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

摘要

Radical coupling reactions of Co-2(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 a-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 sitea-to-gamma 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 alpha-alcohols and gamma-Me ethers, affording respective gamma,gamma-radical dimers in excellent yields (98-99%).