Oxidation and Oxidative Bromination Reactions Catalyzed Bya Reusable Polymer-Anchored Iron(III) Complex in Waterat Room Temperature

Sk. Manirul Islam • Kajari Ghosh •

Anupam Singha Roy • Noor Salam •

Tanmay Chatterjee

Received: 9 September 2013 / Accepted: 14 December 2013 / Published online: 21 December 2013

� Springer Science+Business Media New York 2013

Abstract A polymer-anchored iron(III) catalyst was

synthesized and characterized. Its catalytic activity was

evaluated for the oxidation of various alkenes, sulfides,

aromatic alcohols and ethylbenzene with 30 % H2O2 as the

oxidizing agent. The catalyst was also effective for the

oxidative bromination reaction with 80–100 % selectivity

of monobrominated products with H2O2/KBr at room

temperature. The above reactions require a minimum

amount of H2O2 and short reaction time. Most importantly,

all the above reactions occur in aqueous medium. The

catalyst can be facilely recovered and reused six-atimes

without significant decrease in its activity and selectivity.

Keywords Polymer anchored � Iron(III) complex �Oxidative bromination, H2O2

1 Introduction

Metal complexes are essential elements in the toolbox of

organic chemists. The demand for environmentally friendly

and recyclable metal complexes, which can be used in

catalytic amount, has also increased in recent years [1]. The

separation and recovery of the products and the catalyst are

to be kept in mind. All these requirements are greatly

achieved by the use of heterogeneous catalysts. Thus, the

preparation of such catalysts with high stability is a matter

of great interest [2].

Transition metals like Mn(III), Fe(III), Cu(II) and

Co(II), which are supported on materials such as alumina,

amorphous silicates, polymers, zeolites and MCM-41, are

commonly used in heterogeneous catalysis [3–8]. Recently,

functionalized polystyrene-anchored catalysts were used to

carry out various catalytic organic transformations [9–11].

Among them chloromethylated polystyrene is widely used

as a support. These polystyrene-anchored transition metal

complexes are inert and reusable catalysts for various

organic functional group conversions [12–14].

Schiff bases play a central role as chelating ligands in

main group and transition metals coordination chemistry

[15] as they are capable of binding with transition, non-

transition, lanthanide and actinide metal ions to give

complexes with useful properties. The chelating ability of

Schiff bases combined with the ease of preparation and

flexibility in varying the chemical environment about the

C=N group make them interesting ligands in coordination

chemistry.

Oxidation of olefins is especially important because

oxidation of alkenes gives oxygen-containing products

such as alcohols, aldehydes, ketones, acids and epoxides,

which are extremely important and useful in both chemical

and pharmaceutical industries [16, 17]. Alkane oxidation is

one of the basic reactions in industrial organic synthesis

because aldehydes and ketones are key intermediates for

the manufacture of wide variety of valuable products [16,

18, 19]. Conversion of alcohols to aldehydes and ketones

are among the most important and widely used oxidation

reactions in organic chemistry. Benzaldehyde is a very

useful chemical that has widespread applications in

Electronic supplementary material The online version of thisarticle (doi:10.1007/s10904-013-0017-5) contains supplementarymaterial, which is available to authorized users.

Sk. M. Islam (&) � K. Ghosh � A. S. Roy � N. Salam

Department of Chemistry, University of Kalyani, Kalyani,

Nadia 741235, W.B., India

e-mail: manir65@rediffmail.com

T. Chatterjee

Department of Organic Chemistry, Indian Association for the

Cultivation of Science, Jadavpur, Kolkata 700032, India

123

J Inorg Organomet Polym (2014) 24:457–467

DOI 10.1007/s10904-013-0017-5

perfumery, dyestuff and agricultural chemical industries

[20, 21]. Sulfoxides and sulfones are also useful synthetic

intermediates for the synthesis of several important organic

compounds. Sulfoxides and sulfones are generally prepared

via oxidation of the corresponding sulfides [22–24].

Bromoaromatics are important synthetic intermediates

for a variety of transformations including carbon–carbon

bond formation and carbon–heteroatom bond formation

[25]. The reported methods involve an electrophilic bro-

mination of aromatic rings using brominating agents other

than molecular bromine [26, 27]; others are based on

oxidative bromination [28–32]. Oxyhalogenation is safer

and greener as low-cost and easy-to-handle metal halides

are used as halogenating agents.

Herein, the preparation, characterization and investiga-

tion of catalytic activities for oxidation and oxidative

bromination reactions of a polymer-anchored iron(III)

complex were carried out using H2O2 as the oxygen source.

The catalytic activity of the anchored complex was tested.

