A biomimetic tris-imidazole/Mn(II) system for homogeneous catalytic epoxidation of olefins with H2O2

Article abstract of DOI:10.1016/j.catcom.2010.10.021

A biomimetic tris-imidazole ligand, herein called L_3imid, has been synthesized. The complexes formed by its association with Mn(II) have been evaluated for catalytic alkene epoxidation with H₂O₂. The catalytic efficiency of Mn(II)/L_3imid system was shown to be switched on by ammonium acetate, with remarkable effectiveness and selectivity towards epoxides. The incorporation of the biomimetic ligand L_3imid which combines structural features as Schiff base imine-groups and imidazole rings is considered to be related with the enhanced catalytic activity of the Mn(II)/L_3imid system.

Full text of DOI:10.1016/j.catcom.2010.10.021

Catalysis Communications 12 (2011) 475–479
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Catalysis Communications
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Short Communication
A biomimetic tris-imidazole/Mn(II) system for homogeneous catalytic epoxidation of
olefins with H O2
2
Ag. Stamatis, Ch. Vartzouma, M. Louloudi ⁎
Department of Chemistry, University of Ioannina, 45110 Ioannina, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 June 2010
Received in revised form 8 October 2010
Accepted 27 October 2010
Available online 3 November 2010
A biomimetic tris-imidazole ligand, herein called L3imid, has been synthesized. The complexes formed by its
association with Mn(II) have been evaluated for catalytic alkene epoxidation with H O . The catalytic
2 2
efficiency of Mn(II)/L3imid system was shown to be switched on by ammonium acetate, with remarkable
effectiveness and selectivity towards epoxides. The incorporation of the biomimetic ligand L3imid which
combines structural features as Schiff base imine-groups and imidazole rings is considered to be related with
the enhanced catalytic activity of the Mn(II)/L3imid system.
Keywords:
©
2010 Elsevier B.V. All rights reserved.
Catalytic epoxidation
Manganese complexes
Hydrogen peroxide activation
Additive
1
. Introduction
2 2
[17–19] as homogeneous catalysts for alkene epoxidation with H O .
These systems showed considerable effectiveness in an acetone–
methanol mixture at room temperature in the presence of ammonium
acetate as additive.
Olefin epoxidation with H
complexes has received considerable attention because a) H O
2 2
2
O
2
catalyzed by non-heme manganese
is
inexpensive, readily available with relatively high oxygen content and
environmentally friendly and b) non-heme manganese complexes are
often easily prepared and handled [1].
In the context of optimizing the oxidation reactions of olefins by
manganese and iron catalysts, we present here the synthesis of a new
biomimetic ligand which a) bears three imidazole residues and
b) offers exclusively nitrogen atoms for association with manganese
(II) centers. The derived tris-imidazole/Mn(II) system, which was
evaluated as homogeneous catalyst for alkene epoxidation with H O ,
2 2
is found to be ammonium acetate-dependent and shows remarkable
effectiveness and selectivity.
2 2
Activation of H O by non-heme manganese-enzymes is well
known [2,3]. Thus, the challenge in the field of synthetic biomimetic
chemistry is to construct optimized functional, low-cost biomimetic
catalysts.
Mn-salen-type complexes have been extensively studied as
epoxidation catalysts, with special attention to asymmetric catalysis
[
4–7]. For the epoxidation of simple olefins, the Jacobsen-type
catalysts with in situ addition of imidazole or N-methylimidazole
have been also evaluated [8,9]. The use of Mn-Me TACN complexes
TACN:1,4,7 triazacyclononane), as promising oxidation catalysts has
2. Experimental
3
(
All substrates were purchased from Aldrich, in their highest
commercial purity, stored at 5 °C and purified by passing through
a column of basic alumina, prior to use. Hydrogen peroxide was 30%
(w/w) aqueous solution.
Elemental analyses (C, H, N) were obtained using a Perkin Elmer
Series II 2400 elemental analyzer. Infrared spectra were recorded on a
Spectrum GX Perkin Elmer FT-IR System and UV–Vis spectra were
recorded using a UV/VIS/NIR JASCO Spectrophotometer. Mass spectra
were measured on an Agilent 1100 Series LC-MSD-Trap-SL spectrom-
eter and solution NMR spectra were recorded with a Bruker AMX-
been explored for commercial application e.g. by Unilever for washing
detergents [10]. This system was further improved by the work of
Bein et al. [11]. Very recently, it was shown that the high activity and
selectivity of these systems is due to the in situ formation of bis(μ-
carboxylato)-bridged dinuclear manganese(III) complexes [12,13]. An
efficient dinuclear manganese catalyst based on polypyridine ligands
has been also synthesized [14].
