Cooperative Catalytic Activation of Si?H Bonds: CO2-Based Synthesis of Formamides from Amines and Hydrosilanes under Mild Conditions

Article abstract of DOI:10.1002/cssc.201601490

A simple cooperative catalytic system was successfully developed for the solvent-free N-formylation of amines with CO₂ and hydrosilanes under ambient conditions, which was composed of a Zn(salen) catalyst and quaternary ammonium salt. These commercially available binary components activated the Si?H bonds effectively, owing to the intermolecular synergistic effect between Lewis base and transition metal center (LB–TM), and subsequently facilitated the insertion of CO₂ to form the active silyl formats, thereby leading to excellent catalytic performance at a low catalyst loading. Furthermore, the bifunctional Zn(salen) complexes, with two imidazolium-based ionic-liquid (IL) units at the 3,3′-position of salen ligand, acted as intramolecularly cooperative catalysts, and the solvent-regulated separation resulted in facile catalyst recycling and reuse.

Full text of DOI:10.1002/cssc.201601490

Accepted Article
Title: Cooperative Catalytic Activation of Si−H Bonds: CO2-based
Synthesis of Formamides from Amines and Hydrosilanes under
Mild Conditions
Authors: Rongchang Luo, Xiaowei Lin, Yaju Chen, Wuying Zhang,
Xiantai Zhou, and Hongbing Ji
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To be cited as: ChemSusChem 10.1002/cssc.201601490
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10.1002/cssc.201601490
ChemSusChem
FULL PAPER
Cooperative Catalytic Activation of Si−H Bonds: CO2-based Synthesis of
Formamides from Amines and Hydrosilanes under Mild Conditions
Rongchang Luo, Xiaowei Lin, Yaju Chen, Wuying Zhang, Xiantai Zhou and Hongbing Ji*[a]
Abstract: The simple cooperative catalytic system had been
successfully developed for the solvent-free N-formylation of
amines with CO2 and hydrosilanes at ambient conditions, which
was composed of the Zn(salen) catalyst and quaternary
ammonium salt. These commercially available binary
components could activate the Si−H bonds effectively
originated from the intermolecularly synergistic effect between
Lewis base-transition metal center (LB-TM), and subsequently
facilitate the insertion of CO2 to form the active silyl formats,
thereby leading to obtain the excellent catalytic performance at
a low catalyst loading. Furthermore, the bifunctional Zn(salen)
complexes owning two imidazolium-based ionic liquid (IL) units
at the 3,3’-position of salen ligand were acted as
intramolecularly cooperative catalysts and the feature of
solvent-regulated separation resulted in the ease of catalyst
recycle and reuse.
a mild reduction potential was kinetically more reactive.[13]
Therefore, the reduction of CO2 to formamides using amines
and hydrosilanes is an attractive method for incorporating CO2
into valuable organic compounds because they often operate
under mild conditions with remarkable chemoselectivity.
However, the promising process has been investigated less
compared to other CO2 reductive emission.[14]
Scheme 1. The chemoselective reduction of CO2 to formamides with amines
over various reductive agents
In 2012, Cantat et al first discovered an organocatalytic
version of this reaction with organic bases (e.g. TBD) as
catalysts, but the diagonal transformation only proceeds at high
temperatures (>=100 °C) with solvent and presents the limited
Introduction
substrate scope.[15] They also reported
a highly active
organocatalytic system based on N-heterocyclic carbenes
(NHCs) that has been designed for the N-formylation of various
amines at ambient conditions.[16] Subsequently, the
mechanisms were comprehensively studied with DFT
calculations.[17] Nevertheless, these catalytic systems with a
high catalyst loading to achieve high efficiency were sensitive to
air and moisture. Although a rhodium-based bis(NHC) complex
could exhibit the high activity at a low catalyst loading and a
wide substrate scope were compatible, the reported routes
generally suffered from drawbacks such as requiring noble
metal catalysts, inert gas atmosphere, and complicated route of
catalyst synthesis.[18] Some toxic organic phosphine ligands[5d,
19] or in combination with copper[12, 20] or iron[13] salts were also
used to promote the reductive functionalization of CO2 at
ambient conditions, which was inaccordance to the requirement
of green chemistry. Recently, Liu et al reported the simple
imidazolium-based ionic liquid (IL) can directly catalyze the N-
formylation of amines using CO2 and hydrosilane originated
from the cooperative catalysis between anion and cation within
IL moiety.[21] Additionally, some strong polar solvents including
DMSO[22] and γ-valerolactone[23] might also promote this
transformation. Alkaline metal carbonates (Cs2CO3) can
facilitate the selective formylation and methylation of amines
efficiently in line with “cesium effect”, but the reaction required
not green organic solvent (MeCN) to achieve the catalytic
cycle.[24] Likewise, a handful of heterogeneous catalysts had
been successfully applied for preparing the formamide products
on the basis of their characteristics of easy recycling.[5b, 25]
In view of the fact that the common intermediate in metal-
catalyzed hydrosilylation reactions of CO2 is the metal hydride
complex (M−H form),[26] as well as our previous works on the
metal-catalyzed cycloaddition reaction of CO2/epoxides,[27] we
envisioned that the strong electron-donating ability of
Recently, carbon dioxide (CO2) is widely used as an abundant,
inexpensive and bio-renewable C1 building block for chemical
synthesis.[1] Converting waste CO2 into useful products or fuels
is usually considered as an attractive method, which is also
known as “carbon capture and utilization” (CCU).[2] The major
challenge in this area may be the lack of effective catalysts to
facilitate its activation and subsequent transformation due to its
thermodynamic and kinetic stability.[3] Among these, an
interesting route for the synthesis of formamides is the N-
formylation of amines with CO2 in the presence of various
reducing agents (Scheme 1)[4]
because formamides are
versatile chemicals with wide applications in industry as
important solvents and platform compounds.[5] Although
hydrogen gas (H2) is the cleanest and most atom economical
reductant, hydrogenation of CO2 in the presence of an amine
often requires noble metal catalysts (Ru,[6] Pd,[7] Pt,[8] Au,[9] Ir,[10]
etc[11]) working at harsh reaction conditions, such as high
reaction temperature and pressure, thereby leading to obtain
the low activity and prevent its broad application. Metal
hydrides and hydroboranes are generally sensitive to air and
moisture aroused from the limitation in the substrate scope.[12]
Hydrosilanes are used as a kind of cheap, nontoxic and easy-
to-handle reducing agent, even the polar Si−H bond as well as
[a]
Dr. R.C. Luo, X.W. Lin, Y.J. Chem, W.Y. Zhang, Dr. X.T. Zhou, Prof.
