Liquid phase and microwave assisted oxidation of some hydrocarbons, aromatic aldehydes, and phenols by cerium(IV) catalyzed by iridium(III) in acidic medium

Article abstract of DOI:10.1080/00397910802028796

Addition of traces of iridium(III) chloride with cerium(IV) sulphate (catalyst-substrate ratio 1:75757 to 1:151515) in traditional water-bath heating resulted in the oxidation of propyl benzene, naphthalene, dimethoxy benzaldehyde, trimethoxy benzaldehydes, cresol, and quinol dissolved in acetic acid to give 70, 33, 96, 74, 49, and 66% yields respectively of the products, while phenol and resorcinol polymerized. Reactants adsorbed on alumina under solventless conditions in a microwave oven resulted in the decrease in yield. Oxidation of both hydroxyl groups takes place in quinol, whereas it was selective at the α-carbon in the side chain and at the methyl group in the case of propyl benzene and cresol, respectively. Conditions were tested for the highest yields under the experimental conditions. Copyright Taylor & Francis Group, LLC.

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Liquid Phase and Microwave
Assisted Oxidation of Some
Hydrocarbons, Aromatic
Aldehydes, and Phenols by
Cerium(IV) Catalyzed by
Iridium(III) in Acidic Medium
a
a
Praveen K. Tandon , Manish Srivastava , Santosh
a
a
B. Singh & Satpal Singh
a
Department of Chemistry, University of Allahabad,
Allahabad, India
Published online: 18 Aug 2008.
To cite this article: Praveen K. Tandon , Manish Srivastava , Santosh B. Singh
&
Satpal Singh (2008): Liquid Phase and Microwave Assisted Oxidation of Some
Hydrocarbons, Aromatic Aldehydes, and Phenols by Cerium(IV) Catalyzed by
Iridium(III) in Acidic Medium, Synthetic Communications: An International Journal for
Rapid Communication of Synthetic Organic Chemistry, 38:13, 2125-2137
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1
Synthetic Communications , 38: 2125–2137, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802028796
Liquid Phase and Microwave Assisted Oxidation
of Some Hydrocarbons, Aromatic Aldehydes, and
Phenols by Cerium(IV) Catalyzed by
Iridium(III) in Acidic Medium
Praveen K. Tandon, Manish Srivastava,
Santosh B. Singh, and Satpal Singh
Department of Chemistry, University of Allahabad, Allahabad, India
Abstract: Addition of traces of iridium(III) chloride with cerium(IV) sulphate
(
catalyst–substrate ratio 1:75757 to 1:151515) in traditional water-bath heating
resulted in the oxidation of propyl benzene, naphthalene, dimethoxy benzal-
dehyde, trimethoxy benzaldehydes, cresol, and quinol dissolved in acetic acid to
give 70, 33, 96, 74, 49, and 66% yields respectively of the products, while phenol
and resorcinol polymerized. Reactants adsorbed on alumina under solventless
conditions in a microwave oven resulted in the decrease in yield. Oxidation of
both hydroxyl groups takes place in quinol, whereas it was selective at the a-car-
bon in the side chain and at the methyl group in the case of propyl benzene and
cresol, respectively. Conditions were tested for the highest yields under the experi-
mental conditions.
Keywords: Catalyst; Cerium(IV) sulphate; Iridium(III) chloride; Microwave;
Oxidation
INTRODUCTION
Oxidation of organic compounds with cerium(IV) is quite interesting
because cerium(IV) is an unusually strong, one-electron-transfer oxidant.
Iridium(III) chloride shows sluggish catalytic activity in alkaline medium
[
1,2]
and thus it has been given little attention until now.
studies, we observed
During kinetic
that a iridium(III) chloride–cerium(IV) sulphate
[3–6]
Address correspondence to Praveen K. Tandon, Department of Chemistry,
University of Allahabad, Allahabad 211002, India. E-mail: ptandonk@yahoo.
co.in
2
125

