Transition-Metal-Controlled Inorganic Ligand-Supported Non-Precious Metal Catalysts for the Aerobic Oxidation of Amines to Imines

Article abstract of DOI:10.1002/chem.201703185

Most state-of-art transition-metal catalysts usually require organic ligands, which are essential for controlling the reactivity and selectivity of reactions catalyzed by transition metals. However, organic ligands often suffer from severe problems including cost, toxicity, air/moisture sensitivity, and being commercially unavailable. Herein, we show a simple, mild, and efficient aerobic oxidation procedure of amines using inorganic ligand-supported non-precious metal catalysts 1, (NH₄)_n[MMo₆O₁₈(OH)₆] (M=Cu²⁺; Fe³⁺; Co³⁺; Ni²⁺; Zn²⁺, n=3 or 4), synthesized by a simple one-step method in water at 100 °C, demonstrating that the catalytic activity and selectivity can be significantly improved by changing the central metal atom. In the presence of these catalysts, the catalytic oxidation of primary and secondary amines, as well as the coupling of alcohols and amines, can smoothly proceed to afford various imines with O₂ (1 atm) as the sole oxidant. In particular, the catalysts 1 have transition-metal ion core, and the planar arrangement of the six Mo^VI centers at their highest oxidation states around the central heterometal can greatly enhance the Lewis acidity of catalytically active sites, and also enable the electrons in the center to delocalize onto the six edge-sharing MO₆ units, in the same way as ligands in traditional organometallic complexes. The versatility of this methodology maybe opens a path to catalytic oxidation through inorganic ligand-coordinated metal catalysis.

Full text of DOI:10.1002/chem.201703185

A Journal of
Accepted Article
Title: Transition-metal-Controlled Inorganic-ligand Supported Non-
precious Metal Catalysts for the Aerobic Oxidation of Amines to
Imines
Authors: Yongge Wei, Han Yu, Yongyan Zhai, Guoyong Dai, Shi Ru,
and Sheng Han
This manuscript has been accepted after peer review and appears as an
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201703185
Link to VoR: http://dx.doi.org/10.1002/chem.201703185
Supported by

10.1002/chem.201703185
Chemistry - A European Journal
COMMUNICATION
Transition-Metal-Controlled Inorganic-ligand Supported Non-
precious Metal Catalysts for the Aerobic Oxidation of Amines to
Imines
Han Yu,*[a], [b] † Yongyan Zhai,[a]† Guoyong Dai,[a] Shi Ru,[a] Sheng Han,*[a] and Yongge Wei*[b], [c]
polyoxometalates (POMs),[4,5] we envisioned introduction of the
Abstract: Most of the state-of-art transition-metal catalysis usually
need organic ligands which are essential for controlling the reactivity
B-type Anderson-structured POMs,[6,7] is very important since
and selectivity of reactions catalyzed by transition metals. However,
organic ligands often suffer from severe problems including cost,
toxicity, air/moisture sensitivity and being commercially unavailable.
Herein, we show a simple, mild and efficient aerobic oxidation
procedure of amine using inorganic-ligand supported non-precious
metal catalysts 1, (NH4)n[MMo6O18(OH)6] (M = Cu2+; Fe3+; Co3+; Ni2+;
Zn2+, n = 3 or 4), synthesized by a simple one-step method in water
at 100oC, showing the catalytic activity and selectivity can be
significantly improved by changing the central metal atom. In the
presence of these catalysts, the catalytic oxidation of primary and
secondary amines as well as the coupling of alcohols and amines
can smoothly proceed to afford various imines with O2 (1 atm) as the
sole oxidant. In particular, the catalysts 1 has a transition metal ions
core and easy synthesized by a simple one-step method in water at
100°C with high yield, which the planar arrangement of six MoVI
centers at their highest oxidation states around the central
heterometal will on one hand greatly enhance the Lewis acidity of
catalytically active sites and on other hand enable the electrons of
the center to delocalize onto the six edge-sharing MO6 unit
analogous to the role of ligand in traditional organometallic
complexes catalysts. The generality of this methodology maybe
opens a way for the catalytic oxidation reaction via inorganic-ligand
coordinated metal catalysis.
