Abstract
Mechanistic studies of the catalyst [Pd2(dba)3/1,1'-bis(tert-butyl(pyridin-2-yl)phosphanyl)ferrocene, L2] for olefin alkoxycarbonylation reactions are described. X-ray crystallography reveals the coordination of the pyridyl nitrogen atom in L2 to the palladium center of the catalytic intermediates. DFT calculations on the elementary steps of the industrially relevant carbonylation of ethylene (the Lucite α-process) indicate that the protonated pyridyl moiety is formed immediately, which facilitates the formation of the active palladium hydride complex. The insertion of ethylene and CO into this intermediate leads to the corresponding palladium acyl species, which is kinetically reversible. Notably, this key species is stabilized by the hemilabile coordination of the pyridyl nitrogen atom in L2. The rate-determining alcoholysis of the acyl palladium complex is substantially facilitated by metal-ligand cooperation. Specifically, the deprotonation of the alcohol by the built-in base of the ligand allows a facile intramolecular nucleophilic attack on the acyl palladium species concertedly. Kinetic measurements support this mechanistic proposal and show that the rate of the carbonylation step is zero-order dependent on ethylene and CO. Comparing CH3OD and CH3OH as nucleophiles suggests the involvement of (de)protonation in the rate-determining step.
Highlights
Transition metal complexes play a crucial role in homogeneous catalysis both for basic organic synthesis and on a large scale in the chemical industry.[1]
Elementary catalytic steps occur at a given metal center and the stabilizing ligands remain untouched during the reaction
The initial rate of this process was estimated by an online gas drop of ethylene and CO
Summary
New avenues to synergistically activate small molecules and develop new and potentially ‘greener’ catalytic processes.[3]. In the past two decades, several types of ligand have been developed for this purpose Substituents such as amino, carboxylate and hydroxyl groups are introduced at a speci c position of the ligand.[4] Most of these systems have been favorably applied for catalytic (de)hydrogenation reactions.4a–f,5. Drent et al showed the superiority of diphenylphosphinopyridine (Ph2P(2-Py)) in the carbonylation of propyne with CO.[8] Here, the 2-pyridyl moiety in this ligand is suggested to promote the nucleophilic attack of an alcohol on the key palladium acyl species, which is o en rate-limiting, via metal–ligand cooperativity.[9] Inspired by this seminal work, very recently we developed highly efficient palladium catalysts for 2510 | Chem.
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