The product was not obtained under Ar
atmosphere instead of CO2 (entry 5). In the Cu(OAc)2
3
3
H2Oꢀ1 system, the formic acid yield increased to 95% at
60 °C (entry 6). Other phosphine ligands were not effective
under the reaction conditions (entries 7 and 8).
Transformation of CO2 to formic acid through silyl
formate was confirmed by the reaction using C18O2 and
13CO2. Scheme 2 shows the results of the reaction using
C18O2. The reaction of diphenylmethylsilane with C18O2
Table 1. Formic Acid Synthesis from CO2 with PMHSa
afforded diphenylmethylsilyl formate with >95% of 18
O
content: m/z [Mþ]: 246, [Mþ ꢀ CH3]: 231 (100), 229 (4.4),
227 (<1.0) (Figure S1, Supporting Information). The
successive hydrolysis of the silyl formate proceeded by
addition of H2O, giving formic acid with >95% of 18O
entry
catalyst
ligand conv of SiꢀHb (%) yieldb (%)
1
Cu(OAc)2 H2O
1
>99
11
8
91
0
3
2
Cu(OAc)2 H2O none
3
(7) Matsuo, T.; Kawaguchi, H. J. Am. Chem. Soc. 2006, 128, 12362.
(8) Riduan, S. N.; Zhang, Y.; Ying, J. Y. Angew. Chem., Int. Ed. 2009,
48, 3322.
3
none
none
1
0
4
none
19
17
96
30
<1
0
5c
6d
7d
8d
Cu(OAc)2 H2O
1
1
0
3
(9) Recent examples: (a) Mitsudome, T.; Arita, S.; Mori, H.; Mizugaki,
T.; Jitsukawa, K.; Kaneda, K. Angew. Chem., Int. Ed. 2008, 47, 7938. (b)
Mitsudome, T.; Noujima, A.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K.
Chem. Commun. 2009, 5302. (c) Ison, E. A.; Corbin, R. A.; Abu-Omar,
M. M. J. Am. Chem. Soc. 2005, 127, 11938. (d) Corbin, R. A.; Ison, E. A.;
Abu-Omar, M. M. Dalton Trans. 2009, 2850. (e) Ishimoto, R.; Kamata, K.;
Mizuno, N. Angew. Chem., Int. Ed. 2009, 48, 8900. (f) Kikukawa, Y.;
Kuroda, Y.; Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2012, 51,
2434.
(10) (a) Chandrasekhar, V.; Boomishankar, R.; Nagendran, S.
Chem. Rev. 2004, 104, 5847. (b) Murugavel, R.; Walawalkar, M. G.;
Dan, M.; Roesky, H. W.; Rao, C. N. R. Acc. Chem. Res. 2004, 37, 763.
(c) Zhou, Q.; Yan, S.; Han, C. C.; Xie, P.; Zhang, R. Adv. Mater. 2008,
20, 2970.
Cu(OAc)2 H2O
95
18
trace
3
3
Cu(OAc)2 H2O dpppe
f
Cu(OAc)2 H2O PPh3
3
a Reaction conditions: CO2 (1 atm, balloom), PMHS (SiꢀH: 1.0
mmol), Cu catalyst (5.0 ꢁ 10ꢀ3 mmol), ligand (7.5 ꢁ 10ꢀ3 mmol), 1,4-
dioxane (2.0 mL), 100 °C, 4 h. After the hydrosilylation, H2O (0.2 mL)
was added, and the mixture was stirred at rt for 1 h. b Determined by 1H
NMR using an internal standard. Based on SiꢀH. Conv of SiꢀH was
measured before the addition of H2O. c Under 1 atm of Ar. d 60 °C,
30 min. e 1,3-Bis(diphenylphosphino)propane. f 1.5 ꢁ 10ꢀ2 mmol of PPh3.
(11) (a) Hirabayashi, K.; Nishihara, Y.; Mori., A.; Hiyama, T.
Tetrahedron Lett. 1998, 39, 7893. (b) Hirabayashi, K.; Kawashima, J.;
Nishihara, Y.; Mori, A.; Hiyama, T. Org. Lett. 1999, 1, 299. (c) Denmark,
S. E.; Werner, N. S. J. Am. Chem. Soc. 2008, 130, 16382.
(5) Eisenschmid, T. G.; Eisenberg, R. Organometallics 1989, 8, 1822.
(6) (a) Jansen, A.; Pitter, S. J. Mol. Catal. A 2004, 217, 41. (b)
Deglmann, P.; Ember, E.; Hofmann, P.; Pitter, S.; Walter, O. Chem.;
ꢀ
ꢁ
ꢀ
(12) (a) Baker, B. A.; Boskovic, Z. V.; Lipshutz, B. H. Org. Lett. 2008,
10, 289. (b) Lipshutz, B. H. Synlett 2009, 509.
€
Eur. J. 2007, 13, 2864. (c) Suss-Fink, G.; Reiner, J. J. Organomet. Chem.
