A Solvent-Free Process for Synthesis of Imines by Iron-Catalyzed Oxidative Self- or Cross-Condensation of Primary Amines Using Molecular Oxygen as Sole Oxidant

Article abstract of DOI:10.1007/s10562-016-1789-3

Abstract: The synthesis of imines by oxidative self- or cross-condensation of primary amines has been achieved using iron(II) bromide as the catalyst. The protocol is ligand-free, additive-free, solvent-free, and high-yielding and utilizes renewable oxygen as sole oxidant. The reaction tolerated a wide range of functionalities. Graphical Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-016-1789-3

Catal Lett
DOI 10.1007/s10562-016-1789-3
A Solvent-Free Process for Synthesis of Imines by Iron-Catalyzed
Oxidative Self- or Cross-Condensation of Primary Amines Using
Molecular Oxygen as Sole Oxidant
1
1
Kovuru Gopalaiah
•
Anupama Saini
Received: 6 June 2016 / Accepted: 7 June 2016
Ó Springer Science+Business Media New York 2016
Abstract The synthesis of imines by oxidative self- or
cross-condensation of primary amines has been achieved
using iron(II) bromide as the catalyst. The protocol is
ligand-free, additive-free, solvent-free, and high-yielding
and utilizes renewable oxygen as sole oxidant. The reaction
tolerated a wide range of functionalities.
1 Introduction
Oxidative condensation reactions are important transfor-
mations in organic synthesis, which are largely promoted
by metals such as Pd, Ru, Cu, and Ni catalysts [1, 2].
Recently, we reported analogous C–C, C–N, C–O, and C–S
bond-forming processes using iron compounds as the cat-
alysts [3, 4]. In general, iron compounds are of great syn-
Graphical Abstract
Keywords Oxidative condensation Á Iron(II) bromide Á
Molecular oxygen Á Imines Á Primary amines
thetic interest because they possess benign character and
abundant [5–11].
Imines are versatile intermediates for the synthesis of
pharmaceutically and biologically active compounds, and
fine chemicals [12–14]. The C=N bond in imines is also
widely used in organic transformations such as addition,
reduction, cyclization, and aziridination reactions [15–20].
Imines have traditionally been synthesized through con-
densation of primary amines with aldehydes or ketones, but
alternative mild and practical catalytic methods [21–24],
including oxidative coupling of alcohols and amines
th
Dedicated to Professor Pavan Mathur on the occasion of his 65
birthday.
&
Kovuru Gopalaiah
gopal@chemistry.du.ac.in
[
[
25–30] together with oxidation of secondary amines
31–33] have also been developed. For a long time, little
1
Organic Synthesis and Catalysis Laboratory, Department of
Chemistry, University of Delhi, Delhi 110007, India
123

