Efficient oxidative coupling of amines to imines catalyzed by manganese(III) meso-tetraphenylporphyrin chloride under ambient conditions

Article abstract of DOI:10.1016/j.catcom.2010.09.009

Highly efficient oxidative coupling of amines to imines by tert-butyl hydroperoxide (t-BuOOH) in the presence of metalloporphyrins has been reported. Manganese porphyrin showed excellent activity and selectivity for the oxidative coupling of amines to imines under ambient conditions. Moreover, a plausible mechanism involving high valence oxo intermediate has been proposed.

Full text of DOI:10.1016/j.catcom.2010.09.009

Catalysis Communications 12 (2010) 202–206
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Catalysis Communications
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Short Communication
Efficient oxidative coupling of amines to imines catalyzed by manganese(III)
meso-tetraphenylporphyrin chloride under ambient conditions
a
b
b,
Qiu-Lan Yuan , Xian-Tai Zhou , Hong-Bing Ji ⁎
a
School of Chemistry and Materials Science, Longyan University, Longyan, FuJian 364000, China
School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 June 2010
Received in revised form 31 August 2010
Accepted 4 September 2010
Available online 16 September 2010
Highly efficient oxidative coupling of amines to imines by tert-butyl hydroperoxide (t-BuOOH) in the
presence of metalloporphyrins has been reported. Manganese porphyrin showed excellent activity and
selectivity for the oxidative coupling of amines to imines under ambient conditions. Moreover, a plausible
mechanism involving high valence oxo intermediate has been proposed.
©
2010 Elsevier B.V. All rights reserved.
Keywords:
Amines
Imines
Oxidative coupling
Metalloporphyrins
1
. Introduction
oxidant [18]. Subsequently, an efficient oxidative coupling of amines
to imines by tert-butyl hydroperoxide (t-BuOOH) in the presence of
metalloporphyrins has been developed (Scheme 1) in this paper. Low
amount of manganese porphyrin catalyst (0.03 mol%, based on
substrate) was used for the oxidative coupling of amines to the
corresponding imines under ambient conditions. Furthermore, high
valence oxo intermediate was verified by in situ UV–vis spectroscopy
in the oxidative coupling system, and a plausible mechanism has been
proposed.
Imines have been used as versatile substrates for nucleophilic
addition, cycloaddition and reduction [1]. They can also be applied as
lipoxygenase inhibitors, anti-inflammatory and anti-cancer agents
[
2]. Traditionally, imines are prepared by condensation of carbonyl
compounds with amines in the presence of Lewis acid catalysts e.g.
TiCl [3], Al [4], MgSO [5,6] or K-10 [7,8]. The methodologies
4
O
2 3
4
reported have some disadvantages e.g. high reaction temperature,
prolonged reaction period and requiring excessive costly dehydrating
reagents. Therefore, direct synthesis of imines from amines by
oxidative dehydrogenation attracted wide attention [9]. Among
such procedures, metal-catalyzed oxidations by hydroperoxide or
dioxygen were found to be efficient and widely applicable for
converting amines to imines. Ruthenium complexes [10,11], dirho-
2
. Experimental
Amines were obtained from Aldrich or Fluka without further
purification unless indicated. Pyrrole was purified before using it. All
the reagents were of analytical grade. Metalloporphyrins catalysts
were prepared and characterized according to the procedures
reported previously [18]. UV–vis spectra were recorded on a HITACHI
U-3010 spectrophotometer. Elemental analysis data were obtained
on Vario EL III. FT-IR spectra were recorded on a Bruker VERTEX70
spectroscopy.
dium caprolactamate [12], PdCl
II) oxide-iodine [15] have been used as the catalysts.
As model catalysts of cytochrome P-450, metalloporphyrins could
2 3 4
/PPh [13], MnSO [14] and mercury
(
be used as an intermediate of oxygen carrier for biological systems.
Metalloporphyrins have been widely applied as catalysts for various
biomimetic oxidations [16]. However, only few investigations have
been reported on metalloporphyrins-catalyzed oxidation of amines
[
17].
2.1. General procedure for the catalytic oxidative coupling of amines
to imines
In our previous studies on biomimetic oxidations, metallopor-
phyins exhibited highly catalytic performance for oxidation of olefins,
sulfides, alcohols and ketones with molecular oxygen as the sole
Oxidant (t-BuOOH, 2 mmol) was added to 5 mL toluene containing
−
4
benzylamine (1 mmol), catalyst (3.0×10 mmol) and 0.8 mmol
naphthalene (as inert internal standard) at room temperature. The
consumption of amines were monitored by GC (Shimadzu GC14C)
and GC-MS (Shimadzu GCMS-QP2010 plus). The products were
⁎
Corresponding author. Tel.: +86 20 84113658; fax: +86 20 84113654.
E-mail address: jihb@mail.sysu.edu.cn (H.-B. Ji).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.09.009

