Hydrodehalogenation of polyhalogenated aromatics with Pd/C catalyst carried out in the presence of a quaternary onium salt follows zero-order kinetics in the substrate and first-order kinetics in the Pd/C catalyst; the related rate constants were determined for o-, m-and p-bromotoluenes, 0-, m-and p-chloroalkylbenzenes (methyl, ethyl, and propyl derivatives), and other aryl halides. Reaction rates, depending on the aromatic to be reduced, may be strongly enhanced by the presence of quaternary onium salts: the isomeric chloroethylbenzenes were reduced 50 times faster when operating in the presence of Aliquat 336 (1). Also the hindered 2-chloro-m-xylene easily yielded m-xylene. The cocatalyst onium salts operate by being adsorbed on the Pd/C surface, as shown when kinetic constants are reported by varying the onium salt amount: classical Langmuir adsorption isotherms are observed. The presence of the onium salt may also influence selectivity in the reduction of isomeric aryl halides: when 1 is present, p-dichlorobenzene reacts in diethyl ether at 20 °C, 5-fold slower than the ortho isomer; whereas the reduction rates of the two compounds are almost the same in its absence.

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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.

Both arylacetonitriles and methyl arylacetates react with dimethyl carbonate (DMC) (20 molar excess) at 180-200 "C in the presence of K, CO, to produce monomethylated 2-arylpropionitriles and methyl 2-arylpropionates, respectively, with a selectivity > 99.5%. The reaction, with wide application, proceeds by D M C acting as a methoxycarbonylating agent towards the ArCH -X anion (X = CN, C0,Me) and as a methylating agent to Arc-(C0,Me)X. D M C also proved to be the best solvent for such reactions.Direct base-promoted mono-alkylation of methylene-active compounds, is not easy since the reaction usually proceeds to dialkylation, especially so in the case of methylation. Essentially, this is because of the small difference in acidity between the reagent and the monoalkylated compound: the pK, values of compounds 1 and 2 are comparable as illustrated for the arylacetonitriles in eqn. (1).Since the monomethylated arylacetonitriles 2 (R = Me) are important precursors of 2-arylpropionic acids, the well known anti-inflammatory drugs, compound 1 was treated with a variety of alkylating agents (e.g. alkyl halides, dialkyl sulfates) with the hope of inducing monomethylation.2 Even under phase-transfer catalysis (PTC) conditions, however, highly selective monomethylation was e l~s i v e .~ Of the new methods for alkylation under safe conditions and with non-toxic reagent^,^ gas-liquid phase-transfer catalysis (GL-PTC) t conditions with dimethyl carbonate (DMC) as a methylating agent is of particular interest. Under such conditions, operating in a catalytic bed composed of K2C03 and polyethylene glycols as PT catalysts, DMC methylated phenols, amines, ' phenylacetonitrile and (p-isobutylpheny1)-acetonitrile with high selectivity.* Interestingly, methylation with non-toxic DMC (not now prepared from phosgene but by oxidative carbonylation of methanol)' produced no waste, the base being catalytic, the methanol recyclable and the CO, involving no disposal problems. Only those compounds having a relatively high vapour tension, however, react under such conditions.We report here the highly selective batchwise reaction of arylacetonitriles and arylacetic esters with DMC. ResultsThe reactions, carried out in an autoclave, are reported in detail in the Experimental section whilst the results for the methylation of phenylacetonitrile by different methods are GL-PTC is a continuous-flow procedure for carrying out organic reactions: gaseous reagents are allowed to flow through molten phasetransfer catalysts (e.g. polyethylene glycols, PEGS, onium salts, etc.) supported on a solid. The catalytic bed is fitted into a plug-flow reactor, thermostatted at the reaction temperature. In the case of DMC, methylation requires the presence of a base as co-catalyst.5

~ ~Polyhalogenated benzenes in the presence of a Pd/C catalyst with sodium hypophosphite in a multiphase system consisting of a hydrocarbon solvent, concentrated aqueous alkali and a quaternary onium salt undergo reduction with rapid and progressive displacement of all their chlorine atoms. The onium salt, being insoluble in both the organic and the aqueous phases, coats, as a third liquid phase, the Pd/C catalyst. The strong alkaline medium and the phase-transfer agent are synergic.Operating at 50 "C, 1,2,4,5-tetrachlorobenzene gives after 2.5 h a 99% yield of benzene.Enhancement of the reaction rate, compared with already k n o w n methods, has been attributed to the transfer of phosphite anion from the aqueous solution to the catalyst surface, t o partition of halogenated compounds between the hydrocarbon solution and the liquid phase of the phasetransfer agent and to the rapid removal of HCI adsorbed on Pd/C by the alkaline medium.The reaction is also effective with aryl bromides and deactivated p-methoxyaryl halides.Hydrodehalogenation of aromatic halides, especially chlorides, is of interest because it allows transformation of potential pollutants of the environment into their corresponding aromatic hydrocarbons without production of waste. Such reactions may also have synthetic potential, where regioselective dehalogenation of polyhalogenated benzenes could afford compounds difficult to synthesise directly. Further, where chlorination of benzene (or toluene) produces considerable amounts of polychlorobenzenes (or polychlorotoluenes), because of the comparable rate constants of the related consecutive reactions, reactions of these polychloro compounds under the conditions described may produce the desired lesshalogenated compounds. Our initial study of the palladium-catalyzed hydrodehalogenation of polychlorobenzenes was aimed at improving the reduction rate by developing new reaction conditions. Use of a classic reduction catalyst such as palladium on carbon (Pd/C) gave, as reported herein, reduction of tri-and tetra-chlorobenzenes to less chlorinated compounds and to benzene. This problem is of interest because four chlorine atoms have to be removed at once.Chlorobenzene can be reduced to benzene by a variety of methods: l a hydrodehalogenation with supported palladium catalysts,'" Pd/C,'b platinum catalysts on pin el,^ and reactive aluminium powder^.^ Hypophosphite has also been reported as a hydrogen s o~r c e ,~"~~ the same compound has also been used also for the hydrogenation of functionalized organic compounds, 5d,e for the hydrogenolysis of phenolic bonds,5f and aliphatic CBr, CCI bonds;5g [Et,NH][H, PO,] has been used for the reduction of nitro derivatives and carbonyl compounds. 5 k Formate salts have also been described as hydrogen donors although in the reactions reported, high temperatures, high pressures, or long reaction times are involved. In particular, Sasson and co-workers6 have recently reported an efficient reduction system for monochlorobenzenes that employs formate salts and Pd/C a...