Ceria nanoparticles as an efficient catalyst for oxidation of benzylic CH bonds

Article abstract of DOI:10.1016/j.molcata.2012.01.020

Catalytic oxidation of benzylic CH bonds with potassium bromate to carbonyl compounds was studied in the presence of ceria nanoparticles (NPs). Aldehydes and ketones in high yields were obtained when the oxidation was conducted in water/1,4-dioxane/acetic acid (AcOH) by ratio 5/1/1 (v/v/v). Benzyl esters were also yielded as the main products from the oxidation of benzylic CH bonds with potassium bromate in the presence of ceria NPs in glacial acetic acid. In comparison with other methods reported in the literature, ceria NPs as an efficient catalyst in oxidation of benzylic CH bonds have advantageous such as selectivity, recyclability, high reaction rate, and high yield of product because of their large specific surface area to volume ratio.

Full text of DOI:10.1016/j.molcata.2012.01.020

Journal of Molecular Catalysis A: Chemical 357 (2012) 67–72
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Ceria nanoparticles as an efficient catalyst for oxidation of benzylic C H bonds
∗
Batool Akhlaghinia , Hossein Ebrahimabadi, Elaheh K. Goharshadi, Sara Samiee, Soodabeh Rezazadeh
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad 9177948974, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Catalytic oxidation of benzylic C H bonds with potassium bromate to carbonyl compounds was studied
in the presence of ceria nanoparticles (NPs). Aldehydes and ketones in high yields were obtained when
the oxidation was conducted in water/1,4-dioxane/acetic acid (AcOH) by ratio 5/1/1 (v/v/v). Benzyl esters
were also yielded as the main products from the oxidation of benzylic C H bonds with potassium bromate
in the presence of ceria NPs in glacial acetic acid. In comparison with other methods reported in the
literature, ceria NPs as an efficient catalyst in oxidation of benzylic C H bonds have advantageous such
as selectivity, recyclability, high reaction rate, and high yield of product because of their large specific
surface area to volume ratio.
Received 5 November 2011
Received in revised form 15 January 2012
Accepted 23 January 2012
Available online 1 February 2012
Keywords:
Ceria nanoparticles
Benzylic C H bond
Aldehydes
©
2012 Elsevier B.V. All rights reserved.
Ketones
Benzyl esters
1
. Introduction
cheaper catalyst than CAN in the presence of sodium bromate can
oxidize benzylic C H bonds to aldehydes or ketones [14]. Again,
this method has some limitations such as long reaction times and
low yields.
Nanomaterials are more effective than conventional catalysts
because of their extremely small size and tremendous surface area-
to-volume ratio. In continuation of our research program to develop
more efficient synthetic methodologies and to remove the draw-
backs of the previous methods, the present study suggests a novel
and efficient nanocatalyst, ceria NPs, for oxidation of benzylic C H
bonds.
In synthetic organic chemistry, oxidation is an important
methodology for the introduction and modification of functional
groups [1–4]. During the last two decades, there has been a spec-
tacular development in this field and a large number of novel
and useful oxidation reagents and methods have been discovered.
The oxidation of benzylic C H bonds to their corresponding car-
bonyl compounds is of significant importance in organic chemistry
both for fundamental research and industrial manufacturing [5–8].
The world-wide annual production of carbonyl compounds is over
1
oxidation of alkyl arenes and alcohols [8].
The use of cerium salts with stoichiometric amounts have
been reported by several research groups [9,10]. Oxidations of
organic compounds with cerium (IV) are potentially interesting
since cerium (IV) is an unusually strong and one-electron oxidant.
The unique reactions of cerium (IV) with organic compounds can
be expected because of its specific coordination properties with
various organic and inorganic ligands. However, using cerium (IV)
in organic synthesis has been limited since the large quantities of
the reagent are required. To overcome this drawback, Amer et al.
