Hydrodehalogenation of Halogenated Aryl Ketones under Multiphase Conditions. 5. Chemoselectivity toward Aryl Alcohols over a Pt/C Catalyst

Halogenated aryl ketones (XC6H4COR: X ) Cl, F; R ) Me, Et, Ph) and benzyl alcohols (XC6H4CH(OH)R: X ) Cl; R ) Me, Et) have been reduced catalytically over Pt/C at 50 °C, by bubbling H2 at atmospheric pressure into a multiphase system consisting of a hydrocarbon solvent (isooctane) and an alkaline aqueous phase (KOH 50%) in the presence of a phase-transfer (PT) agent (onium salts or polyoxyethylene monoalkyl ethers). The chemoselectivity of the reaction is sharply modified by the PT-agent: in its absence, a wide range of products is observed, including those deriving from the complete reduction of both the aromatic ring and carbonyl functions. Instead, the presence of the PT-agent allows several different halo ketones to be transformed into their corresponding benzylic alcohols: that is, the hydrodechlorination occurs only with the partial hydrogenation of the CO group to the CH(OH) one, while further reductions are totally prevented. Under such conditions, both ionic (onium salts) and nonionic (polyoxyethylene monoalkyl ethers: Brij 35, 52, 56, and 58) compounds have been tested, and results seem to suggest that the higher their hydrophilicity is, the better the chemoselectivity toward the formation of dehalogenated alcohols is as well. Moreover, under multiphase conditions, also halo aryl alcohols lead to the corresponding dechlorinated alcohols in quantitative yields although a slower reaction is observed with respect to ketones.