Efficient and selective oxidation of alcohols with tert-BuOOH catalyzed by a dioxomolybdenum(VI) Schiff base complex under organic solvent-free conditions

Article abstract of DOI:10.1007/s11164-016-2531-x

The catalytic activity of dioxidobis{2-[(E)-p-tolyliminomethyl]phenolato}molybdenum(VI) complex was studied, for the first time, in the selective oxidation of various primary and secondary alcohols using tert-BuOOH as oxidant under organic solvent-free co

Full text of DOI:10.1007/s11164-016-2531-x

Res Chem Intermed
DOI 10.1007/s11164-016-2531-x
Efficient and selective oxidation of alcohols with tert-
BuOOH catalyzed by a dioxomolybdenum(VI) Schiff
base complex under organic solvent-free conditions
Mehdi Hatefi-Ardakani1 Samira Saeednia1
•
•
Zahra Pakdin-Parizi1 Mahdieh Rafeezadeh1
•
Received: 5 December 2015 / Accepted: 11 March 2016
Ó Springer Science+Business Media Dordrecht 2016
Abstract The catalytic activity of dioxidobis{2-[(E)-p-tolyliminomethyl]pheno-
lato}molybdenum(VI) complex was studied, for the first time, in the selective
oxidation of various primary and secondary alcohols using tert-BuOOH as oxidant
under organic solvent-free conditions at room temperature. The effect of different
solvents was studied in the oxidation of benzyl alcohol in this catalytic system. It
was found that, under organic solvent-free conditions, the catalyst oxidized various
primary and secondary alcohols to their corresponding aldehyde or ketone deriva-
tives with high yield. The effects of other parameters such as oxidant and amount of
catalyst were also investigated. Among different oxidants such as H2O2, NaIO4,
tert-BuOOH, and H2O2/urea, tert-BuOOH was selected as oxygen donor in the
oxidation of benzyl alcohol. Also, it was found that oxidation of benzyl alcohol
required 0.02 mmol catalyst for completion. Dioxomolybdenum(VI) Schiff base
complex exhibited good catalytic activity in the oxidation of alcohols with tert-
BuOOH under mild conditions. In this catalytic system, different primary alcohols
gave the corresponding aldehydes in good yields without further oxidation to car-
boxylic acids.
Keywords Schiff base Á Dioxomolybdenum(VI) complex Á Alcohol Á Catalytic
oxidation Á TBHP Á Solvent-free
Electronic supplementary material The online version of this article (doi:10.1007/s11164-016-2531-x)
contains supplementary material, which is available to authorized users.
&
Mehdi Hatefi-Ardakani
m_hatefi_chem@yahoo.com; m.hatefi@vru.ac.ir
1
Department of Chemistry, Faculty of Science, Vali-e-Asr University of Rafsanjan,
Rafsanjan 77188-97111, Iran
123

M. Hatefi-Ardakani et al.
Introduction
Aldehydes and ketones represent an important class of products and intermediates in
the fine chemicals and specialties industry [1]. Therefore, selective oxidation of
alcohols to their corresponding aldehydes and ketones is one of the most important
fundamental reactions in organic chemistry at both laboratory and industrial level,
and numerous methods have been developed using a variety of reagents for this
purpose [2–8]. In traditional oxidation processes, large amounts of toxic and volatile
organic solvents and inorganic oxidants (i.e. CrO3, KMnO4, NaClO, MnO2, etc.)
were extensively used [9]. However, these oxidants are toxic, hazardous to handle,
expensive, and lack selectivity, and large amounts of waste are generated [10]. From
the environmental friendliness point of view, development of clean and green
oxidants such as TBHP, H2O2 or O2 has attracted increasing attention [11, 12]. In
recent years, solvent-free organic reactions have attracted great interest because of
their various advantages such as high efficiency and selectivity, easy separation and
purification, mild reaction conditions, reduction in waste, simplicity in progress and
handling, and benefits to the industry as well as the environment [13–15].
The ability of molybdenum to form stable complexes with Schiff base ligands led
to the development of molybdenum Schiff base complexes which are efficient
catalysts in both homogeneous and heterogeneous reactions [16–19]. In this area,
dioxomolybdenum(VI) complexes have been particularly investigated because of
their good catalytic activity for selective oxidation [20]. Recently, several
dioxomolybdenum(VI) complexes have been used as catalysts in various oxidation
reactions, such as epoxidation of olefins and oxidation of sulfides [21–25].
However, studies on oxidation of alcohols with dioxomolybdenum(VI) complexes
as catalysts are still very limited [26].
In the present work, selective oxidation of alcohols to aldehydes and ketones with
tert-BuOOH (TBHP) in the presence of a dioxomolybdenum(VI) Schiff base
complex as catalyst under organic solvent-free conditions is reported (Scheme 1). In
this catalytic system, the effect of different solvents, oxidants, and amount of
catalyst was also studied.
Experimental
Materials and preparation of catalyst
All materials were of commercial reagent grade and obtained from Merck or Fluka.
The Schiff base ligand 2-(p-tolyliminomethyl)phenol was prepared by the standard
procedure of refluxing ethanolic solution of salicylaldehyde and p-tolylamine in 1:1
molar ratio. The dioxo-Mo(VI) Schiff base complex was prepared according to the
published procedure by adding the Schiff base ligand and MoO2(acac)2 (molar ratio
2:1) to dry methanol (25 ml), followed by refluxing for 1 h [27]. Light-yellow
crystals of the complex were filtered off and recrystallized from absolute methanol
123

