Transition Metal Complexes of Schiff Base Ligands as ... · catalytic activity of metal complexes of different ... reaction mechanism is unclear, the peroxo titanium species activated

Imperial Journal of Interdisciplinary Research (IJIR)

Vol-2, Issue-12, 2016

ISSN: 2454-1362, http://www.onlinejournal.in

Imperial Journal of Interdisciplinary Research (IJIR) Page 448

Transition Metal Complexes of Schiff Base Ligands

as Efficient Catalysts for Epoxidation of Alkenes

Aniket A. Pawanoji1 & Bipin H. Mehta

2

1 K.J. Somaiya College of Science and Commerce, Mumbai,

2University of Mumbai,

Mumbai

Abstract: Chiral Schiff base complexes are highly

selective in reactions such as epoxidation of alkenes.

Transition metal complexes of Schiff bases possess

high catalytic activity in comparison to their

unsupported analogues. In the present study

catalytic activity of metal complexes of different

types of Schiff bases for epoxidation of alkenes was

analyzed in this review. The Schiff base complexes

of manganese(II), iron(II), cobalt(II), nickel(II)

Chromium(III) and other ions are used as catalysts

in the epoxidation. The manganese(III) Schiff base

complexes exhibited high catalytic activity in the

epoxidation of alkenes both in homogeneous and

heterogeneous conditions. Polymer-supported

Schiff base complexes have demonstrated higher

activity than unsupported Schiff base complexes of

metal ions. The oxidation of styrene, indene, stilbene

and its alkyl derivatives was catalyzed significantly

by polymer-supported Schiff base complexes of

different metal ions. The recyclability of polymer-

supported Schiff base complexes is evaluated in this

review.

Keywords: Schiff base complexes, epoxidation,

catalyst, transition metal.

1. Introduction

Schiff base complexes have a high potential to

act as versatile catalysts for organic synthesis

resulting in a remarkable development in recent

times. Several families of transition metal

compounds are extensively used in a variety of

chemical transformations such as hydration [1],

hydrogenation [1], isomerization [2],

decarbonylation [3], cyclopropanation [4], olefin

metathesis [5], oxidation of hydrocarbon [6-12],

epoxidation [13-16], nitro aldol reaction [17], Diels-

Alder reaction [18], enol-ester synthesis [19] and

other related catalytic process [1]. Schiff base metal

complexes are important systems in asymmetric

catalysis [20]. The versatility of the steric and

electronic properties can be fine-tuned by choosing

appropriate amine precursors and ring substituents.

Metal complexes of Schiff bases derived from

aromatic carbonyl compounds have received a great

deal of attention in connection with studies on

asymmetric catalysis and metalloprotein modeling

[21-36]. Over the last few years, several reports have

appeared on synthesis of Schiff base complexes and

evaluation of their catalytic activities in epoxidation

reaction [37-45] hence; the need of such review

article highlighting the catalytic activity of Schiff

base complexes was comprehended. In this review,

the report on the Schiff base complexes in

epoxidation reactions is presented for the period of

1980-2016.

Catalytic epoxidation of alkenes is a significant

industrial reaction for the production of a wide

variety of fine chemicals [46] derived directly from

alkenes, a primary petrochemical source.

Researchers worldwide investigated olefin

epoxidation reactions that are catalyzed by a metal

complex and this area is an important research topic

in organic synthesis [47-52]. The last few decades

witnessed the complexes of early transition metals

such as rhenium [53], titanium [54, 55], manganese

[56] and molybdenum [57-59] used as homogeneous

catalysts in the alkene epoxidation.

In recent years, styrene oxide has been used as a

preparatory material for epoxy resins, flavoring

agents in food, tobacco, soap, cosmetic essences,

perfumes, and pharmaceuticals [60, 61]. Catalytic

pathways via the formation of high-valent metal-oxo

intermediates involving some of the transition metal

ions are well established [46]. Chiral salen Mn(III)

complexes are found to be highly enantioselective

for the asymmetric epoxidation of conjugated cis

disubstituted and trisubstituted olefins [62-64].

Heterogeneous systems as compared to homogenous

counterparts have the inherent advantages of easy

separation, enhanced handling properties and higher

stability; therefore, heterogenization of such

complexes is widely considered [65-68]. As an

application, Janssen et al [69] synthesized a dimeric

form of Mn(III) salen ligand and retained metal

complex in the cross-linked polymer membrane to

use as a catalyst for epoxidation. Asymmetric

epoxidation of alkenes is a powerful method for the

synthesis of chiral intermediates in the

pharmaceutical and agrochemical fields [70].

Among the most beneficial systems for the

asymmetric epoxidation of non-functionalized

olefins is the Jacobsen–Katsuki Salen(Mn)–catalyzed reaction [71, 72]. The (salen)Mn catalyst

http://www.onlinejournal.in/



Vol-2, Issue-12, 2016



system has been shown to be effective for the

epoxidation of an impressive variety of

unfunctionalized conjugated olefins [73]. Olefins

could be effectively epoxidized by molecular

oxygen, catalyzed by porous silica support metal

Schiff base complexes [74].

2. Schiff base ligands

Schiff base ligands are formed by the

condensation of aromatic or aliphatic primary

amines with aldehydes. The resultant imines

participate in binding with metal ions via nitrogen

lone pair electrons [75, 76]. Like aldehydes, the

ketones are also able to form Schiff base ligands,

although Schiff base ligand with ketones are formed

less readily than with aldehydes. The mono, di-, tri-

and multi-dentate chelating Schiff base ligands are

designed according to the binding environments of

metal ions [77, 78].

2.1 Epoxidation

The epoxidation of hydrocarbons is an

important reaction in chemical industry. Metal

complex catalysis plays a significant role in the

selective, partial oxidation of both saturated and

unsaturated hydrocarbons into useful products. OO

Scheme 1.Epoxidation reaction

Epoxidation (Scheme 1) is an important

reaction in organic synthesis because the formed

epoxides are intermediates that can be converted to a

variety of products. This reaction is widely used in

asymmetric cases since it can lead to two chiral

carbons in one step [79].

2.2 Schiff base catalyzed

epoxidations

Matsumoto et al [80] developed another

approach to M(salalen) complexes: the

transformation of Ti(salen) complexes 1 to the

corresponding di-m-oxo Ti(salalen) complexes 2 via

Meerwein–Ponndorf–Verley reduction (Scheme 2).

In the above complex, salalen ligands adopted a cis

β conformation. Ti(salalen) complex 2 catalyzed

various cyclic and acyclic conjugated olefins into

corresponding epoxides with high to excellent

enantioselectivity (Table 1, Scheme 3). Although the

reaction mechanism is unclear, the peroxo titanium

species activated by an intramolecular hydrogen

bonding with the amino proton has been proposed to

be the active species, on consideration of the

difference in catalysis between Ti(salen) and

Ti(salalen) complexes.

Table 1. Asymmetric epoxidation of olefins

catalyzed by Ti(salalen) complex 2.

OR1

R2

R3

30% H2O2+2 (1 mol%)

CH2Cl2, rt

R1

R2

R3 Entry Product yield (%) ee (%)

1a O

87 99

2 O

90 93

3

O

75 95

4 O

Ph

64 88

5b O

n-C6H13

70 82

aAcOEt was used as the solvent.

b3 mol% of 2 was used.

O

O

O O

NN

M

O

O

O O

NN

M

peroxo - inactive

with Ti(salen) hydrogen-bonded peroxo - active

with Ti(salalen)

1 2

OHOH

NN

Ph Ph Ph Ph

OO

N

Ti

NH

(S or R) (S or R) (S or R) (S or R)

1) Ti(Oi-Pr)4 CH2Cl2

2) H2O

2 (possessing (S,S)-

cyclohexanediamine moiety)

2

O

ON

ON

Ti

O

O

H

ON

ON

Ti

O

O

i-Pr i-Pr

Scheme 2. Synthesis of di-µ-oxo Ti(salalen)

complex 2 via MPV reduction




Vol-2, Issue-12, 2016



O

+ 30% H2O2

(1.01 equiv.)

2 (1 mol%)

CH2Cl2, rt

99%, >99% ee

Scheme 3.Asymmetric epoxidation of 1,2-

dihydronaphthalene by Ti(salalen) complex 2

Soriente et al [81] synthesized a new octahedral

titanium complex bearing a bulky binaphthyl

bridged Schiff base ligand. This complex has shown

to be an effective catalyst for the epoxidation of

allylic alcohols.

The vanadyl salen complexes (VOLx; x = 7–18)

were tested as catalysts (3, Fig. 1) for aerobic

oxidation of hydrocarbons by Boghaei and Mohebi

[82]. The unusual catalytic properties were observed

with simple salen ligand systems. Under typical

conditions, cyclohexene oxidised to mixture of

cyclohexene oxide, 2-cyclohexene-1-ol and 2-

cyclohexene-1-one in presence of major complexes.

