Ionic liquid-grafted Mn(III)-Schiff base complex: A highly efficient and recyclable catalyst for the epoxidation of chalcones

Article abstract of DOI:10.1055/s-2005-872226

A new Mn(III)-Schiff base complex with imidazolium phase tag was synthesized and employed as an efficient and recyclable catalyst for the epoxidation of chalcones with MCPBA/NMO. It can be recovered and reused (at least five times) without any loss of activity. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2005-872226

LETTER
2147
Ionic Liquid-Grafted Mn(III)–Schiff Base Complex: A Highly Efficient and
Recyclable Catalyst for the Epoxidation of Chalcones
I
onic-Liquid-Graft
a
edMn(III)–
n
Schiff Base
qComplex ing Peng, Yueqin Cai, Gonghua Song,* Jing Chen
Shanghai Key Laboratory of Chemical Biology, Institute of Pesticides & Pharmaceuticals, East China University of Science & Technology,
Shanghai 200237, P. R. China
Fax +86(21)64252603; E-mail: ghsong@ecust.edu.cn
Received 15 June 2005
Task-specific ionic liquids have recently been introduced
Abstract: A new Mn(III)–Schiff base complex with imidazolium
as ligands of recoverable metal catalysts used in a series
phase tag was synthesized and employed as an efficient and recycla-
of chemical transformations, such as olefin metathesis,10
ble catalyst for the epoxidation of chalcones with MCPBA/NMO. It
hydrogenation,11 cyanosilylation of aldehydes,12 hydro-
can be recovered and reused (at least five times) without any loss of
activity.
formylation,13 Negishi cross-coupling reaction,14 Heck re-
action,15 Suzuki and Stille coupling reactions.16 Herein,
we report an efficient and mild epoxidation of chalcones
with m-chloroperoxybenzoic acid (MCPBA) as the oxy-
gen source and N-methylmorpholine N-oxide (NMO) as
the co-oxidant, catalyzed by a new Mn(III)–Schiff base
complex with covalently attached imidazolium tags. The
recycled catalyst showed comparable activity even after 5
runs.
Key words: Mn(III)–Schiff base complex, imidazolium, catalyst
The metal-catalyzed epoxidation of olefins continues to
be an area of interest since epoxides are well known as one
of the most valuable building blocks in organic synthesis.1
Particularly, the epoxidation reactions of electron-defi-
cient alkenes such as a,b-unsaturated carbonyl com-
pounds, which are widely applied in the preparation of
intermediates in pharmaceutical and fine chemicals, are
currently the focus of many investigations.2 Efficient cat-
alysts are usually needed because of the poor reactivity of
those unsaturated compounds. In various metal-contain-
i
+
N
N
N
N
NH2
_
PF6
1
2
3
ing epoxidation catalysts, Jacobsen’s catalyst based on
+
N
N
Mn(III)–salen complex represents one of the most elegant
and effective members for these important transforma-
tions, including the epoxidation of electron-deficient
enones.
N
OH
_
PF6
ii
3
On the other hand, the major disadvantage of homoge-
neous catalysts is the frequently cumbersome catalyst sep-
aration from the reaction medium. Numerous efforts have
been devoted to the heterogenization of homogeneous cat-
alysts.4 For example, Jacobsen-type catalysts have been
attached on polymeric supports,5 occluded in mem-
branes,6 or entrapped in zeolites.7 However, this immobi-
lization often leads to a decrease in activity and selectivity
in comparison with those of the parent homogeneous sys-
tem. To integrate the advantages of homogeneous and het-
erogeneous catalysis into one process, a conceptual
solution involves the use of ‘separation tags’, which are
chosen such that they dominate over tagged molecules in
a complementary separation technique. Pozzi and co-
workers has employed fluorous-tagged Jacobsen catalyst
to facilitate recovery work up.8 Another major advance in
this area was the use of unmodified Jacobsen’s catalyst
dissolved in ionic liquid; however, the reusability of this
catalyst system was unsatisfactory.9
iii
N
_
PF6
O
N
N
N
+
Mn Cl
O
+
N
N
_
PF6
4
Scheme 1 Reagents and conditions: i: a) BrCH2CH2NH2·HBr,
MeCN, reflux; b) NaOH; c) KPF6; ii: salicylaldehyde, [bmim][PF6],
r.t.; iii: a) Mn(OAc)2·4H2O, EtOH, reflux; b) LiCl, reflux.
