Ethanol-mediated N-formylation of amines with CO2/H2 over cobalt catalysts

Article abstract of DOI:10.1039/c8nj03047b

The CO₂-involved synthesis of chemicals is of great significance from a green and sustainable point of view. Herein, we present an efficient Co-based catalytic system composed of a commercially available Co salt, the tetradentate phosphine ligand P-(CH₂CH₂PPh₂)₃, and a base, denoted as [Co]/PP₃/base, for the synthesis of formamides via the formylation of amines with CO₂/H₂. It was indicated that the selectivity of products (i.e., formamide or methylamine) could be tuned to some extent via changing the solvent and the base. Using ethanol as the solvent, the Co(ClO₄)₂·6H₂O/PP₃/K₂CO₃ system showed high activity for the production of formamides, affording product yields of 82-95%, together with its broad substrate scope. Exploration of the reaction mechanism indicated that formamide was formed with HCOOH as the intermediate, while the methylamine byproduct was produced with HCHO as the intermediate via the hydrogenolysis of dialkylaminomethane.

Full text of DOI:10.1039/c8nj03047b

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Ethanol-mediated N-formylation of amines with
CO2/H2 over cobalt catalysts†
Cite this:DOI: 10.1039/c8nj03047b
Zhenghui Liu,‡ab Zhenzhen Yang,‡a Zhengang Ke,ab Xiaoxiao Yu,ab Hongye Zhang,a
ab
Bo Yu,a Yanfei Zhaoa and Zhimin Liu
*
The CO2-involved synthesis of chemicals is of great significance from a green and sustainable point of view.
Herein, we present an efficient Co-based catalytic system composed of a commercially available Co salt, the
tetradentate phosphine ligand P-(CH2CH2PPh2)3, and a base, denoted as [Co]/PP3/base, for the synthesis of
formamides via the formylation of amines with CO2/H2. It was indicated that the selectivity of products
(i.e., formamide or methylamine) could be tuned to some extent via changing the solvent and the base. Using
ethanol as the solvent, the Co(ClO4)2ꢀ6H2O/PP3/K2CO3 system showed high activity for the production of
formamides, affording product yields of 82–95%, together with its broad substrate scope. Exploration of the
reaction mechanism indicated that formamide was formed with HCOOH as the intermediate, while the methyl-
amine byproduct was produced with HCHO as the intermediate via the hydrogenolysis of dialkylaminomethane.
Received 19th June 2018,
Accepted 15th July 2018
DOI: 10.1039/c8nj03047b
rsc.li/njc
From the sustainable development point of view, utilization of
cheaper, more abundant, less toxic and easily available first-row
Introduction
Formamides are a class of chemicals with widespread applications transition metals, e.g., cobalt, for the selective formation of for-
in industry as solvents (e.g. N,N-dimethylformamide (DMF)) and mamides in the presence of CO2/H2 is more significant.
raw materials for organic synthesis.1 Traditionally, the carbonyl
On the other hand, the reaction of amines with CO2/H2
moiety is introduced into formamides using relatively hazardous could result in the formation of methylamines.14 Compared to
reagents such as chlorals, CO, acetic formic anhydrides and formic the synthesis of formamides from amines/CO2/H2, the for-
acid.2 In comparison to these synthesis routes, utilization of mation of N-methylamines with a higher CO2 reduction level
carbon dioxide (CO2) as a renewable C1 building block is of is difficult to realize. In addition, selective regulation of the
great significance in the aspect of reducing the dependence on formylation or methylation products by simply changing the
petrochemical feedstocks.3 CO2-involved reductive formylation solvents or additives is also highly desired.15
of amines provides a green approach for the synthesis of
Herein, we developed an efficient, cobalt-based catalytic system,
formamides, in which reductants such as H2, hydrosilanes or composed of a commercially available cobalt precursor, a tetra-
hydroboranes are required.3a,4 From the viewpoint of green dentate phosphine ligand P(CH2CH2PPh2)3 (named as PP3) and an
chemistry, H2 is more attractive for the reductive formylation of inorganic base, denoted as [Co]/PP3/base, for the formylation of
amines with CO2 because the byproduct is water.
