Palladium acetoxylations

It should be noted that heterogeneous palladium acetoxylation catalysts do not contain copper cooxidants, presumably because the support stabilizes the resulting palladium(II) hydride such as (136) and prevents the formation of metallic palladium. The stabilized palladium hydride (136) may react with 02 to give the hydroperoxide (137), which is probably an important intermediate for the regeneration of the initial Pd11 catalyst. 

Pd-hydroquinone-mediated electrochemical 1,4-diacetoxylation of cyclohexa-1,3-diene (118), leading to 1,4-diacetoxycyclo-hex-2-ene (119), has been investigated (Scheme 46) [156]. Palladium-catalyzed indirect electrochemical monoacetoxylation of olefins has been attained in an MeCN/Ac0H-NaC104/Ac0Na/Pd(0Ac)2-Cu(OAc)2-(C) system. The acetoxylation of cyclohexene produces unsaturated esters with less current efficiency, giving a 1 1 mixture of allylic and vinylic products [118]. 

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-x-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast... 

Palladium-catalyzed addition of oxygen nucleophiles to alkenes dates back to the Wacker process and acetoxylation of ethylene (Sects. 1 and 2). In contrast, catalytic methods for intermolecular oxidative amination of alkenes (i.e., aza-Wacker reactions) have been identified only recently. Both O2 and BQ have been used as oxidants in these reactions. 

Scheme 14 Possible outcomes for the palladium-catalyzed oxidative acetoxylation of alkenes...

Highly selective formation of phenyl acetate was observed in the oxidation of benzene with palladium promoted by heteropoly acids.694 Lead tatraacetate, in contrast, usually produces acetoxylated aromatics in low yields due to side reac-tions. Electrochemical acetoxylation of benzene and its derivatives and alkoxylation of polycyclic aromatics789 790 are also possible. Thermal or photochemical decomposition of diacyl peroxides, when carried out in the presence of polycyclic aromatic compounds, results in ring acyloxylation.688 The less reactive... 

The homogeneous palladium-catalyzed process for acetoxylation was never commercialized because of low selectivity and the difficulty in separating the catalyst from the reaction mixture. Heterogeneous palladium catalysts applied in the gas phase, in turn, quickly lose activity caused by buildup of polybutadiene. The Mitsubishi process uses a Pd-Te-on-active-carbon catalyst in the liquid phase. Tellurium apparently prevents palladium elution to acetic acid. 

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). 

First discovered by Moiseev et a/.,416 the palladium-catalyzed acetoxylation of ethylene to vinyl acetate has been the subject of very active investigations, particularly in industry, as shown by the considerable number of patents existing in this area. Vinyl acetate is an extremely important petrochemical product which is used for the synthesis of polymers such as poly(vinyl acetate) and poly(vinyl alcohol). Most of its annual production ( 2.6 Mt) results from the acetoxylation of ethylene (equation 160). 

A widely accepted mechanism for acetoxylation of ethylene is shown in equation (161) and consists of the nucleophilic attack of the acetate anion on the coordinated ethylene, followed by acetoxypalladation and /3-hydride elimination, giving vinyl acetate and palladium hydride.367... 

Such a stabilization of the palladium catalyst can also be achieved in homogeneous liquid phase by the use of appropriate ligands. Thus, it has recently been shown that palladium(II) hydroxamates are effective catalysts for the acetoxylation of ethylene with high selectivity and a high turnover (>200) (equation (162), whereas Pd(OAc)2 rapidly becomes deactivated and precipitates in the form of metallic palladium.419 It is probable that the bidentate hydroxamate ligand stabilizes the hydride Pd—H species and prevents palladium from precipitating. 

When palladium-catalyzed acetoxylation is carried out in the presence of nitrate or nitrite ions, ethylene glycol monoacetate (EGMA) results as the major product of the reaction (equation 163).420... 

Acetoxylation of propene to allyl acetate can be performed in the liquid phase with high selectivity (98%) in acetic acid in the presence of catalytic amounts of palladium trifluoroacetate. The stability and activity of this catalyst can be considerably increased by adding copper (II) trifluoroacetate and sodium acetate as cocatalysts (100 °C, 15 bar, reaction time = 4 h, conversion = 70%, selectivity = 97%). Gas-phase procedures for the manufacture of allyl acetate are described in several patents and use conventional palladium catalysts deposited on alumina or silica, together with cocatalysts (Au, Fe, Bi, etc.) and sodium acetate. The activity and selectivity reported for these catalysts are very high (100-1000 g l-1 h-1, selectivity = 90-95% ).427 A similar procedure has been used for the synthesis of methallyl acetate from 2-methylpropene.428... 

Acetoxylation of toluene using a Pd(OAc)2-Sn(OAc)2-charcoal catalyst selectively produces benzyl acetate with high turnover numbers ( 100).373,434 The active catalyst presumably contains Pd—Sn bonds. Tin ligands are known to increase the 7r-acceptor ability of palladium, and may favor the coordination of the toluene in the form of a benzylic 7r-allyl complex (141) which is nucleophilically attacked by the acetate anion.435... 

Allylic acetoxylation of cyclohexene can be selectively effected by palladium acetate in the presence of Mn02 and p-benzoquinone (bq) (equation 298).640... 

ALLYLIC ACETOXYLATION Palladium(II) chloride-Silver(I) acetate. 

Pd-catalysed chelate-directed acetoxylation of meta -substituted arenes has been studied.61 Many substituted groups are tolerated by this process and the reaction shows a high degree of regioselectivity for the less sterically hindered ortfto-position. For example, 2-(3-nitrophenyl)pyridine forms 2-(2-acetoxy-3-nitrophenyl)pyridine. Finally, density functional calculations62 on the palladium acetate-promoted cyclomet-allation of dimethylbenzylamine suggest that reaction occurs via an agostic C-H complex rather than a Wheland intermediate. An intramolecular H-transfer to a coordinated acetate via a six-membered transition state follows. 

In the first step of the process (Fig. 1), the acetoxylation of propylene is carried out in the gas phase, using solid catalyst containing palladium as the main catalyst at 160 to 180°C and 70 to 140 psi (0.49 to 0.98 MPa). The reactor effluents from the reactor are separated into liquid components and gas components. The liquid components containing allyl acetate are sent to the hydrolysis process. The gas components contain unreacted gases and... 

The asymmetric allylic acetoxylation of cycloalkenes has also been reported. In this case, the catalyst is a bimetallic palladium(II) complex bearing a chiral bisox-azoline or a chiral diphosphine (DIOP). The reaction is performed in acetic acid/ sodium acetate under oxygen atmosphere at room temperature. Under these conditions, acetoxylation products of cyclohexene and cydopentene are obtained with 55 % and 78 % ee, respectively, albeit in low yields [39a]. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

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