Thermoresponsive chiral salen Mn(III) complexes as efficient and reusable catalysts for the oxidative kinetic resolution of secondary alcohols in water

Article abstract of DOI:10.1016/j.apcata.2013.03.015

A series of novel chiral salen Mn(III) complexes possessing a thermoregulated phase-transfer function was prepared for the first time. The preparation involved the introduction of the "smart" poly(N-isopropylacrylamide) (PNIPAAm) group to the framework of the chiral salen Mn(III) complex by covalently connecting to one side of the 5-position in the salen ligand (complex 1) or by axially grafting onto the metal center of the complex (complex 2). Characterization results suggested the presence of the PNIPAAm moiety and active sites in the complexes. The thermoresponsive PNIPAAm groups imparted inverse temperature-dependent water solubility to the PNIPAAm-based complexes, which allowed them to undergo thermoregulated phase-separable catalysis during oxidative kinetic resolution (OKR) in water. Excellent enantioselectivity (up to 96%) with high kinetic resolution efficacy (13.1) was achieved over complex 1 in the aqueous OKR of α-methylbenzyl alcohols, and this efficacy was significantly higher than that obtained over neat complex. The PNIPAAm-based complexes can also be easily recovered by simply regulating the temperature and be reused for four times without obvious loss of enantioselectivity and activity.

Full text of DOI:10.1016/j.apcata.2013.03.015

Applied Catalysis A: General 458 (2013) 1–10
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Thermoresponsive chiral salen Mn(III) complexes as efficient and
reusable catalysts for the oxidative kinetic resolution of secondary
alcohols in water
Rong Tana,∗, Yan Donga, Ming Penga, Weiguo Zhenga, Donghong Yina,b,∗∗
a Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal University, Changsha, Hunan
410081, China
b Technology 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:
A series of novel chiral salen Mn(III) complexes possessing a thermoregulated phase-transfer function
was prepared for the first time. The preparation involved the introduction of the “smart” poly(N-
isopropylacrylamide) (PNIPAAm) group to the framework of the chiral salen Mn(III) complex by
covalently connecting to one side of the 5-position in the salen ligand (complex 1) or by axially grafting
onto the metal center of the complex (complex 2). Characterization results suggested the presence of the
PNIPAAm moiety and active sites in the complexes. The thermoresponsive PNIPAAm groups imparted
inverse temperature-dependent water solubility to the PNIPAAm-based complexes, which allowed them
to undergo thermoregulated phase-separable catalysis during oxidative kinetic resolution (OKR) in water.
Excellent enantioselectivity (up to 96%) with high kinetic resolution efficacy (13.1) was achieved over
complex 1 in the aqueous OKR of ˛-methylbenzyl alcohols, and this efficacy was significantly higher than
that obtained over neat complex. The PNIPAAm-based complexes can also be easily recovered by simply
regulating the temperature and be reused for four times without obvious loss of enantioselectivity and
activity.
Received 16 December 2012
Received in revised form 9 March 2013
Accepted 11 March 2013
Available online xxx
Keywords:
Chiral salen Mn(III) complex
Thermoregulated phase-separable catalysis
Oxidative kinetic resolution
Racemic secondary alcohol
Water
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
by chiral salen Mn(III) complexes. However, their success is hin-
dered by the limited solubility of this catalyst in aqueous media
Optically active alcohols are extremely useful starting materials
and intermediates in synthetic organic chemistry and the phar-
maceutical industry [1]. The oxidative kinetic resolution (OKR) of
racemic secondary alcohols catalyzed by chiral salen Mn(III) com-
plexes is one of the most attractive methods of producing optically
active alcohols together with corresponding carbonyl compounds
[2–4]. Excellent product enantioselectivity and high kinetic reso-
lution efficacy can be achieved when the reaction is carried out
in a mixed solution of water and dichloromethane using iodoben-
zene diacetate (PhI(OAc)2) as a cooxidant and potassium bromide
(KBr) as an additive [5–7]. From the standpoint of environmentally
benign organic synthesis, chemists are investigating the possi-
bility of using water as a solvent in the OKR reaction catalyzed
[8,9]. Phase-transfer catalysts (PTCs) are commonly used in aque-
ous systems to enhance the mutual solubility of organic substrate
and catalyst in water [10]. Sun et al. [8] reported an efficient
OKR of secondary alcohols catalyzed by chiral salen Mn(III) com-
plexes in water with tetraethylammonium bromide (TEAB) as a
PTC. Amphiphilic PTC molecules form an emulsion in water, which
provides a micellar-type reaction system for the aqueous OKR reac-
tion and thus produces unexpected kinetic resolution efficacy [11].
However, the catalyst separation and PTC removal from solution
are unfavorable.
Stimuli-responsive catalysts are being developed to solve prob-
lems associated with the mass transfer and separation of catalysts
[12–14]. Thermoresponsive catalysts with inverse temperature-
dependent water-solubility are well exploited by incorporating
thermoresponsive functionalized groups [14,15]. Such “smart”
modifiers often enable a catalyst to become hydrophilic at tem-
peratures lower than the critical solution temperature (CST) and
become hydrophobic at elevated temperatures. The reversible
switching between hydrophobicity and hydrophilicity at the CST
allows thermoresponsive catalysts to perform excellently, which
is called thermoregulated phase-separable catalysis (TPSC). In
∗
Corresponding author. Tel.: +86 731 85559528; fax: +86 731 8872531.
Corresponding author at: Key Laboratory of Chemical Biology and Traditional
∗∗
Chinese Medicine Research (Ministry of Education), Hunan Normal University,
Changsha, Hunan 410081, China.
