efficient allylic oxidation of olefins catalyzed by polymer supported metal schiff base complexes...

Efficient Allylic Oxidation of Olefins Catalyzed by PolymerSupported Metal Schiff Base Complexes with Peroxides

S. M. Islam • Anupam Singha Roy •

Paramita Mondal • Noor Salam

Received: 23 August 2011 / Accepted: 17 February 2012 / Published online: 3 March 2012

� Springer Science+Business Media, LLC 2012

Abstract Three homogeneous Cu(II), Co(II) and Ni(II)

complexes of a Schiff base ligand and their heterogeneous

complexes supported on poly(4-aminostyrene) were pre-

pared and characterized by using elemental analysis, fou-

rier transform infrared spectroscopy, UV–Vis diffuse

reflectance spectroscopy, thermogravimetric analysis and

scanning electron microscopy. The catalytic performance

of both homogeneous and heterogeneous complexes was

evaluated in the liquid phase oxidation of cyclohexene,

styrene and trans-stilbene in acetonitrile with tert-butyl-

hydroperoxide or hydrogen peroxide as the oxidant. All

types of catalyst were active in oxidation; and, the com-

plexes produce allylic oxidation products in all cases.

Immobilized complexes are slightly more active than their

homogeneous complexes. The polymer-supported Cu(II)

complex shows a higher catalytic activity than the other

metal species. The activities of the immobilized catalysts

remained nearly the same after five cycles, suggesting the

true heterogeneous nature of the catalyst.

Keywords Polymer supported � Schiff base � Metal

complexes � Oxidation � H2O2

1 Introduction

Oxygenation of organic substrates has been extensively

studied; and, olefin oxidation is especially interesting

because of its industrial importance. Allylic oxidation of

olefins into unsaturated ketones and alcohols is an

important reaction in organic synthesis [1, 2]. In particular,

the oxidation products of cyclohexene and their deriva-

tives, viz. 2-cyclohexen-1-one, 1-methylcyclohex-1-en-3-

one, etc., are important in organic synthesis owing to the

presence of a highly reactive carbonyl group, which is

utilized in cycloaddition reactions [3, 4]. The benzalde-

hyde, allylic oxidation product of styrene and trans-stil-

bene is an important intermediate in organic synthesis in

the manufacture of perfumery, pharmaceuticals, dyestuffs

and agrochemicals [5, 6].

Normally, manganese dioxide, selenium dioxide or

chromium trioxide were used as oxidants and homoge-

neous catalysts for allylic oxidations [7, 8]. Good activities

and selectivities have been highlighted as the main

advantages of homogeneous catalysts. However, after

excessive use of these oxidants, some problems such as

corrosion, difficulty in recovery and separation of the cat-

alyst from reaction mixtures are observed with these

homogeneous catalysts and make such systems environ-

mentally unsuitable.

In recent years, the design and synthesis of catalytically

active supported metal complexes have received consid-

erable interest because of easy separation of the product

from the reaction medium, and the recovery and reuse of

these expensive catalysts. Many effective and recyclable

heterogeneous catalysts have been studied for liquid phase

allylic oxidation [9–13]. Various approaches have been

focused on the incorporation of metal-based catalysts onto

or into inert supports by different methods; e.g., zeolites

[14–16], grafting on organic supports including polybenz-

imidazole [17], ion exchange resins [18] and epoxy resins

[19]. Modified MCM-41 [20] and alumina [21] or inter-

calations in clays [22] have been used for immobilization

of homogeneous complexes and applied as catalysts for

alkene oxidation.

S. M. Islam (&) � A. S. Roy � P. Mondal � N. Salam

Department of Chemistry, University of Kalyani, Kalyani,

Nadia 741235, West Bengal, India

e-mail: [email protected]

123

J Inorg Organomet Polym (2012) 22:717–730

DOI 10.1007/s10904-012-9666-z

Application of polymer-supported catalysts in oxida-

tion reactions has received attention in recent years

[23–26] due to their potential advantages over the

homogeneous catalysts. Schiff base ligands derived from

aldehydes and amines and their complexes with transition

metals have been widely used in many organic trans-

formations. They can be prepared simply and cheaply for

industrial applications. The development of environmen-

tally friendly technologies has promoted much research

in heterogeneous catalysis and, in particular, the heter-

ogenization of known active homogeneous Schiff base

complexes for oxidation. Heterogeneous Schiff base

complexes containing donor atoms such as oxygen and

nitrogen have been used for oxidation [27–32]. The

activity of polymer-supported Schiff base complexes of

transition metal ions varies with the type of Schiff base

ligands, coordination sites and metal ions used in their

formation.

