Halogenated Aromatic Ketones Heterogeneous Catalytic Hydrogenation Reaction

The hydrogenation of halogenated aromatic ketones are very important in syntheses . Their hydrogenated products may be aromatic ketones or aromatic alcohols ,both are key intermediates in the syntheses of nonsteroidal antiinflammatiry drugs (NSAID). For example , Naproxen can be prepared by rearrangement from 6-methoxy-2-propionylnaphthalene which can be obtained through the dehydrogenation of 5-halo-6-methoxy-2-propionylnaphthalene .This method is better than the direct Friedel-Crafts propionylization of 2-methoxynathphalene which will produce 2-methoxy-1 -propionylnathphalene as byproduct ,because the 1st position is more active than the 6th position. Although the synthetic route through l-halo-2-methoxynaphthalene has one more step than the direct Friedel-Crafts propionylization of 2-methoxynaphthalene, the overall yield is increased greatly.For the same reason 1 -(6-methoxy-naphthalene) ethanol was prepared through the hydrogenation of 5-halo-6-methoxy-2-acetylnaphthalene instead of hydrogenation of 6-methoxy-2-acetylnaphthalene.Halogenated aromatic ketones may have three possible hydrogenation products: aromatic ketones , only when hydrodehalogenation takes place .aromatic alcohols and aromatic hydrocarbons when deep reduction of carbonyl group is carried out.In this paper , the multiphase system including organic solvent and KOH aqueous solution was investigated ,in which 5-halo-6-methoxy-2-acylnaphthalene was reduced by H2 catalyzed by Pd/C in the presence of phase-transfer agent at atmospheric pressure .And the optimum conditions of preparing aromatic ketones or aromatic alcohols were found. The effects of agents on the hydrogenation in the multiphase system were also discussed.All reactions were carried out in a four-neck flask equipped with bubble pipe, thermometer, stirring apparatus and condenser .KOHaq was added after substrate solved in solvent, then added Pd/C and PTC. At last hydrogen was bubbled into the system at atmospheric pressure and at certain temperature.The reaction mixture was filtered and the filtrate was separated into two phases.Let out KOH aqueous solution and wash organic phase with water. Then the phase was concentrated in vacuo until solid material appeared .The solid material was dried and was analyzed by HPLC.In the presence of EtaN , l-(6-methoxy-2-naphthalene) ethanol (propanol) were also obtained in high yields by the hydrogenation of 5-halo-6-methoxy-2-acyl naphthalene.