FACILE HYDRODEHALOGENATION WITH H-2 AND PD C CATALYST UNDER MULTIPHASE CONDITIONS .3. SELECTIVE REMOVAL OF HALOGEN FROM FUNCTIONALIZED ARYLKETONES .4. ARYL HALIDE-PROMOTED REDUCTION OF BENZYL ALCOHOLS TO ALKANES/

Operating in a two-phase system [apolar organic solvent and 50% KOH(aq )] at 50 degrees C, using 5% Pd/C and Hz at atmospheric pressure, the reduction of halogenated aromatic ketones and benzylic alcohols is con sidered. In the case of the ketones, the reaction is strongly influenc ed by the presence of an onium salt, while for the alcohols, the reduc tion occurs only if an aromatic halide is added as a promoter. In part icular, in the presence of Aliquat 336 (methyltricaprylammonium chlori de), halogenated ketones are selectively dehalogenated: for instance, in isooctane solvent, 4-chloropropiophenone, 4-bromoacetophenone, and 4,4'-dichlorobenzophenone give the corresponding ketones with selectiv ity greater than or equal to 95%. No reduction of the carbonyl group i s observed. If no onium salt is used, alkylbenzenes are the sole produ cts as occurs when the reduction is carried out over Pd/C in an ethano lic solution under classical conditions, Likewise, the reduction of pr imary, secondary, and tertiary benzylic alcohols to their correspondin g hydrocarbons expressly requires the presence of aromatic halides; th ese latter undergo a simultaneous hydrodehalogenation to the respectiv e hydrocarbon (for example, o-dichlorobenzene to benzene). In the abse nce 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 quickl y removed from the substrate, so that the corresponding aromatic hydro carbons are the sole final products.