One-pot imine synthesis from methylarenes and anilines under air over heterogeneous Cu oxide-modified CeO2catalyst

Article abstract of DOI:10.1039/d0cc02969f

Cu oxide-modified CeO2 (CuOx-CeO2) with 2 wtpercent Cu loading amount was the most effective and reusable heterogeneous catalyst for selective one-pot imine synthesis from methylarenes and anilines via direct oxidation of the sp3 C-H bond in the methylare

Full text of DOI:10.1039/d0cc02969f

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One-pot imine synthesis from methylarenes
and anilines under air over heterogeneous Cu
Cite this:DOI: 10.1039/d0cc02969f
oxide-modified CeO catalyst†
2
Received 24th April 2020,
Accepted 26th May 2020
a
b
b
Masazumi Tamura, * Yingai Li and Keiichi Tomishige
DOI: 10.1039/d0cc02969f
rsc.li/chemcomm
Cu oxide-modified CeO
2
(CuO
x
2
–CeO ) with 2 wt% Cu loading
Imines are an important class of organic compounds due to
amount was the most effective and reusable heterogeneous the versatility for transformations, and conventionally synthesized
catalyst for selective one-pot imine synthesis from methylarenes by condensation of aldehydes or ketones and amines with an acid
3
6
and anilines via direct oxidation of the sp C–H bond in the catalyst. Various effective methods have been investigated such
methylarenes with atmospheric air (0.1 MPa) as an oxidant.
as oxidation or dehydrogenation of secondary amines, dimeriza-
tion of amines, hydroamination of alkynes with amines, and
3
Direct, catalytic and selective aerobic oxidation of the sp C–H direct coupling of alcohols and amines and others. As more
bond in alkylarenes to the corresponding oxygen-containing economical and greener methods, hydrogenative coupling of
functional groups such as alcohols, aldehydes, ketones and nitroarenes and aldehydes has been actively investigated because
7
carboxylic acids using O , particularly air, as an oxidant is an nitroarenes are more primitive chemicals than anilines. Simi-
2
1
important and challenging reaction in organic syntheses. One- larly, aerobic oxidative coupling of alkylarenes and amines
pot reactions composed of partial aerobic oxidation of alkyl- (Scheme 1) is also promising because alkylarenes are more
arenes and C–N, C–O or C–C bond formation using alcohols, primitive and cheap chemicals than other starting ones such as
amines or other chemicals have attracted much attention as aldehydes, alcohols and alkynes. However, selective oxidative
effective, economical and environmentally-benign methods for coupling of alkylarenes and anilines is generally difficult due to
direct synthesis of more functionalized chemicals such as the reactions of oxidation of anilines and over-oxidation to
2
–4
esters, amides, imines, and so on.
Generally, for one-pot carboxylic acids, which are often a poison for solid catalysts.
reactions, multiple functions (often multiple active sites) which Pd–Au/SiO was reported to be an effective heterogeneous catalyst
2
5
3b
can act without interference each other are required, and for the oxidative coupling of alkylarenes and amines, which
heterogeneous catalysts are a suitable candidate. However, provided high yields of the target imines but suffered from use of
3
effective heterogeneous catalyst systems enabling both sp
2 3
strong base (K CO ), high pressure of pure oxygen (Z1 MPa), use
C–H bond activation and one-pot reactions are very limited of noble metals and cumbersome catalyst preparation. Combi-
3
4
2 2
such as Au–Pd bimetallic catalysts and MnO : for examples, nation of mesoporous MnO and tert-butyl hydroperoxide (THBP)
4b
Hutchings and co-workers showed a seminal work that sup- was also effective, however THBP, an organic oxidant, was
ported Au–Pd alloy nanoparticles on C or TiO showed high necessary for high yield. Therefore, development of effective
2
3
activity for the oxidation of the sp C–H bond in toluene heterogeneous catalysts for the aerobic oxidative coupling of
derivatives and the consecutive coupling to the corresponding alkylarenes and amines under mild reaction conditions is highly
3
a
esters. Mizuno and co-workers demonstrated that MnO
2
was desirable.
