Ultraviolet-responsive self-assembled metallomicelles for photocontrollable catalysis of asymmetric sulfoxidation in water

Article abstract of DOI:10.1039/c7ra11022g

Self-assembled metallomicelles with ultraviolet (UV)-controlled morphologies were constructed from a synthesized azobenzene-containing amphiphilic chiral salen Ti^IV catalyst. The morphologies of the metallomicelles could be well adjusted by changing the UV irradiation time, and this was confirmed by TEM analyses. The UV-induced change in morphology potentially adjusted the catalyst concentration and/or accessibility in real-time, allowing photocontrollable catalysis of asymmetric sulfoxidation in water. UV-responsive catalytic activities with excellent selectivities were observed over the metallomicelles for a wide range of alkyl aryl sulfides. Moreover, a thermo-responsive NIPAAm block in the catalyst enables it to be easily recovered for steady reuse by thermo-controlled separation. This work constructed a photo-responsive metallomicellar system to carry out metallomicellar catalysis in a controllable way.

Full text of DOI:10.1039/c7ra11022g

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Ultraviolet-responsive self-assembled
metallomicelles for photocontrollable catalysis of
Cite this: RSC Adv., 2017, 7, 54570
asymmetric sulfoxidation in water†
Mengqiao Gao, Rong Tan, * Pengbo Hao, Yaoyao Zhang, Jiang Deng
and Donghong Yin
Self-assembled metallomicelles with ultraviolet (UV)-controlled morphologies were constructed from
IV
a synthesized azobenzene-containing amphiphilic chiral salen Ti catalyst. The morphologies of the
metallomicelles could be well adjusted by changing the UV irradiation time, and this was confirmed by
TEM analyses. The UV-induced change in morphology potentially adjusted the catalyst concentration
and/or accessibility in real-time, allowing photocontrollable catalysis of asymmetric sulfoxidation in
water. UV-responsive catalytic activities with excellent selectivities were observed over the
metallomicelles for a wide range of alkyl aryl sulfides. Moreover, a thermo-responsive NIPAAm block in
the catalyst enables it to be easily recovered for steady reuse by thermo-controlled separation. This work
constructed a photo-responsive metallomicellar system to carry out metallomicellar catalysis in
a controllable way.
Received 7th October 2017
Accepted 22nd November 2017
DOI: 10.1039/c7ra11022g
rsc.li/rsc-advances
response to local temperature. At room temperature which was
its lower critical solution temperature (LCST), catalysts were
Introduction
Metallomicellar systems, which exhibit similar structural and amphiphilic and self-assembled to form metallomicelles,
kinetic properties to natural enzymes, have been extensively accelerating the aqueous sulfoxidation by “concentration
investigated as effective biomimetic systems for asymmetric effect”. Upon heating above their LCST, they switched to
1
catalysis in water. The ability to carry out metallomicellar hydrophobic compounds, and thus precipitated from aqueous
catalysis in a controllable way has profound importance in system for facile recovery. The switchable behavior turned the
chemical synthesis, and this area has garnered intense interest. enantioselective sulfoxidation on or off at will by adjusting local
The key to achieving this controllable catalysis was developing temperature. Nevertheless, in the metallomicellar catalysis,
a metallomicellar system in which the concentration and/or chemical processes were typically limited to a xed rate and/or
accessibility of catalytic sites in reaction media could be selectivity once the catalyst and reaction condition are chosen,
2
adjusted in a controlled manner. Stimuli-responsive metal- control of catalytic activity as the reaction proceeds was difficult
lomicelles were thus developed to control catalytic reactions via to achieve. Logically, if the morphology and/or architecture of
3
external signals. The metallomicelles oen undergo instanta- the metallomicelles can be adjusted in real-time by external
neous morphological and/or architectural changes upon expo- stimuli, controlled catalysis of asymmetric sulfoxidation should
sure to an external stimulus, consequently altering their be realized.
4
catalytic properties. Lately, we have reported thermo-
Light irradiation, a green and sustainable energy which
responsive metallomicelles for enantioselective sulfoxidation could be precisely manipulated in a clean environment without
IV
at room temperature in water by using a chiral salen Ti
pollution to reaction system, was considered as an ideal
complex as the catalytic site and NIPAAm as the thermo-trig- external stimulus for controllable catalysis. A variety of light-
3
c
ger. The metallomicelles underwent reversible switching responsive moieties was introduced in catalysts to give the
2
a,5
between self-assembly and precipitation in aqueous system in catalytic system response to light. Among them, azobenzene
has received much attention in recent years due to its well-
known reversible photoisomerization from trans- to cis-cong-
Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research,
Ministry of Education, National & Local Joint Engineering Laboratory for New uration. Amphiphilic catalysts containing azobenzene group
Petro-chemical Materials and Fine Utilization of Resources, Hunan Normal oen undergo conformational and electronic changes upon
University, Changsha 410081, P. R. China. E-mail: yiyangtanrong@126.com; Tel:
6
light irradiation. It offered an attractive alternative to modulate
+
86-731-8872576
morphology and/or architecture of metallomicelles in real-time.
With these points in mind, we decided to incorporate azo-
benzene as the hydrophobic block of amphiphilic chiral salen
†
Electronic supplementary information (ESI) available: Preparation and identity
of the catalysts, NMR and HPLC analysis for the chiral sulfoxides. See DOI:
0.1039/c7ra11022g
1
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IV
Ti catalyst to develop photo-controlled metallomicellar cata- reversible switching between hydrophilicity and hydrophobicity
lytic system of asymmetric sulfoxidation in water. We envi- at its LCST was incorporated as the “smart” hydrophilic block to
sioned that in aqueous media, the azobenzene-containing ensure not only self-assembly of catalyst for metallomicellar
amphiphilic catalyst should self-assemble through hydrophobic catalysis, but also recovery of catalyst for efficient reuse in the
interaction and p–p staking of azobenzene group, forming aqueous system.
IV
micelle having hydrophobic azobenzene core. Catalytically
active Ti(salen) was graed on azobenzene segment to ensure catalyst of PN427
the effective shielding of catalytic sites in the hydrophobic dened diblock copolymer of poly(NIPAAm-co-azo) (denoted
compartment, mimetic of enzyme. The self-assembled metal- as PN427 10), featuring a terminated amino (–NH ) group on
The synthesis of thermo/photo-responsive chiral salen Ti
A
10-C was illustrated in Scheme 1. First, well-
A
2
lomicelles would behave as biomimetic nanoreactor, acceler- azobenzene block, was prepared via ATRP by using N,N-azobi-
ating asymmetric sulfoxidation in water. In particular, the s(isobutyronitrile) (AIBN) as a radical initiator and 2-amino-
salient feature of azobenzene, reversible structural changes in ethanethiol hydrochloride (AET) as the chain transfer agent.
the photoisomerization (trans/cis), should be benecial for Successive N-alkylation of the terminal amino group
2 2
temporal control of the metallomicelles morphology using (–NH ) with benzyl chloride group (–CH Cl) at the 5-position
light. This unique property allowed one to control the metal- of an reported asymmetric chiral salen ligand (CL) of
0
lomicellar catalysis of aqueous asymmetric sulfoxidation by (R,R)-N-(3,5-di-tert-butylsalicylidene)-N -[3-tert-butyl-5-chlor-
9
external light irradiation. Thermo-responsive NIPAAm, which omethylsalicylidene]-1,2-cyclohexane-diamine
resulted in
underwent hydrophilic-to-hydrophobic transformation at its covalently appending chiral salen ligand on the hydrophobic
7
LCST, was selected as the “smart” hydrophilic block to control segment of PN427A10, as shown in Scheme 1. The strategy
self-assembly and precipitation of Ti(salen)-containing metal- ensured shielding of the catalytic motif in hydrophobic domain
lomicelle in the aqueous system.