1981, 221, C36. (d) Koinuma, H.; Kawakami, F.; Kato, H.; Hirai, H.
€
J. Chem. Soc., Chem. Commun. 1981, 213. (e) Jansen, A.; Gorls, H.;
(13) For Stryker’s reagent, see: (a) Mahoney, W. S.; Brestensky,
D. M.; Stryker, J. M. J. Am. Chem. Soc. 1988, 110, 291. (b) Mahoney,
W. S.; Stryker, J. M. J. Am. Chem. Soc. 1989, 111, 8818.
Pitter, S. Organometallics 2000, 19, 135. (f) Jessop, P. G. Top. Catal.
1998, 3, 95.
Org. Lett., Vol. 14, No. 10, 2012
2643

content: m/z [Mþ]: 50, [Mþ
ꢀ
18OH]: 31, and diphenyl-
methylsilanol with <1% of 18O (Figure S2 and S3,
Supporting Information). This result indicates that all
the oxygen atoms in the formic acid product originated
from CO2. The use of 13CO2 instead of C18O2 afforded
formic acid with >95% of 13C content: m/z [Mþ]: 47,
[Mþ ꢀ OH]: 30 (Figure S4, Supporting Information).
Thus, the carbon atom of CO2 is also incorporated into
the formic acid.
The present system was applicable to 50 mmol scale
synthesis of formic acid from CO2. The reaction of
50 mmol of SiꢀH in PMHS occurred in the presence of
5 μmol of Cu catalyst under 1 atm of CO2 to afford an 81%
yield of formic acid (eq S1, Supporting Information). The
TON (turnover number) value was calculated to be 8100
(reaction time: 6 h). This value is much higher than those of
reported catalysts for hydrosilylation of CO2, such as
RuCl3 nH2O6a and Ru2Cl5(MeCN)7,6a as shown in Table 2.
3
It has been reported that the reaction between Cu(OAc)2
3
H2O and PMHS in the presence of 1 affords the Cu(I)ꢀ
hydride complex having ligand 1.12 In order to clarify the
formation of the Cu(I)ꢀH species and the catalytic CO2
hydrosilylation pathway, in situ NMR analysis was con-
Figure 1. 13C{1H} NMR spectra of the reaction between 13CO2
and PMHS in the presence of the copper catalyst. Reaction
conditions: Cu(OAc)2 H2O (Cu: 5.0 ꢁ 10ꢀ2 mmol), 1 (7.5 ꢁ
3
10ꢀ2 mmol), PMHS (SiꢀH: 1.1 mmol), and benzene-d6 (1.5
mL), Ar, rt, 1 h. Then, 0.4 mL of the reaction mixture was
transferred to the NMR tube under Ar. After the measurement
of the spectrum (a), the sample was treated with 13CO2 at room
temperature. Spectra bꢀe are (b) 2, (c) 10, (d) 20, and (e) 60 min
after the treatment with 13CO2. The spectra in bꢀe show the
conversion of 13CO2 (*) to silyl formate (0) thorough copper
formates (b, O).
ducted in the presence of Cu(OAc)2 H2O, 1 (1.5 equiv), and
PMHS (22 equiv of SiꢀH to Cu) in benzene-d6 solution.14
1
peared in the H NMR spectrum at 1.42 and 3.57 ppm
3
After the reaction at room temperature, new signals ap-
(Figure S6, Supporting Information). These signals are
assignable to the reported 1ꢀCu(I)ꢀH complex and a
presumed analogue of Stryker’s reagent,12 respectively,
indicating the formation of Cu(I)ꢀH complexes having
ligand 1. The NMR sample was then treated with 13CO2
at room temperature. The intensity of these Cu(I)ꢀH
signals decreased, indicating a reaction between CuꢀH
and CO2. Figure 1 shows the 13C{1H} NMR spectra of
the solution before and after the addition of 13CO2. After the
addition of 13CO2, new signals appeared immediately at 124,
159, and 168 ppm, which were assigned to CO2, SiꢀOC(O)-
H product, and a copper formate complex,15 respectively
(Figure 1b). After 10 min, the signals of CO2 and the copper
formate disappeared, whereas the signal of the SiꢀOC(O)H
increased (Figure 1c). This result indicates the formation of
Table 2. List of Catalysts for Hydrosilylation of CO2
Figure 2. 1H NMR spectra of the reaction between 13CO2
and PMHS in the presence of the copper catalyst (a) 5 and
(b) 180 min after treatment with 13CO2. Reaction conditions are
shown in Figure 1.
catalyst
CO2 (atm) time (h) TON
ref
Cu(OAc)2 H2O
1
6
8100 this work
980 6a
3
RuCl3 nH2O
69ꢀ87
20
3
Ru2Cl5(MeCN)7
[N(PPh3)][HRu3(CO)11
RuCl2(PPh3)3
14
2 ꢁ 10a 4619a 6a
]
50
24
20
292
14
6c
6d
6f
8
silyl formate from CO2 through the copper formate com-
plex. Additionally, a new signal appeared at 169 ppm
which is assignable to another copper formate complex.
