MnO2/graphene oxide: A highly efficient catalyst for imine synthesis from alcohols and amines

Article abstract of DOI:10.1002/aoc.3659

α-MnO₂ nanocrystals supported on graphene oxide (α-MnO₂/GO) was prepared through a soft chemical route and evaluated for the first time as a novel, eco-friendly and efficient catalyst in the coupling reaction of alcohols and amines to imines. The well-organized α-MnO₂/GO was characterized using various techniques. The results show that MnO₂ nanocrystals are highly dispersed on the GO sheets and interconnected with each other, leading to large available surface area, which greatly enhances the catalytic performance of conventional MnO₂. Under mild conditions, the catalyst exhibits excellent catalytic activity and selectivity with O₂ serving as terminal oxidant. Various imines can be smoothly obtained in good to excellent yield. Importantly, the catalyst is easily recovered and can be reused six times with no significant loss of activity.

Full text of DOI:10.1002/aoc.3659

Received: 7 July 2016
DOI 10.1002/aoc.3659
Revised: 1 September 2016
Accepted: 20 September 2016
F U L L PA P E R
MnO2/graphene oxide: A highly efficient catalyst for imine
synthesis from alcohols and amines
Shengxian Cheng | Xiaoxia Ma | Yongke Hu | Bindong Li
School of Chemical Engineering, Nanjing
α‐MnO2 nanocrystals supported on graphene oxide (α‐MnO2/GO) was prepared
through a soft chemical route and evaluated for the first time as a novel, eco‐friendly
and efficient catalyst in the coupling reaction of alcohols and amines to imines. The
well‐organized α‐MnO2/GO was characterized using various techniques. The results
University of Science and Technology, 210094
Nanjing, People's Republic of China
Correspondence
Bindong Li, School of Chemical Engineering,
Nanjing University of Science and Technology,
show that MnO2 nanocrystals are highly dispersed on the GO sheets and intercon-
210094 Nanjing, People's Republic of China.
Email: libindong@mail.njust.edu.cn
nected with each other, leading to large available surface area, which greatly
enhances the catalytic performance of conventional MnO2. Under mild conditions,
the catalyst exhibits excellent catalytic activity and selectivity with O2 serving as ter-
minal oxidant. Various imines can be smoothly obtained in good to excellent yield.
Importantly, the catalyst is easily recovered and can be reused six times with no
significant loss of activity.
KEYWORDS
GO, heterogeneous catalyst, imine synthesis, α‐MnO2/GO
1
|
INTRODUCTION
as catalysts for the coupling reaction of amines and alcohols,
such as Ru,[15,16] Os[17] and Co.[18] Although these methods
Imines, also known as Schiff bases, are an important class of
nitrogen source in the synthesis of a variety of useful organic
compounds such as pharmaceuticals and fine chemicals.[1]
Also, imines are widely applied as electrophiles in many
organic transformations such as cyclization, reduction, addi-
tion, condensation, aziridination and so on.[2] Therefore, syn-
thesis of imines is a consistently demanding research topic,
and making them efficiently is of paramount importance.
The conventional protocol for the synthesis of imines is
through condensation reaction of amines with aldehydes or
ketones.[3] In the past few years, several alternative methods
involving green synthesis have been developed, including
oxidation of primary amines,[4–7] oxidation or dehydrogena-
tion of secondary amines[8,9] and direct coupling of alcohols
and amines/ammonia.[10–13] In comparison with the former
two approaches, the aerobic oxidative coupling of alcohols
and amines is considered to be most promising from both
environmental and economic viewpoints, because alcohols
are readily available and low‐cost starting materials, and
water or hydrogen is generated as sole byproduct.
