Liquid-phase and solventless oxidation of cyclohexane, benzene, and other hydrocarbons by cerium(IV) catalyzed by iridium(III) in acidic medium

Article abstract of DOI:10.1080/00397910802109265

Oxidation of some hydrocarbons dissolved in acetic acid by cerium(IV) sulphate at 100°C in the presence of traces of iridium(III) chloride (catalyst-substrate 1:56818 to 151515) in the solution phase resulted in good to excellent yields of corresponding c

Full text of DOI:10.1080/00397910802109265

This article was downloaded by: [University of Bath]
On: 06 October 2014, At: 03:45
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Synthetic Communications: An
International Journal for Rapid
Communication of Synthetic
Organic Chemistry
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/lsyc20
Liquid-Phase and Solventless
Oxidation of Cyclohexane,
Benzene, and Other
Hydrocarbons by Cerium(IV)
Catalyzed by Iridium(III) in
Acidic Medium
Praveen K. Tandon a , Manish Srivastava a , Santosh
B. Singh a & Nidhi Srivastava a
a Department of Chemistry, University of Allahabad ,
Allahabad, India
Published online: 09 Sep 2008.
To cite this article: Praveen K. Tandon , Manish Srivastava , Santosh B. Singh & Nidhi
Srivastava (2008) Liquid-Phase and Solventless Oxidation of Cyclohexane, Benzene,
and Other Hydrocarbons by Cerium(IV) Catalyzed by Iridium(III) in Acidic Medium,
Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:18, 3183-3192
To link to this article: http://dx.doi.org/10.1080/00397910802109265
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no

representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or
indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Synthetic Communications1, 38: 3183–3192, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910802109265
Liquid-Phase and Solventless Oxidation of
Cyclohexane, Benzene, and Other
Hydrocarbons by Cerium(IV) Catalyzed by
Iridium(III) in Acidic Medium
Praveen K. Tandon, Manish Srivastava, Santosh B. Singh,
and Nidhi Srivastava
Department of Chemistry, University of Allahabad, Allahabad, India
Abstract: Oxidation of some hydrocarbons dissolved in acetic acid by cerium(IV)
sulphate at 100 ꢀC in the presence of traces of iridium(III) chloride (catalyst-
substrate 1:56818 to 151515) in the solution phase resulted in good to excellent
yields of corresponding carbonyl compounds. In the cases of cyclohexane and
benzene, 44% and 51.8% yields of corresponding carbonyl compounds were
obtained, whereas in other cases, yield ranged from 34.9 to 99.8%. Yield
decreased when reactions were performed in a microwave oven by adsorbing reac-
tants (except acetic acid) on alumina. Decrease in the yield was probably due to
the high temperature generated during the course of the reaction, resulting in the
loss of organics from evaporation. Conditions were optimized for the highest
yields under ambient conditions.
Keywords: Cerium(IV), iridium(III) chloride, microwave, oxidation
INTRODUCTION
Cerium(IV) is an unusually strong, one-electron oxidant. Catalysis by iri-
dium(III) chloride in the oxidation of organic compounds has been given
little attention because of its sluggish catalytic activity in alkaline
medium.[1,2] During kinetic studies, we observed[3–6] that iridium(III)
chloride is a more efficient catalyst compared to even ruthenium(III)
chloride[7] or osmium teroxide.[8] Surprisingly, when tried from the
Received January 24, 2008.
Address correspondence to Praveen K. Tandon, Department of Chemistry,
University of Allahabad, Allahabad 211002, India. E-mail: ptandonk@yahoo.co.in
3183

