The oxidative halogenations of arenes in water using hydrogen peroxide and halide salts over an ionic catalyst containing sulfo group and hexafluorotitanate

Article abstract of DOI:10.1016/j.molcata.2013.01.023

An ionic compound, bis[1-methyl-3-(3′-sulfopropyl)imidazolium] hexafluorotitanate (1), was proved to be the efficient and recyclable catalyst for the oxidative halogenations of arenes in water using H₂O ₂ as the oxidant and halide salts as the halogenation sources. The mono-halogenated products were obtained selectively by this method. The synergetic catalytic effect coming from the two incorporated functionalities of SO₃H and [TiF₆]^2- was manifested in 1. The halogenation rate catalyzed by 1 was in the ranking of NaBr NaCl > KI. The UV-vis and FT-IR analyses indicated that the successful formation and regeneration of the active peroxo-Ti species (1A) with the aid of proton acid guaranteed the recycling uses of 1.

Full text of DOI:10.1016/j.molcata.2013.01.023

Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
The oxidative halogenations of arenes in water using hydrogen peroxide and
halide salts over an ionic catalyst containing sulfo group and hexafluorotitanate
a
a
b,∗∗
a,∗
Ling Wang , Sa-Sa Wang , Giang VO-Thanh
, Ye Liu
a
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Chemistry Department of East China Normal University, Shanghai 200062, PR China
UMR 8182, Bat. 420, Université Paris-Sud 11, Orsay Cedex, France
b
a r t i c l e i n f o
a b s t r a c t
ꢀ
Article history:
An ionic compound, bis[1-methyl-3-(3 -sulfopropyl)imidazolium] hexafluorotitanate (1), was proved to
be the efficient and recyclable catalyst for the oxidative halogenations of arenes in water using H2O2 as
the oxidant and halide salts as the halogenation sources. The mono-halogenated products were obtained
selectively by this method. The synergetic catalytic effect coming from the two incorporated functionali-
Received 23 October 2012
Received in revised form 21 January 2013
Accepted 26 January 2013
Available online 4 February 2013
2
−
ties of SO3H and [TiF6] was manifested in 1. The halogenation rate catalyzed by 1 was in the ranking of
NaBr ꢁ NaCl > KI. The UV–vis and FT-IR analyses indicated that the successful formation and regeneration
of the active peroxo-Ti species (1A) with the aid of proton acid guaranteed the recycling uses of 1.
Keywords:
Oxidative halogenation
Ionic catalyst
©
2013 Elsevier B.V. All rights reserved.
Hexafluorotitanate
Hydrogen peroxide
Halide salt
1
. Introduction
From green chemistry perspectives, halide salts are much safer
and easier to be handled and can be oxidized to the correspond-
+
+
+
−
Halogenation is an important electrophilic substitution reac-
ing positive halogens (Cl , Br , I ) or hypohalous acids ([XO] ,
+
+
+
tion and halogenated organic compounds are essential in organic
synthesis as useful synthetic intermediates for preparation of
pharmaceuticals, pesticides, agricultural chemicals, and bioactive
compounds [1,2]. They are also useful and important substrates
for various cross-coupling reactions [3]. In the laboratory, how-
ever, halogen-based oxidants such as elemental halogens, organic
and inorganic hypohalites, hypervalent iodine (III) and (V) reagents,
X = Cl , Br , I ) by H O , which is an environmentally friendly oxi-
2 2
dant [2,7,8]. Although the oxidation of these halides with H O2
2
is thermodynamically favored, it is kinetically slow [9]; hence it
is not a practical process. However, At lower pH, the oxidation
of the halides with H O2 can be accelerated and the hypohalous
2
acids generated from hydrochloric or hydrobromic acid and H O2
2
have been used for the successful halogenation of a variety of
organic substrates although acid-sensitive functionality will not
tolerate the reaction conditions [6,10]. In order to mimic the func-
tion of the haloperoxidases to catalyze the halide ion to form a
halogenating reagent, chemists with growing ecological awareness
ꢀ
dihalo-5,5 -dimethylhydantoins and N-halosuccinimides are often
used for the selective halogenation of unsaturated hydrocarbons.
Halogenations using such hazardous, toxic, and corrosive regents
result in serious environmental problems with respect to the hand-
ling, transportation, storage, and stoichiometric amounts of waste
have sought catalysts to activate H O2 and to provide environ-
2
[
4,5].
