Liquid-phase oxidation of some aromatic aldehydes, hydrocarbons, and alcohols by cerium(IV) catalyzed by Iridium(III) in acidic medium

Article abstract of DOI:10.1080/00397910802621707

Addition of traces of iridium(III) chloride with cerium(IV) sulfate (catalyst-substrate ratio (1:2994 to 1:10,000) in traditional water-bath heating resulted in the oxidation of p-chlorobenzaldehyde, p-nitrobenzaldehyde, benzyl alcohol, p-methoxy benzyl alcohol, p-xylene, and p-nitrotoluene dissolved in acetic acid to give 77%, 90%, 21.7%, 88.6%, 86.2%, and 18% yields of the products, respectively, while catechol and resorcinol polymerized. Oxidation of aldehydes and alcohols resulted as usual in the corresponding acids and aldehydes, respectively, while p-xylene and p-nitrotoluene gave p-tolualdehyde and p-nitrobenzoic acid. Conditions were obtained for getting the highest yields under the experimental conditions.

Full text of DOI:10.1080/00397910802621707

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Liquid-Phase Oxidation of
Some Aromatic Aldehydes,
Hydrocarbons, and Alcohols
by Cerium(IV) Catalyzed by
Iridium(III) in Acidic Medium
Praveen K. Tandon a , Priy B. Dwivedi a b & Satpal
Singh a
a Department of Chemistry, University of Allahabad,
Allahabad, India
b Department of Chemical Engineering, SVKM'S
NMIMS University's Mukesh Patel School of
Technology Management & Engineering, Mumbai,
India
Version of record first published: 27 Apr 2009
To cite this article: Praveen K. Tandon, Priy B. Dwivedi & Satpal Singh (2009):
Liquid-Phase Oxidation of Some Aromatic Aldehydes, Hydrocarbons, and Alcohols by
Cerium(IV) Catalyzed by Iridium(III) in Acidic Medium, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 39:11,
1920-1928
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Synthetic Communications1, 39: 1920–1928, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397910802621707
Liquid-Phase Oxidation of Some Aromatic
Aldehydes, Hydrocarbons, and Alcohols by
Cerium(IV) Catalyzed by Iridium(III)
in Acidic Medium
Praveen K. Tandon, Priy B. Dwivedi, and Satpal Singh
Department of Chemistry, University of Allahabad,
Allahabad, India
Abstract: Addition of traces of iridium(III) chloride with cerium(IV) sulfate
(catalyst–substrate ratio (1:2994 to 1:10,000) in traditional water-bath heating
resulted in the oxidation of p-chlorobenzaldehyde, p-nitrobenzaldehyde, benzyl
alcohol, p-methoxy benzyl alcohol, p-xylene, and p-nitrotoluene dissolved in
acetic acid to give 77%, 90%, 21.7%, 88.6%, 86.2%, and 18% yields of the
products, respectively, while catechol and resorcinol polymerized. Oxidation of
aldehydes and alcohols resulted as usual in the corresponding acids and alde-
hydes, respectively, while p-xylene and p-nitrotoluene gave p-tolualdehyde and
p-nitrobenzoic acid. Conditions were obtained for getting the highest yields under
the experimental conditions.
Keywords: Catalyst, cerium(IV) sulphate, iridium(III) chloride, oxidation
INTRODUCTION
Cerium(IV) is an unusually strong, one-electron-transfer oxidant in the
oxidation of organic compounds. The most widely used reagent for organic
oxidation is diammonium hexakis(nitrato-O-) cerate, commonly known
as ceric ammonium nitrate (CAN). Use of transition-metal ions as
Received September 15, 2008.
The present affiliation for P.B.D. is Department of Chemical Engineering,
SVKM’S NMIMS University’s Mukesh Patel School of Technology Manage-
ment & Engineering, Mumbai, India.
Address correspondence to Praveen K. Tandon, Department of Chemistry,
University of Allahabad, Allahabad 211002, India. E-mail: ptandonk@yahoo.co.in
1920