7
× 10 tons and most of these compounds are produced from the
2. Experimental
2.1. General
The products were purified by column chromatography. The
purity determinations of the products were accomplished by TLC
on silica gel polygram STL G/UV 254 plates. The melting points of
products were determined with an Electrothermal Type 9100 melt-
ing point apparatus. The FT-IR spectra were recorded on an Avatar
3
70 FT-IR Therma Nicolet spectrometer. The NMR spectra were pro-
[
(
11–13] used the catalytic amounts of cerium ammonium nitrate
CAN) which is continuously regenerated by a less expensive oxi-
vided on Brucker Avance 100 and 400 MHz instruments in CDCl3.
All products were known compounds and characterized by IR and
dant such as KBrO . In Amer’s method, purification of products
was very difficult because of formation of by-products. Ceria as a
3
1
H NMR spectra and comparison of their melting points (or those
of the derivatives) with known compounds.
The phases of ceria NPs were determined by means of a
◦
Bruker/D8 Advanced diffractometer in the 2ꢀ range from 20 to
◦
◦
∗ Corresponding author. Tel.: +98 511 8795457; fax: +98 511 8795457.
E-mail address: akhlaghinia@um.ac.ir (B. Akhlaghinia).
80 , by step of 0.04 , with graphite monochromatic Cu K␣ radiation
(ꢁ = 1.541 A˚ ). In order to obtain a TEM image of the ceria NPs, the NPs
1
381-1169/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2012.01.020

6
8
B. Akhlaghinia et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 67–72
600
500
400
300
200
30
40
/deg
50
60
2
Fig. 1. The XRD pattern of ceria NPs.
were dispersed in ethanol using a sonocating bath. The TEM anal-
ysis of the catalyst was performed on a LEO 912 AB transmission
electron microscope and the electron beam accelerating voltage
was 120 kV. The surface area was measured by an ASAP-2010 sys-
tem from Micromeritics. The catalyst, ceria NPs was prepared and
purified by the method described in the literature [15].
Fig. 2. TEM image of ceria NPs.
2
.4. The oxidation of ethyl benzene to 1-phenylethyl acetate
To a solution of KBrO3 (0.167 g, 1 mmol) in anhydrous acetic
−4
acid (3 ml), NaOAc (0.082 g, 1 mmol) and ceria NPs (5 × 10 g,
0
.003 mmol) was added at room temperature with continuous stir-
ring. Ethyl benzene (0.104 g, 1 mmol) was added with stirring for 1 h
at 110 C. Then, the red brown suspension was filtered to remove
2.2. Characterization of the catalyst
◦
the catalyst. The filtrate was extracted with cold H O/CH Cl
2
2
2
Fig. 1 illustrates the XRD pattern of ceria NPs. All the diffraction
peaks can be indexed to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), (4 0 0),
3 3 1), and (4 2 0) reflections corresponding to the face-centered
cubic phase with the lattice parameter of a = b = c = 0.5410 nm. No
additional peak is seen in the XRD pattern of the catalyst which
reveals the high purity of the prepared ceria NPs.
The average crystallite size, D, can be calculated by the well-
known Scherrer’s equation:
(
3 × 10 ml). The organic layer was dried with anhydrous Na SO
2
4
and passed through a short silica-gel column using n-hexane/ethyl
acetate (8/1) as an eluent. After removing the solvent under reduced
pressure, 1-phenylethyl acetate was obtained in 95% yield.
(
3. Results and discussion
Herein, we report a novel and efficient method for oxidation of
h × l
benzylic C H bonds to the corresponding aldehydes and ketones
(Scheme 1).
Dh k l
=
(1)
ˇh k l × cos ꢀ
h k l
The optimum reaction parameters, shown in Scheme 1, were
chosen after examining a variety of reaction factors. The oxida-
tion of ethyl benzene was initially performed in the presence of
where Dhkl is the crystallite size perpendicular to the normal line
of (hkl) plane, k is a constant (0.9), ˇhkl is the full width at half
maximum of the (hkl) diffraction peak (FWHM), ꢀhkl is the Bragg
angle of (hkl) diffraction peak, and ꢁ is the wavelength of X-ray.
The average crystallite size of ceria NPs is about 10 nm.