E-mail addresses: yiyangtanrong@126.com (R. Tan),
yindh@hunnu.edu.cn, dhyin99@yahoo.com (D. Yin).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.03.015

2
R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
aqueous TPSC, catalysts completely dissolve in water for homoge-
neous reaction and can achieve rapid precipitation from solution
by increasing temperature up to the CST. Thus, TPSC is a poten-
tial solution to problems associated with the poor activity and
recovery of chiral salen Mn(III) complexes in aqueous reac-
tions. Polymeric N-isopropylacrylamide (PNIPAAm) is an attractive
thermoresponsive polymer because its lower critical solution
temperature (LCST) in water (32 ◦C) is close to room tempera-
ture [16–19]. Given this unique LCST behavior of PNIPAAm, it
is widely used in the fields of “smart” biomaterials, such as
controllable drug-delivery systems [20], column-packing materi-
als [21], and cell-culture substrates [22]. The thermosensitivity
also has potential in “smart” applications for fabricating ther-
moresponsive catalysts for aqueous reaction systems. Modifying
a catalyst with the thermoresponsive PNIPAAm moiety confer the
catalyst with a thermoregulated phase-transfer function, which
typically realizes a homogeneous reaction coupled with biphasic
separation in aqueous reactions. Notably, the LCST of PNIPAAm
in water (32 ◦C) is close to the reaction temperature of OKR
(room temperature). Thus, PNIPAAm was used in this study as
the “smart” modifier for fabricating thermoresponsive chiral salen
Mn(III) complexes for aqueous OKR reaction. At the reaction
temperature, the catalyst incorporating the hydrophobic chiral
salen Mn(III) complexes along with the hydrophilic PNIPAAm
moiety should be located at the interface of the water–organic
reactants where catalytic conversion occurs, thereby circum-
venting mass transfer in the aqueous reaction. Moreover, the
catalyst can achieve rapid precipitation from solution upon heating
because of the hydrophobicity of the catalyst at high tempera-
tures.
In the present work, the thermoresponsive polymer PNIPAAm
was synthesized by free-radical polymerization and introduced
to the framework of chiral salen Mn(III) complexes by covalently
connecting to one side of the 5-position in the salen ligand or
by axially grafting onto the metal center of the complex. The
new catalysts can achieve TPSC in the OKR of secondary alco-
hols in water. Excellent efficacy of aqueous OKR was observed
over the PNIPAAm-based complexes, and this efficacy was sig-
nificantly higher than that obtained over neat complex. Activity
switching was also repeatable even after four heating/cooling
cycles. Thus, the problems associated with the solubility limita-
tion and separation of catalysts in aqueous reactions can be solved.
In addition, the catalytic performances strongly depended on the
attachment location of chiral salen Mn(III) complexes with PNI-
PAAm.
2.2. Methods
Fourier-transform infrared (FT-IR) spectra were obtained as
potassium bromide pellets with a resolution of 4 cm−1 and 32
scans within the range of 400–4000 cm−1 using an AVATAR 370
Thermo Nicolet spectrophotometer. 1H NMR spectra of samples
were recorded on a Varian-500 spectrometer. Thermogravimet-
ric analysis was conducted with a NETZSCH STA 449C thermal
analyzer. Samples were heated from room temperature to 800 ◦C
in flowing air using alumina sample holders. The sample weight
was ca. 10 mg, and the heating rate was 10 K/min. UV–vis spec-
tra of the complexes were obtained on a UV–vis Agilent 8453
spectrophotometer at 200–800 nm. The LCST of the PNIPAAm-
functionalized chiral salen Mn(III) complexes were measured by
transmittance using a UV–visible spectrophotometer in water.
The complex concentration was 1 wt%, and the heating rate was
5 ◦C/min. The manganese contents of the samples were determined
by inductively coupled plasma-atomic emission spectrometry on a
BAIRD PS-6 analyzer after the samples were dissolved in deion-
ized water. The optical rotation of the catalysts was measured in
dichloromethane on a WZZ-2A Automatic Polarimeter. The molec-
ular weight and molecular weight distribution of all polymers were
obtained by gel permeation chromatograph (GPC). Analyses were
performed on an Alltech Instrument (Alltech, USA) using tetrahy-
drofuran (THF) as the solvent eluted at a flow rate of 1 mL/min
through a Jordi GPC 10,000 A column (300 mm × 7.8 mm) equipped
with an Alltech ELSD 800 detector, and the system was calibrated
with standard polystyrenes. Reaction products were analyzed on an
Agilent Technologies 6890N gas chromatograph equipped with a
19091G-B213 chiral capillary column (30 m × 0.32 mm × 0.25 ␮m)
using an flame ionization detector (FID).
2.3. Preparation of PNIPAAm-NH2
PNIPAAm-NH2 was synthesized by a previously described
procedure [27] (Scheme 1). NIPAAm (50 mmol, 5.65 g), 2-
aminoethanethiol hydrochloride (1 mmol, 0.11 g), and AIBN
(0.5 mmol, 0.08 g) were dissolved in 20 mL of methanol in a Schlenk
tube. The reaction mixture was degassed by bubbling with nitro-
gen gas at room temperature for 30 min, and polymerization was
carried out at 70 ◦C for 5 h under N2 atmosphere. The mixture was
cooled to room temperature and then treated with KOH (1 mmol,
0.05 g). The obtained solution was concentrated in a vacuum and
then poured into diethyl ether to precipitate the polymer. The
crude product was further purified by repeated precipitation from
a THF solution into diethyl ether to remove unreacted monomers
and then dried for 12 h in a vacuum at 25 ◦C to give PNIPAAm-
NH2 as a white solid. FT-IR (KBr): ꢀmax (cm−1) 3438, 3309, 3072,
2974, 2931, 2878, 2825, 1652, 1541, 1458, 1387, 1367, 1261, 1173,
1130, 1055, 987, 928, 881, 839, 633, and 517. The molecular weight
of PNIPAAm-NH2 was ca.11,800 g/mol as measured by GPC. Thus,
the polymer comprised an average of ca. 104 repeating units
(n = 104).
2. Experimental
2.1. Materials and reagents
N-Isopropylacrylamide (NIPAAm) was obtained from Acros,
purified by recrystallization from n-hexane, and dried at 25 ◦C in
vacuo. N,N-Azobis(isobutyronitrile) (AIBN), 2-aminoethanethiol
hydrochloride, l(+)-tartaric acid, 1,2-diaminocyclohexane, and
PhI(OAc)2 were also purchased from Acros. 2-tert-Butyl phenol
was purchased from Alfa Aesar. All solvents were purified by
standard procedures [23]. (R,R)-1,2-Diammoniumcyclohexane,
2.4. Preparation of PNIPAAm-functionalized chiral salen Mn(III)
complexes 1 and 2
The preparation of 1 and 2 is outlined in Scheme 2.
