Diametrically Opposite Trends in Alkene Insertion in Late and Early Transition Metal Compounds:  Relevance to Transition-Metal-Catalyzed Polymerization of Polar Vinyl Monomers

Variable-temperature ¹H NMR studies of the reaction of cationic (α-diimine)Pd-alkyl complexes with alkenes are presented. The studies reveal that vinyl bromide coordinates to the Pd(II)−Me complex followed by migratory insertion and β-bromo elimination, to generate free propene. Propene further reacts to give β-agostic Pd(II)-tert-butyl species. From the reactions with vinyl bromide, stable chloro-bridged dicationic Pd complex was isolated and characterized. For a series of alkenes (CH₂CHX), the rate for migratory insertion decreases as follows: X = CO₂Me > Br > H > Me.