Cerium–Iridium System in Aromatic Oxidations
1921
homogeneous catalysts in the synthesis of organic compounds in aqueous
medium in conjunction with various cerium(IV) species has not been
reported properly. Iridium(III) chloride is not a good catalyst in an alkaline
medium, and thus it has been given little attention until now.[1,2] We
observed[3,4] that iridium(III) chloride, when used in conjunction with
cerium(IV) sulfate, acts as a very effective catalyst for the oxidation of
organic compounds. When tried, this system 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). Oxidation of 1-butanol to 4-
butanone has been reported with CeO2=MgO catalyst with 25.5% yield.[5]
CAN, a versatile oxidant for synthetic organic chemistry, has also been used
as a catalyst in a number of synthetic reactions.[6] Liquid-phase oxidation of
cyclohexane was carried out over mesoporous Ce-MCM-41 catalysts using
aqueous hydrogen peroxide as oxidant and acetic acid as solvent without
adding any initiator.[7] CAN-catalyzed oxidation of alkyl aromatics giving
rise to aldehydes, ketones, acids, and alcohols with potassium bromate has
been reported.[8] Organic synthesis with cerium(IV) sulfate as an oxidant in
the presence of transition-metal ions has not been attended properly. Here
we report liquid-phase oxidation of aromatic aldehydes, alcohols, and
hydrocarbons by cerium(IV) sulfate in the presence of traces of iridium(III)
chloride with the catalyst-substrate ratio in the range of 1:2994 to 1:10,000.
EXPERIMENTAL
Cerium(IV) sulfate (Loba), p-chlorobenzaldehyde, p-nitrobenzaldehyde,
benzyl alcohol, p-nitro toluene, p-xylene, catechol, resorcinol (E. Merck),
and p-methoxy benzyl alcohol (Lancaster) were used as such without
further purification. A solution of iridium(III) chloride was prepared
by dissolving the sample [sodium hexachloroiridate(III)] from Johnson
Matthey & Co. in a minimum amount of concentrated HCl; the final
strengths of acid and catalyst were 0.00624 M and 3.35 ꢀ 10ꢁ3 M respec-
tively. Cerium(IV) sulfate, prepared by dissolving the sample in 1:1 sulfu-
ric acid, was standardized by titrating it with a freshly prepared standard
solution of ferrous ammonium sulfate using ferroin (CDH) as an external
indicator. All other chemicals used were of analytical grade or chemically
pure substances. For getting the maximum yield, five to eight sets were
performed by changing the concentration or conditions of each compo-
nent, which can affect the yield, for example, in the case of synthesis of
p-chlorobenzoic acid from p-chlorobenzaldehyde (Table 1). For perform-
ing the reactions in the solution phase, a calculated amount of organic
substrate dissolved in acetic acid was taken in a round-bottom flask, to

1922
P. K. Tandon, P. B. Dwivedi, and S. Singh
Table 1. Effect of various factors on the yield of p-chlorobenzoic acid (a0) from
p-chlorobenzaldehyde (a) (1.0 mmol) with cerium(IV) sulfate in aqueous acetic
acid medium in the presence of iridium(III) chloride
Cerium(IV) Acetic
sulfate
(mmol)
Yield (%)
Entry
no.
acid IrCl3 ꢀ 104 Temperature Time p-chlorobenzoic
(mmol)
(mmol)
(ꢂC)
(min)
acid (a0)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
2.50
—
204
204
204
350
204
204
204
204
204
204
204
204
204
204
204
204
204
—
100
100
Rt
200
200
24 (h)
200
200
200
200
200
200
200
200
200
200
200
200
150
250
>10
0.0
>5
3.34
3.34
3.34
3.34
3.34
3.34
3.34
1.00
1.67
5.01
3.34
3.34
3.34
3.34
3.34
3.34
2.50
2.50
1.00
2.00
2.50
3.00
1.00
1.00
1.00
2.50
2.50
2.50
2.50
2.50
2.50
100
100
100
100
100
100
100
100
40
60
80
100
100
100
76.50
38.46
64.10
77.00
70.51
12.82
25.64
32.00
25.64
38.46
64.10
77.0
64.00
77.00
which iridium(III) chloride and cerium(IV) sulfate solutions were added.
The flask, fitted with a water condenser, was kept in a water bath at a
fixed temperature for the desired time. The contents were cooled and
extracted with appropriate solvents. The extract was dried over anhy-
drous MgSO4. The solvent was removed under reduced pressure, and
the obtained product was identified by appropriate methods. Infrared
1
(IR; in KBr; Brucker Vector-22 IR spectrophotometer) and H NMR
(Xeol 400 MHz in CdCl3 with tetramethylsilane [TMS] as internal
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, melting
points, neutralization equivalents, 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. Reaction conditions of the various
experiments are summarized in Table 2, and the general procedures for

