Angewandte
Communications
Chemie
3853; b) M. Costas, M. P. Mehn, M. P. Jensen, L. Que, Chem.
a Fenton-like fashion. This step is the rate-determining step,
as confirmed by the kinetic analysis described above. The COH
species initiates a radical chain mechanism by reacting with
the alkane to generate a C-centered radical species. The LCuII
[5] a) S. E. Allen, R. Walvoord, R. Padilla-Salinas, M. C. Kozlowski,
[6] Y. Y. See, A. T. Herrmann, Y. Aihara, P. S. Baran, J. Am. Chem.
state can be regenerated when the LCuIII OH complex is
À
reduced by H2O2, with the generation of a COOH radical. A
similar mechanism was recently proposed for the oxidation of
benzene to phenol with the catalyst [CuII(TMPA)]2+ in H2O2/
acetone mixtures.[22]
[7] D. H. R. Barton, S. D. Bꢀviꢃre, W. Chavasiri, E. Csuhai, D.
The cyclohexyl hydroperoxide product can be generated
by termination of the radical chain (RC + COOH) or by
hydrogen abstraction of an alkane by the ROOC intermediate,
which can be generated by a reaction between the C-centered
radical and the O2 present in solution. When the reaction was
carried out under Ar, a significant decrease in the reaction
yield was observed (34 versus 55%).[21] The selectivity of the
present system towards the formation of the cyclohexyl
hydroperoxide is quite remarkable. We envision that this
selectivity is due to the ability of the CuII complexes to
discriminate between H2O2 (in excess) and the cyclohexyl
hydroperoxide product, thus avoiding the formation of cyclo-
hexanol and cyclohexanone and their overoxidation products.
In the chemical industry, cyclohexane is oxidized to
cyclohexanone with a homogeneous CoIII catalyst under
harsh conditions (1508C, 12 atm O2) with low conversion
(4%).[2b] The copper catalysts described herein constitute
a first step in the development of cheap, environmentally
sustainable, and efficient synthetic methodologies that could
eventually replace traditional oxidation processes.
[8] P. Stavropoulos, R. C¸ elenligil-C¸ etin, A. E. Tapper, Acc. Chem.
[9] D. H. R. Barton, D. Doller, Y. V. Geletii, Mendeleev Commun.
1991, 115 – 116.
[10] A. Sobkowiak, A. Qui, X. Liu, A. Llobet, D. T. Sawyer, J. Am.
[11] a) A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova, M. Haukka,
b) A. M. Kirillov, M. N. Kopylovich, M. V. Kirillova, E. Y.
Karabach, M. Haukka, M. F. C. G. da Silva, A. J. L. Pombeiro,
[12] a) A. Conde, L. Vilella, D. Balcells, M. M. Dꢄaz-Requejo, A.
b) previously, other mononuclear Cu complexes were found to
catalyze the oxidation of cyclohexane with H2O2: C. Shimokawa,
[13] a) G. B. Shulꢁpin, J. Mol. Catal. A 2002, 189, 39 – 66; b) Note:
other reducing agents and metal salts have been found to be
effective in the reduction of alkyl hydroperoxides to alcohols
(see Ref. [13a]).
[15] L. Gꢅmez, I. Garcia-Bosch, A. Company, J. Benet-Buchholz, A.
[17] I. Prat, J. S. Mathieson, M. Gꢆell, X. Ribas, J. M. Luis, L. Cronin,
Experimental Section
See the Supporting Information for experimental details.
graphic data for this paper. These data can be obtained free of charge
[19] L. M. Slaughter, J. P. Collman, T. A. Eberspacher, J. I. Brauman,
[20] H. Marusawa, K. Ichikawa, N. Narita, H. Murakami, K. Ito, T.
[21] a) We observed by UV/Vis spectroscopy (see the Supporting
Information) that mixing CuI and the ligand led to fast oxidation
to generate LCuII. However, before the addition of H2O2, some
of the LCuI complex might still be present in solution, and could
react with H2O2 to generate LCuII and COH. The peroxidation of
cyclohexane can also be carried out with LCuII sources with
a slight decrease in the oxidation yields (see the Supporting
Information). b) It has been proposed that the oxidation of
alkanes with Cu can occur by a radical mechanism, whereby
LCuII is reduced by H2O2 to LCuI, which reacts with H2O2 to
generate COH and LCuII: M. V. Kirillova, A. M. Kirillov, M. F. C.
Acknowledgements
This research was supported by The Robert A. Welch
Foundation (Grant N-1900).
Keywords: alkanes · bioinorganic chemistry · copper ·
homogeneous catalysis · oxidation
[1] Modern Oxidation Methods (Ed.: J.-E. Bꢂckvall), Wiley-VCH,
Weinheim, 2004.
[2] a) R. A. Sheldon, J. K. Kochi, Metal-Catalyzed Oxidation of
Organic Compounds, Academic, New York, 1981; b) M. T.
Musser in Ullmannꢀs Encyclopedia of Industrial Chemistry,
Wiley-VCH, Weinheim, 2000.
[3] a) E. I. Solomon, D. E. Heppner, E. M. Johnston, J. W. Gins-
bach, J. Cirera, M. Qayyum, M. T. Kieber-Emmons, C. H.
Received: July 26, 2016
Published online: && &&, &&&&
4
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 5
These are not the final page numbers!