Direct arylations silver® carbonate

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. 

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],... 

Silver carbonate catalyzes the carbon phosphonation of aryl propiolates with dialkyl H-phosphonates in order to prepare 3-phosphonated conma-rins (140L3356). Other propiolates undergo copper(II)-mediated direct trifluoromethylation with Togni s reagent 19 providing 3-trifluoromethyl-coumarins (Scheme 54) (14OL4240). 

Silver(I) carbonate was the best choice for oxidant, outperforming AgOAc. Pivalic acid was utilized as an additive. Different arenes could be employed, providing separable mixtures of arylated and diarylated products. The resulting products were converted to phenanthroline derivatives and tested in transition metal-free direct arylation and Mizoroki-Heck reactions. 

The first direct coupling of terminal alkynes with aryl iodides or bromides without palladium was reported by Wang and Li in 2006.135 Silver iodide and triphenylphos-phane in polar solvents proved to be the best catalyst combination, while potassium carbonate proved to be the better base, giving diarylacetylenes in high yields (Table 10.8). 

We have seen (Sec. 14.24) that an alkyl halide is conveniently detected by the precipitation of insoluble silver halide when it is warmed with alcoholic silver nitrate. The reaction occurs nearly instantaneously with tertiary, allyl, and benzyl bromides, and within five minutes or so with primary and secondary bromides. Compounds containing halogen joined directly to an aromatic ring or to a doubly-bonded carbon, however, do not yield silver halide under these conditions. Bromo-benzene or vinyl bromide can be heated with alcoholic AgN03 for days without the slightest trace of AgBr being detected. In a similar way, attempts to convert aryl... 

Fused thiophene-cyclopentanes 39 can be synthesized by intramolecular dual C-H activation of 2-arylthiophenes 38 (Scheme 17, Table 10) [67]. The cross-coupling proceeds moderately well using palladium(ll) acetate as catalyst and silver (1) carbonate as oxidant. When the thiophene moiety is not substituted at positimi 2, homocoupling occurs easily. However, in contrast to this direct route, a two-step sequence consisting of a prior bromination and a subsequent palladium-catalyzed arylation is much more effective forming the fused thiophene-cyclopentane 39 in a yield of 83%. 

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