In this work, we have prepared three heterogeneous

oxidation catalysts. First, homogeneous complexes were

synthesized by reacting salicylaldehyde with o-aminophe-

nol followed by metal chloride. Then these complexes

were encapsulated into a polymer matrix. By heterogeni-

zation of the homogeneous catalyst using a polymer, the

following drawbacks can be avoided under heterogeneous

condition: (i) difficulty in recovery and separation of the

catalyst from the reaction mixture, (ii) oxidative self-

destruction of the catalyst in the oxidizing media, and (iii)

deficiency of the recycling property. The catalytic effi-

ciency of polymer-supported metal complexes was tested

in the oxidation of cyclohexene, styrene and trans-stilbene

using tert-butylhydroperoxide (TBHP) and hydrogen per-

oxide (H2O2) as oxygen sources. The polymer-supported

copper catalyst shows excellent catalytic activity and

selectivity. These heterogeneous catalysts produce both

allylic (major) and epoxides (minor) oxidation products

with cyclohexene, styrene and trans-stilbene. The catalytic

activities were also tested with recycled catalysts. Com-

parison of homogeneous and supported catalysts for oxi-

dation reaction was studied. The catalytic activity and

selectivity of products did not decrease during the recy-

cling experiments.

2 Experimental

2.1 Materials

Analytical grade reagents and freshly distilled solvents

were used. Liquid substrates were pre-distilled and dried

over appropriate molecular sieves. Distillation and purifi-

cation of the solvents and substrates were done by standard

procedures [33]. Poly(styrene divinyl benzene) (2% cross-

linked) and olefins were supplied by Sigma-Aldrich

chemicals Company, USA. Metal salts, salicylaldehyde, o-

aminophenol, H2O2 (30% aq) and TBHP (70% aq) were

obtained from Merck Co.

2.2 Physical Measurements

A Perkin-Elmer 2400 C elemental analyzer was used to

collect microanalytical data (C, H and N). The metal

content of the samples was measured by Varian AA240

atomic absorption spectrophotometer (AAS). Infrared

spectra were recorded on a Perkin-Elmer FT-IR (fourier

transform infrared spectroscopy) 783 spectrophotometer as

KBr pellets. Diffuse reflectance spectra (DRS) were

obtained with a Shimadzu UV/3101 PC spectrophotometer.

Magnetic moments were measured on an EG&G PARC

vibrating sample magnetometer. Mettler Toledo thermo-

gravimetric analysis (TGA)/SDTA 851 instrument was

used for TGA. The morphology of functionalized poly-

styrene and complexes were analyzed using a scanning

electron microscope (SEM) (ZEISS EVO40, England)

equipped with energy dispersive spectroscopy analysis of

X-rays (EDX) facility. NMR spectra were obtained with a

Bruker AMX-400 NMR spectrophotometer using

DMSO.d6 as solvent and tetramethylsilane as an internal

standard.

2.3 Synthesis of Catalyst

The preparation of homogeneous and polymer supported

complexes is given in Scheme 1.

2.3.1 Synthesis of Schiff Base Ligand and Metal Complex

The Schiff base ligand, N-(hydroxyphenyl)salicyldimine,

was prepared by mixing o-aminophenol and salicylaldehyde

in a stoichiometric ratio [34]. The M–amp-Cl complexes

were prepared by reacting a metal chloride with the Schiff

base ligand in a stoichiometric ratio. To a methanolic solu-

tion (20 mL) of Schiff base ligand (0.01 mol) containing

KOH (0.01 mol) was added metal chloride (0.01 mol). The

resultant mixture was stirred for 1-h at room temperature in

air and a solid precipitated. The mixture was filtered and

washed thoroughly with water and ether, and dried over

fused CaCl2: NMR data for Ni–amp-Cl complex: 1H NMR

(DMSO.d6, 400 MHz), d (ppm): 9.02 (–CH=N), 6.88 (1H, t),

6.95 (3H, t), 7.13 (1H, t), 7.35 (1H, d), 7.39 (1H, d), 7.62 (H,

d), 13.91 (phenolic –OH); 13C NMR (DMSO-d6, 100 MHz)

d (ppm): 115.5, 117.8, 119.4, 120.6, 121.5, 122.7, 128.7,

132.2, 132.6, 140.8 (C-aryl), 161.2 (C–O), 156.5 (C–OH),

161.3 (CH=N).

718 J Inorg Organomet Polym (2012) 22:717–730

123

2.3.2 Synthesis of Immobilized Catalyst

The immobilized catalysts were readily prepared in three

steps. First, poly(4-aminostyrene) was prepared according

to the literature [35]. The poly(4-aminostyrene) was stirred

with an excess aqueous solution of metal chloride for 12-h

and then washed thoroughly with water. The metal-loaded

poly(4-aminostyrene) samples were then reacted with

solutions of Schiff base ligand in methanol to get the

immobilized metal complex (PS–M–amp-Cl).

2.4 General Procedure for the Heterogeneous

Oxidation

A mixture of catalyst (1.02 9 10-5 mol), acetonitrile

(ACN, 10 mL) and 5 mmol substrate was stirred under a

nitrogen atmosphere in a 50 mL two-necked round-bottom

flask equipped with a condenser. The reaction vessel was

placed in an oil bath at 50 �C and vigorously stirred. TBHP

(10 mmol, 70% in water) or H2O2 (30% in water) was

added. The resulting mixture was then stirred for 6-h under

N2. After filtration and washing with solvent, the filtrate

was concentrated and analyzed by gas chromatograph

(Varian 3400 equipped with a 30 m CP-SIL8CB capillary

column and a Flame Ionization Detector). The concentra-

tion of products was determined using cyclohexanone as an

internal standard. All reaction products were identified with

an Agilent GC–MS.