3184
P. K. Tandon et al.
synthetic point of view, the system proved to be highly efficient in the oxi-
dation of many hydrocarbons including cyclohexane and benzene also,
which otherwise are very difficult to oxidize. Oxidation of cyclohexane
has been attempted by many workers with mononuclear iron(III) com-
plexes with H2O2 and hydroperoxides,[9] with H2O2 in acetonitrile using
Bi-VPO catalyst at 60 ꢀC for 12 h with 38% and 36% yields of cyclohex-
anone and cyclohexanol,[10] and with oxygen in 8 h at 140 ꢀC and pressure
of 2.5 Mpa in the presence of iron porphyrin with yield of 21.4%[11] of adi-
pic acid. 1,4-Diketones were prepared by cerium-catalyzed oxidative coup-
ling of styrene with molecular oxygen and 1,3-dicarbonyl compounds.[12]
Ceric ammonium nitrate (CAN)-catalyzed oxidation of alkyl aromatics
with potassium bromate affords aldehydes, ketones, acids, or alcohols.[13]
Aerial and peroxide (30%) oxidation of cyclohexane with Au-MCM 41[14]
or mesoporous Ce-MCM-41 catalyst[15] gives a mixture of cyclohexanone
and cyclohexanol. To the best of our knowledge, 44% yield of cyclohexa-
none directly from cyclohexane in a one-pot reaction has not been
reported. Here we report the oxidation of cyclohexane, benzene, toluene,
ethyl benzene, anthracene, and phenanthrene by cerium(IV) sulphate in
an aqueous acetic acid medium catalyzed by iridium(III) chloride (with
catalyst–substrate ratio of 1:56818 to 151515) synthetic purposes.
EXPERIMENTAL
Cerium(IV) sulphate, anthracene (Loba Chemie Indaustranal Co.), cyclo-
hexane, benzene, toluene, ethyl benzene, and phenanthrene (E. Merck)
were used as such without further purification. Solution of iridium(III)
chloride was prepared by dissolving the sample (sodium chloroiridate)
(Johnson Matthey & Co.) in a minimum amount of concentrated HCl
(analytical reagent [A.R.]), and the final strengths of acid and catalyst
were 0.00624 M and 3.35 ꢁ 10ꢂ3M, respectively. Cerium(IV) sulphate,
prepared by dissolving the sample in 1:1 sulphuric acid, was standardized
with a standard solution of ferrous ammonium sulpahate using ferroin
(CDH) as an external indicator. All other chemicals used were either
A.R. or chemically pure substances. A Kenstar (model OM-20 ESP,
800 W, Aurangabad, India) domestic microwave oven was used for
studying the reactions under microwave irradiations, in which control
of temperature was not possible. For getting the maximum yield, five
to eight sets were performed by changing the concentration or conditions
of each component, which can affect the yield, for example in the syn-
thesis of cyclohexanone from cyclohexane (Table 1). For performing
the reactions in the solution phase, a calculated amount of hydrocarbon
dissolved in a requisite amount of acetic acid was mixed with iridium(III)

