Redox transfer

This is one of the steps in the copper-catalyzed redox-transfer chain addition of arenesulfonyl chlorides to styrenes (vide infra). The p-value of + 0.56 indicates the involvement of a simple atom transfer as well as a polar contribution to the transition state. 

Although sulfonyl chlorides add readily to unactivated olefins, with vinylic monomers telomeric and/or polymeric products were observed. This difficulty has been overcome by carrying out the addition in the presence of catalytic amounts of CuCl2, so as to provide a general and convenient synthesis of /5-chlorosulfones (Asscher-Vofsi reaction)63. For the copper-catalyzed system a redox-transfer mechanism has been suggested in which the... 

Arenesulfonyl chlorides77 as well as alkenesulfonyl chlorides78 react with vinylarenes in the presence of RuCl2(PPh3)3 and 1 molar equiv. of Et3N to form a,/ -unsaturated sulfones in 70-90% yields. The reaction mechanism for the ruthenium(II) catalyzed reaction involves a free-radical redox-transfer chain process as outlined below77 ... 

Ruthenium complexes are capable of catalyzing halogen atom transfer reactions to olefins. This has been illustrated in the enantioselective atom transfer reactions of alkane and arene-sulfonyl chlorides and bro-motrichloromethanes to olefins using chiral ruthenium complexes. Moderate ee s up to 40% can be achieved for these transformations [74-77]. These specific reactions are believed to follow a radical redox transfer chain process. 

Some radical reactions occur under the control of transition metal templates. The first example of asymmetric creation of an asymmetric carbon with a halogen atom is shown by the a DIOP-Rh(I) complex-catalyzed addition of bromotrichloromethane to styrene, which occurs with 32% enantioselectivity (Scheme 99) (233). Ru(II) complexes with DIOP or BINAP ligands promote addition of arenesulfonyl chlorides to afford the products in 25-40% ee (234). A reaction mechanism involving radical redox transfer chain process has been proposed. 

Modification of the electrochemical properties of a redox centre surrounded by dendritic fragments [93] can lead to two different dendritic effects. The first one is manifested in a shift of the redox potentials, the extent and direction depending upon the dendritic architecture and the solvent. Such behaviour was observed in dendritic iron-porphyrins [94]. The second effect is apparent in a delay of redox transfer kinetics and is characterised by a stepwise increase in the distance between the peaks in a cyclovoltammogram with increasing dendrimer generation number. 

Asscher and Vofsi649,650 have discovered that copper(I,II) and iron (II,III) strikingly enhance the activity of CC14 and of CHC13. The former completely suppresses telomeriza-tion and the latter only partially so. A new path (redox transfer) has been proposed for the above termination step (equation 86). 

At present we are conducting experiments with XIX, XX, and other flavins with crude hydrogenase (quite air labile) kindly provided by Wolfe. Initial results demonstrate ready reduction of biological and synthetic Fo (XX) at approximately 200 nmol/min/mg while the 7-methyl compound, XIX, is reduced tenfold more slowly (at equivalent 20fiM concentration). Studies are underway to investigate ratedetermining steps, flavin specificity, stereochemistry, and mechanism of redox transfer between H2 and F420 as well as its reoxidation by NADP. 

The first term is usually very high and it demonstrates perfectly the step of termination/redox transfer. Under these conditions, it is believed that Bamford observed such a process in his investigation since the value CMe is 75 and Ccci4 0.013. From such a mechanism, numerous bistelomerizations were performed in our laboratory to produce either monoadducts or diblock cotelomers [107,108]. Thus, difunctionalization by allyl acetate of telomers which exhibit trichloromethyl end-groups was successfully achieved as follows ... 

C. Cyclopropane Models for NADH Redox Transfer Mechanisms... 

In contrast to mediators, also redox polymers can be used. These polymers are mainly characterized by the presence of specific electrochemically active sites. There are different possibilities to facilitate redox transfer by shuttling electrons via redoxactive groups in non-conductive or conductive polymers. In general, a redox polymer consists of a system where a redoxactive molecule is covalently bound to a polymer backbone which may or may not be electroactive. Erequently, electroactive polymers are formed by the electropolymerization of suitable monomer complexes. A few representative examples of electron shuttle molecules are shown in Eig. 1. 

The essential aspects of mechanism are the dissociative adsorption of water on ultrafine gold particles, followed by the spillover of active OH groups onto adjacent sites of the ferric oxide. The formation and decomposition of intermediate species are accompanied by redox transfer of Fe Fe in Fe304 and the reverse step during the dissociation of water molecule. 

FIGURE 1. Metabolic pathways of redox transfer between subcellular compartments... 

A somewhat simpler rate law is observedjji oxidation of [Cofedta)] -, In this system there is kinetic evidence for an inner-sphere process involving the complex [(edta)Co i—which undergoes an intramolecular redox transfer with the formation of a cobalt(ra)-edta complex which ring-closes to the sexadentate [Co(edta)] ion. 

But in mammalian species, several mechanisms exist for transferring NADH reducing equivalents, i.e., the malate-aspartate cycle (10) and the a-glycerophosphate-dihydroxy acetone phosphate cycle (16). It seems unlikely that the magnitude of NADH redox transfers due to the interconversions of proline - P5C would alter the N AD+/NADH balance. The oxidation of NADPH by P5C reductase, on the other hand, may play a major role in regulating NADP+/NADPH. Importantly, the of P5C reductase for NADPH is markedly lower than that for NADH (86,117). Although the V .. activities are higher with NADH than with NADPH the conversion of P5C to proline by PC reductase would affect NADP / NADPH more than NAD+/NADH if one considers the respective in vivo concentrations and the respective redox ratios of the two pyridine nucleotides. 

B. Theoretical Redox Transfers Mediated by the Interconversions of Proline, Ornithine, and Glutamate... 

Because the proposed transfers of redox mediated by the metabolism of P5C can be catalyzed by several enzyme mechanisms, inborn deficiencies of any one of the mechanisms would not necessarily result in a total absence of P5C-mediated redox transfers. In fact, pathophysiologic manifestations may be limited to those tissues in which the deficient mechanism plays a major physiologic role. For example, in gyrate atrophy of the choroid and retina with absent ornithine aminotransferase, the pathology is restricted to ocular tissues. Within the proposed scheme for P5C-mediated redox transfers, this tissue specificity may be due to the absence or relative deficiency of proline oxidase and P5C synthase in ocular tissues (61). The proposed redox transfer mechanisms would then be deficient since there is no source of P5C. We can speculate that the deficiency in the transfer of cytosolic NADPH (Table H) is related to the pathogenesis of the ocular pathology. 

The proton conduction properties of layered matrices HM(III)(S04)2.nH20, M=Fe, In and n = 1 and 4, have previously been reported (16), and the redox transfer characteristics of the ferric tetrahydrate member have been demonstrated. Facile insertion of monovalent or divalent ions occurs, with retention of the layered structure, and reduction of Fe(III) centres (17). The negatively charged layers of HFe(S04)2-4H20 contain octahedral Fe(III), the coordination sphere of which is formed by 4 oxygen atoms from sulphate groups and two from water molecules (18) (Figure 1). 

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