The vanadium catalyzed aerobic oxidation of

cyclohexene proceeds with a moderate selectivity

for epoxidations. (>50% for VOLx; x = 8, 16, 17)

B

A

N

O

VO

O

D

N

C

(CH2)n

VOLx (x = 7-18)

n = 2,3A,C = H, OHB,D = H, CH3

Figure 1.Structures of Vanadyl salen complexes 3

Chirally modified vanadium complexes [83] are

used as catalysts in enantioselective epoxidations of

allylic alcohols and asymmetric sulfide to sulfoxide

oxidations. For both reactions various combinations

of chiral ligands and vanadium sources were tested.

Rayati et al [84] have synthesized

oxovanadium(IV) complexes (4, Fig. 2) of three

N2O2 tetradentate Schiff base ligands (H2L1–3

) by

reaction of 1,2-propylenediamine and appropriate

aldehyde and ketone. These complexes are used as

catalyst for the selective epoxidation of olefins.

High selectivity of epoxidation for cyclooctene is

observed from oxidation data. The catalytic activity

increases as the number of electron donor groups

increases and the catalytic selectivity is varied by

changing the substituents on the ligands. The

catalytic system described here is an efficient and

inexpensive method for the oxidation of olefins with

the advantages of high activity, selectivity, re-

usability and short reaction time. Rayati et al [85-87]

have also studied epoxidation of various alkenes

using oxo-vanadium(IV), Dioxo-molybdenum(VI)

Schiff bases complexes as catalysts.

O O

N N

O

V

X

Y

R

X

Y

R

Complex

VOL1

VOL2

VOL3

R

H

H

Me

X, Y

XY: -CH=CH-CH=CH

Y: H, X: Br

X,Y: H

Figure 2.Structure of oxovanadium(IV) complexes

4

Yaul et al [88] synthesized vanadium

complexes with quadridentate Schiff bases. The

catalytic activity of these complexes was evaluated

using H2O2 as oxidant for styrene oxidation.

Bezaatapour et al [89-90] synthesized Ni(II) and

vanadyl Schiff base which showed good catalytic

activity for the epoxidation of cyclooctene using

TBHP as oxygen source in acetonitrile. Mandal et al

[91] synthesized homogeneous and heterogeneous

mononuclear and dinuclear oxido vanadium Schiff

base complexes which were found to be effective

catalysts for epoxidation of styrene. Catalytic

activity of the metal complexes was greatly

influenced by the nature of the ligand and redox

potential of the metal complexes. It was discovered

that metal complexes with ligands having an

electron pushing group or having higher conjugation

show lesser catalytic activity then the unsubstituted

or those having less conjugated ligand system.

Parida et al [92] carried out covalent

functionalization of vanadium Schiff base complex

onto the CP-MCM-41. These catalysts showed

pronounced efficiency with NaHCO3 as co-catalyst

and H2O2 as oxidant in a CH3CN medium at room

temperature. This method is green and economical

from environment point of view. Zamanifar and

Farzaneh [93] prepared vanadium complexes of N-

salicylidene-L-histidine immobilized on Al-MCM-

41 designated as VO(Sal-His)/Al-MCM-41 which

were found to catalyze the epoxidation of trans-2-

hexen-1-ol, cinnamyl alcohol and geraniol with

TBHP.

Grivani et al [94] synthesized oxovanadium(IV)

Schiff base complex, VIV

OL2 (Scheme 4), by the

reaction of a bidentate Schiff base ligand L and

VO(acac)2 (L=N-salicylidin-2-chloroethylimine).

The catalytic activity of the complex 5 was tested in

the epoxidation of cyclooctene. The results showed

that the complex 5 was highly active and selective

catalyst in optimized conditions in the epoxidation

of cyclooctene. The proposed mechanism for the

epoxidation of alkenes by the oxovanadium complex

VOL2 in the presence of TBHP is shown in Scheme

5. Grivani et al [95-97] have observed similar results

in the case of vanadium complexes.




Vol-2, Issue-12, 2016



OH

O

HCl.NH2

Cl

Cl

N

OH

Cl

Cl

O

NO

ON

V

+CH3OH/NaOH

Reflux

VO(acac)2

CH3OH/NaOH

5

Scheme 4. Synthesis of Schiff base ligand L and its

oxovanadium(IV) complex

Jana Pisk et al [98] reported a series of

dinuclear and mononuclear oxovanadium(V)

complexes containing tridentate Schiff base ligands

derived from pyridoxal and appropriate

thiosemicarbazide or hydrazide. All vanadium

complexes have been used as (pre) catalysts at

0.05% loading for epoxidation reactions of cis-

cyclooctene under “green” reaction conditions. Che and Huang [99] synthesized (R)-

[CrIII

(L4)Cl]-MCM-41(m) which are used for

epoxidation of alkenes 6-9 (Scheme 6) is an unique

example of heterogenized metal catalysts with chiral

binaphthyl Schiff base ligands.

OHO

OHO

O

OO

OH

LVIVO

I

LVVO

II

LVV

III

Scheme 5. The proposed mechanism for the

epoxidation of alkenes by the oxovanadium complex

VOL in the presence of TBHP

PhIO

O

Cl

(R)-[CrIII(L4)Cl]-MCM-41(m)

+

6-9 ee, 42- 73%

6 7 8 9

Scheme 6. Epoxidation of alkenes with

Chromium(III) metal complexes

A. Aziz [100] studied the interaction of the

tetradentate Schiff base N,N’-bis(salicylidene)4,5-

dichloro-1,2-phenylenediamine (H2L) with M(CO)6

(M = Cr and Mo) under different conditions and

afforded the dicarbonyl precursors [Cr(CO)2(H2L)],

[Mo(CO)2(L)], the oxo complex [Cr(O)(L)] and the

dioxo complex [Mo(O2)(L)]. The reported

complexes were investigated in the epoxidation of

cyclooctene, cyclohexene, 1-octene and 1-hexene.

Chromium(III) salen complexes [101]

asymmetrically epoxidised E--methylstyrene with

up to 83% ee in the presence of phosphine oxides,

DMSO or DMF. In all cases the analogous reaction

with Z--methylstyrene gives lower ee.

Horwitz and coworkers [102] studied

electrochemical reduction of [MnIII

(salen)]+ (salen =

N,N'-ethylenebis(salicylaldiminato)) complexes in

acetonitrile solution in the presence of benzoic

anhydride, 1-methylimidazole, dioxygen, and an

olefin. Manganese-peroxo complex is formed in

which the O-O bond is subsequently heterolyzed by

reaction with benzoic anhydride to yield a Mn(V)-

oxo complex. Cis cyclooctene and trans-2-octene

gave a cis/trans reactivity ratio of 10 for the

electrolytic epoxidation. Electrolysis of cyclohexene

yields primarily cyclohexene oxide and significant

quantities of the allylic oxidation products 2-

cyclohexen-1-one and 2-cyclohexen-1-ol.

Chiral manganese(II) complexes [103] of 1,2-

bis(salicylideneamino) cyclohexane entrapped in

zeolite showed catalytic activity in the

enatioselective epoxidation of alkenes. The

manganese(II) complexes of bis(2-pyridinaldehyde)

ethylenediamine, bis(2-pyridinaldehyde) propylene

diamine ligands were used in epoxidation of olefins

but epoxide selectivity was observed only in the

presence of PhIO oxidant.

Skarzewski et al [104] carried out two-phase

epoxidation of olefins with sodium hypochlorite,

catalyzed by the salen-type Mn(III) complex 10,

(Fig. 3) was examined in the presence of various

additional ligands and 4-dodecyloxypyridine-1-

oxide was found the most effective for epoxidation.

Researchers [105, 106] studied similar epoxidations

using Mn(III)-tridentate complexes as a catalyst.

t-Bu

t-But-Bu

t-Bu OCl

O

N

Mn

N

Figure 3. Structure of salen-type Mn(III) complex

10

Minutolo et al [107] studied polymer-supported

manganese(III) salen complexes for heterogeneous

asymmetric epoxidation of unfunctionalized alkenes




Vol-2, Issue-12, 2016



in presence of mCPBA/NMO (65–72%). The

polymer-supported chiral manganese(III) salen

complexes [108] were used in the epoxidation of

styrene derivatives in the presence of PhIO, which

produced 16–46% ee. Li et al [109] Mono-Schiff

base Mn(III) complexes with pendant aza-crown or

morpholino substituents, and studied aerobic

oxidation of p-xylene to p-toluic acid.

