Aerobic Oxidation of Primary Amines to Imines in Water using a Cobalt Complex as Recyclable Catalyst under Mild Conditions

Article abstract of DOI:10.1002/chem.201803251

Oxidative coupling of primary amines to imines has been achieved by using a water soluble cobalt complex as catalyst and air as the oxidant at near ambient conditions. Aromatic, heteroaromatic and aliphatic amines were successfully converted to the corresponding imines with yields of up to 96 %. A 20 gram scale reaction for the synthesis of imine from benzylamine in good yield is also demonstrated with this method. The catalyst has been found to be reusable with up to five cycles. It is highly efficient, gives a turnover number (TON) of up to 128, and shows chemoselectivity with the only byproducts being water and ammonia. Control experiments and mechanistic studies indicate that the Co^II/Co^III catalytic cycle is responsible for these oxidative transformations. Some of the reactive intermediates of this reaction have also been isolated and structurally characterized.

Full text of DOI:10.1002/chem.201803251

A Journal of
Accepted Article
Title: Aerobic oxidation of primary amines to imines in water using a
cobalt complex as recyclable catalyst under mild conditions
Authors: Susanta Hazra, Priti Pilania, Mayukh Deb, Ajay Kishor
Kushawaha, and Anil Jacob Elias
This manuscript has been accepted after peer review and appears as an
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To be cited as: Chem. Eur. J. 10.1002/chem.201803251
Link to VoR: http://dx.doi.org/10.1002/chem.201803251
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Aerobic oxidation of primary amines to imines in water using a
cobalt complex as recyclable catalyst under mild conditions
Susanta Hazra, Priti Pilania, Mayukh Deb, Ajay Kishor Kushawaha and Anil J. Elias*
Abstract: Oxidative coupling of primary amines to imines has been
achieved by using a water soluble cobalt complex as catalyst and air
as the oxidant at near ambient conditions. Aromatic, hetero aromatic
and aliphatic amines were successfully converted to the
corresponding imines with yields up to 96%. We also demonstrate a
20 gram scale reaction for the synthesis of imine from benzylamine
in good yield using our method. The catalyst has been found to be
reusable up to five cycles. It is highly efficient, giving TON up to 128,
showing chemoselectivity with only by-products being water and
ammonia. Control experiments and mechanistic studies indicate that
the Co(II)/Co(III) catalytic cycle is responsible for these oxidative
transformations. Some of the reactive intermediates of this reaction
have also been isolated and structurally characterized.
these challenges have also been addressed recently by
employing metal based catalytic systems.7, 8. 9
Imines are versatile synthetic intermediates in a wide variety of
organic synthesis and are essential in the production of many
pharmaceuticals,
heterocyclic
and
biologically
active
compounds.1 During the past decade, considerable efforts have
been made for the direct synthesis of imines, in particular via
operationally simple one-pot procedures through oxidative
processes to reduce energy consumption and waste emission.2
Considerable progress has been made in imine synthesis by
catalytic oxidation of secondary amines, primary amines and
direct synthesis from alcohols and amines. A range of transition-
metal based catalytic systems have been established, that are
capable of efficient oxidation of primary amines to imines
(Scheme 1).2, 3, 4, 5 Despite good yields and selectivity, many of
these catalytic systems require the use of organic solvents, often
have cumbersome preparation methods or use expensive
metals, often require pure oxygen, elevated temperatures and
oxidative promoters. Naturally occurring metalloenzymes such
as copper amino oxidases (CuAOs) are well known for aerobic
oxidation of amines with complete chemo-selectivity under mild
conditions in aqueous medium.6a Therefore, bioinspired
catalysts using benzoquinone cofactor have received particular
attention and attempts have been made to mimic these catalytic
systems using metal based catalysts and organo-catalysts
(Scheme 1).6 As pointed out recently by Fleury and Largeron,
many challenges remain in the oxidation of amines to imines.6a
These include the use of ambient air rather than pure molecular
oxygen as oxidant, reactions carried out at room temperature,
high TON, high selectivity, catalyst recyclability, use of
