Design of Lewis base functionalized ionic liquids for the N-formylation of amines with CO2 and hydrosilane: The cation effects

Article abstract of DOI:10.1016/j.cattod.2020.01.030

A series of functionalized ionic liquids (ILs) were developed for the reductive functionalization of CO₂ with amine and hydrosilane to afford formamides under mild conditions. It was found that 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-based IL i.e. [DBUC₁₂]Br showed high efficiency for the N-formylation reaction of amines without using any organic solvents or additives. Furthermore, control experiments suggested the cations with active hydrogen may weaken the nucleophilicity of anions through ion pairing interactions, thereby affecting the activation of hydrosilane. The reaction mechanism was then investigated by Density Functional Theory (DFT) calculations. This protocol represents a highly efficient and environmentally friendly example for catalytic conversion of CO₂ into value-added chemicals such as formamide derivatives by employing DBU functionalized ILs.

Full text of DOI:10.1016/j.cattod.2020.01.030

Journal Pre-proof
Design of Lewis base functionalized ionic liquids for the N-formylation of
amines with CO2 and hydrosilane: The cation effects
Xiao-Ya Li, Hong-Chen Fu, Xiao-Fang Liu, Shu-Han Yang, Kai-Hong
Chen, Liang-Nian He
PII:
S0920-5861(20)30034-1
DOI:
https://doi.org/10.1016/j.cattod.2020.01.030
CATTOD 12649
Reference:
To appear in:
Catalysis Today
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Revised Date:
Accepted Date:
13 June 2019
10 January 2020
24 January 2020
Please cite this article as: Li X-Ya, Fu H-Chen, Liu X-Fang, Yang S-Han, Chen K-Hong, He
L-Nian, Design of Lewis base functionalized ionic liquids for the N-formylation of amines with
CO2 and hydrosilane: The cation effects, Catalysis Today (2020),
doi: https://doi.org/10.1016/j.cattod.2020.01.030
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Design of Lewis base functionalized ionic liquids for the
N-formylation of amines with CO2 and hydrosilane: The cation
effects
Xiao-Ya Li, Hong-Chen Fu, Xiao-Fang Liu, Shu-Han Yang, Kai-Hong Chen*,
Liang-Nian He*
State Key Laboratory and Institute of Elemento-Organic Chemistry, College of
Chemistry, Nankai University, Tianjin 300071, China
*Corresponding authors
e-mail addresses: khchen@nankai.edu.cn (K.-H. Chen), heln@nankai.edu.cn (L.-N.
He), Tel: +86-22-23503878
Graphical Abstract
Highlights


[DBUC12]Br is developed as an efficient catalyst for the N-formylation of amines with CO2
[DBUC12]Br can be reused over 5 times without significant loss of its activity.
1



Control experiments indicate the active hydrogen on the IL’s cation hinders the
N-formylation
DFT calculations show that amine activation is not the rate-determining step to this
transformation.
Abstract
A series of functionalized ionic liquids (ILs) were developed for the reductive
functionalization of CO2 with amine and hydrosilane to afford formamides under mild
conditions. It was found that 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)-based IL i.e.
[DBUC12]Br showed high efficiency for the N-formylation reaction of amines without
using any organic solvents or additives. Furthermore, control experiments suggested
the cations with active hydrogen may weaken the nucleophilicity of anions through
ion pairing interactions, thereby affecting the activation of hydrosilane. The reaction
mechanism was then investigated by Density Functional Theory (DFT) calculations.
This protocol represents a highly efficient and environmentally friendly example for
catalytic conversion of CO2 into value-added chemicals such as formamide
derivatives by employing DBU functionalized ILs.
Keywords
Carbon dioxide fixation, Formylation, Ionic liquid, Metal-free catalysis, Reduction
1. Introduction
As carbon dioxide (CO2) is a non-toxic, abundant, easily available, and
environmentally benign feedstock, a series of thriving research fields have been built
around its chemical modification and conversion [1-6]. In view of green and
sustainable chemistry, designing recyclable catalysts for the transformation of CO2
into various value-added chemicals under mild conditions is interesting and
significant both in academia and society [7-13]. Ionic liquids (ILs), consisting of
2