amines with CO2/H2. It was indicated that the product selectivity
Although synthesis of DMF from dimethylamine and CO2/H2 has (formamide or methylamine) could be tuned to some extent via
attracted immense interest since 1970,5 the N-formylation of other changing the solvent and the base. Using ethanol as the solvent,
bulky amines using CO2/H2 has been achieved only in recent years the Co(ClO4)2ꢀ6H2O/PP3/K2CO3 system showed high catalytic activ-
over homogeneous ruthenium-pincer-type complexes,6 Cu(OAc)2/ ity for the production of formamides, affording product yields of
4-dimethylaminopyridine,7 (glycinato)copper(II) complexes,8 82–95%, together with a broad substrate scope. Exploration of the
and cobalt-pincer-type complexes,9 as well as heterogeneous reaction mechanism indicated that formamide was formed with
Pd/Al2O3,10 Au/TiO2,11 Pd–Au bimetallic catalysts12 and Co0/ZnCl2.13 HCOOH as the intermediate, while the methylamine byproduct
was produced with HCHO as the intermediate via the hydrogeno-
a Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloid,
Interface and Thermodynamics, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. E-mail: liuzm@iccas.ac.cn
lysis of dialkylaminomethane.
b University of Chinese Academy of Sciences, Beijing 100049, China
† Electronic supplementary information (ESI) available: General experimental
methods and procedures, Tables S1–S8, and characterization (NMR and EI-MS) of
the intermediates and products. See DOI: 10.1039/c8nj03047b
‡ These authors contributed equally to this work.
Results and discussion
In an initial set of experiments, morpholine (1a) was selected as
the substrate for the reaction with CO2/H2, and the results are
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a
Table 1 Cobalt-catalyzed N-formylation of morpholine (1a) with CO2/H2
decreased without base (entry 7). The selectivity of 2a was also
good using a mixed solvent of EtOH/THF (entry 8). Exploration
of other solvents showed that both 2a and 3a were detected,
although with lower yields, in the reaction mixtures (entries 9–15).
From the above results, it was obvious that ethanol was more
favorable to the formylation while THF to the methylation,
which was probably owing to the stabilization of the HCOOH
intermediate by the protic solvent through hydrogen bond
formation, thus inhibiting further hydrogenation of HCOOH
to HCHO for methylation, as shown in the mechanism of
investigation results. Co species with various anions were
examined (entries 16–20), and the highest 2a yield of 80% was
obtained with Co(ClO4)2ꢀ6H2O/PP3/K3PO4. Using EtOH as the
solvent, the detailed investigation of other reaction parameters,
including bases and temperature, showed that 95% yield of 2a
was obtained with Co(ClO4)2ꢀ6H2O/PP3/K2CO3 under the optimal
reaction conditions (entry 21) (for screening of detailed reaction
parameters, see Tables S2 and S3 in the ESI†).
Subsequently, the N-formylation of other amines was inves-
tigated in the presence of Co(ClO4)2ꢀ6H2O/PP3/K2CO3 in EtOH.