Q.-L. Yuan et al. / Catalysis Communications 12 (2010) 202–206
203
Table 2
Effect of solvent on the oxidative coupling of benzylamine catalyzed by MnTPPCl a.
Entry
Solvent
Dielectric constant
Conv. (%)
Yieldb (%)
1
2
3
4
5
6
Toluene
2.4
9.2
2.0
9.1
18.3
37.5
N99
N99
78
21
18
91
98
76
21
18
13
Benzotrifluoride
Cyclohexane
Dichloromethane
Isopropanol
Acetonitrile
13
a
MnTPPCl (3.0×10−4 mmol), benzylamine (1 mmol), solvent (5 mL), t-BuOOH
(
2 mmol), 15 min, r.t.
Based on benzylamine, determined by GC and GC-MS.
b
(
entry 3). Although cyclohexane is a commonly used model substrate
Scheme 1. Oxidative coupling of benzylamine catalyzed by metalloporphyrins
catalysts.
for metalloporphyrin catalyzed oxidations, no oxidized products from
cyclohexane were determined for this catalytic system. Other solvents
with large dielectric constant e.g. dichloromethane, acetonitrile and
isopropanol were used in the oxidative coupling of benzylamine, the
product could be obtained with rather low yields (entries 4–6).
purified according to the reported procedures and identified by GC
retention times or the comparison of their mass spectra [7,19].
3
. Results and discussion
3.3. Oxidative coupling of various substrates catalyzed by MnTPPCl
3
.1. Oxidative coupling of benzylamine catalyzed by
To evaluate the scope of the catalytic system, various amines
various metalloporphyrins
were subjected to the reaction system at room temperature (Table 3).
As shown in Table 3, most substrates can be smoothly converted to
corresponding imines with high conversion rate and excellent
selectivity.
Most primary amines could be smoothly coupled to corresponding
imines through oxidation by t-BuOOH in the presence of MnTPPCl
under mild conditions. Small amount of aldehyde, direct oxidation
product of amines, were detected for all the substrates in the oxidative
coupling reactions. It is noteworthy that N-benzylidene-2-pheny-
lethanamine was detected for the oxidation of phenylethylamine
The catalytic activity and selectivity of different metalloporphyrins
for the oxidative coupling of amines to imines by t-BuOOH were
investigated with benzylamine as a model substrate and the results
are summarized in Table 1.
The oxidative coupling of benzylamine catalyzed by metallopor-
phyrins catalysts produces N-benzylbenzaldimine as the main
product. As shown in Table 1, the catalytic activity of the metallopor-
phyrins appears to be dependent on the nature of their central ions.
Manganese porphyrin was more effective compared with iron, cobalt
and ruthenium porphyrins for the oxidative coupling of benzylamine
under mild condition (entries 1–4). In addition, only 12% benzylamine
(
entry 2). It could be attributed to the nucleophilic reaction between
phenylethylamine and benzaldehyde [21], in which benzaldehyde
was generated from the cleavage oxidative of C–C bond of 2-
phenylacetaldehyde. Our previous works demonstrated that benzal-
dehyde could be produced in the cleavage oxidative of cinnamalde-
hyde catalyzed by metalloporphyrins [22].
could be converted when the catalyst was replaced by MnCl
same catalytic amount as metalloporphyrins (entry 5).
2
with the
3
.2. Effect of solvent on the oxidative coupling
With MnTPPCl as catalyst, the oxidative coupling of benzylamine
Furthermore, the electronic and steric effects on the oxidative
coupling of benzylamine were also well investigated with various
substituents on the benzene ring (entries 3–6). The electronic
property of substrates affects the selectivity of product, in which the
electron-withdrawing group seems more favorable to the formation
of imines products. The electron-donating groups (entries 3 and 4)
and electron-withdrawing groups (entries 5 and 6) retard the
oxidative coupling so that slightly longer reaction times are required.
The influence of steric effects can be illustrated from the oxidative
coupling of (2,4-dichlorophenyl) methanamine, in which its conver-
sion only reaches 70% after reacted for 3 h (entry 7). In addition,
oxidative coupling of non-aromatic primary amines e.g. cyclohex-
ylmethanamine and 1-hexanamine also proceed smoothly with high
conversion and yield (entries 8 and 9).
in different solvents was investigated. The results are summarized in
Table 2.
As shown in Table 2, solvent also plays an important role for the
oxidation system. It seems that solvent with large dielectric constant
is not suitable for the oxidative coupling of benzylamine except
benzotrifluoride. Toluene and benzotrifluoride were more favorable
for the oxidative coupling reaction compared with cyclohexane,
dichloromethane, isopropanol and acetonitrile (entries 1 and 2).
Toluene is often applied for radical reactions because of its reluctance
to undergo radical addition, and benzotrifluoride is commonly used in
metal-catalyzed oxidation [20]. Moderate yield of N-benzylbenzaldi-
mine (76%) was obtained when cyclohexane was used as solvent
3
.4. Plausible mechanism for oxidative coupling of benzylamine
Table 1
Oxidative coupling of benzylamine catalyzed by various metalloporphyrins a.
catalyzed by MnTPPCl
b
Entry
Catalyst
Conv. (%)
Yield (%)
The results from the controlled experiment have demonstrated
that manganese porphyrin is crucial for the oxidative coupling of
benzylamine. Meanwhile, it was found that the reaction was
completely inhibited when the radical trap was used, which suggested
that the oxidative coupling of benzylamine with manganese porphy-
rin should involve radical species. Generally for the metalloporphyr-
ins-catalyzed oxidations, high valence intermediate is accepted as the
active species, which could transfer oxygen to substrate directly.
1
2
3
4
5
MnTPPCl
FeTPPCl
CoTPPCl
RuTPPCl
N99
86
77
73
12
91
80
72
69
10
MnCl
2
a
Benzylamine (1 mmol), catalyst (3.0×10−4 mmol), toluene (5 mL), t-BuOOH
(
2 mmol), 15 min, r.t.
b
Based on benzylamine, determined by GC and GC-MS.