Seyedi et al [110] elaborated the effect of crown

ether ring bonded to Mn(III) Schiff base catalysts on

the efficiency of these catalysts for the epoxidation

of cyclohexene and cyclooctene by KHSO5. The

catalysts catalytic activity emerges completely when

an aromatic nitrogen base such as pyridine as axial

ligand and potassium nitrate when added to the

reaction mixture. Thatte et al [111] prepared four

unique, heterogeneous, chitosan-based Schiff base

substituted with triazene, ethylene diamine and

salicylaldehyde and their manganese, copper, cobalt

and nickel complexes. Efficiency of these catalysts

was tested with L-carvone, Limonene, cis-stilbene,

trans-stilbene, α-terpeneol, α-methyl styrene.

Katia et al [112] synthesized new chiral Schiff

base complexes obtained by condensation of 2,2’-diamino-1,1’-binaphthalene or 1,2-diaminocyclo-

hexane and differently substituted salicylaldehydes

(bearing tert-butyl, H, Cl, or Br). The manganese

complexes containing the diiminobinaphthyl residue

have been tested as catalysts for the epoxidation of a

cis-disubstituted prochiral olefin, 1,2-dihydro-

naphthalene. In spite of a chiral manganese

environment, the catalytic activity and

enantioselectivity of these complexes were found to

be lower as compared to those of their

diiminocyclohexyl analogues. This low activity is

probably due to extensive decomposition of the

manganese binaphthyl based complexes under the

oxidation reaction conditions. One possible

explanation for the instability of these manganese

complexes is the highly distorted geometry of the

Salbinapht ligands which are highly compatible with

tetrahedral rather than octahedral or square

pyramidal coordination.

Che and co-workers [113-115] studied the

epoxidation of 1,2-dihydronaphthylene 11, cis--

methylstyrene 12, 4-chlorostyrene 13, styrene 14, 3-

chlorostyrene 15, 3-nitrostyrene 16 and 4-methyl-

styrene 17 (Fig. 4) with PhIO by employing

manganese catalysts formed in situ from

Mn(OAc)3.xH2O with different ee values. (Scheme 7

Reaction 1-5). Che and co-workers [113-115]

studied similar epoxidations using manganese,

palladium and chromium complexes as a catalyst.

Netalkar et al [116] prepared a series of aromatic

spaced bipalladium center complexes from Schiff

bases derived from 4,6-diacetylresorcinol and 2,6-

dialkyl substituted anilines. These complexes

exhibited good catalytic activity for epoxidation of

alkenes.

Cl

Cl O2N

Me

11 1312 14

15 16 17

Figure 4. Structures of various alkenes used for

epoxidation reaction by

Che and co-workers [113-115]

Manganese(II) Schiff base complexes [117] of

1,3-diamino-2-hydroxypropane with

salicylaldehyde, 2-hydroxynaphthaldehyde or 2-

hydroxyacetophenone ligands were excellent

catalysts in the epoxidation of olefins in the

presence of PhIO as an oxidant. The extent of

olefins epoxidation was dependent on OH

substitution on Schiff base ligand. Sabater et al [4]

used zeolite encapsulated chiral manganese(II)

complexes of trans-(R,R’)-1,2-(salicylidene amino)

cyclohexane as active catalysts in the epoxidation of

alkenes and highlighted the importance of chiral

catalysts in the epoxidation of unfunctionalized

alkenes.

O

O

O

O

O

+ NaClO(S)-[MnII(Ln)]

(n = 1, 3, 4)

ee: 13-15%11

+ PhIO

Mn(OAc)3.xH2O + (S)-H2Ln

(n = 3, 11, 29, 30)

Yield : 34-79%

ee: 19-58%

12-17

+ PhIO

(R)-[MnII(Ln)X]

X = OAc : n = 3

X = acac : n = 3 Yield : 59-91%

ee: 34-76%

12-14

+ tBuOOH

(R)-[PdII(L48)]

Yield : 37% (26), 51% (30)

ee: 17% (26, 71% (30)

14

+ PhIO(R)-[CrIII(L4)Cl]-MCM-41(m)

Yield : 37% (26), 51% (30)

ee: 17% (26, 71% (30)

11-14 ...(5)

...(4)

...(3)

...(2)

...(1)

Scheme 7. Epoxidation reactions studied by Che

and co-workers [113-115]

Ogunwumi and Bein [118] synthesized intra-

zeolite chiral salen complexes of manganese(III)

which provided the benefit of shape selectivity of

zeolites and stereo control of chiral manganese(III)

complexes in the epoxidation of olefin. This intra

zeolite combination showed high catalytic activity

than the corresponding homogeneous conditions of

these complexes. The activity of the catalysts in

homogeneous conditions decreased due to the

collapse of the mesoporous skeleton [119]. The




Vol-2, Issue-12, 2016



micelle templated silica (MTS)-supported

complexes of manganese(III) were also used as

catalysts. The 3-chloropropylsilane grafted MTS

reacted with Salpr or tert-Salprpentadentate ligands

[120] to produce suitable support for anchoring of

Schiff base complexes (Scheme 8).

O

Si

Si

Si

Si

MTS

(EtO)3Si(CH

2)3Cl

Si

Si

Si

Si

OEt

OEt

ClSi

O

O

OH

Si

Si

Si

Si

R

R

R

ROH

OH

N

N

N

O

O

O

OH

Si

Si

Si

Si

Si

Si

Si

Si

Si

O

O

O

OH

Si Cl

O

O

R

R

R

RN

N

N

O

O

O

OH

Si Mn

Cl-MTS

Salpr or tSalpr

-Et2O

Salpr(tSalpr)1) Mn(II)

2) H2O, NaCl, Au

Salpr(Cl) or t-Salpr(Cl)-MTS Mn-Salpr(Cl) or Mn-t-Salpr(Cl)-MTS

Scheme 8. Manganese(III) Salpr complex grafting

on MTS materials

Diaz and Balkus [121] successfully prepared

and characterized a series of MCM-41 grafted

metal-amine complexes, including ligands based on

ethylenediamine (M41ED), diethylenetriamine

(M41DET), and ethylenediaminetriacetic acid

(M41EDT). Ethylenediamine grafted to amorphous

silica (SilED) was compared with the MCM-41

grafted metal complexes. Cobalt(II) complexes of

M41ED and M41DET bind oxygen reversibly in

contrast to the silica-supported complexes. The

anchoring of chiral ligands on these surfaces

produced chiral catalysts for enantioselective

epoxidation of olefins.

Piaggio et al [122] showed that Mn-exchanged-

Al-MCM-41 when modified by a chiral salen ligand

can be an effective enantioselective heterogeneous

epoxidation catalyst. The epoxidation of (Z) and (E)-

stilbene was catalyzed using chiral (R,R’)-(-)-N,N’-bis-(3,5-di-tert-butylsalicylidene) cyclohexane 1,2-

diamine ligand modified and manganese exchanged

Al-MCM-41 in the presence of PhIO as oxygen

donor. The enantioselectivity of (Z)-stilbene to (E)-

epoxide with heterogeneous catalyst was low (70%

ee) in comparison to homogeneous catalyst (77%

ee).

Tang et al [123] has synthesized a series of

catalysts for epoxidation of styrene by immobilizing

salicylaldimine transition metal complexes of

copper, manganese and cobalt on mesoporous silica

nanoparticles (MSNs) with diameters of 120–150

nm. These catalysts possess excellent catalytic

efficiency in epoxidation of styrene in the presence

of TBHP as oxidant. Styrene shows a high

conversion (~99%) as well as epoxide selectivity

(~80%) over Cu-MSN catalysts, and high

conversion (~99%) and moderate epoxide selectivity

(~65%) over Mn-MSN and Co-MSN catalysts. The

catalytic activity of recycled catalyst is summarized

in Table 2. The mesoporous silica nanoparticle-

supported catalyst provides higher surface area of

the support causes a higher concentration of

catalytically active sites and thus show higher

conversion. Results indicate that MSNs can serve as

better catalyst supports. Massomeh Ghorbanloo

[124-126] prepared silica gel-immobilized Mn(II)

and Mo(VI) Schiff base complexes and studied its

catalytic epoxidation of various alkenes.

Li et al [127] developed an innovative method

to the epoxidation of cyclohexene under mild

conditions using molecular oxygen as oxidant. The

Mn(II) complexes of Schiff base ligands are

supported on montmorillonite by ion exchange

method. The montmorillonite catalysts performed

high activity and epoxide selectivity for aerobic

epoxidation of cyclohexene under Mukaiyama

conditions.

