Cerium(III) bromate as a new reagent in oxidation of organic compounds

Article abstract of DOI:10.1081/SCC-120020194

Cerium(III) bromate, prepared by the reaction of barium bromate with cerium(III) sulfate, can be used for the oxidation of alkylarenes, alcohols, and sulfides. Products are obtained in high yields under mild conditions. Ce(BrO₃)₃ is an unique, dual property reagent in which the anion is the oxidant and the cation a catalyst. Stoichiometric studies indicate that bromate is reduced to bromide under these conditions, making the process highly efficient.

Full text of DOI:10.1081/SCC-120020194

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Cerium(III) Bromate as a New Reagent in Oxidation of
Organic Compounds
Ahmad Shaabani a & Donald G. Lee b
a Chemistry Department, Shahid Beheshti University, Tehran, Iran
b Department of Chemistry, University of Regina, Regina, Canada
Version of record first published: 17 Aug 2006
To cite this article: Ahmad Shaabani & Donald G. Lee (2003): Cerium(III) Bromate as a New Reagent in Oxidation of Organic
Compounds, Synthetic Communications: An International Journal for Rapid Communication of Synthetic Organic Chemistry,
33:11, 1845-1854
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SYNTHETIC COMMUNICATIONSÕ
Vol. 33, No. 11, pp. 1845–1854, 2003
Cerium(III) Bromate as a New Reagent in
Oxidation of Organic Compounds
2
Ahmad Shaabani1, and Donald G. Lee
*
1Chemistry Department, Shahid Beheshti University,
Tehran, Iran
2Department of Chemistry, University of Regina,
Regina, Canada
ABSTRACT
Cerium(III) bromate, prepared by the reaction of barium bromate
with cerium(III) sulfate, can be used for the oxidation of alkylarenes,
alcohols, and sulfides. Products are obtained in high yields under
mild conditions. Ce(BrO3)3 isan unique, dual property reagent
in which the anion isthe oxidant and the cation a catalyts.
Stoichiometric studies indicate that bromate is reduced to bromide
under these conditions, making the process highly efficient.
*Correspondence: Ahmad Shaabani, Chemistry Department, Shahid Beheshti
University, P.O. Box 19396-4716, Tehran, Iran; Fax: þ98-21-2403041; E-mail:
a-shaabani@cc.sbu.ac.ir.
1845
DOI: 10.1081/SCC-120020194
Copyright & 2003 by Marcel Dekker, Inc.
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1846
Shaabani and Lee
INTRODUCTION
Cerium(IV) ammonium nitrate (CAN) hasbeen used extensively as
an oxidant in organic chemistry. It has been employed both as a stoichio-
metric oxidant and asa catalyst in combination with other co-oxidants
for the oxidation of toluene and substituted toluenes to the correspond-
ing benzaldehydes,[1] ethylbenzene and diphenylmethane to acetophe-
none and benzophenone, respectively,[1,2] benzylic alcoholsto
[4]
aldehydes,[3] phenolsto quinones
and sulfides to sulfoxides.[5] When
coupled with a co-oxidant such as sodium bromate, cerium(IV) has
also been as a catalyst for the oxidation of alcohols,[6–8] sulfides,[9] and
aromatic side chains.[10] The oxidation reactionsof cerium(IV) have been
reviewed by several authors.[11–13]
Although very useful for many purposes, CAN suffers from a pair
of disadvantages: (i) because it is a high molecular weight, one-electron
transfer agent, large quantities are required for certain applications.
In particular, the oxidation of alkylarene, to the corresponding alpha-
ketones, which is a four electron process, requires over 2 kg of oxidant
for each mole of reductant; and (ii) since acidic conditions are required,
acid sensitive compounds cannot be oxidized without causing undesirable
degradations. Some of these disadvantages can be overcome by using
catalytic amounts of CAN coupled with a less expensive multi-electron
transfer agent such as sodium bromate; however, rather acidic conditions
are still required. For example, both CAN and CeðSO4Þ2 Á 2H2SO4,[7]
which produce acidic aqueous solutions are active catalysts, while
corresponding neutral compounds such as cerium(III) chloride,[7]
cerium(IV) oxide,[7] and cerium(IV) fluoride[14] are much less active.
In continuation of our studies on the oxidation of organic compounds
with bromate ion,[15–19] we wish to report that the above mentioned
problemscan be overcome by the use of cerium(III) bromate, which gives
excellent yields in the oxidation of alkylarenes, alcohols, and sulfides under
neutral conditions. Ce(BrO3)3 isa unique oxidant that containsboth the
catalyst, Ce3þ, and the co-oxidant, BrOÀ3 , in a single compound. Although
the preparation of Ce(BrO3)3 was first described about a century ago,[20]
a search of literature failed to uncover any previous reports of its use as
an oxidant for organic compounds. However, as described in the following
report, we have found that it isa very convenient reagent for the oxidation
of alkylarenes, alcohols, and sulfides in aqueous acetonitrile.

