Catalyst-free selective: N -formylation and N -methylation of amines using CO2 as a sustainable C1 source

Article abstract of DOI:10.1039/c9gc03637g

We herein describe catalyst-free selective N-formylation and N-methylation of amines using CO₂ as a sustainable C1 source. By tuning the reaction solvent and temperature, the selective synthesis of formamides and methylamines is achieved in good to excellent yields using sodium borohydride (NaBH₄) as a sustainable reductant.

Full text of DOI:10.1039/c9gc03637g

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Catalyst-free selective N-formylation and N-methylation of
‡
amines using CO2 as a sustainable C1 source
Qizhuang Zou,†,a,b Guangcai Long,†,a,b Tianxiang Zhao,*a,b and Xingbang Hu*c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
We herein describe a catalyst-free selective N-formylation and N- products is also essential.5 Many pioneering works in this area have
methylation of amines using CO2 as a sustainable C1 source. By achieved great results. In 2016, Fu et al.5a established Cs2CO3 system
tuning reaction solvent and temperature, the selective synthesis of to display remarkable reactivity for the selective N-formylation and
formamides and methylamines is achieved in good to excellent N-methylation using CO2 and hydrosilanes. Whereafter,
a
yields using sodium borohydride (NaBH4) as
reductant.
a sustainable hierarchical reduction of CO2 with amine and diphenylsilane
(Ph2SiH2) was acquired by using glycine betaine as a catalyst.5b,c
Recently, Xia et al.5d reported DBU-catalyzed selective N-formylation
and N-methylation of amines with CO2 and polymethylhydrosiloxane
(PMHS). Almost simultaneously, He and coworkers developed a
pressure-switched and K2WO4-catalyzed as a catalyst strategy for
selective N-formylation and N-methylation of amines using CO2 and
phenylsilane (PhSiH3).5e The above processes usually use
hydrosilanes as the reductants, but the inherent disadvantages of
using hydrosilanes will result in relatively low atomic economy. And
the recovery of the catalyst is also difficult. In addition, for some
catalyst-free examples, selective N-formylation and N-methylation of
amines have not been achieved concurrently using the same
reducing agent.6 Hence, the development of the catalyst-free and
high atom efficient methodology is essential for CO2 fixation to
formamides and methylamines in a perspective of green chemistry.
CO2, as an abundant and green sustainable C1 feedstock, is used
widely to build value added chemicals.1 Especially, synthesis of
formamides and methylamines by reductive functionalization of CO2
is very attractive in recent years.2 However, this process is
challenging because CO2 molecules are highly stable. Therefore, the
boranes2a-c and hydrosilanes2d-h are used as highly active reductants
in above processes. Even so, many processes still require metal or
non-metal catalysts, especially when hydrogen is used as
a
reductant.3 For all we know, metal catalysts with remarkable activity,
such as Ru,3a Rh,3b Pd,3c Cu,3d Zn,3e and Fe3f, have been reported.3 In
contrast, a number of organocatalysts, involving superbases,4a N-
heterocyclic carbenes (NHCs),4b,c B(C6F5)3,4d and ionic liquids (ILs)4e
have also revealed moderate activity.4
Although above results are encouraging,3,4 most of these
processes still require high temperature and pressure (H2 as
reductant), and low atomic economy (hydrosilanes as reductants).
Otherwise, how to control the chemoselectivity of reduction
The metal borohydrides are inexpensive reductants, which can
react with CO2 to form formate borohydride species.7 By comparison,
a more attractive strategy is replacing hydrosilanes with the
inexpensive metal borohydrides. In this respect, several examples
reveal the feasibility of CO2 reduction functionalization using NaBH4
and NaBH(OAc)3.8 In our previous work, we developed a catalyst-free
N-formylation of amines using CO2 and NH3BH3,9 however, the
reaction does not selectively obtain methylated products, and the
expensive price of NH3BH3 is not conducive to large-scale synthesis.
