Cobalt metallacycles. Part 5. Synthesis of pyridines from nitriles and acetylenes via cobaltacyclopentadienes

The complexes, [[graphic omitted]R4)(cp)(PPh3)](cp =η-cyclopentadienyl) react with nitriles to give highly substituted pyridines. When R4= CO2Me, nitrites react regioselectively such that the nitrogen atom in the resulting pyridine ring occupies a position away from the carbon to which the CO2Me is attached. The reaction with 2-methylacrylonitrile gives, beside the pyridine, a cyclohexadiene derivative and its complex. The complex [Co(cp)(PhC⋮CPh)(PPh3)] does not react with nitriles to form a nitrogen-containing cobalt metallacycle. The mechanism of the stoicheiometric and catalytic reaction is discussed.