Kinetic Studies of Migratory Insertion Reactions at the (1,3-Bis(diphenylphosphino)propane)Pd(II) Center and Their Relationship to the Alternating Copolymerization of Ethylene and Carbon Monoxide

Abstract

The quantitative details of three migratory insertion processes relevant to the copolymerization of ethylene and CO by the highly active (dppp)Pd(II)-based catalyst have been examined. Cationic Pd(II) intermediates generated at low temperatures and characterized by 1 H, 13 C and 31 P NMR spectroscopy include (dppp)Pd(C 2 H 4 )(R) + (R = −CH 3, −CH 2 CH 3 ), 6 and 7; (dppp)Pd(CO)(R) + (R = −CH 3, −CH 2 CH 3 ), 8 and 11; and (dppp)Pd(C 2 H 4 )(COCH 3 ) +, 13 . Kinetic studies of the migratory insertion reactions of these species established the following activation parameters: 6 (Δ G ⧧ = 16.6(1) kcal/mol (−45.6 °C), Δ H ⧧ = 15.2(7) kcal/mol, Δ S ⧧ = −6.2(2.9) eu), 7 (Δ G ⧧ = 16.3(1) kcal/mol (−33.7 °C), Δ H ⧧ = 15.9(8) kcal/mol, Δ S ⧧ = −1.6(3.7) eu), 8 (Δ G ⧧ = 14.8(1) kcal/mol (−81.7 °C), Δ H ⧧ = 14.8(7) kcal/mol, Δ S ⧧ = 0.1(3.0) eu), 11 (Δ G ⧧ = 13.4(1) kcal/mol (−94.2 °C)), 13 (Δ G ⧧ = 12.3(1) kcal/mol (−103.1 °C)). On the basis of the relative binding affinities of CO and C 2 H 4 to the Pd(II) center and the relative rates of alkyl to CO and alkyl to ethylene migratory insertion reactions, it can be estimated that sequential ethylene insertions occur only once in ∼10 5 turnovers, thus corroborating the exceptionally high fidelity of the alternating copolymerization.