K. Gopalaiah, A. Saini
attention had been given to the oxidation of primary amines
probably due to poor product selectivity [34]. The dis-
covery of oxidative catalytic systems that operate effec-
tively with molecular oxygen or air as the sole oxidant has
contributed to revitalize interest in this area [35]. Recently,
non-precious transition-metal catalysis [36–42], metal-free
catalysis [43–51], bioinspired catalysis [47–51], and pho-
tocatalysis [52–59] have led to homo-coupled imines in
high yields and selectivity. However, these coupling reac-
tions suffer from two major draw backs that limits the
synthetic applications of imines. Firstly, most of these
reactions are performed in solvent media. Solvents are
perhaps the most crucial parameters in industrial chemical
synthesis; they represent an important challenge because
solvents often account for the vast majority of mass waste
in the processes [60]. Secondly, the condensation reactions
of amines are not efficient methods to prepare cross-cou-
pled imines due to the intrinsic self-coupling properties of
the substrates [36, 43, 61, 62]. To the best of our knowl-
edge, only a few efficient and selective ([95 %) oxidative
cross-coupling protocols for diverse amines have been
reported to date [48, 63–65].
7,10). Gratifyingly, the iron(II) bromide reaction in the
absence of solvent produced the highest yield (97 %, entry
11; Table 1). The yield of imine 2a was decreased when
the reaction was carried out in air atmosphere or at low
temperature due to the incomplete reaction (entries 12, 13;
Table 1). Investigation of the catalyst loading showed that
5 mol% of iron(II) bromide was required. The yield of 2a
was reduced to 71 % when 2 mol% iron(II) bromide was
used (entry 14; Table 1). In the absence of iron catalyst, the
reaction proceeded very slowly, and the conversion of
benzylamine to N-benzylidenebenzylamine reached 53 %
in seven days (entry 15; Table 1). Furthermore, the reac-
tion failed to give the desired imine compound when the
process was carried out under a N atmosphere. The best
2
catalytic activity of FeBr under solvent-free conditions,
2
compared with the other iron salts, is possibly associated
with the fact that the bromide is a better leaving group than
chloride. This leads to a vacant coordination site on iro-
n(II), thus allowing easy coordination of substrate to the
iron(II) centre. Furthermore, the catalytic system requires a
one electron change in oxidation state from iron(II) to
iron(III), which is more feasible than the conversion of
iron(III) to iron(IV), thus making the catalyst iron(III)
bromide less useful than iron(II) bromide. Therefore, we
decided to perform the subsequent reactions of the ben-
zylamines in the presence of iron(II) bromide as catalyst
(5 mol%) and molecular oxygen as oxidant at 110 °C
under solvent-free conditions.
In the course of our research on the development of new
methods for nitrogen heterocycles syntheses [3, 4], we
found that N-benzylidenebenzylamines can readily be
obtained from the iron-catalyzed oxidative self-condensa-
tion (homo-coupling) of benzylamines. We then, intended
to explore a generalized method for the preparation of
various functionalized imines. Herein, we report an effi-
cient solvent-free process for the synthesis of imines by
oxidative self and/or cross-condensation of primary amines
using iron(II) bromide as catalyst and molecular oxygen as
sole oxidant.
The scope of the oxidative self-condensation of benzy-
lamines under optimized conditions is outlined in Table 2.
Substituents at the ortho, meta, and para positions of the
phenyl ring of the amine (entries 1–7; Table 2) are easily
accommodated. The reaction is also tolerant of electronic
perturbation: oxidative self-condensation of benzylamines
bearing electron-donating (entries 1–3; Table 2) and elec-
tron-withdrawing substituents (entries 4–7; Table 2) pro-
ceeded in excellent yields in short times. Functional groups
such as methoxy (entry 2; Table 2), acetal (entry 3;
Table 2), halogens (entries 4–6; Table 2), and trifluo-
romethyl (entry 7; Table 2) were shown to be highly
compatible with the present reaction conditions. It is
noteworthy that the halogen-substituted benzylamines
performed well, leading to the halogen-substituted imine
compounds (entries 4–6; Table 2), which could be used for
further transformations along with the imine functionality
[66]. Sterically hindered substrate such as (± )-a-methyl-
benzylamine also underwent efficient oxidative self-con-
densation (entry 8; Table 2). Furthermore, the bulkier
1-naphthalenemethylamine could be used as the substrate,
leading to formation of the corresponding imine product 2j
in 89 % yield (entry 9; Table 2). Heterocyclic methy-
lamines namely furfurylamine and 2-thiophenemethy-
lamine, could also be converted into the corresponding
2
Results and Discussion
The oxidative self-condensation of benzylamine was cho-
sen as a model reaction to optimize the reaction conditions
(
Table 1). Taking into account the observations made in
the oxidative condensation of benzylamines with
o-phenylenediamines [3], a reagent system of iron(II)
bromide, molecular oxygen and chlorobenzene was tested
(
entry 1; Table 1). To our delight, amine 1a self-coupled
smoothly at 110 °C to give N-benzylidenebenzylamine
2a) in 90 % yield. Comparing with many other transition
(
metal-catalyzed reactions [36–42], this reaction system was
quite simple because no ligand or additives were required.
A quick survey of different iron salts indicated that iron(II)
bromide was superior to other catalysts (entries 2–10;
Table 1). Strikingly, the yields of imine 2a were slightly
higher in solvent-free conditions than the chlorobenzene
solution by simply testing for some iron salts (entries 2,
1
23