Efficient and selective oxidation of alcohols with tert…
Scheme 1 Selective oxidation of alcohols by dioxo-Mo(VI) Schiff base complex
with yield of 76 % (0.57 g). Anal. calcd. for C28H24MoN2O4 (%): C, 61.32; H, 4.41;
N, 5.11. Found: C, 61.10; H, 4.15; N, 4.94.
Physical measurements
Fourier-transform infrared (FT-IR) spectrum (KBr pellets) was obtained with a
Thermo SCIENTIFIC model NICOLET iS10 spectrophotometer in the range of
400–4000 cm-1. The ultraviolet–visible (UV–Vis) spectrum of the complex was
recorded in methanol solution using a T80 UV–Vis spectrophotometer. Elemental
analysis was carried out on a Vario EL III CHNS elemental analyzer. Single-crystal
X-ray data for the complex were collected at 296 K on a STOE IPDS II
˚
diffractometer with Mo Ka (k = 0.71073 A) radiation. The purity of the products
and the progress of reaction were measured by thin-layer chromatography (TLC)
using silica-gel Polygram SILG/UV254 plates. Gas chromatography (GC) exper-
iments were performed with a Philips GC-PU 4600 instrument using 2 m column
packed with silicon DC-200 with a flame ionization detector (FID).
General procedure for selective oxidation of alcohols
A typical experimental procedure was as follows: alcohol (1 mmol) as substrate was
added to the dioxo-Mo(VI) Schiff base complex (0.02 mmol). Then, tert-BuOOH
(1 mmol, 70 % aqueous solution) was added and the mixture was stirred at room
temperature (25 °C). The progress of the reaction was monitored by thin-layer
chromatography (TLC) (eluent n-hexane:ethyl acetate, various ratios). After the
reaction was completed, the residue was washed with a minimal volume of CH2Cl2.
123