they are made up of a single metal atom coordinated by a
polymolybdate or poly-tunstate, which consists of six edge-
sharing XO6 (X = W or Mo) octahedral surrounding a central,
edge-sharing metal heteroatom octahedron (MO6) with six
protons. In particular, the structure of Anderson POMs, featured
with easily accessible central metal heteroatom, a smaller
molecular structure and the well-defined nature of their active
sites, allows for more rational development of the catalyst
through structure-activity relationships. Furthermore, their
specific geometry with redox and charge-transfer properties
provide a quick and stable state of atomic valence could be used
to solve the instability and activity problems of organometallic
complex catalysts, thus providing a new potential alternative of
aerobic oxidation pathway. Very recently, our research group
has first introduced Anderson-type POMs as an inorganic-ligand
coordinated metal catalyst for the aerobic oxidation of aldehyde
to carboxylic acids in water under mild conditions,[8] which
demonstrate excellent catalytic activity in oxidation reaction,
supporting this idea. These methods may open a way for the
catalytic oxidation reaction catalyzed by inorganic-ligand
coordinated metal catalysis with environmentally benign O2 as
the sole oxidant.
Organometallic catalysis has greatly facilitated the development
of organic synthesis chemistry, providing a powerful bonding
method for the preparation of a variety of valuable compounds.[1]
In such catalytic system, organic ligands have been established
as a defining feature of transition-metal catalysts, providing
access to additional parameters that can be used to optimize the
rate and selectivity of a reaction.[2] However, there are several
concerns, for example, cost, toxicity, air/moisture sensitivity and
being commercially unavailable.[1,2] Therefore, the use of other
supported ligands as a way to intelligently tackle these problems
is highly desired.
In classical coordination chemistry, catalytically active, stable
complexes involving low-valence metals ions coordinated by
metal oxides are commonplace. A close inspection of these
compounds reveals that many of them are in fact inorganic-
ligand coordinated complexes containing dispersed metal ions
with a variable number of metal–oxygen bonds between the
support and the metal sites, states that are intrinsically more
stable than organometallic complexes.[3] Inspired by this idea
and according to our long-term efforts in the chemistry of
Figure 1. Oxidation of Amines to Imines by Inorganic-ligand Supported Metal-
catalyst
Imines and their derivatives are important building blocks for
the synthesis of fine chemicals, pharmaceuticals and also are
regarded as important electrophilic intermediates in organic
synthesis.[9] Generally, imine synthesis which involves
a
condensation reaction often requires unstable aldehydes and
acid catalysts. Several procedures use the stoichiometric
amount of harmful or explosive reagents with emission of the
corresponding amount of pollutants.[10] As an alternative
approach, the direct oxidation of amines has attracted
considerable attention for the synthesis of imines in recent
years.[11-14] However, key challenges still exist, as follows: the
limited symmetric imines products, prolonged reaction times,
elevated temperatures, environmentally unfriendly oxidants, or
low selectivity. Nowadays, some established transition-metal-
based catalytic systems such as Au, Pd, Ru, Cu, and Fe
capable of efficient and selective aerobic oxidation of amines to
imines have been developed.[15] Despite good yields and
selectivity, the durability and recyclability of transition-metal
catalysts still remain a major challenge. Therefore, the design of
an effective catalyst system with high stability and activity for the
oxidation of amines under mild conditions is high desirable. We
herein report a simple, mild and efficient aerobic oxidation of
amine procedure using inorganic-ligand coordinated metal
catalysts,[16] (NH4)n[MMo6O18(OH)6] (M = Cu2+; Fe3+; Co3+; Ni2+
Zn2+, 1, Figure 1), synthesized by a simple one-step method in
[a]
Dr. H. Yu, Y. Zhai, G. Dai, S. Ru, Dr. H. Lin, and Prof. S. Han
School of Chemical and Environmental Engineering
Shanghai Institute of Technology
100 Haiquan Road, Shanghai 201418, P. R. China
E-mail: hansheng654321@sina.com
These authors contributed equally to this work.
