Radical Reactions of the Cobalt-Complexed Propargyl Acetals: Inter- and Intramolecular Variants

Propargyl acetals are used as a new
type of substrate in cobaltocene-induced
radical reactions stereodirected by a π-bonded Co2(CO)6 metal core. The experimental protocol involves treatment
of the cobalt-complexed acetals with triflic anhydride, an in situ
generation of ionic propargyl triflates, and a single-electron, low-temperature
reduction of α-alkoxy-stabilized, Co2(CO)6-complexed propargyl cations. Inter- and intramolecular reactions
provide a facile access to polyfunctional, topologically diverse 3,4-dialkoxy-1,5-alkadiynes
and 3,4-dialkoxy-1,5-cyclodecadiynes with a predominant formation
of the respective d,l-diastereomers (73–100% d,l). The stereo- and chemoselectivities are found to be
dependent upon the (1) bulkiness of the gamma substituent (1,3-steric induction; d,l- 73–100%),
(2) presence of a phosphorus ligand altering the π-bonded metal
core (axial ligand induction; d,l- 91–97%), and (3) steric hindrance created by an alpha alkoxy
group (1,1-steric induction; d,l- 83–92%).