Aryl trifluoroborates

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],... 

Murphy JM, Tzschucke CC, Hartwig JF (2007) One-pot synthesis of arylboronic acids and aryl trifluoroborates by Ir-catalyzed borylation of arenes. Org Lett 9 757... 

Instead of aryl halides, arenediazonium salts are also excellent arylating agents in the Suzuki coupling, although more hindered arylboronic acids did not react The reaction is catalysed by several sources of ligand-free palladium such as Pd(OAc)2, Pd2(dba)3 and Pd/C at room temperature in dioxane without any added base [98]. Use of potassium aryl trifluoroborate salts also allowed the introduction of more sterically hindered aryl groups [99]. 

The first Pd -catalyzed P-arylation of dialkyl phosphites with arylboro-nic acids or aryl trifluoroborates has been carried out by using palladium acetate, the rigid bidentate ligand dmphen (dmphen = 2,9-dimethyl-1,10-phenanthroline), and without addition of a base or acid. The arylated phosphonates (512) were isolated in 44-90% yields (Scheme 132). 

PCy3 also proved to be the ligand of choice for a wide-ranging study of Suzuki couplings of heteroaryl chlorides and heteroarylboronic acids (Equation 2.18) where aryl boronate esters and aryl trifluoroborates could also be employed [33]. 

Copper-mediated arylation of various nucleophilic groups, including phenols, anilines and thiophenols, with boronic acids has been reported by several groups (Scheme 2.178) As an organometallic species is used as the aryl donor, the copper(II) acts as a net oxidant, and must be used stoichiometrically. Inclusion of an oxidant, such as molecular oxygen as in the Wacker reaction (Section 6.1) can allow a system that is catalytic in copper. Aryl trifluoroborates have also been used as the aryl donor. ... 

More recently, it was reported that potassium alkenyl and aryl-trifluoroborates undo go similar addition to enones in the presence of a Rh(I) catalyst (Schemed) (ii). It was found that the reaction proceeds more rapidly than with the corresponding boronic acids, and the choice of catalyst ligand does not signifrcantly influence the overall catalyst efficiency. 

Batey and Quach have investigated alkenyl and aryl trifluoroborate salts as coupling agents. Their earlier publication [47], disclosed a C-O cross-coupling protocol... 

Synthesis of Alkyl/Aryl Trifluoroborates Using KHF2.749... 

Sanford has also described a fluorination of aryl trifluoroborates using Cu(OTf)2 with potassium fluoride as the source of fluoride (Scheme 15.73). The copper is proposed to play a dual role in not only being the mediator for the aryl-F coupling but also as the oxidant to access a proposed Cu (aryl)(F) intermediate. The yields tend to be somewhat lower than the (tBuCN)2CuOTf mediated protocol, though several heterocycles are reported as successful substrates in this system. 

In 2008, Ackerman and co-workers applied these methods for the synthesis of fully decorated triazoles [259]. Their studies revealed that a single copper system based on Cul and DMEDA L26 ligand was able to catalyze in a one-pot procedure the arylation of sodium azide and the subsequent in situ arylation in position 5 of the resulting triazole, via a sustainable C-H functionalization. Based on the same type of strategy, various organo-[l,2,3]-triazol-l-aryl-trifluoroborates were recently prepared by Ham and subsequently used as substrates in Suzuki-Miyaura cross-coupling reactions [260],... 

Scheme 3.20 Synthesis and cross-coupling of functionalized potassium aryl trifluoroborates.

Batey has reported Pd-catalyzed cross-coupling reactions of tetraalkylammo-nium aryl trifluoroborate salts bearing functional groups such as acetyl, nitro and chloro with functionalized aryl bromides under mild conditions 35]. These tetra-alkylammonium organofluoroborates are prepared from the respective boronic acids using counter ion exchange protocol. They are air and moisture stable and are soluble in various organic solvents (Scheme 3.21). 

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