Aerobic oxidation of amines to imines catalyzed by a ruthenium complex under solvent-free conditions

Article abstract of DOI:10.1016/j.catcom.2016.03.019

A ruthenium complex [Et₃NH]₂[Ru(dipic)Cl₃] showed high efficiency in the homo-coupling of primary amines and cross-coupling of benzylamine with anilines and aliphatic amines to the corresponding imines under air and solvent-free conditions. This protocol is an atom-economical green process and tolerates various substrates bearing both electron-donating and electron-withdrawing substituents.

Full text of DOI:10.1016/j.catcom.2016.03.019

Aerobic oxidation of amines to imines catalyzed by a ruthenium complex
under solvent-free conditions
Yuecheng Zhang, Fei Lu, Rong Huang, Hongyu Zhang, Jiquan Zhao
PII:
DOI:
Reference:
S1566-7367(16)30111-X
doi: 10.1016/j.catcom.2016.03.019
CATCOM 4621
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
31 January 2016
25 March 2016
26 March 2016
Please cite this article as: Yuecheng Zhang, Fei Lu, Rong Huang, Hongyu
Zhang, Jiquan Zhao, Aerobic oxidation of amines to imines catalyzed by a ruthe-
nium complex under solvent-free conditions, Catalysis Communications (2016), doi:
10.1016/j.catcom.2016.03.019
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ACCEPTED MANUSCRIPT
Aerobic Oxidation of Amines to Imines Catalyzed by a Ruthenium
Complex under Solvent-free Conditions
Yuecheng Zhang, Fei Lu, Rong Huang, Hongyu Zhang, Jiquan Zhao*
School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin 300130,
People's Republic of China
Corresponding author: Jiquan Zhao
Hebei University of Technology
School of Chemical Engineering
Guangrongdao, Hongqiao District
Tianjin,
CHINA
+8602260202926
FAX: +8602226564733
zhaojq@hebut.edu.cn
ABSTRACT
A ruthenium complex [Et3NH]2[Ru(dipic)Cl3] showed high efficiency in the homo-coupling of
primary amines and cross-coupling of benzylamine with anilines and aliphatic amines to the
corresponding imines under air and solvent-free conditions. This protocol is an atom-economical
green process and tolerates various substrates bearing both electron-donating and
electron-withdrawing substituents.
Keywords: Ruthenium complex, imines, benzylamines, aerobic oxidation

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1. Introduction
Imines are key synthetic intermediates in many reactions such as reductions, condensations,
additions, and cycloadditions, and also important class of compounds used in the synthesis of
numerous biologically and pharmaceutically active compounds [1-4]. Traditionally, imines are
synthesized by the condensation reaction of carbonyl compounds with primary amines in the
presence of an acid catalyst. Due to the need for organic and pharmaceutical synthesis, exploring
new methods for the synthesis of various imines is still an essential task faced by chemists.
In recent years, a number of methods for the synthesis of imines have been developed such as
oxidative cross-coupling between amines and alcohols [5-7], oxidation of secondary amines [8, 9],
and self‐condensation of primary amines with oxidant [10, 11]. The self‐condensation of primary
amines to imines has the advantage of ready availability of amines. Therefore, a wide variety of
catalysts such as metal (especially non-noble biocompatible metals copper, cobalt and iron)
[12-17], metal-free[18-21], photo-[22-26], and bioinspired [27-30]catalysts have been developed
and bring big developments to this approach. Among the catalytic systems the transition metal
based catalytic systems, which tolerate important substrate classes, afford imines in good yields
and with high selectivity. However, some of the metal-based catalytic systems still have some
disadvantages such as requiring pure oxygen, high oxygen pressure, solvent and (or) other
auxiliaries to obtain high yields. Therefore, it is essential to develop simple catalytic systems with
high efficiency for the aerobic oxidation of primary amines to imines under mild conditions. We
focus on ruthenium complexes as catalysts to get this goal because some ruthenium based
catalysts proved to be very active in the cross-coupling between amines and alcohols [31, 32],
oxidation of secondary amines to imines [33], and homo-coupling of amines to imines via
hydrogen transfer without oxidant [34]. We found that a mononuclear ruthenium complex
[Et3NH]2[Ru(dipic)Cl3] (dipic = 2,6-pyridinedicarboxylate, dipicolinate) which is easily
synthesized from RuCl3, triethylamine and dipicH2 showed excellent performance in the
self‐condensation of primary amines to imines with air as oxidant under solvent-free and
atmospheric pressure.
2. Experimental
2.1. Materials and apparatus
The primary amines and 2, 6-pyridinedicarboxylic acid (dipicH2) were obtained from Alfa Aesar
China (Tianjin) Co., Ltd. All reagents were used as received without further purification. The
ruthenium complex [Et3NH]2[Ru(dipic)Cl3] was prepared as described in the literature [35].
1H MNR spectra of imines were recorded with TMS as internal standard on a Bruker AC-P 400
spectrometer. Oxidation reaction samples were analyzed on a Shandong Lunan Ruihong Gas
Chromatograph (SP-6800A) equipped with a FID detector and a SE 30 column (30 m × 0.5 μm).