cations and anions, have attracted intense interest in the areas of chemistry and
industry as efficient catalysts and solvents in the past few decades owing to their
outstanding characters, such as thermal and chemical stability and non-volatility
[14-17]. The property of ILs can be adjustable for a variety of desired reactions
through designing the structures of cations and anions [18]. Particularly, task-specific
ILs such as amino-, amino acid-, azolate-, phenolate-, pyridine-containing ILs and so
on have been also reported as efficient catalysts in CO2 capture or utilization by
rational designing their structures [12, 19-22].
The reductive functionalization of CO2 combining both CO2 reduction and C–N,
C–C or C–O bond formation in one pot has boomed recently, which significantly
broadens the range of chemicals accessible from CO2 [23-29]. In this context, selective
2-, 4-, 6-electron reduction of CO2 with a reducing reagent e.g. hydrogen, hydrosilane
or hydroboron and different nucleophiles to give various products such as formamides
[30-34], formamidine derivatives [35], formates [36], benzothiazoles [37-38], aryl
aldehydes [39-41], aminals [42-45], methylene [46], homoallylic alcohols [47],
dimethoxymethane (DMM) [48-50], dithioacetals [51], methylamine [52-55] and so on
(Scheme 1). In recent years, intensive investigations focus on the 2-electron reductive
functionalization of CO2 especially the N-formylation reaction of amines into
formamide derivatives, since formamides are a kind of versatile chemicals and
important building blocks for the production of drug molecules and agrochemicals, for
instance, fluoroquinolines, 1,2-dihydroquinolines, substituted aryl imidazoles,
nitrogen-bridged heterocycles, etc [30-32, 56]. In this context, Cantat and co-workers
firstly reported the novel diagonal transformation pathway for CO2 recycling with
1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD)-catalyzed system to produce formamides
employing hydrosilane as reducing reagent in combination with amines as nucleophile
in 2012 [23]. Following this initial work, diverse efficient transition-metal-free catalysis
systems based on N-heterocyclic carbenes (NHCs) [30, 57-59], 1,3,2-diazaphospholene
(NHP-H) [60], inorganic bases [61-63], organic bases [43, 45, 64-65], as well as
catalyst-free [66-68] protocol have been developed for the N-formylation reaction.
3

Scheme 1. The examples of reductive functionalization of CO2.
Recently, imidazolium-based ILs [69], acetylcholine-carboxylate ILs [70],
ILs-decorated COFs (Covalent Organic Frameworks) catalysts [71], and
ILs-functionalized Zn(salen) catalysts [72] have showed good activity for the
N-formylation of amines with CO2 to form formamides under mild conditions. Notably,
theses ILs exhibited outstanding catalytic performance in reusability, which can reduce
cost and avoid pollution. Moreover, it is found that the ILs serve as bifunctional
catalysts, anions play a significant role for the activation of hydrosilane and cations can
help to activate substrate e.g. amine or CO2 simultaneously. However, the role and
effect of the cation of ILs in this N-formylation reaction has not been studied in detail.
Therefore, we decided to design some functionalized ILs and further investigated the
role of cation and the possible mechanism, which is essential to develop efficient
IL-based catalysts for the N-formylation reaction as depicted in Scheme 2. We choosed
moderate nucleophilic bromide or chloride as anions [73] and various cations with or
without active hydrogen, for example, imidazolium, guanidinium, quaternary
ammonium, DBU and DBN-based ILs. The DBU-based ILs i.e. [DBUC12]Br and
4

[DBUC4]Br showed excellent activity for catalyzing the N-formylation reaction to
afford formamides at room temperature, and had excellent reusability through washing
with hexane from the reaction mixture.
Scheme 2. Design of functionalized ILs for the N-formylation reaction from CO2 and
the structures of ILs employed in this work.
2. Experimental section
2.1 Materials and methods
The starting materials including amines and hydrosilanes were commercially
available from Innochem, Energy Chemical or J&K Company and were used without
further purification. Ionic liquids were prepared according to the methods of
literatures. Products were isolated by column chromatography on silica gel (200-300
5

