N-formylation of amine using graphene oxide as a sole recyclable metal-free carbocatalyst

Article abstract of DOI:10.1007/s13738-018-1471-3

Abstract: Graphene oxide (GO), an inexpensive, environment-friendly, and metal-free carbocatalyst, used for the N-formylation of amines is developed. In this reaction, GO shows good activity, selectivity, and recyclability. This strategy has an array of advantages, such as being metal free, without additive, wide-scope protocol, scalable with a low catalyst loading of 3?wt%, use of readily available and recyclable carbocatalyst, and DMF as a readily available formyl source. Furthermore, this strategy provides an avenue for the convenient hydroformylation of various amines. Graphical abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s13738-018-1471-3

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1471-3
ORIGINAL PAPER
N-formylation of amine using graphene oxide as a sole recyclable
metal-free carbocatalyst
Juan Ma1 · Jingyu Zhang1 · Xiao Zhou1 · Jiawei Wang1 · Hang Gong1
Received: 5 May 2018 / Accepted: 9 August 2018
© Iranian Chemical Society 2018
Abstract
Graphene oxide (GO), an inexpensive, environment-friendly, and metal-free carbocatalyst, used for the N-formylation of
amines is developed. In this reaction, GO shows good activity, selectivity, and recyclability. This strategy has an array of
advantages, such as being metal free, without additive, wide-scope protocol, scalable with a low catalyst loading of 3 wt%,
use of readily available and recyclable carbocatalyst, and DMF as a readily available formyl source. Furthermore, this strategy
provides an avenue for the convenient hydroformylation of various amines.
Graphical abstract
Keywords Synthetic methodology · Graphene oxide · Hydroformylation · C–N bond formation · Recyclability
Introduction
that have controllable electronic states, large π-conjugated
structures, rich oxygen groups, and some structural defects,
has gained increasing interest in the field of catalysis [9–13].
GO-catalyzed/promoted reactions mainly include oxida-
tion reactions (Scheme 1, A) [14–21], oxidative coupling
reactions of amines (Scheme 1, B) [22, 23], Friedel–Crafts
alkylations (Scheme 1, C) [24, 25], C–H arylations of aro-
matics (Scheme 1, D) [26, 27], epoxidations of styrene
(Scheme 1, E) [28], hydration of alkyne (Scheme 1, F) [14],
aldol condensations (Scheme 1, G) [29], and other types
of reactions [9–13]. However, the N-formylation catalyzed
by GO was unprecedented based on the reported literature.
Based on our investigation, the formamides can be achieved
easily from this strategy catalyzed by GO (Scheme 1, H).
Currently, formamides, such as formoterol [30], leu-
covorin [31], tetrahydrolipstatin [32], and other biologi-
cally active compounds (Fig. 1) [33], are widely applied
in pharmaceuticals. These formamides are also used to
protect a group of amines [34–36], as precursor or inter-
mediate for nitrogenous compounds [37–47], and as Lewis
The development of an efficient, environmentally friendly,
and metal-free catalyst is substantially relevant to the sus-
tainable development of the chemical industry [1, 2]. Car-
bon, one of the most readily available and cheap materials on
earth, has numerous applications as a catalyst. Carbon mate-
rials can act as a catalyst [3–6] despite being widely used
as a metal catalyst carrier [7, 8]. Graphene oxide (GO), one
of the outstanding representatives of carbon nanomaterials
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s13738-018-1471-3) contains
supplementary material, which is available to authorized users.
* Hang Gong
hgong@xtu.edu.cn
1
The Key Laboratory for Green Organic Synthesis
and Application of Hunan Province, Xiangtan University,
Xiangtan 411105, China
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Scheme 1 Graphene oxide-
catalyzed reactions and our
investigation
Fig. 1 Formamide-containing
pharmaceuticals
base catalysts for organic conversion processes [48–52].
Formic acid and its derivatives [53–56], methanol [57–59],
aldehydes [60–65], CO [66, 67], and CO2 [68–73] have
been applied as formyl sources. However, these strategies
are significantly limited because of the use of expensive
metal catalysts, water-sensitive reagents, toxic gases, and
complex workups, and thus, are inconsistent with the prin-
ciples of environmentally benign chemistry.
