Direct synthesis of imines by 9-azabicyclo-[3,3,1]nonan-N-oxyl/KOH-catalyzed aerobic oxidative coupling of alcohols and amines

Article abstract of DOI:10.1016/j.cclet.2017.10.006

A simple and efficient method for preparation of imines by the oxidative coupling of benzyl alcohols with aromatic amines or aliphatic amines was developed. The reaction was catalyzed by 9-azabicyclo[3.3.1]nonan-N-oxyl (ABNO)/KOH with air as the economic and green oxidant. Under the optimal reaction conditions, a variety of imines were obtained in 80%-96% isolated yields.

Full text of DOI:10.1016/j.cclet.2017.10.006

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CCLET 4279 No. of Pages 4
Chinese Chemical Letters xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Chinese Chemical Letters
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Communication
Direct synthesis of imines by 9-azabicyclo-[3,3,1]nonan-N-oxyl/KOH-
catalyzed aerobic oxidative coupling of alcohols and amines
Yan Wan, Jia-Qi Ma, Chao Hong, Mei-Chao Li, Li-Qun Jin, Xin-Quan Hu, Bao-Xiang Hu,
*
Wei-Min Mo, Nan Sun, Zhen-Lu Shen
College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, China
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple and efficient method for preparation of imines by the oxidative coupling of benzyl alcohols with
aromatic amines or aliphatic amines was developed. The reaction was catalyzed by 9-azabicyclo[3.3.1]
nonan-N-oxyl (ABNO)/KOH with air as the economic and green oxidant. Under the optimal reaction
conditions, a variety of imines were obtained in 80%-96% isolated yields.
Received 15 June 2017
Received in revised form 4 September 2017
Accepted 11 October 2017
Available online xxx
© 2017 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences.
Published by Elsevier B.V. All rights reserved.
Keywords:
Imines
Benzyl alcohols
Amines ABNO
Air
With the reactive C
¼
N bond, imines are widely used in various
methods; (2) Not easily accessed catalysts, noble metal complexes,
or sensitive and expensive ligands were essential in some cases; (3)
Harsh reaction conditions could not be avoided sometimes; (4)
Dangerous pure oxygen was used in some aerobic oxidation
reactions. These drawbacks restricted the practical applications of
imines formation from alcohols and amines.
Recent years, nitroxyl radicals have been applied to synthesis of
imines. Several catalyst systems such as CuI/Bipy/TEMPO [9], Cu
(OAc)2/DMAP/TEMPO [10], Pd(OAc)2/Et3N/TEMPO/t-BuOK [11] and
Fe(NO3)3/TEMPO/KOH [12] were successfully developed for the
oxidative coupling of alcohols with amines. Nevertheless, these
catalytic systems still require transition metal catalysts, which
might thereby increase the risk for traces of metals in the products,
and limit their applications in the field of pharmaceutical
chemistry. Hence, the development of a transition-metal-free
and practical catalytic system for the oxidative coupling of alcohols
with amines is still highly desirable.
Inspired by hereinbefore work, as a part of our research
program aimed at transition-metal-free nitroxyl-catalyzed oxida-
tion reactions [13], we reported an efficient methodology for
synthesis of imines from alcohols and amines with air as the
oxidant and the synthetically accessible 9-azabicyclo[3.3.1]nonan-
N-oxyl (ABNO) as the catalyst.
ABNO, keto-ABNO and ABNOH were prepared in our laboratory
according to the literatures [14,15]. Other chemicals were
purchased from supplier and used without any further treatment.
1H NMR spectra were recorded on 500 MHz; 13C NMR spectra were
recorded on 125 MHz. CDCl3 was used as the solvent with
tetramethylsilane (TMS) as the internal standard. GC analyses
organic transformations such as reduction, addition, cyclization
and aziridination reactions [1]. They are also versatile intermedi-
ates for the synthesis of pharmaceuticals, particularly for the
preparation of anti-inflammatory agents, non-natural amino acids
and anticancer agents [2]. Therefore, synthesis and application of
imines is essentially ever-appealing research field in chemistry.
The typical process for the preparation of the imines involves
the condensation of primary amines with carbonyl compounds,
especially odorous and unstable aldehydes, whereas this protocol
requires acid-promotes in many situations [3]. During the past
decade, new methodologies for the synthesis of imines have been
developed, such as the oxidative coupling of amines with the
greener and more available alcohols, the self-coupling of primary
amines and the oxidative dehydrogenation of secondary amines. A
wide variety of catalysts including metal catalysts [4], metal-free
catalysts [5], photo-catalysts [6] and bioinspired catalysts [7] have
been extensively explored and bring significant developments to
these new methodologies. In these reported methods, the direct
synthesis of imines by oxidative coupling of alcohols and amines is
very attractive because diverse symmetric and asymmetric imines
can be easily synthesized by choosing different starting substrates
[8]. However, some drawbacks still remained to be solved
satisfactorily: (1) Stoichiometric oxidants, which will produce
large amounts of undesired waste, were needed in the early
* Corresponding author.
E-mail address: zhenlushen@zjut.edu.cn (Z.-L. Shen).
https://doi.org/10.1016/j.cclet.2017.10.006
1001-8417/© 2017 Chinese Chemical Society and Institute of Materia Medica, Chinese Academy of Medical Sciences. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Wan, et al., Direct synthesis of imines by 9-azabicyclo-[3,3,1]nonan-N-oxyl/KOH-catalyzed aerobic
oxidative coupling of alcohols and amines, Chin. Chem. Lett. (2017), https://doi.org/10.1016/j.cclet.2017.10.006