3-tert-buty-2-hydroxybenzaldehyde,
and
3-tert-butyl-5-
chloromethyl-2-hydroxybenzaldehyde were prepared according
to the procedures described in [24–26], respectively. The neat
complex of [(R,Rꢀ)-(N,Nꢀ-bis(3,5-di-tert-butylsalicylidene))-1,2-
cyclo-hexanediaminato] Mn(III) chloride was prepared according
to a previously described procedure [24]. All racemic secondary
alcohols used were prepared by the reduction of the corresponding
ketones with NaBH4.
2.4.1. Preparation of PNIPAAm-functionalized chiral salen Mn(III)
1
2.4.1.1. Synthesis of asymmetric chiral salen ligand (CL). The
asymmetric CL (R,R)-N-(3,5-di-tert-butylsalicylidene)- Nꢀ-[3-tert-
butyl-5-chloromethylsalicylidene]-1,2-cyclo-hexanediamine was
synthesized by a previously described procedure [28]. Hydro-
gen chloride was used to generate diamine monoammonium salt

R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
3
CH
C
H2C
H3C
H2
C
H
C
*
SHCH2CH2NH2
1. AIBN, N2, MeOH, 70 oC
O
H2 H2
104
HS
C
C
NH2
HCl
+
C
O
NH
2. KOH, MeOH
NH
CH
CH
CH3
CH3
H3C
PNIPAAm-NH2
Scheme 1. Preparation of PNIPAAm-NH2.
that can undergo stepwise condensations. This salt was prepared
in almost quantitative yield from a 1:1 molar ratio of (R,R)-
diaminocyclohexane and 2.0 M hydrogen chloride in ether at 0 ◦C.
(R,R)-1,2-Diaminocyclohexane mono(hydrogen chloride) (8 mmol,
1.2 g), 3,5-di-tert-butyl-2-hydroxybenzaldehyde (8 mmol, 1.87 g),
2.4.1.2. Synthesis of PNIPAAm-functionalized chiral salen Mn(III) 1.
Under nitrogen protection, CL (3.2 mmol, 1.73 g) was reacted with
PNIPAAm-NH2 (3 mmol) in 15 mL of dry chloroform at 60 ◦C for
48 h. Then, Mn(OAc)2·4H2O (10 mmol, 2.45 g) dissolved in 15 mL
of dry chloroform was added dropwise to the above solution. The
reaction was continued at 60 ◦C for 8 h, and the reaction mixture
was cooled to ambient temperature. LiCl (30 mmol, 1.26 g) dis-
solved in 10 mL of dry chloroform was added to the suspension.
The mixture was further stirred at 50 ◦C for 3 h while exposed to
air overnight. Removal of the dry chloroform gave a crude product,
which was purified by precipitation from diethyl ether (3× 25 mL)
˚
and 4 A molecular sieves (1 g) were placed in a 250 mL flask.
Anhydrous ethanol (30 mL) and anhydrous methanol (30 mL)
were added, and the bright yellow solution was stirred at room
temperature for 4 h. A solution of 3-tert-butyl-5-chloromethyl-
2-hydroxybenzaldehyde (8 mmol, 1.81 g) in anhydrous CH2Cl2
(20 mL) and anhydrous NEt3 (13.4 mmol, 1.35 g) was added. The
mixture was stirred at room temperature for an additional 4 h
and filtered through a short pad of dry silica gel. The silica gel
was flushed with CH2Cl2. The solvent was removed under reduced
pressure. The residue was purified by column chromatography
on silica gel (ethyl acetate/hexanes 1:5) to afford CL as a pale-
yellow powder. Calc. for C33H47ClN2O2: C, 73.51; H, 8.79; N, 5.20%.
Found: C, 73.52; H, 8.72; N, 5.18%. 1H NMR (CDCl3, 500 MHz):
ıH ppm: 14.29 (s, 1 H, OH (p-CH2Cl)), 13.67 (s, 1 H, OH (p-t-
Bu)), 8.44–8.31 (s, 2 H, CH = N), 7.30–6.90 (s, 4 H, ArH), 4.43 (s,
2 H, CH2Cl), 3.55–3.32 (m, 2 H, CH = N CH), 1.97–1.46 (m, 8 H,
cyclohexyl-H), 1.40 (s, 9 H, t-butyl (C-5-position)), 1.23 (s, 18 H,
t-butyl (C-3,3ꢀ-positions)). FT-IR (KBr): ꢀmax (cm−1) 3446, 2954,
2862, 1629, 1591, 1479, 1469, 1439, 1391, 1361, 1271, 1252, 1241,
1201, 1174, 1144, 1086, 1040, 981, 934, 879, 828, 803, 772, 731,
711, and 644.
and dried in a vacuum at 25 ◦C to afford 1. FT-IR (KBr): ꢀmax (cm−1
)
3309, 3072, 2974, 2931, 2878, 1770, 1652, 1541, 1458, 1387, 1365,
1256, 1173, 1130, 1036, 991, 928, 879, 837, 634, 568, 492, and 416.
UV–vis (CH2Cl2): ꢁmax (nm) (εmax (L mol−1 cm−1)) 327 (126,088),
433 (27,772), 508 (8850). Mn ion content: 0.076 mmol/g (theoreti-
cal value = 0.0807 mmol/g). ˛D25 = +125 (C = 0.02, CH2Cl2). The LCST
of 1 in its aqueous solution was ca. 35 ◦C, which was determined
by transmittance measurements using a UV–vis Agilent 8453 spec-
trophotometer.