Table 2. The catalytic activity of recycled catalysts

Run Catalyst Conversion

(%)

Selectivity

(%)

1 Cu-MSN 99 80

2 Cu-MSN 97 80

3 Cu-MSN 98 78

1 Co-MSN 99 64

2 Co-MSN 98 60

3 Co-MSN 98 61

1 Mn-MSN 99 65

2 Mn-MSN 98 62

3 Mn-MSN 96 64

The manganese(III) complexes [128] of N,N’-bis (salicylideneethylenediamine) catalyzed

epoxidation of alkenes. These catalysts were used to

study the formation of oxygen transfer agents

[(salen)Mn=O]+

and [PhIO(salen)Mn–OMn(salen)OIPh]

2+, which were intermediates in

the epoxidation of alkenes in the presence of PhIO.

(Scheme 9)

Chiral manganese(III) salen complexes [129] in

the presence of chiral additives were studied for

epoxidation of 6-acetoamino-7-nitro-2,2-dimethyl

chromene with moderate yield (5–65%) and 73%

ee. The electrocatalytic activity and stability of

differently substituted manganese Schiff base

complexes, towards the oxidation by dioxygen of

various substrates was analyzed by Moutet and

Ourari [130]. Results obtained during the

electrocatalytic epoxidation of cyclooctene have

shown that Mn(III) complexes gave higher turnovers

than the already described electrocatalytic system

based on the unsubstituted Mn(III) Salen complex.

The best results were obtained in the presence of 2-




Vol-2, Issue-12, 2016



methylimidazole (as the co-catalyst) and benzoic

anhydride containing a small amount of benzoic acid

as an activator.

O

PhIO[(salen)MIII]X

+ PhI+

L

N

O O

N

M

O

L

N

O O

N

MPhIO

- PhI

+ +

O

[O=MV(salen)L]+ + [MIII(salen)L]+ +

M = Cr, Mn

Scheme 9. Mechanism of alkene epoxidation

catalysed by chromium(III) and Manganese(III)

salen complexes

Manganese salen complexes [131] have been

investigated in alkene epoxidations with H2O2,

largely from the perspective of asymmetric catalysis.

Katsuki [132] developed unfunctionalized Mn salen

complex and obtained high ee (96%) but low yields

(17%) in imidazole solution. Pietikainen [133]

obtained higher yields and ee’s using caboxylates as

additives. Jacobson et al [134] used another

manganese(III) schiff base complex in epoxidation

of olefins in the presence of additives such as 4-

phenylpyridine-N-oxide, N-methylmorpholine-N-

oxide. Mansuy et al [135] developed a system to

obtain high enantioselectivities (92%) and yields

(86%) by adding ammonium acetate to Jacobsen’s catalysts. Durak et al [136] synthesized an

unsymmetrical tetradentate Schiff base by the

reaction of 3-methoxysalicylaldehyde, o-

phenylenediamine, and salicylaldehyde. Its Co(II)

and Mn(III) complexes were prepared and

immobilized on 3-aminopropyltriethoxysilane

functionalized silica gel. The immobilized materials

were found to be efficient catalysts for epoxidation

of styrene in the presence of TBHP in acetonitrile at

40oC.

Patel et al [137] successfully carried out

epoxidation of alkene in the presence of

calix[4]arene Schiff base Co(II) complexes as

catalyst. A new Co(III) Schiff base complex was

synthesized by Saha et al [138] which is capable of

activating dioxygen in air to catalyze the

epoxidation of various alkenes using

isobutraldehyde as a co-reductant under

homogeneous conditions. Among the series of

Schiff base complexes 18-21 (Fig. 5) synthesized by

Lu et al [139] Mn complex 18 resulting from N,N’-bis(2-hydroxy-1-naphthalidene)

cyclohexanediamine ligand was considerably active

for the catalytic epoxidation of styrene under mild

conditions in which the highest yield of styrene

oxide reached 91.2 mol%, notably higher than those

achieved from simple manganese salt catalysts.

Erdem and Guzel [140] also carried out similar

epoxidation reactions.

OAc

Mn

OO

NN

52-55

18 - M - Mn 19 - M - Co20 - M - Cu 21 - M - Fe

Figure 5. Structures of salen-metal complexes 18-21

Polymer-supported manganese(II) salen

complexes [141] were used as catalysts in the

enantioselective epoxidation of 1,2-dihydro-

naphthalene, cis-β-methyl styrene and styrene. The

heterogenized catalyst was more selective to form

trans-epoxide with cis-β-methyl styrene. The

supported salen complexes were produced by

sequential treatment of polymer with 2,4,6-

trihydroxy benzaldehyde and chiral trans-1,2-

diamino cyclohexane and 3,5-di-tert-butyl

salicylaldehyde and then manganese(II) ions were

loaded.

Canali et al [142] has prepared polymer-

supported salen ligand by polymerization of 4-(4-

vinyl benzyloxy) salicylaldehyde with styrene and

vinyl benzene then reacting subsequently with 1,2-

diaminocyclohexane. The prepared polymer-

supported salen after reacting with manganese(II)

was used as a catalyst in the asymmetric epoxidation

of dihydroxy naphthalene and indene in the presence

of m-chlorobenzoic acid as activator. The

immobilized catalyst showed high chemical activity

and modest enantioselectivity. Canali et al [143]

also examined polymer-supported analogue of

Jacobsen’s catalyst selectively as the homogeneous species. These catalysts were used in the

epoxidation of 1,2-dihydronaphthalene, indene, 1-

phenylcyclohex-1-ene and 1-phenyl-3,4-

dihydronaphthalene using m-chloroperbenzoic acid

as the oxidant and 4-methylmorpholine N-oxide as

the co-oxidant but the catalysts show a very rapid

fall in both activity and selectivity in the first and

second cycles.

New unsymmetrical chiral salen complexes

were synthesized by Kim and Shin [144] and the

efficiency of Mn(III), Ti(IV), Co(II) and Co(III)

type catalysts were examined in the enantioselective

epoxidation of styrene and α-methylstyrene. The

unsymmetrical (salen) complexes showed




Vol-2, Issue-12, 2016



comparatively high enantioselectivity as compared

to conventional symmetrical salen complexes, which

were prepared mainly from salicylaldehyde and 2-

formyl-4,6-di-tert-butylphenol derivatives. Several

variables that affect the enantioselectivity in the

epoxidation of unfunctionalized alkenes by amino

acid based Schiff base complexes as catalysts were

investigated by Kureshy et al [145]. Complexes 22-

26 (Scheme 10) derived from L histidine are better

catalysts than the other complexes synthesized by

Kureshy et al [145]. On the basis of chemical yield

and ee values, effectiveness of the solvents may be

arranged in the following order, fluorobenzene >

dichloromethane > acetonitrile which is in the

reverse order of increasing polarity. As far as

oxidant is concerned, only PhIO was able to perform

epoxidation reaction. Other commonly used

oxidants like H2O2, NaOCl, O2 and mCPBA were

either inactive or gave product other than epoxides.

H

OH

COOK

N

O

O

N

O

NH

N

H2C

R1

R2

R3

R1

R2

Ru

OH2

PPh3

H2O

R3

H

RuCl2(PPh3)3+

22-26

R3=

22 = R1 = R2 = NO2

23 = R1 = R2 = Cl

24 = R1 = R2 = H

25 = R1 = C(CH3)3, R2 = H

26 = R1 = R2 = C(CH3)3

(L - histidine)

Scheme 10. Synthesis of the complexes 22-26

Sivadasan and Sreekumar [146] synthesized

cross linked polystyrene-supported Schiff's base

complexes of metal ions such as Fe(III), Co(II),

Ni(II) and Cu(II). The catalytic activity of these

metal complexes is studied in the epoxidation of

cyclohexene and styrene. The reactions showed first

order dependence on the concentration of both the

substrates and the catalyst. Fe(III), Co(II) and Cu(II)

complexes showed significant catalytic activity

towards epoxidation of cyclohexene and styrene, but

Ni(II) complex was found to be inactive. The metal

complexes showed low catalytic activity at low

crosslink density (2% and 5%) but 10% crosslink

resins show higher activity. Erkenez and Tumer

[147] prepared three polymer anchored Schiff base

ligands from the reaction of 2,4-

dihydroxybenzaldehye with diamines in the ethanol

solution. Cu(II), Co(II) and Ni(II) transition metal

complexes of these Schiff bases showed low

catalytic activity.

Thomas and Janla [148] used manganese(II)

salen complexes on soluble and insoluble supports

as catalysts in various asymmetric epoxidation

reactions. Poly(ethylene glycol) monoethyl ether

and non-cross linked polystyrene were used as

soluble support, whereas Janda Jel and Merrifield

resins were used as insoluble supports. These

supports were linked to salen complexes through a

glutarate spacer. The soluble supports were

recovered by precipitation with suitable solvents,

while insoluble catalysts were simply filtered from

the reaction mixture. These catalysts were used in

epoxidation of styrene, cis-β-methylstyrene and

dihydronaphthalene.

