A peroxotungstate-ionic liquid brush assembly: An efficient and reusable catalyst for selectively oxidizing sulfides with aqueous H2O 2 solution in neat water

Article abstract of DOI:10.1590/S0103-50532012005000012

An efficient and reusable heterogeneous catalytic assembly of peroxotungstate held in a ionic liquid (IL) brush was synthesized and an environmentally-friendly procedure was developed for selective oxidation of sulfides at room temperature using 30 wt.percent hydrogen peroxide as the terminal oxidant and water as a sole solvent. No organic co-solvent or other additive was needed. A 1.5-2.0 molpercent (based on W atom) loading catalyst was found to be sufficient for a smooth and clean reaction. Both aliphatic and aromatic sulfides were efficiently and selectively transformed into their respective sulfoxides or sulfones by simply controlling of equivalents of hydrogen peroxide. In addition to the high catalytic activity, the catalyst exhibits excellent chemoselectivity. Sensitive functional groups, such as double bond and hydroxyl, remained under the oxidation conditions the reaction even with an excess hydrogen peroxide. The catalyst was easily recovered (via simple filtration) and reused at least eight times without a noticeable loss of activity. ?2012 Sociedade Brasileira de Qui?mica.

Full text of DOI:10.1590/S0103-50532012005000012

J. Braz. Chem. Soc., Vol. 23, No. 8, 1536-1542, 2012.
Printed in Brazil - ©2012 Sociedade Brasileira de Química
0
103 - 5053 $6.00+0.00
Article
A
A Peroxotungstate-Ionic Liquid Brush Assembly: an Efficient and Reusable Catalyst for
Selectively Oxidizing Sulfides with Aqueous H O Solution in Neat Water
2
2
Xianying Shi,* Wenjuan Ma, Hui Ou, Xiaoyan Han, Congmin Lu, Yan Chen and Junfa Wei*
School of Chemistry and Chemical Engineering, Shaanxi Normal University and Key Laboratory for
Macromolecular Science of Shaanxi Province, Xi’an, 710062, P. R. China
Uma sistema catalítico heterogêneo de peroxotungstato realizado em líquido iônico (IL) tipo
escova foi sintetizado e aplicado na oxidação seletiva de sulfetos a temperatura ambiente e usando
peróxido de hidrogênio 30 wt.% como oxidante terminal e água como solvente. Nenhum cosolvente
orgânico ou outro aditivo foi necessário e a reação ocorre com uma quantidade de 1,5-2,0 mol%
(
baseado no W) de catalisador. Sulfetos alifáticos e aromáticos foram transformados de maneira
eficiente e seletiva em seus respectivos sulfóxidos ou sulfonas pelo controle dos equivalentes de
peróxido de hidrogênio. Grupos funcionais sensíveis à oxidação, tais como ligações duplas e
hidroxila, não foram afetados mesmo com excesso de peróxido de hidrogênio. O catalisador foi
recuperado após a reação via filtração e reutilizado pelo menos oito vezes sem perda significativa
de atividade.
An efficient and reusable heterogeneous catalytic assembly of peroxotungstate held in a ionic
liquid (IL) brush was synthesized and an environmentally-friendly procedure was developed
for selective oxidation of sulfides at room temperature using 30 wt.% hydrogen peroxide as the
terminal oxidant and water as a sole solvent. No organic co-solvent or other additive was needed.
A 1.5-2.0 mol% (based on W atom) loading catalyst was found to be sufficient for a smooth and
clean reaction. Both aliphatic and aromatic sulfides were efficiently and selectively transformed into
their respective sulfoxides or sulfones by simply controlling of equivalents of hydrogen peroxide.
In addition to the high catalytic activity, the catalyst exhibits excellent chemoselectivity. Sensitive
functional groups, such as double bond and hydroxyl, remained under the oxidation conditions
the reaction even with an excess hydrogen peroxide. The catalyst was easily recovered (via simple
filtration) and reused at least eight times without a noticeable loss of activity.
Keywords: green catalysis, ionic liquid brush, clean oxidation, hydrogen peroxide
Introduction
Tungsten based catalysts, especially peroxotungstates,
have been observed to be efficient catalysts using hydrogen
peroxide as the oxidant and heterogeneous peroxotungstate
catalysts are of significant industrial interest since they can
Sulfoxides and sulfones have interested organic
chemists for a long time because of their importance as
synthetic intermediates in C–C bond formation and various
20,21
be easily separated and recycled.
