Polymer supported vanadium complexes as catalysts for the oxidation of alkenes in water

Article abstract of DOI:10.1007/s10562-010-0410-4

Polymer supported vanadium complexes (denoted as c-PMA _n -V) were prepared by the complexation of vanadium ions onto a cross linked polyacrylate. c PMA _n -V can catalyze the oxidative cleavage of olefins with a large excess of t-butyl hydroperoxide (TBHP) or bishydroxylation of olefins with 4 eq. of TBHP. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-010-0410-4

Catal Lett (2010) 139:61–66
DOI 10.1007/s10562-010-0410-4
Polymer Supported Vanadium Complexes as Catalysts
for the Oxidation of Alkenes in Water
•
Ming-Chieh Hsiao Shiuh-Tzung Liu
Received: 21 May 2010 / Accepted: 8 July 2010 / Published online: 22 July 2010
Ó Springer Science+Business Media, LLC 2010
Abstract Polymer supported vanadium complexes
(denoted as c-PMAn-V) were prepared by the complexa-
tion of vanadium ions onto a cross linked polyacrylate.
cPMAn-V can catalyze the oxidative cleavage of olefins
with a large excess of t-butyl hydroperoxide (TBHP) or
bishydroxylation of olefins with 4 eq. of TBHP.
relatively less toxic reagents such as titanium based cata-
lysts, heteropolyacids on oxide supports, metallo-porphy-
rin, gold(I)- and indium(III)-based catalysts have been
reported [12–18]. In addition to these procedures, we report
that vanadium complexes immobilized onto cross-linked
polyacylates (c-PMAn-V) are effective catalysts for bishydr-
oxylation and oxidative cleavage of alkenes in water. Fur-
thermore, these catalysts can catalyze the epoxidation of
cyclooctene and can be recycle-used without losing the
activity.
Keywords Oxidation of olefins Á Vanadium complex Á
Polyacrylate Á Cross-linked polymer Á Bishydroxylation Á
Epoxidation
1 Introduction
2 Experimental
The oxidative cleavage of olefins into the corresponding
carboxylic acids or ketones is a practical methodology in
organic synthesis [1, 2]. The typical reagents for this
transformation adopted in most textbooks include ozone
and some metal oxides such as OsO4 and RuO4 in com-
bination with a co-oxidant [1–8], but the major drawback
for these processes is the concern of chemical wastes and
for safety. Thus, the search for alternative catalytic systems
for bishydroxylation and/or oxidative cleavage of C=C has
received considerable attention particularly with the use of
ruthenium or vanadium-based catalysts [7–9]. However,
development of efficient and green protocols for this oxi-
dation is an area of growing interest due to the environ-
mental concerns [5–9]. A combination of Na2WO4 and
H2O2 does provide a good catalytic system for the oxida-
tion of alkenes [10, 11]. Other examples involving
All reaction steps for polymerization were performed under
a dry nitrogen atmosphere. Dichoromethane was dried with
CaH2 and distilled under nitrogen. Nuclear magnetic res-
onance spectra were recorded on a Bruker AVANCE 400
spectrometer. Infrared spectra were measured on a Nicolet
Magna-IR 550 spectrometer (Series-II) as KBr pellets. Gel
permeation chromatography (GPC) data were obtained
from a Waters Model 590 liquid chromatograph installed
with a Lab Allience RI 2000 detector using THF as eluant
(at the rate 1 mL/min) at 40 °C and polystyrene calibration
curve for analyses.