A Solvent-Free Process for Synthesis of Imines by Iron-Catalyzed Oxidative Self- or Cross-…
Table 1 Screening of catalysts
and reaction conditions for the
oxidative self-condensation of
benzylamine
a,b
Entry
Catalyst
Solvent
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
9
FeBr
FeCl
FeF
Fe(OAc)
FeSO
Á7H
Fe(ClO
ÁxH
FeBr
FeCl
Fe(acac)
Fe(NO
2
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
chlorobenzene
–
6
90
2
24
24
24
24
24
24
24
24
24
4
76 (81)
51
2
2
39
4
2
O
trace
67
4
)
2
2
O
3
58 (60)
43
3
3
24
1
1
1
1
1
1
0
1
2
3
4
5
3
)
3
71 (75)
97
FeBr
FeBr
FeBr
FeBr
–
2
2
2
2
c
d
e
–
24
24
24
7 days
74
–
59
–
71
–
53
Reaction conditions: 1a (3.0 mmol), catalyst (5 mol %), chlorobenzene (2 ml), 110 °C, molecular oxygen
balloon) atmosphere unless otherwise noted
(O
2
a
Yield of product after chromatography
Values in parenthesis belongs to the yields at solvent-free conditions
Reaction under air atmosphere
b
c
d
e
Reaction was performed at 80 °C
Using 2 mol % of catalyst
self-coupled imines in high yields indicating a good tol-
erance of heteroatoms (entries 10, 11; Table 2). In addition,
aliphatic amines such as n-octylamine and cyclohexy-
lamine were quite effective in the reaction, affording the
desired products 2m and 2n in 53 and 61 % yields,
respectively (entries 12, 13; Table 2). Interestingly, FeBr2/
oxidation of ortho-substituted benzylamines, the yields for
the imine products were not notable. However, a longer
reaction time for all the conversions was required com-
pared with the system reported here. Besides the above
limitations, an expensive TEMPO was used as a co-cata-
lyst, which is not recovered and reused.
O catalytic system could efficiently oxidize the secondary
2
Encouraged by excellent yields of the oxidative self-
condensation of benzylamines, our efforts were next focused
to explore the scope of cross-condensation between benzy-
lamines 1 and primary amines 3 (Table 3). First, the reaction
between various benzylamines and aniline was investigated
under standard conditions (entries 1–6; Table 3). All the
cross-condensation reactions were carried out with one
molar equivalent of benzylamines and 1.2 molar equivalents
of primary amines. To our delight, benzylamines reacted
smoothly with aniline furnished the N-benzylideneanilines
4a–f in good to excellent yields. As previously reported by
our group [3], it is likely that the self-condensation products
N-benzylidenebenzylamines were formed first and then
reacted with aniline to form N-benzylideneanilines by
amines viz, dibenzylamine and 1,2,3,4-tetrahydroiso-
quinoline to the corresponding imines in high yields
(
92–95 %), which represents the versatility of the present
reaction protocol (entries 14, 15; Table 2).
Recently, Xu and co-workers reported Fe(NO ) /
3
3
TEMPO-catalyzed aerobic oxidation of amines to imines in
toluene with 5–10 mol% catalyst loading.[40] This [Fe]/
TEMPO catalyst system could oxidize benzylic amines to
imines efficiently, but insignificant reaction or failed to
oxidize hetero aryl methylamines and aliphatic amines.
Moreover, in this catalytic system the reaction efficiency
was largely affected by substituents on the phenyl ring of
amine, which is not observed in our method. Also, for the
123

K. Gopalaiah, A. Saini
Table 2 Iron-catalyzed
oxidative self-condensation of
amines
a
Reaction conditions: amine 1 (3.0 mmol), FeBr
sphere, 110 °C
2 2
(5 mol%), molecular oxygen (O balloon) atmo-
a
Isolated yield
1
23

A Solvent-Free Process for Synthesis of Imines by Iron-Catalyzed Oxidative Self- or Cross-…
Table 3 Iron-catalyzed
oxidative cross-condensation of
primary amines
1
R (1)
2
R (3)
a
Yield (%) (4)
Entry
Time (h)
1
2
3
4
5
6
7
8
9
C
6
H
5
C
C
C
C
C
C
6
6
6
6
6
6
H
H
H
H
H
H
5
5
5
5
5
5
6
95 4a
95 4b
99 4c
92 4d
89 4e
94 4f
96 4g
91 4h
95 4i
97 4j
90 4k
86 4l
85 4m
80 4n
91 4o
78 4p
69 4q
75 4r
4–MeC
4–MeOC
4–FC
2–ClC
3,4–Cl
6
H
4
6
6
H
4
6
6
H
4
10
12
8
6
H
4
2 6 3
C H
C
C
C
C
C
C
C
C
C
C
C
C
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
H
6 5
4-MeC
2-MeC
6
H
H
4
4
6
6
6
4-iPrC
4-MeOC
4-FC
4-ClC
4-BrC
3,5-Cl
6
H
4
5
10
11
12
13
14
15
16
17
18
6
H
4
5
6
H
4
8
6
H4
8
6
H
4
8
2
C
6
H
3
9
1-naphthyl
n-octyl
12
18
18
18
n-butyl
cyclohexyl
Reaction conditions: amine 1 (3.0 mmol), amine 3 (3.6 mmol), FeBr
oxygen (O
balloon) atmosphere, 110 °C
2
(5 mol %), molecular
2
a
Isolated yield
transimination. The yields of the reactions of benzylamines
bearing electron-donating groups with aniline were slightly
higher in comparison with electron-withdrawing amines.
Next, we turned our attention to study the effect of the
reactions of benzylamine with various primary amines
under standard conditions (entries 7–18; Table 3). In these
reactions, the electronic properties of groups on the phenyl
ring of anilines had some effects on the catalytic activities.
Both electron-donating (e.g., Me, iPr, OMe) and electron-
withdrawing (e.g., F, Cl, Br) substituents were tolerated in
the reaction. Generally, anilines having an electron-do-
nating substituent on the phenyl moiety gave slightly
higher yield of cross-coupled imine products than the
electron-withdrawing ones (entries 7–15; Table 3). This is
likely due to the reduced basicity of the anilines containing
electron-withdrawing groups which makes the transimina-
tion step sluggish. Steric effects of substituents had little
effect on the yield of the reaction. For example, the reac-
tion of 4- and 2-methylanilines with benzylamine resulted
in the formation of N-benzylidene-4-methylaniline (4g)
and N-benzylidene-2-methylaniline (4h) in 96 and 91 %
yields, respectively (entries 7, 8; Table 3). Furthermore,
aliphatic amines such as n-octylamine, n-butylamine and
cyclohexylamine were performed well in the cross-con-
densation reaction, furnished the corresponding imines 4p–
r in 78, 69 and 75 % yields, respectively (entries 16–18;
Table 3).
To examine the feasibility of a large-scale experiment,
the reaction of 4-fluorobenzylamine with aniline was
investigated. The reaction could afford 1.81 g of imine 4d
in 91 % yield without any significant loss of its efficiency
(Scheme 1). Therefore, this protocol could be used as a
practical method to synthesize the precursors of some
important biologically active molecules, such as triaryl-
1,2,4-oxadiazole 5, which shows significant inhibitory
cytotoxicity against MCF7 and K562 cell lines [67].
To explore the reaction mechanism, the following
experiments were performed as shown in Scheme 2. Ben-
zylamine treated with iron(II) bromide and molecular
oxygen to provide homo-coupled imine 2a in 97 % yield
(Scheme 2A). Whereas, only a trace amount of 2a was
observed in the absence of Fe source (Scheme 2B). This
experiment demonstrates the essential role of the iron
catalyst for this reaction. Next, the reaction of imine 2a
with aniline in the presence of iron catalyst afforded the
cross-coupled imine 4a in 98 % yield in 3 h (Scheme 3A).
123