mesh) using petroleum ether (60-90 oC) and ethyl acetate. All compounds were
characterized by 1H NMR, 13C NMR and mass spectroscopy, which are consistent
with those reported in literature. 1H NMR spectra was recorded on 400 MHz
spectrometers using CDCl3 as solvent referenced to tetramethylsilane (TMS, 0 ppm).
The 13C NMR chemical shifts were reported in ppm relative to the carbon resonance
of CDCl3 (central peak is 77.0 ppm). GC-MS data were performed on Shimadzu
GCMS-QP2010 with an ionization voltage of 70 ev and a capillary column model of
RTX-5MS. High-resolution mass spectrometry (HRMS) was conducted using Agilent
6520 Q-TOF LC/MS Infrared. Melting points are measured on an X4 apparatus and
uncorrected.
2.2 General procedure for the N-formylation reaction of amines with CO2
The formylation reaction was conducted in a stainless steel autoclave (25 mL inner
volumes). In a typical reaction, the reactor was charged with [DBUC12]Br (20.0 mg,
20 mol% relative to amine), N-methylaniline (27 μL, 0.25 mmol) and phenylsilane
(62 μL, 0.5 mmol) successively at room temperature. Then, CO2 was introduced into
the reactor and the pressure was adjusted to 5 bar at room temperature for 6 h. After
the reaction was completed, the excess of CO2 was carefully vented and then
1,3,5-trimethyoxybenzene (21.0 mg) was added as an internal standard for 1H NMR
yield determination. The residue was purified by column chromatography with ethyl
acetate–petroleum ether as the eluent to afford the desired formylated product. The
characterization data of the products (1b-15b) can be found in supporting information.
2.3 General procedure for the recovery of the IL catalyst
After the reaction was completed under the optimal conditions, the reaction mixture
was washed with hexane five times to extract the formylated product remained in the
organic phase. Nevertheless, the residual IL catalyst at the bottom of the bottle was
separated with product due to its insoluble in hexane.
2.4 Computational methods
Theoretical calculations were performed with the Gaussian 09 program [74].
6

Geometry optimizations were carried out at the B3LYP [75-78]/6-31+G(d) level.
Frequency calculations were carried out at the same level to conform that there was
no imaginary frequency for ground state structures and only one imaginary frequency
for transition state structures. The transition state was confirmed by examining the
vibrational mode of the imaginary frequency. Some important transition states were
further checked by intrinsic reaction coordinate (IRC) calculations. Single point
energy calculations for optimized structures were performed at M06-2X
[79-81]/cc-pvtz level with SMD [82] solvation model used to account for solvent
(dichloroethane) effects, because dichloroethane had similar dielectric properties with
ILs [83]. In addition, we utilized a correction of (n-m)*5.4 kcal/mol for a process
from m- to n-components [84].
3. Results and discussion
3.1 Catalyst screening for the N-formylation
In our initial investigation, the model reaction of N-formylation using N-methylaniline
(1a), CO2 and phenylsilane (PhSiH3) was established to evaluate the catalytic activity
of ILs, and the results were summarized in Table 1. To our surprise, catalytic amount
(20 mol%) of [MImC12]Br and [TMGC12]Br, which could be regarded as bifunctional
catalysts, only obtained 1b in 9% and 42% yields respectively in the presence of CO2
at 0.5 MPa and room temperature without any solvent (entry 1 and 2). However, 1a
was almost completely converted when using [N222C12]Br, indicating it’s catalytic
activity is much higher than [MImC12]Br and [TMGC12]Br (entry 3). To our delight,
[DBUC12]Br allowed the desired reaction to afford 1b in 99% yield (entry 4).
[DBNC12]Br, provided 1b in 95% yield, possibly due to a similar structure with
[DBUC12]Br (entry 5). With a shorter chain, [DBUC4]Br was also effective for this
reaction, indicating the chain length had no effect on its activity (entry 6). We didn't
choose [DBUC4]Br, though it is cheaper than [DBUC12]Br, when considering the
toxicity of 1-bromobutane. However, [DBUC4]Cl exhibited lower catalytic activity
comparing with [DBUC4]Br, maybe owing to the difference nucleophilicity of anions
7

