Aryltrifluoroborate salts

Azolecarbene ligands are shown to be favorable to Suzuki coupling involving polyfluori-nated aryltrifluoroborate salts (e.g., C6F5BF3K) catalyzed by (Ph3P)2PdCl2 under anaerobic conditions. 

Recently, Zhang and coworkers reported a direct palladium-catalyzed C-2 arylation of indoles with potassium aryltrifluoroborate salts [204]. Remarkably, the direct arylation took place at room temperature when acetic acid was used as the solvent. 

Phenylboronic acid (17) and its derivatives are widely used. Boronic acids are sometimes difficult to purify because they undergo cyclotrimerizadon with loss of water to form boroxines. On the other hand, organotrifluoroborate salts 18 are easily prepared, purified, and handled [23]. Aryltrifluoroborate salts 18 are prepared by the reaction of arylboronic acid with HF and base [24]. Alkenyltrifluorobo-rates 19 are prepared by hydroboration of 1-alkynes, followed by treatment with KHF2 [25]. 

The use of potassium aryltrifluoroborate salts as useful bench-stable alternatives to arylboronic acids has been demonstrated in O—H bond arylations (vide supra),... 

In a later study, Zhao reported the arylation of indolizines with aryltrifluoroborate salts and used this same Pd(OAc)2/AgOAc catalyst system to affect C3-alkynylation with phenylpropiolic acids via a decarboxylative pathway. ... 

Potassium aryltrifluoroborate salts are excellent coupling partners for free or protected NH-indoles at the C-2 position the palladium- and copper-catalyzed transformation takes place under mild conditions, in air, at room temperature. Furthermore, conditions were developed for the coupling of indoles with aryl boronic acids, employing Pd(OAc)2 as the catalyst and a copper salt or O2 as the oxidant. ... 

Direct Arylation ofindolizines with Aryltrifluoroborate Salts. 

SCHEME 1.3 Synthesis of alkyl aryl ethers using aryltrifluoroborate salts [7],... 

SCHEME 2.30 Oxidation of aryltrifluoroborate salts using Oxone [3]. 

Copper compounds also promoted the coupling between aryltrifluoroborate salts and alcohols (Scheme 2.51) [76]. A vast array of primary, secondary, benzyl, and aUyUc alcohols were arylated using a catalyst system comprised of copper acetate and DMAP. Heterocyclic substrates were also well tolerated by this approach. The reactions were... 

This has been established for several organoboron compounds. Perhaps the most attractive are the aryltrifluoroborate salts due to their stabihty [77]... 

Aryltrifluoroborate salts are significantly more stable than boronic acids or boronates. Will they still react in N-vinylation reactions ... 

A number of successful coupling reactions between aryltrifluoroborate salts with nitrogen nucleophiles have been reported [127-129]... 

Aryltrifluoroborates are extremely valuable compounds for synthetic applications, and the vast majority of aryltrifluoroborates have been prepared by treatment of the arylboronic acids with KHF [37], This method typically generated good to excellent yields of the aryltrifluoroborate salts and was tolerant to a wide range of functional groups. Even arylboronates were effectively converted into the ArBE K species upon tfeatment with KHFj in a biphasic solvent system [38],... 

Can the iridium chemistry tolerate substrates with primary alcohols Need to generate an aryltrifluoroborate salt Yes, a version of the iridium-catalyzed C—H borylation is tolerant to the presence of primary alcohols [75] A convenient precursor for the direct synthesis is not readily available. Try generating an arylboronic acid or arylboronate, and treat it with a solution of KHP [37, 38]. For arylboronates, a 4 1 THF/water mixture was successful [38]... 

Another approach to the conversion of aryltrifluoroborate salts into aryl fluorides also used a common copper salt to promote the reactions however, this chemistry used an electrophilic source of fluorine (Scheme 7.58) [84]. The most effective source of F was found to be A-fluoro-2,4,6-trimethylpyridinium ttiflate. Similar to the authors previous work with arylstannanes, the key to the synthesis was prestirring the F source and the copper complex prior to the introduction of the aryltrifluoroborate salts. Using this system, a wide range of aryltrifluoroborate salts bearing ethers, halogens, aldehydes, ketones, amides, and esters were converted into aryl fluorides. 

Example 7.17 Palladium-Catalyzed Fluorination of Aryltrifluoroborate Salts [95],... 

Scheme 26 Copper acetate-catalyzed coupling of arylboronic acids or potassium aryltrifluoroborate salts with N-nucleophiles...

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