Recyclable ionic liquid-bridged chiral dimeric salen Mn(III) complexes for oxidative kinetic resolution of racemic secondary alcohols

Article abstract of DOI:10.1016/j.catcom.2011.08.009

Novel chiral dimeric salen Mn(III) complexes bridged with N,N-dialkylimidazolium ionic liquids (ILs) at the C-5 position of the salicylaldehyde moieties were designed and synthesized. The ILs-bridged complexes presented excellent enantioselectivity and catalytic activity in the oxidative kinetic resolution of α-methyl benzyl alcohol, using diacetoxyiodobenzene as oxidant. The complexes combined the high catalytic efficiency of the active centers and the special solubility of the ILs, thus exhibiting notable efficacy of oxidative kinetic resolution and facile recovery from the reaction system by changing the solvent.

Full text of DOI:10.1016/j.catcom.2011.08.009

Catalysis Communications 15 (2011) 27–31
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Recyclable ionic liquid-bridged chiral dimeric salen Mn(III) complexes for oxidative
kinetic resolution of racemic secondary alcohols
a
a
Chengyong Li a, Jiangfeng Zhao a, Rong Tan a, , Zhigang Peng , Rongchang Luo ,
⁎
a,b,⁎⁎
Ming Peng a, Donghong Yin
a
Institute of Fine Catalysis and Synthesis, Hunan Normal University, Changsha 410081, Hunan, China
b
R & D Center, China Tobacco Hunan Industrial Corporation, NO.426 Laodong Road, Changsha, Hunan, 410014, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel chiral dimeric salen Mn(III) complexes bridged with N,N-dialkylimidazolium ionic liquids (ILs) at the C-5
position of the salicylaldehyde moieties were designed and synthesized. The ILs-bridged complexes presented
excellent enantioselectivity and catalytic activity in the oxidative kinetic resolution of α-methyl benzyl alcohol,
using diacetoxyiodobenzene as oxidant. The complexes combined the high catalytic efficiency of the active centers
and the special solubility of the ILs, thus exhibiting notable efficacy of oxidative kinetic resolution and facile
recovery from the reaction system by changing the solvent.
Received 26 January 2011
Received in revised form 19 July 2011
Accepted 8 August 2011
Available online 16 August 2011
Keywords:
Chiral dimeric salen Mn(III) complexes
Ionic liquids (ILs)
© 2011 Elsevier B.V. All rights reserved.
Catalysis
Secondary alcohols
Oxidative kinetic resolution
1. Introduction
special ionophilicity and polarity of the IL moiety played positive roles in
increasing the enantiomeric excess (ee) and the efficacy of kinetic
Optically active secondary alcohols are useful chiral auxiliaries and
intermediates in the pharmaceutical, agrochemical, and fine chemical
industries [1]. The oxidative kinetic resolution (OKR) of racemic
secondary alcohols is an efficient method to obtain chiral secondary
alcohols and their variants [2]. During the last 10 years, significant
progress in nonenzymatic catalytic methods, such as those based on
sparteine-Pd(II) complexes [3,4] and on several iridium or ruthenium
complexes [5,6], had been designed for the OKR of racemic secondary
alcohols. Recently, homogeneous chiral salen Mn(III) complexes have
been found to be effective catalysts for OKR [7,8], but the separation of the
catalysts have remained difficult. To solve this issue, immobilization of the
chiral salen Mn(III) complexes onto solid supports have been attempted.
This strategy produces recyclable catalysts for the OKR of racemic
secondary alcohols [9–12]. Unfortunately, the low accessibility of the
substrates often causes mass transfer limitations, and thereby reduces
catalytic activity. Recently, ionic liquids (ILs) bonded to the chiral salen
Mn(III) complex have been found to enhance catalysts efficiency and
recovery [13,14]. Thus, immobilization of the complex using ILs might
provide a means to develop reusable catalysts with high activity.
resolution (krel) in the OKR ofracemic secondary alcohols. Moreover, the
IL-bridged dimeric salen Mn(III) complexes were readily recovered by
simple precipitation in n-hexane and efficiently reused.
2. Experimental
2.1. Materials
L(+)-Tartaric acid, 1,2-diaminocyclohexane, 3-bromopropylamine
hydrobromide, diacetoxyiodobenzene (PhI(OAc)2) and 1-methylimida-
zole were purchased from Acros. 2-tert-Butyl phenol was purchased from
Alfa Aesar. All the racemic secondary alcohols used in the present study
were prepared by the reduction of the corresponding ketones with NaBH4.
Other commercially available chemicals were obtained from local
suppliers. (R,R)-1,2-diaminocyclohexane mono (hydrogen chloride) was
prepared according to a previously described procedures [15].
2.2. Preparation of IL-bridged chiral dimeric salen Mn(III) complexes of
1A, 1B and 1C
Herein, ILs with imidazolium were used as the bridge to bond
covalently at the C-5 position of two salicylaldehyde moieties. Novel IL-
bridged chiral dimeric salen Mn(III) complexes were synthesized. The
Synthesis of the complexes of 1A, 1B, and 1C are presented in
Scheme 1. The synthesis and identification of the IL-bridged salen Mn
(III) complex 1A and its corresponding intermediates are presented in
detail in the supplementary information. A strategy to bridge covalently
chiral ligand (CL) with various IL moieties to prepare IL-bridged salen
ligands was designed (Scheme 1). The dimeric salen Mn(III) complexes
⁎
Corresponding author.
⁎⁎ Corresponding author. Tel.: +86 731 8872576; fax: +86 731 8872531.
E-mail addresses: yiyangtanrong@126.com (R. Tan), yindh@hunnu.edu.cn (D. Yin).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.08.009

