Oxidation of Benzylic Methylene Compounds to Ketones
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of oxidation of diphenylmethane to benzophenone. In each experiment the sup-
ported 4-aminobenzoic acid was recovered quantitatively.
In summary, mild reaction conditions, selectivity, good yields, ease of work up,
and reusability of the supported reagent are the most significant aspects of this
method.
Experimental
Chemicals were purchased form Merck, Aldrich, and Riedel de Haen AG and were used without further
purification. IR and NMR spectra were recorded on FT-IR Unicam Mattson 1000 and Bruker AC-80
(80 MHz). All products are known compounds and they were characterized by comparison of their
physical and spectral data with those of authentic samples. All yields refer to pure isolated products.
Activation of Silica Gel
Chromatographic grade silica gel (30 g, 70–230 mesh) was heated in an electric furnace at 700ꢁC for
3 h. Then it was cooled to room temperature to obtain 27.9 g of activated silica gel.
Supporting of 4-Aminobenzoic Acid on Activated Silica Gel
To a stirred solution of 6.9g of 4-aminobenzoic acid (50 mmol) in 200 cm3 of ethylacetate 27.9g of
activated silica gel were added and the mixture was refluxed under magnetic stirring for 16h. Then it
was cooled to room temperature, filtered, and the solid matter was continuously extracted with
ethylacetate in a Soxhlet apparatus for 18h. Then it was dried at 80ꢁC for 3 h to yield 33.7 g of silica
gel supported 4-aminobenzoic acid. 4-Aminobenzoic acid (5.8g, 42mmol) was supported on 27.9 g of
activated silica gel.
Oxidation of Silica Gel Supported 4-Aminobenzoic Acid
to the Corresponding Peroxyacid
Silica gel supported 4-aminobenzoic acid (33.7 g) containing 5.8g of 4-aminobenzoic acid was added
to a stirred solution of a commercial 30% solution of H2O2 at room temperature and the reaction
mixture was stirred for 8 h. Then it was filtered, washed with distilled H2O (5ꢂ20cm3), and dried over
anhydrous CaCl2 at room temperature in vacuum (40 torr) to obtain 34.3g of silica gel supported
4-aminoperoxybenzoic acid.
Typical Procedure for the Oxidation of Diphenylmethane to Benzophenone
Diphenylmethane (3.4g, 20mmol) was dissolved in 100 cm3 of CH2Cl2, then 33g of silica gel
supported 4-aminoperoxybenzoic acid (containing 40mmol of 4-aminoperoxybenzoic acid) were
added and the mixture was stirred at room temperature for 9 h during which time oxygen was bubbled
at a rate of 10 cm3 minꢃ1 into the solution. Progress of the reaction was monitored by TLC (petroleum
ether:ethylacetate ¼ 4:1). Then the reaction mixture was filtered to recover the solid material. The solid
material was washed with 2ꢂ10cm3 of CH2Cl2. The solvent was evaporated and the product purified
by column chromatography using silica gel. The pure benzophenone was obtained in 98% yield (3.6g).
Similar conditions were examined for the use of air instead of molecular oxygen. The reaction was
run for 16 h to give benzophenone in 95% yield (3.5g).
Reusability of Silica Gel Supported 4-Aminoperoxybenzoic Acid
After using silica gel supported 4-aminoperoxybenzoic acid as oxidant for the oxidation of benzylic
methylene compounds it can be reconverted to the corresponding peroxyacid by treatment with