Iridium-Catalyzed [2 + 2 + 2] Cycloaddition of α,ω-Diynes with Nitriles

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Abstract
[Ir(cod)Cl]2/DPPF or BINAP efficiently catalyzed the cycloaddition of α,ω-diynes with nitriles to give pyridines. The reaction can accommodate a very wide range of nitriles. Both aliphatic and aromatic nitriles smoothly reacted with α,ω-diynes to give pyridines. Ten equivalents of unactivated aliphatic nitrile were enough to give the product in high yield. Aliphatic nitriles bearing an acetal or amino moiety could be used for the reaction. The highly regioselective cycloaddition of unsymmetrical diyne bearing two different internal alkyne moieties was achieved. The observed regioselectivity could be reasonably explained by considering the different reactivities of the α-position in iridacyclopentadiene. Regioselective cycloaddition was successfully applied to the synthesis of terpyridine and quinquepyridine. This chemistry was extended to a new and efficient synthesis of oligoheteroarenes. Five aromatic or heteroaromatic rings were connected in a single operation. [Ir(cod)Cl]2/chiral diphosphine catalyst can be applied to enantioselective synthesis. Kinetic resolution of the racemic secondary benzyl nitrile catalyzed by [Ir(cod)Cl]2/SEGPHOS gave a central carbon chiral pyridine in 80% ee. The mechanism was analyzed on the basis of the B3LYP level of density functional calculations.

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