2.5 General Procedure for the Homogeneous Oxidation

The neat metal complex (1.02 9 10-5 mol) in ACN

(10 mL) and TBHP or H2O2 (10 mmol) were added to a

solution of the substrates (5 mmol). The resulting mixture

was then stirred at 50 �C for 6-h under N2. The solvent was

evaporated under reduced pressure. The crude product was

analyzed by GC and GC–MS. The concentration of prod-

ucts was determined using cyclohexanone as an internal

standard.

3 Results and Discussion

3.1 Characterization of Metal Schiff Base Complexes

Due to the insolubility of the polymer supported metal

complexes in all common organic solvents, their structural

investigation was limited to physicochemical properties,

chemical analysis, SEM, TGA, IR and UV–Vis spectral

data. Table 1 provides elemental analysis from which the

obtained values of the soluble metal complexes are quite

comparable to the calculated values. Elemental analysis

indicates that all of the complexes are formed by coordi-

nation of 1 mol of metal ion and 1 mol of Schiff base

ligand. The metal content of the polymer supported cata-

lysts was estimated by atomic absorption spectroscopy.

The chemical composition confirmed the purity and stoi-

chiometry of the neat and polymer supported complexes.

3.1.1 SEM and Energy Dispersive X-ray Analyses (EDAX)

Field emission-SEM for poly(4-aminostyrene) and polymer

supported complexes were recorded to examine the mor-

phological changes that occur in the polystyrene beads

during various stages of synthesis. The SEM images of

poly(4-aminostyrene) (A) and the immobilized copper (B),

cobalt (C) and nickel (D) complexes on the functionalized

polymer are shown Fig. 1. The pure poly(4-aminostyrene)

bead has a smooth surface. After metal loading on the

OH

NH2

HO

OHC

O

N CH

O

H

Cl

N CH

OH HO

M

MeOH

MCl2KOH

MeOH

+Reflux

M= Cu, Co and Ni

M-amp-Cl

(a)

N CH

O O

M

H

ClNH2P

CH CH2

n

M= Cu, Co and Ni

P = polystyrene framework =

(b)

Scheme 1 Synthesis of a homogeneous and b polymer supported

metal complexes

J Inorg Organomet Polym (2012) 22:717–730 719

123

polymer, a change in morphology of the polymer surface is

observed. As expected, the smooth and flat surface of the

polymer shows a slight roughening on complexation. The

presence of metals along with oxygen and chlorine can be

further surmised by EDX (Fig. 2), which suggests the

immobilization of the metal complexes on poly(4-

aminostyrene).

3.1.2 Fourier Transform Infrared Spectroscopy Spectra

A partial list of the IR spectral bands of the polymer-sup-

ported complexes and their respective neat complexes is

given in Table 1. The formation of Schiff base moiety is

indicated by the appearance of an absorption band at

1639 cm-1 due to the C=N stretching vibration. In neat

Table 1 Chemical composition

and IR stretching frequencies of

homogeneous and polymer

supported metal complexes

Calculated values are given in

parentheses

PS = amino-polystyrene

framework = –(C8H9N)n–a Infrared spectra measured as

KBr pellets

Compound C H N Cl M mC=Na mM-

OamM-

NamM-

Cla(%) (%) (%) (%) (%)

Cu–amp-Cl

(C13H10O2NClCu)

50.22

(50.16)

3.25

(3.21)

4.52

(4.50)

11.56

(11.41)

20.35

(20.42)

1,592 652 544 315

PS–Cu–amp-Cl–(C8H9N)n–

(C13H10O2NClCu)

58.49 4.35 6.44 8.21 1.55 1,641 651 543 314

Co–amp-Cl

(C13H10O2NClCo)

50.96

(50.91)

3.31

(3.26)

4.62

(4.57)

11.64

(11.59)

19.16

(19.23)

1,596 686 580 330

PS–Co–amp-Cl–(C8H9N)n–

(C13H10O2NClCo)

59.19 4.41 6.55 8.29 2.19 1,640 684 578 328

Ni–amp-Cl

(C13H10O2NClNi)

50.97

(50.95)

3.31

(3.27)

4.62

(4.57)

11.63

(11.60)

19.11

(19.16)

1,605 632 521 300

PS–Ni–amp-Cl– (C8H9N)n–

(C13H10O2NClNi)

59.20 4.41 6.52 8.30 1.49 1,641 630 520 298

Fig. 1 FE SEM images of

poly(4-aminostyrene) (a) and

polymer-supported complexes

of Cu(II) (b), Co(II) (c) and

Ni(II) (d)


123

Fig. 2 EDX plots of poly(4-

aminostyrene) (a) and polymer-

supported complexes of Cu(II)