2.1 Synthesis (Scheme 1) and Characterization
2.1.1 Preparation of PMAn
A mixture of 1-bromoethylbenzene (1.75 g, 9.4 mmol) and
CuBr (541 mg, 3.7 mmol) was placed in a flask sealed with a
septum. The flask was evacuated and flashed with nitrogen
three times. Methyl acrylate (20 mL, 222 mmol) was syr-
inged into the above mixture with stirring. N(CH2CH2NMe2)3
M.-C. Hsiao Á S.-T. Liu (&)
Department of Chemistry, National Taiwan University, Taipei
106, Taiwan, ROC
e-mail: stliu@ntu.edu.tw
123

62
M.-C. Hsiao
Me
N
OMe
O
H
OMe
CO2Me
N
O
MeN(CH2CH2CH2NH2)2
O
N
H
Δ
Ph
Ph
Ph
Br
ATRP
y
n
x
PMAn
c-PMA50 (x=43, y= 7)
c-PMA28 (x=25, y= 3)
c-PMA15 (x=11, y= 4)
(VO)SO4
NaOH
H2O/MeOH
c-PMAn-V
n = 50, 28, and 15
Scheme 1
2.1.3 Preparation of c-PMAn-V
(56 lL, 0.2 mmol) was then added and the polymerization
took place immediately and the reaction flask was immersed
into a water bath to keep the reaction temperature at 26 °C.
After stirring 12 min, the reaction was quenched by the
addition of THF (30 mL). The reaction mixture was filtered
through silica gel to remove the catalyst. Upon concentration
and re-precipitation in ether/methanol, the obtained copoly-
c-PMA28 (5 g, 1.42 mmol) dissolved in a mixture of
methanol (75 mL) and 5% NaOH aqueous solution
(100 mL) was heated at 40 °C for 72 h. Hydrochloric acid
was added to adjust the pH = 5 and then brown solids
appeared, which were collected by filtration and washed
with ethanol and acetone. Upon drying under vacuum, light
yellow solids were obtained (3.75 g, 84%). This solid was
dissolved in a water solution under pH = 6.5 and an
aqueous solution of (VO)SO4 (0.34 M, 10 mL, 3.4 mmol)
was added. Dark-green solid appeared slowly and the
mixture was stirred for another 6 h. The dark-green solid
was collected and washed with water and ethanol. After
drying under vacuum for 24 h, c-PMA28-V was obtained
(91%). IR (KBr): 2951(tC–H), 1711(tC=O), 1563(tC=O asym),
1396 (tC=O sym), and 982 (tV=O) cm-1. ICP mass analysis
showed that one gram of solid contains 1.48 mmol of
vanadium metal ions. c-PMA15-V, c-PMA50-V, and
PMA28-V were prepared in a similar manner.
1
mer PMA28 was dried under vacuum overnight (12 g). H
NMR (400 MHz, CDCl3) d 7. 5-7.05 (m, 5H, Ar-H), 3.8-3.55
(br,(3n)H,COOCH3),2.65(m, 1H, –ArCHCH3),2.45-2.2(br,
(1n)H), 2-1.3 (br, (2n)H), 1.2 (d, 3H, –ArCHCH3); IR (KBr)
2951 (tC–H), 1737 (tC=O), 1438 (tC=C) cm-1. Table 1 sum-
marizes the results of polymerization.
2.1.2 Preparation of c-PMAn
A mixture of PMA28 (5.4 g, 2.0 mmol) and CH3N(CH2
CH2CN)2 (2.9 g, 20 mmol) [19] in 1,2-dichloroethane
(75 mL) was heated at 90 °C for 72 h. After removal of
solvents, the residue was dissolved in a small amount of
methanol. Upon slow addition of ether, the brown solid
precipitated out from the solution, which was the desired
cross linked polymer c-PMA28 (5 g, 71%): 1H NMR
(400 MHz, CDCl3) d 7.35-7.05 (m, 5H, Ar-H), 3.8-3.55
(br, 75 H, –COOCH3), 3.35-3.00 (br, 24 H), 2.9-2.72 (br,
9H, CH3N-), 2.65 (m, 1H, –ArCHCH3), 2.45-2.2 (br, 28 H),
2.2-1.8 (br, 12H), 2-1.3 (br, 56H), 1.2 (d, 3H, –ArCHCH3).
c-PMA15 and c-PMA50 were prepared in a similar manner.
