Graphite oxide as an efficient and durable metal-free catalyst for aerobic oxidative coupling of amines to imines

Article abstract of DOI:10.1039/c2gc16681j

Graphite oxide was found to be a highly efficient, reusable and cost-effective heterogeneous catalyst for the direct metal-free transformation of amines to afford the corresponding imines under mild and neat conditions with molecular oxygen as the terminal oxidant. This method is simple, economic and environmentally benign, resulting in practical advantages for the convenient synthesis of imines and their derivatives.

Full text of DOI:10.1039/c2gc16681j

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COMMUNICATION
Graphite oxide as an efficient and durable metal-free catalyst for aerobic
oxidative coupling of amines to imines†
Hai Huang, Jun Huang, Yong-Mei Liu,* He-Yong He, Yong Cao* and Kang-Nian Fan
Received 29th December 2011, Accepted 16th February 2012
DOI: 10.1039/c2gc16681j
6
Graphite oxide was found to be a highly efficient, reusable
and cost-effective heterogeneous catalyst for the direct metal-
free transformation of amines to afford the corresponding
imines under mild and neat conditions with molecular
oxygen as the terminal oxidant. This method is simple, econ-
omic and environmentally benign, resulting in practical
advantages for the convenient synthesis of imines and their
derivatives.
and extremely high surface area. Until recently, the catalytic
application of graphene and related carbon nanostructures has
focused primarily on the use of these materials as supports for
7
catalytically active transition metals. Very recently, Bielawski
and co-workers has, however, shown that pristine graphite oxide
GO), a common precursor to graphene-like materials, can be
directly used as an efficient catalyst which is capable of oxidiz-
ing a broad array of alcohols to their corresponding carbonyl
compounds. Following this seminal discovery, metal-free GO
has emerged as a highly versatile material that can be applied to
(
8
a
Catalytic oxidation of organic compounds using molecular
8
a
oxygen (O ) as the terminal oxidant has attracted an enormous
a variety of synthetic processes including alkyne hydration,
2
8
c
8d
8b
amount of attention from the viewpoint of green sustainable
thiol oxidation, C–H oxidation, C–C and carbon–heteroa-
1
8e
chemistry. In particular, considerable efforts have been devoted
tom bond-forming reactions. The unusual metal-free reactivity
to the development of mild and efficient methods for the oxi-
dation of alcohols to carbonyl products. In contrast, the conver-
of GO has prompted us to investigate the possibilities offered by
GO for direct transformation of amines by catalytic oxidation.
Herein, we report the use of GO as a readily available, inexpen-
sive, applicable, efficient and reusable catalyst for the direct syn-
thesis of imines from amines under mild aerobic conditions. To
the best of our knowledge, this is the first convenient meta-free
catalytic process ever reported for an efficient imine synthesis
from various amines using molecular oxygen as the oxidant
under neat conditions.
2
sion of corresponding amines to imines by catalytic oxidation
has remained largely undeveloped, despite great utility of imines
in the synthesis of industrial materials, biologically active com-
3
a,b
pounds and their intermediates, such as chiral amines,
hydro-
3c–e
3f,g
3h,i
xyamines,
nitrones
and oxazolidines.
As an attractive
alternative to the traditional amine–carbonyl condensation, cata-
lytic amine oxidation can deliver direct synthesis of imines in a
4
a
green and more efficient manner. While a number of procedur-
The GO was prepared from natural graphite powder using a
4
b–g
9
es
have been developed for such purposes, a persistent chal-
modified Hummers method. To ensure complete removal of the
lenge is the control of reaction pathways to avoid undesired by-
products, such as nitriles or aldehydes. Thus far, a range of tran-
sition-metal-based systems capable of efficient and selective oxi-
residual metallic impurities, the resulting GO suspension was
extensively dialyzed for 5 days. Based on an inductively coupled
plasma mass spectrometry (ICP-MS) analysis, the Mn content in
the GO thus obtained was found to be lower than 30 ppb. The
other metals like Fe, Co, Cu, Pb, etc. were below the detection
limit. The chemical nature and structural morphology of the final
GO material was then thoroughly characterized by a range
of spectroscopic methods including transmission electron
microscopy (TEM), X-ray diffraction (XRD), Fourier transform
infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy
(XPS) analysis. The TEM image showed that the GO nanosheets
have a thickness of several layers (Fig. 1a). The XRD pattern of
GO displayed a well-defined peak at 2θ = 11.6° (Fig. 1b), which
corresponds to an interplanar distance of ca. 0.76 nm. The
characteristic d002 reflection of graphite at 26.6° could not be
observed, which gave evidence that complete oxidation was
5
dation of amines to imines has been reported. Despite their
usefulness, many of these metal-mediated processes are not
applicable in pharmaceutical or biotechnological applications
due to toxicity concerns and separation problems. Therefore, the
development of a simple, efficient and cost-effective process that
can enable direct access to imines by metal-free amine oxidation
utilizing O as the sole oxidant is of high actual interest.
