5-Chloro-7-iodo-8-quinolinolatomanganese(III) with the feature of pH-regulated molecular switches as a highly efficient catalyst for epoxidation of olefins with hydrogen peroxide

Article abstract of DOI:10.1002/adsc.200800054

A simple 5-chloro-7-iodo-8-quinolinolato-manganese(III) complex with the feature of pH-regulated molecular switches was conveniently synthesized and found to be a novel and highly efficient catalyst for the epoxidation of olefins with aqueous hydrogen peroxide in water-acetone media containing ammonium acetate and acetic acid as additives. This should be due to its special hexadentate binding structure that could be easily converted to the corresponding pentadentate form with a pendanthy droxy group via the opening of an axial Mn-O bond in the reaction media, as supported by in situ cyclic voltammetry and UV-vis spectra characterization.

Full text of DOI:10.1002/adsc.200800054

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DOI: 10.1002/adsc.200800054
5
-Chloro-7-iodo-8-quinolinolatomanganese(III) with the Feature
A
H
R
U
G
of pH-Regulated Molecular Switches as a Highly Efficient
Catalyst for Epoxidation of Olefins with Hydrogen Peroxide
a
a,
a
a,
a
Sheng Zhong, Zaihui Fu, * Yueming Tan, Qingji Xie, * Fang Xie,
a
a
a
a
Xiaoping Zhou, Zhengpei Ye, Guosheng Peng, and Dulin Yin
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China),
College of Chemistry and Chemical Engineering, Hunan Normal University, Changsha, Hunan 410081, Peopleꢀs Republic
of China
a
Fax : (+86)-731–8872531; e-mail: fzhhnu@tom.comxieqj@hunnu.edu.cn
Received: January 24, 2008; Published online: March 31, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
Abstract: A simple 5-chloro-7-iodo-8-quinolinolato-
manganese(III) complex with the feature of pH-
Recently, there has been increasing interest in devel-
oping highly efficient catalysts for the epoxidation of
ACHTREUNG
[
1,2]
regulated molecular switches was conveniently syn-
thesized and found to be a novel and highly effi-
cient catalyst for the epoxidation of olefins with
aqueous hydrogen peroxide in water-acetone media
containing ammonium acetate and acetic acid as ad-
ditives. This should be due to its special hexaden-
tate binding structure that could be easily converted
to the corresponding pentadentate form with a
pendanthydroxy group via the opening of an axial
MnÀO bond in the reaction media, as supported by
olefins.
Particularly, the Mn-mediated epoxidation
with aqueous H O has been appreciated as a green
2
2
[
3]
process, and the efficient catalysts reported to date
are mainly tetradentate-binding manganese(III) com-
plexes. This work reports a hexadentate-binding 5-
chloro-7-iodo-8-quinolinolatomanganese(III) complex
[Q Mn(III) in Scheme 1] that can highly efficiently
catalyze the epoxidation of olefins with H O in ace-
AHCTREUNG
[
4]
AHCTREUNG
AHCTREUNG
3
2
2
tone-water media containing ammonium acetate
(NH OAc) and acetic acid (HOAc). In situ cyclic vol-
4
in situ cyclic voltammetry and UV-vis spectra
tammetry (CV) and UV-vis spectroscopy techniques
are introduced to identify the structural transition of
characterization.
ACHTREUNG
Q Mn(III) from a hexadentate to a pentadentate
A
H
R
U
G
3
Keywords: catalysis; epoxidation; hydrogen perox-
ide; molecular switch; olefins; 8-quinolinolatoman-
ganese(III)
structure in the catalytic process.
Table 1 lists the data for the epoxidation of differ-
entolefins wh it aqueous
Q Mn(III). The effects of additives were studied in
entries 1–5 for the epoxidation of styrene. In the ab-
sence of additives, the Q Mn(III) showed a relatively
H
2O2 catalyzed by
AHCTREUNG
3
AHCTREUNG
3
low catalytic efficiency (entry 1). Among the additives
used, weak acidic additives such as NH Cl and espe-
4
Scheme 1. Proposed catalytic cycle.
802
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 802 – 806

