Pyridine anion-radicals

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... 

The preceding results and Ashby s observation (81JOC2429) that LDA acts as the single-electron donor towards aromatic hydrocarbons, stimulated Newkome (82JOC599) to study the reaction of pyridine with LDA (HMPA at 0°C) (Scheme 149). The products of this reaction, 2,4 - (77) and 4,4 - (494) bipyridines, obtained in the ratio 1 8, were rationalized as being formed from a pyridine radical-anion 493. In support of this proposal, complex ESR signals corresponding to both pyridine radical-anion and the diisopropylamino radical were observed under these conditions. 

Bipyridyl (20) (63AHC(2)179) is synthesized by oxidative dimerization of pyridine over hot Raney nickel, while 4,4 -bipyridyl (21) (B-79MI10705) is made by free radical coupling of the pyridine radical anion generated by sodium in liquid ammonia, followed by air oxidation (Scheme 6). Quaternization of (21) with methyl chloride gives paraquat while reaction of (20) with 1,2-dibromoethane gives diquat. 

Unfortunately, ( PDI)Mn(THF)2 exhibited little activity for the hydrogenation of alkenes or [2 + 2] cyclization of dienes. In an attempt to synthesize an active bis(imino)pyridine manganese precatalyst, alternative reduction conditions were explored. Sodium amalgam reduction of ( PDI)MnCl2 in pentane yielded the red bis(chelate) complex ( PDI)2Mn. X-ray diffraction established a cis-divacant octahedral compound where one imine arm on each of the chelates is dissociated from the metal center. The metrical parameters from X-ray diffraction in conjunction with SQUID magnetic and EPR spectroscopic data established that the overall S = 3/2 compound is best described as a high-spin Mn(II) species (Sy = 5/2) with two bis(imino)pyridine radical anions. 

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