Facile Hydrodehalogenation with HP and Pd/C Catalyst under Multiphase Conditions. 3. Selective Removal of Halogen from Functionalized Aryl Ketones. 4. Aryl Halide-Promoted Reduction of Benzyl Alcohols to Alkanes

Operating in a two-phase system [apolar organic solvent and 50% KOH(aq)l at 50 "C, using 5% Pd/C and Hz at atmospheric pressure, the reduction of halogenated aromatic ketones and benzylic alcohols is considered. In the case of the ketones, the reaction is strongly influenced by the presence of an onium salt, while for the alcohols, the reduction occurs only if an aromatic halide is added as a promoter. In particular, in the presence of Aliquat 336 (methyltricaprylammonium chloride), halogenated ketones are selectively dehalogenated: for instance, in isooctane solvent, 4-chloro- propiophenone, 4-bromoacetophenone, and 4,4'-dichlorobenzophenone give the corresponding ketones with selectivity L 95%. No reduction of the carbonyl group is observed. If no onium salt is used, alkylbenzenes are the sole products as occurs when the reduction is carried out over Pd/C in an ethanolic solution under classical conditions. Likewise, the reduction of primary, secondary, and tertiary benzylic alcohols to their corresponding hydrocarbons expressly requires the presence of aromatic halides; these latter undergo a simultaneous hydrodehalogenation to the respective hydrocarbon (for example, o-dichlorobenzene to benzene). In the absence of the halide promoter, the reaction takes place only if a halogen is present in the aromatic ring of the alcohol: that is, if haloaryl alcohols are the starting reagents. In this case the halogen is quickly removed from the substrate, so that the corresponding aromatic hydrocarbons are the sole final products.