CeO has been investigated in various research fields due to
an effective heterogeneous catalyst for the oxidative amidation
of methylarenes with urea to the corresponding amides.
2
4
a
8
the unique physical and chemical properties. The prominent
acid/base and redox properties have attracted much attention
a
Research Center for Artificial Photosynthesis, Advanced Research Institute for
Natural Science and Technology, Osaka City University, 3-3-138, Sugimoto,
Sumiyoshi-ku, Osaka, 558-8585, Japan. E-mail: mtamura@osaka-cu.ac.jp
Department of Applied Chemistry, School of Engineering, Tohoku University,
b
6
-6-07, Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cc02969f
Scheme 1 Direct imine formation from methylarenes and anilines.
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Table 1 Performance comparison of metal oxides in the direct imine
a
formation from mesitylene and aniline
Selectivity/%
Metal
Conv./ 1 yield/
Entry oxide
%
%
1
Azobenzene Azoxybenzene Others
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
CeO
MnO
Pr
MgO
2
25
12
6
2
5
4
3
8
1
6
3
1
1
10
9
2
39 50
9
5
2
3
2
77 15
36 31
70 20
6
O
11
33
10
14
14
15
o1
o1
3
o1
o1
19
—
o1
o1
o1
o1
o1
o1
o1
o1
o1
o1
o1
—
1
Y
2
O
3
o1
o1
o1
o1
o1
o1
o1
o1
o1
o1
o1
7
9
79
76
Eu
Sm
Al
ZnO
ZrO
2
O
3
2
Fig. 1 Performance comparison of metal oxide-modified CeO in the direct
2
O
3
3
10 76
97
19 81
92
imine formation from mesitylene and aniline. J: conversion, bars: selectiv-
ities (red: 1, blue stripe: azobenzene, green dotted: azoxybenzene, black:
2
O
3
0
1
2
3
4
5
2
4
others). Reaction conditions: aniline 1 mmol, mesitylene 1.5 g, MO
x 2
–CeO
Nb
2
O
5
o1 499
o1 499
19 62
(M: 1 wt%, 873 K) 50 mg, air, 393 K, 24 h.
TiO
2
La
CaO
SiO
Al
2
O
3
o1
—
—
—
—
Co, W and Mn species increased the selectivity to 1, and among
these metal species Cu provided the highest selectivity of 75%,
while the conversion slightly decreased in comparison with
2
–
o1
—
—
2
O
3
1
1
6
7
SiO
—
2
o1
o1
o1
o1
—
—
—
—
—
—
—
—
2
only CeO . In terms of the selectivity, Cu is the most effective
Reaction conditions: aniline 1 mmol, mesitylene 1.5 g, catalyst 50 mg, additive for CeO
in the reaction.
2
a
air, 393 K, 24 h. Conversion and selectivity were calculated on aniline
The effect of Cu loading amount in CuO –CeO catalysts was
studied (Fig. 2). The conversion gradually decreased with
increasing the Cu loading amount up to 2 wt% and became
x
2
basis.
9,10
in liquid-phase organic reactions,
and recently we firstly almost constant at over 2 wt%. On the other hand, the selectiv-
2
reported on the unique redox property of CeO at low tempera- ity to 1 drastically increased from 39% to 85% with increasing
ture, which can function as a heterogeneous catalyst for one-pot the Cu loading amount up to 2 wt% and became almost
imine formation from benzyl alcohols and anilines under air at constant (B87%) at more than 2 wt%. Therefore, 2 wt% CuOx
10
3
33 K. Other researchers also vigorously investigated the redox loaded CeO is the optimum catalyst.
2
property of CeO
2
with different morphologies or synthesis
The calcination temperature dependence of CuO –CeO was
x 2
11
methods in the same reaction. Herein, we found that Cu investigated in the range of 473–1073 K (Table S1, ESI†). The
oxide-modified CeO (CuO –CeO ) was an effective and reusable conversion increased from 10 to 15% with decreasing the
2
x
2
heterogeneous catalyst for the selective oxidative imine formation calcination temperature from 1073 to 673 K and became almost
from methylarenes and anilines under atmospheric air without constant at lower calcination temperature (473–673 K), while
any additives, giving high yields of the target imines.
