Wacker oxidation scope

Direct palladation of C-H bonds can be achieved by treatment of, for example, electron-rich arenes with Pd(II) salts (see also Section8.11). After cross-coupling via reductive elimination the resulting Pd(0) must be reoxidized to Pd(II) if Pd-catalysis is the aim [85], Reoxidation of Pd(0) with Cu or Ag salts (as in the Wacker process) is not always well suited for C-C bond-forming reactions [86], but other oxidants, for example peroxides, have been used with success (Scheme8.9). The required presence of oxidants in the reaction mixture limits the scope of these reactions to oxidation-resistant starting materials. 

If carboxylic acids and alcohols act as nucleophiles in the Wacker reaction, the products are vinyl esters or acetals, respectively. As mentioned, the substrate scope of these oxidations is usually limited to olefins not bearing hydrogen in allylic position, because of competing allylic oxidation. 

When media other than water are used, different but related processes operate. Thus, the oxidation of ethylene in acetic acid can be directed to give vinyl acetate, ethylene glycol acetate, or 2-chloroethyl acetate [9]. Similarly, the synthesis of acetals or ketals can be achieved in an alcoholic medium [10]. Although the oxidation of alkenes in such a medium is closely parallel to the Wacker process, the chemistry of these reactions is far beyond the scope of this section, which is limited to Wacker-type reactions in aqueous media, and will not be discussed here. 

Although the detailed treatment of Wacker chemistry is beyond the scope of this book, it seems appropriate to briefly summarize the main features of Pd(II) catalyzed olefin oxidation on the example of ethylene oxidation, which can be given by the equation ... 

The Wacker process, an oxidative addition reaction, performed with Pd(II)-catalyst in the presence of an oxidant is out of scope of this review. Nevertheless, some representative references concerning Wacker reaction are given [65-72]. 

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