Electrochemical reduction pyridinium salts

The pyridinium salt NAD 19a and its reduced form NADH 20a are important co-factors for many enzymes, fhe reduced form is involved in enzyme mediated reductions where it is converted to NAD. In natural systems, NAD is converted back to NADH by another enzyme-controlled process. Attempts to effect the direct electrochemical conversion of NAD to NADH are not very successful. Reduction on a mercury cathode at -1.1 V see on the first one-electron reduction wave leads to the radical-zwitterion, which reacts further to give dimers. Three stereoisomers of the 4,4 -dimer account for 90 % of the mixture and three 4,6 -dimers form the remainder [78]. Reduction at -1.8 V on the second reduction wave produces only 50 % of enzymatically active 1,4-NADH. The NAD analogue 19b shows related behaviour and one-electron reduction affords two diastereoisomers... 

SET-induced chain reactions are usually much faster than Sn2 reactions, and if both pathways are accessible for a given pair of starting materials, products resulting from SrnI will usually predominate. This is, for example, observed for the first reaction sketched in Scheme4.4 [11, 16]. Electrochemical or zinc-mediated [17] reduction of the starting pyridinium salt yields a carbanion which, on reaction with Me2tBuS+, can either be methylated via Sn2 or tert-butylated via SrnI. Because the nucleophile... 

One-electron electrochemical reduction of pyridinium salts 181 yields mixtures of four isomeric dimers 182-185. The two most abundant products are 182 and 183 (Equation 95) <2002J(P1)542>. 

Exhaustive reduction of an exactly known amount of pyridinium salt (weight, absorption spectrum) leads to accurate absorption coefficients which agree with those obtained by alternative means. Electrochemical techniques have been used to probe some of the chemical properties of both mono- and bis-pyridinyl radicals (see also Sect. 4.5). 

UV photolysis of CpMn(CO)3 in toluene leads to loss of CO and formation of CpMn(CO)2( ] -toluene). Kinetic studies suggest that the binding energy of the toluene is ca. 60kJmor. The binding of H2 to CpMn(CO)2 has been studied in supercritical CO2 solvent. It has been proposed that pyrylium and pyridinium salts such as (35) can be used to label proteins and thereby aid in the detection and characterization of receptor sites. Cymantrene bound to lysine residues of bovine serum albumin (BSA) has been used as a redox label. Electrochemical reduction of the label established an impressive BSA detection limit of 2 x 10 M. 

Electrochemical oxidation of the reduced form (NADH) of nicotinamide adenine dinucleotide and its analogs has been investigated [262] best results are obtained using a carbon felt electrode. The results are consistent with an ECE mechanism yielding the pyridinium salt. Indirect oxidation of NADH (and reduction of NAD" ) is treated in Chapters 27 and 29. 

The Hupp group utilized electrochemical oxidation of [Ru(phen)3]2 + in the presence of tert-butyl-4-pyridine, 4-phenylpyridine, or 4,4 -bipyridine to prepare the pyridinium derivatives, (29), (30), and (31), respectively.130 It is believed that activation of coordinated (1) is facilitated by oxidation of ruthenium(II) to ruthenium(III). Subsequent elimination of an H atom from (1) by the substituted pyridines is likely accomplished by spontaneous reduction of ruthenium(III) to ruthenium(II), producing the pyridinium salt. 

The pioneering work of Conant showed that the one-electron reduction of pyridinium ion by low-valent vanadium produces the corresponding carbon radical, which dimerizes to give a homocoupling product. Saveant studied the electrochemical reduction of stable iminium salts, and observed two reduction waves in the polarograms (Scheme 5.27). The first wave corresponds to the one-electron reduction process for which dimerization occurs. This process presumably involves formation of the carbon centered radical. The second wave is concerned with the formation of the amine by two-electron reduction. Wayner performed extensive work on oxidation and reduction potentials of carbon radicals.A modulated... 

Fig. 26. When the dibromide salt 19-2PF6 is reacted with bis (pyridine) ethylene in the presence of BPP34C10 in acetonitrile, the resultant [2]catenane 18-4PF6 is formed in 43% yield. The crown ether resides preferentially around the bipyridinium site in a 92 8 ratio with respect to occupancy around the bis(pyridinium) ethylene site - co-conformer A. a Upon electrochemical reduction, the best electron donor - the bipyridinium site - is reduced first. This reduction leads b to the unfavorable situation in which the bipyridinium radical cation is located within the cavity of the crown ether, and so the cyclophane circumrotates c to locate the bis(pyridinium)ethylene site within the crown ether cavity - co-conformer B. This process is reversible, in that d reoxidation of the bipyridinium radical cation leads to e circumrotation of the cyclophane to yield the [2]catenane in its original state - co-conformer A...

One-third of all pyridine electrochemical citations deal with the electrolysis of quaternary salts of pyridines two out of five cathodic reports are concerned with them. Moreover, the products of reduction and the salts themselves are commercially valuable. A whole class of biochemical transformations depends on the reactivity of pyridinium ions. Agricultural products are also derived from these salts, and the value of bipyridiniiim herbicides is directly linked to their redox chemistry. 

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