Initially, catalyst 4 was synthesized as illustrated in
Scheme 1. Starting from the commercially available 1-
methylimidazole (1), quaternization with equimolar
amounts of 2-bromoethylamine hydrobromide, followed
by recrystallization, neutralization and standard anion
metathesis with KPF6 afforded the desired amino-func-
tionalized ionic liquid 2 (67% yield) as a yellowish liq-
uid.17 Condensation of salicylaldehyde with functional
ionic liquid 2 led to the formation of Schiff base ligand 3
with two coordinating sites.18 The ligand 3 was then
SYNLETT 2005, No. 14, pp 2147–2150
0
2
.0
9
.2
0
0
5
Advanced online publication: 20.07.2005
DOI: 10.1055/s-2005-872226; Art ID: U18405ST
© Georg Thieme Verlag Stuttgart · New York

2148
Y. Peng et al.
LETTER
treated with Mn(OAc)2·4H2O and LiCl to deliver the air-
Table 1 Effect of Temperature on the Epoxidation of Chalcone
stable complex 4 as a brownish powder, which was char-
1
Temp (°C)
–10
Time (h)
Yield (%)a
acterized by H NMR, FT-IR and elemental analysis.19
This catalyst was air-stable and thermally stable to allow
easy use and storage without any precautions. In agree-
ment with the expectations according to its structure,
complex 4 is insoluble in a range of common organic sol-
vents such as toluene, hexane, diethyl ether, THF, dichlo-
romethane, ethyl acetate and ethanol, but is soluble in
water, DMF, DMSO, acteonitrile, methanol and ionic liq-
uids {e.g. [bmim][BF4] and [bmim][PF6]}.
8
77
86
91
92
92
93
–
–15
6
–20
3
–25
1.5
1.5
1.5
5
–35
–45
The thermal behavior of ionic liquid was characterized by
thermogravimetric analysis (TGA). The sample was ana-
lyzed in a platinum pan under air atmosphere and the tem-
perature was linearly increased at 10 °C/min over a
temperature range of 25–600 °C. The thermal decomposi-
tion profile shows a catastrophic weight loss around 200–
300 °C, followed by gradual 20% weight loss between
200 °C and 500 °C (Figure 1).
–55
a Amount of catalyst 4: 1 mol%.
The control reaction, run without catalyst, gave no prod-
uct after ten hours at –25 °C. In comparison, the addition
of tagged complex 4 (1 mol%) led to nearly quantitative
conversion within one hour. The Ni- and Co-coordinated
counterparts of 4 were also prepared in analogous fash-
ions as described above and tested in the model reaction.
However, no reaction was observed even after prolonged
reaction time (10 h).
To optimize the amount of catalyst 4 required, model re-
actions were carried out using various catalyst stoichiom-
etries at –25 °C in MeCN. It is noteworthy that smaller
amounts of catalyst could be used in this process, though
the reaction becomes more sluggish. For example, where-
as the epoxidation is complete within 90 minutes in the
presence of 1 mol% of 4 under standard conditions (92%
yield), about six hours are required to afford even lower
yield (84%) if only 0.5 mol% of catalyst was employed.
Moreover, increasing the amount of catalyst to 5 mol%
could not improve the efficiency (1.5 h, 93% yield).
Figure 1 Thermogravimetric analysis of catalyst 4.
With the designed catalyst in hand, epoxidation of chal-
cone with MCPBA/NMO (1.5 equiv/1.5 equiv) in the
presence of catalyst 4 was selected as a model reaction to
optimize the reaction conditions. Acetonitrile was the sol-
vent of choice in virtue of its significant solvency to both
catalyst and substrate. It is worthy to note that the explo-
ration of this reaction in neat ionic liquid [bmim][PF6]
failed completely due to the remarkably increased viscos-
ity of ionic liquids at low temperature. Moreover, in the
absence of NMO, poor and erratic results were obtained
due to the formation of unidentified by-products.
Using conditions optimized in model reactions, we turned
our attention to establish the substrate generality of the ep-
oxidation with MCPBA/NMO using 1 mol% of ionic liq-
uid-grafted Mn(III)–Schiff base complex 4 as catalyst. As
shown in Table 2, all of the chalcones were converted to
the desirable products in 1.5–2.5 hours with excellent iso-
lated yields (ranged from 90–94%) and purities (97–99%,
GC) without further purification.20 The reaction is more
facile in the presence of electron-donating substituents,
however, substituents on the phenyl rings have only a
slight influence on the yields and purities. Although two
electron-withdrawing groups were placed in the phenyl
rings of 4-chloro-4¢-nitrochalcone 5f, very good yield
(91%) was also obtained when the reaction time was pro-
longed to 2.5 hours.