G Model
CCLET 4279 No. of Pages 4
2
Y. Wan et al. / Chinese Chemical Letters xxx (2017) xxx–xxx
were conducted with a flame ionization detector (FID) and a DB-5
or FFAP capillary column. GC–MS (EI, 70 eV) was performed with
DB-5 MS capillary column. Melting points were measured using
melting point instrument and are uncorrected. General procedure
for the synthesis of imines: To a sealed tube equipped with a
magnetic stir, was added aniline (1.0 mmol), alcohol (1.2 mmol, 1.2
equiv.), ABNO (4.2 mg, 0.03 mmol, 3 mol%), KOH (16.8 mg,
0.3 mmol, 30 mol%) and toluene (0.5 mL). Then air was introduced
via a balloon. The mixture was stirred at 80 ꢀC until the reaction
was complete (determined by GC or TLC). The mixture was directly
purified through column chromatography using petroleum and
Et3N (100:1) as eluent to afford the pure product.
We started to optimize the reaction conditions for the oxidative
coupling of alcohols and amines to form the corresponding imines
using benzyl alcohol (1a) and aniline (2a) as the model substrates
(Table 1). Initially, the reaction was carried out in toluene with
3 mol% of ABNO under atmosphere pressure of air in balloon at
80 ꢀC, and about 1.3% yield of N-benzylideneaniline (3a) could be
observed (entry 1). According to Zhao et al. [16], stoichiometric
KOH could promote imines formation from alcohols and amines. To
our delight, when 30 mol% of KOH was added, the yield of 3a was
increased to 92% (entry 2). Other nitroxyl radicals such as TEMPO,
4-OH-TEMPO, 4-AcNH-TEMPO and keto-ABNO were also investi-
gated, and the results displayed that ABNO/KOH was the most
efficient catalyst combination (entries 2–6). Then different ABNO/
base combinations were tested. The reactions in the presence of
NaOH and t-BuONa provided moderate yields (entries 7 and 8),
while very low yield was obtained in the presence of K2CO3 (entry
10). Though good result could be achieved with t-BuOK (entry 9), it
was not good as KOH from an economic and environmental point
of view. On the other hand, desired product almost could not be
observed with Et3N or DBU (entries 11 and 12). Screening of a range
of solvents showed that the reaction did not proceed in DMF or
DMSO (entries 13 and 14). Moderate to good yields were recorded
in dioxane, PhCl and xylene, but the best result was obtained in
toluene (entries 2 and 15–17). In addition, the reaction tempera-
ture was optimized. When the reaction temperature was decreased
from 80 ꢀC to 70 ꢀC, the yield of 3a was dramatically dropped to 41%
(entry 18). However, the yield remained almost unchanged by
increasing the temperature to 90 ꢀC (entry 19). Further optimiza-
tion showed that the optimum amounts of ABNO and KOH were 3
mol% and 30 mol%, respectively (entries 2, 20 and 21).
