MECHANISTIC STUDIES OF THE PALLADIUM(II)-CATALYZED COPOLYMERIZATION OF ETHYLENE WITH CARBON-MONOXIDE

The microscopic steps responsible for the perfectly alternating copoly merization 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-(CF3)(2)C6H3; R, L = CH3, CO (2); CH3, C2H4 (3); CH2CH3, C2H4 (7); C(O)CH3, CO (8); C(O)CH3, C2H4 (13); CH2CH2C(O) CH3, C2H4 (15); CH2CH2C(O)CH3, CO (16); C(O)CH2CH2C(O)CH3, C2H4 (17); C(O)CH2CH2C(O)CH3, CO (18)); and the beta- and gamma-keto chelate comp lexes (phen)PdCH2CH2C(O)CH3+ (14) and (phen)PdC(O)CH2CH2C(O)CH3+ (19)] have been prepared. An X-ray structure of the carbonyl acyl complex ( phen)Pd(C(O)CH3)(CO)Ar-+(4)'B-. CH2Cl2 (8 . CH2Cl2) has been obtained. The migratory insertion reactions of 2, 3, 7, 13, 16, and 17 have bee n studied by low-temperature NMR techniques. The barriers for insertio n increase in the following order: Delta G(R-->CO double dagger) appro ximate to 15 kcal/mol (-66 degrees C) (Delta G(Ac-->C2H4)double dagger approximate to 17 kcal/mol (ca. -45 degrees C) < Delta>G(R-->C2H4)dou ble dagger approximate to 19 kcal/mol (-25 degrees C). The relative bi nding affinities of ethylene and CO to Pd methyl, acyl, and chelate co mplexes have been determined by combining stepwise measurements of the binding affinities of ligands with intermediate strength to (phen)Pd( CH3)(L)(+) (CO > MeSPh > CH3CN approximate to C2H4 > C6H5CN >> OEt(2)) with relative equilibrium constants for ethylene/CO binding between a cyl and alkyl complexes. The copolymerization mechanism has been deter mined from the kinetic and thermodynamic data. The catalyst resting st ate is a carbonyl acyl complex which is in equilibrium (K-5(25 degrees C) = (7.1 +/- 3.5) x 10(-4)) with a less stable ethylene acyl interme diate which undergoes beta-acyl migratory insertion to generate a Pd a lkyl species followed by rapid reaction with 2 equiv of CO to reform t he resting state. This model is tested by comparing calculated and exp erimental turnover frequencies.