Mechanistically, the authors suggested the generation of photoactive
electron-donor-acceptor complexes between the redox-active electrophile
(NHPI ester or pyridinium salt), Hantzsch ester, and LiI (or
Et3N) are central to the success of the developed
approaches, which was supported by spectroscopy experiments. A detailed
mechanism was proposed, as described in Scheme 31. The photoinduced
single-electron transfer enables the generated EDA complex to undergo
decarboxylative or deaminative fragmentation, affording alkyl radical
species 222 . In a concurrent nickel-catalyzed cycle, a
Ni(0)-mediated halogen atom abstraction enabled the formation of
glycosyl radical from a glycosyl halide 215 . The resulting
glycosyl radical diastereoselectively re-associates with the nickel
center to produce a glycosyl-Ni(II) intermediate 228 , followed
by trapping of alkyl radical 222 generated from EDA, furnishing
a glycosyl-Ni(III)-alkyl species 229 . Subsequent reductive
elimination of this crucial intermediate produces the desired alkyl
C-glycoside adduct 218 . The active nickel catalyst 226is regenerated by a single-electron reduction of 227 .
Scheme 31 Plausible mechanism