Cross-Coupling of Cobalt-Complexed Propargyl Radicals:  Metal Core- and α- and γ-Aryl-Induced Chemo- and Diastereoselectivity

Chemo- and diastereoselectivities of homo- and cross-coupling reactions of Co2(CO)6-complexed propargyl radicals were studied. The alternative radical generation methodsreduction of respective cations with Zn or Cp2Co and one-step mediation with THF or Tf2Owere employed. In the case of cobalt complexes with terminal triple bonds, the product distribution is nearly statistical and dependent upon the reducing agent with the concentration of the cross-coupling product 8 falling in the range 38−49%. The diastereoselectivity varied widely (de 40−92%) with the preponderant formation of d,l-diastereoisomers 7−9 in both homo- and cross-coupling reactions. The highest level of stereocontrol was achieved in THF- and Tf2O-mediated reactions (de up to 92%), while the reductions with Cp2Co were inferior in both homo- and cross-coupling processes (de 40−56%). An introduction of a γ-aromatic ring revealed a kinetic differentiation at the radical generation step that, in turn, resulted in a nonstatistical distribution of homo- and cross-dimers. The d,l-diastereoselectivity is found to be systematically lower for homo-dimer 16, containing a γ-phenyl substituent at the triple bond (de: 16 14−64%; 7 52−84%). The observed chemoselectivities are accounted for on the basis of the computed values of charge distribution in the requisite cations.