Song and Roh [149] have developed a practical

recycling procedure for Jacobsen’s chiral (salen) Mn(III)

catalyst, using N,N’-bis(3,5-di-tert-

butylsalicylidene)-1,2-cyclohexanediamine and

manganese(III) chloride by use of ionic liquid 28

(Fig. 6). Results showed that the catalyst (R,R) 27 in

reaction medium containing ionic liquid 28

exhibited comparable enantioselectivity (96% ee for

2,2-dimethylchromene) in asymmetric epoxidation

of alkenes as those obtained without ionic liquid and

moreover, showed an increase in activity. At the end

of reaction the immobilized catalyst in ionic liquid

28 could be easily recycled.

Cl

H H

OO

NN

Mn

But But

ButBut

(R,R)-27

NN

PF6

+

[bmim][PF6] 28 Figure 6. Structures of Jacobsen’s chiral (salen)

Mn(III) catalyst 27 and Ionic liquid 28

Pouralimardan et al [150] synthesized five

dissymmetric tridentate hydrazone Schiff base

ligands including HL1–4

and L5 (HL

1 = benzoic acid

(2-hydroxy-3-methoxy-benzylidene)-hydrazide, HL2

= benzoic acid (2,3-dihydroxy-benzylidene)-

hydrazide, HL3 = benzoic acid (2-hydroxy-

benzylidene) hydrazide, HL4 = benzoic acid (5-

bromo-2-hydroxy-benzylidene) hydrazide, L5 =

benzoic acid pyridine-2-yl methylene hydrazide) and

their manganese complexes. It was demonstrated

that these dissymmetric hydrazone Schiff base

manganese(II) complexes (Mn–L1–5

) (Scheme 11)

are highly selective catalysts for oxidation of

cyclohexene by PhIO under mild conditions. The

only product of the reaction was cyclohexene oxide

in the presence of imidazole. Adding nitrogenous




Vol-2, Issue-12, 2016



donor imidazole increased the catalytic activity.

These kinds of complexes are potentially important

catalysts in quantitative cyclohexene oxidation or

similar oxidation processes of hydrocarbons.

Star et al [151] synthesized D-2,3-bis

(arylideneamino)-1,4-butanediol derivatives 31-34

from 29 and 30 constituted a new class of salen

based Schiff base ligands to form complexes with

transition and heavy metal ions. The manganese(III)

complexes (Scheme 12) 35-38 corresponding to

Schiff base ligands 31-34 were used in asymmetric

epoxidation of indene (Scheme 13).

z

X

Y

O

NH2

NH

O

NNH

Oz

X

Y

+method b:MnCl2.4H2O

method a:MnCl2.4H2O

[MnCl(L1)(H2O)2]

[Mn(L2)2(H2O)2]

[MnCl(L3)]

MnCl(L4)]

[MnCl2(H2O)(L5)]

HL1 : Z = C-OH, Y = OCH3, X = H

HL2 : Z = C-OH, Y = OH, X = H

HL3 : Z = C-OH, Y = H, X = H

HL4 : Z = C-OH, Y = H, X = Br

L5 : Z = N, Y = H, X = H

Scheme 11. Synthesis of Schiff base manganese(II)

complexes (Mn–L1–5

)

NH2

NH2

H

H

CH2OR

CH2OR

OHC

R'

R'

OH

RO

OH OH

R'

R'

OR

NN

R'

R'

RO

Cl

MnOO

R'

R'

OR

NN

R'

R'

29 R = H 30 R = CH2Ph

31 R = R’ = H 32 R = CH2Ph, R’ = H 33 R=H R’= t-Bu 34 R=CH2Ph, R’= t-Bu

35 R = R’ = H 36 R= CH2Ph, R’=H

37 R=H R’= t-Bu 38 R=CH2Ph, R’= t-Bu

Scheme 12. Synthesis of D-2,3-

bis(arylideneamino)-1,4-butanediol derivatives (32-

34) and of their manganese(III) complexes (35-38)

RO

Cl

MnOO

R'

R'

OR

NN

R'

R'

ONaoCl/H2SO

4

Scheme 13. The epoxidation of indene using the

manganese(III) complexes of D-2,3-bis(di-t-butyl-

salicylideneamino)-1,4-butanediol and dibenzyl

ether

The ScCO2 manganese(II) salen complexes

[152] were used as catalyst in the epoxidation of

olefins using TBHP as terminal oxidant. Similarly,

the substituted Mn[(5,5-NO2)2salen] catalyst in

CH3CN was used in the epoxidation of alkenes in

the presence of PhIO as terminal oxidant resulted in

corresponding epoxides and other products. Schuster

et al [153] synthesized immobilized sterically

demanding complexes which were tested in the

diastereoselective epoxidation of (-)-α-pinene in the

liquid phase in an autoclave at room temperature and

at elevated pressure using O2 as oxidant. Best results

so far are 100% conversion, 96% epoxide

chemoselectivity and 91% diastereomeric excess

obtained in the presence of the entrapped [(R,R)-

(N,N’)-bis(3,5-di-tert-butyl salicylidene)-1,2-

diphenylethylenediamino] cobalt(II) = Co(salen)

complex (Fig. 7). Suggested mechanism of

epoxidation is shown in Scheme 14.

M

O O

N N

R1R1

R2R2

Salen-39: R

1 = t-Bu, R

2 = t-Bu

Salen-40: R1 = t-Bu, R

2 = Me

Salen-41: R1 = t-Bu, R

2 = H

Figure 7. Salen complexes derived from (R,R)-

diphenylethylenediamine

O

CHO COOH

[M(salen)-complex]-zeolite

O2, RT

(-)-α-pinene

(-)-α-pinene oxide

Scheme 14. Mechanism of epoxidation suggested by

Schuster et al [90].

Kureshy et al [154] synthesized dimeric

Mn(III)-Schiff base complex 42 (Fig. 8) which

successfully catalyzed the enantioselective

epoxidation of the non-functionalized alkenes.

Excellent ee of 100% was obtained in the

epoxidation of nitro- and cyanochromene. The rate

of reaction increased when ammonium acetate was

used as co-catalyst. The activity of the recycled

catalyst gradually decreased upon successive use,




Vol-2, Issue-12, 2016



possibly due to minor degradation during the

reaction.

Cl

H H

Mn

OO

NN

CH2Cl

H H

Mn

OO

NN

t-Bu

t-Bu

t-Bu t-Bu

t-Bu

t-Bu

Figure 8. Structure of dimeric Mn(III)-Schiff base

complex 42

Morries et al [155] described the use of

supramolecular complexation to enhance (salen)Mn

catalyst stability in the asymmetric epoxidation of

unfunctionalized conjugated olefins namely styrene

and 2,2-dimethylchromene as substrates (Scheme

15). (Salen)Mn(III) catalysts (43, Fig. 9) show

increased turnover numbers in the catalytic

asymmetric epoxidation of conjugated olefins upon

addition of bulky Lewis acids such as zinc

tetraphenylporphyrin (ZnTPP). It is observed that

supramolecular complex formation enhanced the

catalyst’s stability without compromising its enantioselectivity.

O O

O

O

Mn catalyst

PhIO

or or

Scheme 15. Asymmetric epoxidation of conjugated

olefins

N

ClOO

N

MntBu

tBu tBu

tBu

Figure 9. (Salen)Mn(III) catalysts 43

Paulsamy et al [156] synthesized a series of new

chiral binol based [1+1] macrocyclic Schiff bases

and used it for the enantioselective epoxidation of

unfunctionalized olefins that resulted high yields

and good enantioselectivity.

Krishnan and Vencheshan [157] synthesized

polymeric, polynuclear Schiff base Mn complexes,

with repeating salen-like cores. Unfortunately they

proved to be moderate catalysts for the epoxidation

of simple alkenes and additives such as imidazole

were required to achieve catalytic activity with

H2O2. Das and Cheng [158] has synthesized a

mononuclear complex of Mn(III) with a Schiff base

ligand L [L = N,N’-ethylenebis(3-formyl-5-

methylsalicylaldimine)]. A series of binuclear

complexes, MnML’Clx solventy [M = Zn(II), Cu(II),

Ni(II), Co(II), Fe(III), and Mn(III); x = 3 or 4

depending on the oxidation state of M; L’ is the aniline condensation product of L; y = 1, 2 or 3 and

solvent = H2O or CH3OH] have been prepared by

the reaction of same mononuclear complex of

Mn(III), aniline and M(II/III) chloride with a 1:3:1.2

molar ratio in methanolic media. The results of

magnetic susceptibility measurements indicate that

all the metal ions are in their high spin states. The

catalytic properties have been investigated using

PhIO as a terminal oxidant. The mononuclear

Mn(III) complexes are more efficient epoxidation

catalysts than the binuclear Mn(III) complexes. The

reduced activities of the bimetallic complexes may

be due to both the low rates of formation of a

Mn(V)=O intermediate and the transfer of oxygen

from this intermediate to the alkene.