2
Graphene, a flat monolayer of carbon atoms arranged in a hon-
eycomb network, has attracted tremendous attention owing to its
outstanding electronic, optical, thermal, mechanical properties
Department of Chemistry, Shanghai Key Laboratory of Molecular
Catalysis and Innovative Materials, Fudan University, Shanghai
10
achieved. The FTIR measurements showed several intense,
broad absorption features centered at 3400, 1726, 1577, 1227
2
6
†
00433, P.R. China. E-mail: yongcao@fudan.edu.cn; Fax: (+86-21)
5643774
Electronic supplementary information (ESI) available: Experimental
−1
and 1061 cm , confirming the presence of carboxyl-, hydroxyl-
and epoxy-based functional groups on the GO surface (Fig. S1
in ESI†). The presence of a high abundance of oxygen-
section, FTIR, XPS and temporal reaction profiles data. See DOI:
1
0.1039/c2gc16681j
930 | Green Chem., 2012, 14, 930–934
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11
runs (Table 1, entries 1 and 2). Compared to previously
reported oxidative self-coupling of amines using metal catalysts
(
e.g., high surface area iron-based metal–organic frameworks;
4
g
MOFs), appreciably less time was required (4 h versus several
days) to achieve quantitative reaction using GO under similar
temperatures and catalyst loadings. Subsequent experiments per-
formed at 80 °C or with lower oxygen pressures under otherwise
identical reaction conditions showed that the reaction rate was
significantly decreased (Table 1, entries 3–5). Furthermore, it
was found that variations in the GO loadings (ranging from
2
0–200 wt%) had a significant effect on the isolated yield of the
imine product (Table 1, entries 6–8). While 20 wt% GO afforded
70% of the target product (Table 1, entry 6), increasing the
loading to 50 wt% GO was found to be sufficient to drive the
oxidation reaction to quantitative conversion (Table 1, entry 1).
The high efficiency of GO for the direct transformation of
Fig. 1 (a) TEM image of graphite oxide powders. (b) X-Ray diffrac-
tion patterns pristine graphite and graphite oxide.
neat 1a with O relative to other pure carbon nanostructures was
2
shown clearly (Table 1). For example, the use of natural flake
graphite (NG), activated carbon (AC), multi-walled nanotubes
(
MWCNTs) or hydrazine-reduced GO (RGO) in place of GO
Scheme 1 Oxidation of benzylamine (1a) to N-benzylidene benzyla-
mine (1b) catalyzed by GO.
results in only very poor yields of the imine product (Table 1,
entries 10–13). Moreover, it is important to note that aerobic oxi-
dation of 1a to corresponding 1b ,in absence of catalyst, can
only deliver a very low conversion (Table 1, entry 9), and with
GO as the catalyst the reaction does not proceed in the absence
of oxygen (Table 1, entry 3). These experiments clearly demon-
strate the indispensable role of GO in facilitating the selective
oxidation of 1a to 1b in the presence of oxygen. At this point, it
should also be stressed here that the solventless conditions tested
in this study are essentially different from precedents reported
with gold catalysts that require organic solvents.
of straightforward comparison with an earlier study,
additional test using gold nanoparticles supported on TiO2
Table 1 Oxidation of benzylamine a(1a) into N-benzylidene
benzylamine (1b) under various conditions
b
Entry
1
Catalyst
Loading [wt%]
Con. [%]
Yield [%]
GO
GO
GO
GO
GO
GO
GO
GO
No catalyst
NG
AC
MWCNTs
RGO
50
50
50
50
50
20
100
200
—
50
50
50
50
50
99
92
n.r.
40
30
71
99
99
2.5
5
12
10
27
75
98
90
—
38
27
70
97
96
2.5
5
c
2
3
4
5
d
e
f
5a–d
For the sake
6
7
8
9
5a
an
(Au/TiO ), which is among the best gold catalysts reported for
2
1
0
this aerobic oxidation, was performed under our solventless con-
ditions. In this case, conversion of 75% of 1a to 1b with 81%
selectivity was attained (Table 1, entry 14). This relatively low
1
1
12
9.5
25
1
1
1
2
3
4
g
h
Au/TiO
2
61
selectivity of 1b with Au/TiO as compared to that with GO may
2
a
be attributed to the occurrence of undesired reactions arising
by the oxidative cleavage of imine product under oxygen
atmosphere.
2
Reaction conditions: 1a (5 mmol), 5 atm O , 100 °C, 4 h, n.r. = no
b
reaction. Conversion of 1a to 1b, as determined by GC with anisole as
an external reference. After five runs. 5 atm N
c
d
e
f
2
.
