Hydrodehalogenation of halogenated aryl ketones under multiphase conditions. 6. pH Effect on the chemoselectivity and preliminary mechanistic investigation

A model halogenated aryl ketone (4-chloroprbpiophenone 1) was subjected to mild catalytic hydrodehalogenation (p[H-2] = 1 atm, T = 50 degrees C) in a multiphase system consisting of isooctane and an alkaline aqueous phase, in the presence of onium salts as phase-transfer agents. The chemoselectivity of the reaction was sharply influenced by the KOH concentration of the aqueous phase: thus, the formation of 1-phenyl-1-propanol 4 was favored in a highly alkaline pH range (>13.5 by 50% to 5% KOHaq). Instead, up to 95% selectivity toward phenyl ethyl ketone 2 was observed if the pH was controlled (8 < pH < 13) by adjusting the KOHaq concentration in the range between 5 and 1%. Further investigation of the pH effect indicated that an acidic aqueous phase directed the reaction chemoselectivity toward the formation of products arising from aromatic ring reduction (i.e., cyclohexyl ethyl ketone 7 and 1-cyclohexyl-1-propanol 8) rather than from carbonyl hydrogenation or hydrogenolysis. Also aliphatic amines, used in place of the PT onium salt, afforded good selectivities toward 2, although not as high as the ones obtained with Aliquat 336.