As shown in Scheme 1, the N-formylation reaction proceeded
smoothly for primary and secondary aliphatic amines, selectively
affording the corresponding formamides 2a–2t in good to excellent
yields (82–95%). Besides 1a, other cyclic aliphatic amines also
performed well, producing the corresponding formamides 2a–2e
in 487% yields. Linear aliphatic amines, di-n-propylamine 1f and
di-n-hexylamine 1g, showed good reactivity, affording 93% and
89% yields of 2f and 2g, respectively. For N-methylbenzylamine or
its derivatives with electro-donating or electro-withdrawing groups
(e.g. –Me or –Cl) (1h–1j), no significant differences in the reactivity
were observed, and all the corresponding formylated products were
obtained in excellent yields (493%). When di-benzylamine 1k with
Yieldb/%
Entry
Catalyst
Solvent
2a
3a
1cd
2c
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(BF4)2ꢀ6H2O
Co(ClO4)2ꢀ6H2O
Co(NO3)2ꢀ6H2O
Co(OAc)2ꢀ4H2O
Co(acac)2
THF
THF
THF
THF
THF
EtOH
EtOH
EtOH/THF
CF3CH2OH
2-MeTHF
CH3CN
Toluene
1,4-Dioxane
Diglyme
Xylene
EtOH
EtOH
EtOH
EtOH
EtOH
7
3
9
9
3
3
3
3cde
4d
5
14
52
o1
12
o1
7
14
22
26
30
19
2
15
10
18
32
17
o1
69
68
69
46
65
17
62
59
23
26
11
27
41
22
80
68
62
48
70
95
28
27
29
6
7d
8f
9
10
11
12
13
14
15
16
17
18
19
20
21g
22
23h
24i
CoCl2ꢀ6H2O
Co(ClO4)2ꢀ6H2O
Co(ClO4)2ꢀ6H2O
Co(ClO4)2ꢀ6H2O
Co(ClO4)2ꢀ6H2O
EtOH
THF
THF
THF
a
Reaction conditions: 1a, 1 mmol; Co species, 5 mol%; PP3, 5 mol%;
K3PO4, 1 mmol; CO2, 3 MPa; H2, 3 MPa; solvent, 3 mL; oil bath, 140 1C,
24 h. Determined by GC using dodecane as an internal standard.
b
c
d
e
f
Triphos as the ligand. No base. HNTf2 15 mol%. EtOH 2 mL +
g
h
i
THF 1 mL. K2CO3, 1.5 mmol. 48 h. 72 h.
provided in Table 1. As reported, the tridentate phosphine ligand, relatively high steric hindrance was used as the substrate, 92% yield
triphos, complexing with ruthenium species was very efficient in of 2k could be obtained. Formylation of primary amines with
CO2 hydrogenation for the formation of methanol, methylamines linear chains afforded products 2l–2n in 492% yield, while
and so on,16 and the combination of Co(BF4)2ꢀ6H2O and triphos sec-butylamine with higher steric hindrance afforded product
could catalyze CO2 hydrogenation to methanol.17 Thus, we first 2o in a relatively low yield (82%). Cyclopentylamine 1p and
examined the activity of Co(BF4)2ꢀ6H2O/triphos for the reaction cyclohexylamine 1q also showed good reactivity, affording 2p
of morpholine with CO2/H2, which however only afforded very and 2q in 88% and 92% yields, respectively. Formylation
low yields of N-formylmorpholine (2a) (7%) and N-methyl- of benzylamine and its derivatives was performed well, with
proline (3a) (3%) using THF as the solvent (entry 1). Additives 90–95% yields of 2r–2t.
such as K3PO4 or HNTf2 showed no obvious enhancement
Subsequently, the reaction mechanism was explored to gain
effect (entries 2 and 3). The tetradentate phosphine ligand more insights into the formation of both formamides and
PP3 together with Co(BF4)2ꢀ6H2O or Fe(BF4)2 was reported to methylamines, and to find the reason for solvent regulation
be capable of effectively catalyzing the hydrogenation of CO2 to of the product selectivity. From the results given in Table 1, it
produce HCOOH.18 The Co(BF4)2ꢀ6H2O/PP3 system was tested seems that THF is a good solvent for the methylation of amines
for the reaction of amines with CO2/H2, which showed very low with CO2/H2. Up to now, only Ru-based homogeneous catalysts,
yields of 2a (9%) and 3a (14%) as well (entry 4). To our delight, such as [Ru(triphos)(tmm)], (triphos = 1,1,1-tris(diphenylphos-
the addition of 1 equiv. K3PO4 to the Co(BF4)2ꢀ6H2O/PP3 phinomethyl)ethane and tmm = trimethylene methane)16a and
catalytic system significantly increased the product yields, with ([Ru(acac)3]/triphos)16b have been reported.
