Palladium catalysts electrochemical

Reduction of l,2,4-triazin-3-ones (84) with Raney nickel, zinc and acetic acid, lithium aluminum hydride, sodium borohydride, titanium(III) chloride, p-toluenethiol, hydrogen and a palladium catalyst, or electrochemically, produces 4,5-dihydro-l,2,4-triazin-3-ones (268) (78HC(33)189, p. 246, 80JHC1237), which may be further reduced to 1,4,5,6-tetrahydro-l,2,4-triazin-3-ones (269). l,2,4-Triazin-3-ones (84) with hydriodic acid and phosphorus yielded imidazoles (05LA(339)243). 3-Alkoxy-l,2,4-triazines (126) and sodium borohydride gave the 2,5-dihydro derivatives (270) (80JOC4594). 

Catalytic hydrogenation of 3-phenylpyrido[3,4-e]-l,2,4-triazine (45b) over a palladium catalyst gave the 1,4-dihydro compound (29) in a 90% yield this is an unusual result as the position of the pyridine nitrogen atom does not seem to affect the formation of 1,4-dihydro derivatives (see also Section 7.17.5.4). Electrochemical reduction of the pyrido-l,2,4-triazine (45b) in both protic and... 

Another approach to reducing fraui-acids involves electrochemical hydrogenation with a palladium catalyst in a solid-state electrolyte reactor (10) where about 50% reduction in fraui-was observed compared with nickel at iodine values of about 90. 

Palladium, in the form of palladium(II) acetate, has also been used to catalyze biaryl formation directly from aryl iodides (R3N 100 °C), especially P-NO2 and p-Cl derivatives. As usual, ortho substituents severely hinder this type of coupling. Related reductive couplings of aryl halides have been achieved using hydrazine and a Pd-Hg catalyst, electrochemically generated Pd° catalysts, or a palladium on carbon catalyst in the presence of aqueous sodium formate, sodium hydroxide and, crucially, a catalytic amount of a surfactant. The first two procedures look to be particularly selective and efficient, while the latter, rather different, method is not so efficient but does look amenable to large scale work. 

This is done readily by hydrogenation over a palladium catalyst, but electrochemical reduction has also been used. Both processes are illustrated in the following examples. 

In a more specialized approach, IL phases have been immobilized in membrane materials. Although the primary driver of this work was the use of these materials as electrochemical devices, they have also been investigated for catalytic applications [20]. Membrane materials composed of air-stable, room-temperature ILs and poly(vinylidene fluoride)-hexafluoropropene copolymers were prepared with the incorporation of the active catalyst species in the form of palladium on activated carbon. Optical imaging revealed that the prepared membranes contained a high dispersion of the palladium catalyst particles. Studies on the materials included evaluating their gas permeability and their catalytic activity for the hydrogenation reaction of propene. 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

The active site responsible for the aerobic oxidation of alcohols over Pd/AljO, catalysts has long been debated [96-lOOj. Many reports claim that the active site for this catalyst material is the metallic palladium based on electrochemical studies of these catalysts [100, 101]. On the contrary, there are reports that claim that palladium oxide is the active site for the oxidation reaction and the metalhc palladium has a lesser catalytic activity [96,97). In this section, we present examples on how in situ XAS combined with other analytical techniques such as ATR-IR, DRIFTS, and mass spectroscopic methods have been used to study the nature of the actual active site for the supported palladium catalysts for the selective aerobic oxidation of benzylic alcohols. Initially, we present examples that claim that palladium in its metallic state is the active site for this selective aerobic oxidation, followed by some recent examples where researchers have reported that ojddic palladium is the active site for this reaction. Examples where in situ spectroscopic methods have been utilized to arrive at the conclusion are presented here. For this purpose, a spectroscopic reaction cell, acting as a continuous flow reactor, has been equipped with X-ray transparent windows and then charged with the catalyst material. A liquid pump is used to feed the reactants and solvent mixture into the reaction cell, which can be heated by an oven. The reaction was monitored by a transmission flow-through IR cell. A detailed description of the experimental setup and procedure can be found elsewhere [100]. Figure 12.10 shows the obtained XAS results as well as the online product analysis by FTIR for a Pd/AljOj catalyst during the aerobic oxidation of benzyl alcohol. 

In addition to the palladium catalysts mentioned, other transitional metal catalysts were explored and applied in this transformation as well. In 1995, Dunach and Olivero studied the nickel-catalyzed electrochemical reductive deprotection of allyl ethers. Among the various substrates, allyl-o-halogenophenols were tested as well. 2-Chlorophenol was produced from allyl-o-chlorophenol phenol was produced from allyl-o-bromophenol while 3-methyl-2,3-dihydrobenzofuran (33%) was produced from allyl-o-iodophenol together with phenol (52%). 

Chetty R, Sott K. Characterization of thermally deposited platinum and palladium catalysts for direct formic acid fuel cells. J New Mat Electrochem Systems 2007 10 135-42. 

From these investigations, during the electrochemical studies, was been possible to value the applicability of the palladium electrochemical reoxidation to develop a system, that not use conventional chemistry to report palladium zero in oxidation form. A selective and enviroiunent friendly methodology, based on the use of electrochemistry and palladium catalysts, for fine chemical preparation is here reported. 

Table 4. Electrochemical synthesis of aliphatic and aromatic ureas using palladium catalyst ...

Savadogo O, Lee K, Oishi K, Mitsushimas S, Kamiya N, Ota K-I. 2004. New palladium alloys catalyst for the oxygen reduction reaction in an acid medium. Electrochem Commun 6 105 109. 

The stereochemistry of electrochemical reduction of acetylenes is highly dependent upon the experimental conditions under which the electrolysis is carried out. Campbell and Young found many years ago that reduction of acetylenes in alcoholic sulfuric acid at a spongy nickel cathode produces cis-olefins in good yields 126>. It is very likely that this reduction involves a mechanism akin to catalytic hydrogenation, since the reduction does not take place at all at cathode substances, such as mercury, which are known to be poor hydrogenation catalysts. The reduction also probably involves the adsorbed acetylene as an intermediate, since olefins are not reduced at all under these conditions and since hydrogen evolution does not occur at the cathode until reduction of the acetylene is complete. Acetylenes may also be reduced to cis olefins in acidic media at a silver-palladium alloy cathode, 27>. 

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

The reduction is usually made in a multi-compartment electrochemical cell, where the reference electrode is isolated from the reaction solution. The solvent can be water, alcohol or their mixture. As organic solvent A,A-dimethyl form amide or acetonitrile is used. Mercury is often used as a cathode, but graphite or low hydrogen overpotential electrically conducting catalysts (e.g. Raney nickel, platinum and palladium black on carbon rod, and Devarda copper) are also applicable. 

The electrochemical Wacker-type oxidation of terminal olefins (111) by using palladium chloride or palladium acetate in the presence of a suitable oxidant leading to 2-alkanones (112) has been intensively studied. As recyclable double-mediatory systems (Scheme 43), quinone, ferric chloride, copper acetate, and triphenylamine have been used as co-oxidizing agents for regeneration of the Pd(II) catalyst [151]. The palladium-catalyzed anodic oxidation of... 

The a-substitution product from oxidation of methylbenzenes in acetic acid can be eliminated by electrochemical hydrogenolysis at the cathode. An undivided cell is used and a palladium on carbon catalyst is suspended in the medium. The necessary hydrogen is generated by reduction of protons at the cathode. In this way, the... 

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