biocompatible catalysts rather than precious metals and most
importantly, use of pure water as the only solvent. Some of
Scheme 1: Methods for oxidation of primary amines to imines
Metalloporphyrins, as the model of cytochrome P-450, have
been used recently as bio-inspired catalysts for amine
oxidation.7 However, this method requires high reaction
temperature, use of organic solvents and high pressure of
oxygen or an external oxidant. Suib and co-workers reported
oxidation of amines to imines using cesium-promoted
mesoporous manganese oxide as catalysts and air as the
oxidant. This method requires high reaction temperature (80-110
oC), use of organic solvents and higher catalyst loading.8 Recent
reports by Wei and co-workers on amine oxidation using non-
precious metal catalysts supported by inorganic ligands have
shown recyclability of the catalysts.9 However, this method still
requires pure O2 gas and organic solvents. Kobayashi and co-
workers reported a co-operative catalytic system using Pt/Ir
alloyed nanoclusters as catalyst along with 4-tert-butylcatechol
in a mixture of CHCl3 and H2O (9:1, v/v).4a-4b Report also exist on
co-catalytic oxidation of amines to imines using gold
nanohybrids as catalyst in aqueous/methanol medium.6h It may
be noted that recyclable catalytic systems that operate at
ambient or near ambient temperature and aqueous medium
using air or oxygen as sole oxidant are still scarce.6g
[a]
Department of Chemistry, Indian Institute of Technology, Delhi, Hauz
Khas, New Delhi-110016, India, Email: eliasanil@gmail.com, Fax:
+91-11-26581102; Tel: +91-11-26591504.
Supporting information for this article is given via a link at the end of
the document.
Herein, we report for the first time, the oxidation of primary
amines to imines using the water soluble cobalt complex of N,N'-
bis(2'-pyridinecarboxamide)-1,2-benzene with air as the oxidant
under near ambient conditions and using water as the only
solvent. The catalyst has been found to be reusable, highly
efficient, giving high TON, showing chemoselectivity and
efficiently mimicking metalloenzyme catalyzed amine oxidations
with water and ammonia as the only by-products.
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Recent report from our group has shown the utility of
picolinamide as an useful functionality for sp3 C-H bond
oxidation of organometallic compounds.10 Complexes of early
transition metals with the bispicolinamide ligand [N,N'-bis(2'-
pyridinecarboxamide)-1,2-benzene] (bpb) were first reported by
Vagg and co-workers in 1979 and many of its complexes have
been structurally characterized as well.11 σ organo cobalt (III)
complexes made with this tetradentate dianionic pyridine-amido
ligand have been found to be convenient air-stable models for
cobalamine chemistry.12 In 2015, Ye and co-workers have also
reported a water splitting reaction catalyzed by the cobalt(II)
complex supported by the bpb ligand with high turnover
frequency.13 However, no reports exist so far on the use of bpb
based complexes for oxidation of amines. The high water
solubility and structural similarities with porphyrin based
catalysts made us explore oxidation reactions of primary amines
using the cobalt complex of this ligand. We began our
investigation by examining the oxidation of benzylamine in
purely aqueous medium using an equivalent amount of
After optimizing the reaction conditions, we explored the
substrate scope of the reaction (Table 1). This reaction was
extended to compounds having electron donating as well as
electron withdrawing substituents on the aryl ring of
benzylamines and both type of substrates gave >85% yields of
the desired imines (Table 1). The electronic properties of the
substituents on the aryl ring had little influence on the efficiency
of the oxidation reaction. It should be mentioned that
heteroaromatic amines such as 2-thiophenemethylamine, which
has the potential to deactivate the catalyst by coordination with
the metal ion, also underwent the oxidation resulting in a
moderate yield of the desired imine (Table 1, 14a). C-α methyl
benzylamine was also converted to the corresponding the imine
in good yield (Table 1). The catalyst was able to oxidize
relatively bulky 1-napthylmethylamine also to the corresponding
imine (13a) in 80% yield. Under identical reaction conditions,
aliphatic amines showed a lower percentage of conversion with
65-66% isolated yields. Perhaps due to poor water solubility, the
yield of imines is lesser in case of such amines.
