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

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′-functionalization in propargyl systems. Heterolytic bond dissociation energy (BDE) values are used to quantitate the impact of alpha substituents, to identify the “ionic” and “covalent” domains for electronically diverse propargyl triflates (ionic: BDEhet 238–271 kcal/mol; covalent: BDEhet 277–315 kcal/mol), and also to make predictions for new types of substituents and new classes of organic compounds.