An ionic liquid-functionalized amphiphilic Janus material as a Pickering interfacial catalyst for asymmetric sulfoxidation in water

Article abstract of DOI:10.1039/C8CC08292H

Ionic liquid-functionalized amphiphilic Janus chiral salen Ti^IV catalysts were prepared by partial hydrophobic modification of silica with a chiral salen Ti^IV complex through an imidazolium ionic liquid (IL) linker. By optimizing their hydrophobic/hydrophilic balance, the IL-functionalized JNP materials exhibited excellent interfacial activity, significantly accelerating asymmetric sulfoxidation in water through the formation of stable Pickering emulsions. Moreover, catalyst recovery was readily achieved using centrifugation.

Full text of DOI:10.1039/C8CC08292H

Volume 55 Number 5 16 January 2019 Pages 559–718
ChemComm
Chemical Communications
rsc.li/chemcomm
ISSN 1359-7345
COMMUNICATION
Rong Tan et al.
An ionic liquid-functionalized amphiphilic Janus material
as a Pickering interfacial catalyst for asymmetric
sulfoxidation in water

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An ionic liquid-functionalized amphiphilic Janus
material as a Pickering interfacial catalyst for
asymmetric sulfoxidation in water†
Cite this: Chem. Commun., 2019,
5, 592
5
Received 17th October 2018,
Accepted 21st November 2018
Mingjie Zhang, Zhiyang Tang, Wenqin Fu, Weiying Wang, Rong Tan * and
Donghong Yin
DOI: 10.1039/c8cc08292h
rsc.li/chemcomm
IV
Ionic liquid-functionalized amphiphilic Janus chiral salen Ti cata- as colloid surfactants. If chiral catalysts are deposited on one
lysts were prepared by partial hydrophobic modification of silica side of the JNPs, they will stabilize emulsions and simulta-
IV
with a chiral salen Ti complex through an imidazolium ionic liquid neously catalyze asymmetric reactions at the oil–water interface,
7
(
IL) linker. By optimizing their hydrophobic/hydrophilic balance, making asymmetric catalysis highly efficient in water. These
IV
the IL-functionalized JNP materials exhibited excellent interfacial fascinating features inspired us to reason that chiral salen Ti
activity, significantly accelerating asymmetric sulfoxidation in water complex-containing amphiphilic JNPs could undergo PIC and
through the formation of stable Pickering emulsions. Moreover, allow for efficient asymmetric sulfoxidation in water. Herein,
IV
catalyst recovery was readily achieved using centrifugation.
we report the design of amphiphilic JNP-type chiral salen Ti
catalysts as Pickering interfacial catalysts for the aqueous asym-
Asymmetric catalytic reactions in water are of current interest, metric sulfoxidation. To achieve the amphiphilic surface, we
not only due to the environmentally benign process, but also modified the partial surface of silica nanoparticles with hydro-
because water often exerts a positive effect on chemical reac- phobic Ti(salen) units and left the rest free. An imidazolium IL
1
IV
tivity and selectivity. However, low catalytic efficiency is always was introduced as a flexible linker between the chiral salen Ti
present in these aqueous systems due to mass transport resis- complex and the SiO surface to ensure necessary conformation
2
2
tance resulting from limited organic/aqueous interfacial areas.
Surfactants are commonly used to enhance mass transfer.
freedom of active sites and diminish undesired steric effects of
the matrix on the accessibility of active sites. It was expected
3
8
Unfortunately, the systems often encounter difficulty in product that in aqueous systems, the amphiphilic JNP catalyst should
purification and catalyst recycling, which impedes the practical be located at the oil (sulfide)–water interface, forming stable
3
application of the technology. Constructing Pickering emul- Pickering emulsions to promote mass transfer and in turn the
sions by using solid catalysts, called Pickering interfacial cata- reactivity. A hydrophobic catalytic hemisphere should orient
lysis (PIC), has recently been demonstrated as an alternative itself to the oil phase at the interface, allowing for high local
4
strategy to improve the catalytic efficiency of aqueous systems.
concentration of reactants around the active sites. Moreover, the
Apart from maximizing oil/water interfacial surface area, the IL linker assembled at oil–water interfaces along with Ti(salen)
Pickering emulsion relying on solid particles allows for in situ units may further enhance the accessibility of the reaction
5
9
separation and recycling of catalysts. More importantly, effective interface for reactants due to its unique compatibility.
IV
separation of products from a reaction mixture is also achieved by
differences in the water–oil solubility.