2.2 Catalysis
2.2.1 Typical Procedure for Oxidation of Olefins
A mixture of substrate (1 mmol), c-PMAn-V (30 mg),
oxidant, and water (1 mL) was placed in a 5 mL flask. The
solution was stirred at a certain temperature for a certain
period of time. After the reaction, the organic products
were separated from the reaction mixture by extraction
with dichloromethane (2 mL 9 2). The combined organic
portions were dried and concentrated. Product analysis was
Table 1 Polymerization of methyl acrylate via ATRP
Entry
Methyl acrylate
(mmol)
MN(PDI)a
MA
unitsb
Abbreviation
1
1
2
3
132.3
63.5
35.1
4900 (1.19)
2760 (1.09)
1650 (1.22)
50
28
15
PMA50
PMA28
PMA15
performed by GC and H NMR spectroscopic methods,
which were all essentially identical to those reported in
literature.
Reaction conditions: 1-bromoethylbenzene (2.7 mmol), CuBr
(0.54 mmol), Metren (N[CH2CH2NMe2]3, 0.54 mmol) at 26 °C for
15 min
2.2.2 1H NMR Data of Products
a
1
Based on GPC analysis
Adipic acid: H NMR (400 MHz, DMSO-d6) d 11.5 (br,
2H), 2.2 (m, 4H), 1.72-1.30 (m 4 H).
b
Based on NMR integration
123

Polymer Supported Vanadium Complexes
63
1
Trans-1,2-cyclohexandiol: H NMR (400 MHz, CDCl3)
were subjected to undergo the hydrolysis. On treatment with
basic water, the polymers c-PMAn slowly became water
soluble, indicating that hydrolysis of –CO2CH3 into
–CO2Na was accomplished. Finally, a solution of vanadyl
sulfate (VOSO4) was added to the solution containing the
hydrolyzed polymers under pH = 6–7. The desired mate-
rials (c-PMAn-V) were precipitated immediately as dark-
green solids.
d 3.34 (m, 2H), 1.97 (m, 2H), 1.70 (m, 2H), 1.25 (m, 4H).
1-Phenylpropane-1,2,3-triol: 1H NMR (400 MHz,
CDCl3) d 7.35-7.2 (m, 5H), 4.68 (d, 1H), 3.85-3.3 (m, 3H).
1-Phenyl-1,2-cyclohexanediol: 1H NMR (400 MHz,
CDCl3) d 7.5-7.2 (m, 5H), 3.97 (m, 1H), 1.9-1.3 (br, 8H).
6-Oxo-6-phenyl-hexanoic acid: 1H NMR (200 MHz,
CDCl3) d 7.98-7.92(m, 2H), 7.61-7.26(m, 3H), 3.01 (t, J =
6.8 Hz, 2H), 2.43 (t, J = 6.7 Hz, 2H), 2.30-1.40 (m, 4H).
2,3-Pinanediol: 1H NMR (400 MHz, CDCl3) d 3.96
(d,1H), 3.1-1.4 (m, 6H), 1.3-0.9 (9H).
Characterization of the polymer-supported complexes
was performed by infrared spectroscopy, EPR and ICP-Mass
analysis. The infrared carbonyl stretching frequencies of
c-PMA28-V appear at 1563 and 1396 cm-1, which are
consistent with the pattern of carboxylate-bridged vanadium
species [21]. Furthermore, the V=O stretching vibrations
(tV=O) of c-PMA28-V was observed at 982 cm-1, showing
that the metal center is a typical five-coordinated oxo-
vanadium species. EPR spectrum of c-PMA28-V shows
g// = 1.936, A// = 188 G and g\ = 1.973, A\ = 76 G,
confirming that the metal center is seated in a square pyra-
midal geometry [22]. The content of vanadium species in
the polymers was estimated to be 1.48 mmol per gram of
the material (by ICP-MS) versus 1.49 and 1.66 mmol for
c-PMA50-V and c-PMA15-V, respectively. In addition to the
above complexes, we have prepared the complex of vana-
dium with the hydrolyzed PMA28 (denoted as PMA28-V).
