Oxidation with palladium

The key intermediate 124 was prepared starting with tryptophyl bromide alkylation of 3-acetylpyridine, to give 128 in 95% yield (Fig. 37) [87]. Reduction of 128 with sodium dithionite under buffered (sodium bicarbonate) conditions lead to dihydropyridine 129, which could be cyclized to 130 upon treatment with methanolic HC1. Alternatively, 128 could be converted directly to 130 by sodium dithionite if the sodium bicarbonate was omitted. Oxidation with palladium on carbon produced pyridinium salt 131, which could then be reduced to 124 (as a mixture of isomers) upon reaction with sodium boro-hydride. Alternatively, direct reduction of 128 with sodium borohydride gave a mixture of compounds, from which cyclized derivative 132 could be isolated in 30% yield after column chromatography [88]. Reduction of 132 with lithium tri-f-butoxyaluminum hydride then gave 124 (once again as a mixture of isomers) in 90% yield. 

The proposed mechanism is given in Scheme 15. Initially the dissociation of water, maybe trapped by the molecular sieve, initiates the catalytic cycle. The substrate binds to the palladium followed by intramolecular deprotonation of the alcohol. The alkoxide then reacts by /i-hydride elimination and sets the carbonyl product free. Reductive elimination of HOAc from the hydride species followed by reoxidation of the intermediate with dioxygen reforms the catalytically active species. The structure of 13 could be confirmed by a solid-state structure [90]. A similar system was used in the cyclization reaction of suitable phenols to dihydrobenzofuranes [92]. The mechanism of the aerobic alcohol oxidation with palladium catalyst systems was also studied theoretically [93-96]. 

Oxidation with Palladium in the Homogeneous Phase. The most thoroughly studied reaction concerning the transformation of alkenes to carbonyl compounds is their oxidation catalyzed by palladium in homogeneous aqueous media.243 244 494-503 As a rule, ethylene is oxidized to acetaldehyde, and terminal alkenes are converted to methyl ketones.504 505... 

Insertion Reactions of Compounds of Metals and Metalloids Involving Unsaturated Substrates M. F. Lappert and B. Prokai Olefin Oxidation with Palladium (II) Catalyst in Solution A. Aguil6... 

A Mdssbauer investigation of the reduction of iron oxide (0.05 wt % Fe) and iron-oxide-with-palladium (0.05 wt % Fe, 2.2 wt % Pd), carried upon 7 -Al203, reveals that supported ferric ion alone, under hydrogen, yields ferrous ion only at 500—700 °C this reduction takes place at room temperature with the bimetallic catalyst and proceeds to form a PdFe alloy at 500 °C. Similar effects are found in reduction by carbon monoxide, which yields iron-palladium metal clusters at 400 °C. The view is taken that migration over T7-A1203 is not involved but that activated hydrogen transfers only at bridgeheads on the contact line between the metal and iron oxide. 

Kinetics and Mechanism of the Oxidation with Palladium Complexes... 

It has already been mentioned (Section 9.2) that the Wacker reaction occurs regiospecifically. Thus, 1-olefins give oxidation with palladium chloride to methylketones according to Markovnikov s rule. The synthetic reactions were carried out stoichiometrically as well as catalytically. 

Colloids of alloys have been made by the chemical reduction of the appropriate salt mixture in the solution phase. Thus, Ag-Pd and Cu-Pd colloids of varying composition have been prepared by alcohol reduction of mixtures of silver nitrate or copper oxide with palladium oxide (Vasan and Rao 1995). Fe-Pt alloy nanocrystals have been made by thermal decomposition of the Fe and Pt acetylac-etonates in high-boiling organic solvents (Sun et al. 2000). Au-Ag alloy nanocrystals have been made by co-reduction of silver nitrate and chloroauric acid with sodium borohydride (Sandhyarani et al. 2000 He et al. 2002). 

AOMC321R A. Aguilo, Adv. Organomet. Chem., 1967, 5, 321-352. Olefin Oxidation with Palladium(ll) Catalyst in Solution. VIII... 

The rate-determining process is considered to involve the conversion of the initial intermediate complex into a c-bonded 1,3-oxypalladium adduct which undergoes further rapid reactions possibly involving two specific, consecutive 1,2-hydride shifts. In the reaction with cyclohexene and other unsaturated hydrocarbons of this type under acidic conditions (I.5M-HCIO4), a fast oxidation with palladium acetate is observed, the products being cyclohexanone and other acetate-containing species. A slower oxidation is reported, however, in the presence of oxygen. 

In the Wacker two-stage process, the ethylene was completely oxidized with palladium chloride/cupric chloride solution at 110°C and 10 bar, with the stoichiometric volume of air in the reaction vessel. Acetaldehyde was recovered from the circulating solution and the palladium oxidized with air at lOO C and 100 bar pressme in a second vessel before the catalyst solution was letumed to the reactor. 

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