A review on the assembly of multi-substituted pyridines via Co-catalyzed [2 + 2 + 2] cycloaddition with nitriles

In recent years, many methods for the facile synthesis of pyridines and their derivatives have been developed. The [2 + 2 + 2] cycloaddition reaction of alkynes and nitriles catalyzed by transition metals has emerged as the most straightforward and efficient method to obtain pyridine derivatives. Recently, Earth-abundant cobalt has been employed as a versatile and economical catalyst for the synthesis of functionalized molecules, as compared to other transition metals. This review mainly focuses on the recent research and development of the Co-catalyzed intramolecular [2 + 2 + 2] cycloaddition of diynes-nitriles or intermolecular [2 + 2 + 2] cycloaddition reaction of alkynes or diynes with nitriles for the construction of chiral or achiral multi-substituted pyridines. Meanwhile, brief mechanistic insights are also discussed here to explain the observed regioselectivity.