Copper(II)-catalyzed C-H oxidation of alkylbenzenes and cyclohexane with hydrogen peroxide

Article abstract of DOI:10.1016/j.tetlet.2003.10.016

Copper(II) complex 1 efficiently catalyses the oxidation of alkylbenzenes and cyclohexane into the corresponding ketones in moderate to high yields in the presence of 30% H₂O₂. This protocol is simple, clean and generates water as the only by-product.

Full text of DOI:10.1016/j.tetlet.2003.10.016

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 8955–8957
Copper(II)-catalyzed CꢀH oxidation of alkylbenzenes and
cyclohexane with hydrogen peroxide
Subbarayan Velusamy and T. Punniyamurthy*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India
Received 27 June 2003; revised 22 September 2003; accepted 3 October 2003
Abstract—Copper(II) complex 1 efficiently catalyses the oxidation of alkylbenzenes and cyclohexane into the corresponding
ketones in moderate to high yields in the presence of 30% H2O2. This protocol is simple, clean and generates water as the only
by-product.
© 2003 Elsevier Ltd. All rights reserved.
Transition-metal-catalyzed oxidation of organic com-
pounds with atom efficient oxidants such as O2 or H2O2
is rapidly gaining importance as a viable alternative to
the environmentally hazardous metal promoted stoi-
chiometric oxidations.1,2 Recent studies have shown
that transition metals in combination with various oxi-
dizing agents convert a wide range of hydrocarbons to
the corresponding oxidized products.3 These oxidations
are modeled on certain enzymes that perform the oxi-
dation of hydrocarbons in natural systems.4 In spite of
many studies, there are very few which use H2O2 as a
source of oxygen in the CꢀH oxidation of alkanes.5
More recently, we have reported the oxidation of alco-
hols to carbonyl compounds with a cobalt(II) complex
in the presence of 30% H2O2 in high yields.6 This
Scheme 1.
protocol functions in the absence of additive and gener-
ates water as the only by-product. Herein, we wish to
report the oxidation of alkylbenzenes and cyclohexane
to the corresponding ketones with copper(II) complex 1
Table 1. Copper(II)-catalyzed benzylic oxidation of
in the presence of 30% H2O2 in moderate to high yields
a
diphenylmethane to benzophenone with 30% H2O2
(Scheme 1).7
Entry
Catalyst
(0.1 mol%)
Time
(h)
Conv.b
(%)
Selectivityb
(%)
The oxidation of diphenylmethane was first examined
as a standard substrate with 30% H2O2 in the presence
of a catalytic amount of 1 in acetonitrile under ambient
conditions (Table 1).8 As expected the oxidation took
place to afford a 1:1 mixture of diphenylmethanol and
benzophenone in 34% yield in the presence of a 1 and
10 equiv. of 30% H2O2 in 16 h. Alternatively, the
reaction can be driven to the formation of benzophe-
1c,d
2
3
4
5
1
1
16
5
16
5
18
87
–
8
58
>99
>99
–
>99
>99
None
Cu(OAc)2
CuIIsalen
5
a A solution of the catalyst (0.1 mol%), diphenylmethane (2 mmol),
30% H2O2 (2.26 mL, 20 mmol) and acetonitrile (2 mL) was stirred
under atmospheric oxygen at ca. 80°C.
Keywords: oxidation; alkanes; catalyst; hydrogen peroxide; copper(II)
complex.
* Corresponding author. Fax: +91-361-2690762; e-mail: tpunni@
iitg.ernet.in
b Determined by GC.
c Reaction was performed at ambient temperature.
d Diphenylmethanol (16%) obtained.
0040-4039/$ - see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2003.10.016

8956
S. Velusamy, T. Punniyamurthy / Tetrahedron Letters 44 (2003) 8955–8957
none in 87% yield by heating the reaction mixture at
80°C for 5 h (entry 2). A control experiment without
catalyst 1 using the same reaction conditions showed no
oxidation. Cu(OAc)2 and CuIIsalen were also examined
as catalysts for this reaction, however, they were less
effective compared to 1 and afforded benzophenone in
8 and 58% yields, respectively (entries 4 and 5).
To evaluate the scope of the reaction, the oxidations of
ethylbenzene, butylbenzene, ethyl phenylacetate, tetra-
lin and cyclohexane were further studied (Table 2). As
above, all the substrates consistently underwent oxida-
tion selectively to the corresponding ketones in high
yields. The substrates, butylbenzene, ethylbenzene and
ethyl phenylacetate, were oxidized to the respective
ketones in 82–88% yields (entries 1–3). Similarly, cyclo-
hexane underwent oxidation to cyclohexanone in 18%
yield and tetralin was converted to a-tetralone in 89%
yield (entries 5 and 4). It is noteworthy to mention that
no oxidation was observed in the aromatic ring of the
Figure 1. Progress of the oxidation of ethylbenzene to aceto-
phenone with complex 1 and 30% H2O2.
alkyl benzenes. Likewise, in the case of butylbenzene,
the oxidation took place selectively at the benzylic CꢀH
bond and no oxidation was observed in the remaining
CꢀH bonds. The profile of the oxidation of ethylben-
zene to acetophenone is shown in Figure 1.
Table 2. Oxidation of alkylbenzenes and cyclohexane to
the corresponding ketones with catalyst 1 in the presence
a
of 30% H2O2
In conclusion, we have described a simple, clean and
efficient catalytic oxidation procedure that allows the
transformation of alkylbenzenes and cyclohexane into
the corresponding ketones in moderate to high yields.
Acknowledgements
This work was supported by the Department of Science
and Technology (Sanction No. SR/S1/OC-092002),
New Delhi and by the Council of Scientific and Indus-
trial Research (Sanction No. 01(1804)/02/EMR-II),
New Delhi.
References
1. (a) Edwards, J. O.; Curci, R. Catalytic Oxidations with
Hydrogen Peroxide as Oxidant; Strukul, G., Ed.; Kluwer:
Dordrecht, 1992; Chapter 4, pp. 97–151; (b) Sheldon, R.
A.; Kochi, J. K. Metal-Catalyzed Oxidation of Organic
Compounds; Academic Press: New York, 1991.
2. (a) Notari, B. Catal. Today 1993, 18, 163; (b) Reddy, J. S.;
Khire, U. R.; Ratnasamy, P.; Mitra, R. B. Chem. Com-
mun. 1992, 1234; (c) Butler, A.; Clague, M. J.; Meister, G.
E. Chem. Rev. 1994, 94, 625; (d) Jacobson, S. E.; Tang, R.;
Mares, F. Chem. Commun. 1978, 888; (e) Campestrini, S.;
Furia, F. D. J. Mol. Catal. 1993, 79, 13; (f) Herrmann, W.
A.; Fischer, R. W.; Correia, J. D. G. J. Mol. Catal. 1994,
94, 213; (g) Aubry, J.-M.; Bouttemy, S. J. Am. Chem. Soc.
1997, 119, 5286; (h) Shilov, A. E.; Shul’pin, G. B. Chem.
Rev. 1997, 97, 2879; (i) Dickman, D. H. Chem. Rev. 1994,
94, 569.
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.