In 2008, our group
1-3
molecular rearrangements. The oxidation of sulfides is
a straightforward and frequently method used to obtain
sulfoxides and sulfones.Among various oxidants employed
reported the application of peroxotungstates immobilized
on mono-layer ionic liquid-modified silica in the selective
22
oxidation of sulfides. Although, these catalysts displayed
excellent activities, the reactions can only take place
well in the mixed solvent of CH OH and CH Cl , which
4
-14
in the reactions, hydrogen peroxide is an attractive and
environmentally-friendly oxidant owing to its low cost,
3
2
2
15-19
high availability and cleanliness.
peroxide alone exhibits a weak oxidative ability and a
catalytic activation is required to access its full potential.
However, hydrogen
can bring about environmental pollution. In view of
economic and environmental problems, the replacement of
environmentally-friendly reaction medium to the organic
solvent is important research topic to perform oxidation
reaction. Water as a medium for organic reactions has
attracted great attention due to its low cost and safety to
*
e-mail: shixy@snnu.edu.cn, weijf@snnu.edu.cn

Vol. 23, No. 8, 2012
Shi et al.
1537
2
3-26
industrial processes and to environment. Whereas water
is still not commonly used as sole solvent in oxidation
reaction because of its poor compatibility with organic
substrates.
oxidant. More importantly, as a supported bi-positively
charged organic salt, our bis-layer ionic liquid brush,
bearing two imidazolium cations on each of its side
chains, would tightly associate with the peroxotungstate
dianions via a strong electrostatic interaction. Therefore,
the loss of active components or the contamination of metal
ions in the product should be significantly minimized or
even eliminated. Herein, it is presented our studies on
a peroxotungstate-in-brush catalytic assembly for the
selective oxidation of sulfides to sulfoxides or sulfones in
neat water under mild conditions.
Our group has been involved in the development of
green catalysts and environmentally-friendly procedures to
organic transformations. Recently, it was synthesized a set
of silica-supported multilayer ionic liquids (ILs) bearing
27,28
two or more imidazolium ions on each of the side chains.
They were called “ionic liquid brushes” because of their
brush-like feature. In comparison with the monolayer ILs
previously reported, it was expected that our immobilized
multiple layer ILs should act as highly active catalysts
on the basis of the following characteristics: (i) the brush
extends the immobilized ILs to the upper spaces outside the
surface of the supports, offering a deeper 3D environment
that closely resembles the liquid-phase character of ionic
liquids for both hydrophilic and hydrophobic reactants,
Results and Discussion
The heterogeneous catalytic assembly was prepared
by immobilizing the active peroxotungstate onto inorganic
materials, SiO modified with the ionic liquid by ion
2
exchange (Scheme 1). The results of elemental analysis,
IR (infrared) spectra and SEM-EDAX (scanning electron
microscopy-energy dispersive X-ray spectroscopy)
studies of the catalyst and intermediates displayed that
(
ii) more flexibility of the longer branched chains for more
substrates and reagents to be accommodated and to access
the catalytic species, and (iii) more catalytic species can be
loaded on the unit surface by exchanging with anion of IL.
Our group proved their high efficiency and recyclability for
-
the ionic liquid anion PF was not completely exchanged
6
2
−
by the binuclear peroxotungstate anion [W O (O ) ] .
2
3
2 4
27
some organic reactions in neat water.
Consequently, the feature of the homogeneous reaction of
the ionic liquid was shifted to the heterogeneous catalyst.
Meanwhile, the grafted ionic liquid avoids the disadvantage
that a little loss in the homogeneous system would lead to
an enhancement in cost.
Encouraged by our previous research, it was developed
a catalytic assembly of dianion peroxotungstates
held in bis-layer ionic liquid brushes-modified silica
SiO -BisILs[W O (O ) ] (Figure 1). It was envisioned that
2
2
3
2 4
this catalytic assembly could work well as a green and
reusable catalyst in on-water reactions using H O as
The oxidation of methyl phenyl sulfide was selected as a
prototypical reaction for the assessment of suitable reaction
2
2
H O
2
2
S
R
R
R'
R'
SiO2
O
S
O
O
or
S
R
R'
H O
2
=
N
N
-
1
2-
=
PF , / [W O (O ) ]
6
2
2
3
2 4
Figure 1. Peroxotungstate held in bislayer ionic liquid brushes and its role in the reaction.

