Preparation of chiral oxovanadium (IV) Schiff base complex functionalized by ionic liquid for enantioselective oxidation of methyl aryl sulfides

Article abstract of DOI:10.1016/j.catcom.2011.06.006

Chiral oxovanadium (IV) Schiff base complex covalently grafted with ionic liquid (IL: 1-(3-aminopropyl)-3-methylimidazolium tetrafluoroborate) has been reported first time. The IL-functionalized complex was found to be an efficient catalyst in the enantioselective oxidation of methyl aryl sulfides to sulfoxides with hydrogen peroxide as an oxidant. Especially, the IL-functionalized complex could be recovered conveniently by simple precipitation with addition of hexane and reused at least six cycles without loss of activity and enantioselectivity.

Full text of DOI:10.1016/j.catcom.2011.06.006

Catalysis Communications 12 (2011) 1488–1491
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Preparation of chiral oxovanadium (IV) Schiff base complex functionalized by ionic
liquid for enantioselective oxidation of methyl aryl sulfides
Rong Tan a, Chengyong Li a, Zhigang Peng a, Dulin Yin a, Donghong Yin a,b,
⁎
a
Institute of Fine Catalysis and Synthesis, Hunan Normal University, Changsha 410081, Hunan, China
b
Technology Center, China Tobacco Hunan Industrial Corporation, Changsha 410014, Hunan, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral oxovanadium (IV) Schiff base complex covalently grafted with ionic liquid (IL: 1-(3-aminopropyl)-3-
methylimidazolium tetrafluoroborate) has been reported first time. The IL-functionalized complex was found
to be an efficient catalyst in the enantioselective oxidation of methyl aryl sulfides to sulfoxides with hydrogen
peroxide as an oxidant. Especially, the IL-functionalized complex could be recovered conveniently by simple
precipitation with addition of hexane and reused at least six cycles without loss of activity and
enantioselectivity.
Received 1 March 2011
Received in revised form 27 May 2011
Accepted 4 June 2011
Available online 13 June 2011
Keywords:
© 2011 Elsevier B.V. All rights reserved.
Chiral oxovanadium (IV) Schiff base complex
Ionic liquid
Enantioselective oxidation
Sulfide
Sulfoxide
1. Introduction
oxovanadium (IV) Schiff base complex. The amphiphilic IL-functionalized
chiral oxovanadium (IV) Schiff base complex can improve the yield
The optically active sulfoxides are of great important chiral
auxiliaries in organic synthesis and bioactive ingredients in the
pharmaceutical industry [1,2]. The catalytic asymmetric oxidation of
prochiral sulfides is often the efficient methodology to synthesize
enantiopure sulfoxides [3]. Bolm and Bienewald first described
enantioselective oxidation of sulfides catalyzed by the in situ formed
vanadyl complex of the substituted salicylidene derivatives of
(S)-tert-leucinol (1.5 mol%) utilizing aqueous 30% H2O2 as stoichio-
metric oxidant [4]. However, the recovery of the in situ formed
vanadyl complex often becomes difficulty. To address the issue, much
effort has been made to immobilize the chiral oxovanadium (IV) Schiff
base complex onto insoluble solid supports [5,6]. Unfortunately, the
nature of the insoluble supports often suffers from poor activity and
low accessibility of the substrates. Recently, ionic liquids (ILs) bonded
to the chiral salen Mn(III) complex have been found to enhance
catalysts efficiency and recovery [7,8]. Thus, immobilization of the
complex using ILs might provide a means to develop reusable
catalysts with high activity.
and enantioselectivity of sulfoxides in the asymmetric oxidation of
methyl aryl sulfides. Furthermore, a facile product isolation and
catalyst recycle were also enjoyed upon introduction of the IL moiety
into the chiral oxovanadium (IV) Schiff base complex.
2. Experimental
2.1. Reagents
2-tert-butyl phenol, L-leucinol and methyl aryl sulfides were
purchased from Alfa Aesar. Other laboratory grade reagents were
obtained from the local suppliers. 1-(3-aminopropyl)-3-methylimi-
dazolium tetrafluoroborate [7], 3-tert-butyl-2-hydroxybenzaldehyde
[9], 5-chloromethyl-2-hydroxy-3-nitrobenzaldehyde [10] and 3-tert-
butyl-5-chloromethyl-2-hydroxybenzaldehyde [11] were prepared
by the procedures described in the literature, respectively.
Herein, a hydrophilic IL of 1-(3-aminopropyl)-3-methylimidazolium
tetrafluoroborate was first applied to functionalize the chiral
2.2. Characterizations
Fourier transform infrared (FT-IR) spectra were obtained at 400–
4000 cm−1 region on an AVATAR 370 Thermo Nicolet spectrophotom-
eter, usingKBr pellets. UV–visspectra were obtained ona UV–vis Agilent
8453 spectrophotometer at 200–800 nm. The contents of vanadium in
the samples were determined by inductively coupled plasma-atomic
emission spectrometry (ICP-AES) using a BAIRD PS-6 analyzer.
⁎
Corresponding author at: Institute of Fine Catalysis and Synthesis, Hunan Normal
University, Changsha 410081, Hunan, China. Tel.: +86 731 88872576; fax: +86 731
8872531.
E-mail address: yindh@hunnu.edu.cn (D. Yin).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.06.006