Solventless Oxidation of Hydrocarbons
3185
Table 1. Effect of various factors on the yield of cyclohexanone (a0) from cyclo-
hexane (a) (2.0 mmol) by cerium(IV) sulphate catalyzed by iridium(III) chloride in
aqueous acetic acid medium in solution phase and in the absence of acetic acid
under microwave irradition
Yield
Acetic acid
(for water
Time in
h (for
MW
(%)
MW (yield
power with
(%) MW)
Temp. (ꢀC)
Ce(IV) bath only) Ir(III) ꢁ 106 for water
Entry (mmol) (mmol)
(mmol)
bath
in min)
1
2
3
4.0
—
4.0
250
250
250
—
17.6
17.6
100
100
Room
temp.
100
100
100
100
100
100
70
3.0
3.0
48
—
—
—
0.0
0.0
0.0
4
5
6
7
8
4.0
4.0
4.0
4.0
3.0
5.0
4.0
4.0
4.0
4.0
250
250
250
350
250
250
250
—
15.4
17.6
19.8
17.6
17.6
17.6
17.6
17.6
17.6
17.6
3.0
3.0
3.0
3.0
3.0
3.0
3.0
—
—
—
—
—
—
—
35.9
44.0
42.0
44.0
22.4
31.0
15.2
35.9
19.9
20.6
9
10
11
12
13
—
—
—
3.0 (min) 40
3.0 (min) 20
3.0 (min) 80
—
—
chloride and cerium(IV) sulphate in a round-bottomed flask. The flask,
fitted with a water condenser, was kept in a water bath at a fixed tem-
perature for the desired time. Contents were cooled and extracted with
appropriate solvents. The extract was dried over anhydrous MgSO4.
Solvent was removed under reduced pressure, and hydrazone of the
product was prepared. For performing the reaction in solid phase in
a microwave oven, the same amounts of reactants except acetic acid
[cerium(IV) sulphate, cyclohexane, and iridium(III) chloride] as in the
solution-phase reaction were mixed in 25 g of alumina. The paste was
kept in an alumina bath was exposed in the microwave oven at the
desired power for the desired time. The workup procedure was the same
as given in the solution-phase reactions performed in a water bath. IR
1
(in KBr; Brucker Vector-22 IR spectrophotometer) and H NMR (Xeol
400 MHz in CdCl3 with TMS as internal standard) spectra were
recorded, and reactions were monitored with thin-layer chromato-
graphy, TLC (Merck GF254 silica-gel-coated plates). Purity and identi-
fication of products were confirmed by TLC, mp of the derivatives,
and their IR and NMR spectra.

3186
P. K. Tandon et al.
RESULTS
In all the cases, running TLC plates with hydrazone of the authentic sam-
ple showed only one spot. In the cases of a to d and f, a mixture was
extracted (15 ml ꢁ 3) with ether, whereas in the case of e, it was extracted
with benzene. Reaction conditions in which various experiments were
performed are summarized 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 derivatives of the products were prepared by the standard
method.[16] Hydrazone of cyclohexanone (a0) from cyclohexane (a) was
recrystallized with ethyl alcohol. Hydrazone derivative (245 mg) was
obtained, corresponding to 44% yield of cyclohexanone; mp of hydra-
zone was 160 ꢀC (reported as 162 ꢀC). NMR: d 7.96 to 7.99 (1H, d), d
8.27 to 8.31 (1H, dd), d 9.12 to 9.13 (1H, d), d 1.71 to 2.5 (10H, m). Sur-
prisingly under microwave irradiation (40% power for 3.0 min), the yield
decreased to 35.9% (200 mg of hydrazone was obtained). Benzoquinone
(b0) from benzene (b) was prepared as in a. Hydrazone (485 mg, 51.8%
yield) was obtained, which was recrystallized with ethyl alcohol. Under
microwave irradiation (40% power, 3.0 min), yield decreased to 22.4%
(210 mg of hydrazone was obtained). Hydrazone mp was 182 ꢀC (repor-
ted as 186 ꢀC). IR nmaxꢃ1619 nm (nC N), 1301 nmꢃ(nAr-NO2), 1491 nm
=
(nꢃbenzene ring), 3320–3362 (nNꢂH str.). NMR: d 6.8 (4H, s), d 6.5
(4H, s).
Benzaldehyde (c0) from toluene (c) prepared as before gave 200 mg of
hydrazone (34.9% yield). In the microwave oven (40% power, 3.0 min),
yield decreased to 24.4% (140 mg of hydrazone was obtained). Hydra-
zone mp was 232 ꢀC (reported as 241 ꢀC). NMR: d 11.2 (1H, s), d 5.29
(1H, s), d 9.1 (1H, d), d 8.1 to 8.3 (2H, m), d 7.2 to 7.9 (5H, m).
Acetophenone (d0) from ethyl benzene (d) prepared as before gave
540 mg of hydrazone (90% yield). On conducting the reaction in micro-
wave oven (60% power, 3.0 min), yield decreased to 65% (390 mg of
hydrazone). Hydrazone mp was 244 ꢀC (reported as 250 ꢀC). NMR: d
7.26 (1H, s), d 4.0 (1H, s), d 1.55 (3H, s), d 8.77 to 8.76 (1H, d), d 8.42
to 8.41 (2H, d), d 8.39 to 8.38 (1H, d), d 7.88 to 7.82 (2H, d), d 7.48 to
7.46 (1H, t).
Anthraquinone (e0) from anthracene (e) prepared as before gave
207 mg (99.8% yield). On conducting the reaction in a microwave oven
(60% power, 2.0 min), the yield decreased to 79.8% (166 mg of
anthraquinone). Anthraquinone mp was 284 ꢀC (reported as 286 ꢀC). IR
nmax 1673 cm ꢂ1 (nC O), 935–809 cm ꢂ1 (nsubs.benzene ring), NMR: d 7.049
=
to 8.12 (8 Hm). Phenanthraquinone (f0) from phenanthrene (f) gave
200 mg, (96.1% yield). In a microwave oven (60% power, 2.0 min), the