1
538
A Peroxotungstate-Ionic Liquid Brush Assembly: an Efficient and Reusable Catalyst
J. Braz. Chem. Soc.
Cl-
OEt
Si
Cl-
O
N
N
C H
8
17
O
N
N
OH
Ionic liquid brushes SiO -BisILs[Cl]
2
PF -
OEt
Si
PF -
6
6
O
KPF rt,
6,
N
N
C H
8
17
24 h, CH CN
O
N
N
3
OH
-
SiO -BisILs[PF ]
2
6
OEt
Si
2-
[
W O (O ) ]
K [W O (O ) ]
O
2
3
2 4
2
2
3
2 4
N
N
C H
8 17
O
OH
N
N
H O
2
SiO -BisILs[W O (O ) ]
2
2
3
2 4
Scheme 1. Preparation of peroxotungstate-ionic liquid brush assembly.
conditions and the examination of the catalytic activity in
neat water.All reactions were carried out by simply stirring
a mixture of the catalyst, methyl phenyl sulfide, and H O in
Moreover, the optimum molar ratio of sulfide and H O is
2 2
1:1.2.Although the conversion of the sulfide improves with
the increasing amount of the H O , the yield of the sulfoxide
2
2
2
2
neat water at room temperature, using different amounts of
H O and catalyst. No organic co-solvents or additives were
decreases owing to a reduced selectivity for the sulfoxide.
To explore the scope of our protocol for the selective
oxidation of sulfides to sulfoxides, various functionalized
sulfides were employed under the optimized conditions
using the SiO -BisILs[W O (O ) ] and H O system. The
2
2
employed. The results are listed in Table 1 (entries 1-9).
It can be seen from Table 1 that the brush catalyst is
efficient for the oxidation of methyl phenyl sulfide with
H O in neat water. In sharp contrast, the control experiment
2
2
3
2
4
2
2
results are summarized in Table 1 (entries 10-18). In
most cases, aryl-alkyl, aryl-aryl, alkyl-alkyl sulfides were
successfully oxidized to the corresponding sulfoxides in
excellent yield and high selectivity. Only trace levels of
their over-oxidation sulfone products were detected by GC.
Similarly, the catalyst provided excellent chemoselectivity
towards sulfides containing oxidation sensitive functional
groups. More notably, allylic sulfides gave sulfoxides
selectively in high yield with short reaction times, leaving
the double bonds unaffected (entries 11, 12). This is
important, since tungsten-based catalysts are well known
to promote epoxidation reactions or carbon-carbon double
2
2
(
with no catalyst) led to a very low yield of 3.2% (entry 1).
This demonstrated that the catalyst plays a critical role
in the reaction. Meanwhile, using SiO -IL[Cl] and
2
-
SiO -IL[PF ] as catalyst directly, the poor yields of methyl
2
6
phenyl sulfoxide were obtained, which displayed that
peroxotungstate is the effective catalytic center to active
hydrogen peroxide. Moreover, the catalyst prepared by the
chloride anion directly exchanged with peroxotungstate
also gave lower yield of product, which showed that to
synthesize high efficiency catalyst, the exchangement
-
-
between Cl and PF6 is important and necessary. A
catalyst:sulfide molar ratio of 1.5:100 is recommended.
If the ratio of the catalyst is reduced to 1.0 mol%, levels
of oxidation are significantly reduced (entry 2). A catalyst
ratio of 2 mol% also led to unsatisfactory yield (entry 5).
These results may be rationalized by the over-oxidation
of sulfoxide. In fact, the minimal sulfone was detected by
GC (gas chromatography) as the over-oxidation product.
2
9-33
bond cleavage hydrogen peroxide based processes.