o
[Co(II)(bpb)] complex as catalyst under air at 50 C for 24h. We
observed conversion to imine with 55 % yield along with
benzonitrile (40 %) as the main by-product. Benzonitrile is
formed possibly due to the over oxidation of imine under this
condition as similar observation has been reported earlier.3i
Hence to get better selectivity, we carried out the reaction using
Table 1. Substrate scope for aerobic oxidation of symmetrical amines
CoL* (0.75 mol%)
R2
R1= Aromatic
hetetromatic,
aliphatic
R2
R2
H2O, 35-40 o
C
Air bubbler ( 120 bubbles/min)
12-15h
R1
NH2
R1
N
R1
R2= H, Me
1-18
1a-18a
o
catalytic amount of the complex (2 mol%) under air at 50 C for
N
N
3a
N
1a
MeO
OMe
24h. Interestingly, we were able to isolate the desired imine in
92% yield with 100% selectivity. We observed that atmospheric
O2 is crucial for the oxidation as the reaction did not proceed
under N2 atmosphere and use of external oxidants such as aq.
H2O2 and aq.TBHP gave mixture of products (Table S4).The
oxidation when carried out under a balloon of pure O2 gas
resulted in the formation of the desired imine in 96% yield and
the reaction proceeded even at room temperature (25 °C) (Table
S4). This result prompted us to carry out the reaction using an
air bubbler at room temperature and we observed a significant
increase in the yield as well as TON compared to reactions
carried out in simple open reaction vials (Table S2). We noticed
that bpbH2 ligand is also crucial for this reaction as use of
CoCl2.6H2O or Co(OAc)2 alone did not give significant yields of
the imine (Table S1). We explored the use of other transition
metal [Cu(II), Ni(II), Fe(II)] complexes with bpbH2, and a related
ligand N,N'-bis(2'-pyridinecarboxamide)-1,2-ethane (bpeH2).
However, all these complexes were not as efficient or recyclable
as the cobalt complex to achieve the required conversion and
selectivity (Table S1).We carried out the reaction by varying the
reaction parameters and after a detailed exploration (Table S1,
S2, S3, S4 and S5), it was found that 0.75 mol% of catalyst,
water as solvent and continuous air flow through the reaction
mixture over a period of 12h at 40°C are the best conditions
suited for maximizing the yields of this oxidation (Scheme 2).
2a
(126.6)
F
F
95%
90% (120)
96% (128)
Me Me
N
5a
N
4a
N
6a
82% (46)*
H3C
CH3
F3C
Cl
CF3
(117.3)
88%
90% (120)
MeO
Cl
OMe
Br
Br
N
7a
N
8a
N
9a
84% (84)$
87% (116)
93% (120)
O
O
O
N
F3C
CF3
N
N
10a
Cl
Cl
O
11a
12a
86% (114.6)
75% (100)
88% (117.3)
N
N
N
14a
S
S
15a
66% (66)#
80% (106.6)
72% (98.6)
()5
()5
N
16a
65 (65)#
Reaction conditions: amine (2.5 mmol), catalyst (0.018 mmol, 0.75 mol%), 1
mL deionized H2O, 35-40 oC for 12-15h, isolated yields are given. $For
RCH2NH2.HCl (10a) 1.5 equiv. Et3N was used. *For compound 6a 2 mol%
catalyst was used. # For compound 15a and 16a 1 mol% catalyst was used.
TON in parenthesis.
Excellent activity of our catalytic system for oxidation of primary
amines to imines promoted us to check the feasibility of this
protocol for the synthesis of unsymmetrical imines (Table 2).
Reactions between benzylamine and 2-phenylethylamine were
performed under different molar ratios. The reaction having ½
molar ratio of benzylamine/2-phenylethylamine showed the
highest selectivity towards the formation of unsymmetrical
imines (70% yield). Interestingly, a reaction of benzylamine with
Scheme 2.Optimized reaction conditions for oxidation of amines
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1, 2-diaminobenzene under this reaction condition resulted in
the formation of 2-phenylbenzimidazole in 65% yield. This
observation got us interested in the synthesis of 2-substituted
benzimidazole derivatives as benzimidazole and its derivatives
are important building blocks for the pharmaceutical industry.15
Proton pump inhibitors and several drugs that exhibit significant
activity against viruses such as HIV, herpes (HSV-1), or
influenza contain the benzimidazole moiety in the active drug
molecules. Therefore, to illustrate a potential application of our
catalyst was performed by using benzylamine as substrate and
the catalyst showed its activity up to five catalytic cycles with
only slight reduction (96 to 85%) in its activity (Figure 1).
methodology,
we
have
synthesized
2-substituted
benzimidazoles in 60-70% yields (Table 2, 10-14xy).