Our group has developed imidazole based-acetamide/Mn(II)
systems [15,16] and acetylacetone-based Schiff bases/Mn(II) systems
4
00 MHz spectrometer with external TMS as reference. GC analysis
was performed using an 8000 Fisons chromatograph with a flame
ionization detector and a Shimadzu GC-17A gas chromatograph
coupled with a GC-MS-QP5000 mass spectrometer.
⁎
Corresponding author. Tel.: +30 26510 08418; fax: +30 26510 08786.
E-mail address: mlouloud@uoi.gr (M. Louloudi).
1566-7367/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2010.10.021

476
A. Stamatis et al. / Catalysis Communications 12 (2011) 475–479
c
b
a
2
.1. Synthesis and characterization of L3imid
d
N
i
N
HC
N
N
CH
(
1H-Imidazol-4-ylmethylene)-[2-(3-{2-[(1Η-imidazol-4-
ylmethylene)-amino]-ethyl}-2,3,4,5-tetrahydro-1′Η-[2,4′]biimidazolyl-
-yl)-ethyl]amine (C18 10, L3imid): a solution of triethylentetramine
0.774 ml, 5.2 mmol) in methanol (50 ml) was added to an ice-cold
e
1
(
24
Η Ν
f
NH h
g
NH
NH
solution of 4(5)-imidazolecarboxaldehyde (1.0 g, 10.4 mmol) in meth-
anol (100 ml). The resulting pale-yellow solution was stirred for 15 min
at 0 °C. Then a new solution of 4(5)-imidazolecarboxaldehyde (0.5 g,
N
N
N
5
.2 mmol) in methanol (50 ml) was added into the reaction mixture.
L3imid
The yellow solution was stirred at 0 °C for 15 min, and finally at 30 °C
for 23 h. The solution was then evaporated to dryness. The crude
product was treated with hexane and acetone, and finally the yellow
Fig. 1. The ligand (1H-imidazol-4-ylmethylene)-[2-(3-{2-[(1Η-imidazol-4-ylmethylene)-
amino]-ethyl}-2,3,4,5-tetrahydro-1′Η-[2,4′]biimidazolyl-1-yl)-ethyl]amine (L3imid
) used
solid was dried under reduced pressure over fused CaCl
p. 72–75 °C. Anal. Calcd. for C18 10.CH OH (%): C, 55.3; N, 34.0; H,
.8. Found (%): C, 55.0; N, 34.4; H, 7.1.
IR (KBr, cm−1, selected peaks) 3120: ν(NH); 1649: ν(C=N); 1558:
δ(NH); 1496: ν(C=C); 1447: ring stretching modes (imidazole); 625:
2
. Yield 81%. m.
herein.
24
H N
3
6
consistent with the proposed formulation. For example, in the 13C
NMR spectrum, the signal at 154 ppm was attributed to the imine-
carbon atoms. In the 1H NMR spectrum the imine N=CH resonance
was located at 8.13 ppm.
1
δ(NH). H NMR ((CD
CH NCH CH ; 3.48 (m, 4H): HCNCH
Η): C=CΗ–N; 7.3 (m, Η): N=CH–NH; 7.56 (Η): C–NH–CH; 8.13(H):
3
)
2
SO, δ) 2.2 (m, 4Η): CH
2
NCH
2 2
CH ; 3.2 (m, 4Η):
2
2
2
2
CH NCH
2
2
; 3.9 (H): Ν–C–Ν; 6.95
(
2 2
3.2. Catalytic epoxidation with H O
13
C=N. C NMR ((CD
0.5: HCNCH CH NCH
NH; 138: CO–CH –C C=C–NH N; 154: C=N. UV (MeOH, λmax (nm), ε
cm )) 251 (119000); 308 (19900); 437 (5900). ESI-MS (m/z)
)
3 2
2 2 2 2 2 2
SO, δ) 51.6: CH NCH CH ; 54.3: CH NCH CH ;
6
2
2
2
; 82.6: Ν–C–Ν; 130: C=CΗ–N; 136: N=CH–
According to the data in Table 1, the MnCl
L3imid systems showed significant catalytic activity within 3 h.
