Alkenyl potassium trifluoroborate

A formal total synthesis of oximidine II was achieved by G.A. Molander et al., using an intramoiecuiar Suzuki-type cross-coupiing between an alkenyl potassium trifluoroborate and an alkenyl bromide to construct the highly strained, polyunsaturated 12-membered macrolactone core of the natural product. " The stability of potassium trifluoroborates was exploited in order to establish the best conditions for the macrocyclization. 

The Molander group reported a formal total synthesis of oximidine II, in which an intramolecular Suzuki cross coupling between an E-alkenyl potassium trifluoroborate and aZ,Z-dienyl bromide constructed the highly strained 12-membered macrolactone core of the natural product (Fig. 7) [22]. Importantly, the stereochemistry of the starting partners was conserved to deliver the ,Z,Z-conjugated triene in the natural product. 

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. 

The reactions take place under mild conditions with good to high yields. Another important advantage is the use of relatively nontoxic reagents. In the synthesis of the alkenyl fluorides, mixtures of Z/E isomers are obtained. The substrate trifluoroborates can be prepared from the corresponding boronic acids by reaction with potassium hydrogen fluoride/water. ... 

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]. 

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. 

Molander, G. A., Bernard , C. R. 2002. Suzuki-Miyaura cross-coupling reactions of potassium alkenyl-trifluoroborates. J. Org. Chem. 67 8424—8429. 

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