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

Abstract

Chemo- and diastereoselectivities of homo- and cross-coupling reactions of Co2(CO)6-complexed propargyl radicals were studied. The alternative radical generation methodsreduction of respective cations with Zn or Cp2Co and one-step mediation with THF or Tf2Owere 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.