Table 2. Oxidation of organic substrates (dissolved in acetic acid) by cerium(IV) sulfate in the presence of iridium(III) chloride
(amounts of organic substrate in all the cases is 1.0 mmol)
Acetic
acid
(mmol)
Ce(SO4)2
(mmol)
Catalyst ꢀ 104 Temp. Time
Yield
(%)
Organic substrate
Product
(mmol)
(ꢂC)
(min)
p-Chlorobenzaldehyde (a0) p-Chlorobenzoic acid (a)
2.5
2.7
3.0
2.5
204
290
170
170
3.34
3.34
3.34
—
100
100
100
100
200
200
180
180
77
90
21.73
88.60
p-Nitrobenzaldehyde (b0)
Benzylalcohol (c0)
p-Methoxybenzyl
alcohol (d0)
p-Nitrobenzoic acid (b)
Benzaldehye (c)
p-Methoxybenzaldehyde (d)
p-Xylene (e0)
p-Tolualdehyde (e)
p-Nitrobenzoic acid (f)
Polycatechol (g)
4.0
6.5
2.5
2.0
170
255
170
170
1.0
100
100
—
240
200
30
86.20
18
Polymerized
Polymerized
p-Nitrotoluene (f0)
Catechol (g0)
Resorcinol (h)
3.34
3.34
3.34
Polyresorcinol (h)
—
30

