Oxidative decarboxylation, silver-mediated

V. SILVER-MEDIATED OXIDATION OF ALCOHOLS, ALDEHYDES, IMINES, AND DECARBOXYLATION 17... 

In 1970, Anderson and Kochi (99) reported a silver-mediated oxidative decarboxylation reaction with peroxydisulfate as the oxidant. Kinetic studies showed that the reaction is first order in both silver and peroxydisulfate and zero order in carboxylic acid. Silver(II) species and alkyl radicals are considered intermediates. 

Figure 16. Formation of silver(II) species and silver-mediated oxidative decarboxylation.

Although silver-mediated oxidative decarboxylation was known for years, its application in synthetic chemistry was very limited (107-110). Systematic studies of this chemistry and other silver-mediated oxidation chemistry in homogeneous solution is rare. This result may be due to the inherent difficulties in working with silver catalysts, which include sensitivity to ligand environment and relative inertness toward oxidation. However, these drawbacks may be overcome with carefully tuned reaction conditions and/or supporting ligand systems. Some of the recent successes with silver nitrene and carbene-transfer reactions will be discussed in detail in Sections VI and Vll. 

Silver-mediated C—X bond formation was known for many years as the Hunsdiecker reaction, in which substrates bearing carboxylic acids are oxidatively decarboxylated to give alkyl halides in the presence of halogens (142). If olefins are used with silver benzoate and I2, the Prevost reaction occurs to yield diols (143). 

The decarboxylative alkenylation of electron-deficient benzoic acids was found to occur by using a cheaper oxidant, benzoquinone, in place of the silver salt (Scheme 4.22) [27]. A CuF2/benzoquinone oxidant system can mediate the reaction of electron-rich benzoic acids [28]. The decarboxylative alkenylation of both electron-rich and electron-deficient benzoic acids using dioxygen as the terminal oxidant can be performed (Scheme 4.23) [29]. 

This first plan for a decarboxylative cross-coupling carried with it certain weaknesses for potential industrial applications. It was to be expected that the salt metathesis between alkali metal carboxylates and late transition metal halides would be thermodynamically disfavored. We expected the formation of a palladium benzoate complex i from palladium bromide complexes c and potassium benzoate (g) to proceed well only in the presence of a stoichiometric quantity of silver to capture bromide ions [27]. However, we did not like the idea of using stoichiometric quantities of silver salts or of expensive aiyl triflates in the place of aryl halides. Finally, the published substrate scope of the oxidative Heck reactimi led to concerns that palladium catalysts mediate the decarboxylation rally of a narrow range of carboxylates, precluding use of this reaction as a general synthetic strategy. 

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