S. Coseri
SHORT COMMUNICATION
tene reaction with NHPI oxidized with lead tetraacetate and so-
dium periodate and H NMR spectrum for the products of cyclo-
hexene reaction with NHPI oxidized with sodium periodate.
monoadduct was the only detected product for this reac-
tion, since in the H NMR spectra (see Supporting Infor-
1
1
mation) the diadduct’s characteristic peak at δ = 3.972 ppm
for the two protons, bonded to the sp3-hybridized carbon
atom bearing the radical moieties, is not present. As we
had expected, the monoadduct was the sole product for the
cyclohexene/NHPI/sodium periodate system, according to
Acknowledgments
The author thanks NATO, for a NATO Reintegration Grant,
CBP.EAP.RIG 982044. Thanks are also due to Prof. Bogdan Cr.
Simionescu for his full support and helpful discussions.
1
the H NMR spectra of the reaction product (see Support-
ing Information), since in the cyclohexene/NHPI/Pb(OAc)4
system only 3% of diadduct was detected.[22]
The following points are found to determine a successful
completion of the reaction:
(a) The T1 ratio (mass of silica gel/volume of NaIO4
solution) was the most critical parameter. According to the
literature,[39–41] water was absolutely required, but in our
case, the suspension stability was directly dependent on the
amount of aqueous solution. A T1 ratio larger than or
equal to 1 ensured a suspension stability for 1 h, suitable
for most reactions.
(b) Solvent polarity was also of importance; dichloro-
methane was the solvent of choice, since more polar sol-
vents (ethyl acetate, ether, etc.) increased hydrate formation,
and less polar solvents decreased the yields.
(c) Vigorous stirring avoided formation of colloidal silica
gel.
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We have found a new method to generate the phthal-
imide N-oxyl (PINO) radical from its precursor N-hydroxy-
phthalimide (NHPI), using the heterogeneous system so-
dium periodate/wet silica gel in the presence of dichloro-
methane. Thus generated PINO reacts by a “pure” radical
mechanism with cycloalkenes, the corresponding monoad-
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Experimental Section
General: 1H and 13C NMR spectra were recorded in CDCl3 (in-
ternal standard was CHCl3: δ = 7.283 ppm). The assignment of
proton chemical shifts was based on decoupling experiments. Ana-
lytical thin-layer chromatography (TLC) was performed on alumi-
num plates precoated with Silica Gel 60 F254 as the adsorbent.
Typical Experimental Procedure for the NHPI/NaIO4/Cycloalkene
Reaction: To a vigorously stirred suspension of chromatography-
grade silica gel (15 g) in CH2Cl2 (100 mL) in a 250 mL Erlenmeyer
flask, were added a 0.35 aq. solution of NaIO4 (10 mL) dropwise
with stirring and NHPI (7 mmol). After 5 min, the cycloalkene
(70 mmol) in CH2Cl2 (20 mL) was added, and the reaction was
monitored by TLC, until it was complete, generally in less than
15 min. The mixture was then filtered through a sintered-glass fun-
nel, and the silica gel was thoroughly washed with CH2Cl2
(3ϫ30 mL). Evaporation of the solvent and excess cycloalkene af-
forded the crude product, pure enough for most purposes; yield:
88–92%. GC analysis and/or 1H NMR spectroscopy revealed no
diadduct formation.
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Supporting Information (see also the footnote on the first page of
1
this article): H and 13C NMR spectra for the products of cyclooc-
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