Transition Metal Complexes of Schiff Base Ligands as Efficient Catalysts for Epoxidation of Alkenes

26 Pages·2016·0.86 MB·English

Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-12, 2016
ISSN: 2454-1362, http://www.onlinejournal.in

Transition Metal Complexes of Schiff Base Ligands
as Efficient Catalysts for Epoxidation of Alkenes

1 2
Aniket A. Pawanoji & Bipin H. Mehta
1 K.J. Somaiya College of Science and Commerce, Mumbai, 2University of Mumbai,
Mumbai

Abstract: Chiral Schiff base complexes are highly asymmetric catalysis and metalloprotein modeling
selective in reactions such as epoxidation of alkenes. [21-36]. Over the last few years, several reports have
Transition metal complexes of Schiff bases possess appeared on synthesis of Schiff base complexes and
high catalytic activity in comparison to their evaluation of their catalytic activities in epoxidation
unsupported analogues. In the present study reaction [37-45] hence; the need of such review
catalytic activity of metal complexes of different article highlighting the catalytic activity of Schiff
types of Schiff bases for epoxidation of alkenes was base complexes was comprehended. In this review,
analyzed in this review. The Schiff base complexes the report on the Schiff base complexes in
of manganese(II), iron(II), cobalt(II), nickel(II) epoxidation reactions is presented for the period of
Chromium(III) and other ions are used as catalysts 1980-2016.
in the epoxidation. The manganese(III) Schiff base Catalytic epoxidation of alkenes is a significant
complexes exhibited high catalytic activity in the industrial reaction for the production of a wide
epoxidation of alkenes both in homogeneous and variety of fine chemicals [46] derived directly from
heterogeneous conditions. Polymer-supported alkenes, a primary petrochemical source.
Schiff base complexes have demonstrated higher Researchers worldwide investigated olefin
activity than unsupported Schiff base complexes of epoxidation reactions that are catalyzed by a metal
metal ions. The oxidation of styrene, indene, stilbene complex and this area is an important research topic
and its alkyl derivatives was catalyzed significantly in organic synthesis [47-52]. The last few decades
by polymer-supported Schiff base complexes of witnessed the complexes of early transition metals
different metal ions. The recyclability of polymer- such as rhenium [53], titanium [54, 55], manganese
supported Schiff base complexes is evaluated in this [56] and molybdenum [57-59] used as homogeneous
review. catalysts in the alkene epoxidation.
In recent years, styrene oxide has been used as a
preparatory material for epoxy resins, flavoring
Keywords: Schiff base complexes, epoxidation,
agents in food, tobacco, soap, cosmetic essences,
catalyst, transition metal.
perfumes, and pharmaceuticals [60, 61]. Catalytic
pathways via the formation of high-valent metal-oxo
1. Introduction
intermediates involving some of the transition metal
ions are well established [46]. Chiral salen Mn(III)
Schiff base complexes have a high potential to complexes are found to be highly enantioselective
act as versatile catalysts for organic synthesis for the asymmetric epoxidation of conjugated cis
resulting in a remarkable development in recent disubstituted and trisubstituted olefins [62-64].
times. Several families of transition metal Heterogeneous systems as compared to homogenous
compounds are extensively used in a variety of counterparts have the inherent advantages of easy
chemical transformations such as hydration [1], separation, enhanced handling properties and higher
hydrogenation [1], isomerization [2], stability; therefore, heterogenization of such
decarbonylation [3], cyclopropanation [4], olefin complexes is widely considered [65-68]. As an
metathesis [5], oxidation of hydrocarbon [6-12], application, Janssen et al [69] synthesized a dimeric
epoxidation [13-16], nitro aldol reaction [17], Diels- form of Mn(III) salen ligand and retained metal
Alder reaction [18], enol-ester synthesis [19] and complex in the cross-linked polymer membrane to
other related catalytic process [1]. Schiff base metal use as a catalyst for epoxidation. Asymmetric
complexes are important systems in asymmetric epoxidation of alkenes is a powerful method for the
catalysis [20]. The versatility of the steric and synthesis of chiral intermediates in the
electronic properties can be fine-tuned by choosing pharmaceutical and agrochemical fields [70].
appropriate amine precursors and ring substituents. Among the most beneficial systems for the
Metal complexes of Schiff bases derived from asymmetric epoxidation of non-functionalized
aromatic carbonyl compounds have received a great olefins is the Jacobsen–Katsuki Salen(Mn)–
deal of attention in connection with studies on catalyzed reaction [71, 72]. The (salen)Mn catalyst
Imperial Journal of Interdisciplinary Research (IJIR) Page 448
Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-12, 2016
ISSN: 2454-1362, http://www.onlinejournal.in

system has been shown to be effective for the difference in catalysis between Ti(salen) and
epoxidation of an impressive variety of Ti(salalen) complexes.
unfunctionalized conjugated olefins [73]. Olefins
could be effectively epoxidized by molecular Table 1. Asymmetric epoxidation of olefins
oxygen, catalyzed by porous silica support metal catalyzed by Ti(salalen) complex 2.
Schiff base complexes [74]. R2 R2
O
2. Schiff base ligands R1 + 30% H2O2 2 (1 mol%) R1
CHCl, rt
R3 2 2 R3
Entry Product yield (%) ee (%)
Schiff base ligands are formed by the
condensation of aromatic or aliphatic primary 1a O 87 99
amines with aldehydes. The resultant imines
participate in binding with metal ions via nitrogen

lone pair electrons [75, 76]. Like aldehydes, the 2 O 90 93
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-
3 75 95
and multi-dentate chelating Schiff base ligands are
O
designed according to the binding environments of
metal ions [77, 78].