With the optimized conditions in hand, the scope and
limitations of the present method were examined (Table 2).
Firstly, 1a was chosen to react with a series of anilines bearing
electron-donationing or electron-withdrawing substituents on
ring under the optimized reaction conditions. It was observed that
the presence of a substituent with different electronic properties
on benzene ring of anilines had no significant influence on the
reaction. All of the anilines could react with 1a smoothly to give the
desired imine products in excellent yields (3a–3h). Moreover, it
was found that the reaction was sensitive to the steric hindrance of
the substituents on the phenyl ring, and meta- and para-
substituted anilines exhibited higher activities than ortho-substi-
tuted isomer (3b–3d).
The scope of this catalytic method was further explored, and
various substituted benzyl alcohols such as 4-chlorobenzyl
alcohol, 4-methylbenzyl alcohol, 4-methoxybenzyl alcohol and
4-methylthiobenzyl alcohol were subjected to the oxidative
coupling with 2a. It was found that imine products 3i–3l could
be isolated in 92%–95% yields. Similarly, substituted benzyl
alcohols could react smoothly with different anilines and afforded
their corresponding substituted imine derivatives 3m–3r in good
to excellent yields (80%–96%).
In addition, the representative polycyclic amine substrate, 1-
naphthylamine tolerated this catalytic system, which oxidative
coupled with 1a and 4-methoxybenzyl alcohol afforded 3s and 3t
in 90% and 88% yields, respectively. What’s more, 2-
Table 1
Optimization of reaction conditions.a
Entry
Nitroxyl radical (3 mol%)
Base (30 mol%)
Solvent
T (ꢀC)
Yield (%)b
1
2
3
4
5
6
7
8
ABNO
ABNO
–
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
DMF
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
70
90
80
80
1.3
92
27
12
16
21
64
62
86
1.2
trace
trace
trace
trace
50
79
84
41
93
KOH
KOH
KOH
KOH
KOH
NaOH
t-BuONa
t-BuOK
K2CO3
DBU
Et3N
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOHc
KOH
keto-ABNO
TEMPO
4-OH-TEMPO
4-AcNH-TEMPO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
ABNO
9
10
11
12
13
14
15
16
17
18
19
20
21
DMSO
Dioxane
PhCl
Xylene
Toluene
Toluene
Toluene
Toluene
ABNO
77
79
ABNOd
a
Reaction conditions: 1a (1.2 mmol), 2a (1.0 mmol), solvent (0.5 mL), air balloon, 4 h.
Yields were determined by GC using an internal standard method with biphenyl as the internal standard substance.
KOH is 20 mol%.
ABNO is 2 mol%.
b
c
d
Please cite this article in press as: Y. Wan, et al., Direct synthesis of imines by 9-azabicyclo-[3,3,1]nonan-N-oxyl/KOH-catalyzed aerobic
oxidative coupling of alcohols and amines, Chin. Chem. Lett. (2017), https://doi.org/10.1016/j.cclet.2017.10.006

