Assembling Contiguous Quaternary Carbon Atoms: Regio- and Stereoselective Rearrangements in Cobalt-Directed Radical Reactions of 1,4-Enynes

Radical coupling reactions of Co2(CO)6-complexed
1,4-enynes occur in a regio- and stereoselective fashion, providing
access to 3E,7E-decadiene-1,9-diynes
in excellent yields (84–99%). The formation of contiguous quaternary
carbon atoms follows a tandem allylic rearrangement that projects
an original reaction site gamma to the metal core. Propargyl alcohols
with an α-alkenyl group as a substituent are treated with HBF4, followed by the reduction of the highly conjugated propargyl
cations with zinc. The scope of the reaction is expanded to include
1,4-enyne complexes with cyclic and acyclic substituents gamma to
the metal core, as well as aliphatic and aromatic substituents attached
to the acetylenic termini. The alternative design includes relocation
of the cation generation siteα-to-γprior
to the reduction step, employing either the cation isolation technique
with HBF4 or an in situ generation of
ionic propargyl triflates with Tf2O. Retention of the reaction
site in 1,3-enynes is observed in both γ-alcohols and γ-Me
ethers, affording respective γ,γ-radical dimers in excellent
yields (98–99%).