2.4.2. Preparation of PNIPAAm-functionalized chiral salen Mn(III)
2
A solution of neat chiral salen Mn(III) complex (2.7 mmol,
1.71 g), PNIPAAm-NH2 (2.5 mmol), and adequate amount of sodium
hydroxide in THF (50 mL) was vigorously stirred for 24 h under
N
N
N
O
N
O
ClH2C
OH HO
t-Bu
Mn
Cl
H2
C
H2 H2
H
N
*
CH
C
S
C
C
C
t-Bu
104
O
H2
CL
1) Mn(CH3COO)2·4H2O ⁄ Ethanol
2) LiCl ⁄ Ethanol
NH
CH
Dry chloroform, 60 oC, 48 h
H2
C
Complex 1
H
C
CH3
H3C
*
SHCH2CH2NH2
104
C
O
t-Bu
N
O
N
O
NH
CH
Mn
Cl
t-Bu
t-Bu
CH3
H3C
O
O
N
N
H2
H2 H2
Mn
*
C
CH
C
S
C
C
N
H
PNIPAAm-NH2
104
O
Neat complex
NaOH, THF, reflux, 24 h
NH
CH
t-Bu
CH3
H3C
Complex 2
Scheme 2. Synthesis of the catalysts 1 and 2.

4
R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
reflux. After removing THF, the residue was purified by precip-
itation from diethyl ether (3× 50 mL) and dried in a vacuum at
25 ◦C to obtain 2. FT-IR (KBr): ꢀmax (cm−1) 3309, 3072, 2974,
2931, 2878, 1652, 1541, 1458, 1387, 1365, 1254, 1173, 1130, 928,
879, 837, 644, 572, 507, and 420. UV–vis (CH2Cl2): ꢁmax (nm)
(εmax (L mol−1 cm−1)) 327 (138,661), 424 (22,211), 500 (7122).
Mn ion content: 0.079 mmol/g (theoretical value = 0.081 mmol/g).
complex was readily oxidized by oxygen, thereby affording 1. On
the other hand, the PNIPAAm segment was axially grafted onto the
metal center of 2, which was prepared using the terminated amino
group ( NH2) of PNIPAAm-NH2 as an axial linkage. The amino-
terminated PNIPAAm directly reacted with the neat chiral salen
Mn(III) complex under a basic condition, thereby affording 2.
The PNIPAAm-functionalized chiral salen Mn(III) complexes
were found to possess a temperature-responsive property and
exhibited an LCST at 35 ◦C. As shown in the typical photographs
in Fig. 1, an effective model of separation and redispersion of the
PNIPAAm-based complexes by thermoregulation was established.
At <35 ◦C, the polymer-based complexes were completely soluble
in aqueous media and formed a homogeneous solution (Fig. 1A-
a and B-a). However, the solution became opaque with increased
temperature, and the complexes completely precipitated out of
the aqueous solution when the temperature exceeded the LCST
(Fig. 1A-b and B-b). The thermoregulated phase-transfer capa-
bility induced controllable switching between efficient reactions
and facile separation in aqueous OKR by regulating the tempera-
ture.
˛
D
25 = +128 (C = 0.02, CH2Cl2). The LCST of 2 in its aqueous solution
was ca. 35 ◦C, as determined by transmittance measurements using
the UV–vis Agilent 8453 spectrophotometer.
2.5. Typical procedure for the OKR of racemic secondary alcohols
Racemic secondary alcohols (0.25 mmol), catalyst (1.5 mol% of
substrate), and KBr (0.02 mmol, 0.0024 g) were added to H2O
(1.5 mL). The mixture was stirred for 10 min at room temper-
ature, and then PhI(OAc)2 (0.175 mmol, 0.056 g) was added in
four equal parts. The reaction progress was monitored by gas
chromatography (GC). After achieving the desired oxidation level,
the reaction mixture was heated to 40 ◦C. The catalyst was pre-
cipitated out of the reaction system, washed with diethyl ether
(3× 5 mL), dried in a vacuum, and finally recharged with fresh
substrate, additive, and oxidant for the next catalytic cycle. The
supernatants separated from the reaction system were extracted
with ether three times. The collected organic phase was dried
over sodium sulfate and concentrated in a vacuum. The resultant
mixture was purified by column chromatography on silica gel as
the stationary phase (petroleum ether/ethyl acetate, 90/10). Enan-
tioselectivity was determined by a Agilent Technologies 6890N
GC system equipped with a 19091G-B213 chiral capillary column
(30 m × 0.32 mm × 0.25 ␮m) with an FID.
3.2. Characterization of samples
3.2.1. FT-IR spectra
FT-IR spectroscopy was used to characterize and compare 1, 2,
and neat chiral complex, and the results are shown in Fig. 2. The FT-
IR spectrum of PNIPAAm exhibited characteristic vibration bands
at around 3309, 3072, 2974, 2931, 2878, 1652, and 1541 cm−1
,
which were associated with the stretching vibration modes of
NH of CONH CH of CH CH CH3 of CH(CH3)2, CH of
,
,
CH(CH3)2, typical amide I, and typical amide II [31] (Fig. 2a). These
characteristic bands also distinctly appeared in the FT-IR spectra
of 1 and 2, suggesting the presence of the PNIPAAm segment in
the framework of 1 and 2 (Fig. 2b and c vs. a). Furthermore, new
characteristic bands at around 1256, 568, and 416 cm−1 appeared
in the FT-IR spectrum of 1 and were assigned to the character-
istic vibrations of Ph–O, Mn–O, and Mn–N in the salen Mn(III)
unit, respectively [10,32,33] (Fig. 2b vs. a). The characteristic bands
were consistent with those of neat chiral salen Mn(III) complex
(Fig. 2b vs. d). Thus, the PNIPAAm moiety was anchored on the
salen backbone, which provided 1 with an intact active center.
Meanwhile, the characteristic vibrations of Mn–O and Mn–N in the
FT-IR spectrum of 2 showed a blue shift from 568 and 416 cm−1
to 572 and 420 cm−1, respectively, compared with those of neat
chiral salen Mn(III) complex (Fig. 2c vs. d). The slight shifts of the
characteristic bands suggested the presence of electrostatic inter-
actions between the manganese center and the amino-terminated
PNIPAAm. Hence, the PNIPAAm moiety was axially grafted suc-
cessfully onto the metal center of the chiral salen Mn(III) complex
by the terminated amino group ( NH2) as an axial linkage in
2.