M. Hatefi-Ardakani et al.
The solvent was then evaporated, and the pure product was obtained after
chromatography on a silica gel column.
Results and discussion
Characterization of catalyst
In the FT-IR spectrum of the dioxo-Mo(VI) Schiff base complex (Fig. S1), the
azomethene (C=N) band was observed at 1604 cm-1 and the band at 1500 cm-1
can be attributed to the aromatic rings in the Schiff base ligand. Also, the presence
of two bands at 906 and 956 cm-1 is due to Mo=O vibrations.
In the UV–Vis spectrum of the dioxo-Mo(VI) Schiff base complex (Fig. S2),
peaks at 230 and 270 nm are assigned to benzene p ? p* and imine p ? p*
transitions, respectively. The third peak at 340 nm is attributed to the O(pp) ?
Mo(dp) charge-transfer transition [28, 29].
The crystal structure of the dioxo-Mo(VI) Schiff base complex was also
confirmed by single-crystal X-ray diffraction (CCDC 792575) [27].
Catalytic oxidation of alcohols
To explore the catalytic activity of the dioxo-Mo(VI) Schiff base complex, benzyl
alcohol was used as model substrate. First, different solvents were examined in the
presence of dioxo-Mo(VI) Schiff base complex and tert-BuOOH (70 % aqueous
solution) (Table 1). The results showed that use of a less-polar solvent such as
toluene or chloroform afforded high yields of benzaldehyde (entries 1, 2), but lower
yield of benzaldehyde was obtained using polar solvents such as methanol, acetone,
and acetonitrile (entries 3–5). Interestingly, it was found that the highest yield of
benzaldehyde was obtained under organic solvent-free conditions (entry 6).
Table 1 Effect of solvent on oxidation of benzyl alcohol with tert-BuOOH catalyzed by dioxo-Mo(VI)
Schiff base complex
Entry Solvent
Dipole
Time Alcohol
Benzaldehyde Benzaldehyde TON TOF
(h-1
moment (h)
(D)
conversion selectivity (%) yield (%)a
(%)
)
1
2
3
4
5
6
Toluene
CHCl3
0.36
1.15
1.69
2
2
2
2
2
2
73
55
47
18
12
92
96
96
94
83
83
98
70
53
45
15
10
90
35
17.5
26.5 13.25
22.5 11.25
CH3OH
CH3COCH3 2.88
7.5
5
3.75
2.5
CH3CN
3.92
–
Solvent free
45
22.5
Reaction conditions: benzyl alcohol (1 mmol), tert-BuOOH (1 mmol, 70 % aqueous solution), catalyst
(0.02 mmol), solvent (1 ml), at room temperature (25 °C)
a
Isolated yield
123

Efficient and selective oxidation of alcohols with tert…
Table 2 Optimization of catalyst amount in oxidation of benzyl alcohol with tert-BuOOH catalyzed by
dioxo-Mo(VI) Schiff base complex under organic solvent-free conditions
Entry Catalyst amount Time Alcohol
(mmol)
Benzaldehyde
selectivity (%)
Benzaldehyde
yield (%)a
TON TOF
(h-1
)
(h)
conversion
(%)
1
2
3
4
0
2
2
2
2
8
75
92
93
62
86
98
98
5
65
90
91
–
–
0.01
0.02
0.03
65
45
32.5
22.5
30.5 15.15
Reaction conditions: benzyl alcohol (1 mmol), tert-BuOOH (1 mmol, 70 % aqueous solution), catalyst,
at room temperature (25 °C)
a
Isolated yield
Table 3 Effect of various oxidants on oxidation of benzyl alcohol with tert-BuOOH catalyzed by dioxo-
Mo(VI) Schiff base complex under organic solvent-free conditions
Entry Oxidant
Time Alcohol
(h)
Benzaldehyde Benzaldehyde TON TOF
(h-1
)
conversion selectivity (%) yield (%)a
(%)
1
2
3
4
NaIO4
2
2
2
2
35
50
78
92
86
96
96
98
30
48
75
90
15
24
7.5
12
H2O2
H2O2/urea (UHP)
37.5 18.75
tert-BuOOH (TBHP)
(70 % aqueous
solution)
45
22.5
5
No oxidant
2
12
83
10
5
2.5
Reaction conditions: benzyl alcohol (1 mmol), oxidant (1 mmol), catalyst (0.02 mmol), at room tem-
perature (25 °C)
a
Isolated yield
The effect of reaction parameters was examined by performing the reaction in
organic solvent-free conditions. We optimized the amount of catalyst in the
oxidation of benzyl alcohol. The results showed that, when 0.02 mmol (11 mg)
catalyst was used in the oxidation of benzyl alcohol, the highest yield of
benzaldehyde was obtained, whereas trace amount of products (5 %) were detected
when the same reaction was carried out in the absence of catalyst (Table 2).
The effect of different oxidants such as NaIO4, H2O2, H2O2/urea (UHP), and
tert-BuOOH (70 % aqueous solution) was also investigated in the oxidation of
benzyl alcohol. The results showed that tert-BuOOH (TBHP) is the best oxygen
source (Table 3).
Under the optimized reaction conditions, different primary and secondary alcohols
were oxidized by this catalytic system. The results are summarized in Table 4.
It was found that various types of primary benzylic alcohols including those
bearing electron-withdrawing and electron-donating groups were selectively
converted to their corresponding aromatic aldehydes in good to high yields. In
comparison with the electron-withdrawing groups at para-position for benzylic
123