The TEM image of ceria NPs was shown in Fig. 2. All the particles
display the uniform cubic morphology with the average particle
size of about 8 nm which is in a good agreement with the results
deduced from the XRD. The BET specific surface area of NPs was
−4
a catalytic amount of ceria NPs (5 × 10 g, 0.003 mmol) and a vari-
ety of oxidants, different molar ratios of substrate/oxidant, and
various solvents (Table 1). As Table 1 (entries 6–14) shows the
use of ceria NPs/potassium bromate is the most efficient mixed
reagent system for the oxidation of ethylbenzene to acetophe-
none. The high surface area-to-volume ratio of ceria NPs is mainly
responsible for their high catalytic activity. Replacing KBrO3 with
H5IO , NaClO , H O , and NaIO do not produce any acetophe-
2
−1
.
9
2 m g
6
3
2
2
3
none (Table 1, entries 1–4) since these oxidants cannot reproduce
Ce (IV). Another mixed reagent system, ceria NPs/KMnO , pro-
duces acetophenone in only 20% yield (entry 5). The effect of
2.3. The oxidation of ethyl benzene to acetophenone
4
different molar ratios of PhCH CH /oxidant (entries 6–8), vari-
ous temperatures (entries 9–11), and different ratios of (v/v/v)
To
dioxane/glacial acetic acid (5/1/1, v/v/v 3 ml), ceria NPs (5 × 10 g,
.003 mmol) were added at room temperature with continuous
a
solution of KBrO3 (0.167 g, 1 mmol) in H O/1,4-
2
3
2
−
4
H O/1,4-dioxane/AcOH (entries 11–15) were also examined. By
0
2
diluting the reaction media by distilled water, the reaction rate
stirring. The reaction mixture was stirred for 5 min. Ethyl benzene
◦
(
0.104 g, 1 mmol) was added with stirring for 1 h at 95 C. Then,
the red brown mixture was filtered to remove the catalyst. The fil-
O
o
CeO (nanoparticles) / KBrO , 95 C
trate was extracted with CH Cl2 (2 × 5 ml). The organic layer was
2
3
2
Ar
R
dried with anhydrous Na SO4 and passed through a short silica-
Ar
R
2
gel column using n-hexane/ethylacetate (8/1) as an eluent. After
removing the solvent under reduced pressure, acetophenone was
obtained in 80% yield.
H O / 1,4-dioxane / AcOH
2
Scheme 1.

B. Akhlaghinia et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 67–72
69
Table 1
The oxidation of ethylbenzene with various oxidants, different molar ratios of substrate/oxidant, different ratios of (v/v/v) H2O/1,4-dioxane/AcOH, and various temperatures
in the presence of catalytic amount of ceria NPs.
◦
Entry
Oxidant
Substrate/oxidant
H2O/1,4-dioxane/AcOH
Temperature ( C)
Time (h)
Conversion (%)
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
H5IO6
NaClO3
H2O2
1:1
1:1
1:1
1:1
1:1
1:2
1:1
1:0.5
1:1
1:1
1:1
1:1
1:1
1:1
1:1
5:1:1
5:1:1
5:1:1
5:1:1
5:1:1
5:1:1
5:1:1
5:1:1
5:1:1
5:1:1
10:1:1
15:1:1
20:1:1
5:1:0
H2O
95
95
95
95
95
95
95
95
70
50
95
95
95
95
95
12
12
12
12
12
1
0
0
0
0
NaIO3
KMnO4
KBrO3
KBrO3
KBrO3
KBrO3
KBrO3
KBrO3
KBrO3
KBrO3
KBrO3
KBrO3
20
100
100
50
100
30
100
100
100
100
0
1
12
10
12
75 min
1.5
2
1
1
1
1
1
1
2.5
12
Table 2
benzyl acetates obtained from the oxidation of various arenes
were shown in Table 4. In most cases, excellent yields were
obtained.
The oxidation of diphenylmethane to benzophenone under different molar ratios of
diphenylmethane/ceria NPs.
