WO3/70% TBHP/aqueous NaOH: An efficient catalytic combination for the selective oxidation of methylarenes and alkyl aryl ketones to benzoic acids

Article abstract of DOI:10.1002/ejoc.200800664

A new solvent-free, reusable catalytic combination consisting of WO ₃/70% TBHP/aqueous NaOH has been described for the direct oxidation of methylarenes and acetophenones to the corresponding benzoic acids in high yields. Alkylarenes are oxidized to the corresponding aromatic ketones or benzylic alcohols depending upon whether NaOH is used or not. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejoc.200800664

SHORT COMMUNICATION
DOI: 10.1002/ejoc.200800664
WO3/70% TBHP/Aqueous NaOH: An Efficient Catalytic Combination for the
Selective Oxidation of Methylarenes and Alkyl Aryl Ketones to Benzoic Acids
Tanveer Mahammad Ali Shaikh[a] and Arumugam Sudalai*[a]
Keywords: Tungsten / Oxidation / Arenes / Ketones / Benzoic acids
A new solvent-free, reusable catalytic combination con-
sisting of WO3/70% TBHP/aqueous NaOH has been de-
scribed for the direct oxidation of methylarenes and aceto-
phenones to the corresponding benzoic acids in high yields.
Alkylarenes are oxidized to the corresponding aromatic
ketones or benzylic alcohols depending upon whether NaOH
is used or not.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
oxidation of methylarenes or alkyl aryl ketones to the corre-
sponding benzoic acids by using 70% tert-butyl hydroper-
oxide (TBHP) as oxidant in the presence of 40% NaOH as
additive (Schemes 1 and 2).
Introduction
The oxidation of benzylic C–H bonds to the correspond-
ing oxy-functionalized products constitutes one of the most
fundamental transformations in organic synthesis.[1] In par-
ticular, the direct conversion of methylarenes to the corre-
sponding benzoic acids assumes greater industrial impor-
tance, as such carbonyl derivatives are versatile building
blocks in pharmaceutical and polymer industries.[2] A vari-
ety of oxometal oxidants[3] such as KMnO4, Na2Cr2O7,
CrO3, CeO2, TiO2, or NaIO4 in stoichiometric amounts
Scheme 2. Reagents and conditions (i): (a) WO3 (10 mol-%), 70%
TBHP (12 mmol), 40% NaOH (12 mmol), 80 °C, 8 h; (b) acidified
with 6  HCl.
[7]
[8]
and Co(OAc)2,[4] Cu–Fe,[5] ZnO,[6] MnCO3
AlBr3
FeCl3,[9] and more recently NiII(TPA),[10] RuCl3,[11]
CuCl,[12] and Bi salts[13] in catalytic amounts have been em-
ployed for such oxidative processes. Among non-oxometal
oxidants, urea hydrogen peroxide, HNO3, HBr, and CBr4–
PPh3 are commonly used.[14] However, stoichiometric use of
metallic oxidants generally cause problems associated with
either environmental pollution or difficult separation of the
metal reagent from the products. The development of cata-
lytic benzylic oxidative process with a readily available and
inexpensive catalyst with no toxicity would be of consider-
able relevance for academia and industries. We wish to re-
port herein tungsten(VI) oxide (WO3)-catalyzed solvent-free
Results and Discussion
Oxidation of Methylarenes
Table 1 shows the results of WO3-catalyzed oxidation of
4-bromotoluene as a model substrate to the corresponding
4-bromobenzoic acid with 70% TBHP as oxidant in the
presence of NaOH as additive. Thus, when 70% TBHP
(1 equiv.) and 40% NaOH (8 equiv.) was used, the corre-
sponding 4-bromobenzoic acid was obtained only in 27%
yield. A larger amount of TBHP (4 equiv.) resulted in an
improved yield up to 36%. When the reaction was per-
formed with an even larger concentration of TBHP
(8 equiv.) and WO3 (5 mol-%), 4-bromobenzoic acid was
obtained in moderate yield (42%). Also, when the quantity
of NaOH was reduced to 9 or 15 mmol, the product was
obtained in 15 or 57% yield, respectively. Thus, optimal re-
action conditions comprised WO3 (20 mol-%), 70% TBHP
(8 equiv.), and 40% NaOH (8 equiv.) as additive. This gave
4-bromobenzoic acid in 89% isolated yield. The use of
other tungsten-based catalysts like Na2WO4 and H2WO4
gave poor yields of the product (Ͻ15%). Also, the reaction
failed when H2O2 (aq. 30% or 50%) was used as the oxi-
dant. Control experiments showed that, in the absence of
NaOH, no reaction took place.
