Catalyst-free: N -formylation of amines using BH3NH3 and CO2 under mild conditions

Article abstract of DOI:10.1039/c7cc03860g

The catalyst-free N-formylation of amines using CO₂ as the C₁ source and BH₃NH₃ as the reductant has been developed for the first time. The corresponding formylated products of both primary and secondary amines are obtained in good to excellent yields (up to 96% of isolated yield) under mild conditions.

Full text of DOI:10.1039/c7cc03860g

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
Reactivity differences of Sc3N@C2n (2n 68 and 80). Synthesis of the
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Catalyst-free N-formylation of amines using BH3NH3 and CO2 at
mild conditions
Received 00th January 20xx,
Accepted 00th January 20xx
Tian-Xiang Zhao, Gao-Wen Zhai, Jian Liang, Ping Li, Xing-Bang Hu,*and You-Ting Wu*
DOI: 10.1039/x0xx00000x
www.rsc.org/
great significance.
Catalyst-free N-formylation of amines using CO2 as the C1 source
and BH3NH3 as the reductant is developed for the first time. The
corresponding formylated products of both primary and
secondary amines are obtained in good to excellent yields (up to
96% of isolated yield) under mild conditions.
Ammonia borane (BH3NH3) is known as the excellent hydrogen
storage material because of its high hydrogen capacity.3 The
pyrolytic dehydrogenation of BH3NH3 usually require noble metal
catalyst and high temperature.3 Recently, we developed a method
for the room temperature dehydrogenation of BH3NH3.4 However,
catalyst is still vital for this process. Hence, it is not surprise that big
challenge may exist in the production of useful chemicals using CO2
as the C1 source and BH3NH3 as the reductant.5 As an ongoing effort
in the capture and utilization of CO2,6 herein, we report the
successful development of catalyst-free N-formylation of amines
using CO2 as the C1 source and BH3NH3 as the reductant at mild
conditions. The direct synthesis of formamides from CO2 and
amines under mild conditions (50oC) enables excellent conversion of
amines and good selectivity of formamides.
Carbon dioxide,
a notorious greenhouse gas, has been a
worldwide focus due to its serious effect in the atmosphere. The
capture and utilization of CO2 become one of the greatest scientific
and technological challenges of the 21st century.1 The CO2 capture
and storage (CCS) approach, which is based on the idea of capturing
CO2 into adsorbents such as solids1a and solutions,1b,c or with
membranes,1d is obvious. In comparison with CCS, the approach of
CO2 capture and utilization (CCU) is more attractive if economical
processes are available for the conversion of CO2 into useful
products.1e-h
A
promising methodology in this area is the reductive
functionalization of CO2 with amines and reductants to produce
formamides.2 Formamides are a kind of versatile chemicals or
important building blocks that are generally produced via the N-
formylation of amines with carboxylic acid or CO in the presence of
a catalyst.2j A green and attractive development is to use CO2
instead of toxic CO. As we all know, CO2 is characteristic of it
inherent stability and inertness, so that the conversion of CO2 is full
of challenge.1e-h Hence, chemists have been committed to
developing many efficient methods for the conversion of CO2 into
formamides.2 However, the reported routes generally require
expensive or complex catalysts (i.e., compounds containing Ru,2d,j
Rh,2i Ir,2h,j Pd,2v Au,2v Cu,2c,e,m Co,2a Fe,2t,x, Zn,2u NHC,2l NHP-H,2r
NHO,2b GVL,2f TBD,2w GB,2y ILs,2g,k,n,o or alkali carbonate2p,q). It is still
difficult to recycle these catalysts.2 In addition, most of these
processes suffer from high temperature and pressure (H2 as
reductant),2a,c,d,j or low atomic economy (hydrosilanes as
reductants)2b,f-i,k-s,w-y (Scheme 1). Therefore, the exploration of
catalyst-free and atom-economic methods at mild conditions is of
Precedents
O
H
hydrosilane or H2
Cat
N
N
CO2
+
R1
R2
R1
R2
Cat = Ru; Rh; Ir; Pd; Au; Cu; Co; Zn; Fe; Cs2CO3; K2CO3; NHC; NHP-H;
NHOs; [Et4NBr]-Py-OFs; [BMIm]Cl; TBAF; GVL; GB; TBD
This work
O
H
BH3NH3
Cat.-free
N
N
CO2
+
R1
R1
R2
R2
Scheme 1. The difference between literatures and this work in the
N-formylation of amines.