At first, performance of metal oxides was compared in
oxidative imination of mesitylene with aniline under air at
3
93 K as a model reaction (Table 1). Without metal oxides, no
conversion was observed (entry 17). Metal oxides except CeO
and MnO2 showed low conversion (r8%), and the yield of
3,5-dimethylbenzylidene)-phenyl-amine (1), the target imine,
showed higher conversion (25%)
2
(
was r2% (entries 4–16). CeO
than other metal oxides, however, the selectivity to 1 was not so
high (39%, entry 1), and the coupling products of anilines such
as azobenzene and azoxybenzene were obtained. On the other
2
hand, MnO
although the conversion (12%) was half of that over CeO
entry 2). In terms of the yield of 1, CeO has a comparative
2
showed high selectivity to the target imine,
2
(
2
3
potential to MnO2 in the oxidation of the sp C–H bond.
Therefore, CeO was selected for further investigation.
Next, addition of various metal species (1 wt%) to CeO
investigated (Fig. 1). The conversion was not improved by
x 2
Fig. 2 Effect of Cu loading amount in CuO –CeO catalyst in the direct
2
imine formation from mesitylene and aniline. J: conversion, bars: selec-
tivities (red: 1, blue stripe: azobenzene, green dotted: azoxybenzene,
black: others). Reaction conditions: aniline 1 mmol, mesitylene 1.5 g,
2
was
addition of any metal species, in contrast, addition of Cu, Hf, CuO
x
–CeO
2
50 mg, air, 393 K, 24 h.
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smoothly proceeded to reach 499% conversion at 120 h. The
selectivity to 1 increased at the low conversion level (o10%),
and high selectivity (B93%) was maintained at the high con-
version level. The maximum imine yield of 93% was obtained at
120 h. Focusing on the other products from mesitylene, for-
mation of 3,5-dimethyl benzyl alcohol was not observed, but a
small amount of 3,5-dimethyl benzaldehyde was detected
(Table S3, ESI†). This result indicates that 3,5-dimethyl benzal-
dehyde is one possible intermediate, which may be formed
from the corresponding benzyl alcohol. The catalyst reusability
of CuO –CeO was investigated (Fig. 4(b)). The conversion was
x 2
Fig. 3 XRD patterns of CuO –CeO catalysts (a) and STEM and STEM-
x
2
x 2
EDX of CuO –CeO (Cu: 2 wt%, 673 K) (b).
not changed in all reuse tests and high selectivity to 1 was
maintained during five-time reaction tests, although the selec-
tivity increased at first reuse test. Moreover, the filtrate of the
reaction mixture was measured by ICP-AES, showing no leach-
ing of Ce and Cu species (below the detected level (o0.1%)).
The XRD patterns of the catalysts before and after reactions
the selectivity to 1 was not so changed. Therefore, lower
calcination temperature (473–673 K) was preferable for high
activity, and the calcination temperature was fixed at 673 K in
the following study.
x 2
were not changed (Fig. S2, ESI†). CuO –CeO was a robust and
reusable heterogeneous catalyst in this reaction.
x 2
The CuO –CeO catalysts were characterized by XRD and
STEM analyses (Fig. 3). XRD analyses showed that CuO species
were not detected up to 3 wt% Cu loading amount but were
observed at higher Cu loading amount (Fig. 3(a), the expanded
figure is in Fig. S1, ESI†). Therefore, CuO species can be highly
dispersed on CeO2 in CuO –CeO catalysts with Cu loading
x
The scope of anilines and methylarenes with CuO –CeO2
catalyst is shown in Table S4 (ESI†). Various p-substituted
anilines with electro-withdrawing groups such as halogens
(entries 2–4) and an electron-donating methyl group (entry 5)
x
2
were converted to the imines in high conversions and high
selectivities. o-, m-, p-Xylenes also reacted to give the corres-
ponding imines in high conversions and selectivities (entries 6–8).