†
These authors contributed equally to this work.
aSchool of Chemistry and Chemical Engineering, Guizhou University,
Guiyang, Guizhou 550025, P. R. China
bKey Laboratory of Green Chemical and Clean Energy Technology, Guiyang,
Guizhou 550025, P. R. China
Inspired by above results, we want to realize here a catalyst-free
selective N-formylation and N-methylation of amines using CO2 and
inexpensive metal borohydrides. In the course of our study on the
conversion of CO2, we unexpectedly discovered that formamide and
methylamine can be produced simultaneously by using NaBH4 as
reductant. The reaction solvent and temperature can significantly
affect the selectivity of the product. So we performed initially the
reaction in various solvents using N-methylaniline (1a) as a model
cSchool of Chemistry and Chemical Engineering, Nanjing University,
Nanjing, Jiangsu 210093, P. R. China
E-mail: txzhao3@gzu.edu.cn (T. Zhao), huxb@nju.edu.cn (X. Hu)
‡Electronic supplementary information (ESI) available: Details on synthesis
of formamides and methylamines, mechanism research, and NMR information of
formamides and methylamines.
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substrate at 50oC to investigate the selective N-formylation and N- temperature along with 2a in a 11% yield, while poor selectivity of 3a
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DOI: 10.1039/C9GC03637G
methylation (See SEI ‡). To our delight, the best result with a 94% was observed in DMF, CH3CN, or THF under same conditions (Entry
yield of 2a was obtained in DMF (Table 1, Entry 1), suggesting that 9-11), indicating that the solvent and reaction temperature played
solvent can activate the N-H bond of 1a through polarization and the critical role in the selectivity and reactivity. Shortening reaction
solvation.6,10 Both KBH4 and NaBH(OAc)3 are also effective for N- time or decreasing NaBH4 led to reduction of 3a yield (Entry 13 and
formylation, affording 2a in 91% and 89% yield (Entry 2 and 3), 14).
respectively. Next we tried to enhance the selectivity of the
With the optimized reaction conditions in hand, the substrate
methylated product. Before this, some previous works11 supported
scope of N-formylation with various amines (Scheme 1) was
further reduction of formamide to form methylamine. Thus, we are
evaluated firstly. We found that both aromatic and aliphatic
just trying to increase the amount of NaBH4 to get more methylation
secondary amines converted into the desired formamides in
products. However, only 18% yield of 3a was obtained by using 4 eq.
excellent yields. For instance, 2a was achieved in an isolate yield of
of NaBH4 (Entry 11). To further clarify the mechanism, 2a was also
88%. Most of the N-methylanilines with both electron-withdrawing
tested to react with the NaBH4 (Eq.1). The result showed that the
and electron-donating groups in the phenyl ring were well tolerated
reaction cannot take place meaning that formamide was unlikely to
(2b-2h), giving the yields of up to 94%. Interestingly, the electronic
be the intermediate for the N-methylation.
property of the functional group on the amines affected observably
the reaction activity. The substrate of N-methyl-4-nitroaniline was
N
N
O
NaBH4 (2.5 eq.)
unreactive (2i) because a strong electron-withdrawing effect that
weakens greatly the nucleophilicity of the amine. The N-alkyl
substituent on the secondary monoaromatic amines led to the
declining yields (2j-l), presumably because of the steric hindrance.
The aliphatic amines also converted to the corresponding
formamides in moderate yields (2m-p). In addition, this protocol
could expand to several heterocyclic amines (2q-t), affording the
yields of up to 95%. However, only the trace of the product was
detected using aniline as reactant, presumably because weak
nucleophilic aniline cannot trap the weak electrophilic formate
intermediate.12
DMF (3 mL), 50 oC, 24 h
H
3a
0%
2a 1 mmol
(Eq. 1)
Table 1 Optimization of reaction conditions.
