Cross-coupling reactions nucleophile transmetallation

The following mechanism is suggested for the cross-coupling of alkenylsilanes. Nucleophilic attack of a fluoride ion to the silicon atom of alkenylsilanes is first assumed to afford a pentacoordinate silicate and enhance both nucleophilicity of the silicon-substituted carbon and Lewis acidity of silicon to assist transmetalation effectively through a four-centered transition state (Scheme 2). Lewis acidity on silicon is critical as evidenced by the fact that hexacoordinate pentafluorosilicates that are fully coordinated and thus should have sufficient nucleophilicity are much less effective for the cross-coupling reaction (Eq. 2, vide supra). 

The catalytic cycle for the Suzuki cross-coupling reaction involves an oxidative addition (to form RPd(II)X)-transmetalation-reductive elimination sequence. The transmeta-lation between the RPd(lI)X intermediate and the organoboron reagent does not occur readily until a base, such as sodium or potassium carbonate, hydroxide or alkoxide, is present in the reaction mixture. The role of the base can be rationalized by its coordination with the boron to form the corresponding ate-complex A, thereby enhancing the nucleophilicity of the organic group, which facilitates its transfer to palladium. Also, the base R O may activate the palladium by formation of R-Pd-OR from R-Pd-X. 

One can dso make use of the directing effect of the 3-bromine to chlorosulfonate directly in the 2 position (3-bromotiiiophene 35). Amination with ammonia give the free sulfonamide, while rm-butyl amine treatment gives a protected sulfonamide that can be used for the cross coupling reaction and nucleophilic displacements already described for the other isomers. The bromide 36 can also be used to direct transmetalation to the 3 position for the introduction of a variety of electrophiles. 

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