N-Formylation of Amines with CO2 and H2 by Using NHC–Iridium Coordination Assemblies as Solid Molecular Catalysts

Article abstract of DOI:10.1002/asia.201800953

One of the NHC–iridium coordination assemblies containing 1,5-cyclooctadiene (COD) and iodide ion has been demonstrated as robust, efficient, recyclable solid molecular catalyst for N-formylation of diverse primary and secondary amines with CO₂ and H₂ under mild reaction conditions. Remarkably, in the case of N,N-dimethylformamide production, even at 0.1 mol % catalyst loading under solvent-free conditions, the solid catalyst can be readily recovered by simply filtration and reused more than 10 runs without noticeable loss of activity.

Full text of DOI:10.1002/asia.201800953

CHEMISTRY
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Accepted Article
Title: N-Formylation of Amines with CO2 and H2 by Using NHC-Iridium
Coordination Assemblies as Solid Molecular Catalysts
Authors: Tao Tu, Yang Zhang, Jiaquan Wang, and Haibo Zhu
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Chemistry - An Asian Journal
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COMMUNICATION
N-Formylation of Amines with CO and H by Using NHC-Iridium
2
2
Coordination Assemblies as Solid Molecular Catalysts
[a]
[a]
[a]
[a,b]
Yang Zhang, Jiaquan Wang, Haibo Zhu and Tao Tu*
[
32]
Abstract: One of the NHC-iridium coordination assemblies
containing 1,5-Cyclooctadiene (COD) and iodide ion has been
demonstrated as robust, efficient, recyclable solid molecular catalyst
2 2
under CO /H atmosphere (35 atm for each, Scheme 1a).
Remarkably, a TON of up to 599000 has been attended
with high selectivity for DMF production. In addition, even
as a homogenous case, the pincer catalyst could be reused
for 12 runs without significant loss of activity by a simple
recycling process. In 2017, Milstein and coworkers realized
for N-formylation of diverse primary and secondary amines with CO
and H under mild reaction conditions. Remarkably, in the case of
2
2
N,N-dimethyl-formamide production, even at 0.1 mol% catalyst
loading under solvent free conditions, the solid catalyst can be
readily recovered by simply filtration and reused more than 10 runs
without obviously loss of activity, highlighting the feasibility in
industry.
the selective N-formylation of amines with CO
2 2
and H
using Co-PNP pincer complex as catalyst. A number of
amines were converted to their corresponding formamides
o
[34]
at 150 C.
Despite of these achievements, air-sentive
luxury phosphine ligands, high reaction temperature and
high gas pressure are still hampered their pratical
application in industry. As heterogeneous catalytic system
As one of the most inexpensive, renewable and abundant
C1 resource, the utilization of carbon dioxide (CO
2
) for
[35-37]
is more suitable for the industrial DMF manufacturing,
syntheses of feedstock and value-added chemicals has
attracted intensive attentions from both laboratory and
in 2014, Shi and coworkers demonstrated Pd/Al
catalyst by depositing palladium on a shape controllable
Al support exhibiting good activity in the catalytic N-
formylation of amines with CO and H (1 MPa CO and 2
, Scheme 1a) at 130 C. And the Pd/Al -NR-RD
2 3
O -NR-RD
[
1-9]
industry.
Therefore, in recent years, great efforts have
2
O
3
been devoted to explore and develop efficient catalysts for
this purpose. Among all diverse direct transformations of
2
2
2
o
MPa H
2
2 3
O
CO
2 2 2
, N-formylation of amines with CO and H constitutes a
catalysts are readily seperated via centrifugation, and 83%
straightfoward, atom-economic and sustainable approach,
[35]
yield was maintained after reused for three times.
[
10]
in which water is the only by-product.
Remarkably,
valued-added formamides are usually considered as useful
chemicals and precursors for syntheses of a number of
[
11-14]
biological active compounds.
For instance, N,N-
dimethylformamide (DMF) is not only one of the most
important solvents, but also considered as a key precursor
to access many adhesives, pesticides, preservatives and
[
15,16]
drugs.