K. Gopalaiah, A. Saini
Scheme 1 The large-scale
reaction for the preparation of
N-(4-fluorobenzylidene)aniline
octylamine in the presence of FeBr2 led to the corre-
sponding cross-coupled imine in 57 % yield in 12 h,
whereas no cross-imine compound was detected in the
absence of catalyst at the same interval. Thus, the iron
catalyst would be essential to promote both the self- and
cross-condensation reactions of amines. We presumed that
the cross-condensation reaction take place through the
homo-coupled imine.
Scheme 2 Oxidative self-condensation of benzylamine
According to the above observations, a tentative mech-
anism for the formation of homo and cross-coupled imines
was proposed based on previous reports in the litera-
ture [3, 63–65]. As shown in Scheme 4, the first step may
involve the oxidative addition of iron to the amine forming
the complex 6. Further oxidation of 6, provides ben-
zylimine (7) and regenerate the iron(II) bromide for
Unfortunately, the transimination reaction to form imine 4a
did not occur in the absence of catalyst in 3 h (Sche-
me 3B). However, after a prolonged reaction time (40 h),
investigating the reaction mixture showed that the imines
1
a and 4a are present in 3:1 ratio by H NMR analysis
Scheme 3B). Similarly, the transimination of 2a with n-
2
(
Scheme 3 Reaction of N-
benzylidenebenzylamine with
aniline
Scheme 4 Proposed reaction
mechanism for the iron-
catalyzed oxidative
condensation of primary amines
to imines
1
23

A Solvent-Free Process for Synthesis of Imines by Iron-Catalyzed Oxidative Self- or Cross-…
catalytic cycle. Reaction of imine 7 with another benzy-
lamine leads to the homo-coupled product 2a with libera-
tion of ammonia. Next, the nucleophilic addition of aniline
to imine 2a takes place in the presence of iron catalyst to
give iron-diamine complex 8. Finally, diamine complex 8
breaks down to the cross-imine product 4a and iron-amine
complex 6 (regenerated).
23. Qin W, Long S, Panunzio M, Biondi S (2013) Molecules
8:12264
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1
2
2
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3
Conclusions
In summary, a practical and convenient synthetic method
in solvent-free conditions using iron(II) bromide as the
catalyst (5 mol%) and molecular oxygen as the oxidant has
been developed for the facile synthesis of functionalized
homo- and cross-coupled imines from primary amines. The
operational simplicity, broad substrate scope, and excellent
yields of the products are the main advantages of this
method. The tolerance of diverse functional groups makes
the present protocol more attractive. Furthermore, this
procedure is cheap, safe and environmentally benign.
34. Sch u¨ mperli MT, Hammond C, Hermans I (2012) ACS Catal
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Acknowledgments This work was supported by the University of
Delhi through R & D Grant and DU/DST-Purse Grant for promoting
the Basic Research in the University. A.S. give thanks the University
Grants Commission for fellowship.
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