Aryl potassium trifluoroborate

Besides the one-pot process described above, the White Reagent catalyzes a chelate-controlled oxidative Heck arylation between a wide range of a-olefins and organoborane compounds in good yields and with excellent regio-and stereoselectivities (Figure 6). Unlike other Heck arylation methods, no Pd-H isomerization is observed under the mild reaction conditions. Aryl boronic acids, styrenylpinacol boronic esters, and aryl potassium trifluoroborates (activated with boric acid) are all compatible with the general reaction conditions. 

Scheme 15.67 Pd-mediated fluorination of aryl potassium trifluoroborate salts.

The optimal reaction conditions were applied with 59d in the addition of various aryl boronic acids and potassium trifluoroborates to several cyclic and acyclic enones (Fig. 8). Arylboronic acids added to cyclic enones in high yields (89-97%) and with good to excellent selectivities (85-98% ee). Under these conditions, the potassium trifluoroborate reagents reacted at faster rates, but with slightly lower selectivities (83-96% ee). The reactions of acyclic enones with aryl boron reagents gave also excellent yields (83-96%). 

Regarding the use of other metals for this transformation, Shirai and co-workers reported that a system constituted by palladium(II) complex [Pd(p-Cl)(r -aUyl)]2 and thioether-imidazolium chloride 19 achieved the arylation of aldehydes with boronic acids [33] and potassium trifluoroborates in good to excellent yields (Scheme 7.5) [34], More recently, Buffard and Itami showed that a NKcod) / IPr-HCl system could catalyse the reaction of arylboronate esters and inactivated aldehydes and ketones (Scheme 7.5) [35]. 

The Suzuki Coupling, which is the palladium-catalysed cross coupling between organoboronic acid and halides. Recent catalyst and methods developments have broadened the possible applications enormously, so that the scope of the reaction partners is not restricted to aryls, but includes alkyls, alkenyls and alkynyls. Potassium trifluoroborates and organoboranes or boronate esters may be used in place of boronic acids. Some pseudohalides (for example triflates) may also be used as coupling partners. 

Potassium aryl- and 1-alkenyltrifluoroborates (ArBFsK and RBF3K) are easily prepared from organoboronic acids or esters. In general, the trifluoroborates have greater air stability and greater nucleophilicity " when compared to the... 

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]. 

In 2009 Castanet, Sauthier and their colleagues reported on a useful protocol for aryl vinyl ketone synthesis [37]. The carbonylative cross-coupling of potassium vinyl trifluoroborate or 2,3,6-trivinylcycloboroxane with aryl iodides afford... 

Another solution is to employ trifluoroborate salts 2.215, which can be prepared from the boronic acid by treatment with potassium hydrogen fluoride (Scheme 2.74). They can also be prepared from alkyl bo-ranes in a similar way (Scheme 2.75). These stable salts can be efficiently employed in Suzuki coupling reactions (Scheme 2.T6)P One application is in the synthesis of the unusual amino acid, trityrosine 2J26 (Scheme 2.77). In this synthesis an aryl diiodide 2.222 was coupled with a trifluoroborate salt 2.224 that had been formed by Miyaura borylation of iodide 2.223 followed by KHF2 treatment. Global debenzylation of coupling product 2.225 gave the product 2.226. Another example of the use of a trifluoroborate salt can be found in Scheme 2.88. 

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. 

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. 

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

---