204
Q.-L. Yuan et al. / Catalysis Communications 12 (2010) 202–206
Table 3
Oxidative coupling of various amines catalyzed by MnTPPCl a.
Entry
Amines
Products
Time (h)
0.25
Conv. (%)
Yieldb (%)
91
NH2
N
1
N99
NH2
c
N
N
2
3
0.5
1.0
N99
N99
89
NH2
N
73
MeO
MeO
OMe
NH2
4
5
1.0
0.5
N99
88
95
NH2
N
97
Cl
F
Cl
F
Cl
NH2
N
6
0.5
3.0
N99
96
57
F
Cl
Cl
Cl
NH2
N
7
8
70
Cl
Cl
Cl
NH2
N
2.0
1.0
N99
N99
91
96
9
NH2
N
a
MnTPPCl (3.0×10−4 mmol), substrate (1 mmol), toluene (5 mL), t-BuOOH (2 mmol), rt.
Based on amines, determined by GC and GC-MS, residual product was aldehyde.
Residual product was benzaldehyde.
b
c
While for the present system, it seems that no oxygen transformation
to substrate is involved.
0.6
High valence manganese porphyrin in the catalytic system was
verified from the in situ UV–vis spectroscopy as shown in Fig. 1. The
characteristic absorption peak of MnTPPCl was at 476 nm. By adding
t-BuOOH and benzylamine into the reaction system, in situ measure-
ment revealed a new absorption peak at 520 nm, while the
characteristic absorption peak of MnTPPCl was weakened gradually.
These evidence indicated that a new metalloporphyrins species with
absorption peak at 520 nm is an active catalytic intermediate
expected as high valence manganese porphyrin [23–26].
0
0
0
.4
.2
Benzylamine could be oxidized in two stages to the corresponding
N-benzylbenzaldimine via benzaldehyde intermediate with metal
oxides such as copper compound [27], HgO–I [28]. Benzaldehyde was
2
.0
4
60
480
500
520
540
Wavelength/nm
also detected in the present manganese porphyrin catalyzed oxidative
coupling reaction. Attempts to explore whether aldehyde was the
intermediate for the oxidative coupling reaction by the reaction of
benzaldehyde with benzylamine in the presence of t-BuOOH was
Fig. 1. In situ UV–vis spectra of manganese porphyrin for the oxidative coupling reaction
system (time scan with 3 min intervals), MnTPPCl (3.0×10
(1 mmol), toluene (5 mL), t-BuOOH (2 mmol), rt.
−4
mmol), benzylamine