Patel et al [159] have synthesized polymer-

supported Mn(II) Schiff base complexes from cross

linked chloromethylated poly(styrene-

divinylbenzene) copolymer beads by sequential

modification in to a Schiff base bearing ligand.

These Schiff base bearing polymer on treatment

with a solution of manganese salt gave the

corresponding metal complexes. This Mn(II)

complex bound to poly(styrene-divinylbenzene)

copolymer catalyzed the epoxidation of norbornene

and cis-cyclooctene in the presence of alkyl

hydroperoxide under mild conditions. Kinetic

experiments reveal that at elevated temperature the

activity of the catalysts toward the epoxidation is

enhanced. The catalysts can be recycled several

times without any loss in selectivity. The

mechanism of epoxidation is shown in Scheme 16.

O(a) t-BuooH + P Mn+2Mn+4 + t-BuOH

O

O

O

P Mn+4 +

+ P Mn+2

(b) cat.

oxidant Scheme 16. Mechanism of olefin epoxidation

Louloudi et al [160] prepared Schiff base

ligands by template induced

macrocyclization/condensation of

diethylenetriamine (H2NCH2CH2NHCH2CH2NH2)

with eitherpentane-2,4-dione or 1,3-diphenyl-

propane-1,3-dione. The binuclear manganese

complexes (47, Fig. 10) produced was moderately

active in epoxidations, giving yields from 9% to




Vol-2, Issue-12, 2016



82% for isoprene and limonene in the presence of

ammonium acetate. Researchers [161,162]

synthesized phenoxo bridged dinuclear Mn(III)

complex that acts as an efficient catalyst in the

epoxidation of alkenes.

Costas et al [163] observed that a number of

iron complexes have been applied as catalysts in

alkene oxidations. The almost unexplored

mechanism of the iron catalyzed olefin epoxidation

with H2O2–acetic acid was studied by Mas-Balleste

and Que [164]. In addition iron complexes such as

novel tetra- [165] or pentadentated amino–pyridine

ligands [166]

N,N,O-ligands and (m-oxo)diiron

complexes [167] are also used in epoxidation.

Researchers have [168, 169] synthesized iron(III)

complex and used as an alkene epoxidation catalyst.

H

NH

N

O O

N N

Mn

tBu

tBu

Ph4B-

+

44

Ar

O O

N N

Mn

Ar

+

F6P-

45

tBuAr =

O O

N N

Mn

CltBu

tBu

tBu

tBu

46

O O

N N

Mn

O O

N N

Mn

Cl Cl

tBu

tBu

tBu

tBu

tButBu

47

NN

O O

N N

Mn

Cl

OHHO

48

n

Figure 10. Manganese-complexes synthesized by

Louloudi et al [160]

Goldani et al [170] prepared a series of Fe(III)

Schiff base complexes immobilized on MCM-41.

The immobilized complexes proved to be effective

catalysts and exhibited much higher catalytic

performance than their homogeneous analogue.

Among all the alkenes, those containing π-electron-

withdrawing groups and trans-orientations exhibited

lower tendency for oxidation.

Masoud et al [171] have also synthesized

Cobalt(II) Schiff base complex on multi-wall carbon

nanotubes (MWCNTs) by covalently grafted

method. The catalytic activity of the novel

nanotubes based materials was tested in the

epoxidation of cyclohexene. Kureshy et al [172]

have presented the aerobic enantioselective

epoxidation of 1-octene, 1-hexene, trans-4-octene

and indene using Ni(II) symmetrical and non-

symmetrical chiral Schiff base complexes in

presence of molecular oxygen using

isobutyraldehyde as sacrificial reductant. The

catalyst 49, 51 and 52, 54 (Fig. 11) forms a

conversion with 1-octene and trans-4-octene and

catalyst 50 and 51 favor the formation of 1-hexene

oxide in higher yield. The symmetric catalysts 49-51

gave the maximum enantioinduction with indene

though the chemical conversions were low in

comparison to the long chain olefins. As expected S

form of the catalyst gave predominantly R form of

the product. R

H HCH3

CH3

CH3

OO

O

CH3

OO O

N N

Ni1

2 3

456

7

8

9

1

23

45 6

7

8

9

1' 2'

49-51

R

H HCH3

CH3

O

O O

N

Ni

1

2 3

456

7

8

9

1' 2'

OH2

NH2CH3COO-

52-54

+

CH3

PhPh

1' 2'

3'

4'5'

6'R = 1' 2' 1' 2'

Figure 11. Representative structures of the catalysts

49-54

Nickel(II) and Manganese(III) metal complexes

of Schiff bases such as N,N’-bis(2-hydroxyphenyl)

ethylenediimine) 55 and N-((2-hydroxyphenyl)

acetylaldimine)-N-(2-hydroxyphenyl) acetamide) 56

(Fig. 12) are prepared by Chatterjee et al [173] and

Kureshy et al [174] in good yield by direct

interaction of 2-aminophenol,

glyoxal/methylacetatotate and NiCl2. Catalytic

ability of NiL complexes were examined and found

that both the complexes can effectively catalyze the

epoxidation of olefins namely cyclohexene, 1-

hexene, cis- and trans-stilbenes, indene with NaOCl.




Vol-2, Issue-12, 2016



OH

NN

OH

O

N

OH

NH

OH

55 56 Figure 12. Structures of NiL

1 and NiL

2 complexes

Bhunia and Koner [175] observed similar

results by tethering nickel(II) Schiff base complex

via post-synthesis modification of mesoporous

silica, MCM-41. The Schiff base is derived from

salicylaldehyde and 3-aminopropyltriethoxysilane

(3-APTES). Immobilized nickel catalyst was found

to be catalytically more active and selective as

compared to the similar type of nickel(II) complex

as well as Ni(NO3)2.6H2O in homogeneous media.

Mavrogiorgou et al [176, 177] developed a new

functional catalytic material prepared via chemical

modification of mesoporous silica SBA-15, MCM-

41 or carbon nanomaterials CMK-3. All the

catalysts were evaluated for alkene epoxidation with

H2O2 as oxidant and CH3COONH4 as additive.

Jana et al [178] have anchored the copper(II)

Schiff base complex immobilized into Si-MCM-41

(Scheme 17) matrix via covalent bond. The Cu-

MCM-41 is porous but the mesoporous character of

Si-MCM-41 was not retained after modification.

The catalyst Cu-MCM-41 has shown excellent

catalytic activities in epoxidation reaction toward

various olefins including allylic alcohol. An

unprecedented high conversion (97%) as well as

selectivity of epoxide (89%) has been observed in

styrene epoxidation. Notably, epoxidation of allylic

alcohol showed considerably high turnover

frequency (166 h-1

) in this study.

Three (II) Schiff base complexes,

[Cu(L1)(H2O)](ClO4), [Cu(L

2)] and [Cu(L

3)] have

been synthesized by Adhikary et al. [46, 179] All of

them are found to be catalytically active in

industrially important epoxidation reactions in

different solvent media under homogeneous

conditions. The efficiency of the catalysts, in

general, followed the order: acetonitrile >

chloroform > dichloromethane > methanol. Ray et al

[180] synthesized dicopper(II) complexes of

tridentate pyrimidine derived Schiff base ligands

which have presented excellent catalytic activities in

epoxidation reactions towards various olefins. Shen

et al [181] prepared a Copper(II) complex derived

from the Schiff base ligand 2-hydroxy-5-

methoxybenzaldehyde oxime (HL) and tested for its

catalytic oxidation property. Abbasi et al [182]

reported the results of solvent free epoxidation

studies of cyclooctene to cycloocteneoxide using

Cu(II) Schiff base complexes (Scheme 18). Higher

catalytic activities, epoxide selectivity and lower

reaction times were achieved.

OH

OH

OH

Si(CH2)3NH

2

EtO

EtO

EtOSi(CH

2)3NH

2

O

O

O

OH

CHO

O

O

O

Si(CH2)3N

OH

O

O

O

Si(CH2)3N

O

O

O

O

O

Si(CH2)3N

+ +

channel wall

of Si-MCM-41 3-APTES

-3EtOH

a

MCM-41-(SiCH2CH2CH2NH2)n

b

Cc

C

Cu2+

C

Cu-MCM-41

Scheme 17.

