Nonmetallic catalysts reduction

Asymmetric catalytic reduction reactions represent one of the most efficient and convenient methods to prepare a wide range of enantiomerically pure compounds (i.e. a-amino acids can be prepared from a-enamides, alcohols from ketones and amines from oximes or imines). The chirality transfer can be accomplished by different types of chiral catalysts metallic catalysts are very efficient for the hydrogenation of olefins, some ketones and oximes, while nonmetallic catalysts provide a complementary method for ketone and oxime hydrogenation. 

Asymmetric Reduction of Ketones Using Nonmetallic Catalysts... 

Table 11.4 Catalytic reduction of ketones by nonmetallic catalysts (results according to the relevant publications).

There are also nonmetallic catalysts, of which the best known are the various forms of activated carbon. For example, isomerization of inert, chiral cobalt(in) complexes is accelerated significantly by the presence of carbon this is assigned to reduction of cobalt(III) on the surface of the carbon to labile cobalt(II), allowing rapid ligand rearrangement, with air subsequently rapidly re-oxidizing the cobalt(II) to cobalt(III) before ligand dissociative processes can become involved. 

It is important to compare these nonmetallic catalysts with the performance of zeolite catalysts because few catalysts have been able to successfully activate CH4 for NOx reduction. One obvious difference between these two catalyst systems is that all the zeolite catalysts have shown a volcano-like (or bending-over) activity dependence on temperature, with the activity maximum falling between 670 and 770K. Li and Armor believe that the decrease in NO conversion at high temperature may be the result of significantly lowered CH4 concentrations due to combustion (8). However, Petunchi... 

Attempts to use nonmetallic catalysts for the catalysis of the oxygen reduction reaction in acid solutions have been unsuccessful. Tungsten-sodium bronzes or tungsten oxides, when used for this purpose, have to be... 

In addition, catalysts for C02 reduction based on nonmetallic compounds have also been reported. Taniguchi et al100 reported... 

Nonmetallic systems (Chapter 11) are efficient for catalytic reduction and are complementary to the metallic catalytic methods. For example lithium aluminium hydride, sodium borohydride and borane-tetrahydrofuran have been modified with enantiomerically pure ligands161. Among those catalysts, the chirally modified boron complexes have received increased interest. Several ligands, such as amino alcohols[7], phosphino alcohols18 91 and hydroxysulfoximines[10], com-plexed with the borane, have been found to be selective reducing agents. 

The coverage in this chapter is not comprehensive and no tabulation of rate data has been attempted. A more extensive account of the material will appear in Volume 9 of the series the presentation of the material also differs slightly from previous volumes. There have been many important developments in this general area. A comprehensive review of one-electron reduction potentials for nonmetallic substrates has appeared. Detailed kinetic studies have been reported for several important reactions where metal ions serve as catalysts in the transformation of organic substrates.Catalytic oxidation reactions involving metal complexes and macrocyclic metal complexes have been reviewed. Continued interest centers on catalysis by metalloporphyrins, and the role of metal complexes in electrocatalytic reductions has been reviewed. ... 

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