Arylboronic acids silver® oxide

In a process developed by Myers et al., aromatic carboxylic acids were directly employed as substrates for Heck olefinations, albeit in the presence of a large excess of silver carbonate [38]. This base both facilitates the decarboxylation step and acts as an oxidant, generating arylpalladium(II) intermediates. In related processes, arylphosphonic [39] and arylboronic acids [40] were used as aryl sources in the presence of an oxidant. 

In 2007, Wu et al. showed that arylboronic acids and the corresponding boronates reacted with terminal alkynes in the presence of a palladacycle as catalyst and silver oxide (Scheme 10.80).132 As in the Mori reaction using basic silver salts, stoichiometric amounts of silver ion were required for high efficiency. 

Shi has reported a method for /V-alkylanilidc arylation by simple arenes [53], The reaction conditions include heating the anilide with excess arene in propionic acid in the presence of catalytic palladium acetate and copper triflate under oxygen atmosphere. Use of monosubstituted arenes leads to the formation of isomer mixtures however, from the point of atom economy C-H/C-H couplings are the most efficient way for formation of C-C bonds if oxygen is used as the terminal oxidant. Shi has also reported that anilides can be coupled with arylboronic acids and trialkoxyaryl-silanes [54, 55], Silver and copper salts are used as terminal oxidants. 

A new approach for direct arylation of pyridine W-oxides with arylboronic acids through C-H functionalization has been developed (Scheme 45) [99]. This reaction can be performed at room temperature using catalytic silver (I) nitrate in the presence of potassium persulfate, thus giving 2-aryl derivatives of pyridine W-oxides. 

Kinetic and spectroscopic studies of the silver-catalyzed single-electron oxidation process for coupling arylboronic acids with pyridines demonstrated the complexity of the reaction mechanism (13JA4672). The ratedetermining step involves pyridine coordinating to Ag(I) to enable persulfate oxidation. The pyridine also participates in the side reaction by protodebo-ronating the arylboronic acid. 

O/t/20-arylation of benzoic acids is often preferable to ortho-arylation of benzamides if conversion of the amide moiety to other functional groups is desired. However, only a few reports have dealt with the orf/io-functionalization of free benzoic acids due to challenges that involve such transformations. The reactions can be complicated by decarboxylation of the product and the starting material. Despite those difficulties, several methods for direct o/t/io-arylation of benzoic acids have been developed. Yu has shown that arylboronates are effective in arylation of benzoic acids under palladium catalysis [59], The reactions require the presence of palladium acetate catalyst, silver carbonate oxidant, and benzoquinone. Even more interestingly, the procedure is applicable to the arylation of unactivated sp3 C-H bonds in tertiary carboxylic acids such as pivalic acid (Scheme 13) if aryl iodide coupling partner is used. Aryl trifluoroborates can also be used [60],... 

---