The IL-functionalized amphiphilic Janus chiral salen Ti
catalysts were prepared based on a wax-in-water emulsion
5
7b,10
Amphiphilic JNPs that have two opposite wetting surfaces method
(Scheme 1). Common silica nanoparticles were
have recently received increasing concerns in PIC due to their prepared according to the modified St o¨ ber method, and then
1
1
6
excellent interfacial activity. Thermodynamically stable emul-
sions can be formed using JNPs, especially, spherical JNPs,
National & Local Joint Engineering Laboratory for New Petro-chemical Materials
and Fine Utilization of Resources, Key Laboratory of Chemical Biology and
Traditional Chinese Medicine Research (Ministry of Education), Hunan Normal
University, Changsha 410081, P. R. China. E-mail: yiyangtanrong@126.com
†
Electronic supplementary information (ESI) available: Synthetic details and
characterization of materials and the evaluation of their catalytic activity. See
DOI: 10.1039/c8cc08292h
2
Scheme 1 Schematic representation of HO-SiO -IL-Ti(salen) by the wax
emulsion method.
5
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2
hydroxylated in piranha solution to give SiO –OH. These
obtained SiO –OH nanoparticles were used to form a Pickering
2
emulsion in a wax/water solution at 75 1C with the assistance of
CTAB. In this case, the silica nanoparticles were located at the
wax-in-water emulsion interface, leading to the formation of
2
silica-covered wax droplets (wax@SiO –OH). After rapidly cooling
to room temperature, the nanoparticles were locked uniformly
on the surfaces of wax droplets. A hydrophobic modifier of
Fig. 2 WCA measurements of SiO
HO-SiO -IL-Ti(salen)0.07 (C), HO-SiO
-IL-Ti(salen) (F), and HO-SiO
2
–OH (A), HO-SiO
2
-IL-Ti(salen)0.05 (B),
2
2 2
-IL-Ti(salen)0.09 (D), HO-SiO -IL-
IL/Ti(salen)-containing organosiloxane ((C
2
H O)
3 3
Si-IL-Ti(salen)) Ti(salen)0.11 (E), SiO
2
2
-Ti(salen)0.07 (G).
was then synthesized through N-alkylation of N-[3-(triethoxysi-
lyl)propyl]-4,5-dihydroimidazole with the benzyl chloride group
IV with a hydrophilic surface possessed an apparent contact angle
of 221 (Fig. 2A). The WCA value increased upon hydrophobic
modification. When the content of Ti(salen) increased from
(
–CH Cl) at the 5-position of the asymmetric chiral salen Ti
2
complex. It was used to decorate the exposed faces of locked
SiO –OH through the silylation of alkoxy with the hydroxyl
groups (–OH) on the SiO –OH surface. The exposed surface size
of SiO –OH was controlled by the concentration of the CTAB
2
ꢀ
1
0
.05 to 0.11 mmol g , the WAC of the IL-functionalized JNPs
was controlled from 301 to 721 accordingly (Fig. 2B–E). Notably,
with similar Ti(salen) content, HO-SiO -Ti(salen)0.07 gave a WCA
value (661) higher than that of HO-SiO -IL-Ti(salen)0.07 (531)
Fig. 2G vs. 2C). This indicated that the imidazolium-IL moiety
2
2
2
surfactant, implying that the surface coverage of Ti(salen) on the
silica surface was modulated by changing the CTAB concen-
tration. Removal of the wax gave approximately spherical silica
JNPs of HO-SiO -IL-Ti(salen) (where x represents the titanium
2
(
tuned the surface wettability of amphiphilic JNPs in water. When
2
x
the SiO –OH surface was uniformly modified with IL/Ti(salen),
2
content in samples, x = 0.05, 0.07, 0.09, and 0.11 mmol/g) with a
diameter of ca. 40 nm (see TEM images in Fig. 1A and Fig. S2,
ESI†). Their representative structure is shown in Chart S1 (ESI†).
Modification of the IL-Ti(salen) moiety on the silica surface
resulted in an increase in their hydrodynamic diameter (Fig. S3,
the WCA was up to 821 (Fig. 2F). Notably, all WCA values were
smaller than 901, suggesting the preferred oil-in-water (o/w)
1
2
emulsions over the NPs. It may provide large interfacial surface
areas favorable for aqueous catalysis.