2.1 Epoxidation 4 O 64 88
Ph
The epoxidation of hydrocarbons is an
important reaction in chemical industry. Metal 5b O 70 82
n-C H
complex catalysis plays a significant role in the 6 13

selective, partial oxidation of both saturated and
aAcOEt was used as the solvent.
unsaturated hydrocarbons into useful products.
b3 mol% of 2 was used.
O
O
O O
Scheme 1.Epoxidation reaction
N N N N
M M
Epoxidation (Scheme 1) is an important
O O O O
reaction in organic synthesis because the formed
epoxides are intermediates that can be converted to a O O
variety of products. This reaction is widely used in peroxo - inactive hydrogen-bonded peroxo - active
asymmetric cases since it can lead to two chiral with Ti(salen) with Ti(salalen)
carbons in one step [79]. 1 2
2.2 Schiff base catalyzed
(S or R) (S or R) (S or R) (S or R)
epoxidations N N N NH
Ti
OH HO 1) Ti(Oi-Pr)4 CH2Cl2 O O
Matsumoto et al [80] developed another Ph Ph Ph Ph
2) HO
approach to M(salalen) complexes: the 2
transformation of Ti(salen) complexes 1 to the O
corresponding di-m-oxo Ti(salalen) complexes 2 via 2
2 (possessing (S,S)-
Meerwein–Ponndorf–Verley reduction (Scheme 2). cyclohexanediamine moiety)
In the above complex, salalen ligands adopted a cis
β conformation. Ti(salalen) complex 2 catalyzed
various cyclic and acyclic conjugated olefins into O H O
corresponding epoxides with high to excellent ONTiON ONTiON
enantioselectivity (Table 1, Scheme 3). Although the Oi-Pr Oi-Pr
reaction mechanism is unclear, the peroxo titanium
Scheme 2. Synthesis of di-µ-oxo Ti(salalen)
species activated by an intramolecular hydrogen
complex 2 via MPV reduction
bonding with the amino proton has been proposed to
be the active species, on consideration of the
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O
2 (1 mol%) X X
+ 30% H2O2 O O O
(1.01 equiv.)
CHCl, rt V
2 2 Y Y
99%, >99% ee N N
Scheme 3.Asymmetric epoxidation of 1,2- R R
dihydronaphthalene by Ti(salalen) complex 2
Complex R X, Y
Soriente et al [81] synthesized a new octahedral VOL1 H XY: -CH=CH-CH=CH
VOL H Y: H, X: Br
titanium complex bearing a bulky binaphthyl 2
bridged Schiff base ligand. This complex has shown VOL3 Me X,Y: H
Figure 2.Structure of oxovanadium(IV) complexes
to be an effective catalyst for the epoxidation of
4
allylic alcohols.
The vanadyl salen complexes (VOL; x = 7–18)
x Yaul et al [88] synthesized vanadium
were tested as catalysts (3, Fig. 1) for aerobic
complexes with quadridentate Schiff bases. The
oxidation of hydrocarbons by Boghaei and Mohebi
catalytic activity of these complexes was evaluated
[82]. The unusual catalytic properties were observed
using H O as oxidant for styrene oxidation.
with simple salen ligand systems. Under typical 2 2
Bezaatapour et al [89-90] synthesized Ni(II) and
conditions, cyclohexene oxidised to mixture of
vanadyl Schiff base which showed good catalytic
cyclohexene oxide, 2-cyclohexene-1-ol and 2-
activity for the epoxidation of cyclooctene using
cyclohexene-1-one in presence of major complexes.
TBHP as oxygen source in acetonitrile. Mandal et al
The vanadium catalyzed aerobic oxidation of
[91] synthesized homogeneous and heterogeneous
cyclohexene proceeds with a moderate selectivity
mononuclear and dinuclear oxido vanadium Schiff
for epoxidations. (>50% for VOL; x = 8, 16, 17)
x base complexes which were found to be effective
A C
catalysts for epoxidation of styrene. Catalytic
activity of the metal complexes was greatly
O O VOLx (x = 7-18) influenced by the nature of the ligand and redox
VO n = 2,3 potential of the metal complexes. It was discovered
N N A,C = H, OH that metal complexes with ligands having an
B (CH ) D B,D = H, CH3 electron pushing group or having higher conjugation
2n
show lesser catalytic activity then the unsubstituted
Figure 1.Structures of Vanadyl salen complexes 3
or those having less conjugated ligand system.
Parida et al [92] carried out covalent
Chirally modified vanadium complexes [83] are
functionalization of vanadium Schiff base complex
used as catalysts in enantioselective epoxidations of
onto the CP-MCM-41. These catalysts showed
allylic alcohols and asymmetric sulfide to sulfoxide
pronounced efficiency with NaHCO as co-catalyst
oxidations. For both reactions various combinations 3
and H O as oxidant in a CH CN medium at room
of chiral ligands and vanadium sources were tested. 2 2 3
temperature. This method is green and economical
Rayati et al [84] have synthesized
from environment point of view. Zamanifar and
oxovanadium(IV) complexes (4, Fig. 2) of three
Farzaneh [93] prepared vanadium complexes of N-
N O tetradentate Schiff base ligands (H L1–3) by
