Bioinspired manganese complex for room-temperature oxidation of primary amines to imines by t-butyl hydroperoxide

Article abstract of DOI:10.1016/j.ica.2021.120282

A sustainable method is developed for the selective and additive-free synthesis of imines from primary amines with TBHP catalyzed by bioinspired manganese complex (MnCl₂(TPA)) at room temperature. Use of 0.2 mol % MnCl₂(TPA) was efficient enough for this transformation by offering excellent conversions up to 98.2% along with 93.4% product yield within 1 h. The influence of reaction parameters (catalyst dosage, solvent, reaction temperature, time, etc.) on the catalytic performance was also investigated in detail. Building on these results, the selected MnCl₂(TPA) was further employed to transform various primary amines into corresponding imines and exhibited good compatibility even for the challenging aliphatic amine. The high efficiency, combining with a large substrate scope and ambient reaction conditions, makes the developed bioinspired Mn complex/TBHP system a promising pathway to produce imines. This work also paves a way to the expansion of non-heme metal catalysts as efficient platforms for various oxidation reactions.

Full text of DOI:10.1016/j.ica.2021.120282

Inorganica Chimica Acta 519 (2021) 120282
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Inorganica Chimica Acta
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Research paper
Bioinspired manganese complex for room-temperature oxidation of
primary amines to imines by t-butyl hydroperoxide
,
Lin Leia, Yaju Chena *, Zhenfeng Fenga, Chunyan Denga, Yepeng Xiaob
a School of Chemistry, Guangdong University of Petrochemical Technology, Maoming 525000, PR China
b School of Chemical Engineering, Guangdong University of Petrochemical Technology, Maoming 525000, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
A sustainable method is developed for the selective and additive-free synthesis of imines from primary amines
with TBHP catalyzed by bioinspired manganese complex (MnCl2(TPA)) at room temperature. Use of 0.2 mol %
MnCl2(TPA) was efficient enough for this transformation by offering excellent conversions up to 98.2% along
with 93.4% product yield within 1 h. The influence of reaction parameters (catalyst dosage, solvent, reaction
temperature, time, etc.) on the catalytic performance was also investigated in detail. Building on these results,
the selected MnCl2(TPA) was further employed to transform various primary amines into corresponding imines
and exhibited good compatibility even for the challenging aliphatic amine. The high efficiency, combining with a
large substrate scope and ambient reaction conditions, makes the developed bioinspired Mn complex/TBHP
system a promising pathway to produce imines. This work also paves a way to the expansion of non-heme metal
catalysts as efficient platforms for various oxidation reactions.
Bioinspired metal complex
Oxidative coupling
Imines
Amines
TBHP
1. Introduction
or harsh reaction conditions, such as high temperature, the addition of
oxidative promoters, and light irradiation. For example, a vanadium
–
–
Imines containing high-activity unsaturated C N double bonds are
catalyst (VO(Hhpic)2) was developed for the aerobic oxidation of amines
to imines at 120 ◦C in acetonitrile [24]. Liang and co-workers reported
salicylic acid complexed with TiO2 for visible light-driven selective
oxidation of amines into imines with air using TEMPO as a co-catalyst
[18]. Moreover, some reported protocols are often limited in lower ac-
tivity and unruly selectivity for aliphatic amines [25]. In view of the
above, the exploration of simple catalytic system with high efficiency
and good substrate expansibility for the oxidative coupling of amines
under mild even ambient conditions is highly desirable.
among the most significant intermediates for the synthesis of a variety of
pharmaceuticals, agrochemicals and molecular motors [1–3]. As a
consequence, extensive attention from both academic and industrial
field has been attracted toward the synthesis of imines. The condensa-
tion reaction between amines and carbonyl compounds is a traditional
strategy to acheive imines fabrication. This process involves the use of
unstable aldehydes, dehydrating agents and acid catalysts, which re-
strains its industrial application from the practical and environmental
point of view [4]. The direct oxidative coupling of amines to the cor-
responding imines provides a promising and attractive alternative to the
conventional approach.