3. Results and discussion
3.1. Preparation and characterization of 1 and 2
PNIPAAm is a fascinating thermosensitive polymer because it
exhibits reversible phase transition in aqueous solutions at around
32 ◦C (LCST) [16]. The modification of catalysts with this smart poly-
mer is very useful for fabricating thermoresponsive catalysts that
possess inverse temperature-dependent water solubility [29] and
can thus meet the demands of aqueous reactions at room tempera-
ture. Accordingly, PNIPAAm was used as a functional switching unit
in the current work to modify chiral salen Mn(III) complexes and
make thermoregulated phase-separable catalysts out of the result-
ing PNIPAAm-based complexes for aqueous OKR reaction. Given
that the presence of bulky tert-butyl groups at the C-5(5ꢀ)-positions
in the CL is essential for high enantioselectivity in the OKR reac-
tion [2,30,31], at least one tert-butyl substituent was maintained
in CL upon modification. Two strategies were designed to intro-
duce the temperature-responsive PNIPAAm unit to the framework
of chiral salen Mn(III) complex: one is by covalently appending the
PNIPAAm unit to one side of the C-5-position in the salen ligand
(1), and the other is by axially grafting the PNIPAAm unit to the
metal center of the complex (2). The synthesis routes for 1 and 2
are outlined in Scheme 2.
3.2.2. UV–vis spectra
UV–vis spectra of the neat and PNIPAAm-based complexes
provided evidence of successful grafting and information on the dif-
ferent grafting locations of the PNIPAAm mioety in the framework
of 1 and 2, as shown in Fig. 3. The neat complex exhibited char-
acteristic peaks at around 433 and 508 nm, which were assigned
to the ligand-to-metal charge transfer transition in the chiral salen
Mn(III) complex and the metal-to-metal charge transfer between
the chiral salen Mn(III) complexes, respectively [10] (Fig. 3a). The
spectrum of 1 had features similar to those of neat chiral salen
Mn(III) complex, indicating that no change in the local environment
of the Mn(III) coordination center in the complex occurred during
modification (Fig. 3b vs. a). Thus, the PNIPAAm moiety had been
covalently grafted onto one side of the C-5-position in the salen
One PNIPAAm segment was present as a side chain in 1, which
was prepared by the reaction between the terminated amino group
(
NH2) of PNIPAAm-NH2 and chloromethyl groups ( CH2Cl) at one
side of the C-5-position of CL. First, the amino-terminated PNIPAAm
prepared by radical polymerization using thiol compounds as a
chain transfer agent directly reacted with CL. Further treatment
with Mn(II) acetate tetrahydrate under nitrogen gave the dianionic
PNIPAAm-based complex. The acetic ion was replaced by chlo-
ride ion with lithium chloride at room temperature. The dianionic

R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
5
Fig. 1. Photographs of 1 (A) and 2 (B) in aqueous medium (a, soluble in aqueous media at <35 ◦C; b, precipitated out of the aqueous solution at >35 ◦C).
ligand, providing chiral 1 with an intact catalytic active site. In the
case of 2, the characteristic peaks shifted from 433 and 508 nm to
424 and 500 nm, respectively (Fig. 2c vs. a). The blue shift suggested
the presence of an interaction between the metal central of the
chiral salen Mn(III) complex and the amino-terminated PNIPAAm,
further confirming the axial grafting of the amino-terminated PNI-
PAAm onto the metal center in 2 [34–36]. The position at which
PNIPAAm was tethered to the complex may play a crucial role in
the asymmetric induction of the resulting complex because of the
different contributions to the complex stereogenic ability of each
substituent depending on the bulkiness.
3.2.3. Thermogravimetry (TG)–differential thermogravimetry
(DTG) analysis
Thermal analysis was used to monitor the decomposition pro-
files of the neat and PNIPAAm-functionalized complexes, and the
results are depicted in Fig. 4. The neat complex showed two steps of
weight loss in the combined TG–DTG curves when heated at room
temperature to 800 ◦C in flowing air (Fig. 4a). The first loss in weight
(ca. 5.7%) was centered at 125 ◦C. As aforementioned, this weight
c
a
1541
1652
c'
b
424
416
b
568
c
500
b'
a
420
572
3072
2878
2931
3309
2974
d
568
433
400
416
500
508
1256
350
450
500
550
600
4000
3500
3000
2500
2000
1500
1000
Wavelength (nm)
Wavenumbers (cm-1)
Fig. 3. UV–vis spectra of neat chiral salen Mn(III) complex (a), fresh 1 (b), 1 after the
fourth reaction (bꢀ), fresh 2 (c) and 2 after the fourth reaction (cꢀ).
Fig. 2. FT-IR spectra of PNIPAAm (a), 1 (b), 2 (c), and neat chiral complex (d).

6
R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
c
B
A
b
a
125oC
125oC
c
b
a
100 200 300 400 500 600 700
100 200 300 400 500 600 700
Temperature (ºC)
Temperature (ºC)
Fig. 4. Thermogravimetric (A) and differential thermogravimetric (B) analysis results of neat complex (a), 1 (b), and 2 (c).
loss was due to the release of chloride ions as hydrogen chloride
[10,37]. A second large weight loss (ca. 35%) appeared at 339 ◦C
and was assigned to the successive cleavage of the salen ligand moi-
eties. The decomposition profiles of neat complex became complete
at ca. 600 ◦C (weight loss = ca. 50.4%) with the residues amount-
ing to manganese oxides [38,39]. Notably, 1 also exhibited weight
loss corresponding to the release of chloride ions (ca. 125 ◦C) in
the TG–DTG curves (Fig. 4b). This finding indicated that the posi-
tive charge of 1 was balanced by chloride ion but not by the amine
group of the amino-terminated PNIPAAm. This result further con-
firmed the covalent linkage of the PNIPAAm unit to one side of the
C-5-position in the salen ligand. Furthermore, the temperature of
the successive cleavage of the salen ligand moieties coupled with
the PNIPAAm unit (weight loss = ca. 93.6%) in 1 increased to 385 ◦C
because of the mutual stabilization of the salen ligand part and
PNIPAAm moiety (Fig. 4b). This finding was an indirect proof of
the successful grafting of the PNIPAAm moiety onto CL. Different
from 1, 2 showed only the decomposition profile associated with
the successive cleavage of the salen ligand moieties coupled with
the PNIPAAm unit (at ca. 385 ◦C; weight loss = ca. 92.4%) without
any defined weight loss below 200 ◦C (Fig. 4c). Thus, the chlo-
ride ion was replaced with the amino-terminated PNIPAAm in 2,
thereby providing further evidence that the PNIPAAm moiety was
axially grafted successfully onto the metal center of the chiral salen
Mn(III) complex. The decomposition profiles of the PNIPAAm-based
complexes became complete at ca. 610 and 550 ◦C for 1 and 2,
respectively, with the residues amounting to manganese oxides.
of the chiral salen Mn(III) complex. The LCST of 1 and 2 was inves-
tigated by cloud-point measurement using UV–vis spectroscopy.