1 atm O
2 2
h. Au/TiO (type A, lot no. Au/TiO no. 02-1) were supplied by the
h
2
. 80 °C,
g
5
Given the observed high activity and selectivity, it is of interest
to clarify the catalytic origin of the present GO–amine oxidation
system. In line with the broad literature documenting metal-free
World Gold Council (WGC). The sole by-product was benzaldehyde.
8d,e,12
carbocatalysis using molecular O as the oxidant,
a facile
2
containing functional groups on the GO surface was further
confirmed by XPS analysis, which showed a pronounced
shoulder on the higher binding energy tail of the C 1s peak
oxidative dehydrogenation of the reactant molecules by highly
functionalized GO could be the key factor for achieving efficient
amine oxidation under mild conditions. On the other hand, it is
known that the oxidation of primary amines can proceed by two
different oxidative dehydrogenation routes (Scheme 2) both
(Fig. S2 in ESI†).
Having established the essential nature of the GO material, the
5
c
conversion of benzylamine (1a) was chosen as a model to
explore the feasibility of the proposed reaction (Scheme 1). In a
preliminary experiment, neat 1a was heated to 100 °C in the
involve the formation of a RCHvNH intermediate. In path A,
this intermediate is attacked by a second molecule of the primary
amine to give an aminal which loses NH to give the coupled
3
presence of 50 wt% GO for 4 h under 5 atm O (Table 1). To our
imine product RCHvNCH R. In path B, the initially formed
2
2
delight, the corresponding imine, N-benzylidene benzylamine
imine reacts with trace amounts of H O to give the aldehyde
2
(
1b), can be obtained in an excellent yield of ca. 98% (Table 1,
RCHvO, which subsequently reacts with a second molecule of
the amine to give the final imine product. In the GO-catalyzed
solventless reaction described herein, a phenomenon worthy of
note is the transient formation of notable amount of aldehyde at
entry 1). Of yet further interest is that the GO sample was stable
and can be easily reused in next reaction without an appreciable
loss in either activity or selectivity between the first and fifth
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a
Table 2 Oxidation of amines to imines catalyzed by GO
b
Time
[h]
Con.
[%]
Yield
[%]
c
Entry Substrate
1
4
99
98(91)
97
2
3
4
5
3.5
99
3
3
3
99
91
84
98(92)
88
Scheme 2 Possible mechanisms for the GO-catalyzed aerobic oxi-
dation of primary amines to imines according to ref. 5c.
80
1
3
the initial stage of the reaction (Fig. S3a in ESI†). Considering
the hygroscopic nature of GO, it is likely that a reasonable
amount of water occluded in the GO nanosheets could play a
key role in facilitating the target product formation. This is
further supported by the fact that a deliberate introduction of a
6
7
8
9
5
6
2
95
96
97
89
substantial amount of H O can greatly accelerate the reaction
2
88
(Fig. S3b in ESI†). Therefore, it seems that the present GO-
mediated amine oxidation may occur mainly through path B.
Based on these results the scope and limitations of GO-cata-
lyzed amine oxidation were explored, using 5 atm oxygen
pressure at 100 °C. As revealed in Table 2, various structurally
diverse benzylamines, regardless of the presence of electron-
donating or electron-withdrawing functionalities, could be
directly coupled to give the corresponding imines in excellent
yields. The reaction rate depended qualitatively on the Hammett
96(90)
2
4
8
93
n.r.
98
88
1
4
1
0
—
constant of the substituent (see Fig. S4 in ESI†), with the oxi-
dation being favored by the presence of electron-donating substi-
tuents (Table 2, entries 2, 3, and 6, 7). Moreover, higher reaction
rates for the para-substituted benzylamines relative to the meta
and ortho isomers reveal the presence of a steric effect (Table 2,
entries 3–5). It is noteworthy that halo-substituted benzylamines
performed well, leading to halo-substituted imine compounds
11
88(81)
1
1
2
3
8
8
95
97
83
75
(
Table 2, entries 6, 7), which could be used for further transform-
1
5
ations along with the imine functionality. Furthermore, hetero-
cyclic amines containing nitrogen and sulfur atoms, which
usually poison most metal catalysts,
4
b,16
could also be converted
into the corresponding imines in excellent yields (Table 2,
entries 8 and 9). Note that the oxidation of both pyridyl- and
thiophene-methylamine appeared much more efficient than that
of their phenyl counterparts. However, under similar conditions,
the oxidation of aromatic amines lacking a hydrogen atom at the
α-carbon position, such as aniline, did not proceed at all
14
15
16
5
4
4
97
95(90)
—
n.r.
n.r.
(Table 2, entry 10).
To further investigate the catalytic ability of the present GO
—
system, we next applied this system to the selective oxidation of
secondary amines. As exemplified in the entries 11–14 of
Table 2, a number of secondary amines could be selectively oxi-
dized to their corresponding imines with high to excellent yields.