Experimental
General remarks
Preparative thin-layer chromatography was performed
for product purification using Sorbent Silica Gel 60 F254
TLC plates and visualized with ultraviolet light. IR spec-
tra were recorded on a new Fourier transform infrared
spectroscopy.1H, 13C and 19F NMR spectra were recorded
on 400, 100, 377 MHz NMR spectrometer using CDCl3
as solvent unless otherwise stated. HRMS were made by
means of ESI. Melting points were measured on micro-
melting point apparatus and uncorrected. Unless otherwise
noted, all reagents were weighed and handled in air, and
all reactions were carried out in a sealed tube under an
atmosphere of argon. Unless otherwise noted, all reagents
were purchased from reagent company, and used without
further purifications. Notably, graphene oxide (GO) is
purchased from Hunan Fenghua Material Development
Co., Ltd.
Alternatively, N-formylation has been considerably
developed by Blanchet [74], Feng [75], Xu [76], Wang
[77], Sheppard [78], Williams [60–62], Shankarling [79],
and other groups [80–83]. Despite the major advances,
most of these strategies require a metal-containing catalyst
or large amounts of non-metal catalyst, long reaction time,
and limited substrate scope. In addition, the separation and
recycling of organocatalyst and the metal residue remain
challenges in these catalyst systems. In this context, we
reported the GO-catalyzed N-formylation of amines. This
strategy has an array of advantages, such as being metal
free, without additive, short reaction time, wide-scope
protocol, scalable (with a low catalyst loading of 3 wt%),
and use of readily available and recyclable carbocatalyst.
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Journal of the Iranian Chemical Society
then DMF was removed by distillation under reduced pres-
sure. The residue was purified by silica gel column chroma-
tography with petroleum ether and ethyl acetate (v/v=3/1)
to achieve the pure product 2-formyl-1,2,3,4-tetrahydroiso-
quinoline (yield: 85%, 1.02 g).
General procedure and characterization data
for products
A solution of N-heterocycle compound (0.2 mmol) and
graphene oxide (GO) (8 mg) in N,N-dimethylformamide
(1.0 mL) was stirred in a sealed tube under an atmosphere
of argon at 150 °C for 18 h. After being cooled to room tem-
perature, the reaction mixture was filtered and washed with
ethyl acetate (20 mL). Afterward, 10 mL water was added to
the solution and extracted with ethyl acetate (3×15 mL), the
combined organic layers were dried over anhydrous Na2SO4.
The solvent was evaporated under vacuum and the crude
product was purified by preparative thin-layer chromatog-
raphy (TLC) on silica gel with petroleum ether and ethyl
acetate to achieve the pure product. The example of charac-
terization data (2a) 2-formyl-1,2,3,4-tetrahydroisoquinoline:
purified by TLC (petroleum ether/ethyl acetate=3/1); yel-
Experimental procedure (Scheme 2, Eq. 2)
A solution of 1,2,3,4-tetrahydroisoquinoline (1 g, 7.5 mmol)
and graphene oxide (GO) (30 mg) in DMF (8 mL) was
stirred in a sealed tube under an atmosphere of argon at
150 °C for 7 days. After being cooled to room temperature,
the reaction mixture was filtered and washed with ethyl ace-
tate. Afterward, the solution was evaporated under vacuum.
Yield was detected by 1H NMR using CH3NO2 as internal
standard (yield: 80%, 0.96 g).
1
low oil; H NMR (400 MHz, CDCl3) δ 8.23 (minor rota-
Results and discussion
mer, s, 0.36H), 8.17 (major rotamer, s, 0.62H), 7.21–7.08
(m, 4H), 4.67 (major rotamer, 1.29H), 4.52 (minor rotamer,
0.76H), 3.76 (minor rotamer, t, J = 6.2 Hz, 0.76H), 3.63
(major rotamer, t, J= 5.8 Hz, 1.29H), 2.90–2.84 (m, 2H);
13C NMR (100 MHz, CDCl3) δ 161.62 (major rotamer),
161.11 (minor rotamer), 134.28 (minor rotamer), 133.46
(major rotamer), 132.15 (minor rotamer), 131.62 (major
rotamer), 129.08 (minor rotamer), 128.84 (major rotamer),
126.98, 126.61–126.39 (m), 125.81, 47.19 (minor rotamer),
43.12 (major rotamer), 42.17 (major rotamer), 37.87 (minor
rotamer), 29.59 (major rotamer), 27.81 (minor rotamer); IR
(neat) 3151, 3025, 2931, 2862, 1672, 1584, 1498, 1439,
1400, 1343, 1318, 1282, 1228, 1197, 1164, 1109, 1049,
930, 882, 814, 751, 710, 675, 606, 477 cm−1; HRMS (ESI)
m/z [M + Na]+ calcd for C10H11NNaO 184.07329, found
184.07325.