R. Tan et al. / Catalysis Communications 12 (2011) 1488–1491
1489
2.3. Preparation of IL-functionalized chiral oxovanadium (IV) Schiff base
complex 1, 2, and the neat chiral oxovanadium (IV) Schiff base complex 3
purified by chromatography on silica gel (petroleum ether/ethyl
acetate, 1.5/1, V/V) using ethyl acetate as the eluent. The e.e value of
methyl aryl sulfoxides was determined by HEWLETT PACKARD SERIES
1050 HPLC analysis with chiral columns of Daicel Chiracel OD-H or
OB-H (0.46 cm i.d. ×25 cm) using hexane/ i-PrOH as eluent according
to the method presented in supplementary information.
The procedure for synthesis of the complex 1 and 2 was outlined in
Scheme 1.
To a stirred solution of L-leucinol (1.17 g, 10 mmol) in dry ethanol
(20 mL), an equivalent amount of 3-tert-butyl-5-chloromethyl-2-
hydroxybenzaldehyde or 3-nitro-5-chloromethyl-2-hydroxybenzal-
dehyde in dry ethanol (20 mL) was added dropwise. The mixture was
stirred for 48 h at 0 °C, and then the solvent was evaporated under
vacuum. Recrystallization from ethanol gave a yellow solid of CL1
(2.96 g, yield of 91%) or CL2 (2.74 g, yield of 87%).
3. Results and discussions
3.1. Preparation and characterization of samples
A solution of the CL1 (2.61 g, 8 mmol) or CL2 (2.52 g, 8 mmol) and
an equivalent amount of IL in dry toluene (20 mL) was refluxed for
48 h under nitrogen protection. After cooled to 5 °C overnight, the
obtained compound was collected by removal of toluene, washed
completely with hexane. Drying in vacuum gave a deep yellow solid of
IL-CL1 (3.4 g, yield of 82%) or IL-CL2 (3.09 g, yield of 79%).
To a stirred solution of the IL-CL1 (2.58 g, 5 mmol) or IL-CL2 (2.52 g,
5 mmol) in dry methnol (20 mL), VO(acac)2 (1.22 g, 3.5 mmol) in dry
methnol (15 mL) was added dropwise at 40 °C under nitrogen
protection. The mixture was stirred for 1 h, and then heated to reflux
for additional 8 h. After exposed to air overnight, the above mixture was
cooled to 5 °C for 2 h, filtered and washed with 50 mL of water and
50 mL of hexane. The obtained solid was dried under vacuum at 40 °C to
give a light brown powder of IL-functionalized chiral oxovanadium (IV)
Schiff base complex 1 (2.51 g, 86%) or complex 2 (2.74 g, 83%).
The structure identification of the synthesized ligands of CL1, CL2,
IL-CL1, IL-CL2, as well as the complex 1 and 2 were presented in
supplementary information.
As described in Scheme 1, the tridentate chiral Schiff base ligand (CL)
was prepared by the condensation of the substituted salicylaldehydes
with optical L-leucinol. Then, the –CH2Cl group in the resultant CL was
reacted with –NH2 of the IL to give the IL-functionalized chiral Schiff
base ligand (IL-CL). Lastly, the IL-CL was treated with VO(acac)2 under
