Copper-triethanolamine complex as efficient and active catalyst for selective oxidation of alkylarenes to phenyl ketones by tert-butylhydroperoxide

Article abstract of DOI:10.1016/j.tetlet.2007.10.135

Several types of alkylarenes were selectively oxidized to the corresponding α-phenyl ketones in moderate to good selectivities and conversions using tert-butylhydroperoxide in the presence of a tetranuclear copper-triethanolamine complex under mild reaction conditions.

Full text of DOI:10.1016/j.tetlet.2007.10.135

Available online at www.sciencedirect.com
Tetrahedron Letters 48 (2007) 9108–9111
Copper-triethanolamine complex as efficient and active catalyst
for selective oxidation of alkylarenes to phenyl ketones
by tert-butylhydroperoxide
Mingxia Zhu, Xin Wei, Bodong Li and Youzhu Yuan*
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering,
Department of Chemistry, Xiamen University, Xiamen 361005, China
Received 7 July 2007; revised 22 October 2007; accepted 26 October 2007
Available online 30 October 2007
Abstract—Several types of alkylarenes were selectively oxidized to the corresponding a-phenyl ketones in moderate to good selec-
tivities and conversions using tert-butylhydroperoxide in the presence of a tetranuclear copper-triethanolamine complex under mild
reaction conditions.
ꢀ 2007 Elsevier Ltd. All rights reserved.
Selective oxidation of alkylarenes to aromatic ketones is
one of the most important fundamental reactions, with
respect to both academic and industrial purposes in
the manufacture of various perfumes, drugs, and phar-
maceuticals. The earlier processes, such as the oxidation
reaction of the methylene group attached to the aro-
matic ring using stoichiometric quantities of oxidizing
agents like KMnO4 or K2Cr2O7 and by Friedel–Crafts
acylation reaction of aromatics with acid halide/anhy-
dride using stoichiometric amounts of corrosive AlCl3
as catalyst,1,2 for the production of aromatic ketones
suffered from their corrosive and environmentally
unfriendly nature. Recently there has been an increased
interest in the development of clean and economical pro-
cesses for the selective oxidation of alkylarenes to higher-
value added phenyl ketones. In this line the catalytic
systems heterogeneously and homogeneously in combi-
nation with environmentally benign oxidants like O2,
H2O2 and tert-butylhydroperoxide (TBHP) are highly
desirable systems to be tested and developed.3,4 It is
known that copper species are widely present in nature
and in many enzymes, some multinuclear copper com-
plexes have been applied as catalyst for C–H oxidation
in the presence of H2O2 and TBHP.5 Pomberio and
his co-workers synthesized several copper(II)-triethanol-
amine complexes as catalysts for the oxidation of cyclo-
hexane and alkanes. The reactions showed potentially
higher yields.6 In this Letter, we report the results
obtained with copper(II)-triethanolamine complexes 1–4,
that is, dimeric [Cu2(H2tea)2(C6H5COO)2]Æ2H2O 1,
trimeric [Cu3(H2tea)2(4-OC6H4COO)2(H2O)]Æ4H2O 2,
tetrameric [OꢀCu4(tea)4(BOH)4][BF4]2 3, and polymeric
[Cu2(H2tea)2{l-C6H4(COO)2-1,4}]nÆ2nH2O 4, for the
selective oxidation of alkylarenes to the corresponding
aromatic ketones using TBHP as oxidant under mild
conditions. The results showed, in particular, that the
alkylarenes like ethylbenzene and 1-bromo-4-n-alkyl-
benzenes could be converted to the corresponding
a-phenyl ketones with selectivity higher than 90% in
moderate to good conversions at 343 K by the tetra-
nuclear complex 3.
The Cu(II) atoms in complexes 1 and 4 are hexa-coordi-
nated with triethanolamine ligand and carboxylate
ligand (for 1) or terephthalic acid ligand (for 4), respec-
tively. While the Cu(II) species in complexes 2 and 3 are
penta-coordinated with triethanolamine ligand. Com-
plex 3 is a cage structure composed of centro-symmetric
subunits [OꢀCu4(tea)4(BOH)4] and BFꢁ4 per unit, where
an oxygen atom is embedded into the center of four Cu
atoms and each Cu atom coordinates with one tetra-
dentate triethanolamine ligand and the central oxygen
atom. Moreover, four B–OH groups connect three
oxygen atoms of different triethanolamine ligand.6 The
geometries of complexes 1–4 are shown in Figure 1S
(see Supplementary data).
*
Corresponding author. Tel.: +86 592 2184565; fax: +86 592
2183047; e-mail: yzyuan@xmu.edu.cn
0040-4039/$ - see front matter ꢀ 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.135