In a first experiment, the formylation of N-methylaniline 1 with
CO2 as the C1 source and BH3NH3 as the reductant was performed
in various solvents (Table 1). Inferior conversions of 1 are obtained
in benzene, toluene, and CHCl3 (entries 1-3), which are due to the
poor solubility of BH3NH3 in these solvents. Weakly polar solvents
such as 1,4-dioxane, THF, and acetone are also not suitable for this
reaction because of unsatisfactory conversion and selectivity
(entries 4-6). Although CH3CN and DMSO lead to considerable
conversion of 1 (entries 7 and 8), they are also abandoned due to
the inferior selectivity of the product 1a. Almost complete
School of Chemistry and Chemical Engineering, Separation Engineering Research
Center, Nanjing University, Nanjing 210093 (P. R. China).
E-mail: huxb@nju.edu.cn (X.-B. Hu); ytwu@nju.edu.cn (Y.-T. Wu)
Electronic Supplementary Information (ESI) available: Experiment methods,
characterization of products and original NMR spectra, See DOI:
10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 1
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conversion of 1 with a 94% yield of 1a is achieved in DMF at 80oC negligible yield of 3% of N,N-dimethylaniline (Eq. 1). It indicates that
for 24 h (entry 9). It can probably be rationalized with the good the reductive methylation through the formation of formamide is
solubility of CO2 and BH3NH3 in DMF, and the appropriate basicity quite difficult in the absence of catalyst, which enables the high
and nucleophilicity of amines.2s The reaction is also tested under selectivity to the N-formylation products.
solvent-free conditions, resulting in poor conversion of 1 and low
yield of the desired product (entry 10). It can be seen that the
As for the effect of temperature on the reaction, it is found that
reducing the temperature to 30oC results in an apparent decrease
solvent has a prominent effect on the N-formylation reaction,
in the product yield (entry 13), whereas the reaction at 120 oC leads
namely, intensive polar solvent can activate the N-H bond in N-
methylaniline through solvation and polarization.2f,s
to a slightly lower yield compared with the case at 80°C (entries 9
and 15). The most appropriate temperature is around 50oC in which
Table 1. Optimization of reaction conditions.
a yield of 96% for 1a can be obtained (entry 14). Previous literature
had reported that the high temperature may favors the methylated
product 1b. 2p It is not the case in the present study since the yield
of 1b only shows a negligible increase at elevated temperatures
(entries 9, 14 and 15). This may be due to the lack of catalyst to
activate the methylation. Various borane-amine complexes are also
employed to find out more suitable reductants. The use of
morpholineborane as the reductant leads to a yield of 51% of 1a
(entry 16), and the presence of dimethylaminoborane results in a
yield of only 30 % of 1a (entry 17). Therefore, BH3NH3 is regarded as
the effective reductant and used in the following investigations.
Some previous researches reported the transformylating of amine
with DMF to formamide in the presence of Ni(II) metal complex or
CeO2.8 To eliminate the possibility of N-formylation using DMF as
reactant, reactions in the absence of BH3NH3 or CO2 were
performed and no products were obtained (entries 18 and 19). The
effect of CO2 pressure on the yield is also examined. The yield of 1a
decreases to 78% at 0.5 MPa (entry 20). In contrast, at higher
pressures such as 1 MPa, more CO2 can be dissolved in the system,
resulting in higher yields of 1a (entries 9 and 14). BH3NH3 dissolves
in water quite well. If the N-formylation can be performed in water,
the reaction will be more environmentally friendly. However,
because the acid environment induced by the reaction between
CO2 and H2O can rapidly promote the hydrolysis of BH3NH3,9 no N-
formylation of amines happens in water (Entry 21).