(entry 7). According to those results, we found that cations have a great influence on
this N-formylation reaction. The less steric hindrance of imidazolium than other ILs
will lead to a stronger interaction between imidazolium and Br-. In addition,
guanidinium with active hydrogen resulted in lower catalytic activities comparing
with quaternary ammonium, DBU and DBN. It was also proposed that strong
cation-anion interaction could lead to a weaker nucleophilic of ion pairs [73, 85].
Then being reduced the pressure of CO2 to 0.2 MPa, 1b could be obtained with 90%
yield (entry 8). Notably, as the amount of [DBUC12]Br was increased to 50 mol%, 1b
yield could attain to 89% at atmospheric pressure (entry 9). Moreover, the reaction did
not occur without CO2 and IL, implying that CO2 acted as C1 source and IL acted as
catalyst was indispensable for this reaction (entry 10 and 11). Subsequently, various
hydrosilanes were also investigated and PhSiH3 was the best reductant (see Table S1).
8

Table 1.
N-formylation of amines to formamides with CO2 .
a
Entry ILs
CO2
(MPa)
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.2
0.1
-
Conv. of Yield of
1ab (%)
11
1bb (%)
9
1
[MImC12]Br
2
[TMGC12]Br
[N222C12]Br
[DBUC12]Br
[DBNC12]Br
[DBUC4]Br
[DBUC4]Cl
[DBUC12]Br
[DBUC12]Br
[DBUC12]Br
-
42
99
99
99
99
86
92
92
0
42
77
99
95
99
80
90
89
0
3
4
5
6
7
8
9c
10d
11
0.5
0
0
aConditions: 1a (0.25 mmol, 27 μL), IL (20 mol%, relative to 1a), PhSiH3 (0.5 mmol,
62μL). bDetermined by 1H NMR using 1,3,5-trimethyoxybenzene as an internal
standard. c IL (20 mol%, relative to 1a). dArgon instead of CO2 atmosphere.
3.2 Catalytic activity towards other amine substrates
Using a catalytic amount of [DBUC12]Br (20 mol% relative to substrate), we then
evaluated the scope of this N-formylation reaction with various amines in the presence
of PhSiH3 as reductant at room temperature without any solvent. As shown in Scheme
3, secondary aromatic amines with both electron-donating and electron-withdrawing
substituents on the phenyl ring including 4-methyl, 4-chloro, 4-methoxyl were all
suitable substrates, delivering N-methylformanilide derivatives in 89~99% yields
9

(2b-5b). However, N-methyl-4-nitroaniline (6a) was inactive, possibly due to the
strong electron-withdrawing effect. N-ethyl-, N-isopropyl-, N-benzyl- and
N-allylanilines proceeded efficiently to furnish the corresponding formylated products
in 52~94% yields (7b-10b). Due to steric hindrance, bulk substrates gave relatively
low product yields (9b-10b). Secondary aliphatic amines, such as morpholine,
pyrrolidine and diethylamine, showed good reactivity and afforded the desired
products in 64~94% yields (11b-13b). Notably, this N-formylation reaction was also
applicable to cyclic amines such as 1,2,3,4-tetrahydroquinoline and indoline (14a and
15a).
Scheme 3. N-formylation of various amines with CO2. a Reaction were performed
with amine (0.25 mmol), [DBUC12]Br (20 mol%, 20.0 mg), PhSiH3 (2 mmol, 62 μL),
CO2 (0.5 MPa) at room temperature for 12 h. Yields were determined by 1H NMR
using 1,3,5-trimethyoxybenzene as internal standard and the isolated yields were
given in parentheses. b Reaction time is 6 h.
10

3.3 Catalyst Recyclability
Reusability was also a significant criteria for IL catalysis. To investigate the
recyclability of [DBUC12]Br, the reaction was performed under the optimal reaction
conditions (Fig. 1). [DBUC12]Br was recovered from the reaction mixture through
washing with hexane. It was shown that the yield of 1b almost stayed constant during
five successive recycles, indicating that its activity could maintain under this
conditions.
120
100
80
60
40
20
0
1
2
3
4
5
Cycle
Fig. 1. The recyclability of [DBUC12]Br.
3.4 Possible mechanism
Clearly, DBU functionalized ILs in this study was found to be a better catalyst than
others, particularly [MImC12]Br (entry 4 vs entry 1, Table 1), and this superiority
drives us to find out the reason. At first, DFT calculations were conducted to
investigate the mechanism as shown in Fig. 2. Firstly, Br‾ nucleophilic attacks PhSiH3
and then CO2 inserts into the Si−H bond to form silyl formate IM2, which has to
overcome 17.8 kcal/mol barrier. To our surprise, the barrier for nucleophilic attacking
from 1a to IM3 is only 10.4 kcal/mol, indicating that the amine activation is not a
rate-determining step (RDS) to this transformation. Subsequently, with the assistance
of H2O, IM3 undergoes a 6-membered ring transition state TS3 and then realize
11