2 Experimental

2.1 Materials and Instruments

Analytical-grade reagents and freshly distilled solvents

were used throughout the experiment. Chloromethylated

poly(styrene-divinyl benzene) was supplied by Sigma-

Aldrich chemicals Company, USA. Other reagents were

obtained from Merck Co. Liquid substrates were predis-

tilled and dried with appropriate molecular sieves. Distil-

lation and purification of the solvents and substrates were

done by standard procedures [33].

A Perkin-Elmer 2400 C elemental analyzer was used to

collect micro-analytical data (C, H and N). FTIR spectra of

the samples were recorded on a Perkin-Elmer FTIR 783

spectrophotometer using KBr pellets. A Mettler Toledo

TGA/SDTA 851 instrument was used for the

thermogravimetric (TGA) analysis. The morphology of the

functionalized polystyrene and complex was analyzed

using a scanning electron microscope (SEM) (ZEISS

EVO40, England) equipped with EDAX facility. The metal

content in the catalyst was determined using a Varian

AA240 atomic absorption spectrophotometer (AAS).

Electron paramagnetic resonance (EPR) spectra of the

fresh and used catalyst were recorded at room temperature

with a JES-FA200 ESR spectrometer (JEOL).

2.2 Synthesis of Catalyst

The outline for the preparation of polymer anchored iro-

n(III) complex is given in Scheme 1.

2.2.1 Preparation of Schiff Base Compound (A)

Triethylenetetramine was reacted with salicylaldehyde (1:2

molar ratio) in methanol solvent at 70 �C with stirring for

24-h to prepare the yellow Schiff base compound. The

mixture was cooled to room temperature and then filtered.

The residue was washed with ethanol until the filtrate

became colorless and dried under vacuum.

C20H26N4O2 (Yield, 70 %). Calcd. (%): C 67.80, H

7.35, N 15.82; found: C 67.85, H 7.43, N 15.89. Mass

spectrum (EI): m/z 354 (M? = L?); Infrared spectrum

(cm-1, KBr disk): m(phenolic OH) 3,415(w); m(C=N)

1,632(s); m(phenolic C–O) 1,370(s); m(CH2) 858(m);

m(aromatic CH) 754(m). 1H NMR (CDCl3, ppm):

d = 13.21 (s, 2 H), 8.29 (s, 2H) 7.26–7.45 (m, 8 H),

3.65–3.72 (m, 4 H), 2.79 (t, 4 H), 2.62 (t, 4 H), 2.21 (s, 2H).

2.2.2 Preparation of Polymer-Bound Ligand (B)

The polymer-anchored ligand (B) was prepared as fol-

lows: Chloromethylated polystyrene (1.2 g) was added to

a methanolic solution of Schiff base compound (1 g) and

N NH

NH

N

OH HONH2 NH

NH

NH2

CHO

OH

MeOH, 500C, reflux(A)

OO

N

N

N=CH

N=CH

HO

HO

N

N

N C

N C

FeFeCl3

EtOH

(B)

RefluxMerrifield resign

MeOH, Reflux, 24h

(A)

Cl

OH2

(C)

Scheme 1 Synthesis of

polymer anchored Fe(III)

complex

458 J Inorg Organomet Polym (2014) 24:457–467

123

heated to reflux with continuous stirring. After refluxing,

the mixture was cooled to room temperature and filtered.

The yellow beads were washed with ethanol until the

filtrate became colorless. It was then dried under

vacuum.

2.2.3 Preparation of Polymer-Bound Fe(III) Complex (C)

To the polymer-anchored Schiff base ligand (1 g) in eth-

anol (20 mL), 5 mL of 1 % (w/v) ethanolic solution of

FeCl3 was added drop-wise over a period of *30-min

under constant stirring. The reaction mixture was refluxed

for 24-h. The complex [PS-Fe(III)teta] thus formed was

filtered and washed thoroughly with ethanol and dried at

room temperature under vacuum.

2.3 General Procedure for Oxidation Reaction

Catalyzed by [PS-Fe(III)teta]

Substrates (5 mmol) were dissolved in water (5 mL) for

different sets of reactions together with the catalyst

(50 mg) and H2O2 (10 mmol, 30 % in aq.). After reaction,

the organic products were separated from the mixture by

extraction with dichloromethane (5 mL 9 2). The com-

bined organic portions were dried and concentrated.

Product analysis was performed with a Varian 3400 gas

chromatograph equipped with a 30 m CP-SIL8CB capil-

lary column and a flame ionization detector using cyclo-

hexanone as internal standard. All reaction products were

identified by using Trace DSQ II GC–MS.

2.4 General Procedure for Oxidative Bromination

Reaction of Organic Substrates Catalyzed by [PS-

Fe(III)teta]

In a typical reaction, aqueous H2O2 (30 %, 20 mmol) was

added to a mixture of substrates (10 mmol) and KBr

(20 mmol) dissolved in 10 mL of water. The catalyst

(50 mg) and conc. H2SO4, (36 N, 5 mmol) were added and

the reaction mixture was stirred at room temperature.

