Reoxidants palladium-catalysts

During the reaction, the palladium catalyst is reduced. It is reoxidized by a co-catalyst system such as cupric chloride and oxygen. The products are acryhc acid in a carboxyUc acid-anhydride mixture or acryUc esters in an alcohoHc solvent. Reaction products also include significant amounts of 3-acryloxypropionic acid [24615-84-7] and alkyl 3-alkoxypropionates, which can be converted thermally to the corresponding acrylates (23,98). The overall reaction may be represented by ... 

Ca.ta.lysis, The most important iadustrial use of a palladium catalyst is the Wacker process. The overall reaction, shown ia equations 7—9, iavolves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-cataly2ed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or ia a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chloriaated by-products. 

From a chemistry standpoint a dehydration agent, which can give controlled alcohol release and remove water formed during catalytic reoxidation of palladium(O), to palladiumCII), is key in obtaining a high product yield. After one hour at 100 C, 1800 psig total carbon monoxide/air pressure and 1500 ppm palladium catalyst concentration, conversion based on butadiene is 30 mole %. Selectivity to linear unsaturated diester carbonylation product is 79 mole %. About 10 mole % methyl, 4-pentadienoate is formed along with 11% various other by-products (Table II.). 

Palladium-catalyzed, Wacker-type oxidative cycHzation of alkenes represents an attractive strategy for the synthesis of heterocycles [139]. Early examples of these reactions typically employed stoichiometric Pd and, later, cocat-alytic palladium/copper [140-142]. In the late 1970s, Hegedus and coworkers demonstrated that Pd-catalyzed methods could be used to prepare nitrogen heterocyles from unprotected 2-allylanilines and tosyl-protected amino olefins with BQ as the terminal oxidant (Eqs. 23-24) [143,144]. Concurrently, Hosokawa and Murahashi reported that the cyclization of allylphenol substrates can be accomplished by using a palladium catalyst with dioxygen as the sole stoichiometric reoxidant (Eq. 25) [145]. 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

In fact, the role of copper and oxygen in the Wacker Process is certainly more complicated than indicated in equations (151) and (152) and in Scheme 10, and could be similar to that previously discussed for the rhodium/copper-catalyzed ketonization of terminal alkenes. Hosokawa and coworkers have recently studied the Wacker-type asymmetric intramolecular oxidative cyclization of irons-2-(2-butenyl)phenol (132) by 02 in the presence of (+)-(3,2,10-i -pinene)palladium(II) acetate (133) and Cu(OAc)2 (equation 156).413 It has been shown that the chiral pinanyl ligand is retained by palladium throughout the reaction, and therefore it is suggested that the active catalyst consists of copper and palladium linked by an acetate bridge. The role of copper would be to act as an oxygen carrier capable of rapidly reoxidizing palladium hydride into a hydroperoxide species (equation 157).413 Such a process is also likely to occur in the palladium-catalyzed acetoxylation of alkenes (see Section 61.3.4.3). 

A certain disadvantage of this method is that stoichiometric amounts of the palladium catalyst have to be employed. While it is possible to reoxidize the in situ generated Pd(0) in the above-mentioned alkoxycarbonylations, the development of efficient catalytic procedures for the alkoxypalladation/y5-hydride elimination reactions still represents an unsolved problem. However, first successes [12] in special systems suggest that this problem should not be insoluble. 

The oxidation of propylene has been chosen as a probe reaction to study the catalytic activity of Cu Pd -TSM. The olefin oxidation in an acidic solution of Cu(II) and Pd(U) chlorides, well known as the Wacker reaction, is achieved when olefins are selectively oxidized to ketones or aldehydes by hydrated Pd, leaving Pd . The Pd is oxidized back to Pd by 2Cu, and the resulting Cu is reoxidized by dissolved oxygen. Because the corrosive nature of the catalyst solution is a serious disadvantage for practical use, supported copper-palladium catalysts have been proposed to operate the reaction in a gas flow reactor (40). 

