Palladium catalysts alcohol oxidation with

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]. 

Grunwaldt et al. (2003b) reported XAFS measurements recorded during palladium-catalyzed alcohol oxidation in supercritical CO2. A commercial shell-impregnated catalyst consisting of 0.5 wt% Pd on alumina was used for benzyl alcohol oxidation (to benzaldehyde) in supercritical CO2 with pure O2 as oxidant. The conditions were 353 K and 150 bar. The results are summarized in Table 8. The authors reported only Pd XANES data, not EXAFS data, and thus the analysis is limited to information about the average oxidation state of the palladium. 

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

Wilkinson s catalyst has also been utilized for the hydroboration of other alkenes. Sulfone derivatives of allyl alcohol can be hydroborated with HBcat and subsequently oxidized to give the secondary rather than primary alcohol. This reactivity proves to be independent of substituents on the sulfur atom.36 Similarly, thioalkenes undergo anti-Markovnikoff addition to afford a-thioboronate esters.37 The benefits of metal-catalyzed reactions come to the fore in the hydroboration of bromoalkenes (higher yields, shorter reaction times), although the benefits were less clear for the corresponding chloroalkenes (Table 3).38,39 Dienes can be hydroborated using both rhodium and palladium catalysts [Pd(PPh3)4] reacts readily with 1,3-dienes, but cyclic dienes are more active towards [Rh4(CO)i2].40... 

The most characteristic reaction of butadiene catalyzed by palladium catalysts is the dimerization with incorporation of various nucleophiles [Eq. (11)]. The main product of this telomerization reaction is the 8-substituted 1,6-octadiene, 17. Also, 3-substituted 1,7-octadiene, 18, is formed as a minor product. So far, the following nucleophiles are known to react with butadiene to form corresponding telomers water, carboxylic acids, primary and secondary alcohols, phenols, ammonia, primary and secondary amines, enamines, active methylene compounds activated by two electron-attracting groups, and nitroalkanes. Some of these nucleophiles are known to react oxidatively with simple olefins in the presence of Pd2+ salts. Carbon monoxide and hydrosilanes also take part in the telomerization. The telomerization reactions are surveyed based on the classification by the nucleophiles. 

We showed that the application of PEG/CO2 biphasic catalysis is also possible in aerobic oxidations of alcohols [15]. With regard to environmental aspects it is important to develop sustainable catalytic technologies for oxidations with molecular oxygen in fine chemicals synthesis, as conventional reactions often generate large amoimts of heavy metal and solvent waste. In the biphasic system, palladium nanoparticles can be used as catalysts for oxidation reactions because the PEG phase both stabilises the catalyst particles and enables product extraction with SCCO2. 

The transformation of alcohols to the corresponding carbonyl compounds or carboxylic acids is one of the few examples in which a heterogeneous (solid) catalyst is used in a selective, liquid phase oxidation (7,2). The process, which is usually carried out in an aqueous slurry, with supported platinum or palladium catalysts and with dioxygen as oxidant, has limited industrial application due to deactivation problems. 

Tetrahydro- (133) and trcm.s-decahydronaphthyridines (134) can be selectively prepared from the parent ring systems by reduction in the presence of either platinum oxide or palladium or by reduction with sodium in ethanol or amyl alcohol.79"8- Hydrogenation in acetic acid with platinum catalyst affords a cis- and traas-decahydro mixture (135 and 134).81... 

A remarkable number of palladium-catalyzed aerobic oxidation reactions of alcohols have been reported to date [15]. Unfortunately, although some progress has been made with heterogeneous Pd catalysts, such as Pd on activated carbon [16], Pd on pumice [17], Pd-hydrotalcite [18], Pd on Ti02 [19] and Pd/SBA-15 [20], most of these systems suffer from low catalytic activities and a limited substrate scope. 

Leitner et al. have synthesized the PEG-modified silica stabilized and immobilized palladium nanoparticles for aerobic alcohol oxidation in combination with scC02 as reaction medium under mild conditions, which show high activity and excellent stability under continuous-flow operation [68], ScC02 could diffuse the substrates and products from the active nanoparticles in a gas-like manner. This allows rapid chemical transformation at the active center, ensures efficient removal of the products from the surface, and minimizes the mobility of solid-supported catalytically active species [69]. In this way, catalysts based on palladium nanoparticles together with PEG as stabilizing matrix could avoid aggregating and forming less active and selective Pd-black [20, 60, 70]. 

Deoxygenation of pyridine A-oxides has been achieved using dimethyldioxiran <95CC1831> and palladium with sodium hypophosphite <95GCI(124)385>. Pyridine A-oxides, with ruthenium porphyrin catalysts, have been used as an oxidant of aromatic compounds <95JA(117)8879> or olefins, alcohols, sulfides and alkanes <95FI(40)867>. 

This work was expanded in 2006, when the benzyl alcohol oxidation was conducted with cyclohexane as the solvent (Grunwaldt et al., 2006). The authors set out to address whether the active phase of the catalyst was oxidic or metallic palladium. The reaction conditions were as follows temperature 323 K reactant mixture, 200 p.1 of benzyl alcohol in 100 ml of cyclohexane saturated with 02. The catalytic activity was determined online by monitoring the intensity of the C=0 band in the benzaldehyde with IR spectroscopy. If the catalyst was not reduced in the XAFS reactor (by the H2-saturated cyclohexane), then no catalytic activity was measured, and the palladium remained oxidic. If the catalyst was prereduced, a highly active catalyst was obtained, and the XAFS data were found to be consistent with the presence of only metallic palladium. Thus, the conclusion was reached that palladium oxide exhibits hardly any activity at 323 K, whereas metallic palladium particles are much more active. 

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