2 2 2 salicylidene-L-histidine immobilized on Al-MCM-
reaction of 1,2-propylenediamine and appropriate
41 designated as VO(Sal-His)/Al-MCM-41 which
aldehyde and ketone. These complexes are used as
were found to catalyze the epoxidation of trans-2-
catalyst for the selective epoxidation of olefins.
hexen-1-ol, cinnamyl alcohol and geraniol with
High selectivity of epoxidation for cyclooctene is
TBHP.
observed from oxidation data. The catalytic activity
Grivani et al [94] synthesized oxovanadium(IV)
increases as the number of electron donor groups
Schiff base complex, VIVOL (Scheme 4), by the
increases and the catalytic selectivity is varied by 2
reaction of a bidentate Schiff base ligand L and
changing the substituents on the ligands. The
VO(acac) (L=N-salicylidin-2-chloroethylimine).
catalytic system described here is an efficient and 2
The catalytic activity of the complex 5 was tested in
inexpensive method for the oxidation of olefins with
the epoxidation of cyclooctene. The results showed
the advantages of high activity, selectivity, re-
that the complex 5 was highly active and selective
usability and short reaction time. Rayati et al [85-87]
catalyst in optimized conditions in the epoxidation
have also studied epoxidation of various alkenes
of cyclooctene. The proposed mechanism for the
using oxo-vanadium(IV), Dioxo-molybdenum(VI)
epoxidation of alkenes by the oxovanadium complex
Schiff bases complexes as catalysts.
VOL in the presence of TBHP is shown in Scheme
2
5. Grivani et al [95-97] have observed similar results
in the case of vanadium complexes.
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Cl A. Aziz [100] studied the interaction of the
O tetradentate Schiff base N,N’-bis(salicylidene)4,5-
Cl CHOH/NaOH N
+ 3 dichloro-1,2-phenylenediamine (H L) with M(CO)
OH 2 6
HCl.NH2 Reflux OH (M = Cr and Mo) under different conditions and
afforded the dicarbonyl precursors [Cr(CO) (H L)],
2 2
Cl [Mo(CO)2(L)], the oxo complex [Cr(O)(L)] and the
dioxo complex [Mo(O )(L)]. The reported
O 2
NVO VO(acac)2 complexes were investigated in the epoxidation of
O N CHOH/NaOH cyclooctene, cyclohexene, 1-octene and 1-hexene.
3
Chromium(III) salen complexes [101]
Cl
asymmetrically epoxidised E--methylstyrene with
5
up to 83% ee in the presence of phosphine oxides,
Scheme 4. Synthesis of Schiff base ligand L and its DMSO or DMF. In all cases the analogous reaction
oxovanadium(IV) complex with Z--methylstyrene gives lower ee.
Horwitz and coworkers [102] studied
Jana Pisk et al [98] reported a series of electrochemical reduction of [MnIII(salen)]+ (salen =
dinuclear and mononuclear oxovanadium(V) N,N'-ethylenebis(salicylaldiminato)) complexes in
complexes containing tridentate Schiff base ligands acetonitrile solution in the presence of benzoic
derived from pyridoxal and appropriate anhydride, 1-methylimidazole, dioxygen, and an
thiosemicarbazide or hydrazide. All vanadium olefin. Manganese-peroxo complex is formed in
complexes have been used as (pre) catalysts at which the O-O bond is subsequently heterolyzed by
0.05% loading for epoxidation reactions of cis- reaction with benzoic anhydride to yield a Mn(V)-
cyclooctene under “green” reaction conditions. oxo complex. Cis cyclooctene and trans-2-octene
Che and Huang [99] synthesized (R)- gave a cis/trans reactivity ratio of 10 for the
[CrIII(L )Cl]-MCM-41(m) which are used for electrolytic epoxidation. Electrolysis of cyclohexene
4
epoxidation of alkenes 6-9 (Scheme 6) is an unique yields primarily cyclohexene oxide and significant
example of heterogenized metal catalysts with chiral quantities of the allylic oxidation products 2-
binaphthyl Schiff base ligands. cyclohexen-1-one and 2-cyclohexen-1-ol.
Chiral manganese(II) complexes [103] of 1,2-
bis(salicylideneamino) cyclohexane entrapped in
OH
O zeolite showed catalytic activity in the
enatioselective epoxidation of alkenes. The
manganese(II) complexes of bis(2-pyridinaldehyde)
OH OH ethylenediamine, bis(2-pyridinaldehyde) propylene
O
diamine ligands were used in epoxidation of olefins
LVIVO LVVO LVV O O
but epoxide selectivity was observed only in the
I II III
presence of PhIO oxidant.
Skarzewski et al [104] carried out two-phase
epoxidation of olefins with sodium hypochlorite,
O
catalyzed by the salen-type Mn(III) complex 10,
(Fig. 3) was examined in the presence of various
Scheme 5. The proposed mechanism for the additional ligands and 4-dodecyloxypyridine-1-
epoxidation of alkenes by the oxovanadium complex oxide was found the most effective for epoxidation.
VOLin the presence of TBHP Researchers [105, 106] studied similar epoxidations

