Potassium trifluoroborates,

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%). 

Enantiomeric excesses of 81-91% were obtained with the boronic acids and of 73-81% with potassium trifluoroborates. 

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. 

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. 

Molander has effected a Suzuki reaction between 7-bromoindole and potassium trifluoroborates (e.g. 155) to give the corresponding coupled product 156 [163]. 

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. 

For Petasis borono-Mannich reactions of formaldehyde, methylation of the amine reportedly occurs as a side-reaction, presumably by iminium ion reduction [19]. The hydride source under these conditions may be formic acid, as in the Eschweiler-Clarke methylation reaction. This problem was overcome by the use of potassium trifluoroborate salts and Lewis acids in toluene at 90 °C (the reaction in highly polar solvents such as acetonitrile, DMF and DM SO gives the reduction product). 

B) Copper-mediated fluorination of aiyl potassium trifluoroborates using potassium fluoride. 

The utility of potassium organotrifluroborate salts in natural product synthesis was demonstrated by Molander through a formal total synthesis of the macrolide oximidine II. Alkyne 68 was selectively hydroborated with di(isopropylprenyl)borane and then converted to the potassium trifluoroborate salt 69. Formation of the macrocyclic ring was achieved through intramolecular Suzuki coupling of 69, which generated 70 in a 42% yield. Intermediate 70 was transformed to 71 in two steps to complete the formal synthesis. 

Asymmetric SOMO vinylation of aldehydes was achieved with the vinyl potassium trifluoroborate salt of terminal alkenes, such as benzylvinylene, under SOMO-conditions, using catalyst 1 and CAN as one-electron oxidizing agent (Scheme 39.10) [16]. In this reaction the radical intermediate of the addition product was oxidized by a second equivalent of CAN, providing the cationic intermediate, from... 

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. 

Organo-SOMO catalysis has been successfully exploited to achieve the first asymmetric a-vinylation of aldehydes using vinyl trifluoroborate salts and the commercial Kim and MacMillan catalyst 191. " ° Vinyl potassium trifluoroborate salts participate in enantioselective and regioselec-tive carbon-carbon bond formation with the aldehyde-derived radical cation 192 to form a (3-borato-stabilized radical 193 (Scheme 25.90), which in the presence of a suitable oxidant will undergo rapid electron transfer to render the (3-cation (not shown). Subsequent Peterson elimination of the trifluoroborate group with tran.s-selectivity followed by iminium hydrolysis of 194 would then reveal an optically enriched R-( )-vinyl aldehyde (e.g., 195). 

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

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