28
C. Li et al. / Catalysis Communications 15 (2011) 27–31
Scheme 1. Synthesis of the IL-bridged chiral dimeric salen Mn(III) complexes of 1A, 1B and 1C.
of 1A, 1B, and 1C with (R,R)-configuration were obtained by reaction of
IL-bridged salen ligands with Mn(CH3COO)2 followed by air oxidation.
LiCl was added just before air oxidation.
was measured by the complexometric method with ethylenediamine
tetraacetic acid, as described previously [16]. Reaction products were
analyzed on an Agilent Technologies 6890 N gas chromatograph
equipped with
a 19091 G-B213 chiral capillary column
2.3. Characterization methods
(30 m×0.32 mm×0.25 μm) and a flame ionization detector.
Fourier transform infrared (FT-IR) spectra were obtained at 400–
4000 cm− 1 region on an AVATAR 370 Thermo Nicolet spectropho-
tometer, using KBr pellets. UV–vis spectra were obtained on a UV–vis
Agilent 8453 spectrophotometer at 200–800 nm. The Mn ion content
2.4. Typical procedure for the OKR of racemic secondary alcohols
The racemic secondary alcohols (0.25 mmol), catalyst (0.0025 mmol,
1 mol% of substrate, based on monomeric unit of the complex), KBr
Table 1
Results of the OKR of α-methyl benzyl alcohol over various catalysts.a
OH
OH
O
(R, R) chiral salen Mn(III) complex, PhI(OAc)2
+
CH2Cl2 : H2O/ KBr, 20 oC
(+)
(R)
Conversionb (%)
eeb (%)
krel
TONd
c
Entry
Catalyst
Run times
1
2
3
4
5
6
7
8
∕
∕
60
62
62
62
62
62
63
63
62
62
62
62
59
64
0
∕
/
Complex 1A
Fresh
2nd
3rd
99
99
98
98
98
99
99
98
98
98
98
56
97
20.0
20.0
17.2
17.2
17.2
18.3
18.3
17.2
17.2
17.2
17.2
3.8
62
62
62
62
62
63
63
62
62
62
62
59
64
4th
5th
Complex 1B
Complex 1C
Fresh
2nd
5th
Fresh
2nd
5th
9
10
11
12
13
14
Complex 2
Complex 3
Fresh
Fresh
13.2
a
Reaction conditions: Catalyst (1 mol% of substrate, based on monomeric unit of the complex), KBr (6 mol% of substrate), α-methyl benzyl alcohol (0.25 mmol), PhI(OAc)2
(0.175 mmol), H2O/CH2Cl2 (1 ml/0.5 ml), 0.5 h, 20 °C.
b
Determined by GC.
c
krel value is calculated by the expression of ln[(1−C)(1−ee)]/ln[(1−C)(1+ee)], where C is the conversion of the secondary alcohol and ee is the enantiomeric excess of the
secondary alcohol.
d
Turnover number (TON): total moles of oxidized alcohol per mole of active metal centre.

