Selective oxidations Subject

The reactions of olefins with peracids to form epoxides allows for the selective oxidation of carbon-carbon double bonds in the presence of other functional groups which may be subject to oxidation (for example, hydroxyl groups). The epoxides that result are easily cleaved by strong acids to diols or half-esters of diols and are therefore useful intermediates in the synthesis of polyfunctional compounds. 

Reversed micelles have also shown to be useful not only in bioconversions, but also in organic synthesis. Shield et al. (1986) have reviewed this subject and brought out its advantages in peptide synthesis, oxidation or reduction of steroids, selective oxidation of isomeric mixtures of aromatics, etc. In the oxidation of aromatic aldehydes to carboxylic acids with enzymes hosted in reverse micelles, the ortho substituted substrates react much more slowly than other isomers. 

Oxidative, degradative reactions of maltose have been studied,136-138 and reviewed in this Series,139-142 and this subject will not, therefore, be discussed. The selective oxidation of primary52,143-146 and secondary66,147 hydroxyl groups in maltose has been investigated. The synthesis143 of maltouronic acid, 4-0-(a-D-glucopyranosyluronic... 

As shown by Table 3, most of the Group VIII metal-peroxo complexes are obtained from the direct interaction of dioxygen with the corresponding reduced forms. A considerable effort has been devoted to this subject in the last decade with the hope that selective oxidations of hydrocarbons could be achieved by the activation of molecular oxygen under mild conditions12,56 133,184 and several such examples have actually been shown to occur. 

Among common alcohol oxidants, TEMPO-mediated oxidations have been the subject of a close scrutiny, aimed at finding optimum conditions for the selective oxidation of primary alcohols. In fact, TEMPO-mediated oxidations, that is oxidations in which an oxoammonium salt acts as a primary oxidant, have a great tendency to operate quicker with primary alcohols, regardless of the secondary oxidant employed and the exact experimental conditions. 

In 1976, Ueno and Okawara highlighted the fact that no oxidation of primary saturated alcohols to aldehydes via tin alkoxides had been reported in the literature and published a procedure for the selective oxidation of secondary alcohols.25 Interestingly, rather than performing the oxidation on pre-formed tin alkoxides, these researchers subjected a mixture of the diol and (Bu3Sn)20 in CH2C12 to the action of Br2. Regardless of the fact that no complete formation of tin alkoxides is secured and no HBr quencher is added, this method may provide useful yields of hydroxyketones during the selective oxidation of diols.26... 

The subject of heterogeneously catalyzed selective oxidation has been reviewed many times. Under the keyword combination selective catalytic oxidation the ISI database reports about 5400 papers. Over 100 reviews on the topic have been published. In the present discussion, the subjects of methane activation and model studies of unselective CO oxidation, which represent large fields, are excluded. Homogeneously or biologically catalyzed selective oxidation, a combined field that is about 10-fold larger in scientific coverage, is also excluded from this chapter. 

A convenient HPLC technique known as the Fukushima-Nixon method has been widely used for selective analyses of tetrahydrobiopterin and tetrahy-droneopterin in biological samples [49]. This method allows the estimation of concentrations of tetrahydropterins based on difference in the concentrations of the corresponding aromatic pterins in the samples, which are prepared in situ by treatment with iodine under acidic and basic conditions. The Fukushima-Nixon method does not require special techniques or equipment for the chemical reaction the sample is simply subjected to oxidation just before its injection into the HPLC column. For example, tetrahydrobiopterin (43) was selectively oxidized to biopterin (30) by iodine in the presence of... 

The chemical behavior of 02 after coordination, that is the activation of 02, is a diverse and complex subject because of the many pathways available for 02 reactions. The search for more selective oxidations, especially using 02, is a prime goal that has immense potential on an industrial scale (4, 30), and is thus an important research area. The ultimate aim would be to mimic the 02-activation ability of enzyme systems, for example the ability of cytochrome P 450 to catalyze Reaction 1, where R is a hydrocarbon molecule. 