Herein, well-dened diblock copolymer of poly(NIPAAm-co- water, which was a salient feature of enzyme catalysis. The
azo) (NIPAAm, N-isopropylacrylamide; azo, azobenzene) was resultant PN427 10-modied chiral salen ligand was coordi-
(protected by the hydrophilic corona) upon self-assembly in
A
i
synthesized by copolymerizing NIPAAm with N-azo-acrylamide nated with Ti(O Pr)
4 2
and further treated with H O, giving
8
IV
i
via atom transfer radical polymerization (ATRP). Covalently PN427
A
10-modied chiral salen Ti complex. Excess Ti(O Pr)
IV
appending chiral salen Ti complex on the azobenzene block was hydrolyzed into TiO and then was removed by ltration,
2
IV
IV
provided an amphiphilic chiral salen Ti catalyst responsive to affording the thermo/photo-responsive chiral salen Ti catalyst
temperature and UV irradiation. At the reaction temperature of PN427A10-C (where 427 and 10 represented the repeated units
ꢀ
(
25 C), the catalyst was amphiphilic and self-assembled into number of NIPAAm and azobenzene in copolymer, which were
1
metallomicelles in water, accelerating the aqueous asymmetric determined by H NMR spectrum, as shown in ESI†).
IV
i
10
sulfoxidation by “concentration effect”. UV irradiation
provoked the morphological change of metallomicelles, which (denoted as neat complex, as shown in Chart 1) that was
thus controlled the metallomicellar catalysis of asymmetric completely insoluble in water, the obtained PN427 10-C pre-
2
Unlike traditional chiral ((R,R)-salen)Ti (O pr) complex
A
sulfoxidation in water. Aer reaction, the catalyst became sented temperature-controlled water-solubility with the LCST of
ꢀ
hydrophobic upon heating above its LCST, and precipitated 26 C. The LCST which was approximate to room temperature
ꢀ
from aqueous system for recovery. The activity switching was (25 C) made PN 27A10-C self-assemble at room temperature and
4
repeatable even aer seven cycles.
separate also at mild temperature (slightly higher than room
temperature) in the aqueous asymmetric sulfoxidation. Actu-
ally, to regulate the LCST close to room temperature, various
rations of NIPAAm to azobenzene blocks have been modulated
Results and discussion
Preparation of catalysts
Stimuli-responsive metallomicellar catalysis, in which intrinsic
activity could be nely modulated in real-time by external
2,5e
stimulus, was highly desirable in biomimetic catalysis. Light
was attractive stimulus for this stimuli-responsive system, since
it offered excellent temporal and spatial resolution, and could
be precisely controlled with an appropriate source. The key to
achieving the light-controlled catalysis was developing the
metallomicelle whose morphology and/or architecture in reac-
tion media could be instantaneously induced by light stim-
2a,5
uli.
Azobenzene with reversible E/Z photoisomerization was
an ideal photosensitive group for constructing the light-
6
responsive metallomicelle. With these points in mind, we
decided to incorporate hydrophobic azobenzene into amphi-
IV
philic chiral salen Ti catalyst to fabricate a Ti(salen)-
containing metallomicelle for photocontrollable catalysis of
asymmetric sulfoxidation in water. PNIPAAm which underwent Scheme 1 Synthesis of PN427
A
10-C.
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structural changes in PN427
A
10-C were then evaluated through
spectral studies. The change in UV-vis absorption spectra of
PN427A10-C upon UV irradiation (365 nm) were shown in Fig. 2.
Before UV irradiation, a strong absorption band at 348 nm was
observed which was ascribed to the p–p* transition of trans-azo
12
(
Fig. 2A). The absorption band gradually decreased with UV
Chart 1 Structure of neat complex.
irradiation, and showed an obvious blue shi to 334 nm.
Concomitantly, a new peak ascribed to n–p* band of cis-azo
1
3
IV
appeared at around 443 nm, and gradually increased with
prolonged irradiation time (Fig. 2A). The observation suggested
the progressive isomerization of azobenzene from tans to cis
in the chiral salen Ti -containing amphiphiles. The detail
synthesis and identication of the controlled copolymers were
available in the ESI.†
13
upon UV irradiation. Aer 220 s of UV light exposure, a pho-
tostationary state enriched by cis isomer was reached, and
longer irradiation time caused no change in the UV-vis spec-
Characterization of samples
LCST determination. Thermo-responsive behavior of trum (Fig. 2A). The trans-azo could be obtained again when the
10-C was investigated by monitoring the optical trans- irradiated sample was then kept in dark for 72 h (Fig. 2A). It
mittance of typical PN427 10-C solution at 440 nm using UV-vis indicated reversible photoisomerization of the azobenzene in
PN427
A
A
spectrophotometry (Fig. 1). Obviously, PN427A10-C was well PN427A10-C between the trans- and cis-forms. Notably, the trans–
soluble in water, giving clear aqueous solutions at temperature cis photoisomerization of azobenzene could be repeated many
ꢀ
below 26 C (Fig. 1A), although neat complex was practically times without fatigue and decomposition of the component
insoluble in water. When the local temperature was raised to (Fig. 2B).
ꢀ
1
2
6 C, the aqueous solution became turbid, and transmittance
H NMR. Photosensitivity of PN 27A10-C was also investi-
4
1
decreased dramatically (Fig. 1A). It suggested a hydrophobic gated in detail by H NMR spectroscopy carried out in DMSO-d .
