RSC Advances
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Acknowledgements
We acknowledge the American Chemical Society Petroleum
Research Fund for a New Investigator Award (#54247-UNI3), the
Research Foundation of the City University of New York for a
PSC-CUNY award (#67312-0045),
a
CUNY Collaborative
Research Grant (CIRG #80209-06), and the Jingchu University of
Technology for support.
Notes and references
Fig. 1 The X-ray structure of tetranuclear complex 1 (a) and the
Cu
4
(m-O)Cl
6
core found in 1 (b).
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3
I. W. C. A. Arends and R. A. Sheldon, Modern Oxidation
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observation of 1 has important implications in the possible
active catalytic species, considering that CuCl also served as a
2
good metal source for the aerobic oxidation in water (entry 14,
Table 1). 1 was also tested as a catalyst in the presence of added
TEMPO. It was observed that when 1 mol% of 1, 5 mol% of
DMAP and 5 mol% TEMPO were employed for the oxidation of
benzylic alcohol in water, 97% conversion was detected.
However, if the reaction was run without additional DMAP, the
conversion was only 19% (entries 21 and 22, Table 1). This
indicates that 1 was likely to be an active intermediate during
the catalysis, which is in good agreement with the fact that a
4
(a) R. A. Sheldon and I. W. C. E. Arends, Adv. Synth. Catal.,
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1
: 2 Cu/DMAP ratio was required to ensure a complete
conversion (entry 12, Table 1). Therefore, we assume that during
the reaction DMAP acts bifunctionally as both a monodentate
ligand and a base to promote the reaction, similar to the best-
performing Cu/bpy/base systems previously reported by the
7,8,11–13
Sheldon, Koskinen and Stahl groups.
Although it is diffi-
cult to conclude that tetranuclear 1 was the real reactive inter-
mediate during the catalytic process, it has been previously
indicated that dimeric copper species could be active interme-
diates in Cu/bpy/TEMPO catalytic aerobic alcohol oxidations,
on the basis of both kinetic and spectroscopic studies, in rele-
130, 14462; (e) A. Gabrielsson, P. van Leeuwen and
W. Kaim, Chem. Commun., 2006, 4926.
(a) S. K. Hanson, R. Wu and L. A. Pete Silks, Org. Lett., 2011,
12,13d
vant work by both the Koskinen and Stahl groups.
A recent
7
report by Waymouth and coworkers also revealed that a trinu-
13, 1908; (b) C. Michel, P. Belanzoni, P. Gamez, J. Reedijk
II
3 2
clear Pd O cluster was an active intermediate for Pd -catalysed
and E. J. Baerends, Inorg. Chem., 2009, 48, 11909; (c)
N. Jiang and A. J. Ragauskas, J. Org. Chem., 2006, 71, 7087;
18
aerobic oxidation of alcohols. In addition, our own work also
suggested that tetranuclear copper cluster was a more active
(
d) P. Gamez, I. W. C. E. Arends, R. A. Sheldon and
19
catalyst for the same reaction than a dinuclear analog.
J. Reedijk, Adv. Synth. Catal., 2004, 346, 805; (e) P. Gamez,
I. W. C. E. Arends, R. A. Sheldon and J. Reedijk, Chem.
Commun., 2003, 2414; (f) I. E. Marko, P. R. Giles,
M. Tsukazaki, S. M. Brown and C. J. Urch, Science, 1996,
In conclusion, we have developed a Cu/DMAP/nitroxyl
radical catalyst system that successfully converted in water a
variety of primary and secondary alcohols into corresponding
aldehydes and ketones with high yields and selectivities. A novel
274, 2044.
4 6 4
tetranuclear complex, Cu (m-O)Cl (DMAP) was isolated from a
8
(a) B. A. Steinhoff, I. A. Guzei and S. S. Stahl, J. Am. Chem.
Soc., 2004, 126, 11268; (b) B. A. Steinhoff and S. S. Stahl, J.
Am. Chem. Soc., 2006, 128, 4348; (c) X. Ye, M. D. Johnson,
T. Diao, M. H. Yates and S. S. Stahl, Green Chem., 2010, 12,
closely related reaction and structurally determined by X-ray
crystallography, and the catalytic results indicated it was likely
to be the reactive intermediate during the catalytic oxidation.
We are currently seeking to observe more efficient copper-based
catalyst systems for aerobic alcohol oxidations with a broad
substrate scope.
1180; (d) M. N. Kopylovich, K. T. Mahmudov, M. Haukka,
P. J. Figiel, A. Mizar, J. A. L. da Silva and A. J. L. Pombeiro,
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61910 | RSC Adv., 2014, 4, 61907–61911
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