(b), Co(II) (c) and Ni(II) (d)


123

metal complexes, this band shifts to lower frequency and

appears at 1,588–1,606 cm-1 and indicates coordination of

azomethine nitrogen atoms to the metal [36]. The phenolic

(C–O) stretching frequency is observed at 1,270 cm-1

(ligand), which shifts to a higher frequency region

(1,294–1,315 cm-1) in complexes; the shift indicates

coordination through phenolic oxygen atoms [37, 38]. In

addition, the interaction of metal ions with the ligand dis-

plays a new absorption band at 520–580 cm-1, which is

due to the formation of M–N bond between metal ions and

Schiff base in metal complexes, [39]. The complexation of

Cu(II), Co(II) and Ni(II) metals also results in absorption

bands at *630–688 cm-1 due to the formation of a bond

between the metal and the phenolic oxygen (M–O) [39].

The homogeneous metal (Cu(II), Co(II) and Ni(II)) com-

plexes show a characteristic M-Cl frequency at 315, 330

and 300 cm-1 [40], respectively. There is also a weak band

at 3,435 cm-1, which indicates the presence of –OH.

The FT-IR spectra of poly(4-aminostyrene) and poly-

mer-supported immobilized complexes were studied. The

intensity of the polymer-supported metal complexes is

weak due to the low concentration. The IR spectra of the

supported complexes are quite similar to those of the neat

metal complexes; i.e., poly(4-aminostyrene) exhibits strong

multiple bands at 3,400–3,200 cm-1(tNH2) and a shoulder

at 1,625 cm-1 (dNH2). The band for the primary amine in

poly(4-aminostyrene) is shifted to 1,590–1,600 cm-1 in the

supported complexes. The presence of Schiff base moiety

in the immobilized complexes is indicated by the appear-

ance of a band at 1,641 cm-1 due to the t(C=N) stretch.

3.1.3 TGA–DTA

The thermal stability of complexes was investigated by

TGA (heating rate, 10 �C/min in air from 30 to 500 �C,

Fig. 3). Poly(4-aminostyrene) decomposes from 190 to

200 �C. After complexation with metal ions, the thermal

stability of the immobilized complexes improves. The

polymer-supported Cu(II) Schiff base complex decom-

poses at 220 �C; the Co(II) and Ni(II) complexes decom-

pose at 330 and 240 �C. The polymer-supported metal

complexes degrade at a considerably higher temperature;

the Co(II) complex is the most stable.

3.1.4 NMR Spectra

In the 1H-NMR spectrum of the Schiff base ligand two

signals of the phenolic proton appear at 13.78 and

9.73 ppm, respectively. One signal at 9.73 ppm is not

present in the spectrum of the nickel complex, but a signal

is present at 13.91 ppm, which indicates that one O-atom

covalently is bonded to the Ni-atom and another O-atom

has a coordinate bond. The azomethine proton signal for

the ligand due to CH=N is observed at 8.96 ppm and is

shifted to downfield to 9.02 ppm in the complex. This

indicates that the azomethine nitrogen is coordinated with

the metal. Multiplets in the region 6.88–7.61 ppm are

assigned to the aromatic ring protons in the ligands and in

the Ni(II) complex.

3.1.5 Electronic Spectral Studies and Magnetic Moment

The electronic spectra of the Schiff base ligand and homo-

geneous metal complexes were measured in ACN (Fig. 4).

The ligand displays two absorption bands at 265 and

348 nm, which are assigned to the n ? p* and p ? p*

transitions, respectively. The Cu–amp-Cl complex exhibits a

100 200 300 400 500

Wei

ght l

oss

(%)

Temperature (°C)

PS-Cu-amp-Cl PS-Co-amp-Cl PS-Ni-amp-Cl

Fig. 3 Thermogravimetric weight loss for polymer-supported com-

plexes of Cu(II), Co(II) and Ni(II), respectively

250 300 350 400 450 500 550 600

d

c

b

a

Abs

orba

nce

Wavelength (nm)

Fig. 4 Electronic spectra of the Schiff base (a), Cu–amp-Cl (b), Co–

amp-Cl (c), and Ni–amp-Cl (d)


123

band at 425 nm (sh), which is assigned to the d ? d tran-

sition [41]. The band at 275 nm is presumably caused by

charge-transfer. The Co–amp-Cl complex exhibits band at

442 nm (sh), which indicates a d ? d transition [42]. The

band at 272 nm is assigned to charge transfer transition. The

Ni–amp-Cl complex exhibits three bands; i.e., the 245 and

268 nm bands are due to charge transfer and the band at

421 nm (sh) is a d ? d transition [43].

The magnetic moment for Cu(II) complex is 1.73 BM.

This value suggests a square planar geometry for the Cu(II)

ion. The Co(II) complex has a magnetic moment of 2.3

BM, which indicates that a low spin square planar complex

[44]. The Ni(II) complex is diamagnetic, which suggests a

square planar geometry.