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2.2. Catalytic oxidation
In a typical process, into a 5 ml two-necked, round-bottom flask equipped with a magnetic stirrer
were added [Et3NH]2[Ru(dipic)Cl3] (0.005 mmol 0.01 equiv.) and benzylamine (5 mmol 1
equiv.) at room temperature successively. After which the resulting reaction mixture was heated at
100 ℃ for 10 h in an air atmosphere. The final reaction conversion and selectivity towards the
corresponding imines were determined by 1H NMR. After completion of the reaction, the product
was received by column chromatography of the reaction mixture on neutral alumina using
hexanes/EtOAc (95:5) as eluent.
3. Results and discussion
The complex [Et3NH]2[Ru(dipic)Cl3] being active in self‐condensation of primary amines to
imines was found occasionally in our study of aerobic oxidation of benzylamine to benzonitrile
using this complex as catalyst in acetonitrile. The major product identified was
N‐(benzylidene)benzylamine. Therefore, we turned our attention to the self‐condensation of
benzylamine to N‐(benzylidene)benzylamine. First, the reaction was run in solvent and
solvent-free conditions under air, respectively. Reaction was not efficient in acetonitrile (Table 1,
entry 1), only moderate yield of target product was obtained. The reaction was more efficient in
trifluorotoluene (Table 1, entry 2) but still with about 10% of benzonitrile as byproduct. To our
delight,
the
reaction
proceeded
smoothly
under
solvent-free
conditions,
N‐(benzylidene)benzylamine was obtained almost quantitatively (Table 1, entry 4). This is
attractive from the point of view of green chemistry.
The effect of reaction parameters on the reaction was investigated under solvent-free conditions,
and the results are shown in Table 1. As can be seen from Table 1, increasing the complex loading
from 0.5 mol% to 1.5 mol% led to fast reaction, the reaction time required to finish the reaction
decreased from 18 h to 8 h. Meanwhile, the yield increased from 97% to higher than 99% (Table 1,
entries 3-5). Increasing reaction temperature can accelerate the reaction, and increase the yield of
N‐(benzylidene)benzylamine (Table 1, entries 4, 6, 7). It is unexpected that pure oxygen (balloon
on the top) as oxidant gave low yield compared to air (Table 1, entries 4, 6, 7), which could be
ascribed to impossible discharge of ammonia generated in the self‐condensation of benzylamine
due to the plugging of the balloon.
Control experiments were performed to evaluate the necessity of the complex in the reaction. As
shown in Table 1, the use of complex [Et3NH]2[Ru(dipic)Cl3] is essential. No reaction was
observed when no [Et3NH]2[Ru(dipic)Cl3] was charged. RuCl3 instead of [Et3NH]2[Ru(dipic)Cl3]
as catalyst led to the yield of N‐(benzylidene)benzylamine as low as 30% in 15 h (Table 1, entry
11). Under the nitrogen atmosphere, very low yield (13%) of N‐(benzylidene)benzylamine was