nitrogen, and then exposed to air overnight to afford the expected
IL-functionalized chiral oxovanadium (IV)-Schiff-base complexes
1 and 2.
The synthesized IL-functionalized complexes 1 and 2, as well as the
neat complex 3, were characterized by FT-IR and UV–vis spectra (Figs. 1
and 2). It was found that the IL-functionalized complex 1 displayed the
υ(V=O) stretching band at 996 cm−1, the υ(O–Ph) stretching band at
the range of 1290–1320 cm−1, and the υ(C=N) stretching band at
1628 cm−1 , respectively [12], which were in agreement with the neat
complex 3 (Fig. 1b vs.Fig. 1a). While, in the case of the complex 2,
theυ(C=N) and the υ(V=O) stretching band shifted to 1634 cm−1 and
974 cm−1 due to the electron-withdrawing effect of the nitro group in
the aryl group (Fig. 1c). It has been reported that the oxovanadium (IV)
complexes with coordination numbers 5 and 6 haveυ(V=O) values
higher than and lower than about 990 cm−1, respectively [13]. As
shown in Fig. 1, the complex 1 in the solid state should be penta
coordinated oxovanadium (IV) complex due to theυ(V=O) stretching
band at 996 cm−1. The complex 2, whose υ(V=O) value is around
974 cm−1, should be hexa-coordination in the solid state. The neat
complex 3 in the solid state exhibited another characteristic band at
820 cm−1, which assigned toV–O–V bond[13]. The result indicatedthat
the neat complex 3 should be dimer in the solid state [14]. Furthermore,
the IL-functionalized complex 1 and 2 showed additional signals at 621
and 3389 cm−1, assigned to the characteristic bands of imidazole
fragment and stretching vibration of N–H group of the second amine
group, respectively (Fig. 1a and c) [7]. The second amine should be
The neat chiral oxovanadium (IV) Schiff base complex 3 was
synthesized according to the above similar method.
2.4. Asymmetric oxidation of the methyl aryl sulfides
Methyl aryl sulfide (1 mmol) was added into the mixture of
catalyst (0.02 mmol) and dichloromethane (1 mL) at 20 °C under
stirring. Aqueous 30% H2O2 (0.17 g, 1.5 mmol) was added dropwise.
Gas chromatography (Agilent Technologies 6890) was employed to
monitor the progress of the reaction. After the reaction, the mixture
was extracted with hexane to make the catalyst precipitate out from
the reaction system. The combined extract layer was dried over
anhydrous Na2SO4, and concentrated in vacuo. The crude product was
CHO
ClH2C
OH
NH2
N
N
HC
R
N
OH
R
BF4
Toluene, Reflux, N2, 48 h
ClH2C
OH
H2N
OH
EtOH, 0 oC, 48 h
CL
HC
R
N
OH
HC
N
O
O
VO(acac)2
H2
C
H2
C
H
N
H
N
BF4
N
V
N
N
OH
N
O
BF4
Methnol, Reflux, 7 h
R
IL-CL
Complex
CL1
IL -CL1
R= t-Bu,
R= t-Bu,
CL2
IL -CL2
R= NO2;
R=NO2;
R= NO2.
Complex 1 R= t-Bu, Complex 2
Scheme 1. Synthesis and the idealized structure of the complexes 1 and 2.