Scheme 1. Reagents and conditions (i): (a) methylarene (3 mmol),
WO3 (20 mol-%), 70% TBHP (24 mmol), 40% NaOH (24 mmol),
80 °C, 10 h; (b) acidified with 6  HCl.
[a] Chemical Engineering
& Process Development Division,
National Chemical Labotratory,
Pune 8, India
Fax: +91-020-25902174
E-mail: a.sudalai@ncl.res.in
Eur. J. Org. Chem. 2008, 4877–4880
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T. M. A. Shaikh, A. Sudalai
SHORT COMMUNICATION
Table 1. WO3-catalyzed oxidation of 4-bromotoluene to 4-bromo- ylacetophenone was subjected to oxidation, p-toluic acid
benzoic acid.[a]
was obtained selectively in 83% yield, whereas propiophen-
Yield[b]
[%]
one gave benzoic acid in 95% yield (Table 3, Entry 3).
Entry
WO3
[mol-%]
70% TBHP 40% NaOH
[mmol]
[mmol]
1
2
3
4
5
6
20
20
5
20
20
20
3
24
24
15
9
15
24
27
36
42
15
57
89
Table 3. WO3-catalyzed oxidative C–C bond cleavage of alkyl aryl
12
24
24
24
24
ketones.[a]
Entry
Alkyl aryl ketones (3)
Benzoic acids (4)
Yield[b]
[%]
1
2
3
4
5
6
7
8
9
acetophenone
4-methylacetophenone
propiophenone
4-bromoacetophenone
2,4-dichoroacetophenone 2,4-dichlorobenzoic acid
4-nitroacetophenone
4-methoxyacetophenone
4-fluoroacetophenone
4-chloroacetophenone
benzoic acid
p-toluic acid
benzoic acid
4-bromobenzoic acid
89
83
95
95
92
84
85
93
94
[a] (i) 4-Bromotoluene (3 mmol), WO3, 70% TBHP, 40% NaOH,
80 °C, 10 h; (ii) acidified with 6  HCl. [b] Isolated yield and prod-
uct was characterized by m.p. and 1H and 13C NMR spectroscopy.
4-nitrobenzoic acid
4-methoxybenzoic acid
4-fluorobenzoic acid
4-chlorobenzoic acid
For understanding the scope and generality of this oxi-
dation method, several methylarenes were subjected to oxi-
dation under the optimized conditions, and the results are
presented in Table 2. Alkylarenes with electron-withdrawing
as well as electron-donating substituents underwent oxi-
dation readily to produce the corresponding benzoic acids
in excellent yields. It is noteworthy that for substrates with
polymethyl groups, one of the methyl groups is selectively
oxidized to give monocarboxylic acid (Table 2, Entries 4
and 9). However, α-picoline failed to undergo oxidation un-
der the reaction conditions.
[a] (i) Alkyl aryl ketone (3 mmol), WO3 (10 mol-%), 70% TBHP
(1.5 mL, 12 mmol), 40% NaOH (12 mmol), 80 °C, 8 h; (ii) acidified
with 6  HCl. [b] Isolated yields and products were characterized
1
by m.p. and H and 13C NMR spectroscopy.
Upon oxidation, acetophenones with both electron-with-
drawing and electron-donating substituents gave the corre-
sponding benzoic acids in high yields. The halo-substituted
acetophenones also underwent facile oxidation to give the
benzoic acids in high yields.
