The oxidation of ketones by a chiral bimetallic palladium(II) complex in the presence of CuCl2 in THF–water solvents gave an enantioselective synthesis of α-hydroxyketones in catalytic oxidation utilizing an atmosphere of oxygen. The ee’s ranged from 61% to 92%. The reaction was accelerated by addition of strong acid that presumably increases the rate of enolization.

Chiral bimetallic palladium (II) complex B was used in a catalytic air oxidation process to convert ketones directly into optically active α-hydroxyketones. The reaction was carried out in an aqueous solution of THF or dioxane, and in the presence of a catalytic amount of an acid. The enantiomeric excess (e.e.) of produced -hydroxyl ketones ranged from 61% to 91%.

This manuscript describes the extension of Stille's palladium-catalyzed olefin dicarbonylation reaction to chiral allylic alcohols with chirality transfer to afford the corresponding chiral alcohol functionalized with bis-carbomethoxy esters, containing three contiguous chiral centers, in good to excellent diastereoselectivities (78-98%). (C) 2010 Elsevier Ltd. All rights reserved.

AbstractDieser Kurzaufsatz gibt einen Überblick über die wichtigsten Untersuchungen zur Aufklärung des komplexen Mechanismus des Wacker‐Prozesses, also der homogenen Olefinoxidation mit Palladium(II)‐Katalysatoren. Details der nucleophilen Addition und anderer Schritte des Wacker‐Prozesses sind über vier Jahrzehnte hinweg in zahlreichen Studien behandelt worden. Eine Zusammenfassung und kritische Analyse der bisherigen Arbeiten zeigt uns den Stand der Forschung auf und kann helfen, ungelöste Fragen herauszuarbeiten.

We present a concise review on the most pertinent investigations that illuminate the complicated and elusive mechanism for the Wacker process, homogeneous olefin oxidation by palladium(II) catalysts. For more than four decades, multitudes of creative and elegant studies detailing the nucleophilic addition and other steps of the Wacker process have appeared contradictory, while in fact modern perspective has shown an intricate and colorful picture of the "textbook" organometallic reaction. A summary and critical analysis of previous studies is of great importance to explain resolved and highlight unresolved questions about this frequently misunderstood reaction.

Recently, a paper (“Mechanism and Kinetics of the Wacker Process: A Quantum Mechanical Approach” by S. Ali Beyramabadi, Hossein Eshtiagh-Hosseini, Mohammed R. Housaindokht, and Ali Morsali) appeared in Organometallics which concludes that anti-nucleophilic attack is the rate-determining step for the Wacker process (aerobic olefin oxidation via PdCl_2 and CuCl_2 in aqueous hydrochloric acid solution) under standard conditions.1 This paper also claims that these conclusions are consistent with experimental observations. In fact, as shown below, these conclusions dramatically contradict both experimental observations and other more complete calculations. Because of the importance of the Wacker process to the chemical industry and the many controversies and inconsistencies that confused the understanding of this system, we felt it important to bring to the attention of the Organometallics community that ethylene and olefin oxidation via PdCl_2 and CuCl_2 under standard conditions (low [Cl^-] and low [CuCl_2]) almost certainly proceeds via syn-nucleophilic attack.

The oxidation of cyclohexadiene in a mixed aqueous solvent in the present of azide ion produced 4-azido-2-cyclohexene-1-one, 1, and 2-azido-3-cyclohexene-1-one, 2, in a four electron transfer in about a 70% yield. The overall reaction is a net air oxidation. These enantioselectivies are higher than any obtained previously for addition to 1,3-dienes. The conversion of 1 to 2 involves a [3,3]sigmatropic rearrangement.

