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.