2126
P. K. Tandon et al.
system, in an acidic medium, is more efficient than ruthenium(III) chlor-
[8]
ide in an acidic medium or osmium teroxide in an alkaline medium.
[7]
This system, when tried from the synthetic point of view, proved to be
highly efficient in the oxidation of many organic compounds that were
otherwise difficult to oxidize. Various functional groups in the organic
compounds have been oxidized with different species of cerium(IV). Oxi-
dation of 1-butanol to 4-butanone has been reported with CeO =MgO
2
[9]
catalyst with 25.5% yield. Ceric ammonium nitrate, (CAN) a versatile
oxidant for synthetic organic chemistry, has been used as a catalyst also
[10]
in a number of synthetic reactions.
Liquid-phase oxidation of cyclo-
hexane was carried out over mesoporous Ce-MCM-41 catalysts using
aqueous hydrogen peroxide as oxidant and acetic acid as solvent without
[
11]
adding any initiator.
aldehydes, ketones, acids, or alcohols with potassium bromate has been
CAN-catalyzed oxidation of alkyl aromatics to
[
12]
reported. Most of the synthetic work related to cerium(IV) deals with
uncatalyzed oxidations, whereas catalyzed organic synthesis with ceriu-
m(IV) as an oxidant has not been studied properly. Here we report
liquid-phase and microwave-assisted oxidation of propyl benzene, naph-
thalene, dimethoxy and trimethoxy benzaldehydes, cresol, quinol, phe-
nol, and resorcinol by cerium(IV) sulphate in the presence of traces of
iridium(III) chloride with catalyst–substrate ratio in the range of
1
:75757 to 1:151515.
EXPERIMENTAL
Cerium(IV) sulphate (Loba), propyl benzene, naphthalene, cresol,
phenol, dimethoxy benzaldehtyde, trimethoxy quinol, and resorcinol
(E. Merck) were used as such without further purification. Solution of
iridium(III) chloride was prepared by dissolving the sample [sodium hex-
achloroiridate(III)] (Johnson Matthey & Co.) in a minimal amount of
concentrated HCl (AnalaR), the final strengths of acid and catalyst were
ꢁ
3
0
.00624 M and 3.35 ꢀ 10 M respectively. Cerium(IV) sulphate, prepared
by dissolving the sample in 1:1 sulphuric acid, was standardized by titrat-
ing it with a freshly prepared standard solution of ferrous ammonium sul-
pahate using ferroin (CDH) as an internal indicator. All other chemicals
used were either Analar or chemically pure substances. Commercially
obtained reagents were used without further purification. For getting
the maximum yield, 5 to 8 sets were performed by changing the concen-
tration or conditions of each component, which can affect the yield, for
example, in the case of synthesis of 1-phenyl 2-propanone from propyl
benzene (Table 1). Under microwave irradiations, reactions were per-
formed at atmospheric pressure in a Kenstar (model OM-20 ESP,

2
127

2128
P. K. Tandon et al.
8
00 W, Aurangabad, India) domestic microwave oven in which control of
temperature was not possible. For performing the reactions in the
solution phase, a calculated amount of organic substrate dissolved in
acetic acid was taken in a round-bottom flask, to which iridium(III)
chloride and cerium(IV) sulphate solutions were added. The flask was fit-
ted with a water condenser and kept in a water bath at a fixed tempera-
ture for the desired time. Contents were cooled and extracted with
appropriate solvents. The extract was dried over anhydrous MgSO4.
The solvent was removed under reduced pressure, and the obtained pro-
duct was identified by an appropriate method. For performing the reac-
tion in a domestic microwave oven, the same amounts of cerium(IV)
sulphate, organic substrate, and iridium(III) chloride as in solution-phase
reaction were mixed in 25 g of alumina. The mixture, kept in an alumina
bath, was exposed in a microwave oven at the desired power for the
desired time. The workup procedure was the same as given in the
water-bath method. IR (in KBr; Brucker Vector-22 IR spectropho-
1
tometer) and H NMR (Xeol 400 MHz in CdCl with TMS as internal
3
standard) spectra were recorded, and reactions were monitored with
thin-layer chromatography (TLC) (Merck GF254 silica gel–coated
plates). Purity and identification of products were confirmed by TLC,
mp of the derivatives, and their IR and NMR spectra.
RESULTS
On running the TLC plate with the organic part, only one spot corre-
sponding to the product was obtained. In the cases of b, g, and h, the
reaction mixture was extracted (15 ml ꢀ 3) with chloroform, whereas in
the cases of a, c, d, e, and f it was extracted (15 ml ꢀ 3) with ether. Reac-
tion conditions in which various experiments were performed are sum-
marized in Table 2, and the general procedures for performing the
reactions in solution phase and under microwave irradiation are given
in the Experimental section.
2,4-Dinitrophenyl hydrazone derivative in the case of propyl benzene
a) was prepared by a standard method
[
13]
(
ethyl alcohol. Hydrazone derivative (220 mg) was obtained correspond-
and was recrystallized with
ꢂ
ing to 70.8% yield of 1-phenyl 2-propanone: mp of hydrazone 155 C
ꢂ
(
reported 156 C); NMR d 2.48 (2H, s), d 1.55 (3H, s), d 7.26 (1H, s), d
4
(
.0 (1H, s), d 8.77 to 8.76 (2H, d), d 8.42 to 8.41 (2H, d), d 7.48 to 7.46
1H, t), d 8.39 to 8.38 (1H, d), d 7.88 to 7.82 (1H, d). Surprisingly under
microwave irradiation (20% power for 3.0 min), the yield decreased to
7.7 % (150 mg of hydrazone was obtained).
4