Catalytic oxidation methodologies based on bioinspired metal com-
plexes and peroxides, which can be regarded as biologically inspired, are
gradually highlight their remarkable advantages of mild reaction con-
ditions, excellent reactivity and high selectivity [26,27]. Among them,
the biologically important elements-based complexes such as iron and
manganese attract particular attention. Iron porphyrins, as the mimics of
the cytochrome P450 active site, has been the most extensively studied
in various oxidation reactions. For eaxmple, fluorinated Fe-porphyrins
were found to be highly efficient for catalyzing the double epoxidation
of divinylbenzene with H2O2 under mild conditions [28]. Metal com-
plexes with porphyrin-inspired tetradentate nitrogen (N4) ligands
Over the past decade, considerable efforts have been made for this
transformation, thus yielding plentiful catalytic systems based on metal
catalysts, such as Cu [2,5,6], Fe [7], Mn [8–10], V [11], Co [12], Pd
[13], Au [14], Ru [15], Cd [16], and Ti [17,18]; and other catalysts,
such asquinone (TBHBQ) [19], ionic liquids [20], graphene oxide [21],
mesoporous carbon [22], and salicylic acid derivative [23]. Despite
good catalytic performance, most of theses methodologies involve
complicated catalyst preparation process, cumbersome reaction setup,
* Corresponding author.
E-mail address: chenyaju970@126.com (Y. Chen).
https://doi.org/10.1016/j.ica.2021.120282
Received 19 January 2021; Received in revised form 30 January 2021; Accepted 31 January 2021
Available online 9 February 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

L. Lei et al.
Inorganica Chimica Acta 519 (2021) 120282
Table 1
Oxidative coupling of benzylamine catalyzed by various non-heme metal com-
plexes.[a]
Entry
Catalyst
Conversion[b] (%)
Yield[b] (%)
1
2
3
4
5
6
7
8
9
–
5.4
23.8
22.0
34.5
24.0
95.8
98.2
96.8
96.4
3.1
21.3
18.9
32.2
20.5
87.5
93.4
89.6
89.0
CoCl2(TPA)
NiCl2(TPA)
CuCl2(TPA)
ZnCl2(TPA)
FeCl3(TPA)
MnCl2(TPA)
MnBr2(TPA)
Mn(ClO4)2(TPA)
Scheme 1. Room-temperature oxidation of primary amines to imines catalyzed
by non-heme metal complexes.
[a]
Reaction conditions: 1a (1.0 mmol), TBHP (2.0 mmol), catalyst (0.2 mol%),
acetonitrile (5 mL), room temperature (30 ◦C), 1 h.
known as non-heme metal complexes possess of particular appeal
because of their fine tunable and easily variable structural and electronic
properties comparing to heme (porphyrin)-based catalysts [29,30]. Talsi
and coworkers reported Fe and Mn aminopyridine catalysts for the
olefin epoxidations with H2O2/carboxylic acid with high efficiency (up
to 1000 turnover number (TON)) and selectivity (up to 100%) [31].
Over the past decades, Que [27,32] and Nam [33] have made significant
efforts to achieve abiological oxidation reactions with non-heme Fe
(Mn)/peroxides catalytic systems and reveal their catalytic mechanism.
Romero and Rodríguez have also developed a series of catalytic systems
based on non-heme metal complexes for alkene epoxidation [34–38].
Despite significant progress, most of these studies focused on olefin
epoxidation and alkane hydroxylation reactions. Given the importance
of oxidative coupling of amines to imines, this transformation has been
actively pursued, but non-heme Fe(Mn)/peroxides catalytic systems for
this reaction exhibiting high reactivity and working under a green
process remain scarce.
[b]
Conversion and yield determined by GC of the crude reaction mixture using
naphthalene as the internal standard.
70% aq.), cumene hydroperoxide (CHP, purity 80%), and urea-
hydrogen peroxide (UHP, purity 98%) were obtained from Energy
Chemical and TCI. Hydrogen peroxide (H2O2, 30% aq.) and other
commercially available chemicals (analytical grade) were available
from local suppliers.
2.2. Characterization
Solution NMR spectra were recorded with a Bruker Avance III HD
400 MHz spectrometer at ambient temperature in deuterated chloro-
form (CDCl3) using TMS as an internal standard. Elemental analysis of C,
H, and N was performed on a Vario EL cube instrument. Fourier trans-
form infrared spectroscopy (FT-IR) spectra of the samples were obtained
under ambient conditions at a resolution of 4 cmꢀ 1 in the wave number
range of 4000–400 cmꢀ 1 by using an EQUINOX 55 spectrometer. Mass
spectrometry analyses were recorded in a Shimadzu LC-MS 2010 liquid
chromatography-tandem mass spectrometry (ESI-MS in methanol) with
negative mode. The ultra-violet-visible light (UV–vis) spectra were
recorded on a Shimadzu UV-2450 spectrophotometer. The solution of
samples in dichloromethane (ca. 1.0 mM) was poured into a 1 cm quartz
cell for UV–vis spectroscopy with dichloromethane as the reference. Gas
chromatographic (GC) analysis was performed on a Shimadzu GC 2010
gas chromatograph equipped with a flame ionization detector and a
capillary column (Rtx-5, 30 m × 0.32 mm × 0.25 mm).