are efficient and atom‐economic, some intrinsic drawbacks
still exist. They suffer from harsh reaction conditions, the
use of expensive ligands and unavailable noble metals. On
the other hand, transition metals such as Fe,[11] Cu[19] and
Pd[20] catalyzing aerobic oxidative synthesis of imines from
alcohols and amines have attracted much attention, because
the reactions use molecular oxygen or air as the oxidant with
water as the only byproduct. But the disadvantages of these
homogeneous processes include the difficulty in recycling
the catalysts and the possibility of leaving traces of toxic
heavy metals in the products. As to the aerobic oxidative pro-
tocols, heterogeneous catalysts have also been widely investi-
gated due to their reusability and durability. For example,
recently Tamura and Tomishige[21] reported a CeO2‐catalized
protocol, through which imines can be obtained at low tem-
perature (333 K) with no solvent, although its applicability
was limited by long reaction time (24 h). The group of
Xia[22] have developed a solvent‐free catalytic system using
Co2+‐exchanged zeolites as catalyst, although it requires base
(ButOK) and high temperature (443 K) to promote the reac-
tion, which is not environmentally friendly. Moreover, there
are many other heterogeneous catalysts such as gold
Since the ground‐breaking work of Milstein and co‐
workers,[14] a wide range of metal complexes have been used
Appl Organometal Chem 2016; 1–8
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
CHENG ET AL.
nanoparticles,[23] Au–Pd alloy,[24] Pd/ZrO2,[25] etc. But
most of them have some defects such as high price, time‐
consuming preparation and requirement for addition of
base. Therefore, we expected to look for a heterogeneous
catalyst which is efficient, cheap, readily synthesized and
does not require any promoter like base.
water (350 ml) was added to stop the reaction. An amount
of 6.5 ml of 30 vol.% H2O2 was then added into the slurry
to react with remaining KMnO4, resulting in a tawny suspen-
sion. The suspension was washed with 3.8 vol.% HCl solution
three times and then with deionized water two times. The
product after washing was put into a dialysis tube to eliminate
residue ions. After being dried, GO was obtained.
To the best of our knowledge, manganese is one kind of
non‐noble metal contained in heterogeneous catalysts which
are used in the synthesis of imines from alcohols and amines.
Blackburn and Taylor[26] have reported MnO2‐mediated in
situ oxidation–imine formation from alcohols and amines,
but the method has a serious drawback in that it requires a
large excess of MnO2 (10 equiv.) as the oxidant and 4 Å
molecular sieves as the dehydrating agent. Furthermore, two
other manganese‐catalyzed systems were reported using a
catalytic amount of K‐OMS‐2[27] and MnOx/HAP,[28] respec-
tively, but either long reaction time (24 h) or large excess of
amines are required to achieve high yields. Accordingly, we
want to search for a better manganese‐containing catalyst.
Well‐organized manganese dioxide/graphene oxide (MnO2/
GO) is well known as being used in electrochemistry,[29–31]
but it recently has been utilized in organic synthesis indicat-
ing its efficient catalytic activity,[32,33] which benefits from
the large surface area (up to 400–1500 m2 g−1) and synergis-
tic charge delocalization of the GO support.[33,34] MnO2/GO
is easy to prepare, and over which a variety of imines can be
obtained in a short time with molecular oxygen serving as the
terminal oxidant (Scheme 1). To our delight, base is not nec-
essary in this system. Importantly, the catalyst is easily sepa-
rated and can be used at least six times without significant
loss of catalytic activity. This is the first time that MnO2/
GO has been used for imine formation from alcohols and
amines under mild conditions.
2.2 | Synthesis of α‐MnO2/GO
When the feed ratio of α‐MnO2 to GO was 3:1, the detailed
procedure was as follows.[29] GO (0.33 g) and MnCl2⋅4H2O
(1.35 g) were dispersed in isopropyl alcohol (250 ml) by
ultrasonication for 2 h. Then, the slurry was heated to approx-
imately 80 °C with vigorous stirring, after which KMnO4
(0.75 g) dissolved in deionized water (250 ml) was rapidly
added. The slurry was cooled to room temperature after
refluxing for 120 min. After filtering, washing and finally
drying in air at 50 °C, the nanocomposite labeled as
α‐MnO2/GO was obtained. For comparison, nano‐MnO2
was synthesized in the absence of GO via a similar method.
