Wacker-type catalysts

Pd(II) Wacker-type catalysts were also studied.146 Selective oxidation of benzene to phenol by molecular oxygen in the presence of Pd and heteropolyacids have been published.147... 

The catalytic oxycarbonylation of benzene and naphthalene to benzoic or naphthoic acid in the presence of Wacker-type catalysts has been reported in several patents,376,448 but difficulties in reoxidizing the reduced palladium have inhibited industrial use of this chemistry. 

Deactivation Effects in the Synthesis of Methyl Ethyl Ketone by Selective Oxidation over Solid Wacker-type Catalysts... 

The objective of the present study was to analyze the reasons for the deactivation effects observed in solid Wacker-type catalysts for 1-butene oxidation. For this purpose the catalytic behavior and characteristics of Pd-V205 on alumina catalysts, prepared using either a N PdC or a PdS04 salt, were compared with those of alternative catalysts prepared by substituting the V-oxide with Ce02 in order to obtain a better understanding of the role of V-oxide. In addition, the behavior of a Pd-doped V-heteropolyacid also is discussed to further extend the analysis. 

The catalytic system for this entire class of reactions is quite similar, being a Wacker type catalyst, such as a palladium salt, in the presence of a cooxidant (for instance CuClj). 

Tang, H.-G. and Sherrington, D.C., Polymer-supported Pd(ll) Wacker-type catalysts. 1. Synthesis and characterization of the catalysts. Polymer, 34, 2821, 1993. 

Centi, G., Stella, G. Synthesis of 2-butanone by selective oxidation on solid Wacker-type catalysts. Chem. Ind. 1995, 62, 319-329. 

For a detailed treatment of various aspects of Wacker chemistry, the reader is referred to the monograph of Henry [137]. Factors influencing vinylic vs. allylic oxidation catalyzed by Pd(II) complexes have been treated by Lyons [132]. Recently, supported Wacker type catalysts have been developed [145]. 

Other than the palladium-alkyl nitrite system, the usual Wacker-type catalyst, which utihzes molecular oxygen as stoichiometric oxidant, is commonly employed.It has superior catalyst efhciency but shows lower productivity than that of the nitrite system. 

Pd—COPh moiety rather than to neutralize the acidity generated during the reaction. A similar sequence is proposed to explain the rhodium-catalysed carbonylatlon of methanol to acetic acid. Full details of the carbonylatlon of cis- and trans-bai-l-ene in methanol with a Pd -Cu Wacker-type catalyst have now appeared the stereospecificity of the resulting methoxypalladation trans) was found to be similar to that involving chelating di-olefins which until now has been considered anomalous. ... 

The results of carbonylation of cis- and trans-hvA-l-enc in methanol in the presence of palladium(u)-copper(n) Wacker type catalyst show that in the initial stages of reaction, stereospecific fra/w-methoxypalladation is observed giving exclusively the threo- and ery/Aro-methoxyesters, respectively, ... 

Betzemeier et al. (1998) have used f-BuOOH, in the presence of a Pd(II) catalyst bearing perfluorinated ligands using a biphasic system of benzene and bromo perfluoro octane to convert a variety of olefins, such as styrene, p-substituted styrenes, vinyl naphthalene, 1-decene etc. to the corresponding ketone via a Wacker type process. Xia and Fell (1997) have used the Li salt of triphenylphosphine monosulphonic acid, which can be solubilized with methanol. A hydroformylation reaction is conducted and catalyst recovery is facilitated by removal of methanol when filtration or extraction with water can be practised. The aqueous solution can be evaporated and the solid salt can be dissolved in methanol and recycled. 

In 1997, Uozumi and Hayashi found high enantioselective Wacker-type cycUza-tion of o-allylphenols or o-homoaUylphenols by using Pd(II) catalysts coordinated with chiral bis(oxazoline) ligands based on the 1,1 -binaphthyl backbone (Eq. 6.36)... 

A survey of Wacker-type etherification reactions reveals many reports on the formation of five- and six-membered oxacycles using various internal oxygen nucleophiles. For example, phenols401,402 and aliphatic alcohols401,403-406 have been shown to be competent nucleophiles in Pd-catalyzed 6- TZ /fl-cyclization reactions that afford chromenes (Equation (109)) and dihydropyranones (Equation (110)). Also effective is the carbonyl oxygen or enol of a 1,3-diketone (Equation (111)).407 In this case, the initially formed exo-alkene is isomerized to a furan product. A similar 5-m -cyclization has been reported using an Ru(n) catalyst derived in situ from the oxidative addition of Ru3(CO)i2... 