Entry
Solvent
T (oC) t (h)
Con. of
1 (%)a
21
Yield of
1a (%)a
13
13
8
Yield of
1b (%)a
1
2
toluene
benzene
CHCl3
1,4-dioxane
THF
80
80
80
80
80
80
80
80
80
80
80
80
30
50
120
80
80
80
80
80
80
24
24
24
24
24
24
24
24
24
24
24
8
7
9
22
3
11
3
4
75
60
79
75
78
76
94
17
82
82
58
96
93
51
30
0
15
16
25
22
15
6
5
95
6
acetone
CH3CN
DMSO
DMF
> 99
> 99
91
7
8
9
> 99
18
10b
11c
12
13
14
15
16d
17e
18f
19g
20h
21
1
—
DMF
> 99
88
18
6
DMF
DMF
24
24
24
24
24
24
24
24
24
64
6
DMF
> 99
> 99
88
4
DMF
7
(Eq. 1)
(Eq. 2)
DMF
37
2
DMF
32
DMF
0
0
DMF
0
0
0
DMF
81
78
0
3
The substrate scope of this reaction was investigated (Scheme 2).
A series of secondary amines are tested firstly. Both steric and
electronic effect can control the reactivity of these substrates. The
substrate 1 gives 1a with an isolated yield of 91%. The methyl
substitutions position on the benzene ring of N-methylanilines has
obvious influence on the activity (2a-4a). The substrate 4 that has a
para-methyl group possesses a higher reactivity than 2 and 3. Due
to the steric hindrance, the ortho-methyl substitution of N-
methylaniline leads to a low product yield of 2a (yield: 76%), and
similar substrates that have branched chains in the vicinity of amine
group also suffer from low yields (64 and 45% for 13a and 21a,
respectively). Both electron-donating and electron-withdrawing
groups are tolerated, affording the desired products in good to
excellent yields (up to 95%, 5a-8a). However, N-methyl-4-
nitroaniline is inert with a yield of 16% (9a), due to a strong
electron-withdrawing effect that weakens greatly the
nucleophilicity of the amine. When ethyl, isopropyl, and phenyl
H2O
0
0
Conditions: N-methylaniline (1 mmol), solvent (5 mL), BH3NH3 (3 mmol),
CO2 (1 MPa). [a] The yields were determined by GC/MS using dodecane as
the internal standard. [b] solvent-free, N-methylaniline (5 mmol), BH3NH3
(15 mmol). [c] BH3NH3 (5 mmol). [d] morpholineborane (3 mmol). [e]
dimethylaminoborane (3 mmol). [f] without BH3NH3. [g] without CO2. [h]
CO2 (0.5MPa).
Some previous works2o,p,u,7 suggested that formamide was the
main intermediate for reductive methylation. It implies that
increasing the amount of reductant supports to the reductive
methylation. The amount of BH3NH3 is thus raised to 5 mmol
justifying the improvement in the selectivity of methylated product
(entry 11). However, the yield of N'N-dimethylaniline (1b) is still
quite low. To elucidate the mechanism, N-methylformanilide 1a
was further tested to react with the reductant, resulting in a
2 | J. Name., 2012, 00, 1-3
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substituted amines are used as the substrates, the product yields presence of stoichiometric catalyst. Besides, both CH3OH and
are located in the range of 44 to 85% (10a-14a). The use of HCOOH can serve as the C1 source for N-formylation or
10
dibenzylamine results in the desired formamide 15a with a perfect methylation promoted by catalysts.
A control experiment
yield (96%). In particular, the reaction can be conveniently scaled replacing CO2 by CH3OH or HCOOH was performed and no any
up to grams with an isolated yield of 81% (Eq. 2).
product was detected (Eq. 3).
The non-aromatic amines, such as N-methylcyclohexylamine,
morpholine, piperidine, and dipropylamine, can also be converted
to the corresponding formamides in high yields (16a-19a), whereas
the hydroformylation of sterically hindered amines, such as
diisopropylamine 20 and 2,2,6,6-tetramethylpiperidine 21, is
relatively sluggish. Several heterocyclic compounds have also been
employed. When 1,2,3,4-tetrahydroquinoline 22 is used, the
corresponding formamide 22a is obtained in a yield of 88 %. In the
case of Indoline 23, the product 23a is generated to have an
excellent yield of 95%. However, indole or pyrrole (24 and 25) are
completely ineffective for the formylation reaction, primarily due to
a strong conjugative effect. To further expand the scope of the
reaction, we also tested primary amines. All of the primary amines
work well and the corresponding mono-formamides are produced
with yields ranging from 74 to 90% (26a-30a).
