Iron-catalyzed hydrosilylation of CO2: CO2 conversion to formamides and methylamines

Article abstract of DOI:10.1039/c4cy00130c

Catalytic hydrosilylation of CO₂ is an efficient and selective approach to form chemicals. Herein, we describe the first iron catalysts able to promote the reductive functionalization of CO₂ using hydrosilanes as reductants. Iron(ii) salts supported by phosphine donors enable the conversion of CO₂ to formamide and methylamine derivatives under mild reaction conditions. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00130c

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Iron-catalyzed hydrosilylation of CO2: CO2
conversion to formamides and methylamines†
Cite this: Catal. Sci. Technol., 2014,
4, 1529
*
Xavier Frogneux, Olivier Jacquet and Thibault Cantat
Received 31st January 2014,
Accepted 15th February 2014
DOI: 10.1039/c4cy00130c
www.rsc.org/catalysis
Catalytic hydrosilylation of CO2 is an efficient and selective
approach to form chemicals. Herein, we describe the first iron
catalysts able to promote the reductive functionalization of CO2
using hydrosilanes as reductants. Iron(II) salts supported by
phosphine donors enable the conversion of CO2 to formamide
and methylamine derivatives under mild reaction conditions.
methylamines (Scheme 1).6 These new advances have moti-
vated the search for novel efficient catalysts able to facilitate
the hydrosilylation of CO2.5b–e,g,i,j,7 From another standpoint,
remarkable efforts have recently demonstrated the potential
of iron complexes as earth abundant and cost efficient metal
catalysts in hydrosilylation reactions.8 For example, Sortais,
Darcel et al. have utilized well-defined iron carbene complexes
for the chemoselective reduction of esters to aldehydes.9 In
2009, Beller et al. and Nagashima et al. showed independently
that iron carbonyl complexes were potent hydrosilylation cata-
lysts for the reduction of amides to amines.10 Recently, our
group reported the first examples of urea reduction to
formamidines, using iron complexes as hydrosilylation
catalysts.3k Yet, so far, iron catalysts have never been utilized
in CO2 hydrosilylation reactions and, herein, we describe the
first iron complexes able to promote the reductive
functionalization of CO2 using hydrosilanes. In this contribu-
tion, FeII salts supported by phosphine donors are shown to
catalyze the conversion of CO2 to formamide and methylamine
derivatives under mild reaction conditions.
Catalytic hydrosilylation reactions are attractive alternatives
to classical reduction methods with hydrogen or metal
hydrides because they usually operate under mild conditions
with superior chemoselectivity.1 Indeed, hydrosilanes possess
a reduction potential similar to H2 and a Si–H bond that is
kinetically more reactive because of its polarity and lower bond
dissociation energy (92 kcal mol−1 in SiH4 vs. 104 kcal mol−1
in H2).2 In addition, they circumvent the problematic
sensitivity of aluminium and boron hydrides to moisture. As a
result, catalytic hydrosilylation can achieve highly chemo- and
regio-selective transformations of a wide range of carbonyl
groups such as ketones, carboxylic acids, esters, amides
and ureas.3 Importantly, in 1981, Hirai et al. extended
hydrosilylation strategies to reduce CO2 using RuCl2(PPh3)3 as
a catalyst,4 and a variety of organic and organometallic
catalysts have been shown to promote the direct hydrosilylation
of CO2 since then.5 CO2 reduction to formic acid and methanol
has limited economical interest because these molecules are
produced at low cost and on large scales that are incompatible
with the availability of hydrosilanes. In contrast, CO2 conver-
sion to fine and bulk chemicals has the advantage of creating
added value for niche applications. In this respect, the unique
reducing properties of hydrosilanes have been exemplified,
over the last 4 years, with the design of novel catalytic transfor-
mations to convert CO2 to carboxylic acids, formamides and
Using CO2 and hydrosilanes for the formylation of amines
affords an attractive route to formamides and this transfor-
mation was unveiled for the first time in 2012 in our
CEA, IRAMIS, SIS2M, CNRS UMR 3299, CEA/Saclay, 91191 Gif-sur-Yvette, France.