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obtained, which indicated that the formation of N‐(benzylidene)benzylamine was through an
oxidative dehydrogenation but not through a direct dehydrogenation of benzylamine. From the
results we found the optimal amount of the ruthenium complex to be 1 mol% of benzylamine, and
the suitable reaction temperature to be 100 oC.
Table 1
Optimization of the conditions for the oxidative coupling of benzylamine to imines
Catalyst
(mol%)
1.0
Temperature Time
Yielda
(%)
77
Entry
Solvent
(℃)
(h)
24
10
18
10
8
1
MeCN
PhCF3
—
reflux
reflux
100
100
100
80
2
1.0
90
3
0.5
97
4
1.0
—
>99
>99
84
5
1.5
—
6
1.0
—
20
8
7
1.0
—
120
100
100
100
100
>99
0
8
0
—
10
6
9b
10c
11d
1.0
—
79
1.0
—
10
15
13
1.0
—
30
Reaction conditions: benzylamine (5 mmol), running reaction under open air atmosphere.
a Yield was determined by GC based on area%, the selectivity of imine was 100%.
b Reaction was performed under an oxygen balloon.
c Reaction was performed under an nitrogen atmosphere.
d RuCl3 in stead of [Et3NH]2[Ru(dipic)Cl3] was used as catalyst.
For evaluating the versatility of this novel catalytic system, the oxidative homo-coupling of
various amines to the corresponding imines was explored under the optimized reaction conditions,
and the results are shown in Table 2. As shown in Table 2, most of benzyl amines, including those
bearing both electron-withdrawing and electron-donating groups para or meta to aminomethyl
group, were converted to the corresponding imines in good to excellent yields under the optimized
reaction conditions (Table 2, entries 1-8, 10). But the time required to finish the reaction was
different. Generally, the electron-withdrawing substituents led to longer reaction time. (Table 2,

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entries 2, 6, 8). These results demonstrated that electronic properties of the substituents seem to
have some effects on the reaction. It is obvious that the position of the substituent on the benzene
ring has certain effects on the reactivity of benzyl amines, too. The substrate with an o-substituent
showed poor reactivity compared to that with an m- or p-substituent due to the steric hindrance
(Table 2, entries 9, 11). For instance, only 50% yield of the corresponding imine was obtained in
18 h in the case of o-fluorobenzyl amine as a substrate (Table 2, entry 11). Aldehydes as
by-products were observed in the cases that the selectivity of imines was lower than 100%. The
aldehyde by-products were derived from the hydrolysis of imine intermediates in the reaction.
The complex also showed high activity in the oxidative coupling of heterocyclic amines such as
2-thiolyl methylamine. 2-Thiolyl methylamine was converted to the corresponding imine in a
yield as high as 95% in 15 h. Generally, heterocyclic amines were poor substrates in the oxidative
homo-coupling of amines to imines catalyzed by copper-based [13]. Moderate yields of the
corresponding imines were received in the homo-coupling of non-activated aliphatic amines at 1%
of catalyst loading (Table 2, entries 14-17). When the catalyst loading was increased to 1.5%, the
yields of the corresponding imines were improved largely (Table 2, entries 14-21). For instance, in
the case of cyclohexylmethylamine as a substrate, the yield of the corresponding imine reached
81% (GC yield) (Table 2, entry 21).
Table 2
[Et3NH]2[Ru(dipic)Cl3]-catalyzed oxidation of different amines
Time Conv Selecta Yieldb
Entry Substrate
Product
(h)
10
(%)
100
(%)
100
(%)
1
100(85)
2
17
100
92
92(82)
3
4
5
12
12
12
100
100
100
94
99
99
94(86)
99(89)
99(80)