2.3 | Synthesis of CuO/GO
The detailed procedure was as follows. Cu(AcO)2⋅H2O
(0.39 g) and GO (0.05 g) were dispersed in 30 and 50 ml of iso-
propyl alcohol, respectively, by ultrasonication for 30 min.
The Cu(AcO)2⋅H2O solution was then added into the GO
slurry, after which it was heated to approximately 83 °C with
vigorous stirring. After the slurry was kept at 83 °C for
30 min, deionized water (5 ml) was quickly added into it,
and then the temperature was maintained at 83 °C for
30 min. After cooling to room temperature, the slurry was cen-
trifuged, washed with ethanol and finally dried in air at 70 °C.
2.4 | Synthesis of imines
2
| EXPERIMENTAL
Typically, the procedure for the formation of imine from ben-
zyl alcohol and aniline was as follows. Benzyl alcohol
(2 mmol), aniline (2.4 mmol), catalyst (0.010 g), diphenyl
ether (internal standard) and toluene (2 ml) were added into
a 25 ml reaction vessel. The reaction mixture was vigorously
stirred at 110 °C under oxygen atmosphere for 6–8 h. After
the reaction was completed, the mixture was cooled to room
temperature. Then the catalyst was separated by filtration
and washed several times with ethyl acetate. The filtrate
was dried using MgSO4 and concentrated in vacuum, and
then subjected to purification by silica gel column chroma-
tography or recrystallization.
2.1 | Synthesis of GO
GO was prepared using a modified Hummers method,[35] and
the typical route was as follows. Preliminarily oxidized graph-
ite powder (2.5 g) was slowly added into concentrated sulfuric
acid (115 ml) in a flask which was immersed in an ice bath.
After being stirred for 10 min, KMnO4 (9 g) was slowly added
into the slurry. The slurry reacted for 10 min with stirring, dur-
ing which the temperature of the slurry was kept below 20 °C.
The slurry was then heated to 35 °C, and maintained for 3.5 h.
Then, 230 ml of deionized water was slowly added into the
slurry, and stirred for 15 min at 98 °C, and then deionized
2.5 | Recycling of catalyst
The catalyst was separated from the reaction mixture by fil-
tration and washed with ethyl acetate and deionized water.
After drying at 50 °C overnight, it could be used for the next
reaction run.
SCHEME
1
MnO2/GO‐catalyzed oxidative coupling of alcohols and
amines

CHENG ET AL.
3
2.6 | Characterization of α‐MnO2/GO
X‐ray diffraction (XRD) patterns were recorded with a Bruker
D8 Advance diffractometer with Cu Kα radiation (1.54 Å).
Transmission electron microscopy (TEM) images were
obtained using a Tencnai 12 (Philips). Scanning electron
microscopy (SEM) measurements were conducted with a
Hitachi S‐4800II. Energy‐dispersive X‐ray spectroscopy
(EDX) was performed with a Thermo Noran 7. Fourier trans-
form infrared (FT‐IR) spectra were recorded with a Nicolet
IS‐10 spectrometer equipped with a DTGS detector. Raman
spectra were recorded with an inVia (Renishaw). Nitrogen
adsorption was carried out at 77 K using an ASAP2020.
X‐ray photoelectron spectroscopy (XPS) was carried out
with a PHI Quantera II.
3
| RESULTS AND DISCUSSION
3.1 | Characterization of α‐MnO2/GO
The morphology of as‐synthesized MnO2/GO hybrid was
investigated using TEM and SEM. Figure 1(a) shows that
pure GO sheet is a virtually transparent film. After growing
MnO2 on its surface, GO sheet is covered with numerous
clusters of MnO2 nanocrystals, which interconnect with each
other forming a porous structure (Figure 1b). From high‐
magnification TEM image (Figure 1c) and SEM image
(Figure 1d), we can see α‐MnO2 nanocrystal is like needle
shape attached on the GO sheet. Additionally, it is clear from
Figure 1(b) and (d) that needle‐like α‐MnO2 nanocrystals
with diameters of 20–50 nm and lengths of 50–500 nm are
highly dispersed on the surface as well as on the edge of
GO sheets. The assembly of α‐MnO2 nanocrystals on the sur-
face of GO sheets to some extent prevents the stacking of GO
sheets due to van der Waals interactions, leading to a large
available surface area for transferring reactants and prod-
ucts.[32] Nitrogen adsorption–desorption isotherms are shown
in Figure 2. Noticeably, MnO2/GO (100 m2 g−1) exhibits
higher surface area than MnO2 (76 m2 g−1).
