Harold Wiener

Palladium-catalyzed aryl-aryl coupling in water using molecular hydrogen: kinetics and process optimization of a solid-liquid-gas system

Abstract Coupling of substituted chlorobenzenes to the respective biphenyls is effected in water, using hydrogen gas and NaOH in the presence of catalytic PEG-400 and Pd/C. The catalyst can be efficiently recycled. The competing reduction process ( e.g. of chlorobenzene to benzene) can be minimized by altering reaction conditions. The roles of the hydrogen, the hydroxide, the Pd catalyst, and the PEG are discussed.

Palladium-catalyzed decomposition of aqueous alkali metal formate solutions

Abstract Aqueous sodium and potassium formate solutions undergo catalytic decomposition, under mild conditions, into molecular hydrogen and hydrogen carbonate in the presence of palladium on carbon. Both the formate and water hydrogen atoms are involved in the process and undergo fast exchange. Some kinetic and mechanistic features of the decomposition reaction are reported.

The heterogeneous catalytic hydrogenation of bicarbonate to formate in aqueous solutions

Abstract The kinetics of the hydrogenation of aqueous solutions of HCO 3 − to HCO 2 − over Pd on C catalysts was studied. It was shown that the rate rose with increasing H 2 pressure in accordance with a Langmuir isotherm law. With increasing HCO 3 − concentration the rate passed through a maximum. Measurements of the equilibrium at 35 °C indicated a Gibbs free energy change of about −2.2 kcal/ mole. The highest concentration of HCO 2 − obtainable at 6 atm H 2 was limited by the common ion effect to 5.8 M .

Solid–solid palladium-catalysed water reduction with zinc: mechanisms of hydrogen generation and direct hydrogen transfer reactions

Facile generation of hydrogen gas from water takes place under moderate conditions in the presence of zinc powder and catalytic palladium on carbon; 82% conversion of zinc is obtained. An unusually large kinetic isotope effect is observed using D2O (kH/kD=14), which may reflect the cleavage of both O–H bonds in the rate-determining step. Experiments using D2O–H2O mixtures evidence that water molecules adsorbed on the catalyst surface undergo H–D exchange reactions (with molecules from the solvent bulk) that are approximately 100 times faster than the hydrogen generation reaction. The primary factors in this system appear to be palladium–hydrogen and zinc–oxygen interactions. Conversely, in the presence of an organic hydrogen acceptor, such as benzaldehyde, a different course is realised, consisting of direct hydrogen transfer from ‘‘zinc-activated ’’ water to the substrate, without the participation of Pd–H intermediates. Quantitative hydrogenation of benzaldehyde to benzyl alcohol, and of aromatic nitro compounds to the corresponding amines, is obtained. Another application of the above system is the specific deutero-dehalogenation of aromatic halides. Possible mechanisms and the implications of a chemical reaction involving two macroscopic solid particles are discussed.

Pyridines as bifunctional co-catalysts in the CrO3-catalyzed oxygenation of olefins by t-butyl hydroperoxide

Abstract t -Butyl hydroperoxide (TBHP) oxidizes olefins to epoxides and allylic oxidation products in the presence of a Cr(VI) catalyst. A concurrent decomposition of the oxidant occurs. Pyridine-derived additives alter the behavior of this catalytic system: monodentate pyridines and trans -chelated bidentate bipyridines retard the decomposition of TBHP, and arrest the epoxidation reaction, shifting the product selectivity towards allylic oxidation. Adversely, cis -chelated bipyridines accelerate the decomposition of TBHP. Depending on ligand nature and concentration, the initial decomposition rate can be slowed down to 1/8th, or accelerated up to two orders of magnitude, (relative to CrO 3 catalysis). The allylic oxidation and the TBHP decomposition are free-radical reactions, but the epoxidation is evidently not. A reaction mechanism is proposed, where the diverse role of the pyridine ligands is attributed to specific complex formations.