Dr. H.B. Ji
School of Chemistry, Key Laboratory of Low-Carbon Chemistry &
Energy Conservation of Guangdong Province, Sun Yat-sen
University, Guangzhou, 510275, China
E-mail: jihb@mail.sysu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201601490
ChemSusChem
FULL PAPER
macrocyclic salen ligand may increase the nucleophilicity of the
metal hydride species and facilitate the reduction of the weakly
electrophilic CO2. In addition, the Lewis base-transition metal
center (LB-TM) catalytic system could activate hydrogen
molecular (H2) effectively inspired by the activation mechanism
of hydrogenase in nature.[28] Herein, the efficient binary catalytic
system, i.e. simple Zn(salen) complex together with quaternary
ammonium salts, were successfully developed for N-formylation
reaction of amines using CO2 and hydrosilanes under solvent-
free conditions. Moreover, several IL-functionalized Zn(salen)
catalysts were also synthesized for promoting the
transformation on account of the mechanism of the synergistic
catalysis as described in Scheme 2. To the best of knowledge,
this is the first example for the metallosalen-catalyzed selective
reduction of CO2 in a homogenous medium, leading to the
production of formamides with excellent yield and
unprecedented chemoselectivity under mild conditions. Thus,
inter- or intramolecularly cooperative catalysis could activate
Si−H bonds of hydrosilane leading CO2 inserting and thereby
implementing the activation of this molecule.
directly catalyze the N-formylation reaction owing to the
formation of the highly active Zn−H species. The consequences
were accessible that both can make it more favorable for the
insertion of CO2. Because the high reaction temperature might
result in the formation of methylamines from the N-methylation
of amines, it was necessary to perform the N-formylation
reaction under mild conditions in order to control the
chemoselectivity. When the CO2 pressure (from 1.5 MPa to 0.5
MPa) and the reaction temperature (from 40 °C to 25 °C) were
decreased, only a negligible yields could be obtained using
Zn(salen) or TBAB as a sole catalyst (Table 1, entries 1−3).
However, it was also found that the reaction catalyzed by both
one-component catalysts required the longer reaction time (24-
48 h) to obtain the excellent yields at 40 oC and 1.5 MPa.
Therefore, on the basis of the promising results, we observed
these simple one-component catalytic systems required high
catalyst loading or higher reaction temperature/pressure or
longer reaction time to obtain a better catalytic performance.
Thus, we hypothesized to use
a new catalytic system
constituted by Zn(salen) and TBAB that would take advantage
of both the catalysts properties. As expected, only 0.5 mol%
Zn(salen) in combination with 1.0 mol% TBAB represented an
efficient binary catalytic system for the production of
formamides and an excellent yield of 99 % was obtained for 7 h
at 25 oC and 0.5 MPa without adding any solvent (Table 1,
entry 4). Furthermore repeating the reaction at 40°C and at 1.5
MPa it was possible to obtain the product in 99 % yield after
only 3 hours as shown in Figure 1(B). The by-product N,N’-
dimethylaniline (2b) could not be detected, and the 100 %
chemo-selectively was obtained. On the basis of the promising
results of the kinetic investigation, it was believed that the
synergistic effect between Lewis base and transition metal
promotes the reaction with low catalyst loading and
consequently it was possible to operate under low CO2
pressure and room temperature. Decreasing the TBAB loading
from 1.0 mol% to 0.5 mol%, a longer reaction time (15 h) was
required (Table 1, entry 5). Moreover, when reducing the CO2
pressure to 0.1 MPa, the efficiency of the N-formylation
decreased (88 % yield) in spite of the prolongation of reaction
time (Table 1, entry 6).
Scheme 2. The N-formylation of amines using CO2 and hydrosilanes
catalyzed by the binary or bifunctional catalysts in this paper
100
80
60
40
20
0
100
80
60
40
20
0
Results and Discussion
Zn(salen)
TBAB
Intermolecularly cooperative catalysis
40 oC, 1.5 MPa
In our preliminary experiments, the reductive N-formylation of
N-methylaniline (1a) with CO2 and phenylsilane (PhSiH3) as a
representative reaction to produce the corresponding N-methyl
25 oC, 0.5 MPa
40 oC, 1.5 MPa
o
formanilide (2a) was carried out without solvent at 40 C and
1.5 MPa CO2 pressure. As shown in Figure 1(A), when using
1.0 mol% tetrabutylammonium bromide (denoted as TBAB) as
an one-component catalyst, the yield of 80 % was achieved
after 24 h. The experimental results were in consistence with
that reported by Liu et al in their recent works related to the
imidazolium-based ILs used in stoichiometric amount.[21] It was
believed that the cooperative catalysis between the cation and
anion of IL might lead to activate the Si−H bond of phenylsilane.