3187

3188
P. K. Tandon et al.
Scheme 1. Oxidation of hydrocarbons (a–d).
Scheme 2. Oxidation of anthracene (e).
Scheme 3. Oxidation of phenanthrene (f).
yield decreased to 72.1% (150 mg of phenanthraquinone). Phenanthraqui-
none mp was 206 ꢀC (reported as 208 ꢀC). IR nmaxꢃ3053 cm ꢂ1 (nArꢃCꢂH),
1674 cm ꢂ1 (nC¼O), and 732 cm ꢂ1 (nsubs.benzene ring).
DISCUSSION
The study was performed mainly to determine the efficiency and econ-
omy of the novel, simple, one-pot cerium(IV)–iridium(III) reaction on
to oxidize various organic compounds. Five to seven sets were performed
to obtain the maximum yield by changing concentration or conditions of
each component. Change in the concentration of acetic acid did affect the
yield (Table 1, entries 5 and 7), indicating that it acts as a solvent to

Solventless Oxidation of Hydrocarbons
3189
dissolve the hydrocarbons. Yields in all the cases reached a maximum and
then started to decrease with further increase in catalyst (entries 4 to 6,
Table 1) and oxidant concentrations (Table 1, entries 5, 8, and 9), indicat-
ing that optimum conditions are required to get the highest yield or that
some nonreactive complex is formed at higher concentrations of the cata-
lyst and oxidant. Lack of formation of the desired products, after pro-
longed heating in the absence of oxidant (Table 1, entry 2) or leaving
the mixture for 48 h at room temperature (Table 2, entry 3), indicates that
aerial oxidation was not taking place at higher temperatures and also that
temperature control was essential for getting the product with the desired
yield. It was also observed that the system was incapable of to oxidizeing
cyclohexane in the absence of catalyst (Table 2, entry 1), indicating that it
is the catalyst that is responsible for the oxidation, at least in the case of
cyclohexane. Probably the catalyst facilitates the decomposition of the
complex formed between oxidant and the organic substrate. During the
kinetic study of the oxidation of cyclic alcohols and aromatic aldehydes,
we observed[4–6] that it is not the catalyst but cerium(IV) that forms the
complex with the organic substrate in the first step, and iridium(III) facil-
itates the decomposition of the complex formed between cerium(IV) and
organic substrate. It is known that IrCl3 in HCl medium gives IrCl63ꢂ
species.[17] It has also been reported that IrIII and IrI ions are the sta-
ble species of iridium.[18] Further, the aquation of [IrCl6]3ꢂ gives
[IrCl5H2O]2ꢂ, [IrCl4(H2O)2]1ꢂ and [IrCl3(H2O)3] species.[19,20] In the
present case, also for preparing the catalyst, sodium hexachloroirida-
te(III) was dissolved in aqueous hydrochloric acid, and the concentration
of acetic acid did not affect the yield, indicating that acetic acid probably
does not change the species of catalyst in the medium. Therefore, an
active species of iridium taking part in the solution-phase reaction may
well be considered to be IrCl5(H2O)2ꢂ, which has been considered pre-
viously also.[1–6] 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
radicals or free radicals are generated.[21] Normally, these intermediates
undergo rapid oxidation to afford neutral products by electron transfer
(outer-sphere reaction) or by ligand transfer (inner-sphere reaction). As
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 CꢂH, CꢂC bond cleavage or hydrogen trans-
fer, depend on their structure. In the oxidation of alcohols[22] and aro-
matic aldehydes[23] 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 the presence of Ir(III) probably facilitates the removal of

