Transformation promoter selection

In connection with reactions where a solvent is required, it must be noted that all transformations promoted by chiral Mo catalysts may be carried out in toluene (in addition to benzene) or various alkanes (e.g., n-pentane) with equal efficiency (see below for specific examples). Moreover, although 5 mol % catalyst is typically used in our studies, 1-2 mol % loading often delivers equally efficient and selective transformations. 

Abstract A description of selected types of reactions catalyzed by heme peroxidases is given. In particular, the discussion is focused mainly on those of potential interest for fine chemical synthesis. The division into subsections has been done from the point of view of the enzyme action, i.e., giving emphasis to the mechanism of the enzymatic reaction, and from that of the substrate, i.e., analyzing the type of transformation promoted by the enzyme. These two approaches have several points in common. 

Table 1.1 Selected biochemical key transformations promoted (catalyzed) by metal ions comparison... 

The literature is filled with various processes and catalyst compositions and systems for these transformations. Promoted platinum and sulfided platinum are the most selective group VIII metal catalysts but depending on reaction conditions and the nature of the halogenonitrobenzene, some undesirable halo-azo and azoxy compounds are left in the product (refs. 3, 11). 

Examination of the Enzyme Commission (EC) divisions of enzymes (2) reveals a large number of groups that promote carbon-carbon bond formation or cleavage. These are classified by the types of functional groups involved in the reaction rather than by the reaction pattern or the mechanism of the transformation. A selected list of enzymes involved in carbon-carbon bond formation illustrating this impression includes the following groups ... 

Iron salts are easily accessible, inexpensive and abundant and the metal itself is non-toxic. Their use should therefore become attractive from an economic and environmental point of view in a wide variety of carbohydrate transformations, in either stoichiometric applications or as a catalyst. As stated in the introduction, this review concentrates on a few transformations promoted by ferric salts used as Lewis acids in our laboratories and does not present exhaustive work done in carbohydrate chemistry with these salts. Many more other applications have been reported. However, their uses could be far more developed for fast and selective transformations of carbohydrates to useful new molecular constructs. Besides the acidic properties of iron(iii) presented here, iron chemistry is rich and could be particularly fruitful with carbohydrates in generating new types of complexes for regioselective transformations or in carbon-carbon forming reactions based on iron-catalyzed cross-coupling reactions. The glycochemistry community should certainly expect many more useful accomplishments in the near future. 

It is the aim of this chapter to present in detail a few selected examples of useful organic transformations promoted by Group 4-11 (Ti-Cu) metals rather than to give a comprehensive listing of all possible transformations, as this information is available in several other excellent books. - The protocols are selected to demonstrate the most common oxygenation (addition of O atoms) or oxidation (removal of H atoms) pathways encountered in transition metal-promoted reactions of organic substrates. 

Promoters are sometimes added to the vanadium phosphoms oxide (VPO) catalyst during synthesis (129,130) to increase its overall activity and/or selectivity. Promoters may be added during formation of the catalyst precursor (VOHPO O.5H2O), or impregnated onto the surface of the precursor before transformation into its activated phase. They ate thought to play a twofold stmctural role in the catalyst (130). First, promoters facilitate transformation of the catalyst precursor into the desired vanadium phosphoms oxide active phase, while decreasing the amount of nonselective VPO phases in the catalyst. The second role of promoters is to participate in formation of a soHd solution which controls the activity of the catalyst. 

The surface properties of three types of methanation catalysts obtained by oxidation of selected Intermetallics were examined In relation to their CO conversion activity. The first type (Ni Si, N1 A1 ) which corresponds to active phase-supporl iX the coXventionally prepared catalyst Is little affected by the oxidation treatment. The surface Nl is oxidized and relatively more abundant In the active solids. The second type (active phase-promoter ex Ni Th ) is extensively decomposed on oxidation. The transformation of these alloys Is accompanied by a surface enrichment in Nl. 

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