E-mail: thibault.cantat@cea.fr; Fax: +33 1 6908 6640; Tel: +33 1 6908 4338
† Electronic supplementary information (ESI) available: General experimental
details, synthetic procedures and data for 2r and 2v. See DOI: 10.1039/
c4cy00130c
Scheme
1 Reductive functionalization of CO2 to α,β-unsaturated
carboxylic acids, formamides and methylamines using hydrosilane
reductants.
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laboratories (eqn (2) in Scheme 1).6b,d This catalytic reaction
was found to be robust and a large scope of N–H bonds in
amines, anilines, hydrazines and N-heterocycles were success-
fully formylated with hydrosilanes, such as PhSiH3, Ph2SiH2,
(EtO)3SiH or polymethylhydrosiloxane (PMHS). Interestingly,
while organic catalysts (guanidines and N-heterocyclic
carbenes (NHCs)) were originally utilized, Baba et al. showed
that copper(II) diphosphine complexes were also active
catalysts in this transformation.7 As such, the formylation
of N-methylaniline (1a) with CO2 and phenylsilane was
selected as a benchmark reaction to test the catalytic activity
of a variety of iron complexes in CO2 hydrosilylation. In the
presence of a catalytic amount of FeCl2, FeCl3, Fe(SO4)·7H2O,
Fe(acac)2 or Fe(acac)3 (5.0 mol%), addition of 1 equiv. of PhSiH3
to a THF solution of 1a under an atmosphere of CO2 (1 bar) led
to no reaction and the starting materials were recovered
unreacted after 18 h at 100 °C. Notably, Beller et al. have shown
that iron(II) phosphine complexes are able to promote the
hydrogenation of the kinetically stable CO2 molecule to formate
derivatives11 and we have found recently that Fe(acac)2 in com-
bination with tris[2-(diphenylphosphino)ethyl]phosphine (PP3)
can catalyze the hydrosilylation of organic ureas to
formamidines.3k Supporting phosphine ligands were therefore
screened so as to form complexes with Fe(acac)2 and generate
active catalysts in the formylation of 1a (entries 1–6, Table 1
and ESI†). While PPh3, 1,3-bis(diphenylphosphino)propane
(dppp),
1,1′-bis(diphenylphosphino)ferrocene
(dppBz)
(dppf),
and
1,2-bis(diphenylphosphino)benzene
4,5-bis(diphenylphosphino)-9,9-dimethylxanthene (Xantphos)
did not improve the reactivity of Fe(acac)2, an equimolar
mixture of PP3 (5.0 mol%) and Fe(acac)2 allowed for the quanti-
tative conversion of 1a to its formamide 2a at RT after 18 h
(entry 1, Table 1). After usual work-up aimed at eliminating the
siloxanes by-products, 2a was successfully isolated in 92%
yield. Importantly, the presence of both Fe(acac)2 and the
supporting ligand is necessary to obtain catalytic activity in the
conversion of 1a to 2a (entries 2 and 3). Replacing Fe(acac)2
with Fe(BF4)2·6H2O lowers the conversion yield of 2a to 13%
(entry 7, Table 1).
It is noteworthy that the polarity of the solvent has a
significant impact on the activity of the iron catalytic
system. While toluene and 1,4-dioxane (ε0 < 2.4) impair the
formylation of 1a, polar solvents with a dielectric constant ε0
greater than 7.5 (THF, CH2Cl2, and CH3CN) lead to the
quantitative formation of 2a (entries 1, 8–11 in Table 1). CO2
reductive functionalization to 2a also depends on the
nature of the reductant and less reactive hydrosilanes, such
as Et3SiH, 1,1,4,4-tetramethyldisiloxane (TMDS) and PMHS,
are unreactive in eqn (4), even at 70 °C (entries 12–14,
Table 1). As a result, Fe(acac)2 + PP3 is superior to 1,5,7-
triazabicyclo[4.4.0]dec-5-ene (TBD) which operates at 100 °C
and affords 2a in a modest 39% yield after 24 h with 1 equiv.
