Trifluoroborates, cross-coupling reactions

Scheme 2.24 Cross-coupling reactions of trifluoroborate salts to provide biaryls.

Scheme 2.25 Suzuki-Miyaura cross-coupling reactions of heteroaromatic trifluoroborates.

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

Typically, boronic acids/boronates, dialkylboranes, or trifluoroborates can all be used for Suzuki-Miyaura cross-coupling reaction. The following section details several protocols reported in the literature for the synthesis of these boron congeners. 

In 2008, Ackerman and co-workers applied these methods for the synthesis of fully decorated triazoles [259]. Their studies revealed that a single copper system based on Cul and DMEDA L26 ligand was able to catalyze in a one-pot procedure the arylation of sodium azide and the subsequent in situ arylation in position 5 of the resulting triazole, via a sustainable C-H functionalization. Based on the same type of strategy, various organo-[l,2,3]-triazol-l-aryl-trifluoroborates were recently prepared by Ham and subsequently used as substrates in Suzuki-Miyaura cross-coupling reactions [260],... 

Synthesis of Functionalized Trifluoroborates and their Palladium-catalyzed Suzuki-Miyaura Cross-coupling Reactions... 

Batey has reported Pd-catalyzed cross-coupling reactions of tetraalkylammo-nium aryl trifluoroborate salts bearing functional groups such as acetyl, nitro and chloro with functionalized aryl bromides under mild conditions 35]. These tetra-alkylammonium organofluoroborates are prepared from the respective boronic acids using counter ion exchange protocol. They are air and moisture stable and are soluble in various organic solvents (Scheme 3.21). 

Scheme 3.22 Palladium-catalyzed cross-coupling reactions of perfluoroorgano trifluoroborates.

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.

Scheme 3.83 Suzuki cross-coupling reactions of functionalized alkyl trifluoroborates with triflates.

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. 

Quach and Batey299 reported the coupling of primary amines with phenylboronic acids catalyzed by Cu(OAc)2 (10 mol%) without base or ligand but in the presence of 4 A molecular sieves and air in CH2C12 solvent (equation 73). Reactions of potassium phenyl-trifluoroborate also occurred, but in lower yields than reactions of boronic acids. They showed that these reactions occur with a variety of functional groups on the amine, including alkenes, esters, ketones and ketals. a-Amino acid derivatives underwent reaction without detectable epimerization. Anilines were poorer cross-coupling partners under these conditions than were primary and secondary cyclic aliphatic amines. 

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