Transmetallation in the Hiyama Reaction

The lower reactivity of the Si-C bond requires the use of activating reagents to enhance the reactivity of silanes and to promote the Si-Pd transmetallations. Fluoride is the common additive, although other nucleophiles such as hydroxide, metal oxides, and aUcoxides are also effective [30, 293, 294]. Fluoride converts the starting silanes into pentacoordinate fluorosilicates, which are the actual transmetallation reagents. 

Hiyama studied the stereoselectivity of alkyl transmetallation in the [Pd(PPh3)4]-catalyzed reaction of aryl triflates with enantiomerically enriched (S)-l-phenyl-ethyltrifluorosilane in the presence of TBAF (tetrabutylammonium fluoride) [297]. 

In polar solvents or at higher temperatures, the fluorine-silicon bridge would be cleaved to switch the transition-state model to the Sg2(open) (72), thus resulting in inversion. On the other hand, open and cyclic Sg2 mechanisms have been proposed to justify the observed stereochemistry in the y-selective cross-coupling of allylsilanes [298]. Allylic silanolates undergo the palladium-catalyzed Hiyama cross-coupling with aromatic bromides with excellent stereoselectivity through a syn SE transmetallation [299]. 

The Kumada-type couplings catalyzed by copper probably involve Cu(I)/Cu(III) redox catalytic cycles [180, 206, 303-305]. Indeed, the oxidative addition of aryl halides to copper(I) to form well-characterized aryl-Cu(III) complexes has recently been reported [306]. 

The complex [AuCl(tht)] (tht, tetrahydrothiophene) can be used instead of Cu(I) as cocatalyst in the Sonogashira reactions catalyzed by [PdCl2(PPh3)2], although this Au(I) complex is inactive in the absence of palladium [320]. Gold and palladium have also been used as catalysts in the Sonogashira reactions of arenediazonium salts [321]. 

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