Epoxidation of a wide range of olefins proceeded with high
conversion and selectivity for the epoxide product (the mass balance
is 98–100%) in most of the cases (Table 1).
2 2
/L3imid, and Mn(acet) /
2
−1
−1
(
3
M
+
+
81.3 ([MH] ); 403.3 ( [M+Na] ).
2
.2. Catalytic reactions
Oxidation of cyclooctene, catalysed by MnCl
2
/L3imid and Mn
(
acet) /L3imid, provided a 100% selectivity for cis-cyclooctene epoxide
2
Manganese catalysts were prepared in situ, by mixing appropriate
with 77.7% and 74.9% yield corresponding to 777 and 749 TONs and
259 and 250 TOF respectively (Figs. 2 and 3). Hexene-1, which is a
rather hard oxidation substrate, showed maximum epoxide yield
21.3% and 100% selectivity for the cis-epoxide. Cyclohexene oxidation
amounts of MnCl
stock solution of the catalysts was prepared by dissolving 1.1 mmol of
3imid in a 0.110 M solution of MnCl ·4H O or Mn(CH COO) ·4H O in
0 ml CH OH.
Typical conditions employed in catalytic reactions were: 1 equiv.
of catalyst, 2000 equiv. of H 30% (w/w) and 1000 equiv. of
2 3 2
or Mn(CH COO) with L3imid ligand. A 0.055 M
L
2
2
2
3
2
2
3
Table 1
2 2
O
Alkene epoxidations catalyzed by MnCl
presence of H O .
2 2
/L3imid and Mn(acet) /L3imid systems in the
substrate and additive. The alkene (1 mmol), acetophenone or
bromobenzene (internal standard, 1 mmol), catalyst solution
2
2
Substrate
Products
Yield (%)a TON TOF (h−1
MnCl Mn(acet)
77.7 777 259 74.9 749 250
)
(
(
1 μmol) and CH
450 μl/400 μl) solvent mixture were cooled to 0 ° C. H
3
COONH
4
(additive, 1 mmol) in a acetone/MeOH
(2 mmol)
2
/L3imid
2
/L3imid
2 2
O
was added by a digitally controlled syringe pump, type SP101IZ WPI,
over 1 h under stirring. 10 min later, the test tube was removed from
the ice bath and allowed to warm to room temperature 26± 1 °C.
The progress of the reaction was monitored by GC–MS, by
removing small samples of the reaction mixture. GC analysis of the
solution provided the substrate conversion and product yield relative
to the internal standard integration. To establish the identity of the
epoxide product unequivocally, the retention time and spectral data
were compared to those of an authentic sample. Blank experiments
Cyclooctene
Hexene
cis-Epoxide
-
1
cis
-
Epoxide
21.3 213
49.1
4.9
71 17.6 176
59
Cyclohexene
cis-Epoxide
59.9
4.7
2
2
-Cyclohexenone
-Cyclohexenol
3.1
3.4
cis-Diol
2.0
1.8
591 197
698 233
646 215
576 192
Styrene
Epoxide
Phenyl acetaldehyde
58.1
1.8
63.1
1.5
5
99 200
39 213
trans-β-Methylstyrene trans-Epoxide
trans-Methyl-ketone
62.5
1.4
56.3
1.3
showed that without Mn-catalyst or CH
reactions do not take place.
3 4
COONH , epoxidation
6
Limoneneb
1,2-Epoxides
Z−/E−)
,9-Epoxides
Alcohol
71.7
(40.3/31.4)
13.4
2.2
69.4
(37.4/32.0)
14.5
2.4
(
8
-
3
. Results and discussion
3
.1. Synthesis of the ligand
-Ketone
di-Epoxide
1.4
1.3
1.5
1.0
9
4
00 300
888 296
The biomimetic ligand L3imid was prepared by a stepwise conden-
sation of trien with three equivalents of 4(5)-imidazolecarboxaldehyde
Fig. 1).
