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.