Hydroxyl groups also remained intact under these
conditions (entry 16) and the corresponding sulfoxide was
obtained in 93.9% of yield. The reaction times required
for sulfoxide formation ranged from 1.5 to 4 h for all
substrates except for diphenyl sulfide, which took 7 h and
led to the lowest yield (entry 14). This may be due to the

Vol. 23, No. 8, 2012
Shi et al.
1539
a
Table 1. The brush catalyzed on-water oxidation of sulfides to sulfoxides with H O2
2
O
S
Cat., H O
2
S
R1
R2
+ H O2
R1
R2
2
1
2
b
c
d
entry
R
R
catalyst (eq.)
NO
H O (eq.)
time / h
Conversion / %
Yield / %
3
2
2
1
2
3
4
5
6
7
8
C H
Me
1.2
1.2
1.2
1.2
1.2
1.1
1.2
1.3
1.4
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
3
3
4
6
5
0.010
0.013
0.018
0.020
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
0.015
88
85
88
91
88
90
93
90
85
9
3
92
3
94
3
99
3
91
3
100
100
100
10
3
9
3
e
10
11
12
13
14
15
16
17
18
19
20
3
f
3
14
14
55
84
87
91
92
71
94
94
76
g
3
69
87
n-butyl
allyl
n-butyl
allyl
Me
3
1.5
2
100
93
allyl
p-NO C H
Me
4
92
2
6
4
C H
C H
7
76
6
5
6
5
ClCH CH
Et
2
96
2
2
HOCH CH
Et
4
99
2
2
p-BrC H
Me
3.5
84
6
4
2
1
Me
0.015
1.2
3.5
93
90
O
a
b
c
d
Conditions: sulfides(5 mmol), H O (5 mL), were stirred at room temperature; based on the content of tungsten; determined by GC; isolated yields(average
2
e
f
-
g
in two runs); use SiO -BisILs[Cl] as catalyst; use SiO -BisILs[PF ] as catalyst; the catalyst was prepared by the chloride anion exchanged directly with
2
2
6
peroxotungstate.
steric influence of phenyl ring. Moreover, the presence
of electron withdrawing substituents on the phenyl ring
required slightly longer reaction time compared with
methyl phenyl sulfoxide (entry 13).
afforded diphenyl sulfone in 94.3% yield with a prolonged
reaction time of 18 h (entry 13). Similar to the oxidation
of sulfides to sulfoxides, functional group tolerance was
excellent, even when hydrogen peroxide was used in
great excess. Substrates containing sensitive functional
groups such as alkenes and alcohols (entries 10, 11 and
16) were selectively oxidized at the sulfur atom. These
results further exemplify the catalytic oxidation system
excellent chemoselectivity.
Convenient recyclability and a prolonged retention of
catalytic activity are two very important features for the
practical and efficient application of catalytic processes.
The reusability of our catalyst was established by carrying
out the repeated oxidation of methyl phenyl sulfide, with
each successive catalyst recovery, under the optimum
conditions. It is particularly worthy to point out that the
By simply increasing the amount of H O , the sulfides
2
2
can be selectively oxidized to sulfones. Methyl phenyl
sulfide was again chosen as model substrate for reaction
optimization (Table 2, entries 1-8). The results show that
the most suitable molar ratio of the catalyst, sulfide and
H O is 1.5:100:280 (entry 5). To explore the scope and
2
2
the limitations of our catalyst, the oxidation of a variety
of sulfides was investigated with the optimized reaction
conditions (Table 2, entries 9-18). The catalyst works well
for all substrates. All reactions resulted in 100% sulfide
conversion and all sulfone yields exceeded 85%. Even
with the highly hindered diphenyl sulfide, the oxidation

1
540
A Peroxotungstate-Ionic Liquid Brush Assembly: an Efficient and Reusable Catalyst
J. Braz. Chem. Soc.
a
Table 2. The brush catalyzed on-water oxidation of sulfides to sulfones with H O2
2
O
O
Cat., H O
2
S
S
R1
R2
+ H O2
R1
R2
2
1
2
b
c
entry
R
R
Catalyst (eq.)
