Organotin reagents transmetalation

In the direct coupling reaction (Scheme 30), it is presumed that a coordinatively unsaturated 14-electron palladium(o) complex such as bis(triphenylphosphine)palladium(o) serves as the catalytically active species. An oxidative addition of the organic electrophile, RX, to the palladium catalyst generates a 16-electron palladium(n) complex A, which then participates in a transmetalation with the organotin reagent (see A—>B). After facile trans- cis isomerization (see B— C), a reductive elimination releases the primary organic product D and regenerates the catalytically active palladium ) complex. 

There has been controversy on the stereochemistry of the transmetallation of alkylmetals. Transmetallation from Hg to Pd is reported to proceed in THF with retention of configuration [27]. By contrast, cross-coupling reactions of organotin reagents are assumed to take place with inversion in HMPA [28] but with retention in toluene [29]. We consider that the organosilicon-based coupling reaction would be a useful model for examining the stereochemistry of the transmetallation process. 

The mechanism of the coupling was not explored in detail, but likely involves ligand exchange at the rhodium center and coordination by the pyridine substrate, directed cyclometalation, transmetalation of the organotin reagent, and reductive elimination to afford the ortho arylated products (Scheme 28). The nature of the reoxidation process is more equivocal. Trichloroethylene is generated during the reaction when 1,1,2,2-tetrachloroethane is chosen as solvent, which may be... 

A variety of substituents R can be transferred from tin, including typically unre-active alkyl substituents. The coupling proceeds much more slowly in the absence of fluoride, which appears to play at least two useful roles in this chemistry (i) it activates the organotin reagent for transmetallation and (ii) it produces insoluble BusSnF, which can be easily separated from the product. Difficulties in eliminating tin impurities are commonly encountered during purifications of Stille reactions [78]. 

Open and closed transition states proposed for transmetallation from organotin reagents to palladium(ll). 

The transmetallation of oxidative addition products of alkenyl halides and zerovalent palladium species by organotin reagents is an important step in Stille type reactions and its mechanism depends on many variables. A recent review on this topic accounts for transmetallation of alkenylpalladium(ii) species.The involvement of 14-electron T-shaped species [PdX(R)(L)] in Stille reactions involving alkenyl and other substrates has been refuted as unrealistic. Instead, associative ligand substitution at palladium is advocated, which has been corroborated by full and quantitative evaluation of kinetic data as well as MO considerations. ... 

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