of PhSiH3.6b For comparison, low catalyst loadings of
Cu(OAc)2 + 1.5 dppBz (0.07 mol%) were shown to convert 1a
to 2a in 87% yield after 30 h at 80 °C. In fact, the
catalytic activity of the iron(II) system resembles that of free
NHCs which are able to promote the formylation of N–H
bonds of amines, anilines, hydrazines and hydrazones at
room temperature.6d
Table 1 Iron-catalyzed formylation of 1a using CO2
The scope of active amine substrates in the iron(II)
catalyzed formylation reaction was then explored (eqn (5) and
Table 2). Using 5.0 mol% of Fe(acac)2 + PP3 with PhSiH3,
aliphatic secondary amines 1b, 1d, 1e and 1h proved to be
highly active in this reaction, providing quantitative conver-
sions to the desired formamides after 18 h at RT under 1 bar
CO2 (entries 1, 3, 4, 7, Table 2). Under the same conditions,
the sterically hindered di-iso-propylamine 1c was successfully
converted to 2c in a modest 40% yield determined by GC/MS
(entry 2, Table 2). Nonetheless, while 1a is an active
substrate, the presence of two aromatic rings on the nitrogen
atom completely shuts down the formylation of the N–H bond
in 1g (entry 6, Table 2). This reaction can also be applied with
good success to convert primary amines and formanilide 2i
was obtained in good conversion (79%) from aniline 1i
(entry 8, Table 2). Despite the presence of two iso-propyl
substituents at the α-position, 1j was transformed to 2j in
38% conversion. Interestingly, the introduction of an electron
donating group at the para position of aniline hampers the
formylation rate and p-anisidine (1k) afforded 2k in a modest
34% yield, while conversions greater than 62% were observed
starting from aniline (1i) or p-chloroaniline (1l). In contrast
Entrya Catalysta
Solvent
R3SiH (n) Yieldb (%)
1
Fe(acac)2 + PP3 (1 : 1)
THF
THF
THF
THF
THF
PhSiH3 (1) >95 (92)c
PhSiH3 (1) <1
PhSiH3 (1) <1
PhSiH3 (1) <1
PhSiH3 (1) <1
PhSiH3 (1) <1
PhSiH3 (1) 13
PhSiH3 (1) >95
PhSiH3 (1) >95
PhSiH3 (1) 63
2
Fe(acac)2
3
PP3
4
5
Fe(acac)2 + PPh3 (1 : 4)
Fe(acac)2 + dppp (1 : 2)
6
Fe(acac)2 + dppBz (1 : 2) THF
7
Fe(BF4)2·6H2O + PP3 (1:1) THF
8
9
Fe(acac)2 + PP3 (1 : 1)
Fe(acac)2 + PP3 (1 : 1)
Fe(acac)2 + PP3 (1 : 1)
Fe(acac)2 + PP3 (1 : 1)
Fe(acac)2 + PP3 (1 : 1)
Fe(acac)2 + PP3 (1 : 1)
Fe(acac)2 + PP3 (1 : 1)
CH3CN
CH2Cl2
Toluene
1,4-Dioxane PhSiH3 (1) 48
THF
THF
THF
10
11
12
13
14
Et3SiH (3) <1
TMDS (1.5) <1
PMHS (3) <1d
a
Reaction conditions: N-methylaniline (1a, 0.250 mmol), hydrosilane
R3SiH (3 eq. Si–H), catalyst (0.0125 mmol, 5.0 mol%), solvent (0.7 mL),
CO2 (1 bar), 18 h, RT. b Determined by GC/MS using mesitylene as the
internal standard after calibration. c Isolated yield. d 70 °C.