One aspect that deserves mention is that, compared to the case
with benzylamines as described above, a much longer reaction
time is required to complete the reaction of these secondary
a
2
Reaction conditions: amine (5 mmol), 50 wt% loading GO, 5 atm O ,
00 °C, n.r. = no reaction. Conversion of amine to the corresponding
b
1
imine, as determined by GC with anisole as external reference.
c
Numbers in parenthesis refer to yields of isolated products.
932 | Green Chem., 2012, 14, 930–934
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amines. These results are again indicative of the presence of a
steric effect. In the case of dibenzylamine, a comparably low
selectivity to 1b (ca. 90%) for the imine was observed (Table 2,
entry 11), with the main by-product being benzaldehyde; the for-
mation of which was ascribed to the oxidative cleavage of the
CvN bond. The same phenomenon also occurred in the case
of N-benzylaniline, with by-products of benzaldehyde and
aniline being formed (Table 2, entry 13). On the other hand,
Table 3 Oxidative cross coupling of benzylamine (1a) with anilines or
aliphatic amines (2a)
a
Yield
[%]
b
17
Entry
Amine (2a)
Time [h]
4
Con. [%]
2b
1b
c
1
97
23
74
1
,2,3,4-tetrahydroisoquinoline was quantitatively dehydroge-
nated to 3,4-dihydroisoquinoline (Table 2, entry 14). Attempts
were also made to oxidize simple aliphatic amines such as piper-
idine and n-hexylamine, but they were unsuccessful (Table 2,
entries 15, 16).
Encouraged by excellent yields of the oxidative self-coupling
of benzylamines, an effort was initiated to explore the feasibility
of using the GO-mediated protocol to produce asymmetrically
substituted imines by the oxidative cross condensation of ben-
zylamines and amines lacking hydrogen atoms at the α-carbon
2
3
4
5
4
4
4
4
98
97
99
99
89
90
95
94
9
7
5
6
(Scheme 3). In a preliminary experiment, the reaction was
carried with equimolar amount of 1a and aniline, the former
amine was completely oxidized to form a mixture of imines,
namely 1b and N-benzylidene aniline (2b) in 72 and 24% yield,
respectively (Table 3, entry 1). Formation of 2b corresponds to
the dehydrative coupling of benzaldehyde, the proper and most
6
5
6
99
99
84
92
16
8
18
reasonable reaction according to the literature, with aniline.
7
8
Meanwhile, the lack of aniline oxidation favored the preferential
trapping of benzylamine in residual aniline. To optimize the
reaction further, the reaction was carried out in the presence of
four equivalents of aniline. In this case, the selectivity towards
the target asymmetric N-benzylidene aniline increased remark-
ably with minimal self-condensation of 1a (Table 3, entry 2).
6
8
99
99
88
91
12
9
19
9
By using this newly established procedure, a variety of anilines
bearing electron-withdrawing and electron-donating substituents
can be converted into the corresponding benzylidene anilines in
excellent yields under neat conditions as depicted in Table 3.
Finally, to further expand the variety of asymmetrically substi-
tuted imines that can be obtained through the GO-mediated pro-
cedure, we have carried out analogous experiments with
different simple aliphatic amines such as piperidine and n-hexy-
lamine, and the results are summarized in entries 7 and 8 of
Table 3. In all cases, high yields of the corresponding benzyli-
dene alkylamines are obtained with GO, a result indicating that
the proposed methodology is applicable for the production of a
wide diversity of asymmetrically substituted imines.
a
Reaction conditions: 1a (1 mmol), 2a (4 mmol), 50 wt% loading GO
b
(base on 1a + 2a), 5 atm O
Numbers in parenthesis refer the present of 5 ml toluene for the amine
2
, 100 °C. Conversion based on 1a.
c
(2a) is solid. 1a (1 mmol), 2a (1 mmol), 50 wt% loading GO (base on
1
a + 2a).
asymmetrical and cyclic imines can be conveniently prepared by
this method. The clean and mild synthetic procedure described
herein is expected to contribute to its utilization for the develop-
ment of sustainable products and processes.
This work was financially supported by the National Natural
Science Foundation of China (20873026, 20803012 and 2107-
In summary, we have demonstrated a simple, efficient and
eco-friendly protocol for the direct synthesis of useful imines
from amines via a facile GO-mediated metal-free catalysis pro-
cedure, which proceeds under mild and neutral conditions with
3
042), New Century Excellent Talents in the University of China
(
(
NCET-09-0305), the State Key Basic Research Program of PRC
2009CB623506), and Science & Technology Commission of
molecular O without using any cosolvent. Both symmetrical,
2
Shanghai Municipality (08DZ2270500).
Notes and references
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