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Y. Wan et al. / Chinese Chemical Letters xxx (2017) xxx–xxx
3
Table 2
ABNO/KOH-catalyzed aerobic oxidative coupling of alcohols with amines to
imines.a
Scheme 2. Plausible reaction pathways.
Entry
R1
R2
Time (h)
Product
Yield (%)
1
2
3
4
5
6
7
8
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
4
12
8
8
8
6
8
8
6
6
8
5
6
6
6
12
6
4
8
8
6
5
4
4
4
4
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
3p
3q
3r
90
85
90
95
92
93
91
89
95
94
92
94
92
95
94
80
90
96
90
88
91
90
90
91
90
90
when the reaction was carried out in the absence of KOH
(Scheme 1, (2)). In addition, it was found that benzyl alcohol
(1a) could be converted to benzaldehyde with 55% GC yield in 18 h
in the presence of 1 equiv. of ABNO under air (Scheme 1, (3)).
Based on the above observation and relevant literatures, we
propose a probable mechanism for the formation of imines
(Scheme 2). Initially, ABNO abstracts the hydrogen atom from
2-MeC6H4
3-MeC6H4
4-MeC6H4
4-MeOC6H4
4-BrC6H4
4-ClC6H4
4-FC6H4
Ph
9
4-ClC6H4
4-MeC6H4
4-MeOC6H4
4-MeSC6H4
4-MeOC6H4
4-ClC6H4
4-MeC6H4
4-MeOC6H4
4-MeC6H4
4-ClC6H4
Ph
4-MeOC6H4
2-thienyl
2-thienyl
Ph
4-MeOC6H4
4-ClC6H4
Ph
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
Ph
Ph
Ph
benzyl alcohol 1 to form the corresponding
a-hydroxyl carbon
radical 4, and ABNO is converted to ABNOH. ABNOH can be
reoxidized into ABNO by air in the presence of KOH. The radical 4
subsequently reacts with dioxygen to give the a-hydroxyl peroxyl
4-MeOC6H4
4-ClC6H4
4-MeC6H4
4-ClC6H4
4-MeOC6H4
4-MeC6H4
1-naphthyl
1-naphthyl
Ph
4-MeOC6H4
n-C4H9
n-C4H9
n-C4H9
cyclohexyl
radical 5, from which the carbonyl product 6 is formed by release of
hydroperoxyl radical [18,19]. The condensation of the amine 2 with
the aldehyde 6 affords the product imine 3. In addition, this
condensation reaction can promote the transformation of 1 to 6.
In summary, we have developed a simple and efficient method
for preparation of imines by the oxidative coupling of alcohols and
amines. The reaction was catalyzed by ABNO/KOH with air as the
economic and green oxidant. Under the optimal reaction
conditions, a wide range of products were synthesized in good
to excellent yields.
3s
3t
3u
3v
3w
3x
3y
3z
a
Reaction conditions: 1 (1.2 mmol), 2 (1.0 mmol), ABNO (3 mol%), KOH (30 mol%),
Acknowledgments
toluene (0.5 mL), air balloon, 80 ꢀC.
This work was supported by the National Natural Science
Foundation of China (No. 21376224), the Natural Science Founda-
tion of Zhejiang Province (No. LY17B060007), and Hangzhou
Qianjiang Distinguished Experts Project.
thiophenemethanol,
a heteroatom-containing benzyl alcohol,
successfully reacted with aniline and 4-methoxyaniline to give
the corresponding imines 3u and 3v in excellent yields. To our
delight, the reactions of aliphatic amines such as butan-1-amine
and cyclopentanamine with different benzyl alcohols gave 3w–3z
in 90%–91% yields. The reactions of aliphatic alcohols or 1-
phenylethanol with aniline were also tested under the same
conditions. Unfortunately, no desired imine products were
detected.
To gain some insight into the possible reaction mechanism,
several control experiments were carried out. According to our
recently report, KOH could accelerate the regeneration of ABNO
from ABNOH under O2 atmosphere [17]. When ABNOH was reacted
with KOH under air for 0.5 h, 70% yield of ABNO was isolated
(Scheme 1, (1)). However, trace amount of ABNO was obtained
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oxidative coupling of alcohols and amines, Chin. Chem. Lett. (2017), https://doi.org/10.1016/j.cclet.2017.10.006

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Please cite this article in press as: Y. Wan, et al., Direct synthesis of imines by 9-azabicyclo-[3,3,1]nonan-N-oxyl/KOH-catalyzed aerobic
oxidative coupling of alcohols and amines, Chin. Chem. Lett. (2017), https://doi.org/10.1016/j.cclet.2017.10.006