A Highly Strained Planar-Chiral Platinacycle for Catalytic Activation of Internal Olefins in the Friedel-Crafts Alkylation of Indoles

摘要

Platinum(II) and gold(I) catalysis have experienced significant growth over the past five years, as these late transition metals have the ability to catalyze atom economical reactions of unactivated alkynes, olefins, or allenes, creating a significant increase in the molecular complexity of a single synthetic step by using simple starting materials. The catalysts are compatible with most functional groups because of their low oxophilicity, and they are usually robust towards moisture or air. Platinum(II)-olefin complexes are reported to be highly reactive toward outer-sphere attack by nucleophiles, and the resulting platinum(II)-alkyl complexes undergo rapid protonolysis with Bronsted acids rather than β-hydride elimination, which is the preferred pathway for palladi-um(II)-alkyl complexes. In contrast, π-ligand exchange is relatively slow for platinum(II) complexes. Catalysts allowing a more rapid ligand exchange could lead to enhanced activity of this expensive metal and potentially expand the scope of the reaction to additional applications. Due to the reactivity issue and despite considerable progress made in this field, the asymmetric activation of π ligands by gold or platinum complexes is still an area with the potential for development.