Additional H2SO4 (15 mmol) was added to the reaction

mixture in three equal portions at 30-min intervals under

continuous stirring. After a specified time of reaction, the

catalyst was filtered and the solid was washed with ether.

The combined filtrates were washed with saturated sodium

bicarbonate solution and then shaken with ether in a se-

paratory funnel. The organic extract was dried over anhy-

drous sodium sulfate. The products were analyzed

chromatography equipped. The identity of the products was

confirmed by using Trace DSQ II GC–MS.

3 Results and Discussion

3.1 Characterization of Polymer Anchored Fe(III)

Complex

The Schiff base ligand obtained from salicylaldehyde and

triethylenetetramine was characterized and established

earlier [34]. Due to the insolubilities of the polymer-sup-

ported metal complex in all common organic solvents, its

structural investigation was limited to their physicochem-

ical properties, chemical analysis, SEM, TGA, FTIR, EPR

and UV–Vis spectral data. Table 1 provides the elemental

analysis data. The metal content of the polymer-supported

catalyst was estimated by atomic absorption spectroscopy.

The attachment of metal to the support was confirmed

by comparison of the FT-IR spectra (Fig. 1) (4,000–400

and 600–50 cm-1 regions) of the polymers before and after

loading with metals. The IR spectrum of pure chlorome-

thylated polystyrene has an absorption band at 1,261 cm-1

due to the C–Cl bond, which was absent in the ligand and

in the catalyst. The IR spectrum shows a stretching

vibration for –CH2 at 2,918 cm-1 for the polymer-bound

ligand and its complex. The C=N bond stretching vibration

appears at 1,655 cm-1 for the polymer-bound Schiff base

ligand and is lowered to 1,632 cm-1 in the metal complex.

This indicates the coordination of azomethine nitrogen to

the metal [35, 36]. Another band at 3,325 cm-1 is assigned

to O–H stretching, which is shifted to lower region for the

metal complex as the –OH group coordinates with the

metal through oxygen. The band for Fe–O in the metal

complex is observed at *556 cm-1. Bands due to the Fe–

N stretching vibrations at *460 cm-1 establishes the for-

mation of the metal complex through Fe–N bond. Another

peak at 1,366 cm-1 due to the C–O bond decreased to

1,315 cm-1 in (PS-Fe(III)teta) and confirms metal oxygen

coordination [37]. The stretching vibrations do not change

significantly, which suggests the existence of all the

properties in the recycled catalyst (Fig. 1c).

The electronic spectra (Fig. S1, Supporting information)

of the polymer-supported ligand and metal complex were

recorded in diffuse reflectance spectrum mode as MgCO3/

BaSO4 disks. In the UV spectrum of the ligand, two

absorption bands are observed at 235-265 and 330 nm,

Table 1 Chemical composition polymer-anchored ligand and its

iron(III) complex

Compound Colour C

(%)

H

(%)

N

(%)

Cl

(%)

Metal

PS-teta Yellow 78.70 6.99 7.99 3.37 –

PS-[Fe(III)teta] Grey 68.60 6.23 7.12 4.37 3.66

(3.65)a

a Reused catalyst

J Inorg Organomet Polym (2014) 24:457–467 459

123

which are assigned to the n–p* and p–p* transitions,

respectively. The band at 415 nm is presumably caused by

an intraligand charge transfer. These absorptions were also

present in the spectrum of the iron complex, but shifted

showing further evidence for complexation.

Field emission-scanning electron micrographs of a single

bead of pure chloromethylated polystyrene, polymer-

anchored ligand (PS-teta) and its complex [PS-Fe(III)teta]

were recorded. Morphological changes occurred on the

polystyrene beads at various stages of the synthesis. The

SEM images of pure chloromethylated polystyrene (A),

polymer-anchored ligand (B) and the immobilized Fe(III)

complex (C) are shown in Fig. 2. The pure chloromethylated

polystyrene bead has a smooth surface while the polymer-

anchored ligand and complex show a slight roughening of the

outer layer of the bead. This roughening appears relatively

complex. Also, the presence of iron is supported by EDAX

(Fig. S2, Supporting information), which suggests the for-

mation of the metal complex with the polymer-anchored

ligand.

The thermal stability (TGA–DTA) of the complex was

investigated in air from 30 to 600 �C at a heating rate of

10 �C/min. TGA curves of polymer anchored ligand and

iron complex are shown in Fig. S3 (Supporting informa-

tion). The polymer-anchored ligand decomposed from 320

to 330 �C. After complexation iron, the thermal stability

of the immobilized complex increased by 40–70 �C. The

polymer-anchored Fe(III) complex decomposed from 360

to 400 �C; i.e., at considerably higher temperature than

the precursor.