In contrast with the conventional industrial
) with
with safe
for DMF syntheses is challenging and less-
studied, probably due to the high thermodynamic stability
protocols by carbonylation of dimethylamine (NHMe
2
[
17]
toxic CO,
CO and H
the direct N-formylation of NHMe
2
2
2
[
6]
[
18]
of CO
The first example of catalytic N-formylation of
dimethylamine with CO and H gas was reported by
Haynes and co-workers, which provided a sustainable and
2
and the possible further reduction of formamides.
2
2
[
19]
Scheme 1. N-Formylation of Amines with CO2 and H2
。
general approach to access DMF. After this seminal work,
a number of homogeneous and heterogeneous catalytic
[
20-37]
systems have been reported.
In general, homogeneous
N-heterocyclic carbenes (NHCs) constitute one of robust
complexes are more efficient catalysts and exhibit higher
selectivity in a number of transformations. In 2015, in the
presence of 0.002 mol% Ru-PNP pincer complex, Ding and
co-workers have realized the efficient straightforward and
[
38,39]
and environmental-benign ligands,
which have been
widely applied in the diverse transition-metal-catalyzed
[
40,41]
reactions due to their high reactivity.
knowledge, there is no example on N-formylation of amines
with CO and H involving NHC ligands. Recently, we have
To our best
o
practical N-formylation of dimethylamine in DMF at 120 C
2
2
demonstrated a series of NHC-Ir coordination assemblies
functioned as highly efficient solid molecular catalysts in the
[
a]
b]
Y. Zhang, J. Wang, Dr. H. Zhu and Prof. T. Tu
Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Department of Chemistry, Fudan University, 2205 Songhu
Road, Shanghai, 200438, China
E-mail: taotu@fudan.edu.cn
Prof. T. Tu
[
42]
oxidative hydrogenation of glycerol to potassium lactate,
[43]
hydrogenation of levulinic acid to γ-valerolactone
and
selective monomethylation of anilines via reductive
[
44]
[
amination pathway.
Remarkably, these solid molecular
State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, China
catalysts could be reused dozens of runs without obviously
loss of activity and selectivity under mild conditions.
Following our recent research interests in the development
of molecular assemblies based on functional organometallic
Supporting information for this article is given via a link at the end of
the document.(Please delete this text if not appropriate)
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complexes and their potential applications in sensor and
entries 11-13). Disappointingly, assemblies 1b-d all
exhibited lower catalytic activity; inferior yields with lower
TONs were presented even with extended reaction time
(37-45%, Table 1, entries 9-11), which clearly indicated
[
45-48]
catalysis,
activity and reusability in N-formylation of diverse primary,
secondary and even chiral amines with CO and H
Scheme 1b), especially, at low temperature, low gas
pressure and solvent-free or base-free reaction conditions.
herein, we would like to explore their catalytic
2
2
-
(
auxiliary ligands (COD and CO) and counter-ions (I and
-
BF
4
) affected the transformation. For direct comparison, a
was also
homogeneous analogue, NHC-Ir complex
2
Table 1. Optimization of reaction conditions for N-formylation of
involved in the model reaction under otherwise identical
reaction conditions. However, only a 80% yield with 800
TON was obtained (Table 1, entry 14), which may be
caused by possible inactive Ir species formed by
[
a]
2 2
morpholine with CO and H
[
46]
dimerization of NHC-Ir
2 during the reaction, highlighting
o
[b]
the advantage of solid assemblies 1a
.
Entry
[Cat.]
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
2
Solvent
THF
Base
T ( C)
Yield (%)
TON
230
260
210
230
410
400
760
990
450
330
370
800
[
a]
1
2
3
4
5
6
7
8
9
t-BuOK
120
120
120
120
120
100
100
100
100
100
100
100
23
26
21
23
41
40
2 2
Table 2. NHC-Ir-catalyzed N-formylation of amines with H and CO
MeOH
EtOH
t-BuOK
t-BuOK
i-PrOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
t-BuOK
/
/
/
/
/
/
/
/
[
c]
76
[
d]
99
45
33
37
80
[
d]
d]
d]
[
[
10
11
12
[
d]
[
a] Reaction was carried out with 0.1 mol% catalyst, 10 mmol morpholine
in 2 mL solvent at PH2 = PCO2 = 30 bar for 2 hrs. [ ] Yield was determined
b
by GC-MS with tridecane as an internal standard and 0.1 mol% t-BuOK
was used. [c] For 10 hrs. [d] For 20 hrs.