The LCST was the temperature at which the transmittance of the
PNIPAAm solutions markedly decreased. The turbidimetry curve
was drawn by measuring the transmittance of the aqueous polymer
solution (ꢁmax = 450 nm) at a fixed concentration (5 mg mL−1) as a
function of temperature, as shown in Fig. 5. The rate of temperature
increase was 5 ◦C/min, and the polymer solution was stirred and
kept at the same temperature for 1 min before each measurement.
The transmittance of 1 almost remained steady at <35 ◦C, indi-
cating that the solution was homogeneous. Beyond 35 ◦C, the
clear solution instantaneously became opaque, and the complex
100
90
80
70
60
50
b
40
30
20
a
3.3. LCST determination
20
25
30
35
40
45
50
55
60
65
70
Temperature (oC)
Aqueous solutions of 1 and 2 showed turbidity upon heat-
ing and became clear and homogeneous once the temperature
decreased. Such thermosensitive-controlled behavior suggested
that PNIPAAm was successfully incorporated into the framework
Fig. 5. Plot of changes in solution transmittance as a function of temperature for
aqueous solutions of 1 (a) and 2 (b). The mid-point of the transition curve is the
LCST. The solution concentration in water was 5 mgmL−1
.

R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
7
Table 1
Results of the OKR of ˛-methyl benzyl alcohol over various catalysts.a
HO
OH
O
(R, R) chiral salen Mn(III) complexes, PhI(OAc)2
+
H2O/ KBr, 25 oC
(R)
(+)
Conversion (%)b
ee (%)c
krel
d
Entry
Catalyst
Catalyst amount (mol%)
1
2
3
4
5
6
7
8
Neat complex
1
1.5
0.5
1.0
1.5
2.0
1.5
–
54
60
64
63
64
58
53
38
56
51
84
96
90
71
0
4.8
3.2
6.9
13.1
8.7
6.3
–
2
–
PNIPAAm-NH2
1.5
0
–
a
Reaction conditions: KBr (6 mol% ˛-methyl benzyl alcohol), ˛-methyl benzyl alcohol (0.25 mmol), PhI(OAc)2 (0.175 mmol), H2O (1 mL), 2 h, 25 ◦C.
Determined by GC.
b
c
Determined by GC on a 19091G-B213 chiral capillary column (30 m × 0.32 mm × 0.25 ␮m).
d
krel was calculated by the expression ln[(1 − C)(1 − ee)]/ln[(1 − C)(1 + ee)], where C is the conversion of secondary alcohol and ee is the enantiomeric excess of secondary
alcohol.
precipitated out of the aqueous solution. At this point, the transmit-
tance dramatically decreased and became constant with increased
temperature. The measurements were repeated during cooling and
a superimposable curve was obtained, indicating a reversible phase
transition with ꢂT = 10 ◦C under the aforementioned measurement
conditions. Apparently, the LCST of 1 was determined as ca. 35 ◦C
(Fig. 5a). Different attachment locations showed no relevant dif-
ferences in the LCST, and 2 whose PNIPAAm moiety was axially
graft onto the metal center also exhibited an LCST at around 35 ◦C
(Fig. 5b). The special LCST behaviors of 1 and 2 was due to the ther-
moregulated H-bonding interaction between the PNIPAAm moiety
and surrounding water molecules. Below the LCST, the PNIPAAm
moiety formed intermolecular hydrogen bonds with surrounding
water molecules, resulting in a hydrophilic state. With increased
temperature, these hydrogen bonds weakened and the PNIPAAm
moiety preferred to form intramolecular H-bonding interaction
inside the chains or with one another. Consequently, the hydropho-
bic groups were exposed, a hydrophobic state ensued, and the
PNIPAAm-based complexes were partially dehydrated and precip-
itated out of water. Notably, the LCST of 1 and 2 (ca. 35 ◦C) was
slightly higher than that of pure PNIPAAm-NH2 microgels (32 ◦C),
which may be due to the fact that the interaction between the chi-
ral salen Mn(III) complex moiety and PNIPAM component changed
the hydrophilicity–hydrophobicity balance in the PNIPAAm-based
complexes [40].
3.4. Catalytic performances
The LCST and inverse temperature-dependent water solubility
of
1 and 2 endowed them with a thermoregulated phase-
transfer function that enabled the occurrence of TPSC in aqueous
OKR reaction. Racemic ˛-methylbenzyl alcohol was used as the
A
B
100
80
60
40
20
0
100
80
60
40
20
0
b
a
b
a
1
2
3
4
5
1
2
3
4
5
Run times
Run times
Fig. 6. Reuse of 1 (A) and 2 (B) in the aqueous OKR of ˛-methylbenzyl alcohol (a, conversion; b, ee value).

8
R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
Table 2
Results of the OKR of ˛-methyl benzyl alcohol using 1 in the presence of various additives.a
d
Entry
Additives
Additive amount (mol%)
Conversion (%)b
ee (%)c
krel
1
2
3
4
5
6
7
8
9
–
KBr
KBr
KBr
–
4
6
8
10
6
6
6
6
56
58
63
67
68
65
61
56
12
0
78
96
96
94
92
91
93
2
–
7.9
13.1
9.9
8.3
9.1
11.6
9.0
1.0
KBr
N(C4H9)4Br
NH4Br
CaBr2
KCl
N
N
Cl
10
11
12
13
6
6
6
6
26
35
28
24
7
11
9
1.6
1.8
1.7
1.6
NH4Cl
CaCl2
KI
6
a
Reaction conditions: 1 (1.5 mol% of ˛-methyl benzyl alcohol), ˛-methyl benzyl alcohol (0.25 mmol), PhI(OAc)2 (0.175 mmol), H2O (1 mL), 1 h, 25 ◦C.