1-Phenyl-1,2-ethanediol: 1H NMR (400 MHz, CDCl3) d
7.4-7.2 (m, 5H), 4.82 (dd, 1H), 3.8-3.6 (m, 2H).
1,7-Heptanedioic acid: 1H NMR (400 MHz, DMSO-d6)
d 2.35 (t, J = 7 Hz, 4 H), 1.78-1.20 (m, 6H).
1
Cycloctene oxide: H NMR (400 MHz, CDCl3) d 3.02-
2.83 (m, 2 H), 2.33 -2.01 (m, 2 H), 1.73-1.03 (m, 10 H).
3 Results and Discussion
3.1 Preparation of Catalysts
The polymer supported vanadium complexes (denoted as
c-PMAn-V) were prepared by the complexation of vana-
dium ions with acrylate moieties from a cross linked
polyacrylate (Scheme 1). The atom transfer radical poly-
merization (ATRP) of methyl acrylate was carried out in
bulk at 26 °C, using 1-bromoethylbenzene as the initiator
and CuBr/N(CH2CH2NMe2)3 as the catalyst [20]. By
manipulating the molar ratio of methyl acrylate versus
initiator, various chain lengths of polymers (PMAn) were
prepared (Table 1). The 1H NMR chemical shifts at
d = 3.7 ppm can be assigned to the protons of the methyl
groups from the poly(methyl acrylate), whereas that at
d = 1.2 ppm (doublet) is due to the methyl group from the
end group (Ph-CH–CH3). Resonances at d = 2.45-2.20 and
2.01-1.32 can be assigned to the –CH–CH2– group of the
polymer chain. Additionally, the peak area ratio of 3.7
versus 1.2 ppm provided the information for the calcula-
tion of the degree of polymerization, which is fairly con-
sistent with the results from GPC analysis.
3.2 Catalytic Oxidation of Olefins
In the first stage of the investigation for the catalysis, we
tried to seek the optimal conditions for the bishydroxyla-
tion or the oxidative cleavage of cyclohexene catalyzed by
the polymer-supported oxo-vanadium complex (Table 2).
After several attempts, we have learned that t-butyl
hydroperoxide (TBHP) is the best oxidant for the oxidation
of olefins. Typically, a mixture of cyclohexene (1 mmol),
TBHP (8 mmol), and c-PMA28-V (30 mg) in water was
heated to reflux for 32 h and adiptic acid as the desired
product was isolated in 67% yield. By lowering the reac-
tion temperature and the amount of oxidant, bishydroxy-
lation of cyclohexene leading to trans-cyclohexanediol
became the major pathway. It is noticed that there is no
significant difference by carrying out the reaction under
either oxygen or nitrogen atmosphere, i.e., molecular
oxygen does not play any role in the oxidation process
(entries 2 vs. 3). Under the similar conditions, it is found
that the 1, 2-cyclohexane-diol was converted into the cor-
responding dicarboxylic acid (entry 4), suggesting that the
oxidative cleavage of olefins might go through the diol
intermediate followed by the cleavage. Based on the above
observations, we tentatively concluded that this oxygena-
tion process might involve the epoxide intermediate, which
was subsequently hydrolyzed to yield the 1,2-diol and then
C–C cleavage (Scheme 2).