H O (eq.)
time / h
Yield / %
2
2
1
2
3
4
5
6
7
8
9
C H
Me
0.02
0.02
0.02
0.02
0.02
0.02
0.15
0.25
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
0.02
2.0
9
9
58
63
64
70
98
96
89
97
98
100
99
99
94
100
86
94
93
6
5
2.2
2.4
2.6
2.8
3.0
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
2.8
9
9
9
9
9
9
n-butyl
allyl
n-butyl
allyl
Me
9
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
9
allyl
9
p-NO C H
Me
11
18
7
2
6
4
C H
C H
6
5
6
5
Et
Me
ClCH CH
Et
Et
13
13.5
24
2
2
HOCH CH
2
2
p-BrC H
Me
6
4
1
8
Me
0.02
2.8
16
95
O
a
b
c
Condition: sulfides (5 mmol), H O (5 mL) were stirred at room temperature; based on the content of tungsten; isolated yields (average in two runs).
2
a
Table 3. The recycling of catalyst in the selective oxidation of methyl phenyl sulfide
Cycle
1
2
3
4
5
6
7
8
b
Yield / %
93
92
91
92
92
91
90
89
a
Condition: methyl phenyl sulfide (5 mmol), H O (6 mmol), H O (5 mL) and catalyst (0.075 mmol, based on the content of tungsten) were stirred at room
2
2
2
b
temperature for 3 h; isolated yields (average in two runs).
use of water as solvent allows easy recycling of the catalyst
since the catalyst can be recovered by a simple filtration.
After the first use, the catalyst was filtered, washed with
EtOAc and dried under vacuum before reuse. The recovered
catalyst is then applied in the next cycle. Eight consecutive
preparations of the methyl phenyl sulfoxide led to no
significant loss in yields (Table 3), meaning the catalytic
activity of the catalyst did not significantly change, as
compared with fresh catalyst.
significance in the synthesis of sulfoxide or sulfone drugs,
in which the content of heavy metal is strictly limited.
Conclusions
In summary, a “real” on-water procedure and an
environmentally benign catalyst for the synthesis of
sulfoxides and sulfones, with a 30 wt.% aqueous H O
2
2
solution at room temperature, was developed. It avoids the
use of any organic co-solvent or addictive. Sulfides were
easily and selectively converted to sulfoxides or sulfones
in high yield by simply controlling the equivalents of
hydrogen peroxide. The catalyst combines the advantage
of ionic liquids and peroxotungstate, and differentiates
As expected, no significant leaching of metal was
observed. ICP-MS (inductively coupled plasma mass
spectrometry) analysis showed that after filtration of the
−
1
catalyst the reaction stream contained only 0.50 mg L of
W. This result makes this catalytic system possess great

Vol. 23, No. 8, 2012
Shi et al.
1541
itself from other peroxotungstate-based catalysts with its
high efficiency, simple isolation of product and catalyst,
outstanding recyclability and organic solvent-free
conditions. Our group believes that this oxidation system
will find applications in both academic and industrial
laboratories.
ν(O-O), 731 ν(W O), 560 ν(W(O )); specific surface area
2
-1
2
2
(SBET) of 103 m g . The loading amount of tungsten was
-1
0.334 mmol g .
General procedure for the selective oxidation of sulfides
to sulfoxides
Experimental
In a typical experimental procedure, to a 25 mL flask, the
mixture of catalyst (1.5 mol% with respect to substrate) and
5 mL water was stirred at room temperature for about
General
1
0 min. Then, sulfide (5 mmol) and 30 wt.% aq. H O
2 2
Unless otherwise stated, all reagents were purchased
from commercial sources and used without purification.
Reaction progress was monitored by TLC (thin layer
chromatography) using Merck silica gel 60 F-254
with detection by UV or concentrated sulfuric acid
as chromogenic reagent. Flash chromatography was
performed with freshly distilled solvents. Columns
were typically packed as slurry and equilibrated with
the appropriate solvent system prior to use. ICP-MS
(6 mmol) were successively added. The resulting mixture
was electromagnetically stirred at room temperature and
monitored by TLC.After the completion of the reaction, the
catalyst was separated by filtration under reduced pressure,
washed with EtOAc (3 × 5 mL), and dried in vacuum.
The combined organic layer was dried with anhydrous
MgSO , and concentrated under reduced pressure to give
4
the crude product which was purified by silica gel column
chromatography using petroleum ether-EtOAc as eluent.
1
1
was measured by X series Π mass spectrometer. H and
The products were confirmed by H NMR.
