Air Oxidation of Benzoin Catalyzed by Metal-Salen Complexes

Article abstract of DOI:10.1134/S002315841801010X

Metal-Salen (M = Fe, Co, Ni, Cu, Zn) complexes were synthesized to catalyze the oxidation of benzoin to benzil with air as the “green” oxidizing reagent. The effects of temperature, base and solvent on M-Salen catalyzed oxidation of benzoin had been examined and optimized. The catalysis results showed that Co-Salen and Fe-Salen complexes were efficient to catalyze the oxidation reaction with yields over 80% while the catalytic activities of other three M-Salen complexes were poor. Moreover, the Co-Salen catalysis system could be reused three times with satisfied yield.

Full text of DOI:10.1134/S002315841801010X

ISSN 0023-1584, Kinetics and Catalysis, 2017, Vol. 58, No. 6, pp. 758–762. © Pleiades Publishing, Ltd., 2017.
Air Oxidation of Benzoin Catalyzed by Metal-Salen Complexes1
Linjun Shao*, Qiuling Chen, and Chenze Qi
Zhejiang Key Laboratory of Alternative Technologies for Fine Chemicals Process, Shaoxing University, Zhejiang Province,
312000 People’s Republic of China
*e-mail: shaolinjun@usx.edu.cn
Received May 5, 2017
Abstract⎯Metal-Salen (M = Fe, Co, Ni, Cu, Zn) complexes were synthesized to catalyze the oxidation of
benzoin to benzil with air as the “green” oxidizing reagent. The effects of temperature, base and solvent on
M-Salen catalyzed oxidation of benzoin had been examined and optimized. The catalysis results showed that
Co-Salen and Fe-Salen complexes were efficient to catalyze the oxidation reaction with yields over 80% while
the catalytic activities of other three M-Salen complexes were poor. Moreover, the Co-Salen catalysis system
could be reused three times with satisfied yield.
Keywords: Metal-Salen complex, DFT calculation, catalytic oxidation, benzoin, mechanism
DOI: 10.1134/S002315841801010X
INTRODUCTION
Salen and Co-Salen complexes exhibited high catalytic
activity towards the oxidation of benzoin to benzil.
Inspired by the enzymatic systems, such as cyto-
chrome P-450 and methane monooxygenase, transi-
tion metal complexes incorporating ligands have
attracted significant interest in redox-active reaction
[1, 2]. Salicylidene Schiff base (Salen) as bidentate
ligand can be synthesized readily by the condensation
of salicylaldehyde derivatives with amine derivatives.
Salen complexes with various transition metals (M-
Salen complexes) [M = Cu, Co, Ni, Zn, Fe, Mn, et al.]
have been reported and used in the fields of biology,
medicine industry and catalysis [3–9]. For example,
Os(VI)-Salen nitride complex has been synthesized
and used as the oxidation catalyst of alkanes with H2O2
as the oxidant [10]. Mn- and Co-Salen complexes
have been immobilized on copolymer and used as sup-
ported catalysts for the asymmetric epoxidation of ole-
fins and the hydrolytic kinetic resolution of epoxides
[11, 12]. Cu-Salen complex have been immobilized on
the graphene oxide and used for the catalytic epoxida-
tion of olefins [13]. Co-Salen has been successfully
used for the diastereoselective and enantioselective
Baeyer-Villiger oxidation of cyclobutanones in micel-
lar environment [14]. However, the inner factors con-
cerning the catalytic activities of M-Salen complexes
with different transition metals in catalysis are still not
very clear.
N
N
M
O
O
M = Fe2+, Co2+, Ni2+, Cu2+, Zn2+
Molecular structure of M-Salen complexes.
EXPERIMENTAL SECTION
Materials
Salicylaldehyde and ethylenediamine were pur-
chased from Sinopharm Chemical Reagent Co. Ltd.
(Shanghai, China). Benzoin was bought from Aladdin
Chemical Reagent Co. Ltd (Shanghai, China). All
other chemicals and solvents used were of AR grade.
Synthesis of Salen Ligand
Salicylaldehyde (10.84 g, 89 mmol) was added to
80 mL absolute ethanol and stirred magnetically at
80°C. Then, ethylenediamine (2.22 g, 37 mmol) was
dropped slowly and the solution became yellow. The
solution was allowed to stir at 80°C for another 1.5 h.
The reaction mixture was cooled down to room tem-
perature after completion (20°C), and then filtered.