M. Zhu et al. / Tetrahedron Letters 48 (2007) 9108–9111
9109
It has been reported that high catalytic activity
and excellent selectivity in the oxidation of ethylbenzene
to acetophenone were possibly obtained by using
cobalt(III) complexes and Co/SBA-15 as catalysts in
the presence of O2 or air without the need of solvent.3b,7
But the reaction was carried out under pressure and
higher temperatures. Other catalyst systems such as
anthraquinone derivatives/N-hydroxyphthalimide/zeo-
lite HY biomimetic system, complexes of Cu, Ti, and
Fe, o-phenanthroline-mediated metal-free catalytic
system, MnOꢁ4 -exchanged MgAl hydrotalcite, NiAl
hydrotalcite, Mn-MCM-41, Ba-, and V-containing com-
pounds were also found to be active and efficient for the
selective oxidation of ethylbenzene to acetophenone
with dioxygen, TBHP, or H2O2 as oxidants.4,5,8 Table
1 lists the oxidation of ethylbenzene in the presence of
different Cu compounds as the catalysts with TBHP
under mild reaction conditions. All these Cu compounds
afforded acetophenone as major product. Among them
complex 3 exhibited the highest acetophenone yield,
while complexes 1, 2, 4, Cu(NO3)2, and CuCl2 were less
effective than 3 (see entries 1–4 and 15, 16). A controlled
experiment without any copper compounds under iden-
tical conditions gave negligible conversion (entry 17).
The influences of different parameters (the ratio of
TBHP to ethylbenzene, temperature, and time profile)
on the performance of 3 for the oxidation of ethylbenzene
are also incorporated in Table 1. It was found that the
ratio of TBHP to ethylbenzene could significantly
affect the conversion and acetophenone selectivity
(entries 5–7). Higher conversion associated with higher
selectivity was obtained when the ratio of TBHP to
ethylbenzene was set at 3:1. When the reaction temper-
ature was increased from 313 to 343 K, the conversion
of ethylbenzene increased drastically from 9.2% to
63.7%, where the selectivity to main product acetophe-
none was also increased from 70.9% to 91.7% (entries
8–10). However, when the reaction temperature
increased to 353 K, a considerable amount of unknown
tar-like products was formed, probably due to extensive
carbonization which occurred. By prolonging the reac-
tion time from 1 to 7 h, both the conversion and aceto-
phenone selectivity increased gradually and reached
almost saturation by 7 h of reaction. No remarkable
increase in the conversion and selectivity were obtained
by further extending the reaction time to 10 h.
When using the protocol for the oxidation of other
alkylarenes with complex 3, the corresponding phenyl
ketones were also formed as the major products with
moderate to good selectivities (Table 2). However, a
different reactivity was observed with the substrates
having different side-chains and electron-withdrawing
groups in the aromatic ring. In general, the longer is
the side-chain, the lower is the selectivity to ketone.
For instance, n-propylbenzene and n-butylbenzene were
oxidized to propiophenone and butyrophenone with
selectivities of 76.3% and 61.5% in conversions of
Table 1. Selective oxidation of ethylbenzene by several copper compounds using TBHP oxidanta
O
OH
O
Copper complex
H
+
Others
+
+
CH3CN, TBHP
313~343 K
III
II
I
Entry
Catalyst
T (K)
Time (h)
TBHP/substrate
(molar ratio)
Conversionb (%)
Selectivityc (%)
II
Yieldd (%)
I
III
2.1
1.9
1.3
2.3
1
2
3
4
1
2
3
4
333
333
333
333
7
7
7
7
2
2
2
2
48.7
44.7
61.7
30.7
68.5
59.7
82.6
39.1
11.2
4.5
5.1
8.1
32.9
26.7
51.0 (41.9)
12.0
5
6
7
3
3
3
333
333
333
7
7
7
0
1
3
2.3
29.8
66.3
35.7
43.6
90.2
6.3
6.8
3.6
16.3
2.2
0.6
0.8
13.0
59.8 (47.8)
8
9
10
3
3
3
313
323
343
7
7
7
2
2
2
9.2
45.7
63.7
70.9
75.6
91.7
7.2
5.6
3.7
1.1
1.7
1.8
6.5
34.5
58.4
11
12
13
14
3
3
3
3
343
343
343
343
1
3
4
5
2
2
2
2
37.0
47.8
50.0
54.3
96.8
93.4
94.0
93.2
1.6
2.2
2.5
1.8
0.5
1.3
1.0
1.6
35.8
44.6 (32.3)
47.0
50.6
15
16
17
Cu(NO3)2
CuCl2
—
333
333
333
7
7
7
2
2
2
51.2
28.5
1.7
72.3
53.8
99.0
2.2
6.7
0
3.2
0.7
0
37.0
15.3
1.7
a Ethylbenzene (6 mmol) and copper compound (0.015 mmol) in CH3CN (5 mL).
b Average conversion of two runs based on GC results (DM-Wax 30 m · 0.25 mm · 1.0 lm). The reaction mixture was analyzed twice, before and
after treatment with PPh3.8d,e No differences from each other were obtained.
c The other products (mainly benzoic acid) have not been incorporated for conciseness.
d The data in parenthesis are the isolated yields of acetophenone.