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Cerium(III) Bromate in Oxidation
EXPERIMENTAL SECTION
Oxidant
1847
Ce(BrO3)3 is easily prepared in situ by the reaction of cerium(III)
sulfate with barium bromate, as in Eq. 1. Barium sulfate precipitates,
leaving a solution of the oxidant that can be used without further
modification.
3BaðBrO3Þ2 þ Ce2ðSO4Þ3 À! 2CeðBrO3Þ3 þ 3BaSO4 #
ð1Þ
Reductants
The reductants used in this study were obtained commercially
(Aldrich) and used without additional purification.
Products
All productsare known compounds, which were identified by com-
parison of their physical and spectral data with literature values. Melting
pointswere meausred on an Electrothermal 9100 apparatusand are
uncorrected. Infrared psectra were recorded asneat filmsor asKBr
pellets on a Perkin Elmer FT-IR 1600 spectrophotometer. NMR spectra
were recorded on a Bruker AC-200 spectrometer at 200 MHz with CDCl3
asoslvent and (CH 3)4Si as internal standard. Yields refer to isolated
[21]
productsor 2,4-dinitrophenylhydrazone (2,4-DNP) derivatives.
A Typical Procedure for the Oxidation of Alkylarenes
Cerium(III) sulfate hydrate (0.133 g, 0.23 mmol) and barium bromate
(0.295 g, 0.75 mmol) were added to indan (0.177 g, 1.50 mmol) in aqueous
acetonitrile (CH3CN:H2O ¼ 7:3; 10 mL). The reaction mixture was
refluxed and the progress of the reaction monitored by TLC (eluent;
hexane:ethyl acetate ¼ 2:1). When the reaction wascompleted (after 2 h),
the product mixture was filtered through a sinter glass funnel and the
residue washed with methylene chloride (10 mL). The filtrate was treated
with saturated aqueous sodium thiosulfate (5 mL) in order to reduce any
residual oxidant. The resulting mixture was extracted with methylene
chloride (2 Â 10 mL) and the combined organic layerswere dried over

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1848
Shaabani and Lee
anhydrous magnesium sulfate. Evaporation of the solvent gave
1-indanone (0.176 g, 1.33 mmol, 89%). Thisproduct wasidentified by
comparison with authentic 1-indanone using TLC, melting point
(41–42ꢀC; Lit.[22] 42ꢀC) and 1H NMR (200 MHz, CDCl3): ꢀ 2.63
(t, J ¼ 6.10 Hz, 2H), 3.09 (t, J ¼ 6.10 Hz, 2H), 7.08–7.80 (m, 4H).
The stoichiometry of the reaction was determined by monitoring
the extent to which indan wasconverted to indanone by limited
amounts of oxidant. Indan (0.177 g, 1.5 mmol) was dissolved in aqueous
acetonitrile (CH3CN:H2O ¼ 7:3; 10 mL), cerium(III) sulfate hydrate
(0.133 g, 0.230 mmol) and barium bromate (0.197 g, 0.500 mmol) were
added and the reaction wasallowed to proceed at reflux temperature
until all of the oxidant had been consumed (20 h). The extent of reaction
1
was then established by comparing peak integrals in the H NMR spec-
trum for indan (ꢀ 2.03 and 2.88 ppm) and 1-indanone (ꢀ 2.63 and
3.09 ppm). The experiment was repeated two more times using less
barium bromate (0.375 and 0.25 mmol). From a plot (r2 ¼ 0.98) of
unreacted indan vs. the amount of bromate reduced it was found that
1.50 mmol of indan reacted with 1.06 mmol BrOÀ3 (or 0.35 mmol
Ce(BrO3)3).
A Typical Procedure for the Oxidation of Sulfides
Cerium(III) sulfate hydrate (0.100 g, 0.17 mmol) and barium bromate
(0.200 g, 0.51 mmol) were added to methyl p-tolyl sulfide (0.138 g,
1.00 mmol) in aqueousacetonitrile (CH 3CH:H2O ¼ 7:3; 10 mL). The
reaction mixture wastsirred at room temperature and the progresof
the reaction monitored by TLC (eluant; chloroform:ethyl acetate ¼ 1:1).
When the reaction wascomplete (after 50 min), the product mixture
was filtered through a sinter glass funnel and the residue washed with
methylene chloride (10 mL). The filtrate wastreated with asturated
aqueous sodium thiosulfate (2.5 mL) in order to reduce any residual
oxidant. The resulting mixture was extracted with methylene chloride
(2 Â 10 mL) and the combined organic layerswere dried over anhydrous
magnesium sulfate. Evaporation of the solvent gave methyl p-tolyl
sulfoxide (0.150 g, 0.99 mmol, 97%). This product was identified by
comparison with authentic methyl p-tolyl sulfoxide using TLC, melting
point (45–46ꢀC; Lit.[23] 44–46ꢀC) and 1H NMR (200 MHz, CDCl3): ꢀ 2.43
(s, 3H), 3.70 (s, 3H), 7.43 (d, J ¼ 8.51 Hz, 2H), 7.55 (d, J ¼ 8.50 Hz, 2H).
The stoichiometry of this reaction was established as described above
for the oxidation of indan to indanone. The results showed each mmol of
sulfide reacted with 0.35 mmol BrO3 (or 0.12 mmol Ce(BrO3)3).