The filtrate cake was washed by H2O and ethanol. Sol-
vent was removed under a reduced pressure to obtain
bright yellow product with yield of 92%. Melting
In this paper, we synthesized five M-Salen com-
plexes [M = Fe(II), Co(II), Cu(II), Ni(II), Co(II)]
and their catalytic activities were evaluated by the cat-
alytic oxidation of benzoin to benzil with air as the oxi-
dizing reagent. Catalysis results demonstrated that Fe-
1
point: 126–128°C. H NMR (400 MHz, CDCl3) δ
13.23 (s, 2H), 8.34 (s, 2H), 7.33 – 7.18 (m, 4H), 6.94
1
The article is published in the original.
758

AIR OXIDATION OF BENZOIN CATALYZED
759
(dd, J = 8.3, 0.4 Hz, 2H), 6.85 (td, J = 7.5, 1.1 Hz, tion became dark green. The reaction was carried out
2H), 3.91 (s, 4H). m/z: 269 (M + H)+. Element anal-
ysis: C: 70.64%; H: 5.11%; N: 9.87%; Calc. for Salen:
C: 71.64%; H: 5.97%; N: 10.45%.
at 80°C with magnetic stirring and monitored by thin layer
chromatography (TLC). The air flow rate was 60 mL/min.
The solution became reddish brown slowly from dark
green. After completion, 25 mL H2O was added to the
solution and the pH value of solution was adjusted to
3–4 by dropping 4 mol/L HCl aqueous solution. The
crude benzil was separated by filtration and the filtrate
was stored for the followed reuse experiment. The
crude benzil was recrystallized from 80% ethanol.
Melting point of benzil: 95.2–96.8°C. 1H NMR
(400 MHz, CDCl3, TMS): 7.98 (d, J = 7.5 Hz, 4H), 7.66
(t, J = 7.1 Hz, 2H), 7.52 (t, J = 7.5 Hz, 4H) ppm.
Synthesis of M-Salen complexes
Salicylaldehyde (10.84 g, 89 mmol) was added to
80 mL absolute ethanol and stirred magnetically at
80°C. Then, 2.22 g ethylenediamine (37 mmol) was
dropped slowly and the solution became yellow. The
solution was allowed to stir at 80°C for another 1.5 h.
Co(CH3COO)2 ⋅ 4H2O (9.30 g, 37 mmol) aqueous
solution (50 mL) was dropped slowly into the reaction
mixture and the solution became red and appeared
dark red precipitation. After reacted for 1 h, the reac-
tion mixture was filtrated and washed by H2O and eth-
anol three times (3 × 20 mL). The product (Co-Salen
complex) was dried under a reduce pressure at 60°C
for 12 h with yield of 73% based on the amount of eth-
yldiamine. MS(+ESI) of Co-Salen (%): m/z = 325
(100) (M + H)+; Element analysis for Co-Salen, C:
58.87%, H: 3.85%, N: 8.11%; Calculated value for Co-
Salen: C: 59.09%, H: 4.34%, N: 8.61%.
Recovery and Reuse of Co-Salen Complex
The filtrate (from “General procedure for the M-Salen
complex catalyzed oxidation of benzoin”) was
extracted three times by CH2Cl2 (3 × 10 mL) to
recover Co-Salen complex catalyst. Then, the solvent
in the extract liquid was removed by rotary evapora-
tion and the recovered Co-Salen complex was used for
the next run with the same procedure.
Ni-Salen complex, Cu-Salen complex, and Zn-
Salen complex were synthesized with the same proce-
dure as that of Co-Salen complex and the correspond-
ing used metal salts were Ni(CH3COO)2 · 4H2O,
Cu(CH3COO)2 · H2O and ZnCl2, respectively. The
procedure of synthesis of Fe-Salen was the same with
that of Co-Salen but under nitrogen atmosphere and
the FeSO4 was used as the ferrous ion source. The
yields of Fe-Salen complex, Ni-Salen complex, Cu-
Salen complex, and Zn-Salen complex were 73, 78,
62, and 83%, respectively. Fe-Salen complex:
MS(+ESI), m/z (%) = 322 (100) (M + H)+. Element
analysis value, C: 57.42%, H: 3.64%, N: 7.62% and
Calculated value, C: 59.66%, H: 4.35%, N: 8.70%.