In nature, electrophilic halogenation mainly occurs by oxida-
mentally friendly means for the halogenation of organic substrates
[2,5–8,11]. Many transition metal compounds (such as vanadium,
molybdenum, cerium, tellurium, etc.) have been used as catalysts
for oxidative halogenation of organic substrates with halide salts
tive halogenation through the catalyzed oxidation of the halide ion
to form a halogenating reagent [1,4,6]. An exception is fluorina-
tion, since it is too difficult to oxidize fluoride [1]. The increased
understanding of oxidative halogenation in biological systems has
boosted research in the field of green oxidative halogenations.
and H O2 [2,5–8,12–21]. However, TiF62 -based catalysts were
−
2
rarely reported in these examples.
On the other hand, the use of polar organic solvents in most
cases, such as CH Cl , THF, and CH CN largely defeats the environ-
2
2
3
mental and economic advantages of oxidative halogenation with
H O and halide salts. The use of water as the reaction medium
affords many benefits [8,22]. Water is a cheap and abundant, non-
toxic, non-flammable and green solvent. In addition, in water,
phase separation is easier because most organic compounds are
2
2
∗
Corresponding author. Tel.: +86 21 62232078; fax: +86 21 62233424.
Corresponding author. Fax: +33 1 69154680.
E-mail addresses: giang.vo-thanh@u-psud.fr (G. VO-Thanh),
∗
∗
yliu@chem.ecnu.edu.cn (Y. Liu).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.01.023

L. Wang et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
57
not soluble in water and can be easily separated from the aqueous
phase.
yield the solids as the product of 1 (Yield 79%). TG/DTA (in air
flow): 366 C (thermal decomposition). FT-IR (KBr disc, cm ):
◦
−1
However, until now the inexpensive and environmentally
friendly methods for the oxidative halogenations with halide salts
ꢀ = 3150 (m), 3104 (m), 1562 (m, C N), 1457 (w, C C), 1267 (s,
C
N), 1176 (vs, broad, SO H), 1033 (s, SO H), 752 (m), 624 (vs,
3 3
2
−
TiF6 ). H NMR (D O, ppm): ı = 8.6 (H, s, NC(H)N ), 7.3–7.4 (2H,
2
1
+
and H O in water, which are catalyzed by the recyclable catalysts,
2
2
+
+
are scarce in the literature and are of interest from economical and
environmental concerns.
NC(H)C(H)N ), 4.2 (2H, t, J = 7, N C(H )C(H )C(H )SO H), 3.8 (3H,
s, C(H )N), 2.8 (2H, t, J = 7, N C(H )C(H )C(H )SO H), 2.2 (2H, m,
N C(H )C(H )C(H )SO H). CHN-elemental analysis (wt%): C 29.02,
2 2 2 3
+
3
2
2
2
3
+
Herein, we introduced a new catalytic method using an ionic
compound (1) containing two functional moieties of sulfo group
2 2 2 3
H 4.88, N 9.23 (Calcd.: C 29.38, H 4.55, N 9.78).
2
−
SO H) and hexafluorotitanate (TiF6 ) as the efficient and
3
(
recyclable catalyst for the oxidative halogenations of aromatic com-
pounds. In this method, water was used as the solvent; the cheap
and non-corrosive halide salts (NaBr, NaCl, and KI) were applied
as the halogenation sources; the clean, non-toxic and inexpen-
sive H O was applied as the oxidant. The purpose of using halide
2
.2.2. Bis-[1-butyl-3-methylimidazolium] hexafluorotitanate (2)
To the aqueous solution (100 mL) of 1-butyl-3-
methylimidazolium chloride ([Bmim]Cl, 0.1 mol), an aqueous
solution of H TiF6 (0.05 mol, 60%) was added. After vigorous
2
2
2
salts as the halogenations sources instead of hydrohalic acids (HX,
stirring at ambient temperature for 24 h, the resultant solution
was stripped of solvent on a rotary evaporator. The obtained
residues were added with CH Cl . After vigorous stirring, the
mixtures were filtered to give the clear filtrate, which was
concentrated under vacuum to afford a sticky liquid as the
product of 2 (Yield 82%). FT-IR (KBr disc, cm ): ꢀ = 3155 (m),
3
TiF6 ). H NMR (D O, ppm): ı = 8.6 (H, s, NC(H)N ), 7.3–7.4
+
+
+
X = Cl , Br , I ) was to elucidate the possible synergetic catalytic
2−
effects coming from SO H and [TiF ] in 1 and rule out the over-
lapping effect coming from the H in HX against that in sulfo group.
