Silver -mediated radical reaction

Compared to iron(III), copper(ll), and especially manganese(III) and cerium(IV) other metals have found less application for the oxidative generation of radicals [1]. An exception is cobalt(III)-mediated radical reactions, based on the pioneering work of Iqbal et ah, which was recently reviewed [20] (see also Volume 1, Chapter 1.8). Some examples of oxidative couplings of silyl enol ethers 44 in the presence of silver(I) oxide were developed [21]. However, there is no advantage over copper(II)-mediated radical reactions, since the reagent is more expensive and the 1,4-diketones 45 are isolated in only moderate yield (Scheme 15). 

Primary alkyl halides can also be alkynylated by silver acetylides. Isabelle and coworkers reported the reaction of methyl iodide, ethyl iodide and <7rmethyl iodide with several silver acetylides to give disubstituted alkynes.116 The authors preferred a non-radical-mediated mechanism for this reaction, as neither methane nor ethane, expected byproducts of a radical reaction, were observed. 

In 1970, Anderson and Kochi (99) reported a silver-mediated oxidative decarboxylation reaction with peroxydisulfate as the oxidant. Kinetic studies showed that the reaction is first order in both silver and peroxydisulfate and zero order in carboxylic acid. Silver(II) species and alkyl radicals are considered intermediates. 

In 2001, Rai and co-workers (114) reported a silver-mediated aziridination of olefins in THF with Chloramine-T. In their case, aprotic solvents gave better yields versus protic solvents. Then, in 2003, Komatsu and co-workers (115) used similar conditions and found no reaction in THF (solvent) while they detected 70% conversion in CH2CI2. Silver nitrate (AgNOs) was required stoichiometrically in this transformation. Komatsu proposed a nitrene-radical mechanism based on the fact that the reaction shut down in the presence of oxygen. They designed a model reaction using 1,6-dienes, and as they expected, bicyclic pyrrolidines were isolated as products instead of aziridines. The role of silver in this reaction is not clear and most likely a free nitrene radical is released with the precipitation of silver(I) chloride (Fig. 18). 

Quantum efficiencies for photoxidation were improved by platinization of the Ti02 photocatalyst, with the observed yield increasing linearly with platinum coverage [117-121], Acetone was the major product in the presence of air or silver ion, a result that was similar to those observed upon y irradiation or in the Fenton reaction. This similarity was interpreted as implying hydroxyl radical mediation of the observed oxidations. 

Direct, nonmediated electrochemical reduction of NADIP)" " at modified electrode surfaces has been used to produce the en2ymatically active NAD(P)H and even to couple the NAD(P)H regeneration process with some biocatalytic reactions [228]. The modifier molecules used for these purposes are not redox active and they do not mediate the electron-transfer process between an electrode and NAD(P)+ however, they can effectively decrease the required overpotential and prevent formation of the nonenzymatically active dimer product [228]. For example, the efficiency of the direct electrochemical regeneration of NADH from NAD" " was enhanced by the use of a cholesterol-modified gold amalgam electrode that hinders the dimerization of the NAD-radicals on its modified-surface [228]. This direct electrochemical NAD+ reduction process was used favorably to drive an enzymatic reduction of pyruvate to D-lactate in the presence of lactate dehydrogenase. The turnover number for NAD" " was estimated as 1400 s k Other modifiers that enhance formation of the enzymatically active NAD(P)H include L-histidine [229] and benzimidazole [230], immobilized as monolayers on silver electrodes. CycKc voltammetric experiments demonstrated that these modified electrodes can catalyze the reduction of NAD+ to enzymatically active NADH at particularly low overpotentials. 

---