using Mn(III)-tridentate complexes as a catalyst.
(R)-[CrIII(L4)Cl]-MCM-41(m) O
+PhIO
N N
6-9 ee, 42- 73%
Mn
t-Bu O O t-Bu
Cl
Cl
t-Bu t-Bu
6 7 8 9
Figure 3. Structure of salen-type Mn(III) complex
10
Scheme 6. Epoxidation of alkenes with
Chromium(III) metal complexes Minutolo et al [107] studied polymer-supported
manganese(III) salen complexes for heterogeneous
asymmetric epoxidation of unfunctionalized alkenes
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in presence of mCPBA/NMO (65–72%). The
polymer-supported chiral manganese(III) salen
Cl
complexes [108] were used in the epoxidation of
11 12 13 14
styrene derivatives in the presence of PhIO, which
produced 16–46% ee. Li et al [109] Mono-Schiff Cl O2N
base Mn(III) complexes with pendant aza-crown or Me
morpholino substituents, and studied aerobic 15 16 17
oxidation of p-xylene to p-toluic acid.
Seyedi et al [110] elaborated the effect of crown Figure 4. Structures of various alkenes used for
ether ring bonded to Mn(III) Schiff base catalysts on epoxidation reaction by
the efficiency of these catalysts for the epoxidation
of cyclohexene and cyclooctene by KHSO . The Che and co-workers [113-115]
5
catalysts catalytic activity emerges completely when Manganese(II) Schiff base complexes [117] of
an aromatic nitrogen base such as pyridine as axial 1,3-diamino-2-hydroxypropane with
ligand and potassium nitrate when added to the salicylaldehyde, 2-hydroxynaphthaldehyde or 2-
reaction mixture. Thatte et al [111] prepared four hydroxyacetophenone ligands were excellent
unique, heterogeneous, chitosan-based Schiff base catalysts in the epoxidation of olefins in the
substituted with triazene, ethylene diamine and presence of PhIO as an oxidant. The extent of
salicylaldehyde and their manganese, copper, cobalt olefins epoxidation was dependent on OH
and nickel complexes. Efficiency of these catalysts substitution on Schiff base ligand. Sabater et al [4]
was tested with L-carvone, Limonene, cis-stilbene, used zeolite encapsulated chiral manganese(II)
trans-stilbene, α-terpeneol, α-methyl styrene. complexes of trans-(R,R’)-1,2-(salicylidene amino)
Katia et al [112] synthesized new chiral Schiff cyclohexane as active catalysts in the epoxidation of
base complexes obtained by condensation of 2,2’- alkenes and highlighted the importance of chiral
diamino-1,1’-binaphthalene or 1,2-diaminocyclo- catalysts in the epoxidation of unfunctionalized
hexane and differently substituted salicylaldehydes alkenes.
(bearing tert-butyl, H, Cl, or Br). The manganese
complexes containing the diiminobinaphthyl residue (S)-[MnII(Ln)] O
+ NaClO
have been tested as catalysts for the epoxidation of a
(n = 1, 3, 4) ...(1)
cis-disubstituted prochiral olefin, 1,2-dihydro- 11 ee: 13-15%
naphthalene. In spite of a chiral manganese
O
environment, the catalytic activity and + PhIO Mn(OAc)3.xH2O + (S)-H2Ln
enantioselectivity of these complexes were found to (n = 3, 11, 29, 30)
be lower as compared to those of their 12-17 Yield : 34-79% ...(2)
ee: 19-58%
diiminocyclohexyl analogues. This low activity is
O
probably due to extensive decomposition of the + PhIO (R)-[MnII(Ln)X]
manganese binaphthyl based complexes under the X = OAc : n = 3
oxidation reaction conditions. One possible 12-14 X = acac : n = 3 Yield : 59-91% ...(3)
ee: 34-76%
explanation for the instability of these manganese
O
complexes is the highly distorted geometry of the + tBuOOH (R)-[PdII(L48)]
Salbinapht ligands which are highly compatible with
tetrahedral rather than octahedral or square 14 Yield : 37% (26), 51% (30) ...(4)
ee: 17% (26, 71% (30)
pyramidal coordination.
O
Che and co-workers [113-115] studied the +PhIO (R)-[CrIII(L4)Cl]-MCM-41(m)
epoxidation of 1,2-dihydronaphthylene 11, cis--
methylstyrene 12, 4-chlorostyrene 13, styrene 14, 3- 11-14 Yeei:e l1d7 %: 3 7(2%6, (7216%), 5 (13%0) (30) ...(5)
chlorostyrene 15, 3-nitrostyrene 16 and 4-methyl- Scheme 7. Epoxidation reactions studied by Che
styrene 17 (Fig. 4) with PhIO by employing and co-workers [113-115]
manganese catalysts formed in situ from
Mn(OAc) xH O with different ee values. (Scheme 7 Ogunwumi and Bein [118] synthesized intra-
3. 2
Reaction 1-5). Che and co-workers [113-115] zeolite chiral salen complexes of manganese(III)
studied similar epoxidations using manganese, which provided the benefit of shape selectivity of
palladium and chromium complexes as a catalyst. zeolites and stereo control of chiral manganese(III)
Netalkar et al [116] prepared a series of aromatic complexes in the epoxidation of olefin. This intra
spaced bipalladium center complexes from Schiff zeolite combination showed high catalytic activity
bases derived from 4,6-diacetylresorcinol and 2,6- than the corresponding homogeneous conditions of
dialkyl substituted anilines. These complexes these complexes. The activity of the catalysts in
exhibited good catalytic activity for epoxidation of homogeneous conditions decreased due to the
alkenes. collapse of the mesoporous skeleton [119]. The
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micelle templated silica (MTS)-supported conversion (~99%) and moderate epoxide selectivity
complexes of manganese(III) were also used as (~65%) over Mn-MSN and Co-MSN catalysts. The
catalysts. The 3-chloropropylsilane grafted MTS catalytic activity of recycled catalyst is summarized
reacted with Salpr or tert-Salprpentadentate ligands in Table 2. The mesoporous silica nanoparticle-
[120] to produce suitable support for anchoring of supported catalyst provides higher surface area of
Schiff base complexes (Scheme 8). the support causes a higher concentration of
R catalytically active sites and thus show higher
Si Si O OEt
SSii O (EtO)3Si(CH2)3Cl SSii OOESti Cl Salpr or tSalpr Si O N OHR cboetntveer rscioanta. lRysets usltusp ipnodritcsa. te Mthaasts MomSeNhs cGanh osrebravnel oaos
Si Si OH -Et2O Si O Si N [124-126] prepared silica gel-immobilized Mn(II)
Si O N OH
MTS Cl-MTS Si OH R and Mo(VI) Schiff base complexes and studied its
catalytic epoxidation of various alkenes.
R
Li et al [127] developed an innovative method
R
to the epoxidation of cyclohexene under mild
R conditions using molecular oxygen as oxidant. The
Si O Si O N O
Si O Si Salpr(tSalpr) 1) Mn(II) Si O Si NMn Cl Mn(II) complexes of Schiff base ligands are
Si O 2) H2O, NaCl, Au Si O N O supported on montmorillonite by ion exchange
Si OH Si OH R
method. The montmorillonite catalysts performed
R high activity and epoxide selectivity for aerobic
Salpr(Cl) or t-Salpr(Cl)-MTS Mn-Salpr(Cl) or Mn-t-Salpr(Cl)-MTS epoxidation of cyclohexene under Mukaiyama