2
129

2130
P. K. Tandon et al.
0
a-Naphthol (b ) from naphthalene (b) (1.0 mmol) was prepared simi-
larly as in (a). The product was obtained as a brown solid (33.5% yield),
which was recrystallized with chloroform. Under microwave irradiation
(
40 % power, 3.0 min), yield decreased to 18.8 % (27 mg of a-naphthol
ꢂ
ꢂ
was obtained): mp of a-naphthol 90 C (reported 94 C). NMR d 7.7 to
8
.0 (7H, m), d 7.1 (1H, s).
Dimethoxy benzoic acid, obtained from dimethoxy benzaldehyde
c, 1.0 mmol) and prepared as described previously, gave 90 mg of the
(
product (34.9% yield). In a microwave oven (60 % power, 4.0 min) yield
decreased to 27.4 % (30 mg product was obtained). Mp of dimethoxy
ꢂ
ꢂ
benzoic acid was 179 C (reported 181 C). NMR d 9.88 (1H, s), d 3.96
6H, s), d 6.92 to 6.94 (2H, dd), d 7.87 to 7.6 (1H, dd); IR nmax
(
2
1
944 nm (no-H str.), 2847 nm (nCH3 str.), 1686 nm (nC=O), 1589 nm (nC=C),
506 nm (nbenzene ring), 1236 nm (nC-O), and 845.9 nm (nC-C str.).
In the case of trimethoxy benzoic acid prepared from trimethoxy
benzaldehyde (d, 1.0 mmol), 66 % yield (140 mg of product) was
obtained. On conducting the reaction in a microwave oven (20 % power,
3
.0 min), yield decreased to 37.7 % (80 mg of product was obtained). Mp
ꢂ
ꢂ
of trimethoxy benzoic acid was 141 C (reported 144 C). NMR d 9.88
(
1H, s), d 7.14 (2H, s), d 3.94 (9H, s).
0
p-Hydroxy benzaldehyde (e ) from cresol (e, 1.0 mmol) prepared as
described previously, gave 235 mg product (96.3 % yield). On conducting
the reaction in a microwave oven (80% power, 3.0 min), the yield
decreased to 53.2% (130 mg of p-hydroxy benzaldehyde was obtained):
ꢂ
ꢂ
mp 115 C (reported 117 C). IR nmax3397 nm (nOH), 1218 nm(n
2
),
CꢁO
924 nm (nC-H aldehyde), 1661.8 nm (nC=O), 1605 nm (nbenzene ring),
1
456.8 nm (nC=C), and 813.8 nm (n
Benzoquinone (f ) from quinol (f, 1.0 mmol) prepared as described
).
CꢁC
0
previously gave 80 mg of product (74% yield). In a microwave oven
40% power, 3.0 min), the yield decreased to 27.7% (30 mg of benzaqui-
(
none was obtained): mp of benzaquinone 113 C (reported 116 C). NMR
ꢂ
ꢂ
d 6.8 (2H, s), d 6.5 (2H, s).
Phenol (g) and resorcinol (h) polymerized under the reaction
conditions.
DISCUSSION
Catalyst–substrate ratio ranging from 1:75757 to 1:151515 was enough
for the conversion of aromatic hydrocarbons (a and b, Table 2) whereas
1
:75757 to 1:113636 ratio was required for good yields in the case of
aromatic aldehydes (c and d, Table 2). For cresol and quinol, cata-
lyst–substrate ratio was found to be 1:90909 and 1:151515, respectively.

