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1443
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the solubility properties of catalyst 4 and 5 in some traditional
solvents were first investigated. They are completely soluble in po-
lar solvents such as water, MeOH, acetone, MeCN, but insoluble in
non-polar solvents such as ether and hexane. This could be benefi-
cial since it can provide an easy route to separate the product and
catalyst. After the reaction, the mixture was first diluted with
water, followed by extraction with ether or hexane, which led to
the separation of product from the catalyst. The reusability of the
catalyst was investigated by the oxidation of 1-phenylethanol. It
was found that when catalyst 4 was used as a catalyst, there was
no apparent loss of catalytic activity, and only 0.8% loss of weight
was observed after five times recycling.
Although, so far, we cannot be certain of the actual role of HFIP
in this reaction, a tentative mechanism is depicted in Scheme 2.
The oxoammonium cation I formed from catalyst 4 is the actual
oxidant in this reaction, responsible for the oxidation of alcohols
to the corresponding carbonyl compounds. The addition of 2,6-
2. For a recent review on applications of hypervalent iodine compounds in the
total synthesis of natural products, see: Silva, L. F.; Olofsson, B. Nat. Prod. Rep.
2011, 28, 1722–1754.
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Ishihara, K. Chem. Commun 2009, 2086–2099; (c) Uyanik, M.; Ishihara, K.
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185–191.
di-tert-butylphenol,
a free radical scavenger, to the reaction
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mixture has led to a drastic decrease of the conversion of 1-phen-
ylethanol (Table 1, entry 16), which provides an experimental
evidence for the generation of oxoammonium cation during the
reaction. The iodoarene moiety of II is reacted with PAA to form
the hypervalent iodine species III, which could further generate
the more electrophilic intermediate IV with HFIP via ligand ex-
change19; the generation of the more electrophilic intermediate
IV is responsible for the rate-accelerating effect. The role of IV is
to intramolecularly oxidize the hydroxylamine moiety to I for re-
use in this cycle.
In conclusion, we have designed and synthesized novel bifunc-
tional ionic liquid supported iodoarene-TEMPO catalysts bearing
two catalytic sites, the iodoarene and TEMPO moieties. These
catalysts were demonstrated to be useful for the efficient and envi-
ronmentally benign oxidation of alcohols to the corresponding
carbonyl compounds using PAA as a co-oxidant. A key feature of
this protocol is its inherent simplicity, there is no need for rigorous
exclusion of air or moisture in order to afford a clean oxidation, the
reaction conditions and reagents are easy to handle. Moreover,
these bifunctional catalysts could be easily recycled and reused
without the loss of activity.
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Acknowledgments
Chenjie Zhu thanks the China Scholarship Council for support-
ing his visit to the University of Minnesota Duluth, and Project sup-
ported by the Research and Innovation Plan for Graduate Students
of Jiangsu Higher Education Institutions (No. CX2211_0266). This
work was supported by research grants from the National Science
Foundation (CHE-1009038 and MRI CHE-0922366).
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Supplementary data
Supplementary data (experimental procedures and 1H and 13C
NMR spectra) associated with this article can be found, in the on-
References and notes
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