First, the optimization reactions were conducted (Table 1),
and the optimized reaction conditions were obtained as fol-
lows: tetrahydroisoquinoline (0.2 mmol), DMF (1.0 mL),
GO (8 mg), reaction temperature 150 °C, and reaction time
18 h under an argon atmosphere (Table 1, entry 13).
The scope of the substrates and synthetic applications
was examined with the available reliable N-formylation
protocol. The reaction was conducted well when tetrahy-
droisoquinoline and its analogs were used as substrates;
moderate to excellent yield was achieved (Table 2, 2a–g).
However, the electron-withdrawing group on the benzene
ring is not good at this transformation (2c–d). When other
secondary amines, either the cyclic or linear amine, were
used as substrates, the desired products could be obtained
at good to excellent yields in most cases (2h–r). The steric
hindrance of the substitute on nitrogen will be discussed
carefully (Tables 2, 2n–o and Table S1 in SI, s2a–g). From
the results, when the substituent on nitrogen was large, the
yield obtained would be low. Fluoxetine, an antidepressant,
could be hydroformylated with moderate yield (2r, 46%)
through this strategy. The N-formyl compounds 2r could be
used as markers for fluoxetine [84]. The low yield may be
attributed to the big skeleton of fluoxetine, which blocks its
access to the surface of GO.
Gram‑scale synthesis of 2a
Experimental procedure (Scheme 2, Eq. 1)
A solution of 1,2,3,4-tetrahydroisoquinoline (1 g, 7.5 mmol)
and graphene oxide (GO)(100 mg) in DMF (30 mL) was
stirred in a sealed tube under an atmosphere of argon at
150 °C for 96 h. After being cooled to room temperature, the
reaction mixture was filtered and washed with ethyl acetate.
Afterward, the solution was evaporated under vacuum, and
The primary amine is also compatible with this strategy,
and moderate to excellent yield could be obtained (2s–za).
Interestingly, the distance between the amino and phenyl
groups has an obvious effect on this reaction (2s–u). When
they are separated by three methylenes, an excellent yield
of 97% could be achieved. The hydroformylation of pri-
mary amine has an important application value in organic
synthesis. For example, the hydroformylation product
(2v) of homoveratrylamine is an important precursor for
Scheme 2 Gram reactions of 2a
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Journal of the Iranian Chemical Society
Table 1 Selected optimization results
Entry
GO/mg
Time/h
T/℃
Yield/%
1
2
3
4
5
6
7
8
8
5
3
8
8
8
8
8
8
8
8
8
8
8
0
18
18
18
3
150
150
150
150
150
150
150
60
83
80
82
58
61
71
83
2
27
48
65
81
91(86)c
46
12
6
12
24
18
18
18
18
18
18
18
18
9
80
10
11
12a
13b
14d
15
100
120
150
150
150
150
Unless otherwise noted, all reactions were conducted at 0.2 mmol scale in a sealed tube in 1.5 mL DMF under argon atmosphere. Yields are
detected by 1H NMR using CH3NO2 as internal standard
a0.5 mL DMF was used, b1 mL DMF was used, cisolated yield, dunder air atmosphere
synthesizing the natural product pseudopalmatine, 8-oxop-
seudopalmatine, and ilicifoline B [85, 86].
of GO is necessary [14–17, 22]. The combination of a
metal-free catalysis and a low catalyst loading is a good
model for practical application.
The N-formylation reaction has no adverse effects on the
hydroxyl in the substrates (2y and 2za). Particularly, ani-
line and its derivatives are not suitable for this transforma-
tion; only a trace amount of the product could be detected
(Table S1 in SI, s2h–l) because the conjugation of nitrogen
and benzene ring decreased the nucleophilicity of nitrogen.
Particularly, formamide could also act as the formyl source,
and an excellent yield of the desired product 2zb could be
obtained. Moreover, the acetylated product 2zc could be
achieved with high yield when acetamide was used. Other
more complex amides such as propionamide, 2-furamide,
2-thiophenecarboxamide and benzamide also could act as
substrates and the corresponding products could be achieved
by moderate yield (2zd-zg).