9110
M. Zhu et al. / Tetrahedron Letters 48 (2007) 9108–9111
Table 2. Oxidation of alkylarenes catalyzed by complex 3 using TBHP oxidanta
Entry
1
Substrate
Major product
Time (h)
5
Conversionb (%)
63.1
Selectivityc (%)
76.3
Yieldd (%)
48.1 (44.3)
O
O
2
3
4
5
3
52.6
46.8
79.2
61.5
42.8
93.8
32.3 (31.2)
20.0 (18.3)
74.3 (65.5)
O
O
10
Br
Br
Br
O
5
6
7
10
3
55.8
72.7
52.1
92.3
51.5 (50.3)
47.8
Br
O
65.8
OH
O
7
45.6 and 23.4
23.8 (21.6)e
and
a Substrate (6 mmol), complex 3 (0.015 mmol), and TBHP (12 mmol) in CH3CN (5 mL) at 343 K.
b Average conversion of two runs based on GC results.
c The other products such as carboxylic and alcoholic species have not been incorporated for conciseness.
d The data in parenthesis are the isolated yields of the corresponding major product.
e The yield of cyclohexanone.
63.1% and 52.6% at 343 K for 5 h, respectively (entries 1
and 2). The 1-bromo-4-n-alkylarenes were converted to
the corresponding phenyl ketones with selectivities
above 90% (entries 4 and 5). A yield as high as 74.3%
for 40-bromoacetophenone can be obtained by the
oxidation of 1-bromo-4-ethylbenzene at 343 K for 10 h
(entry 4). The reaction mechanism of iso-propylbenzene
to acetophenone by the protocol is not clear enough at
present (entry 3), but it would undergo the cleavage of
dant. The process exhibited advantages of mild reaction
conditions and higher selectivities, providing a simple
method to synthesize aromatic ketones starting from
alkylarene substrates.
Acknowledgments
The authors gratefully acknowledge the financial
supports from the NSFC (20473065 and 20433030),
the Key Project of Fujian Province (2005HZ01-3) and
the Research Fund for the Doctoral Program of Higher
Education (20050384011) and the Key Project of
Chinese Ministry of Education (106099).
a
C–C bond. When tetrahydronaphthalene was
employed as a substrate, a-tetralone was obtained with
65.8% selectivity in a reaction conversion of 72.7% at
343 K for 3 h (entry 6). In the case of cyclohexane, a
conversion of 52.1% was obtained at 343 K for 7 h,
affording cyclohexanone and cyclohexanol as the major
products with selectivities of 45.6% and 23.4%,
respectively.
Supplementary data
The oxidations from alkylarenes to ketones were con-
ceivably passing through the aromatic alcohols as inter-
mediates. We have used a-phenylethanol as a substrate
and carried out the oxidation reaction under identical
conditions. The results showed that the acetophenone
was obtained in a yield higher than 99%.
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tetlet.
2007.10.135.
References and notes
In summary, we have reported that the tetranuclear cop-
per-triethanolamine complex 3 catalyzed the oxidation
of alkylarenes to phenyl ketones using TBHP as oxi-
1. (a) Culis, C. F.; Ladbury, J. W. J. Chem. Soc. 1955, 2850;
(b) Clark, J. H.; Kybett, A. P.; London, P.; Macquarrie, D.
J.; Martin, K. J. Chem. Soc., Chem. Commun. 1989, 1355.