The chemical components of as‐prepared catalyst were
examined using XRD patterns and Raman, FT‐IR, EDX
and XPS spectra. Figure 3 shows powder XRD patterns of
GO and MnO2/GO. As shown in Figure 3, the most intense
peak at 2θ = 10.2° corresponds to the (001) reflection of
GO. The interlayer spacing (0.86 nm) of GO is much larger
than that of pristine graphite (0.34 nm) because of the local-
ization of oxygen‐containing functional groups on the graph-
ite sheets.[36] In the pattern of α‐MnO2/GO in Figure 3, all
diffraction peaks indicate the formation of α‐MnO2 (JCPDS
44–0141), and the (001) reflection peak of GO almost disap-
pears due to the insertion of MnO2.[37,38]
FIGURE 1 TEM images of (a) GO and (b, d) MnO2/GO. (c) SEM image of
MnO2/GO. (e) EDX spectrum of MnO2/GO
to the carboxyl O─H vibrations. And the broad peak at
3206 cm−1 is ascribed to stretching of coupling O─H
bond.[39,40] In the case of α‐MnO2/GO, the main characteris-
tic peaks of GO still remain and two peaks appear at 589 and
698 cm−1, which can be ascribed to the Mn─O vibrations in
MnO2.[41] After deposition of α‐MnO2, all peaks of GO
except the peak at 3206 cm−1 are intensified. This is because
The FT‐IR spectra of GO and α‐MnO2/GO composite are
displayed in Figure 4. In the spectrum of GO, the stretching
frequencies of C═O and C─O are observed at 1715 and
1045 cm−1, respectively. The peak at 1417 cm−1 is attributed

4
CHENG ET AL.
shown in Figure 5. It is clear from the spectrum of GO that
the characteristic Raman peaks for GO structure are G and
D bonds at around 1361 and 1592 cm−1, corresponding to
the presence of defects/disorders and well‐defined sp2
graphitized structure, respectively.[30] In the spectrum of
α‐MnO2/GO, the band at around 643 cm−1 can be attributed
to the symmetric stretching vibration (Mn─O) of MnO2.
Obviously, α‐MnO2/GO exhibits characteristics of both GO
and α‐MnO2, indicating that GO is not reduced under the
applied conditions. In addition, the presence of C, O and
Mn in the sample was studied using EDX (Figure 1e), the
result of which is consistent with XRD, FT‐IR and Raman
results.
XPS spectra of GO and α‐MnO2/GO are shown in
Figure 6. By comparing Figure 6(a) and (b), we can see that
the oxidized carbon of α‐MnO2/GO (44.65%) is lower than
that of GO (58.66%). This is because of the decrease of
oxygen‐containing groups, which are expected to capture
Mn2+ ions, partially reduced by Mn2+ ions and anchored
MnO2 nanoparticles on the surface of GO.[32,42] In the O
1 s spectrum (Figure 6c), the dominant peak at 529.7 eV is
ascribed to Mn─O bond in MnO2, while the other peak at
531.9 eV is attributed to oxygen‐containing groups of
GO.[43] In the Mn 2p spectrum (Figure 6d), the Mn 2p3/2
peak is centered at 642.2 eV and the Mn 2p1/2 peak is at
653.9 eV, with a spin‐energy separation of 11.7 eV, suggest-
ing the formation of MnO2.[44]
FIGURE
2
Nitrogen adsorption–desorption isotherms of MnO2 and
MnO2/GO
3.2 | Catalytic performance
Initially, the catalytic activity of various catalysts for imine
synthesis from benzyl alcohol and aniline were studied with
molecular O2 as oxidant at 80 °C. As evident from Table 1,
there is no formation of imine in the absence of catalyst
(entry 1). When GO and α‐MnO2 (prepared via a procedure
similar to that for α‐MnO2/GO) are added separately, the
yield of benzylidene aniline is found to be only 5 and 17%
(entries 2 and 3). Whereas the α‐MnO2/GO composite shows
FIGURE 3 XRD patterns of GO and MnO2/GO
FIGURE 4 FT‐IR spectra of GO and MnO2/GO
of the destruction of some regular layered GO sheets caused
by growth of α‐MnO2 crystals. However, the peak at
3206 cm−1 is weakened due to the interlayer hydrates in α‐
MnO2.[41] The Raman spectra of GO and α‐MnO2/GO are
FIGURE 5 Raman spectra of GO and MnO2/GO

CHENG ET AL.
