Molybdenum(VI)-peroxo complex catalyzed oxidation of alkylbenzenes with hydrogen peroxide

Article abstract of DOI:10.1016/S0040-4039(03)01126-2

The molybdenum(VI) peroxo complex 1, synthesized from the reaction of MoO₃ with H₂O₂ and 3,5-dimethylpyrazole (dmpz), selectively oxidizes benzylic C-H bonds of alkylbenzenes to the corresponding alcohols and ketones in moderate to good yields in the presence of H₂O₂ in acetonitrile under reflux (ca. 80°C).

Full text of DOI:10.1016/S0040-4039(03)01126-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 4915–4917
Molybdenum(VI)-peroxo complex catalyzed oxidation of
alkylbenzenes with hydrogen peroxide
Subhabrata Das, Tandra Bhowmick, T. Punniyamurthy,* Deepa Dey, Jayashree Nath and
Mihir K. Chaudhuri*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
Received 16 December 2002; revised 23 April 2003; accepted 2 May 2003
Abstract—The molybdenum(VI) peroxo complex 1, synthesized from the reaction of MoO3 with H2O2 and 3,5-dimethylpyrazole
(dmpz), selectively oxidizes benzylic C-H bonds of alkylbenzenes to the corresponding alcohols and ketones in moderate to good
yields in the presence of H2O2 in acetonitrile under reflux (ca. 80°C). © 2003 Elsevier Science Ltd. All rights reserved.
Metal catalyzed oxidation of organic compounds with
hydrogen peroxide is rapidly gaining importance as a
viable alternative to the environmentally hazardous
metal promoted stoichiometric oxidations.1,2 Recent
studies have shown that transition metals in combina-
tion with various oxidizing agents can convert a wide
range of hydrocarbons into the corresponding oxidized
products.3 These metal-catalyzed oxidations are mod-
eled on certain enzymes which perform the oxidation of
hydrocarbons in natural systems.4 In spite of many
studies, to the best of our knowledge, there is only one
report which uses hydrogen peroxide, in addition to
molecular oxygen, as the source of oxygen for the
oxidation of the benzylic CꢀH bond of alkyl benzenes
with a molybdenum complex under homogeneous con-
ditions.5 We now report that the new molybdenum(VI)
oxo-diperoxo complex6 1, synthesized from the reaction
of MoO3 with H2O2 and dmpz, has been found to
catalyze the oxidation of the benzylic CꢀH bond of
alkylbenzenes with hydrogen peroxide to the corre-
sponding alcohols and ketones in moderate to good
yields. This is the first example of a molybdenum
catalyzed oxidation reaction that employs a molybde-
num complex as the catalyst with H2O2 as the oxidant
under atmospheric oxygen for the oxidation of
alkylbenzenes.
catalytic amount of 1 in acetonitrile under atmospheric
oxygen (Table 1). As anticipated the oxidation occurred
selectively at the benzylic CꢀH bond and afforded a 3:2
mixture of diphenylmethanol and benzophenone in
62% yield when the reaction was allowed to reflux at ca.
80°C in the presence of 5 mol% of 1 for 12 h.7 The
reaction conditions with catalyst 1 and H2O2 are sum-
marized in Table 1.
Table 1. Oxidation of diphenyl methane with 1 and H2O2
Entry
Catalyst 1
H2O2 (mmol) Products (yield, %)a,b
(mol%)
Ph2CHOH
Ph2CO
1
2
3
5
5
5
20
40
60
30
38
43
9
24
25
The oxidation of diphenylmethane was first examined
as a standard substrate with H2O2 in the presence of a
a A solution of the catalyst 1, diphenylmethane (1 mmol), H2O2 (30%)
and acetonitrile (1 ml) was refluxed at ca. 80°C for 12 h under
atmospheric oxygen.
Keywords: alkylbenzene; selective oxidation; molybdenum peroxo
complex; hydrogen peroxide.
* Corresponding authors. E-mail: tpunni@iitg.ernet.in; mkc@
iitg.ernet.in
b Isolated yield.
0040-4039/03/$ - see front matter © 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0040-4039(03)01126-2

