σ-Alkylpalladium intermediates in intramolecular Heck Reactions : isolation and catalytic activity

The isolation of σ-alkylpalladium Heck intermediates, possible when β-hydride elimination is inhibited, is a rather rare event. Performing intramolecular Heck reactions on Nallyl-2-halobenzylamines in the presence of [Pd(PPh3)4], we isolated and characterized a series of stable bridged palladacycles containing an iodine or bromine atom on the palladium atom. Indolyl substrates were also tested for isolation of the corresponding com- plexes. X-ray crystallographic analysis of one of the indolyl derivatives revealed the presence of a five-membered palladacycle with the metal center bearing a PPh3 ligand and an iodine atom in a cis position with respect to the nitrogen atom. The stability of the σ-alkylpalladium complexes is probably a consequence of the strong constraint resulting from the bridged junction that hampers the cisoid conformation essential for β-hydride elimination. Subsequently, the thus obtained bridged five-membered palladacycles were proven to be effective precatalysts in Heck reactions as well as in cross-coupling processes such as Suzuki and Stille reactions.