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
b
c
d
representative substrate for evaluating the catalytic efficiency of
alone also showed negligible catalytic activity toward aqueous OKR
reaction of ˛-methylbenzyl alcohol, giving poor conversion of ˛-
methylbenzyl alcohol (38%) with no enantioselectivity (Table 1,
entry 8). Based on the results, 1 was successfully designed as an
efficient thermoregulated phase-transfer catalyst for aqueous OKR
reaction at room temperature. Notably, only 1.5 mol% of 1 was
sufficient for the smooth catalysis of the aqueous OKR reaction
presented here (Table 1, entry 4). Further decreased or increased
amount of the catalyst decreased the ee and krel values.
the PNIPAAm-based complexes in aqueous OKR reaction at room
temperature using PhI(OAc)2 as an oxidant and KBr as an addi-
tive. The results are summarized in Table 1. For comparison, the
water-insoluble neat chiral salen Mn(III) complex was also inves-
tigated under the same reaction conditions. Moreover, the kinetic
resolution efficacy (krel) of the catalysts representing the relative
oxidation rates of the enantiomers of the racemic substrates in
aqueous OKR reaction was calculated [30].
As expected, the PNIPAAm-based complexes efficiently pro-
moted the OKR reaction of ˛-methylbenzyl alcohol, producing
higher enantioselectivity (ee) and krel values than neat complex
(Table 1, entries 4 and 6 vs. 1). In particular, 1 whose PNIPAAm
moiety was covalently attached to the C-5-position of the chiral
salen Mn(III) complex structure gave ee = 96% and krel = 13.1 in the
OKR of ˛-methylbenzyl alcohol at 25 ◦C within 2 h (Table 1, entry
4). By contrast, the analogous resolution using neat chiral salen
Mn(III) complex was found to have krel = 4.8 only (Table 1, entry
1). The significant increase in the chiral recognition and the effi-
cacy of the kinetic resolution was due to the unique micellar-type
reaction system originating from 1 at low temperatures. Indeed,
the PNIPAAm-based complexes were observed to be completely
soluble in aqueous medium and able to catalyze homogeneously
the OKR reaction at the reaction temperature (T < LCST). Although
still amphiphilic, 2 whose PNIPAAm moiety was axially anchored
onto the metal center of the chiral salen Mn(III) complex was far
less active than 1 (Table 1, entry 6 vs. 4). Only 71% ee and 6.3 krel
were achieved under identical reaction conditions (Table 1, entry
6). The decrease in the efficacy of the kinetic resolution may be
due to the space handicap orignating from the axial PNIPAAm moi-
ety. The steric handicap contrary to the desired steric hindrance
at the 5,5ꢀ-positions did allow close contact of the substrate with
the catalytically active metal center of the chiral 2 [2]. Eventually,
the formation of the alkoxy complex [salen-Mn(V)-Br-OR]+ as the
key reaction intermediate and precursor of the ketonic product
in OKR reaction decreased [10], resulting in the reduced catalytic
reactivity of 2. Furthermore, the chiral (R,R)-salen Mn(III) complex
preferred to integrate with (S)-˛-methylbenzyl alcohol to form the
alkoxy complex [salen-Mn(V)-Br-OR]+ and was further converted
to the respective ketone, yielding ˛-methylbenzyl alcohol enriched
with an R configuration. Therefore, the presence of the chiral salen
Mn(III) complex was essential to chiral recognition. No enantiose-
lectivity was observed when the chiral salen Mn(III) complex was
absent in aqueous OKR reaction (Table 1, entry 7). PNIPAAm-NH2
Apart from forming the favorable micellar-type reaction system
for smooth catalysis, another salient feature of the PNIPAAm-based
complexes in aqueous OKR reaction was the easy recovery through
thermocontrolled separation. After reaction, the PNIPAAm-based
complexes were separated from the aqueous medium by heating
the reaction solution to >35 ◦C and then used for subsequent cat-
alytic runs without further activation. Fig. 6A shows the results
of the recovery and reusability of 1. Similar catalytic activity and
enantioselectivity were observed in all four recycles, suggesting
the reusability of the PNIPAAm-based complex during aqueous OKR
oxidation. UV–vis spectra gave evidence of the stability of 1 based
on the fact that no significant change in the catalyst occurred even
after reusing for four times (Fig. 3b vs. bꢀ). Furthermore, 2 was found
a
a'
568 416
b
b'
420
3072
2974
572
420
500
3309
2878
2931
1652
2000
1541
1500
4000
3500
3000
2500
1000
Wavenumbers (cm-1)
Fig. 7. FT-IR spectra of fresh 1 (a), 1 after the fourth reaction (aꢀ), fresh 2 (b), and 2
after the fourth reaction (bꢀ).

R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
9
Table 3
to be reusable and stable in the aqueous OKR oxidation presented
Results of the OKR of various secondary alcohols over 1.a
here (Figs. 6B and 3c vs. cꢀ), although, it was less effective than 1.
FT-IR spectra of 1 and 2 (fresh and reused four times) further con-
firmed the perfect stability of the PNIPAAm-based complexes since
because no significant changes occurred in the FT-IR spectra of 1
and 2 even after four times of reuse (Fig. 7). The reusability of the
PNIPAAm-based complexes confirmed that the PNIPAAm group as
a functional switching unit for modifying the chiral salen Mn(III)
complex indeed realized the homogeneous reaction coupled with
biphasic separation in aqueous reaction. Further reuse of the com-
plexes led to a marginal decrease in the conversion of ˛-methyl
benzyl alcohol without any change in the enantioselectivity of the
product, which was probably due to the slight solubility loss of the
amphiphilic catalysts in the aqueous phase [10].
