Abstract
Pd/C catalyst in aqueous alcohol with molecular oxygen, sodium borohydride, and potassium carbonate efficiently oxidized benzylic and allylic alcohols. The use of a reducing reagent, sodium borohydride, for the reactivation of active sites of the Pd surface proved to be remarkable. The utilization of room temperature reaction condition and aqueous alcohol solvent as well as the recyclability of Pd/C makes this manipulation very interesting from an economic and environmental perspective.
Key word
palladium - oxidations - alcohols - sodium borohydride - oxygen
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9
General Procedure for the Oxidation of Benzylic or Allylic Alcohols : A catalytic amount of Pd/C (0.025 equiv) was added to H2 O (2 volume). NaBH4 (0.1 equiv) was slowly added to this suspension followed by K2 CO3 (3.0 equiv). After addition of the starting materials, benzylic or allylic alcohol (1.0 equiv) was added to the resulting suspension followed by addition of MeOH, EtOH, or i -PrOH (1 volume) at r.t. The resulting reaction mixture was stirred vigorously under an oxygen atmosphere (maintained with a balloon). Alternatively, oxygen gas was bubbled into the reaction mixture through a long needle with the use of several balloons. The progress of the oxidation reaction was monitored by silica gel TLC. The resulting reaction mixture was neutralized with a dilute HCl solution and the aqueous layer was extracted with sufficient EtOAc. The organic layer was separated, dried over anhyd MgSO4 , and concentrated via rotary evaporation. The residue was purified by silica gel flash column chromatography (EtOAc-n -hexane = 1:3-1:7). Reactions were typically run on a 2-3-mmol scale.
