Stille coupling ligand influences

It is reported in literature that additives like LiCl or Cu(I) salts can have a dramatic influence on the coupling. The copper effect in Stille coupling reactions was investigated by Farina and Liebeskind and coworkers. For instance in the reaction of iodobenzene and vinyltributyltin in dioxane at 50 °C catalyzed by Pd2(dba)3 in presence of a strong ligand like PPhs, it was found that the addition of 2 molar equivalents of Cul per mol of catalyst led to a > 100 fold increase in reaction rate. 

As a mechanistic hypothesis, the authors assumed a reduction of the Fe(+2) by magnesium and subsequent coordination of the substrates, followed by oxidative coupling to form alkyl allyl complex 112a. A ti—c rearrangement, followed by a syn p-hydride elimination and reductive elimination, yields the linear product 114 with the 1,2-disubstituted ( )-double bond (Scheme 29). This hypothesis has been supported by deuterium labeling experiments, whereas the influence of the ligand on the regioselectivity still remains unclear. 

Cross-coupling Reactions. Many cross-coupling reactions are critically influenced by the ligand used to form a reactive complex with the metal catalyst. Often, the use of di-iert-butyl(methyl)phosphine as a ligand leads to positive results. As an example, the Stille cross-coupling between a vinyl stannane and a primary alkyl bromide with -hydrogens, and therefore potentially subjected to undesired /3-hydride elimination, gives the best results with di-ferf-butyl(methyl)phosphine (eqs 1 and 2) ... 

Pd(PPh3)4 is a commonly used catalyst for the Stille cross<oupling reaction. However, several studies demonstrated that the Pd/phosphine ratio exerts a dramatic influence on the kinetics, in the fact that an excess of ligand would inhibit the coupling by slowing the formation of the coordinatively unsaturated PdLj active species. Pd(PPh3)4 can be replaced by the correct combination of Pdjdbaj/phos-... 

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