The characterization and identification of the peroxo-Ti species
3
6
2
2
+
(
1A) derived from 1 were studied by means of UV–vis and FT-IR
−1
spectroscopies.
100 (m), 1632 (m, C = N), 1460 (w, C = C), 747 (m), 568 (vs,
2−
1
+
2
+
+
(2H, NC(H)C(H)N ), 4.1 (2H, t, J = 7, N C(H )C(H )C(H )C(H )), 3.8
2
. Experimental
2 2 2 3
+
(3H, s, C(H )N), 1.7 (2H, m, J = 7, N C(H )C(H )C(H )C(H )),
3
m,
2
2
0.8
2
3
m,
+
1
.2
(2H,
N C(H )C(H )C(H )C(H )),
(3H,
2.1. Reagents and analysis
2
2
2
3
+
N C(H )C(H )C(H )C(H ). CHN-elemental analysis (wt %): C 44.24,
2
2
2
3
H 7.27, N 12.33 (Calcd.: C 43.64, H 6.87, N 12.72).
The chemical reagents were purchased from Shanghai Aladdin
Chemical Reagent Co. Ltd. and used as received. FT-IR spectra
were recorded on a Nicolet NEXUS 670 spectrometer. UV–vis spec-
tra were monitored on a SHIMADZU-UV 2550 spectrophotometer
at ambient temperature. 1H NMR spectra were recorded on a
Bruker Avance 500 spectrometer. The Ti amount in the sample
was quantified using an inductive coupled plasma atomic emis-
sion spectrometer (ICP-AES) on an IRIS Intrepid II XSP instrument
ꢀ
2.2.3. Bis[1-methyl-3(3 -sulfopropyl)imidazolium] sulfate (3)
ꢀ
Bis[1-methyl-3(3 -sulfopropyl)imidazolium] sulfate (3) was
prepared according to the similar procedures as described
for 1, but H SO aqueous solution was used in place of
H TiF . FT-IR (KBr disc, cm ): ꢀ = 3155 (m), 3111 (m),
2
4
−
1
2
6
(
Thermo Electron Corporation). CHN-element analysis was per-
1579 (m, C = N), 1452 (w, C = C), 1108 (vs, broad,
SO H).
3
1
+
formed on a Vario EL III Element Analyzer. TG/DTA was performed
in air flow with a temperature ramp of 10 C min between 50
and 800 C, using a Mettler TGA/SDTA 851e instrument and STARe
H
NMR (D O, ppm): ı = 8.5 (H, s, NC(H)N ), 7.2–7.3 (2H,
2
◦
−1
+
+
NC(H)C(H)N ), 4.1 (2H, t, J = 7.0, N C(H )C(H )C(H )SO H), 3.7
2 2 2 3
◦
+
(3H, s, C(H )N), 2.7 (2H, t, J = 7, N C(H )C(H )C(H )SO H), 2.2 (2H,
3 2 2 2 3
thermal analysis data processing system. Gas chromatography (GC)
was performed on a SHIMADZU-2014 equipped with a DM-1 cap-
illary column (30 m × 0.25 mm × 0.25 ␮m). GC–mass spectrometer
m, N + C(H )C(H )C(H )SO H). CHN-elemental analysis (wt%): C
32.69, H 5.97, N 10.74 (Calcd.: C 33.19, H 5.17, N 11.06).
2 2 2 3
(
GC–MS) was recorded on an Agilent 6890 instrument equipped
with an Agilent 5973 mass selective detector.
2.3. General procedures for bromination of arenes using NaBr
and H O catalyzed by 1
2
2
2
.2. Synthesis
.2.1. Bis[1-methyl-3-(3 -sulfopropyl)imidazolium]
For the typical experiment, to 3 mL of water, anisole (or the other
substrate, 5 mmol), NaBr (10 mmol), 1 (2.0 mmol), and 30% aque-
ꢀ
2
ous solution of H O (10 mmol) were mixed sequentially in a single
hexafluorotitanate (1)
2
2
ꢀ
addition. The obtained mixture was stirred vigorously at room tem-
perature. Upon completion of the reaction, the upper organic phase
was separated by decantation and the left mixture was extracted
by diethyl ether repeatedly (2 mL × 3 mL). The combined organic
phase was analyzed by GC and GC–MS. The conversion of anisole
was based on GC analysis with n-dodecane as the internal standard.
The selectivity of the product was based on GC analysis using the
normalization method. The GC yield was obtained on the basis of
conversion × selectivity. The products were further identified by
GC–MS.