The EPR spectra of the fresh and the used iron catalysts

were recorded for the solid sample at room temperature

Fig. 1 FT-IR spectra of polymer anchored ligand (a), polymer anchored iron(III) catalyst (b) and reused catalyst (c)

Fig. 2 FE SEM images of chloromethylated polystyrene (a), polymer anchored ligand (b) and polymer anchored Fe(III) complex (c)

460 J Inorg Organomet Polym (2014) 24:457–467

123

(Fig. 3). The g values of the fresh catalyst are g1 = 7.84,

g2 = 4.19 and g3 = 1.99 and those of the used catalyst are

g1 = 8.19, g2 = 4.20 and g3 = 2.00. These data indicate

that iron remains in the 3? oxidation state in both the fresh

and used catalyst [38–40].

3.2 Catalytic Activity

The catalytic activity of the polymer-anchored iron(III)

complex was investigated for the oxidation of alkenes,

alkanes, alcohols and sulfides. The activity of the metal

complex as a catalyst was also tested for oxidative bromin-

ation. The reactions were carried out at room temperature

using water as a green solvent.

3.2.1 Oxidation of Alkenes with H2O2 Catalyzed by [PS-

Fe(III)teta]

Oxidation of olefins was carried out in water with H2O2 as

the oxygen source to test the catalytic activity of the metal

complex. For optimization of reaction conditions, styrene

was chosen as the alkene. In this case, styrene was selec-

tively converted to benzaldehyde (Scheme 2).

The effect of various oxidants on the conversion of

styrene and the selectivity of benzaldehyde using [PS-

Fe(III)teta] as a catalyst is summarized in Table S1 (Sup-

porting information). The ability of various oxidants like

TBHP, H2O2, NaIO4, PhIO, KHSO5 and molecular oxygen

for styrene oxidation was investigated (Table S1, entries

2–7). Styrene oxidation did not occur in the absence of an

oxidant (Table S1, entry 1). The reaction shows almost no

conversion when molecular oxygen is used as oxidant

(Table S1, entry 2). Table S1 shows that among the six

oxidants used in styrene oxidation reaction, H2O2 performs

best (Table S1, entry 3). Oxidants such as TBHP, PhIO and

NaIO4 are less efficient as evident from the percentage

conversion (Table S1, entries 4-6). The use of KHSO5 is

not favored because a buffer is needed (Table S1, entry 7).

The activity of the catalyst was also evaluated using three

different styrene-to-H2O2 molar ratios (Table S1, entries 3

and 8–9). The conversion of styrene increased from 69 to

97 % on increasing the molar ratio from 1:1 to 1:2, The

CHO

O

)b()a(enerytS

[PS-Fe(III)teta]

H2O2, H2O, rt

Scheme 2 Oxidation products of styrene

Table 2 Olefins oxidation using H2O2 catalyzed by polymer-

anchored Fe(III) catalyst

Entry Substrate Conversion

(%)aProduct (selectivity

%)aTON

1 Cyclohexene 82 2-Cyclohexene-1-one

(63)

2-cyclohexene-1-ol

(28) Cyclohexene

epoxide (9)

125

2 trans-Stilbene 66 Benzaldehyde (77)

Benzil (16) trans-

Stilbene oxide (7)

101

3 a-Pinene 50 Verbenol (31)

verbenone (55) a-

pinene oxide (14)

76

4 4-

Methylstyrene

82 4-Methyl

benzaldehyde (82)

4-methylstyrene

epoxide (18)

125

5 4-

Chlorostyrene

88 4-Chloro

benzaldehyde (85)

4-chlorostyrene

epoxide (15)

135

6 4-Nitrostyrene 85 4-Nitro

benzaldehyde (82)

4-Nitro styrene oxide

(18)

130

7 a-Methyl

styrene

80 Acetophenone (100) 122

Reaction conditions: catalyst (0.05 g), substrate (5 mmol), water

(5 mL), H2O2 (10 mmol), time (6 h)a Determined by GC

Fig. 3 X-band EPR spectra of Fe in (a) fresh catalyst and (b) used

catalyst

J Inorg Organomet Polym (2014) 24:457–467 461

123

conversion was barely affected by a further increase in

ratio to 1:3. Thus, a 1:2 ratio is sufficient for good con-

version of styrene. A control experiment (Table S1, entry

10) in the presence of an oxidant under the same reaction

conditions was also investigated and showed that H2O2

alone has poor oxidizing ability.

The above optimized reaction conditions can be applied

to the oxidation reaction of other olefins catalyzed by the

polymer-anchored Fe(III) catalyst. The results are shown in

Table 2. This catalyst efficiently converts olefins to their

corresponding allylic products with H2O2 in aqueous

medium. In the oxidation of cyclohexene, allylic products,

2-cyclohexene-1-one and 2-cyclohexene-1-ol, were pro-

duced. Substituted styrene was selectively converted to

their corresponding aldehydes. Acetophenone was detected

in the oxidation of a-methyl styrene as a major product.

trans-Stilbene was also oxidized by this heterogeneous

catalyst in high yields. trans-Stilbene gave benzaldehyde as

a major product with benzil. This catalytic system also

showed good activity in the case of a-pinene. In the oxi-

dation of a-pinene, the major products were verbenone and

verbenol.