C. Li et al. / Catalysis Communications 15 (2011) 27–31
29
to extract the product. The catalyst was separated from the reaction
mixture as a precipitate and subsequently used without further
purification.
a
621
570
412
1613
1613
3. Results and discussion
3408
a'
3.1. Catalytic activity of the chiral salen Mn(III) complexes for the OKR of
α-methyl benzyl alcohol
412
621
570
3408
The chiral salen Mn(III) complex with (R,R)-configuration could
selectively oxidize the (S)-secondary alcohol to produce the correspond-
ing ketone, leaving secondary alcohol enriched with (R)-configuration
isomer [8]. Table 1 summarizes the catalytic activity and enantioselec-
tivity of the chiral salen Mn(III) complexes in the OKR of α-methyl
benzyl alcohol. No enantioselectivity was observed without the catalyst
(entry 1). This confirms the necessity of the chiral salen Mn(III) complex
in the OKR of α-methyl benzyl alcohol (Table 1). The complex 1A
produced higher ee (N99%) of (R)-α-methyl benzyl alcohol, with a krel
value (20.0) higher than that of the neat chiral salen Mn(III) complex 3
(krel=13.2) (entry 2 vs. 14). The enhanced ee and krel values may be due
to the readily solubility of 1A in the solvent of dichloromethane, and the
synergy of the two catalytic active sites of 1A [17]. Changing the counter
ion of the IL moiety from Br− to BF4− (1B) and PF6− (1C) did not improve
the ee of the reaction and the krel value (entries 2, 7, and 10). To
investigate the steric hindrance caused by the substituents of the chiral
salen ligand on the catalytic performance, the chiral salen Mn(III)
complex 2 (Chart 1), prepared according to our previous method [14]
was also used as a catalyst for the OKR. This complex was functionalized
by the IL 1-propylamine-3-methylimidazolium bromide at the C-5, 5′
positions of the salicylaldehyde moiety of the salen ligand. Complex
2 was less effective than complex 1A, as evidenced by the low ee and krel
(entry 13 vs. 2). This may be due to the lower steric hindrance of the –
CH2-IL substituent group in complex 2. Therefore, tert-Butyl group
substituents, due to the bulk steric hindrance and strong electron-
donating abilities at C-5′, improve the enantioselectivity [7]. In addition,
bromide salts such as additives were also absolutely crucial for fast and
highly enantioselective OKR, since the bromide ion could activate the Mn
(III)-salen complex through forming an active dibromo-Mn(V) species
[18]. Low conversion and poor enantioselectivity resulted when the
additive was not used even over the complexes of 1A, 1B and 1C. The
results were presented in the supplementary information (see Table S1
and Table S2).
b
412
500
570
1613
4000
3500
3000
2500
2000
1500
1000
Wavenumbers (cm-1)
Fig. 1. FT-IR spectra of the fresh complex 1A (a), complex 1A after 5th reuse (a′), and
the fresh complex 3 (b).
a
439
a'
439
510
510
b
433
508
350
400
450
500
550
600
Wavelength (nm)
The interesting soluble feature of the 1A, 1B, and 1C complexes
depend on their inherent tendency to precipitate in n-hexane, since the IL
moiety has selective solubility in the solvent. After one catalytic run, the
catalysts could be easily separated from the reaction mixture by the
addition of hexane, and reused for the subsequent catalytic runs by
adding fresh reactants. The IL-bridged salen Mn(III) complexes with
various anions could be reused at least five times without significant loss
of activity and enantioselectivity (Table 1). The maintenance of catalytic
Fig. 2. UV–vis spectra of the fresh complex 1A (a), complex 1A after 5th reuse (a′), and
the fresh complex 3 (b).
(0.015 mmol, 1.78 g), CH2Cl2 (0.5 ml), and H2O (1.0 ml) were stirred for
10 min at room temperature. Afterward, PhI(OAc)2 (0.175 mmol,
0.056 g) was added. Gas chromatography was employed to monitor
the progress of the reaction. After complete reaction, hexane was added
N
O
N
O
Mn
Cl
H2C
HN
CH2
NH
N
O
N
O
Mn
Cl
t-Bu
t-Bu
N
N
N
N
Br
Br
Complex 3
Complex 2
Chart 1. Structure of the IL-functionalized chiral salen Mn(III) complex 2, and the neat chiral salen Mn(III) complex 3.