In this contribution, we developed a facile and sustainable protocol
based non-heme metal catalysts bearing TPA (tris(2-pyridylmethyl)-
amine) employing TBHP (t-butyl hydroperoxide) as an oxidant for the
selective synthesis of imines from amines (Scheme 1). It was found that
MnCl2(TPA) was the optimal catalyst. Reactions were performed in
short reaction times at low catalysts loading (0.2 mol %). The remark-
ably high conversion of benzylamine (98.2%) was obtained with 93.4%
yield for benzylidenebenzylamine within 1 h at room temperature
(30 ◦C). Further, various amines are smoothly converted into the desired
imines in good yields (up to 99.0%) with high selectivity under optimal
reaction conditions. The observed excellent performace provided a
promising methodology based on no-heme Mn/TBHP system to offer
various valuable imines under a sustainable way.
2.3. Synthesis of TPA and non-heme metal complexes
TPA, [MnX2(TPA)](X = Cl, Br, ClO4), [FeCl3(TPA)], [CoCl2(TPA)],
[NiCl2(TPA)], [CuCl2(TPA)] and [ZnCl2(TPA)] were synthesized ac-
cording to published procedures [39–42], and the synthetic procedure
routes of them were outlined in Scheme 2. Further details of these ma-
terials are given in the Supplementary Information.
2. Experimental section
2.1. Materials
2-(Aminomethyl)pyridine (purity, 99%) and 2-(chloromethyl)pyri-
dine hydrochloride (purity, 97%) were purchsed from J&K Scientific
Ltd. All primary amines (purity, >97%), metal salts (purity, >98%),
benzylidenebenzylamine (purity, 99%), t-butyl hydroperoxide (TBHP,
2.4. Catalyst testing
All the oxidative coupling reactions of amines were conducted in a
Scheme 2. Synthetic route for the non-heme metal complexes.
2

L. Lei et al.
Inorganica Chimica Acta 519 (2021) 120282
Table 2
The results of the benzylamine oxidative coupling reaction in different solvent
catalyzed by MnCl2(TPA).[a]
Entry
Solvent
Conversion[b] (%)
Yield[b] (%)
1
2
3
4
5
6
water
53.5
80.6
98.2
62.8
22.5
25.2
42.6
67.4
93.4
51.8
19.3
20.1
methanol
acetonitrile
dichloromethane
toluene
cyclohexane
[a]
Reaction conditions: benzylamine (1.0 mmol), TBHP (2.0 mmol), catalyst
(0.2 mol%), 30 ◦C, 1 h.
[b]
The same as Table 1.
25 mL round-bottom flask. Typically, benzylamine (1 mmol), TBHP (2
mmol), and catalyst (0.2 mol%, bsed on the mol of benzylamine) in
CH3CN (5 mL) were quickly added into the flask. Then, the reaction
Fig. 1. Effect of oxidant on benzylamine oxidative coupling reaction (Reaction
conditions: benzylamine (1.0 mmol), oxidant (2.0 mmol), MnCl2(TPA) (0.2 mol
%), acetonitrile (5 mL), 30 ◦C, 1 h).
◦
mixture was stirred at room temperature (30 C). After reaction, 2–3
drops of reaction mixture were added to 2 mL acetonitrile, and dried by
anhydrous magnesium sulfate. The product was analyzed by GC using a
Shimadzu GC 2010 gas chromatograph equipped with a flame ionization
detector and a capillary column (Rtx-5, 30 m × 0.32 mm × 0.25 mm).
The conversion and selectivity were determined using the calibration
curves obtained with naphthalene as an internal standard and each
authentic imine product. All reactions were repeated at least three times,
and the average values were used.
100
80
60
40
20
0
100
80
60
40
20
0
b
a
b
a
(B)
(A)
1.0 1.5 2.0 2.5 3.0
TBHP dasage (mmol)
0.10 0.15 0.20 0.25 0.30 0.35
Catalyst dosage (mol%)
3. Results and discussion
100
80
60
40
20
0
100
80
60
40
20
0
b
b
The oxidative coupling of benzylamine (1a) with TBHP in acetoni-
trile was chose as a model reaction to evaluate the catalytic performance
of as-perpared non-heme metal complexes. The results were listed in the
Table 1.