An improved method for the catalytic preparation of t-butyl esters of carboxylic and fatty acids

Abstract Esterification of various acids such as fatty acids, simple carboxylic acids, and Nα blocked amino acids by t-butanol in very high yield is described. The one-pot esterification is achieved by dicyclohexylcarbodiimide (DCC) and catalyzed by dimethylaminopyridine (DMAP). This procedure overcomes the excessive formation of the undesired side product, N-acyl urea obtained normally when a DCC/DMAP-mediated esterification of fatty acids is attempted. The procedure is based on studies of reaction rate and product distribution under various reaction conditions. A suggested mechanism for the esterification of fatty acids is presented.

Copper-catalyzed homolytic and heterolytic benzylic and allylic oxidation using tert-butyl hydroperoxide

Allylic and benzylic alcohols were oxidized in good yields to the respective ketones by tert-butyl hydroperoxide (TBHP) in the presence of copper salts under phase-transfer catalysis conditions. This dehydrogenation was found to proceed via a heterolytic mechanism. CuCl2, CuCl, and even copper powder were equally facile as catalysts, as they were all transformed in situ to Cu(OH)Cl which was extracted into the organic phase by the phase-transfer catalyst (PTC). Deuterium labeling experiments evidenced the scission of the benzylic C–H bond in the rate-determining step. Nonproductive TBHP decomposition was not observed in the presence of the alcohol substrates. Conversely, the oxygenation of π-activated methylene groups in the same medium was found to be a free radical process, and the major products were the appropriate tert-butyl peroxides. Catalyst deactivation, solvent effects, and extraction effects are discussed. By applying Minisci’s postulations concerning the relative reactivity of TBHP molecules towards tert-butoxyl radicals in protic and nonprotic environments, the coexistence of the homolytic and the heterolytic pathways can be explained. A complete reaction mechanism is proposed, wherein the free-radical oxidation obeys Kochi’s mechanism, and the heterolytic dehydrogenation is based on either a high-valent CuIVO species or a [Cu(OH)Cl]2 species.

Kinetics and mechanism of heterogeneous palladium-catalyzed coupling reactions of chloroaryls in water

Coupling reactions of substituted chlorobenzenes to biphenyls catalyzed by palladium on carbon are performed in water using sodium hydroxide and sodium formate in the presence of a surface active agent. Thus, chlorobenzene, p-chloro-o-xylene, p-chloro-1,1,1-trifluorotoluene, p-chloroanisole, and p-chlorotoluene are coupled under moderate conditions to the respective biaryls. A competing reduction process occurs (e.g. chlorobenzene is reduced to benzene), which can be minimized by altering conditions. The relationship of product selectivity to reaction temperature, formate concentration, base concentration, and surfactant type is examined. The roles of formate, Pd catalyst, and surfactant are discussed. It is proposed that the reduction is dependent on the participation of palladium hydride [Pd2+(H–)2], while the coupling occurs via single electron-transfer from Pd0 to the substrate, with subsequent decomposition of the chloroaryl radical anions to obtain aryl radicals and chloride anions. This mechanism is supported by experiments with stoichiometric and sub-stoichiometric amounts of palladium which indicate that selective coupling can occur also in the absence of hydrogen (providing that reduced palladium Pd0, is present in sufficient amount), and by kinetic investigations which indicate that the coupling is actually a first-order reaction, for which the rate-determining step may be the dissociation of the chloroaryl radical anion.

The Role of Water in Phase Transfer Catalysis

Abstract Water is shown to be an essential component in anion exchange process between metal salts and quaternary onium salts in organic solvents. Hydration of ion pairs in organic phases, on the other hand, reduces the nucleophilicity of anions in displacement reactions. These and other effects of water in liquid-liquid and solid-liquid phase transfer catalyzed substitution reactions are combined into the complex role of water in phase transfer catalysis. The subject is discussed in view of some specific examples.

Novel synthesis of alkali and quaternary onium hydroxides via liquid anion exchange; an alternative concept for the manufacture of KOH and other hydroxide salts

Alcohols enhance the extraction of a basic potential through a liquid membrane in the presence of quaternary ammonium salts, thus enabling preparation of caustic solutions without direct transport of hydroxide ions.