H
CO (10 bar),
2
NaBH4
+
Catalyst-free
N
N
N
O
H
1a
2a
3a
Entry
Solvent
T (oC)
NaBH4
(mmol)
Yield of
Yield of
3a
2a (%)a
(%)a
6
H
1
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
CH3CN
THF
50
50
50
50
2.5
2.5
2.5
4
94
91
89
82
61
77
42
0
87
33
25
11
10
6
NaBH4 (2.5 eq.), 50 oC, 24 h
R1
R1
2b
3c
4
7
CO2
NH
R2
N
O
+
DMF , Catalyst-free
R2
10
18
Trace
12
Trace
0
13
57
66
85
H
5
50
50
50
50
100
100
100
1
H
H
H
6d
7e
8f
2.5
2.5
2.5
4.0
4.0
4.0
4.0
4.0
2.5
N
O
N
O
N
O
N
O
2a
(88%)
2b
2c
2d
2h
2l
91%
92%
94%
O
F
Cl
Br
H
9
H
H
H
O
10
11
12
13e
14
N
O
N
O
N
O
O
N
O
2e
(90%)
2f
90%
2g
88%
84%
1,4-dioxane 100
1,4-dioxane 100
1,4-dioxane 100
O2N
H
72
55
H
H
H
H
N
N
N
O
N
N
O
O
Ph
Reaction conditions: amine 1a (1.0 mmol), solvent (3 mL), CO2 (10
a
20%
2j
2k 52%
2i
79%
0%
bar), reaction time (24 h). The yields were determined by GC/MS
using dodecane as the internal standard. b2.5 mmol of KBH4. 2.5
c
H
H
N
O
mmol of NaBH(OAc)3. dReaction time 12 h. eCO2 (1 bar). fCO2-free.
N
H
N
O
O
Reducing the amount of NaBH4 and reaction time led to a decrease
in yield of 2a (Entry 5 and 6). The pressure of CO2 also affected the
yield of 2a. Specifically, the yield of 2a decreased to 42% at 1 bar of
CO2 (Entry 7). In contrast, at higher pressure of 10 bar of CO2, more
CO2 can be dissolved in the liquid phase to react with NaBH4 and
amine, resulting in higher yield of 2a (Entry 1 vs Entry 7). In addition,
to exclude the possibility of DMF participating in the reaction, the
reaction was performed in the absence of CO2, and no product was
detected (Entry 8). Noteworthy, when the reaction was carried out
in 1,4-dioxane under the same conditions, the methylated product
3a was obtained in 33% yield (Table S1, Entry 6). The yield of 3a
increased observably to 85% (Entry 12) with increasing reaction
2o
2p
88%
O
86%
H
2m
2q
2n
(76%)
85%
H
H
H
O
N
N
O
N
O
N
O
94%
2r
(95%)
2s
2t
(95%)
(90%)
Scheme 1 Substrate scope of the N-formylation. Reaction conditions:
amine (1.0 mmol), CO2 (10 bar), and DMF (3 mL). The yields were
determined by the H NMR using 1,3,5-trimethyoxybenzene as an
internal standard. Isolated yields are given in parentheses.
1
We evaluated subsequently substrate scope of the N-methylation
of amines by tuning the reaction temperature and solvent. The
2 | J. Name., 2012, 00, 1-3
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summary of results is presented in Scheme 2. We found that the
To study the mechanism of the reaction, several control
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secondary monoaromatic amines, both electron-withdrawing and experiments were designed respectivelyDOuI:n1d0e.1r039th/Ce9GsCta0n36d3a7rGd
electron-donating group on the aromatic ring, could be methylated, conditions. Considering that CO2 can react with primary or secondary
affording the corresponding methylamines in 64-93% yields (3a-g). to produce carbamates,9a and CO2 and amine-CO2 adduct coexist in
The weak nucleophilicity N-methyl-4-nitroaniline is relatively inert, equilibrium,9b reducing CO2 and CO2-amine adduct with NaBH4 were
giving the desired 3h in a 53% yield. Some representative aliphatic carried out separately. The experimental results indicated that
amines were further investigated, but they showed lower selectivity reaction did not undergo amine-CO2 adduct intermediate pathway in
for the corresponding methylated products (3i-3m), preferring the our study (Eq. 3). Similar result was also obtained by reducing
formation of formylation products. Particularly, this protocol could morpholine (1r) carbamate, thereby further confirming that
be also applicable to heterocyclic amines, such as 2,3-dihydro-1H- carbamates is not an intermediate in the reaction (See SEI‡).
