Pyridinium ions reductive dimerization

An unusual thermal dimerization has been reported for the cyano-l,2-dihydropyridines. The preparation of (54), by the sodium borohydride reduction of the corresponding pyridinium ion, gave only the [2 + 2] dimer (55) (75JOC559). Heating this compound to 85 °C resulted in isomerization to the [2 + 4] dimer (56). This type of dimer appears to be restricted to the cyano-substituted dihydropyridines (74HCA1204). 

Initial electron transfer to a pyridinium ion forms a neutral radical (94) the mechanistic pathways open for reduction should be (and are) different from those for neutral pyridines where the initial electron transfer forms a radical anion (see Scheme 1). Radical 94 can dimerize, abstract hydrogen, transfer an electron to another substrate, undergo a crossed hydrocoupling, or be further reduced to the anion 95 (Scheme 33). Protonation or hydrocoupling reactions can ensue from 95. Most of these reaction pathways seem to have been experimentally observed. 

The electrochemistry of N-substituted nicotinamides (101) dominates the reports on ring-substituted pyridinium ions. There is also some controversy about the products of reduction. Elving et al. have reviewed this area up to about 1975.158 Apparently there is little doubt that a dihydro product, either 102 or 103, and dimer, either 104 or 105, are formed (Scheme 36). The latest... 

Pyridinyl radicals are prepared primarily by reduction of the corresponding pyridi-nium ions, but may also be generated for special types of experiments by dissociation of dimers and disproportionation of pyridinium ions and dihydropyridines... 

New electrochemical techniques have made possible the measurement of reliable one-electron reduction potentials for pyridinium ions. The shifts in potential introduced by subsequent reactions at the electrode (chiefly, dimerization) have lead to a variety of useful kinetic and thermodynamic properties of the pyridinyl radicals. The potentials are listed in Table 5, along with some values for the one-electron reduction potentials of pyridinyl radicals. 

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... 

Pyridinium ions can be reductively dimerized. Thus 1-methylpyridinium chloride yields the dimer 136 either with Na/Hg or by cathodic reduction via the radical 135. The dimer 136 can be oxidized to the bipyridinium dication 137 known as the herbicide paraquat [62]. 

Simple pyridinyl radicals, Py, like those derived from 1-ethyl-4-carbomethoxy-pyridinium ion through l reduction, have proven useful for the investigation of properties of radicals, including complexes, and the study of the mechanisms of their reactions. Discoveries include (a) distinction between atom-transfer and electron-transfer reactions by means of solvent effect on rate constants, (b) intramolecular radical complexes (ir-mers) within pyridinyl diradicals and pyridinyl diradical metal halide complexes, (c) occurrence of either electron-transfer or dimerization between pyridinyl radicals in water according to structure and (d) con5)lexation between pyridinium ions and pyridinyl radicals. Consideration of actual and potential biological roles for pyridinyl radicals is aided by these discoveries. 

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