Palladium-promoted reaction Suzuki-Miyaura

Bromo- and iodoimidazoles are useful intermediates for further functionalization. 4(5)-Aryl- I //-imidazoles 57 can be efficiently and selectively prepared by palladium-catalyzed Suzuki-Miyaura reaction of commercially available 4(5)-bromo-l//-imidazole 56 with arylboronic acids under phase-transfer conditions, which then underwent highly selective palladium-catalyzed and copper(I) iodide mediated direct C-2-arylation with a variety of aryl bromides and iodides under base-free and ligandless conditions to produce 2,4(5)-diaryl-l//-imidazoles 58 in modest to good yields <07JOC8543>. A new procedure for the synthesis of a series of substituted 2-phenylhistamines 60 utilizing a microwave-promoted Suzuki... 

Under the reaction conditions, phenylacetylene was found to be a much more reactive coupling partner than arylboronic acids in the analogous Suzuki-Miyaura coupling, as in addition to the desired product (38), alkynylation and further addition reactions occurred with a variety of transient palladium(II) species (Scheme 27). Despite these undesired side reactions, Catellani was able to fine-tune the reaction conditions to form predominantly product 38 or 39. The formation of the desired product 38 (and suppression of product 39) is promoted by acceleration of norbomene carbopalladation by KOAc [47] and by using an excess of alkyl halide affording several structurally similar unsymmetrical alkyne products in good yields (Scheme 28). 

Polyurea-encapsulated palladium catalysts promote the phosphine-free Mizoroki-Heck reaction, which results in a high yield of cinnamates 46 (Scheme 86). The catalyst, which is easily recovered by filtration, is also applicable to the Suzuki-Miyaura and Stifle coupling reactions. 

Due to their widespread application in the Suzuki-Miyaura reaction, arylboronic acids are attractive aryl transfer precursors. However, in the zinc-promoted reaction, an excess of Et2Zn (up to 7equiv.) must be added in order for efficient transmetallation to occur. The application of a reactive aryl metal species necessitating only a catalytic amount of metal would thus be advantageous. In recent years, several rhodium-catalyzed enantioselective protocols have been described, as well as processes involving palladium and nickel. 

The three basic steps in the palladium-catalysed Suzuki-Miyaura reaction involve oxidative addition, transmetalation, and reductive elimination. A systematic study of the transmetalation step has found that the major process involves the reaction of a palladium hydroxo complex with boronic acid, path B in Scheme 3, rather than the reaction of a palladium halide complex with trihydroxyborate, path A. A kinetic study using electrochemical techniques of Suzuki—Miyaura reactions in DMF has also emphasized the important function of hydroxide ions. These ions favour reaction by forming the reactive palladium hydroxo complex and also by promoting reductive elimination. However, their role is a compromise as they disfavour reaction by forming of unreactive anionic trihydroxyborate. A method for coupling arylboronic acids with aryl sulfonates or halides has been developed using a nickel-naphthyl complex as a pre-catalyst. It works at room temperature in toluene solvent in the presence of water and potassium carbonate. ... 

Palladium nanoparticles and other heterogeneous catalysts are often invoked as catalysts in cross-coupling processes [194, 195). Direct evidence in support of an oxidative-addition-promoted leaching mechanism has been recently obtained in the Suzuki-Miyaura reactions with nanoparticle catalysts, suggesting that true surface catalysis remains largely unknown with these heterogeneous catalysts [196]. 

A detailed study of the transmetaUation in the Suzuki-Miyaura reaction by the group of Amatore and Jutand shows that hydroxide [261] and fluoride anions [262] form the key trans-[ArPdX(L)2] complexes that react with the boronic acid in a rate-determining transmetaUation. In addition, the anions promote the reductive elimination. Conversely, the anions disfavor the reaction by formation of nonreactive anionic [Ar B(OH)3 X ] (7t=l-3). Countercations M" " (Na" ", K" ", and Cs+) of anionic bases in the palladium-catalyzed Suzuki-Miyaura reactions decelerate the transmetaUation step in the following decreasing reactivity order nBu4NOH > KOH > CsOH > NaOH this is due to the complexation of the hydroxy ligand in [ArPd(OH)(PPh3)2] by M+[263]. 

A fluorous 1,4-disubstituted [l,2,3]-trizole 26 was prepared from fluoroalkyl tosylate to replace the air- and moisture-sensitive phosphine ligand as a recyclable ligand for the palladium-catalyzed Suzuki-Miyaura reaction and Mizoroki-Heck reaction (Scheme 7.23) [36]. As expected, the fluorous ligand, together with palladium acetate, promoted these coupling reactions and the fluorous ligand was conveniently... 

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