Radical species reductive dimerization

The 1,5-, 1,6-, 1,7-, 1,8-, 2,6-, and 2,7-naphthyridines have been electro-chemically reduced to afford radicals that decay by a slow shift of hydrogen from nitrogen to carbon.143 The resulting radicals dimerize readily. In acid media the first reduction step produces a radical-cation that is relatively stable in the 1,7- and 2,6-naphthyridines, whereas in 2,7-naphthyridine, the species is stable for a few minutes only. All of these radical-cations undergo a hydrogen shift from nitrogen to carbon to form unstable radicals that react with original cation radicals to form dimers. The process is an acid- or base-catalyzed first-order reaction.144... 

Similarly, phenothiazine may be oxidized to the cation radical species which then dimerizes forming the 3,10 -diphenothiazinyl species (Tsujino, 1969). The product of the electron-transfer step may react, via a second-order process, with a species in solution to form a new product. An example of this type of mechanism involves the reduction of anthraquinone and its derivatives in the presence of oxygen (Jeziorek etal., 1997). To understand quantitatively an EC and EC2 process, the concentration and scan-rate dependence of the associated cyclic voltammograms is matched with theory deriving from the mass transport/kinetic equations for each species. 

In deference to their biological significance much work has been done on this class of compound a model studies to analyze or mimic the behavior of pyridine nucleotides and bipyridinium herbicides, py ridinium ions (132 Scheme 26) being more susceptible to reduction. Dimerization is a common fate o the neutral radical formed. The most thermodynamically stable product, the 4,4 -bipyridyl species pre dominate, and other dimers can sometimes rearrange to the 4,4 -bipyridyl isomer. Radicals with mere... 

A radical species may also be generated by reduction of an electron-deficient compound and a classical entry to 4,4 -bipyridines is the reduction of a pyridine by sodium and subsequent rearomatization. Figure 8a illustrates the use of such a reduction in order to prepare the precursor of a sodium-ion molecular switch [30], A more general route, derived from the ancient copper-catalyzed Ullmann coupling, is the metal-induced dimerization of an aryl halide. The key step is a reductive elimination within the coordination sphere of the metal. A nickel(O) complex, in stoichiometric quantities, is usually selected for this purpose. Constable and Ward have used such a reaction to prepare a bis-terpyridine from an interesting synthon, which would have otherwise required a more specialized strategy with dedicated intermediates (Figure 8b) [31]. 

Reduction in dry aprotic solvents allows the formation of ion pairs between the radical anion and a countercation. Where the cation can interact simultaneously with two radical species, dimerization favors the ( )-product because there are fewer steric interactions, in the corresponding transition state, between groups attached to the reacting radicals. Solvent and supporting electrolyte combinations that give a high yield of ( )-pinacol are acetonitrile with tetraethylammonium bromide [82], dimethylformamide with sodium perchlorate [83], and dimethylformamide with europium(III) chloride. Europium(II) is formed in the last... 

Reduction of acetophenone in acetonitrile containing (5)-trimethyl-l-phenylethyl-ammonium salts shows no optical induction in the 1-phenylethanol formed, but the ( )-pinacol is obtained with up to 26% excess of one enantiomer [95]. These results further confirm the importance of some counterions in orienting radical-ion and radical species during the dimerization step to form the pinacols. 

Functionally substituted benzylic, allylic, and vinylic compounds containing alkoxides, esters, ethers, nitriles, or amides can be reacted with halosilanes under Barbier conditions using HMPT to yield C- and O-silylated products, 1,2- or 1,4-addition products, as well as reductive dimers. Radical and anionic intermediates are postulated, based on SET reactions from the metal, and multiple silated species can be obtained. The use of the TMSCl-Mg-HMPT system has been extensively investigated by Galas group [85] at the University of Bordeaux, and their work has greatly advanced the science of the Barbier reaction with silanes. 

Stereoselective radical addition on the exo-face of levoglucosenone 250 leads to the C-linked dimer 251 in a modest 26% yield. The expected competing reaction is the direct hydrogen abstraction by the initial radical species. Stereoselective hemiketal and ketone reductions afford the 1,6-anhydro derivative 252, which is deprotected and opened to give the final C-ana-log of a /3-(1 4) disaccharide 253. 

Detailed meehanistic studies have demonstrated that the reduction process for [(CO)3Fe(/t-Se)2Fe(CO)3], 2, ultimately leading to the corresponding dianion 5, is stepwise and not unimolecular. The added electrons are unpaired at the bridging atoms and the radical species combine in a tetrameric intermediate whose scission finally yields the dianonic dimer 5 with Fe-Fe but no Se-Se linkage. Apparently, inconsistent with the latter pathway, the electrochemical oxidation of the iw-sulfido dianion 5 has been quoted to lead directly to the uncharged species 1... 

A biomimetic route for the synthesis of dihydrobenzo[b]furan heterocycles was investigated via a reagent-based electrochemical transformation of 2-(2 -hydroxyethyl)-quinine precursors. It was believed that a putative oxonium ion was generated by treatment of the quinone with PPTS, followed by reduction with dihydroquinone (DHQ) to generate a radical species which underwent either further reduction by DHQ to give the product or a direct dimerization <01JOC4965>. 

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