indole (1n) and 1,2,3,4-tetrahydroquinoline (2n) to afford the Whereafter, we designed a stepwise reaction of NaBH4, CO2 and 1a
corresponding products in 71% (3n) and 75% (3o) yield, respectively. (Eq. 4). It is interesting to find that the formate borohydride species
In addition, the N-methylation was carried out using aniline as the [NaH4-nB(OCOH)n] was formed preferentially during the reduction of
substrate under the above optimized conditions (See SEI‡), but CO2 with NaBH4 in DMF, which was confirmed by NMR (Fig. S1) and
dimethylated product was only obtained in 11% yield (3p). The yield defined to Int.1 (Scheme 3).7 Especially, it can further react with 1a
of 3p could be increased to 25% by lengthening of reaction time, to produce 2a in a 53% yield. The result suggested that Int.1 was a
indicating that the aniline was also compatible with the optimized key intermediate in the formylation reaction of 1a and CO2 using
conditions, despite low reactivity of the aniline.
NaBH4 as the reductant. Indeed, if NaBH4 was instead with NaBD4 in
the reaction of 1a and CO2, the containing deuterium 2a was
obtained in a yield of 86% (Eq. 5), supported by NMR analysis (Fig.
S2), further proving that NaBH4 acted as the reductant in the
formylation reaction of amine and CO2.
NaBH4(4.0 eq.), 100 oC, 24 h
R1
R1
R1
CO2
NH
R2
N
+
N
or
1,4-dioxane , Catalyst-free
(H)
R2
N
N
N
N
H
N
O
N
(1) CO2 (10 bar), DMF (3 mL), 24 h, rt.
(2) NaBH4 (2.5 eq.) 24 h, 50 o
3c
3d
(82%)
(93%)
3b
3f
3a
(85%)
(80%)
H
C
Cl
O
F
O2N
2a
0%
1a
1 mmol
(Eq. 3)
(Eq. 4)
(Eq. 5)
N
N
N
N
N
O
(1) CO2 (10 bar), DMF (3 mL), 24 h, 50oC
3h
3g
(53%)
3e
3i
(64%)
(72%)
N
(78%)
NaBH4
o
1a
(2) N-Methylaniline
(1 mmol) 24 h, 50 C
H
2.5 mmol
N
38%
N
2a
53%
O
N
N
3j
3l (
48%)
(54%)
N
3k (
34%)
N
H
N
N
CO2 (10 bar),
NaBD4 (2.5 eq.)
DMF (3 mL), 50 oC, 24 h
D
1a
1 mmol
2a'
86%
N
a
3m (
41%)
3n
3o
(75%)
(71%)
3p
11%/25%
Analogously, to insight into the reaction mechanism of
methylation, the reaction of CO2 with NaBH4 was performed in 1,4-
dioxane for 24 h at 100°C. The results of the H NMR and 13C NMR
1
Scheme 2 Substrate scope of the N-methylation. Reaction conditions:
amine (1.0 mmol), CO2 (10 bar), and 1,4-dioxane (3 mL). The yields
were determined by the 1H NMR using 1,3,5-trimethyoxybenzene as
an internal standard. Isolated yields are given in parentheses.