6
state of PN427
LCST of PN427
A
A
10-C at this temperature range. Therefore, the Dark-adapted sample was irradiated directly in the NMR tubes,
10-C was determined as ca. 26 C. We noticed by placing it 4 cm below 4 W 365 nm UV-lamp. Aer irradiation,
ꢀ
that the temperature was much lower than LCST of pure PNI- the tube was put imminently into the NMR magnet, and the
ꢀ
1
PAAm microgel (32 C). This difference conrmed the success- spectra were taken. As shown in H NMR spectra (Fig. 3A),
ful incorporation of hydrophobic azobenzene block and discernible aromatic proton signals associated with azobenzene
Ti(salen) unit into PNIPAAm, which changed the hydrophi- group underwent obvious changes upon UV irradiation. Before
licity–hydrophobicity balance in copolymer, and thus lower the UV treatment, PN427A -C exhibited only the proton signals in
10
1
1
1
LCST. It was the combined hydrophobic block that made the 7.90–7.79 ppm range in H NMR spectrum, which was
0
PN427
PN427
A
A
10-C amphiphilic and ensured the self-assembly of the assigned as the 6- and 6 -position protons of trans-azo group
10-C in water. By continuous cooling to below 26 C, the (Fig. 3A(a)). Irradiation of UV light on the PN427A10-C solution
ꢀ
PN427A10-C aqueous solution reverted to the initial clear state, induced a characteristic upeld shi of the typical signals
suggesting the returned amphiphilic state of PN427A10-C. (Fig. 3A(b–e)). Since overlapped protons in the bent cis-azo were
1
Signicantly, the water-solubility switch could be reversibly shown at higher eld than normal aromatic protons in the H
repeated for several times by controlling local temperature NMR spectra owning to the shielding effect of the overlapping
(Fig. 1B). The salient feature allowed for catalysis and separa- benzene ring, the observed upeld shi was attributed to pho-
tion of PN427 10-C in aqueous system in a mild temperature- toisomerization of trans-azo in PN4 A -C to the cis-isomer
A
27
0
10
5a,c
controllable way.
which was overlapped at the 6- and 6 -positions. The trans to
UV-vis. Apart from thermo-responsivity, PN427
A
10-C also cis isomerization also reduced the intramolecular distance
exhibited UV stimuli-responsive behavior due to the presence of
photosensitive azobenzene moiety. The photoresponsive
Fig. 2 UV-vis spectra of PN427
A
10-C aqueous solution (concentration:
10-C aqueous solutions 0.5 mg mL ) upon UV irradiation (365 nm) for different times (A), and
concentration: 0.8 mg mL , (A)), and optical transmittance at 440 nm optical absorbance at 443 nm of PN427 10-C aqueous solution
10-C solution observed upon several cycles under heating at observed upon several cycles under irradiation at 365 nm for 140 s and
ꢁ1
Fig. 1 Transmittance curves of the PN427
(
A
ꢁ
1
A
of PN427
A
ꢀ
ꢀ
3
2 C and then cooling to 25 C (B).
keeping in dark for 72 h (B).
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1
Fig. 3 Changes in H NMR spectra of PN427
A10-C upon photo-
isomerization in DMSO-d (A) (at dark-adapted state (a), after 1 min of
6
Fig. 4 Plots of surface tension against logarithm of concentration for
UV irradiation (b), after 3 min of UV irradiation (c), after 5 min of UV
irradiation (d), after 10 min of UV irradiation (e), and the UV-treated
ꢀ
PN427
A
10-C in water at 25 C (at dark-adapted state (A), after 5 min of
1
UV irradiation (B)).
samples kept in dark for 72 h (f)); H NMR spectra of dark-adapted
2 6
PN427A10-C (B) in D O (a) and in DMSO-d (b).
The CMC value of PN427A -C at dark-adapted state was thus
10
ꢁ
1
between aromatic rings of azobenzene and slowed the rotation determined to be 0.196 mmol L (Fig. 4A). Notably, the CMC
ꢁ
1
around C–N bonds, leading to better separation of the proton value slightly increased to 0.210 mmol L when the dark-
14
signals. Therefore, multiple upeld signals at 6.75, 6.80, and adapted PN427A10-C aqueous solution was exposed to UV irra-
.84 ppm progressively appeared when PN427 10-C was irradi- diation for 5 min (Fig. 4B). Photoisomerization of azobenzene
ated by UV light (Fig. 3A(b–e)). The composition of cis-isomer from trans-form to more hydrophilic cis-form should account
6
A
16
increased with the irradiation of UV light and reached the for the increased CMC value. The results suggested the UV-
photostationary state aer irradiation for 10 min (Fig. 3A(e)). controlled self-assembly behavior of PN427A10-C.
1
Then, the H NMR spectra could revert to the initial state, aer
TEM. TEM images gave direct information of the self-
the UV-treated PN427A10-C was held in dark for 72 h (Fig. 3A(f)). assembly of PN427A10-C in water, as well as the morphological
It demonstrated the reversible trans–cis photoisomerization of change induced by UV irradiation (Fig. 5). As expected, driven by
azobenzene moiety in PN427
A
10-C.
hydrophobic interaction and the p–p stacking of azobenzene
block, PN427A10-C self-assembled in water, forming metal-
Interestingly, dark-adapted PN427
A
6 2
10-C in DMSO-d and D O
1
1
gave different H NMR spectra, as shown in Fig. 3B. H NMR lomicelles in a nanometer scale. Light-induced changes in
spectrum of PN427 10-C in DMSO-d exhibited the distinct H azobenzene conformation adjusted the hydrophilic–hydro-
signals of PNIPAAm moiety (d ¼ 3.84 ppm, O]CNHCH(CH ), phobic balance of PN427A10-C. As a result, morphology and/or
A
6
3 2
)
azobenzene group (d ¼ 7.84 ppm, N]N–Ph–H), and Ti(salen) size of the metallomicelles were controlled by UV irradiation.
unit (d ¼ 1.24, cyclohexyl-H). It indicated the presence of In the absence of UV irradiation, dark-adapted PN427A10-C self-
hydrophilic PNIPAAm block, hydrophobic azobenzene block, assembled into spherical nanoparticular (diameter of ca. 2 nm)
and catalytically active Ti(salen) unit in PN427A10-C. Whereas, in water exposing the hydrophilic NIPAAm to water, while the
the H signals associated with azobenzene and Ti(salen) moie- hydrophobic azobenzene and Ti(salen) units packed together to
1
ties almost disappeared when H NMR spectrum was carried form a hydrophobic core (Fig. 5a). The nanoassemblies pre-
out in D
2
O. The difference gave an evidence for the effective sented a micelle to vesicle transition behavior upon UV treat-
shielding of azobenzene and Ti(salen) groups from the aqueous ment (Fig. 5b–d). Vesicles were observed when the sample was
13d,15
environment upon self-assembly of PN427A10-C in water.
exposed to UV light for 10 s (Fig. 5b). The morphological tran-
Hydrophobic shielding of active site made the PN427A10-C an sition accomplished aer UV irradiation for 30 s, giving almost
efficient enzyme-mimetic metallomicelle for asymmetric catal-
ysis in water.
CMC determination. CMC determination provided strong
evidence for the self-assembly behaviors of PN427A10-C, since
amphiphiles would self-assemble in water when the concen-
tration of solutions were above the CMC value. Surface tension
measurements over a wide range of concentrations were used to
determine the CMC value of PN427A10-C. Fig. 4 showed the plots
of surface tension as a function of logarithm of concentration
for PN427
A
10-C at dark-adapted state or under UV irradiation in
ꢀ
water at 25 C. Clearly, the surface tension decreased dramati-
cally as the concentration of catalyst increased, and then it
didn't change substantially as a function of concentration of
catalyst. The inection point at characteristic concentration
gave the information on CMC value of corresponding sample.
Fig. 5 TEM images of PN427
UV irradiation for 10 s (b), 30 s (c), 40 s (d), 60 s (e), 70 s (f), 90 s (g), and
the UV-treated PN427 10-C kept in dark for 72 h (h).
