Yang et al.
cycloaddition with olefins.20 The unstable dioxetane
analogues could decompose under the reaction conditions
to yield the corresponding carbonyl products and nitrite.14
Indeed, we detected the formation of nitrite ion by using
acidic ferrous sulfate after the reaction was accom-
plished.21
In the case of Nafion-supported 1 as the sensitizer, the
oxygen in the outside dichloromethane solution can
diffuse into the D2O-swollen Nafion and undergo energy
transfer with the triplet state of sensitizer 1. The
generated 1O2 diffuses back to the outside solution to
react with the substrate to form the corresponding
carbonyl compound. Thus, although the Nafion-incorpo-
rated 1 cannot undergo photoinduced electron transfer
with the oxime in solution outside, the photosensitized
oxidation for this sensitizer system can efficiently oper-
ate.
SCHEME 1
pendant chains terminated by sulfonic groups.16 When
swollen in water or methanol, the structure of Nafion is
believed to resemble that of an inverse micelle. The
-
hydrated SO3 headgroups are clustered together in a
water- or methanol-containing pocket ca. 40 Å in diam-
eter, and the pockets are interconnected with each other
by short channels within the perfluorocarbon matrix.
Complex 1 can be easily adsorbed into Nafion membranes
by immersing the polymer in a well-stirred solution of 1
in methanol. The sample incorporating 1 was removed
from the methanol solution, and the methanol adsorbed
in the Nafion membranes was evaporated. Then, we used
a small amount of D2O to swell the 1-incorporated Nafion
membranes and immersed them in oxygen-saturated
dichloromethane where the oxime substrate was dis-
solved. Since dichloromethane cannot swell Nafion and
is insoluble in water, the water in Nafion cannot be
extracted into the solution. On the other hand, the oxime
substrate in dichloromethane cannot diffuse into the
Nafion because it is insoluble in water. Photoirradiation
of the 1-incorporated Nafion in the manner described
above efficiently resulted in the deprotection of the oxime
in the outside solution. For example, in the case of 8 as
the substrate, 3 h of irradiation led to ca. 20% conversion,
and the yield of the acetophenone based on the consumed
8 was ca. 74%. Since the photosensitizer in Nafion is
isolated from the oxime in the solution outside, electron
transfer between the excited state of 1 and the substrate
should be inhibited. The observation of the efficient
deprotection suggests that the photosensitization must
not proceed via an electron-transfer mechanism.
Con clu sion
Singlet oxygen, generated by photosensitization, reacts
with aldoximes and ketoximes to produce their corre-
sponding carbonyl compounds. This deprotection of oximes
employs molecular oxygen as the oxidant, and the process
is environmentally benign. Although the reactions have
not been optimized, the yields of the carbonyl compounds
are generally good or even excellent. Thus, these reac-
tions may serve as a potential synthetic method for
deoximes. Particularly in the case of 1 as the sensitizer,
after the reaction is completed, the sensitizer can be
easily separated from the products and unreacted start-
ing material and can be reused many times without loss
1
of O2-generation capacity.
Exp er im en ta l Section
Ma ter ia ls a n d In str u m en ta tion . Oximes 2-10 were
prepared from their corresponding aldehydes or ketones
according to a standard oximation method.22 The sol-
vents, acetonitrile and dichloromethane, used in this
work were of analytic grade. Nafion membrane 117 in
acid form (Nafion-H+) with an equivalent weight of 1100
and a thickness of 0.0175 cm was a product of DuPont.
Prior to use, the membrane was cleaned by boiling in
concentrated nitric acid for 4 h and then thoroughly
washed with distilled water and finally immersed in
water for 24 h. The membrane in sodium form (Nafion-
Na+) was obtained by immersing the pretreated Nafion-
H+ membrane in 1 N NaOH aqueous solution. Excess
base was then removed by stirring the sample in water.
Syn t h esis of P la t in u m (II) Ter p yr id yl Acet ylid e
Com p lex (1). Complex 1 was prepared in two steps.
First, the starting material, [Pt(4′-(p-methoxyphenyl)-
trpy)Cl]Cl (trpy ) 2,2′:6′,2′′-terpyridine), was prepared
by a literature method,23 using 4′-(p-methoxyphenyl)-
terpyridine and K2PtCl4 as the reagents. This starting
We believe that our photosensitized deoxime involves
singlet oxygen, because in the absence of oxygen the
reaction cannot occur. Furthermore, we have demon-
1
strated by EPR spectroscopy that O2 was indeed gener-
ated upon irradiation of the photosensitizer.17 It has been
established that 2,2,6,6-tetramethylpiperidine (TMP)
reacts with 1O2 to give the stable free radical nitroxide
(TMPO), which can be readily detected by EPR spectro-
scopy.18 We dissolved TMP and 1 in oxygen-saturated
acetonitrile and irradiated the solution using light with
wavelengths above 400 nm. After 100 s of irradiation of
the solution, the nitroxide free radical was clearly
detected (see Supporting Information). Obviously, inter-
action between oxygen and the triplet state of 1 results
in energy transfer yielding 1O2. We have determined the
quantum yield of the 1O2 for 1 as the sensitizer in
acetonitrile by a 9,10-diphenylanthracene bleaching
method19 and found that this quantum yield was close
1
to 1.0. The generated O2 would undergo [2 + 2] cycload-
dition with the CdN double bond of the oximes to give
dioxetane analogues (Scheme 1) as in the case of the
(20) Ranby, B.; Rabek, J . F. In Singlet Oxygen Reactions with
Organic Compounds and Polymers; J ohn Wiley & Sons, Ltd.: Chich-
eser, 1979.
(16) Lee, P. C.; Meisel, D. J . Am. Chem. Soc. 1980, 102, 5477.
(17) Zhang, D.; Wu, L.-Z.; Yang, Q.-Z.; Li, X.-H.; Zhang, L.-P.; Tung,
C.-H. Org. Lett. 2003, 5, 3221.
(18) Zang, L. Y.; van Kuijk, F. J .; Misra, B. R.; Misra, H. P. Biochem.
Mol. Biol. Int. 1995, 37, 283.
(21) Slowinski, E. J .; Masterton, W. L. In Qualitative Analysis and
the Properties of Ions in Aqueous Solution; W. B. Saunders: Philadel-
phia, PA, 1971; p 144.
(22) Vogel, A. In Textbook of Practical Organic Chemistry, 4th ed.;
Longman: New York, 1981; p 1113.
(19) Zahir, K. O.; Haim, A. J . Photochem. Photobiol. A: Chem. 1992,
63, 167.
(23) Yip, H. K.; Cheng, L. K.; Cheung, K. K.; Che, C. M. J . Chem.
Soc., Dalton Trans. 1993, 2933.
4790 J . Org. Chem., Vol. 69, No. 14, 2004