In situ Formation of NO_x and Br Anion for Aerobic Oxidation of Benzylic Alcohols without Transition Metal

Abstract

The reaction of KBrO₃ and NH₂OH˙HCl in situ generates NO_x and Br anion, which combine with 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) to construct a NO-activating dioxygen, Br-assisted, TEMPO-catalyzed aerobic oxidation of alcohols. Catalyzed by KBrO/NH₂OH˙HCl/TEMPO, various benzylic alcohols can be oxidized quantitatively to their corresponding carbonyl compounds under mild conditions. The easy handling and simple product separation make the process an attractive candidate for the oxidation of alcohols.

oxidation - alcohols - catalyst - bromate - hydroxylammonium - 2,2,6,6-tetramethylpiperidine-N-oxide

References and Notes

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General Typical Procedure for the Oxidation
The reaction was carried out in a 70 mL autoclave, and the general procedure is described typically with benzyl alcohol as follows: To a reactor were added benzyl alcohol (2 mL, 19.3 mmol), NH₂OH˙HCl (174.8 mg, 10 mol%), KBrO₃ (161.2 mg, 5 mol%), TEMPO (15.1 mg, 0.5 mol%), and CH₂Cl₂ (10 mL). The closed autoclave was charged with O₂ to 0.3 MPa and warmed to 80 ˚C under stirring. The pressure of O₂ was kept under 0.4 MPa for 2 h. After cooled to r.t., 20 mL CH₂Cl₂ were added to the autoclave. Then the solution was analyzed by gas chromatography, which was conducted using an Agilent Technologies 6890N Network GC System with a flame ionization detector and a DB-1 capillary column (30 m × 0.535 mm × 3.0 µm).

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General Isolation Procedure for the Oxidation Product
After GC showed the reaction to be complete, the reaction mixture was diluted with CH₂Cl₂ and transferred into a separation funnel. The CH₂Cl₂ solution was washed with 15 mL of a sat. solution of Na₂CO₃, followed by brine. The organic layer was dried over anhyd Na₂SO₄, and the solvent was evaporated to yield the product without further purification.