30
[RuH2(PMe3)4]
200ꢀ220
62
N-heterocyclic carbeneb
zirconium complexc
[Ir(CN)(CO)(dppe)]
1
1
1
72
1840
225
2.3
1.5
7
5
(14) The copper-catalyzed hydrosilylation of CO2 effectively pro-
ceeded in benzene solvent as well as 1,4-dioxane.
a Total TON value is reported during the 10 times reuse experiment.
b For methanol synthesis. c For methane synthesis.
(15) The 13C chemical shift of a previously reported Cu(I) formate
€
complex was 168.5ppm; see: Lang, H.; Shen, Y.; Ruffer, T.; Walfort, B.
Inorg. Chim. Acta 2008, 361, 95.
2644
Org. Lett., Vol. 14, No. 10, 2012

The high utility of the silanols, the coproduct of the
formic acid synthesis, encouraged us to examine the
transformation of several hydrosilanes to silanols with
the Cu catalyst using CO2 as an oxidant. Diphenyl-
methylsilane reacted with CO2 to afford 90% yield of
the corresponding silanol accompanied by 87% yield of
formic acid (eq 1).18 Oxidation of tetramethyldisiloxane
also proceeded, affording 73% yield of disilanol and
94% yield of formic acid (eq 2). The reaction of triethyl-
silane and triethoxysilane also occurred. The results
are summarized in Table S1 (Supporting Information).
Unfortunately, this catalyst system is not applicable
toward hydrosilanes having carbonꢀcarbon double
bond and alkylchloride functions due to side reactions,
such as hydrogenation.
Scheme 3. Proposed Catalytic Reaction Pathway
After 60 min, the second copper formate decreased and the
SiꢀOC(O)H species increased (Figure 1e). This result in-
dicates that both of these copper formate species are inter-
mediates to the silyl formate product. The formation of two
copper formates is also supported by the 1H NMR analysis:
the sample after the addition of 13CO2 showed two broad
doublet signals centered at 8.9 and 9.2 ppm with 13CꢀH
coupling constants of 191.0 and 192.3 Hz, respectively
(Figure 2a).16 These coupling constant values are compar-
able to the reported Rh formate species (204.6 Hz).2e After
180 min, only one, sharp doublet signal centered at 9.2 ppm
(192.3 Hz) was observed (Figure 2b).17 The correlation
between the 1H NMR doublet resonance and the 13C
In summary, this study has provided the first copper-
catalyzed formic acid synthesis from CO2.19 The Cu(OAc)2
3
H2Oꢀ1 system is highly effective for the formic acid
synthesis under 1 atm of CO2. The catalytic reaction
pathway was revealed by in situ NMR analysis and
isotopic experiments using C18O2 and 13CO2. Several
hydrosilanes were converted to the corresponding silanols
in high yields accompanied by formic acid.
1
NMR signal at 169 ppm was confirmed by 13Cꢀ H corre-
lation spectroscopy analysis (Figure S8, Supporting
Information). On the basis of these results, these two copper
formates are suggested to be an unstable and reactive
monodentate [Cu(η1-OCHO)] species and a relatively stable
bidentate [Cu(η2-OCHO)] complex. The proposed reac-
tion pathway involving two copper formates is shown in
Scheme 3. As shown in Figure 1, the 13C NMR signal of the
monodentate species rapidly disappeared due to the CO2
consumption and thermodynamic equilibrium. The forma-
tion of two equilibrium Rh formate species was also
reported.2e The monodentate Rh formate species was also
detectable only under high pressure CO2.2e
Acknowledgment. This work was supported by a
Grant-in-Aid for Young Scientists (A) (24686092) and
The UBE Foundation. The authors thank Center for
Advanced Materials Analysis (Suzukakedai), Technical
Department, Tokyo Institute of Technology.
Supporting Information Available. Experimental de-
tails and spectra. This material is available free of charge
via the Internet at http://pubs.acs.org.
(16) Two broad singlet signals at 9.0 and 9.2 ppm were detected by 1H
NMR analysis after treatment with 12CO2.
(19) Formic acid derivatives (formamides) have been made via for-
mic acid by CO2 hydrogenation with heterogeneous and homogeneous
copper catalysts; see: (a) Farlow, M. W.; Adkins, H. J. Am. Chem. Soc.
1935, 57, 2222. (b) Haynes, P.; Slaugh, L. H.; Kohnle, J. F. Tetrahedron
Lett. 1970, 11, 365.
(17) In proton-coupled 13C NMR analysis, a doublet signal centered
at 169 ppm with a 13CꢀH coupling constant of 192.4 Hz was observed
(Figure S7, Supporting Information), indicating the carbon atom has a
single, directly bonded H (formate species).
(18) The yields of formic acid and silanol from C16O2 are much higher
than those of the reaction of C18O2 shown in Scheme 2, because a large
excess of C16O2 could be used in the balloon.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 10, 2012
2645