a
a
(C)
First, the control experiment was carried out at 30 ◦C in the absence
of any catalyst, giving a negligible yield of 3.1% to benzylidenebenzyl-
amine (2a) within 1 h (Table 1, entry 1). Then, several metal comlexes
including CoCl2(TPA), NiCl2(TPA), CuCl2(TPA), ZnCl2(TPA),
FeCl3(TPA) and MnCl2(TPA) were examined under the same reaction
conditions (Table 1, entries 2 ~ 7). It was found that MnCl2(TPA) and
FeCl3(TPA) showed highly effective of these, and gave a good yields of
87.5 and 93.4% to 2a, respectivly (Table 1, entries 6 and 7). Besides, the
catalytic activity of manganese complexes was not very sensitive to the
anions of these catalyts, such as Cl-, Br- and (ClO4)-, and the coores-
ponding complexes exhibited satisfactory performance (Table 1, entry 7
~ 9). Benzaldehyde is the main by-product in this transformation.
Moreover, various solvents, such as water, acetonitrile, methanol,
dichloromethane, toluene, and cyclohexane were explored in the
oxidative coupling reaction of benzylamine in different solvent cata-
lyzed by MnCl2(TPA). As listed in Table 2, the catalytic activity strongly
influenced by the solvent polarity. Water, known as the strong polar
green solvent, has been attracted continuous attention in some organic
reactions. This transformation could proceed in water with a moderate
conversion (Table 2, entry 1). When the slightly lower polarity methanol
was used, the catalytic activity was obviously increased to give a 80.6%
conversion (Table 2, entry 2). This probably due to strong interaction
between the solvent and the metal center of catalyst, suppressing the
ability of the formation of the active high-valence metal species in the
oxidative progress [43]. Nearly complete conversion of benzylamine to
the imine was observed in acetonitrile (Table 2, entry 3). Decreasing the
polarity of the solvent showed a decrease in the conversion to benzyl-
amine (Table 2, entry 4 ~ 6). The lower activity in nonpolar solvents
may be attributed to the limited compatibility of the benzylamine and
catalyst in the aqueous solution of the oxidant [44].
(D)
15
10 20 30 40 50
Temperature ( C)
30
45
60
75
Time (min)
Fig. 2. Effect of the catalyst dosage (A), TBHP dosage (B), reaction temperature
(C) and time (D) on catalytic performance of MnCl2(TPA) (a: conversion; b:
selectivity). Reaction conditions: (A) benzylamine (1 mmol), TBHP (2 mmol),
30 ◦C, 1 h; (B) MnCl2(TPA) (0.2 mol%), benzylamine (1 mmol), 30 ◦C, 1 h; (C)
MnCl2(TPA) (0.2 mol%), benzylamine (1 mmol), TBHP (2 mmol), 1 h; (D)
MnCl2(TPA) (0.2 mol%), benzylamine (1 mmol), TBHP (2 mmol), 30 ◦C.
hydrogen peroxide (UHP) and were investigated. As shown in Fig. 1,
Use of H2O2 as an oxidant gave relatively poor conversion of 18.2% with
a selectivity of 81.7%. During this reaction process, H2O2 was decom-
pounded rapidly with the observation of generation of oxygen bubbles,
thereby reducing the oxidization capability. Further, high conversions
(>92.2%) of amine with good selectivities (90.0%) of desired imine
were achieved by using other organic oxidants (TBHP, CHP and UHP). In
particular, a complete conversion of benzyl amine to the imine was
observed in the presence of TBHP. Therefore, we selected TBHP as the
oxidant in the following studies. Moreover, a reaction carried out under
N2 atmosphere yielded only traces of the desired imine demonstrating
the necessity for oxidizing conditions in this transformation.
To screen the optimal reaction conditions for this transformation, the
effect of various reaction parameters (catalyst dosage, TBHP dosage,
temperature and time) was investigated in detail and the results were
summarized in Fig. 2. First, the influence of the dosage of Mn(TPA)2 was
studied as represented in Fig. 2A. The higher activity of the catalyst was
observed with the increasing amount of catalyst from 0.1 to 0.2 mol%
where the benzylamine conversion increased from 62.2 to 98.3% with a
selectivity of 95.1%. A further increase in catalyst mount to 0.3 mol%
had little effect on conversion and selectivity, while the use of 0.35 mol
% led to a clear decrease in both conversion and selectivity. Similarly, a
To standardize the oxidant needed for this reaction, common ter-
minal oxidants, including aqueous hydrogen peroxide (H2O2), t-butyl
hydroperoxide (TBHP), cumene hydroperoxide (CHP) and urea-
3

L. Lei et al.
Inorganica Chimica Acta 519 (2021) 120282
Scheme 3. Oxidation of various primary amines to imines catalyzed by MnCl2(TPA) (the conversion and selectivity (in parenthesis) were determined by GC; reaction
conditions: benzylamine (1.0 mmol), TBHP (2.0 mmol), catalyst (0.2 mol%), 30 ◦C, 1 h).