Based on their alkyl nitrite technology, Ube developed their own new proeess for the manufacture of dialkyl oxalates by oxidative carbonylation of alcohols. This process is a two-step reaction, in which alkyl nitrite acts as an reoxidant for the palladium catalyst system, similarly to the situation in the preparation of dialkyl carbonates mentioned above. The published patent literature does not make it possible to give exact details about the Ube industrial plant in Yamaguchi, Japan, which has produced several thousands tons of dibutyl oxalate annually since 1978. The first step of the manufacturing process for dialkyl oxalates is the preparation of the gaseous alkyl nitrite from NO, oxygen, and... 

Ito and his coworkers have reported that palladium-catalyzed oxidative coupling reactions of Grignard reagents in the presence of V-substituted isocyanide dichloride afford diynes (equation 17). Isocyanide dichloride may serve as a reoxidant of the palladium catalyst in this sequence via a catalytic cycie. In addition Kiji and his coworkers have described the oxidative coupling of phenylacetylene by a Pd-Cu catalyst in the presence of 4-iodo-(3//) phenothiazin-3-one. ... 

As reoxidant of palladium catalysts The oxygen/DMSO couple recycles the... 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use is made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant, our catalyst combination is completely regioselective for alicyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

Acetaldehyde is produced by the liquid phase oxidation of ethylene in the presence of a palladium chloride-cupric chloride catalyst. The reaction takes place in aqueous solution at approximately 125-130°C (255-265°F). It is carried out either as a one-step process using high purity oxygen or as a two-step process using air as the source of oxygen to reoxidize the catalyst in a separate reactor. 

Palladium oxidizes methyl acrylate in the presence of methanol to form 3-methoxy methyl acrylate and equimolar amounts of water with a reaction rate R3. Reduced Pd is reoxidized by the Fe/Gu cocatalyst with a reaction rate R2 producing a reduced Fe/Gu cocatalyst, which is reoxidized by oxygen. The palladium catalyst is also responsible for the formation of by-products from oxidation of methanol (methyl formate, dimethoxymethane). 3-Methoxy methyl acrylate... 

It is worth noting that the palladium catalyst could successfully be recycled palladium in reduced form was allowed to precipitate on added celite over the course of the reaction, the solid was later removed by centrifugation, and the palladium reoxidized with the corresponding amount of bromine. This version of the Mizoroki-Heck reaction may not qualify as an attractive alternative to standard protocols for everyday laboratory use, but it can be advantageous especially for companies with an in-house supply of carboxylic acids and propyne gas. 

In the last six chapters we discussed the transition metal catalyzed carbonylative activation of organohalogen (C-X, X = I, Br, Cl, OTf, etc.) compounds. They all have one common point in their reaction mechanism taking a palladium catalyst, for example, the reactions start with Pd(0) and then go to Pd(II) after an oxidative addition. To summarize, the reactions all go through Pd(0) to Pd(II) and a Pd(0) cycle. But for oxidative carbonylation reactions, the reactions go through Pd(ll) to Pd(0) and a Pd(II) cycle. Clearly, oxidative carbonylations need additional oxidants to reoxidize the Pd(0) to Pd(II), and various organic nucleophiles were applied as substrates in the presence of CO. One of the most obvious advantages for oxidative carbonylation reactions is the oxidative addition step can be avoid which is more reluctant under CO atmosphere. 

The production of another important chemical and polymer intermediate, acetic acid, was revolutionized by the Wacker process that was introduced in 1960. It was a simple, high yield process for converting ethylene to acetaldehyde, which replaced the older process based on ethanol and acetylene. In the Wacker reaction, the palladium catalyst is reduced and then reoxidized. Ethylene reacts with water and palladium chloride to produce acetaldehyde and palladium metal. The palladium metal is reoxidized by reaction with cupric chloride, which is regenerated by reaction with o gen and hydrochloric acid. In 1968, BASF commercialized an acetic acid process based on the reaction of carbon monoxide and methanol, using carbonyl cobalt promoted with an iodide ion (74). Two years later, however, Monsanto scored a major success with its rhodium salt catalyst with methyl iodide promoter. Developed by James F. Roth, this new catalyst allowed operation at much milder conditions (180°C, 30-40 atm) and demonstrated high selectivity for acetic acid (75). 