(a) Modification of Si-MCM-41 Channel Wall:

APTES/CHCl3;

(b) Condensation with Salicylaldehyde in Methanol;

(c) Metal Complex Formation : Cu(NO3)2.3H2O/

MeOH

X Y

OH

R

N

N

R

OH

XY

z

z

z

R

XY

O

N

Cu

X Y

R

N

O

Ph Ph

z

Ph

Ph

O

N

O

N

Cu

O

N

O

N

Cu

OMe

OMe

Ph

Ph

+ Cu(OAc)2

MeO

MeO

Cu2(L5)2

CuLn

Ligands and

Complexes

R X Y Z

H2L1, CuL

1 H H H H

H2L2, CuL

2 H H H Br

H2L3, CuL

3 H NO2 H Br

H2L4, CuL

4 CH3 H H H

H2L5, Cu2(L

5)2 H OCH3 H H

Scheme 18. Synthetic procedure for the preparation

of ligands and complexes

Ethanol was used as solvent

Ambroziak et al [183] synthesized

dioxomolybdenum(VI) complexes 57-62 of

tetradentate Schiff base derivatives (Scheme 19) of

trans-1,2-diaminocyclohexane were used for the

epoxidation of cyclohexene and 1-octene. The best

results were obtained for [trans-N,N’-bis-(4,6-

dimethoxysalicylidene)-1,2-cyclohexanediaminato]-

dioxomolybdenum(VI) and [trans-N,N’-bis-(2-

hydroxynaphthylidene)-1,2-cyclohexanediaminato]-

dioxomolybdenum. An increase in the reaction




Vol-2, Issue-12, 2016



temperature may lead to an increase in the yield of

the epoxide.

Researchers [184, 185] synthesized cis-dioxo-

Mo(VI) complexes (Scheme 20) containing simple

ONO tridentate Schiff base ligands in the

epoxidation of various olefins using TBHP with

excellent chemo- and stereoselectivity.

NN

OH OH

R R

OO

NN

Mo

O OR R

OO

NN

Mo

O O

C C C C

EtOH

Reflux 2h

Mo2(acac)2 +

46-50

C C

62

57 R = H58 R = 3,5-diCl59 R = 3,5 diBr60 R = 5-NO2

61 R = 4,6-diOCH3

Scheme 19. Synthesis of dioxomolybdenum(VI)

complexes

O

H

MeHO

Y

X

OOO

Mo

O

CH3

N

TBHP, DCE, 80oC

X, Y = H MoL1

X= NO2, Y=H MoL2

X= OMe, Y=H MoL3

X, Y = t-Bu MoL4

Scheme 20. Epoxidation of olefins catalyzed by the

MoVI

Schiff base complexes

Gao et al [186] prepared a new bidentate Schiff

base dioxomolybdenum(VI) complex immobilized

on chloromethlated cross linked polystyrene

microspheres via two stepwise polymer reactions.

The solid catalyst used in the epoxidation reaction of

cyclohexene with TBHP exhibited high catalytic

activity and excellent catalytic selectivity. Katkar et

al [187] carried out green epoxidation of 1-hexene

using [hydrazine-N-salicylidene-N’-salicyloyl-] cis-

dioxomolybdenum(VI) and its zeolite-Y composite

as catalyst in DMF in the presence of molecular

oxygen as oxidant and in the temperature range

333–363 K. The ‘green’ factor of olefin oxidation catalysis using the Mo(VI) dioxo complex is its

ability to use atmospheric oxygen which is non-

hazardous and inexpensive and produces no harmful

byproducts.

Sui et al [188] synthesized five kinds of

dioxomolybdenum(VI) complexes with Schiff base

ligands derived from tris(hydroxymethyl)amino

methane (63 Fig. 13). These complexes show good

catalytic activities and selectivity in the epoxidation

of cyclohexene with TBHP, especially for complex

[MoO2(H2L)(H2O)], which could give a nearly

100% of epoxidation conversion and selectivity. The

electron withdrawing group on the salicylidene ring

of complex is advantageous over electron donating

one on the effectiveness of a catalyst but

disadvantageous on the redox stability of a complex.

OHH

H

OH

OH

OH

N

O2N

Figure 13.Structure of tris(hydroxymethyl)amino

methane 63

Grivani et al [189] produced polymer-bound

molybdenum carbonyl Schiff base catalyst 64 as

shown in Scheme 21. This supported catalyst shows

high activity in epoxidation of various alkenes in the

presence of TBHP. The supported molybdenum

catalyst can be recovered and reused eight times

without loss in its activity. Grivani and Akherati

[190] have synthesized polymer-supported bis(2-

hydroxylanyl) acetylacetonato MoO2 Schiff base

complex which were used for selective in

epoxidation of various alkenes.

Cl NaHCO3

O

Cl

NH2

N

NH2

N

CO

COCO

CO

Mo

+DMSO

155oC/6 h

Ethylene diamine / Methanol Reflux / 8 h

Mo(CO)6

i ) Dioxane/reflux 1h

ii) 2/ reflux 45 min

64

Scheme 21. Preparation procedure of catalyst 64

Bagherzadeh et al [191, 192] synthesized

Mo(VI) complexes of Schiff bases and used as

catalyst for epoxidation of alkenes. Mirzaee et al

[193] used boehmite nanoparticles (BNPs) grafted

with 3-(trimethoxysilyl)propylamine (MSPA) to

anchor Schiff base and to support two complexes of

molybdenum and vanadium. Then, these supported

catalysts were utilized in the epoxidation of various

olefin substrates (Scheme 22 and 23).

Masoud and Mehdi [194] synthesized three

dimeric cis-dioxomolybdenum(VI) complexes of

Schiff bases. These bis-bidentate Schiff base ligands

derived from aromatic nitrogen–nitrogen linkers

(bis[(N,O-salicylidene)-4,4’-diaminodiphenyl]

methane, H2L1; bis[(N,O-salicylidene)-4,4-




Vol-2, Issue-12, 2016



diaminodiphenyl]ether, H2L2; bis[(N,O-

salicylidene)-4,4’-diaminodiphenyl] sulfone, H2L3)

exhibit good catalytic activities and selectivity in the

epoxidation of cyclohexene with TBHP forming six-

coordinated cis-dioxo Mo(VI) complexes especially

for complex [MoO2L3]2, which could give a nearly

88% of epoxidation conversion and 94% of

selectivity. The electron withdrawing group on the

aromatic nitrogen–nitrogen linkers of complex is

advantageous over electron donating one on the

effectiveness of a catalyst but disadvantageous on

the redox stability of a complex. The addition of

single wall carbon nanotubes (SWCNT) can enhance

the activity of the Mo complexes and the selectivity

toward epoxide.

Scheme 22. Proposed mechanism for the

epoxidation of olefin with TBHP catalyzed by Mo-

IFBNPs

Mohamed Shaker S. Adam [195] studied

homogeneous oxidation processes of various

aliphatic and cyclic alkenes using

dioxomolybdenum(VI) and dioxouranium(VI)

homobimetallic bis-ONO tridentate (2-hydroxy-1-

benzylidene)malonyl-, succinyl-, and terephthalo-

dihydrazone complexes. Mo(VI)O2 complexes were

five times more selective than U(VI)O2 complexes.

Scheme 23. Proposed mechanism for the

epoxidation of olefin with TBHP catalyzed by V-

IFBNPs

M. Masteri-Farahani and S. Abednatanzi [196,

197] have prepared covalently attached

molybdenum–Schiff base complex on the surface of

MWCNTs as a new hybrid catalyst (Scheme 24).

The prepared hybrid nanomaterial was found to be

an efficient and stable heterogeneous catalyst in the

selective epoxidation of olefins with TBHP and

CHP as oxidants.

Scheme 24. The sequence of events in the

preparation of MoO2(acac)sal-MWCNT’s nanomaterial

Li et al [198] immobilized a series of tridentate

Schiff base dioxomolybdenum(VI) complexes on to

a promising organic–inorganic hybrid zirconium

poly(styrene-phenylvinylphosphonate)-phosphate

via covalent bond. The prepared heterogeneous

catalysts are found to be highly reactive in the

epoxidation of unfunctionalized olefins. Moreover,

these catalysts are of higher stability and reusability

in the oxidation reactions. Excellent enantiomeric

excess was obtained for the epoxidation of α-

methylstyrene in this oxidant system. Similar

dioxomolybdenum(VI) complexes [199-205] of

Schiff bases is used as a catalyst for epoxidation of




Vol-2, Issue-12, 2016



various olefins. S. Ding and W. Li [206] synthesized

tridentate Schiff base ligand and its

dioxomolybdenum(VI) complexes.

Molybdenum is octahedrally coordinatedin the

complexes. The complexes showed effective

catalysis in oxidation of cyclohexene, vinylbenzene,

1-butylene, and 1-pentene, to their corresponding

epoxides. Zeinab et al [207] prepared heterogeneous

nanocatalyst via covalent anchoring of

dioxomolybdenum(VI) Schiff base complex

(Scheme 25) on core–shell structured Fe3O4@SiO2.