Table 2. Substrate scope for the aerobic oxidation of un-symmetrical amines
Figure 1. Recycling studies for the catalyst. Reaction conditions: amine (0.5
gm), catalyst (0.75 mol%), 1mL of H2O at 40 oC.
To investigate the possible reaction mechanism and to identify
the reactive intermediates for this transformation, several control
experiments were carried out (Scheme 4). For a reaction using
benzylamine and [Co(II)(bpb)] under N2 atmosphere there was
no imine formation (Table S4, entry 4). However, when we used
aq. H2O2 or aq. TBHP as the oxidant under inert conditions
(Table S4, entry 5, 6), we observed imine formation with
maximum 75% conversion. These observations indicate the
requirement of O2 for oxidizing Co(II) to Co(III) species under
these reaction conditions. Therefore, we carried out a reaction of
[Co(II)(bpb)] only with air in aqueous medium using an air
bubbler for 6h which resulted in the formation of a green
complex B (Scheme 4, entry 1). The complex B contains a
Co(III) peroxy moiety which was confirmed by UV-visible
Reaction conditions: amine (x 2.5 mmol and y 5 mmol), catalyst (0.018 mmol,
0.75-1 mol%), 1 mL deionized H2O, 40oC for 10-15h. a 1H-NMR yield is given
using ferrocene as internal standard.15e
spectroscopy as
a similar alkylperoxy complex was also
reported by Weiss and co-workers.12 The complex B was
separately used as a catalyst for the oxidation of benzylamine
under N2 atmosphere and we were able to isolate the imine in
55% yield (Scheme 4, entry 2). To further confirm the formation
of Co(III) intermediates, we carried out reactions of
benzylamines with Co(II)(bpb) in 4:1 molar ratio. We observed
full conversion of amines to imines within 10h. During a reaction
of 3-trifluoromethyl benzylamine under identical conditions,
NH4SCN (1 equiv.) was added after 2h into the reaction mixture
To demonstrate the applicability of our methodology, we have
also carried out a gram scale reaction of benzylamine 1 (0.18
mol, 20 gm) under identical reaction conditions (Scheme 3). The
reaction proceeded smoothly and resulted in the formation of the
desired imine 1a in 90.5 % isolated yield.
+
and we were able to isolate [Co(bpb)(3-CF3C6H4CH2NH2)2 SCN–
] along with corresponding imine (Scheme 4, entry 3). From UV-
visible spectra and single crystal X-ray diffraction studies (Figure
2), it was confirmed that Co(III) species (complex C) was formed
during the reaction.16 The complex C is stable to air and
moisture as we were unable to isolate imine from the complex C,
when it was reacted with air for 15h (Scheme 4, entry 4a).
Interestingly, when 1 equiv. FeCl3 was added to the reaction
mixture under identical reaction conditions, the reaction mixture
turned blood red in color (due to ferric thiocyanate formation)
and we were able to isolate the imine 10a in 50% yield (Scheme
4, entry 4b). It may be noted that thiocyanate stabilized complex
Scheme 3. Gram scale synthesis of imine 1a. Reaction conditions: amine (20
gm, 0.18 mmol), catalyst (0.75 mol%), 5 mL of H2O at 40 oC for 15h.
The catalyst was insoluble in most of the organic solvents like
CHCl3, DCM and EtOAc. Therefore, it was very easy to isolate
the catalyst from the reaction mixture by simple filtration
(Whatman, grade 1) after the reaction. The reusability of the
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On the basis of above experiments and supported by previous
C is stable at room temperature while analogous complex with
7, 8, 9, 10
Cl as counterion (C') was found to be stable only below -45 oC.
literature,
a plausible reaction mechanism for the
oxidative coupling of benzylamine with the catalyst has been
proposed as shown in Scheme 5.
