Recyclable Oxofluorovanadate-Catalyzed Formylation of Amines by Reductive Functionalization of CO2 with Hydrosilanes

Article abstract of DOI:10.1002/cssc.202100117

An efficient method has been developed for the reductive amination of CO₂ by using readily available and recyclable oxofluorovanadates as catalysts. Various amines are transformed into the desired N-formylated products in moderate to excellent yields at room temperature in the presence of phenylsilane. Mechanistic studies based on in situ infrared spectroscopy suggest a reaction pathway initiated through F?Si interactions. The activated phenylsilane allows for CO₂ insertion to produce phenylsilyl formate, which undergoes attack by the amine to generate the target product.

Full text of DOI:10.1002/cssc.202100117

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Recyclable Oxofluorovanadate-Catalyzed Formylation of
Amines by Reductive Functionalization of CO2 with
Hydrosilanes
Shanxuan Wu+,[a, b] Zijun Huang+,[b, c] Xiaolin Jiang,[b] Fachao Yan,[b] Yuehui Li,*[b] and
Chen-Xia Du*[a]
An efficient method has been developed for the reductive
amination of CO2 by using readily available and recyclable
oxofluorovanadates as catalysts. Various amines are trans-
formed into the desired N-formylated products in moderate to
excellent yields at room temperature in the presence of
phenylsilane. Mechanistic studies based on in situ infrared
spectroscopy suggest a reaction pathway initiated through FÀ Si
interactions. The activated phenylsilane allows for CO2 insertion
to produce phenylsilyl formate, which undergoes attack by the
amine to generate the target product.
Introduction
numerous reports on reduction of CO2 and reductive amination
using metal catalysts such as Pt,[11] Ru,[12] Re,[13] Ir,[14] Fe,[15] W,[16]
Cu,[17] among others. In addition, the use of some organic
catalysts such as TBD (1,5,7-triazabicyclo[4.4.0]dec-5-ene),[18]
ionic liquids,[19,20] NHCs (N-heterocyclic carbene)[21] and
betaine[22] have also been reported. Although these catalysts
have high efficiency and generate products in shorter reaction
times, homogeneous nature of these catalysts are less favored
by the industry, and hence new and suitable catalytic systems
need to be further explored.
Nonmetallic element fluorine, with a small ionic radius and
strong electronegativity, is widely distributed in nature. Organic
F-containing compounds and F-containing polymers exhibit
excellent physical and chemical properties. The introduction of
F atom in drug molecules can fine-tune the molecular structure,
easily block metabolized sites, improve the molecular metabolic
stability and increase the binding affinity of the
compounds.[23,24] Fluorine and silicon are often engaged in
strong interaction, which results in silicon-hydrogen bonds
activation to achieve phenylsilane hydride transfer accelerating
the reductive reaction. The increased nucleophilicity of the
hydride ion in the presence of fluoride ions with TBAF
(tetrabutylammonium fluoride) or cesium fluoride was reported
by the groups of Baba[25] and He[26] using dimethylphenylsilane
and triethoxysilane as reducing agents, respectively. However,
heterogeneous fluorine-containing materials are rarely used in
formamide synthesis reactions involving carbon dioxide,
although they are more suitable catalysts for industrial
applications.
The intensification of CO2 greenhouse effects in recent years
has attracted considerable research activities to mitigate climate
change through CO2 utilization as inherently nontoxic, abun-
dant, and renewable chemical feedstock.[1–5] Carbon dioxide can
be reduced to carbon monoxide, formic acid, formaldehyde,
methanol and methane, or can undergo a reduction functional-
ization process to build CÀ O, CÀ C, and CÀ N bonds; and hence
generate a multitude of chemical products.[6–9] Compounds
featuring CÀ N bond are widely present in agrochemicals and
pharmaceuticals, and moreover CÀ N bond assumes an impor-
tant functionality in the field of organic synthesis.[10] Forma-
mides and amides are representative of CÀ N bond compounds,
and often play the role of solvents and substrates in organic
synthesis. Formamides were generally prepared from carbon
monoxide as a carbon source requiring high pressures and
temperatures and longer reaction times.[10] Recently, with
advent of CO2 utilization technologies, reductive functionaliza-
tion of carbon dioxide with amines is regarded an effective and
a safer synthetic method to access formamides. There are
[a] S. Wu,+ Prof. Dr. C.-X. Du
College of Chemistry
Zhengzhou University
Zhengzhou 450001 (P.R. China)
E-mail: dcx@zzu.edu.cn
[b] S. Wu,+ Z. Huang,+ X. Jiang, F. Yan, Prof. Dr. Y. Li
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Suzhou Research Institute of LICP
Center for Excellence in Molecular Synthesis
Lanzhou Institute of Chemical Physics
Chinese Academy of Sciences
Vanadate-type compounds have interesting properties
(ionic environments with strong polar distortions and Lewis
acidity and magnetic properties) that can contribute to small
molecule activation and catalysis.[27–30] To our knowledge, the
use of oxofluorovanadate materials as catalysts for reductive
Lanzhou 730000 (P. R. China)
E-mail: yhli@licp.cas.cn
[c] Z. Huang+
College of Chemistry and Chemical Engineering
Hunan Institute of Engineering
Xiangtan, 411104 (P. R. China)
amination of carbon dioxide has not been reported to date. We
[31–34]
investigated K2VOF4
as a heterogeneous catalyst for the
[+] These authors contributed equally to this work.