O
HO
OH
Complex 1, PhI(OAc)2
+
R
R'
R
R'
H2O/ KBr, 25 oC
R
R'
(R)
(+)
d
Entry
Substrate
Conversion (%)b
ee (%)c
krel
OH
1
2
63
54
96
7
13.1
1.2
OH
Additives play an important role in accelerating the reaction rate
and enantioselectivity of the OKR catalyzed by chiral salen Mn(III)
complexes [2,4]. Various halide salts were investigated as additives
in the OKR of ˛-methylbenzyl alcohol catalyzed by 1, and the results
are listed in Table 2. In the absence of an additive, the OKR reaction
carried out with 1 as the catalyst was nonenantioselective, although
56% conversion was achieved (Table 2, entry 1). The ee and krel val-
ues significantly increased upon the addition of bromide salts, such
as potassium bromide, TEAB, ammonium bromide, and calcium
bromide (Table 2, entries 2–8). The crucial influence of bromide
salts was readily understandable on the basis of the OKR pathway
of secondary alcohols as proposed by Brown et al. [41], in which
HOBr or Br2/H2O generated by the oxidation of bromide ion with
PhI(OAc)2 converted the Mn(III)-salen complex into a Mn(V)-salen
dibromide complex. This complex then reversibly reacted with
the secondary alcohol to form the alkoxy-Mn(V) species [salen-
Mn(V)-Br-OR]+ as the key reaction intermediate and precursor of
the ketonic product. Further intramolecular hydrogen transfer of
[salen-Mn(V)-Br-OR]+ from the carbinol C H to the neighboring
phenolic oxygen along a five-membered cyclic pathway provided
the corresponding ketonic product, optical pure secondary alcohol,
and recovered Mn(III) species. Notably, the inexpensive inorganic
salt KBr gave excellent ee and krel values comparable with those
observed by TEAB (Table 2, entry 3 vs. 6) in the aqueous reaction
under identical conditions. The ee and krel values obtained were
96% and 13.1, respectively, in the presence of 6 mol% KBr in just
1 h (Table 2, entry 3). The results provided further proof that the
phase-transfer capability of 1 originated from the PNIPAAm moi-
ety. Meanwhile, chloride salts were only marginally active for OKR
reaction. Low krel and ee values were obtained when the reactions
were carried out in the presence of chloride salts (Table 2, entries
9–12). Iodine salts (such as potassium iodide) as additives also gave
very poor results in terms of ee and krel values (Table 2, entry 13).
The results were in accordance with those reported in literature
[2,4,6].
OH
H3C
3
4
63
17
90
8
9.4
2.5
CH3 OH
OH
Cl
5
52
56
5.5
OH
Cl
6
7
40
15
29
5
3.3
1.9
Cl OH
OH
Br
8
9
59
63
92
94
14.2
11.5
OH
The scope of OKR of various racemic secondary alcohols was
investigated with 1 (1.5 mol%) under the optimized reaction con-
ditions, and the results are summarized in Table 3. In general, 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 Rꢀ = CH3 to
Rꢀ = CH2CH3) resulted in a dramatic decrease in the ee value from
96% to 7% and in the krel value from 13.1 to 1.2 (Table 3, entry 1 vs. 2).
The decreases in the ee and krel values were presumably due to the
steric repulsion between alcohol and catalyst. This phenomenon
prevented the formation of the alkoxy complex [salen-Mn(V)-Br-
OR]+ as the key reaction intermediate and precursor of ketonic
products. Additionally, substituents at the 2-position of the phenyl
group severely affected the ee and krel values. The OKR of ˛-methyl-
O
OH
10
Trace
–
–
a
Reaction conditions: 1 (1.5 mol% of substrate), KBr (6 mol% of substrate), sec-
ondary alcohol (0.25 mmol), PhI(OAc)2 (0.175 mmol), H2O (1 mL), 2 h, 25 ◦C.
b
Same as in Table 1.
Same as in Table 1.
Same as in Table 1.
c
d
was in accordance with the view that the undesired steric hin-
drance caused by the substituted group did not allow close contact
of the substrate with the catalytically active metal center of 1. The
OKR of the chlorinated ˛-methylbenzyl alcohols were also in agree-
ment with this view. ˛-Methyl-4-chlorobenzyl alcohol gave higher
ee (56%) and krel (5.5) values than ˛-methyl-3-chlorobenzyl alco-
hol (29% ee and 3.3 krel), whereas ˛-methyl-2-chlorobenzyl alcohol
gave the poorest results (5% ee and 1.9 krel) (Table 3, entries 5–7).
4-methylbenzyl alcohol gave 90% enantioselectivity and 9.4 krel
,
whereas ˛-methyl-2-methylbenzyl alcohol used as the substrate
gave only 8% ee and 2.5 krel (Table 3, entry 4). This observation

10
R. Tan et al. / Applied Catalysis A: General 458 (2013) 1–10
Furthermore, ˛-methyl-4-bromobenzyl alcohol was resolved with
excellent ee of 92% and krel of 14.2 (Table 3, entry 8). Moreover, 1
was effective for the OKR of fatty alcohols such as 2-pentanol, where
94% ee and 11.5 krel were obtained in the OKR reaction of 2-pentanol
(Table 3, entry 9). However, 1 was inactive for 2-furylethanol. No
oxidation of the heterocyclic compounds was observed under this
condition (Table 3, entry 10).
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PNIPAAm-functionalized chiral salen Mn(III) complexes pos-
sessing a thermoregulated phase-transfer function were prepared
by introducing a “smart” PNIPAAm unit to the framework of the
chiral salen Mn(III) complex. The thermoresponsive catalysts were
efficient in OKR reaction in pure water with no need for any organic
solvent. Excellent ee and krel values were obtained over 1 in the
aqueous reaction, and these values were significantly higher than
those obtained over neat complex. The catalysts were also easily
recovered for reuse by simply adjusting the temperature of the
aqueous medium, exhibiting not only excellent TPSC property but
also good recycling efficiency. The attractive catalytic process of
TPSC realized homogenous reaction coupled with biphasic separa-
tion in aqueous reaction.
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This project was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21003044 and 20973057),
the Natural Science Foundation of Hunan Province (10JJ6028), and
the Program for Science and Technology Innovative Research Team
in Higher Educational Institutions of Hunan Province.
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