3190
P. K. Tandon et al.
hydrogen from the complex formed in the first step between the oxidant
and the organic substrate by forming another complex.[3–6] Homolysis of
the CꢂH bond may give rise to H&, which may transfer its electron to
Ir(III), giving rise to [HIrCl26ꢂ], which in turn may dissociate back into
the original Ir(III) species, although homolysis of water may provide
OH&, giving rise to alcohol, which is further oxidized to a carbonyl com-
pound. The presence of alcohol in the system could not be detected, but
its formation as an intermediate cannot be ruled out.
The reported novel one-pot system is highly efficient, easy, and also
can be used to oxidize a variety of other functional groups, for which
studies are being done. Above all, it oxidizes cyclohexane and benzene,
which are otherwise difficult to oxidize. High yield of the carbonyl
group makes the system very attractive from a synthetic point of view.
Solventless synthesis under MW irradiation is also important from an
environmental point of view.
ACKNOWLEDGMENT
The authors are grateful to University Grants Commission [32–240=2006
(S.R.)] and Council of Scientific and Industrial Research [01(2150)=07=
EMR-II], New Delhi, India, for financial assistance. Help from Central
Drug Research Institute, Lucknow, India, is acknowledged for spectral
studies.
REFERENCES
1. Manibala; Singh, H. S.; Krishna, B.; Tandon, P. K. Iridium(III) chloride–
catalyzed oxidation of propan-2-one by hexacyanoferrate(III) in aqueous
alkaline medium. J. Indian Chem. Soc. 1985, 62, 434–437.
2. Singh, M. P.; Tandon, P. K.; Singh, R. M.; Mehrotra, A. Iridium(III)
chloride–catalyzed oxidation of some alcohols by alkaline hexacyanoferra-
te(III). J. Indian Chem. Soc. 1990, 67, 458–462.
3. Tandon, P. K.; Singh, A. K.; Baboo, R.; Dwivedi, P. B. Iridium(III)-cata-
lyzed oxidation of cyclic ketones by cerium(IV). Transition Met. Chem.
2004, 29, 663–670.
4. Tandon, P. K.; Sahgal, S.; Singh, A. K.; Gayatri; Purwar, M. Oxidation of
ketones by ceric perchlorate catalyzed by iridium(III). J. Mol. Catal. A:
Chem. 2005, 232, 83–88.
5. Tandon, P. K.; Sahgal, S.; Gayatri; Purwar, M.; Dhusia, M. Oxidation of
ketones by cerium(IV) in presence of iridium(III) chloride. J. Mol. Catal,
A: Chem. 2006, 250, 203–209.
6. Tandon, P. K.; Sahgal, S.; Singh, A. K.; Kumar, S.; Dhusia, M. Oxidation of
cyclic ketones by cerium(IV) in presence of iridium(III) chloride. J. Mol.
Catal, A: Chem. 2006, 258, 320–326.