The interfacial activity of HO-SiO
2 x
-IL-Ti(salen) (x = 0.05,
2
ESI†). The more IL-Ti(salen) moiety on the SiO –OH surface, the
0.07, 0.09, 0.11) was then examined by testing their emulsifying
larger the hydrodynamic diameter (D ) became (Fig. S3, ESI†).
h
capacity in a methyl phenyl sulfide/H O system. Amphiphilic
2
To confirm the Janus characteristic of HO-SiO -IL-Ti(salen) ,
2
x
HO-SiO -IL-Ti(salen) gave a highly dispersed Pickering emul-
2
x
we further modified the hydroxyl side of typical HO-SiO -IL-
2
sion upon emulsification (Fig. 3Aa–Da). The average sizes of
these droplets varied with their degree of hydrophobization,
as evidenced by the optical microscopy image (Fig. 3A–D).
Ti(salen)0.07 with 3-aminopropyltriethoxysilane through silyla-
tion, and labelled the amino group with Pd nanoparticles (NPs).
The TEM image of the Pd-labelled sample showed that Pd NPs
indeed mainly adsorbed on one side of the silica JNPs (Fig. 1B).
2
HO-SiO -IL-Ti(salen)0.07 gave the smallest emulsion droplets
with a diameter of ca. 12 mm (Fig. 3B). Decreased droplet size
was accompanied by an increase in the interfacial contact area,
which was favorable for the mass transfer in PIC. The emul-
sions were stable for at least 24 h without any obvious change
in physical appearance (Fig. 3Ab–Db). Benefiting from the
2
An uniform counterpart of SiO -IL-Ti(salen) (Chart S1, ESI†),
whose surface was uniformly modified by IL/Ti(salen) units,
was prepared as the control catalyst. The D value (342 nm) was
h
larger than that of HO-SiO -IL-Ti(salen) (190–325 nm), as shown
2
x
in Fig. S3 (ESI†). Furthermore, an IL-free Janus counterpart of
HO-SiO -Ti(salen) (Chart S1) was also prepared according to the
procedure similar to that of HO-SiO -IL-Ti(salen). During the
process, IL-free organosiloxane of (C O) Si-NH -Ti(salen), which
2
2
2
H
5
3
2
was obtained by N-alkylation of 3-aminopropyltriethoxysilane with
IV
an asymmetric chiral salen Ti complex, was used to partially
modify the SiO –OH surface.
2
The content of IL-Ti(salen) directly affected the wettability of
HO-SiO
2
-IL-Ti(salen)
x
. We quantified their surface wettability
-OH particles
by the water contact angle (WCA). Pristine SiO
2
Fig. 3 Optical microscopy image of the sulfide-in-H
by HO-SiO -IL-Ti(salen)0.05 (A), HO-SiO -IL-Ti(salen)0.07 (B), HO-SiO
Ti(salen)0.09 (C), HO-SiO -IL-Ti(salen)0.11 (D), SiO -IL-Ti(salen) (E), and HO-
SiO -Ti(salen)0.07 (F) (scale bar: 500 mm). Chiral salen Ti catalyst (0.1 g) and
2
O emulsions stabilized
2
2
2
-IL-
2
2
IV
2
methyl phenyl sulfide (0.2 mL) were suspended in water (1 mL) and stirred
(10000 rpm) for 10 min, before the images were taken. The inset shows the
photos of the emulsions after standing for different times (a: 0 h; b: 24 h).
Fig. 1 TEM images of HO-SiO
2
-IL-Ti(salen)0.07 (A) and Pd NP-labelled
HO-SiO -IL-Ti(salen)0.07 (B) in methanol.
2
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IV
Janus characteristic, IL-free JNPs of HO-SiO
2
-Ti(salen)0.07 as Table 1 Results of asymmetric sulfoxidation over different chiral salen Ti
a
catalysts
emulsifiers also gave stable emulsions with a droplet diameter
of ca. 65 mm (Fig. 3F). Different from JNP-stabilized emulsions
which are thermodynamically stable, the Pickering emulsions Entry Catalyst
b
b
c
d
Conv. Sel. ee
Substrate (%)
TOF
1
ꢀ
(%) (%) (h )
stabilized with uniform SiO -IL-Ti(salen) particles are only
kinetically stable and thus can undergo destabilization via
coalescence (Fig. 3E). Stable Pickering emulsions together with
2 x
controllable interfacial area made HO-SiO -IL-Ti(salen) the
desirable Pickering interfacial catalyst for asymmetric sulfoxi-
dation in water.