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Cerium(III) Bromate in Oxidation
A Typical Procedure for the Oxidation of Alcohols
1849
Cerium(III) sulfate hydrate (0.110 g, 0.186 mmol) and barium bromate
(0.393 g, 1.0 mmol) were added to 1-indanol (0.268 g, 2.0 mmol) in aqueous
acetonitrile (CH3CN:H2O ¼ 7:3; 10 mL). The reaction mixture was
refluxed and the progress of the reaction monitored by TLC (eluant;
hexane:ethyl acetate ¼ 2:1). When the reaction wascomplete (after 2 h),
the product mixture was filtered through a sinter glass funnel and the
residue washed with methylene chloride (10 mL). The filtrate was treated
with saturated aqueous sodium thiosulfate (5 mL) in order to reduce
any residual oxidant. The resulting mixture was extracted with methylene
chloride (2 Â 10 mL) and the combined organic layerswere dried
over anhydrous magnesium sulfate. Evaporation of the solvent gave
1-indanone (0.264 g, 2.0 mmol, 100%). Thisproduct wasidentified by
comparison with authentic 1-indanone using TLC, melting point
(41–42ꢀC; Lit.[22] 42ꢀC) and 1H NMR (200 MHz, CDCl3): ꢀ 2.63
(t, J ¼ 6.10 Hz, 2H), 3.09 (t, J ¼ 6.10 Hz, 2H), 7.08–7.80 (m, 4H).
The stoichiometry of this reaction was then determined as described
above for the oxidation of indan to indanone. The results showed 2 mmol
of alcohol reacted with 0.69 mmol BrOÀ3 (or 0.23 mmol Ce(BrO3)3).
RESULTS ANDDISCUSSION
The Oxidation of Alkylarenes
From a quantitative study of the reaction of indan with limited
amountsof oxidant it wasdetermined that 1.5 mmol of alkylarene reacts
with 0.35 mmol Ce(BrO3)3, giving the stoichiometry expressed in Eq. 2.
9ArCH2R þ 2CeðBrO3Þ3 À! 9ArCOR þ 2CeBr3 þ 9H2O
ð2Þ
The observed stoichiometry clearly indicates that the process is
highly efficient; very little oxidant is lost to side reactions.
The results summarized in Table 1 indicate that the reaction can be
used effectively to synthesize alpha-ketonesfrom a wide variety of
alkylarenes. Both cyclic and acyclic side chains are efficiently
oxidized. Interestingly, the reaction appears to fail when there is a
methoxy substituent on the aromatic ring as in Entries 6 and 8. This
rather unusual observation suggests that the reaction does not likely
involve a rate limiting hydride or hydrogen atom transfer because
the resulting intermediate carbocations or free radicals would be