S. Velusamy, T. Punniyamurthy / Tetrahedron Letters 44 (2003) 8955–8957
8957
Commun. 1993, 139; (d) Tateiwa, J.-I.; Horiuchi, H.;
Uemura, S. Chem. Commun. 1994, 2567; (e) Punniya-
murthy, T.; Kalra, S. J. S.; Iqbal, J. Tetrahedron Lett.
1995, 36, 8497; (f) Dura-Vila, V.; Mingos, D. M. P.; Vilar,
R.; White, A. J. P.; Williams, D. J. Chem. Commun. 2000,
1525; (g) Elemans, J. A. A. W.; Bijsterveld, E. J. A.;
Rowan, A. E.; Nolte, R. J. M. Chem. Commun. 2000,
2443.
6.7–7.2 (m, 8H), 3.9 (s, 4H), 2.85 (s, 4H); IR (KBr) 3288,
2909, 2868, 2827, 1608, 1565, 1398, 1260, 999 cm−1. Anal.
calcd for C16H20N2O2: C, 70.56; H, 7.40; N, 10.29. Found:
C, 70.42; H, 7.39; N, 10.31. The salen-H4 (150 mg, 0.55
mmol) was then reacted with Cu(OAc)2 (200 mg, 1 mmol)
in methanol (10 mL) at 50°C for 2.5 h under a nitrogen
atmosphere. Evaporation of the solvent on a rotary evapo-
rator gave a powder which was purified on silica gel
(60–120 mesh) column chromatography using EtOAc and
MeOH (15:5) as eluent to afford complex 1 as a green
powder in 70% yield: UV–vis (CH3CN) umax 328, 584 nm;
FAB MS: m/z 334 (M+).
4. Groves, J. T.; Krishnan, S.; Avaria, G. E.; Nemo, T. J.
Adv. Chem. Ser. 1980, 191, 277.
5. (a) Battioni, P.; Renaud, J. P.; Bartoli, J. F.; Reina-Artiles,
M.; Fort, M.; Mansuy, D. J. Am. Chem. Soc. 1988, 110,
8462; (b) Robert, A.; Loock, B.; Momenteau, M.; Meu-
nier, B. Inorg. Chem. 1991, 30, 706; (c) Bandyopadhyay,
R.; Biswas, S.; Guha, S.; Mukherjee, A. K.; Bhat-
tacharyya, R. Chem. Commun. 1999, 1627.
8. A homogeneous solution of the alkane (2 mmol), complex
1 (0.1 mol%) and 30% H2O2 (2.26 mL, 20 mmol) was
stirred in acetonitrile (2 mL) at ca. 80°C for the appropri-
ate time (see Table 2). The reaction mixture was then
cooled to room temperature and dimethyl sulphide (50 ml)
was added. After GC analysis of the reaction mixture, the
aqueous solution was removed on a rotary evaporator and
the residue was treated with diethyl ether (15 mL). The
organic solution was washed with saturated brine. Drying
(Na2SO4) and evaporation of the solvent on rotary evapo-
rator afforded a residue which was passed through a short
pad of silica gel using ethyl acetate and hexane as eluent to
afford the analytically pure products.
6. Das, S.; Punniyamurthy, T. Tetrahedron Lett. 2003, 44,
6033.
7. The reaction of salicylaldehyde (122 mg, 1 mmol) with
ethylenediamine (30 mg, 0.5 mmol) in methanol (5 mL)
afforded salen-H2 as a lemon yellow powder in 81% (217
mg), which was further reacted with NaBH4 (32 mg) in
methanol (5 mL) for 2 h at ambient temperature. Removal
of the solvent on a rotary evaporator followed by treat-
ment with water afforded salen-H4 as a colorless powder
in 72% (162 mg) yield. 1H NMR (CDCl3, 90 MHz) l