Suzuki-Miyaura coupling centers

Suzuki-Miyaura Coupling. The Suzuki-Miyaura coupling reaction is very useful for construction of carbon-carbon bonds under mild conditions. Because it is tolerant of many functional groups, it has been employed widely in the synthesis of many natural products and related compounds. The stereochemistry of such coupling reactions proceeds with retention of configuration at both the C-X and C-(9-BBN) centers, confirmed recently hy independent studies from the groups of Woerpel and Soderquist. ... 

The air-stable phosphine-borane la was evaluated in the Pd-catalyzed Suzuki-Miyaura reaction. Typically, 4-bromoanisole and phenyl boronic acid were efficiently coupled using 1 mol% of PdCOAc) and 2 mol% of ligand la (Scheme 6). The BMes moiety is compatible with the cross-coupling and it actually improves catalytic activity (under the same conditions, PPh gives a notably lower yield). Interestingly, a catalytically competent Pd(0) complex 39 was isolated and fully characterized. The phosphine-borane adopts a new coordination mode besides phosphorus, the Pd center is coordinated by one of the Mes groups at B interaction). 

Palladium NPs confined in hoUow graphite nanofibers exhibit attractive catalytic properties in Suzuki-Miyaura cross-coupling reactions [209]. There also, confinement of NPs at the step-edges facilitates retention of catalytic centers and recycling without any significant loss of activity or selectivity over multiple catalytic cycles. Furthermore, careful comparison of the catalytic properties of Pd NPs either on or in graphite nanofibers reveals that nanoscale confinement of catalysts fundamentally affects the pathways of the Suzuki-Miyaura reaction. The yield and selectivity for the cross-coupled product are altically dependent on the steric properties of the aryl iodide reactant, whereas no effects of confinement are observed for aryl boronic add reactants possessing substituents in different positions. 

The nature of ligands, such as Boxs, phosphines, and N-heterocyclic carbenes (NHC), bound to a transition metal center usually afiects the selectivities and reaction rates in the organic reactions. Styring et al. described the preparation of a Pd (Il)-salen complex immobilized on Merrifield resin (42). The polymer catalyst facilitated the Suzuki-Miyaura cross-coupling reaction in a continuous-flow system (Scheme 7.32) [130]. The reactor continuously afforded biaryl adducts (43) from aryl... 

Thus the proton abstraction mechanism seems to be the favored pathway in the metalation step of transition-metal-catalyzed arylations. This may seem surprising because a relatively weak base as carbonate is taking away an arylic proton, which has very low acidity. The presence of the metal is in this case key to stabilize the development of a negative charge in the carbon center, as proved by the structure of the transition states, where the metal-carbon bond is practically already formed. It is worth mentioning that external bases are key in both direct arylation and the Suzuki-Miyaura cross-coupling, but their roles are completely different. 

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