conditions.
Scheme 8. Manganese(III) Salpr complex grafting
on MTS materials
Table 2. The catalytic activity of recycled catalysts
Diaz and Balkus [121] successfully prepared
Run Catalyst Conversion Selectivity
and characterized a series of MCM-41 grafted
(%) (%)
metal-amine complexes, including ligands based on
1 Cu-MSN 99 80
ethylenediamine (M41ED), diethylenetriamine
(M41DET), and ethylenediaminetriacetic acid 2 Cu-MSN 97 80
(M41EDT). Ethylenediamine grafted to amorphous 3 Cu-MSN 98 78
silica (SilED) was compared with the MCM-41 1 Co-MSN 99 64
grafted metal complexes. Cobalt(II) complexes of 2 Co-MSN 98 60
M41ED and M41DET bind oxygen reversibly in 3 Co-MSN 98 61
contrast to the silica-supported complexes. The 1 Mn-MSN 99 65
anchoring of chiral ligands on these surfaces 2 Mn-MSN 98 62
produced chiral catalysts for enantioselective 3 Mn-MSN 96 64
epoxidation of olefins.
Piaggio et al [122] showed that Mn-exchanged- The manganese(III) complexes [128] of N,N’-
Al-MCM-41 when modified by a chiral salen ligand bis (salicylideneethylenediamine) catalyzed
can be an effective enantioselective heterogeneous epoxidation of alkenes. These catalysts were used to
epoxidation catalyst. The epoxidation of (Z) and (E)- study the formation of oxygen transfer agents
stilbene was catalyzed using chiral (R,R’)-(-)-N,N’- [(salen)Mn=O]+ and [PhIO(salen)Mn–
bis-(3,5-di-tert-butylsalicylidene) cyclohexane 1,2- OMn(salen)OIPh]2+, which were intermediates in
diamine ligand modified and manganese exchanged the epoxidation of alkenes in the presence of PhIO.
Al-MCM-41 in the presence of PhIO as oxygen (Scheme 9)
donor. The enantioselectivity of (Z)-stilbene to (E)- Chiral manganese(III) salen complexes [129] in
epoxide with heterogeneous catalyst was low (70% the presence of chiral additives were studied for
ee) in comparison to homogeneous catalyst (77% epoxidation of 6-acetoamino-7-nitro-2,2-dimethyl
ee). chromene with moderate yield (5–65%) and 73%
Tang et al [123] has synthesized a series of ee. The electrocatalytic activity and stability of
catalysts for epoxidation of styrene by immobilizing differently substituted manganese Schiff base
salicylaldimine transition metal complexes of complexes, towards the oxidation by dioxygen of
copper, manganese and cobalt on mesoporous silica various substrates was analyzed by Moutet and
nanoparticles (MSNs) with diameters of 120–150 Ourari [130]. Results obtained during the
nm. These catalysts possess excellent catalytic electrocatalytic epoxidation of cyclooctene have
efficiency in epoxidation of styrene in the presence shown that Mn(III) complexes gave higher turnovers
of TBHP as oxidant. Styrene shows a high than the already described electrocatalytic system
conversion (~99%) as well as epoxide selectivity based on the unsubstituted Mn(III) Salen complex.
(~80%) over Cu-MSN catalysts, and high The best results were obtained in the presence of 2-
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methylimidazole (as the co-catalyst) and benzoic for the catalytic epoxidation of styrene under mild
anhydride containing a small amount of benzoic acid conditions in which the highest yield of styrene
as an activator. oxide reached 91.2 mol%, notably higher than those
achieved from simple manganese salt catalysts.
O Erdem and Guzel [140] also carried out similar
[(salen)MIII]X
+PhIO + PhI epoxidation reactions.

