Ferrites catalyzed aerobic oxidation of benzoin and its extension to enantioselective version

Article abstract of DOI:10.1016/j.catcom.2015.01.019

Ferrites have been used as efficient catalysts for the oxidation of α-hydroxy ketones in the presence of air. The enantio selective version of the benzoin oxidation gave 30% yield with 43% ee using (L)-tartaric acid as chiral modifier on magnetite (inverse spinel).

Full text of DOI:10.1016/j.catcom.2015.01.019

Ferrites Catalyzed Aerobic Oxidation of Benzoin and its Extension to Enan-
tioselective Version
Melad Shaikh, M. Sahu, Pramod K. Gavel, Kalluri V.S. Ranganath
PII:
DOI:
Reference:
S1566-7367(15)00027-8
doi: 10.1016/j.catcom.2015.01.019
CATCOM 4201
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
21 November 2014
19 January 2015
22 January 2015
Please cite this article as: Melad Shaikh, M. Sahu, Pramod K. Gavel, Kalluri V.S.
Ranganath, Ferrites Catalyzed Aerobic Oxidation of Benzoin and its Extension to Enan-
tioselective Version, Catalysis Communications (2015), doi: 10.1016/j.catcom.2015.01.019
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Ferrites Catalyzed Aerobic Oxidation of Benzoin and its Extension to
Enantioselective Version
Melad Shaikh, M. Sahu, Pramod K. Gavel and Kalluri V.S. Ranganath*
E-mail: rangakvs@gmail.com or kallurivsr@gmail.com
Guru Ghasidas University, Bilaspur, India-495009
Abstract: Ferrites have been used as efficient catalysts for the oxidation of α-
hydroxy ketones in the presence of air. The enantio selective version of the benzoin
oxidation gave 30% yield with 43% ee using (L)-tartaric acid as chiral modifier on
magnetite (inverse spinel)
Keywords: Ferrites, Heterogeneous, Modifier, Oxidation, Spinel

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An interesting family of mixed transition metal oxides, typically in a spinel structure
has recently attracted research interest. The ferrites (designated as MFe2O4, where
M= Fe, Zn, Co, Mg) with magnetic properties are useful in electronic devices,
magnetic fluids, and magnetic recording media and also in catalysis [1-3]. In
addition, ferrites are also used as heterogeneous catalysts or catalyst support for
organic reactions, adsorption and sensors [4-6]. Heterogeneous catalysis has been
around for a long time, but has still much to groom. Industry favors a heterogeneous
catalysis over homogeneous due to the advantage of recovery and recycling. The
oxidation of alcohols to ketones is an important organic transformation since the
resultant compounds are used in a variety of drugs, agro-chemicals and fragrances
[7-8]. The oxidation of benzoin to benzil is an important organic transformation since
benzils have been using as ligands, biologically active compounds and also as
photosensitive agent [9-11]. In view of significant drawbacks, such as toxic and
expensive reagents, high temperature, stiochiometric amount of reagents, and low
yield of these methods, more efficient catalytic system is highly demanded [12-15].
As a part of our interest in heterogeneous catalysis, we have been in hunt for
development of new catalytic system [16]. The spinels have been used successfully
in catalytic reactions; however this is our first attempt to utilize ferrites as catalyst in
asymmetric reaction.
Herein, we report the aerobic oxidation of benzoin to 1,2-diphenylethane-1,2-dione
in high yields in the presence of air using cobalt ferrite as a heterogeneous catalyst
(up to 94%) (Scheme 1). Initially various ferrites (MFe2O4, M= Fe, Zn, Mg and Co)
were evaluated in the oxidation of benzoin in the presence of air. The easy removal
of the catalyst from the reaction mixture makes the procedure very simple and eco-
friendly.

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Scheme 1 Aerobic oxidation of benzoin using cobalt ferrite at 80 C.
2. Experiment
2.1. Catalyst preparation
The magnetite was prepared by the so called wet-impregnation method as reported
in the literature [17]. The substituted ferrites were synthesized by co-precipitation
method depends mostly on parameters such as reaction temperature, pH of
suspension, initial molar concentration, etc. The ultra fine particles of Fe3O4 are
prepared by co-precipitating aqueous solutions of ferrous ammonium sulfate
(NH4)2Fe(SO4)2 and ferric chloride (FeCl3) mixture, respectively, in alkaline medium.
(NH4)2Fe(SO4)2 and FeCl3 solutions are mixed in their respective stoichiometry (i.e.,
ratio Fe2+: Fe3+ = 1:2). The mixture is kept at 80oC followed by the mixture is added
in 80.0 mL of water. To this boiling solution, NH4OH is added drop by drop till the
precipitation occurs under constant stirring. Magnetite formed by conversion of
metals into hydroxides, which takes place immediately, and transformation of
hydroxides into ferrites. The solution is maintained at 80 C for 2 h. The precipitate,
thus formed is then washed several times by distilled water. The precipitate is then
o
separated by the help of external magnet and is then kept for drying at 80 C.
Various ferrites were also synthesized using literature procedure [17].

