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

[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.