5
TABLE 1 Imine synthesis from benzyl alcohol and aniline over various
catalysts
Entry
Catalyst
None
Conv. (%)a
Yield (%)a
1
2
—
5
—
5
GO
3
α‐MnO2
α‐MnO2/GOb
19
84
68
85
83
84
84
81
17
84
65
84
82
84
83
81
4
5
Cu/GO
6
α‐MnO2/GOc
α‐MnO2/GOd
α‐MnO2/GOe
α‐MnO2/GOf
α‐MnO2/GOg
7
8
9
10
Reaction conditions: benzyl alcohol (2 mmol), aniline (2.4 mmol), catalyst
(0.02 g), O2 balloon, toluene (2 ml), 80 °C.
aDetermined by GC using diphenyl ether as the internal standard.
bFeed ratio of α‐MnO2/GO is 3/1.
cFeed ratio of α‐MnO2/GO is 4/1.
dFeed ratio of α‐MnO2/GO is 2/1
eCatalyst (0.015 g).
fCatalyst (0.010 g).
gCatalyst (0.005 g).
excellent activity under the same conditions, achieving
highest alcohol conversion and imine yield (entry 4). The
Cu/GO composite also has good catalytic activity for this
reaction, but it is inferior to the α‐MnO2/GO composite both
in alcohol conversion and imine yield (entry 5). Catalysts
with different α‐MnO2 loading were also tested. It is found
that when the feed ratio of α‐MnO2 to GO increases from
3:1 to 4:1, the result remains virtually unchanged (entry 6).
But, the imine yield decreases (entry 7) as α‐MnO2 loading
decreases to 2:1. Moreover, with the amount of catalyst
reducing to 0.015 or 0.010 g, the yields of benzylidene
aniline show no noticeable decline (entries 8 and 9). How-
ever, when the amount continued to decrease to 0.005 g,
the imine yield obviously decreases (entry 10).
After selecting α‐MnO2/GO as catalyst for this reaction,
the reaction conditions were optimized further. Firstly, the
influence of solvent was studied. Non‐polar and aprotic
toluene was preferred here, which gives the desired imine in
84% yield (Table 2, entry 1). Other solvents, such as
dimethylformamide (DMF), ethanol, tetrahydrofuran (THF)
and MeCN, were also tested and result in the formation of
imine in poor to moderate yields (entries 2–5). Subsequently,
the effect of temperature was investigated. With the tempera-
ture increases to 110 °C, the yield of benzylidene aniline is
dramatically increased from 84 to 97% (entry 6). It is notice-
able that the reaction time is greatly reduced from 12 to 6 h.
By contrast, the imine yield is only 65% (entry 7) when reac-
tion temperature and time are 80 °C and 6 h, indicating the
significant influence of temperature. When the temperature
FIGURE 6 XPS spectra of the C 1 s region of (a) GO and (b) MnO2/
GO; (c) O 1 s region of GO and MnO2/GO; and (d) Mn 2p region of
MnO2/GO

6
CHENG ET AL.