A10-C in a dark-adapted state (a) and after
A
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uniform vesicles with size of ca. 6 nm (Fig. 5c). UV-induced a large number of very small vesicles (Fig. 5g vs. f). Similar
isomerization of azo-moiety from trans- to cis-form was observations of the photo-induced process including fusion,
responsible for this morphological transition due to the disintegration and rearrangement have also been reported in
19
differences in geometry and polarity between the two isomers. the literature. Upon keeping the UV-treated PN427A10-C in dark
As we all know, the bend cis-azo occupied a larger volume than for 72 h, spherical nanoparticular emerged again with a size
13d,17
the rod-like trans-form.
This implied that the UV-induced close to 2.0 nm (Fig. 5h). Such morphological changes indicated
isomerization of azo-moiety increased the steric hindrance the responsiveness of the metallomicelles towards UV light,
between azo-molecule in the hydrophobic core, which weak- which has the potential to adjust catalyst concentration and/or
ened their p–p staking interaction. Furthermore, hydropho- accessibility in real-time, allowing for ne control of the asym-
bicity of azo-molecule also decreased when trans-form were metric sulfoxidation in water.
5
e
transformed into cis-form under UV light. The reduced p–p
staking and hydrophobic interactions made the azobenzene assemblies under the inuence of UV irradiation were also
groups have no ability to accumulate tightly in PN427 10-C self- investigated by DLS. The hydrodynamic diameter and size
assemblies. As a result, the micellar structure with a mono- distribution under different irradiation conditions were shown
layered arrangement of hydrophilic and hydrophobic chains in Fig. 6. Before UV irradiation, PN427 10-C aqueous solution
was unable to stabilize the self-assemblies. The vesicle structure showed the aggregate with hydrodynamic diameter (D ) of ca.
DLS. Size and morphological changes of PN427A10-C nano-
A
A
h
thus occurred with the formation of the bilayer, in which 23.5 nm (polydispersity indexes, PDI ¼ 0.143). When the
hydrophobic segments stacked inside and the remaining PN A -C aqueous solution was exposed to UV light for 30 s,
4
27 10
hydrophilic NIPAAm block dangle in the surrounding aqueous the size of the metallomicelles increased to 44.1 nm (PDI ¼
solution. 0.164). Great change in the metallomicelle size reected
Interestingly, the adjacent vesicles underwent close contact a morphological transformation from micelle to vesicle, as
and mergence to become the bigger vesicles, when the UV conrmed by TEM. The D increased continually to 58.3 nm
h
irradiation time was prolonged from 30 to 60 s (Fig. 5d and e). (PDI ¼ 0.179) when the UV irradiation time was prolonged to
Larger vesicles with size greater than 20 nm were observed when 60 s, due to photo-induced fusion of vesicles during the irra-
the sample was exposed to UV light for 60 s. Increasing cis- diation. Aer UV treatment for 90 s, the size of PN427A10-C
isomer in vesicular membrane was the ultimate cause of this metallomicelle decreased to 32.7 nm (PDI ¼ 0.236) which
fusion process. As the cis-isomer increasing, a larger volume indicated a disintegration and rearrangement of the larger
was needed for the cis-form to exist in stable form. It thus vesicle. Relatively higher PDI value (0.236) was probably due to
20
brought a loose stacking formation in the vesicular membrane, the relative inhomogeneity of rearranged vesicle. These
or even disrupted the bilayer of vesicles. In addition, the surface observations were in complete agreement with the results ob-
area of vesicles should expand as a result of the transition from tained in TEM images.
tight to loose packing, leading to an increase in instantaneous
surface free energy. Therefore, fusion was the most likely Catalytic performances
pathway for the vesicles to become stable and reduce their
surface free energy. Furthermore, fusion proceeded through
Encouraged by the photo-induced change in morphology of
PN427A10-C, the self-assembled Ti(salen)-containing metal-
lomicelles was employed as the photo-sensitive bio-nanoreactor
for controlled catalysis of asymmetric sulfoxidation in water.
Phenyl methyl sulde was used as a substrate to investigate the
a highly bent stalk intermediate, which was formed when
hydrophobic tails from two apposing membranes came into
contact via a uctuation and made a hydrophobic bridge
18
between them. The formation of the stalk structure required
overcoming the free energy barriers. In fact, the change from
tight to loose packing could increase the entropy without
adversely affecting the enthalpy. It thus lowered the free energy
18
barriers, which beneted the fusion process. In other words,
the defects in loose packing state with more structural exibility
and plasticity may facilitate the restructuring of the PN427A10-C
bilayer in the fusion process. Notably, the regions of defects
were not large enough to cause the rupture of individual vesicle,
since the rupture would overcome the entanglement of polymer
chains and increase the surface free energy. The disruption
extent of membranes gradually increased with continuous
irradiation. When the vesicles with large disruption areas
appeared, rupture of vesicular membrane took place. Subse-
quently, the disintegrated fragments rearranged to a majority of
small vesicles by virtue of the hydrophobic interaction. Indeed,
aer the sample was irradiated for 60 s, those larger vesicles
with damages and defects severely ruptured until almost
Fig. 6 Size distribution of self-assembled PN427A10-C in water at
ꢁ1
complete disintegration occurred, and then rearranged to form a concentration of 0.5 mg mL under different UV irradiation time.
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photo-mediated catalytic performance of PN427
A
10-C. Before by the hydrophobic compartment from the surrounding
reaction, dark-adapted PN427A10-C in which all the azobenzene aqueous environment, creating a highly concentrated environ-
chromophores were in trans-state was exposed to UV light (l ¼ ment for efficient catalysis. Furthermore, synergistic effects
365 nm) for a certain time, and then wrapped with aluminum between the metal-catalyzed center and the hydrophobic
foil to avoid the disturbance of natural light. The results are microenvironment in the metallomicelle led to high
IV
23
presented in Table 1. Traditional chiral salen Ti complex selectivity.
denoted as neat complex, Chart 1) was also employed for Photo-responsive behavior of the metallomicelles made
catalytic efficiency of PN427 10-C sensitive to UV irradiation.
10-C-based metallomicelles signicantly Indeed, the catalytic activity of PN427 10-C was adjusted by UV
(
comparison.
A
As expected, PN427
A
A
accelerated the asymmetric oxidation of methyl aryl suldes in irradiation in the aqueous asymmetric sulfoxidation. Before UV
water using H O as oxygen source, giving high activity and treatment, dark-adapted PN427A10-C gave 82% conversion of
2
2
remarkable selectivity (in terms of chemo- and enantiose- phenyl methyl sulde in water within 75 min (Table 1, entry 1).