Entry
Molar ratio of
diphenylmethane/ceria NPs
Time (min)
Conversion (%)
The results of Tables 3 and 4 suggest that the oxidation reaction
proceeds via the formation of a benzylic carbocation which reacts
with water and acetic acid to give the corresponding hydroxyl or
ester derivatives, respectively. The oxidation of benzylic C H bonds
to carbonyl compounds in aqueous acetic acid probably proceeds
through the alcohol since the alcohol is present in small amounts
at the end of the reaction. The alcohols were oxidized to the corre-
sponding carbonyl groups rapidly by Ce (IV) [33]. The Ce (IV) and
bromate salts act as efficient oxidants. The bromate salt oxidizes Ce
1
2
3
4
5
6
7
100
200
300
350
400
500
600
30
30
30
30
30
30
30
100
100
100
93
82
45
20
(
III) ion. It is now generally accepted that the side-chain oxidation of
was decreased (entries 11–15). Employing the 1/1 molar ratio of
arenes by Ce (IV) occurs via an electron-transfer mechanism lead-
ing to the formation of carbocations as the reaction intermediates
◦
PhCH CH /oxidant at 95 C in 5/1/1 (v/v/v) H O/1,4-dioxane/AcOH
2
3
2
gave the best results and produced acetophenone after 1 h in a
quantitative yield (entry 7). Whereas, Shi et al. [14] obtained ace-
tophenone from ethylbenzene in 85% yield after 24 h by using bulk
ceria and applying our similar conditions.
[
34–37]. The suggested mechanism was given in by the following
reactions [35] (Scheme 3).
As it was mentioned before, subsequent reaction of the carbo-
cation with water and acetate ion leads to formation of alcohol
and acetate ester, respectively. The presence of alcohol in the pro-
posed mechanism was investigated by performing the oxidation
reaction on 1-phenylethanol in aqueous and anhydrous conditions.
The oxidation of benzylic C H bonds with KBrO in the presence
3
of catalytic amounts of ceria NPs exhibited high efficiency. In a set of
experiments, we studied the oxidation of diphenylmethane to ben-
zophenone with different molar ratios of diphenylmethane/ceria
NPs. The results of this study were shown in Table 2.
On the basis of the data in Table 2, the best molar ratio for
oxidation of diphenylmethane to benzophenone is 300. Using the
results of Table 2, it is possible to calculate turnover number for
this transformation (Scheme 2) [16]:
◦
By employing 1/1 molar ratio of substrate/oxidant at 95 C in 5/1/1
(v/v/v) H O/1,4-dioxane/AcOH, the alcohol was oxidized to ace-
2
tophenone completely after 30 min whereas ethylbenzene after
6
0 min produced acetophenone at the same reaction conditions.
Various structurally arenes were rapidly oxidized to the cor-
responding carbonyl compounds (aldehydes and ketones) with
excellent conversions in a short reaction time (Table 3). Oxidation
of toluene leads to formation of benzaldeyde as the chief prod-
uct (entry1). 1-Ethyl-4-methyl-benzene, as a dialkyl benzene, was
oxidized selectively to 1-p-tolyl-ethanone in high yield (entry 3).
Also, 2-phenyl propane-2-ol and acetophenone by 78/17 molar
ratio were obtained from the oxidation of iso-propyl benzene (entry
.
ArCHR2 + Ce+4
ArCR + Ce + H+
+3
2
.
+
+
4
+3
ArCR2 + Ce
ArCR2 + Ce
+
ArCH(OAC)R + H
2
8
). 3,4-Dihydro-10H-antracene-9-one and 2-benzyl pyridine were
+
ArCR
2
oxidized to the corresponding ketones in excellent yields (entries
7
and 9).
ArC(OH)R2 + H+
Ce+4
We observed that in anhydrous acetic acid as solvent and
in the presence of sodium acetate and ceria NPs/KBrO3 system,
arenes can be converted to benzyl acetates. The yields of the
mole of prudact
ArCOR + Ce+3 + H+
+
turn over number =
= 300
_
mole of catalyst
4Ce+3 + BrO + 5H
4Ce+4 + HOBr + 2H O
2
3
Scheme 2.
Scheme 3.

7
0
B. Akhlaghinia et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 67–72
Table 3
The oxidation of benzylic C H bond catalyzed with ceria NPs/KBrO3.
Producta
Time (min)
2 (h)
Isolated yield (%)
90
m.p. C (lit [Ref.])