M. Hatefi-Ardakani et al.
Table 4 Selective oxidation of alcohols with tert-BuOOH catalyzed by dioxo-Mo(VI) Schiff base
complex under organic solvent-free conditionsa
Entry
Alcohol
Productb
Time Alcohol
Selectivity (%) Yield (%)c
TON
TOF
(h-1
(h)
conversion (%)
)
1
2
92
98
95
98
97
97
90
95
92
45
22.5
23.75
23
2
3
2
2
47.5
46
4
5
2
2
90
78
94
96
85
75
42.5
37.5
21.25
18.75
6
7
2
3
75
78
93
70
35
39
17.5
13
100
78d
8
3
82
100
82d
41
13.67
9
3
3
75
70
100
100
75d
70d
37.5
35
12.5
10
11.67
Reaction conditions: alcohol (1 mmol), tert-BuOOH (1 mmol, 70 % aqueous solution), catalyst
(0.02 mmol), at room temperature (25 °C)
a
All products identified by comparison of their physical and spectral data with those of authentic samples
b
Isolated yield of corresponding aldehyde or ketone
c
Determined by GC
alcohols, the electron-donating groups seemed to be more favorable for the
formation of carbonyl compounds (entries 1–5, Table 4). Furthermore, furfuryl
alcohol, a heteroaromatic alcohol which is considered to be a highly challenging
substrate in most transition-metal catalyst systems, was oxidized to furfural in good
yield in this catalytic system (entry 6, Table 4). As shown in Table 4, this system
was found to be efficient for the oxidation of secondary alcohols. In this oxidation
reaction the corresponding ketones were obtained in relatively good yields
123

Efficient and selective oxidation of alcohols with tert…
Scheme 2 Proposed mechanism for oxidation of benzyl alcohol with TBHP catalyzed by
dioxomolybdenum(VI) Schiff base complex
(entries 7–9, Table 4). The catalytic oxidation can also be successfully performed
with sterically hindered alcohols such as diphenylmethanol (entry 10, Table 4).
Based on experimental results and related literature [30–34], a plausible mechanism
for oxidation of benzyl alcohol with tert-BuOOH (70 % aqueous solution) catalyzed
by dioxomolybdenum(VI) Schiff base complex is proposed in Scheme 2.
The first step of the catalytic cycle involves transfer of a TBHP proton to one of
the terminal oxygen atoms of MoO2 group and coordination of tert-BuOO- to the
Lewis-acidic metal center, leading to the formation of the peroxo intermediate in the
first step. Then, the peroxo intermediate species oxidizes benzyl alcohol to release
one tert-BuOH molecule and give an intermediate that leads to benzyl alcohol and
H2O and regenerates the dioxomolybdenum(VI) Schiff base complex.
Conclusions
The catalytic activity of a dioxomolybdenum(VI) Schiff base complex in the
selective oxidation of alcohols to aldehydes and ketones with tert-BuOOH as
oxidant under solvent-free conditions is explored. Easy preparation, mild reaction
123

