Liquid-Phase Selective Oxidation of Organic Compounds

Liquid Phase Selective Oxidation of Organic Compounds 

Liquid-phase selective oxidations are of industrial importance for fine chemical syntheses. However, stoichiometric oxidizing agents have been mostly used and the generation of unwanted by-products ecceeds that of desired products in quantity. To 

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Polyoxometalates undoubtedly have enormous catalytic potential in liquid phase selective oxidation of organic compounds. Various strategies for immobilization of POMs on solid matrices have been developed during the past two decades and opened new opportunities for practical applications. The most developed and widely used technique is electrostatic... 

Liquid-Phase Selective Oxidation of Organic Compounds 111... 

There are many chemical sytheses where high selectivities can only be obtained at low degrees of conversion. This is the case when the reactions are not very rapid, and when the product may react further (consecutive reactions see section 3.4.3). Many partial oxidations of organic compounds in the liquid phase are of this category. It may then be sensible to operate a reactor at a low degree of conversion, separate the unconverted reactants from the reaction product and recycle these. Let us consider the reaction scheme ... 

In both the synthetic organic laboratory and industry, the first and foremost procedure for the preparation of oxiranes is the direct oxidation of alkenes. Significant new results have been achieved in the development of methods of oxidizing alkenes in the liquid phase. The major aim is the attainment of an oxidation reaction under the mildest possible experimental conditions, which allows an increase in the selectivity of oxirane formation and permits the selective oxidation of more sensitive compounds. Since the various methods of preparing oxiranes were reviewed quite recently, the individual oxidation procedures will be mainly illustrated here with some more recent examples. Surveys concentrating on stereo-controlled epoxidations and assymmetric synthetic methods have been published. "... 

Much has been published on the selective electrochemical oxidation of a large variety of organic compounds, among which the higher alcohols (e.g. ethanol to acetic add, propanol-2 to acetone). The interesting point in the present context is that some of these conversions have also been studied purely catalytically in the liquid phase, employing catalysts such as Pt/C, with O2 or air as the oxidant. It has been remarked [152] that such systems should also be considered from an electrochemical point of view. Indeed, it stands to reason that the overall oxidation reaction is essentially the sum of the two constitutive electrochemical half-reactions [153]. In the case of alcohol oxidation we would then have... 

The selection of an efiicient catalyst and the choice of the reaction conditions are the key steps to realize an ideal oxidation procedure. Many of the heterogeneous catalysts used in liquid-phase oxidation are mixed oxides with one or more transition metak. For these catalysts, the active site is directly involved in successive redox cycles, which underlines the fundamental role of the electronic fiic-tor [30]. Transition metal oxides also exhibit surface acid-base properties. Many authors have attempted to relate these properties with the activity or selectivity in oxidation reactions [31] and photo-oxidative degradation of organic compounds. 

One of the exciting results to come out of heterogeneous catalysis research since the early 1980s is the discovery and development of catalysts that employ hydrogen peroxide to selectively oxidize organic compounds at low temperatures in the liquid phase. These catalysts are based on titanium, and the important discovery was a way to isolate titanium in framework locations of the inner cavities of zeolites (molecular sieves). Thus, mild oxidations may be run in water or water-soluble solvents. Practicing organic chemists now have a way to catalytically oxidize benzene to phenols alkanes to alcohols and ketones primary alcohols to aldehydes, acids, esters, and acetals secondary alcohols to ketones primary amines to oximes secondary amines to hydroxyl-amines and tertiary amines to amine oxides. 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

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