hydrogen transformation step between N and O to form IM4, which is the RDS for
this reaction (19.4 kcal/mol). After that, a new Si−O bond is formed accompanying
with the obtaining of 1b.
Fig. 2. Potential energy surface for the formation of 1b.
According to previous reports [44] and our DFT calculations, the activation of
PhSiH3 is crucial to initiate the reaction. Therefore, we wanted to find out the
difference between [DBUC12]Br and [MImC12]Br in the ability to activate PhSiH3. It
was disappointing that only a tiny shift for the hydrogen on PhSiH3 was found in 1H
NMR, after mixing [DBUC12]Br with PhSiH3 (see Fig. S1). Inspired by previous
reports [86] that CO2 could facilitate the hydrogen activation, we performed these
control experiments under 0.5 MPa CO2. With [DBUC12]Br as catalyst, the reaction
between PhSiH3 and CO2 afforded the key intermediate IM2 in 18% yield, but it did
12

not occur by using [MImC12]Br as catalyst or in the absence of catalyst (Scheme 4).
In other words, the poor ability to activate PhSiH3 is the reason for the poor activity of
[MImC12]Br. The consequence could also support our hypothesis that cations with
active hydrogen may weaken the nucleophilicity of anions through ion pairing
interactions, thereby affecting the activation of hydrosilane.
Scheme 4. Control experiments to explore the role of the ILs in activating
phenysilane.
On the basis of our experimental results and previous reports [44, 69, 73, 87], a
tentative reaction mechanism for the [DBUC12]Br-catalyzed N-formylation of amines
with CO2 was proposed in Scheme 5. Initially, Br‾ of [DBUC12]Br nucleophilic
attacks PhSiH3 forming the active hypervalent silicon intermediate IM1. Then CO2
inserts into the Si−H bond to form silyl formate IM2. Next, IM2 is nucleophilically
attacked by 1a generating IM3 under the activation of Br‾. Subsequently, with the
assistance of H2O, IM3 undergoes a 6-membered ring transition state TS3 to realize
hydrogen transformation step between N and O to form IM4. Finally, C−O bond on
IM4 cleavages providing the target product 1b.
13

Scheme 5. Possible reaction mechanism for the [DBUC12]Br-catalyzed N-formylation
of amines with CO2.
4. Conclusions
In summary, DBU-based ILs, [DBUC12]Br, was proven to be a highly efficient and
recoverable catalyst for efficient CO2 conversion into formamides under mild
conditions. The poor activity for [MImC12]Br should be ascribed to its ion pairing
interactions weakening the nucleophilic of Br‾ for the activation of hydrosilane, as
identified by control experiments. The protocol was found to be applicable to different
kinds of secondary amines giving the corresponding formamide products in moderate
to excellent yields. Moreover, the catalyst was easy recovery and could be reused over
5 times without significant loss of its activity. This work will undoubtedly shed light
on the direction for designing catalyst on reductive functionalization of CO2.
14

Declaration of interests
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Credit Author Statement
X.-Y. Li, H.-C. Fu and S.-H. Yang performed the catalyst preparation, CO2 reduction
experiments. X.-Y. Li, X.-F. Liu, K.-H. Chen, and L.-N He provided helpful
discussion. K.-H. Chen perform the DFT calculation. X.-Y. Li, K.-H. Chen, and L.-N
He analyzed the data, and wrote the paper. K.-H. Chen, and L.-N He designed the
study.
Acknowledgements
This work was financially supported by National Key Research and Development
Program (2016YFA0602900), National Natural Science Foundation of China
(21672119), and the China Postdoctoral Science Foundation (2018M641624).
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