Asymmetric induction has also been achieved in the cyclization of aliphatic alcohol substrates where the catalyst derived from a spirocyclic ligand differentiates enantiotopic alcohols and alkenes (Equation (114)).416 The catalyst system derived from Pd(TFA)2 and (—)-sparteine has recently been reported for a similar cyclization process (Equation (115)).417 In contrast to the previous cases, molecular oxygen was used as the stoichiometric oxidant, thereby eliminating the reliance on other co-oxidants such as GuCl or/>-benzoquinone. Additional aerobic Wacker-type cyclizations have also been reported employing a Pd(n) system supported by A-heterocyclic carbene (NHC) ligands.401,418... 

Wacker cyclization has proved to be one of the most versatile methods for functionalization of olefins.58,59 However, asymmetric oxidative reaction with palladium(II) species has received only scant attention. Using chiral ligand 1,1 -binaphthyl-2,2 -bis(oxazoline)-coordinated Pd(II) as the catalyst, high enantioselectivity (up to 97% ee) has been attained in the Wacker-type cyclization of o-alkylphenols (66a-f) (Scheme 8-24). 

The reaction is highly exothermic as one might expect for an oxidation reaction. The mechanism is shown in Figure 15.1. Palladium chloride is the catalyst, which occurs as the tetrachloropalladate in solution, the resting state of the catalyst. Two chloride ions are replaced by water and ethene. Then the key-step occurs, the attack of a second water molecule (or hydroxide) to the ethene molecule activated towards a nucleophilic attack by co-ordination to the electrophilic palladium ion. The nucleophilic attack of a nucleophile on an alkene coordinated to palladium is typical of Wacker type reactions. 

A development of the last two decades is the use of Wacker activation for intramolecular attack of nucleophiles to alkenes in the synthesis of organic molecules [9], In most examples, the nucleophilic attack is intramolecular, as the rates of intermolecular reactions are very low. The reaction has been applied in a large variety of organic syntheses and is usually referred to as Wacker (type) activation of alkene (or alkynes). If oxygen is the nucleophile, it is called oxypalladation [10], Figure 15.4 shows an example. During these reactions the palladium catalyst is often also a good isomerisation catalyst, which leads to the formation of several isomers. 

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... 

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]. 

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). 

The oxidative carbonylation of arenes to aromatic acids is a useful reaction which can be performed in the presence of Wacker-type palladium catalysts (equation 176). The stoichiometric reaction of Pd(OAc)2 with various aromatic compounds such as benzene, toluene or anisole at 100 °C in the presence of CO gives aromatic acids in low to fair yields.446 This reaction is thought to proceed via CO insertion between a palladium-carbon (arene) allyl chloride, but substantial amounts of phenol and coupling by-products are formed.447... 

Supported liquid-phase catalysts (SLPCs) combine the salient features of both homogeneous and heterogeneous catalysis for enhanced catalytic and/or process efficiency (337). SLPC catalysts, in which a liquid-phase (homogeneous) catalyst is dispersed within a porous support, have been used in Wacker-type ethylene oxidation for acetaldehyde and vinyl acetate production (337, 338). In the former case, a traditional homogeneous Wacker catalyst (vide supra) consisting of a chlorinated solution of Pd and Cu chlorides retained on a support with monomodal pore size distribution... 

The field of homogeneous palladium catalysis traces its origin to the development of the Wacker process in the late 1950s (Eq. 7) [83]. Since this discovery, palladium-catalyzed reactions have evolved into some of the most versatile reactions for the synthesis of organic molecules [84,85]. Palladium-catalyzed Wacker-type oxidation of alkenes continues to be an active field of research [86-88], and several recent applications of NHC-coordinated Pd catalysts have been reported for such reactions. 

The Pd-catalyzed conversion of terminal alkenes to methyl ketones is a reaction that has found widespread use in organic chemistry [87,88]. These reactions, as well as the industrial Wacker process, typically employ CuCh as a co-catalyst or a stoichiometric oxidant. Recently Cu-free reaction conditions were identified for the Wacker-type oxidation of styrenes using fBuOOH as the oxidant. An NHC-coordinated Pd complex, in-situ-generated (I Pr)Pd(OTf)2, served as the catalyst (Table 5) [101]. These conditions min-... 

The reaction media for Wacker-type reactions are highly corrosive. This is due to the presence of free acids (acetic acid for vinyl acetate), ions like Cl, and dioxygen. For any successful technology development, the material of construction for the reactors is a major point of concern (see Section 3.1.4). Some progress in this respect has recently been made by the incorporation of heteropolyions such as [PV14042]9 in the catalytic system. The heteropolyions probably act as redox catalysts. A seminonaqueous system is used for this modified catalytic system, and the use of low pH for dissolving copper and palladium salts is avoided. 

A solvent combination of. vcC02- C4Ciim PF6 was found to afford superior selectivity in the Wacker-type oxidation of 1-hexene with PdCl2-CuCl as catalyst to afford 2-hexanone as the main product as shown in Scheme 5.16.[73] In the absence of either scC02 or ionic liquid, considerably lower selectivity for 2-hexanone was observed. Catalyst solutions were recycled five times with a low, but steady, decrease in activity. 

---