Additionally, it was also found that the catalyst Zn(salen) could
0
6
12
18
24
0
1
2
3
4
5
6
7
(B)
(A)
t /h
t /h
Figure 1 Kinetic curves of the N-formylation reaction from N-methylaniline,
CO2 and PhSiH3 catalyzed by the one-component (A) or Zn(salen)/TBAB
bicomponent catalysts (B). Reaction conditions: 10 mL stainless-steel
autoclave, N-methylaniline (1.0 mmol), PhSiH3 (1.0 mmol), Zn(salen) (0.5
mol%), TBAB (1.0 mol%), CO2 pressure (0.5 MPa or 1.5 MPa), reaction
temperature (25 °C or 40 oC).
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10.1002/cssc.201601490
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Moreover,
although
both
Cu(OAc)2/dppe[12]
and
hydrosilane required the participation of the metal center to
form the highly active metal hydride complex, which was well
known to be one of the key steps in the metal-catalyzed
Fe(acac)2/PP3[13] binary catalytic systems enable the conversion
of CO2 to formamide derivatives under mild reaction conditions
(Table 1, entries 7-8), there are some significant drawbacks
such as the instability of iron(II) salts, difficult preparation of
phosphine donors, environmental unfriendliness and even
using the poisonous organic solvent or raw materials. In
addition, Fe-catalyzed formylation with CO2 and hydrosilanes
often requires high catalyst loading (5.0 mol%) and longer
reaction time (18 h) to obtain the excellent catalytic
performance.[13] Han et al have reported Cu-dppe complex
could efficiently catalyze this transformation at ambient
condition with only a 0.1 mol% catalyst loading.[12] However, the
binary catalytic system does not meet the criteria of green
chemistry and could not be recovered and reused in the
reductive conditions via a simple separation method, and the
recyclability of these bifunctional catalysts is discussed later in
detail in the article.
hydrosilylation reaction of CO2.[29] As
a result, if using
aluminium, copper or cobalt instead of zinc as metal active
center, the low-to-moderate yields were obtained with Cu(salen)
and Co(salen) catalyst, whereas Al(salen) complex could not
catalyze the reaction effectively at the same conditions. The
interesting phenomenon signified the nucleophilicity and
stability of the metal hydride intermediate had significant
influence on the catalytic performance in the reduction N-
formylation of amines.
100
100
80
60
35
40
16
20
Table 1. The results of N-formylation reaction of N-methylaniline with CO2
<1
1
1
1
1
<1
and PhSiH3 catalyzed by various catalytic systems[a]
0
Figure 2 The N-formylation reaction from N-methylaniline, CO2 and PhSiH3
promoted by various catalysts using TBAB as cocatalyst. Reaction conditions:
10 mL stainless-steel autoclave, N-methylaniline (1.0 mmol), PhSiH3 (1.0
mmol), CO2 pressure (1.5 MPa), reaction temperature (25 °C), Catalyst (0.5
mol%), TBAB (1.0 mol%).
Loading
/mol%
−
Cocatalyst
/mol%
T
CO2
/oC /MPa /h
40 1.5 12
Time Conv. Yield (2a)
Entry
Ref.
/%[b]
/%[b]
1
2
3
4
5
−
−
n.d.
n.d.
Our work
Our work
Our work
−
TBAB (1.0) 25 0.5
25 0.5
12
12
7
<3
<1
>99
99
92
99
95
2
Obviously, the experimental findings also indicated that the
catalytic activity of the binary catalytic system strongly
depended on the kind of cocatalyst as reported in Figure 3.
Replacing TBAB with organic nitrogen base such as DMAP,
Et3N, DBU, or Ph3P as cocatalyst, the poor results were
observed due to the lack of bifunctionality between cation and
anion.[21] However, all of the familiar quaternary ammonium
SZ (1.0)
SZ (0.5)
SZ (0.5)
n.d.
TBAB (1.0) 25 0.5
TBAB (0.5) 25 0.5
TBAB (0.5) 25 0.1
99 (96) Our work
99 (95) Our work
92 (88) Our work
15
30
2
6[c] SZ (0.5)
7[c] Cu(OAc)2
8[c] Fe(acac)2
dppe
PP3
25 0.1
25 0.1
99
95
[12]
[13]
18
−
salts (TBA+BF4 , TBA+Br−, TBA+I−) achieved the moderate-to-
[a] Reaction conditions: N-methylaniline (1.0 mmol), catalyst (0.5 or 1.0 mol%),
excellent yields, and bromide anion (Br−) having the moderate
nucleophilicity was the best choice. The phenomenon was also
demonstrated by Liu group using NMR as an analytical tool.[21]
In fact, the presence of both Zn(salen) and quaternary
ammonium salts was necessary to obtain excellent catalytic
activity in the selective reduction of CO2.
cocatalsyt (0.5 or 1.0 mol%), PhSiH3 (1.0 mmol), reaction temperature (25
[b]
oC), CO2 pressure (0.5 MPa), Zn(salen) was abbreviated to SZ;
Determined by GC and the values in parentheses were the isolated yields of
[c]
products;
balloon).