5
-Chloro-7-iodo-8-quinolinolatomanganese(III)
A
H
R
U
G
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[a]
Table 1. Epoxidations of representative olefins catalyzed by Q Mn(III) complex.
A
H
R
U
G
3
Entry
Substrates
Additive (mmol)
No additive
Time [h]
Conversion [%]
Selectivity [%]
1
2
3
4
5
5
4
2
2
2
4
2
27
90
100
5
53
99
100
96
97
98
90
95
98
98
NH Cl (0.2)
4
NH OAc-HOAc
4
Imidazole (0.05)
NaHCO (0.2)
3
[
[
b]
c]
6
7
NH OAc-HOAc
4
NH OAc-HOAc
4
[
d,e],
8
24
95
93
9
3
100
95
[
f]
10
3
98
98
1
1
1
2
2
2
90
90
90
NH OAc-HOAc
4
100
[
f]
13
3
95
83
1
1
4
5
3
4
100
95
98
85
[
d,g]
[
a]
Unless otherwise specified, the reactions were performed using substrate (1 mmol), catalyst (2 mol percent), NH OAc
4
(
0.2 mmol), HOAc (0.1 mmol), 10% H O (1.5 mmol) and water/acetone (1/3, 3 mL) at 108C; H O was added dropwise
2
2
2
2
to the solution. Conversion and selectivity for olefins were determined by GC using an internal standard technique.
Catalyst was generated in situ.
[
[
[
[
[
[
b]
[5]
c]
d]
e]
f]
1
0
00-fold scale-up (yield: 10.7 g, 89%).
.05 mmol NH OAc and 0.05 mmol HOAc were added.
4
Using 3 mmol 30% H O and water/acetone (v/v=1/4, 3 mL).
2
2
3
0
.5 mol percentof ca ta lystwas used.
.5% mol percentof ca ta lystwas used.
g]
cially NH OAc-HOAc were highly effective in in- imply that the pH value of the reaction media should
4
creasing the epoxidative efficiency of Q Mn
A
H
R
U
G
ACHTREUNG
3
3
2
2
3
zole, which has been proven to be an effective on showed a positive effect (entry 5), probably attributa-
[
6]
salen-Mn
A
C
H
T
R
E
U
N
G
(III) and porphyrin-Mn
A
H
R
U
G
À
[7]
a quite disappointing result (entry 4). These facts (HCO ) in the presence of H O . Notably, the cata-
Adv. Synth. Catal. 2008, 350, 802 – 806
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asc.wiley-vch.de
803