3.2.2 Oxidation of Ethylbenzene with H2O2 Catalyzed

by [PS-Fe(III)teta]

Oxidation of ethylbenzene catalysed by [PS-Fe(III)teta]

using H2O2 as oxidant was performed; and, the products

were benzaldehyde (a), phenylacetic acid (b), acetophe-

none (c) and 1-phenylethane-1,2-diol (d) (Scheme 3).

The effect of the amount of oxidant (moles H2O2/moles

ethylbenzene) was studied. It was observed that the con-

version was best using a substrate:oxidant mole ratio of

1:3. The influence of H2O2 concentration on the oxidation

of ethylbenzene was studied considering ethylbenzene:

aqueous 30 % H2O2 molar ratios of 1:2, 1:3 and 1:4, while

keeping ethylbenzene (10 mmol), catalyst (0.050 g), H2O

(10 mL) constant. Thus, the conversion improved from

18 % at a molar ratio of 1:2 to 52 % at 1:3 and finally to

53 % at a molar ratio of 1:4 in an 8-h reaction time

(Fig. 4). However, at the expense of the oxidant, a mini-

mum molar ratio of 1:3 is necessary to observe maximum

oxidation.

The effect of the amount of catalyst on the oxidation of

ethylbenzene is illustrated in Fig. 5. Here the reaction

proceeded slowly to give only 16 % conversion with

0.040 g of catalyst using the above reaction conditions.

Increasing the catalyst amount to 0.050 g improved this

O

OH

OO HO

OH

[PS-Fe(III) teta]H2O2, H2O, rt

Ethylbenzene (a) (b) (c) (d)

+ + +

Scheme 3 Oxidation products

of ethyl benzene

0 100 200 300 400 5000

5

10

15

20

25

30

35

40

45

50

55

60

% c

onve

rsio

n

time/min

1:2 1:3 1:4

Fig. 4 Effect of amount of H2O2 on the oxidation of ethylbenzene.

Reaction conditions: ethylbenzene (10 mmol), PS-[Fe(III)teta]

(0.050 g), H2O (10 mL) and at room temperature

0 100 200 300 400 5000

5

10

15

20

25

30

35

40

45

50

55

60

% c

onve

rsio

n

time/min

0.040 g 0.050 g 0.060 g

Fig. 5 Effect of amount of catalyst on the oxidation of ethylbenzene.

Reaction conditions: ethylbenzene (10 mmol), H2O2 (30 mmol), H2O

(10 mL) and at room temperature

462 J Inorg Organomet Polym (2014) 24:457–467

123

conversion to 52 %. Further addition of catalyst showed no

improvement in oxidation. It was noticed that the products

formed are independent of the oxidant and catalyst amount

and follow the order: benzaldehyde (71.6 %) [ phenyla-

cetic acid (20.5 %) [ acetophenone (4.7 %) [ 1-phenyle-

thane-1,2-diol (3.2 %). A very small amount of an

unidentified product was also noticed. No significant

improvement in the oxidation of ethylbenzene was

observed beyond an 8-h reaction time. A blank experiment

under the optimized experimental conditions in the pre-

sence of oxidant and in the absence of the catalyst was also

investigated. The results showed that, without the catalyst,

H2O2 has poor ability to oxidize the alkane.

3.2.3 Oxidation of Aromatic Alcohols with H2O2

Catalyzed by [PS-Fe(III)teta]

Catalytic oxidation of alcohols generally needs an organic

solvent and addition of base. Thus in line with the

requirements of ‘‘green chemistry’’, we chose water as a

reaction medium to conduct the oxidation of alcohols in the

absence of base (Scheme 4).

The catalytic activity of the polymer anchored iron(III)

catalyst was examined for the oxidation of benzyl alcohol by

changing the various reaction conditions. The effect of the

amount of H2O2 on the oxidation of benzyl alcohol to

benzaldehyde was studied (Table 3). The percent conversion

of benzyl alcohol to benzaldehyde is dependent on the

quantity of H2O2 used to a considerable amount. The benzyl

alcohol-to-H2O2 molar ratio of 1:1 resulted in 67 % con-

version when the amount of catalyst was 50 mg (reaction

time, 6-h). When the molar ratio was increased to 1:2, the

conversion increased to 97 % (same reaction conditions).

However, the conversion was found to be almost constant

when the molar ratio was further increased to 1:3. Therefore,

a 1:2 molar ratio of benzyl alcohol-to-H2O2 is optimum.