The reaction was performed at ambient conditions (25 oC,
In order to further understand the synergistic effect between
Zn(salen) and TBAB, we investigated the influence factors for
the catalytic performance, including the type of ligand, metal
center, and the type of cocatalysts, which was elaborated in
Figure 2. It was found that the conversion of N-methylaniline
was negligible in the case of salen ligand or ZnBr2 salt as the
alternative catalyst under identical conditions. Additionally, the
completely negligible yields was acquired by the Zn-based
100
99
100
80
60
40
20
0
92
3
<1
2
1
<1
<1
catalysts
containing
N4-chelating
ligand,
such
as
tetraphenylporphyrin (ZnTPP) and tris(2-pyridylmethyl)amine
(ZnTPA). The promising observations suggested that the ligand
owning the appropriate structure was crucial for obtaining the
excellent catalytic activity. Surely, the catalytic activity of
metallosalen catalysts was closely related with the type of a
metal active center since the activation of Si−H bond within the
Figure 3 The N-formylation reaction from N-methylaniline, CO2 and PhSiH3
promoted by Zn(salen) catalyst and various cocatalysts. Reaction conditions:
10 mL stainless-steel autoclave, N-methylaniline (1.0 mmol), PhSiH3 (1.0
mmol), CO2 pressure (1.5 MPa), reaction temperature (25 °C), Zn(salen) (0.5
mol%), cocatalyst (1.0 mol%).
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Intramolecularly cooperative catalysis
Table 2. The results of N-formylation reaction of N-methylaniline with CO2
and PhSiH3 over various bifunctional catalysts.[a]
On the basis of the mechanism of the intermolecularly
cooperative catalysis, the binary Zn(salen)/TBAB catalytic
system could obtain the excellent catalytic performance on the
reductive N-formylation reaction of N-methylaniline at ambient
conditions. However, there were some disadvantages such as
the difficulty for catalyst recycle and the complexity of product
separation. Recently, the IL-based metallocatalysts might
present the special characterization on the catalyst reuse in
view of the concept of “one-phase catalysis and two-phase
Entry
Catalyst
T /oC
CO2 /MPa t /h
Conv. [b] /% Yield [b] /%
1
2
−
40
40
40
40
40
40
40
40
40
40
40
40
40
25
25
25
25
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
0.5
1.5
0.5
0.5
0.1
12
3
−
n.d.
5
−
n.d.
<3
Zn(salen) (1.0)
IL (2.0)
3
3
4
Zn(salen)/IL
ILSA1 (1.0)
ILST1 (1.0)
ILSC1 (1.0)
ILSZ1 (1.0)
ILSZ2 (1.0)
ILSZ3 (1.0)
ILSZ4 (1.0)
ILSZ5 (1.0)
ILSZ1 (1.0)
ILSZ1 (1.0)
ILSZ1 (1.0)
ILSZ1 (0.5)
ILSZ1 (2.0)
3
90
<1
<2
21
>99
>99
20
35
12
88
94
90
63
99
90
5
3
n.d.
1
6
3
7
3
20
8
3
99 (95)
99 (95)
19
9
3
separation”.
Our
groups
reported the metallosalen
10
11
12
13
14
15
16
3
complexes[27a] or 2,2’-bipyridine metallocatalysts[27b] having two
IL moieties with built-in nucleophilicity could effectively catalyze
the cycloaddition reaction of CO2/epoxides under mild
conditions, and these catalysts should also be capable of
recycling from the reaction system. In the initial experiments,
the imidazolium-based IL [BMIm]Br exhibited the similar
catalytic properties with respect to the cocatalyst TBAB under
identical conditions, which was described in Figure 3. Since
bifunctional catalysts might show high catalytic activity, the
imidazolium-based ILs were successfully introduced into salen
zinc complexes by covalent attachment (Scheme 3), and this
kind of catalyst design encouraged us to perform the N-
formylation reaction of amines under mild conditions and make
the recycling of catalyst much easily. In addition, the
introduction of the long alkyl chain (n-octyl) was beneficial to
improving the moisture resistance of these bifunctional
catalysts. Therefore, the catalytic performances of the novel
catalysts were measured in the reduction N-formylation reaction
of N-methylaniline with CO2 and PhSiH3, which was conducted
in a batch operation under mild conditions (see Table 2). The
synthetic route of these bifunctional catalysts was reported by
our previous work.[27a] At first, N-octylimidazole was directly
reacted with benzyl halides (CH2X) modified salicyclaldehyde to
afford IL-substituted salicyclaldehyde. The successive Schiff-
base condensation between the aldehyde (–CHO) group and
the amino (–NH2) groups was used to form the corresponding
salen ligand, and subsequently the IL-functionalized
metallosalen complexes were obtained with high yields via the
metallization process under nitrogen protection (Scheme 3).
3
35
3
10
6
88
9
94 (90)
90
12
12
12
63
17
98(95)
[a]
Reaction conditions: N-methylaniline (1.0 mmol), catalyst (0.5−2.0 mol%),
PhSiH3 (1.0 mmol), CO2 pressure (0.1−1.5 MPa), temperature (25−40 oC),
solventless; [b] Determined by GC or GC-MS, the values in parentheses are
the isolated yields.