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1850
Shaabani and Lee
a
Table 1. Oxidation of alkylarenesby cerium(III) bromate.
Reaction
time (h)
Yield
(%)
Entry
Reactant
Ethylbenzene
Product
1
2
Acetophenone
4-Methylacetophenone
4-Fluoroacetophenone
4-Nitroacetophenone
Butyl phenyl ketone
No reaction
2
94b
92b
94b
95b
97
4-Ethyltoluene
4-Ethylfluorobenzene
4-Ethylnitrobenzene
Pentylbenzene
4-Ethylanisole
Indan
1.3
2.2
3
4
24
5
6
10
2
6
—
89
7
1-Indanone
No reaction
8
5-Methoxyindan
Phthalan
15
2
—
90
9
Phthalide
No reaction
1-Tetralone
10
11
12
13
14
15
16
17
18c
19d
20e
2,3-Dihydrobenzofuran
Tetralin
20
—
95
3.5
Isochroman
Diphenylmethane
Fluorene
2-Chromanone
Benzophenone
9-Fluorenone
Benzyl phenyl ketone
Anthraquinone
Xanthone
2
3
90
80
24
6
50b
85
1,2-Diphenylethane
Anthrone
Xanthene
20
24
24
8
97
90
Indan
Indan
Indan
No reaction
1-Indanone
1-Indanone
—
10
8
15
aCerium(III) sulfate (0.23 mmol) and barium bromate (0.75 mmol) were reacted
with alkylarene (1.5 mmol) in aqueousacetonitrile (10 mL) under reflux.
bA small amount (3–6%) of the corresponding alcohol was also obtained.
cNo cerium sulfate was used.
dThe amount of cerium sulfate was reduced to 0.023 mmol.
eCerium(III) chloride (2 mmol) and sodium bromate (1.5 mmol) were reacted with
indan (1.5 mmol) in aqueousacetonitrile (10 mL).
stabilized by methoxy substituents. The unreactivity of 2,3-dihydrobenzo-
furan (Entry 10) is also consistent with these observations. However, one
apparent exception to thispattern isxanthene (Entry 17) where the ortho
oxygen does not seem to greatly inhibit the reaction. Although these
observations are currently not well understood from a mechanistic
point of view, the selectivity exhibited by the reaction may be of empirical
value when this reaction is used for organic synthesis.
The low yield obtained when cerium(III) chloride and sodium bromate
were used under the same conditions (Entry 20) indicates that Ce(BrO3)3 is
superior to the use of other cerium(III) compounds as catalysts in the
presence of sodium bromate.

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Cerium(III) Bromate in Oxidation
Table 2. Oxidation sulfides by cerium(III) bromate.a
Reaction
1851
Yield
(%)
Entry
Reactant
Product
time (min)
1
2
3
Thioanisole
Methyl p-tolyl sulfide
4-Fluorothioanisole
Methyl phenyl sulfoxide
Methyl p-tolyl sulfoxide
4-Fluorophenyl methyl
sulfoxide
60
50
50
97
99
98
4
5
6
7
4-Chlorothioanisole
4-Bromothioanisole
4-Nitrothioanisole
4-Chlorophenyl methyl
sulfoxide
45
45
97
98
98
97
4-Bromophenyl methyl
sulfoxide
4-Nitrophenyl methyl
sulfoxide
60
4-Methoxy-(methylthio)
benzene
4-Methoxyphenyl methyl
sulfoxide
120
8
9
Phenyl sulfide
Benzyl phenyl sulfide
Benzyl sulfide
Ethyl sulfide
Phenyl sulfoxide
Benzyl phenyl sulfoxide
Benzyl sulfoxide
Ethyl sulfoxide
180
120
80
99
98
95
98
96
97
98
99
—
—
98c
92d
4
10
11
12
13
14
15
16b
17b
18
19
20
21
45
45
Butyl sulfide
Hexyl sulfide
Octyl sulfide
Butyl sulfoxide
Hexyl sulfoxide
Octyl sulfoxide
60
60
Pentamethylene sulfide
Thioanisole
Butyl sulfide
Pentamethylene sulfoxide
No reaction
No reaction
60
1400
1400
1400
1400
1400
1400
Thioanisole
Butyl sulfide
Methyl phenyl sulfoxide
Butyl sulfoxide
Methyl phenyl sulfoxide
Butyl sulfoxide
Methyl phenyl sulfone
Butyl sulfone
9
aCerium(III) sulfate (0.17 mmol) and barium bromate (0.50 mmol) were added to
the sulfide (1.0 mmol) in aqueous acetonitrile (10 mL) and the reaction was
allowed to proceed at room temperature.
bNo cerium sulfate was used.
cPlus2% methyl phenyl sulfone.
dPlus8% butyl sulfone.
The Oxidation of Sulfides
Alkyl and aryl sulfides are converted into sulfoxides in quantitative
yieldswhen treated with cerium(III) bromate in aqueousacetonitrile.
Asthe data in Table 2 (Entries20 and 21) indicates, the rate of oxidation
of sulfoxides under these conditions is so slow that the products obtained
after a reasonable reaction time are not contaminated by sulfones.