formylation of several amines with CO2 and phenylsilane. We
discovered that K2VOF4 was able to catalyze formylation
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cssc.202100117
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reactions at room temperature and at atmospheric pressure of
CO2 (1 bar) producing formamides in excellent yields.
Table 2. Substrate scope study.
Results and Discussion
Based on our recent work in developing fluoride-type catalysts
for aminocarbonylation[35] and arylborate catalysts for CO2
reductive amination,[36] we commenced by investigating inor-
ganic fluorides as catalysts for N-formylation reactions. Initially,
various fluoride salts were tested with N-methylaniline (1a) as a
model substrate for the optimization of the reaction conditions
(Table 1). A blank experiment without catalyst resulted in
unsuccessful formamide formation (Table 1, entry 1). Various
common fluoride salts such as FeF2, FeF3, K2TiF6 have been
tested and also resulted in no product formation (Table 1,
entries 2–4). In contrast, when K2NbF7 catalyst was used, N-
methylformanilide was afforded in 64% yield at room temper-
ature (Table 1, entry 5). To our delight, an excellent yield of
90% was obtained when K2VOF4 was used as catalyst with
phenylsilane (Table 1, entry 6). Encouraged by this result we
turned to optimize the reducing agents through simple silane
screening. Unfortunately, most silanes were ineffective in the
formation of the product (Table 1, entries 8–11), with the
exception of diphenylsilane, which gave a reduced yield of 22%
(Table 1, entry 7).
With the optimal reaction conditions in hand, we then
started to explore the scope and limitations of K2VOF4-catalyzed
N-formylation reaction as shown in Table 2. p-Substituted
anilines were transformed smoothly to give the corresponding
products in excellent yields (2b–2d), with the exception of p-
iodo-aniline substrate providing N-formylated product in only
35% yield (2e). N-methylaniline compounds with various aryl
substituents were also investigated and provided the N-
formylated in good yields (2f–2j). Both electron withdrawing or
donating substituents have little effect on the reaction yield
with chloride, methyl, methoxy and acetyl para-substituents
[a] Reaction conditions: 1 (0.2 mmol), CO2 (1 bar), K2VOF4 (10 mg) PhSiH3
(1.2 equiv.) acetonitrile (0.5 mL), room temperature, 10 h; yields refer to
isolated products. [b] Yield determined by GC.
gave excellent yields of 83% (2f), 95% (2g, 2h), 87% (2i) and
77% (2j), respectively. Under optimized conditions, N-butylani-
line can also be N-formylated in 92% yield (2k). Furthermore,
benzylamine compounds (2l–2r), heterocyclic amines (2s, 2t),
and aliphatic amines (2u, 2v) were also investigated with
moderate to excellent yields obtained (Table 2). N-meth-
ylbenzylamine with different substituents provided the N-
formylated products with comparable yields, 78~83% (2m–
2o). N-ethylbenzylamine provided an excellent yield of 97%
(2q). N-tBu-benzylamine with bulky tertiary butyl group gave in
lower yield of 76% (2p) while GC determined yield for
dibenzylamine was up to 87% (2r). Indoline and 1,2,3,4-
tetrahydroquinoline were formylated in 87% (2s) and 94% (2t)
yield (isolated products), respectively. The yields of aliphatic
hexylamine (2u) and N-[2-(dimethylamino)ethyl]-N-meth-
ylformamide (2v) were determined by GC, and the results were
satisfactory.
Table 1. Optimization of the reaction conditions.[a]
Entry
Cat./mol%
SiÀ H/equiv.
