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DOI: 10.1002/cctc.201200459
Magnetic Polystyrene Nanosphere Immobilized TEMPO: A
Readily Prepared, Highly Reactive and Recyclable Polymer
Catalyst in the Selective Oxidation of Alcohols
[
a]
Zhi Zheng, Jianli Wang,* Miao Zhang, Lixin Xu, and Jianbing Ji
The 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) radical immo-
bilized on the magnetic polystyrene nanospheres (MPNs) was
used as a recyclable catalyst in the oxidation of various alco-
hols. The new and simply prepared heterogeneous TEMPO/
MPNs exhibits both similar versatility and efficiency to homo-
geneous TEMPO under basic Montanari conditions. The excel-
lent stability of the MPNs enables the TEMPO/MPNs to be recy-
cled more than 20 times without significant leaching of immo-
bilized TEMPO radicals or degradation of Fe O nanoparticles.
3
4
Moreover, the magnetic response ensures the rapid separation
and quantitative recycling of TEMPO/MPNs by simple magnetic
decantation.
Introduction
To achieve both economic and environmental benefits, a cata-
lyst should be stable enough to tolerate several iterative reac-
tions, and its high activity and facile recovery are also extreme-
ly desirable, especially if the reactions are performed on
944). Analogously to the non-immobilized TEMPO, this catalyst
allowed the efficient conversion of primary and secondary al-
cohols into their carbonyl compounds (ꢀ1.25 equiv. NaClO,
1 mol% catalyst, 10 mol% KBr, t=20 min), and precipitated se-
lectively from dichloromethane by addition of (tert-butyl)-
methylether, thereby enabling catalyst recycling 2 cycles with
[
1]
a
large scale.
Since 2,2,6,6-tetramethylpiperidine-1-oxyl
(
TEMPO) was first reported by Golubev et al. in 1965 as an oxi-
[
2]
[11]
dation catalyst, and further developed by Montanari et al. in
a minor loss of activity (5%). Along these lines, Tsang et al.
[
3]
1
987, it has been used widely over the past 20 years in the
developed a polyethylene glycol (10 KDa>M >5 KDa) teth-
W
catalytic oxidation of alcohols owing to its low toxicity, reversi-
ered TEMPO. Separation of this catalyst was achieved easily
[4]
[12]
ble redox behavior, and high efficiency and selectivity. How-
ever, TEMPO is quite expensive; therefore, its efficient recycling
is highly desirable.
(ꢀ>95% each cycle) by precipitation from diethyl ether. Cat-
alyst recycling was also possible for 6 iterative oxidation
[13]
cycles. In the meantime, Pozzi et al. synthesized the perfluor-
oalkyl tag functionalized TEMPO with peculiar solubility prop-
erties, thus enabling efficient catalyst separation by biphasic
(fluorous phase/organic phase) extraction (ꢀ >95%, each
cycle). However, to enhance the affinity for the fluorous extrac-
tion, this recycle approach required that the TEMPO catalyst
Recently, several strategies have emerged to conquer the
problem of TEMPO recycling through the design of various
[
5]
types of supported TEMPO. One approach is the immobiliza-
[
6]
tion of TEMPO onto insoluble supports, including saponite,
[
7]
[8]
mesoporous silica, silica sol–gels, cross-linked polystyrene
[
9]
[10]
[14]
resins, and polyolefin fibers, which allowed the catalyst to
be recycled out of the reaction media by simple filtration.
However, owing to the heterogeneous nature of the reaction,
most of these systems were far less versatile and efficient
have a “heavy” fluorine content (ꢀ60 wt%). Fortunately, as
documented by Reiser, the silica gel could be applied to sepa-
rate the “light” fluorous TEMPO (fluorine content 46%), which
had a catalyst recovery of >70% and could be recycled
[
15]
(
ꢀ1.25 equiv. NaClO, 1–5 mol% catalyst, 10–33 mol% KBr, t=
3 times with slightly diminished conversions (ꢀ4%).
Gao
[
6–9]
0
.5–5 h) than homogeneous TEMPO.
Another approach was
et al. prepared the imidazolium functionalized TEMPO, and the
catalyst recycle was based on the imidazolium bound TEMPO
and ionic liquid association, thereby achieving product separa-
tion by diethyl ether extraction. The ionic liquid phase contain-
ing the catalyst could be recycled 2 times with a small loss in
[
11–16]
to anchor TEMPO onto soluble supports,
which combined
the benefits of both homogeneous and heterogeneous cataly-
sis. Sheldon et al. first demonstrated this approach in the prep-
À27
aration of polyamine [M ꢀ3000 Da (1 Da=1.6605ꢀ10 kg)]
W
[16]
bound TEMPO analogues from light stabilizer (Chimassorb
efficiency (ꢀ2%).
So far, many chemists have paid great attention to the
design of a new improved protocol on TEMPO immobilization
with emphasis on the catalyst activity. Recently, magnetic
nanoparticles are used increasingly as supports for catalysts,
and such small and magnetically separable catalysts could
combine the advantages of high reactivity and easy separa-
[
a] Z. Zheng, Prof. J. Wang, M. Zhang, L. Xu, Prof. J. Ji
Zhejiang Province Key Laboratory of Biofuel
The State Key Laboratory Breeding Base
of Green Chemistry-Synthesis Technology
Zhejiang University of Technology
1
8 Chaowang Road, Hangzhou, 310014 (P.R. China)
[17]
[18]
[19]
tion. Reiser et al. and Garrell et al. have demonstrated
the convenient separation enabled by anchoring TEMPO onto
Fax: (+86)571-88320832
E-mail: wangjl@zjut.edu.cn
ꢁ
2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 307 – 312 307