Bis[1-methyl-3-(3 -sulfopropyl)imidazolium]
hexafluo-
rotitanate (1) was prepared according to our previously
published method [23], with some modifications. The mix-
tures of 1,3-propanesultone (0.15 mol) in 100 mL acetone and
N-methylimidazole (0.15 mol) in 100 mL acetone were stirred
vigorously at ambient temperature. Gradually, the white solids
were precipitated from the reaction solution. Upon completion
of the reaction for 3 h, the precipitated white solids, through
washing with acetone and drying under vacuum, were collected
with the yield of 91%. The obtained white solids (0.10 mol) were
added with 0.05 mol of aqueous hexafluorotitanic acid (H TiF ,
The left mixtures, containing the aqueous phase and the pre-
2
6
2
cipitated yellow solids, were added with the saturated Na SO3
6
0% aqueous solution, commercial). The resultant solution after
solution gradually to destroy the unreacted H O until the KI-starch
stirring at ambient temperature for 24 h was stripped of solvent
on a rotary evaporator. The obtained residues were treated with
ethanol. After vigorous stirring, the mixtures were filtered to
give the clear filtrate, which was concentrated under vacuum to
2
2
test paper changed to blue color. Afterwards, the obtained mix-
2
tures were recharged with anisole (5 mmol), NaBr (5 mmol), H O2
(
10 mmol), and H SO4 (2.5 mmol) if required, for next run.
2

5
8
L. Wang et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
3
. Results and discussion
4
4
3
2
1
3
63
b
d
354
3.1. Investigation on the catalytic performance of 1 for the
c
oxidative halogenation
368
3
2
1
0
e
354
363
The bromination of anisole with H O and NaBr was selected as
2
2
0
3
the model reaction (Table 1). It was found that, without the pres-
ence of 1 as a catalyst, the reaction did not happen (Entry 1), while
the conversion of anisole increased along with the amount of 1
00 350 400 450 500 550 600
Wavelength (nm)
◦
at R.T. (25 C) (Entries 2–4). In these cases, although excess NaBr
was used as the bromination source, 4-bromoanisole was found
to be the only mono-brominated product and no di-brominated
product was formed at all. Once 4-bromoanisole was formed, the
nucleophilicity of aryl ring got weakened due to the strong elec-
tronegativity of Br atom, leading to the unfavorable electrophilic
a
−
attack by the second [BrO] and then the inhibited di-bromination.
Under the similar conditions, when less NaBr (5 mmol) or H O2
300
350
400
450
500
2
(
5 mmol) was applied, the yield of 4-bromoanisole decreased dra-
Wavelength (nm)
matically (Entries 5 and 6). It was found that the self-decomposition
of H O2 under the applied conditions (no anisole, NaBr 10 mmol,
Fig. 1. The evolving processes of 1 in H2O recorded by UV–vis spectra under differ-
ent conditions: (a) 1; (b) 1 + H2O2; (c) 1 + H2O2 + NaBr +anisole; (d) 1 + H2O2 + NaBr
2
H O 10 mmol, H O 3 mL, R.T., time 3 h) was ca. 18% in the presence
2
2
2
+anisole + H2SO4; (e) 1 + H2O2 + NaBr +anisole + H2SO4 + H2O2 (inset: the UV–vis
of 1 (2.0 mmol) or 15% in the presence of H SO4 (2.5 mmol), which
2
spectra of the yellow solids (1B) precipitated from water upon completion of the
1st run reaction in Table 2).
was measured by permanganimetric titration (standard KMnO4
aqueous solution, 0.1 M), accounting for the requirement for the
excess of H O (2 mmol) for the complete oxidative bromination of
2
2
anisole.
Actually, it was noted that in 1, two catalytic sites were located.
In order to elucidate the individual contribution of SO H and
and then reused in the 2nd run, only 19% yield of 4-bromoanisole
was obtained, indicating the dramatic activity loss of 1. Fortunately,
if the left mixtures were further treated with the proton acid like
3
2
−
[
TiF6]
to the activation of the substrate, the oxidative bromina-
H SO , the yellow precipitates disappeared to afford a clear light-
2
4
tion of anisole catalyzed by 2 and 3 was investigated in parallel. As
shown in Entries 7 and 8, the absence of any function moiety led
to much lower yields of 4-bromoanisole (2: 41%; 3: 39%) respec-
tively, indicating that the cumulative contribution of 2 and 3 to the
activation of anisole is less than that of 1 individually. These results
yellow aqueous solution, in which the oxidative bromination of
anisole with H O and NaBr could be carried out as well as the fresh
2
2
1
-catalyzed reaction. The cumulative Na SO and H O showed neg-
2 4 2
ligible effect on the performance of 1.