10
lectivity) (Table 1, entries 1–10). Only 0.1 mol% of PN427 10-C UV irradiation mediated the catalytic performance of PN427A -
A
was sufficient for giving almost quantitative yield (>99%) of C in the aqueous asymmetric sulfoxidation (Table 1, entries 1–
phenyl methyl sulfoxide with outstanding chiral induction 10). In particular, almost quantitative yield (>99%) of (R)-phenyl
10
(99%) when catalyst was treated with UV irradiation for 30, 60, methyl sulfoxide was obtained when the dark-adapted PN427A -
or 90 s (Table 1, entries 4, 7, and 10). In contrast, extremely low C was exposed to UV light for 30 s (Table 1, entry 4). Enhanced
conversion (8%) with disappointing chemoselectivity (86%) and activity should be related to the micelle to vesicle transition
ee value (83%) was obtained under identical conditions when behavior of PN427A10-C upon UV treatment, as shown in TEM
the reaction was catalyzed by neat complex (Table 1, entry 11). images. Despite both conned catalysis, vesicles represented
The results demonstrated the advantages of metallomicellar bubble-like structures in which a water-containing interior was
catalytic approach over self-assembled PN427A10-C. The formed enclosed by a double-layer membrane. The bilayer membrane
metallomicelles not only showed good water-solubility, but also was a more conducive structure for sequestering substrates
possessed a hydrophobic compartmentalized structure which from the surrounding aqueous environment, creating a highly
allowed effectively shielding active sites from aqueous envi- concentrated environment favorable for efficient sulfox-
21
IV
3c
ronment, reminiscent of enzyme. Dense chiral salen Ti
idation. Varied UV irradiation time resulted in the variation of
complexes conned in the hydrophobic domain may enforce catalytic efficiency of PN427A10-C (Table 1, entries 5–10). Quan-
a cooperative reaction pathway favourable for the asymmetric titative yield of (R)-phenyl methyl sulfoxide was also observed
3
c,20,22
sulfoxidation.
Substrates were also effectively sequestered when PN427A10-C was treated by UV irradiation for 60 or 90 s.
IV
a
2 2
Table 1 Results of the asymmetric sulfoxidation of alkyl aryl sulfides with H O over chiral salen Ti catalysts under different UV-irradiation time
b
c
d
Entry
Catalyst
Substrates
Products
UV-irradiation time (s)
T (min)
Conv. /%
Sel. /%
ee /%
1
2
3
4
5
6
7
8
9
PN427
PN427
PN427
PN427
PN427
PN427
PN427
PN427
PN427
PN427
A
A
A
A
A
A
A
A
A
A
10-C
10-C
10-C
10-C
10-C
10-C
10-C
10-C
10-C
10-C
0
75
75
75
75
75
75
75
75
75
75
75
100
100
100
82
76
88
99
76
85
99
78
86
98
8
95
99
99
99
99
99
99
99
99
99
86
90
91
66
99 (R)
99 (R)
99 (R)
99 (R)
99 (R)
99 (R)
99 (R)
99 (R)
99 (R)
99 (R)
83 (R)
97 (R)
97 (R)
82 (R)
10
20
30
40
50
60
70
80
90
—
0
10
11
12
13
14
Neat complex
PN427
PN427
A10-C
10-C
54
90
35
A
60
—
Neat complex
15
16
17
PN427
PN427
Neat complex
A
A
10-C
10-C
0
60
—
75
75
75
90
94
23
92
92
78
94 (R)
97 (R)
70 (R)
18
19
20
PN427
PN427
Neat complex
A
A
10-C
10-C
0
60
—
105
105
105
75
91
43
98
99
91
99 (R)
99 (R)
76 (R)
a
2 2 2
Catalyst (0.1 mol% of substrate, based on titanium content), substrate (1.0 mmol), H O (30 wt%, 1.2 mmol, slowly added within 15 min), H O
d
ꢀ
b
c
(2.0 mL), 25 C. Determined by GC. Chemoselectivity to sulfoxide (determined by GC). Determined by HPLC (Daicel Chiralpak AD column).
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Various activities should be related to the photo-induced despite also homogeneous system, traditional neat complex in
change in morphology of the metallomicelles. During the irra- ethanol was far less efficient than PN427A10-C under identical
diation of UV light, the vesicles underwent photo-provoked conditions (Fig. 7e vs. d). Extremely low kobs value was observed
disruption, fusion, disintegration and rearrangement in water. over the traditional neat complex (Fig. 7B(e)). It was reasonable
Relatively integrated double-layer membrane structure of the that traditional neat complex without azobenzene moiety could
vesicles was observed when PN427
for 60 or 90 s, which was favourable for the metallomicellar catalytic sites and organic substrate. Low local concentration of
catalysis. The UV-responsive catalytic efficiency of PN427 10-C catalytic species and suldes were insufficient for the efficient
made controlled catalysis of asymmetric sulfoxidation in water sulfoxidation. The results were consistent with our hypothesis
come true. that the high local concentrations of catalytic species and
A10-C was treated by UV light not gather together in ethanol system to achieve concentrated
A
Kinetics was used to further investigate the advantages of the substrate were crucial for this system to be active, and this was
metallomicellar catalysis approach in aqueous reaction, as well only achieved in the metallomicellar system in water. Notably,
as the photo-controlled catalytic performance of PN427
based metallomicelle. Corresponding kinetic curves and rate varied with the UV irradiation time, and highest efficiency was
curves of asymmetric sulfoxidation of methyl phenyl sulde observed when PN427 10-C was treated with UV light for 60 s
over PN427 10-C in water (aer 60 s of UV irradiation, aer 40 s (Fig. 7a). The observations did agree with the ability to photo-
A10-C- despite all metallomicellar catalysis, the kobs over PN427A10-C
A
A
of UV irradiation, and at dark-adapted state) were shown in modulate the activity of PN427A10-C in asymmetric sulfox-
Fig. 7. Obviously, gradient increase in conversion of sulde was idation in water.
observed over PN427A10-C either at dark-adapted state or upon
UV treatment, when asymmetric sulfoxidations were performed also noticeable for other aryl methyl suldes, such as ethyl
in water (Fig. 7A(a–c)). The corresponding rate constant (kobs phenyl sulde, methyl p-methoxyphenyl sulde, and methyl o-
initially rose rapidly due to the increasing concentration of methoxyphenyl sulde. PN427 10-C aer 60 s of UV irradiation
substrate in hydrophobic microenvironment, went through always gave high activity than the dark-adapted PN427 10-C in
a maximum, and then drastically decreased due to a dilution corresponding sulfoxidations (Table 1, entry 13 vs. 12, entry 16
The UV-responsive catalytic efficiency over PN427A10-C was
)
A
A
24
effect (Fig. 7B(a–c)). The featured kinetics demonstrated vs. 15, entry 19 vs. 18). All the aryl methyl suldes get quanti-
conned catalysis of asymmetric sulfoxidation in water over tatively oxidized to the corresponding sulfoxides with excellent
PN427A10-C. To evaluate the advantages of metallomicelle, we chemo- (91–99%) and enantioselectivity (97–99%) within
carried out the reaction with dark-adapted PN427A10-C in 105 min in water with the employment of UV-treated PN427A10-C
ethanol, a solvent in which PN427
be well dissolved. As expected, at the same catalyst concentra- chiral induction to (R)-sulde (>99%) was observed for the
tion, dark-adapted PN427 10-C in ethanol was less efficient than methyl o-methoxyphenyl sulde over the PN427 10-C aer 60 s of
that in water, although the ethanol-based system was homo- UV irradiation (Table 1, entry 16). The results suggested the
geneous (Fig. 7d vs. c). Logically, in ethanol, PN427 10-C may universality of PN427 10-C in green asymmetric sulfoxidation.