TABLE 2 Optimization of reaction conditions
TABLE 3 Synthesis of imines from various alcohols and amines
Entry
Alcohol
Amine
Yield (%)a
Entry
Solvent
Temp. (°C)
Conv. (%)a
Yield (%)a
1
94
1
Toluene
DMF
80
80
84
Trace
67
84
Trace
64
2
2
3
94
93
3
Ethanol
THF
80
4
80
20
17
5
MeCN
80
58
31
6
Tolueneb
Tolueneb
Tolueneb
Toluenec
Toluened
Toluenee
Toluenef
110
80
96
96
7
66
65
8
120
110
110
110
110
97
96
9
98
96
4
94.
10
11
12
96
96
67
62
Trace
Trace
Reaction conditions: benzyl alcohol (2 mmol), aniline (2.4 mmol), catalyst
(0.010 g), O2 balloon, solvent (2 ml), 12 h.
aDetermined by GC using diphenyl ether as the internal standard.
5
6
96
90
b6 h.
c0.1 eq. KOH, 6 h.
d0.1 eq. K2CO3, 6 h.
eAir balloon.
fN2 balloon.
7
85
81
76
—
—
97
94
96
93
87
8
increases to 120 °C the imine yield no longer increases (entry
8). In order to verify whether base can further promote this
reaction, KOH and K2CO3 were introduced separately. As
evident from entries 9 and 10, there is no obvious improve-
ment of both alcohol conversion and imine yield, suggesting
that base is not required for this reaction. Finally, the reaction
was carried out under nitrogen atmosphere and air. Compared
with the reaction under oxygen atmosphere, the reaction
under air gives lower imine yield and longer reaction time
(entry 11). When the reaction is carried out under nitrogen,
the desired product is not observed, which indicates the
importance of atmospheric molecular oxygen as the terminal
oxidant (entry 12).
Having smoothly optimized the reaction conditions, we
explored the substrate scope and generality of this catalytic
system. Firstly, various primary alcohols were tested with
aniline. As evident from Table 3, benzyl alcohols containing
either electron‐withdrawing or electron‐donating groups are
smoothly transformed into the desired imine in excellent
yield (entries 1–5). p‐Methylbenzyl alcohol gives a slightly
higher yield and shorter reaction time than m‐methylbenzyl
alcohol and o‐methylbenzyl alcohol presumably because of
the steric hindrance (entries 5–7). However, to our surprise,
1‐naphthalene methanol that bears a more steric hindering
group converts well with good yield, which might be attrib-
uted to the benefit from the rigid ring (entry 8).[28] Further-
more, heterocyclic alcohol can also be tolerated with our
catalyst and gives a moderate yield of the corresponding
imine (entry 9). But n‐octyl alcohol and cyclohexanol fail
9
10
11
12
13
14
15
16
(Continues)

CHENG ET AL.
7
TABLE 3 (Continued)
Entry
Alcohol
Amine
Yield (%)a
17
81
18
82
Reaction conditions: alcohol (2 mmol), amine (2.4 mmol), catalyst (0.010 g), tol-
uene (2 ml), O2 balloon, 110 °C.
aIsolated yield.
FIGURE 8 XRD patterns of fresh and recovered MnO2/GO
to react with aniline in this catalytic system (entries 10, 11).
Next, the reactions between benzyl alcohol and a series of
amines were examined. Similarly, aniline derivatives bearing
a substituent with different electronic properties also react to
the corresponding imines in excellent yields (entries 12–15).
Due to steric hindrance, the meta‐ and ortho‐substituted ani-
lines have lower conversion and imine yields than para‐
substituted ones (entries 15–17). Furthermore, the aliphatic
amine cyclohexylamine can also be converted to imine with
a good yield (entry 18).
The recyclability of the catalyst was examined in the
oxidative coupling of benzyl alcohol and aniline. As
recycling data (Figure 7) show, there is no significant
decrease of catalytic activity after six runs. Moreover, we
can see from Figure 8 that there is no obvious change
between the XRD patterns of fresh α‐MnO2/GO and that
of α‐MnO2/GO recovered after six runs, proving that the
catalyst retains robust stability.