100
80
60
40
20
0
100
80
60
40
20
0
(B)
(B)
a
b
1.5 MPa
1.0 MPa
0.5 MPa
b
4
a
0
1
2
3
4
5
6
0
1
2
3
5
t /h
t /h
Figure 4 (A) Increase in N-methylformanilide formation amount with time
using ILSZ1 as catalyst at the different CO2 pressure. Reaction conditions: N-
methylaniline (1.0 mmol, if use), PhSiH3 (1.0 mmol), ILSZ1 (1.0 mol%), CO2
(0.5-1.5 MPa), Temperature (40 oC); (B) Decrease in PhSiH3 remaining
amount with time using ILSZ1 as catalyst without N-methylaniline (a) or
adding N-methylaniline as substrate (b). Reaction conditions: N-methylaniline
(1.0 mmol, if use), PhSiH3 (1.0 mmol), ILSZ1 (1.0 mol%), CO2 (1.0 MPa),
Temperature (40 oC).
As expected, the Zn-based catalyst ILSZ1 exhibited the
excellent catalytic performance with a catalyst loading of 1.0
mol% for 3 h at 40 C and 1.5 MPa, which was superior to the
o
binary Zn(salen)/IL catalytic system (Table 2, entry 8 vs entry 3).
The satisfying results might be due to the intramolecularly
cooperative catalysis between metal center and the IL moieties
within one molecule, which was similar to the binary catalytic
system. Surely, if employed the other IL-based metallocatalysts
owing the same structure and the different metal centers such
as aluminum (ILSA1), tin (ILST1), cobalt (ILSC1), the low
yields were also obtained under identical conditions (Table 2,
entries 5-7). In order to further understand the reaction process
from the kinetic perspective, the kinetic curves were conducted
to discuss the proposed reaction pathway. Notably, the
hydrosilylation of CO2 were converted smoothly to the
corresponding silyl formats in the absence of amine at the
different CO2 pressure as well as the amount of PhSiH3
decreased quickly, which was elaborated in Figure 4. When
adding N-methylaniline into the above reaction mixture, the
Scheme 3. Synthetic route of various bifunctional Zn(salen) catalyst
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10.1002/cssc.201601490
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yield of N-methylformanilide increased obviously. This finding
demonstrated that the reaction might involve the two-steps
reaction process and the formation of silyl formates
intermediate was very fast. Additionally, if adding all of the
reactants (including CO2, amine, PhSiH3) at the beginning of N-
formylation reaction, the consumption rate of PhSiH3 reduced to
a certain extent compared to the previous results. Thus, the
rate limiting step might be the amine N-formylation step for the
active formoxysilane, which was consistent with the proposed
reaction mechanism.
reduction of CO2 to formanilide at room temperature and
atmospheric pressure ascribed to the excellent intramolecularly
synergistic effect. In addition, it was noting that the reduction of
CO2 might also depend on the nature of the reductant and less
reactive hydrosilanes, such as Ph2SiH2, PhMe2SiH, Et3SiH,
(EtO)3SiH, TMDS and PMHS, were unreactive under the same
conditions owning to the steric and electronic effect (see Figure
5).[21]
100
80
60
40
20
0
Next, a series of bifunctional Zn-based catalysts were also
synthesized for comparison via covalent linkage following the
similar methods, such as ILSZ2-ILSZ5. Based on the
investigation to the catalytic performance of these catalysts, we
discovered the structure of these bifunctional Zn(salen)
catalysts had a huge influence on the catalytic activity. As
expected, the bifunctional Zn(salophen) catalyst ILSZ4 was
inferior to the Zn(salen) catalyst ILSZ1 on the catalytic activity
(Table 2, entry 11 vs entry 8). Moreover, the Zn(salen) catalyst
ILSZ3 functionalized by ILs at the 5,5’-position of salen ligand
also exhibited the lower catalytic performance compared to that
at 3,3’-position (Table 2, entry 10 vs entry 8). The interesting
experimental results implied the Zn(salen) complexes having
the proper space structure might require the excellent catalytic
activity in the reduction of CO2. More surprisingly, the Cl-based
catalyst ILSZ2 showed the considerable activity than that of Br-
based catalyst ILSZ1 (Table 2, entry 9 vs entry 8). In order to
further investigate the difference between the two, i.e. ILSZ1
and ILSZ2, both of the kinetic curves were conducted to
demonstrate the catalyst ILSZ2 owing the highest catalytic
activity. We speculated the phenomenon was due to the
nucleophilic ability of halogen anion (Cl－>Br－) because the N-
formylation reaction might proceed via nucleophilic attack on
the hydrosilane to promote the formation of a hypervalent
silicon species.[30] Recently, He et al also reported TBAF
(tetrabutylammonium fluoride) as an excellent organocatalyst
for the formation of formamides from amines, CO2 and
hydrosilanes, and the mechanism involving the fluoride effect
was proposed because the fluoride could promote hydride
transfer from the hydrosilane to CO2, which was originated from
its strong nucleophilicity.[31] From another angle, the catalyst
ILSZ2 presented the higher activity relative to catalyst ILSZ3
even though both of them contained chloride anion within ILs
(Table 2, entry 9 vs entry 10). It followed that the structure of
catalyst was a key factor for the catalytic performance. Finally,
it was understandable for us the Zn(salen) complex having
additional quaternary ammonium salts ILSZ5 presented the bad
catalytic activity probably due to the poor solubility (Table 2,
entry 12). It was well-known that a significant drawback of using
CO2 as a raw material may be the potential dangers associated
with high-temperature and high-pressure operations. Then, we
investigated the catalytic performance of the bifunctional
catalyst ILSZ1 at selected different conditions (Table 2, entries
13-17). More specifically, the N-formylation of amine using CO2
and PhSiH3 proceeded efficiently even at atmospheric CO2
pressure (0.1 MPa) and at room temperature (25 oC). Therefore,
the bifunctional catalytic system was capable of catalyzing the
Figure 5 The N-formylation reaction from N-methylaniline and CO2 over
catalyst ILSZ1 using various hydrosilanes as a reductive agent. Reaction
conditions: 10 mL stainless-steel autoclave, N-methylaniline (1.0 mmol),
hydrosilane (3 eq. Si−H), ILSZ1 (1.0 mol%), reaction temperature (40 °C),
CO2 pressure (1.5 MPa).
