Arenes, oxidation acetoxylation

Bis-(2,2-dipyridyl)-silver(II) peroxydisulfate, Ag(dipy)2S2 0g. Mol. wt. 684.44. The reagent is prepared in the same way as the corresponding pyridine complex. Oxidative acetoxylation of arenes. This Ag(II) salt oxidizes arenes in acetic acid containing sodium acetate to acetoxyarenes usually in high yield. The... 

Acetoxylation of arenes. Arenes are acetoxylated by acetic acid (sodium acetate can be added) with potassium persulfate as oxidant and palladium(II) acetate as catalyst. The reaction is unusual in that wie/a-acetoxylation predominates this selectivity can be enhanced by addition of a complexing amine such as 2,2 -bipyridine. Side-chain acetoxylation can be effected with some arenes. Thus mesitylene and durene arc acetoxylated mainly in the a-position of the substituents. ... 

Nuclear597 or side-chain588,598 acetoxylation of arenes can be performed with good yields by persulfate and copper(II) salts in acetic acid (equations 268 and 269). As previously shown for cyclohexene (equation 263), persulfate oxidizes the aromatic ring to a radical cation which loses a proton to give a carbon radical, which is further oxidized by copper(II) acetate to the final acetoxylated product. 

The same workers590,591 also showed that, in the Pd(II)-catalyzed acetoxylation of substituted arenes, a complete reversal of the usual pattern of isomer distribution for electrophilic aromatic substitution or anodic oxidation of aromatics is observed. To explain these results it was suggested that acetoxylation by Pd(OAc)2 takes place via the following addition-elimination sequence ... 

In addition to the industrial apphcations, in Scheme 8.1, other reactions have been the focus of extensive research and development. For example. Chapter 12 surveys the research efforts directed toward Pd-catalyzed oxidative carbonylation of phenol affords the important monomer, diphenyl carbonate (Scheme 8.2a). Other reactions of potential industrial significance highlighted in this chapter include the oxidation of alcohols to aldehydes and ketones (Scheme 8.2b), oxidative coupling of arenes and carboxylic acids to afford aryl esters (Scheme 8.2c), benzylic acetoxylation (Scheme 8.2d), and oxidative Heck reactions (Scheme 8.2e). The chapter concludes by highlighting a number of newer research developments, including ligand-modulated catalytic oxidations, Pd/NO cocatalysis, and alkane oxidation. 

Oxidative esterification of arenes with carboxylic acids produces aryl esters, which can be used as precursors to valuable phenol derivatives (Scheme 8.6). Commercial production of phenol involves the aerobic oxidation of cumene to cumene hydroperoxide, followed by conversion to acetone and phenol under acidic conditions (Hock process) [49]. Aerobic acetoxylation of benzene to phenyl acetate provides a potential alternative route to phenol, and Pd-catalyzed methods for this transformation have been the focus of considerable effort. None ofthese methods are yet commercially viable, however. 

Early observations of benzylic acetoxylation were made in the study of arene acetoxylation and biaryl coupling when toluene was used as a substrate. In 1966, the reaction between stoichiometric Pd(OAc)2 and toluene to give benzyl acetate as the major product was disclosed [72]. Two years later, acetoxylation of toluene with catalytic Pd salts was reported by Union Carbide by using phosphines or a combination of Sn(OAc)2, charcoal, and air as oxidant to give 96TONs [73]. Additional metal acetates such as KOAc are beneficial for the reaction [74]. These acetoxylation methods were further applied to other arenes [75] (e.g., benzene, cyclohexene) and the synthesis of benzyl diacetate [76] (a precursor to benzalde-hyde). 

Nitrogen oxide (NO, ) cocatalysts [120] have received industrial interest in Pd-catalyzed aerobic oxidations such as oxidative carbonylation (see Section 8.2.2) [18], alkene oxidation [121], and arene acetoxylation [55]. Recent studies from academic literature have provided new insights into the roles of NO in these reactions. Pd-catalyzed aerobic alkene oxidation (Wacker reaction) typically affords methyl ketones arising from Markovnikov addition of water (or hydroxide) to an... 

Reactions involving electrophilic substitution of hydrogen in arenes are known for both nontransition [Hg(II), Tl(III), Pb(IV)] and transition metals [Au(III), Pd(II), Pt(IV)] [49]. Pd(II)-catalyzed acetoxylation involves arene activation via formation of an organometallic aryl-Pd c-complex followed by oxidative addition of oxidant and reductive elimination to restore Pd(II) and release the product [11, 50]. Without oxidant, coupling reactions predominate, suggesting arylpalladium(IV) and arylpalladium(II) intermediates in the routes leading to aryl acetates and biaryls, respectively (Scheme 14.10). 

Pd-catalyzed orf/to-acetoxylations involve coordination of an arene functional group followed by cyclopalladation, two-electron oxidation to Pd(lV) intermediate, reductive C—O elimination, and, finally, ligand exchange to release the product [12,13]. 

While this method represents an important advance in the field of arene oxygenations, the use of PhI(OAc)j as a terminal oxidant is a significant limitation. Phl(OAc)j is expensive and leads to the formation of nndesired iodobenzene as a stoichiometric by-product. In order to address this drawback, the use of K S Og in place of Phl(OAc)j has been explored [45]. Potassium persulfate (KjSjOg) is more than an order of magnitude less expensive than Phl(OAc)j and leads to easily separable water soluble by-products. Based on previous results using Phl(OAc)j (Scheme 24.50), the use of 1 1 Pd/pyr catalytic system was investigated for the acetoxylation of benzene with as... 

SCHEME 24.51 Ligand effects for arene acetoxylations using as the oxidant... 

Pyridine and oxime-directed acetoxylation has been reported by Sanford (Scheme 3.14). Treatment of the arenes below with Pd(OAc)2 and bis-acetoxyiodobenzene (BAIB) afforded the desired acetoxylated arenes in good yields with high levels of regioselectivity, greater than 20 1 in most cases, for the more steri-cally accessible ortfto-position. The transformation displayed wide functional group tolerance with respect to the meto-substituent on the arene. The utilization of oxone as the terminal oxidant displayed comparable reactivity to BAIB and provides a safe, greener alternative. Tandem acetoxylation processes have also been described. ... 

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