The results for the oxidation of benzyl alcohol in the

presence of different amounts of catalyst are presented in

Fig. S4. All the tests were conducted with 5 mmol of

substrate, 10 mmol of H2O2, 5 mL of water under base free

condition at room temperature. A conversion of 25 %

within 6-h was obtained when 0.02 g of polymer-anchored

catalyst was used. No by-products such as benzoic acid

were detected. When the amount of the catalyst was

increased to 0.04 g, the conversion increased to 80 %

conversion. In this experiment, a small amount of benzoic

acid was detected. A further increase if catalyst amount to

0.05 g led to an increase in the conversion to 97 %.

However, when the catalyst amount was increased to 0.06

and 0.07 g, the conversion remained almost constant; but,

the selectivity for benzaldehyde was reduced. These find-

ings indicate that the oxidation of benzyl alcohol is highly

dependent on the amount of the catalyst.

In order to study the catalytic activity of the catalyst for

the oxidation of aromatic alcohols, a series of benzylic

alcohols was used. Substituted benzyl alcohols are also

oxidized to their corresponding aldehydes. In all the above

cases, trace amounts of acids as products were observed.

3.2.4 Oxidation of Sulfides with H2O2 Catalyzed by [PS-

Fe(III)teta]

Sulfide oxidation with the [PS-Fe(III)teta] catalyst was also

studied. Thus, when methyl phenyl sulfide was treated with

CH2OH CHO COOH

[PS-Fe(III) teta]

H2O2, H2O, rt

Benzyl alcohol (a) (b)

+

Scheme 4 Oxidation products of benzyl alcohol

Table 3 Oxidation of aromatic alcohols with 30 % H2O2 catalyzed

by Fe(III) catalyst

Entry Aromatic alcohol Conversion

(%)aProduct

Selectivity (%)aTON

1b Benzyl alcohol 67 Benzaldehyde (90)

Benzoic acid (10)

103

2c Benzyl alcohol 97 Benzaldehyde (94)

Benzoic acid (6)

149

3d Benzyl alcohol 95 Benzaldehyde (71)

Benzoic acid (29)

146

4 4-Methoxybenzyl

alcohol

80 4-Methoxy

benzaldehyde

(84)

4-Methoxy benzoic

acid (16)

123

5 4-Methylbenzyl

alcohol

85 4-methyl

benzaldehyde

(91)

4-methyl benzoic

acid (9)

130

6 4-Nitrobenzyl

alcohol

79 4-Nitro

benzaldehyde

(89)

4-Nitro benzoic

acid (11)

121

7 1-Phenyl ethanol 78 Acetophenone

(100)

120

Reaction condition: aromatic alcohol (5 mmol), 30 % H2O2 (10 mmol),

water (5 mL), catalyst (50 mg), time (6 h), room temperaturea Determined by GCb Benzyl alcohol:H2O2 = 1:1c Benzyl alcohol:H2O2 = 1:2d benzyl alcohol:H2O2 = 1:3

J Inorg Organomet Polym (2014) 24:457–467 463

123

[PS-Fe(III)teta] and 30 % H2O2 in water at room temper-

ature, methyl phenyl sulfoxide was obtained (Scheme 5).

Methyl phenyl sulfide as a model substrate was subjected to

different reaction conditions (Table S2). The effect of tem-

perature for the oxidation of methyl phenyl sulfide is given in

Table S2. The amount of H2O2 has a significant effect on the

conversion and selectivity the oxidation reaction. Conversion

increased when amount of H2O2 was increased from 5 mmol

(Table S2, run 2) to 10 mmol (Table S2, run 1). However,

there was no significant change in conversion when the

amount of H2O2 was increased to 15 mmol (Table S2, run 3,

same reaction conditions). Moreover, the selectivity of oxi-

dation of sulfide to sulfoxide decreased with the increase in the

amount of H2O2. This is due to further oxidation of sulfoxide

to the corresponding sulfone.

The reaction was also monitored using different quantities

of catalyst. On using a lower catalyst amount The reaction

takes a longer time and is incomplete with a smaller amount

of catalyst is used (Table S2, 40 mg, run 4). However, with

50 mg of catalyst (Table S2, run 1), the reaction complete

within 4-h, yielding 95 % sulfoxide with a small amount of

sulfone (5 %). On increasing the amount of catalyst to 60 mg

(Table S2, run 5), the reaction gives more sulfone along with

sulfoxide. Based on the above observations it is concluded

that 50 mg of polymer-anchored catalyst, 10 mmol 30 %

H2O2 with water as a solvent are the best conditions for the

selective oxidation of sulfide to sulfoxide. It was also found

that without H2O2, [PS-Fe(III)teta] alone cannot oxidize

sulfide (Table S2, run 6).

