Oxidation of olefins by palladium(II). 11. Kinetics and mechanism of the oxidation of allyl alcohol by PdCl42- in aqueous solution

Abstract

With use of a potentiometric procedure with quinone as reoxidant for Pd(O), the rate expression for allyl alcohol oxidation was determined to be rate = k[PdC1,2-][C6H60]/[H+] [C1-l2, an expression identical in form with that found previously for ethene and other acyclic olefin oxidations indicating similar mechanisms. The two trans attack mechanism suggested for acyclic olefin oxidation involve trans equilibrium hydroxypalladation and external hydroxide ion attack. In regard to the first, with most acyclic olefins the reversibility of the process cannot be determined. With allyl alcohol, however, the reversibility of hydroxypalladation can be tested by determining if allyl-1 , I d z alcohol (5a) isomerizes into an equilibrium mixture of 5a and dyl-3,3-d2 alcohol (5b) during the course of the oxidation. It was found that isomerization of 5a did not occur during oxidation so hydroxypalladation is nonreversible and the equilibrium hydroxypalladation mechanism cannot be operative. Next the possibility of external hydroxide was tested. The equilibrium formation of the intermediate r-complex PdC13(C6H60)-was studied by spectral means, and a value of 5 was found for the equilibrium constant, K,. As with other acyclic olefins, the value of K 2 for formation of the second intermediate PdC12(C3H60)(H20) was too small to be measured W0.025). With use of the experimental values of k, K,, and K2, it could be calculated that hydroxide attack would have to be faster than a diffusion-controlled reaction and thus impossible. Thus both reasonable trans attack mechanisms can be eliminated for allyl alcohol.

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

“…However, the isotope scrambling experiments with allyl-1,1-d2 15 alcohol indicates that hydroxypalladation is not an equilibrium step. 18 As discussed before, earlier theoretical calculations6…”

“…4). 18 The isomerisation between H 2 C=CHCD 2 OH and D 2 C=CHCH 2 OH was indeed not observed (Scheme 5). The lack of isotopic scrambling indicated that the nucleophilic addition is not an equilibrium process.…”

“…Henry's group also performed stereochemical investigations by measuring the rates of isomerisation and oxidation of a set of 10 isotopically substituted olefins. For the case of [D 2 ]allylic alcohol, 18 this species is capable of showing isotopic scrambling. Thus, such studies should assess whether nucleophilic attack is an equilibrium process.…”

Oxidation of allyl alcohol by alkaline periodate in the presence of micro amounts of palladium(II)

“…This species of palladium (II) in the presence of chloride has also been observed earlier. 15 The complex between the active form of the catalyst, PdCl , and the substrate, allyl alcohol, is a type of p-complex 16 which may be of the form.…”

PdCl2 Catalyzed and Uncatalyzed N-Oxidation of Naratriptan with Alkaline Chloramine-T: Delineation of Reaction Mechanisms and Kinetic Modelings

“…The mechanisms of catalysis to some extent is complicated by the formation of different intermediate complexes, free radicals and different oxidation states [16,17]. Palladium (II) chloride (Pd(II)) has been used as a catalyst in the oxidation of several substrates in alkaline medium and some of them have been proved to be suitable for kinetic analysis [11,[18][19][20][21][22]. This background instigated us to carry out the detailed study on the kinetics and mechanism of NTT-CAT redox system in alkaline medium with and without Pd(II) catalyst.…”