these side products were never observed, which also illustrates
that ceric triflate has superior chemoselectivity towards benzylic
oxidation as compared to CAN or ceric trifluoroacetate. In
oxidation reactions with ethyltoluenes there is a strong prefer-
ence for benzylic methylene oxidation as compared to benzylic
methyl oxidation.
reduced pressure to afford a yellow residue which was then
dried as before to furnish a light-yellow solid. Typical yields in
independent preparations: 9.98 g (93%; 12.8% H2O); 9.42 g
(78%, 14% H2O); 10.82 (90%, 22% H2O), 10.21 g (91%; 16.4%
H2O); Among various batches synthesized by this method there
was a sample whose H2O content was estimated as 1.5%.
Samples purchased commercially exhibited low oxidation
power. Attempts to activate them by adjusting the % H2O to
“optimal level” were only partially successful since only
benzylic alcohol (not the ketone) could be produced in high
yield. By comparison, ceric triflate does not perform well for
nitroarenes in rt reactions, however, on the whole, cerium tri-
flate appears to exhibit a broader substrate tolerance than
CAN. A noteworthy feature of the present work is the ease of
operation in room temperature oxidaton reactions requiring no
special equipment or precaution, provided Ce(OTf)4 is
correctly prepared and the water content is kept in the optimal
range.
General procedure for arene oxidation
Ceric triflate (2 equivalents; ∼0.680 mmol) was added all at once
to a vigorously stirred solution of the arene substrate in dry
MeCN (10 mL) at rt, initially resulting in a clear yellow (or
orange) solution depending on the substrate. Upon stirring,
color changes are observed depending on the substrate (see
Discussion). After the indicated reaction time (see tables) the
mixture was poured into a separatory funnel along with dis-
tilled water (2 × 15 mL). The organic layer was extracted into
CH2Cl2 (2 × 15 mL), washed with water (2 × 15 mL) and dried
(MgSO4 s). After filtration, the solvent was gently removed
under vacuum and the residue was analyzed by GC. The oxid-
ation products were all known compounds whose identities
were established by coinjection with authentic samples, by
GC-MS and in selected cases by isolation and NMR analysis.
Experimental
Starting materials
Triflic acid (Aldrich) was distilled under a dry nitrogen atmos-
phere in an all-glass distillation unit and stored in a Nalgene
bottle with a Teflon seal. 1,1,2-Trichlorotrifluoroethane (Freon-
113), CAN and cerium trifluoromethanesulfonate (2 × 1 g
samples) were purchased from Aldrich and used as received.
Acknowledgements
We thank Dr Takao Okazaki for NMR assistance
Preparation of cerium(IV) triflate
References
(Method 1). A solution of potassium carbonate (4.24 g, 30.6
mmol) in distilled water (24 mL) was added all at once to a
vigorously stirred solution of cerium ammonium nitrate (CAN)
(7.00 g, 12.8 mmol) in distilled water (21 mL) whereby the
initial clear orange solution formed a pale-yellow precipitate
(gas evolution). After 30 min stirring at rt the precipitate was
collected by filtration and washed thoroughly with water (350
mL) to remove excess potassium carbonate. The resulting moist
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flask equipped with a magentic stirrer bar and a septum. The
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mmol) was added dropwise via a syringe over a 30 minute
period (CO2 evolution was observed). After stirring for an
additional 1 h, the resulting clear orange solution was evapor-
ated under reduced pressure to afford a yellow residue which
was dried in a vacuum desiccator to yield a canary-yellow solid.
Typical yields in independent preparations: 8.21 g (88%;
16.06% H2O from TGA exp.); 10.02 g (92%; 14.4% H2O from
TGA exp.).
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Laali, Kenneth K.