Dr. H. Yu, and Prof. Y. Wei
[†]
[b]
Key Lab of Organic Optoelectronics & Molecular Engineering of
Ministry of Education,Department of Chemistry, Tsinghua University
Beijing 100084, P. R. China
E-mail: yonggewei@tsinghua.edu.cn
[c]
Prof. Y. Wei
State Key Laboratory of Natural and Biomimetic Drugs, Peking
University, Beijing 100191, P. R. China.
E-mail: ygwei@pku.edu.cn
Supporting information for this article is given via a link at the end of
the document.
;
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10.1002/chem.201703185
Chemistry - A European Journal
COMMUNICATION
water at 100oC. With the help of these pure inorganic small
molecular catalysts, the aerobic oxidation self-coupling of
primary, secondary, and coupling of alcohols and amines to
imines reaction can be conducted at a low-pressure (1 atm)
using molecular oxygen as the terminal oxidant under mild
conditions (Figure 1). The catalysts can be recycled and used in
successive reactions without any degradation in catalyst
performance.
[a] Reaction conditions: Cat. 1Cu (1.0 mol %), amine (2.0 mmol), O2 (1 atm),
and solvent (2 mL) at 60oC, unless otherwise noted. [b] Conversions and
[e]
yields were by GC and confirmed by GC-MS. [c] at 70 °C. [d] at 30 °C.
Reactions were carried out under atmospheric air. [f] Reactions were carried
out under Nitrogen atmosphere.
Yield
98%
96%
96%
95%
93%
100%
80%
60%
40%
20%
0%
90%
Table 1. Investigation of catalyst 1[a]
Entry
Cat.
Sel.(%)[b]
Yield (%)[b]
1
2
3
4
5
6
-
-
-
1Cu
1Fe
1Co
1Ni
1Zn
99
96
94
87
84
98
95
92
85
78
1
2
3
4
5
6
Cycles
Figure 2. Recycling Experiments of Catalyst. [a]
[a] Reaction conditions: Cat. 1 (1.0 mol %), amine (2.0 mmol), O2 (1 atm),
and solvent (2 mL) at 60oC for 16h, unless otherwise noted. [b] Selectivity
and yields were determined by GC and confirmed by GC-MS.
[a] Reaction conditions: Cat. 1Cu (1.0 mol%), amines (2.0 mmol), O2 balloon
(1.0 atm), CH3CN (2.0 mL) at 60oC for 16h
At the beginning of our studies, to explore the activity of the
inorganic-ligand supported metal catalysts 1 in the oxidative
coupling of amines, oxidative coupling of 4-methoxybenzylamine
was chosen as the model reaction in the acetonitrile solution
with O2 balloon as the sole oxidant at 60 °C for 16h (Table 1). A
trace product was detected in the absence of catalysts 1 within
48 h (entry 1). To our delight, the inorganic-ligand supported
metal catalyst 1 showed high activity and excellent selectivity
towardthe reaction, and Cu-catalyst 1Cuexhibited the highest
efficiency affording the desired imine in 98% yield with excellent
selectivity (99%) after the reactive mixture was heating for 16h
(Table 1, entry 2). Compared to 1Cu, Fe-catalyst 1Fe resulted in
95% yield for 16h, it also showed highest activity and selectivity
toward the reaction but took a longer time. Interestingly,
performing the reaction with the Co-catalyst 1Co or Ni-catalyst
1Ni resulted in 92% and 85% yield with 94% and 87% selectivity,
respectively (entries 4 and 5). Upon employing Zn-catalyst 1Zn,
the yield was decreased to 84% with 78% selectivity (Table 1,
entry 6). These results showed that the catalytic
can significantly be improved by adjusting the central metal atom.