Vanadium phosphates have been established as selective hydrocarbon oxidation catalysts for more than 40 years. Their primary use commercially has been in the production of maleic anhydride (MA) from n-butane. During this period, improvements in the yield of MA have been sought. Strategies to achieve these improvements have included the addition of secondary metal ions to the catalyst, optimization of the catalyst precursor formation, and intensification of the selective oxidation process through improved reactor technology. The mechanism of the reaction continues to be an active subject of research, and the role of the bulk catalyst structure and an amorphous surface layer are considered here with respect to the various V-P-O phases present. The active site of the catalyst is considered to consist of V and V couples, and their respective incidence and roles are examined in detail here. The complex and extensive nature of the oxidation, which for butane oxidation to MA is a 14-electron transfer process, is of broad importance, particularly in view of the applications of vanadium phosphate catalysts to other processes. A perspective on the future use of vanadium phosphate catalysts is included in this review. 

The complex selective oxidation of an n-butane molecule to MA involves 14 electrons and occurs entirely on the surface of the catalyst. No intermediates have been detected in the effluent product imder conditions of continuous flow operation. Mechanisms of the reaction have been proposed on the basis of a variety of experimental and theoretical findings. The description of the active site is linked to the mechanism and is the subject of considerable debate in the literature. 

Although a number of different reagents have been discovered for the selective oxidation of ethers, e.g. halogens, iodine tris(trifluoroacetate), trichloroisocyanuric acid, UFs, A(,N-dibromobenzenesul-fonamide and lead tetraacetate, few have assumed any synthetic importance. Of these, the most significant are the metallic oxidants chromic acid and ruthenium tetroxide. DDQ has also been widely used for the oxidative d rotection of benzyl ethers. It is the aim of this chapter to review the latest developments in ether oxidation by these, and other reagents, with particular emphasis on chemo- and regio-selectivity. Several reviews on the subject have appeared previously. " The related oxidation of acetals has been reviewed recently" and will not be dealt with here. 

The oxidation of -butane to maleic anhydride is a 14-electron oxidation. It involves the abstraction of eight hydrogen atoms, the insertion of three oxygen atoms, and a multi-step polyfunctional reaction mechanism that occurs entirely on the adsorbed phase. No intermediates have been observed under standard continuous flow conditions, although mechanisms for this process have been proposed based on a variety of experimental and theoretical findings. The description of the active site is linked to the mechanism and is the subject of considerable debate in the literature. The mechanisms are linked to the researchers hypotheses of the active site, which will be discussed in a separate section in this chapter. It is widely accepted that the (100) plane of vanadyl pyrophosphate, (VO)2P207, (referred to as the (020) plane by certain authors) plays an important role in the selective oxidation of butane. 

This volume consists of reviews devoted to a range of important subjects. Vadim Guliants and Moises Carreon (University of Cincinnati) review the selective oxidation of butane. This is an excellent example of a catalytic process designed to add value to an inexpensive raw material, and is the only vapor phase selective oxidation of an alkane that is practiced industrially. This process also avoids the use of benzene, which eliminates the risk of handling this carcinogenic compound. The authors review the synthesis, activation, and mechanism of this reaction on V-P-O catalysts. 

Although several mixed oxide catalysts have been developed commercially for the selective oxidation of propylene, the investigation of their fundamental physical and chemical properties has resulted in only a slow and steady accumulation of information. It also appears that attempts to correlate data from different investigations have frequently resulted in unsatisfactory interpretations. It seems that some of this uncertainty arises from correlations between results obtained from different catalysts subjected to different pretreatments and assessed under different evaluation conditions. Hence, the comprehensive description of the bulk and surface properties of a single catalyst, their interdependence, and their influence on catalytic performance is in most cases quite unclear. 

Studies of the selective oxidation of propylene, which is an important application of the tin-antimony oxide catalyst, have resulted in the description of several mechanisms and the subject was recently reviewed by Keulks et al. (7). It is not the purpose of this article to give a similar in depth consideration of this aspect of the catalytic properties of tin-antimony oxides. However, it is important that the improved knowledge of the bulk and surface properties of the catalyst and their relationship with the catalytic character should be considered in terms of the formulation of reaction mechanisms. 

---