PALLADIUM-CATALYZED COUPLING OF ARYLSTANNANES WITH ORGANIC SULFONATES- A COMPREHENSIVE STUDY

The effect of ligands and lithium chloride on the rates of the palladi um catalyzed coupling between organic triflates and arylstannanes was studied. The dependence of the rate on the ligand is similar to the on e previously reported for the coupling of vinylstannanes, but in the p resent case triphenylarsine is shown to be superior to both triphenylp hosphine and tri(2-furyl)phosphine. The effect of added chloride is co mplex and varies depending on solvent and ligand used. Ortho-substitut ed arylstannanes tend to transfer alkyl moieties to a substantial exte nt , and therefore rates and efficiencies of aryl vs alkyl transfer we re quantitated. When ortho substituents that are potentially coordinat ing to tin are used, no rate acceleration in the alkyl transfer proces s was observed, which is in contrast with two recently reported studie s that suggest nucleophilic assistance at tin to be important in the t ransmetalation step. An important side reaction in the coupling of poo rly reactive vinyltriflates and most aryltriflates is the Pd-induced h omocoupling of the stannane to form biaryls. The experimental factors that control this process were evaluated.