Trifluoroborates, alkenyl coupling

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. 

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 use of trifluoroborate salts in couplings, which are very easily prepared from boronic acids by reaction with KHF2, is a useful variant of the Suzuki reaction. These salts have the advantage of enhanced (often considerably) stabihty compared to boronic acids and this is particularly notable for alkenyl compounds, which can be stored for a considerable time. The coupling conditions are very similar to those for boronates and are apphcable to a wide range of heterocyclic substrates, ... 

Scheme 2.26 Suzuki-Miyaura cross-coupling reactions of alkenyl substrates involving trifluoroborate salts.

Batey and Quach have investigated alkenyl and aryl trifluoroborate salts as coupling agents. Their earlier publication [47], disclosed a C-O cross-coupling protocol... 

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

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. 

Synthesis and Palladium-catalyzed Cross-coupling Reactions of Functionalized Alkenyl Trifluoroborates... 

Cross-coupling reactions of functionalized alkenyl trifluoroborates 124 with 4-bro-mobenzonitrile in the presence of catalytic amounts of PdCl2(dppf) has been studied by Molander [97,98], Herein functionalized 4-cyanostyrenes of type 125 were obtained in satisfactory yields (Scheme 3.64). 

Scheme 3.64 Preparation and cross-coupling reactions of functionalized alkenyl trifluoroborates.

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