Amino acid Schiff base complex catalyst for effective oxidation of olefins with molecular oxygen

Article abstract of DOI:10.1016/S1381-1169(99)00143-0

Molecular oxygen catalyzed by amino acid Schiff base Mn complex (Sal-Phe-Mn) effectively oxidized cyclohexene. 2-Cyclohexen-1-ol (-OH), 2-cyclohexen-1-one (C = O) and 2-cyclohexen-1-hydroperoxide (-OOH) were the major products of the reaction, which were different with typical oxidation of cyclohexene. The selectivity of 2-cyclohexen-1-hydroperoxide varied with reaction time and different additives. Rapid oxygen consumption was observed in the presence of acetate acid at 343 K. The cyclohexene oxidation rate was 1.1 times with that of no additive. The addition of pyridine into the oxidation system reduced the rate of O₂ consumption quickly, which was 24% with that of no additive. The allylic hydroperoxide was obtained as an essential product, suggesting a clear allylic pathway of oxidation of cyclohexene.

Full text of DOI:10.1016/S1381-1169(99)00143-0

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Journal of Molecular Catalysis A: Chemical 147 1999 173–178
www.elsevier.comrlocatermolcata
Amino acid Schiff base complex catalyst for effective
oxidation of olefins with molecular oxygen
a
a
b
Rong-Min Wang a,b,), Cheng-Jun Hao , Yun-Pu Wang , Shu-Ben Li
a
Department of Chemistry, Northwest Normal UniÕersity, Lanzhou, 730070, China
b
State Key Laboratory for Oxo Synthesis and SelectiÕe Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
Lanzhou 730000, China
Abstract
The amino acid Schiff base manganese complex Sal–Phe–Mn was prepared with L-phenylalanine, salicylaldehyde and
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Mn OAc 2P4H2O. The ligand and the complex were characterized by the infrared spectra, small area X-ray photoelectron
spectroscopy, and ICP-AES. In the presence of the manganese complex, cyclohexene was effectively oxidized by molecular
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oxygen without reductant. The major products of the reaction were 2-cyclohexen-1-ol –OH , 2-cyclohexen-1-one C5O
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and 2-cyclohexen-1-hydroperoxide –OOH , which was different with typical oxidation of cyclohexene. The influence of
reaction temperature and additive for oxidation had been studied. The selectivity of 2-cyclohexen-1-hydroperoxide varied
with reaction time and different additives. The mechanism of cyclohexene oxidation had also been discussed. q 1999
Elsevier Science B.V. All rights reserved.
Keywords: Aerobic oxidation; Amino acid Schiff base manganese complex; Cyclohexene
1. Introduction
that are able to reproduce the heme-enzyme
mediated oxygenation and oxidation reactions,
at least in terms of reaction types, mechanisms
The finding of efficient catalysts for the se-
lective insertion of one oxygen atom from oxy-
gen donors, like dioxygen, hydrogen peroxide,
alkylhydroperoxide, sodium hypochlorite or io-
dosobenzene into various organic molecules,
under mild conditions, remains a difficult chal-
lenge in the fields of chemical and biological
w
x
and often rates 6,7 . However, in olefin epoxi-
dations or alkane hydroxylation, the oxygen
source has mainly come from H2O2, alkylhy-
droperoxide, NaOCl or PhIO. When dioxygen
was used as an oxidant, in most cases, the
reductant should be added into the reactive sys-
tem. Halogenated iron porphyrins can catalyze
oxidation of hydrocarbons by molecular oxygen
w x
catalysis 1 . The oxidation of hydrocarbons by
transition metal complexes has been studied
w
x
extensively 2–5 . The current progress of the
research on synthetic metalloporphyrin catalysts
has led to the development of several systems
w x
without reductant 8 . When cobalt Schiff base
w
x
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complex 9,10 or bis 1,3-diketonato metal
w
x
complex 11 was used as catalyst in the
oxidation of organic substrates by dioxygen, the
reductant, such as isobutyraldehyde, isovalver-
aldehyde, acetaldehyde, or ethyl 2-oxocyclo-
)
Corresponding author. Tel.: q86-931-7972081; Fax: q86-
931-7663356; E-mail: wangrm@nwnu.edu.cn
1381-1169r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S1381-1169 99 00143-0