In order to further understand the reaction pathway, a
kinetic experiment of imine formation from benzyl alcohol
and aniline was conducted under the optimized conditions.
Benzylidene aniline is the major product during the whole
course of the reaction, and the benzaldehyde formed by
selective oxidation of alcohol is observed at the initial
stage and its amount slowly decreases with time, which
indicates that benzaldehyde is an intermediate. Based on
these observations and previous reports, the reaction
SCHEME 2 Proposed pathway of α‐MnO2/GO‐catalyzed imine formation
from alcohols and amines
possibly
proceeds
by
two
consecutive
steps
(Scheme 2):[12,21] (i) oxidative dehydrogenation of alcohol
to aldehyde or ketone and (ii) imine formation of aldehyde
or ketone with amine.
4
| CONCLUSIONS
In summary, a reusable and efficient heterogeneous catalyst
(α‐MnO2/GO) for oxidative coupling reaction of alcohols
and amines was prepared through an easily controlled chem-
ical route. Due to the large surface area, α‐MnO2/GO exhib-
ited excellent catalytic activity without the need for base or
any other promoter. Diverse types of substrates can be con-
verted into corresponding imines in good to excellent yields
under mild conditions. Importantly, the catalyst is readily
available and can be reused six times without significant
loss of catalytic activity. These features make our method
attractive for imine synthesis, and could lead to extensive
application of this catalyst in the preparation of fine
chemicals.
REFERENCES
[1] P. G. Cozzi, Chem. Soc. Rev. 2004, 33, 410.
[2] J. Gawronski, N. Wascinska, J. Gajewy, Chem. Rev. 2008, 108, 5227.
[3] A. K. Chakraborti, S. Bhagat, S. Rudrawar, Tetrahedron Lett. 2004, 45,
7641.
[4] A. N. Campbell, S. S. Stahl, Acc. Chem. Res. 2012, 45, 851.
[5] M. Largeron, M. B. Fleury, Angew. Chem. Int. Ed. 2012, 51, 5409.
[6] Z. Hu, F. M. Kerton, Org. Biomol. Chem. 2012, 10, 1618.
[7] B. Govinda Rao, P. Sudarsanam, A. Rangaswamy, B. M. Reddy, Catal. Lett.
2015, 145, 1436.
FIGURE 7 Recycling studies of the catalyst
[8] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. Engl. 2003, 42, 1479.

8
CHENG ET AL.
[9] L. Liu, Z. Wang, X. Fu, C. H. Yan, Org. Lett. 2012, 14, 5692.
[32] R. F. Nie, J. J. Shi, S. X. Xia, L. Shen, P. Chen, Z. Y. Hou, F. S. Xiao,
J. Mater. Chem. 2012, 22, 18115.
[10] E. Balaraman, D. Srimani, Y. Diskin‐Posner, D. Milstein, Catal. Lett. 2014,
145, 139.
[33] M. M. Kadam, K. B. Dhopte, N. Jha, V. G. Gaikar, P. R. Nemade, New J.
Chem. 2016, 40, 1436.
[11] E. Zhang, H. Tian, S. Xu, X. Yu, Q. Xu, Org. Lett. 2013, 15, 2704.
[34] G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth, R. Mulhaupt,
[12] L. Zhang, W. Wang, A. Wang, Y. Cui, X. Yang, Y. Huang, X. Liu, W. Liu,
J. Am. Chem. Soc. 2009, 131, 8262.
J. Y. Son, H. Oji, T. Zhang, Green Chem. 2013, 15, 2680.
[35] W. S. Hummers Jr., R. E. Offeman, J. Am. Chem. Soc. 1958, 80, 1339.
[13] J. Xu, R. Zhuang, L. Bao, G. Tang, Y. Zhao, Green Chem. 2012, 14, 2384.
[36] M. M. Kadam, O. R. Lokare, K. V. M. K. Kireeti, V. G. Gaikar, N. Jha, RSC
Adv. 2014, 4, 62737.
