Ni(NHC)]-Catalyzed Cycloaddition of Diynes and Tropone: Apparent Enone Cycloaddition Involving an 8π Insertion

A Ni/N-heterocyclic
carbene catalyst couples diynes to the C­(α)–C­(β)
double bond of tropone, a type of reaction that is unprecedented for
metal-catalyzed cycloadditions with aromatic tropone. Many different
diynes were efficiently coupled to afford [5–6–7] fused
tricyclic products, while [5–7–6] fused tricyclic compounds
were obtained as minor byproducts in a few cases. The reaction has
broad substrate scope and tolerates a wide range of functional groups,
and excellent regioselectivity is found with unsymmetrical diynes.
Theoretical calculations show that the apparent enone cycloaddition
occurs through a distinctive 8π insertion of tropone. The initial
intramolecular oxidative cyclization of diyne produces the nickelacyclopentadiene
intermediate. This intermediate undergoes an 8π insertion of
tropone, and subsequent reductive elimination generates the [5–6–7]
fused tricyclic product. This initial product undergoes two competing
isomerizations, leading to the observed [5–6–7] and
[5–7–6] fused tricyclic products.