Yield (2a) [%][b]
Yield (3a) [%][b]
1
2
3
4
5
6
7
8
none
FeF2/10
FeF3/10
K2TiF6/10
K2NbF7/10
K2VOF4/10[c]
K2VOF4/10[c]
K2VOF4/10[c]
K2VOF4/10[c]
K2VOF4/10[c]
K2VOF4/10[c]
PhSiH3/2
PhSiH3/2
PhSiH3/2
PhSiH3/2
PhSiH3/2
PhSiH3/2
Ph2SiH2/2
(EtO)3SiH/2
PMHS[d]/2
Et3SiH/2
0
0
0
5
64
90
22
0
0
0
0
0
0
0
0
trace
0
0
0
0
0
9
10
11
Ph2MeSiH/2
0
To gain more information about the mechanism for this
transformation, we studied the interactions between K2VOF4,
phenylsilane and CO2 by NMR and in situ infrared spectroscopy.
[a] Reaction conditions: 1a (0.2 mmol); CO2 pressure: 1 bar; acetonitrile
(0.5 mL). [b] GC yield with n-dodecane as internal standard. [c] 10 mg.
[d] PMHS=polymethylhydrosiloxane.
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A new characteristic peak at 1139 cmÀ 1 can be observed, which
is red-shifted from the signal of phenylsilane at 1118 cmÀ 1,
when adding the catalyst to the acetonitrile solution of phenyl-
silane under carbon dioxide atmosphere (see the Supporting
Information). When the amine was added to the reaction
mixture, we observed that the peak continues to increase
(Figure 1). We speculate that the alkalinity of the reaction
system increases which increases the solubility of carbon
dioxide and also increases the interaction between the silane
and the catalyst.
The solubility of the catalyst in the reaction solution was
tested by ICP-MS. The test results showed that a very small
amount of vanadium was present in the reaction solution
(25.942 ppb). The fluorine-containing inorganic salt catalyst was
evaluated on a 10 mg catalyst scale in batch mode over several
cycles in the N-formylation of N-methylaniline, affording N-
methyl-N-phenylformamide in >90% yield (GC yield) over 5
batches recycling (Figure 2). After the initial reaction, the
catalyst was retained by centrifugation, then the catalyst was
washed twice with a small amount of acetonitrile and reused in
the next cycle without drying.
Based on NMR and IR spectroscopy results, we have
proposed a plausible reaction mechanism (Scheme 1). At first,
intermediate B was formed through interactions of A with
phenylsilane, owing to the Lewis acidity of vanadium, which Conclusion
was consistent with previous reports.[37] The activated SiÀ H
bond was easily attacked by carbon dioxide accompanied by
hydride transfer, leading to the formation of intermediate C.
This is followed by CÀ O bond cleavage[19,38] and amine/formyl
condensation giving product E with the release of silanol. On
the surface of oxofluorovanadate catalysts, VÀ H interactions
might also be involved to activate hydrosilanes, considering the
strong Lewis acidity of VIV center.
In summary, we have reported oxofluorovanadate K2VOF4 as an
effective catalyst in the N-formylation of primary and secondary
amines in the presence of phenylsilane and CO2. The catalyst is
readily available, cost effective, and reusable. Aromatic amines,
benzylamines, NÀ H bond-containing heterocycles, and aliphatic
amines were all efficiently converted into the corresponding N-
formamides under standard conditions. This new approach for
N-formylation of amines uses CO2 with inorganic oxofluorova-
nadate catalysts proceeded at room temperature and at
atmospheric pressure of CO2 with high efficiency and catalyst
recyclability. The work also highlighted the reaction pathway
initiated through FÀ Si interactions and the utility of oxofluor-
ovanadate heterogeneous catalysts in N-formylation reactions.
Experimental Section
Preparation of K2VOF4
K2VOF4 was prepared according to a reported method,[31] with slight
modifications. V2O5 (0.364 g, 2.00 mmol) was weighed into a 50 mL
poly(tetrafluoroethylene) (PTFE)-lined stainless steel autoclave and
dissolved in 48% HF (1 mL) at room temperature. Then, water
(10 mL) and ethylene glycol (10 mL) were mixed and added and the
Figure 1. In situ infrared spectrum showing the characteristic peaks
of PhSiH3.
Figure 2. Recycling of the K2VOF4 catalyst in the N-formylation of N-meth-
ylaniline to N-methyl-N-phenylformamide. Yields determined by GC using
n-dodecane as an internal standard.
Scheme 1. Proposed mechanism for the transformation.
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Keywords: amides · carbon dioxide · fluorides · heterogeneous
catalysis · silanes
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