Ionic Liquid-Promoted Regioselective Catalysis by Palladium

Palladium catalysed arylation and vinylation of olefins by aryl or vinyl halides, that is, the Heck reaction, is now one of the most important tools in synthetic chemistry. However, the reaction works well generally only with olefins bearing electron-withdrawing substituents such as CO2R, CN, and Ph. With electron-rich olefins such as acyclic enol ethers, silanes, and enol amides, a mixture of linear and branched regioisomers usually results, thus limiting the wider applicability of the reaction in synthetic chemistry (Scheme 1). This is so, because it is believed that the reaction proceeds via two pathways, one ionic leading to the branched product and the other neutral giving rise to the linear variant. Scheme 1 shows the key intermediates involved in the two pathways proposed in the literature. The problem can be addressed by using aryl triflates or stoichiometric halide scavengers [Ag(OTf) and Tl(OAc) being most often used]. However, triflates are thermally labile and in general not commercially available and the inorganic additives create new problems, i.e. waste salts, toxicity and added cost.