Under refluxing conditions, the ester functionality in the
PMAn reacted with CH3N(CH2CH2CH2NH2)2 I success-
fully to yield the cross-linked materials c-PMAn as shown
in Scheme 1. The amount of incorporation of I can be
1
estimated by the H NMR integration. The protons ratio
of –N–CH3 to –O–CH3 was about 25:3 in c-PMA28, indi-
cating that approximately 10% of ester groups were con-
verted into amide groups. Infrared absorptions for c-PMA28
appearing at 1734 and 1645 cm-1 in the carbonyl stretching
region are associated with ester and amide functionalities in
c-PMA28, respectively. The obtained polymers c-PMAn
123

64
M.-C. Hsiao
Table 2 Oxidation of olefins catalyzed by c-PMA28-V in water
Entry
Substrate
Temp (°C)
TBHP (mmol)
Time (h)
Conv. (%)a
Products
1
Cyclohexene
Cyclohexene
Cyclohexene
OH
105
60
8
4
4
4
32
16
16
9
100
95
1,6-hexanedioic acid (67%)
2
3b
Trans-1,2-cyclohexanediol (70%)
Trans-1,2-cyclohexanediol (67%)
1,6-Hexanedioic acid (70%)
60
94
4
105
100
OH
Ph
5
6
60
4
8
4
8
100
100
1-Phenyl-1,2-cyclohexanediol (62%)
6-oxo-6-phenylhexanoic acid (15%)
Ph
105
1-Phenyl-1,2-cyclohexanediol (62%)
7
8
60
60
4
4
24
14
90
2,3-Pinanediol (70%)
100
1-Phenyl-1,2-ethanediol (65%)
benzoic acid (22%)
9
60
8
8
14
14
100
100
Benzoic acid (64%)
10
100
Benzoic acid (84%)
11
60
4
12
100
1-Phenyl-1,2,3-propanetriol (82%)
OH
OH
12
13d
14
15
16
60
60
80
80
80
8
16
16
8
12
16
16
16
16
100
100
100
0
Benzoic acid (75%)
Trans-stilbene
Cycloheptene
1-Octene
Benzoic acid (78%)
1,7-Heptanedioic acid (56%)
–
–
Me3CH=CH2
8
0
Reaction conditions: substrate (1 mmol), c-PMA28-V (30 mg), TBHP in water
a
Yield based on NMR integration with internal standard
b
Due to solubility of the substrate, chloroform was used as the co-solvent
Scheme 2 Mechanistic
pathway for the oxidation of
alkenes
R
oxidative
cleavage
R
R
R
R
c-PMA28-V
R
R
O
TBHP
OH
OH
HO
O
epoxide
diol
3.3 Epoxidation of Cyclooctene
Catalyst c-PMA28-V was used for screening different
substrates (Table 2). It showed good activities for most of
olefins, but not terminal ones. Again, we have checked the
influence of the amount of oxidant on the oxidation of
styrene. As shown in the entries 8 and 9, when the molar
ratio of substrate/TBHP was increased from 4 to 8, the
oxidative cleavage product became the single product. As
observed earlier, the performance of the catalyst on the
oxidative cleavage is much better at 100 °C than 60 °C
with achieving the maximum yield of benzoic acid. Simi-
larly, oxidation of cinnamyl alcohol with the use of four
equivmolar amount of TBHP provided the bis-hydroxyl-
ation product (1-phenyl-1,2,3-propanetriol) (entry 11),
indicating that the primary –OH functionality remained
unchanged. Of course, the cleavage of cinnamyl alcohol
can be achieved by using a higher amount of oxidant (entry
12).
Besides the above studies, we have also investigated the
possibility of epoxidation of cycloolefins using c-PMA28-V.
Thus the epoxidation of cyclooctene catalyzed by
c-PMA28-V was first screened and the results are summa-
rized in Table 3.