Graphene oxide demonstrates good recyclability and
can be reused by simple precipitation. The catalytic activ-
ity and selectivity exhibited no obvious change (Table 3)
from the first to the forth cycle tested. When the fifth cycle
experiment was conducted, the yield of 2a was decreased
to 71% because of the cumulative loss of catalyst. It is
worth noting that the percentage of oxygen of GO before
and after the first cycle reaction were 44 and 18%, respec-
tively. It may be caused by the side reaction of oxidized
amine to imine [22, 23, 87]. However, the recycle experi-
ment shows the catalytic active for this N-amine reaction
is retained.
Other carbocatalysts, such as natural graphite, acetylene
black, graphite oxide and GO with a low oxidation degree,
were attempted to understand the catalytic mechanism of
this transformation. When natural graphite, acetylene black
and graphite oxide were used, the desired product could only
be detected at a low yield (Table 4, entries 3–5). However,
when GO with a low oxidation degree was used as a catalyst,
a good yield of 73% could still be achieved. These results
Importantly, the gram-scale preparation of 2a was con-
ducted (Scheme 2), and a good yield of 85% (10 wt% GO)
was achieved with an extended reaction time (96 h). A
high yield of 80% could be obtained even when only 3 wt%
of GO was used as a catalyst. The low catalyst loading is
more attractive compared with the conventional GO-cata-
lyzed reaction, in which the high loading of 50–100 wt%
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Journal of the Iranian Chemical Society
Table 2 GO-catalyzed
N-formylation of various
amines and synthetic
applications
Reaction conditions: amine (0.2 mmol); GO: 8 mg, DMF 1 mL, in a seal tube, argon atmos-
1
phere, 150 °C, 18 h. Isolated yield was showed out parentheses, H NMR yield was showed
a
b
c
d
in parentheses; from formamide; from acetamide; from propionamide; from 2-furamide
e
(0.5 g), dioxane 0.5 mL, reaction time is 30 h; from 2-thiophenecarboxamide (0.5 g), diox-
ane 0.5 mL, reaction time is 30 h; ffrom benzamide (1 g), reaction time is 30 h
indicated that the oxygen group on carbocatalysts may play
a pivotal role in the N-formylation reaction.
The desired product was achieved with a moderate yield
of 51% in the presence of benzoic acid (entry 5). So, the
carboxylic acid moiety on GO plays an important role in
the N-formylation reaction. Then, we assumed that the
π-conjugated system has certain effects on the catalytic
activity. Subsequently, the carboxylic acid with a large
π-conjugated system was tested, and a good yield of 89%
was obtained (entry 9). Moreover, the ketone, alcohol, and
epoxide combined with pyrene nearly showed a consid-
erably higher catalytic activity compared with the cor-
responding benzene derivatives (entries 6–8). Thus, the
carboxylic acid group and large π-conjugated system are
At least four kinds of oxygen species are present on the
surface of GO, containing a carbonyl (C=O), hydroxyl
(–OH), epoxide (C–O–C), and carboxylic acid (–CO2H)
[88]. To investigate the catalytic activity of different oxy-
gen species, four kinds of oxygen-containing compounds,
namely ketone, alcohol, epoxide, and carboxylic acid,
were tested as a “catalyst” in this N-formylation reaction
(Table 5, entries 2–5). The results indicated that ketone,
alcohol, and epoxide nearly could not catalyze this reac-
tion; the yield of the desired product is poor (entries 1–4).
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Journal of the Iranian Chemical Society
Table 3 Recyclability of GO in transamidation reaction
Cycle
Yield/%
1
2
3
4
5
87
94
92
86
71
Unless otherwise noted, all reactions were conducted at 0.2 mmol scale in a sealed tube in 1.0 mL DMF under argon atmosphere. Yields are
detected by 1H NMR using CH3NO2 as internal standard
presumed essential for the catalytic activity of GO in the
N-formylation reaction.
the catalytic activity of GO is probably dependent on the
carboxyl group and independent of hydroxyl and epoxide
groups. Furthermore, the influence of metal impurities in
GO was investigated using EDTA as a metal-shielding
agent (Table 6, entry 1), and a good yield of 72% still
could be obtained. The results indicated that the metal
impurities have a limited effect on the catalytic activity
of GO.
