Potassium alkyl trifluoroborate

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]. 

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. 

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. 

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