Pyridines cathodic reduction

A further difficulty arises during preparative electrolyses in aprotic solvents because of the bulk pH change which commonly occurs. Thus cathodic reductions often require proton abstraction from the solvent in order to yield stable products, while many anodic oxidations, mcluding those of aromatic and aliphatic hydrocarbons, give rise to a quantitative yield of proton and the consequent changes in the pH. of the electrolysis media would be expected to lead to a variation in the products with the duration of the electrolysis. Unfortunately, the pH can be a very difficult parameter to control in aprotic solvents and most work reported in the literature has been carried out in unbuffered conditions. In the case of oxidations, organic bases, e.g. pyridine, have... 

Selective cathodic reduction of pyridines is a process of great industrial significanee [171]. As an example, dimethyl pyridinedicarboxylates undergo a highly selective electroreduction in methanol by use of a divided cell. The product obtained depends on the position of the substituents. Thus, the 2,3- and 2,5-dicarboxylates give the 1,2-dihydropyridines whereas the 2,6-, 3,4- and 2,4-dicarboxylates the 1,4-dihydropyridines [172]. Many other ring reductions of azines (often with dimerization as a side-reaction, see Section 6.4.3) are discussed in electrochemistry texts and reviews [116]. Attempts have been made to rationalize the herbicidal properties of dipyridinium salts in terms of their cathodic behavior [173]. 

It has been demonstrated by Bhadani and Parravano that pyridine anion radicals, formed by electrochemical reduction, are able to initiate the polymerization of 4-vinylpyridine, if the monomer is added to the yellow or blue pre-electrolyzed pyridine solution54. The yellow color is attributed to the Py and the blue one to the 4,4 -Bipy anion radicals. 4-Vinylpyridine, if present during the electrolysis undergoes direct cathodic reduction giving rise to anionic polymerization. 

A similar procedure led to polymerization of a-methyl styrene297), while Bhadani and Parravano208) reported a rapid and quantitative polymerization of 4-vinyl-pyridine. The latter reaction was initiated by cathodic reduction of the monomer in hexamethyl-phos-phorictriamide with NaBPh4 as the supporting electrolyte. 

In subsequent years, electropolymerization was definitely the established procedure. Polymerization by cathodic reduction, even exploiting redox mediation at the electrode, has been carried out on a wide series of Fe(II), Ru(II), and Os(II) vinyl-containing complexes based on differently substituted pyridine, and the relevant polymerization mechanism was extensively discussed in the same article [13]. An element of complexity of the RPs electrochemical growth hes in the presence, in the monomer, of additional electroactive groups. 

Another example concerns the initial electronic reduction of a-nitrostilbene (Todres et al. 1982, 1985, Todres and Tsvetkova 1987, Kraiya et al. 2004). The reduction develops according to direction a in Scheme 2.9 if the mercury cathode as well as cyclooctatetraene dianion are electron sources and according to direction b if the same stilbene enters the charge-transfer complexes with bis(pyridine)-tungsten tetra(carbonyl) or uranocene. For direction b, the charge-transfer bands in the electronic spectra are fixed. So the mentioned data reveal a great difference in electrochemical and chemical reduction processes a and b as they are marked in Scheme 2.9. 

Electrolytic reduction using a lead cathode in 20% sulfuric acid converted pyridine a-carboxaldehyde to a mixture of 41% of a-picoline, 25% of a-pipecoline and 11% of 2-methyl-1,2,3,6-tetrahydropyridine [443]. 

Miller and his co-workers60) reported surprisingly high optical yields, close to 50 %, in the reduction of 2-acetylpyridine in the presence of strychnine. They also prepared chemically modified electrodes with optically active amino acids and attempted asymmetric induction in both reduction and oxidation61 . The best optical yield, only 14.5 %, seemed to be obtained in the reduction of 4-acetyl-pyridine on a graphite cathode modified with (S)-phenylalanine methyl ester. 

Pentafluoropyridine (292) is reduced440 in dry DMF at a mercury cathode to perfluoro-4,4 -bipyridyl (293) in the presence of hydroquinone as proton donor, 2,3,5,6-tetrafluoropyridine (294) is obtained [Eq. (150)]. Pentachloro-pyridine gives on reduction in dry DMF very little bipyridyl derivative, but tetrachloropyridine and bis(tetrachloropyridyl) mercury. 

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. 

Since bromo(pyridine)cobaloxime(III) was not commerically available and its synthesis was not convenient36, we utilized chloro(pyridine)bis(dimethylglyoximato)-cobalt(III) (Equation 3) (also known as chloro(pyridine)cobaloxime (III)) instead. It has four cathodic waves in polarography when observed in acetonitrile. Its half wave potentials are located at -0.65, -1.45, -2.42, and -2.92 volts vs the Ag/AgNC>3 electrode, corresponding to the reduction of the cobalt from +3 to +2, +1, and 0, and the reduction of the ligand, respectively. 

Reduction of pyridine by electrolytic methods is the oldest reported industrial process involving pyridine compounds. Merck patented this process in 1896 however, catalytic hydrogenation has supplanted this process for virtually every piperidine manufacturer.8,14,13 The earliest report was by Ahrens, who described a process that others could not repeat.16 Up to 1934 the technology was to use an aqueous sulfuric acid electrolyte and a lead cathode. Many of these reports are conflicting.17-19 The interacting nature of electrochemical variables may be responsible in part for these discrepancies. Thus experimentation by an approach that attempts to hold all but one variable constant is bound to lead to different results depending on where the starting point was chosen or whether an important variable was, or was not,... 

The production of bipyridyls (1) and bipiperidyls (2) was observed on reduction of pyridine. Schering AG has a patent on a process for producing 4,4 -dipyridyls at the cathode of a divided or undivided cell using liquid ammonia as the solvent.26 The same bipyridyl was also formed during electrolysis of bromobenzene in pyridine solvent, using Mg electrodes.27 Bipiperidyls (2) were observed as products of pyridine reduction as early as... 

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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