The base-treated GO (b-GO) and base–acid-treated
GO (ba-GO) were used as catalysts to further validate the
catalytic activity of oxygen-containing groups (Table 6,
entries 2–3). Loh’s research showed that the conjugate
degree of b-GO and ba-GO recovered partly, whereas the
hydroxyl groups nearly lost, and epoxides reduced signifi-
cantly [23]. When b-GO was used as a catalyst, only 3%
product yield was detected, because the carboxyl group
was converted to carboxylate when GO was treated with a
base. When the b-GO was acidified with HCl, the catalytic
activity of ba-GO nearly recovered, and a good yield of
82% could be achieved. These results also indicated that
The kinetic isotope effect (KIE) experiments were then
attempted (Scheme 3a, d) to clarify the detailed reaction
process. The desired product was obtained with a kH/kD
ratio of 4.3, indicating that the activation of the N–H
bond in this transformation is the rate-determining step.
Meanwhile, the hydrogen source of the aldehyde group
Table 4 Investigation of various carbocatalysts
Entry
Carbocatalyst
Yield/%
1a
2a
1
2
3
4
5
GO
0
0
58
49
28
91(86)a
73
15
24
41
*GOb
Natural graphite
Acetylene black
Graphite oxide
Unless otherwise noted, all reactions were conducted at 0.2 mmol scale in a sealed tube in 1.0 mL DMF under argon atmosphere. Yields are
detected by 1H NMR using CH3NO2 as internal standard; aisolated yield; bGO with a lower oxidation degree
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Journal of the Iranian Chemical Society
Table 5 Preliminary screening
of catalytic activity of oxygen-
containing group
Recovery yield/%
Yield/%
2a
Entry
catalyst
none
1a
1
2
75
12
35
47
12
25
3
4
5
35
7
ND
51
6
7
12
31
59
45
8
9
ND
0
61
89
Unless otherwise noted, all reactions were conducted at 0.1 mmol scale in a sealed tube in 1.0 mL DMF
under argon atmosphere. Yields are detected by 1H NMR using CH3NO2 as internal standard
was examined (Scheme 3b, c). When the d-1a and DMF
were used in this reaction, no d-2a was detected, whereas
in the case of using 1a and d-DMF [(CH3)2NCDO], only
d-2a could be detected. These results indicated that the
hydrogen of the aldehyde group in the desired product
originated from DMF. Afterward, the radical inhibition
test indicated that this reaction is not a radical process
(Scheme 3d).
Based on these control experimental results and other
literature studies [44, 75, 82], we proposed a reasonable
mechanism for this N-formylation reaction (Scheme 4).
First, the carboxyl group on the shell of GO activates the
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Journal of the Iranian Chemical Society
Table 6 Treated GO-catalyzed reactions of tetrahydroisoquinoline with DMF
Entry
Cat.
Yield%
1a
2a
1
2
3
GOa
1
61
2
72
3
82
b-GOb
ba-GOc
Unless otherwise noted, all reactions were conducted at 0.2 mmol scale in a sealed tube in 1.0 mL DMF under argon atmosphere. Yields are
detected by 1H NMR using CH3NO2 as internal standard; aEDTA (3 mg) was added; bGO was refluxed with 30% NaOH for 2 h; cthe b-GO was
acidified with HCl to pH 4–5
Scheme 3 Control experiments
of N-formylation reaction
(A)
(B)
(C)
(D)
amino and formyl groups to generate the intermediate
complex (I), wherein the carbonyl moieties of the car-
boxyl group function as the Brønsted bases to activate the
nucleophilicity of the amino group, while the hydroxyl
group combined with the formyl via a hydrogen bond and
works as the Lewis acid center for activating the carbonyl
of DMF. Subsequently, the nucleophilic attack of the acti-
vated amino to the activated carbonyl group occurred,
and the sp3-hybridized tetrahedral intermediate (II) was
formed. Meanwhile, the active species were regenerated.
The sterically congested intermediate (II) disintegrated,
followed by a proton transfer process; the target molecule
(T.M.) and amine were then released.
Conclusion
In summary, the GO-catalyzed N-formylation of amines
was described. GO showed good activity, selectivity, and
recyclability when it was used as a metal-free carbocata-
lyst for the N-formylation reaction. This strategy has an
array of advantages, such as a metal free, without addi-
tive, wide-scope protocol, scalable with a lower catalyst
loading as 3 wt%, the use of readily available and recycla-
ble carbocatalyst, and use of DMF as a readily available
formyl source. Therefore, the proposed strategy provided
an avenue for the convenient hydroformylation of various
amines.
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Journal of the Iranian Chemical Society
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