+ +
N N NO N
M PhIO M
O L O O L O N N
- PhI Mn

O O
O OAc
[O=MV(salen)L]+ + [MIII(salen)L]+ +
52-55
M = Cr, Mn 18 - M - Mn 19 - M - Co
20 - M - Cu 21 - M - Fe
Figure 5. Structures of salen-metal complexes 18-21
Scheme 9. Mechanism of alkene epoxidation
catalysed by chromium(III) and Manganese(III) Polymer-supported manganese(II) salen
salen complexes complexes [141] were used as catalysts in the
enantioselective epoxidation of 1,2-dihydro-
Manganese salen complexes [131] have been naphthalene, cis-β-methyl styrene and styrene. The
investigated in alkene epoxidations with H2O2, heterogenized catalyst was more selective to form
largely from the perspective of asymmetric catalysis. trans-epoxide with cis-β-methyl styrene. The
Katsuki [132] developed unfunctionalized Mn salen supported salen complexes were produced by
complex and obtained high ee (96%) but low yields sequential treatment of polymer with 2,4,6-
(17%) in imidazole solution. Pietikainen [133] trihydroxy benzaldehyde and chiral trans-1,2-
obtained higher yields and ee’s using caboxylates as diamino cyclohexane and 3,5-di-tert-butyl
additives. Jacobson et al [134] used another salicylaldehyde and then manganese(II) ions were
manganese(III) schiff base complex in epoxidation loaded.
of olefins in the presence of additives such as 4- Canali et al [142] has prepared polymer-
phenylpyridine-N-oxide, N-methylmorpholine-N- supported salen ligand by polymerization of 4-(4-
oxide. Mansuy et al [135] developed a system to vinyl benzyloxy) salicylaldehyde with styrene and
obtain high enantioselectivities (92%) and yields vinyl benzene then reacting subsequently with 1,2-
(86%) by adding ammonium acetate to Jacobsen’s diaminocyclohexane. The prepared polymer-
catalysts. Durak et al [136] synthesized an supported salen after reacting with manganese(II)
unsymmetrical tetradentate Schiff base by the was used as a catalyst in the asymmetric epoxidation
reaction of 3-methoxysalicylaldehyde, o- of dihydroxy naphthalene and indene in the presence
phenylenediamine, and salicylaldehyde. Its Co(II) of m-chlorobenzoic acid as activator. The
and Mn(III) complexes were prepared and immobilized catalyst showed high chemical activity
immobilized on 3-amino- propyltriethoxysilane and modest enantioselectivity. Canali et al [143]
functionalized silica gel. The immobilized materials also examined polymer-supported analogue of
were found to be efficient catalysts for epoxidation Jacobsen’s catalyst selectively as the homogeneous
of styrene in the presence of TBHP in acetonitrile at species. These catalysts were used in the
40oC. epoxidation of 1,2-dihydronaphthalene, indene, 1-
Patel et al [137] successfully carried out phenylcyclohex-1-ene and 1-phenyl-3,4-
epoxidation of alkene in the presence of dihydronaphthalene using m-chloroperbenzoic acid
calix[4]arene Schiff base Co(II) complexes as as the oxidant and 4-methylmorpholine N-oxide as
catalyst. A new Co(III) Schiff base complex was the co-oxidant but the catalysts show a very rapid
synthesized by Saha et al [138] which is capable of fall in both activity and selectivity in the first and
activating dioxygen in air to catalyze the second cycles.
epoxidation of various alkenes using New unsymmetrical chiral salen complexes
isobutraldehyde as a co-reductant under were synthesized by Kim and Shin [144] and the
homogeneous conditions. Among the series of efficiency of Mn(III), Ti(IV), Co(II) and Co(III)
Schiff base complexes 18-21 (Fig. 5) synthesized by type catalysts were examined in the enantioselective
Lu et al [139] Mn complex 18 resulting from N,N’- epoxidation of styrene and α-methylstyrene. The
bis(2-hydroxy-1-naphthalidene) unsymmetrical (salen) complexes showed
cyclohexanediamine ligand was considerably active
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Vol-2, Issue-12, 2016
ISSN: 2454-1362, http://www.onlinejournal.in

comparatively high enantioselectivity as compared as catalysts in various asymmetric epoxidation
to conventional symmetrical salen complexes, which reactions. Poly(ethylene glycol) monoethyl ether
were prepared mainly from salicylaldehyde and 2- and non-cross linked polystyrene were used as
formyl-4,6-di-tert-butylphenol derivatives. Several soluble support, whereas Janda Jel and Merrifield
variables that affect the enantioselectivity in the resins were used as insoluble supports. These
epoxidation of unfunctionalized alkenes by amino supports were linked to salen complexes through a
acid based Schiff base complexes as catalysts were glutarate spacer. The soluble supports were
investigated by Kureshy et al [145]. Complexes 22- recovered by precipitation with suitable solvents,
26 (Scheme 10) derived from L histidine are better while insoluble catalysts were simply filtered from
catalysts than the other complexes synthesized by the reaction mixture. These catalysts were used in
Kureshy et al [145]. On the basis of chemical yield epoxidation of styrene, cis-β-methylstyrene and
and ee values, effectiveness of the solvents may be dihydronaphthalene.
arranged in the following order, fluorobenzene > Song and Roh [149] have developed a practical
dichloromethane > acetonitrile which is in the recycling procedure for Jacobsen’s chiral (salen)
reverse order of increasing polarity. As far as Mn(III) catalyst, using N,N’-bis(3,5-di-tert-
oxidant is concerned, only PhIO was able to perform butylsalicylidene)-1,2-cyclohexanediamine and
epoxidation reaction. Other commonly used manganese(III) chloride by use of ionic liquid 28
oxidants like H O , NaOCl, O and mCPBA were (Fig. 6). Results showed that the catalyst (R,R) 27 in
2 2 2
either inactive or gave product other than epoxides. reaction medium containing ionic liquid 28
O exhibited comparable enantioselectivity (96% ee for
R3 R3 O 2,2-dimethylchromene) in asymmetric epoxidation
H COOK H of alkenes as those obtained without ionic liquid and
N N OH
2 moreover, showed an increase in activity. At the end
Ru
R2 OH + RuCl2(PPh3)3 R2 O PPh3 of reaction the immobilized catalyst in ionic liquid
28 could be easily recycled.
R1 R1 H2O
H 22-26 H H
R3= H2C N (L - histidine) N N
N Mn
But O O But
22 = R1 = R2 = NO2 Cl
23 = R = R = Cl
1 2
24 = R1 = R2 = H But But
25 = R = C(CH), R = H
1 33 2
26 = R1 = R2 = C(CH3)3 (R,R)-27