The complex c-PMA28-V is very effective for the
epoxidation of cyclooctene in water with TBHP as the
oxidant. With four equimolar amounts of TBHP, the oxi-
dation of cyclooctene proceeded smoothly at 60 °C,
yielding to 100% conversion within 12 h. (Table 3, entry
1). The corresponding diol was formed as a minor
byproduct. Among the screening of various solvents, it
appears that water is the best choice as compared to other
organic solvents. Toluene favors good conversion with
highly selectivity toward cyclooctene oxide, but it is not
123

Polymer Supported Vanadium Complexes
65
Table 3 Conversion of
Entry
Oxidant
Solvent
Temp (°C)
Time (h)
Conv. (%)
Yield (%)
cyclooctene into its epoxide
1
2
3
4
5
TBHP (4 mmol)
TBHP (2 mmol)
TBHP (2 mmol)
NaOCl
Water
Water
Water
Water
Water
CH3CN
Toluene
60
60
40
60
60
60
60
12
12
12
12
12
24
24
100
75
92
68
42
40
25
25
H2O2
\5
65
Trace
62
Reaction conditions: substrate
(1 mmol), c-PMA28-V (30 mg),
and oxidant in water
6
7
TBHP (4 mmol)
TBHP (4 mmol)
82
80
easy for the separation of product from that solvent (entry 7
of Table 3). Other oxidants appeared to be less efficient as
compared to TBHP (entries 4 and 5). Experiments were
also carried out to determine the effects of the amount of
oxidants on the conversion. For these experiments, the
substrate and c-PMA28-V were held constant in 1 mL
water at 50 °C while the relative amounts of TBHP were
varied. As shown in Fig. 1, the observed conversion rate
increases with TBHP concentration.
100
80
60
40
20
TBHP (equivmol.)
4
2
1
0
0
Unlike cyclooctene, the oxidation of cyclohexene read-
ily provided the corresponding diol under similar reaction
conditions. The hydrolysis of the intermediate epoxide by
water catalyzed by vanadium complexes is presumably for
the production of 1,2-cyclohexanediol. Indeed, treatment of
cyclohexene oxide with water in the presence of c-PMA28-
V yielded the diol directly. This observation is also
applicable to a-pinene, styrene, 1-phenylcyclohexene and
cinnamyl alcohol (Table 2, entries 6, 7, 8, and 11).
4
8
12
16
20
time (h)
Fig. 1 Influence of the amount of TBHP on epoxidation of cyclo-
octene at 50 °C
Table 4 Epoxidation of cyclooctene by various catalysts
Entry
Catalysts
Metal contenta
Yield (%)b
1
2
3
4
c-PMA15-V
c-PMA28-V
c-PMA50-V
PMA28-V
1.66
1.48
1.49
0.39
73
73
78
25
In order to investigate the effect of various catalysts
prepared in this work, epoxidation of cyclooctene catalyzed
by c-PMAn-V and PMA28-V were studied and results are
summarized in Table 4. As shown in the table, there is no
significant difference among oxo-vanadium complexes
binding onto various cross-linked polymers, but the activ-
ities of c-PMAn-V (n = 15, 28, 50) are much better than
that of PMA28-V. The unsatisfactory performance of
PMA28-V might be due to the decomplexation of metal
ions from the polymer. The results clearly highlight the
potential of highly cross-linked polymers as supports for
the metal complexes, because the metal ions are fixed in a
rigid matrix. As for the metal leaching, ICP-MS analysis of
the filtered solution from the reaction catalyzed by
c-PMA28-V did not show any trace of vanadium presented
in the solution, indicating the advantage of the cross-linked
polymers on supporting metal ions.
Reaction conditions: substrate (1 mmol), c-PMA28-V (30 mg), and
TBHP (94 mmol) in water (1 mL) at 50 °C for 8 h
a
ICP-MASS analysis. mmol of vanadium per gram of material
b
Based on GC analysis
period of five reaction cycles under the same conditions, no
significant loss of activity was observed and the conversion
for each run is 98, 95, 94, 90 and 92%, respectively.