Reaction time (48 h).
suggested that the CO2 was reduced to the Int.2 (Fig. S3), and then
3a was detected in a yield of 42% when equimolar 1a was added into
the reaction solution of CO2 and NaBH4 at 100°C (Eq. 6), meaning that
Int.2 was the key intermediate of methylation. Accordingly, the 11B{H}
NMR spectrum signals at 19.5 and 1.4 ppm also implied generation
of Int.2 as major and Int.1 as minor.13 In addition, we performed a
control experiment using NaBH4 to reduce 2a under the harsher N-
methylation optimized conditions (Eq. 7). The result shows that the
reaction cannot occur, implying that formamide was not an
intermediate for the synthesis of methylamine.
a
To verify the applicability of the present methodology, the
reactions of the N-formylation and N-methylation were performed
on the gram scale (Eq. 2). The desired products 2d and 3d were
achieved in 85% and 78% isolated yields, respectively. It suggests
that this catalyst-free and efficient methodology has a potential
application for large-scale synthetic chemistry.
(1) CO2 (10 bar), 1,4-dioxane (3 mL), 24 h, 100oC
N
N
O
NaBH4 (2.5 eq.)
DMF (20 mL), 50 oC, 24 h
CO2 (10 bar),
NaBH4
o
1a
(2) N-Methylaniline
(1 mmol) 24 h, 100 C
H
H
N
4 mmol
2d
3d
3a
42%
85%
(Eq. 6)
N
1d
10 mmol (1.21 g)
CO2 (10 bar),
NaBH4 (4 eq.)
100 oC, 24 h
1,4-dioxane (20 mL),
78%
(Eq. 2)
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2. (a) W. Chen, J. Shen, T. Jurca, C. Peng, Y. Lin, Y. Wang, W. Shih,
H
NaBH4(4.0 eq.), 100 oC, 24 h
1,4-dioxane , Catalyst-free
View Article Online
G. P. A. Yap and T. Ong, Angew. Chem. Int. Ed., 2015, 54,
DOI: 10.1039/C9GC03637G
N
O
N
15207-15212; (b) V. B. Saptal and B. M. Bhanage,
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2a
3a
No reaction
(Eq. 7)
Based on the experimental results and previous reports,3-7
a
possible reaction mechanism is proposed (Scheme 3). Firstly, CO2
reacts with NaBH4 in DMF to produce the Int.1. Then, amine as a
nucleophilic reagent attacks the carbon atom of intermediate Int.1
to form the N-formylation product. For N-methylation process, CO2
reacts with NaBH4 in 1,4-dioxane to produce Int.1, followed by
further reaction with NaBH4 affords intermediate Int.2. Finally, the
nucleophilic reagent of amine attacks the carbon atom of
intermediate Int.2 to form the N-methylated product. In fact, the
effect of solvent is still not clear in above processes, we speculated
that both DMF and 1,4-dioxane coordinated to sodium ion,7a,14
making the insertion of CO2 much easier to form the intermediate,
and led to the reaction with selectivity to afford desired formamides
and methylamines.
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NaH4-nB(OH)n
R1
Int. 2
N
NaH4-nB(OCH3)n
R2
R1R2NH
NaBH4
NaBH4
R1
R1R2NH
NaBH4
CO2
N
O
NaH4-nB(OCOH)n
NaBH4
R2
H
Int. 1
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Scheme 3 The possible mechanism of formylation and methylation
of amine using CO2 and NaBH4.
Conclusions
In conclusion, we have developed a catalyst-free and efficient
selective N-methylation and N-formylation of amines for the
synthesis of formaides and methylamines with CO2 as a sustainable
C1 source and inexpensive NaBH4 as a reductant. By tuning solvent
and reaction temperature, both selective N-methylation and N-
formylation of amines can be controlled smoothly, affording desired
products of formaides and methylamines in good yields.
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Acknowledgement
This work was supported by Natural Science Special Foundation of
Guizhou University (No. X2019065 Special Post A) and Scientific and
Technological Innovation Talents Team Project of Guizhou Province
(NO.20185607).
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Conflicts of interest statement
We have no conflicts of interest to declare.
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Green Chemistry
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Journal Name
COMMUNICATION
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DOI: 10.1039/C9GC03637G
TOC: Catalyst-free selective N-formylation and N-methylation of amines using
CO2 as a sustainable C1 source
O
R1
NH
R2
NaBH4
NaBH4
R2
N
R1
CO2
H
N
+
R1
1,4-dioxane
DMF
R2