Alcohols, catalytic oxidation substrate

The titanosilicate version of UTD-1 has been shown to be an effective catalyst for the oxidation of alkanes, alkenes, and alcohols (77-79) by using peroxides as the oxidant. The large pores of Ti-UTD-1 readily accommodate large molecules such as 2,6-di-ferf-butylphenol (2,6-DTBP). The bulky 2,6-DTBP substrate can be converted to the corresponding quinone with activity and selectivity comparable to the mesoporous catalysts Ti-MCM-41 and Ti-HMS (80), where HMS = hexagonal mesoporous silica. Both Ti-UTD-1 and UTD-1 have also been prepared as oriented thin films via a laser ablation technique (81-85). Continuous UTD-1 membranes with the channels oriented normal to the substrate surface have been employed in a catalytic oxidation-separation process (82). At room temperature, a cyclohexene-ferf-butylhydroperoxide was passed through the membrane and epoxidation products were trapped on the down stream side. The UTD-1 membranes supported on metal frits have also been evaluated for the separation of linear paraffins and aromatics (83). In a model separation of n-hexane and toluene, enhanced permeation of the linear alkane was observed. Oriented UTD-1 films have also been evenly coated on small 3D objects such as glass and metal beads (84, 85). 

The first chapter concerns the chemistry of the oxidation catalysts, some 250 of these, arranged in decreasing order of the metal oxidation state (VIII) to (0). Preparations, structural and spectroscopic characteristics are briefly described, followed by a summary of their catalytic oxidation properties for organic substrates, with a brief appendix on practical matters with four important oxidants. The subsequent four chapters concentrate on oxidations of specific organic groups, first for alcohols, then alkenes, arenes, alkynes, alkanes, amines and other substrates with hetero atoms. Frequent cross-references between the five chapters are provided. 

A major problem in noble metal catalyzed liquid phase alcohol oxidations -which is principally an oxidative dehydrogenation- is poisoning of the catalyst by oxygen. The catalytic oxidation requires a proper mutual tuning of oxidation of the substrate, oxygen chemisorption and water formation and desorption. When the overall rate of dehydrogenation of the substrate is lower than the rate of oxidation of adsorbed hydrogen, noble metal surface oxidation and catalyst deactivation occurs. 

Redox potential is a catalytically relevant property of heme peroxidases, as in theory, sets the limit for the oxidative ability of the enzyme. An inverse correlation was found between the activity and the redox potential of methoxybenzenes and methoxy-substituted benzyl alcohols for lignin peroxidase (LiP) and horseradish peroxidase (HRP) [42, 43]. These enzymes were able to catalyze the oxidation of methoxybenzenes with redox potential as high as 1.45 V and 1.12 V, respectively [42]. In the case of methoxy-substituted benzyl alcohols, the maximum substrate redox potential was 1.39 V for both enzymes [43]. This type of correlation has allowed ranking enzymes from the more oxidant to the less oxidant. The inverse... 

Catalytic oxidations with dioxygen can also proceed via heterolytic pathways which do not involve free radicals as intermediates. They generally involve a two-electron oxidation of a (coordinated) substrate by a metal ion. The oxidized form of the metal is subsequently regenerated by reaction of the reduced form with dioxygen. Typical examples are the palladium(II)-catalyzed oxidation of al-kenes (Wacker process) and oxidative dehydrogenation of alcohols (Fig. 4.6). 

The palladium(II) complex of sulfonated bathophenanthroline was used in a highly effective aqueous biphasic aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes or carboxylic acids and ketones respectively (Fig. 7.15) [52, 53]. No organic solvent was necessary, unless the substrate was a solid, and turnover frequencies of the order of 100 h-1 were observed. The catalyst could be recovered and recycled by simple phase separation (the aqueous phase is the bottom layer and can be left in the reactor for the next batch). The method constitutes an excellent example of a green catalytic oxidation with oxygen (air) as the oxidant, no organic solvent and a stable recyclable catalyst. 

Chromium(VI) oxide can be used as a catalytic oxidant for alcohols with r-butyl hydroperoxide as the cooxidant. This reagent appears to be selective for allylic and benzylic over saturated alcohols, though ( )/(Z)-isomerization has been observed during the preparation of a,3-unsaturated aldehydes. This reagent is also a good oxidant for allylic and ben lic C—bonds these may be competing pathways in more sophisticated substrates. ... 

A similar mechanism was formulated for the catalytic oxidation of hydrocarbons and for the photo-sensitized oxidation of 2-propanol. (2) A pure dehydrogenation can be depicted in which the platinum cleaves the hydrogen from the substrate alcohol. 

A second area of MnP model chemistry has dealt with Mn11 oxidation by Fe-porphyrin systems. An early example suggested that Mn11 in the presence of excess pyrophosphate could be catalytically oxidized to Mn1" by an Fe-porphyrin system. In these experiments, sulfo-nated porphyrins were utilized, and the co-oxidant was potassium monopersulfate. Co-substrates, such as veratryl alcohol, were reported to enhance the rate of Mn11 oxidation (468). 

In 2001, Marko et al. reported a neutral variant of their Cu Cl(phen)-DBADH2-base system 91). A catalytic amount of base was used, i.e., 5mol% potassium er -butoxide, which was advantageous for pH-sensitive substrates and products. The order of addition of the different reactants turned out to be crucial for the reactivity. The best catalytic procedure was obtained when the base was added to the [Cu Cl(Phen)]2 complex in the presence of the alcohol, followed by the addition of DEAD. A number of sensitive and/or sterically hindered alcohols were oxidized at 80°C using this optimized protocol and the results are shown in Table IX 91). (lS,2S,5i )-Neomenthol was converted to menthone without any epimerization to isomenthone. A rather hindered decaline derivative could be smoothly oxidized as well as... 

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