The proton acid such as 3 possessing sulfo group has been
found to be able to catalyze the bromination of anisole to afford
2−
revealed that the catalytic behaviors of SO H and TiF6
were
3
reinforced each other when they were combined together in 1 as
one unit, and that the observed cooperative effect in 1 was caused
4
-bromoanisole (%) with the yield of 39% (Entry 8 of Table 1). Simi-
larly, when 2.5 mmol of H SO was used in place of 1, anisole could
2
4
2−
by a specific synergetic interaction between SO H and TiF6
,
3
be brominated to 4-bromoanisole with the yield of 44% under the
but not attributable to pure statistic reasons. On the other hand,
same conditions. Hence, when H SO4 (2.5 mmol) was added addi-
2
when the commercial inorganic acid of H TiF6 (60% aqueous solu-
2
tionally in each run in Table 2 (Runs: 3rd–6th), it not only played
the role in regenerating the deactivated 1, but also acted as the
acid-catalyst to further spur the oxidative bromination, leading
to the successful reuses of 1 along with the excellent yields of
4-bromoanisole.
tion, 2 mmol) was used as the catalyst, the relatively lower yield of
8
0% was obtained in comparison with that over 1 (Entry 9). Rea-
sonably, the catalytic nature for H TiF6 and 1 are identical which
2
2−
are both featured with a combination of proton acid and TiF6
counter-anion, but H TiF6 is absolutely hydrophilic, whereas 1 is
2
amphipathic. Probably, the oxidation of NaBr by H O catalyzed
2
2
−
by 1 to yield [OBr] and the subsequent electrophilic bromination
of the lipophilic anisole by hydrophilic [OBr] are more favored
−
H TiF
2
6
over 1 than H TiF . The amphipathicity of 1 might more adapt to
2
6
the oxidative bromination of lipophilic anisole by hydrophilic NaBr
and H O . On the other hand, 1 as a kind of organic acid is less cor-
2
2
1
rosive than the inorganic strong acid of H TiF , which facilitates
2
6
1
458
the experimental manipulation.
For the practical application, the request for easily separable
and recyclable catalysts is driven by economic consideration and
3101
1569
3
152
1B
environmental concerns. Since catalyst 1, H O2 and NaBr were all
2
6
5
20
39
very soluble in water, the separation of the lipophilic substrate of
anisole and the brominated product from the aqueous phase could
be conducted very easily through decantation. The left mixtures
were reused for next run. The recycling uses of the recovered 1 for
the bromination of anisole were given in Table 2. It was found that,
upon completion of the 1st run, 100% yield of 4-bromoanisole was
obtained along with the precipitation of the yellow solids from the
aqueous phase (which were characterized in Figs. 1 inset and 2).
When the left mixtures containing the aqueous phase and the yel-
low solids were treated with Na SO to destroy the unreacted H O ,
1267
1
037
1173
1126
9
05
3000
2500
2000
1500
-1
1000
500
Wavenumber (cm )
2
3
2
2
Fig. 2. The FT-IR spectra of H2TiF6, 1, and the yellow solids (1B).

L. Wang et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
59
Table 1
The oxidative bromination of anisole using H2O2 and NaBr catalyzed by 1
a
OCH3
TiF62-
1
2
OCH3
Br
N
N
N
N+
N+
N+
SO3H
2
NaBr/H O2
2
Cat.
TiF62-
2
SO42-
SO H
3
3
2
Entry
Catalyst (mmol)
NaBr (mmol)
H2O2 (mmol)
Conv. (%)b
Sel. (%)b,c
Yield (%)b,c
1
2
3
4
5
6
7
8
9
None
10
10
10
10
5
10
10
10
10
10
10
10
10
10
5
10
10
10
–
30
78
–
100
100
100
100
100
100
100
100
–
1 (1.0)
1 (1.5)
1 (2.0)
1 (2.0)
1 (2.0)
2 (2.0)
3 (2.0)
H2TiF6 (2.0)
30
78
100
56
62
41
39
84
100
56
62
41
39
84
a
b
c
◦
Anisole 5 mmol, H2O 3 mL, room temperature (R.T., 25 C), time 3 h.
Determined by GC.
To 4-bromoanisole.