As expected, the amphiphilic Janus catalysts significantly
accelerated asymmetric sulfoxidation in water through the for- 10
2
1
2
3
4
5
6
7
8
9
HO-SiO -IL-Ti(salen)
84
99
83
77
71
80
45
98
76
79
33
99
61
85
17
96
97
96
89
80
94
70
96
79
90
73
95
89
89
76
92 67.2
98 79.2
95 66.4
87 61.6
82 56.8
94 65.6
70 36
97 78.4
84 60.8
88 63.2
75 26.4
98 79.2
87 48.8
90 68.0
77 13.6
2
0.05
HO-SiO
HO-SiO
HO-SiO
2
2
2
-IL-Ti(salen)0.07
-IL-Ti(salen)0.09
-IL-Ti(salen)0.11
SiO
HO-SiO
Neat complex
HO-SiO -IL-Ti(salen)0.07
SiO -IL-Ti(salen)
HO-SiO -Ti(salen)0.07
Neat complex
HO-SiO -IL-Ti(salen)0.07
SiO -IL-Ti(salen)
HO-SiO -Ti(salen)0.07
Neat complex
2
-IL-Ti(salen)
2
-Ti(salen)0.07
2
2
2
1
1
1
1
2
3
mation of stable sulfide/water emulsion droplets, as shown in
Table 1 (Table 1, entries 1–4). In particular, up to 99% con-
version with 97% chemoselectivity and 98% ee of (R)-methyl 14
2
2
2
15
2
phenyl sulfoxide was achieved when HO-SiO -IL-Ti(salen)0.07
was used as a catalyst (Table 1, entry 2). Comparatively, a neat
complex (Chart S1, ESI†) which was insoluble in water gave only
1
1
6
7
HO-SiO
SiO -IL-Ti(salen)
HO-SiO -Ti(salen)0.07
Neat complex
2
-IL-Ti(salen)0.07
99
55
87
11
98
85
95
74
97 79.2
89 44
94 69.6
2
4
5% conversion with 70% chemoselectivity and 70% ee under 18
2
1
9
65
8.8
identical conditions (Table 1, entry 7). A great difference in
catalytic efficiency was due to the fact that Pickering emulsions
over HO-SiO
2
-IL-Ti(salen)
x
could provide high oil–water interfa- 20
HO-SiO
SiO -IL-Ti(salen)
HO-SiO -Ti(salen)0.07
Neat complex
HO-SiO -IL-Ti(salen)0.07
SiO -IL-Ti(salen)
HO-SiO -Ti(salen)0.07
Neat complex
HO-SiO -IL-Ti(salen)0.07
SiO -IL-Ti(salen)
HO-SiO -Ti(salen)0.07
Neat complex
2
-IL-Ti(salen)0.07
95
78
91
67
99
79
90
81
88 76.0
69 62.4
81 72.8
49 53.6
21
22
23
2
cial areas. Furthermore, substrates were effectively sequestered
from the surrounding environment during emulsification,
creating a highly concentrated environment for confined
2
24
2
90
78
84
47
69
63
65
31
86
81
83
78
90
83
89
80
97 72.0
90 62.4
92 67.2
71 37.6
95 55.2
68 50.4
85 52.0
38 24.8
4
a,13
catalysis.
Simultaneously, HO-SiO
2
-IL-Ti(salen)
x
orientated 25
2
26
27
28
2
their hydrophobic Ti(salen) hemisphere to the oil phase (sulfide),
ensuring sufficient interfacial contact between reactants and
2
IV
active species at the o/w interface. Dense chiral salen Ti
29
2
3
3
0
1
2
complexes confined in the hydrophobic domain may enforce
a cooperative reaction pathway resulting in an enhanced reac-
a
8
b,14
Catalyst (1.25 mol% of substrate, based on titanium content), sub-
(1.2 mmol, added in 15 min), H O (1.0 mL),
0 min, 25 1C. Determined by GC. Determined by HPLC (Daicel
tion rate and higher selectivity.
reaction space excluded excess aqueous H
mizing the undesired over-oxidation of sulfoxide to sulfone. For chiralpak AD column). Turnover frequency (TOF) is calculated by
Moreover, the hydrophobic
strate (1.0 mmol), H
6
2
O
2
2
2
O
2
, thereby mini-
b
c
d
ꢀ
1
[
product]/[catalyst] ꢁ time (h ).