[14] B. Gnanaprakasam, J. Zhang, D. Milstein, Angew. Chem. Int. Ed. Engl.
2010, 49, 1468.
[37] M. N. Patel, X. Wang, D. A. Slanac, D. A. Ferrer, S. Dai, K. P. Johnston,
[15] J. W. Rigoli, S. A. Moyer, S. D. Pearce, J. M. Schomaker, Org. Biomol.
Chem. 2012, 10, 1746.
K. J. Stevenson, J. Mater. Chem. 2012, 22, 3160.
[38] J. Su, M. Cao, L. Ren, C. Hu, J. Phys. Chem. C 2011, 115, 14469.
[39] M. R. Nabid, Y. Bide, F. Dastar, Catal. Lett. 2015, 145, 1798.
[16] A. Maggi, R. Madsen, Organometallics 2012, 31, 451.
[17] M. A. Esteruelas, N. Honczek, M. Oliván, E. Oñate, M. Valencia, Organo-
metallics 2011, 30, 2468.
[40] H. Su, S. Wu, Z. Li, Q. Huo, J. Guan, Q. Kan, Appl. Organometal. Chem.
2015, 29, 462.
[18] G. Zhang, S. K. Hanson, Org. Lett. 2013, 15, 650.
[19] Q. Kang, Y. Zhang, Green Chem. 2012, 14, 1016.
[41] L. Kang, M. Zhang, Z. H. Liu, K. Ooi, Spectrochim. Acta A 2007, 67, 864.
[42] Z. Li, S. Wu, D. Zheng, J. Liu, H. Liu, H. Lu, Q. Huo, J. Guan, Q. Kan, Appl.
Organometal. Chem. 2014, 28, 317.
[20] L. Jiang, L. Jin, H. Tian, X. Yuan, X. Yu, Q. Xu, Chem. Commun. 2011, 47,
10833.
[43] R. I. Jafri, N. Rajalakshmi, S. Ramaprabhu, J. Mater. Chem. 2010,
20, 7114.
[21] M. Tamura, K. Tomishige, Angew. Chem. Int. Ed. Engl. 2015, 54, 864.
[22] Y. W. Sun, X. H. Lu, X. L. Wei, D. Zhou, Q. H. Xia, Catal. Commun. 2014,
43, 213.
[44] L. Mao, K. Zhang, H. S. On Chan, J. Wu, J. Mater. Chem. 2012, 22, 18.
[23] P. Liu, C. Li, E. J. Hensen, Chemistry 2012, 18, 12122.
SUPPORTING INFORMATION
[24] W. Cui, Q. Xiao, S. Sarina, W. Ao, M. Xie, H. Zhu, Z. Bao, Catal. Today
2014, 235, 152.
Additional Supporting Information may be found online in
the supporting information tab for this article.
[25] W. Cui, B. Zhaorigetu, M. Jia, W. Ao, H. Zhu, RSC Adv. 2014, 4, 2601.
[26] L. Blackburn, R. J. K. Taylor, Org. Lett. 2001, 3, 1637.
[27] S. Sithambaram, R. Kumar, Y. Son, S. Suib, J. Catal. 2008, 253, 269.
[28] B. Chen, J. Li, W. Dai, L. Wang, S. Gao, Green Chem. 2014, 16, 3328.
[29] S. Chen, J. Zhu, X. Wu, Q. Han, X. Wang, ACS Nano 2010, 4, 2822.
How to cite this article: Cheng, S., Ma, X., Hu, Y.,
and Li, B. (2016), MnO2/graphene oxide: A highly
efficient catalyst for imine synthesis from alcohols
and amines, Appl Organometal Chem, doi: 10.1002/
aoc.3659
[30] T. Gan, Z. Shi, Y. Deng, J. Sun, H. Wang, Electrochim. Acta 2014,
147, 157.
[31] H. Yang, J. Jiang, W. Zhou, L. Lai, L. Xi, Y. M. Lam, Z. Shen, B. Khezri, T.
Yu, Nanoscale Res. Lett. 2011, 6, 531.