Finally, the recyclability and reusability of catalyst ILSZ1 was
further investigated as well at 40 oC and 1.5 MPa, as described
in Figure 6, In a typical catalytic cycle, the catalyst was
separated as a solid by centrifugation after adding organic
solvent (e.g. ether or ester), and the yield of N-
methylformanilide remained basically unchanged after reused
for five times, indicating that the bifunctional catalyst was quite
stable in this catalytic system and ILSZ1 should be denoted as
an easily recyclable catalyst by the solvent-regulated
separation techniques. The FT-IR analysis of the recovered
ILSZ1 was indirectly confirmed the stability of catalyst under
mild conditions. The reason was that the FT-IR spectra show
characteristic vibration bands at around 1635 cm−1 and 1547
cm−1, which were associated with the stretching vibration
modes of C=N and C–O at the salen ligand, respectively. Thus,
it was proved that the imine group of salen backbone might be
tolerated at the reductive conditions.
100
80
60
40
1547
20
0
1452
1635
1
2
3
4
5
3000
2000
1000
(B) 4000
Wavenumbers (cm-1)
(A)
Run times
Figure 6. (A) Recyclability and reusability of the catalyst ILSZ1 in the N-
formylation reaction of N-methylaniline from CO2 and PhSiH3; Reaction
conditions: 10 mL stainless-steel autoclave, N-methylaniline (1.0 mmol),
PhSiH3 (1.0 mmol), ILSZ1 (1.0 mol%), CO2 pressure (1.5 MPa), reaction
temperature (40 °C), reaction time: 3 h.
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Substrate scope
compounds. For example, morpholine with ether functional
group achieved the best yield of 99 % (2u) in a relatively short
period of time, and N-substituted piperazine derivatives also
exhibited the high activity under the optimized conditions (2v
As listed in Table 3, it was further demonstrated that different
kinds of amines including aliphatic, alicyclic and aromatic
amines with CO2 (0.5 MPa) and PhSiH3 were N-formylated to
the corresponding formamides in moderate to excellent yields
at 25 °C within 15 h with 0.5 mol% Zn(salen) catalyst and
and 2w). However,
a
second N-formylation of 2,6-
dimethylpiperazine did not occur at the hindered site, with only
mono-formylation product 2x being obtained in a moderate
yield. Similarly, using 1,2-diphenylhydrazine as a selected
substrate, the mono-formylation product 2y was also acquired
under identical conditions. Additionally, the reaction did not
proceed in the case of DL-proline following the recovered
substrate presumably because of the sterically hindered and
deactivating COOH group on the adjacent carbon atom of the
amine (2z). Likewise, the by-product with carboxyl group being
reduced could not be afforded under mild conditions. Thus, not
only linear and cyclic secondary amines, but also aliphatic (2A),
alicyclic (2B) and aromatic primary amines (2C) were found to
be reactive substrates. For most of primary amines, both N−H
bonds of the amines were reactive for N-formylation, and mono-
and diformylated products were obtained under identical
conditions.[21] Delightfully, 4- ester or carbonyl group substituted
phenylamine (2D and 2E) were suitable for this transformation
and these substituents could be tolerated, which were obtained
only mono-formylation products. Then, we chose two
substrates containing the C=N bond (2F and 2G) to investigate
the stability of imine group in this reductive conditions. As
expected, benzophenone imine (2F) and benzophenone
hydrazine (2G) were converted to their corresponding
formamides without the reduction of the unsaturated bond. The
similar phenomenon was also reported by Prof. Paul J. Dyson’s
group using the TBAF•3H2O as a sole catalyst.[30] Additionally,
Gao et al had recently reported a series of ionic mesoporous
o
equivalent TBAB cocatalyst (or at 40 C for 6 h with 0.5 mol%
ILZ1), reflecting excellent substrate generality and tolerance to
various functionalities.[32] As expected, for each secondary
amine, sole N-formylated product was obtained with
a
negligible yield of methylamine products. Notably, both steric
and electronic effects play an important role in the N-
formylation reaction of amines. For N-alkyl phenylamine
derivatives, the decreased catalytic activity was observed as
the alkyl length of N-substituted substrate increased in the
following order, i.e., methyl>ethyl>isopropyl (2a>2b>2c).
Similarly, N,N’-dibenzylamine present the low activity compared
to N-methylbenzylamine due to the steric hindrance (2e<2d).