increasing dosage of oxidant TBHP promoted the conversion of ben-
zylamine in a certain range, while the selectivity decreased with further
increase of TBHP amount as the amount exceeds 2 mol (Fig. 2B). It could
be clearly observed that the reaction temperatture showed a strong ef-
fect on this reaction (Fig. 2C). With the rise of temperature from 10 to
30 ◦C, the benzylamine conversion enhanced sharply from 31.7 to
98.2%, and the temperature showed slightly negative influence on the
desired imine selectivity beyond 30 ◦C. Furthermore, the reaction profile
of MnCl2(TPA) with time is shown in Fig. 2D. The benzylidenebenzyl-
amine synthesis from benzylamine with TBHP proceeded rapidly, and
more than 98.2% benzylamine comversion was obtained within 60 min
at 30 ◦C. During the whole process, the benzylidenebenzylamine selec-
tivity remains as high as >95.1%. The benzylidenebenzylamine yield
experienced a continuing growth up to 99.0% with a slightly decreased
selectivity of 92.1% within 75 min. Considering the yield of desired
product, the optimal reaction time can be locked at 60 min in this
reaction.
selectivity by prolonging the reaction time. These results indicate the
developed catalyytic protocol shows wide substrate scope for this amine
oxidative coupling reaction.
4. Conclusions
In summary, a series of bioinspired metal complexes bearing TPA
ligands were firstly employed for the oxidative coupling of primary
amines to imines. These complexes showed high-efficiency and selec-
tivity in this transformation by using aqueous TBHP as an oxidant at
room temperature under additive-free conditions. Especially, at a low
catalyst loading (0.2 mol %) of MnCl2(TPA), a 98.2% conversion of
benzylamine with 93.4% yield to benzylidenebenzylamine were ob-
tained within 1 h. Under the optimal reaction conditions, MnCl2(TPA)
presented a wide substrate scope to give the desired products in 70.9 ~
96.2% yields. Particularly, 1-heptylamine, a challenging aliphatic
amine, was also amenable to this protocol and gave a good selectivty of
92.7%. This work provides a sustainable approach to the oxidative
synthesis of imines from amines. Further investigations are underway to
elucidate the reaction mechanism and expand application of the catalyst
to other oxidation reactions.
Consequently, the optimal reaction conditions for the oxidative
coupling reaction was identified as: benzylamine (1 mmol), MnCl2(TPA)
(0.2 mol%) and TBHP (2 mmol) in acetonitrile at 30 ◦C for 1 h.
With the optimal reaction conditions obtained, we inwestigated the
general applicability of the catalyst MnCl2(TPA) using various aromatic
amines with either electron-donating or electron-withdrawing sub-
stituents and aliphatic amines as substrates. As summarized in Scheme 3,
all the above amines could be smoothly oxidized by MnCl2(TPA) to the
corresponding imines in good yields (76.5~>99.0%) and excellent
selectivity (>91.0%). Clearly, the catalytic performance for this oxida-
tive coupling reaction was modest influenced by electronic properties of
the substituents on phenyl ring (Scheme 3, 1a-1f), indicating the good
tolerance of functional-groups. With the increase of electron-
withdrawing property of the substituent, the corresponding imine was
obtained in a relatively lower yield (2d vs. 2e vs. 2f). However, with the
increasing of steric hindrance of substituents (1g and 1h), apparently
lower conversion of the related product was obtained under the same
CRediT authorship contribution statement
Lin Lei: Investigation, Data curation, Writing - original draft, Writing
- review & editing. Yaju Chen: Conceptualization, Writing - review &
editing, Supervision, Resources, Validation. Zhenfeng Feng: Investiga-
tion, Methodology. Chunyan Deng: Formal analysis, Software. Yepeng
Xiao: Visualization, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
conditions. For α-methyl benzylamine (1h), long reaction time (3.0 h)
was required to achieve higher conversion (87.0%). Notably, hetero-
aromatic amine (2i aand 2j) were also suitable substrates in this trans-
formantion, affording the desired products with high efficiency. In terms
of aliphatic amines 1k and 1i, which might show inertness to the
oxidation reaction, can also provide a good coversion with high
Acknowledgements
This work was financially supported by the Guangdong Basic and
Applied Basic Research Foundation (2019A1515010612), Characteristic
4

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