1924
P. K. Tandon, P. B. Dwivedi, and S. Singh
performing the reactions are given in the experimental section. In the case
of aromatic aldehydes (Scheme 1), after completion of the reaction
contents were extracted three times with diethyl ether (3 ꢀ 25 ml). The
organic part was washed three times with ice-cooled double-distilled
water to remove excess acetic acid and was again extracted with diethyl
ether. After evaporating the solvent, NaOH (2N) was added to the crude
product, and the solution was filtered. Filtrate, which was devoid of
unreacted aromatic aldehyde, was acidified with HCl (2N). The precipi-
tate was filtered with ice-cold water, dried, and weighed. p-Chlorobenzoic
acid (a0) (120.5 mg; 77% yield) was obtained from p-chlorobenzaldehyde
(a). Melting point of the product was found to be 240ꢂC (reported
242ꢂC), while neutralization equivalent was found to be 154.00 (156.50
of pure acid). Finally, the identity of the product was confirmed by taking
its IR spectrum in which following peaks were obtained: IR nmax, 3100–
2554 cmꢁ1 (broad nO-H str.), 1685.2 cmꢁ1 (nC¼O), 759.6 cmꢁ1 (nC-Cl).
p-Nitro benzoic acid (b) from p-nitrobenzoic acid (b0) was prepared in
the same manner as before. The product was obtained as white solid,
150 mg, 90% yield. Melting point of the product was found to be 240ꢂC
(reported 243ꢂC), and neutralization equivalent was found to be 165.00
(167.00 of pure acid). IR nmax, 3128–2500 cmꢁ1 (nO-Hstr.), 1685cmꢁ1
(nC¼O), 1284cmꢁ1 (nC-N), 1557cmꢁ1, and 1350cmꢁ1 (nN-Oasy & sym).
Benzaldehyde (c0) from benzyl alcohol (c) (Scheme 2) was prepared
similarly and was extracted with diethyl ether. 2,4-Dinitrophenyl hydra-
zone derivative in the case of benzaldehyde was prepared by the standard
method[9] and was recrystallized with ethyl alcohol. Hydrazone derivative
(62.14 mg) was obtained, corresponding to 21.73% yield. Hydrazone mp
239ꢂC (reported 241ꢂC); NMR: d 11.3 (1H, s), d 9.1 (1H, d), d 8.1–8.3
(2H, m), d 7.2–7.9 (5H, m).
4-Methoxy benzaldehyde (d0) from 4-methoxy benzyl alcohol (d)
(Scheme 2) was prepared similarly as for (c0). Hydrazone derivative
(359.72 mg) was obtained corresponding to 88.66% yield. Hydrazone
was identified by running TLC plates. Hydrazone mp 250ꢂC (reported
254ꢂC); NMR: d 11.3 (1H, s), d 9.1 (2H, d), d 8.0–8.3 (2H, m), d 7.0–
7.8 (4H, m), d 3.9 (3H, s).
p-Tolualdehyde (e0) from p-xylene (e) (Scheme 3) was prepared as
before and gave 336.18 mg of the product (86.20% yield). Its hydrazone
was separated and identified by TLC; hydrazone mp 235ꢂC (reported
239ꢂC). NMR d 11.2 (1H, s), d 9.1 (1H, d), d 8.0–8.3 (2H, m), d 7.2–7.9
(4H, m), d 2.3 (3H, s).
p-Nitro benzoic acid (f0) from p-nitro toluene (f) (Scheme 3) was pre-
pared as before. Product (30.06 mg) was obtained corresponding to 18%
yield. The compound was identified by its neutralization equivalent,
which was found to be 165 (167 for pure compound); melting point of

Cerium–Iridium System in Aromatic Oxidations
1925
Scheme 1. Conditions for the conversion of aromatic aldehydes into acids.
the product was found to be 240ꢂC (reported 243ꢂC for pure compound).
IR nmax 3115–2548 cmꢁ1 (nO-H), 1690.2 cmꢁ1 (nC¼O), 1281.2 cmꢁ1 (nC-N),
1580.7 cmꢁ1, and 1350.9 cmꢁ1 (nN-O asym & sym).
Catechol (g) and resorcinol (h) polymerized under the reaction
conditions.
DISCUSSION
It was observed that catalyst–substrate ratio ranging from 1:2994 to
1:2997 was enough for the conversion of aromatic aldehydes (a and b,
Table 2), whereas 1:2994 ratio was required in case the case of benzyl
alcohol (c). Surprisingly, it was observed that in the case of p-methoxy
benzyl alcohol (d0), the presence of iridium(III) chloride decreased the
yield. Thus sets were performed in the absence of catalyst to obtain the
maximum yield of p-methoxy benzaldehyde (d) (Table 2). For p-xylene
and p-nitrotoluene, catalyst–substrate ratios were found to be 1:10,000
and 1:2994, respectively. The study was performed mainly to determine
the efficiency and selectivity of the novel, simple, and one-pot cerium(IV)
sulfate–iridium(III) chloride system to oxidize various organic com-
pounds. Four to five sets were performed to obtain the maximum yield
by changing concentration 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 7, Table 1), indicating that it acts as a
solvent to dissolve the organic substrates. Yields in all the cases (except in
Scheme 2. Conditions for the conversion of aromatic alcohols into aldehydes.

