HYDRODEHALOGENATION OF POLYHALOGENATED AROMATICS UNDER MULTIPHASE CONDITIONS WITH H-2 AND METAL CATALYST - KINETICS AND SELECTIVITY

Aryl halides have been catalytically hydrodehalogenated by bubbling H- 2 at atmospheric pressure into a biphasic system constituted by an org anic apolar solvent and an aqueous solution tag., KOH 50%), in the pre sence of a metal catalyst (Pd/C or Raney-Ni) and a quaternary onium sa lt as a phase-transfer (PT) catalyst. Under such new conditions, the h ydrodehalogenation of aromatic halides, even of sterically hindered on es, proceeds quickly at low temperatures (20-50 degrees C) and affords the corresponding non-halogenated hydrocarbons in substantially quant itative yields. The addition of a quaternary onium salt to the multiph ase system may induce remarkable effects both in the enhancement of th e reduction rate and in the regio- and chemo-selectivity of the reacti on. For instance, when Aliquat 336 (tricaprylmethylammonium chloride) is used, the isomeric chloroethylbenzenes react 50 times faster, para- dichlorobenzene is reduced slower than the ortho isomer, and halogenat ed aromatic ketones can be effectively dehalogenated, without any redu ction of the carbonyl group. Under the same reaction conditions, the a ddition of an onium salt is required for the Raney-Ni catalyst to beco me effective as a hydrodehalogenation catalyst.