3.1.6 UV–Visible DRS Spectroscopy

The electronic spectra of the polymer-supported metal

complexes were recorded in diffuse reflectance spectrum

mode using a MgCO3/BaSO4 disc as a reference. The Schiff

base showed absorption maxima at 267 and 348 nm,

whereas the DRS of the Cu/Co/Ni complexes after immo-

bilization on polymer showed shifts in the absorption max-

ima depending upon the metal–ligand interactions. This

result suggests that the metal-Schiff base complexes are

successfully grafted at the surface of the polymer support. In

the UV–Vis spectrum of polymer-supported Cu(II) complex,

a very weak, broad absorption band at 485 nm is observed.

This band may be attributed to the d ? d transition and

indicated a square-planar structure of the complex in the

polymer support [45]. Co- and Ni-containing materials also

showed similar structural features. Based on the above

results of elemental analysis, FT-IR, electronic spectra,

magnetic moment and thermal analysis, the structures of the

Cu(II), Co(II) and Ni(II) complexes at the polymer support

are suggested (Scheme 1).

3.2 Catalytic Activity

Since polymer-supported metal complexes exhibit catalytic

activity in a wide range of the industrially important

reactions, we have investigated the catalytic activity of

polymer-supported Cu(II), Co(II) and Ni(II) complexes in

the oxidation of cyclohexene, styrene and trans-stilbene at

50 �C using TBHP or H2O2 as an oxidant and ACN as

solvent under a N2 atmosphere. The activity and selectivity

of these heterogeneous complexes were compared with the

respective homogeneous metal complexes. Oxidation of

cyclohexene, styrene, trans-stilbene produce allylic oxi-

dation products as shown in Scheme 2.

The catalytic activity and selectivity of the polymer-

supported and homogeneous catalysts in the oxidation of

cyclohexene in the presence of TBHP or H2O2 after a 6-h

reaction time are given in Table 2. The catalytic activity of

the homogeneous complexes and control experiment

(without catalyst, Table 2, entry 1) under the identical

reaction conditions are also given. The control experiment

shows negligible activity. When homogeneous complexes

were supported on the polymer, the catalytic activity of the

complexes increases. This result demonstrates that the

catalytic activity is linked to the supported polymer matrix.

After separating the catalyst and the reaction products

from the reaction mixture, the resulting solution was ana-

lyzed by UV–Vis spectroscopy. No characteristic band of

metal was observed. This result suggests that the homo-

geneous complexes of Cu(II), Co(II) and Ni(II) are strongly

bonded to the polymer. Higher catalytic activity of the

polymer-supported catalysts relative to the homogeneous

counterpart is attributed to the fine distribution of the iso-

lated catalytic site in the supported polymer matrix.

For the oxidation of cyclohexene, allylic oxidation

occurs with the formation of 2-cyclohexene-1-one and

2-cyclohexene-1-ol in all cases. The formation of

O OH

O

OCHO

CHO

OO

O

+ +

+

+

(a)

(b)

(c)+

Scheme 2 Oxidation products

of a cyclohexene, b styrene and

c trans-stilbene


123

2-cyclohexene-1-one and 2-cyclohexene-1-ol suggests the

preferential attack of the activated allylic *C–H bond over

the olefinic C=C bond. Clearly, TBHP promotes the allylic

oxidation pathway and epoxidation is minimized. When the

oxidant is H2O2, the selectivity of epoxide is higher for all

the olefins. In the oxidation of cyclohexene, although

selectivity is different, both oxidants, TBHP and H2O2, in

the presence of the polymer-supported metal complexes

gives a mixture of allylic oxidation products and cyclo-

hexene-oxide under similar reaction conditions. These

results indicate that oxidation reactions occur via a com-

mon intermediate for both oxidants.

The effect of various solvents was studied in the oxi-

dation of cyclohexene with PS-M–amp-Cl catalysts

(Fig. 5). The oxidation reactions were carried out in protic

and aprotic solvents. In all cases, 2-cyclohexene-1-one was

formed as the major product. When the reaction was car-

ried out in a coordinating solvent like ACN, conversion is

increased. This might be due to the polarity of the solvent

and solubility of substrates and oxidants. The efficiency of

these catalysts for the oxidation of cyclohexene in different

solvents decreases in the following order: ACN [ metha-

nol [ chloroform [ dichloromethane. The polymer-sup-

ported Cu(II) catalyst was used in the presence of TBHP to

oxidize cyclohexene at room temperature (Table 2, entry

8) and 50 �C (Table 2, entry 5). The results suggested a

considerable increase in conversion and selectivity with

rise in temperature.

Benzaldehyde is formed in the oxidation of styrene

using TBHP or H2O2 in ACN over polymer-supported

metal complexes. The catalytic activity of these polymer-

supported complexes were compared to those of the

corresponding non-polymer bonded complexes. Table 3

summarizes the activity and selectivity details of various

products obtained in a 6-h reaction. The catalytic effect

of the non-polymer-bonded complexes are also good.

