Ligand effects on chain length Propanoate

The hydride cycle starts with palladium hydride and a fast migratory insertion of ethene. Insertion of CO is also a last reaction, enhanced by the preferential coordination of CO with respect to ethene. The product is formed by reaction of propanoylpalladium with methanol which coordinates to palladium cis to the acyl group. A palladium zero complex may form that regenerates palladium hydride via an oxidative addition with a proton. 

There is a lot of evidence and general agreement that the catalytic cycle of the new, fast catalysts starts with a palladium hydride species [45,56], with perhaps one exception [57], 

Now we will try to answer the question why at very large cone angles the cis bidentate ligands give ester formation rather than polymer. Initially, increasing steric bulk leads to faster polymerisation, but at some point it is overtaken by the termination reaction giving propanoates. 

The resting state of the propanoate catalysts may well be an acyl complex [60,61], while the attack of alcohol at the acylpalladium complex is considered to be the rate-determining step. It is probably more precise to say that fast preequilibria exist between the acyl complex and other complexes en route to it and that the highest barrier is formed by the reaction of alcohol and acylpalladium complex. The precise course of the reaction is still not known presumably deprotonation of the coordinating alcohol and the migratory elimination are concerted processes, accelerated by the steric bulk of the bidentate ligand. Toth and Elsevier showed that the reaction of an acetylpalladium complex and sodium methoxide is very fast and occurs already at low temperature to give methyl acetate and a palladium(I) hydride dimer [46]. 

---