Enantioselective Synthesis of Six-Membered Palladacycles Having Metal-Bound Stereogenic Carbons: Isolation and Reactivity of Palladacycles Containing Readily Accessible β-Hydrogens

Five enantiopure palladacycles containing palladium bonded to a stereogenic carbon and an N-coordinated oxindole were synthesized by the reaction of alkenyl aryl triflates 2 and 9 with Pd(0) bisphosphine complexes. Two palladacyclic complexes, 3β and 10α, were characterized by single-crystal X-ray crystallography. The reactivity of neutral palladacycles 3β and 10β was studied in detail. These unusual palladium alkyls, which have three accessible β-hydrogens, are thermally stable at temperatures as high as 120 °C. At higher temperature, or at room temperature in the presence of weak acids, these complexes epimerize at the stereogenic carbon bonded to palladium. The mechanism of the acid-promoted epimerization was studied in detail. During this epimerization, cationic palladium alkyls 13/14 and 33 and cationic palladium hydride alkene complexes 31 and 32 are in rapid equilibrium.