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Table 2 Formylation of various amines with the described system
Table 2 (continued)
Entrya
1
Substrate (1)
1b,
Product (2)
2b,
Yieldb (%)
Entrya
13
Substrate (1)
Product (2)
2q,
Yieldb (%)
>95
1q,
1r,
<1
14
15
16
17
18
2r,
2s,
2t,
2u,
24
8
2
3
4
5
6
1c,
1d,
1e,
1f,
2c,
2d,
2e,
2f,
40
1s,
1t,
1u,
>95
>95
>95
<1
26
65
1v,
2v,
58
1g,
1h,
2g,
2h,
19
1w,
2w,
<1
7
8
>95
a
Reaction conditions: amine (0.250 mmol), PhSiH3 (0.250 mmol), catalyst
b
(0.0125 mmol), solvent (0.7 mL), CO2 (1 bar), 18 h, RT. Determined
by GC/MS using mesitylene as the internal standard after calibration.
to the results obtained with NHCs, the bis-formylated products
are not observed when aniline derivatives are reacted with
PhSiH3 and CO2 in the presence of Fe(acac)2 + PP3.6d Yet,
starting with aliphatic primary amines, a competition between
mono- and bis-formylation appears and, although the mono-
formamides 2m and 2o are obtained as major products from
benzylamine (1m) and n-heptylamine (1o), respectively, signifi-
cant amounts of 2m′ and 2o′ were also detected (up to 25%)
(entries 9 and 11, Table 2). This product distribution was left
unchanged after longer reaction times (36 h). For sterically
hindered substrates such as tert-butylamine 1n, no trace of
bis-formylated products were detected (entry 10) and the
formamide was obtained in a good 70% GC yield.
79
38
34
62
9
1m,
2m,
2m′,
76/17
10
11
1n,
1o,
2n,
70
2o,
45/25
The N–H bonds in less basic substrates such as imidazoles
(1q) or indoles (1p) are resistant to formylation (Entries 12
and 13). Benzophenone imine (1s) and aliphatic and aromatic
hydrazines (1r and 1t) display a low reactivity and the
corresponding formyl products were obtained in low yields,
ranging from 8 to 26% (entries 14–16, Table 2). An important
advantage of hydrosilylation over classical reduction
methods (with hydrogen or metal hydrides) is the enhanced
2o′,
12
1p,
2p,
<1
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chemoselectivity, enabled by the use of a mild and polarized
hydrosilane reductant. This benefit translates well into the
present iron-catalyzed formylation of amines and 1u and 1v
are successfully formylated to 2u and 2v, respectively, with no
reduction of the additional ketone or ester functionality
(entries 17 and 18, Table 2). Nevertheless, the system is
incompatible with the presence of a hydroxyl group (entry 19,
Table 2).
directly 1a to N,N-dimethylaniline (3a) in 23% yield after 18 h
(eqn (7), Scheme 2). As such, the iron catalyst exhibits a some-
what lower activity than the zinc carbene or ruthenium phos-
phine complexes utilized previously by Cantat et al. and
Beller et al., respectively.6c,12 As expected, formamide 2a accu-
mulates in the methylation of 1a and its reduction to 3a is rate
limiting. Increasing the catalyst loading to 10.0 mol% is benefi-
cial to the conversion to methylamines and 3a, 3x and 3y are
obtained in good yields, ranging from 51 to 84%, from 1a, 1x
and 1y, respectively (eqn (7), Scheme 2). Under the same condi-
tions, the aliphatic dibenzylamine (1h) yields selectively form-
amide 2h. Although modest, the catalytic activity of Fe(acac)2 +
PP3 in the methylation of amines establishes the potential of
iron complexes to promote the 6-electron reduction of CO2 and
further efforts are underway in our laboratories to improve the
catalytic activity of the iron system and to utilize inexpensive
hydrosilanes, such as PMHS and TMDS, in this transformation.
In 2013, we designed a novel catalytic reaction to utilize
CO2 as a C1-building block in the methylation of amines.6c
Using zinc catalysts and hydrosilanes as reductants, CO2 was
shown to undergo a complete deoxygenation via a 6-electron
reduction pathway coupled to the formation of a C–N bond
(eqn (3) in Scheme 1). Shortly afterwards, Beller et al.