2 2
Initially, N-formylation of morpholine with CO and H
was selected as a model reaction to optimize the reaction
conditions. In the presence of 0.1 mol% NHC-Ir
coordination assemblies 1a and 0.1 mol% t-BuOK, the
[
(
a] Conditions: amines (10 mmol), 1a (0.1 mol %), PH2 = PCO2 = 30 bar, MeOH
o + -
reactions were carried out in 10 mmol scale under 40 bar
2 2 2
2 mL), 100 C, 20 h, isolated yield. [b] [Me NH ] •[Me NCOO] was used.
o
pressure (20 bar for CO
2
and H
2
, respectively) at 120 C for
Subsequently, the scope of various amines was then
2
hours. When THF, MeOH, EtOH and i-PrOH were
explored under the optimized reaction conditions. As compiled in
Table 2, diverse secondary and primary amines were selected
for this purpose. Cyclic amines including pyrrole, piperidine,
azepane and 1,4-piperazine were all well tolerated to produce
corresponding formamides in good to excellent yields (85-94%,
selected as solvents, similar but low yields of product
3
were usually obtained (21-26%, Table 1, entries 1-4). Other
inorganic and organic bases hardly enhanced yields (Table
S1, see the Supporting Information). When the total
pressure was increased to 60 bar, a 41% yield was
presented without external base addition (Table 1, entry 5).
4a-c). Similar as cyclic cases, acyclic symmetric and asymmetric
o
secondary amines are also compatible and readily converted
into the desired products in good to excellent yields (75-93%, 5-
At 100 C, a similar yield was still obtained (40%, entry 6).
Further decreasing the reaction temperature, inferior
outcomes were obtained (Table S1, see the Supporting
Information). A good yield could be achieved by extending
the reaction time to 10 hours (76%, Table 1, entry 7).
Finally, a quantitative yield with 990 TON was obtained by
extending the reaction time to 20 hours (Table 1, entry 8).
With the optimized reaction conditions in hand, the catalytic
activity of other solids 1b-d was then investigated (Table 1,
1
3). In the case of volatile dimethylamine (for safety reason,
+
-
[
2 2 2
Me NH ] •[Me NCOO] was applied instead), a moderate yield
was still presented to produce DMF (53%, 8). Other dialkyl
secondary amines with higher boiling points, good to excellent
yields were usually observed even with bulky substitutes and
unprotected OH groups (80-93%, 7-13). In the case of primary
amines, even better outcomes were presented (77-97%, 15-23),
whenever bearing aliphatic acyclic and cyclic chains, aromatic
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and heterocyclic substitutes, further highlighting the protocol
efficiency. Heterocyclic substrate, such as 2-picolylamine and
furan-2-ylmethanamine still delivered the corresponding
products in excellent yields (77-89%, 18-19). Aminoalcohols are
very important building blocks, which are also suitable
substrates for the N-formylation with CO and H under the
2 2
standard reaction conditions and excellent yields are observed
both for protected and unprotected aminoalcohols (81%-94%,
1a act as a molecular catalyst in the reaction. In addition, no
obvious changes were observed in the SEM and TEM
morphologies of freshly prepared and recovered (after 10 runs)
solid catalyst 1a (Figure 2 a, b vs d, e). Energy dispersive X-ray
spectroscopy (EDS) mapping images of fresh and recovered
solids 1a indicated no alternation of distribution of Ir species in
the polymer matrix (Figure 2 c vs f), further confirming the
stability of the solid molecular catalyst.
20-23). It was notable that when the chiral aminoalcohol was
involved, unprotected chiral amide 23 was produced in 81%
yield.
Figure 2. a-c): SEM, TEM, EDS images of the freshly prepared NHC-Ir
coordination polymer 1a; d-f): images of the recovered 1a after the 10th run.