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6
7
15
15
100
100
95
93
95(81)
93(83)
8
15
16
16
18
88
86
77
50
94
95
99
99
83(73)
82(69)
77(52)
50(35)
9
10
11
12
13
15
13
95
85
90
86(70)
85(71)
100
14d
19
19
47
68
100
98
47(29)
67(49)
15c,d
16d
17
17
18
18
18
18
58
80
70
87
61
81
99
58(32)
78 (60)
70(57)
87(76)
61(42)
81(64)
17c, d
18d
97
100
100
100
100
19c,d
20d
21c,d
Reaction conditions: primary amine (5 mmol), [Et3NH]2[Ru(dipic)Cl3] (0.05 mmol), reaction
temperature 100 ℃, open air atmosphere.
a Conversions and selectivity determined by 1H NMR.
b The data in the parentheses is the isolated yields.
c [Et3NH]2[Ru(dipic)Cl3] (0.075 mmol).
d Conversions and selectivity determined by GC.

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In subsequent studies, the complex was evaluated in the synthesis of unsymmetrically substituted
imines, which were carried out by reacting benzylamines with various substituted anilines under
conditions obtained above. Anilines bearing both electron-donating and weak
electron-withdrawing substituents afforded good yields of cross-coupled imines (Table 3, entries
1-5). Thus, except in the case of the p-nitro group, the presence of substituents in para or meta
position with respect to the amino group increased the selectivity in favor of the unsymmetrical
imine A (Table 3, entries 2-7). Increasing the molar ratio of 4-methyl phenylamine to benzylamine
increased the selectivity of the cross-coupling product (Table 3, entries 2, 3). The reaction of
benzylamine with p-nitroaniline bearing a strong electron-withdrawing nitro group, however,
produced only the symmetrical imine B (Table 3, entry 6), which can be ascribed to the low
nucleophilicity of p-nitroaniline compared to benzylamine. This was also observed in the copper
catalyzed cross-coupling of amines [10].
The catalyst was also evaluated in the synthesis of scarcely stable aliphatic imines. Symmetrical
imine B (10%) was observed at the molar ratio of hexylamine to benzylamine of 2:1 (Table 3,
entry 10). Increasing the molar ratio of hexylamine to benzylamine remarkably improved the
selectivity to the unsymmetrical imine A. Complete selectivity to A at quantitative conversion of
benzylamine was achieved at the molar ratio of hexylamine to benzylamine of 3:1 (Table 3, entry
11). Then the cross-coupling of benzylamine with other aliphatic amines was carried out at the
molar of aliphatic amine to benzylamine of 3:1. All the tested aliphatic amines gave the
corresponding unsymmetric imines in high yield (Table 3, entries 12-14).
Table 3
Synthesis of unsymmetrical iminesa
Time
(h)
Yieldb
(%)
Imine selectivity
Entry
1
Coupled amine
A:B
11
12
12
88
80
85
82:18
92:8
2 c
3
100:0

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4
5
6
7
8
9
12
12
15
14
14
13
80
82
84
83
80
81
100:0
100:0
0:100
98:2
79:21
100:0
10d
11e
15
15
85
85
90:10
100:0
12 e
17
80
100:0
13 e
14 e
16
16
83
82
100:0
100:0
Reaction conditions: anilines (10 mmol), benzylamine (5 mmol), [Et3NH]2[Ru(dipic)Cl3] (0.05
mmol), reaction temperature 100 ℃, open air atmosphere.
a Conversions of benzylamine was 100%.
b Isolated yield of imine.
c Benzylamine (5 mmol), 4-methylaniline (5 mmol).
d Benzylamine (5 mmol), n-hexylamine (10 mmol).
e Benzylamine (5 mmol), aliphatic amine (15 mmol).
4. Conclusions
In conclusion, we have found that [Et3NH]2[Ru(dipic)Cl3] is an efficient catalyst for the
homo-coupling of primary amines and cross-coupling of benzylamine with anilines and aliphatic
amines to the corresponding imines with air as oxidant under solvent-free conditions. This
atom-economical green process tolerates substrates bearing both electron-donating and

