Mechanistic Studies of the Palladium(II)-Catalyzed Copolymerization of Ethylene with Carbon Monoxide

The microscopic steps responsible for the perfectly alternating copolymerization of ethylene and CO catalyzed by 1,10-phenanthroline (phen) based palladium complexes have been studied. Palladium carbonyl alkyl, carbonyl acyl, ethylene alkyl, and ethylene acyl complexes (phen)Pd(R)(L) + Ar‘ 4 B - (Ar‘ = 3,5-(CF 3 ) 2 C 6 H 3; R, L = CH 3, CO ( 2 ); CH 3, C 2 H 4 ( 3 ); CH 2 CH 3, C 2 H 4 ( 7 ); C(O)CH 3, CO ( 8 ); C(O)CH 3, C 2 H 4 ( 13 ); CH 2 CH 2 C(O)CH 3, C 2 H 4 ( 15 ); CH 2 CH 2 C(O)CH 3, CO ( 16 ); C(O)CH 2 CH 2 C(O)CH 3, C 2 H 4 ( 17 ); C(O)CH 2 CH 2 C(O)CH 3, CO ( 18 )); and the β- and γ-keto chelate complexes (phen)PdCH 2 CH 2 C(O)CH 3 + ( 14 ) and (phen)PdC(O)CH 2 CH 2 C(O)CH 3 + ( 19 ) have been prepared. An X-ray structure of the carbonyl acyl complex (phen)Pd(C(O)CH 3 )(CO) + Ar‘ 4 B - ·CH 2 Cl 2 ( 8 ·CH 2 Cl 2 ) has been obtained. The migratory insertion reactions of 2, 3, 7, 13, 16, and 17 have been studied by low-temperature NMR techniques. The barriers for insertion increase in the following order: Δ G ⧧ R → CO ≈ 15 kcal/mol (−66 °C) MeSPh > CH 3 CN ≈ C 2 H 4 > C 6 H 5 CN ≫ OEt 2 ) with relative equilibrium constants for ethylene/CO binding between acyl and alkyl complexes. The copolymerization mechanism has been determined from the kinetic and thermodynamic data. The catalyst resting state is a carbonyl acyl complex which is in equilibrium ( K 5 (25 °C) = (7.1 ± 3.5) × 10 - 4 ) with a less stable ethylene acyl intermediate which undergoes β-acyl migratory insertion to generate a Pd alkyl species followed by rapid reaction with 2 equiv of CO to reform the resting state. This model is tested by comparing calculated and experimental turnover frequencies.