[(bpb)Co(II)]
A
O2
[(bpb)Co(RCH2NH2)2]+Cl-
[(bpb)Co(III)-O-O-Co(III)(bpb)]
B
C'
O.
(bpb)Co
D
O2
H2NCH2R
- RCH2NH2
Reactive intermediate
-H2O
R
HN
G
O
H2O
RCH2NH2
RCH2NH2
N
R
R
R
R
H
R
NH2
NH
-NH3
Under neat
condition
[Co]/-H2O
path a
Imine
F
E
path 1
path 2
-NH3
Scheme 4. Control experiments for mechanistic studies.
path b
The oxidation of amines to imines have been reported to be
proceeding through a radical mechanism.7, 8, 9, 10 Therefore, to
find out the possibility of a radical pathway, we carried out
reactions with 2, 2, 6, 6-tetramethyl-1-piperidinyloxy (TEMPO)
free radical and butylated hydroxytoluene (BHT) (Scheme 4,
entry no 5). There was no significance change in the yield of the
imine when TEMPO was used as radical scavenger (Scheme 4,
entry 5c). This may be due to its catalytic activity for oxidation
reactions.3i However, when BHT was used as radical scavenger,
there was drastic reduction in the yield of imine (10%, Scheme
4, entry 5d). As reported by Satish and co-workers,14 BHT can
inhibit the formation of complex B due to formation of a stable
peroxy-p-quinalato-cobalt complex and also can form a stable
complex with intermediate D (Scheme 5).
Scheme 5.Possible catalytic cycle for the oxidation of amines to imines in
water.
Compound A, (bpb)Co(II) reacts with oxygen to generate the
active intermediate B, which coordinates with benzylamine to
9
form the intermediate D.7,
Intermediate D can also be
generated via intermediate C'. It is subsequently converted to a
Schiff base intermediate E and H2O.7a, 9 We would also like to
mention that formation of a superoxo intermediate that abstracts
a hydrogen atom from the amine to form an imine could not be
rule out.12d, 12e Hydrolysis of this imine intermediate E induced by
H2O with the elimination of a molecule of NH3 results in the
formation of benzaldehyde F, which reacts with another
benzylamine molecule to generate the desired imine product
9
with the release of one molecule of H2O.7a, Kinetic studies
indicate that the reaction is first order w.r.t amines which
supports the formation of intermediate D from B. Evidence for
benzaldehyde F was obtained from 1H-NMR studies where a
chemical shift value at 9.87-10.03 ppm was obtained. Imine
intermediate E can also react with free amine in absence of
water to form hemi-aminal intermediate G (path 2), which can
also lead to the formation of the desired imine with the
elimination of one molecule of NH3.2g E/Z ratios of the imine
products when analyzed by NOESY NMR experiments indicated
preference of E isomer formation. This can be explained based
on the proposed mechanism as elimination process in such
reactions normally favour trans geometry.
In conclusion we report for the first time, catalytic activity of the
water soluble N,N'-bis(2'-pyridinecarboxamide)-1,2-benzene
cobalt complex for oxidation of amines to imines. Reactions
Figure 2.Molecular structure of compound C. Thermal ellipsoids are drawn at
the 30% probability level. Selected hydrogen atoms have been omitted for
clarity.
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The authors thank DST, India for financial assistance in the form
of research grant to A. J. E [DST EMR/2015/000285]. S. H and
M. D thank UGC, India, for research fellowship. We thank DST-
FIST and IIT D for funding the single crystal X-ray diffraction at
IIT Delhi.
Conflicts of interest
The authors declare no competing financial interest.
Keywords: aerobic oxidation•water soluble complex•recyclable
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Susanta Hazra, Priti Pilania, Mayukh
Deb, Ajay Kishor Kushawaha and Anil J.
Elias*
Page No. – Page No.
Aerobic oxidation of primary amines
to imines in water using a cobalt
complex as recyclable catalyst under
mild conditions
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