Moreover, the combination of easy recovery of the

anchored catalysts, no leaching and recycling ability

make these materials better than the homogeneous

complexes. The selectivity for different oxidation prod-

ucts varies in the following order: benzaldehyde [ sty-

rene oxide. Benzaldehyde was obtained in the highest

yield in all cases.

The control experiment (Table 3, entry 1) was tested at

first. In the absence of a catalyst, the oxidant was unable to

oxidize styrene to a significant extent. After adding the

catalyst, a significant conversion of styrene occurred.

The polymer-supported Cu(II) catalyst was used to

study the temperature effect in styrene oxidation using

TBHP (Table 3, entries 6 and 8). Oxidation of styrene was

slow at room temperature, but proceeded at 50 �C. Com-

parison of the activity of the polymer-supported transition

metal complexes was also carried out. In all cases benz-

aldehyde was the major product with small amounts of

styrene oxide. When H2O2 was used, the selectivity for

benzaldehyde decreased slightly with an increase in styrene

oxide.

Both homogeneous and polymer-supported metal com-

plexes were also used for the oxidation of trans-stilbene

where benzaldehyde, benzil and trans-stilbene oxide are

obtained (Scheme 2). The catalytic activity of the polymer-

supported metal complexes is compared to the homogeneous

complexes; the results are given in Table 4. The results show

that polymer-supported metal complexes give higher

Table 2 Oxidation of

cyclohexene catalyzed by

homogeneous and polymer-


Reaction conditions:

cyclohexene (5 mmol), oxidant

(10 mmol), catalyst

(1.02 9 10-5 mol), ACN

(10 mL), temperature (50 �C)a Determined by GCb 2-Cyclohexene-1-onec 2-Cyclohexene-1-old Cyclohexe oxidee Reaction carried out at room

temperature

S.

no.

Catalyst Reaction time

(h)

Oxidant Conversion

(%)aSelectivity (%)a TOF

(h-1)Ketoneb Alcoholc Oxided

1 Blank 6 TBHP 6 70 24 6 –

H2O2 4 58 28 14 –

2 Cu–amp-Cl 6 TBHP 61 79 18 3 49.84

H2O2 45 67 23 10 36.76

3 Co–amp-Cl 6 TBHP 57 74 22 4 46.57

H2O2 39 62 28 10 31.86

4 Ni–amp-Cl 6 TBHP 52 69 27 4 42.48

H2O2 36 56 32 12 29.41

5 PS–Cu–

amp-Cl

6 TBHP 82 83 10 7 66.99

H2O2 68 70 16 14 55.55

6 PS–Co–

amp-Cl

6 TBHP 73 75 17 8 59.64

H2O2 60 61 26 13 49.02

7 PS–Ni–amp-

Cl

6 TBHP 64 69 21 10 52.29

H2O2 53 54 30 16 43.30

8e PS–Cu–

amp-Cl

6 TBHP 31 72 17 11 25.33


123

conversion. The control experiment suggests that a catalyst

is required for the reaction; i.e., oxidant alone slowly oxi-

dizes the substrates (Table 4, entry 1). All the complexes

produced benzaldehyde as the major product; and, the

selectivity of the different oxidation products varied in the

following order: benzaldehyde [ benzil [ trans-stilbene

oxide.

The efficiency of different polymer-supported transition

metal complexes was examined. Polymer supported Cu(II)

complex exhibited better catalytic activity than the other two

complexes. The effect of temperature on the conversion of

trans-stilbene using polymer-supported Cu(II) complex

suggested that conversion and selectivity increased with

temperature (Table 4, entries 5 and 8).

3.3 Influence of Catalyst Support

To demonstrate the effect of support on the catalytic

activity of homogeneous metal complexes in the oxidation

of alkenes using TBHP or H2O2, all the oxidation reac-

tions under the optimized reaction conditions and with

polymer-supported catalysts were repeated. The results are

given in Tables 2, 3 and 4 and show that in alkene oxi-

dation the conversions, turnover frequencies, selectivity

and stability of the heterogeneous catalysts are better than

the homogeneous catalysts. In alkene oxidation, the for-

mation of by-products decreased in the heterogeneous

relative to homogeneous systems. The greatest disadvan-

tage of homogeneous complexes is catalysts degradation

in the presence of oxidant while heterogeneous complexes

can be reused several times without significant loss of its

activity.

3.4 Comparison Between Polymer-Supported Metal

Complexes

Efficient oxidation catalysts are obtained by immobilizing

homogeneous metal complexes on poly(4-aminostyrene).

The conversion of cyclohexene, styrene and trans-stilbene

using TBHP is given in Fig. 6. The percent of conversion

of cyclohexene follows the order: 82% (for PS–Cu–amp-

Cl) [ 73% (for PS–Co–amp-Cl) [ 64% (for PS–Ni–amp-

Cl) where all the catalysts produce allylic oxidation

products. All the catalysts follow the same selectivity

pattern of products. However, PS–Cu–amp-Cl produces

comparatively higher allylic products. In the case of sty-

rene and trans-stilbene, all the catalysts follow the same

pattern.