Scheme 10. Synthesis of the complexes 22-26
N N
+ PF
6
Sivadasan and Sreekumar [146] synthesized

cross linked polystyrene-supported Schiff's base
complexes of metal ions such as Fe(III), Co(II), [bmim][PF6] 28
Figure 6. Structures of Jacobsen’s chiral (salen)
Ni(II) and Cu(II). The catalytic activity of these Mn(III) catalyst 27 and Ionic liquid 28
metal complexes is studied in the epoxidation of
cyclohexene and styrene. The reactions showed first
Pouralimardan et al [150] synthesized five
order dependence on the concentration of both the
dissymmetric tridentate hydrazone Schiff base
substrates and the catalyst. Fe(III), Co(II) and Cu(II)
ligands including HL1–4 and L5 (HL1 = benzoic acid
complexes showed significant catalytic activity (2-hydroxy-3-methoxy-benzylidene)-hydrazide, HL2
towards epoxidation of cyclohexene and styrene, but
= benzoic acid (2,3-dihydroxy-benzylidene)-
Ni(II) complex was found to be inactive. The metal hydrazide, HL3 = benzoic acid (2-hydroxy-
complexes showed low catalytic activity at low benzylidene) hydrazide, HL4 = benzoic acid (5-
crosslink density (2% and 5%) but 10% crosslink
bromo-2-hydroxy-benzylidene) hydrazide, L5 =
resins show higher activity. Erkenez and Tumer
benzoic acid pyridine-2-yl methylene hydrazide) and
[147] prepared three polymer anchored Schiff base
their manganese complexes. It was demonstrated
ligands from the reaction of 2,4-
that these dissymmetric hydrazone Schiff base
dihydroxybenzaldehye with diamines in the ethanol manganese(II) complexes (Mn–L1–5) (Scheme 11)
solution. Cu(II), Co(II) and Ni(II) transition metal
are highly selective catalysts for oxidation of
complexes of these Schiff bases showed low
cyclohexene by PhIO under mild conditions. The
catalytic activity.
only product of the reaction was cyclohexene oxide
Thomas and Janla [148] used manganese(II)
in the presence of imidazole. Adding nitrogenous
salen complexes on soluble and insoluble supports
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Vol-2, Issue-12, 2016
ISSN: 2454-1362, http://www.onlinejournal.in

donor imidazole increased the catalytic activity. Scheme 13. The epoxidation of indene using the
These kinds of complexes are potentially important manganese(III) complexes of D-2,3-bis(di-t-butyl-
catalysts in quantitative cyclohexene oxidation or salicylideneamino)-1,4-butanediol and dibenzyl
similar oxidation processes of hydrocarbons. ether
Star et al [151] synthesized D-2,3-bis
(arylideneamino)-1,4-butanediol derivatives 31-34 The ScCO manganese(II) salen complexes
2
from 29 and 30 constituted a new class of salen [152] were used as catalyst in the epoxidation of
based Schiff base ligands to form complexes with olefins using TBHP as terminal oxidant. Similarly,
transition and heavy metal ions. The manganese(III) the substituted Mn[(5,5-NO ) salen] catalyst in
2 2
complexes (Scheme 12) 35-38 corresponding to CH CN was used in the epoxidation of alkenes in
3
Schiff base ligands 31-34 were used in asymmetric the presence of PhIO as terminal oxidant resulted in
epoxidation of indene (Scheme 13). corresponding epoxides and other products. Schuster
et al [153] synthesized immobilized sterically
X demanding complexes which were tested in the
O
H diastereoselective epoxidation of (-)-α-pinene in the
z + H2N N method b: liquid phase in an autoclave at room temperature and
O
Y MnCl2.4H2O at elevated pressure using O2 as oxidant. Best results
so far are 100% conversion, 96% epoxide
[MnCl(L1)(HO)]
[Mn(L2)(HO2)]2 chemoselectivity and 91% diastereomeric excess
2 2 2
X NH method a: [MnCl(L3)] obtained in the presence of the entrapped [(R,R)-
N MnCl2.4H2O M[MnnCCl(lL(4H)]O)(L5)] (N,N’)-bis(3,5-di-tert-butyl salicylidene)-1,2-
z O 2 2
diphenylethylenediamino] cobalt(II) = Co(salen)
HL1 : Z = C-OH, Y = OCH, X = H
3
Y HL2 : Z = C-OH, Y = OH, X = H complex (Fig. 7). Suggested mechanism of
HL3 : Z = C-OH, Y = H, X = H
epoxidation is shown in Scheme 14.
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)
N N
OHC OH M
R' R' R' R2 O O R2
CHOR
2
H2HN NHH2 R' R' NOH HNO R' Salen-39: RR11 = t-BuR1, R2 = t-Bu
CHOR
2 Salen-40: R1 = t-Bu, R2 = Me
RO OR
R' R' Salen-41: R1 = t-Bu, R2 = H
R' O Cl O R' Figure 7. Salen complexes derived from (R,R)-
Mn diphenylethylenediamine
N N
RO OR [M(salen)-complex]-zeolite O