1
3
C MNR (nuclear magnetic resonance) spectra were
recorded on a Bruker AVANCE300 MHz spectrometer
tetramethylsilane (TMS) as an internal standard). GC
General procedure for the selective oxidation of sulfides
to sulfones
(
analysis was performed on an Agilent 6890N using a
capillary column (HP-5, 30 m × 0.25 mm × 0.53 µm) with
a flame ionization detector (FID).
The methods of oxidizing sulfides to sulfones were
similar to that of sulfoxides except that the ratio of
catalyst, H O , and sulfide was changed from 1.5:120:100
2
2
1
Synthesis of catalyst
to 2:280:100. The products were confirmed by H NMR.
The ionic liquid brushes SiO -BisILs[Cl] was
Reusability of the catalyst
2
synthesized via a 3-step route according to the procedure
2
7
that our group previously reported. The exchange between
The selective oxidation of methyl phenyl sulfide to
sulfoxide as a model reaction was carried out under the
optimum conditions to examine the recyclability of the
catalyst.After completion of the reaction in the first run, the
catalyst was separated by filtration under reduced pressure,
washed with EtOAc (3 × 5 mL).After complete drying, the
catalyst was charged again into the vessel containing water,
the post-processing was the same as above and the reaction
was performed under the same conditions.
−
−
Cl and PF was performed by the direct reaction of the
SiO -BisILs[Cl] and KPF in CH CN at room temperature
for 24 h. After the reaction, the obtained material was
washed consecutively with water and acetone, then, dried
under vacuum at 70 °C over night. The bis-layered ionic
liquid modified SiO was obtained. Elemental analysis:
found C 12.48%, H 1.80%, N 2.96%; IR (KBr disk)
νmax/cm 3162, 2938 and 2860 ν(C-H), 1636cm , ν(C=N),
32 ν(PF ); specific surface area (S ) of 117 m g .
6
2
6
3
2
-1
-1
-
2
-1
8
6
BET
The ionic liquid brushes SiO -BisILs [PF ] (1 g) was
Supplementary Information
2
6
suspended in a solution of K [W O (O ) ]·2H O (1.316 g,
2
2
3
2
4
2
2
mmol) in water (20 mL), and the mixture was stirred
Supplementary information (Figures S1-S21) is
available free of charge at http://jbcs.sbq.org.br as PDF file.
at room temperature for 24 h. After filtration and dry
in vacuo, the catalyst of peroxotungstate-ionic liquid
brush assembly was prepared according to the previous
reference. Elemental analysis: found C 12.06%, H 1.71%,
N 2.64%, IR (KBr disk) νmax/cm 3166, 2937 and 2864
Acknowledgements
34
-1
The authors are grateful to the National Foundation of
Natural Science (Grant No. 20906059 and 21072123), the
-
ν(C-H), 1638 ν(C=N), 831 ν(PF ), 935 ν(W=O), 844
6

1
542
A Peroxotungstate-Ionic Liquid Brush Assembly: an Efficient and Reusable Catalyst
J. Braz. Chem. Soc.
Fundamental Research Funds for the Central Universities
Grant GK201102005), the Foundation of Science and
16. Giuseppe, A. D.; Crucianelli, M.; Angelis, F. D.; Crestini, C.;
Saladino, R.; Appl. Catal., B 2009, 89, 239.
(
Technology Planning Project of Xi’an City (Grant
No.CXY1121) and the Innovation Foundation of Graduate
Cultivation of Shaanxi Normal University (Grant No.
17. Liu, P.; Wang, C. H.; Li, C.; J. Catal. 2009, 262, 159.
18. Saisaha, P.; Pijper, D.; van Summeren, R. P.; Hoen, R.; Smit, C.;
de Boer, J. W.; Hage, R.;Alsters, P. L.; Feringaa, B. L.; Browne,
W. R.; Org. Biomol. Chem. 2010, 8, 4444.
2
012CXS041) for providing financial support for this
research.
19. Liu, F. L.; Fu, Z. H.; Liu, Y. H.; Lu, C. L.; Wu, Y. Y.; Xie, F.;
Ye, Z. P.; Zhou, X. P.; Yin, D. L.; Ind. Eng. Chem. Res. 2010,
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