Selective aerobic oxidation of allyl phenyl ether to methyl ketone by palladium–polyoxometalate hybrid catalysts

Abstract

In this study, we report that selective aerobic oxidation of allyl phenyl ethers is attained by a Pd catalyst/polyoxometalate hybrid system to yield corresponding methyl ketones in water-enriched acetonitrile. The Pd(OAc)2/H5PV2Mo10O40 system exhibits higher conversions and yields of corresponding methyl ketone by Wacker-type oxidation of allyl phenyl ether as compared with the conventional PdCl2/CuCl2 system. The higher yields are attributed to the efficient re-oxidation of Pd0 to Pd2+ by H5PV2Mo10O40 using O2 as an oxidant as evidenced by electrochemical measurements. A reduced species of H5PV2Mo10O40 by Pd0 during the catalytic oxidation is revealed by UV–vis spectral measurements. The use of PdCl2 in place of Pd(OAc)2 in combination with [PV2Mo10O40]5− bearing tetraalkylammonium counter cations has also exhibited comparable conversions and product yields in the Wacker-type oxidation of allyl phenyl ethers. Para-substituted allyl phenyl ether derivatives are successfully oxidized in the Pd catalyst/polyoxometalate system to yield corresponding methyl ketones. The initial rate of products of para-substituted methyl ketones depended on the electronic effect of the substituents in which allyl phenyl ethers with electron-donating groups have accelerated the initial rate in the Pd catalyst/polyoxometalate system.