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.
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