11874
J. Am. Chem. Soc. 1998, 120, 11874-11879
Remarkable Product Selectivity in Photosensitized Oxidation of
Alkenes within Nafion Membranes
Chen-Ho Tung* and Jing-Qu Guan
Contribution from the Institute of Photographic Chemistry, Chinese Academy of Sciences,
Beijing 100101, China
ReceiVed May 15, 1998
Abstract: 9,10-Dicyanoanthracene (DCA)-sensitized photooxidation of trans,trans-1,4-diphenyl-1,3-butadiene
(DPB), trans-stilbene (TS), and R-pinene (PE) incorporated in Nafion membranes has been investigated. While
the oxidation in homogeneous solution yields the products derived from both the energy transfer and the
electron transfer pathways, that within Nafion membranes selectively gives either the singlet oxygen mediated
or the superoxide radical anion mediated products depending on the status and location of the substrate and
sensitizer molecules in the reaction media. Upon incorporating the alkene within the water-swollen Nafion
membranes and dissolving DCA in the surrounding CH2Cl2 solution, the sensitizer is isolated from the substrate
during the irradiation time and the electron transfer between them is prevented. The singlet oxygen produced
in the surrounding solution diffuses into the Nafion clusters and reacts with the alkenes. Thus, only the singlet
oxygen oxidation products are obtained and no product derived from superoxide radical anion is detected. In
contrast, incorporating both the substrate and the sensitizer within the water-swollen Nafion membranes leads
to the possibility of situating the two dissimilar molecules near one another within the restricted space of the
Nafion cluster. Thus, the electron transfer from the alkenes to the singlet excited state of DCA is enhanced,
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and the efficiency of the intersystem crossing from DCA* to DCA* is reduced. The photosensitized oxidation
in this case only gives products derived from the electron-transfer pathway, and no singlet oxygen mediated
product is observed.
Introduction
radical cation and sensitizer radical anion, which subsequently
-
•
reduces O2 to give superoxide radical anion (O2 ). The
generated superoxide radical anion reacts with the alkene radical
cation to yield the oxidation products. Unfortunately, in many
cases the two types of photooxidation occur simultaneously,
and the selectivity of the oxidation reactions is poor.
Selective oxidation of alkenes by molecular oxygen is one
of the current challenges in the manufacture of organic building
blocks and industrial intermediates. The key problem in such
reactions is the product specificity, and here photosensitized
oxidations hold special promise.2 There are two well-established
types of dye-sensitized photooxidation: energy-transfer pathway
and electron-transfer pathway.3 The energy-transfer pathway
involves the energy transfer from the triplet sensitizer to the
1
To gain the selectivity in photosensitized oxidation of alkenes,
various efforts have been made in the past decades, and
remarkable control of the reaction pathway has been obtained
6-14
by use of organized and constrained media.
Whitten and
1
ground-state oxygen to generate singlet oxygen ( O2), then the
generated singlet oxygen reacts with the substrate. For
6
co-workers reported that the oxidation of protoporphyrin IX
4
example, Diels-Alder reaction of conjugated dienes, “ene”
reaction of olefins with allylic hydrogen, and dioxetane reaction
of olefins that do not feature an allylic hydrogen belong to this
type. In the electron-transfer photosensitized oxidation electron-
deficient sensitizers are generally used.5 Electron transfer from
alkene to the sensitizer in its excited states results in alkene
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0.1021/ja9816842 CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/06/1998