2 2
A10-C, sulde and H O could (Table 1, entries 13, 16, and 19). Especially, almost quantitative
A
A
A
A
gather together just via p–p stacking of azobenzene species Traditional neat complex was also inactive in the aqueous sul-
from neighbouring catalyst, rather than packing to form the foxidations systems due to water-incompatibility (Table 1,
conned hydrophobic pocket. It thus couldn't sequester entries 14, 17, and 20).
organic substrate from surrounding environment through
Apart from photo-responsive catalytic activity, PN427A10-C
hydrophobic effect. Low local concentration of substrate in the also exhibited the reversibility of thermal-driven water-
ethanol system resulted in the unsatised activity. Interestingly,
Fig. 7 Kinetic curves (A) and rate curves (B) of asymmetric sulfox-
idation of methyl phenyl sulfide over PN427D10-C in water (after 60 s of
UV irradiation (a), after 40 s of UV irradiation (b), and at dark-adapted Fig. 8 Reuse of PN427
A
10-C after 60 s of UV treatment (A) or in dark-
state (c)), over dark-adapted PN427
in ethanol.
D
10-C (d) and over neat complex (e) adapted state (B) in asymmetric sulfoxidation of methyl phenyl sulfide
ꢀ
in water using aq. H
2
O
2
as an oxidant at 25 C.
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solubility switching, as shown in Fig. 1. The salient features Methods
allowed for the facile recovery of PN427A10-C from the aqueous
UV-vis spectroscopy was carried out on a UV-vis Agilent 8453
spectrophotometer. Molecular weight of the synthesized
copolymers were obtained by gel permeation chromatography
phase by thermo-controlled separation. Especially, the LCST
close to room temperature made the recovery of catalyst for
reuse under mild local temperature (at 32 C). Fig. 8 showed the
reusability of PN427A10-C aer 60 s of UV treatment or in dark-
adapted state in asymmetric oxidation of methyl phenyl
sulde in water.
To our delight, the catalyst either at dark-adapted state or
upon UV treatment could be reused up to seven times without
signicant loss in activity and selectivity. Leaching test of
catalyst revealed negligible leaching loss of titanium species
ꢀ
(GPC) on an Alltech Instrument (Alltech, America) equipped
with an Alltech ELSD 800 detector. NMR spectrum of samples
was recorded on a BRUKER AVANCE-500 spectrometer with
TMS as an internal standard. The measurements were repeated
for at least three times to ensure good reproducibility. The
critical micellization concentration (CMC) was investigated
using the surface tension method. The surface tension of an
aqueous solution of catalyst was measured as a function of
(
less than 0.1 ppm by ICP) to reaction medium during the
oxidation. Chemical analysis of the recovered PN427 10-C gave
the titanium content (0.0172 mmol g ) almost identical to that
ꢁ3
ꢁ1
catalyst concentration (10 ꢂ 10 to 1.0 mmol L ) on a Kr u¨ ss
A
ꢀ
K12 tensiometer using the Wilhelmy plate method at 25 C. The
ꢁ1
surface tension vs. catalyst concentration plot gave information
on the CMC. Morphologies of the self-assembled aggregates
were observed by TEM on a Microscope JEM-2100F at an
accelerating voltage of 200 kV. Samples were prepared by
ꢁ
1
of the fresh one (0.018 mmol g ). Furthermore, oxidative
decomposition of PN427 10-C, a main reason for the deactiva-
A
IV
25
tion of chiral salen Ti catalyst in H O -based oxidation, also
2
2
did not occur during the reaction. The excellent stability of
ꢁ1
depositing aqueous solution (0.5 mg mL ) onto a carbon-
IV
catalyst should arise from the shielding of the chiral salen Ti
coated copper grid, followed by removal of excess solution by
blotting the grid with lter paper. The samples were dried for
complex in a hydrophobic pocket, which protected the catalytic
site from oxidative decomposition by excluding excess H
from the hydrophobic compartment.
2 2
O
72 h at room temperature in a desiccator containing dried silica
gel. Aer that, the samples were negatively stained by phos-
photungstic acid and dried for another 72 h before examina-
tion. Dynamic light scattering (DLS) was performed using
a MS2000 Laser Particle Size Analyzer (Malvern, UK). The
sample solutions for measurements were prepared for concen-
Conclusions
In conclusion, controllable metallomicellar catalysis of aqueous
asymmetric sulfoxidation has been realized by using photo-
responsive self-assembled metallomicelle which contained
ꢁ1
trations of 0.5 mg mL followed by ltering through a 0.45 lm
disposable polyamide (PA) membrane to free it from dust
particles. Light transmittance was xed at 633 nm with the
IV
azobenzene and chiral salen Ti complexes in the hydrophobic
ꢀ
scattering angle of 90 . The mean diameters of self-assemblies
compartment. Catalytic activity of the metallomicelle could be
precisely controlled by UV irradiation time, due to the UV-
induced change in morphology which adjusted catalyst
concentration and/or accessibility in real-time. Furthermore,
the catalysts could be facilely recovered from the aqueous
system for efficient reuse by adjusting the local temperature.
The photo-controlled catalytic efficiency, as well as outstanding
reusability, made the metallomicelle highly promising for
controllable catalysis of various organic reactions in water. And
also, the responsive self-assembly approach inspired
researchers to develop other controllable metallomicellar
catalysis system for a much wider range of application.
were obtained from the number distribution curves produced
by the particle analyzer. The titanium content in samples was
determined by inductively coupled plasma mass spectrometry
(ICP-MS) on a NexION 300X analyzer (Perkin-Elmer Corp.).
Optical rotation of samples was measured in dichloromethane
on
a
WZZ-2A Automatic Polarimeter. The dark-adapted
PN427
A
10-C was obtained by the irradiation of visible light (l ¼
410 nm) for 30 min, and then being kept in the dark for 72 h to
ensure that all the azobenzene chromophores were in trans-
state.
Preparation of PN427A -C
10
Synthesis
of
N-azo-acrylamide.
Aminoazobenzene
(
3.0 mmol, 0.592 g), triethylamine (3.0 mmol, 0.303 g) and
Experimental section
Materials and reagents
benzenediol (0.1 mmol, 0.011 g) were dissolved in 10 mL mixed
solvents of acetonitrile and dichloromethane (1 : 1, v/v) at room
Tetra-isopropyl titanate, aminoazobenzene and methyl aryl temperature. Acryloyl chloride (4.5 mmol, 0.410 g) in acetoni-
suldes were obtained by J&K. N,N-Azobis(isobutyronitrile) trile (5 mL) was then dropwise added into the solution. The
ꢀ
(
AIBN), NIPAAm, 2-aminoethanethiol hydrochloride, and L(+)- mixture was stirred at 55 C for 4 h, and the amidation progress
tartaric acid were purchased from Acros. 2-tert-Butyl phenol was was monitored by thin-layer chromatography. Aer cooling to
purchased from Alfa Aesar. Other commercially available room temperature, the reaction mixture was ltered through
chemicals were laboratory grade reagents from local suppliers. a 0.2 mm PTFE lter. The orange residue was washed with water
All solvents were puried by standard procedures. NIPAAm was and puried by recrystallization in ethanol to get N-azo-
ꢁ
1
puried by recrystallization from n-hexane and dried in vacuo acrylamide. FT-IR (KBr): gmax/cm
3280, 3199, 3134, 3068,
1
before use.