Q.-L. Yuan et al. / Catalysis Communications 12 (2010) 202–206
205
t-BuOOH
H O
PorMnIII
2
PhCH=NH2
(a)
OH
Ph
C
H
NH2
H
C
H
O
O
MnV
Ph
N
H
PorMnV
Ph
O
PhCH NH
2
2
OH
PhCH NH
2
2
Ph
N
H
Ph
(b)
Ph
N
Ph
Fig. 2. A plausible reaction mechanism for oxidative coupling of benzylamine catalyzed by MnTPPCl in presence of t-BuOOH.
successful. The formation of N-benzylbenzaldimine was the result of
the nucleophilic attack of benzaldehyde by a second molecule of
benzylamine followed with the removal of a water molecule. This
reaction course in the present system could be further confirmed from
the result using phenylethylamine as substrate (entry 2 in Table 3).
The product with loss of one methylene group could be attributed
to the reaction between phenylethylamine and benzaldehyde,
in which benzaldehyde was generated from the cleavage oxidative
of C–C bond of 2-phenylacetaldehyde. In addition, no benzaldehyde
was determined in the stirring solution with catalyst, oxidant and N-
benzylbenzaldimine, which indicated that benzaldehyde could not be
generated from the imines hydrolysis in this catalytic system.
On the basis of previous observations, a plausible reaction
mechanism for the oxidative coupling of benzylamine with MnTPPCl
as catalyst has been proposed as shown in Fig. 2. Through the reaction
with benzylamine, the role of high valence manganese intermediate
in the present system is to produce Schiff-base imine intermediate
As the results listed in Table 3, solvent has a great influence on the
oxidative coupling reaction. Generally in those reactions involves
ions, large dielectric constant of solvent has negative effect on the
reaction rate, which consists with the presence of Schiff-base imine
intermediate (a) in the present system. In addition, the formation of
N-benzylbenzaldimine is related with the nucleophilic attack of
benzaldehyde by a second molecule of benzylamine, which may be
influenced by the frontier orbital energy of the benzaldehyde and
benzylamine according to the frontier molecular orbital theory.
Based on the fact that the electronic property of substrates affects
the selectivity of product as shown in Table 3, calculations were
performed with the Gaussian03W package using the density
functional theory (DFT) [32]. Full geometry optimizations of sub-
strates with different electronic property (e.g. 4-chloro-benzylamine
and 4-methyl-benzylamine, 4-chloro-benzaldehyde and 4-methyl-
benzaldehyde) were performed employing Beck's three parameter
Lee–Yang–Parr correlation functions (B3LYP) combined with 6–31+
G (d, p) basis set. The optimized structures and frontier orbital energy
for the substrates are presented in Fig. 3. As shown in Fig. 3, the
frontier orbital energy gap of 4-methyl-benzaldehyde and 4-methyl-
benzylamine was 0.252 a.u. (5.33 eV), while the gap for the 4-chloro-
benzaldehyde and 4-methyl-benzaldehyde was 0.196 a.u. (6.86 eV).
According to the frontier molecular orbital theory, the stronger
(
a). Then, benzaldehyde was generated from the hydrolysis of Schiff-
base imine intermediate with the elimination of molecule ammonia
29]. The nucleophilic attack of benzaldehyde by a second molecule of
[
benzylamine gives the intermediate (b). The coupled imine product
was generated by the removal of a water molecule from the
intermediate (b) [30,31].
Fig. 3. The calculated frontier molecular orbital energy and energy gap of amines with different electronic property.

206
Q.-L. Yuan et al. / Catalysis Communications 12 (2010) 202–206
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Acknowledgments
The authors thank the National Natural Science Foundation of
China (Nos. 21036009 and 20976203), China Postdoctoral Science
Foundation (20080440792) and the Fundamental Research Funds for
the Central Universities for providing financial support to this project.
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