◦
Entry
Substrate
O
O
1
H
O
Oil [17]
Oil [18]
Oil [19]
2
3
60
60
80
85
O
4
15
90
51–52(50–53 [18])
Br
Br
O
H
5
6
5 (h)
75
95
105–107(106–108 [20])
O N
2
O N
2
O
O
O
45
Oil [21]
O
7
8
30
95
285–287(285–288 [22])
OH
O
120
78/17
Oil [23,18]
O
9
4 (h)
95
Oil [24]
N
N
a
The products was identified by the comparison of its physical constants, IR and NMR spectral data with those of an authentic sample.
According to the results, it is plausible to conclude that formation
of carbonyl compounds from the oxidation of arenes in aqueous
conditions passes through the formation and then oxidation of
alcohol.
On the basis of this study, in anhydrous media, formation of
acetate esters only proceeds through the reaction of carbocation
with acetate ion and not through the formation of alcohol.
Oxidation of iso-propyl benzene leads to formation of 2-phenyl-
propan-2-ol as a tertiary alcohol and acetophenone (Table 3, entry
When the oxidation of 1-phenylethanol was performed in anhy-
drous conditions, acetic acid 1-phenylethyl ester and acetophenone
were produced in 50% and 20% yield, respectively after 60 min. The
oxidation reaction was not completed even after prolonged reac-
tion time (3–4 h) and 30% of 1-phenylethanol was remained intact.
Acetic acid 1-phenylethyl ester may be obtained from dehydration
of 1-phenylethanol and the subsequent reaction of the correspond-
ing carbocation with acetate ion or via from the esterification
reaction of 1-phenylethanol with acetic acid. Also, acetophenone
was obtained from oxidation of 1-phenylethanol in acidic and
anhydrous media (Scheme 4).
H
+
H
H / OAc
50%
OAc
-
H O
2
OH
Ce+4
+
2
0%
30%
O
Scheme 4.

B. Akhlaghinia et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 67–72
71
Table 4
The oxidation of benzylic C H bonds catalyzed with ceria NPs/KBrO3 in anhydrous acetic acid.
Producta
◦
Time (min) Conversion (%) Isolated yield% m.p. C (lit [Ref.])
Entry Substrate
OAc
1
120
60
100
100
92
95
Oil [25]
Oil [26]
OAc
2
3
OAc
60
45
100
100
80
81
Oil [27]
Oil [28]
OAc
4
Br
Br
OAc
5
6
30
100
100
80
55
Oil [29]
OAc
300
78–79 (78 [30])
O N
2
O N
2
O
O
O
O
7b
30
100
5/90
105 (107–109 [31])/287(285–288 [22])
OAc
OAc
8
120
60
100
100
85
92
Oil [32]
Oil [33]
OAc
N
9
N
a
The products were identified by the comparison of its physical constants, the IR and NMR spectral data with those of an authentic sample.
The reaction was performed at 65 C.
b
◦
8
). According to the proposed mechanism in Scheme 3, forma-
water, and drying it under vacuum. The recovered ceria NPs can
be used again for subsequent reactions. In a set of experiments, we
investigated the recyclability of the catalyst (Table 5).
The complete conversion of diphenylmethane to benzophenone
was obtained in the presence of ceria NPs which recovered for
several times. For all time, the recycled catalyst gave 100% con-
version. In this study, ceria NPs as an interesting catalyst provide
a novel method for oxidation of benzylic C H bonds. This catalyst
shows the excellent selectivity and high reusable capacity which is
tion of tertiary carbocation and subsequent reaction with water
produces 2-phenyl propane-2-ol as the major product of the oxi-
dation reaction. Also, the tertiary-formed radical can abstract
methyl radical from iso-propyl benzene which produced iso-propyl
benzyl radical and t-butyl benzene. The oxidation of iso-propyl
benzyl radical and its subsequent reaction with water leads to
formation of 1-phenylethanol which then produced acetophe-
none as minor product after oxidation with ceria NPs/KBrO3
[
14,34] (Scheme 5).
The oxidation of 10H-anthracen-9-one (Table 4, entry 7) pro-
duces acetic acid 10-oxo-9, 10-dihydro-anthracen-9-yl ester with
% and anthraquinone with 90% isolated yield. The oxidation reac-
Table 5
The oxidation of diphenylmethane to benzophenone in the presence of reused ceria
NPs.