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R.-M. Wang et al.rJournal of Molecular Catalysis A: Chemical 147 1999 173–178
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pentanecarboxylate was necessary in the cat-
alytic system. In this paper, we found that cy-
clohexene could be effectively oxidized by
molecular oxygen catalyzed by amino acid
Schiff base complexes. The allylic hydroper-
oxide was obtained as an important product,
which suggested a clear allylic pathway of oxi-
dation of cyclohexene.
solution of Mn OAc P4H2O 10 mmol was
2
slowly added. The color of solution changed to
deep yellow, and the solution was continuously
stirred for 2 h. Left out for a night, the complex
settled out. The precipitate of manganese com-
plex was collected by filtration, washed with a
small amount of water and ethanol, respectively,
and dried under vacuum.
2.3. Oxidation of cyclohexene
2. Experimental
Typical procedure: a glass flask is charged
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2.1. Materials and equipment
with Sal–Phe–Mn 1 mg and cyclohexene 1
.
ml . The dry oxygen was filled from the gauge
L-phenylalanine and salicylaldehyde were
both purchased from Aldrich. Cyclohexene was
purified by fractionating distillation just before
use. All other reagents commercially available,
such as ethyl alcohol, metals and salts were of
the highest grade and were used as received.
The metal contents were analyzed on a model
ARL-3520 Inductively Coupled Plasmas Atomic
glass and the atmosphere was discharged out of
the glass reactor with the gas outlet tube. The
gas outlet tube was closed. The reactor was put
into a heating bath whose temperature was 708C,
and stirring was started. The consumption of
oxygen was measured and calculated by gauge
glass. After reacting for 12 h, the products were
analyzed by gas chromatograph, GCrMS sys-
tem.
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Emission Spectrometry of USA. XPS small
area X-ray photoelectron spectroscopy data
were recorded with the PHI-5702 Multi-Tech-
nique System, Power Source By MgKa line and
3. Results and discussion
Ag 3d5 FWHMF0.48 eV. IR spectra were
r2
3.1. Characterization
recorded in KBr disks with an Alpha-centauri
FT-IR spectrophotometer. The reaction products
of oxidation were determined and analyzed by
using Shimadzu QP-1000A GCrMS system,
GL-16A gas chromatograph with a 5 m=3 mm
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IR spectra Table 1 show that asymmetric
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stretching vibration absorption band nas of
COO appear at 1613 cmy1, nsCOO y at 1409
cmy1, nC
at 1632 cmy1, and nPh – O at 1250
5 N
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.
1
cmy for amino acid Schiff base ligand Sal–
OV-17 column, 808C–2008C 108Crmin , Inj.
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2208C Dect. 2208C . The products were deter-
mined by comparing with the standard mass
spectrometry of organic compounds and frag-
mentation pattern.
.
Phe . The stretching vibrations of C5N, COO
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and Ph–O bonds of complex Sal–Phe–Mn are
different because the backbones are influenced
by the metal ion. The Mn–N bond and Mn–O
bond appear in the complex.
2.2. Synthesis of amino acid Schiff base man-
In order to confirm the structure of the ligand
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ganese complexes Sal–Phe–Mn
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Sal–Phe and the coordination of the complex
Ten millimoles of L-phenylalanine and 10
mmol of sodium hydroxide were stirred in 75
ml 95% EtOH. When the L-phenylalanine and
NaOH were dissolved, 75 ml of the alcohol
which dissolved 10 mmol of salicylaldehyde
was added. After stirring for 15 min, the solu-
tion became yellow. In stirring, 75 ml of the
Table 1
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The IR data of the amino acid Schiff base Sal–Phe and its
complex Sal–Phe–Mn KBr, cm
y1
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. Ž
.
Compound
nC 5 N nasCOO nsCOO nPh – O nMn – N nMn – O
Sal–Phe
Sal–Phe–Mn 1628 1602
1632 1613
1409 1250
–
–
436
1388 1244 532