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2.2. Base Treatment of the Fe3O4 Particles:
A 5 g sample of Fe3O4 particles was dispersed in 100 mL of NaOH solution (1.0
mol/L). . The base treatment of the Fe3O4 particles in a NaOH solution was
performed for 8 h. The particles were then washed repeatedly by deionized water
until the filtrate was neutral. The Fe3O4 particle cake was dried at 60 C for 20 h and
crushed into dispersed particles. Then, the base-treated Fe3O4 particles were ready
for catalytic application.
2. 3. Base Titration of the Fe3O4 Particle Suspension:
Base titrations were carried out in a flask equipped with a pH meter. A blank
experiment was carried out by titrating a NaOH solution (0.010 mol/L) with a mixture
of 5 mL of oxalic acid solution (0.010 mol/L) and 70 mL of deionized water to pH 10.
The base titrations for the Fe3O4 particles were performed as follow: (1) 50 mg of
Fe3O4 particles was dispersed into 70 mL of deionized water to form a suspension;
(2) 5 mL of oxalic acid solution (0.010 mol/L) was uniformly mixed into the
suspension; (3) NaOH solution (0.010 mol/L) was titrated into the suspension until
pH 10.
2.4. Catalytic Reaction:
An oven dried flask was charged with respective ferrites (0.030 g), benzoin (0.127 g,
0.6 m mol). To the flask toluene (3.0 mL) was added and heated to 80 C with
magnetic stirring. After completion of the reaction (monitored by TLC and UV-Visible
spectroscopy), the reaction mixture was cooled to rt, and catalyst was magnetically
removed and washed several times with toluene. The reaction mixture was
monitored by using TLC with eluent of 9:1 Hexane: EtOAC

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2.5. Characterization: The ferrites have been characterized by using X-ray Photo
Electron Spectroscopy (XPS), Scanning Electron Microscopy Energy Dispersive X-
ray analysis (SEM-EDX) and Thermo Gravimetric Analysis (TGA).
3. Results and Discussion
Initially catalytic application of benzoin was evaluated using inverse spinel like
magnetite, Fe3O4. We found that magnetite performed as a versatile catalyst in the
oxidation reaction of benzoin, furnishing the corresponding product in 90 % yield in
15 h. The magnetite has an inverse cubic spinel structure; it possesses both Fe2+
and Fe3+ ions. The activity and selectivity of the oxide catalysts depends upon the
oxidation state of the metal ions and their co-ordination in the lattice, the surface
cation-oxygen bond strength, the extent of total oxygen (both surface and total) and
morphology of the material.
Various
ferrites
possessing
magnetic
properties
like
paramagnetic,
antiferromagnetic, ferromagnetic and electromagnetic materials such as iron, zinc,
magnesium, nickel and cobalt ferrites have been used in the oxidation of benzoin.
The rate of reaction with zinc and nickel ferrites was almost equal to that of
magnetite (Table 1, entry 3 & 4) and low with magnesium ferrite (Table 1, entry 5). It
was observed that cobalt ferrite which possesses electromagnetic property, the rate
of reaction is quite high (up to 94% in 6 h). The catalyst was recycled for three times
without loss of activity (Table 1, entry 5-8).

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Fig 1. XPS spectrum of a) Fresh magnetite b) (L)-TA modified magnetite.

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Table 1
Aerobic oxidation of benzoin in the presence of various ferritesa
Entry
Spinel
Time
(h)
Yield
(%)
Fe
(%) (%)
Metal
1
2
3
4
5
6
7
8
Fe3O4
15
90
90
90
82
94
92
95
93
69.1 -
ZnFe2O4
NiFe2O4
16
16
52.4 24.8
56.8 25.2
54.6 28.2
51.6 27.2
MgFe2O4 18
CoFe2O4 06
CoFe2O4 06
CoFe2O4 06
CoFe2O4 06
-
-
-
-
-
-
-
-
-
-
9
Fe3O4
16
6
88
90
10
CoFe2O4
a All the reactions were carried out in toluene (3.0 mL), metal oxide (30.00 mg)
Entry no: 6-8 are 1st, 2nd and 3rd cycles respectively, Entry no: 9 and 10 are with base
treated catalysts
It is known that there exist a number of -OH groups on oxide particle surfaces in
addition to acidic and basic catalytic sites [17 c]. The NaOH solution titrations for
characterizing the acid/base properties of the -OH groups on the Fe3O4 and
CoFe2O4 particle surfaces are shown in Figure 2. At a fixed pH, the difference in the
amount of titrated NaOH solution between the Fe3O4 samples and the blank reflects