reported an efficient ruthenium phosphine catalyst for this
transformation.12 Both the Zn and Ru catalytic systems operate
at 100 °C with PhSiH3. From a mechanistic standpoint, it was
shown that the zinc-catalyzed methylation of N–H bonds
involves two steps with opposite electronic demand at the
nitrogen centre and the amine substrate is first converted to its
formamide, which is subsequently hydrosilylated to the
corresponding methylamine. In order to evaluate the potential
of Fe(acac)2 + PP3 in the catalytic methylation of amines with
CO2, the reduction of formamide 2a was first tested in the
presence of a stoichiometric amount of PhSiH3. As depicted in
eqn (6) (Scheme 2), the iron catalyst can promote the quantita-
tive hydrosilylation of formamide 2a to 3a, albeit at 100 °C. As
a consequence, raising the reaction temperature to 100 °C
enables the utilization of the iron catalyst in the direct methyla-
tion of N-methylaniline with CO2. In fact, using 1 bar CO2 and
4 equiv. of PhSiH3, Fe(acac)2 + PP3 (5.0 mol%) is able to convert
Conclusions
In the search for earth abundant and cost efficient catalysts
for the reduction of CO2, we have reported herein the first
examples of iron catalysts able to promote the hydrosilylation
of CO2. Iron(II) salts supported by a tetraphosphine ligand
are able to transform CO2 to formamides in the presence of
amines and PhSiH3 at room temperature. The reaction is
chemoselective and tolerant to ketone and ester functionalities.
At 100 °C, the catalytic system is also active in the methylation
of aniline derivatives.
Acknowledgements
For financial support of this work, we acknowledge the CEA,
the CNRS, the CHARMMMAT Laboratory of Excellence and
the European Research Council (ERC Starting Grant Agreement
no. 336467). T.C. thanks the Fondation Louis D. – Institut de
France for its formidable support.
Notes and references
1 (a) Modern Reduction Methods, ed. E. A. Andersson and
I. J. Munslow, Wiley-VCH, Weinheim, 2008; (b) Hydrosilylation:
A Comprehensive Review on Recent Advances, ed. B. Marciniec,
Springer, New York, 2009; (c) Silicon in Organic, Organometallic
and Polymer Chemistry, ed. M. A. Brook, Wiley, New York,
2000; (d) J. Y. Corey, Chem. Rev., 2011, 111, 863–1071; (e)
G. L. Larson and J. L. Fry, Organic Reactions, ed. S. E. Denmark,
Wiley, Hoboken, 2008; ( f ) B. Marciniec, Coord. Chem. Rev.,
2005, 249, 2374–2390; (g) B. Marciniec, J. Gulinsky,
W. Urbaniak and Z. W. Kornetka, The Chemistry of Organic
Silicon Compounds, ed. B. Marciniec, Pergamon Press, Oxford,
1992; (h) V. B. Pukhnarevich, E. Lukevics, L. T. Kopylova and
M. G. Voronkov, Perspectives of Hydrosilylation, ed. E. Lukevics,
Institute of Organic Synthesis, Riga, 1992.
Scheme 2 Iron-catalyzed reduction of 2a to 3a and methylation of
N-methylanilines.
2 R. Walsh, Acc. Chem. Res., 1981, 14, 246–252.
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3 (a) D. Addis, S. Das, K. Junge and M. Beller, Angew. Chem.,