30
C. Li et al. / Catalysis Communications 15 (2011) 27–31
Table 2
method, and no Mn leaching was found in the supernatant. The results
suggest that 1A, 1B, and 1C complexes were stable toward the organic
solvent, water, and oxidizing agents during the OKR of α-methyl benzyl
alcohol. The FT-IR and UV–vis absorption characteristics of fresh and
recovered 1A and 3 complexes are presented in Figs. 1 and 2. No
significant changes in the FT-IR and UV–vis spectra of the 1A complex
were observed even after 5 cycles of reuse. Furthermore, the main
spectroscopic characteristics of 1A complex resembled those of 3,
indicating that the active site in 1A was intact during IL bonding.
Results of the OKR of various secondary alcohols over the complex 1A.a
Entry
1
Substrate
Conversion (%)b
62
ee (%)b
krel
TONd
62
c
3.2. Catalytic activity for the OKR of various secondary alcohols
N99
20.0
2.03
10.0
2.85
1.30
13.8
3.16
10.3
Table 2 summarizes the OKR of other secondary alcohols over 1A. The
steric hindrance around the hydroxyl group of the substrates played a
significant role in the overall efficiency of the kinetic resolution. Extension
of the alkyl chain of the substrate (from R′=CH3 to R′=CH2CH3) resulted
in the sharp decrease of the ee value from 99% to 27%, and of the krel value
from 20.0 to 2.03 (entry 1 vs. 2). α-methylbenzyl alcohols substituted at
the ortho-position underwent relatively poor OKR compared with the
para-substituted counterpart. The substituents –Cl, –CH3, and –Br at the
para-position of α-methylbenzyl alcohol produced high ee values (94%,
93%, and 92%, respectively), and krel values of 10.0, 13.8, and 10.3,
respectively (entries 3, 6, and 8, respectively, Table 2). While, if α-methyl-
o-chlorobenzyl alcohol and α-methyl-o-methylbenzyl alcohol were used
as the substrates, only 5% and 8% of the enantioselectivity, with 1.30 and
3.16 krel values (entries 5 and 7) were obtained, respectively. The
substituent of Cl at the meta-position of α-methylbenzyl alcohol was also
tested. The enantioselectivity (32%) and krel value (2.85) were lower than
those of the para-substituted counterpart but higher than those of the
ortho-substituted counterpart (entry 4). The results suggested that the
bulk steric hindrance caused by the substituted groups prevented the
hydroxyl group in the substrate to approach the catalytic metal center of
1A, and therefore decreased the ee and krel values. Furthermore, the
complex 1A was effective for the OKR of 2-pentanol, where 95% ee with
14.4 krel value could be obtained (entry 9). However, the chiral salen Mn
(III) complex 1A was inactive for heterocyclic compounds such as 2-
furylethanol (entry 10).
2
3
4
5
6
7
8
54
65
47
32
60
14
63
27
94
32
5
54
65
47
32
60
14
63
93
4. Conclusions
The novel IL-bridged chiral dimeric salen Mn(III) complexes were
synthesized and used as catalysts for the OKR of racemic secondary
alcohols. The IL moiety conferred tunable solubility and improved
catalytic activity of the complexes. High chiral purity (ee N99%) and krel
value were achieved for the OKR of α-methylbenzyl alcohol over 1 mol%
of the catalysts for 0.5 h. The complexes were easily recycled for five
successive catalytic runs. The observations suggest that the complexes
are promising chiral catalysts for the OKR of secondary alcohols.
8
92
Acknowledgements
9
61
95
/
14.4
/
61
/
The project was financial supported by the National Natural
Science Foundation of China (Grant Nos. 20973057 and 21003044)
and the Natural Science Foundation of Hunan Province (10JJ6028).
10
trace
Appendix A. Supplementary data
Supplementary data to this article can be found online at doi:10.
1016/j.catcom.2011.08.009.
a
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
b
c
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