To examine further the reactivity of the catalyst, oxi-

dation of other sulfides were investigated (Table 4). The

substrate was extended to diphenyl sulfide, ethyl phenyl

sulfide and dibutyl sulfide. The sulfoxides were selectively

obtained in all cases. A series of substrates, aryl alkyl, aryl

allyl and dialkyl sulfides, can be oxidized to their corre-

sponding sulfoxides. The reactivity and conversion were

dependent on the nature of the substituent. In the case of

allyl sulfides, oxidation of the carbon–carbon double bond

was observed to a smaller extent. Similarly, benzylic sul-

fides can be oxidized to their corresponding sulfoxides with

a little amount of oxidation at the benzylic C–H position.

3.2.5 Oxidative Bromination Catalyzed by [PS-Fe(III)teta]

In the present study, oxidative bromination was optimized

using salicylaldehyde as a substrate (Scheme 6).

Two different parameters; e.g., volume of sulfuric acid and

amount of H2O2, were varied. The results showed that, for a

fixed amount of salicylaldehyde (10 mmol), catalyst

(0.050 g), KBr (20 mmol) in 10 mL water, at least 20 mmol of

H2SO4 and 20 mmol of H2O2 were sufficient to give maximum

conversion. The conversion of salicylaldehyde significantly

improved from 60 to 98 % with an increase on H2SO4 from 10

to 20 mmol. The presence of excess acid may decompose the

catalyst under the above reaction conditions. H2SO4 was added

to the reaction mixture in definite time intervals to prevent

decomposition of catalyst. Under optimized reaction condi-

tions, the performance of the polymer-anchored catalyst is

summarized in Table 5. Approximately 98 % conversion of

salicylaldehyde was obtained with ca. 100 % selectivity in the

formation of 5-bromosalicylaldehyde.

The effect of added salt was studied using H2O2 as an

oxidant and MBr (M = Li, Na, K) as a bromine source

(Table 5). Among the three salts, KBr was the most efficient

bromine source and a monoselective product is obtained.

The use of sodium bromide did not produce regioselective

products. LiBr, though efficient, was less than KBr.

SCH3

SCH3

O

SCH3

O

O[PS-Fe(III) teta]

H2O2, H2O, rt

Methyl phenyl sulfide Methyl phenyl sulfoxide Methyl phenyl sulfone

Scheme 5 Oxidation products

of methyl phenyl sulfide

OH

CHO

OH

CHOBr

[PS-Fe(III) teta]

H2O2, KBr, H2SO4,H2O, rt

Scheme 6 Oxidative bromination of salicylaldehyde

Table 4 Oxidation of sulfides using 30 % H2O2 catalyzed by poly-

mer-anchored Fe(III) catalyst

Entry Substrates Conversion

(%)a/time

(h)

Selectivity

of sulfoxide/

sulfone (%)a

TON

(approx)

1 C4H9SC4H9 89/5 78/22 137

2 PhSPh 94/4 90/10 144

3 PhSC2H5 92/4 89/11 71

4 PhSC6H13 85/5 89/11 141

5 PhSCH2Ph 85/4 83/17 130

6 PhSCH2CH=CH2 70/5 86/14 107

7 p-ClC6H4SCH3 88/4 70/30 134

8 p-CH3C6H4SCH3 93/4 90/10 143

Conditions: Sulfide (5 mmol); 30 % aq H2O2 (10 mmol); water

(5 mL); 50 mg catalyst at room temperaturea Conversion and selectivity were determined by GC

464 J Inorg Organomet Polym (2014) 24:457–467

123

The influence of temperature was also studied; there was

no effect on the conversion of salicylaldehyde. With an

increase in temperature, the conversion of salicylaldehyde

was almost the same.

The role of hydrogen peroxide was confirmed by con-

ducting a control experiment in which the formation of the

bromo compound was not observed.

Oxidative bromination of other organic substrates was

studied under optimized reaction conditions (Table 5). All the

substrates were selectively converted to their mono-bromi-

nated products. Substrates like phenol, resorcinol, anisole and

N,N-dimethylaniline showed high conversion and excellent

para-selectivity. Inactive substrates, like benzene, showed a

low conversion and required long reaction times. On the other

hand, deactivated aromatic rings gave no conversion at these

reaction conditions. 4-Substituted aromatics are selectively

converted to 2-bromo derivatives and vice versa.

4 Test for Heterogeneity

The leaching of iron from polymer-anchored iron(III) com-

plex was confirmed by carrying out analysis of the used

catalyst (EDX, IR) and the product mixtures (AAS, UV–

Vis). Analysis of the used catalyst did not show appreciable

loss of iron content relative to the fresh catalyst. An IR

spectrum of the recycled catalyst is similar to that of fresh

sample indicating the heterogeneous nature of the complex.