Sheng Zhong etal.
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lystgenera et d in situ gave almost equivalent catalytic
performances as the pre-made one (entry 6), and a
100-fold scale-up reaction for styrene showed the
same results as the small-scale one (entry 7), indicat-
ing highly potential for future industrial applications.
Other olefins were also highly selectively oxidized
to their epoxides with satisfactory yields using the cat-
alystQ Mn(III). More specifically, 1-octene which is a
A
C
H
T
R
E
U
N
G
3
non-activated alkene, here could give >95% conver-
sion and 98% selectivity using 3 equivalents of H O
2
2
(
entry 8). Entry 9 illustrates that a-pinene was nearly
quantitatively oxidized to its epoxide without rupture
of the cyclobutane ring.
The epoxidation of aryl-substituted alkenes pro-
ceeded smoothly and almost quantitatively (entries 3,
and 10–12), the rates of these reactions were faster
than that for the aliphatic alkene. Entry 10 illustrates
that even extremely acid-sensitive epoxides could be
formed with 98% selectivity, indicating that the ligand
basicity of Q Mn
A
C
H
T
R
E
U
N
G
(III) can suppress the opening side-
3
reactions of the desired epoxides via lowering the
3+
Lewis acidity of Mn ions. Functionalized alkenes
could be also selectively oxidized to their epoxides
with high yields (entries 13 and 14), but the hydroxy
groups of allylic alcohols could also be partially oxi-
dized to form the corresponding allylic aldehydes as
by-products. Finally, entry 15 shows that cyclohexene
could be also efficiently oxidized to its epoxide even
using 0.5 mole percent of catalyst.
Figure 1. Cyclic voltammograms at
a
GCE in 0.1M
NH OAc +0.1M HOAc acetone-water solutions (v/v=4:1)
4
containing 15 mM Q Mn(III) vs. blank solution. Inset shows
A
H
R
U
G
3
cyclic voltammograms at a GCE in 0.1M NH OAc +0.1M
HOAc acetone-water solutions (v/v=4:1) containing 15 mM
4
À1
5
-Cl-7-I-8-HQ vs. blank solution. Scan rate: 50 mVs .
Why are the catalytic performances of the
Q Mn
3
A
C
H
T
R
E
U
N
G
(III) influenced by the pH value of reaction further showed that this structural transition between
media? Theoretically, because of the Jahn–Teller 1 and 2 was reversible (Figure 2). Here A is the spec-
3+
4 [8]
effectof Mn ions with d , it is anticipated that the trum of Q Mn(III) in acetone where an absorption
A
H
R
U
G
3
Q Mn(III) (1 in Scheme 1) has a distorted octahedral band with lmax ~412 nm should be assigned to the
A
C
H
T
R
E
U
N
G
3
[9]
structure with an axial MnÀO bond that is longer metal to ligand charge transfer (MLCT). After
than the equatorial ones, and the axial MnÀO bond adding aqueous hydrochloric acid to the solution, an
may be more easily cleaved to form 2 in the presence
+
of H ions. As expected, this change was detected by
in situ cyclic voltammetry (CV) tests. Figure 1 illus-
trates the cyclic voltammograms at a glassy carbon
electrode (GCE) for Q Mn(III) and the 5-chloro-8-
A
H
R
U
G
3
hydroxy-7-iodoquinoline (5-Cl-7-I-8-HQ) ligand, re-
spectively, with those in blank solutions as compari-
sons. A pair of quasi-reversible redox waves was ob-
served near (0.302+0.594)/2=0.448 V for Q Mn(III)
A
H
R
U
G
3
and near (0.283+0.581)/2=0.432 V for the ligand, re-
spectively. Also, peak currents for catalyst and ligand
were well comparable with each other. Furthermore,
the CV for the Q Mn
ACHTREUNG
3
[
5]
scan rates and different solution pH values. These
facts suggest that the pristine hexadentate structure of
the Q Mn
A
C
H
T
R
E
U
N
G
(III) (1 in Scheme 1) could be cleaved at
the axial Mn
(III)ÀO bond in the acetone-water solu-
tions containing NH OAc-HOAc to give a pentaden-
3
ACHTREUNG
Figure 2. UV-vis spectra of a solution of Q Mn
A
H
R
U
G
3
À4
4
tone (1.5ꢂ10 M, 5 mL) (A); after addition of aqueous hy-
À2
tate structure with pendant electroactive hydroxy drochloric acid (1.0ꢂ10 M, 0.05 mL) to this solution (B)
À2
group (2 in Scheme 1). In addition, UV-vis spectra and further addition of ammonia (1.0ꢂ10 M, 0.05 mL) (C).
804
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Adv. Synth. Catal. 2008, 350, 802 – 806

