Organic substrates, functionalization transfer

The use of polyethers and quaternary salts as liquid-liquid and solid-liquid phase transfer catalysts has been well-documented in the literature. It has been shown that (1) the catalyst functions as a vehicle for transferring the anion of a metal salt from the aqueous or solid phase into the organic phase where reaction with an organic substrate ensues, (2) the rate of reaction is proportional to the concentration of the catalyst in the organic phase, and (3) small quantities of water have a significant effect on the catalytic process. This Communication specifically addresses the role of cyclic polyethers as phase transfer catalysts and the influence of water with regard to the location of the catalyst. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

Castorina et al (Refs 164 180) studied the surface activity of one-micron size a-HMX as a function of Co60 gamma dose in vacuo and vapors of H20, NO and N02. The production of polar surface adducts suggests that the mechanism of energy transfer from the bulk of the substrate to the surface-vapor phase interface be postulated to apply to crystalline organic substrates. By this mechanism changes in surface properties can be achieved without any serious... 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

Several enzymes such as reductases and dehydrogenases utilize nicotinamide derivatives as reversible carriers of redox equivalents. The reduced dihydronicotinamide moiety NAD(P)H acts by donating a hydride equivalent to other molecules. In the corresponding two-electron oxidized NAD(P) form, the cofactor formally accepts a hydride ion from the substrate. Functional models of such reversible hydride transfer processes are of considerable interest for biomimetic chemistry, and the strategies to regenerate nicotinamide-type cofactors are crucial for the performance of many organic transformations involving biocatalytic key steps 139,140). 

Some hydrogenations can be also carried out under so-called inverse PTC (IPTC) conditions where the function of a PT agent (e.g, cyclodextrin, CD) comprises the transfer of an organic substrate into aqueous phase [44]. Conjugated dienes are reduced with hydrogen to monoolefins in the presence of y9-CD and hydridopentacyanocobaltate anion, generated in situ, in alkaline aqueous solution [45], The same catalytic system is also highly effective for the IPTC reduction of the C=C bond in a,)ff-unsamrated carbonyl compounds [46]. 

Figure 5 Rate of the outer-sphere electron-transfer oxidation of an organic substrate by a POM (oxidation of S S -tetra-Bu biphenyl-TT-diol by a-SiVW11O405 to form a-SiV W O406 and the DPQ in 100% selectivity Equation (12)) as a function of the cation concentration (counterions + added MCI), M+ = K+ (top curve) Na+ (second curve from top) Li+ (third curve from top), and THAN (the nitrate salt of the nonpairing cation, tetra-w-heptylammonium bottom curve) (reprinted with permission from J. Am. Chem.

Scheme 4 General functionalization of organic substrates by "E" transfer with Tp ML catalysts...

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