Table 2
The recycling of 1 as the catalyst for the oxidative bromination of anisole using H2O2 and NaBr
a
Run
Catalyst
H2SO4 (mmol)
NaBr (mmol)
Conv. (%)b
Sel. (%)b,c
Yield (%)b,c
1
2
3
4
5
6
7
8
st (fresh)
1
–
–
–
–
–
–
–
–
–
2.5
2.5
2.5
2.5
2.5
2.5
10
100
19
97
95
99
99
98
99
100
100
100
100
100
100
100
100
100
19
97
95
99
99
98
99
e
nd
rd
th
th
th
th
th
5
d
e
5
d
e
5
d
e
5
5
d
e
d
e
5
d
e
5
a
b
c
◦
2.0 mmol, anisole 5 mmol, H2O2 10 mmol, 3 mL H2O, room temperature (R.T. 25 C), time 3 h.
Determined by GC and GC-MS.
To 4-bromoanisole.
.5 mmol H2SO4 was added additionally in each run.
mmol of NaBr was added in consideration of the cumulative NaBr in the previous run.
1
d
e
2
5
The total loss of 1 in term of Ti amount after eight-runs in the
due to the availability of the abundant iodinated aromatic com-
pounds for accomplishing coupling reactions. Since NaI was very
hygroscopic, KI was used in the case of iodination. However, when
KI was used as the iodination source, no iodinated product was
combined organic phase was below the detection limit (<0.1 ␮g/g
on the basis of the ICP analysis), indicating that the leaching of 1
in the organic phase is negligible in the courses of reaction and
separation.
◦
formed at all even if the reaction temperature increased to 60 C
Next, the oxidative halogenations using NaCl and KI were exam-
ined in Table 3 in parallel. In comparison with chlorination and
bromination, iodination is more concerned by organic chemists
(Entries 2 and 3). It was noted that, in the course of the oxida-
tion of KI by H O2 over 1, the formed non-polar iodine (I ) was
2
2
precipitated completely from the aqueous solution due to its poor
Table 3
a
The oxidative bromination of anisole with H2O2 and other halide salts catalyzed by 1
◦
Conv. (%)b
Sel. (%)b,c
Yield (%)b,c
Entry
Halogenation source
Solvent
Temp. ( C)
1
2
3
4
5
NaBr
KI
KI
KI
KI
H2O
H2O
H2O
R.T.
R.T.
60
R.T.
60
100
–
–
39
68
44
76
100
–
–
100
100
77/23
78/22
100
–
–
39
68
[Bmim]PF6-H2O (v/v = 2:1)
[Bmim]PF6-H2O (v/v = 2:1)
H2O
H2O
d
6
NaCl
NaCl
R.T.
60
34/10
59/17
e
7
a
1
40 mol%, anisole 5 mmol, halide salt 10 mmol, H2O2 10 mmol, solvent 3 mL, time 3 h.
b
c
Determined by GC.
To 4-halogenated anisole.
d
e
7
7
7% selectivity to 4-chloroanisole with the yield of 34%, and 23% selectivity to 2-chloroanisole with the yield of 10%.
8% selectivity to 4-chloroanisole with the yield of 59%, and 22% selectivity to 2-chloroanisole with the yield of 17%.

6
0
L. Wang et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
Table 4
The generality of 1 as a catalyst for the oxidative halogenation of different substrates with H2O2
a
◦
Conv. (%)b
100
Sel. (%)b
100
Yield (%)b
100
Entry
Substrate
Product
X-source
Temp. ( C)
Time (h)
3
OCH3
Br
OCH3
1
NaBr
R.T.
H C
3
OCH3
H C
OCH3
3
2
3
NaBr
NaBr
R.T.
60
3
3
95
92
100
100
95
92
Br
Br
CH3
CH3
Br
Br
Cl
CH3
4
5
6
CH3
NaBr
NaBr
NaCl
60
60
60
3
5
3
65
68
76
100
100
78
65
68
59
Cl
Cl
CH3
CH3
CH3
22
17
Cl
Cl
OCH3
Cl
OCH3
7
NaCl
60
3
71
78
55
16
OCH3
22
Cl
CH3
CH3
8
9
NaCl
NaCl
60
60
3
3
62
87
100
100
62
87
H C
OCH3
3
H C
OCH3
3
Cl
0c
OCH3
OCH3
CH3
I
I
I
OCH3
KI
KI
KI
60
60
70
3
3
4
46
63
62
100
100
100
46
63
62
1
1
1
1c,d
OCH3
2c,d
CH3
I
CH3
CH3
1
3c,d
KI
70
5
10
100
10
a
1
35 mol%, substrate 5 mmol, halide salt 10 mmol, 3 mL H2O, H2O2 10 mmol.
b
c
Determined by GC and GC–MS respectively.