AbstractFor Abstract see ChemInform Abstract in Full Text.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A bimetallic palladium(II) complex containing a triketone ligand and a bridging diphosphine ligand oxidizes olefins in acetic acid to allylic acetates by a direct air oxidation, which does not require intermediate redox systems. When the diphosphine is chiral, an asymmetric reaction occurs which gives enantioselectivities between 52 and 78% for cyclic olefins.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Abstract Previous studies showed that oxidation of α-olefins with monometallic catalysts containing chiral diphosphines and diamines gave chlorohydrins with poor to good enantioselectivites (28–82% ee). The present studies demonstrate that bimetallic catalysts containing a β-triketone and bridging chiral diphosphine and diamines are excellent catalysts for this reaction giving enantioselectivites considerably higher than the monometallic catalysts. Enantioselectivities were more than 50% for most olefins tested. The highest optical purities were 94% ee for propene and 93% ee for allylphenyl ether. A useful feature of this asymmetric synthesis is the fact it is a net air oxidation.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

This paper describes further studies on mono- and bi-metallic catalysts attached to a polymer support by β-di- and tri-ketone surface ligands. The previous two papers described the oxidation of catechol by the heterogeneous catalysts using Cu(II), Fe(III) and Pd(II) as the metal species. The present study expands these studies to a series of mono- and polyfunctional alcohols using Pd(II) as the metal species. The final catalytic surfaces were prepared by treatment of the modified polymer with a very reactive form of Pd(II), [Pd(CH3CN)4]2+. The simple alcohols gave increases in rates of up to 5-fold for the bimetallic systems. As might be expected glycols and α-d-glucose gave even higher increases in rate in going from the mono- to the bi-metallic catalyst. For ethylene glycol the factor was 30. Unsaturated alcohols gave the most dramatic results. With the monometallic catalyst, the products from allyl alcohol consisted of 25% acrolein resulting from direct alcohol oxidation and 75% 3-hydroxypropanal resulting from Wacker-type oxidation of the double bond. With the bimetallic catalyst the overall rate increased by a factor of 10 and the products consisted of 80% acrolein and 20% 3-hydroxypropanal. The actual rate increase for the direct alcohol oxidation is calculated to be a factor of 32. 4-Penten-2-ol and 4-penten-1-ol gave rates that were lower than the monofunctional alcohols. This is attributed to inhibition by olefin π-complex formation with the Pd(II).

Further mechanistic studies on the PdCl3(pyridine)− catalytic system in aqueous solution are described using the tetrasubstituted allylic alcohol, (E)-2-methyl-d3-4-methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-ol, 3a, and the trisubstituted allylic alcohol, (E)-4-Methyl-1,1,1,5,5,5-hexafluoro-3-penten-2-ol, 6, as substrates. At low [Cl−] the PdCl2−4 catalyzed isomerization of 3a, which can only undergo isomerization into its allylic isomer, was previously found to obey the Wacker rate expression: ki[PdCl2−4][3a]/[H+][Cl−]2. In contrast, the rate expression for isomerization of 3a by [PdCl3(Py)−] at low [Cl−] was found to be: ratei = ki[PdCl3(Py)−][3a]/[Cl−]. This rate expression is of the same form as that previously found for the isomerization of 3a by PdCl2−4 at high [Cl−]. This result strongly suggests that the hydroxypalladation by PdCl3(Py)− at low [Cl−] is a trans process as opposed to a cis process with PdCl2−4. This expectation was confirmed by stereochemical studies with chiral 3a. The stereochemistry of addition for PdCl3(Py)− was identical to that for PdCl2−4 at high [Cl−]. Independent stereochemical studies have shown this addition to be trans. With PdCl3(Py)− there are two possible routes for olefin oxidation. A cis process similar to that found for PdCl2−4 or a trans process analogous to that previously proposed to explain the trans stereochemistry found at high [Cl−]. Stereochemical studies with 6, which can undergo oxidation, showed that both processes are operative with PdCl3(Py)− at [Cl−] = 0.05 M. Thus addition of a pyridine to the coordination sphere of Pd(II) causes a profound change in reactivity.

