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