Table 3 Oxidation of styrene

catalyzed by homogeneous and

polymer-supported metal

complexes

Reaction conditions: styrene

(5 mmol), oxidant (10 mmol),

catalyst (1.02 9 10-5 mol),

ACN (10 mL), temperature

(50 �C)a Determined by GCb Benzaldehydec Styrene oxided Reaction carried out at room

temperature

S. no. Catalyst Reaction time

(h)

Oxidant Conversion (%)a Selectivity (%)a TOF (h-1)

Aldehydeb Oxidec

1 Blank 6 TBHP 3 83 17 –

H2O2 2 79 21 –

2 Cu–amp-Cl 6 TBHP 69 84 16 56.37

H2O2 46 72 28 37.58

3 Co–amp-Cl 6 TBHP 60 78 22 49.02

H2O2 41 67 33 33.50

4 Ni–amp-Cl 6 TBHP 54 75 25 44.12

H2O2 39 61 39 31.86

5 PS–Cu–amp-Cl 6 TBHP 87 84 16 71.08

H2O2 69 76 24 56.37

6 PS–Co–amp-Cl 6 TBHP 79 79 21 64.54

H2O2 61 67 33 49.84

7 PS–Ni–amp-Cl 6 TBHP 70 76 24 57.19

H2O2 56 57 43 45.75

8d PS–Cu–amp-Cl 6 TBHP 26 69 31 21.24

Acetonitrile

Methanol

Chloroform

Di-chloromethane

0 10 20 30 40 50 60 70 80 90

Conversion (%)

PS-Ni-amp-Cl PS-Co-amp-Cl PS-Cu-amp-Cl

Fig. 5 Effect of solvent on the conversion of cyclohexene by

polymer supported metal complexes. Reaction conditions: cyclohex-

ene = 5 mmol, [TBHP] = 10 mmol, catalyst (1.02 9 10-5 mol),

temperature (50 �C), time (6 h)


123

Many efficient heterogeneous catalysts have been

reported for the oxidation of alkenes using TBHP or H2O2

[25, 46–49]. The results are summarized in Table 5.

Comparison with previously reported catalysts revealed

that the present systems give higher product yields.

4 Reaction Mechanism

A plausible reaction mechanism for the oxidation of

cyclohexene, catalyzed by polymer-supported metal com-

plexes, is summarized in Schemes 3, 4 and 5 [50]. In all

cases the major product is 2-cyclohexene-1-one

(Scheme 3). Thus, the peroxide replaces chloride and

forms I that reacts with cyclohexene to form II (tert-bu-

tuoxide in case of TBHP or hydroxide in case of H2O2).

Then a proton is abstracted to produce tert-butanol

(detected in GC) and water. II is converted to III. TBHP,

which is present in the medium, helps in hydride abstrac-

tion to form the major product, 2-cyclohexene-1-one, the

hydroxylated metal complex and tert-butanol. After the

formation of III, a proton is abstracted from the oxidant to

give the allylic alcohol and I (Scheme 4). The epoxide is

formed in very small amounts (Scheme 5). II is formed;

and, a nucleophilic attack by the peroxide takes place to

yield an epoxide.

5 Stability and Recycling of Catalysts

The UV–Vis spectra of the reaction solution, in the first

run, does not exhibit absorption peaks characteristic of the

metals, which indicates that leaching of metals does not

occur during the oxidation reaction. To check the leaching

of metals into solution during the cyclohexene oxidation,

the following experiment was carried out under the opti-

mum reaction conditions. The reaction was stopped after

3-h. The catalyst was filtered and the filtrate was allowed to

react for another 3-h under the same reaction conditions.

No further conversion was observed (GC analysis). These

results suggest that the homogeneous complexes remain

supported on the polymer matrix and the catalysts are

heterogeneous.

Table 4 Oxidation of trans-

stilbene catalyzed by

homogeneous and polymer-


Reaction conditions: trans-

stilbene (5 mmol), oxidant

(10 mmol), catalyst

(1.02 9 10-5 mol), ACN

(10 mL), temperature (50 �C)a Determined by GCb Benzaldehydec Benzophenoned trans-Stilbene oxidee Reaction carried out at room

temperature

S.

no.

Catalyst Reaction time

(h)

Oxidant Conversion

(%)aSelectivity (%)a TOF

(h-1)Aldehydeb Ketonec Oxided

1 Blank 6 TBHP 5 71 19 10 –

H2O2 4 57 28 15 –

2 Cu–amp-Cl 6 TBHP 57 76 17 7 46.57

H2O2 44 65 22 13 35.95

3 Co–amp-Cl 6 TBHP 54 73 23 4 44.12

H2O2 39 62 26 12 31.86

4 Ni–amp-Cl 6 TBHP 49 67 25 8 40.03

H2O2 35 55 30 15 28.59

5 PS–Cu–

amp-Cl

6 TBHP 75 78 12 10 61.27

H2O2 63 69 16 15 51.47

6 PS–Co–

amp-Cl

6 TBHP 68 75 16 9 55.55

H2O2 52 59 25 16 42.48

7 PS–Ni–

amp-Cl

6 TBHP 59 69 20 11 48.20

H2O2 48 54 28 18 39.22

8e PS–Cu–

amp-Cl

6 TBHP 28 75 13 12 22.87

Fig. 6 Bar graph showing the conversion of cyclohexene, styrene

and trans-stilbene with polymer supported metal complexes. Reaction

conditions: substrate = 5 mmol, [TBHP] = 10 mmol, catalyst

(1.02 9 10-5 mol), temperature (50 �C), ACN (10 mL), time (6 h)


