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

The quantitative derails 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 H-1, C-13 and P-31 NMR spectroscop y include (dppp)Pd(C2W4)(R)(+) (R = -CH3, -CH2CH3), 6 and 7; (dppp)Pd(CO)(R )(+) (R = -CH3, -CH2CH3), 8 and 11; acid (dppp)Pd(C2H4)(COCH3)(+), 13. Kine tic studies of the migratory insertion reactions of these species establish ed the following activation parameters: 6(Delta G(double dagger) = 16.6(1) kcal/mol(-45.6 degrees C), Delta H-double dagger = 15.2(7) kcal/mol, Delta S-double dagger = -6.2(2.9) eu), 7(Delta G(double dagger) = 16.3(1) kcal/mo l(-33.7 degrees C), Delta H-double dagger = 15.9(8) kcal/mol, Delta S-doubl e dagger = -1.6(3.7) eu), 8(Delta G(double dagger) = 14.8(1) kcal/mol (-81. 7 degrees C), Delta H-double dagger = 14.8(7) kcal/mol, Delta S-double dagg er = 0.1(3.0) eu), 11(Delta G(double dagger) = 13.3(1) kcal/mol (-94.2 degr ees C)), 13(Delta G(double dagger) = 12.3(1) kcal/mol (-103.1 degrees C)). On the basis of the relative binding affinities of CO and C2H4 to the Pd(II ) center and the relative rates of alkyl to CO and alkyl to ethylene migrat ory insertion reactions, it can be estimated that sequential ethylene inser tions occur only once in similar to 10(5) turnovers, thus corroborating the exceptionally high fidelity of the alternating copolymerization.