[Bmim]PF6 2 mL + 1 mL H2O.
d
H2O2 15 mmol.
−
solubility in water. Consequently, the further oxidation of I to [IO]
of 4-iodoanisole was obtained (Entry 5). As for NaCl as the chlori-
nation source, the mono-chlorinated products were obtained with
the yield of 76% at 60 C, including 59% of 4-chloroanisole and 17%
2
by H O2 was depressed, leading to unaccomplished electrophilic
iodination of anisole by [IO] . Therefore, when the hydrophobic IL
2
−
◦
of [Bmim]PF6 was used as the co-solvent, 39% of anisole was con-
verted to the corresponding 4-iodoanisole with 100% selectivity at
R.T. (Entry 4); while the temperature increased to 60 C, 68% yield
of 2-chloroanisole (Entry 7).
It has been proposed that the overall oxidative halogenation
including two steps: the first one is catalytic oxidation of halide
◦

L. Wang et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
61
−
−
(
X ) to hypohalous ion ([XO] ); the second one is electrophilic
N
N
+
SO H
3
−
halogenation with [XO] as the active electrophile (non-catalytic
reaction) [5,7]. Although the oxidation of X happens with increas-
ing ease from Cl to I , the electrophilic activity of the resultant
XO] is in the ranking of [ClO] > [BrO] > [IO] . Consequently, the
F
−
F
F
F
2-
1
−
−
Ti
F
−
−
−
−
[
F
oxidative bromination using NaBr with moderate reactivity both
in catalytic oxidation and electrophilic halogenation exhibits the
fastest reaction rate. It means that the reaction rate of oxidative
halogenation catalyzed by 1 is in the ranking of NaBr ꢁ NaCl > KI.
The generality of 1 as a catalyst for the oxidative halogena-
tion by H O was examined on a series of aromatic compounds
with different electronic and steric effects. As shown in Table 4,
the mono-bromination of the activated aromatic compounds
such as anisole and 1-methoxy-4-methylbenzene were all per-
formed well at room temperature (Entries 1 and 2). 92% of
HO S
N
+
N
3
+
2 2
2H O
N
N+
SO -
3
2
2
2
2 2
2 H O
+
2HF
H
HO
O
Ti
F
F
F
F
2
-methylnaphthalene with bulky steric hindrance was bromi-
-
2
H
2
2
O
2XO-
◦
nated to 1-bromo-2-methylnaphthalene in 3 h at 60 C (Entry 3).
The bromination of toluene required the higher temperature and
prolonged reaction time (Entry 4). As for the bromination of 4-
chlorobenzene with the strong electron-withdrawing effect, the
moderate conversions of 68% were obtained with the excellent
selectivity to the para-mono-brominated product (Entry 5).
The oxidative chlorination of the aromatic compounds with
NaCl was listed in Entries 6–9. The activated substrates such as
anisole, 1-methoxy-4-methylbenzene, and 2-methylnaphthalene
were converted to the corresponding mono-chlorinated prod-
O
H
2X-
OH
2ArH
Peroxo-Ti species: 1A
2
2H O + O
D
e
a
c
ti
va
2ArX + 2OH-
ti
o
n
+
2 2
H O
F
F
O
Ti
O
1B
n
F
F
Polymerized Ti species
(
deactivated catalyst)
◦
ucts at 60 C in good to moderate yields (Entries 6–9). However,
HO
H+
two types of mono-chlorinated products were found in each
F
F
F
on
ti
riza
me
−
Ti
OH
1CC
case, due to the most active electrophilic activity of [ClO] .
epoly
F
D
2
-Chloroanisole and 2-chlorotoluene were found as the minor
products in Entries 6 and 7. 62% of 2-methylnaphthalene was
chlorinated to 1-chloro-2-methylnaphthalene with 100% selectiv-
ity (Entry 8); while 1-methoxy-4-methylbenzene possessing two
1
Scheme 1. The catalytic mechanism for the oxidative halogenations using H O2 and
2
kinds of electron-donating substituents ( OCH3 and CH ) was
halide salts catalyzed by 1.
3
converted to 2-chloro-1-methoxy-4-methylbenzene in the yield of
8
7%, without formation of other by-products (Entry 9).