The effect of reaction temperature on the reaction was
firstly examined in the presence of complex 4, as shown
in Table 1. Interestingly, the catalyst seemed to be very
active when the reaction was performed at low tempera-
tures, whereas the increase of reaction temperatures re-
sulted in a decrease in efficiency. It should be noted that
the epoxidation hardly proceeded at –55 °C, due to the
catalyst separating out of solution. Thus the epoxidation at
–25 °C was the most suitable from the standpoints of effi-
ciency and manipulation.
After obtaining these promising results, we moved on to
test the reusability of the catalyst (Table 3). The recycling
of the catalyst can simply be carried out in the air by first
removing the more volatile acetonitrile under vacuum.
Thus, the significant difference in the solubility of the
residues in diethyl ether should allow one to extract or-
ganic compounds preferentially from the mixture, leaving
Synlett 2005, No. 14, 2147–2150 © Thieme Stuttgart · New York

LETTER
Ionic Liquid-Grafted Mn(III)–Schiff Base Complex
2149
Table 2 Epoxidation of Chalcones with MCPBA/NMO Catalyzed by Catalyst 3
O
O
MCPBA, NMO
Catalyst (1 mol%)
R2
R2
O
R1
R1
MeCN, –25 °C
5a–h
6a–h
Products
6a
R1
R2
H
H
H
H
H
Time (h)
1.5
Yield (%)
Purity (%)a
H
92
91
93
94
92
91
93
90
99
99
99
99
99
98
97
99
6b
4-Cl
3-NO2
2
6c
2.5
6d
3,4-(OCH2O)
3-Br
1.5
6e
1.5
6f
4-Cl
4-NO2
2.5
6g
4-(CH3)2N
3,4-(OCH2O)
4-NO2
4-NO2
2.5
6h
2.5
a Determined by GC.
behind the catalyst as a brown solid. After extraction
work-up, the remaining catalyst was therefore recharged
with fresh starting material, solvent (acetonitrile), and ox-
idants; then reused for the epoxidation of chalcone under
the same conditions as those of the first run. Excellent
yields and purities were obtained consistently over the
five reaction cycles.
Acknowledgment
Financial support from the National Basic Research Program of
China (2003 CB 114402), Shanghai Commission of Science and
Technology and Shanghai Educational Commission are gratefully
acknowledged.
References
Table 3 Recycling of Catalyst
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92
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99
99
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C. J. Am. Chem. Soc. 2003, 125, 9248.
In conclusion, we present a novel ionic liquid-grafted
Mn(III)–Schiff base complex as a highly effective catalyst
for the epoxidation of chalcones with MCPBA/NMO, af-
fording the corresponding epoxides with excellent yields
under mild conditions. This catalyst can be recovered and
recycled for at least five runs without loss in activity. In
addition, ionic liquid grafted catalysts do not suffer from
the extensive mechanical degradation experienced by sol-
id-supported catalysts after operating at the high stirring
rates.
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2150
Y. Peng et al.
LETTER
(11) (a) Lee, S.; Zhang, Y. J.; Piao, J. Y.; Yoon, H.; Song, C. E.;
Choi, J. H.; Hong, J. Chem. Commun. 2003, 2624.
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washed with EtOH (10 mL), and dried to give compound 4
as brown powder (0.43 g, 77% yield). FT-IR (KBr): nmax
=
3167, 3110, 2976, 1611, 1545, 1478, 1439, 1136, 835, 754,
634, 472 cm–1. 1H NMR (500 MHz, DMSO-d6): d = 4.00 (s,
3 H, CH3), 4.15 (t, 2 H, J1 = 5.76 Hz, J2 = 5.70 Hz, NCH2),
4.75 (t, 2 H, J1 = 5.73 Hz, J2 = 5.86 Hz, NCH2), 6.85–7.35
(m, 4 H, Ph), 7.65 (s, 1 H, NCH), 7.75 (s, 1 H, NCH), 8.50
[s, 1 H, N(H)CN], 9.01 (s, 1 H, CH). MS (ESI): m/e (%) =
693 (8), 360 (50), 319 (80), 229 (100). Anal. Calcd for
C26H30N6O2MnClP2F12: C, 37.20; H, 3.60; N, 10.02. Found:
C, 37.13; H, 3.52; N, 9.91.
(20) General Procedure for the Epoxidation of Chalcones.
To a well-stirred solution of chalcone (1 mmol), NMO (0.17
g, 1.5 mmol) and catalyst 4 (8 mg, 0.01 mmol) in MeCN (10
mL) at –25 °C was added MCPBA (0.26 g, 1.5 mmol) in four
equal portions over a 2-min period. The homogeneous
mixture was stirred at –25 °C until chalcone had been
completely consumed (monitored by TLC), followed by
removal of MeCN on a rotary evaporator. The residue was
then extracted with Et2O (3 × 10 mL). The combined extracts
were washed with sat. Na2SO3 (3 × 10 mL), dried over
Na2SO4, filtered, and concentrated to afford essentially pure
products.