5
-Chloro-7-iodo-8-quinolinolatomanganese(III)
A
H
R
U
G
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obvious hypochromic effecton th e MLCT absorbance
was observed (A vs. B in Figure 2), being likely due
Experimental Section
to the opening of the axial MnÀO bond of 1 to form 2 Preparation and Characterizations of 5-Chloro-7-
+
III
in the presence of H ions. On subsequentaddi ti on of
iodo-8-quinolinolato-Mn Complex
ammonia to the solution, the MLCT absorbance soon
rebounds (B vs. C), which suggests that the 2 goes
back to the 1 through loosing a proton. Notably, an-
other absorption band with lmax ~342 nm, which
À1
Five mL of Mn(OAc) aqueous solution (1 molL ) were
A
H
R
U
G
2
added dropwise to 50 mL of stirred tetrahydrofuran (THF)
containing 15 mmol of 5-chloro-8-hydroxy-7-iodoquinoline
(purchased commercially), followed by adding 30 wt% H O2
2
should be assigned to the p-p* transition of the coor- (0.57 g, 5 mmol) into the solution and adjusting the solution
dinated ligands, was hardly affected by acidic or basic to ca. pH 7 with ammonia. Then, the reaction mixture was
additives, indicating that the ligands have not been refluxed for 2 h, and the resulting precipitate was filtered off
and washed with ethanol at least three times. After air-
shed from the Q Mn(III) in the above reversible
A
H
R
U
G
3
drying, a deep-yellow solid was obtained and denoted as
Q Mn(III) (see Scheme 1); yield: 4.46 g (92%). MS (fast
structural transition. These facts indicate that the
Q Mn(III) complex has the feature of pH-regulated
AHCTREUNG
3
ACHTREUNG
3
+
atom bombardment, FAB): m/z=664 for (MÀL) ; Mn and
CHN elemental analyses for Q Mn(III): calcd. for
MnC H O N Cl I : Mn 5.77, C 33.45, H 1.24, N, 4.34%;
molecular switches.
AHCTREUNG
3
Based on the above results, a mechanism was pro-
2
7
12
3
3
3 3
posed as shown in Scheme 1, in which the axial MnÀ
found: Mn 5.76, C 34.00, H 1.47, N 4.32%.
O bond of 1 was opened to form 2 in the presence of
+
H ions, and then, 2, which has a basic structure simi-
Epoxidation Procedures
lar to salen- or porphyrin-Mn(III) with imidazole as
A
T
E
N
[6]
The general procedures for epoxidation were performed as
follows. To a cooled (108C) and stirred of an water-acetone
the axial ligand, may eventually be converted to 3
along a pathway similar to that proposed by Quici
et al. for the Mn-porphyrin-catalyzed epoxidation of
alkenes with H O in the presence of an N-heterocy-
(
v/v 1:3, 3 mL) mixture of olefin (1 mmol), catalyst
Q Mn(III) (0.02 mmol), NH OAc (0.2 mmol), and HOAc
AHCTREUNG
3
4
2
2
(
0.1 mmol), 10% H O (1.5 mmol) was added dropwise
2 2
[
10]
clic ligand and carboxylic acid,
UV-vis spectral characterizations. Moreover, 2 bears total reaction time). After the reaction was completed, the
as supported by
during an appointed period (generally within a half of the
[
5]
3
+
a pendantOH group near ht e Mn
, which may play catalyst was separated from the reaction mixture by filtra-
two important roles: i) acting as a highly active and tion, and then an internal standard (1 mmol) was added to
the filtrate. The filtrate was analyzed on an Agilent 6890N
competitive coordination group to prevent the cata-
[
4]
gas chromatograph (GC) with a HP-5 quartz capillary
lystfrom forming m-oxomanganese dimers, leading
to less deactivation of the catalysts; ii) forming a hy-
drogen bond with H O to activate the OÀO bond
column (30 mꢂ0.32 mmꢂ0.25 mm) and FID detector. Ultra-
À1
pure nitrogen was used as carrier gas (rate 1.0 mLmin );
2
2
the injection port temperature was kept at 2508C and the
column temperature was between 90 and 1708C.
To isolate the epoxides, the filtrate was diluted with water
toward heterolytic cleavage. From this mechanism,
the poor result obtained in entry 4 (Table 1) can be
reasonably explained. As imidazole is a basic additive,
(
10 mL). Then, the mixture was extracted with hexane (5ꢂ
its presence is unfavorable to the transition from 1 20 mL), the organic layer was dried over Na SO , and con-
2
4
to 2.
centrated. The crude residue was purified by flash column
In summary, for the first time we have developed a chromatography on silica gel using hexane/ethyl acetate as
1
eluents. The products were determined by H NMR and also
simple hexadentate-binding Q Mn(III) complex as a
very efficientc aa tlystfor epoxid ai ot n of a broad
range of olefins with aqueous H O and found that
A
H
R
N
3
satisfactorily identified by comparing their mass spectra
with those of the authentic samples.
2
2
this complex has the feature of pH-regulated molecu-
lar switches. Besides that, this system has the follow-
ing advantages: (i) utilization of more cost-effective Acknowledgements
and green water-acetone solvent, (ii) facile operation,
(
(
iii) very high catalytic efficiency and stability, and We acknowledge the financial support for this work by the
National Natural Science Foundation of China (20573035,
iv) capability of in situ generation of highly effective
9
0713018, 20675029, 20335020) and the Natural Science
catalysts. This system may also be exploited in the
future for industrial applications and asymmetric ep-
oxidations.
Foundation of Hunan Province (05JJ40022).
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ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 802 – 806