this reason, it was not surprising that the homogeneous parti-
cles of SiO -IL-Ti(salen), which gave less stable emulsions,
2
afforded poor conversion of methyl phenyl sulfide (71%) with imidazolium-based IL linker played multiple roles in enhancing
lower chemoselectivity (80%) and ee value (82%) under iden- the catalytic performance. It located the chiral complexes away
tical conditions (Table 1, entry 5). Interestingly, although from the silica backbone, enabling all active sites to adapt their
2 x
HO-SiO -IL-Ti(salen) underwent similar PIC, their catalytic preferential conformation for favorable cooperative action. And
efficiency was strongly depended on the corresponding emul- it created a compatible IL environment on the hydrophobic
sion droplet sizes. The smaller emulsion droplets formed were hemisphere, making JNPs more compatible with organic
more favorable for the transformation (Table 1, entries 1–4). sulfides and aqueous H O , facilitating the mass transfer of the
2
2
9
This tendency was probably due to an increased interfacial reagents. In addition, the active sites of Ti(salen) also benefited
contact area, which was governed by emulsion droplet size. A from the local IL microenvironment, since the high polarity and
high interphase contact area compromised diffusion resistance ionic properties of the IL exerted synergistic effects in stabilizing
at the oil–water interface and led to an enhancement in the the formed active metallosalen intermediate in the asymmetric
8
b
catalysis efficiency.
sulfoxidation.
Benefiting from the interfacial activity, as well as the positive
-IL-Ti(salen)0.07
the higher efficiency of HO-SiO -IL-Ti(salen) should also be was also the most efficient in aqueous asymmetric oxidation of
In addition to the fascinating PIC approach, which favored
the compartmentalization and interfacial transfer of reactants, effect of the imidazolium-based IL linker, HO-SiO
2
2
x
related to the flexible IL linker. As expected, with a similar o/w other alkyl aryl sulfides, such as ethyl phenyl sulfide, butyl methyl
Pickering emulsion, the IL-free Janus counterpart of HO-SiO2- sulfide, hexyl phenyl sulfide, methyl p-methoxyphenyl sulfide,
Ti(salen)0.07 gave a moderate conversion of 80% of sulfide with methyl o-methoxyphenyl sulfide, and methyl p-bromophenyl
reduced selectivity (94% chemoselectivity and 94% ee) under sulfide (Table 1, entry 8 vs. entries 9–11, entry 12 vs. entries 13–15,
identical conditions (Table 1, entry 6 vs. 2). In fact, an entry 16 vs. entries 17–19, entry 20 vs. entries 21–23, entry 24 vs.
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asymmetric sulfoxidation in water: (i) an increased interfacial
contact area due to the formation of a stable emulsion, (ii)
enhanced interphase mass transfer due to sufficient interfacial
contact between reactants and active species, (iii) an enforced
cooperative effect due to the high local concentration of active
sites with conformational freedom, and (iv) simplified recovery
for stable reuse. Without any co-solvent, the catalysts showed
excellent catalytic activity and stability under PIC conditions.
The methodologies opened an avenue for the conception of
interfacial catalysts for industrially relevant reactions.
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
Fig. 4 Reuse of HO-SiO
2
-IL-Ti(salen)0.05 (A), HO-SiO
-IL-Ti(salen)0.11 (D) in the asym-
metric oxidation of methyl phenyl sulfide with H in water.
2
-IL-Ti(salen)0.07 (B),
2 2
HO-SiO -IL-Ti(salen)0.09 (C), and HO-SiO
2
O
2
(2016JJ1013), and the Key Laboratory of the Assembly and
Application of Organic Functional Molecules of Hunan Province.
entries 25–27, entry 28 vs. entries 29–31). All the sulfides
get almost quantitatively oxidized to the corresponding sulfoxides
with excellent chemo- (86–99%) and enantioselectivity (88–98%)
within 60 min in water although they were water-incompatible
Conflicts of interest
There are no conflicts to declare.
(Table 1, entries 2, 8, 12, 16, 20, and 24), except for methyl
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(Fig. S1c vs. S1c , ESI†). Their excellent stability should arise
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during emulsification, which protected the complex from
1
1
1
oxidative decomposition by H
2 2
O .
A
novel IL-functionalized amphiphilic JNP-containing
Ti(salen) complex has been synthesized, which formed stable
Pickering emulsions in a sulfide/water system. Benefiting from
the excellent interfacial activity and an unique IL linker, the
JNP catalysts exhibited the following distinct characteristics in
2015, 31, 8818.
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1
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Chem. Commun., 2019, 55, 592--595 | 595