(16) Zhao, D.; Fei, Z.; Geldbach, T. J.; Scopelliti, R.; Dyson, P.
J. J. Am. Chem. Soc. 2004, 126, 15876.
(17) A similar procedure for the synthesis of aminopropyl-type
ionic liquid has been reported: Bates, E. D.; Mayton, R. D.;
Ntai, I.; Davis, J. H. Jr. J. Am. Chem. Soc. 2002, 124, 926.
(18) Synthesis of Imidazolium-Tagged Ligand(3).
A mixture of salicylaldehyde (1.22 g, 10 mmol), 2 (2.71 g,
10 mmol) and ionic liquid [bmim][PF6] (2.84 g, 10 mmol)
was stirred at r.t. for 1 h. After completion of the reaction, as
indicated by TLC, the reaction mixture was diluted with
EtOH (20 mL). The precipitate was then filtered and dried to
afford 3.01 g (80% yield) of ligand 3 as a pale yellow solid;
mp 129–130 °C. FT-IR (KBr): nmax = 3416, 3152, 3093,
2914, 2867, 1637, 1618, 1568, 1505, 1470, 1287, 1163,
1100, 848 cm–1. 1H NMR (500 MHz, CD3COCD3): d = 4.01
(s, 3 H, CH3), 4.18 (t, 2 H, J1 = 5.81 Hz, J2 = 5.49 Hz,
NCH2), 4.80 (t, 2 H, J1 = 5.64 Hz, J2 = 5.83 Hz, NCH2),
6.83–7.33 (m, 4 H, Ph), 7.70 (s, 1 H, NCH), 7.80 (s, 1 H,
NCH), 8.55 [s, 1 H, N(H)CN], 9.10 (s, 1 H, CH), 12.60 (s, 1
H, OH).
Compound 6f: mp 138–139 °C. FT-IR (KBr): nmax 3112,
3045, 1672, 1600, 1519, 1487, 1408, 1358, 1215, 1089,
1036, 864 cm–1. 1H NMR (500 MHz, CD3COCD3): d = 7.43–
7.60 (m, 4 H, Ph), 7.45 (d, J = 15.7 Hz, 1 H, CH-CH), 7.80
(d, J = 15.7 Hz, 1 H, CH-CH), 8.15–8.35 (m, 4 H, Ph). MS
(GC-MS): m/z (%) = 303 (1), 287 (70), 252 (100), 207 (15),
179 (25), 165 (45), 102 (30), 76 (20).
Compound 6g: mp 188–191 °C. FT-IR (KBr): nmax = 3364,
3171, 3045, 2972, 1626, 1540, 1328, 1235, 1149, 844 cm–1.
1H NMR (500 MHz, CD3COCD3): d = 2.95 [s, 6 H,
N(CH3)2], 6.90 (d, J = 15.5 Hz, 1 H, CH-CH), 7.65–7.57 (m,
4 H, Ph), 7.80 (d, J = 15.5 Hz, 1 H, CH-CH), 8.25–8.35 (m,
4 H, Ph). MS (GC-MS): m/z (%) = 312 (1), 296 (100), 250
(25), 207 (3), 179 (25), 165 (20), 105 (10), 76 (10).
Compound 6h: mp 212–213 °C. FT-IR (KBr): nmax = 3112,
2999, 2906, 1646, 1586, 1520, 1493, 1448, 1334, 1202,
1103, 1036, 924 cm–1. 1H NMR (500 MHz, CD3COCD3):
d = 6.01 (s, 2 H, OCH2O), 6.82–7.25 (m, 3 H, Ph), 7.30 (d,
J = 15.5 Hz, 1 H, CH-CH), 7.75 (d, J = 15.5 Hz, 1 H, CH-
CH), 8.15–8.35 (m, 4 H, Ph). MS (GC-MS): m/z (%) = 313
(1), 297 (100), 250 (25), 207(5), 175 (25), 145 (30), 105 (8),
76 (10).
(19) Synthesis of Ionic Liquid-Grafted Mn(III)–Schiff Base
Complex(4).
To a solution of ligand 3 (0.50 g, 1.30 mmol) in EtOH (20
mL) was added Mn(OAc)2·4H2O (0.16 g, 0.65 mmol). After
4 h of refluxing, 0.04 g of LiCl (0.65 mmol) was added and
the reaction mixture was further refluxed for 2 h until the
starting material had been completely consumed as judged
by TLC. On completion of the reaction, the reaction mixture
was cooled to r.t. The precipitate was collected by filtration,
Synlett 2005, No. 14, 2147–2150 © Thieme Stuttgart · New York