The synthesized hybrid material was an efficient

nanocatalyst for selective oxidation of olefins to

corresponding epoxides with TBHP in high yields

and selectivity.

Scheme 25. Synthetic pathways for the synthesis of

Fe3O4@SiO2-NHAlMo nanocomposite

The electrocatalyst, ruthenium(III) Schiff base

complex [RuIII

(Naphdien)Cl](Naphdien = bis(2-

hydroxynaphthaldehyde)) to activate dioxygen for

the epoxidation of olefins in the presence of benzoic

anhydride in DMF was synthesized by Khan et al

[208]. The redox chemistry of the ruthenium

complex, which catalyzes the electrochemical

reduction of dioxygen for metal-oxo formation and

substrate oxidation, was studied using cyclic

voltametry. The results were consistent with an

initial one electron reduction of RuIII

to RuII, which

then reacts with O2 to form an [RuIII–O

2−] adduct.

Further one-electron reduction of the dioxygen

adduct produces a RuIII

-peroxo complex [RuIII–

O22−

]−

as shown in Scheme 26.

[RuIII ]+ e-

[RuII ] + O2 [RuIII (O2)]

+ e-

[RuIII(O2)2-]-[RuIV-O]4-

(C6H5CO)2O2C6H5COO-

olefin

epoxide

Scheme 26. Electron reduction of RuIII

- peroxo

complex

Ali Ourari [209, 210] showed that the Ru(III)-

species develop a particular reactivity towards

molecular oxygen. It was revealed that the formation

of byproducts such as μ-oxodimers are

advantageously avoided with the Ru(III) complexes.

De Sauza et al [211] used ruthenium complexes of

trans-[RuCl2(bpydip)] and trans-

[Ru(OH)2(bpydip)](PF6)2 with tetradentate Schiff

base ligand N,N’-bis(7-methyl-2-pyridylmethylene)-

1,3-diiminopropane as catalysts in the epoxidation

of cyclohexene in the presence of PhIO. Amini et al

[212] synthesized an oxido-peroxido tungsten(VI)

complex [WO(O2)L(CH3OH)] using salicylidene

benzoyl hydrazine as a tridentate ONO donor Schiff

base (H2L) (Scheme 27). The complex was used as a

catalyst for epoxidation of olefins with high yield,

turnover number, and selectivity. Rania et al [213]

carried out thermal reactions of Mo(CO)6 and

W(CO)6 with the different Schiff bases the gave

variety of carbonyl complexes. The two complexes

were tested and found to have epoxidation activity

toward olefins.

Scheme 27. Epoxidation of olefins and oxidation of

sulfides in the presence of [WO(O2)L(CH3OH)]

Sachse et al [214] showed that mononuclear

complexes of the type [ReOLCl2(PPh3)] or

[NBu4][ReOLCl3] (Fig. 14), where L represents

acetylacetone derived β-ketoamine ligands, can be

prepared in high yields. All compounds coordinate

to the rhenium center with the oxygen atom of the

ligands trans to the Re=O group and two chlorine

atoms occupy trans positions to each other.

Compound 65 proved to be a catalyst for the

epoxidation of cis cyclooctene with TBHP at 50oC.

PPh3

Me

Cl

ClN

O

Re

OMe

Figure 14. Mononuclear complexe 65

[ReOLCl2(PPh3)]

Bo Zhang et al [215] prepared Schiff base

complexes of methyltrioxorhenium(VII) and applied

them as catalysts for the epoxidation of cyclooctene

in the ionic liquid with urea hydrogen peroxide as




Vol-2, Issue-12, 2016



oxidant agent at room temperature. The results

showed that the (N-salicylidene) aniline derived

Schiff base complexes of methyltrioxorhenium

(MTO) exhibit higher catalytic activity and

selectivity than di-nitrogen Schiff bases complexes

of MTO. Gao et al [216] also synthesized several di-

nitrogen Schiff bases through the condensation of 2-

pyridinecarboxaldehyde with primary amines. The

Schiff bases as ligands coordinated with

methyltrioxorhenium (MTO) smoothly to afford the

correspondent complexes. Catalytic results indicated

that the complexes as catalysts increased the

selectivity of epoxides remarkably compared with

MTO in the epoxidation. Grunwald et al [217]

synthesized a series of Re(V) complexes bearing

tridentate pyridazine-containing ligands. These oxo

complexes were applied in the catalytic oxidation of

cyclooctene with TBHP, yielding ~60% cyclooctene

oxide after 24h.

Hailiang Su et al [218] Cobalt(II), iron(III) or

oxovanadium(II) Schiff base metal complexes

(Scheme 28) have been covalently grafted onto

graphene oxide (GO) previously functionalized with

3-aminopropyltriethoxysilane. Potential catalytic

behaviors were tested in the epoxidation of styrene,

using air as the oxidant. Co-GO and Fe-GO showed

high styrene conversion (90.8 versus 86.7%) and

epoxide selectivity (63.7 versus 51.4%). VO-GO

showed poor catalytic performance compared to Co-

GO and Fe-GO.

Farzaneh et al [219] prepared magnetic

nanoparticles (MNPs) by co-precipitation method.

The MNPs was then coated by silica due to

formation (SCMNPs) followed by modification with

aminopropyltrimethoxysilane (AmpSCMNPs).

Later, amino acid Schiff base ligands were prepared

from salicylaldehyde and triptophane or histidine

and immobilized on the surface of AmpSCMNPs

through the formation of amide bonds. Finally,

heterogenized vanadium nanocatalysts designated as

VO(Sal-Tryp)/AmpSCMNPs and VO(Sal-

His)/AmpSCMNPs were prepared (Scheme 29).

These catalysts were used for epoxidation of some

allylic alcohols and alkenes. Epoxidation of

geraniol, trans-2-hexene-1ol, 1-octene-3-ol, trans-

stilbene, norbornene and cyclooctene with 100 %

selectivity is observed. High yields, clean reactions,

ease of separation and recyclability of the solid

catalyst are some advantages of this method.

Scheme 28. Schematic outline of synthesis of M-

GO (M = Co, Fe or VO)

Scheme 29.

a. The sequence of events in the preparation of a

AmpSCMNPs,

b. Preparation of Catalyst from AmpSCMNPs

c. Structures of the catalysts

Lu et al [220] prepared Co Schiff base

functionalized MgAlLDH (layered double

hydroxides) by a novel strategy, namely grafting

silylated Co Schiff base on the exfoliated LDH

nanosheets (Scheme 30). The hybrid prepared by

this strategy exhibited a high catalytic activity in

H2O2 decomposition and styrene epoxidation with

O2.




Vol-2, Issue-12, 2016



Scheme 30. Schematic diagram for the preparation

of M(Salen) functionalized LDH

3. Conclusions and future outlook

Catalytic activity of Schiff base complexes

showed significant variations with structure and type

of Schiff base ligands have played a significant role

in epoxidation of alkene to enhance product yield

and selectivity. In the present review catalytic

activity, selectivity and yield of various epoxide

products have been shown. Clearly the oxidation of

the carbon-carbon double bond to the corresponding

epoxide compounds is a highly desirable and

rewarding industrial process. Enantioselectivity in

various reactions was controlled with metal

complexes of chiral Schiff base ligands. The activity

of Schiff base complexes of transition metals

showed significant improvements on their

immobilization on various supports. In addition to

high catalytic activity of Schiff base complexes on

various supports, the supported catalysts also

showed efficient recovery and reuse in various

reactions. Schiff base complexes of transition metal

ions in supercritical carbon dioxide (ScCO2) have

further enlarged the range of substrates, which

earlier were difficult to be oxidized in the presence

of Schiff base complexes of metal ions alone or

other catalysts. Various transition metal Schiff base

complexes covalently grafted with functionalized

SWCNT and GO showed high catalytic activity. The

polarity of reaction media has also influenced the

activity of Schiff base complexes of transition metal

ions as observed in some reactions described in this

review. These studies have clearly demonstrated that

Schiff base complexes of transition metals are

versatile, efficient and suitable to catalyze

epoxidation reactions under mild experimental

conditions. This work may broaden the application

of salen complexes as new asymmetric catalysts.

Although transition metal catalyzed allylic and

benzylic oxidation have met with little success, the

initial studies on this important chemical

transformation are encouraging and it holds a

promising future for its application in the industry.

Finally, transition metal catalyzed oxidation of

organic substrates with molecular oxygen is

becoming an important and highly rewarding

protocol for important feedstock for fine chemical

industries.

4. Acknowledgements

The authors would like to thank the Vidyalankar

Institute of Technology, Mumbai, and K. J. Somaiya

College of Science and Commerce, Mumbai for the

support. The authors also would like to thank

Professor Anil Raghav and Professor Avila Naik for

their timely support.

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