Aryl halides have been catalytically hydrodehalogenated by bubbling H-2 at atmospheric pressure into a biphasic system constituted by an organic apolar solvent and an aqueous solution tag., KOH 50%), in the presence of a metal catalyst (Pd/C or Raney-Ni) and a quaternary onium salt as a phase-transfer (PT) catalyst. Under such new conditions, the hydrodehalogenation of aromatic halides, even of sterically hindered ones, proceeds quickly at low temperatures (20-50 degrees C) and affords the corresponding non-halogenated hydrocarbons in substantially quantitative yields. The addition of a quaternary onium salt to the multiphase system may induce remarkable effects both in the enhancement of the reduction rate and in the regio- and chemo-selectivity of the reaction. 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 halogenated aromatic ketones can be effectively dehalogenated, without any reduction of the carbonyl group. Under the same reaction conditions, the addition of an onium salt is required for the Raney-Ni catalyst to become effective as a hydrodehalogenation catalyst.

Hydrodehalogenation of polyhalogenated aromatics under multiphase conditions with H-2 and metal catalyst: Kinetics and selectivity

SELVA, Maurizio;TUNDO, Pietro
1996-01-01

Abstract

Aryl halides have been catalytically hydrodehalogenated by bubbling H-2 at atmospheric pressure into a biphasic system constituted by an organic apolar solvent and an aqueous solution tag., KOH 50%), in the presence of a metal catalyst (Pd/C or Raney-Ni) and a quaternary onium salt as a phase-transfer (PT) catalyst. Under such new conditions, the hydrodehalogenation of aromatic halides, even of sterically hindered ones, proceeds quickly at low temperatures (20-50 degrees C) and affords the corresponding non-halogenated hydrocarbons in substantially quantitative yields. The addition of a quaternary onium salt to the multiphase system may induce remarkable effects both in the enhancement of the reduction rate and in the regio- and chemo-selectivity of the reaction. 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 halogenated aromatic ketones can be effectively dehalogenated, without any reduction of the carbonyl group. Under the same reaction conditions, the addition of an onium salt is required for the Raney-Ni catalyst to become effective as a hydrodehalogenation catalyst.
1996
126
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/10278/33035
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