M. Hatefi-Ardakani et al.
condition, good to high yields of products, and ecofriendliness can be mentioned as
advantages of this method for oxidizing alcohols.
Acknowledgments Support of this work by Vali-e-Asr University of Rafsanjan is acknowledged. The
authors also thank Prof. Majid Moghadam from University of Isfahan for his valuable help.
References
¨
1. A. Kockritz, M. Sebek, A. Dittmar, J. Radnik, A. Bruckner, U. Bentrup, M.M. Pohl, H. Hugl, W.
¨
¨
Magerlein, J. Mol. Catal. A 246, 85 (2006)
2. S. Zhang, L. Xu, M.L. Trudell, Synthesis 11, 1757 (2005)
3. J.N. Moorthy, N. Singhal, P. Venkatakrishnan, Tetrahedron Lett. 45, 5419 (2004)
4. J.D. Lou, Z.N. Xu, Tetrahedron Lett. 43, 8843 (2002)
5. L. Xu, M.L. Trudell, Tetrahedron Lett. 44, 2553 (2003)
6. Y.Z. Hou, X.T. Ji, G. Liu, J.Y. Tang, J. Zheng, Y. Liu, W.X. Zhang, M.J. Jia, Catal. Commun. 10,
1459 (2009)
7. T. Orbegozo, I. Lavandera, W.M.F. Fabian, B. Mautner, J.G. de Vries, W. Kroutil, Tetrahedron 65,
6805 (2009)
8. A.R. Hajipour, I. Mohammadpoor-Baltork, G. Kianfar, Bull. Chem. Soc. Jpn. 71, 2655 (1998)
9. S.V. Ley, A. Madin, in Comprehensive Organic Synthesis, ed. by B.M. Trost, I. Fleming (Pergamon,
Oxford, 1991), p. 251
10. M. Beller, Adv. Synth. Catal. 346, 107 (2004)
11. X. Li, R. Cao, Q. Lin, Catal. Commun. 69, 5 (2015)
12. W. Wang, T. Vanderbeeken, D. Agustin, R. Poli, Catal. Commun. 63, 26 (2015)
13. T. Kaicharla, S.R. Yetra, T. Roy, A.T. Biju, Green Chem. 15, 1608 (2013)
14. L.Y. Zhu, Z. Lou, J. Lin, W. Zheng, C. Zhang, J.D. Lou, Res. Chem. Intermed. 39, 4287 (2013)
15. J. Kamalraja, P.T. Perumal, Tetrahedron Lett. 55, 3561 (2014)
16. M.M. Farahani, F. Farzaneh, M. Ghandi, Catal. Commun. 8, 6 (2007)
17. M.M. Farahani, F. Farzaneh, M. Ghandi, J. Mol. Catal. A: Chem. 248, 53 (2006)
18. K. Ambroziak, R. Pelech, E. Milchert, T. Dziembowska, Z. Rozwadowski, J. Mol. Catal. A: Chem.
211, 9 (2004)
19. S.N. Rao, A.K. Kathale, N.N. Rao, K.N. Munshi, Inorg. Chim. Acta 360, 4010 (2007)
20. F.E. Kuhn, A.D. Lopez, A.M. Santos, E. Herdweck, J.J. Haider, C.C. Romao, A.G. Santos, J. Mol.
Catal. A: Chem. 151, 147 (2000)
21. M. Bagherzadeh, M. Amini, H. Parastar, M. Jalili-Heravi, A. Ellern, L.K. Woo, Inorg. Chem.
Commun. 20, 86 (2012)
22. M. Bagherzadeh, M.M. Haghdoost, A. Ghanbarpour, M. Amini, H.R. Khavasi, E. Payab, A. Ellern,
L.K. Woo, Inorg. Chim. Acta 411, 61 (2014)
23. Y.D. Li, X.K. Fu, B.W. Gong, X.C. Zou, X.B. Tu, J.X. Chen, J. Mol. Catal. A: Chem. 322, 55 (2010)
24. Y. Yang, Y. Zhang, S.J. Hao, Q.B. Kan, J. Colloid Interface Sci. 362, 157 (2001)
25. S.M. Bruno, J.A. Fernandes, L.S. Martins, I.S. Goncalves, M. Pillinger, P. Ribeiro-Claro, J. Rocha,
A.A. Valente, Catal. Today 114, 263 (2006)
26. C.Y. Lorber, S.P. Smidt, J.A. Osborn, Eur. J. Inorg. Chem. 4, 655 (2000)
27. M. Hatefi, I. Sheikhshoaie, V. Mirkhani, M. Moghadam, R. Kia, Acta Cryst. E 66, 1137 (2010)
28. L.S. Gonzalez, K.S. Nagaraja, Polyhedron 6, 1635 (1987)
29. B. Gao, M. Wan, J. Men, Y. Zhang, Appl. Catal. A 439–440, 156 (2012)
30. M. Bagherzadeh, L. Tahsini, R. Latifi, K. Woo, Inorg. Chim. Acta 362, 3698 (2009)
31. J. Hu, Y. Zou, J. Liu, J. Sun, X. Yang, Q. Kan, J. Guan, Res. Chem. Intermed. 41, 5703 (2015)
32. W.R. Thiel, J. Eppinger, Chem. Eur. J. 3, 696 (1997)
33. S.M. Bruno, C.C.L. Pereira, M.S. Balula, M. Nolasco, A.A. Valente, A. Hazell, M. Pillinger, P.R.
Claro, I.S. Goncalves, J. Mol. Catal. A: Chem. 261, 79 (2007)
34. M. Groarke, I.S. Goncalves, W.A. Herrmann, F.E. Kuhn, J. Organomet. Chem. 649, 108 (2002)
123