M. Zhu et al. / Tetrahedron Letters 48 (2007) 9108–9111
9111
2. (a) Groggins, P. H.; Nagel, R. H. Ind. Eng. Chem. 1934, 26,
1313; (b) Sanders, H. J.; Keag, H. F.; McCullough, H. S.
Ind. Eng. Chem. 1953, 45, 2; (c) Olah, G. A. Friedel–Crafts
and Related Reactions; Wiley-Interscience: New York,
1963.
3. (a) Bandyopadhyay, R.; Biswas, S.; Guha, S.; Mukherjee,
A.; Bhattacharyya, R. Chem. Commun. 1999, 999, 1627; (b)
Wentzel, B. B.; Donners, M. P. J.; Alsters, P. L.; Feiters, M.
C.; Nolte, R. J. M. Tetrahedron 2000, 56, 7797; (c) Qi, J. Y.;
Ma, H. X.; Li, X. J.; Zhou, Z. Y.; Choi, M. C. K.; Chan, A.
S. C.; Yang, Q. Y. Chem. Commun. 2003, 294, 1294; (d)
Cornell, C. N.; Sigman, M. S. J. Am. Chem. Soc. 2005, 127,
2796; (e) Guan, B. T.; Xing, D.; Cai, G. X.; Wan, X. B.; Yu,
N.; Fang, Z.; Yang, L. P.; Shi, Z. J. J. Am. Chem. Soc.
2005, 127, 18004.
4. (a) Yang, G. V.; Ma, Y. F.; Xu, J. J. Am. Chem. Soc. 2004,
126, 10542; (b) Choudhary, V. R.; Indurkar, J. R.;
Narkhede, V. S.; Jha, R. J. Catal. 2004, 227, 257; (c) Jana,
S. K.; Wu, P.; Tatsumi, T. J. Catal. 2006, 240, 268.
5. (a) Velusamy, S.; Punniyamurthy, T. Tetrahedron Lett.
2003, 44, 8955; (b) Bennur, T. H.; Srinivas, D.; Sivasanker,
S. J. Mol. Catal. A: Chem. 2004, 207, 163; (c) Ferguson, G.;
Ajjou, A. N. Tetrahedron Lett. 2003, 44, 9139.
6. (a) Kirillov, A. M.; Kopylovich, M. N.; kirillova, M. V.;
Haukka, M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L.
Angew. Chem., Int. Ed. 2005, 44, 4345; (b) Kirillov, A. M.;
Kopylovich, M. N.; Kirillova, M. V.; Karabach, E. Y.;
Haukka, M.; Guedes da Silva, M. F. C.; Pombeiro, A. J. L.
Adv. Synth. Catal. 2006, 348, 159.
7. Ma, H.; Xu, J.; Chen, C.; Zhang, Q. H.; Ning, J. B.;
Miao, H.; Zhou, L. P.; Li, X. Q. Catal. Lett. 2007, 113,
104.
8. (a) Fujiwara, M.; Xu, Q.; Souma, Y.; kobayashi, T. J. Mol.
Catal. A: Chem. 1999, 142, 77; (b) Tong, X. L.; Xu, J.;
´
Miao, H. Adv. Synth. Catal. 2005, 347, 1953; (c) Menage,
S.; Vincent, J. M.; Lambeaux, C.; Chottard, G.; Grand, A.;
Fontecave, M. Inorg. Chem. 1993, 32, 4766; (d) Evans, S.;
Smith, J. R. L. J. Chem. Soc., Perkin Trans. 2 2000, 1541;
(e) Evans, S.; Smith, J. R. L. J. Chem. Soc., Perkin Trans. 2
2001, 174; (f) Pavan, C.; Julien, J.; Bolm, C. Adv. Synth.
Catal. 2005, 347, 703; (g) Vetrivel, S.; Pandurangan, A. J.
Mol. Catal. 2004, 217, 165; (h) Toribio, P. P.; Campos-
martin, J. M.; Fierro, J. L. G. J. Mol. Catal. 2005, 227, 101;
(i) Wang, X. Q.; Ou, G. N.; Yuan, Y. Z. Acta Chim. Sinica
2004, 62, 1695; (j) Radhika, T.; Sugunan, S. Catal.
Commun. 2007, 8, 150.