Potassium aryltrifluoroborate salts

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. 

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

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. ... 

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

Table 13. Synthesis of biatyls from diaryliodonium salts and potassium aryltrifluoroborates [52]...

The advantage of this method was the low cattilyst loading involved. In fact, just 0.25 mol% of rhodium dimer catalyst afforded the expected benzophenone with full conversion (see Scheme 7.16). So, a straightforward synthetically useful route to congested benzophenone frameworks starting from readily available aryl aldehydes and potassium aryltrifluoroborates was described. Neutral conditions allow the direct formation of di-, tri-, and even tetra-orf/ro-substituted benzophenones under operationally simple conditions in very good yields, thanks to the use of a stable phosphonium salt of P(fBu)3. 

Thiophenediazonium salts 163 can be utiUzed as electrophilic coupling partners for differently functionaUzed alkenes 164 via a Matsuda-Heck (Scheme 64, left) as well as various potassium aryltrifluoroborates via Suzuki-Miyaura (Scheme 64, right) cross-coupling reactions giving rise to 3-substituted thiophenes 165 and 166... 

Molander has published a series of papers demonstrating the utility of potassium alkyl, alkenyl-, alkynyl-, and aryltrifluoroborates in palladium coupling reactions. The crystallinity and air-stability of these trifluoroborate salts make the use of these an interesting alternative to the use of boronic acids or esters. Good yields have been obtained in several related palladium coupling processes, which are most easily classified as Suzuki couplings. The broad applicability of this process is demonstrated by the production of 58 [40], 59 [41], and 60 [42]. 

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