Palladium-Catalyzed Hydrolytic Cleavage of Aromatic C-O Bonds

Metallic palladium surfaces are highly selective in promoting the reductive hydrolysis of aromatic ethers in aqueous phase at relatively mild temperatures and pressures of H₂ . At quantitative conversions, the selectivity to hydrolysis products of PhOR ethers was observed to range from 50 % (R=Ph) to greater than 90 % (R=n-C₄ H₉ , cyclohexyl, and PhCH₂ CH₂ ). By analysis of the evolution of products with and without incorporation of H₂¹⁸ O, the pathway was concluded to be initiated by palladium metal catalyzed partial hydrogenation of the phenyl group to an enol ether. Water then rapidly adds to the enol ether to form a hemiacetal, which then undergoes elimination to cyclohexanone and phenol/alkanol products. A remarkable feature of the reaction is that the stronger Ph-O bond is cleaved rather than the weaker aliphatic O-R bond.