(Eq.3)
(Eq. 4)
Scheme 3. The possible reaction mechanism for the N-
formylation of amines.
Since CO2 and the amine-CO2 adduct coexist in equilibrium, the
preferential reduction of CO2 or the amine-CO2 adduct is
a
controversial subject till now.11 To distinguish these two possible
reaction routes, the reduction of CO2 and the CO2-amine adduct
with BH3NH3 is carried out separately under the standard conditions.
When the morpholine-CO2 adduct is used, the product 17a is
generated in a very low yield (12%) and the methylated product 17b
is undetectable (Eq. 4). However, for other substrates with weaker
alkalinity, no detectable amine-CO2 adduct can be formed (such as
1). Hence, the possibility of reaction via the amine-CO2 adduct can
be eliminated. To explore the reaction mechanism, some
experiments have been further performed. It is interesting to find
that two B peaks (-23.890 and -29.245 ppm) were detected in the
11B-NMR when BH3NH3 was dissolved in DMF (Figure S1(b)),
suggesting there exists an equilibrium between I and II (Scheme 3).
Adding CO2 into the DMF solution of BH3NH3 can produce a white
solid. 1H-, 13C, and 11B-NMR (including dept-90 and -135) analysis
show that the white solid is a boron-formate DMF complex (III)
(Figure S1 (c-f)). The insert of CO2 into B-H bond to generate boron-
formate and B-OCH3 can be found in previous researches.12 The
white solid III can further react with morpholine to produce 17a
Scheme 2. N-formylation of various amines using with BH3NH3 and
CO2. Conditions: substrate (1 mmol), BH3NH3 (3 mmol), DMF (5 mL).
Isolated yield is given unless otherwise stated. [a] 1H NMR yields
(1,3,5-trimethyoxybenzene as an internal standard).
with a good yield of 86%. Considering that the N-formylation also
works in other solvents (Entries 6-8, Table 1), we can image that
17 can replace DMF in III so that the reaction between 17 and
boron-formate will be easier from the viewpoint of transition state
theory. Based on these results, the plausible mechanism was
presented in Scheme 3.
The obtained results indicate that BH3NH3 is a highly effective
reductant for the N-formylation of various amines using CO2 as the
C1 source without any catalyst. To look insight into the reaction
mechanism, several control experiments are performed. It is
confirmed that N-methylaniline does not react with DMF (Entries 18
and 19, Table 1), so that the pathway of transformylating amine
with DMF to formamide can be excluded in the absence of catalyst.8
The previous works presented that BH3NH3 could be used as
reductant for the conversion of CO2 to CH3OH or HCOOH in the
In conclusion, we have developed a green, catalyst-free and
effective procedure for the N-formylation using CO2 as the C1
source and BH3NH3 as the reductant under mild conditions. N-
formylation works quite well in the absence of catalyst whereas the
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N-methylation is highly catalyst dependent, which makes the
reaction reported here affording the formamides in good to
excellent yield. The reported reactions do not require an inert
atmosphere and is applicable to a wide range of substrates. It
provides an easy access to different formamides. Some catalytic
systems using cheap catalysts (e.g. Cs2CO3, K2CO3, Cu, Zn) have been
reported.2m,p,q,u Most of these examples adopted silane as
7
5
5
reductant, which has
a low atomic economy. Besides, the
separation of the N-formylation product and silane by-product is
not easy because they have similar solubility. On the contrary, the
BH3NH3 by-product is water soluble and catalyst-free process
further simplify the separation of product.
3
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ChemComm
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DOI: 10.1039/C7CC03860G
Formamides were produced by catalyst-free N-formylation of amines using CO2 as the C1 source
and BH3NH3 as the reductant at mild conditions with good to excellent yield.