123

The IR spectrum of the recycled catalyst is similar to

that of a fresh sample and indicates the heterogeneous

nature of these complexes. Elemental analysis of the

recovered catalysts indicates no reduction in the amount of

transition metals present. Thus, the complexes may be

recycled for the oxidation reaction without loss in activity.

Table 5 Oxidation of alkenes with TBHP and H2O2 catalyzed by a variety of catalysts

Catalyst Reaction condition Conversion (%) References

Cyclohexene Styrene trans-stilbene

PS–Cu–amp-Cl

PS–Co–amp-Cl

PS–Ni–amp-Cl

ACN, TBHP, 50 �C, 6 h 82

73

64

87

79

70

75

68

59

This

study

ACN, H2O2, 50 �C, 6 h 68

60

53

69

61

56

63

52

48

CuCl16Pc-MCM-41 ACN, TBHP, 40 �C, 8 h – 47 – [46]

[Co(habenzil)]-Al2O3

[Ni(habenzil)]-Al2O3

[Cu(habenzil)]-Al2O3

CH3Cl, TBHP, reflux, 8 h 72.4

42.6

53.2

– – [47]

PS-[Cu(ligand)n] ACN, H2O2, 80 �C, 6 h 51 57 – [48]

[–S2{VO(sal-dach)�DMF}–]n ACN, H2O2, 75 �C, 6 h – – 23 [46]

PS-[VO(fsal-b-ala) DMF] ACN, H2O2, 75 �C, 6 h 35.6 45.6 16.2 [49]

O O

M

NH2 Cl

O O

M

NH2P

O O

M

NH2P O

HH

R

P

O O

M

NH2P O H

O O

M

NH2P OH

ROH

RO

OH

RO

OH

RO

O

+

O-

O

+ +

(I)

(II)

(III)

(IV) R = -H or -tBu

*

Scheme 3 Suggested

mechanism for the preparation

of 2-cyclohexene-1-one


123

O O

M

NH2P

OH

O O

M

NH2 Cl

O O

M

NH2P

O O

M

NH2P O

HH

R

P

O O

M

NH2P O H

RO

OH

RO

OH

RO

RO

O

+

(I)

R = -H or -tBu

O

+

O-

(I)

(II)

(III)

*

Scheme 4 Suggested

mechanism for the preparation

of 2-cyclohexene-1-ol

O O

M

NH2 Cl

O O

M

NH2P

O O

M

NH2P O

R

P

O O

M

NH2P

O R OH

RO

OH

R O

O H

RO

O

RO

O

+

O-

+

(I)

(II)(I)

R = -H or -tBu

Scheme 5 Suggested

mechanism for thew preparation

of cyclohexene oxide


123

The recyclability of a catalyst is important. The recy-

cling experiment confirms the heterogenization of the

complexes on the polymer. To investigate the reusability of

polymer-supported metal complexes, the catalysts were

separated by filtration after the first catalytic reaction. The

catalyst was recovered by washing with solvent and drying

under vacuum. The material was then subjected to the

second run with further addition of substrates under opti-

mum reaction conditions. The nature and yield of the final

products are comparable to he original reaction. Figure 7

illustrates the reusability of the catalysts for the oxidation

of cyclohexene using TBHP for five recycles. The catalytic

activity is essentially unchanged after five repeat runs.

6 Conclusions

An anchored Cu(II), Co(II) and Ni(II) complexes on

polymer matrix was prepared. The heterogeneous catalysts

show high catalytic activity in oxidation of cyclohexene,

styrene and trans-stilbene. Both the heterogeneous and

homogeneous complexes produce allylic oxidation prod-

ucts selectively. The oxidant alone slowly oxidizes the

substrates but heterogeneous and homogeneous complexes

speed up the conversion. The results show that the immo-

bilized catalysts are more active than the homogeneous

catalysts. The heterogeneous catalysts can be reused five-

times without significant decrease in initial activity. The

active sites do not leach from the support and can be reused

without appreciable loss of activity. A leaching test indi-

cates that the catalytic reaction is mainly heterogeneous.

Acknowledgments We thank the Indian Association for the Culti-

vation of Science, Kolkata for providing the instrumental support. MI

acknowledges Department of Science and Technology (DST),

Council of Scientific and Industrial Research (CSIR) and University

Grant Commission (UGC), New Delhi, India for funding.

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efficient allylic oxidation of olefins catalyzed by polymer supported metal schiff base complexes...

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