4916
S. Das et al. / Tetrahedron Letters 44 (2003) 4915–4917
To evaluate the scope of this reaction, the oxidation of
allylbenzene, butylbenzene, ethylbenzene, tetralin, tolu-
ene and xylenes (ortho and para) were studied further
(entries 1–5, 7 and 9, Table 2). As described above, the
oxidation consistently took place at a benzylic CꢀH
bond. The substrates containing secondary CꢀH bonds,
allylbenzene, butylbenzene, ethylbenzene and tetralin,
were oxidized to a mixture of the corresponding alco-
hols and ketones in high yields (entries 4–5, 7 and 9,
Table 2). However, toluene and xylenes were less reac-
tive affording the corresponding alcohols as the prod-
ucts thereby suggesting that the ease of oxidation is a
Table 2. Oxidation of alkylbenzenes with 1 and H2O2

S. Das et al. / Tetrahedron Letters 44 (2003) 4915–4917
2. Dickman, M. H. Chem. Rev. 1994, 94, 569.
4917
direct reflection of the CꢀH bond strength (entries 1–3,
Table 2). In the case of allylbenzene no double bond
oxidation was observed. Similarly, methyl phenylac-
etate afforded the corresponding a-keto ester selectively
as the product, no oxidative cleavage being observed
(entry 6, Table 2).
3. (a) Barton, D. H. R.; Beviere, S. D.; Chavasiri, W.;
Csuhai, E.; Doller, D. Tetrahedron 1992, 48, 2895; (b)
Faraj, M.; Hill, C. L. Chem. Commun. 1987, 1487; (c)
Murahashi, S.-I.; Oda, Y.; Naota, T.; Komiya, N. Chem.
Commun. 1993, 139; (d) Tateiwa, J.-I.; Horiuchi, H.;
Uemura, S. Chem. Commun. 1994, 2567; (e) Dura-Vila, V.;
Mingos, D. M. P.; Vilar, R.; White, A. J. P.; Williams, D.
J. Chem. Commun. 2000, 1525; (f) Elemans, J. A. A. W.;
Bijsterveld, E. J. A.; Rowan, A. E.; Nolte, R. J. M. Chem.
Commun. 2000, 2443; (g) Zhou, X.-G.; Yu, X.-Q.; Huang,
J.-S.; Li, S.-G.; Li, L.-S.; Che, C.-M. Chem. Commun.
1999, 1789; (h) Yoshida, H.; Murata, C.; Hattori, T.
Chem. Commun. 1999, 1551.
Notably, the yields of the oxidized products were sig-
nificantly reduced when the reactions were carried out
under a nitrogen atmosphere. Additionally, the oxida-
tion of diphenylmethanol was carried out separately
and benzophenone (90%) was obtained. This study
suggested that the ketones produced in the oxidation of
the substrates having secondary CꢀH bonds are derived
from in situ oxidation of the respective secondary
alcohols.
4. Groves, J. T.; Krishnan, S.; Avaria, G. E.; Nemo, T. J.
Adv. Chem. Ser. 1980, 191, 277.
5. Bandyopadhyay, R.; Biswas, S.; Guha, S.; Mukherjee, A.
K.; Bhattacharyya, R. Chem. Commun. 1999, 1627.
6. Preparation of 1. MoO3 (22.22 mmol) was dissolved in
30% H2O2 (220 mmol). The solution was cooled in an
ice-bath and then dmpz (44.93 mmol) was added in por-
tions. A red suspension was obtained at this stage which
was stirred for 1 h and then left overnight at ca. 4°C to
afford a lemon yellow colored microcrystalline complex,
[MoO(O2)2(dmpz)2] 1, in 57% yield. Mp 91°C; IR(KBr)
w/cm−1: 935 (MoꢁO), 882 (O-O), 592 (Mo-O2, sym.), 665
(MoꢁO2, asym.), 1568 (C-N), 3323, (N-H), 303 (Mo-N);8
Raman (KBr) w/cm−1: 956 (MoꢁO), 886 (O-O), 320 (Mo-
N), 674 (Mo-O2, asym), 595 (Mo-O2, sym); electronic
In conclusion, the studies described herein indicate that
the molybdenum complex 1 catalyses the oxidation of
alkylbenzenes in the presence of hydrogen peroxide
under atmospheric oxygen in moderate to good yields.
The system is mild and selective in so far as benzylic
CꢀH bond oxidation is concerned. Further investiga-
tion is in progress to determine the detailed mechanism
and the exact chemical nature of the active oxidant in
these oxidations.
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2−
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