While the product of this reaction somewhat resembles a Nazarov cyclization, it is thought that the reaction proceeds via a different mechanistic pathway and depends on the nature of the palladium catalyst. Activation of the olefin by palladium, followed by cyclization gives palladium enolate 41. When Pd(OAc)2 is used as the catalyst, P-hydride elimination of 41 produces cyclopentenone 42, and molecular oxygen reoxidizes the palladium. On the other hand, when PdCl2(MeCN)2 is used as a catalyst, hydrolysis generates HCl which results in rapid protonation of 41, giving cyclopentenone 43 as the product. 

The sluggish reoxidation of atomic palladium(O) by molecular oxygen is greatly facilitated by addition of a reoxidant of copper salt (Scheme 11). The mechanism of the oxidation can be rationalized by assuming Scheme 12. Complexation of alcohol to Pd(II) catalyst forms Pd(II) alkoxide, which undergoes /3-palladiumhydride elimination to give a carbonyl compound. Since palladium catalyst is reduced to Pd(0) by reductive elimination, catalytic reaction must include a combined process, where the Pd(0) species is oxidized to the Pd(II) species (Scheme 12). 

Commercial uses for palladium catalysts include hydrocracking, vinly acetate production, and the Wacker process, which is used to convert olefins into aldehydes or ketones. The best known example is the conversion of ethylene to acetaldehyde. The process is catalyzed by [PdCm, which is reduced to palladium(O) as the olefin is converted fo fhe aldehyde. The mefal is reoxidized with CuCl2. The homogeneous system is attractive here because the product is easy to separate from the catalyst. This is an advantage that heavier products may not enjoy in homogeneous systems in which separation would be more... 

Since, as shown in equation (34), palladium metal is precipitated as a byproduct of the reaction, it is necessary to reoxidize it back to the Pd " " state. This is accomplished with a palladium-copper couple, as depicted in equations (35) and (36), which is driven by oxygen. The reaction is carried out by contacting a mixture of ethylene and oxygen with a mixture of acetic acid, lithium acetate, and the palladium-copper couple at temperatures of 80 to 150 °C. The vapor-phase process is carried out under pressure at high temperatures (120 to 150 °C) using a fixed-bed palladium catalyst [218]. The oxidative acylation of ethylene can also be used for the preparation of the higher vinyl esters, although it is not currently used for that purpose, due to the low demand for those materials. 

A new process is the oxidative carbonylation of ethylene [459,460]. During the reaction the palladium catalyst is reoxidized by a cupric chloride cocatalyst system and oxygen. Selectivity is improved by the addition of a mercury or a tin salt [461]. 

A new mild procedure for the conversion of terminal acetylenes directly into acetylene carboxylates using carbon monoxide at atmospheric pressure and a palladium catalyst has been described (e.g. Scheme 11To enable palladium chloride, the oxidizing species, to be used in catalytic quantities, a stoicheiometric amount of copper(ii) chloride is added as a reoxidant. 

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. 

CO, and methanol react in the first step in the presence of cobalt carbonyl catalyst and pyridine [110-86-1] to produce methyl pentenoates. A similar second step, but at lower pressure and higher temperature with rhodium catalyst, produces dimethyl adipate [627-93-0]. This is then hydrolyzed to give adipic acid and methanol (135), which is recovered for recycle. Many variations to this basic process exist. Examples are ARCO s palladium/copper-catalyzed oxycarbonylation process (136—138), and Monsanto s palladium and quinone [106-51-4] process, which uses oxygen to reoxidize the by-product... 

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