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electron-withdrawing substituents. Studies are underway to investigate further synthetic
applications.
Acknowledgments
The authors are grateful for financial supports from the National Natural Science Foundation of
China (no. 21276061 and 21476057)

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Table 1
Optimization of the conditions for the oxidative coupling of benzylamine to imines
Catalyst
(mol%)
1.0
Temperature Time
Yielda
(%)
77
Entry
Solvent
(℃)
(h)
24
10
18
10
8
1
MeCN
PhCF3
—
reflux
reflux
100
100
100
80
2
1.0
90
3
0.5
97
4
1.0
—
>99
>99
84
5
1.5
—
6
1.0
—
20
8
7
1.0
—
120
100
100
100
100
>99
0
8
0
—
10
6
9b
10c
11d
1.0
—
79
1.0
—
10
15
13
1.0
—
30
Reaction conditions: benzylamine (5 mmol), running reaction under open air atmosphere.
a Yield was determined by GC based on area%, the selectivity of imine was 100%.
b Reaction was performed under an oxygen balloon.
c Reaction was performed under an nitrogen atmosphere.
d RuCl3 in stead of [Et3NH]2[Ru(dipic)Cl3] was used as catalyst.

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Table 2
[Et3NH]2[Ru(dipic)Cl3]-catalyzed oxidation of different amines
Time Conv Selecta Yieldb
Entry Substrate
1
Product
(h)
10
(%)
100
(%)
100
(%)
100(85)
2
17
100
92
92(82)
3
4
5
12
12
12
100
100
100
94
99
99
94(86)
99(89)
99(80)
6
7
15
15
100
100
95
93
95(81)
93(83)
8
15
16
16
18
88
86
77
50
94
95
99
99
83(73)
82(69)
77(52)
50(35)
9
10
11
12
13
15
13
95
85
90
86(70)
85(71)
100
14d
19
19
47
68
100
98
47(29)
67(49)
15c,d

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16d
17
17
18
18
18
18
58
80
70
87
61
81
99
58(32)
78 (60)
70(57)
87(76)
61(42)
81(64)
17c, d
18d
97
100
100
100
100
19c,d
20d
21c,d
Reaction conditions: primary amine (5 mmol), [Et3NH]2[Ru(dipic)Cl3] (0.05 mmol), reaction
temperature 100 ℃, open air atmosphere.
a Conversions and selectivity determined by 1H NMR.
b The data in the parentheses is the isolated yields.
c [Et3NH]2[Ru(dipic)Cl3] (0.075 mmol).
d Conversions and selectivity determined by GC.

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Table 3
Synthesis of unsymmetrical iminesa
Time
(h)
Yieldb
(%)
Imine selectivity
Entry
1
Coupled amine
A:B
11
12
12
12
12
15
14
14
13
88
80
85
80
82
84
83
80
81
82:18
92:8
2 c
3
100:0
100:0
100:0
0:100
98:2
4
5
6
7
8
79:21
100:0
9
10d
11e
15
15
85
85
90:10
100:0
12 e
17
80
100:0
13 e
14 e
16
16
83
82
100:0
100:0
Reaction conditions: anilines (10 mmol), benzylamine (5 mmol), [Et3NH]2[Ru(dipic)Cl3] (0.05

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mmol), reaction temperature 100 ℃, open air atmosphere.
a Conversions of benzylamine was 100%.
b Isolated yield of imine.
c Benzylamine (5 mmol), 4-methylaniline (5 mmol).
d Benzylamine (5 mmol), n-hexylamine (10 mmol).
e Benzylamine (5 mmol), aliphatic amine (15 mmol).

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Graphical Abstract

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Highlights




[Et3NH]2[Ru(dipic)Cl3] showed high efficiency in aerobic oxidation of amines to imines.
The imines can be obtained in high yield under air and solvent-free conditions.
This protocol is an atom-economical green process and tolerates various substrates.
[Et3NH]2[Ru(dipic)Cl3] is readily available.