DFT calculations and experimental observations, a plausible mechanism
was proposed, as depicted in Scheme 27-A. The oxidative addition of
Ni(0) species 199 to 196 followed by reduction and
(net)decarboxylation provides a CO-coordinated Ni(0) complex200 . Subsequent oxidative addition of organoiodides197 affords a Ni(II) species 201 , which is then
converted to a Ni(II)-acyl complex 202 after carbonyl
insertion. Another reduction enables the formation of Ni(I) species203 , which can undergo a stepwise oxidative addition with195 to furnish Ni(III) intermediate 204 . During the
process, halogen atom transfer (XAT) and radical re-association steps
are involved, which provides the rationale for the observed high
selectivity towards the formation of the carbonylative cross-coupling
product. Finally, the reductive elimination of Ni(III) species204 produces the desired glycosyl ketone product 198and Ni(I) species 205 . The reduction of this nickel species
forms active Ni(0) 199 . Moreover, an alternative radical relay
mechanism can be excluded based on calculated energetics.
2.3. Alkyl C-glycoside synthesis
The facile β-elimination of C1-substituted glycosyl metallics may
explain why fully oxygenated and saturated carbohydrate structures are
not typically reachable via transition metal-mediated cross-coupling.[36] Elegant studies of pincer-ligated
organometallic complexes presented that such ligands can effectively
inhibit undesired β-hydrogen elimination by occupying the cis
coordination sites essential for the elimination.[37] In this context, the Gagné group developed a
Negishi cross-coupling approach to access alkyl C-glycosides, where
NiCl2/PyBox was employed to diminish the formation of
undesired glucal (Scheme 28). [38]
Scheme 28 Synthesis of alkyl C-glycoside via Negishi
cross-coupling