Selectivity in hydrodehalogenation of polychloro- and polybromobenzenes under multiphase conditions

Abstract

The competitive hydrodehalogenation of isomeric o-, m-and p-dichloro (and dibromo) benzenes with Hz at atmospheric pressure and Pd/C or Ni-Raney carried out in a multiphase system (organic solvent and 50% KOH aq. solution) is influenced by the presence of a bulky quatemary onium salt where both the catalytic activity and the selectivity change in relation to the halogen to be removed and the metal catalyst used. In particular, the Ni-Raney catalyst becomes effective in the reduction only if an onium salt is added. Similarly, the onium salt effects the catalytic hydrodehalogenation of 1,2&trichloro-and 1,2,4-tribromobenzene.

Multiphase heterogeneous catalytic enantioselective hydrogenation of acetophenone over cinchona-modified Pt/C

“…It can be used to efficiently remove aromatic halogens (from toxic compounds such as PCBs and PCDD/PCDFs) [13][14][15][16][17], or to reduce aromatic carbonyls [18][19][20][21], simply by tuning the conditions. The multiphase system consists of a hydrocarbon solvent, an aqueous phase, a phase-transfer (PT) agent, a supported metal catalyst, hydrogen at atmospheric pressure, at low temperature (T < 50 • C).…”

Mild catalytic multiphase hydrogenolysis of benzyl ethers

“…Previous studies had indicated that Pd/C is a more active reduction catalyst than Raney-Ni under multiphase conditions for the hydrodechlorination of chlorobenzenes, 5 while RaneyNi is more active, provided A336 is present, in the hydrodebromination of bromobenzenes. 5 The observed activity of the Raney-Ni/A336 multiphase system for the hydrogenolysis of benzyl ethers, and the opposite behavior of Pd/C, is another case of changes in the chemoselectivity produced by modifications of a component of the catalytic multiphase system (in this case the catalyst).…”

“…Pd/C, Pt/C) allows a variety of reduction reactions to be conducted under mild conditions (T = 50°C, pH 2 = 1 atm), e.g. ; hydrodehalogenation of aryl halides, [2][3][4][5][6] hydrogenation of carbonyls, 7 hydrogenolysis of hydroxyls, 7 up to aromatic hydrogenation. 7,8 For substrates liable to multiple reductions, selectivity can be achieved by tuning the multiphase system, in order to favor one over the other of the possible reaction pathways.…”

Multiphasic Heterogeneous Catalysis Mediated by Catalyst‐Philic Liquid Phases

“…And-contrary to Pd/C that showed activity in the absence of A336 as well-functioned only when A336 was present. 78,79 Platinum on charcoal provided a further means for directing the selectivity of the multiphasic hydrodehalogenation reactions. For example, the same reaction of Fig.…”

“…[76][77][78][79]81,82 This reaction was investigated from the kinetic standpoint, using kinetic models that allowed an accurate analysis of the reaction selectivity. 83 In particular it was shown that halogen removal was accelerated with respect to carbonyl reduction in the presence of A336.…”