Int. Ed., 2011, 50, 6004–6011; (b) D. Bezier, S. Park and
M. Brookhart, Org. Lett., 2013, 15, 496–499; (c) D. Bezier,
G. T. Venkanna, L. C. M. Castro, J. X. Zheng, T. Roisnel,
J. B. Sortais and C. Darcel, Adv. Synth. Catal., 2012, 354,
1879–1884; (d) L. C. M. Castro, J. B. Sortais and C. Darcel,
Chem. Commun., 2012, 48, 151–153; (e) C. Cheng and
M. Brookhart, Angew. Chem., Int. Ed., 2012, 51, 9422–9424;
( f ) S. Das, D. Addis, S. L. Zhou, K. Junge and M. Beller,
J. Am. Chem. Soc., 2010, 132, 4971–4971; (g) S. Das, Y. H. Li,
K. Junge and M. Beller, Chem. Commun., 2012, 48,
10742–10744; (h) S. Das, K. Moller, K. Junge and M. Beller,
Chem.–Eur. J., 2011, 17, 7414–7417; (i) K. Junge, B. Wendt,
S. L. Zhou and M. Beller, Eur. J. Org. Chem.,
2013, 2061–2065; ( j) K. Matsubara, T. Iura, T. Maki and
H. Nagashima, J. Org. Chem., 2002, 67, 4985–4988; (k)
J. Pouessel, O. Jacquet and T. Cantat, ChemCatChem,
2013, 5, 3552–3556; (l) K. Miyamoto, Y. Motoyama and
H. Nagashima, Chem. Lett., 2012, 41, 229–231.
J. Am. Chem. Soc., 2006, 128, 12362–12363; (g) S. Park,
D. Bézier and M. Brookhart, J. Am. Chem. Soc., 2012, 134,
11404–11407; (h) S. N. Riduan, Y. G. Zhang and J. Y. Ying,
Angew. Chem., Int. Ed., 2009, 48, 3322–3325; (i) W. Sattler
and G. Parkin, J. Am. Chem. Soc., 2012, 134, 17462–17465; ( j)
K. Motokura, D. Kashiwame, A. Miyaji and T. Baba,
Org. Lett., 2012, 14, 2642–2645.
6 (a) T. Fujihara, T. H. Xu, K. Semba, J. Terao and Y. Tsuji,
Angew. Chem., Int. Ed., 2011, 50, 523–527; (b) C. D. Gomes,
O. Jacquet, C. Villiers, P. Thuery, M. Ephritikhine and
T. Cantat, Angew. Chem., Int. Ed., 2012, 51, 187–190; (c)
O. Jacquet, X. Frogneux, C. D. Gomes and T. Cantat, Chem.
Sci., 2013, 4, 2127–2131; (d) O. Jacquet, C. D. Gomes,
M. Ephritikhine and T. Cantat, J. Am. Chem. Soc., 2012, 134,
2934–2937.
7 K. Motokura, N. Takahashi, D. Kashiwame, S. Yamaguchi,
A. Miyaji and T. Baba, Catal. Sci. Technol., 2013, 3, 2392–2396.
8 M. Zhang and A. Q. Zhang, Appl. Organomet. Chem.,
2010, 24, 751–757.
4 H. Koinuma, F. Kawakami, H. Kato and H. Hirai, J. Chem.
Soc., Chem. Commun., 1981, 213–214.
9 H. Q. Li, L. C. M. Castro, J. X. Zheng, T. Roisnel, V. Dorcet,
J. B. Sortais and C. Darcel, Angew. Chem., Int. Ed., 2013, 52,
8045–8049.
10 (a) Y. Sunada, H. Kawakami, T. Imaoka, Y. Motoyama and
H. Nagashima, Angew. Chem., Int. Ed., 2009, 48, 9511–9514;
(b) S. Zhou, K. Junge, D. Addis, S. Das and M. Beller, Angew.
Chem., Int. Ed., 2009, 48, 9507–9510.
11 C. Federsel, A. Boddien, R. Jackstell, R. Jennerjahn,
P. J. Dyson, R. Scopelliti, G. Laurenczy and M. Beller, Angew.
Chem., Int. Ed., 2010, 49, 9777–9780.
12 Y. H. Li, X. J. Fang, K. Junge and M. Beller, Angew. Chem.,
Int. Ed., 2013, 52, 9568–9571.
5 (a) A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem.
Soc., 2010, 132, 10660–10661; (b) A. Berkefeld, W. E. Piers,
M. Parvez, L. Castro, L. Maron and O. Eisenstein, Chem. Sci.,
2013, 4, 2152–2162; (c) O. Jacquet, C. D. Gomes,
M. Ephritikhine and T. Cantat, ChemCatChem, 2013, 5,
117–122; (d) M. Khandelwal and R. J. Wehmschulte, Angew.
Chem., Int. Ed., 2012, 51, 7323–7326; (e) R. Lalrempuia,
M. Iglesias, V. Polo, P. J. S. Miguel, F. J. Fernandez-Alvarez,
J. J. Perez-Torrente and L. A. Oro, Angew. Chem., Int. Ed.,
2012, 51, 12824–12827; ( f ) T. Matsuo and H. Kawaguchi,
This journal is © The Royal Society of Chemistry 2014
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