The five-member imidazolyl ring was formed at the ethylenedia-
cis-Stilbene
cis-Epoxide
trans-Epoxide
-ketone
32.0
4.7
5.5
35.0
5.5
6.1
(
22 141
466 155
mine backbone by imidazolecarboxaldehyde mediation. Thus, a fully
symmetrical molecule was synthesized bearing three imidazole
Conditions-ratio of catalyst: H O :CH COONH :substrate = 1:2000:1000:1000;
2
2
3
4
equivalent of catalyst=1 μmol in 0.85 ml CH
3
3 3
COCH :CH OH (0.45:0.40). Italic data
+
corespond to TONs and TOFs numbers.
residues. In the mass spectra, the molecular peaks for [MH] and
a
+
Yields based on starting substrate and products formed. The mass balance is 98–
[
M+Na] appear at m/z 381.3 and 403.3 (Supporting information).
1
00%. Reactions were completed within 3 h.
Limonene 1,2-oxide was found as a mixture of Z− and E− isomers and limonene
−1
The IR band at 1649 cm
bands, ν(C=N). H and
was attributed to the imine-stretching
b
1
13
C NMR spectral assignments were
8,9-oxide as a mixture of two diastereoisomers.

A. Stamatis et al. / Catalysis Communications 12 (2011) 475–479
477
MnCl
2
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
Mn(acet)
MnCl /L
2
2
3imid
Mn(acet) /L
2
3imid
cyclooctene
cyclohexene
styrene
limonene
cis-stilbene trans-methyl-styrene 1-hexene
Substrates
2 2 2 2 2 2 3 3
Fig. 2. Total turnover numbers for alkene epoxidations catalyzed by MnCl , Mn(acet) , MnCl /L3imid and Mn(acet) /L3imid in the presence of H O . Reaction performed in CH COCH /
3
CH OH (0.45:0.40 ml). See Table 1 and Supporting information for details.
provided mainly epoxide with yields from 49.1 to 59.9%. In addition,
small amounts of the corresponding diol, probably as epoxide ring
opening product, have been also detected (1.8–2.0%). In some extent,
allylic oxidation has also occurred forming 2-cyclohexene-1-ol (3.1–
the corresponding methyl-ketone have been also observed (~1.3%)
(Figs. 4 and 5).
The major products detected from oxidation of limonene, were two
epoxides (Z- and E-) derived from epoxidation of the 1,2-double bond and
two diastereoisomers derived from the exocyclic 8,9-double bond.
Additionally, small amounts of di-epoxide and products formed by allylic
oxidation of the limonene ring have been identified. However, the
epoxidation was clearly the main reaction path resulting mainly in 1,2-
epoxides. The total yield of epoxides (1,2- plus 8,9-) was 85.1% (ratio 1,2-/
3
.4%) and 2-cyclohexene-1-one (4.7–4.9%). The total yield of
cyclohexene oxidation products was 59.1% for MnCl /L3imid and
9.8% for Mn(acet) /L3imid system.
Styrene oxidation produces mainly the corresponding epoxide by
MnCl /L3imid, and Mn(acet) /L3imid with yields 58.1 and 63.1% respective-
2
6
2
2
2
ly, and high selectivities up to 97%; trace amounts of phenyl acetaldehyde
as side product have been detected. The catalysts showed analogous
ability towards the oxidation of trans-β-methylstyrene resulted in 62.5
and 56.3% epoxide yield with N97% retention of its configuration. Traces of
8,9- equal to 5.4) by MnCl
Mn(acet) /L3imid; the total yield of oxidation products raised at 90% with
900 TONs and 300 TOF for MnCl /L3imid and 88.8% with 888 TONs and 296
TOF for Mn(acet) /L3imid system.
2
/L3imid and 83.9% (ratio 1,2-/8,9- equal to 4.8) by
2
2
2
MnCl
2
3
2
2
1
1
00
50
00
50
00
Mn(acet)
MnCl /L
2
2
3imid
Mn(acet) /L
2
3imid
50
0
cyclooctene
cyclohexene
styrene
limonene
cis-stilbene trans-methyl-styrene 1-hexene
Substrates
Fig. 3. TOF for alkene epoxidations catalyzed by MnCl
2 2 2 2 2 2 3 3 3
, Mn(acet) , MnCl /L3imid and Mn(acet) /L3imid in the presence of H O . Reaction performed in CH COCH /CH OH
(0.45:0.40 ml). See Table 1 and Supporting information for details.