5
tion produces only one product which is an acetate ester. The
obtained acetate ester converts to anthraquinone during the aque-
ous work up process (Scheme 6) [38].
One of the advantages of the ceria NPs as the catalyst is its abil-
ity to function as a recyclable reaction medium. We were able to
separate ceria NPs from the reaction medium easily by filtering the
reaction mixture, washing the residue with dichloromethane and
◦
Entry
Time (min)
Temperature ( C)
Conversion%
Isolated yield%
1
2
3
4
5
6
30
30
30
30
30
30
95
95
95
95
95
95
100
100
100
100
100
100
92
89
95
93
96
95

7
2
B. Akhlaghinia et al. / Journal of Molecular Catalysis A: Chemical 357 (2012) 67–72
H
Unlike previously reported methods, ceria NPs requires short
.
reaction time to obtain high yields of the products. In summary,
the catalyst introduced in the present study gives high yields and it
has the operational simplicity and easy recyclability. Further appli-
cations of this reagent combination are under investigation in our
laboratory.
+
Ce+4
+ Ce+3 + H+
H
H
.
.
Acknowledgements
+
+
The authors gratefully acknowledge the partial support of this
study by Ferdowsi University of Mashhad Research Council (Grant
No. p/3/15928) and Dr. N. Ashraf for her intellectual contributions.
H
H
+
.
+
+
Ce+4
+ Ce+3
References
[
[
1] C.N. Satterfield, J.E. Backvall (Eds.), Modern Oxidation Methods, second ed.,
Wiley-VCH, Germany, 2004, and references therein.
2] R.C. Larock, Comprehensive Organic Transformations, second ed., Wiley-VCH,
New York, 1999, p. 1234.
H
H
+
OH
+ H+
[3] R.A. Sheldon, J.K. Kochi, Metal-catalyzed Oxidation of Organic Compounds, Aca-
demic Press, New York, 1981, p. 350.
H O
2
[
4] B.M. Trost, I. Fleming, S.V. Ley, Comprehensive Organic Synthesis, vol. 7, Perg-
amon, Oxford, 1991.
H
[
[
[
5] B.Z. Zhan, A. Thompson, Tetrahedron 60 (2004) 2917–2935.
6] Y. Ishii, S. Sakaguchi, T. Iwahama, Adv. Synth. Catal. 343 (2001) 393–427.
7] M.J. Schultz, M.S. Sigman, Tetrahedron 62 (2006) 8227–8241.
OH + Ce+4
+ Ce+3 + H+
[8] K. Weissermel, H.J. Arpe, C.R. Lindley (Eds.), Industrial Organic Chemistry, third
ed., Willey-VCH, New York, 1997.
O
[
9] L. Eberson, E. Oberrauch, Acta Chem. Scand. B 33 (1979) 343–346.
10] (a) C.M. Selwitz, E.R. Tucci, U.S. Patent, 3, 349, 117 (1967) and 3, 346, 622 (1967);
b) W.S. Trahanovsky, L.B. Young, J. Org. Chem. 31 (1966) 2033–2035.
[
Scheme 5.
(
[
[
11] E. Ganin, I. Amer, J. Mol. Catal. A: Chem. 116 (1997) 323–327.
12] E. Ganin, I. Amer, Synth. Commun. 25 (1995) 3149–3154.
O
[13] G.A. Olah, B.G.B. Gupia, A.P. Fung, Synthesis (1980) 897–898.
14] Q.Z. Shi, J.G. Wang, K. Cai, Synth. Commun. 29 (1999) 1177–1181.
[15] (a) E.K. Goharshadi, S. Samiee, P. Nancarrow, J. Colloid Interface Sci. 356 (2011)
73–480;
b) S. Samiee, E.K. Goharshadi, Mater. Res. Bull., doi:10.1016/j.
O
[
Ceria NPs/ KBrO3
HOAc
4
(
materresbull.2011.12.058, in press.
[16] Y. Wang, H. Zhao, Z. Hu, M. Kimura, Z. Zhiqiang, L. Peng, K. Hiratsuka, Synthesis
OAc
(
2011) 287–292.
