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

The effect of ligands and lithium chloride on the rates of the palladium catalyzed coupling between organic triflates and arylstannanes was studied. The dependence of the rate on the ligand is similar to the one previously reported for the coupling of vinylstannanes, but in the present case triphenylarsine is shown to be superior to both triphenylphosphine and tri(2-furyl)phosphine. The effect of added chloride is complex and varies depending on solvent and ligand used. Ortho-substituted arylstannanes tend to transfer alkyl moieties to a substantial extent , and therefore rates and efficiencies of aryl vs alkyl transfer were quantitated. When ortho substituents that are potentially coordinating to tin are used, no rate acceleration in the alkyl transfer process was observed, which is in contrast with two recently reported studies that suggest nucleophilic assistance at tin to be important in the transmetalation step. An important side reaction in the coupling of poorly reactive vinyltriflates and most aryltriflates is the Pd-induced homocoupling of the stannane to form biaryls. The experimental factors that control this process were evaluated.