Oxidation allylic, heterogeneous catalysis

The allylic oxidation of propene typifies the so-called bimetallic heterogeneous catalysis [4], a terminus technicus to emphasize cooperative effects in catalytic conversions (for multicomponent homogeneous catalysis, see Section 3.1.5). Nevertheless, the SOHIO-type oxidation is included in this book because one can imagine a number of mechanistic implications on a molecular platform, too. Studies on organometallic model compounds and reactions are available in ref. [2]. 

Despite the high mechanical and thermal stability of these stmctures, which are by far sufSdent for short- and medium-term analytical applications as wdl as applications in heterogeneous catalysis, " tert-allylic carbons located at the surface of a monolithic stmcture tend to be oxidized, resulting in a slow change of surface polarity of such monolithic columns. Consequently, the typical long-term stability of NBE-based, ROMP-derived columns is limited to less than 1000 injections. To solve this problem, a novel monomer/ cross-linker system had to be introduced. For these purposes, Buchmeiser et al. devdoped a polymerization system based on... 

The present paper illustrates the versatility of Mo and W exchanged LDHs in heterogeneous oxidation catalysis with three selected examples (a) the W-catalyzed epoxidation of allyl alcohol, (b) the Mo-catalyzed epoxidation of cyclohexene, (c) the bleaching of a typical model dye component with aqueous H202. 

Allylic oxidation, that is, the selective oxidation of olefins at the allylic position, represents a substantial portion of the production of important organic chemicals by heterogeneous oxidation. Furthermore, the development and study of selective catalysts for allylic oxidation has led not only to successful commercial processes, but also to important concepts concerning selective oxidation and the phenomena of catalysis in general. 

Selective allylic oxidation of olefins like isophorone is a very demanding chemical reaction [135].The keto-isophorone obtained by oxidation of (3-isophor-one (Fig. 14) is an important intermediate for the preparation of flavors and fragrances. In homogeneous catalysis, yields superior to 90% are reached [136, 137]. The heterogeneous analogs were always worse catalysts with, for example, a drop of the yield from 67% to rally 6% when the Co(salen) complex was immobilized in silica [138]. 

Indeed, one can easily conclude that selective allylic oxidation of olefins, in the context of fine chemicals, is a largely underdeveloped area of catalysis. Selenium dioxide catalyzes the allylic oxidation of a variety of olefins with TBHP, affording the corresponding allylic alcohols, but the system is homogeneous [3] and, hence, falls outside the scope of this book. The only heterogeneous catalysts for allylic oxidation which seem to have synthetic utility are palladium-based [4]. 

Note at the outset that asymmetric catalysis in the synthesis of fine chemicals is rarely a single-step process that converts a reactant directly to the final product. It is usually one of the steps in a total synthesis but is often the key step. Hence the analysis of the overall yield will be based on the methods described in Chapter 5. There are many types of reactions where asymmetric catalysis can be applied. The most important of these are C-C bond-forming reactions such as alkylation or nucleophilic addition, oxidation, reduction, isomerization, Diels-Alder reaction, Michael addition, deracemization, and Sharpless expoxidation (of allyl alcohols). A few representative examples (homogeneous and heterogeneous) are given in Table 9.6. 

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