Sulfides, 2-pyridyl coupling reactions

Scheme 10 is representative of the mechanism of these coupling reactions involving a captodatively stabhzed glycyl radical 15 from the initial reduction of the pyridyl sulfide group by the divalent lanthanide reagent. Further reduction of this carbon radical by a second equivalent of samarium diiodide leads to a Sm(lII) enolate intermediate 16 of unknown geometry, which ultimately reacts with the carbonyl compound to give 17. 

The preparation of N-alkoxy derivatives (i.e., type V) is somewhat more difficult They can be prepared by treatment of the sodium salt of 2-mercaptopyridine-N-oxide with 1 molar equivalent of the suitable alkyl halide in DMF at 80°C. ° By using this coupling reaction, alkylation occurs at sulfur rather than at oxygen, and the reaction affords pyridyl sulfides as the major product. In such cases, the Mitsonobu method using diethyl azodicarboxylate and triphenylphosphine may provide a solution (Scheme 8). ... 

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