1670, 1600, 1560. H NMR (CDCl , 500 MHz) d (ppm): 7.94–7.89
3
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2
0
ꢁ1
(
3
s, 4H, N]N–Ph–H), 7.76 (s, 2H, O]C–NH–Ph–H), 7.52–7.44 (s, 1.03 (s, 2562H, CH
3
–CH–CH
3
). [a]
D
¼ ꢁ16.0 (C ¼ 0.005 g mL
ꢁ
1
H, aromatic), 6.50–5.81 (s, 3H, O]C–CH]CH2).
in CH Cl ). Titanium content: 0.018 mmol g . The LCST of
2 2
Synthesis of PN427A10. Monomers of NIPAAm (24.5 mmol, PN427A -C is 26 C based on transmittance measurement.
ꢀ
10
2.774 g) and N-azo-acrylamide (0.49 mmol, 0.123 g) were dis-
solved in THF in a Schlenk tube. AIBN (0.125 mmol, 0.021 g)
and 2-aminoethanethiol hydrochloride (0.25 mmol, 0.028 g)
Catalyst testing
were then added into the solution. The reaction mixture was Before reaction, PN427A -C was irradiated by visible light (l ¼
10
degassed by bubbling with nitrogen gas at room temperature 410 nm) for 30 min, and then kept in the dark for 72 h to ensure
ꢀ
for 30 min. Polymerization was carried out at 60 C for 24 h with that all the azobenzene chromophores were in trans-state. The
nitrogen protection. Aer being cooled to room temperature, dark-adapted PN427A10-C (0.1 mol% substrate, based on the
the mixture was treated with solid KOH to liberate the terminal titanium content in catalyst) and methyl aryl suldes
ꢀ
amino group. The obtained solution was concentrated under (1.0 mmol) were added to H O (2 mL) under stirring at 25 C.
2
vacuum. Crude product was puried by repeated precipitating The mixture was exposed to UV light (l ¼ 365 nm) for a certain
from diethyl ether and followed from THF to remove unreacted time, and then wrapped with aluminum foil. H O (30 wt%,
2
2
ꢀ
monomers. Aer being dried under vacuum for 6 h at 40 C, the 1.2 mmol) was dropwise added into the solution within 15 min.
copolymer of poly(NIPAAm-co-azo) was obtained as yellow The resulting mixture was stirred at room temperature until the
powder, which was denoted as PN427
A
10 (where the repeated reaction was judged to be complete based on GC analysis. Then,
ꢀ
units number of NIPAAm and azobenzene in copolymer was 427 the reaction mixture was heated to 32 C. Catalyst was precipi-
1
1
and 10, which were determined by H NMR spectrum). H NMR tated out from the reaction system completely, washed with
500 MHz, DMSO-d ) d (ppm): 7.84 (m, 40H, –N]N–Ph–H), diethyl ether (3 ꢂ 5 mL), dried in a vacuum, and nally
(
6
7.57–7.56 (m, 20H, O]C–NH–Ph–H), 7.52 (m, 30H, N]N–Ph– recharged with fresh substrate and oxidant for the next catalytic
H), 7.30–7.19 (m, 437H, O]C–NH–CH and O]C–NH–Ph), 3.83 cycle. The supernatants separated from reaction system were
(
m, 427H, CH
NH ), 2.63 (m, 2H, S–CH
NH ), 1.96 (m, 437H, CH
acrylamide), 1.44 (m, 874H, –CH –CH– in NIPAAm and N-azo- acetate, 1.5/1) afforded pure chiral sulfoxides. The products
3
–CH–CH
3
in NIPAAm), 2.93 (m, 2H, S–CH
2
–CH
2
–
extracted with dichloromethane thrice. Combined organic
phase was concentrated in vacuum. Further purication of the
2
2
–CH –NH ), 2.36 (m, 2H, S–CH
2
2
2
–CH
2
–
2
2
–CH– in NIPAAm and N-azo- residue by chromatography on silica gel (petroleum ether/ethyl
2
1
13
acrylamide), 1.03 (s, 2562H, CH –CH–CH ). The Mn of have been identied by H NMR and C NMR spectra. The
PN427A10 is about 30 825 g mol based on GPC.
3
3
ꢁ1
conversion and chemoselectivity of chiral sulfoxides were
Synthesis of PN427
A
10-C. The obtained PN427
A
10 (0.18 mmol, measured by a 6890N gas chromatograph (Agilent Co.) equip-
0
5
.54 g), (R,R)-N-(3,5-di-tert-butylsalicylidene)-N -(3-tert-butyl-5- ped with a capillary column (HP19091G-B213, 30 m ꢂ 0.32 mm
chloromethyl-salicylidene)-1,2-cyclohexanediamine (0.25 mmol, ꢂ 0.25 mm) and a FID detector. Ee values of corresponding
0
.137 g) and triethylamine (0.2 mmol, 0.02 g) were mixed in dry chiral sulfoxides were determined by HPLC analysis using the
toluene (30 mL) under room temperature. The mixture was Daicel Chiralpak AD columns. Detailed NMR spectra and HPLC
reuxed for 48 h under nitrogen protection. Aer removal of analysis for the chiral sulfoxides were available in ESI.†
solvent, the residue was dissolved in THF to remove the formed
Methyl phenyl sulfoxide. Chemoselectivity: 99% determined
triethylamine hydrochloride through ltration. Filtrate was by GC, nitrogen was used as the carrier gas with a ow of
i
ꢁ1
concentrated in vacuo, and was then treated with Ti(O Pr)
(
4
30 mL min , the injector temperature and the detector
ꢀ
0.25 mmol, 0.07 g) in dichloromethane (30 mL) for 12 h at temperature were 250 C, the column temperature was pro-
room temperature. The mixture was concentrated under grammed from 80 to 180 C with 6 C min , tmethyl phenyl sulfoxide
ꢀ
ꢀ
ꢁ1
¼
vacuum. Crude product was puried by repeatedly precipitating 6.9 min; ee value: 99% determined by HPLC (i-PrOH/n-hexane ¼
ꢁ1
ꢀ
from tetrahydrofuran using diethyl ether as precipitant. The 5 : 5 (v/v)); ow rate ¼ 1.0 mL min ; 25 C; l ¼ 254 nm; major
1
resulting orange solid was dissolved in chloroform (20 mL), and enantiomer t ¼ 4.4 min, minor enantiomer t ¼ 5.2 min; H
R
S
treated with water (2 mL) to remove any traces of TiO2 by NMR (CDCl , 500 MHz) d (ppm): 2.49 (s, 3H, Me), 7.27–7.29 (m,
3
1
3

ltration. Filtrate was concentrated in vacuum and further 3H, ArH), 7.42–7.44 (m, 2H, ArH); C NMR (CDCl , 125 MHz)
dried in vacuum at 40 C overnight, giving orange powder of d (ppm): 43.1 (SCH ), 122.8, 128.6, 130.3, 144.9 (ArC).
