Regioselective Syntheses of Substituted Pyridines and 2,2/-Bipyridines by Cobalt-Catalyzed (2+2+2) Cycloaddition of alpha,omega-Diynes with Nitriles

摘要

In the presence of a 1,2-bis(diphenylphosphino)ethane-cobalt(II) chloride-zinc catalyst in l-methylpyrrolidin-2-one at room temperature to 50 癈, alpha,omegadiynes reacted with nitriles by a [2+2+2] cycloaddition pathway to give annulated pyridines or 2,2'-bipyridines. The regioselectivity of the reaction is controlled by a combination of steric and electronic effects. The reaction of diynes with a terminal trimethylsilyl group gave the corresponding 3-(tri-methylsilyl)pyridines exclusively; these products could be proto- or halodesilylated to give the corresponding protonated or halogenated pyridines or 2,2'-bipyridines.