478
A. Stamatis et al. / Catalysis Communications 12 (2011) 475–479
aldehyde
diepoxyde
diol
100
alcohol
ketone
8
6
4
2
0
0
0
0
0
epoxide
cyclooctene
cyclohexene
styrene
Cis-stilbene
limonene
1-hexene
trans-m-styrene
Substrates
2 2 2 3 3 3
Fig. 4. Distribution of oxidation products catalyzed by MnCl /L3imid in the presence of H O . Reaction performed in CH COCH /CH OH (0.45:0.40 ml). See Table 1 for further details.
In the oxidation of cis-stilbene, the major product was cis-epoxide
with selectivity ~75% and yield ranging from 32.0 to 35.0%. Moreover,
considerable amounts of trans-stilbene epoxide (4.7–5.5%) and 1,2-
diphenyl ethanone (5.5–6.1) have been also detected.
Competitive reactions show alkene reactivity to increase with the
electron density of the double bond, e.g. hex-1-enebcyclohexeneb1-
methylcyclohexene and the electron-rich trisubstituted double bond
of limonene in 1,2-position gave several times more epoxides than the
more accessible but less electron-rich double bond in 8,9-position
At this point is of relevance to compare the present systems with
other pertinent catalysts. We have developed two sets of homoge-
neous Mn(II)-catalytic systems: a) imidazole based-acetamide
ligands/Mn(II) [15,16] and b) acetylacetone-based Schiff bases/Mn
(II) [17–19]. Generally, the catalytic systems associated with Schiff
2 2
bases showed higher catalytic reactivity with H O for alkene
epoxidations. The present catalytic system incorporates a new
biomimetic ligand which combines structural features of both
previous systems. Thus, it is a Schiff base ligand with two imine-
groups and also it bears three imidazole residues. Comparing the
present catalytic system with that previously reported [15–19], we
observe an enhanced reactivity of the present catalytic system. We
consider that this is attributable to the structure of the ligand which
(
Table 2). These data might be taken as evidence of the electrophilic
nature of oxygen transfer from manganese-oxo intermediate to the
olefinic double bond. The reactivity of various substrates was screened
and the results are listed in Table 2.
100
aldehyde
diepoxyde
diol
8
6
4
2
0
0
0
0
0
alcohol
ketone
epoxide
cyclooctene
cyclohexene
styrene
Cis-stilbene
limonene
1-hexene
trans-m-styrene
Substrates
2 2 2 3 3 3
Fig. 5. Distribution of oxidation products catalyzed by Mn(acet) /L3imid in the presence of H O . Reaction performed in CH COCH /CH OH (0.45:0.40 ml). See Table 1 for further
details.

A. Stamatis et al. / Catalysis Communications 12 (2011) 475–479
479
Table 2
(II) systems. Importantly, the Mn(II)/L3imid catalyst is also ammonium
acetate-dependent and can activate H favouring productive alkene
epoxidations to a remarkable extent with high selectivity.
2 2 2
Chemo- and regio-selectivity in alkene epoxidation with H O catalysed by MnCl /
2 2
O
L
3imid and Mn(acet)
2
/L3imid.
Alkene 1
Alkene 2
Epoxide 1/epoxide 2a
MnCl
2
/L3imid
2
Mn(acet) /L3imid
Appendix A. Supplementary data
Styrene
Styrene
Cyclooctene
Cyclooctene
Cyclohexene
Cyclohexene
trans-Methylstyrene
cis-Stilbene
Styrene
Cyclohexene
Hex-1-ene
0.73
0.70
2.57
0.94
12.00
0.93
5.35
0.79
0.72
2.41
1.02
12.21
0.86
4.79
Supplementary data to this article can be found online at
doi:10.1016/j.catcom.2010.10.021.
1-Methylcyclohexene
b
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Conditions-ratio of catalyst: H
00; equivalent of catalyst=1 μmol in 0.85 ml CH
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2
O
2
:CH
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:substrate
1
:substrate
2
=1:2000:1000:500:
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a biomimetic ligand which combines structural features as Schiff base
imine-groups and imidazole rings. Thus the present Mn(II)/L3imid
system consists an optimised system of our previous biomimetic Mn
2