Analysis of the product mixtures showed that, if any iron was

present, it was below the detection limit. The UV–Vis

spectrum was also used to determine the stability of the

catalyst. Thus, the UV–Vis spectrum of the reaction solution

in the first run did not show absorption peaks characteristic of

iron metal, which indicates that the leaching of metal did not

take place during the course of the reaction. The metal

content of the recycled catalyst was determined by AAS. It

was found that the iron content of the recycled catalyst

remained almost unaltered. These observations strongly

suggest that the catalyst present is heterogeneous in nature.

A heterogeneity test was carried out for the oxidation of

styrene. For a rigorous proof of heterogeneity, a test was

carried out by filtering the catalyst from the reaction

mixture at room temperature after 4-h. The filtrate was

allowed to react up to the completion of the reaction (6-h).

The reaction mixture at 4 h and the filtrate were analyzed

by gas chromatography. No change in the % conversion

selectivity was found, which indicates that the catalyst is

heterogeneous.

Table 5 Oxidative bromination of various organic substrates catalyzed by polymer-anchored Fe(III) catalyst

Entry Substrate Conversion (%)a (time/h) Product selectivitya,b TON

1b Salicylaldehyde 60 (3.0) 5-Bromo-2-hydroxy benzaldehyde (100) 184

2c Salicylaldehyde 98 (3.0) 5-Bromo-2-hydroxy benzaldehyde (100) 300

3d Salicylaldehyde 90 (3.0) 5-Bromo-2-hydroxy benzaldehyde (80) 275

4e Salicylaldehyde 82 (3.0) 5-Bromo-2-hydroxy benzaldehyde (66) 251

5 Phenol 97 (2.5) 4-Bromophenol (100) 298

6 Resorcinol 96 (2.5) 4-Bromo-1,3-dihydroxybenzene (100) 294

7 4-Methylphenol 95 (2.5) 2-Bromo-4-methylphenol (100) 291

8 4-Nitrophenol 83 (6.0) 2-Bromo-4-nitrophenol (100) 254

9 Anisole 94 (3.0) 4-Bromoanisole (100) 288

10 Aniline 96 (2.5) 4-Bromoaniline (85) 294

11 4-Methylaniline 93 (3.0) 2-Bromo-4-methylaniline (87) 285

12 4-Chloroaniline 87 (3.0) 2-Bromo-4-chloroaniline (100) 266

13 2-Methylaniline 89 (3.0) 4-Bromo-2-methylaniline (82) 272

14 2-Chloroaniline 93 (3.0) 4-Bromo-2-chloroaniline (93) 285

15 N,N-Dimethylaniline 94 (3.0) 4-Bromo-N,N-dimethylaniline (100) 288

16 Benzene 12 (6.0) Bromobenzene (100) 37

17 Nitrobenzene – – –

Conditions: substrate (10 mmol); KBr (20 mmol); H2SO4 (20 mmol); 30 % aq H2O2 (20 mmol), water (10 mL), 50 mg catalyst at room

temperaturea Conversion and selectivity were determined by GCb H2SO4 (10 mmol)c KBr as bromine sourced LiBr as bromine sourcee NaBr as bromine source

J Inorg Organomet Polym (2014) 24:457–467 465

123

5 Recycling of Catalyst

The catalyst remained insoluble under the reaction condi-

tions. Hence, it can be easily separated by simple filtration

and washing. The catalyst was washed with dichloro-

methane and dried at 100 �C. Oxidation of styrene, ethyl

benzene, methyl phenyl sulfide and benzyl alcohol was

carried out with the recycled catalyst under the optimized

reaction conditions. The catalyst was recycled to test its

activity and stability. The results are presented in Fig. 6.

The catalyst did not exhibit any appreciable change in

activity, which indicates that it is stable and can be

regenerated for repeated use. Similarly, recycling of the

catalyst was tested for the oxidative bromination of sali-

cylaldehyde. No appreciable change in conversion and

selectivity indicates was observed for the reused catalyst.

6 Conclusions

A chloromethylated polystyrene-anchored iron(III) com-

plex was prepared and characterized. The complex is an

active and reusable catalyst for the oxidation of alkenes,

alkanes, sulfides and alcohols. The catalyst also shows

excellent catalytic activity in oxidative bromination. The

catalyst is stable and recyclable under the reaction condi-

tions used. The heterogeneous catalyst shows no significant

loss of activity in recycling experiments. The active sites

do not leach from the support and the catalyst can be reused

without appreciable loss of activity indicating effective

anchoring. The reusability of this catalyst is high (i.e. six-

times) without significant decrease in activity.

Acknowledgments SMI acknowledges Department of Science and

Technology (DST) and Council of Scientific and Industrial Research

(CSIR), New Delhi, India for funding. ASR acknowledges CSIR,

New Delhi, for providing his senior research fellowship. We

acknowledge Indian Association for the Cultivation of Science,

Kolkata for instrumental support.

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