catalyst, denoted as
) d (ppm): d 9.97 (s, 2H, by GC, nitrogen was used as the carrier gas with a ow of
Ph–HC]N–), 7.90 (s, 2H, Ph–H in Ti(salen)), 7.84 (m, 42H, –N] 30 mL min , the injector temperature and the detector
3
ꢀ
3
IV
PN427
PN427
A
A
10-modied chiral salen Ti
Ethyl phenyl sulfoxide. Chemoselectivity: 91% determined
1
10-C. H NMR (500 MHz, DMSO-d
6
ꢁ1
ꢀ
ꢀ
N–Ph–H), 7.60–7.56 (m, 20H, O]C–NH–Ph–H), 7.52 (m, 30H, temperature were 250 C, the column temperature was 180 C,
N]N–Ph–H), 7.29–7.20 (m, 437H, O]C–NH–CH and O]C– tethyl phenyl sulfoxide ¼ 2.5 min; ee value: 97%, determined by
ꢁ1
NH–Ph), 3.83 (m, 427H, CH –CH–CH in NIPAAm), 3.17–3.16 HPLC (i-PrOH/n-hexane ¼ 2 : 8 (v/v)); ow rate ¼ 1.0 mL min
;
3
3
ꢀ
(
m, 2H, cyclohexyl-H), 2.92–2.90 (m, 2H, NH–CH –Ti(salen)), 25 C; l ¼ 254 nm; major enantiomer t ¼ 6.4 min and minor
2
R
1
2
2
.63 (m, 2H, S–CH
.17 (m, 2H, CH –CH–CH
–CH– in NIPAAm and N-azo-acrylamide), 1.44 (s, 874H, 2H, –CH –), 1.19–1.16 (m, 3H, Me); C NMR (CDCl , 125 MHz)
2
–CH
2
–NH), 2.36 (m, 2H, S–CH
2
–CH
2
–NH), enantiomer t ¼ 8.2 min; H NMR (CDCl , 500 MHz) d (ppm):
S
3
i
3
3
of PrO- in Ti(salen)), 1.95 (m, 437H, 7.59–7.53 (m, 2H, ArH), 7.52–7.47 (m, 3H, ArH), 2.92–2.72 (m,
1
3
CH
CH
cyclohexyl-H), 1.17 (m, 39H, –CH
2
2
3
–
2 3 2
–CH– in NIPAAm and N-azo-acrylamide), 1.22 (m, 8H, d (ppm): 5.91 (CH ), 50.24 (SCH ), 124.13, 129.09, 130.88, 143.25
i
3
of t-Bu and PrO- in Ti(salen)), (ArC).
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Methyl p-methoxyphenyl sulfoxide. Chemoselectivity: 92%
determined by GC, nitrogen was used as the carrier gas with
Chem. Soc. Rev., 2013, 42, 7391; (f) Y. Liu, T. Pauloehrl,
S. I. Presolski, L. Albertazzi, A. R. A. Palmans and
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A. F. Cardozo, C. Julcour, L. Barthe, J. F. Blanco, S. Chen,
F. Gayet and F. D'agosto, J. Catal., 2015, 324, 1.
ꢁ
1
a ow of 30 mL min , injector temperature and detector tem-
ꢀ
perature were 250 C, the column temperature was
ꢀ
ꢀ
ꢁ1
programmed from 80 to 180
C
with
6
C min ,
t
¼ 11.7 min; ee value: 97%, deter-
2 (a) B. M. Neilson and C. W. Bielawski, ACS Catal., 2013, 3,
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methyl p-methoxyphenyl sulfoxide
i
mined by HPLC ( PrOH/n-hexane ¼ 4 : 6 (v/v)); ow rate ¼
ꢁ
1
ꢀ
1
5
5
.0 mL min ; 25 C; l ¼ 254 nm; major enantiomer t ¼
R
1
.5 min and minor enantiomer t
S
¼ 6.6 min; H NMR (CDCl
3
,
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00 MHz) d (ppm): 7.60–7.57 (d, 2H, ArH), 7.03–7.01 (d, 2H,
1
3
ArH), 3.84 (s, 3H, OCH
1
1
3
), 2.69 (s, 3H, SCH
3 3
); C NMR (CDCl ,
25 MHz) d (ppm): 43.9 (SCH
62.0 (ArC).
Methyl o-methoxyphenyl sulfoxide. Chemoselectivity: 99%
3
), 55.5 (OCH ), 114.8, 125.4, 136.6,
3
ꢁ1
nitrogen was used as the carrier gas with a ow of 30 mL min
injector temperature and detector temperature were 250 C, the
column temperature was 180 C, tmethyl o-methoxyphenyl sulfoxide
9
,
ꢀ
ꢀ
¼
.8 min; ee value: 99% determined by HPLC (i-PrOH/n-hexane ¼
ꢁ1
ꢀ
5
: 5 (v/v)); ow rate ¼ 1.0 mL min ; 25 C; l ¼ 254 nm; major
1
enantiomer t
NMR (CDCl , 500 MHz) d (ppm): 7.82–7.80 (m, 1H, ArH), 7.46–
.42 (m, 1H, ArH), 7.19–7.16 (m, 1H, ArH), 6.92–6.90 (m, 1H,
R
¼ 4.8 min and minor enantiomer t
S
¼ 5.7 min; H
3
7
1
3
ArH), 3.87 (s, 3H, OCH ), 2.76 (s, 3H, SCH ); C NMR (CDCl3,
1
1
3
3
25 MHz) d (ppm): 41.16 (SCH ), 55.64 (OCH ), 110.53, 121.65,
3
3
24.58, 131.89, 133.07, 154.76 (ArC).
52, 11156; (c) Y. Shiraishi, K. Tanaka and T. Hirai, ACS
Sulfoxidation reaction for kinetic measurement
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IV
Chiral salen Ti catalyst (0.1 mol% of substrate, based on
titanium ion content) was stirred with methyl phenyl suldes
1.0 mmol) in solvent (2 mL) at 25 C. H
was then added into the stirred solution in one portion. To
determine the rate of sulfoxidation, aliquots at an interval of
0 min were drawn from the reaction mixture, ltrated through
ꢀ
(
2
O
2
(30 wt%, 1.2 mmol)
1
silica gel with ethyl acetate as an eluent and analyzed by GC.
6
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (21676078, 21476069), the Natural Science
Foundation of Hunan Province for Distinguished Young
Scholar (2016JJ1013), the Program for Excellent Talents in
Hunan Normal University (ET14103), and Hunan Provincial
Innovation Foundation For Postgraduate (CX2016B168).
7
8
9
2909.
J. E. Chung, M. Yokoyama, T. Aoyagi, Y. Sakurai and
T. Okano, J. Controlled Release, 1998, 53, 119.
M. Holbach, X. Zheng, C. Burd, C. W. Jones and M. Weck, J.
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