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the amount of acid or base -OH groups on the particle surface at that pH, which also
reflects the activity of the -OH groups. Figure shows that the amounts of acid groups
on the Fe3O4 particles are more than CoFe2O4. Moreover, base treated Fe3O4 and
CoFe2O4 catalysts retain their activity as like fresh catalysts. This result clearly
indicates that –OH groups of magnetite did not have any effect on the catalytic
activity.
11
Blank
Nano Fe3O4
Nano CoFe2O4
10
9
8
7
6
5
4
3
0
2
4
6
8
10
12
14
16
18
Volume of NaOH ( ml )
Fig.2. NaOH titration curves of Fe3O4, CoFe2O4 and blank solution

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Table 2
Effect of various ligands on the oxidation of benzoin to benzil on magnetite
Entry Ligand
Yield (%)
90
1
2
3
-
1,2-diaminoethane 61
1-methyl-3-butyl
imidazolium
94
4
5
6
(R)-BINOL
42
Quinidine
Trace
72
N-methylated
quinidine
7
(L)-tartaric acid
96
All the reactions were carried out in toluene (3.0 mL), magnetite (30.00
mg), ligand (10 mol%)
Kinetic resolution of benzoin is one of the important techniques to get enantiopure
benzoins [18]. Enantiopure benzoins have been reported by using various catalysts
with high enantiomeric excess (ee) [19]. Modification of solid surfaces using chiral
agents is perhaps the most promising avenue for bestowing chirality onto
heterogeneous catalysts, and with those developing cheap and clean processes for
the synthesis of enantiopure compounds [20]
Effect of various ligands has been studied including chiral and achiral to
understand the structure and activity of magnetite. The reaction rate is decreasing in

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the presence of 1,2-diaminoethane and quinidine where as reaction rate is
increasing significantly in the presence of imidazolium salt and tartaric acid (Table 2,
entries 2 & 7). The decrease in activity of magnetite in quinidine and amine ligands
may be due the binding of iron with ligands. The rate of reaction is increasing in the
presence of (L)-tartaric acid (Table 2, entry 7). This result prompted us to prepare
the N-methylated quinidine and interestingly, the rate of the reaction is high with N-
methylated quinidine compared with quinidine in the aerobic oxidation of benzoin
(Table 2, entries 5 & 6). Although enhanced reactivity of magnetite has been
observed in the aerobic oxidation, no enantiomeric excess (ee) was obtained in any
case. Significant decline in the activity of magnetite was observed in the benzoin
oxidation in the presence of (R)-binol ligand.
Effect of temperature played a major role in the asymmetric benzoin oxidation using
magnetite. Quite interestingly, no reaction was observed at room temperature in the
presence of (L)-TA (2.5%) either in toluene or acetonitrile. At 60 C, in the presence
of (L)-TA, gave 30% enantio enriched benzoin with 43% ee using 2.5 mol% of the
(L)-TA (Scheme 2). Interestingly, at 80 C, complete conversion to benzil was
observed in the presence of (L)-TA. The activity and selectivity of the oxide catalysts
depends upon the oxidation state of the metal ions and their co-ordination in the
lattice, the surface cation-oxygen bond strength, the extent of total oxygen (both
surface and total) and morphology of the material. In order to realize the mechanism,
we have tried the reaction with the protected –OH group of tartaric acid and found
that no enenatioselectivity was observed inspite of complete conversion. This result
indicates that –OH groups may play a role in generating enantiomeric excess.
However, no ee was observed at any mol% of N-methyl quinidine and (R)-binol in
the presence of magnetite and other ferrites.

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Scheme 2 Asymmetric oxidation of benzoin using magnetite in the presence of (L)-
tartaric acid.
The magnetite catalyst was characterized by X-ray photo electron spectroscopy
(XPS), in order to understand the change of cations before and after modification
with (L)-tartaric acid. The Fe2p peak of XPS indicates Fe3+ and Fe2+ ratio is nearly
equal before and after modification (Figure 1).
Table 3
XPS data result of Fe3O4 before and after the surface modification
S.No
Catalyst
Fe3+(%) Fe2+ (%)
1
2
Fe3O4
Modified Fe3O4
54.5
57.2
45.5
42.8
The SEM-EDX analysis indicates that iron to metal ratio is nearly same in all cases
(Table 1). Thermo Gravimetric Analysis (TGA) of the ferrites indicates, the spinels
having good thermal stability with initial decompose above 250 C (Supporting
Information).
In conclusion, we have reported the oxidation of benzoin to benzil using
different spinels as a heterogeneous catalyst in the presence of air. Moreover, we
have explored the asymmetric catalytic property first time using various spinels. In

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addition, simple magnetic removal and recycling of the catalyst was shown to
proceed without loss of activity.
Acknowledgement:
KVSR thanks to DST file number: DST-SERB: SB/SI/IC-2013 for financial support.
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Ferrites Catalyzed Aerobic Oxidation of Benzoin and its Extension to
Enantioselective Version
Melad Shaikh, M. Sahu, Pramod K. Gavel and Kalluri V.S. Ranganath*
Graphical abstract

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Highlights
 Catalytic activity of various ferrite spinels have studied in the oxidation.
 Asymmetric version on the surface of magnetite, inverse spinel was studied
first time.
 Cobalt ferrite showed good catalytic activity