46% for 2a and 52% for 3a (entry 5). Other phosphine- or
The reported heterogeneous catalysts for amine methylation
nitrogen-containing ligands showed inferior activity than PP3 using CO2/H2 were mainly metal oxide supported noble metal
(Table S1, ESI†). Subsequently, the influence of solvents on nanocatalysts including Pd/ZrCuOx,19 Pt-MoOx/TiO2,20 Au/Al2O3,21
the 2a/3a selectivity was investigated. Using a polar solvent, Re/TiO222 and CuAlOx.23 It was reported that methylamine products
e.g. EtOH, 65% yield of 2a was obtained, and almost no 3a was were generally obtained with formamides as the intermediates.16a,b
detected (entry 6). But both 2a yield and selectivity were In this way, the formamide intermediates could be converted to
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unchanged by prolonging the reaction time from 24 h to 48 h
and 72 h (Table 1, entries 22–24). In addition, directly con-
sidering 2a as the substrate, no 3a was detectable over
Co(ClO4)2ꢀ6H2O/PP3/K3PO4 in THF in the presence of CO2/H2
(Table 2, entry 1) or only H2 (3 MPa or 6 MPa) (entry 2). That is, the
catalytic activity of Co(ClO4)2ꢀ6H2O/PP3/K3PO4 was insufficient for
the reduction of formamide, and hence the methylamines were
not produced via the hydrogenation of formamide. Another
possible pathway to methylamine formation was from a urea
intermediate, which could be produced through the dehydra-
tion reaction of CO2 with amine.14a,15a In this cobalt-based
system, the urea intermediate 4,40-carbonyldimorpholine (4a)
could not be reduced to 2a or 3a (entry 3), thus excluding the
urea formation pathway. However, N,N0-dimorpholinomethane
(5a) could be converted to 1a (12%), 2a (28%) and 3a (47%) in
the presence of CO2/H2 (entry 4), or to 1a (29%) and 3a (5%)
under a H2 atmosphere (entry 5). Thus, 5a was a possible
intermediate for the formation of methylamine 3a.
The successive reduction of CO2 with H2 to different levels
can afford formic acid, formaldehyde, methanol and methane,
and the formation of formamide from amines and CO2/H2
belongs to the formic acid level.4a The reaction of HCOOH
instead of CO2/H2 with 1a afforded 2a in 70% yield under the
standard conditions (Table 2, entry 6), and it could proceed
smoothly even without any catalyst or additive in THF (2a yield:
61%) and in EtOH (2a yield: 98%) (entries 7 and 8). This
indicates that formamide was formed through the reaction of
the HCOOH intermediate with amine, which was consistent
with the pathway reported previously.6,24 Comparatively, the
reactions of 1a with paraformaldehyde or CH3OH gave no 2a or
3a over the cobalt-based system without any hydrogen source
(entries 9 and 10). In the presence of H2 (3 MPa), the reaction of
Scheme 1 Formylation or methylation of amines catalyzed by cobalt
systems. Reaction conditions: amine 1 mmol, Co(ClO4)2ꢀ6H2O 5 mol%,
PP3 5 mol%, K2CO3 1.5 mmol, CO2 3 MPa, H2 3 MPa, EtOH 3 mL, oil