4 Summary
Polymer-anchored oxo-vanadium complexes have been
prepared and used for the catalytic oxidation of olefins and
epoxidation of cyclcooctene. Various reaction conditions
have been optimized to obtain the maximum yield of the
desired product(s). It has been demonstrated that the
amount of oxidant plays a role in altering the selectivity of
reaction products. With eight equivmolar amount of TBHP,
these catalysts are selective towards the oxidative cleavage
The reusability of the catalyst was tested using
c-PMA28-V as the catalyst. Typically, a mixture of
cyclooctene (1 mmol), catalyst (30 mg) and TBHP
(4 mmol) in water (1 mL) was heated at 60 °C for 12 h.
After each run, the catalyst was recovered by filtration,
followed by washing with ethanol (2 mL 9 3). After dry-
ing, the catalyst was reused directly for the next run. Over a
123

66
M.-C. Hsiao
5. Chen K, Que L Jr (1999) Angew Chem Int Ed 38:2227
6. De Vos DE, De Wildeman S, Sels BF, Grobet PJ, Jacobs PA
(1999) Angew Chem Int Ed 38:980
7. Murahashi SI, Komiya N (2000) In: Meunier B (ed) Biomimetic
oxidations catalyzed by transition metal complexes. Imperial
college press, London, p 563
8. Ho CM, Yu WY, Che CM (2004) Angew Chem Int Ed 43:3303
9. Shul’pin GB, Mishra GS, Shul’pin LS, Strelkova TV, Pombeiro
AJL (2007) Catal Commun 8:1516
10. Noyori R, Aoki M, Sato K (2003) Chem Commun 1977
11. Sato K, Aoki M, Noyori R (1998) Science 281:1646
12. Lee SO, Raja R, Harris KDM, Thomas JM, Johnson BFG, Sankar
G (2003) Angew Chem Int Ed 42:1520
product(s). However, in water, they become more selective
towards epoxide or diols with the use of four equivmolar
amount of TBHP. The procedure in this work offers several
advantages for the oxidation of alkenes such as low loading
of catalyst, mild conditions in aqueous medium, and high
yields, which make it a useful and attractive methodology
for organic synthesis. The simple workup procedure for the
recycle use of catalysts is also beneficial to this method.
Further applications of this catalyst to other transforma-
tions are currently under investigation.
13. Brooks CD, Huang LC, McCarron M, Johnstone RAW (1999)
Chem Commun 37
14. Freitag J, Muchter M, Ondruschka B (2003) Green Chem 5:291
15. Antonelli E, D’Aloisio R, Gambaro M, Fiorani T, Venturello C
(1988) J Org Chem 63:7190
Acknowledgment We thank the National Science Council (NSC-
97-2113-M-002-013-MY3) for financial support.
16. Xing D, Guan B, Cai G, Fang Z, Yang L, Shi Z (2006) Org Lett
8:693
References
17. Ranu BC, Bhadra S, Adak L (2008) Tetrahedron Lett 49:2588
18. Liu ST, Reddy KV, Lai RY (2007) Tetrahedron 63:1821
19. Gerwarth UW, Mu¨ller KD (1977) Z Anorg Allg Chem 433:261
20. Pyun J, Matyjaszewski K (2001) Chem Rev 101:2921
21. Kawabe K, Sasagawa T, Yoshikawa Y, Ichimura A, Kumekawa
K, Yanagihara N, Takino T, Sakurai H, Kojima Y (2003) J Biol
Inorg Chem 8:893
1. Shing TKM (1991) In: Trost BM, Fleming I (eds) Comprehensive
organic synthesis, vol 7. Pergamon Press, Oxford, p 703
2. Hudlicky M (1990) Oxidations in organic chemistry. ACS
monograph 186. American Chemical Society, Washington DC,
p 77
3. Chen K, Costas M, Kim J, Tipton AK, Que L Jr (2002) J Am
Chem Soc 124:3026
4. Costas M, Tipton AK, Chen K, Jo DH, Que L Jr (2001) J Am
Chem Soc 123:6722
22. Prakash AM, Kevan L (2000) J Phys Chem B 104:6860
123