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R.-M. Wang et al.rJournal of Molecular Catalysis A: Chemical 147 1999 173–178
175
Table 2
The XPS data of the amino acid Schiff base Sal–Phe and its
ene oxidation by dioxygen was investigated by
monitoring the O2 consumption. The compo-
nent products were analyzed in the period of
reaction or after the reaction by the gas chro-
matograph and the GC-MS system. The struc-
tures were determined by comparing with the
standard mass spectrometry of organic com-
pounds and fragmentation pattern. The major
products of the reaction were 2-cyclohexen-1-ol
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complex Sal–Phe–Mn
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Compound
Binding energy eV
C1S
O
N
Mn2p3r2
–
1S1 r 2
1S1 r 2
1
r 2
Sal–Phe
531.6
288.6;
284.6
532.8r531.6r 400.0
531.1
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–
.
1r1r1
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.
2
Mn OAc
–
–
640.6
641.8
Sal–Phe–Mn
532.3
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.
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.
–OH , 2-cyclohexen-1-one C5O and 2-
288.6;
284.6
532.5r531.7r 398.9
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.
530.8
cyclohexen-1-hydroperoxide –OOH .
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.
1r1r1
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.
Sal–Phe–Mn , the small area X-ray photo-
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electron spectroscopy data were measured Ta-
ble 2 . It shows that the chemical shift of bind-
.
ing energy of carbon is less than 0.2 eV. The
chemical shifts of nitrogen and metallic element
are more than 0.5 eV. The carbon peaks in the
ligand and the complex could be divided into
As shown in Fig. 1, at 343 K, when the
consumption of dioxygen was more than 20 ml,
the total selectivity for the major products was
more than 80%. The side product was cyclohex-
ene oxide, and its selectivity was between 3%
and 4%.
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two peaks. One 288.6 eV is afforded the
carbon of carboxyl group. The oxygen peak in
the complex could be divided into three peaks.
Comparing with the ligand, the chemical shifts
3.2.1. Effect of temperature
of each O1S in complexes are different. The
The catalytic activity of Sal–Phe–Mn was
investigated as a function of temperature. The
consumption of dioxygen with 2 mg Sal–Phe–
Mn as a function of time at 313, 323, 333, and
343 K are shown in Fig. 2. It is clear that higher
1
atomic concern2tration of the complex showed the
ratio of Mn:N:O(1:1:4, which indicated that a
water molecular was in the complex. The result
of ICP-AES showed that the manganese content
of the complex is 16.11%, which conformed to
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the formula of Sal–Phe–Mn C16H13NO3MnP
.
H2O, Mns16.17% . The structure of amino
acid Schiff base and its complex Sal–Phe–Mn
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could be confirmed.
3.2. Oxidation of cyclohexene
In the presence of the amino acid Schiff base
Fig. 1. Relationship between the selectivity of the major products
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manganese complex Sal–Phe–Mn , cyclohex-
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and reaction time Cat: 2 mg; cyclohexene: 3 ml; 343 K .

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R.-M. Wang et al.rJournal of Molecular Catalysis A: Chemical 147 1999 173–178
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Fig. 3. Effect of the additive 5% in the oxidation of cyclohexene
Fig. 2. Relationship between consumption of O2 with reaction
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.
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1 ml in the presence of Sal–Phe–Mn 1 mg at 343 K.
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time catalyzed by Sal–Phe–Mn in different temperature Cat: 2
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..
mg; Sub: cyclohexene 2 ml .
additive. As shown in Fig. 3, at 343 K, in the
presence of acetate acid, rapid oxygen consump-
tion was observed. The cyclohexene oxidation
rate was 1.1 times that of no additive. The
selectivity of 2-cyclohexen-1-hydroperoxide
the reaction temperature, the faster the oxygen
consumption. The steady reaction rate, conver-
sion and selectivity at the end of the reaction are
collected in Table 3. At 343 K, the rate of
cyclohexene oxidation is 2.8 times than that of
313 K. At 333 K, the rate is 1.7 times than that
of 323 K. When the reaction temperature or
time increased, the reaction rate or conversion
of cyclohexene increased quickly. But the selec-
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.
–OOH was only half of that of no additive,
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.
and the selectivity of 2-cyclohexen-1-ol –OH
and 2-cyclohexen-1-one C5O increased 63%
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.
and 38%, respectively.
When the pyridine was added into the oxida-
tion system, the rate of O2 consumption de-
creased quickly. The rate was only 24% that of
no additive. The selectivity of 2-cyclohexen-1-
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tivity of 2-cyclohexen-1-hydroperoxide –OOH
decreased. It is indicative of a free radical reac-
tion pathway.
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.
hydroperoxide –OOH was 81% that of no
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.
3.2.2. Effect of the additiÕe
additive. The 2-cyclohexen-1-one C5O in-
creased 23% while the 2-cyclohexen-1-ol –OH
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.
The catalytic activity of Sal–Phe–Mn was
investigated in the presence or absence of the
was similar with no additive.
Table 3
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Product distribution and rate of oxidation of cyclohexene 2 ml in the presence of Sal–Phe–Mn at various temperatures
y5
Temperature K
Dn O2 mmol
Conv.b
Ratec =10 molrmin
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.
Ž .
Time h
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.a Ž
.
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.
%
Ž .
Selectivity %
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.
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.
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.
S total
S –OOH
313
323
333
343
10
11
14
14
2.58
3.58
7.73
12.3
33.1
58.5
74.8
99.7
92.7
79.9
75.1
44.5
47.3
40.7
29.6
0.43
0.54
0.92
1.20
10.06
aAmount of oxygen reacted.
bConversion of cyclohexene, analyzed by GC.
c Rate is the steady oxygen consumption rate.

