Acquiring a Prognostic Power in Co2(CO)6-Mediated, Cobaltocene-Induced Radical Dimerizations of Propargyl Triflates

Abstract

Cobalt-complexed propargyl triflates can be generated in situ from methyl propargyl ethers and triflic anhydride and then reduced with cobaltocene to topologically and functionally diverse 1,5-alkadiynes. The electronic effect of an α-substituent is shown to attenuate the ionic nature of an α-C–OTf bond and thus its reducibility with cobaltocene. The powerful π-donors, such as phenyl, naphthyl, alkenyl, alkynyl, and alkoxy groups, provide the ionicity of α-C–OTf bonds and make them suitable recipients for a single-electron delivery from cobaltocene. σ-Donors (alkyl groups), a H atom, and σ/π-acceptors (ester groups) do not sufficiently stabilize propargyl cations, maintaining the covalent nature of α-C–OTf bonds and making them resistant toward the reducing agent. A newly acquired ability to differentiate between the α-C–OTf bonds is used in polyethers for the regioselective reduction and radical dimerization in select propargylic positions, thus paving the way for a long sought after radical-ionic α,α′-f...