Ni-Salen complex: MS(+ESI), m/z (%) = 325 (100)
(M + H)+. Element analysis value, C: 58.22%, H:
3.74%, N: 8.09% and Calculated value: C: 59.13%; H:
4.31%; N: 8.62%. Cu-Salen: MS (+ESI), m/z (%) =
330 (100) (M + H)+. Element analysis: C: 57.27%; H:
3.66%; N: 7.85% and Calculated value, 58.26%, H:
4.25%, N: 8.50%. Zn-Salen complex: MS (+ESI),
m/z (%) = 331 (100) (M + H)+. Element analysis
value, C: 56.36%, H: 4.04%, N: 7.33% and calculated
value, C: 57.94%, H: 4.25%, N: 8.45%.
Characterizations
1H NMR spectra were recorded in CDCl3 (Bruker,
AVANCE III 400 MHz, Switzerland) and the proton
chemical shifts are reported in ppm relative to TMS as
the internal reference. Mass spectrometry was carried
out on LC-MS (Thermo Fisher Scientific, LCQ
Fleet). Product purity analysis was performed on High
Performance Liquid Chromatography (HPLC) (1200
Series, Agilent, USA). Element analysis was per-
formed on element analyzer (EA3000, URO Vector,
Italy). The analysis of the reaction mixture was per-
formed on an Agilent 6890/5975 GC/MS instrument
(Agilent, USA) with a programmable split/splitless
injector.
RESULTS AND DISCUSSION
Catalytic Performance of M-Salen Complexes
Benzil is one of the important organic intermedi-
ates, and has received enormous attention because of
its practical application in organic and pharmaceutical
industry. The benzil could be synthesized readily by
the oxidation of benzoin and numerous oxidizing
regents have been developed for the oxidation of ben-
zoin [15–18]. Among the oxidizing reagents, air was
no doubt the most green and costless oxidizing
reagent. Herein, M-Salen complexes were synthesized
General Procedure for the M-Salen Complex
Catalyzed Oxidation of Bezoin
Benzoin (2.16 g, 12.5 mmol) and N,N-dimethyl- to catalyze the oxidation of benzoin to benzil with air
formamide (DMF) (12.0 mL) were added into the as the oxidizing reagent. Compared with the control
three necked flask equipped with condenser, ther- sample without the air flow into the solution, the ben-
mometer and air conduit. After dissolution of benzoin, zil yield was obviously increased when the air was
M-Salen complex (0.6 mmol) and 0.4 g NaOH forced into the reaction solution with rate of
(10.0 mmol) were added to the solution and the solu- 60 mL/min (Table 1).
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LINJUN SHAO et al.
Table 1. Effect of air flow on the Co-Salen complex cata-
O
C
OH
O
C
O
C
lyzed oxidation reaction of benzoina
Air flow, mL/min
M-Salen
Air
C
H
Yield of benzil, %
The oxidation reaction of benzoin to benzil.
The effect of temperature on the Co-Salen com-
0
60
27
78
plex catalyzed oxidation of benzoin was examined
(Fig. 1). With Co-Salen complex as the catalyst, the
yield was below 60% when the temperature was less
than 65°C. The yield increased to 78% when the reac-
tion temperature increased to 80°C. However, the
yield dropped at higher temperature (90°C). Thus,
80°C was the best temperature for the oxidation of
benzoin to synthesize benzil.
a
Reaction condition: benzoin: 10.0 mmol, DMF: 12.0 mL, Co-
Salen complex: 0.60 mmol, NaOH: 10.0 mmol, reaction tem-
perature: 80°C, reaction time: 60 min.
Table 2. Effect of base and solvent on the Co-Salen complex
catalyzed oxidation reaction of benzoina
Yield
of benzil, %
Entry
Base
Solvent
The effects of base and solvent were also investi-
gated (Table 2). Examination of entry 1–5 in Table 2
showed that the potassium salts was more effective
than the corresponding sodium salts at promoting the
Co-Salen catalyzed oxidation of benzoin due to the
stronger basicity of potassium salts. For the same rea-
son, the potassium hydroxide was more effective than
the corresponding potassium carbonate, which in turn
was better than the corresponding potassium phos-
phate at promoting the oxidation of benzoin (Entry 1–3,
in Table 2). As a result, the potassium hydroxide was
the most effective base to approve the oxidation reac-
tion among the examined bases. As solvent also played
an important role in the reaction, the effect of solvent
was also examined (Entry 1, 6–10 in Table 2). Exam-
ination of entry 6 in Table 2 showed that there was no
product with THF as the solvent. When the reaction
was carried out in CH3CN, the yield was 71%, which
was still lower than that in DMF (Entry 7 in Table 2).
