Electrochemical reduction pyridines

Ravindranath and co-workers studied the electrochemical behavior of 5-amino-2-phenyl-4-arylazo-l,2-dihydro-3//-pyrazol-3-one (90UC864) and 5-methyl-4-arylazo-2-(pyridin-2-ylcarbonyl)-2,4-dihydro-3//-pyrazol-3-(Mie (90IJC895). Similar studies were undertaken by Jain and Damodharan of pyrazol-3-ones 408a-f (95CJC176) (Scheme 94). The underlying rationale for this study on the electrochemical reduction of these biologically important pyrazol-3-ones is that it can lead to information on the reaction routes and mechanisms of biological redox reactions. 

Electrochemical reduction of 1-alkylpyridinium salts 1 leads to Are addition of one electron with the formation of a ii-delocalised radical-zwitterion. This is a formally neutral species. Botlr this species and the N-protonated pyridine radical-anion are essentially n-delocalised radicals. The radical-zwitterion from 1-methylpyridinium shows a long wavelength absorption band in water with 7, ax 750 nm [19]. The nitrogen ring radical-zwitterions take up further electrons at more... 

Formation of 2D phase accompanying electrochemical reduction of 4,4 -pyridine on mercury in the presence of iodide ions occurred via adsorption-nucleation and reorientation-nucleation mechanisms [139]. The first reduction step of Bpy on Hg in the presence of iodide as counterion in acidic medium at 15 involved the BpyH2 " /BpyH couple and led to the formation of a 2D phase. The increased contribution of the reorientation term in the formation of the condensed phase was consistent with the increased adsorption strength of the anion to the electrode surface. 

Infrared spectral studies have been made for a large number of octahedral metal-carbonyl systems of the types LM(CO)5 (L = pyridine and quinoline), L2M(CO)4 [L2 = ethylene diamine, (pyridine)2, and bipyridyl], [LM(CO)4]2 (L = dimethylarsine or dimethylphosphine M = Cr, Mo, and W), and [LFe(CO)3]2 (L = SMe, PMe2) before and after electrochemical reduction. The force constants for the carbonyl groups... 

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Electrochemical reduction of pyridines to piperidines can be achieved using various methods. Piperidines can be obtained in high yield by the electrochemical reduction of pyridine on a lead cathode in the presence of carbon dioxide and Pd — Ni or Cu — Ni catalysts (89KFZ1120). In the absence of catalyst, 4,4 -bipyridine was produced as the major product. 

Electron-attracting substituents facilitate electrochemical reduction. The reduction potentials for the polarographic reduction of quinoline and isoquinoline derivatives are much less negative than those for the pyridine analogues. Diazines are reduced electrochemically stepwise, usually as far as tetrahydro derivatives (70AHC(12)262). 

Electrochemical reduction of the pyridinium compounds (22, R = H, Bu1), prepared by reaction of 6-bromohexan-2-one and the respective pyridine, yields mixtures of the the fused azepines (23) and (24), albeit in low yield <95aG(K)2007>. A new synthesis of claviciptic acid has been published <95Joci486>. 

Catalytic hydrogenation of 3-phenylpyrido[3,4-e]-l,2,4-triazine (45b) over a palladium catalyst gave the 1,4-dihydro compound (29) in a 90% yield this is an unusual result as the position of the pyridine nitrogen atom does not seem to affect the formation of 1,4-dihydro derivatives (see also Section 7.17.5.4). Electrochemical reduction of the pyrido-l,2,4-triazine (45b) in both protic and... 

Reaction of pyridine with 0s02Cl2(PPh3)2 yields Os(py)3(PPh3)CI2, which can be reversibly oxidized to the osmium(III) and osmium(IV) derivatives electrochemical reduction of Os(py) (PPh3)Cl3 gives [Os"(py)2(PPh3)Cl3]-,93... 

Analogous results have been obtained in the electrolysis of the pyridine nucleotides134-141 the dimer has, however, been suggested to be a 4,4 -dimer.139 The coenzyme activity of the reduction product varies with the experimental conditions. Some reductions yielded totally inactive products, whereas others produced partly or fully active material. The highest activity was obtained by an indirect electrolytic reduction of triphosphopyridine nucleotide (TPN).141 Electrochemical reduction of methyl viologen in the presence of ferridoxin-TPN-reductase caused a reduction of TPN to a biochemically active product. 

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