Dibenzylamine
exhibited
good
reactivity,
producing
dibenzylformamide in a yield of 93 % after doubling the catalyst
loading to 1.0 mol%. Undoubtably, the enhanced catalytic
activity of aliphatic secondary amines was observed following
the increased alkyl chains and the order was: ethyl>n-butyl>n-
hexyl (2f>2g>2h). Especially, diallylamine furnished selectively
the N-formylation product 2i in a moderate yield without
reduction of the double bond, reflecting the tolerance to alkenyl
group. Moreover, the yields obtained from N-methylaniline
derivatives with electron-donating groups at the para-position of
phenyl group were generally lower than those with electron-
withdrawing groups under identical conditions, i.e.,
methoxyl>methyl>bromide>chloride>nitro (2j>2k>2l>2m>2n). It
had been verified indirectly the N-formylation of amine from the
second step of mechansim might be the rate limiting step.
Unfortunately, strongly electron-withdrawing substituent (-NO2)
on benzene ring was not well tolerated, and p-nitroaniline (2n)
was unreactive under the identical conditions. It is worthwhile to
mention that the substrate with reducible functional groups (i.e.
bromo, chloro, carbonyl, nitro) could be tolerated quantitatively
without reductive by-products. For example, the 4-acetyl-N-
methylaniline was converted to the corresponding formamide
(2o) without the reduction of carbonyl group. Additionally,
methoxyl substituted N-methylaniline at the ortho-position
presented the lower catalytic activity compared to that at the
para-position of phenyl group due to the steric hindrance
(2p<2j).
Furthermore, cyclic secondary amine including five- and six-
membered ring converted to the corresponding formamides in
excellent yields (2q, 2r, 2s). Thereinto, the activity of 1,2,3,4-
tetrahydroquinoline was superior to indoline owing to the ring
tension (2s>2q). Nevertheless, it was also better than 4-
phenylpiperidine due to the result of comprehensive action from
the steric hindrance and electronic effect (2s>2r). Unfortunately,
the reactivity of piperidine derivatives with four methyl groups at
the 2-position the reaction is negligible ascribed to the steric
bulkiness (2t). This catalytic reaction was also found to be
robust in the N-formylation of complicated N-heterocyclic
imine-linked
pyrene
COFs
([Et4NBr]-Py-COFs)
were
successfully applied for this transformation.[25b] Moreover, the
N-H bonds in heteroaromatic compounds as a less basic
substrate such as 1H-pyrrole (2H) or imidazole (2J) were
resistant to N-formylation, which was ascribed to the rate-
determining step of amine formylation. In a word, both the
binary and bifunctional catalytic system exhibited the excellent
substrate compatibility with oxidizing groups such as alkenes,
halides, ethers, aldehydes, ketones, esters, nitro compounds,
amides and imines. Finally, it was demonstrated that our
methodology could be used for the synthesis of various fine
chemicals.
Table 3. The results of chemoselective reduction of CO2 to formamides using
various amines and PhSiH3 catalyzed by the binary and bifunctional catalytic
systems without solvent.[a]
Entry
Substrate
Product
Number
Cat.1 [b] Cat.2 [c]
1
2
3
4
5
6
7
8
9
2a: R1=Me
2b: R1=Et
2c: R1=i-Pr
2d: R2=Me
2e: R2=Bz
2f: R3=Et
2g: R3=n-Bu
2h: R3=n-Oct
2i: R3=allyl
95
74
56
54
20
>99
48
22
53
88
87
38
73
19
99
70
10
43
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10
11
12
13
14
15
16
2j: R4=4-OMe
2k: R4=4-Me
2l: R4=4-Br
97
90
81
74
<1
80
95
99
97
95
14
<1
91
38
metal catalysts and organocatalysts. Moreover, it was found
that the cation, the anion and metal center also showed an
excellent synergistic effect on promoting the formation of
formamides.[21] To gain deep insight into the reaction pathway,
the control experiment was conducted with isotopically labeled
13CO2 as a C1 resource and the results examined by high
resolution mass spectrometry (HRMS) and 13C NMR spectra
demonstrated that the CO2-reductive process can occur
smoothly, as elaborated in Figure 6.[33]
2m: R4=4-Cl
2n: R4=4-NO2
2o: R4=4-COMe
2p: R4=2-OMe
17
18
2q
2r
53
66
35
80
19
20
21
2s
2t
84
77
n.d.
99
n.d.
>99
2u
Figure 6. (i) The N-formylation of N-methylaniline from PhSiH3 and 13CO2
under standard conditions; (ii) The possible mechanism for the N-formylation
of amine over our catalytic systems
22
23
2v: R5=Me
2w R5=Ph
84
40
95
80
On the basis of the above experimental results and previous
20a]
reports,[13,
a possible mechanism for the N-formylation of
24
2x
19
33
amines using CO2 and PhSiH3 to produce formamides
catalyzed by the cooperative catalytic system was proposed.[34]
It was widely accepted that the metal hydride complexes (such
as Co-H, Cu-H, Zn-H bonds) might exist in the metal-catalyzed
hydrosilylation of CO2.[14] Because of the different coordinated
form of zinc cation (tetrahedron, octahedron …), the Zn-H bond
within the Zn(salen) catalyst could be formed in spite of the
stable four-coordinated structure. The 1H NMR analysis in
DMSO-d6 was conducted to gain deep insight into the role of
25
26
2y
2z
54
61
n.d.
n.d.