Table 2. WO3-catalyzed oxidation of methylarenes to benzoic acids
with 70% TBHP.[a]
The workup procedure for both the methods generally
involves acidification of the reaction mixture with ice-cold
diluted HCl (6 ), followed by extraction of the product
with ethyl acetate so that WO3 is left dispersed in the aque-
ous layer. Concentration of the organic layer gave pure ben-
zoic acid, whereas WO3 in the aqueous layer was recovered
by simple filtration. The recovered WO3 was found to be
effective for oxidation of toluene to benzoic acid (55%) al-
though with reduced activity.
Entry
ArCH3 (1)
ArCO2H (2) Yield [%][b]
1
2
3
4
5
6
7
8
C6H5
4-BrC6H4
4-ClC6H4
4-MeC6H4
85
89
84
78
71
85
74
93
72
75
83
80
70
2-MeC6H4
4-MeOC6H4
4-tBuC6H4
4-NO2C6H4
3,5-Me2C6H3
2,3-Cl2C6H3
2,4-Cl2C6H3
3,4-Cl2C6H3
2-methylnaphthalene
9
10
11
12
13
[a] (i) Methylarene (3 mmol), WO3 (20 mol-%), 70% TBHP (3 mL,
24 mmol), 40% NaOH (24 mmol), 80 °C, 10 h; (ii) acidified with
6  HCl. [b] Isolated yields and products were characterized by
Oxidation of Alkylarenes
1
m.p. and H and 13C NMR spectroscopy.
Interestingly, alkylarenes when subjected to oxidation
with WO3 (20 mol-%) and 70% TBHP (2 equiv.), in the ab-
sence of base, exhibited an unusual product selectivity
(Scheme 3). For example, ethylbenzene on oxidation with
the WO3/TBHP combination gave either acetophenone 7 or
Oxidation of Aromatic Ketones
The catalytic oxidative cleavage of aryl methyl ketones benzylic secondary alcohol 6 in moderate yields (48 and
to the corresponding benzoic acids is a potentially useful 35%, respectively) depending upon whether NaOH was
transformation in organic synthesis.[15] When we extended used or not; the remaining substance was unreacted ethyl-
the present methodology to aryl ketones, we observed that benzene. Other alkyl arenes such as indane, tetraline, fluo-
the corresponding benzoic acids were formed in high yields, rene, and so on have exhibited similar behavior in terms of
and the results are presented in Table 3. Thus, acetophe- product selectivities as well as yields (Table 4). However,
none underwent oxidative cleavage with WO3 (10 mol-%) when 4-ethyltoluene was subjected to oxidation under the
and 70% TBHP (4 equiv.) and 40% NaOH as additive to reaction conditions, in the absence of base, the correspond-
give benzoic acid in 89% yield. The reaction was found to ing 4Ј-methyl-1-phenylethanol was obtained selectively in
be general with other acetophenones as well. When 4-meth- 20% isolated yield.
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Eur. J. Org. Chem. 2008, 4877–4880

Selective Oxidation of Methylarenes and Alkyl Aryl Ketones to Benzoic Acids
secondary alcohols or ketones is dependent upon whether
we use alkali or not. In the present protocol benzoic acids
were isolated in pure form without the need for column
chromatographic purification.
Scheme 3. Reagents and conditions (i): (a) WO3 (20 mol-%), alkyl
arenes (3 mmol), 70% TBHP (6 mmol), 80 °C, 12 h.
Experimental Section
Typical Procedure for Oxidation of Methylarenes: To a mixture of
toluene (3 mmol), WO3 (20 mol-%), and TBHP (70%, 24 mmol)
was added NaOH (40% in water, 24 mmol, 2.4 mL). The reaction
mixture was then heated at 80 °C (using oil bath) for 10 h, cooled
to room temperature, and acidified by using ice-cold HCl (6 ).