bath 140 1C, 24 h. Isolated yields were given. For experimental details,
see the ESI.†
methylamines by prolonging the reaction time. However, in the 1a with paraformaldehyde or CH3OH afforded 74% and 35%
catalytic system developed in this work, 28% yield of 2a and yields of 3a, respectively (entries 11 and 12), maybe through a
69% yield of 3a were detected over Co(ClO4)2ꢀ6H2O/PP3/K3PO4 hydrogen-borrowing like pathway.25 In addition, in the absence
in THF, and the yields of 2a and 3a were almost kept of amines, the reduction of CO2 with H2 over this cobalt-based
a
Table 2 Control experiments in the presence of Co(ClO4)2ꢀ6H2O/PP3/K3PO4
Entry
Substrates
Productsb
1
2
2a, CO2 3 MPa, H2 3 MPa
2a, H2 3 or 6 MPa
No 3a
No 3a
3
4
5
4a, CO2 3 MPa, H2 3 MPa
5a, CO2 3 MPa, H2 3 MPa
5a, H2 3 MPa
1a, HCOOH
1a, HCOOH
1a, HCOOH
1a, paraformaldehyde
1a, CH3OH
1a, paraformaldehyde, H2 3 MPa
1a, CH3OH, H2 3 MPa
CO2 3 MPa, H2 3 MPa
HCOOH
HCOOH, H2 6 MPa
Paraformaldehyde, CO2 3 MPa, H2 3 MPa
No 2a, 3a or 5a
1a: 12%, 2a: 28%, 3a: 47%
1a: 29%, 3a: 5%
2a: 70%, no 3a
2a: 61%, no 3a
2a: 98%, no 3a
No 2a or 3a
6
7c
8cd
9
10
11
12
13
14
15
16
No 2a or 3a
No 2a, 3a: 74%
No 2a, 3a: 35%
HCOOH, CH3OH
CO2, H2
CH3OH
CH3OH: 57%
a
Substrate 1 mmol, Co(ClO4)2ꢀ6H2O 5 mol%, PP3 5 mol%, K3PO4 1 mmol, THF 3 mL, oil bath 140 1C, 24 h. 10 mmol HCOOH, praformaldehyde or
b
CH3OH was added if necessary. Yields were determined by GC (1a, 2a and 3a) or NMR (HCOOH and CH3OH) using dodecane or CH3NO2 as the
internal standard, respectively (for details, see the ESI). Catalyst and base free. EtOH as the solvent.
c
d
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available. All of them were purchased from J&K Chemicals, Sigma-
Aldrich Co. LLC or from Sinopharm Chemical Reagent Co.
H2 (99.99%) and N2 (99.95%) were provided by Beijing Analytical
Instrument Company.
Instrumentation
Liquid 1H NMR spectra were recorded in CDCl3 using the
residual CHCl3 as an internal reference (7.26 ppm) on a Bruker
400 spectrometer. 1H NMR peaks are labeled as singlet (s),
doublet (d), triplet (t), and multiplet (m). The coupling constant
values, J values, are reported in Hertz (Hz). Liquid 13C NMR
spectra was recorded at 100.6 MHz in CDCl3 using the residual
CHCl3 as the internal reference (77.0 ppm). GC analysis for
the reaction system was performed using an Agilent 4890D
with a FID detector and a nonpolar capillary column (DB-5)
(30 m ꢁ 0.25 mm ꢁ 0.25 mm). The column oven was
temperature-programmed with a 2 min initial hold at 323 K,
followed by the temperature increase to 538 K at a rate of
10 K minꢂ1 and kept at 538 K for 10 min. High purity nitrogen
was used as a carrier gas. The gaseous samples were collected
and analyzed using a GC (Agilent 4890D) equipped with a TCD
detector and a packed column (carbon molecular sieve TDX-01,
1 m in length and 3 mm in diameter).