27
28
29
30
2A: R6=nBu
2B: R6=Cy
65:35
37:53
81:19
51
71:25
32:53
70:5
34
1
these Zn(salen) catalysts. From the H NMR spectra of PhSiH3
without any catalysts and its mixture with [BMIm]Br or TBAB or
ILSZ1 in the absence of CO2, it was found that the signal
assigning to Si-H of PhSiH3 was shifted slightly, and the
bifunctional catalyst ILSZ1 has the lower chemical shift values,
suggesting there was the weak interaction between PhSiH3 and
ILSZ1 (see ESI). Understandably, it was the indirect evidence
for the existence of the newly-formed intermediate species
containing Zn-H bond. Thus, we suspected the Si-H bond of
PhSiH3 was activated by catalyst ILSZ1, which might make it
more favorable for the insertion of CO2 to yield formoxysilane.
In order to further explain the cooperative effect between the
metal, cation and anion and the effect the substrate, the 1H
NMR analysis of N-methylaniline without any catalysts and its
mixture with [BMIm]Br or ILSZ1 or Zn(salen)/TBAB in DMSO-d6
was also performed. As expected, the chemical shift of N-H in
2C: R6=Ph
2D: R7=COOBu
31
32
2E: R7=COMe
36
38
38
25
2F
33
34
2G
55
31
2H
2J
n.d.
n.d.
n.d.
n.d.
35
[a]
Reaction conditions: 10 mL autoclave, amine (1.0 mmol), PhSiH3 (1.0
[b]
mmol), CO2 pressure (0.5 MPa); Zn[salen] (0.5 mol%), TBAB (0.5 mol%),
25 oC, 15 h; [c] ILSZ1 (1.0 mol%), 40 oC, 6 h.
1
amine was observed in the H NMR spectra of N-methylaniline
with these catalysts, probably caused by the hydrogen bond
between amine and the bromide anion of ILs (see ESI). In
addition, the Lewis acid-base interaction might be originated
from nitrogen lone pair electron within amine (nucleophilic) and
metal center (electrophilic). In a word, all these results
suggested that the catalyst could activate PhSiH3 to react with
CO2 yielding formoxysilane, and simultaneously activated the
amine to obtain the desired product. Additionally, the major
Mechanism consideration
It was also indicated that it is necessary to promote the
nucleophilic addition of the H atom to the C=O bond of CO2.[4]
The binary or bifunctional catalytic system could activate the
Si−H bond of hydrosilanes to form the formoxysilane
intermediate, reflecting their respective characterization of
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2497-2500 ; (d) H. Zhou, G.-X. Wang, W.-Z. Zhang, X.-B. Lu,
ACS Catal. 2015, 5, 6773-6779.
K. Kobayashi, T. Kikuchi, S. Kitagawa, K. Tanaka, Angew.
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differences between the Cu-based system and our Zn(salen)
system were listed as follows: The copper catalyst activated the
Si-H bond of PhSiH3 to form the active intermediate (Cu-H
bond), which made it more favorable for the insertion of CO2.
Subsequently, the formoxysilane can react with the amine to
produce the final product. However, the Zn(salen) catalyst and
ionic liquid (IL) could activate the Si-H bond of PhSiH3 via
synergistic effect between Lewis base-transition metal center
(LB-TM). Subsequently, the active intermediate (Zn-H bond)
facilitated the insertion of CO2 to form the active silyl formats. In
particular, the reaction might proceed via nucleophilic attack of
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[15]
Conclusions
In summary, the binary and bifunctional Zn(salen) catalytic
system catalyzed the solvent-free N-formylation reactions of
amines using CO2 and hydrosilanes to produce formamide
derivatives for the first time. The reactions can proceed under
mild conditions due to the excellent synergistic effects, which
could activate the Si−H bond of hydrosilane to form the silyl
formats in situ-generated by metal-catalyzed hydrosilylation.
Additionally, the IL-based catalysts can be easily recyclable and
reused for five times without significant loss of activity and
selectivity. In order to understand the reaction mechanism,
further studies are underway in our laboratory.
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Acknowledgements
This work was supported by the National Science for
Distinguished Young Scholars of China (No. 21425627), the
National Natural Science Foundation of China (No. 21676306),
the Natural Science Foundation of Guangdong Province (Nos.
2016A030310211
and
2015A030313104),
and
the
Fundamental Research Funds for the Central Universities of
Sun Yat-sen University. The authors also thank Guangdong
Technology Research Center for Synthesis and Separation of
Thermosensitive Chemicals and Huizhou Research Institute of
Sun Yat-sen University.
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(a) R. C. Luo, X. T. Zhou, S. Y. Chen, Y. Li, L. Zhou, H. B. Ji,
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ACS Catal. 2016, 6, 1655-1662.
Keywords: CO2 Reduction • Salen Zinc • N-formylation •
Cooperative Catalysis • Ionic Liquid
E. A. Jaseer, M. N. Akhtar, M. Osman, A. Al-Shammari, H. B.
Oladipo, K. Garces, F. J. Fernandez-Alvarez, S. Al-Khattaf, L.
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Text for Table of Contents
Rongchang Luo, Xiaowei Lin, Yaju
Chen, Wuying Zhang, Xiantai Zhou,
Hongbing Ji*
The reductive N-formylation of
amines to produce formamides
with CO2 and hydrosilanes at
ambient conditions catalyzed
by the Zn(salen) complexes
with inter- or intramolecular
ionic liquid moieties.
Page No. – Page No.
Cooperative Catalytic Activation of
Si−H Bonds: CO2-based Synthesis of
Formamides from Amines and
Hydrosilanes under Mild Conditions
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