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R.-M. Wang et al.rJournal of Molecular Catalysis A: Chemical 147 1999 173–178
177
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Fig. 4. The allylic peroxide decomposition mechanism for dioxygen reactions with metal complex SP-MsSal–Phe–Mn . Initiated by
radicals in solution, the peroxides thereby generated are catalytically decomposed by the manganese complex. These radicals can then
propagate the reaction further.
Ž .
of 2-cyclohexen-1-ol –OH , 2-cyclohexen-1-
one C5O and 2-cyclohexen-1-hydroperoxide
3.3. Mechanism of oxidation of cyclohexene
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.
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.Ž .
In order to explain the highly catalytic activ-
ity of highly halogenated porphyrin complexes,
–OOH Fig. 1 , and a little of cyclohexene
oxide was observed. The product distribution
and activity varied with reaction time, tempera-
ture and the additive. Considering the similarity
of the major products in two catalytic oxidation
system, especially the appearance of hydro-
peroxide in Sal–Phe–Mn system, it is likely
that they carry out the same radical chain mech-
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.
such as octabromotetra pentafluorophenyl
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.
.
porphyrin ferric complex Fe TFPPBr8 Cl , for
the oxidation of iso-butane to t-butanol by
w x
molecular oxygen, Grinstaff et al.
7
and
w
x
Labinger 12 have proposed a radical-chain
mechanism. This mechanism was also available
for explaining highly catalytic activity of Fe-
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anism, as shown in Fig. 4 SP-MsSal–Phe–
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.
.
TFPPBr8 Cl in the oxidation of cyclohexene
Mn .
Initiated by radicals presented in solution, the
concentration of 2-cyclohexen-1-hydroperoxide
w
x
with dioxygen 13 . In the presence of Sal–
Phe–Mn, cyclohexene oxidation to a mixture
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.
Fig. 5. Effect of the catalyst amount on oxidation of cyclohexene 1 ml at 343 K.

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R.-M. Wang et al.rJournal of Molecular Catalysis A: Chemical 147 1999 173–178
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–OOH increased quickly at the first 5 h, and
the presence of highly halogenated porphyrins
ferric complexes. This kind of catalyst is a
potentially important catalyst in cyclohexene
oxidation or similar oxidation processes of hy-
drocarbons.
then it was catalytically decomposed by the
manganese complex. Therefore, the selectivity
of hydroperoxide decreased after the increase in
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.
reaction time or temperature Fig. 1, Table 3 .
When the acetic acid was added into the oxida-
tion system, the rate of reaction increased and
the selectivity of hydroperoxide decreased be-
cause the proton can prompt decomposition of
hydroperoxide. Since the nitrogen of pyridine
easily coordinate with center metal of amino
acid Schiff base complexes as the fourth ligand
Acknowledgements
We wish to thank the National Natural Sci-
ences Foundation of China and the Gansu Bu-
reau of Environmental Protection for financial
support.
w
x
14 , adding the pyridine into the system made
the catalytic activity of manganese complex de-
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.
crease quickly Fig. 3 .
References
At 343 K, the rate of oxidation increases with
w x
1
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Ž
the increase of the substrate amount cyclohe-
.
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w x
2
catalyst, the rate will increase. But the selectiv-
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Ž
.
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composition of 2-cyclohexen-1-hydroperoxide
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w x
8
J.E. Lyons, P.E. Ellis Jr., H.K. Mayers Jr., J. Catal. 155
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.
base manganese complex Sal–Phe–Mn is an
effective catalyst for cyclohexene oxidation by
O2 without reductant. The major products of the
reaction were 2-cyclohexen-1-ol, 2-cyclohexen-
1-one and 2-cyclohexen-1-hydroperoxide. Add-
ing acetate acid made catalytic activity increase,
and the pyridine made catalytic activity drop.
The mechanism of oxidation is a radical chain
mechanism, which is similar with the system in
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.
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