In the absence of CuCl2, ethene was oxidized to ethanal by PdCl3 (pyridine)− in aqueous solution by the rate expression: −d[C2H4]dt = k′K′[PdCl3(Py)−][C2H4][H+][Cl−]2 where K′ is the equilibrium constant for π-complex formation between ethene and PdCl3(Py)− (Py = pyridine). This rate expression is of the same form as that previously found for the oxidation of ethene by PdCl2−4 in aqueous solution (Wacker reaction). The value of K′ for PdCl3(Py)− was found to be 20.3 which is close to the value of 17.4 previously measured for π-complex formation between ethene and PdCl2−4. However, the value of k for PdCl2−4 was 750 times the value of k′ for PdCl3(Py)−. This result suggests that the hydroxypalladation adduct from PdCl3(Py)− is much more stable towards decomposition to ethanal than the corresponding one from PdCl2−4. A direct result of this higher stability is the expectation that the adduct from PdCl3(Py)− should be more readily intercepted by CuCl2 to produce 2-chloroethanol. At chloride concentrations as low as 0.2 M and [CuCl2] = 4 M, the product was almost 50% 2-chloroethanol. At [CuCl2] = 8 M, the product was 98% 2-chloroethanol. With PdCl2−4, a chloride concentration of 3 M is required before an appreciable amounts of 2-chloroethanol are produced at any cupric chloride concentration. For the reaction of ethene with PdCl2−4, these results are consistent with a mechanism involving cis addition at low [Cl−] and trans addition at high [Cl−]. The pathway for ethanal formation may be different with PdCl3(Py)− than it is with PdCl2−4.

The rate expression for the oxidation of 2-cyclohexenol, rate = k [PdCl42-] [C6H10]/[Cl-], is different from that previously found for the oxidation of acyclic olefins but the same as that found for the nonoxidative isomerization of allyl alcohol at high chloride concentrations. The product distribution under one set of reaction conditions ([Li2PdCl4] = [CI-] = [H+] = 0.1 M) consisted of 19% 2-cyclohexenone (5), 45% cyclohexanone (6), and 36% 3-hydroxycyclohexanone (7). While 5 and 7 were oxidation products, 6 must have arisen from a nonoxidative rearrangement of 2-cyclohexenol. The rate expression is consistent with a mechanism involving equilibrium formation of an olefin pi-complex followed by external attack of water in the slow step to give a hydroxypalladation adduct which decomposed to 7. To obtain further insight into the detailed route, 2-cyclohexenol-1-d (4a) was oxidized. Under the oxidation conditions 4a was isomerized into its allylic isomer 2-cyclohexenol-3-d (4b), at appreciable rates. The rate expression was found to be rate = k(i)[C6H10O] [H+]. When the reaction was run at low acid ([H+] = 0.05 M) and chloride concentrations ([Cl-] = 0.05 M), this complication could be avoided. The isomer of 7 that was formed had the deuterium on the alcoholic carbon, so the hydride shift to form 7 had to have occurred from the carbon to which the hydroxyl was added to form the hydroxypalladation adduct. This is consistent only with the original hydroxyl directing the palladium to the same side of the ring as the hydroxyl followed by trans attack of water. Only the hydrogen on the carbon to which the hydroxyl has been added is in position for a cis hydride migration to give 7. Next, 4a was equilibrated with aqueous acid to give a 50:50 mixture of 4a and 4b. The oxidation of 4b would only give 7 with hydrogens on the alcoholic carbon, so the 7 formed by the oxidation of 4a did not interfere with the analysis of the H-1 NMR spectra. This analysis showed that the alcoholic hydrogen and the hydrogen on the carbon between the alcoholic carbon and the carbonyl were in a cis configuration. This result was also only consistent with trans hydroxypalladation of a pi-complex with the Pd(II) on the same side as the hydroxyl. The 5 formed contained deuterium, which indicated it arose from dehydration of 7 rather than direct deuteride extraction from the alcoholic carbon.