As H2O was known as green and costless solvent, we
also had tried the reaction in aqueous solution, but no
product was isolated (Entry 8 in Table 2). It was found
that the reaction in aqueous solution could be pro-
moted after addition of ethanol (Entry 9 in Table 2).
The yield of benzil reached 76% when ethanol was
used as the solvent (Entry 10 in Table 2). As ethanol
was more green and costless than DMF, ethanol was
chosen for the following experiment.
The catalytic activities of Fe-Salen, Co-Salen, Ni-
Salen, Cu-Salen and Zn-Salen complexes were com-
pared under the optimized reaction condition (Fig. 2).
The catalysis results showed that Fe-Salen and Co-
Salen complexes were very efficient to catalyze the
reaction with yields over 80% while the catalytic activ-
ities of Ni-Salen, Cu-Salen and Zn-Salen complexes
were comparably low. Interestingly, with Co-Salen as
the catalyst, long reaction time (>60 min) and high
reaction temperature (>80°C) would decrease the
benzil yield (Figs. 1 and 2). GC–MS characterization
of the reaction mixtures showed that over oxidation
product of benzoic acid was formed. It has been
reported that the benzil could be further oxidized into
benzoic acid with H2O2 as the oxidizing reagent [19–
21]. Herein, with Co-Salen as the catalyst, the benzil
would be also oxidized into benzoic acid with air as the
1
2
KOH
DMF
78
57
~0
44
~0
~0
71
K2CO3 DMF
K3PO4 DMF
NaOH DMF
Na2CO3 DMF
3
4
5
6
7
KOH
KOH
KOH
KOH
KOH
THF
CH3CN
8
H2O
~0
69
83
9
EtOH–H2O (1 : 1)
EtOH
10
a
Reaction condition: benzoin: 10.0 mmol, solvent: 12.0 mL, Co-
Salen complex: 0.60 mmol, base: 10.0 mmol, air flow rate:
60 mL/min, reaction temperature: 80°C, reaction time: 60 min.
oxidizing reagent under high reaction temperature
(>80°C) and long reaction time (>60 min).
Reuse of catalyst could greatly increase the overall
product and the cost effectiveness while minimize
85
80
75
70
65
60
55
50
30
40
50
60
70
80
90
Temperature, °C
Fig. 1. The effect of temperature on Co-Salen complex
catalyzed oxidation of benzoin. Reaction condition: ben-
zoin 2.16 g, DMF 12.0 mL, Co-Salen complex 0.20 g,
NaOH 0.40 g, air flow rate 60 mL/min, reaction time 60 min.
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AIR OXIDATION OF BENZOIN CATALYZED
761
As shown in Fig. 3, the residue Co-Salen complex
could be reused three times with satisfied yields. How-
ever, the yield of benzil dropped significantly at the
fourth run. Analysis of the reaction mixture of the
fourth run showed that only 32% of initial Co-Salen
complex was remained in the reaction mixture. So, the
decrement of benzil yield was mainly attributed to the
loss of Co-Salen complex catalyst in the reuse proce-
dure.
The effect of catalyst amount on the oxidation of
benzoin was also investigated. It was found that the
yield for the oxidation of benzoin gradually increased
with catalyst loading. Good yield of benzil could be
obtained even with 0.45 mmol Co-Salen. This result
demonstrates that the Co-Salen was efficient to cata-
lyze the oxidation of benzoin.
90
80
70
60
50
40
30
20
1
2
3
4
5
0
20
40
60
80
100 120
Time, min
Fig. 2. The comparison of catalytic activities of different
complexes: (1) Fe-Salen, (2) Co-Salen, (3) Ni-Salen,
(4) Cu-Salen, (5) Zn-Salen. Reaction condition: benzoin
10.0 mmol, ethanol 12.0 mL, M-Salen complex
0.60 mmol, NaOH 10.0 mmol, air flow rate 60 mL/min,
CONCLUSION
o
In summary, five M-Salen complexes were synthe-
sized and their catalytic activities towards the oxida-
tion of benzoin to benzil were examined with air as the
oxidizing reagent. The Fe-Salen and Co-Salen com-
plexes were active to catalyze the reaction with yields
over 80%. Moreover, the catalysis system could be
reused three times with little loss of initial catalytic
activity.
reaction temperature 80 C, reaction time 60 min.
80
70
60
50
40
30
20
10
0
ACKNOWLEDGMENT
This work was supported by the National Natural
Science Foundation of China (no. 11705117).
REFERENCES
1
2
3
4
5
1. Lyons, C.T. and Stack, T.D.P., Coord. Chem. Rev.,
Run times
2013, vol. 257, p. 528.