The mixture was extracted with ethyl acetate (3ϫ40 mL), and the
combined organic phase was washed with saturated brine solution,
dried with anhydrous Na2SO4, and concentrated under reduced
pressure to give pure benzoic acid. Yield: 85%.
Table 4. The effect of base in WO3-catalyzed benzylic C–H oxi-
dation.[a]
Entry Substrates 5
Yield [%][b]
Alcohols 6[c] Ketones 7[d]
1
2
3
4
5
ethylbenzene
4-bromoethylbenzene
indane
tetraline
fluorene
35
35
43
40
48
48
40
58
51
55
General Procedure for Oxidation Aryl alkyl Ketones: To a mixture
of aryl alkyl ketone (3 mmol), WO3 (10 mol-%), and TBHP (70%,
12 mmol) was added NaOH (40% in water, 12 mmol, 1.2 mL). The
reaction mixture was then heated at 80 °C (using oil bath) for 8 h,
cooled to room temperature, and acidified by using ice-cold HCl
(6 ). It was extracted with ethyl acetate (3ϫ40 mL), and the com-
bined organic phase was washed with saturated brine solution,
dried with anhydrous Na2SO4, and concentrated under reduced
pressure to give pure benzoic acid.
[a] Alkylarene (3 mmol), WO3 (20 mol-%), 70% TBHP (6 mmol),
80 °C, 12 h. [b] Isolated yield and products were characterized by
1
m.p. and H and 13C NMR spectroscopy. [c] No base was added.
[d] 40% NaOH (0.6 mL) was used.
Mechanism
The catalytic cycle for the oxidative process is shown in
Scheme 4. In the case of methylarenes and aryl ketones, it
was observed that no oxidation took place in the absence of
base. However, upon the addition of NaOH, WO3 became
homogeneous and readily reacted with TBHP to produce a
metal peroxo species, A; the evidence for its formation came
from its typical IR absorption bands at 495, 640, and
950 cm–1.[16] The metal peroxo species A then probably un-
dergoes C–H insertion at benzylic C–H bond of alkylarenes
to give the benzylic alcohols. Although none of the benzylic
alcohols were isolated during the oxidative process, its for-
mation as an intermediate has been suggested, as 4-ni-
trobenzyl alcohol when subjected to oxidation [WO3
(20 mol-%), 70% TBHP (8 molar equiv.), no base] gave 4-
nitrobenzoic acid (57%). For aryl ketones, it is estab-
lished[17] that formation of α-hydroxyacetophenones fol-
lowed by its facile oxidative cleavage gave benzoic acids.
General Procedure for Oxidation of Alkylarenes: To a mixture of
alkylarenes (3 mmol) and WO3 (20 mol-%), was added TBHP
(70%, 6 mmol). The reaction mixture was then heated at 80 °C
(using oil bath) for 12 h. Progress of the reaction was monitored
by TLC. The reaction mixture was then cooled to room tempera-
ture and extracted with ethyl acetate (3ϫ20 mL), and the com-
bined organic phase was washed with saturated brine solution,
dried with anhydrous Na2SO4, and concentrated under reduced
pressure to give the crude product, which was purified by column
chromatography packed with silica gel (n-hexane/ethyl acetate, 9:1)
to afford pure secondary benzylic alcohol.
Procedure for Oxidation of Alkylarenes to Benzylic Secondary
Ketones: Alkylarenes were subjected to oxidation as mentioned
above with NaOH (40% in water, 0.6 mL, 6 mmol with respect to
substrate) to give benzylic secondary ketones.
Acknowledgments
T. M. S. thanks the Council of Scientific Industrial Research
(CSIR), New Delhi for the award of senior research fellowship. The
authors are also thankful to Dr. B. D. Kulkarni, Head, CE-PD
Division, for his constant encouragement.
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oxidation of methylarenes.
Conclusions
We have developed a new catalytic method consisting of
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T. M. A. Shaikh, A. Sudalai
SHORT COMMUNICATION
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Received: July 4, 2008
Published Online: September 2, 2008
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