Subsequently, the reaction conditions were investigated (Table
2). The optimization conditions of the catalyst 1Cu was
investigated in the oxidation of 4-methoxybenzylamine (1 mmol)
with O2 (1 atm) in different solvents. Among the tested solvents,
acetonitrile gave the highest conversion (99%) with excellent
yield (98%) toward the imine after 16 h at 60°C, probably
because of its excellent electrochemical stability, large dielectric
constant (Table 2 entries 1-3). Furthermore, the impact of
varying the catalyst loading and reaction temperature were
investigated. Lowering the loading of 1Cu from 1 mol% to 0.5 or
0.1 mol% resulted in 92% and 90% yield of the imine product,
respectively (Table 2, entries 4 and 5). Altering the reaction
temperature, the yields of the imine product was significantly
decreased (Table 2, entries 6 and 7). Switching from oxygen to
air gas under the same atmospheric pressure also led to 90%
conversion with 65% yield (Table 2, entry 8). Once the reaction
was performed in nitrogen atmosphere, only 4% yield of the
desired product was obtained, ensuring that O2 as oxidant was
essential for the reaction (Table 2, entry 9). It should be noted
that CuSO4 and (NH4)6Mo7O24•4H2O, used as a catalyst alone,
respectively, no more oxidation product was observed even at
prolonged reaction time up to 48 h.
Table 2. Optimization of reaction conditions[a]
Under optimized conditions, we explored the substrate scope
(Table 3). First, a series of primary amines 2 could be readily
oxidized to produce imines 3 in excellent yields. The oxidation of
benzylamine derivatives bearing electron-donating (compounds
3a-3d) as well as electron-withdrawing groups (compounds 3e-
3i) were successfully converted to the imines in excellent yields
with excellent selectivity. Benzylamines with electron-donating
groups require a longer time to achieve complete conversion,
whereas electron-withdrawing groups reduce the reaction time,
and the yield of the latter imine product was more favor than the
former. Moreover, it was found that the reaction was sensitive to
the steric hindrance of the substituents on the phenyl ring, and
para-substituted amine exhibited a higher activity than meta-
and ortho-substituted isomers (compounds 3c, 3d, 3e and 3f).
The representative of the heterocyclic amines such as 2-
furanmethanamine and 2-thiophenemethylamine, which might
poison the metal catalyst due to the coordination with the metal
active sites, could also be transformed into the corresponding
imines in excellent yields (compounds 3j and 3k). The methyl
group in the α-carbon of benzylamine slightly retarded the
conversion (compound 3l). Dibenzyl amine, a representative of
the secondary amines, is converted to the corresponding imine
in 91% yield (compound 3a’). Utilizing this protocol, we
Entry
Cat. (mol%)
Solvent
Con.(%)[b]
Yield(%)[b]
1
2
1
1
CH3CN
THF
99
90
71
95
92
96
79
90
98
87
52
92
90
95
67
65
3
1
CH3OH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
4
0.5
0.1
1
5
6[c]
7[d]
8[e]
1
1
9[f]
1
CH3CN
<10
<5
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10.1002/chem.201703185
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COMMUNICATION
successfully demonstrate the efficient transformation of 1,2,3,4-
tetrahydroquinoline and indole to the corresponding imine in
89% and 90% yields, respectively, and both of which are
important pharmaceutical intermediates (compounds 3m and
3n). Remarkable catalytic activity of 1Cu was attributed to the
successful oxidation of an inactive aliphatic amine (butan-1-
amine) toward the corresponding imines (compound 3o). We
have recycled and reused the Cu-catalyst 1 Cu by the addition
of diethyl ether (by precipitation). The catalyst maintains its high
activity up to the six cycle when applied in the selective oxidation
of 4-methoxybenzylamine (Figure 2), and the charicterzation of
the recovered catalyst showed that the structure of 1Cu was still
preserved (Figure S1and S2).