The oxidative iodination of the aromatic compounds with KI
with small amount of NaBr and anisole, the absorbance of 363 nm
shifted to 354 nm rapidly, indicating the transformation of 1A to
the other species (defined as 1B) accompanied by the concur-
rent oxidative bromination of anisole to 4-bromoanisole (Fig. 1c).
in [Bmim]PF -H O was listed in Entries 10–13. The correspond-
6
2
ing mono-iodinated product of 4-iodoanisole was obtained with
the yield of 46% (Entry 10). Under the similar conditions, the yield
of 4-iodoanisole was improved up to 63% by using 3 equivalent
of H O (15 mmol, Entry 11). As for toluene, it was iodinated
The subsequent addition of H SO4 (98%) led to the red-shift of
2
354 (1B) to 368 nm instantaneously (Fig. 1d). The band of 368 nm
blue-shifted back to 363 nm (1A) again after the addition of H O ,
2
2
◦
to 4-iodotoluene with the yield of 62% in 4 h at 70 C (Entry
2
2
1
2
2). However, the conversion of 2-methylnaphthalene to 1-iodo-
-methylnaphthalene was quite sluggish even at the increased
indicating the complete regeneration of 1A (Fig. 1e). It has been
found that in the aforementioned results, upon completion of the
1st run reaction over 1 (Entry 1 of Table 2), the precipitated yellow
solid from water also showed a typical UV–vis absorbance band at
354 nm (Fig. 1inset), which was believed to be the same compound
as 1B in Fig. 1c.
temperature and prolonged reaction time, due to the bulky steric
hindrance and the relatively weak electrophilicity of [IO] (Entry
−
1
3).
3
.2. Characterization of peroxo-Ti species
The FT-IR spectra of such yellow solid (1B) were shown in Fig. 2.
In comparison to 1, the disappearance of the vibration bands at
3152 and 3101 (ꢀC H), 1569 and 1458 (ꢀC N, C C), and 1267,
The peroxo-Ti complexes are believed to be the active species
−
in the catalytic oxidations of organic compounds [24–27]. But the
formation and characterization of the peroxo-Ti species in the
oxidative halogenation are rarely reported. In this part, the charac-
terization of the active peroxo-Ti species derived from the oxidation
of 1 by H O was investigated by means of UV–vis spectra.
1173 and 1037 (ꢀSO3 ) revealed that 1B was a kind of inorganic
compounds not holding the SO H-grafted imidazolium tag. The
3
appeared peaks for 1B at 1126 and 905 might be ascribed to the
vibrations of Ti
O
Ti bond [28]. The characteristic vibration of
2−
−1
−1
TiF6 at 620 cm for 1 red-shifted to 539 cm for 1B, indicating
the bond energy of Ti-F in 1B was weakened due to the increased
polarity. According to the FT-IR spectra, 1B was attributed to be the
polymerized derivative of [O TiF4 O]n with poor aqueous solubil-
ity.
2
2
The UV–vis spectra were recorded on a SHIMADZU-UV 2550
◦
spectrophotometer at ambient temperature (ca. 25 C). The con-
centration of 1 in H O was ca. 4.0 × 10 M.
−5
2
As shown in Fig. 1, there was no any absorbance observed for
1
in H O (Fig. 1a). When H O was added to 1-H O, a strong
Based on the results in UV–vis (Fig. 1) and FT-IR analyses (Fig. 2),
a catalytic mechanism for the oxidative halogenations over 1 is
proposed tentatively in Scheme 1. The strong hydrogen-bond inter-
2
2
2
2
absorbance band appeared at ca. 363 nm immediately (Fig. 1b),
which was ascribed to the ligand-to-metal (O-to-Ti) charge transfer
(
CT) transition of the formed peroxo-Ti species (1A), in accordance
action between F. . .H in 1 facilitates the oxidative insertion of H O2
2
to Zecchina’s observations [27]. When the formed 1A was mixed
into the weak Ti F bonds in the axial position due to their relatively

6
2
L. Wang et al. / Journal of Molecular Catalysis A: Chemical 371 (2013) 56–62
longer bond length than those in the equatorial plane [29], lead-
ing to the successful formation of 1A (363 nm), and the consequent
release of HF and the Zwitterionic salt. Subsequently, the formed 1A
undergoes two pathways derivatively, including the catalytic cycle
and the deactivation. In the catalytic cycle, 1A as an active peroxo-
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21273077 and 21076083), Pro-
gram Cai Yuanpei (2011–2013), and 973 Program from Ministry of
Science and Technology of China (2011CB201403).
−
−
Ti species oxidizes X into [OX] , which rapidly reacts with the
−
substrate to afford the halogenated product (ArX) and OH , along
with the regeneration to 1A by H O insertion. Simultaneously, the
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[
2
2