Cerium-Iridium System for Oxidation of Aromatics
2131
The study was performed mainly to determine the efficiency and selec-
tivity of the novel, simple, and one-pot cerium(IV) sulphate–iridium(III)
chloride system to oxidize various organic compounds. Five to seven
sets were performed to obtain the maximum yield by changing concen-
tration or conditions of each component, which can affect the yield
(
Table 1). Change in the concentration of acetic acid does not affect
the yield (entries 4 and 5, Table 1), indicating that it acts as a solvent
to dissolve the hydrocarbons. Yields in all the cases reached a
maximum and then started to decrease with further increase in the con-
centrations of catalyst (entries 5, 6, and 7, Table 1) and the oxidant
(
entries 5, 8, and 9, Table 1) or increasing duration of the experiment
and temperature (entries 5 and 10, Table 1), indicating that optimum
conditions are required to get the highest yield. It may also be possible
that some unreactive complex is formed at higher concentrations of the
catalyst and oxidant. No formation of the desired products, after pro-
longed heating in the absence of oxidant (entry 2, Table 1) or leaving
the mixture for 48 h at room temperature (entry 3, Table 1), excludes
aerial oxidation of the organic compound under experimental con-
ditions and indicates that control of reaction conditions was necessary
for getting the product with the desired yield. IrCl in hydrochloric acid
3
3
medium gives IrCl6 species. It has also been reported that Ir
ꢁ
[14]
III
and
Figure 1. NMR spectrum of 1-phenyl, 2-propanone.

2132
P. K. Tandon et al.
Figure 2. NMR spectrum of a-naphthol.
Figure 3. It spectrum of dimethoxy benzoic acid.

Cerium-Iridium System for Oxidation of Aromatics
2133
Figure 4. NMR spectrum of dimethoxy benzoic acid.
Figure 5. NMR spectrum of trimethoxy benzoic acid.

2134
P. K. Tandon et al.
Figure 6. IR spectrum of p-hydroxy benzaldehyde.
I
[15]
Ir ions are the stable species of iridium.
Further, the aquation of
3
ꢁ 2ꢁ
gives [IrCl H O] , [IrCl (H O) ] , and [IrCl (H O) ] spe-
5 2 4 2 3 2 3
1ꢁ
[IrCl ]
cies.
6
2
[
16,17]
. For preparing the catalyst in the present case, sodium hexa-
chloroiridate(III) was dissolved in a minimal amount of hydrochloric
acid, and no effect of change of the concentration of acetic acid on
the yield was obtained. Therefore, it seems reasonable that the function
of acetic acid was just to dissolve the organic substrates. Considering
our experimental conditions and efficiency of catalyst in the oxidation
2
ꢁ
of various functional groups, [IrCl H O] , which has been considered
5
2
as the reactive species in various kinetic studies, may well be taken as
1–6]
[
the reactive species in the present study too.
It has been reported
that cerium(IV) is a typical one-equivalent oxidant that removes one
electron at a time from the substrate. In this regard, it shares certain
similarities in reaction patterns with Mn(III), Co(III), and V(V).In
one-equivalent oxidation of neutral or anionic organic species, cation
[
18]
radicals or free radicals are generated.
undergo rapid oxidation to afford neutral products by electron transfer
outer-sphere reaction) or by ligand transfer (inner-sphere reaction).
Normally these intermediates
(
Because Ce(IV) oxidations deal most frequently with neutral organic
compounds, radical intermediates are encountered most of the time.
The fates of these intermediates, to undergo C-H, C-C bond cleavage
or hydrogen transfer, depend on their structure. In the oxidation of

Cerium-Iridium System for Oxidation of Aromatics
2135
Figure 7. NMR spectrum of benzoquinone.
[
19]
[20]
alcohols
and aromatic aldehydes
with cerium(IV) in aqueous
acetic acid, involvement of both 1:1 and 2:1 inner-sphere complexes
has been reported.
In the present study, presence of Ir(III) probably facilitates the
removal of hydrogen in the second step by forming another complex with
[
4–6]
C .
1
Homolysis of the C-H bond may give rise to H: which may trans-

2136
P. K. Tandon et al.
2
ꢁ
2ꢁ
fer its electron to Ir(III), giving rise to [HIrCl6 ]. [HIrCl6 ] in turn may
dissociate back into the original Ir(III) species.
CONCLUSION
The reported novel one-pot system is highly efficient and easy, and can be
used to oxidize a variety of other functional groups also, is being studied.
Selectivity for the oxidation of the methyl group in the case of cresol and
a-carbon in propyl benzene makes the system highly attractive from the
synthetic point of view. Solventless synthesis under MW irradiation is
also important from the environmental point of view.
ACKNOWLEDGEMENT
The authors are grateful to University Grants Commission (UGC) (32-
2
40=2006 SR) and Council of Scientific and Industrial Research (CSIR)
01(2150)=07= EMR-II), New Delhi, India, for financial assistance. Help
(
from Central Drug Research Institute (CDRI), Lucknow, India, is
acknowledged for spectral studies.
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