an electron-withdrawing substituent were better substrates, for
they generally gave higher yields of the products (compounds
6l-6n). Reaction of benzylamine with aliphatic alcohol such as
pentan-1-ol or cyclohexylmethanol produced 88% and 82% of
Table 4. Investigation of substrate scope on the cross-coupling of alcohols
and amines [a]
Table 3. Investigation of substrate scope on the self-coupling of amines[a]
[a]Reaction conditions: Cat. 1Cu (1.0 mol%), amines (1.0 mmol), alcohols
(1.0 mmol), O2 balloon (1.0 atm), CH3CN (2.0 mL) at 50oC. [b]Yields were
determined by GC and confirmed by GC-MS.
the corresponding imines, respectively (compounds 6o and 6p).
than the electron-deficient ones. Reaction of benzylamine with
heterocycle alcohol such as thiophen-2-ylmethanol or pyridin-3-
ylmethanol produced 96% and 91% of the corresponding imines,
respectively (compounds 6q and 6r).
Finally, a gram-scale reaction on the self-coupling of amines
was conducted with 1 mol% of catalyst 1a and 2.74 grams (20
mmol) 4-methoxybenzylamine in 5 mL acetonitrile. Analytically
pure 3a was isolated in 95% yield after 16 hours (Figure 3). In
the meanwhile, a gram-scale reaction on the cross-coupling of
benzyl alcohol and benzylamine was also performed well.
(Figure 3). These two examples indicate that our methodology
with cheap catalysts can be readily scaled up to an industrial
level.
[a] Reaction conditions: Cat. 1Cu (1.0 mol%), amines (2.0 mmol), O2 balloon
(1.0 atm), CH3CN (2.0 mL) at 60oC . [b] Isolated yields. [c] Yield is determined by
GC and confirmed by GC-MS.
Excellent activities in the inorganic-ligand coordinated copper-
mediated oxidation of amines to imines encouraged us to check
the feasibility of this aerobic protocol in the synthesis of
unsymmetrical imines. After some optimized conditions (Table
S1) were obtained, we examined the scope of present catalytic
system for various alcohols and amines (Table 4). Heating a
solution of benzyl alcohol with benzylamine using 1Cu at 50 °C
in CH3CN (2 mL) resulted in exclusive formation of the
corresponding imines in 96% yields (compound 6a). Use of
amines with electron-donating substituents generally results in
higher yields than when amines with electron-withdrawing
substituents, probably because of higher nucleophilicity of the
former (compounds 6b-6f).The catalytic reaction of benzyl
alcohol with aniline under analogous reaction conditions yielded
90% of the corresponding imines (compound 6g). When more
challenging aliphatic amines were tested, cyclohexanamine and
butan-1-amine give corresponding imines in 84%, and 73%
yields, respectively (compounds 6h and 6i). As for alcohols part,
substituted benzyl alcohol, cyclic and aliphatic alcohols all
reacted efficiently with benzylamine to produce the imine
products in excellent yields. The p-tolylmethanol and (4-
methoxyphenyl)methanol give corresponding imines in 90%, and
92% yields, respectively (compounds 6j and 6k). Alcohols with
Figure 3. Gram-scale reaction on the self-coupling of amines and the cross-
coupling of alcohols with amines
Based on these experimental results and the enzyme
catalysts reported,[17] a tentative mechanism that rationalizes the
product formation is illustrated in Figure 4. Firstly, the Cu
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10.1002/chem.201703185
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catalyst activates molecular oxygen to afford an active species B,
which undergoes the O-O bond heterolysis to give highly
reactive metal-oxo species C as active oxidants,[18] subsequent
coordination of benzylamine substrate to C would form the
complex D and can be further transformed into NH-imine
intermediate (For the oxidative of alcohol and coupling with
amines: the alcohol substrate first coordination of key
intermediate C form the complex E and give the oxidation of
intermediate aldehydes) after decomposed of this intermediate
and condensation with free benzylamine, the imine product was
finally obtained. It is noteworthy that the signal peaks of
aldehydes were detected by GC-MS during the course of the
experiment. Owing to the strong electron-withdrawing abilities of
the MoO6 groups, which can increase the electrophilicity of C to
further enhance the catalytic activity.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Nos. 21402065, 21471087, 21225103 and
21221062), Doctoral Fund of Ministry of Education of China No.