Figure 1. Recycling and reuse of the solid NHC-Ir catalyst 1a in the DMF
production (20 mmol scale at 0.1 mol% catalyst loading under the
optimized reaction conditions; the average yields of two parallel recycling
sequences are provided).
With all these outcomes in hand, a plausible mechanism
catalyzed by NHC-Ir coordination assemblies 1a was the
proposed in Scheme 2. Initially, the cis-dihydride species A was
In light of the crucial role of DMF in industry, the reaction
generated from catalyst, which may act as the actual catalyst in
+
-
[49]
conditions for the N-formylation of [Me
2
NH
2
] •[Me
2
NCOO] was
the subsequent hydrogenation transformations.
Amines
After direct
[
50]
then fully investigated (Table S2, see the Supporting
Information). To our surprise, solvents hardly show any impacts
on the transformation, similar outcomes were observed under
the standard reaction conditions, which is unlike Ding’s Ru-PNP
readily react with CO
2
to form internal salts.
nucleophilic attacking of the carbonyl group of the salts, and
coordinated species B was generated along with ammonium ion
C. After hydride transfer, active intermediate D could further
reacted with hydrogen to generate intermediate E, which could
[
33]
catalytic system.
higher yield (57%, Table S2, entry 6), which could be further
enhanced to 73% by increasing the pressure to 70 bar (PH2
CO2 = 35 bar, Table S2, entry 7). When the catalyst loading was
In addition, solvent-free conditions led to a
further react with ammonium ion
C leading to the final
=
formamides formation along with corresponding amine and
water production.
P
decreased to 0.05 mol%, a 72% yield was still achieved (Table
S2, entry 12). No obvious change was found by extending the
reaction time to 30 hrs (72% vs. 71%, Table S2, entries 12 vs
13). Again, inferior yields were observed with other NHC-Ir
assemblies 1b-d under such reaction conditions (Table S2,
entries 16-18).
Due to insolubility of solid NHC-Ir 1a in DMF, after reaction
completion, the solid is readily recovered via simple
centrifugation and decantation. Remarkably, the recovered solid
1a could be directly used in the next run without any activation
operation. And up to 10 runs could be recycled for the solid 1a
without any obvious loss in DMF production (Figure 1),
highlighting the feasibility of NHC-Ir solid molecular catalyst 1a.
In order to further understand the catalytic nature of catalyst 1a
in the reaction and corresponding recycling process, the content
of iridium in the filtrate after each run was analyzed by
inductively coupled plasma emission spectroscopy (ICP-AES).
The original Ir content in the NHC-Ir coordination assemblies 1a
was 24.4 % determinded by ICP-AES, which was consistent with
Scheme 2. Plausible mechanism of N-formylation of amines.
5 2 n
the theoretical value (24.0%) of (IrC23H33IN O•DMF•H O) ,
wheraes, the amount of leaching iridium species in the recycling
process was negligible. Notably, no iridium nanoparticles or
nanoclusters were found in the filtrate and recovered solid
catalysts, which may highlight NHC-Ir coordination assemblies
In conclusion, one of NHC-iridium coordination assemblies
containing COD and iodide ion has been demonstrated as
robust, efficient and recyclable solid molecular catalyst for N-
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formylation of various primary and secondary amines with H
and CO under mild reaction conditions. A number of formamide
2
2
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low as 0.1 mol%. Furthermore, the DMF production could be
performed without external solvents at 0.1 mol% catalyst loading.
Remarkably, the catalyst could be readily reused via simple
centrifugation and decantation without any activating treatments.
No significant loss of activity after 10 runs highlights the potential
of protocol in industry.
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Entry for the Table of Contents
One of the NHC-iridium coordination
Yang Zhang, Jiaquan Wang,
assemblies
containing
1,5-
Haibo Zhu and Tao Tu*
Cyclooctadiene (COD) and iodide ion
has been demonstrated as robust,
Page No. – Page No.
efficient
and
recyclable
solid
N-Formylation of Amines
with CO and H by Using
2 2
NHC-Iridium Coordination
Assemblies as Solid
Molecular Catalysts
molecular catalyst for N-formylation of
various primary and secondary
amines with H
reaction conditions
2
and CO
2
under mild
catalyst
at
loadings as low as 0.1 mol %.
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