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Shaabani and Lee
a
Table 3. Oxidation of alcoholsby cerium(III) bromate.
Reaction
time (h)
Yield
(%)
Entry
Reactant
Product
Benzaldehyde
1
2
Benzyl alcohol
3
3.3
3
85
70
4-Fluorobenzyl alcohol
4-Methylbenzyl alcohol
4-Nitrobenzyl alcohol
1-Phenylethanol
Benzhydrol
4-Fluorobenzaldehyde
4-Methybenzaldehyde
4-Nitrobenzaldehyde
Acetophenone
3
90
4
5
36b
90
5
2
6
Benzophenone
Benzil
6
95
7
Benzoin
1-Indanol
24
2
100
100
80
8
1-Indanone
9-Fluorenone
9
9-Hydroxyfluorene
3-Pentanol
4-Heptanol
7
10
11
12
13
14
15
16
17
18
19
20c
21c
3-Pentanone
4-Heptanone
3.3
3.3
3.3
5
100
100
100
90
4-Octanol
Cyclopentanol
Cyclohexanol
4-Octanone
Cyclopentanone
Cyclohexanone
3
100
90
4-Methylcyclohexanol
2-tert-Butylcyclohexanol
Exo-Norborneol
2-Ethyl-1,3-hexanediol
1-Phenyl-1,2-ethannediol
1-Phenylethanol
4-Heptanol
4-Methylcyclohexanone
2-tert-Butylcyclohexanone
Norcamphor
7
7
90
90
7
3-Hydroxymethyl-4-heptanone
Hydroxymethyl phenyl ketone
No reaction
3.3
3.3
24
24
90
90
—
—
No reaction
aCerium(III) sulfate (0.19 mmol) and barium bromate (1.0 mmol) were added to
the alcohol (2.0 mmol) in aqueousacetonitrile (10 mL) and allowed to react under
reflux.
b4-Nitrobenzoic acid (15%) was also isolated.
cNo cerium sulfate was used.
The reagent, therefore, shows excellent selectivity for the formation of
sulfoxides.
Asindicated from the data reported in the experimental esction,
1.0 mmol of sulfide is completely oxidized by 0.12 mmol Ce(BrO3)3,
giving the stoichiometry expressed in Eq. 3.
9RSR0 þ CeðBrO3Þ3À!9RSOR0 þ CeBr3
ð3Þ
The Oxidation of Alcohols
Primary alcoholsare oxidized to aldehydesand secondary alcohols
to ketonesby cerium(III) bromate asindicated by the data in Table 3.

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Cerium(III) Bromate in Oxidation
1853
The yieldsare, in general, better for escondary alcoholsand, in
competitive reactions, secondary alcohols are oxidized preferentially.
For example, the oxidation of 1-phenyl-1,2-ethanediol giveshydroxy-
methyl phenyl ketone in 90% yield (Entry 19). Similarly, 2-ethyl-1,3-
hexanediol gives2-hydroxymethyl-4-heptanone asthe osle product
(Entry 18). The reagent, therefore, shows excellent selectivity for the
oxidation of secondary alcohols.
The results reported in the experimental section show that 2.0 mmol
of secondary alcohol are completely oxidized by 0.23 mmol Ce(BrO3)3,
giving the stoichiometry expressed in Eq. 4.
9RCHOHR0 þ CeðBrO3Þ3À! 9RCOR0 þ CeBr3 þ 9H2O
ð4Þ
SUMMARY
1. Cerium(III) bromate, which may be prepared in situ from the
reaction of barium bromate and cerium(III) sulfate in an
aqueousacetonitrile, isa unique oxidant that containsboth the
catalyst (Ceþ3) and the cooxidant (BrOÀ3 ) in a single compound.
2. Alkylarenesare converted into the corresponding alpha-ketones
in good yields. The reagent does not, however, react with most
alkoxy substituted alkylarenes.
3. Sulfidesare oxidized to the corresponding sulfoxidesin quanti-
tative yields. The products are not contaminated by the presence
of the corresponding sulfones.
4. Primary benzylic and secondary alcohols are oxidized to the
corresponding aldehydes and ketones respectively. In com-
petitive reactionswith diolswhich contain both primary and
secondary functionalities, the secondary hydroxyls are oxidized
preferentially.
5. Stoichiometric studies have shown that bromate is reduced to
bromide in these reactions.
ACKNOWLEDGMENT
Financial assistance from the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Research Council of
Shahid Beheshti University of Iran are gratefully acknowledged.

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1854
Shaabani and Lee
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Received in the NetherlandsAugust 23, 2002