20130002110042, Tsinghua University Initiative Foundation
Research Program No. 20131089204, and the State Key
Laboratory of Natural and Biomimetic Drugs K20160202. The
start-up fund of Shanghai Institute of Technology is also
gratefully acknowledged.
Keywords: Cu(II) catalysts • Polyoxometalates • oxidation of
amines • molecular oxygen
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Figure 4. Proposed mechanism for the Cu-catalysed oxidation of amines
In summary, we have discovered the first example of
catalytic aerobic oxidation of amines to imines with an inorganic-
ligand coordinated copper catalyst using 1 atm oxygen gas as
the oxidant. Various structurally diverse primary and secondary,
as well as the coupling of alcohols and amines have been
smoothly transformed into the corresponding imines with
excellent selectivity and yields. This is protocol is not only more
simple and easy bench operation to the non-expert, but also can
avoid the use of both precious metals and cost, toxicity,
air/moisture sensitivity and being commercially unavailable
organic ligands. Extension to other catalytic reactions with such
pure inorganic small molecular catalysts are undergoing in our
laboratory. The generality of this methodology gives it the
potential to be used on an industrial scale.
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Experimental Section
General procedure for catalytic oxidative coupling of amines: The
catalyst 1Cu (1.0 mol%) and an amine (2.0 mmol) were added in 2.0 ml
of CH3CN with stirring at 60°C for 16h. Meanwhile, an oxygen balloon
was used to charge the reaction tube with oxygen. Afterwards, a small
amount of ethyl acetate was added into the reaction mixture and the
solution was quickly filtered. The filtered solid was washed, dried and
then recycled. Reaction mixture was analyzed by GC-MS analysis.
Finally, the solvent was removed in vacuo, and the corresponding imine
was purified by washing through base-washed silica gel column.
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General procedure for catalytic coupling of alcohols and amines:
The cat. 1Cu (1.0 mol%), an amine (1.0 mmol) and an alcohol (1.0
mmol) were added in 2.0 ml of CH3CN with stirring at 50 °C for 12h.
Meanwhile, an oxygen balloon was used to charge the reaction tube with
oxygen. Afterwards, a small amount of ethyl acetate was added into the
reaction mixture and the solution was quickly filtered. The filtered solid
was washed, dried and then recycled. Reaction mixture were analyzed
by GC-MS analysis.
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10.1002/chem.201703185
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H. Yu,*† Y. Zhai,† G. Dai, S. Ru, S.
Han,* Y. Wei*
Page No. – Page No.
Transition-Metal-Controlled
Inorganic-ligand
Supported
Non-
precious Metal Catalysts for the
Aerobic Oxidation of Amines to
Imines
A mild aerobic oxidative coupling protocol of primary and secondary amines as well as the coupling of alcohols and amines to afford
corresponding imines was developed with inorganic-ligand supported metal-catalyst (NH4)n[MMo6O18(OH)6] (M=Cu2+, Fe3+, Co3+, Ni2+, Zn2+).
This method utilizes mainly an inorganic polymolybdate ligand to support the Cu2+ ion and 1 atm O2 gas as the sole oxidant, which can
smoothly proceed to afford various imines in high yields and selectivity, avoiding the use of cost, toxicity, air/moisture sensitivity and
commercially unavailable organic ligands.
This article is protected by copyright. All rights reserved.