Solventless Oxidation of Hydrocarbons
3191
7. Singh, M. P.; Singh, H. S.; Verma, M. K. Kinetics and mechanism of the
ruthenium(III) chloride catalyzed oxidation of butanone-2 and butanone-3
by cerium(IV) sulfate in aqueous sulfuric acid medium. J. Phys. Chem.
1980, 84, 256–259.
8. Singh, V. N.; Singh, H. S.; Saxena, B. B. L. Kinetics and mechanism of the
osmium tetroxide catalyzed oxidation of acetone and ethyl methyl ketone
by alkaline hexacyanoferrate(III) ion. J. Am. Chem. Soc. 1969, 91,
2643–2648.
9. Carvalho, N. M. F.; Horn, A. Jr.; Antunes, O. A. C. Cyclohexane oxidation
catalyzed by mononuclear iron(III) complexes. Appl. Catal. A: Gen. 2006,
305, 140–145.
10. Ji, H.; Qian, Y.; Luo, S.; He, D. Liquid-phase selective oxidation of cyclohex-
ane with modified-VPO catalysts under mild condition. J. Chem. Ind. Eng.
(China) 2004, 55, 2027–2031.
11. Yuan, Y.; Ji, H.; Chen, Y.; Han, Y.; Song, X.; She, Y.; Zhong, R. Oxidation
of cyclohexane to adipic acid using Fe-porphyrin as a biomimetic catalyst.
Org. Proc. Res. Dev. 2004, 8, 418–420.
12. Rossel, M.; Werner, T.; Frey, W.; Christoffers, J. Cerium-catalyzed, aerobic
oxidative synthesis of 1,2-dioxane derivatives from styrene and their frag-
mentation into 1,4-dicarbonyl compounds. Euro. J. Org. Chem. 2005, 23,
5031–5038.
13. Ganin, E.; Amer, I. Cerium catalyzed selective oxidation of alkylbenzenes
with bromate salts. Synth. Commun. 1995, 25, 3149–3154.
14. Lu, G.; Zhao, R.; Qian, G.; Qi, Y.; Wang, X.; Suo J. A highly efficient cata-
lyst Au=MCM-41 for selective oxidation cyclohexane using oxygen. Catal.
Lett. 2004, 97, 115–318.
15. Yao, W.; Chen, Y.; Min, L.; Fang, H. Yan, Z. Wang, H.; Wang, J. Liquid
oxidation of cyclohexane to cyclohexanol over cerium-doped MCM-41. J.
Mol. Catal. A: Chem. 2006, 246, 162–166.
16. Shriner, R. L.; Hermann, C. K. F.; Morrill, T. C.; Curtin, D. Y.; Fuson R. C.
The Systematic Identification of Organic Compounds, 7th Ed.; John Wiley and
Sons: New York, 1998; pp. 321–322.
17. Chang, J. C.; Garner, C. S. Kinetics of aquation of aquopentachloroirida-
te(III) and chloride anation of diaquotetrachloroiridate(III) anions. Inorg.
Chem. 1965, 4, 209–215.
18. Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. Advanced
Inorganic Chemistry; Wiley-Interscience: New York, 1999; p. 1039.
19. Poulsen, I. A.; Garner, C. S. A thermodynamic and kinetic study of hexa-
chloro and aquopentachloro complexes of iridium(III) in aqueous solutions.
J. Am. Chem. Soc. 1962, 84, 2032–2037.
20. Domingos, A. P. J.; Domingos, A. M. T. S.; Cabral, J. M. P. Kinetics of
aquation of hexachloroiridate(III) and of chloride anation of pentachloro-
aquoiridate(III). J. Inorg. Nucl. Chem. 1969, 31, 2563–2573.
21. Ho, T.-L. Organic Synthesis by Oxidation With Metal Compounds; Plenum:
New York, 1986; p. 570.

3192
P. K. Tandon et al.
22. Young, L. B.; Trahanovsky, W. S. Oxidation of organic compounds with cer-
ium(IV), VII: Formation constants for 1:1 cerium(IV)–alcohol complexes. J.
Am. Chem. Soc. 1969, 91, 5060–5068.
23. Wiberg, K. B.; Ford, P. C. Kinetics of oxidation of benzaldehyde. J. Am.
Chem. Soc. 1969, 91, 124–132.