The direct imine formation from mesitylene and aniline will
proceed via two steps (Scheme 2): (i) oxidation of the methyl group
in mesitylene to corresponding aldehyde. (ii) Imine formation
from the produced aldehyde and aniline. Imine formation from
amount below 3 wt%. In addition, STEM image of 2 wt% CuOx-
modified CeO catalyst showed no particles assignable to CuO
2
x
species over CeO
2
(Fig. 3(b)). STEM-EDX analysis showed Cu
2
species is highly dispersed over CeO (Fig. 3b), which is in good
agreement with the result of XRD analyses.
The reaction temperature dependence was investigated
(Table S2, ESI†) from 373 to 413 K. Higher reaction temperature
x 2
aniline and benzaldehyde was conducted with CuO –CeO catalyst
provided higher conversion, and 51% conversion and 92%
selectivity to 1 were obtained at 413 K and 24 h. It was also
confirmed that the target imine was not formed without
catalysts at 413 K. The time-course of the direct imine for-
mation from mesitylene and aniline was studied with the
(Scheme S1, ESI†) to estimate the reaction rate for the step (ii).
The conversion was 10% at only 5 min, and the formation rate
À1 À1
of the imine was calculated to be 48 mmol h
g , which is
about 440-fold higher than that of the direct imine formation from
À1 À1
mesitylene and aniline over CuO –CeO catalyst (0.11 mmol h
g ).
optimized CuO
x
–CeO
2
catalyst (Cu: 2 wt%, 673 K) at 413 K
x
2
This result indicates that the rate-determining step is not step
ii) but step (i), which can explain the small detected amount of
the aldehyde in the time-course. Moreover, imine formation
from aniline and mesitylene was conducted under N (Scheme S2,
ESI†), showing almost no conversion. Therefore, O in air is
(Fig. 4(a) and the details are in Table S3, ESI†). The reaction
(
2
2
essential for the reaction. As above, the rate-determining step is
the step (i) and is catalyzed by CuO –CeO , and the step (i)
x
2
proceeds with O2 in air over CuO –CeO catalyst. Moreover,
x
2
oxidative coupling of aniline was conducted at 393 K for 6 h over
CeO and CuO –CeO with tert-butylbenzene as an inert solvent
because of no benzylic C–H bond (Scheme S3, ESI†). CuO –CeO
2
x
2
x
2
clearly suppressed formation of aniline coupling products such
Fig. 4 Time-course of the direct imine formation from mesitylene and
aniline over CuO –CeO catalyst (a) (J: conversion, K: selectivity to 1,
: selectivity to azobenzene, ’: selectivity to azoxybenzene), and the
x
2
~
reusability test (b). J: conversion, bars: selectivities (red: 1, blue stripe:
azobenzene, green dotted: azoxybenzene, black: others). Reaction
conditions of (a) aniline 1 mmol, mesitylene 1.5 g, CuO
wt%, 673 K) 50 mg, air, 413 K. (b) Aniline 1 mmol, mesitylene 1.5 g,
CuO –CeO (Cu: 2 wt%, 673 K), 50 mg, air, 393 K, 24 h.
x 2
–CeO (Cu:
2
Scheme 2 Plausible reaction route for the direct imine formation from
mesitylene and aniline.
x
2
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as azobenzene and azoxybenzene (0.6% aniline conversion) com- Notes and references
2 x
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on CeO2 will be effective for suppression of aniline coupling
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2
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2
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(
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À
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We found that CuO
x
–CeO
–CeO
2
2
catalyst.
x
acted as an effective and reusable
heterogeneous catalyst for direct imine formation from methyl-
arenes and anilines with air (0.1 MPa) as an oxidant, giving the
52, 13495.
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This work was supported by Grant-in-Aid for Young
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1
1
Conflicts of interest
There are no conflicts to declare.
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