Based on the above information, a plausible dual-catalytic mechanism was
proposed, as described in Scheme 10-A. First, the tolyl sulfinate is
oxidized by *Ru(bpy)32+[E1/2*II/I (half-wave
potential = +0.77 V versus saturated calomel electrode; τ (lifetime) =
1,100 ns], affording a sulfonyl radical. The resulting tolyl sulfonyl
radical then adds to the terminal alkene group of the glycosyl donor73 , triggering a cascade of bond cleavage and sulfur dioxide
(SO2) loss to produce glycosyl radical 78 , via76 and 77 . Meanwhile, in the nickel catalytic cycle,
oxidative addition of Ni(0) to aryl halide produces Aryl-Ni-X
intermediate 79 . Subsequently, capturing 79 by
glycosyl radical leads to 80 , followed by reductive elimination
to deliver the desired aryl C-glycoside 75 and releases Ni(I).
The resulting Ni(I) species is reduced by Ru(I) to regenerate Ru(II) and
active Ni(0) catalyst, closing the catalytic cycles. An alternative
pathway cannot be ruled out, which involves the addition of glycosyl
radical to Ni(0) species. [14] Moreover, DFT
calculations were performed to provide a deeper understanding of the
origin of the stereochemical outcome.
A key finding was that the radical addition to the aryl nickel complex
is reversible, and the reductive elimination step is the rate-limiting
step. Within the transition structure of reductive elimination step, the
interaction between the C2-OH group and the ligated metal unit plays a
crucial role in determining the stereochemical outcome.
Scheme 11 Photoredox/nickel dual-catalyzed glycosylation