1926
P. K. Tandon, P. B. Dwivedi, and S. Singh
Scheme 3. Conditions for the conversion of aromatic hydrocarbons into
aldehydes and acids.
benzyl alcohol and p-nitro toluene, in which yield became constant after
a certain limit) reached a maximum and then started to decrease with
further increase in the concentrations of catalyst (entries 7, 9–11,
Table 1) and the oxidant (entries 5–8, Table 1). Increase in temperature
(entries 7, and 12–14, Table 1) showed positive effect on the yield, but
increase in temperature could not be studied beyond 100ꢂC because of
practical difficulties and also considering the economy of the system.
Increase in the duration of the experiment increased the yield up to a
certain extent but after a certain limit yield became constant (entries 7,
15–17, Table 1). Results indicated that for a particular organic substrate,
optimum conditions are required to get the highest yield with minimum
expenditure of energy or chemicals. Negligible or no formation of the
desired product, after prolonged heating in the absence of oxidant (entry
2, Table 1) or leaving the mixture for 24 h at room temperature (entry 3,
Table 1), excludes aerial oxidation of the organic compound under
experimental conditions and indicates that control of reaction conditions
is necessary for obtaining the product with the desired yield. It is known
that IrCl3 in hydrochloric acid medium gives IrCl63ꢁ species.[10] It has also
been reported that IrIII and IrI ions are the stable species of iridium.[11]
Further, the aquation of [IrCl6]3ꢁ gives [IrCl5H2O]2ꢁ, [IrCl4(H2O)2]1ꢁ
,
and [IrCl3(H2O)3] species.[12,13] In the present case also, to prepare the
catalyst, sodium hexachloroiridate(III) was dissolved in a minimal
amount of hydrochloric acid, and changing the concentration of acetic
acid had no effect on the yield. Therefore it seems reasonable that the
function of acetic acid was just to dissolve the organic substrates. Consid-
ering our experimental conditions and the efficiency of catalyst in the oxi-
dation of various functional groups, [IrCl5H2O]2ꢁ, which had been
considered as the reactive species in various kinetic studies also, may well
be taken as the reactive species in the present study too.[3,4] It has been
reported that cerium(IV) is a typical 1-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

Cerium–Iridium System in Aromatic Oxidations
1927
1-equivalent oxidation of neutral or anionic organic species, cation
radicals or free radicals are generated.[14] Normally these intermediates
undergo rapid oxidation to afford neutral products by electron transfer
(outer-sphere reaction) or ligand transfer (inner-sphere reaction). 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 CꢀH, CꢀC bond cleavage or hydrogen trans-
fer, depend on their structure. In the oxidation of alcohols[15] and aro-
matic aldehydes[16] with cerium(IV) in aqueous acetic acid, involvement
of both 1:1 and 2:1 inner-sphere complexes has been reported.
PhCHO þ CeðIVÞ Ð PhCHO ½CeðIVÞꢁðComplexC1Þ
PhCHO ½CeðIVÞꢁ Ð PhCO þHþ þ CeðIIIÞ
ð1Þ
ð2Þ
PhCO þCeðIVÞ þ H2O Ð PhCOOH þ Hþ þ CeðIIIÞ
ð3Þ
In the present study, the presence of Ir(III) probably facilitates the
removal of hydrogen in the second step by forming another complex with
C1.[3,4] Homolysis of C-H bond may give rise to H., which may transfer
its electron to Ir(III), giving rise to [HIrCl62ꢀ]. [HIrCl62ꢀ] in turn may
dissociate back into the original Ir(III) species.
CONCLUSION
The reported one-pot system is efficient and easy, and it can be used to
oxidize a variety of other functional groups also, which is being studied.
Selectivity for the oxidation of only one methyl group in the case of p-xylene
makes the system highly attractive from the synthetic point of view.
ACKNOWLEDGMENT
P. B. D. is grateful to the University Grants Commission [F.No. 2-48=
2001(II) EU.II and F-32-240/2006 (SR)], New Delhi, India, for financial
assistance. Help from Central Drug Research Institute, Lucknow, India,
is acknowledged for spectral studies on a payment basis.
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1928
P. K. Tandon, P. B. Dwivedi, and S. Singh
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