1490
R. Tan et al. / Catalysis Communications 12 (2011) 1488–1491
Table 1
996
1293
Results of the asymmetric oxidation of methyl aryl sulfide over various catalystsa.
820
1628
1628
a
Catalyst, H O (30%),
O
2
2
S
b
S
o
3389
CH Cl , 20 C, 4 h
2
2
621
X
996
X
c
X = H, Br, OCH NO
3,
2
Entry
X
Catalyst
No
Run
times
Yield (%)b e.e (%)c TOFd ×10−3
621
(s−1
)
1293
974
3389
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
Br
/
2.7
0
/
1312
The neat Complex 3 fresh 66
Complex 1
39 (S)
43 (S)
43 (S)
42 (S)
38 (S)
40 (S)
39 (S)
32 (S)
2.3
3.0
3.0
2.9
2.6
3.1
2.6
2.3
1522
fresh 85
2nd
6th
fresh 76
fresh 87
fresh 75
fresh 64
d
85
83
1634
2000
Complex 2
Complex 1
621
3424
OCH3 Complex 1
NO2 Complex 1
4000
3500
3000
2500
1500
1000
500
Wavenumbers (cm-1)
a
Catalyst (0.02 mmol), substituted sulfide (1 mmol), dropwise addition of aqueous
30% H2O2 (0.17 g, 1.5 mmol), 1 mL CH2Cl2, 4 h, 20 °C.
b
Determined by GC.
Determined by HLPC.
Turnover frequency (TOF) is calculated by the expression of (product)/[(catalyst)×time].
Fig. 1. FT-IR spectra of the neat complex 3 (a), the complex 1 (b), the complex 2 (c), and
the IL (d).
c
d
formed by the reaction between the end amino group of IL and
chloromethyl group of the chiral Schiff base ligand.
obtained without catalyst (entry 1). In comparison with the neat
complex 3, the complex 1 bearing the hydrophilic IL moiety increased
catalytic activity (TOF) and e.e value (entry 3 vs. 2). The hydrophilic IL
moiety and the hydrophobic catalytic species in the complex structure
were able to transfer the hydrogen peroxide from aqueous phase to
organic phase, which overcame the mass transfer disadvantage.
Furthermore, the IL moiety was thought to activate hydrogen peroxide
and stabilize the catalytic active sites [16], and therefore enhanced the
catalytic activity. A possible mechanism for the catalytic asymmetric
oxidation of sulfide by the complex 1 was presented in supplementary
information.
The influence of the substitutes at the 3 position of the salicylidenyl
moiety of the ligand on the enantioselective oxidation of methyl
phenyl sulfide has also been studied. The complex 1 bearing tert-butyl
group gave better conversion and enantioselectivity than the complex
2 bearing the nitro group (entry 3 vs. 6). The poor performance of
catalyst 2 is due to the electron-withdrawing effect as well as the slight
steric hindrance of nitro group at the 3-position in the complex 2 [17].
Understandably, the complex 1 was more effective than the complex 2
in the enantioselective oxidation reaction.
The UV–vis electronic spectra of the IL-functionalized complex 1
⁎
exhibited the benzene ring π–π transitions at near 274 nm, the d–π
transitions from alkoxys oxygen to vanadium (IV) at 367 nm and the
d–d transitions between the vanadium (IV) and vanadium (IV) of the
complexes at 538 nm (Fig. 2b) [15]. The three distinct peaks were
identical with those of the neat complex 3 (Fig. 2a). The observation
implied that the active site of the complex 1 was intact provided that
the IL was covalently bonded onto chiral Schiff base ligand. However,
in the spectrum of the complex 2, the peaks around 274, 367 and
538 nm shifted to 265, 352 and 443 nm, respectively, due to the
strong electron-withdrawing ability of nitro group in the complex 2
(Fig. 2c).
3.2. Catalytic performances
Table 1 showed the catalytic performances of the IL-functionalized
chiral oxovanadium (IV) Schiff base complexes in the enantioselec-
tivity oxidation of methyl aryl sulfides to sulfoxides. Only 2.7% of
methyl phenyl sulfide conversion with racemic sulfoxide (0% e.e) was
The interesting soluble feature of the complexes depends on their
inherent tendency to precipitate in n-hexane, since the IL moiety has
selective solubility in the solvent. After the reaction, the complex 1
could be recovered conveniently from the reaction system by simply
adding n-hexane and used for the subsequent catalytic runs. Table 1
described the reuse of the complex 1 after six catalytic cycles.
Obviously, the complex 1 could be reused at least 6 times without
significant loss of activity and enantioselectivity (entries 3, 4 and 5).
The recovered complex 1 had been characterized by FT-IR spectra
which were presented in supplementary information. No significant
changes in the FT-IR spectra of the complex 1 had been observed even
after reused for six times. The results indicated the stability of the
obtained complex 1 towards organic solvents, water and oxidative
agents (H2O2). In addition, no vanadium leaching was detected in the
supernatants extracted from the catalyst separation, which clearly
indicated that the complex was readily recyclable from the reaction
system and resistant against leaching during the reaction.
274
274
367
367
538
538
a
265
b
c
352
443
More substrates of other methyl aryl sulfides were used to test the
catalytic activity of complex 1 (entries 3 vs. 7–9). Surprisingly, the
complex 1 was efficient in the oxidation of methyl aryl sulfides having
both electron-donating as well as electron-withdrawing substituents
with good sulfoxides yield and satisfactory ee values, which indicated
the synthetic value of the IL-functionalized complex.
300
400
500
600
700
wavelength (nm)
Fig. 2. UV-vis spectra of the neat complex 3 (a), the complex 1 (b), and the complex 2 (c).

R. Tan et al. / Catalysis Communications 12 (2011) 1488–1491
1491
4. Conclusions
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base complex 1 with unique solubility and improved catalytic activity in
the catalytic asymmetric oxidation of sulfide. Moreover, the complex
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Acknowledgements
The project was financial supported by the National Natural
Science Foundation of China (Grant No. 20973057, 21003044) and the
Natural Science Foundation of Hunan Province (10JJ6028).
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Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.06.006.