Fig. 3. The reuse of Co-Salen complex catalyst. Reaction con-
dition: benzoin 10.0 mmol, ethanol 12.0 mL, Со-Salen com-
plex 0.60 mmol, NaOH 10.0 mmol, air flow rate 60 mL/min,
reaction temperature 80°C, reaction time 60 min.
2. Luca, O.R. and Crabtree, R.H., Chem. Soc. Rev., 2013,
vol. 42, p. 1440.
3. Chiang, L., Herasymchuk, K., Thomas, F., and Storr, T.,
Inorg. Chem., 2015, vol. 54, p. 5970.
4. Luts, T. and Papp, H., Kinet. Catal., 2007, vol. 48,
waste generation. We have recovered the Co-Salen
complex by extracting it from the filtrate. The recov-
ered Co-Salen complex was then used for the next run.
p. 176.
5. Xiong, D.L., Fu, Z.H., Zhong, S., Jiang, X.F., and
Yin, D.L., Catal. Lett., 2007, p. 155.
6. Adão, P., Barroso, S., Avecilla, F., Oliverira, M.C., and
Pessoa, J.C., J. Organomet. Chem., 2014, vol. 760,
p. 212.
Table 3. Dependence of benzil yield with the amount of Co-
Salen complexa
7. Jiang, C.G., Kinet. Catal., 2011, vol. 52, p. 691.
Co-Salen
amount, mmol
Ratio of benzoin/
Co-Salen
8. Weerasiri, K.C. and Gorden, A.E.V., Tetrahedron,
Yield, %
2014, vol. 70, p. 7962.
9. Szajna-Fuller, E., Huang, Y.L., Rapp, J.L., Chaka, G.,
Lin, V.S.Y., Pruski, M., and Bakac, A., Dalton Trans.,
2009, vol. 2009, p. 3237.
0.30
0.45
0.60
33.33
22.22
16.67
59
82
83
10. Chem, M., Pan, Y., Kwong, H.K., Zeng, R.J., Lau, K.C.,
and Lau, T.C., Chem. Commun., 2015, vol. 51, p. 13686.
a
Reaction condition: benzoin: 10.0 mmol, ethanol: 12.0 mL,
11. Holbach, M. and Weck, M., Org. Chem., 2006, vol. 71,
NaOH: 10.0 mmol, air flow rate: 60 mL/min, reaction tempera-
p. 1825.
ture: 80°C, reaction time: 60 min.
KINETICS AND CATALYSIS Vol. 58 No. 6 2017

762
LINJUN SHAO et al.
12. Liu, Z.J., Fu, X.K., Hu, X.Y., and Miao, Q., J. 17. Shaikh, M., Sahu, M., Gavel, P.K., and Ranga-
Organomet. Chem., 2012, vol. 713, p. 157.
nath, K.V.S., Catal. Commun., 2015, vol. 64, p. 18.
13. Zhao, Q.S., Bai, C., Zhang, W.F., Li, Y., Zhang, G.L.,
Zhang, F.B., and Fan, X.B., Ind. Eng. Chem. Res.,
2014, vol. 53, p. 4232.
18. Borthakur, R., Asthana, M., Saha, M., Kumar, A., and
Pal, A.K., RSC Adv., 2014, vol. 4, p. 21638.
19. Imanzadeh, G.H., Zamanloo, M.R., Mansoori, Y.,
and Khodayari, A., Chinese J. Chem., 2007, vol. 25,
p. 836.
20. Maurya, M.R., Chaudhary, N., Kumar, A., Avecilla, F.,
and Pessoa, J.C., Inorg. Chim. Acta, 2014, vol. 420,
p. 24.
14. Bianchini, G. Cavarzan, A., Scarso, A., and Strukul, G.,
Green Chem., 2009, vol. 11, p. 1517.
15. Morales-Serna, J.A., Frontana-Uribe, B.A., Olguín, R.,
Gómez-Vidales, V., Lomas-Romero, L., Garcia-Ríos, E.,
Gaviño, R., and Cárdenas, J., RSC Adv., 2016, vol. 6,
p. 42613.
16. Ruan, L., Shi, M., Li, N., Ding, X., Yang, F., and Tang, J., 21. Maurya, M.R., Arya, A., Kumar, A., Avecilla, F., and
Org. Lett., 2014, vol. 16, p. 733. Pessoa, J.C., Dalton Trans., 2009, vol. 2009, p. 2185.
KINETICS AND CATALYSIS
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No. 6
2017