Abstract
Palladium-catalyzed reactions in general are carried out under an inert atmosphere because the palladium intermediates involved in the catalytic cycles are often known to be sensitive to oxygen. In this paper, we report that various palladium-catalyzed reductive couplings proceed smoothly under an air atmosphere and in aqueous medium. Under the air atmosphere reaction conditions, palladium-triphenylphosphine complex was found to be inactive. By using zinc as the reducing reagent, aryl halides were homo-coupled to give symmetrical biaryls either in aqueous acetone or in water in the presence of a catalytic amount of 18-crown-6 at room temperature. Both aryl iodides and aryl bromides reacted efficiently under the current reaction conditions. The reaction of arylhalosilanes with aryl halides under refluxing conditions in air and water catalyzed by palladium generated unsymmetrical biaryls efficiently in the presence of either KOH or NaF. Such air-stable couplings are also suitable for a Suzuki-type coupling and a Stille-type coupling.
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