O, RT
29 R = H 30 R = CH Ph 2
2
31 R = R’ = H 32 R = CH Ph, R’ = H
2
33 R=H R’= t-Bu 34 R=CH Ph, R’= t-Bu CHO COOH
2

(-)-α-pinene
35 R = R’ = H 36 R= CH2Ph, R’=H (-)-α-pinene oxide
Scheme 14. Mechanism of epoxidation suggested by
37 R=H R’= t-Bu 38 R=CH2Ph, R’= t-Bu Schuster et al [90].
Scheme 12. Synthesis of D-2,3-
Kureshy et al [154] synthesized dimeric
bis(arylideneamino)-1,4-butanediol derivatives (32- Mn(III)-Schiff base complex 42 (Fig. 8) which
34) and of their manganese(III) complexes (35-38)
successfully catalyzed the enantioselective
epoxidation of the non-functionalized alkenes.
R' R' Excellent ee of 100% was obtained in the
R' O Cl O R' epoxidation of nitro- and cyanochromene. The rate
Mn
of reaction increased when ammonium acetate was
N N
used as co-catalyst. The activity of the recycled
RO OR catalyst gradually decreased upon successive use,
NaoCl/H2SO4 O
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Imperial Journal of Interdisciplinary Research (IJIR)
Vol-2, Issue-12, 2016
ISSN: 2454-1362, http://www.onlinejournal.in

possibly due to minor degradation during the mononuclear complex of Mn(III) with a Schiff base
reaction. ligand L [L = N,N’-ethylenebis(3-formyl-5-
methylsalicylaldimine)]. A series of binuclear
complexes, MnML’Cl solvent [M = Zn(II), Cu(II),
x y
Ni(II), Co(II), Fe(III), and Mn(III); x = 3 or 4
H H H H
N N N N depending on the oxidation state of M; L’ is the
Mn Mn
t-Bu O Cl O CH2 O Cl O t-Bu aniline condensation product of L; y = 1, 2 or 3 and
solvent = H O or CH OH] have been prepared by
t-Bu t-Bu t-Bu t-Bu the reaction2 of same3 mononuclear complex of
Mn(III), aniline and M(II/III) chloride with a 1:3:1.2
Figure 8. Structure of dimeric Mn(III)-Schiff base
molar ratio in methanolic media. The results of
complex 42
magnetic susceptibility measurements indicate that
all the metal ions are in their high spin states. The
Morries et al [155] described the use of
catalytic properties have been investigated using
supramolecular complexation to enhance (salen)Mn
PhIO as a terminal oxidant. The mononuclear
catalyst stability in the asymmetric epoxidation of
Mn(III) complexes are more efficient epoxidation
unfunctionalized conjugated olefins namely styrene
catalysts than the binuclear Mn(III) complexes. The
and 2,2-dimethylchromene as substrates (Scheme
reduced activities of the bimetallic complexes may
15). (Salen)Mn(III) catalysts (43, Fig. 9) show
be due to both the low rates of formation of a
increased turnover numbers in the catalytic
Mn(V)=O intermediate and the transfer of oxygen
asymmetric epoxidation of conjugated olefins upon
from this intermediate to the alkene.
addition of bulky Lewis acids such as zinc
Patel et al [159] have synthesized polymer-
tetraphenylporphyrin (ZnTPP). It is observed that
supported Mn(II) Schiff base complexes from cross
supramolecular complex formation enhanced the
linked chloromethylated poly(styrene-
catalyst’s stability without compromising its
divinylbenzene) copolymer beads by sequential
enantioselectivity.
modification in to a Schiff base bearing ligand.
O
These Schiff base bearing polymer on treatment
Mn catalyst
PhIO with a solution of manganese salt gave the
O O corresponding metal complexes. This Mn(II)
or or complex bound to poly(styrene-divinylbenzene)
O
copolymer catalyzed the epoxidation of norbornene
and cis-cyclooctene in the presence of alkyl
hydroperoxide under mild conditions. Kinetic
Scheme 15. Asymmetric epoxidation of conjugated experiments reveal that at elevated temperature the
olefins 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.
N N (a) t-BuooH + P Mn+2 O Mn+4 + t-BuOH
Mn
tBu O O tBu
Cl
P Mn+4 O+
tBu tBu
Figure 9. (Salen)Mn(III) catalysts 43 O + P Mn+2
Paulsamy et al [156] synthesized a series of new
cat.
chiral binol based [1+1] macrocyclic Schiff bases (b) oxidant O
and used it for the enantioselective epoxidation of Scheme 16. Mechanism of olefin epoxidation
unfunctionalized olefins that resulted high yields
and good enantioselectivity.
Louloudi et al [160] prepared Schiff base
Krishnan and Vencheshan [157] synthesized
ligands by template induced
polymeric, polynuclear Schiff base Mn complexes,
macrocyclization/condensation of
with repeating salen-like cores. Unfortunately they
diethylenetriamine (H NCH CH NHCH CH NH )
2 2 2 2 2 2
proved to be moderate catalysts for the epoxidation
with eitherpentane-2,4-dione or 1,3-diphenyl-
of simple alkenes and additives such as imidazole
propane-1,3-dione. The binuclear manganese
were required to achieve catalytic activity with
complexes (47, Fig. 10) produced was moderately
H O . Das and Cheng [158] has synthesized a
2 2 active in epoxidations, giving yields from 9% to
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