Preliminary mechanistic studies revealed the following: 1) the glycosyl
radical is generated during the process; 2) a radical chain mechanism
may be ruled out; 3) the glycosyl radical could be generated directly
from the reduction of furanosyl acetate by zinc in the presence of
TMSBr. Based on these results obtained and Doyle’s elegant studies,[29] the authors proposed a mechanism shown in
Scheme 20. A rapid oxidative addition of Ni(0) catalyst 143 to
an aryl iodide produces the aryl-Ni(II) complex 144 , which is
captured by a glycosyl radical 146 , affording a Ni(III) adduct145 . Subsequent reductive elimination of this intermediate can
furnish the desired aryl C-glycosides 139 and Ni(I) species148 . This Ni(I) species is reduced by Zn to regenerate active
Ni(0) catalyst. Of note, an alternative reaction pathway for the
generation of Ni(III) species 145 cannot be ruled out. The
Ni(II) aryl complex 144 was reduced by Zn, and the resulting
Ni(I) aryl complex 149 reduces the glycosyl bromide toward a
glycosyl radical, companying with the generation of Ni(II) 150 .
Further combining these two species would generate the same Ni(III)
species 145 .
Scheme 20 Plausible mechanism