[17] B.R. Kim, H.G. Lee, E.J. Kim, S.G. Lee, Y.J. Yoon, J. Org. Chem. 75 (2) (2010)
84–486.
[18] T. Nobuta, S.I. Hirashima, N. Tada, T. Miura, A. Itoh, Org. Lett. 13 (2011)
576–2579.
19] J.N. Moorthy, K. Senapati, K.N. Parida, J. Org. Chem. 75 (24) (2010) 8416–
421.
[20] B. Landers, C. Berini, C. Wang, O. Navarro, J. Org. Chem. 76 (5) (2011) 1390–1397.
O
O
O
4
H O/ H +
Ceria NPs/ KBrO3
HOAc
2
2
[
8
AcO
OAc
[
[
[
[
[
[
21] A. Ashnagar, J.M. Bruce, Asian J. Chem. 22 (2010) 4602–4610.
22] M. Hatano, O. Ito, Sh. Suzuki, K. Ishihara, J. Org. Chem. 75 (15) (2010) 5008–5016.
23] H. Li, M. Yang, Y. Qi, J. Xue, Eur. J. Org. Chem. 14 (2011) 2662–2667.
24] A.K. Chakraborti, S.V. Chankeshwara, J. Org. Chem. 74 (2009) 1367–1370.
25] P. Chen, J. Qu, J. Org. Chem. 76 (9) (2011) 2994–3004.
Scheme 6.
easily separated from the reaction mixture. Availability, safety, ease
in handling of the reagents, high yields, and mildness of the reac-
tion conditions make this method novel for oxidation of benzylic
C H bonds to the corresponding carbonyl compounds and esters.
This method is expected to be widely applicable to organic synthe-
sis due to the low cost of the reagents. We believe this protocol will
be a useful method in modern synthetic organic chemistry.
26] L.K. Sydnes, M. Sandberg, Tetrahedron 53 (1997) 12679–12690.
[27] M. Paeivioe, L.T. Kanerva, D. Mavrynsky, R. Leino, Eur. J. Org. Chem. 8 (2011)
452–1457.
1
[
[
28] S. Magens, B. Plietker, J. Org. Chem. 75 (11) (2010) 3715–3721.
29] M. Barbero, S. Bazzi, S. Cadamuro, S. Dughera, Eur. J. Org. Chem. 3 (3) (2009)
430–436.
[
[
30] J. Fleming, H. Shechter, J. Org. Chem. 34 (1969) 3962–3969.
31] M. Hatano, Y. Furuya, T. Shimmura, K. Moriyama, Sh. Kamiya, T. Maki, K. Ishi-
hara, Org. Lett. 13 (2011) 426–429.
[
[
[
32] D. Giomi, R. Alfini, A. Brandi, Tetrahedron 67 (2011) 167–172.
33] W.S. Trahanovsky, L.B. Young, J. Chem. Soc. (1965) 5777–5778.
34] W.J. Mijs, C.R.H.I. De Jonge, Organic Syntheses by Oxidation with Metal Com-
pounds, Plenum Press, New York, 1986.
4
. Conclusions
We developed an efficient and simple procedure for the selec-
[35] (a) M. Orban, E. Koros, R.M. Noyes, J. Phys. Chem. 83 (1979) 3056–3057;
b) R.M. Noyes, R.J. Field, R.C. Thompson, J. Am. Chem. Soc. 93 (1971) 7315–7316.
(
tive oxidation of benzylic C H bonds using ceria NPs and KBrO as
3
[
[
36] E. Baciocchi, L. Mandolini, C. Rol, Tetrahedron Lett. 17 (1976) 3343–3346.
37] D. Benson, Mechanisms of Oxidation by Metal Ions, Elsevier, New York,
1976.
oxidant. Most importantly, the ceria NPs could also be recycled and
reused in subsequent reactions without any significant loss of activ-
ity. This method possesses increased compatibility with different
functional groups in mild conditions.
[38] (a) L.F. Fieser, S.T. Putnam, J. Am. Chem. Soc. 69 (1974) 1038–1041;
b) J. Gronowska, A. Dzielefidziak, Tetrahedron Lett. 23 (1982) 5575–5578.
(