Scheme 2 The proposed reaction mechanism for the formylation or
methylation of amines with CO2/H2.
system could produce HCOOH and CH3OH (entry 13). It was
also found that HCOOH could be decomposed to CO2 and H2 in
the presence of Co(ClO4)2ꢀ6H2O/PP3/K3PO4, and this catalytic
system could catalyze the hydrogenation of HCOOH to form
CH3OH (entries 14 and 15), which would surely go through
the HCHO intermediate, and indirectly proved the existence of
HCHO as the intermediate during the reaction. However,
HCHO was not detected in the reaction mixture, probably
owing to its rapid reaction with amines or hydrogenation with
H2 to form CH3OH (57%) (entry 16).
According to the experimental results and previous reports
related to this [Co]/PP3 system, a possible reaction pathway is
proposed as shown in Scheme 2. Firstly, the cobalt complex
from the coordination of Co(ClO4)2ꢀ6H2O and PP3 catalyzes the
HCOOH formation from CO2 hydrogenation (I) through the
generation of the Co–H active species, which was reported in
the literature18a and HCOOH was also detected in our system.
Then formamide 2a is formed via the dehydration reaction
between 1a and HCOOH (II). 3a was not obtained from 2a or 4a
(III and IV). HCHO is a possible intermediate formed via the
hydrogenation of CO2 (V), which can react immediately with 1a
to produce 5a (VI). Finally, 5a is converted to the methylamine
product 3a through hydrogenolysis (VII).15a
General procedure for the reaction of amines with CO2/H2
All reaction solutions were set up in a N2-filled glovebox, and
the reactions were carried out in a stainless steel autoclave
equipped with a Teflon tube (25 mL inner volume) and a
magnetic stirrer. Reaction temperatures were reported as
the temperature of the oil bath surrounding the autoclave.
In a typical experiment, Co(ClO4)2ꢀ6H2O (0.05 mmol), PP3
(0.05 mmol), K2CO3 (1.5 mmol) or K3PO4 (1 mmol), amines
(1 mmol) and EtOH or THF (3 mL) were successively loaded into
the autoclave under a N2 atmosphere. The autoclave was then
sealed. CO2 (3 MPa) and H2 were charged successively into the
reactor until the total pressure reached 6 MPa at room tem-
perature. The autoclave was moved to an oil-bath of desired
temperature (e.g., 140 1C) and stirred for 24 h. After the
reaction, the autoclave was cooled to 0 1C, and then the excess
amount of gas was vented slowly. Dodecane (internal standard)
and CH2Cl2 (5 mL) were added to the reaction solution, stirred
vigorously and centrifuged. The upper liquid was analyzed by
GC to determine the conversion and yield using the standard
curve established in advance. For the substrate scope investiga-
tion, products 2a–2t were isolated by column chromatography
on silica gel (200–300 mesh) using CH2Cl2/MeOH or petroleum
Conclusions
In summary, a readily available and highly efficient cobalt-
based catalytic system was developed for the formylation of
amines with CO2/H2, and the selectivity of the products could
be regulated by base additives and solvent. The mechanism of
investigation results showed that formamides were formed
through the reactions of amines with HCOOH originated from
CO2 hydrogenation, while methylamines were produced with
dialkylaminomethane intermediates, which was formed through
the reaction of amines with HCHO.
ether/ethyl acetate as the eluent and identified by 1H NMR, 13
C
NMR and EI-MS spectra. Products 2a–2g and 21–2q were
colorated using an iodine cylinder when isolated by column
chromatography, while 2h–2k and 2r–2t were colorated using
an UV lamp.
Experimental
Materials
Co species, ligands, inorganic bases, amine substrates and so
on were purchased from commercial sources and used without
Conflicts of interest
further purification. Solvents were anhydrous and commercially There are no conflicts to declare.
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13 H. Zhenhong, L. Hangyu, L. Huizhen, Q. Qingli, M. Qinglei,
M. Qingqing and H. Buxing, ChemCatChem, 2017, 9, 1947–1952.
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Acknowledgements
We thank the National Natural Science Foundation of China
(No. 21673256, 21533011) and the Chinese Academy of Sciences
(QYZDY-SSW-SLH013-2).
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