Stereochemistry and mechanism for the palladium(II)-catalyzed oxidation of ethene in water (the Wacker process)

Abstract

Palladium(II)-catalyzed oxidation of specifically deuterated ethenes in water in the presence of cupric chloride and chloride ion has been studied. Stereospecific formation of threo-1,2-dideuterio-2-chloroethanol (threo-3) from (£)-1,2-dideuterioethene ((£')-2) indicates that the hydroxypalladation step in the Wacker oxidation is a trans process. Generation of an eryiAro-jS-hydroxyethylpalladium complex (6) from eryzAro-^-hydroxyethylmercuric chloride (5) in the presence of CuCl2-LiCl also results in the formation of threo-3. Analyses of the deuterated products were done by microwave spectroscopy. A new (modified) mechanism is proposed, which involves reversible formation of 11 by external attack of water on 8, followed by a rate-determining dissociation of a chloride ligand from 11.

Migratory Insertion of Alkenes into Metal–Oxygen and Metal–Nitrogen Bonds

“…[28] However, experiments conducted by Akermark, Stille, Bäckvall, and co-workers demonstrated that palladium ligated with substituted or deuterium-labeled alkenes formed products by anti oxypalladation. [29][30][31][32] For example, the addition of CO to the combination of bis[(cis-[D 2 ]ethylene)PdCl 2 ] and H 2 O in a buffered solution of CuCl 2 and NaOAc led to the formation of lactone products, and the configuration of the carbon atoms in the lactone indicated that an anti addition of the oxygen atom and palladium to the ethylene had occurred. [30] In fact, Bäckvall, Siegbahn, and coworkers stated that a hydroxymetal olefin complex is too unreactive to undergo cis migration of an OH ligand to a bound alkene.…”

Challenges in modelling homogeneous catalysis: new answers from ab initio molecular dynamics to the controversy over the Wacker process

“…15 Some authors, however, suggested that these stereochemical studies supporting the anti pathway might not be relevant to the mechanism of the Wacker process because the reaction conditions are not those of the common Wacker process (low [Cl [PdCl 3 (pyridine) -] in aqueous solution (in the presence and absence of CuCl 2 ) showed that the rate law was the same at low [Cl -] as for the regular catalyst used in the Wacker process; the reaction, however, was significantly slower. 17 The product distribution observed was consistent with a mechanism involving 5 an anti addition at high [Cl -] and a syn addition at low [Cl -].…”

“…Thus, such studies should assess whether nucleophilic attack is an equilibrium process. Although quinone was used as reoxidant for Pd, the rate law observed was the same as in the Wacker process 15 (eq. 4).…”

“…Two mechanisms have been postulated for the explanation of the rate law (Scheme 2): a) anti nucleophilic attack by a water molecule from the bulk prior to the rate determining step of the process, b) syn nucleophilic attack by a coordinated hydroxide 15 group located cis to the alkene as rate determining step. They are also described by cis or trans terms, or by inner-or outer-sphere mechanisms in the literature.…”

“…Moreover, the relativistic effects may also become crucial to obtain correct reactivity and energetics. All these factors can result in significant errors in the prediction of structures and 15 energies. 26 Modern theoretical calculations of the Wacker process (reported in the past one decade) were using Generalized Gradient Approximation (GGA) and hybrid density functionals.…”