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Scheme 4 Oxidation of anthracene and 2-ethylanthracene into their
corresponding quinones.
2-ethyl-9,10-anthraquinone in the industrial synthesis of hydrogen
peroxide. Its preparation21 is frequently claimed as a draw-
back in terms of costs. The direct, metal induced oxidation of
2-ethylanthracene has already been reported but always using
a strong acidic reaction medium.20c
6 M. Tani, T. Sakamato, S. Mita, S. Sakaguchi and Y. Ishii, Angew.
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On the basis of the above, we have studied the oxidation of
both anthracene (AN) and 2-ethylanthracene (2EAN) with
hydrogen peroxide. When a series of copper complexes bearing
Tpx ligands were employed as catalysts in those reactions,
nearly quantitative conversions into 9,10-anthraquinone (AQ)
or 2-ethyl-9,10-anthraquinone (2EAQ) were observed for an
array of catalysts upon heating at 80 1C for 2 h (Scheme 4). The
best results were obtained with the complex Tp*,BrCu(NCMe)
as the catalyst, which provided 98% isolated yields of both
quinones. Yields were significantly lower below that tempera-
ture, or longer reaction times were required to enhance
conversion values, although selectivity into the quinones was
not dependent on the temperature (optimization conditions are
given in the ESIw). At variance with the benzene system, in this
case a large excess of H2O2 was employed to assess the desired
oxidation. Attempts to obtain intermediate oxidation products
failed, probably due to their instability under the reaction
conditions. It is worth mentioning that this transformation
lacks the formation of other products derived from the oxidation
of either other aromatic C–H bonds or, more importantly, the
available aliphatic C–H bonds of the ethyl groups. Therefore,
this system operates under a complete selectivity toward the
formers.22 Both the yields and the selectivities observed with
this copper-based catalyst are unprecedented for acid-free
catalytic systems in this oxidation reactions.20c
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13 M. M. Dıaz-Requejo, T. R. Belderraın and P. J. Perez, Chem.
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Commun., 2000, 1853–1854.
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15 Only for the sake of comparison of different catalytic systems and
conditions, it is appropriate to calculate the phenol productivity
as follows: Prod = (mmol of benzene reacted ꢀ selectivity to
phenol ꢀ MW PhOH ꢀ 10ꢁ6)/(mmol of catalyst ꢀ MW catalyst ꢀ
10ꢁ6 ꢀ rxn time in h). The values are the following: iron system,
1.7; this system, 3.6 (in kg PhOH/(kg catalyst ꢀ h)).
In conclusion, we have found that complexes of general
formula TpxCu(NCMe) catalyze the oxidation of benzene into
phenol and of anthracenes into quinones with conversions that
challenge other systems reported in the literature. Particularly,
this system does not require the presence of an acidic medium
and operates under somewhat mild conditions. Studies to
ascertain the mechanism that governs these transformations
are currently underway in our laboratory.
16 D. Bianchi, L. Balducci, R. Bortolo, R. D’Aloisio, M. Ricci,
G. Span, R. Tassinari, C. Tonini and R. Ungarellia, Adv. Synth.
Catal., 2007, 349, 979–986.
17 A. I. Conde, M. M. Dıaz-Requejo and P. J. Perez, Spanish Pat.
´ ´
Appl. P201031109, 2000.
18 A. Vogel, Ullmann’s Encyclopedia of Industrial Chemistry, VCH,
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Chem., 2006, 20, 20; (c) A. E. Gekhman, G. E. Amelichkina,
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We wish to thank DGI (CTQ2008-00042BQU and Consolider
Ingenio 2010, Grant CSD-2006-003) and Junta de Andalucia
(FQM-0914) for funding.
Notes and references
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22 A. I. Conde, M. M. Dıaz-Requejo and P. J. Perez, Spanish Pat.
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c
This journal is The Royal Society of Chemistry 2011
8156 Chem. Commun., 2011, 47, 8154–8156