Tetrahydrofuran-Mediated Stereoselective Radical C−C Bond Formation in Dicobalthexacarbonyl−Propargyl Complexes

An extensive study on the mechanism of novel tetrahydrofuran (THF)-mediated coupling of Co 2 (CO) 6 -complexed propargyl alcohols and cations was carried out. On the basis of the stoichiometry of the process, the measurement of the kinetic isotope effect (KIE) in the competitive and noncompetitive settings (THF- d 0 and THF- d 8 ), ligand substitution experiments with 13 CO, kinetic studies with model compounds Co 2 (CO) 6 -complexed 1-phenyl-2-propyne ( 6 ), tetrahydrothiophene, cobalt−alkyne anchored tetrahydropyran (THP, 21 ) and ab initio calculations, the mechanism of the reaction was proposed. It includes a reversible coordination of two THF molecules with an α-cationic center in a π-bonded propargyl moiety, followed by a single-electron transfer from the THF-sandwiched cobalt complex toward an electron-deficient propargyl cation. Although used in a 2-fold excess, THF acts as a catalyst, altering, both electronically and structurally, the requisite Co 2 (CO) 6 -complexed cations and breaking away from the organometallic scaffold, upon radical generation, in a chemically unchanged form. By triggering disproportionation between cobalt-complexed propargyl cations, THF acts in an unusual capacity of a radical mediator, previously being known in organic chemistry as a Lewis donor and donor of H atoms and hydride ions. The novel process that provides a facile entry to d, l -3,4-diphenyl-1,5-hexadiyne can be expanded toward stereo-, chemo-, and regioselective synthesis of polysubstituted d, l -3,4-diaryl-1,5-alkadiynes otherwise hardly accessible.